Publications

Fourier Transform (FT) has been a mainstay of analytical 13C/15N NMR. On the other hand, it has been shown that Steady-State Free Precession (SSFP) experiments, which depart from this scheme, can, under certain conditions, endow 13C/15N small-molecule NMR with comparable sensitivity and resolution. SSFP is one of the earliest and most widely used NMR pulse sequences, yet its analyses have focused on isolated spin-1/2 ensembles such as water. The present study demonstrates that significant deviations from such isolated spin-1/2 behavior may occur when SSFP is applied in the presence of spinspin couplings. Even in the simplest case supporting such couplings, a single 13C J-coupled to a 1H, departures from the isolated spin-1/2 behavior arise in the 13C SSFP responseboth in the absence and in the presence of 1H spin decoupling. In the former case, deviations are produced by the differential relaxation of antiphase two-spin terms generated by the pulse train; in the latter case, magnified interferences may arise between the SSFP pulses and the coherent perturbation arising upon 1H decoupling. Although both phenomena are also known in FT NMR, the spectral distortions that they will originate may be much larger in the SSFP caseparticularly if interpulse delays in large flip-angle SSFP pulse trains \u201cresonate\u201d with the coupling perturbations. The origins of these effects are here analyzed for heteronuclear spin-1/2 systems and corroborated with 13C NMR SSFP experiments recorded under different conditions. Additional considerations aimed at magnifying or suppressing these effects, as well as extensions to more complex scenarios, are also briefly discussed.

The value of preclinical diffusion MRI (dMRI) is substantial. While dMRI enables in vivo non-invasive characterization of tissue, ex vivo dMRI is increasingly being used to probe tissue microstructure and brain connectivity. Ex vivo dMRI has several experimental advantages including higher SNR and spatial resolution compared to in vivo studies, and enabling more advanced diffusion contrasts for improved microstructure and connectivity characterization. Another major advantage of ex vivo dMRI is the direct comparison with histological data, as a crucial methodological validation. However, there are a number of considerations that must be made when performing ex vivo experiments. The steps from tissue preparation, image acquisition and processing, and interpretation of results are complex, with many decisions that not only differ dramatically from in vivo imaging of small animals, but ultimately affect what questions can be answered using the data. This work represents \u201cPart 2\u201d of a three-part series of recommendations and considerations for preclinical dMRI. We describe best practices for dMRI of ex vivo tissue, with a focus on the value that ex vivo imaging adds to the field of dMRI and considerations in ex vivo image acquisition. We first give general considerations and foundational knowledge that must be considered when designing experiments. We briefly describe differences in specimens and models and discuss why some may be more or less appropriate for different studies. We then give guidelines for ex vivo protocols, including tissue fixation, sample preparation, and MR scanning. In each section, we attempt to provide guidelines and recommendations, but also highlight areas for which no guidelines exist (and why), and where future work should lie. An overarching goal herein is to enhance the rigor and reproducibility of ex vivo dMRI acquisitions and analyses, and thereby advance biomedical knowledge.

This study discusses a potential route for enhancing 1H NMR signals in the liquid phase at high magnetic fields for samples on the 100 mu L volume scale using dynamic nuclear polarization (DNP). The approach involves dispersing an inert powder that is both rich in protons and capable of undergoing DNP with good efficiency at noncryogenic temperatures, and letting the solid 1H polarization thus enhanced pass from the dispersed particles onto the surrounding liquid via spontaneous cross relaxation effects. To this end, BDPA-doped polystyrene (PS) particles in the mu m range were suspended in 30 mu L of heptane, loaded into 3.2 mm sapphire rotors, and spun at approximate to 500 Hz for homogeneity purposes in a 14.1 T magnet. Irradiation with approximate to 13 W at 395 GHz while maintaining temperature in the 185-220 K range thus enhanced the PS proton polarization approximate to 12-fold within approximate to 2 s; after ca. 6 s of irradiation, this resulted in ca. 3-fold enhancements of the heptane proton resonances, while preserving their

Objectives Deuterium (²H) metabolic imaging (DMI) is an emerging magnetic resonance technique to non-invasively map human brain glucose (Glc) uptake and downstream metabolism following oral or intravenous administration of ²H-labeled Glc. The achievable spatial resolution is limited due to inherently low sensitivity of DMI. This hinders potential clinical translation. The purpose of this study was to improve the signal-to-noise ratio (SNR) of 3D DMI via a balanced steady-state free precession (bSSFP) acquisition scheme combined with fast non-Cartesian spatial-spectral sampling to enable high-resolution dynamic imaging of neural Glc uptake and glutamate+glutamine (Glx) synthesis of the human brain at 7T. Materials and Methods Six healthy volunteers (2 f/4 m) were scanned after oral administration of 0.8 g/kg [6,6]-²H-Glc using a novel density-weighted bSSFP acquisition scheme combined with fast 3D concentric ring trajectory (CRT) k-space sampling at 7T. Time-resolved whole brain DMI datasets were acquired for approximately 80 minutes (7 minutes per dataset) after oral ²H-labeled Glc administration with 0.75 mL and 0.36 mL isotropic spatial resolution and results were compared to conventional spoiled Free Induction Decay (FID) ²H-MRSI with CRT readout at matched nominal spatial resolution. Dynamic DMI measurements of the brain were accompanied by simultaneous systemic Glc measurements of the interstitial fluid using a continuous Glc monitoring (CGM) sensor (on the upper arm). The correlation between brain and interstitial Glc levels was analyzed using linear mixed models. Results The bSSFP-CRT approach achieved SNRs that were up to 3-fold higher than conventional spoiled FID-CRT ²H-MRSI. This enabled a 2-fold higher spatial resolution. Seventy minutes after oral tracer uptake comparable ²H-Glc, ²H-Glx, and ²H-water concentrations were detected using both acquisition schemes at both, regular and high spatial resolutions (0.75 ml and 0.36 mL isotropic). The mean Areas Under the Curve (AUC) for interstitial fluid Glc measurements obtained using a CGM sensor was 509 ± 65 mM·min. This is 3.4 times higher than the mean AUC of brain Glc measurements of 149 ± 43 mM·min obtained via DMI. The linear mixed models fitted to assess the relationship between CGM measures and brain ²H-Glc yielded statistically significant slope estimates in both GM (β1 = 0.47, P = 0.01) and WM (β1 = 0.36, P = 0.03). Conclusions In this study we successfully implemented a balanced steady-state free precession (bSSFP) acquisition scheme for dynamic whole-brain human DMI at 7T. A 3-fold SNR increase compared to conventional spoiled acquisition allowed us to double the spatial resolution achieved using conventional FID-CRT DMI. Systemic continuous glucose measurements, combined with dynamic DMI, demonstrate significant potential for clinical applications. This could help improve our understanding of brain glucose metabolism by linking it to time-resolved peripheral glucose levels. Importantly, these measurements are conducted in a minimally invasive and physiological manner.

Purpose: Spatiotemporal encoding (SPEN) MRI offers a unique alternative to address image distortion problems in echo planar acquisition-based techniques, at portable low-field systems that lack multiple receiver coils. However, existing 2-π multislice SPEN schemes fail to keep consistent SNRs and contrasts with different numbers of slice settings. This work proposes a new multislice SPEN scheme (SPENms) to achieve stable quality imaging in portable low-field MRI systems. Methods: The proposed SPENms includes the insertion of one selective π pulse and one non-selective π pulse, closely arranged together, before the frequency-swept π pulse in the original 2D SPEN sequence. Theoretical simulations and experiments on phantoms and human brains were conducted to validate its SNR and contrast performances under different parameters compared to the existing 2-π multislice SPEN scheme. Results: Both simulations and experiments demonstrate the consistent image quality of SPENms with different scanning parameters and targets, as well as good distortion resistance and scan efficiency. Robust diffusion weighted multislice SPEN images of diagnostic value were also highlighted. Conclusion: SPENms provides a robust fast echo planar acquisition approach to obtain multislice 2D images with less distortions, consistent SNRs and contrasts at portable low-field MRI systems.

Deuterium (²H) MRI is an emerging tool for noninvasive imaging. We explore the integration of ²H MRI with deuterated multifunctional nanopolymers for deuterated particle imaging (DPI). To this end, amine-terminated G5-polyamidoamine (PAMAM) dendrimers were labeled with deuterated acetyl surface groups, leading to highly ²H-loaded bioparticles, making them ideal for imaging studies. The accumulation of ∼5 nm PAMAM dendrimers in the kidneys could then be seen by ²H MRI with high submillimeter resolution. The natural abundance HDO signal provided an internal concentration reference to these measurements, leading to quantitative dynamic maps showing distinct nanopolymer uptakes within the renal compartments. Further, these nanopolymers allowed us to obtain in vivo maps of activity in the lymph nodes in an inflammatory rodent leg model, demonstrating these deuterated nanopolymers potential as a novel class of contrast agents for the quantitative mapping of physiological processes.

The ability to obtain quantitative spatial information on subcellular processes of deep tissues in vivo has been a long-standing challenge for molecular magnetic resonance imaging (MRI) approaches. This challenge remains even more so for quantifying readouts of genetically engineered MRI reporters. Here, we set to overcome this challenge with a molecular system designed to obtain quantitative ²H-MRI maps of a gene reporter. To this end, we synthesized deuterated thymidine, d₃-thy, with three magnetically equivalent deuterons at its methyl group (-CD₃), showing a singlet peak with a characteristic ²H-NMR frequency (δ = 1.7 ppm). The upfield 3.0 ppm offset from the chemical shift of the HDO signal (δ = 4.7 ppm) allows for spectrally resolving the two ²H NMR signals and quantifying the concentration of d₃-thy based on the known concentration of a tissues HDO. Following systemic administration of d₃-thy, its accumulation as d₃-thy monophosphate in cells expressing the human thymidine kinase 1 (hTK1) transgene was mapped with ²H-MRI. The data obtained in vivo show the ability to use the d₃-thy/hTK1 pair as a reporter probe/reporter gene system to quantitatively map transgene expression with MRI. Relying on a structurally unmodified reporter probe (d₃-thy) to image the expression of unmutated human protein (hTK1) shows the potential of molecular imaging with ²H-MRI to monitor gene reporters and other relevant biological targets.

Aberrations in metabolism after intracerebral hemorrhage (ICH), particularly lactate metabolism, play a crucial role in the pathophysiology and patient outcome. To date, the evaluation of metabolism relies heavily on invasive methods such as microdialysis, restricting a comprehensive understanding of the metabolic mechanisms associated with ICH. This study proposes a noninvasive metabolic imaging method based on ²H magnetic resonance spectroscopy and imaging (²H-MRS/MRSI) to detect metabolic changes after ICH in vivo. To overcome the low-sensitivity limitation of ²H, we designed a new ¹H-²H double-resonance coil with ²H-channel active detuning and proposed chemical shift imaging based on the balanced steady-state free precession method (CSI-bSSFP). Compared with the volume coil, the signal-to-noise ratio (SNR) of the new coil was increased by 4.5 times. In addition, the SNR of CSI-bSSFP was 1.5 times higher than that of conventional CSI. These two technologies were applied to measure lactate metabolic flux at different phases of ICH. The results show a higher lactate concentration in ICH rats than in control rats, which is in line with the increased expression of lactate dehydrogenase measured via immunohistochemistry staining (AUC_{Lac_area/Glc_area}: control, 0.08 ± 0.02 vs ICH-3d, 0.39 ± 0.05 vs ICH-7d, 0.18 ± 0.02, P 2H-MRS/MRSI shows potential as a useful method for in vivo metabolic imaging and noninvasive assessment of ICH.

Nuclear magnetic resonance (NMR) plays a central role in the elucidation of chemical structures but is often limited by low sensitivity. Dissolution dynamic nuclear polarization (dDNP) emerges as a transformative methodology for both solution-state NMR and metabolic NMR imaging, which could overcome this limitation. Typically, dDNP relies on combining a stable radical with the analyte within a uniform glass under cryogenic conditions. The electron polarization is then transferred through microwave irradiation to the nuclei. The present study explores the use of radicals introduced via γ-irradiation, as bearers of the electron spins that will enhance ¹H or ¹³C nuclides. ¹H solid-state NMR spectra of γ-irradiated powders at 1-5 K revealed, upon microwave irradiation, signal enhancements that, in general, were higher than those achieved through conventional glass-based DNP. Transfer of these samples to a solution-state NMR spectrometer via a rapid dissolution driven by a superheated water provided significant enhancements of solution-state ¹H NMR signals. Enhancements of ¹³C signals in the γ-irradiated solids were more modest, as a combined consequence of a low radical concentration and of the dilute concentration of ¹³C in the natural abundant samples examined. Nevertheless, ca. 700-800-fold enhancements in ¹³C solution NMR spectra of certain sites recorded at 11.7 T could still be achieved. A total disappearance of the radicals upon performing a dDNP-like aqueous dissolution and a high stability of the samples were found. Overall, the study showcases the advantages and limitations of γ-irradiated radicals as candidates for advancing spectroscopic dDNP-enhanced NMR.

Purpose: Demonstrate the potential of spatiotemporal encoding (SPEN) MRI to deliver largely undistorted 2D, 3D, and diffusion weighted images on a 110 mT portable system. Methods: SPEN's quadratic phase modulation was used to subsample the low-bandwidth dimension of echo planar acquisitions, delivering alias-free images with an enhanced immunity to image distortions in a laboratory-built, low-field, portable MRI system lacking multiple receivers. Results: Healthy brain images with different SPEN time-bandwidth products and subsampling factors were collected. These compared favorably to EPI acquisitions including topup corrections. Robust 3D and diffusion weighted SPEN images of diagnostic value were demonstrated, with 2.5 mm isotropic resolutions achieved in 3 min scans. This performance took advantage of the low specific absorption rate and relative long TEs associated with low-field MRI. Conclusion: SPEN MRI provides a robust and advantageous fast acquisition approach to obtain faithful 3D images and DWI data in low-cost, portable, low-field systems without parallel acceleration.

Cancer diagnosis by metabolic MRI proposes to follow the fate of glycolytic precursors such as pyruvate or glucose, and their in vivo conversion into lactate. This study compares the 2H MRI outlooks afforded by these metabolites when targeting a pancreatic cancer model. Exogenously injected [3,3',3-2H3]-pyruvate was visible only briefly; it generated a deuterated lactate signal throughout the body that faded after ~5 min, showing a minor concentration bias at the rims of the tumors. [6,6'-2H2]-glucose by contrast originated a lactate signal that localized clearly within the tumors, persisting for over an hour. Investigations alternating deuterated and nondeuterated glucose injections revealed correlations between the lactate generation and the glucose available at the tumor, evidencing a continuous and avid glucose consumption generating well-localized lactate signatures as driven by the Warburg effect. This is by contrast to the transient and more promiscuous pyruvate-to-lactate transformation, which seemed subject to transporter and kinetics effects. The consequences of these observations within metabolic MRI are briefly discussed.

Quantitative correlations between T2 and ADC values were explored on cancerous breast lesions using spatiotemporally encoded (SPEN) MRI. To this end, T2 maps of patients were measured at more than one b-value, and ADC maps at several echo time values were recorded. SPEN delivered quality, artifact-free, TE-weighted DW images, from which T2-ADC correlations could be obtained despite the signal losses brought about by diffusion and relaxation. Data confirmed known aspects of breast cancer lesions, including their reduced ADC values vs. healthy tissue. Data also revealed an anticorrelation between the T2 and ADC values, when comparing regions with healthy and diseased tissues. This is contrary to expectations based on simple water restriction considerations. It is also contrary to what has been observed in a majority of porous materials and tissues. Differences between the healthy tissue of the lesion-affected breast and healthy tissue in the contralateral breast were also noticed. The potential significance of these trends is discussed, as is the potential of combining T2- and ADC-weightings to achieve an enhanced endogenous MRI contrast about the location of breast cancer lesions.

A recently developed homonuclear dipolar recoupling scheme, Adiabatic Linearly FREquency Swept reCOupling (AL FRESCO), was applied to record two-dimensional (2D) ¹⁵N¹⁵N correlations on uniformly ¹⁵N-labeled GB1 powders. A major feature exploited in these ¹⁵N¹⁵N correlations was AL FRESCOs remarkably low RF power demands, which enabled seconds-long mixing schemes when establishing direct correlations. These ¹⁵N¹⁵N mixing schemes proved efficient regardless of the magic-angle spinning (MAS) rate and, being nearly free from dipolar truncation effects, they enabled the detection of long-range, weak dipolar couplings, even in the presence of strong short-range dipolar couplings. This led to a connectivity information that was significantly better than that obtained with spontaneously proton-driven, ¹⁵N spin-diffusion experiments. An indirect approach producing long-range ¹⁵N¹⁵N correlations was also tested, relying on short (ms-long) ¹H^N1H^N mixings schemes while applying AL FRESCO chirped pulses along the ¹⁵N channel. These indirect mixing schemes produced numerous long-distance N_iN_i±n (n = 2 − 5) correlations, that might be useful for characterizing three-dimensional arrangements in proteins. Once again, these AL FRESCO mediated experiments proved more informative than variants based on spin-diffusion-based ¹H^N1H^N counterparts.

