Publications

Techniques for enhancing the signals arising from low-γ, insensitive (I) nuclei are central to solid-state nuclear magnetic resonance. One of the leading and best-established methods to sensitize these unreceptive species is Hartmann-Hahn cross polarization (HH-CP), a polarization transfer mechanism often executed under MAS. Herein, we explore the possibility of utilizing the ¹H dipolar order created via adiabatic demagnetization in the rotating frame (ADRF), to enhance the unreceptive spins under MAS. It is found that an efficient polarization transfer via ADRF-CPMAS is not only possible but can exceed, at least in some instances involving plastic crystals, the efficiency of an optimized HH-CPMAS transfer. The experiment requires low radiofrequency nutation fields on both the ¹H- and the I-spin channels, and displays unusual matching conditions that are reminiscent of the zero- and double-quantum matching conditions arising under CPMAS, albeit centered at zero frequency and demanding the simultaneous involvement of several spins. The origin of these multi-spin transfer processes is analytically derived and numerically simulated in predictions that compare well with experimental ¹³C and ¹⁵N results collected on model compounds at different spinning speeds. These derivations start from descriptions that depart from traditional thermodynamic arguments, and treat instead the ADRF processes in static and spinning solids on the basis of coherent evolutions. The predictions of these analytical derivations are corroborated by numerical simulations. The effects of additional factors, including chemical shift anisotropies, J-couplings, and radiofrequency inhomogeneities, are also theoretically and experimentally explored.

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.

Deuterium metabolic imaging (DMI) is a promising tool for investigating a tumor’s biology, and eventually contribute in cancer diagnosis and prognosis. In DMI, [6,6′-²H₂]-glucose is taken up and metabolized by different tissues, resulting in the formation of HDO but also in an enhanced formation of [3,3′-²H₂]-lactate at the tumor site as a result of the Warburg effect. Recent studies have shown DMI’s suitability to highlight pancreatic cancer in murine models by [3,3′-²H₂]-lactate formation; an important question is whether DMI can also differentiate between these tumors and pancreatitis. This differentiation is critical, as these two diseases are hard to distinguish today radiologically, but have very different prognoses requiring distinctive treatments. Recent studies have shown the limitations that hyperpolarized MRI faces when trying to distinguish these pancreatic diseases by monitoring the [1-¹³C₁]-pyruvate→[1-¹³C₁]-lactate conversion. In this work, we explore DMI’s capability to achieve such differentiation. Initial tests used a multi-echo (ME) SSFP sequence, to identify any metabolic differences between tumor and acute pancreatitis models that had been previously elicited very similar [1-¹³C₁]-pyruvate→[1-¹³C₁]-lactate conversion rates. Although ME-SSFP provides approximately 5 times greater signal-to-noise ratio (SNR) than the standard chemical shift imaging (CSI) experiment used in DMI, no lactate signal was observed in the pancreatitis model. To enhance lactate sensitivity further, we developed a new, weighted-average, CSI-SSFP approach for DMI. Weighted-average CSI-SSFP improved DMI’s SNR by another factor of 4 over ME-SSFP—a sensitivity enhancement that sufficed to evidence natural abundance ²H fat in abdominal images, something that had escaped the previous approaches even at ultrahigh (15.2 T) MRI fields. Despite these efforts to enhance DMI’s sensitivity, no lactate signal could be detected in acute pancreatitis models (n = 10; [3,3′-²H₂]-lactate limit of detection

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.

INEPT-based experiments are widely used for ¹H→¹⁵N transfers, but often fail when involving labile protons due to solvent exchanges. J-based cross polarization (CP) strategies offer a more efficient alternative to perform such transfers, particularly when leveraging the H^water (Formula presented.) H^N exchange process to boost the ¹H→¹⁵N transfer process. This leveraging, however, demands the simultaneous spin-locking of both H^water and H^N protons by a strong ¹H RF field, while fulfilling the γ_HB_1,H=γ_NB_1,N Hartmann-Hahn matching condition. Given the low value of γ_N/γ_H, however, these demands are often incompatible—particularly when experiments are executed by the power-limited cryogenic probes used in contemporary high field NMR. The present manuscript discusses CP alternatives that can alleviate this limitation, and evaluates their performance on urea, amino acids, and intrinsically disordered proteins. These alternatives include new CP variants based on frequency-swept and phase-modulated pulses, designed to simultaneously fulfill the aforementioned conflicting conditions. Their performances vis-à-vis current options are theoretically analyzed with Liouville-space simulations, and experimentally tested with double and triple resonance transfer experiments.