Deuterium metabolic imaging (DMI) is a promising molecular MRI approach, which follows the administration of deuterated substrates and their metabolization. [6,6-²H₂]-glucose for instance is preferentially converted in tumors to [3,3-²H₂]-lactate as a result of the Warburg effect, providing a distinct resonance whose mapping using time-resolved spectroscopic imaging can diagnose cancer. The MR detection of low-concentration metabolites such as lactate, however, is challenging. It has been recently shown that multi-echo balanced steady-state free precession (ME-bSSFP) increases the signal-to-noise ratio (SNR) of these experiments approximately threefold over regular chemical shift imaging; the present study examines how DMI's sensitivity can be increased further by advanced processing methods. Some of these, such as compressed sensing multiplicative denoising and block-matching/3D filtering, can be applied to any spectroscopic/imaging methods. Sensitivity-enhancing approaches were also specifically tailored to ME-bSSFP DMI, by relying on priors related to the resonances' positions and to features of the metabolic kinetics. Two new methods are thus proposed that use these constraints for enhancing the sensitivity of both the spectral images and the metabolic kinetics. The ability of these methods to improve DMI is evidenced in pancreatic cancer studies carried at 15.2 T, where suitable implementations of the proposals imparted eightfold or more SNR improvement over the original ME-bSSFP data, at no informational cost. Comparisons with other propositions in the literature are briefly discussed.

One of solution-state Nuclear Magnetic Resonance (NMR)s main weaknesses, is its relative insensitivity. J-driven Dynamic Nuclear Polarization (JDNP) was recently proposed for enhancing solution-state NMR's sensitivity, by bypassing the limitations faced by conventional Overhauser DNP (ODNP), at the high magnetic fields where most analytical research is performed. By relying on biradicals with inter-electron exchange couplings J_ex on the order of the electron Larmor frequency ω_E, JDNP was predicted to introduce a transient enhancement in NMR's nuclear polarization at high magnetic fields, and for a wide range of rotational correlation times of medium-sized molecules in conventional solvents. This communication revisits the JDNP proposal, including additional effects and conditions that were not considered in the original treatment. These include relaxation mechanisms arising from local vibrational modes that often dominate electron relaxation in organic radicals, as well as the possibility of using biradicals with J_ex of the order of the nuclear Larmor frequency ω_N as potential polarizing agents. The presence of these new relaxation effects lead to variations in the JDNP polarization mechanism originally proposed, and indicate that triplet-to-singlet cross-relaxation processes may lead to a nuclear polarization enhancement that persists even at steady states. The physics and potential limitations of the ensuing theoretical derivations, are briefly discussed.

Efficient acquisition of wideline solid-state nuclear magnetic resonance (NMR) spectra with patterns affected by large inhomogeneous broadening is accomplished with the use of broadband pulse sequences. These specialized pulse sequences often use frequency-swept pulses, which feature time-dependent phase and amplitude modulations that in turn deliver broad and uniform excitation across large spectral bandwidths. However, the resulting NMR spectra are often affected by complex frequency-dependent phase dispersions, owing to the interplay between the frequency-swept excitations and anisotropic resonance frequencies. Such phase distortions necessitate the use of multi-order non-linear corrections in order to obtain absorptive, distortion-free patterns with uniform phasing. Performing such corrections is often challenging due to the complex interdependence of the linear and non-linear phase contributions, and how these may affect the NMR signal. Hence, processing of these data usually involves calculating the spectra in magnitude mode wherein the phase information is discarded. Herein, we present a fully automated phasing routine that is capable of processing and phase correcting such wideline NMR spectra. Its performance is corroborated via processing of NMR data acquired using both the WURST-CPMG (Wideband, Uniform-Rate, Smooth Truncation with Carr-Purcell Meiboom-Gill acquisition) and BRAIN-CP (BRoadband Adiabatic Inversion Cross Polarization) pulse sequences for a variety of nuclei (i.e., ¹¹⁹Sn, ¹⁹⁵Pt, ³⁵Cl, ⁸⁷Rb, and ¹⁴N). Based on both simulated and experimental NMR datasets, it is demonstrated that automatic phase corrections up to and including second order can be readily achieved without a priori information regarding the nature of the phase-distorted NMR datasets, and independently of the exact manner in which time-domain NMR data are collected and subsequently processed. In addition, it is shown that NMR spectra acquired at both single and multiple transmitter frequencies that are processed with this automated phasing routine have improved signal-to-noise properties than those processed with conventional magnitude calculations, along with powder patterns that better match those of ideal NMR spectra, even for datasets possessing low signal-to-noise ratios and/or affected by spectral artifacts.

Thanks to its increased sensitivity, single-shot ultrahigh field functional MRI (UHF fMRI) could lead to valuable insight about subtle brain functions such as olfaction. However, UHF fMRI experiments targeting small organs next to air voids, such as the olfactory bulb, are severely affected by field inhomogeneity problems. Spatiotemporal Encoding (SPEN) is an emerging single-shot MRI technique that could provide a route for bypassing these complications. This is here explored with single-shot fMRI studies on the olfactory bulbs of male and female mice performed at 15.2T. SPEN images collected on these organs at a 108 µm in-plane resolution yielded remarkably large and well-defined responses to olfactory cues. Under suitable T2* weightings these activation-driven changes exceeded 5% of the overall signal intensity, becoming clearly visible in the images without statistical treatment. The nature of the SPEN signal intensity changes in such experiments was unambiguously linked to olfaction, via single-nostril experiments. These experiments highlighted specific activation regions in the external plexiform region and in glomeruli in the lateral part of the bulb, when stimulated by aversive or appetitive odors, respectively. These strong signal activations were non-linear with concentration, and shed light on how chemosensory signals reaching the olfactory epithelium react in response to different cues. Second-level analyses highlighted clear differences among the appetitive, aversive and neutral odor maps; no such differences were evident upon comparing male against female olfactory activation regions.

Purpose: To characterize the mechanism of formation and the removal of aliasing artifacts and edge ghosts in spatiotemporally encoded (SPEN) MRI within a k-space theoretical framework. Methods: SPEN's quadratic phase modulation can be described in k-space by a convolution matrix whose coefficients derive from Fourier relations. This k-space model allows us to pose SPEN's reconstruction as a deconvolution process from which aliasing and edge ghost artifacts can be quantified by estimating the difference between a full sampling and reconstructions resulting from undersampled SPEN data. Results: Aliasing artifacts in SPEN MRI reconstructions can be traced to image contributions corresponding to high-frequency k-space signals. The k-space picture provides the spatial displacements, phase offsets, and linear amplitude modulations associated to these artifacts, as well as routes to removing these from the reconstruction results. These new ways to estimate the artifact priors were applied to reduce SPEN reconstruction artifacts on simulated, phantom, and human brain MRI data. Conclusion: A k-space description of SPEN's reconstruction helps to better understand the signal characteristics of this MRI technique, and to improve the quality of its resulting images.

Chemical exchange saturation transfer (CEST) NMR is widely used for enhancing the sensitivity of low-abundance exchanging sites in general, and for the water-based detection of labile metabolite protons under in vivo conditions in particular. CEST, however, faces a number of limitations when targeting multiple metabolites, including a radiofrequency (RF)-induced broadening of the detected peaks, and relatively long acquisition times deriving from its continuous-wave nature. Methods have been proposed to overcome these limitations, including a Fourier-encoded version of CEST the Frequency-Labeled EXchange (FLEX) experiment and the incorporation of background gradients during the RF saturation time. This work explores an alternative avenue, based on spatiotemporally encoded ultrafast (UF) 2D NMR. UF NMR can compress the time-domain indirect-dimension encoding of 2D NMR into a single shot; to exploit these potential time savings, an UF version of the FLEX experiment was taken as starting point, and the multiple t1-incremented amplitude modulation cycles that the FLEX experiment normally requires were replaced by a single-shot spatiotemporal encoding. The ensuing UF 2D FLEX experiment was then used to monitor the spectral signatures of multiple moieties as they exchange with the solvent, by imprinting these onto the water resonance as in the original experiment but now all within a single shot. Upon incorporating two-scan phase cycling and quadrature detection, the resulting method showed an experimental performance similar to t1-encoded FLEX, while providing significant time savings plus imaging information that could be of further use in in vivo studies. The main advantages, features and drawbacks observed for UF 2D FLEX are briefly discussed.

J-driven dynamic nuclear polarization (JDNP) was recently proposed for enhancing the sensitivity of solution-state nuclear magnetic resonance (NMR), while bypassing the limitations faced by conventional (Overhauser) DNP at magnetic fields of interest in analytical applications. Like Overhauser DNP, JDNP also requires saturating the electronic polarization using high-frequency microwaves known to have poor penetration and associated heating effects in most liquids. The present microwave-free JDNP (MF-JDNP) proposal seeks to enhance solution NMR's sensitivity by shuttling the sample between higher and lower magnetic fields, with one of these fields providing an electron Larmor frequency that matches the interelectron exchange coupling Jex. If spins cross this so-called JDNP condition sufficiently fast, we predict that a sizable nuclear polarization will be created without microwave irradiation. This MF-JDNP proposal requires radicals whose singlet-triplet self-relaxation rates are dominated by dipolar hyperfine relaxation, and shuttling times that can compete with these electron relaxation processes. This paper discusses the theory behind the MF-JDNP, as well as proposals for radicals and conditions that could enable this new approach to NMR sensitivity enhancement.

The structural and chemical complexities within the brain pose a challenge that few noninvasive techniques can tackle with the dexterity of nuclear magnetic resonance (NMR) spectroscopy. Still, even with the advent of ultrahigh fields and of cryogenically cooled coils for in vivo research, the superposition of metabolic resonances arising from the brain remains a challenge. The present study explores the potential to tackle this milieu using a combination of two-dimensional (2D) NMR techniques, implemented on murine brains in vivo at 15.2 T and ex vivo at 14.1 T. While both experiments were affected by substantial inhomogeneous broadenings conveying distinct elongated lineshapes to the cross-peaks, the ability of increased fields to resolve off-diagonal resonances was clear. A comparison between the corresponding conventional and double quantum-filtered correlated spectroscopy traces enabled an improved assignment of in vivo resonances on the basis of more sensitive ex vivo 2D acquisitions, foremost on the basis of homonuclear cross-relaxation-driven correlations for peaks resonating downfield from water, and of heteronuclear correlations at natural abundance for the upfield protons. With the aid of such 2D correlations approximately 29 metabolites could be resolved and identified. This enhanced resolution was used to explore features related to the metabolites' diffusivities, their exposure to water, and their facility to undergo magnetization transfers to amide/amine/hydroxyl resonances. Cross-peaks from main murine brain biomolecules, including choline, creatine, gamma-aminobutyric acid, N-acetyl aspartate, glutamine, and glutamate, showed enhancements in several of these various features, opening interesting vistas about metabolite compartmentalization as viewed by these 2D NMR experiments.

The benefits of performing locally low-rank (LLR) reconstructions on subsampled diffusion weighted and diffusion kurtosis imaging data employing spatiotemporal encoding (SPEN) methods, is investigated. SPEN allows for self-referenced correction of motion-induced phase errors in case of interleaved diffusion-oriented acquisitions, and allows one to overcome distortions otherwise observed along EPI's phase-encoded dimension. In combination with LLR-based reconstructions of the pooled imaging data and with a joint subsampling of b-weighted and interleaved images, additional improvements in terms of sensitivity as well as shortened acquisition times are demonstrated, without noticeable penalties. Details on how the LLR-regularized, subspace-constrained image reconstructions were adapted to SPEN are given; the improvements introduced by adopting these reconstruction frameworks for the accelerated acquisition of diffusivity and of kurtosis imaging data in both relatively homogeneous regions like the human brain and in more challenging regions like the human prostate, are presented and discussed within the context of similar efforts in the field.

There are currently no methods for the acquisition of ultra-wideline (UW) solid-state NMR spectra under static conditions that enable reliable separation and resolution of overlapping powder patterns arising from magnetically distinct nuclei. This stands in contrast to the variety of techniques available for spin-1/2 or half-integer quadrupolar nuclei with narrow central transition patterns under magic-angle spinning (MAS). Resolution of overlapping signals is routinely achieved in MRI and solution-state NMR by exploiting relaxation differences between nonequivalent sites. Preliminary studies of relaxation assisted separation (RAS) for separating overlapping UWNMR patterns using pseudo-inverse Laplace Transforms have reported two-dimensional spectra featuring relaxation rates correlated to NMR interaction frequencies. However, RAS methods are inherently sensitive to experimental noise, and require that relaxation rates associated with overlapped patterns be significantly different from one another. Herein, principal component analysis (PCA) denoising is implemented to increase the signal-to-noise ratios of the relaxation datasets and RAS routines are stabilized with truncated singular value decomposition (TSVD) and elastic net (EN) regularization to resolve overlapped patterns with a larger tolerance for differences in relaxation rates. We extend these methods for improved pattern resolution by utilizing 3D frequency-R1R2 correlation spectra. Synthetic and experimental datasets, including 35Cl (I = 3/2), 2H (I = 1), and 14N (I = 1) NMR of organic and biological compounds, are explored with both regularized 2D RAS and 3D RAS; comparison of these data reveal improved resolution in the latter case. These methods have great potential for separating overlapping powder patterns under both static and MAS conditions.

NMR spectroscopy is the only method to access the structural dynamics of biomolecules at high (atomistic) resolution in their native solution state. However, this method's low sensitivity has two important consequences: (i) typically experiments have to be performed at high concentrations that increase sensitivity but are not physiological, and (ii) signals have to be accumulated over long periods, complicating the determination of interaction kinetics on the order of seconds and impeding studies of unstable systems. Both limitations are of equal, fundamental relevance: non-native conditions are of limited pharmacological relevance, and the function of proteins, enzymes and nucleic acids often relies on their interaction kinetics. To overcome these limitations, we have developed applications that involve 'hyperpolarized water' to boost signal intensities in NMR of proteins and nucleic acids. The technique includes four stages: (i) preparation of the biomolecule in partially deuterated buffers, (ii) preparation of 'hyperpolarized' water featuring enhanced H-1 NMR signals via cryogenic dynamic nuclear polarization, (iii) sudden melting of the cryogenic pellet and dissolution of the protein or nucleic acid in the hyperpolarized water (enabling spontaneous exchanges of protons between water and target) and (iv) recording signal-amplified NMR spectra targeting either labile H-1 or neighboring N-15/C-13 nuclei in the biomolecule. Water in the ensuing experiments is used as a universal 'hyperpolarization' agent, rendering the approach versatile and applicable to any biomolecule possessing labile hydrogens. Thus, questions can be addressed, ranging from protein and RNA folding problems to resolving structure-function relationships of intrinsically disordered proteins to investigating membrane interactions.

Quadrupolar solid-state NMR carries a wealth of structural information, including insights about chemical environments arising through the determination of local coupling parameters. Current methods can successfully resolve these parameters for individual sites using sample-spinning methods techniques applicable to quadrupolar I ≥ 1 nuclei, provided second-order central transition broadenings do not exceed by much the spinning rate. For large quadrupolar coupling (CQ) values, however, static acquisitions are often preferable, leading to challenges in extracting local structural information. This study explores the use of two-dimensional QUadrupolar Isotope Correlation SpectroscopY (QUICSY) experiments as a means to increase the NMR spectral resolution and enrich the characterization of quadrupolar NMR patterns under static conditions. QUICSY seeks to correlate the solid-state NMR powder line shapes for two quadrupolar isotopes belonging to the same element via a 2D experiment. In general, two isotopes of the same element will have different nuclear quadrupole moments, gyromagnetic ratios, and spin numbers but essentially identical chemical environments. The possibility then arises of obtaining sharp "ridges"in these 2D correlations, even in static samples showing large quadrupolar effects, which lead to second-order line shapes that are several kilohertz wide. Moreover, pairs of quadrupolar isotopes are recurrent in the periodic table and include important elements such as 35,37Cl, 69,71Ga, 79,81Br, and 85,87Rb. The potential of this approach is explored theoretically and experimentally on two rubidium-containing salts: RbClO4 and Rb2SO4. We find that each compound gives rise to distinctive 2D QUICSY line shapes, depending on the quadrupolar and chemical shift anisotropy (CSA) parameters of its sites. These experimental line shapes show good agreement with analytically derived 2D spectra relying on literature values of the quadrupolar and CSA tensors of these compounds. The approach underlined here paves the way toward better characterization of wideline NMR spectra of quadrupolar nuclei possessing different nuclear isotopes.

Both in spectroscopy and imaging, t1-noise arising from instabilities such as temperature alterations, field-related frequency drifts, electronic and sample-spinning instabilities, or motions in in vivo experiments, affects many 2D Magnetic Resonance experiments. This work introduces a post-processing method that aims to attenuate t1-noise, by suitably averaging multiple signals/representations that have been reconstructed from the sampled data. The ensuing Compressed Sensing Multiplicative (CoSeM) denoising starts from a fully sampled 2D MR data set, discards random indirect-domain points, and makes up for these missing, masked data, by a compressed sensing reconstruction of the now incompletely sampled 2D data set. This procedure is repeated for multiple renditions of the masked data some of which will have been more strongly affected by t1-noise than others. This leads to a large set of 2D NMR spectra/images compatible with the collected data; CoSeM chooses out of these those renditions that reduce the noise according to a suitable criterion, and then sums up their spectra/images leading to a reduction in t1-noise. The performance of the method was assessed in synthetic data, as well as in numerous different experiments: 2D solid and solution state NMR, 2D localized MRS of live brains, and 2D abdominal MRI. Throughout all these data, CoSeM processing evidenced 23 fold increases in SNR, without introducing biases, false peaks, or spectral/image blurring. CoSeM also retains a quantitative linearity in the information allowing, for instance, reliable T1 inversion-recovery MRI mapping experiments.

Homonuclear isotropic mixing modules allow J-coupled spins to exchange magnetization even when separated by chemical shift offsets that exceed their couplings. This is exploited in TOtal Correlation SpectroscopY (TOCSY) experiments and its variants, which facilitate these homonuclear polarization exchanges by applying broadband RF pulses. These then establish an effective Hamiltonian in which chemical shift offsets are erased, while J-coupling terms including flip-flop components remain active. The polarization that these non-secular terms will transfer among systems of chemically inequivalent sites over the course of a mixing period, are widely used modules in 1D and in multidimensional liquid-state NMR. Homonuclear correlation experiments are also common in solids NMR, particularly among X = 13C or 15N nuclei. Solids NMR experiments are often challenged by high-power RF demands which have led to a family of homonuclear solid-state correlation experiments that avoid pulsing on the nuclei of interest, and focus instead on the 1Hs that are bonded to them. These solid experiments usually reintroduce/strengthen 1H-X dipolar couplings; these, in conjunction with assistance from rotational resonance effects, bring back the truncated X-X dipolar interactions and facilitate the generation of cross peaks. The present study explores whether a similar goal can be achieved for solution-state counterparts, based on the reintroduction of truncated flip-flop terms in the J-coupling Hamiltonian via the pulsing on other, heteronuclear species. A proposal to achieve this is derived, and the resulting HOmonucleaR Recoupling by hEteroNuclear DecOUplingS (HORRENDOUS) approach to provide correlations between like nuclei without pulsing on them, is demonstrated.

Hadamard encoded saturation transfer can significantly improve the efficiency of NOE-based NMR correlations from labile protons in proteins, glycans and RNAs, increasing the sensitivity of cross-peaks by an order of magnitude and shortening experimental times by ≥100-fold. These schemes, however, fail when tackling correlations within a pool of labile protons for instance imino-imino correlations in RNAs or amide-amide correlations in proteins. Here we analyze the origin of the artifacts appearing in these experiments and propose a way to obtain artifact-free correlations both within the labile pool as well as between labile and non-labile ¹Hs, while still enjoying the gains arising from Hadamard encoding and solvent repolarizations. The principles required for implementing what we define as the extended Hadamard scheme are derived, and its clean, artifact-free, sensitivity-enhancing performance is demonstrated on RNA fragments derived from the SARS-CoV-2 genome. Sensitivity gains per unit time approaching an order of magnitude are then achieved in both imino-imino and imino-amino/aromatic protons 2D correlations; similar artifact-free sensitivity gains can be observed when carrying out extended Hadamard encodings of 3D NOESY/HSQC-type experiments. The resulting spectra reveal significantly more correlations than their conventionally acquired counterparts, which can support the spectral assignment and secondary structure determination of structured RNA elements.

INEPT- and HMQC-based pulse sequences are widely used to transfer polarization between heteronuclei, particularly in biomolecular spectroscopy: they are easy to setup and involve low power deposition. Still, these short-pulse polarization transfers schemes are challenged by fast solvent chemical exchange. An alternative to improve these heteronuclear transfers is J-driven cross polarization (J-CP), which transfers polarization by spin-locking the coupled spins under Hartmann-Hahn conditions. J-CP provides certain immunity against chemical exchange and other T2-like relaxation effects, a behavior that is here examined in depth by both Liouville-space numerical and analytical derivations describing the transfer efficiency. While superior to INEPT-based transfers, fast exchange may also slow down these J-CP transfers, hurting their efficiency. This study therefore explores the potential of repeated projective operations to improve 1H→15N and 1H→15N→13C J-CP transfers in the presence of fast solvent chemical exchanges. It is found that while repeating J-CP provides little 1H→15N transfer advantages over a prolonged CP, multiple contacts that keep both the water and the labile protons effectively spin-locked can improve 1H→15N→13C transfers in the presence of chemical exchange. The ensuing Looped, Concatenated Cross Polarization (L-CCP) compensates for single J-CP losses by relying on the 13Cs longer lifetimes, leading to a kind of \u201calgorithmic cooling\u201d that can provide high polarization for the 15N as well as carbonyl and alpha 13Cs. This can facilitate certain experiments, as demonstrated with triple resonance experiments on intrinsically disordered proteins involving labile, chemically exchanging protons.

Purpose: Deuterium metabolic imaging (DMI) maps the uptake of deuterated precursors and their conversion into lactate and other markers of tumor metabolism. Even after leveraging 2Hs short T1s, DMIs signal-to-noise ratio (SNR) is limited. We hypothesize that a multi-echo balanced steady-state free precession (ME-bSSFP) approach would increase SNR compared to chemical shift imaging (CSI), while achieving spectral isolation of the metabolic precursors and products. Methods: Suitably tuned 2H ME-bSSFP (five echo times [TEs], ΔTE = 2.2 ms, repetition time [TR]/flip-angle = 12 ms/60°) was implemented at 15.2T and compared to CSI (TR/flip-angle = 95 ms/90°) regarding SNR and spectral isolation, in simulations, in deuterated phantoms and for the in vivo diagnosis of a mouse tumor model of pancreatic adenocarcinoma (N = 10).Results: Simulations predicted an SNR increase vs. CSI of 3-5, and that the peaks of 2H-water, 2H6,6-glucose, and 2H3,3-lactate can be well isolated by ME-bSSFP; phantoms confirmed this. In vivo, at equal spatial resolution (1.25 × 1.25 mm2) and scan time (10 min), 2H6,6-glucoses and 2H3,3-lactates SNR were indeed higher for bSSFP than for CSI, three-fold for glucose (57 ± 30 vs. 19 ± 11, P

We show that TOCSY and multiple-quantum (MQ) 2D NMR spectra can be obtained for mixtures of substrates hyperpolarised by dissolution dynamic nuclear polarisation (D-DNP). This is achieved by combining optimised transfer settings for D-DNP, with ultrafast 2D NMR experiments based on spatiotemporal encoding. TOCSY and MQ experiments are particularly well suited for mixture analysis, and this approach opens the way to significant sensitivity gains for analytical applications of NMR, such as authentication and metabolomics.

Glycan structures are often stabilized by a repertoire of hydrogen-bonded donor/acceptor groups, revealing longer-lived structures that could represent biologically relevant conformations. NMR provides unique data on these hydrogen-bonded networks from multidimensional experiments detecting cross-peaks resulting from through-bond (TOCSY) or through-space (NOESY) interactions. However, fast OH/H2O exchange, and the spectral proximity among these NMR resonances, hamper the use of glycans labile protons in such analyses; consequently, studies are often restricted to aprotic solvents or supercooled aqueous solutions. These nonphysiological conditions may lead to unrepresentative structures or to probing a small subset of accessible conformations that may miss \u201cactive\u201d glycan conformations. Looped, projected spectroscopy (L-PROSY) has been recently shown to substantially enhance protein NOESY and TOCSY cross-peaks, for 1Hs that undergo fast exchange with water. This study shows that even larger enhancements can be obtained for rapidly exchanging OHs in saccharides, leading to the retrieval of previously undetectable 2D TOCSY/NOESY cross-peaks with nonlabile protons. After demonstrating ≥300% signal enhancements on model monosaccharides, these experiments were applied at 1 GHz to elucidate the structural network adopted by a sialic acid homotetramer, used as a model for α,28 linked polysaccharides. High-field L-PROSY NMR enabled these studies at higher temperatures and provided insight previously unavailable from lower-field NMR investigations on supercooled samples, involving mostly nonlabile nuclei. Using L-PROSYs NOEs and other restraints, a revised structural model for the homotetramer was obtained combining rigid motifs and flexible segments, that is well represented by conformations derived from 40 μs molecular dynamics simulations.

At the magnetic fields of common NMR instruments, electron Zeeman frequencies are too high for efficient electron-nuclear dipolar cross-relaxation to occur in solution. The rate of that process fades with the electron Zeeman frequency as ω^-2 in the absence of isotropic hyperfine couplings, liquid state dynamic nuclear polarisation (DNP) in high-field magnets is therefore impractical. However, contact coupling and dipolar cross-relaxation are not the only mechanisms that can move electron magnetisation to nuclei in liquids: multiple cross-correlated (CC) relaxation processes also exist, involving various combinations of interaction tensor anisotropies. The rates of some of those processes have more favourable high-field behaviour than dipolar cross-relaxation, but due to the difficulty of their numerical and particularly analytical treatment, they remain largely uncharted. In this communication, we report analytical evaluation of every rotationally driven relaxation process in liquid state for 1e1n and 2e1n spin systems, as well as numerical optimisations of the steady-state DNP with respect to spin Hamiltonian parameters. A previously unreported cross-correlated DNP (CCDNP) mechanism was identified for the 2e1n system, involving multiple relaxation interference effects and inter-electron exchange coupling. Using simulations, we found realistic spin Hamiltonian parameters that yield stronger nuclear polarisation at high magnetic fields than dipolar cross-relaxation.

Multidimensional NOESY experiments targeting correlations between exchangeable imino and amino protons provide valuable information about base pairing in nucleic acids. It has been recently shown that the sensitivity of homonuclear correlations involving RNA's labile imino protons can be significantly enhanced, by exploiting the repolarization brought about by solvent exchanges. Homonuclear correlations, however, are of limited spectral resolution, and usually incapable of tackling relatively large homopolymers with repeating structures like RNAs. This study presents a heteronuclear-resolved version of those NOESY experiments, in which magnetization transfers between the aqueous solvent and the nucleic acid protons are controlled by selecting specific chemical shift combinations of a coupled H-N spin pair. This selective control effectively leads to a pseudo-3D version of HSQC-NOESY, but with cross-peaks enhanced by ∼2-5× as compared with conventional 2D NOESY counterparts. The enhanced signal sensitivity as well as access to both N-H and H-H NOESY dimensions can greatly facilitate RNA assignments and secondary structure determinations, as demonstrated here with the analysis of genome fragments derived from the SARS-CoV-2 virus.

Diffusionweighted imaging (DWI) can improve breast cancer characterizations, but often suffers from low image quality particularly at informative b>1000s/mm2 values. The aim of this study was to evaluate multishot approaches characterizing Gaussian and nonGaussian diffusivities in breast cancer. This was a prospective study, in which 15 subjects, including 13 patients with biopsyconfirmed breast cancers, were enrolled. DWI was acquired at 3T using echo planar imaging (EPI) with and without zoomed excitations, readoutsegmented EPI (RESOLVE), and spatiotemporal encoding (SPEN); dynamic contrastenhanced (DCE) data were collected using threedimensional gradientecho T1 weighting; anatomies were evaluated with T2weighted twodimensional turbo spinecho. Congruence between malignancies delineated by DCE was assessed against highresolution DWI scans with bvalues in the 01800s/mm2 range, as well as against apparent diffusion coefficient (ADC) and kurtosis maps. Data were evaluated by independent magnetic resonance scientists with 320years of experience, and radiologists with 6 and 20years of experience in breast MRI. Malignancies were assessed from ADC and kurtosis maps, using paired t tests after confirming that these values had a Gaussian distribution. Agreements between DWI and DCE datasets were also evaluated using SorensenDice similarity coefficients. Cancerous and normal tissues were clearly separable by ADCs: by SPEN their average values were (1.03±0.17)×10−3 and (1.69±0.19)×10−3 mm2/s (p

Diffusion tensor distribution (DTD) imaging builds on principles from diffusion, solid-state and low-field NMR spectroscopies, to quantify the contents of heterogeneous voxels as nonparametric distributions, with tensor \u201csize\u201d, \u201cshape\u201d and orientation having direct relations to corresponding microstructural properties of biological tissues. The approach requires the acquisition of multiple images as a function of the magnitude, shape and direction of the diffusion-encoding gradients, leading to long acquisition times unless fast image read-out techniques like EPI are employed. While in previous in vivo human brain studies performed at 3 T this proved a viable option, porting these measurements to very high magnetic fields and/or to heterogeneous organs induces B
₀- and B
₁-inhomogeneity artifacts that challenge the limits of EPI. To overcome such challenges, we demonstrate here that high spatial resolution DTD of mouse brain can be carried out at 15.2 T with a surface-cryoprobe, by relying on SPatiotemporal ENcoding (SPEN) imaging sequences. These new acquisition and data-processing protocols are demonstrated with measurements on in vivo mouse brain, and validated with synthetic phantoms designed to mimic the diffusion properties of white matter, gray matter and cerebrospinal fluid. While still in need of full extensions to 3D mappings and of scanning additional animals to extract more general physiological conclusions, this work represents another step towards the model-free, noninvasive in vivo characterization of tissue microstructure and heterogeneity in animal models, at ≈0.1 mm resolutions.

Diffusion-weighted MRI on rodents could be valuable to evaluate pregnancy-related dysfunctions, particularly in knockout models whose biological nature is well understood. Echo Planar Imagings sensitivity to motions and to air/water/fat heterogeneities, complicates these studies in the challenging environs of mice abdomens. Recently developed MRI methodologies based on SPatiotemporal ENcoding (SPEN) can overcome these obstacles, and deliver diffusivity maps at ≈150 µm in-plane resolutions. The present study exploits these capabilities to compare the development in wildtype vs vascularly-altered mice. Attention focused on the various placental layersdeciduae, labyrinth, trophoblast, fetal vesselsthat the diffusivity maps could resolve. Notable differences were then observed between the placental developments of wildtype vs diseased mice; these differences remained throughout the pregnancies, and were echoed by perfusion studies relying on gadolinium-based dynamic contrast-enhanced MRI. Longitudinal monitoring of diffusivity in the animals throughout the pregnancies also showed differences between the development of the fetal brains in the wildtype and vascularly-altered mice, even if these disparities became progressively smaller as the pregnancies progressed. These results are analyzed on the basis of the known physiology of normal and preeclamptic pregnancies, as well as in terms of the potential that they might open for the early detection of disorders in human pregnancies.

Purpose - Diffusion weighted imaging (DWI) is increasingly used in evaluating breast cancer, as complement to DCE measurements of superior spatial resolution. Extracting fine morphological features in DWI is complicated by limitations that sequences such as EPI face, when applied to heterogeneous organs. This study investigates the ability of spatiotemporal encoding (SPEN) MRI to screen breast cancers and define diffusivity features at mm and sub-mm resolutions on a 3T scanner. Methods - Twenty-one patients with biopsy-confirmed breast cancer lesions were examined by T2-weighted and DCE protocols, by EPI-based DWI, and by SPEN-based protocols optimized for SNR, robustness and spatial resolution, respectively. Results - Excellent agreement was found between the diffusivity parameters measured by all SPEN protocols and by EPI, with the lower ADCs characteristic of tumors being readily detected. SPEN provided systematically better SNR and improved qualitative results, particularly when dealing with small lesions surrounded by fatty tissue, or lesions close to tissue/air interfaces. SPEN-derived ADC maps collected at sub-mm in-plane resolutions recapitulated the high-resolution morphology shown by lesions using more sensitive DCE protocols. Conclusion - Measurements on a patient cohort validated SPEN's ability to quantify the diffusivity changes associated with the presence of breast cancers, while imaging the lesions with reduced distortions at sub-mm resolutions.

Hyperpolarized water can be a valuable aid in protein NMR, leading to amide group H-1 polarizations that are orders of magnitude larger than their thermal counterparts. Suitable procedures can exploit this to deliver 2D H-1-N-15 correlations with good resolution and enhanced sensitivity. These enhancements depend on the exchange rates between the amides and the water, thereby yielding diagnostic information about solvent accessibility. This study applied this "HyperW" method to four proteins exhibiting a gamut of exchange behaviors: PhoA((330-471)), an unfolded 122-residue fragment; barstar, a fully folded ribonuclease inhibitor; R17, a 13.3 kDa system possessing folded and unfolded forms under slow interconversion; and drkN SH3, a protein domain whose folded and unfolded forms interchange rapidly and with temperature-dependent population ratios. For PhoA((330-471)) HyperW sensitivity enhancements were >= 300X, as expected for an unfolded protein sequence. Though fully folded, barstar also exhibited substantial enhancements; these, however, were not uniform and, according to CLEANEX experiments, reflected the solvent-exposed residues. R17 showed the expected superposition of >= 100-fold enhancements for its unfolded form, coexisting with more modest enhancements for their folded counterparts. Unexpected, however, was the behavior of drkN SH3, for which HyperW enhanced the unfolded but, surprisingly, enhanced even more certain folded protein sites. These preferential enhancements were repeatedly and reproducibly observed. A number of explanations-including three-site exchange magnetization transfers between water and the unfolded and folded states; cross-correlated relaxation processes from hyperpolarized "structural" waters and labile side-chain protons; and the possibility that faster solvent exchange rates characterize certain folded sites over their unfolded counterparts-are considered to account for them.

Signal enhancements of up to two orders of magnitude in protein NMR can be achieved by employing HDO as a vector to introduce hyperpolarization into folded or intrinsically disordered proteins. In this approach, hyperpolarized HDO produced by dissolution-dynamic nuclear polarization (D-DNP) is mixed with a protein solution waiting in a high-field NMR spectrometer, whereupon amide proton exchange and nuclear Overhauser effects (NOE) transfer hyperpolarization to the protein and enable acquisition of a signal-enhanced high-resolution spectrum. To date, the use of this strategy has been limited to 1D and
¹H-
¹⁵N 2D correlation experiments. Here we introduce 2D
¹³C-detected D-DNP, to reduce exchange-induced broadening and other relaxation penalties that can adversely affect proton-detected D-DNP experiments. We also introduce hyperpolarized 3D spectroscopy, opening the possibility of D-DNP studies of larger proteins and IDPs, where assignment and residue-specific investigation may be impeded by spectral crowding. The signal enhancements obtained depend in particular on the rates of chemical and magnetic exchange of the observed residues, thus resulting in non-uniform hyperpolarization-selective signal enhancements. The resulting spectral sparsity, however, makes it possible to resolve and monitor individual amino acids in IDPs of over 200 residues at acquisition times of just over a minute. We apply the proposed experiments to two model systems: the compactly folded protein ubiquitin, and the intrinsically disordered protein (IDP) osteopontin (OPN).