Static satellite-transitions (ST) NMR line shapes from half-integer quadrupolar nuclei could be very informative: they can deliver information on local motions over a wide range of timescales, and can report on small changes in the local electronic environments as reflected by the quadrupolar parameters. Satellite transitions, however, are typically “invisible” for half-integer quadrupolar nuclei due to their sheer breadth, leading to low signal-to-noise ratio –especially for unreceptive low-gamma or dilute quadrupolar nuclei. Very recently we have introduced a method for enhancing the NMR sensitivity of unreceptive X nuclei in static solids dubbed PROgressive Saturation of the Proton Reservoir (PROSPR), which opens the possibility of magnifying the signals from such spins by repeatedly imprinting frequency-selective X-driven depolarizations on the much more sensitive 1H NMR signal. Here, we show that PROSPR's efficacy is high enough for enabling the detection of static ST NMR for challenging species like 35Cl, 33S and even 17O –all at natural-abundance. The ensuing ST-PROSPR NMR experiment thus opens new approaches to probe ultra-wideline (6–8 MHz wide) spectra; these highly pronounced anisotropies can in turn deliver new vistas about dynamic changes in solids, as here illustrated by tracking ST line shapes as a function of temperature during thermally-driven events.

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.

Purpose Subject head motion is a major challenge in DWI, leading to image blurring, signal losses, and biases in the estimated diffusion parameters. Here, we investigate a combined application of prospective motion correction and spatial-angular locally low-rank constrained reconstruction to obtain robust, multi-shot, high-resolution diffusion-weighted MRI under substantial motion. Methods Single-shot EPI with retrospective motion correction can mitigate motion artifacts and resolve any mismatching of gradient encoding orientations; however, it is limited by low spatial resolution and image distortions. Multi-shot acquisition strategies could achieve higher resolution and image fidelity but increase the vulnerability to motion artifacts and phase variations related to cardiac pulsations from shot to shot. We use prospective motion correction with optical markerless motion tracking to remove artifacts and reduce image blurring due to bulk motion, combined with locally low-rank regularization to correct for remaining artifacts due to shot-to-shot phase variations. Results The approach was evaluated on healthy adult volunteers at 3 Tesla under different motion patterns. In multi-shot DWI, image blurring due to motion with 20 mm translations and 30 degrees rotations was successfully removed by prospective motion correction, and aliasing artifacts caused by shot-to-shot phase variations were addressed by locally low-rank regularization. The ability of prospective motion correction to preserve the orientational information in DTI without requiring a reorientation of the b-matrix is highlighted. Conclusion The described technique is proved to hold valuable potential for mapping brain diffusivity and connectivity at high resolution for studies in subjects/cohorts where motion is common, including neonates, pediatrics, and patients with neurological disorders.

Overhauser dynamic nuclear polarization (ODNP) NMR of solutions at high fields is usually mediated by scalar couplings that polarize the nuclei of heavier, electron-rich atoms. This leaves 1H-detected NMR outside the realm of such studies. This study presents experiments that deliver 1H-detected NMR exper-iments on relatively large liquid volumes (60 ti 100 lL) and at high fields (14.1 T), while relying on ODNP enhancements. To this end 13C NMR polarizations were first enhanced by relying on a mechanism that utilizes e --13C scalar coupling interactions; the nuclear spin alignment thus achieved was then passed on to neighboring 1H for observation, by a reverse INEPT scheme relying on one-bond JCH-couplings. Such 13C ! 1H polarization transfer ported the 13C ODNP gains into the 1H, permitting detection at higher frequencies and with higher potential sensitivities. For a model solution of labeled 13CHCl3 comixed with a nitroxide-based TEMPO derivative as polarizing agent, an ODNP enhancement factor of ca. 5x could thus be imparted to the 1H signal. When applied to bigger organic molecules like 2-13C-phenylacetylene and 13C8-indole, ODNP enhancements in the 1.2-3x range were obtained. Thus, although handicapped by the lower c of the 13C, enhancements could be imparted on the 1H thermal acquisitions in all cases. We also find that conventional 1H-13C nuclear Overhauser enhancements (NOEs) are largely absent in these solutions due to the presence of co-dissolved radicals, adding negligible gains and playing negligible roles on the scalar e -?13C ODNP transfer. Potential rationalizations of these effects as well as extensions of these experiments, are briefly discussed.