NMR sensitivity-enhancement methods involving hyperpolarized water could be of importance for solution-state biophysical investigations. Hyperpolarized water (HyperW) can enhance the
¹H NMR signals of exchangeable sites by orders of magnitude over their thermal counterparts, while providing insight into chemical exchange and solvent accessibility at a site-resolved level. As HyperWs enhancements are achieved by exploiting fast solvent exchanges associated with minimal interscan delays, possibilities for the rapid monitoring of chemical reactions and biomolecular (re)folding are opened. HyperW NMR can also accommodate heteronuclear transfers, facilitating the rapid acquisition of 2-dimensional (2D)
¹⁵N-
¹H NMR correlations, and thereby combining an enhanced spectral resolution with speed and sensitivity. This work demonstrates how these qualities can come together for the study of nucleic acids. HyperW injections were used to target the guanine-sensing riboswitch aptamer domain (GSR
^apt) of the xpt-pbuX operon in Bacillus subtilis. Unlike what had been observed in proteins, where residues benefited of HyperW NMR only if/when sufficiently exposed to water, these enhancements applied to every imino resonance throughout the RNA. The >300-fold enhancements observed in the resulting
¹H NMR spectra allowed us to monitor in real time the changes that GSR
^apt undergoes upon binding hypoxanthine, a high-affinity interaction leading to conformational refolding on a ∼1-s timescale at 36 °C. Structural responses could be identified for several nucleotides by 1-dimensional (1D) imino
¹H NMR as well as by 2D HyperW NMR spectra acquired upon simultaneous injection of hyperpolarized water and hypoxanthine. The folding landscape revealed by this HyperW strategy for GSR
^apt, is briefly discussed.

Purpose: Diffusion MRI is of interest for clinical research and diagnosis. Whereas high-resolution DWI/DTI is hard to achieve by single-shot methods, interleaved acquisitions can deliver these if motion and/or folding artefacts are overcome. Thanks to its ability to provide zoomed, folding-free images, spatially encoded MRI can fulfill these requirements. This is here coupled with a regularized reconstruction and parallel receive methods, to deliver a robust scheme for human DWI/DTI at mm and sub-mm resolutions.Methods: Each shot along the spatially encoded dimension was reconstructed separately to retrieve per-shot phase maps. These shots, together with coil sensitivities, were combined with spatially encoded quadratic phase-encoding matrices associated to each shot, into single global operators. Their originating images were then iteratively computed aided by l(1) and l(2) regularization methods. When needed, motion-corrupted shots were discarded and replaced by redundant information arising from parallel imaging.Results: Full-brain DTI experiments at 1 mm and restricted brain DTIs with 0.75 mm nominal in-plane resolutions were acquired and reconstructed successfully by the new scheme. These 3 Tesla spetiotemporally encoded results compared favorably with EPI counterparts based on segmented and selective excitation schemes provided with the scanner.Conclusion: A new procedure for achieving high-definition diffusion-based MRI was developed and demonstrated.

Purpose: The purpose of the study was to develop an approach for improving the resolution and sensitivity of hyperpolarized C-13 MRSI based on a priori anatomical information derived from featured, water-based H-1 images.Methods: A reconstruction algorithm exploiting H-1 MRI for the redefinition of the C-13 MRSI anatomies was developed, based on a modification of the spectroscopy with linear algebraic modeling (SLAM) principle. To enhance C-13 spatial resolution and reduce spillover effects without compromising SNR, this model was extended by endowing it with a search allowing smooth variations in the C-13 MR intensity within the targeted regions of interest.Results: Experiments were performed in vitro on enzymatic solutions and in vivo on rodents, based on the administration of C-13-enriched hyperpolarized pyruvate and urea. The spectral images reconstructed for these substrates and from metabolic products based on predefined H-1 anatomical compartments using the new algorithm, compared favorably with those arising from conventional Fourier-based analyses of the same data. The new approach also delivered reliable kinetic C-13 results, for the kind of processes and timescales usually targeted by hyperpolarized MRSI.Conclusion: A simple, flexible strategy is introduced to boost the sensitivity and resolution provided by hyperpolarized C-13 MRSI, based on readily available H-1 MR information.

While C-13-based Incredible Natural Abundance DoublE QUAntum Transfer Experiment (INADEQUATE) experiments offer an attractive alternative for establishing molecular structures, they suffer from low sensitivities arising from the scarcity of spin pairs present at natural abundance. Herein we demonstrate that dissolution dynamic nuclear polarization (dDNP) provides sufficient sensitivity to acquire 1D C-13 INADEQUATE spectra in a single scan and at natural abundance. Moreover, if steps are adopted to endow sub-Hertz precision to these measurements, they allow one to measure carbon carbon J couplings over both one and multiple bonds for each chemical site. As these Jcc-couplings are usually sufficiently distinct to enable univocal pairing of the nuclei involved, essentially the same information as in 2D INADEQUATE can be obtained. The feasibility of the method is demonstrated for a range of compounds, including natural products such as a-pinene, menthol and limonene. Features and extensions of this approach are briefly discussed.

Purpose: To develop a rapid, non-CPMG high-resolution volumetric imaging approach, exhibiting a speed and in-plane resilience to field inhomogeneities comparable to RARE/turbo-spin-echo (TSE) while endowed with unique downsampling characteristics.Methods: A multi-scan extension of cross-term spatiotemporal encoding (xSPEN) is introduced and analyzed. The method simultaneously yields k(y)/k(z) data containing low and high frequency components, as well as transposed, low-resolution z/y images. This dual k-/spatial-domain information is captured by a multi-scan procedure that phase-encodes k(y) while simultaneously slice-selecting z. A reconstruction scheme converting this information into high resolution 3D images with fully multiplexed volumetric coverage is introduced and exemplified.Results: Phase-encoded xSPEN was tested by human brain imaging at sub-mm resolutions. The method exceeded 2D TSE's sensitivity by factors of approximate to 3-4, while providing similar resolution and SNR as 3D TSE in approximate to 50% acquisition times. The method's contrast is dominated by T-2 and is free from "bright-fat" effects associated to spin-echo trains. Further acceleration is enabled by the method's downsampling abilities. Tradeoffs between encoding time, number of measurements, spatial resolution, SNR, and artifact levels are also laid out.Conclusion: A new MRI strategy is introduced delivering high in-and through-plane resolutions while enjoying full Fourier multiplexing, leading to fast acquisitions with high SNR.

Cross-relaxation and isotropic mixing phenomena leading to the Nuclear Overhauser Effect (NOE) and to the TOCSY experiment, lie at the center of structural determinations by NMR. 2D TOCSY and NOESY exploit these polarization transfer effects to determine inter-site connectivities and molecular geometries under physiologically-relevant conditions. Among these sequences drawback, particularly for the case of NOEs, are a lack of sensitivity arising from small structurally-relevant cross peaks. The present study explores the application of multiple Zeno-like projective measurements, to enhance the cross-peaks between spectrally distinct groups in proteins in particular between amide and aliphatic protons. The enhancement is based on repeating the projection done by Ramsey or TOCSY blocks multiple times, in what we refer to as Looped, PROjected Spectroscopy (L-PROSY). This leads to a reset of the amide/aliphatic transfer processes; the initial slopes of the NOE- or J-transfer effects thus define the cross-peak growth, and a faster cross-peak buildup is achieved upon looping these transfers over the allotted time T₁. These projections also help to better preserve the magnetization originating in the amides, resulting in an overall improvement in sensitivity. L-PROSY's usefulness is demonstrated by incorporating it into two widely used protein NMR experiments: 2D ¹⁵N-¹H HMQC-NOESY and ¹⁵N-filtered 2D NOESY. Different parameters dictating the overall SNR improvement, particularly the protein correlation times and the amide-water chemical exchange rates, were examined, and L-PROSY's enhancements resulted for all tested proteins. The largest cross-peak enhancements were observed for unstructured proteins, where chemical exchanges with the solvent of the kind that tend to average out NOE cross-peaks in conventional NMR, boost L-PROSY's cross-peaks by replenishing the amide's magnetizations within each loop. Enhanced cross-peaks were also found in extensions involving TOCSY-based experiments when applied to proteins with unfolded segments.

This study demonstrates the usefulness derived from relying on hyperpolarized water obtained by dissolution DNP, for site-resolved biophysical NMR studies of intrinsically disordered proteins. Thanks to the facile amide-solvent exchange experienced by protons in these proteins, 2D NMR experiments that like HMQC rely on the polarization of the amide protons, can be enhanced using hyperpolarized water by several orders of magnitude over their conventional counterparts. Optimizations of the DNP procedure and of the subsequent injection into the protein sample are necessary to achieve these gains while preserving state-of-the-art resolution; procedures enabling this transfer of the hyperpolarized water and the achievement of foamless hyperpolarized protein solutions are demonstrated. These protocols are employed to collect 2D ¹⁵N-¹H HMQC NMR spectra of α-synuclein, showing residue-specific enhancements ≥100× over their thermal counterparts. These enhancements, however, vary considerably throughout the residues. The biophysics underlying this residue-specific behavior upon injection of hyperpolarized water is theoretically examined, the information that it carries is compared with results arising from alternative methods, and its overall potential is discussed.

Placental functions, including transport and metabolism, play essential roles in pregnancy. This study assesses such processes in vivo, from a hyperpolarized MRI perspective. Hyperpolarized urea, bicarbonate, and pyruvate were administered to near-term pregnant rats, and all metabolites displayed distinctive behaviors. Little evidence of placental barrier crossing was observed for bicarbonate, at least within the timescales allowed by ¹³C relaxation. By contrast, urea was observed to cross the placental barrier, with signatures visible from certain fetal organs including the liver. This was further evidenced by the slower decay times observed for urea in placentas vis-à-vis other maternal compartments and validated by mass spectrometric analyses. A clear placental localization, as well as concurrent generation of hyperpolarized lactate, could also be detected for [1-¹³C]pyruvate. These metabolites also exhibited longer lifetimes in the placentas than in maternal arteries, consistent with a metabolic activity occurring past the trophoblastic interface. When extended to a model involving the administration of a preeclampsia-causing chemical, hyperpolarized MR revealed changes in ureas transport, as well as decreases in placental glycolysis vs. the naïve animals. These distinct behaviors highlight the potential of hyperpolarized MR for the early, minimally invasive detection of aberrant placental metabolism.

Nuclear magnetic resonance is a powerful tool for probing the structures of chemical and biological systems. Combined with field gradients it leads to NMR imaging (MRI), a widespread tool in non-invasive examinations. Sensitivity usually limits MRI's spatial resolution to tens of micrometers, but other sources of information like those delivered by constrained diffusion processes, enable one extract morphological information down to micron and sub-micron scales. We report here on a new method that also exploits diffusion-isotropic or anisotropic-to sense morphological parameters in the nm-mm range, based on distributions of susceptibility-induced magnetic field gradients. A theoretical framework is developed to define this source of information, leading to the proposition of internal gradient-distribution tensors. Gradient-based spin-echo sequences are designed to measure these new observables. These methods can be used to map orientations even when dealing with unconstrained diffusion, as is here demonstrated with studies of structured systems, including tissues.

Nuclear magnetic resonance (NMR) spectroscopy has long served as an irreplaceable, versatile tool in physics, chemistry, biology, and materials sciences, owing to its ability to study molecular structure and dynamics in detail. In particular, the connectivity of chemical sites within molecules, and thereby molecular structure, becomes visible by multi-dimensional NMR. Homonuclear correlation experiments are a powerful tool for identifying coupled spins. Generally, diagonal peaks in these correlation spectra display the strongest intensities and do not offer any new information beyond the standard one-dimensional spectrum, whereas weaker, symmetrically placed cross peaks contain most of the coupling information. The cross peaks near the diagonal are often affected by the tails of strong diagonal peaks or even obscured entirely by the diagonal. In this paper, we demonstrate a homonuclear encoding approach based on imparting a discrete phase modulation of the targeted cross peaks and combine it with a site-selective sculpting scheme, capable of simplifying the patterns arising in these 2D correlation spectra. The theoretical principles of the new methods are laid out, and experimental observations are rationalized on the basis of theoretical analyses. The ensuing techniques provide a new way to retrieve 2D coupling information within homonuclear spin systems, with enhanced sensitivity, speed, and clarity.

The use of frequency-swept radiofrequency (rf) pulses for enhancing signals in the magic-angle spinning (MAS) spectra of half-integer quadrupolar nuclides was explored. The broadband adiabatic inversion cross-polarization magic-angle spinning (BRAIN-CPMAS) method, involving an adiabatic inversion pulse on the S-channel and a simultaneous rectangular spin-lock pulse on the I-channel (¹H), was applied to I(1/2) → S(3/2) systems. Optimal BRAIN-CPMAS matching conditions were found to involve low rf pulse strengths for both the I- and S-spin channels. At these low and easily attainable rf field strengths, level-crossing events among the energy levels |3/2〉,|1/2〉,|-1/2〉,|-3/2〉 that are known to complicate the CPMAS of quadrupolar nuclei, are mostly avoided. Zero- and double-quantum polarization transfer modes, akin to those we have observed for I(1/2) → S(1/2) polarization transfers, were evidenced by these analyses even in the presence of the quadrupolar interaction. ¹H-²³Na and ¹H-¹¹B BRAIN-CPMAS conditions were experimentally explored on model compounds by optimizing the width of the adiabatic sweep, as well as the rf pulse powers of the ¹H and ²³Na/¹¹B channels, for different MAS rates. The experimental data obtained on model compounds containing spin-3/2 nuclides, matched well predictions from numerical simulations and from an average Hamiltonian theory model. Extensions to half-integer spin nuclides with higher spins and potential applications of this BRAIN-CPMAS approach are discussed.

Purpose: Evaluate the usefulness of single-shot and of interleaved spatiotemporally encoded (SPEN) methods to perform diffusion tensor imaging (DTI) under various preclinical and clinical settings.Methods: A formalism for analyzing SPEN DTI data is presented, tailored to account for the spatially dependent bmatrix weightings introduced by the sequence's use of swept pulses acting while in the presence of field gradients. Using these b-matrix calculations, SPEN's ability to deliver DTI measurements was tested on phantoms as well as ex vivo and in vivo. In the latter case, DTI involved scans on mice brains and on human lactating breasts.Results: For both ex vivo and in vivo investigations, SPEN data proved less sensitive to distortions arising from Bo field inhomogeneities and from eddy currents, than conventional single-shot alternatives. Further resolution enhancement could be achieved using referenceless methods for interleaved SPEN data acquisitions.Conclusion: The robustness of SPEN-based sequences vis-similar to avis field instabilities and heterogeneities, enables the implementation of DTI experiments with good sensitivity and resolution even in challenging environments in both preclinical and clinical settings.

Purpose: This study seeks to evaluate in vivo T-2 relaxation times of selectively excited stroke-relevant metabolites via H-1 relaxation-enhanced magnetic resonance spectroscopy (RE-MRS) at 21.1T (900 MHz).Methods: A quadrature surface coil was designed and optimized for investigations of rodents at 21.1T. With voxel localization, a RE-MRS pulse sequence incorporating the excitation of selected metabolites was modified to include a variable echo delay for T-2 measurements. A middle cerebral artery occlusion (MCAO) animal model for stroke was examined with spectra taken 24h post occlusion. Fourteen echo times were acquired, with each measurement completed in less than 2min.Results: The RE-MRS approach produced high-quality spectra of the selectively excited metabolites in the stroked and contralateral regions. T-2 measurements reveal differential results between these regions, with significance achieved for lactic acid.Conclusion: Using the RE-MRS technique at ultra-high magnetic field and an optimized quadrature surface coil design, full metabolic T-2 quantifications in a localized voxel is now possible in less than 27min. Magn Reson Med 77:520-528, 2017. (c) 2016 International Society for Magnetic Resonance in Medicine

Purpose: Single-scan two-dimensional MRI has been generally constrained to acquisitions in high quality magnets. This study introduces a methodology, cross-term spatiotemporal encoding (xSPEN), that delivers such images under much poorer external field conditions.Methods: xSPEN departs from conventional k-space scanning, by relying on spatiotemporally encoding the image being sought. Unlike hitherto proposed SPEN methods, however, xSPEN's image readout does not take place using a field gradient along the direction being probed, but rather with the aid of an ancillary source of inhomogeneous frequency broadening. This ancillary dimension was here imposed by an orthogonal field gradient; for example, images along the y axis were read out by application of a z gradient. The principles and characteristics of this new approach, compatible with existing scanners and free from the need to collect auxiliary information such as field maps, are presented and discussed.Results: Single- and multi-slice in vitro, ex vivo, and in vivo MRI experiments, confirmed the unusual resilience of this new single-shot MRI method to multiple chemical sites on phantoms, animals and humans.Conclusion: xSPEN can deliver single-scan MRI with good sensitivity and exceptional resilience to field inhomogeneities. This could enable investigations that have hitherto escaped from MRI's scope. Magn Reson Med 77:623-634, 2017. (c) 2016 International Society for Magnetic Resonance in Medicine

Many neurodegenerative diseases are characterized by misfolding and aggregation of an expanded polyglutamine tract (polyQ). Huntington's Disease, caused by expansion of the polyQ. tract in exon 1 of the Huntingtin protein (Htt), is associated with aggregation and neuronal toxicity. Despite recent structural progress in understanding the structures of amyloid fibrils, little is known about the solution states of Htt in general, and about molecular details of their transition from soluble to aggregation-prone conformations in particular. This is an important question, given the increasing realization that toxicity may reside in soluble conformers. This study presents an approach that combines NMR with computational methods to elucidate the structural conformations of Htt Exon 1 in solution. Of particular focus was Htt's N17 domain sited N-terminal to the polyQ. tract, which is key to enhancing aggregation and modulate Htt toxicity. Such in-depth structural study of Htt presents a number of unique challenges: the long homopolymeric polyQ. tract contains nearly identical residues, exon 1 displays a high degree of conformational flexibility leading to a scaling of the NMR chemical shift dispersion, and a large portion of the backbone amide groups are solvent-exposed leading to fast hydrogen exchange and causing extensive line broadening. To deal with these problems, NMR assignment was achieved on a minimal Htt exon 1, comprising the N17 domain, a polyQ tract of 17 glutamines, and a short hexameric polyProline region that does not contribute to the spectrum. A pH titration method enhanced this polypeptide's solubility and, with the aid of ≤5D NMR, permitted the full assignment of N17 and the entire polyQ. tract. Structural predictions were then derived using the experimental chemical shifts of the Htt peptide at low and neutral pH, together with various different computational approaches. All these methods concurred in indicating that low-pH protonation stabilizes a soluble conformation where a helical region of N17 propagates into the polyQ region, while at neutral pH both N17 and the polyQ become largely unstructured-thereby suggesting a mechanism for how N17 regulates Htt aggregation.

Thanks to their special spatiotemporal encoding/decoding scheme, ultrafast (UF) NMR sequences can deliver arbitrary 2D spectra following a single excitation. Regardless of their nature, these sequences have in common their tracing of a path in the F1t2 plane, that will deliver the spectrum being sought after a 1D Fourier transformation versus t2. This need to simultaneously digitize two domains, tends to impose bandwidth limitations along all spectral axes. Along the t2/F2 dimension this problem is exacerbated by the fact that odd and even time points are not equispaced, and by additional artifacts such as time shifts between time points sampled while under the action of positive and negative decoding gradients. As a result, odd and even t2 points are typically Fourier transformed separately, halving the potential spectral width along this dimension. While this halving of the F2 span can be overcome by an interlaced Fourier transform, this post-processing is seldom used because of its sensitivity to hardware inaccuracies requiring even finer corrections of the even/odd t2 data points. These corrections have so far been done manually, but are challenging to implement when dealing with low signal-to-noise ratio signals like those associated with biomolecular NMR experiments. This study introduces an algorithm for an automatic correction of all even/odd ultrafast NMR inconsistencies, based on the acquisition of a reference scan on the solvent. This algorithm was verified experimentally using an 1H-13C UF-HSQC variant on ubiquitin at 600 MHz. Features of this method as well as of the interlaced Fourier transformation in general, are discussed.

Dynamic nuclear polarization (DNP) is a versatile option to improve the sensitivity of NMR and MRI. This versatility has elicited interest for overcoming potential limitations of these techniques, including the achievement of solid-state polarization enhancement at ambient conditions, and the maximization of ¹³C signal lifetimes for performing in vivo MRI scans. This study explores whether diamond's ¹³C behavior in nano- and micro-particles could be used to achieve these ends. The characteristics of diamond's DNP enhancement were analyzed for different magnetic fields, grain sizes, and sample environments ranging from cryogenic to ambient temperatures, in both solution and solid-state experiments. It was found that ¹³C NMR signals could be boosted by orders of magnitude in either low- or room-temperature solid-state DNP experiments by utilizing naturally occurring paramagnetic P₁ substitutional nitrogen defects. We attribute this behavior to the unusually long electronic/nuclear spin-lattice relaxation times characteristic of diamond, coupled with a time-independent cross-effect-like polarization transfer mechanism facilitated by a matching of the nitrogen-related hyperfine coupling and the ¹³C Zeeman splitting. The efficiency of this solid-state polarization process, however, is harder to exploit in dissolution DNP-enhanced MRI contexts. The prospects for utilizing polarized diamond approaching nanoscale dimensions for both solid and solution applications are briefly discussed.

An initiative to design and build magnetic resonance imaging (MRI) and spectroscopy (MRS) instruments at 14 T and beyond to 20 T has been underway since 2012. This initiative has been supported by 22 interested participants from the USA and Europe, of which 15 are authors of this review. Advances in high temperature superconductor materials, advances in cryocooling engineering, prospects for non-persistent mode stable magnets, and experiences gained from large-bore, high-field magnet engineering for the nuclear fusion endeavors support the feasibility of a human brain MRI and MRS system with 1 ppm homogeneity over at least a 16-cm diameter volume and a bore size of 68 cm. Twelve neuroscience opportunities are presented as well as an analysis of the biophysical and physiological effects to be investigated before exposing human subjects to the high fields of 14 T and beyond.

Purpose: Single-shot imaging by spatiotemporal encoding (SPEN) can provide higher immunity to artifacts than its echo planar imaging-based counterparts. Further improvements in resolution and signal-to-noise ratio could be made by rescinding the sequence's single-scan nature. To explore this option, an interleaved SPEN version was developed that was capable of delivering optimized images due to its use of a referenceless correction algorithm. Methods: A characteristic element of SPEN encoding is the absence of aliasing when its signals are undersampled along the low-bandwidth dimension. This feature was exploited in this study to segment a SPEN experiment into a number of interleaved shots whose inaccuracies were automatically compared and corrected as part of a navigator-free image reconstruction analysis. This could account for normal phase noises, as well as for object motions during the signal collection. Results: The ensuing interleaved SPEN method was applied to phantoms and human volunteers and delivered high-quality images even in inhomogeneous or mobile environments. Submillimeter functional MRI activation maps confined to gray matter regions as well as submillimeter diffusion coefficient maps of human brains were obtained. Conclusion: We have developed an interleaved SPEN approach for the acquisition of high-definition images that promises a wider range of functional and diffusion MRI applications even in challenging environments.

Polarizing nuclear spins is of fundamental importance in biology, chemistry and physics. Methods for hyperpolarizing 13 C nuclei from free electrons in bulk usually demand operation at cryogenic temperatures. Room temperature approaches targeting diamonds with nitrogen-vacancy centres could alleviate this need; however, hitherto proposed strategies lack generality as they demand stringent conditions on the strength and/or alignment of the magnetic field. We report here an approach for achieving efficient electron- 13 C spin-alignment transfers, compatible with a broad range of magnetic field strengths and field orientations with respect to the diamond crystal. This versatility results from combining coherent microwave- and incoherent laser-induced transitions between selected energy states of the coupled electron-nuclear spin manifold. 13 C-detected nuclear magnetic resonance experiments demonstrate that this hyperpolarization can be transferred via first-shell or via distant 13 Cs throughout the nuclear bulk ensemble. This method opens new perspectives for applications of diamond nitrogen-vacancy centres in nuclear magnetic resonance, and in quantum information processing.

A novel method for the rapid acquisition of quality multi-slice 2D images targeting a small number of spectroscopic resonances, is introduced and illustrated. The method exploits the robustness derived from recently proposed spatiotemporal encoding (SPEN) methods, when operating in the so-called "fully refocused" mode. Fully-refocused SPEN provides high-fidelity single-shot images thanks to its refocusing of all offset-derived effects throughout the course of the acquisition. This refocusing, however, prevents exploiting such robustness for spectroscopic imaging. We propose here a solution to this limitation, based on the use of polychromatic refocusing pulses. It is shown that if used to address a series of a priori known resonance positions, these pulses can lead to quality spectroscopic images in a small number of scans - generally equal or slightly larger than the number of targeted peaks. Such strategy is explored in combination with both fully-refocused SPEN and echo-planar-imaging (EPI) acquisitions. The expected SPEN advantages were observed in both phantom-based models, and in in vivo results of fat and water separation in mice at 7 T.

In the Spring of 2013, NMR spectroscopists convened at the Weizmann Institute in Israel to brainstorm on approaches to improve the sensitivity of NMR experiments, particularly when applied in biomolecular settings. This multi-author interdisciplinary Review presents a state-of-the-art description of the primary approaches that were considered. Topics discussed included the future of ultrahigh-field NMR systems, emerging NMR detection technologies, new approaches to nuclear hyperpolarization, and progress in sample preparation. All of these are orthogonal efforts, whose gains could multiply and thereby enhance the sensitivity of solid- and liquid-state experiments. While substantial advances have been made in all these areas, numerous challenges remain in the quest of endowing NMR spectroscopy with the sensitivity that has characterized forms of spectroscopies based on electrical or optical measurements. These challenges, and the ways by which scientists and engineers are striving to solve them, are also addressed. A new spin on bio-NMR: This Review presents a state-of-the-art description of the leading approaches being considered today to improve the sensitivity of NMR spectroscopy, particularly as applied in biomolecular settings. The focus is on the future of ultrahigh-field NMR systems, emerging NMR detection technologies, new approaches to nuclear hyperpolarization, and progress in sample preparation.

Purpose: Evaluating the usefulness of diffusion-weighted spatio-temporal encoding (SPEN) methods to provide quantitative apparent diffusion coefficient (ADC)-based characterizations of healthy and malignant human breast tissues, in comparison with results obtained using techniques based on spin-echo echo planar imaging (SE-EPI). Methods: Twelve healthy volunteers and six breast cancer patients were scanned at 3T using scanner-supplied diffusion-weighted imaging EPI sequences, as well as two fully refocused SPEN variants programmed in-house. Suitable codes were written to process the data, including calculations of the actual b-values and retrieval of the ADC maps. Results: Systematically better images were afforded by the SPEN scans, with negligible geometrical distortions and markedly weaker ghosting artifacts arising from either fat tissues or from strongly emitting areas such as cysts. SPEN-derived images provided improved characterizations of the fibroglandular tissues and of the lesions' contours. When translated into the calculation of the ADC maps, there were no significant differences between the mean ADCs derived from SPEN and SE-EPI: if reliable images were available, both techniques showed that ADCs decreased by nearly two-fold in the malignant lesion areas. Conclusion: SPEN-based sequences yielded diffusion-weighted breast images with minimal artifacts and distortions, enabling the calculation of improved ADC maps and the identification of decreased ADCs in malignant regions.

Purpose: This study quantifies in vivo ischemic stroke brain injuries in rats using ultrahigh-field single-scan MRI methods to assess variations in apparent diffusion coefficients (ADCs). Methods: Magnitude and diffusion-weighted spatiotemporally encoded imaging sequences were implemented on a 21.1 T imaging system, and compared with spin-echo and echo-planar imaging diffusion-weighted imaging strategies. ADC maps were calculated and used to evaluate the sequences according to the statistical comparisons of the ipsilateral and contralateral ADC measurements at 24, 48, and 72 h poststroke. Results: Susceptibility artifacts resulting from normative anatomy and pathological stroke conditions were particularly intense at 21.1 T. These artifacts strongly distorted single-shot diffusion-weighted echo-planar imaging experiments, but were reduced in four-segment interleaved echo-planar imaging acquisitions. By contrast, nonsegmented diffusion-weighted spatiotemporally encoded images were largely immune to field-dependent artifacts. Effects of stroke were apparent in both magnitude images and ADC maps of all sequences. When stroke recovery was followed by ADC variations, spatiotemporally encoded, echo-planar imaging, and spin-echo acquisitions revealed statistically significant increase in ADCs. Conclusions: Consideration of experiment duration, image quality, and mapped ADC values provided by spatiotemporally encoded demonstrates that this single-shot acquisition is a method of choice for high-throughput, ultrahigh-field in vivo stroke quantification.

Speeding up the acquisition of multidimensional nuclear magnetic resonance (NMR) spectra is an important topic in contemporary NMR, with central roles in high-throughput investigations and analyses of marginally stable samples. A variety of fast NMR techniques have been developed, including methods based on non-uniform sampling and Hadamard encoding, that overcome the long sampling times inherent to schemes based on fast-Fourier-transform (FFT) methods. Here, we explore the potential of an alternative fast acquisition method that leverages a priori knowledge, to tailor polychromatic pulses and customized time delays for an efficient Fourier encoding of the indirect domain of an NMR experiment. By porting the encoding of the indirect-domain to the excitation process, this strategy avoids potential artifacts associated with non-uniform sampling schemes and uses a minimum number of scans equal to the number of resonances present in the indirect dimension. An added convenience is afforded by the fact that a usual 2D FFT can be used to process the generated data. Acquisitions of 2D heteronuclear correlation NMR spectra on quinine and on the anti-inflammatory drug isobutyl propionic phenolic acid illustrate the new method's performance. This method can be readily automated to deal with complex samples such as those occurring in metabolomics, in in-cell as well as in in vivo NMR applications, where speed and temporal stability are often primary concerns.

Mammalian models, and mouse studies in particular, play a central role in our understanding of placental development. Magnetic resonance imaging (MRI) could be a valuable tool to further these studies, providing both structural and functional information. As fluid dynamics throughout the placenta are driven by a variety of flow and diffusion processes, diffusion-weighted MRI could enhance our understanding of the exchange properties of maternal and fetal blood pools - and thereby of placental function. These studies, however, have so far been hindered by the small sizes, the unavoidable motions, and the challenging air/water/fat heterogeneities, associated with mouse placental environments. The present study demonstrates that emerging methods based on the spatiotemporal encoding (SPEN) of the MRI information can robustly overcome these obstacles. Using SPEN MRI in combination with albumin-based contrast agents, we analyzed the diffusion behavior of developing placentas in a cohort of mice. These studies successfully discriminated the maternal from the fetal blood flows; the two orders of magnitude differences measured in these fluids' apparent diffusion coefficients suggest a nearly free diffusion behavior for the former and a strong flow-based component for the latter. An intermediate behavior was observed by these methods for a third compartment that, based on maternal albumin endocytosis, was associated with trophoblastic cells in the interphase labyrinth. Structural features associated with these dynamic measurements were consistent with independent intravital and ex vivo fluorescence microscopy studies and are discussed within the context of the anatomy of developing mouse placentas.

In NMR well-logging, the measurement apparatus typically consists of a permanent magnet which is inserted into a bore, and the sample is the rock surrounding the borehole. When compared to the conditions of standard NMR experiments, this application is thus challenged by relatively weak and invariably inhomogeneous B₀ and B₁ fields. Chemical shift information is not generally obtained in these measurements. Instead, diffusivity, porosity and permeability information is collected from multi-echo decay measurements - most often using a Carr-Purcell Meiboom-Gill (CPMG) pulse sequence to enhance the experiment's limited sensitivity. In this work, we explore the consequences of replacing the hard square pulses used in a typical CPMG sequence with chirped pulses sweeping a range of frequencies. The greater bandwidths that for a maximum B₁ level can be excited by chirped pulses translates into marked expansion of the detection volume, and thus significant signal-to-noise improvements when compared to standard CPMG acquisitions using hard pulses. This improvement, usually amounting to signal enhancements ≥3, can be used to reduce the experimental time of NMR well-logging measurements, for measuring T₂ even when B₀ and B₁ inhomogenieties complicate the measurements, and opening new opportunities in the determination of diffusional properties.

Measuring metabolism's time- and space-dependent responses upon stimulation lies at the core of functional magnetic resonance imaging. While focusing on water's sole resonance, further insight could arise from monitoring the temporal responses arising from the metabolites themselves, in what is known as functional magnetic resonance spectroscopy. Performing these measurements in real time, however, is severely challenged by the short functional timescales and low concentrations of natural metabolites. Dissolution dynamic nuclear polarization is an emerging technique that can potentially alleviate this, as it provides a massive sensitivity enhancement allowing one to probe low-concentration tracers and products in a single-scan. Still, conventional implementations of this hyperpolarization approach are not immediately amenable to the repeated acquisitions needed in real-time functional settings. This work proposes a strategy for functional magnetic resonance of hyperpolarized metabolites that bypasses this limitation, and enables the observation of real-time metabolic changes through the synchronization of stimuli-triggered, multiple-bolus injections of the metabolic tracer ¹³C ₁-pyruvate. This new approach is demonstrated with paradigms tailored to reveal in vivo thresholds of murine hind-limb skeletal muscle activation, involving the conversion of ¹³C₁-pyruvate to ¹³C₁-lactate and ¹³C₁-alanine. These functional hindlimb studies revealed that graded skeletal muscle stimulation causes commensurate increases in glycolytic metabolism in a frequency- and amplitude-dependent fashion, that can be monitored on the seconds/minutes timescale using dissolution dynamic nuclear polarization. Spectroscopic imaging further allowed the in vivo visualization of uptake, transformation and distribution of the tracer and products, in fast-twitch glycolytic and in slow-twitch oxidative muscle fiber groups. While these studies open vistas in time and sensitivity for metabolic functional magnetic resonance studies in muscle, the simplicity of our approach makes this technique amenable to a wide range of functional metabolic tracer studies.

A main obstacle arising when using ex situ hyperpolarization to increase the sensitivity of biomolecular NMR is the fast relaxation that macromolecular spins undergo upon being transferred from the polarizer to the spectrometer, where their observation takes place. To cope with this limitation, the present study explores the use of hyperpolarized water as a means to enhance the sensitivity of nuclei in biomolecules. Methods to achieve proton polarizations in excess of 5% in water transferred into the NMR spectrometer were devised, as were methods enabling this polarization to last for up to 30 s. Upon dissolving amino acids and polypeptides sited at the spectrometer into such hyperpolarized water, a substantial enhancement of certain biomolecular amide and amine proton resonances was observed. This exchange-driven ¹H enhancement was further passed on to side-chain and to backbone nitrogens, owing to spontaneous one-bond Overhauser processes. ¹⁵N signal enhancements >500 over 11.7 T thermal counterparts could thus be imparted in a kinetic process that enabled multiscan signal averaging. Besides potential bioanalytical uses, this approach opens interesting possibilities in the monitoring of dynamic biomolecular processes, including solvent accessibility and exchange process.

Purpose To introduce a method that provides simultaneous spatial and spectral selectivity, whose implementation is less demanding than - and quality comparable to - conventional 2D spectral-spatial counterparts. Theory Spatiotemporal encoding concepts lead to a spatially selective, chemical-shift-dependent echo, with simultaneous dephasing of all other off-resonant species. The approach only requires applying a pair of suitable radiofrequency-swept pulses, and allows arbitrary shaping of the spatial profiles. Methods Based on arguments derived for chirp pulses operating in the sequential-sweep approximation, quadratic-phase SLR excitation and refocusing waveforms were designed and used to collect 2D slice- and shift-selective images on a 7 T microimaging system (phantoms). The same strategy was used to obtain multi-slice echo-planar and spin-echo images of breast on human volunteers in a 3 T scanner. Results The method managed to deliver excellent shift-selective multi-slice images in phantoms and human volunteers. Simultaneous water and fat images were also collected in a single, interleaved acquisition mode on both platforms, using straightforward sequence and reconstruction modifications of the basic scheme. Conclusion A new way to achieve chemical shift selectivity with high quality spatial profiling is achieved, without the usual requirements for playing out fast oscillating gradients in conjunction with carefully timed radiofrequency pulses.

Dissolution dynamic nuclear polarization (DNP) enables high-sensitivity solution-phase NMR experiments on long-lived nuclear spin species such as ¹⁵N and ¹³C. This report explores certain features arising in solution-state ¹H NMR upon polarizing low-γ nuclear species. Following solid-state hyperpolarization of both ¹³C and ¹H, solution-phase ¹H NMR experiments on dissolved samples revealed transient effects, whereby peaks arising from protons bonded to the naturally occurring ¹³C nuclei appeared larger than the typically dominant ¹²C-bonded ¹H resonances. This enhancement of the satellite peaks was examined in detail with respect to a variety of mechanisms that could potentially explain this observation. Both two- and three-spin phenomena active in the solid state could lead to this kind of effect; still, experimental observations revealed that the enhancement originates from ¹³C→¹H polarization-transfer processes active in the liquid state. Kinetic equations based on modified heteronuclear cross-relaxation models were examined, and found to well describe the distinct patterns of growth and decay shown by the ¹³C-bound ¹H NMR satellite resonances. The dynamics of these novel cross-relaxation phenomena were determined, and their potential usefulness as tools for investigating hyperpolarized ensembles and for obtaining enhanced-sensitivity ¹H NMR traces was explored.

Interruptions in cerebral blood flow may lead to devastating neural outcomes. Magnetic resonance has a central role in diagnosing and monitoring these insufficiencies, as well as in understanding their underlying metabolic consequences. Magnetic resonance spectroscopy (MRS) in particular can probe ischemia via the signatures of endogenous metabolites including lactic acid (Lac), N-acetylaspartate, creatine (Cre), and cholines. Typically, MRS reports on these metabolites' concentrations. This study focuses on establishing the potential occurrence of in vivo longitudinal relaxation enhancement (LRE) effects - a phenomenon involving a reduction of the apparent T 1 with selective bandwidth excitations -in a rat stroke model at 21.1 T. Statistically significant reductions in Cre's apparent T 1 s were observed at all the examined post-ischemia time points for both ipsi- and contralateral hemispheres, thereby establishing the existence of LREs for this metabolite in vivo. Ischemia-dependent LRE trends were also noted for Lac in the ipsilateral hemisphere only 24 hours after ischemia. Metabolic T 1 s were also found to vary significantly as a function of post-stroke recovery time, with the most remarkable and rapid changes observed for Lac T 1 s. The potential of such measurements to understand stroke at a molecular level and assist in its diagnosis, is discussed.

Nitrogen is an element of utmost importance in chemistry, biology and materials science. Of its two NMR-active isotopes, ¹⁴N and ¹⁵N, solid-state NMR (SSNMR) experiments are rarely conducted upon the former, due to its low gyromagnetic ratio (γ) and broad powder patterns arising from first-order quadrupolar interactions. In this work, we propose a methodology for the rapid acquisition of high quality ¹⁴N SSNMR spectra that is easy to implement, and can be used for a variety of nitrogen-containing systems. We demonstrate that it is possible to dramatically enhance ¹⁴N NMR signals in spectra of stationary, polycrystalline samples (i.e., amino acids and active pharmaceutical ingredients) by means of broadband cross polarization (CP) from abundant nuclei (e.g., ¹H). The BRoadband Adiabatic INversion Cross-Polarization (BRAIN-CP) pulse sequence is combined with other elements for efficient acquisition of ultra-wideline SSNMR spectra, including Wideband Uniform-Rate Smooth-Truncation (WURST) pulses for broadband refocusing, Carr-Purcell Meiboom-Gill (CPMG) echo trains for T₂-driven S/N enhancement, and frequency-stepped acquisitions. The feasibility of utilizing the BRAIN-CP/WURST-CPMG sequence is tested for ¹⁴N, with special consideration given to (i) spin-locking integer spin nuclei and maintaining adiabatic polarization transfer, and (ii) the effects of broadband polarization transfer on the overlapping satellite transition patterns. The BRAIN-CP experiments are shown to provide increases in signal-to-noise ranging from four to ten times and reductions of experimental times from one to two orders of magnitude compared to analogous experiments where ¹⁴N nuclei are directly excited. Furthermore, patterns acquired with this method are generally more uniform than those acquired with direct excitation methods. We also discuss the proposed method and its potential for probing a variety of chemically distinct nitrogen environments.

The metabolic status of muscle changes according to the energetic demands of the organism. Two key regulators of these changes include exercise and insulin, with exercise eliciting catabolic expenditure within seconds and insulin enabling anabolic energy investment over minutes to hours. This study explores the potential of time-resolved hyperpolarized dynamic ¹³C spectroscopy to characterize the in vivo metabolic phenotype of muscle during functional and biochemical insulin-induced stimulation of muscle. Using [¹³C₁]pyruvic acid as a tracer, we find that despite the different time scales of these forms of stimulation, increases in pyruvate label transport and consumption and concomitant increases in initial rates of the tracer metabolism to lactate were observed for both stimuli. By contrast, rates of tracer metabolism to labeled alanine increased incrementally for insulin but remained unchanged following exercise-like muscle stimulation. Kinetic analysis revealed that branching of the hyperpolarized [¹³C]pyruvate tracer between lactate and alanine provides significant tissuespecific biomarkers that distinguish between anabolic and catabolic fates in vivo according to the routing of metabolites between glycolytic and amino acid pathways.

Objective: Recent years have seen an increased interest in combining MRI thermometry with devices capable of destroying malignancies by heat ablation. Expected from the MR protocols are accurate and fast thermal characterizations, providing real time feedback on restricted tissue volumes and/or rapidly moving organs like liver. This article explores the potential advantages of relying on spatiotemporally encoded (SPEN) sequences for retrieving real-time thermometric images based on the water's proton resonance frequency (PRF) shifts. Materials and methods: Hybrid spatiotemporal/k-space encoding single-scan MRI experiments were implemented on animal and human scanners, and their abilities to deliver single- and multi-slice real-time thermometric measurements based on PRF-derived phase maps in phantoms and in vivo, were compared against echo planar imaging (EPI) and gradient-echo counterparts. Results: Under comparable acquisition conditions, SPEN exhibited advantages vis-à-vis EPI in terms of dealing with inhomogeneous magnetic field distortions, with shifts arising due to changes in the central frequency offsets, with PRF distributions, and for zooming into restricted fields-of-view without special pulse sequence provisions. Conclusion: This work confirms the ability of SPEN sequences, particularly when implemented under fully-refocused conditions, to exploit their built-in robustness to shift- and field-derived inhomogeneities for monitoring thermal changes in real-time under in vitro and in vivo conditions.

Nuclear magnetic resonance spectroscopy is governed by longitudinal (T ₁) relaxation. For protein and nucleic acid experiments in solutions, it is well established that apparent T₁ values can be enhanced by selective excitation of targeted resonances. The present study explores such longitudinal relaxation enhancement (LRE) effects for molecules residing in biological tissues. The longitudinal relaxation recovery of tissue resonances positioned both down- and upfield of the water peak were measured by spectrally selective excitation/refocusing pulses, and compared with conventional water-suppressed, broadband-excited counterparts at 9.4T. Marked LRE effects with up to threefold reductions in apparent T₁ values were observed as expected for resonances in the 6-9ppm region; remarkably, statistically significant LRE effects were also found for several non-exchanging metabolite resonances in the 1-4ppm region, encompassing 30-50 % decreases in apparent T₁ values. These LRE effects suggest a novel means of increasing the sensitivity of tissue-oriented experiments, and open new vistas to investigate the nature of interactions among metabolites, water and macromolecules at a molecular level. Relax your mind: Longitudinal relaxation enhancement (LRE) is a phenomenon known in biomolecular NMR spectroscopy, which so far has not been observed for metabolites in tissues. In brain tissues, selective excitation shortens the apparent T₁ of exchanging metabolic resonances by 30-300 %. The ensuing high-fidelity spectra are promising for studying the nature of metabolic interactions within tissues.

A novel method for acquiring and processing quality multislice spectroscopically resolved 2D images in a single shot is introduced and illustrated. By contrast to the majority of single-scan spectroscopic imaging sequences developed so far, the method here discussed is not based on the acquisition of echo planar data in the k/t-space, but rather on the use of recently proposed spatiotemporal encoding methods. These techniques provide a robust alternative to classical techniques, as they can scan two spatial plus one spectral dimension by oscillating a single imaging gradient. This work demonstrates that the use of extended spectral/spatial super-resolution algorithms coupled to new experimental spatiotemporal encoding formulations based on swept inversions rather than on chirped excitations can lead to novel spatiotemporal encoding-based tools for resolving complex multisliced 2D images according to the chemical shifts in subsecond experiments. A number of phantom-based models were explored to clarify the relative merits of this technique and estimate its sensitivity performance. In vivo results of fat and water separation on abdominal imaging of mice at 7 T and on human breast imaging at 3 T are presented.

The understanding and control of spin dynamics play a fundamental role in modern NMR imaging, for devising new ways to monitor an object's density as well as for enabling the tailored excitation of spins in space. It has recently been shown that by relying on spatiotemporal encoding (SPEN), new forms of single-scan multidimensional NMR spectroscopy and imaging become feasible. The present study extends those imaging developments, by introducing a new class of multidimensional excitation pulses that relies on SPEN concepts. We focus in particular on a family of "hybrid" 2D radiofrequency (RF) pulses that operate in both direct and reciprocal excitation space, and which can spatially sculpt the spin magnetization in manners that are beyond the reach of sequential 1D pulse shaping. These SPEN-based 2D pulses are compatible with a majority of single- and multi-scan imaging techniques. Like the corresponding SPEN-based hybrid 2D acquisitions, these pulses can benefit from a high robustness against field inhomogeneities and/or offset effects that affect their k-space-based counterparts. These properties are analyzed, and illustrated with numerical simulations and model experiments.

Object An approach has been recently introduced for acquiring arbitrary 2D NMR spectra or images in a single scan, based on the use of frequency-swept RF pulses for the sequential excitation and acquisition of the spins response. This spatiotemporal-encoding (SPEN) approach enables a unique, voxel-by-voxel refocusing of all frequency shifts in the sample, for all instants throughout the data acquisition. The present study investigates the use of this full-refocusing aspect of SPEN-based imaging in the multi-shot MRI of objects, subject to sizable field inhomogeneities that complicate conventional imaging approaches. Materials and methods 2D MRI experiments were performed at 7 T on phantoms and on mice in vivo, focusing on imaging in proximity to metallic objects. Fully refocused SPEN-based spin echo imaging sequences were implemented, using both Cartesian and back-projection trajectories, and compared with k-space encoded spin echo imaging schemes collected on identical samples under equal bandwidths and acquisition timing conditions. Results In all cases assayed, the fully refocused spatiotemporally encoded experiments evidenced a ca. 50 % reduction in signal dephasing in the proximity of the metal, as compared to analogous results stemming from the k-space encoded spin echo counterparts. Conclusion The results in this study suggest that SPENbased acquisition schemes carry the potential to overcome strong field inhomogeneities, of the kind that currently preclude high-field, high-resolution tissue characterizations in the neighborhood of metallic implants.

Efficient acquisition of ultra-wideline solid-state NMR powder patterns is a continuing challenge. In particular, when the breadth of the powder pattern is much larger than the cross-polarization (CP) excitation bandwidth, transfer efficiencies suffer and experimental times are greatly increased. Presented herein is a CP pulse sequence with an excitation bandwidth that is up to ten times greater than that available from a conventional spin-locked CP pulse sequence. The pulse sequence, broadband adiabatic inversion CP (BRAIN-CP), makes use of the broad, uniformly large frequency profiles of chirped inversion pulses, to provide these same characteristics to the polarization transfer process. A detailed theoretical analysis is given, providing insight into the polarization transfer process involved in BRAIN-CP. Experiments on spin-1/2 nuclei including ¹¹⁹Sn, ¹⁹⁹Hg and ¹⁹⁵Pt nuclei are presented, and the large bandwidth improvements possible with BRAIN-CP are demonstrated. Furthermore, it is shown that BRAIN-CP can be combined with broadband frequency-swept versions of the Carr-Purcell-Meiboom-Gill experiment (for instance with WURST-CPMG, or WCPMG for brevity); the combined BRAIN-CP/WCPMG experiment then provides multiplicative signal enhancements of both CP and multiple-echo acquisition over a broad frequency region.

We show that coupled-spin network manipulations can be made highly effective by repeated projections of the evolving quantum states onto diagonal density-matrix states (populations). As opposed to the intricately crafted pulse trains that are often used to fine-tune a complex network's evolution, the strategy hereby presented derives from the "quantum Zeno effect" and provides a highly robust route to guide the evolution by destroying all unwanted correlations (coherences). We exploit these effects by showing that a relaxationlike behavior is endowed to polarization transfers occurring within a N-spin coupled network. Experimental implementations yield coupling constant determinations for complex spin-coupling topologies, as demonstrated within the field of liquid-state nuclear magnetic resonance.

The power of NMR spectroscopy lies in its ability to obtain information about the structure and dynamics of individual atoms in complex molecules. The narrow frequency ranges of the NMR-active nuclei (₁H, ₁₃C, ₁₅N, ₃₁P) in biological macromolecules such as proteins, nucleic acids, and their complexes result in severe resonance overlap in one-dimensional spectra, making it practically impossible to extract atom-resolved information even for relatively small biopolymers. In order to deal with this problem, multidimensional NMR techniques are commonly used to spread and correlate the signals of individual nuclear spins along different frequency dimensions. A major drawback of conventional multidimensional NMR is the long experimental time needed for recording its data-a reflection of the hundreds or even thousands of experimental repetitions that are inherent to this kind of spectroscopy. While acquisition times for one-and two-dimensional spectra are on the order of seconds and minutes, respectively, typical three-dimensional NMR experimental times could be as long as days. For obtaining the four-and higher-dimensional spectra that may be of particular interest to the study of large molecular systems or of disordered proteins, acquisition times become altogether unreasonable. Moreover, as a large number of different spectra must be recorded during the NMR investigation of a macromolecule, this demands a high stability of the ensuing samples. Long experimental times also represent a major limitation for high-throughput studies as performed in the context of structural proteomics. It follows from all these considerations that reduced acquisition times could make data acquisition more efficient, and greatly enhance the structural and dynamic capabilities of biomolecular NMR. They could also enable real-time investigations of kinetic molecular processes, such as biochemical reactions, protein/RNA folding, and molecular assemblies by multidimensional spectroscopy. In this chapter, we review some of the fast multidimensional NMR data acquisition techniques that have emerged in recent years and their underlying concepts, and we discuss fields of biomolecular NMR research that could benefit from such fast NMR methods.

Recent studies have described some of the new opportunities that have arisen within the context of ultrafast two-dimensional imaging with the advent of spatiotemporal encoding methods. This article explores the potential of integrating these non-Fourier, single-scan, two-dimensional MRI principles, with multi-slice and phase-encoding schemes acting along a third dimension. In unison, these combinations enable the acquisition of complete three-dimensional images from volumes of interest within a 1-s timescale. A number of alternatives are explored for carrying out these very rapid three-dimensional acquisitions, including the use of two-dimensional, slice-selective, spatiotemporal encoding radiofrequency pulses, driven-equilibrium slice-selective schemes, and phase-encoded volumetric approaches. When tested under in vivo conditions, the 'hybrid' schemes combining spatiotemporal encoding with k-encoding imaging principles, proved to be superior to traditional schemes based on echo planar imaging. The resulting images were found to be less affected by field inhomogeneities and by other potential offset-derived distortions owing to a combination of factors whose origin is discussed. Further features, extensions and applications of these principles are also addressed.

This work examines several polycrystalline diamond samples for their potential as polarizing agents for dynamic nuclear polarization (DNP) in NMR. Diamond samples of various origin and particle sizes ranging from a few nanometers to micrometers were examined by EPR, solid-state NMR and DNP techniques. A correlation was found between the size of the diamond particles and the electron spin-lattice relaxation time, the ¹³C nuclear spin-lattice relaxation times in room temperature magic-angle-spinning experiments, and the ability of the diamond carbons to be hyperpolarized by irradiating unpaired electrons of inherent defects by microwaves at cryogenic temperatures. As the size of the diamond particles approaches that of bulk diamond, both electron and nuclear relaxation times become longer. NMR signal enhancement through DNP was found to be very efficient only for these larger size diamond samples. The reasons and implications of these results are briefly discussed, in the light of these EPR, DNP, and NMR observations.

Dynamic nuclear polarization (DNP) followed by sudden sample dissolution, is a topic of active investigation owing to the method's unique prospects for the delivery of NMR spectra and images with unprecedented sensitivity. This experiment achieves hyperpolarization by the combined effects of electron-nuclear irradiation and cryogenic operation; the exploitation of these states occurs following a sudden melting and flushing of the resulting pellet from its original environment into a conventional, liquid-state setting. This melting and flushing usually demands using the equivalent of a few milliliters of hot solvent, a procedure which although well suited for in vivo studies leads to an excessive sample volume when considering typical analytical settings. The present study explores a way of reducing the ensuing dilution of the hyperpolarized analytes, by employing a combination of immiscible liquids for performing the melting and flushing. It is shown that suitable combinations of immiscible solvents - both in terms of their heat capacities and densities - allow one to melt the targeted cryogenic pellet and dissolve the hyperpolarized analytes in a fraction of the solvent hitherto required. By tailoring the resulting volume to the needs of a conventional 5 mm NMR probe, a substantial sensitivity enhancement can be added to the hyperpolarization process. An extra benefit may arise from using radicals that preferentially dissolve in the immiscible organic phase, by way of a lengthening of the relaxation time of the investigated analytes. Examples of these principles are given, and further potential extensions of this approach are discussed.

Typing these lines makes me feel deeply honored, and perhaps somewhat nervous as well. The two preceding Editors of the Journal of Magnetic Resonance were true giants, and their skills were instrumental in guiding the Journal to its current status. It suffices to open an issue of JMR published during the 1970s or 1980s, to appreciate what an outstanding vision Wallace S. Brey had as founder and steward of the Journal over its initial decades. Every issue published in JMR during Breys 27 years as Editor contains at least one and often several ground-breaking papers setting the foundations of EPR, NQR, NMR or MRI. Something similar can be said about the leadership that Stan Opella brought upon becoming the Journals Editor in 1997: despite the increasing complexity that over the intervening years characterized the field of magnetic resonance, Stan managed to endow the Journal with manuscripts of the highest scientific standards; and he succeeded in doing so, while continuously improving the quality and speed of the manuscripts publication process. For all these achievements I believe we all remain deeply grateful to Stan as well as delighted to hear that he has agreed to remain involved with JMR, as member of its Editorial Board.

We experimentally and theoretically demonstrate the purity (polarization) control of qubits entangled with multiple spins, using induced dephasing in nuclear magnetic resonance setups to simulate repeated quantum measurements. We show that one may steer the qubit ensemble towards a quasiequilibrium state of a certain purity by choosing suitable time intervals between dephasing operations. These results demonstrate that repeated dephasing at intervals associated with the anti-Zeno regime leads to ensemble purification, whereas those associated with the Zeno regime lead to ensemble mixing.

Single-scan MRI underlies a wide variety of clinical and research activities, including functional and diffusion studies. Most common among these "ultrafast" MRI approaches is echo-planar imaging. Notwithstanding its proven success, echo-planar imaging still faces a number of limitations, particularly as a result of susceptibility heterogeneities and of chemical shift effects that can become acute at high fields. The present study explores a new approach for acquiring multidimensional MR images in a single scan, which possesses a higher built-in immunity to this kind of heterogeneity while retaining echo-planar imaging's temporal and spatial performances. This new protocol combines a novel approach to multidimensional spectroscopy, based on the spatial encoding of the spin interactions, with image reconstruction algorithms based on super-resolution principles. Single-scan two-dimensional MRI examples of the performance improvements provided by the resulting imaging protocol are illustrated using phantom-based and in vivo experiments.

Conformational transitions and structural rearrangements are central to the function of many RNAs yet remain poorly understood. We have used ultrafast multidimensional NMR techniques to monitor the adenine-induced folding of an adenine-sensing riboswitch in real time, with nucleotide-resolved resolution. By following changes in 2D spectra at rates of approximately 0.5 Hz, we identify distinct steps associated with the ligand-induced folding of the riboswitch. Following recognition of the ligand, long range looploop interactions form and are then progressively stabilized before the formation of a fully stable complex over approximately 2-3 minutes. The application of these ultrafast multidimensional NMR methods provides the opportunity to determine the structure of RNA folding intermediates and conformational trajectories.

Noise measurements of nuclear spin systems using a tuned circuit can reveal the signatures of two different phenomena: Thermal circuit noise absorbed by the spin system, and nuclear spin-noise leading to tiny fluctuating magnetization components. Polarization enhancement can increase the observed noise amplitudes due to an enlarged coupling with the reception circuit. In this work we explore the detection of noise in ¹H NMR of liquid water samples whose spin alignment is enhanced via ex situ dynamic nuclear polarization. A number of ancillary phenomena related to this kind of experiments are also documented.

Intermolecular Multiple-Quantum Coherences (iMQCs) can yield interesting NMR information of high potential usefulness in spectroscopy and imaging - provided their associated sensitivity limitations can be overcome. A recent study demonstrated that ex situ dynamic nuclear polarization (DNP) could assist in overcoming sensitivity problems for iMQC-based experiments on ¹³C nuclei. In the present work we show that a similar approach is possible when targeting the protons of a hyperpolarized solvent. It was found that although the DNP procedure enhances single-quantum ¹H signals by about 600, which is significantly less than in optimized low-γ liquid-state counterparts, the non-linear dependence of iMQC-derived signals on polarization can yield very large enhancements approaching 10⁶. Cleary no practical amount of data averaging can match this kind of sensitivity gains. The fact that DNP endows iMQC-based ¹H NMR spectra with a sensitivity that amply exceeds that of their thermally polarized single-quantum counterpart, is confirmed in a number of simple single-scan 2D imaging experiments.

Scan and deliver: By combining imaging-based spectral/spatial 2D radiofrequency manipulations (see scheme, left) with Hadamard-weighting principles, 2D NMR spectra can be retrieved within a single scan (right). This approach can give homo- or heteronuclear correlations with an enhanced sensitivity over conventional ultrafast 2D NMR spectroscopy.

Multidimensional NMR spectroscopy is a well-established technique for the characterization of structure and fast-time-scale dynamics of highly populated ground states of biological macromolecules. The investigation of short-lived excited states that are important for molecular folding, misfolding and function, however, remains a challenge for modern biomolecular NMR techniques. Off-equilibrium real-time kinetic NMR methods allow direct observation of conformational or chemical changes by following peak positions and intensities in a series of spectra recorded during a kinetic event. Because standard multidimensional NMR methods required to yield sufficient atom-resolution are intrinsically time-consuming, many interesting phenomena are excluded from real-time NMR analysis. Recently, spatially encoded ultrafast 2D NMR techniques have been proposed that allow one to acquire a 2D NMR experiment within a single transient. In addition, when combined with the SOFAST technique, such ultrafast experiments can be repeated at high rates. One of the problems detected for such ultrafast protein NMR experiments is related to the heteronuclear decoupling during detection with interferences between the pulses and the oscillatory magnetic field gradients arising in this scheme. Here we present a method for improved ultrafast data acquisition yielding higher signal to noise and sharper lines in single-scan 2D NMR spectra. In combination with a fast-mixing device, the recording of ¹H-¹⁵N correlation spectra with repetition rates of up to a few Hertz becomes feasible, enabling real-time studies of protein kinetics occurring on time scales down to a few seconds.

Spatial encoding is a particular kind of spin manipulation that enables the acquisition of multidimensional NMR spectra within a single scan. This encoding has been shown to possess a general applicability and to enable the completion of arbitrary nD NMR acquisitions within a single transient. The present study explores its potential towards the acquisition of 2D DOSY spectra, where the indirect dimension is meant to encode molecular displacements rather than a coherent spin evolution. We find that in its simplest form this extension shows similarities with methods that have been recently discussed for the single-scan acquisition of this kind of traces; still, a number of advantageous features are also evidenced by the "ultrafast" modality hereby introduced. The principles underlying the operation of this new single-scan 2D DOSY approach are discussed, its use is illustrated with a variety of sequences and of samples, the limitations of this new experiment are noted, and potential extensions of the methodology are mentioned.

Few analytical techniques rival the capabilities of two-dimensional nuclear magnetic resonance. A scheme enabling 2D NMR acquisitions within a single-scan has been recently demonstrated, based on combined field gradient and radiofrequency manipulations. Distortions were observed upon implementing such 'ultrafast' experiments on solids undergoing magic-angle-spinning, presumably due to interferences arising between the periodic time-dependencies involved in the mechanical and in the spin manipulations. Experimental and numerical setups were designed to investigate these effects, and to find conditions that minimize them. When devoid of these non-idealities, quality 2D NMR spectra could be retrieved from spinning polymers within a single-scan.

We have recently proposed a protocol for retrieving multidimensional magnetic resonance images within a single scan, based on a spatial encoding of the spin interactions. This methodology relies on progressively dephasing spin coherences throughout a sample; for instance, by sweeping a radiofrequency pulse in the presence of a magnetic field gradient. When spins are suitably refocused by a second (acquisition) field gradient, this yields a time-domain signal reflecting in its magnitude the spatial distribution of spins throughout the sample. It is hereby shown that whereas the absolute value of the resulting signals conveys such imaging information, the hitherto unutilized phase modulation of the signal encodes the chemical shift offsets of the present speciae. Spectroscopically-resolved multidimensional images can thereby be retrieved in this fashion at no additional expense in either experimental complexity, sensitivity or acquisition time-simply by performing an additional analysis of the collected data. The resulting approach to single-scan spectroscopic imaging can also incorporate "RF shimming" compensating abilities, capable of providing high-resolution spectral and high-definition imaging data even under the presence of substantial magnetic field inhomogeneities. The principles of these methodologies as applied to spectroscopic imaging are briefly reviewed and compared against the background of traditional Fourier-based single-scan spectroscopic imaging protocols. Demonstrations of these new multidimensional spectroscopic MRI experiments on simple phantoms are also given.

Two-dimensional (2D) NMR is an important tool for elucidating molecular structure and dynamics. However, the method is limited by the low sensitivity inherent to NMR techniques, resulting in typical acquisition times for 2D NMR spectra ranging from minutes to hours. A number of hyperpolarization techniques have been explored to boost NMR's sensitivity, including an ex situ dynamic nuclear polarization method capable of yieldingfor an array of molecules and under conventional observation conditions for liquid samples signals that exceed those currently afforded by the highest-field spectrometers by several orders of magnitude. Whereas this methodology is able to provide the sensitivity equivalent of 10 ⁶ scans, it is constrained to extract its super-spectrum within a single transient, making it a poor starting point for conventional 2D NMR acquisitions. Here, we show that if the ex situ dynamic nuclear polarization approach is suitably merged with spatially encoded ultrafast NMR spectroscopy, 2D NMR spectra of liquid samples at submicromolar concentrations can be acquired within 0.1 s.

Following unidirectional biophysical events such as the folding of proteins or the equilibration of binding interactions, requires experimental methods that yield information at both atomic-level resolution and at high repetition rates. Toward this end a number of different approaches enabling the rapid acquisition of 2D NMR spectra have been recently introduced, including spatially encoded "ultrafast" 2D NMR spectroscopy and SOFAST HMQC NMR. Whereas the former accelerates acquisitions by reducing the number of scans that are necessary for completing arbitrary 2D NMR experiments, the latter operates by reducing the delay between consecutive scans while preserving sensitivity. Given the complementarities between these two approaches it seems natural to combine them into a single tool, enabling the acquisition of full 2D protein NMR spectra at high repetition rates. We demonstrate here this capability with the introduction of "ultraSOFAST" HMQC NMR, a spatially encoded and relaxation-optimized approach that can provide 2D protein correlation spectra at ∼1 s repetition rates for samples in the ∼2 mM concentration range. The principles, relative advantages, and current limitations of this new approach are discussed, and its application is exemplified with a study of the fast hydrogen-deuterium exchange characterizing amide sites in Ubiquitin.

We have recently proposed a protocol for retrieving multidimensional magnetic resonance spectra and images within a single scan, based on a spatial encoding of the spin interactions. The spatial selectivity of this encoding process also opens up new possibilities for compensating magnetic field inhomogeneities; not by demanding extreme uniformities from the B₀ fields, but by compensating for their effects at an excitation and/or refocusing level. This potential is hereby discussed and demonstrated in connection with the single-scan acquisition of high-definition multidimensional images. It is shown that in combination with time-dependent gradient and radiofrequency manipulations, the new compensation approach can be used to counteract substantial field inhomogenities at either global or local levels over relatively long periods of time. The new compensation scheme could find uses in areas where heterogeneities in magnetic fields present serious obstacles, including rapid studies in regions near tissue/air interfaces. The principles of the B₀ compensation method are reviewed for one- and higher-dimensional cases, and experimentally demonstrated on a series of 1D and 2D single-scan MRI experiments on simple phantoms.

Solid-state NMR has been used to analyze the chemical environments of sodium sites in powdered crystalline samples of sodium nucleotide complexes. Three of the studied complexes have been previously characterized structurally by crystallography (disodium deoxycytidine-5-monophosphate heptahydrate, disodium deoxyuridine-5-monophosphate pentahydrate and disodium adenosine-5-triphosphate trihydrate). For these salts, the nuclear quadrupole coupling parameters measured by ²³Na multiple-quantum magic-angle-spinning NMR could be readily correlated with sodium ion coordination environments. Furthermore, two complexes that had not been previously characterized structurally, disodium uridine-3-monophosphate and a disodium uridine-3-monophosphate/disodium uridine-2- monophosphate mix, were identified by solid-state NMR. A spectroscopic assignment of the four sites of an additional salt, disodium adenosine-5-triphosphate trihydrate, is also presented and discussed within the context of creating a general approach for the spectroscopic assignment of multiple sites in sodium nucleotide complexes.

Multidimensional NMR spectroscopy plays an important role in the characterization of molecular structure and dynamics. A new methodology for acquiring this kind of spectra has been recently demonstrated, endowed with the potential to compress arbitrary multidimensional NMR acquisitions into a single scan. This "ultrafast" nD acquisition protocol is based on a spatiotemporal encoding of the indirect-domain spin evolution, followed by a repetitive decoding and reencoding of the information thus stored employing a train of alternating-sign gradients. Such train of switching gradients extending throughout the course of the data acquisition may pose extreme demands on a magnetic resonance system, particularly when dealing with nonshielded gradients, strong eddy currents, or rapidly relaxing spin systems. Limits to the in vivo applicability of such fast-switching scheme may also arise due to gradient-induced perineural stimulation. The present study describes a new approach to ultrafast nD NMR that reduces the number of gradient switchings during the acquisition period to zero, leading in essence to a constant-gradient acquisition scheme. This approach operates on the basis of a novel spatiotemporal encoding including discrete, temporally overlapping, frequency-shifted pulses. Principles and examples of this new approach are given; sensitivity limitations and signal-enhancing prospects of such constant-gradient acquisitions are also discussed and exemplified.

Single-scan multidimensional spectroscopy utilizes spatial dimensions for encoding the indirect-domain internal spin interactions. Various strategies have been hitherto demonstrated for fulfilling the encoding needs underlying this methodology; in analogy with their time-domain counterparts all of them have in common the fact that they proceed monotonically-starting at one end of the sample and concluding at the other. The present manuscript discusses another possibility that arises for the case of amplitude-modulated ultrafast nD NMR, whereby the spatial encoding progresses from both ends of the sample simultaneously towards the center. Such symmetric encoding is compatible with continuous or discrete excitations as well as with homonuclear or heteronuclear correlations, and exhibits a number of advantages vis-à-vis the unidirectional encodings that have been used so far: it originates echoes that are free from large first-order phase distortions, and yields nD peaks possessing a purely-absorptive character. It has the added advantage that for a given indirect-domain spectral resolution it can complete its task in half the time required by a conventional monotonic spatial encoding, leading to potentially important gains in sensitivity. The main features underlying this new spatially symmetric encoding protocol are derived, and its advantages are demonstrated with a series of amplitude-modulated homo- and hetero-nuclear 2D ultrafast NMR examples.

Ultrafast 2D NMR replaces the time-domain parametrization usually employed to monitor the indirect-domain spin evolution, with an equivalent encoding along a spatial geometry. When coupled to a gradient-assisted decoding during the acquisition, this enables the collection of complete 2D spectra within a single transient. We have presented elsewhere two strategies for carrying out the spatial encoding underlying ultrafast NMR: a discrete excitation protocol capable of imparting a phase-modulated encoding of the interactions, and a continuous protocol yielding amplitude-modulated signals. The former is general but has associated with it a number of practical complications; the latter is easier to implement but unsuitable for certain 2D NMR acquisitions. The present communication discusses a new protocol that incorporates attractive attributes from both alternatives, imparting a continuous spatial encoding of the interactions yet yielding a phase modulation of the signal. This in turn enables a number of basic experiments that have shown particularly useful in the context of in vivo 2D NMR, including 2D J-resolved and 2D H,H-COSY spectroscopies. It also provides a route to achieving sensitivity-enhanced acquisitions for other homonuclear correlation experiments, such as ultrafast 2D TOCSY. The main features underlying this new spatial encoding protocol are derived, and its potential demonstrated with a series of phase-modulated homonuclear single-scan 2D NMR examples.

A new methodology capable of delivering complete 2D NMR spectra within a single scan was recently introduced. The resulting potential gain in time resolution could open new opportunities for in vivo spectroscopy, provided that the technical demands of the methodology are satisfied by the corresponding hardware. Foremost among these demands are the relatively short switching times expected from the applied gradient-echo trains. These rapid transitions may be particularly difficult to accomplish on imaging systems. As a step toward solving this problem, we assessed the possibility of replacing the square-wave gradient train currently used during the course of the acquisition by a shaped sinusoidal gradient. Examples of the implementation of this protocol are given, and successful ultrafast acquisitions of 2D NMR spectra with suitable spectral widths on a microimaging probe (for both phantom solutions and ex vivo mouse brains) are demonstrated.

A scheme enabling the acquisition of high-resolution nuclear magnetic resonance (NMR) spectra within inhomogeneous magnetic fields is introduced and exemplified. The scheme is based on the spatial encoding protocol recently introduced for collecting multidimensional NMR data within a single scan, which retrieves spectral information via interference phenomena between spin packets located at distinct positions within the sample. This in turn enables compensating for field inhomogeneities over the sample as a whole by shifting the phases of the radio-frequency pulses involved in the spatial encoding, rather than by demanding an extreme uniformity in the employed magnetic field. The upper tolerable field inhomogeneity limit thus becomes orders of magnitude higher than that in conventional time-domain acquisitions. No particular spatial dependencies are demanded by the new scheme, which can yield its high-resolution results on a single-scan basis.

We have recently demonstrated that the spatial encoding of internal nuclear magnetic resonance (NMR) spin interactions can be exploited to collect multidimensional NMR spectra within a single scan. Such experiments rely on an inhomogeneous spatial excitation of the spins throughout the sample, and lead to indirect-domain peaks via a constructive interference among the spatially resolved spin-packets that are thus created. The shape of the resulting indirect-domain echo peaks approaches a Sinc function when the chemical's distribution is uniform, but will depart from this function otherwise. It is hereby shown that a Fourier analysis of either the diagonal- or the cross-peaks resolved in these single-scan two-dimensional (2D) NMR experiments can in fact provide a weighted spatial distribution of the analyte originating such peak, thus opening up the possibility of completing spatially resolved multidimensional NMR measurements within a fraction of a second. Principles of this new mode of analysis are discussed, and examples where the potential of spatially resolved ultrafast 2D NMR spectroscopy is brought to bear are presented. Potential extensions of this approach to higher dimensions are also briefly addressed.

This study presents a theoretical, numerical, and experimental survey on the nature of homonuclear dipolar couplings in systems of half-integer quadrupolar nuclei undergoing magic-angle-spinning (MAS). Various spin interactions that do not commute with homonuclear dipolar couplings (chemical shift effects, heteronuclear dipolar couplings, quadrupolar interactions) may lead to recoupling effects under MAS, yielding a variety of pathways for transferring magnetization between proximate quadrupole nuclei in 2D correlation experiments. The Hamiltonians underlying this anisotropy-driven recoupling of the dipolar interactions are theoretically derived and their characteristics revealed from theoretical and numerical arguments. To explore when and how these various recoupling mechanisms become relevant, a variety of ²³Na and ¹¹B 2D exchange NMR experiments were performed at different external magnetic fields and MAS frequencies on several compounds: Na₂HPO₄·2H₂O, Na₂SO ₃, disodium deoxycytidine heptahydrate, B₂O₃ and B₁₀H₁₄. The structural information content afforded by these experiments as well as their potential limitations are discussed.

Multidimensional nuclear magnetic resonance (NMR) provides one of the foremost analytical tools available to elucidate the structure and dynamics of complex molecules in their native states. Executing this kind of experiment generally requires collecting an n-dimensional time-domain signal S, from which the spectrum arises via an appropriate Fourier analysis of its various time variables. This time-domain signal is actually measured directly only along one of the time axes, while the effects introduced by the remaining time variables are monitored via a parametric incrementation of their values throughout independent experiments. Two-dimensional (2D) NMR experiments thus usually require longer acquisition times than unidimensional experiments, 3D NMR is orders-of-magnitude more time consuming than 2D spectroscopy, etc. Very recently, we proposed and demonstrated an approach whereby data acquisition in 2D NMR can be parallelized, enabling the collection of complete 2D spectral sets within a single transient. The present paper discusses the extension of this 2D NMR methodology to an arbitrary number of dimensions. The principles of the ensuing ultrafast n-dimensional NMR approach are described, and a variety of homo- and heteronuclear 3D and 4D NMR spectra collected within a fraction of a second are presented.

New multidimensional NMR methods correlating the quadrupolar and heteronuclear dipolar interactions affecting a half-integer quadrupolar spin in the solid state are introduced and exemplified. The methods extend separated-local-field magic-angle spinning (SLF MAS) NMR techniques that have been used successfully in spin-1/2 spectroscopy to the study of S ≥ 3/2 nuclei. In our implementation, these techniques avoid homonuclear proton decoupling requirements by relying on moderately fast MAS rates (6-15 kHz) and use rotor-synchronized constant-time pulse sequences to achieve nearly arbitrary amplifications of the apparent dipolar coupling strengths. The result is a suite of simple 2D NMR experiments, whose line shapes carry valuable information about the structure and dynamics of solids containing quadrupolar and proton nuclei. The potential of these sequences was exploited to gather new insight into the structure and dynamics of a variety of boron-containing samples. These experimental SLF schemes were also extended to 3D NMR experiments that incorporate multiple-quantum MAS, thus enabling the resolution needed to study multiple chemical sites in a solid and providing a useful tool for the assignment of inequivalent sites.

New approaches to the characterization of resonances in the solid-state NMR spectroscopy of half-integer quadrupolar nuclei are explored, on the basis of the acquisition of heteronuclear separate-local-field spectra on rotating solids. In their two-dimensional version, these experiments correlate for each chemical site a second-order quadrupolar MAS powder pattern with the dipolar MAS sideband pattern to nearby heteronuclei. As 3D NMR sequences, such 2D anisotropic correlation spectra become separated for inequivalent chemical sites along a third, isotropic dimension. Extending in such manner separate-local-field NMR approaches to quadrupoles facilitates the assignment of inequivalent resonances to specific structural environments, and provides new tools for the investigation of dynamics in solids. Details about these 2D and 3D NMR experiments are given, and their application is illustrated with ₁H-₂₃Na recoupling experiments on mononucleotides possessing multiple bound cations.

The local ordering, morphology, and dynamics of aromatic cores and flexible alkyl spacers were analyzed for a homologous series of main-chain polymeric liquid crystals. ¹³C NMR experiments showed that the nematic ordering achieved by these synthetic polymers was retained into the solid state if their quenchings occur while remaining within the strong NMR magnetic field. The degree of orientation in the resulting glasses was investigated by variable-angle NMR experiments and found to differ between polymers with an even number of methylene units in the flexible spacer vs those with an odd number. To further discern at a molecular level the nature of these differences, the structures of these polyesters were examined by high-resolution solid-state ¹³C NMR. It was found that while the odd-chained series displayed a conformational annealing upon aligning, even-chained polymers were generally in all-trans conformations both for as-synthesized and for aligned samples. Variable-temperature 1D and 2D NMR experiments also illustrated substantial differences in the degree of motional dynamics between the odd and even polymer series: whereas considerable rigidity was exhibited by the even-numbered series all the way up to 150 °C, a relatively high flexibility displayed by the odd-methylene polymers. In unison, these measurements provide insight into the significant changes that can be imparted into the structure and dynamics of main-chain thermotropic polymers by subtle manipulations of their monomeric structures.

The nature of higher order quadrupolar-shielding effects in solid state nuclear magnetic resonance (NMR) was addressed. These effects were exploited to directly measure the antisymmetric components of the shielding tensor via single-crystal NMR investigations of quadrupolar nuclei. Nonsecular quadrupole-shielding contributions revealed several features, including, the emergence of field-independent terms, that could be directly traced to nonsecular contributions of either the CSA or the antisymmetric chemical shift. A numerical fitting algorithm was derived and used to extract such parameters from single-crystal rotation plots.

A general treatment of the residual 1-S dipolar couplings that arise when heteronuclear quadrupolar spin pairs are subjected to MAS is presented. The resulting model yields analytical expressions for the multiplet structure of 1D S-spin spectra.

The basic features of homonuclear interactions between quadrupolar nuclei undergoing magic-angle spinning (MAS) were studied. The effects of homonuclear spin-pair dipolar interactions at high spinning frequencies on nuclear magnetic resonance (NMR) spectroscopy were analyzed. The quadrupolar driven recouplings were investigated by numerical simulations on half-integer quadrupolar nuclei. A theoretical model of the dipolar recoupling was proposed based on average Hamiltonian theory.

Novel applications of solid state nuclear magnetic resonance (NMR) to the study of small molecules, synthetic polymers, biological systems, and inorganic materials continue at an accelerated rate. Instrumental to this uninterrupted expansion has been an improved understanding of the chemical physics underlying NMR. Such deeper understanding has led to novel forms of controlling the various components that make up the spin interactions, which have in turn redefined the analytical capabilities of solid state NMR measurements. This review presents a perspective on the basic phenomena and manipulations that have made this progress possible and describes the new opportunities and challenges that are being opened in the realms of spin-1/2 and quadrupole nuclei spectroscopies.

A series of uni- and multidimensional variants of the dipolar exchange-assisted recoupling (DEAR) NMR experiment is described and applied to determinations of ¹³C-¹⁴N dipolar local field spectra in amino acids and dipeptides. The DEAR protocol recouples nearby nuclei by relying on differences in their relative rates of longitudinal relaxation, and has the potential to give quantitative geometric results without requiring radiofrequency pulsing on both members of a coupled spin pair. One- and two-dimensional variants of this recoupling strategy on generic I-S pairs are discussed, and measurements of ¹³C-¹⁴N distances and 2D local field experiments sensitive to the relative orientation of CN vectors with respect to the ¹³C shielding tensor are presented. Since these measurements did not involve pulsing on the broad nitrogen resonance, their results were independent of the quadrupolar parameters of this nucleus. High-resolution 3D NMR versions of the 2D experiments were also implemented in order to separate their resulting local field patterns according to the isotropic shifts of inequivalent ¹³C sites. These high-resolution 3D acquisitions involved collecting a series of 2D DEAR NMR data sets on rotating samples as a function of spinning angle, and then subjecting the resulting data to a processing akin to that involved in variable-angle correlation NMR. Once successfully tested on L-alanine this experiment was applied to the analysis of a series of dipeptides, allowing us to extract separate local field ¹³C-¹⁴N spectra from this type of multisite systems.

Nonsecular dipolar couplings between spin-1/2 nuclei that are in close proximity to quadrupolar spins have been extensively documented in solid state nuclear magnetic resonance (NMR), particularly when involving directly bonded S = ¹³C, I = ¹⁴N spin pairs. These couplings arise due to the quadrupole-induced tilting of I's nuclear spin quantization axes, and their most notable characteristic is that they cannot be entirely averaged away by conventional magic-angle-spinning (MAS). Nonsecular dipolar couplings can also be expected to arise when both I and 5 are quadrupolar nuclei, even if these have hitherto not been analyzed in detail due to the interfering effects brought about by first- and second-order quadrupolar anisotropies. Yet, the advent of new high resolution techniques for studying half-integer quadrupole nuclei in solids such as multiple-quantum MAS or dynamic-angle-spinning can change this state of affairs. The present study presents a theoretical and numerical analysis on the results that can be expected from these techniques when applied to the observation of homonuclear or heteronuclear quadrupolar spin pairs in the high field limit. Variable field multiple-quantum MAS NMR results are then presented for a variety of compounds possessing ¹¹B-¹⁴N, ¹¹B-11B and ⁵⁵Mn-⁵⁵Mn spin pairs, that validate these theoretical predictions and illustrate the valuable information that can be extracted from analyzing these residual MAS couplings. The research potential as well as resolution limitations that according to theoretical calculations these effects will impart on MQMAS spectra recorded at low or moderate magnetic fields are thus evidenced.

The acquisition of distortion-free solid-state nuclear magnetic resonance (NMR) powder line shapes from half-integer spin systems possessing large quadrupole couplings is exemplified on cobaltophthalocyanine complexes. The acquisition of ideal-like static line shapes even for megahertz-wide central transition patterns is discussed, with the aid of spin−echo sequences incorporating short and very weak radio frequency (rf) pulses. Under these conditions most of the crystallites within the bandwidth of interest are excited with essentially orientation-independent pulse angles, while the acquisition of several experiments with varying carrier frequency offsets alleviates the limited bandwidth of the excitation given by the rf pulse lengths. After this approach was tuned with the aid of quantum mechanical calculations and model compounds, it was applied to the study of diamagnetic metal centers in hexacoordinated Co(III)phthalocyanines. Solid-state 59Co NMR spectra were acquired as a function of the external magnetic field on complexes with general structure [(L)2CoPc]Br, where Pc denotes the phthalocyanine macrocycle and the axial ligands L were pyridine, methylimidazole, methylpiperidine, and ammonia. Iterative numerical fittings of these data revealed anisotropic coupling parameters that were larger than those observed in cobaltoporphyrin analogues but, which like the latter, deviated from trends traditionally observed for nonaromatic octahedral cobalt complexes. These systematic differences observed for the various 59Co coupling parameters are discussed.

We report here an improved way of doing the multiple-quantum magic-angle spinning (MQMAS) NMR experiment that relies on the use of amplitude modulated pulses. These pulses were found to yield MQMAS NMR signals that are considerably stronger (≈200-300%) than the ones arising from the usual continuous wave pulse schemes by virtue of a superior efficiency of the triple- to single-quantum conversion process. Numerical simulations and experimental results taking ²³Na and ⁸⁷Rb nuclei as examples are presented that corroborate the usefulness of this approach.

Solid state ⁵⁹Co nuclear magnetic resonance (NMR) spectra were recorded on diamagnetic complexes with general structure Co(Por)L₂, where For = tetraphenylporphyrin, tetramethoxyphenylporphyrin, and octaethylporphyrin; and L = imidazole, methylimidazole, pyridine, and isoquinoline. Measurements were carried out at different magnetic field strengths (4.7, 7.1, and 11.7 T) on both static and spinning samples. Iterative fitting of these data yielded the shielding and quadrupolar parameters that characterize the various cobalt(III) sites in the complexes. These results are only in partial agreement with data inferred from previous solution NMR measurements. The values measured for the anisotropic components of the ⁵⁹Co NMR tensors also deviate from traditional correlations observed for simpler nonaromatic solid cobalt complexes. Possible sources for the discrepancies observed between the solution and the solid phase parameters as well as for the anomalous behavior observed with the anisotropic ⁵⁹Co coupling constants are discussed.

We have recently demonstrated that a combination of multiple-quantum NMR and magic-angle spinning can provide high resolution spectra from half-integer quadrupolar nuclei. An interesting feature of these spectra is the appearance of extensive spinning sideband patterns along the indirectly detected dimension, whose origin is here discussed. It is shown that these sidebands arise even in the absence of explicit time modulations of any spin interactions, due to the orientation- and time-dependence characterizing the multiple-to single-quantum conversion process. An analytical formalism is developed to describe these effects, and corroborated by numerical calculations and NMR experiments.

Slow, large-amplitude chain motions play an important role in determining the macroscopic mechanical properties of polymers. Although such motions have been studied quantitatively by two-dimensional (2D) nuclear magnetic resonance (NMR) exchange experiments, overlapping anisotropic patterns hamper spectral analysis, and limit applications. Variable angle correlation spectroscopy (VACSY) has proven useful in resolving such problems for rapidly spinning samples by separating anisotropic spectral patterns according to isotropic chemical shifts. In a previous study [J. Am. Chem. Soc. 115, 4825 (1993)], we described a three-dimensional (3D) NMR experiment that incorporates the VACSY method and a hop of the rotor axis to correlate the isotropic chemical shifts to 2D anisotropic exchange patterns. The hop of the rotor axis, however, presents experimental difficulties and limits the range of motional rates that may be studied. We present in this paper a new 3D VACSY exchange experiment that obtains the same correlations without the need for the rotor axis hop. A series of 2D exchange spectra are recorded with the sample spinning at different rotation axis angles. Then using the scaling of the anisotropic frequency at the different angles, we construct the data onto a 3D matrix so that a Fourier transformation directly yields the desired correlations. The technique is applied to ¹³C exchange NMR to study the slow molecular motion of ordered isotactic polypropylene.