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-R1–R2 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.

The creation of radicals via electrical discharge is a unique approach capable of facilitating dynamic nuclear polarization (DNP) in solids without the need to co-dissolve them with exogenous polarization agents. This method has previously been shown to generate high proton polarization in a variety of organic and inorganic solid compounds. In this work, we extend the scope of applicability for these so-called electrical discharge-induced radicals (EDIRs), by showing their use in DNP of 13C nuclei of glucose. We achieved significant NMR signal enhancement in low temperature solid-state DNP experiments. This enhancement was further improved by the use of a frequency modulated microwave pump instead of conventional DNP with single-frequency microwaves. We demonstrated successful dissolution DNP experiments with such hyperpolarized solid glucose, achieving 13C polarization levels of up to ∼0.6% in the resulting solution (∼0.95% at the exit of the polarizer). This is to be compared with the ∼6% polarization levels achieved with a trityl radical mixed with glucose in a frozen aqueous solution using the same DNP setup and experimental conditions. It is concluded that the use of EDIRs can serve as a suitable polarizing approach for metabolic MRI using glucose, avoiding the need to employ diluted solutions and eliminating the filtration stage of the polarizing agents.

Out of all the magnetic resonance communities active in the world today, few are as vibrant and influential as that of India. India has done well both in terms of generating the basis of scientists that drive magnetic resonance research throughout the world, as well as in terms of its own magneticresonance productivity and impact. Prof. Anil Kumar has played a central role in both of these endeavors. As tribute to Prof. Kumar's 80th birthday (June 2021) and to pay homage to India's blossoming magnetic resonance landscape, we are co-editing a special Golden Open Access issue of Journal of Magnetic Resonance Open (JMRO), entitled “Celebrating Magnetic Resonance in India - Today”.

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 2–3 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.

Chemical exchange saturation transfer (CEST) is widely used for enhancing the solution nuclear magnetic resonance (NMR) signatures of magnetically dilute spin pools, in particular, species at low concentrations undergoing chemical exchanges with an abundant spin pool. CEST’s main feature involves encoding and then detecting weak NMR signals of the magnetically dilute spin pools on a magnetically abundant spin pool of much easier detection, for instance, the protons of H2O. Inspired by this method, we propose and exemplify a methodology to enhance the sensitivity of magic-angle spinning (MAS) solid-state NMR spectra. Our proposal uses the abundant 1H reservoir arising in organic solids as the magnetically abundant spin pool and relies on proton spin diffusion in lieu of chemical exchange to mediate polarization transfer between a magnetically dilute spin pool and this magnetically abundant spin reporter. As an initial test of this idea, we target the spectroscopy of naturally abundant 13C and rely on a Fourier-encoded version of the CEST experiment for achieving broadbandness in coordination with both MAS and heteronuclear decoupling, features normally absent in CEST. Arbitrary evolutions of multiple 13C sites can, thus, be imprinted on the entire 1H reservoir, which is subsequently detected. Theoretical predictions suggest that orders-of-magnitude signal enhancements should be achievable in this manner, on the order of the ratio between the 13C and the 1H reservoirs’ abundances. Experiments carried out under magic-angle spinning conditions evidenced 5–10× gains in signal amplitudes. Further opportunities and challenges arising in this Fourier-encoded saturation transfer MAS NMR approach are briefly discussed.

Nuclear magnetic resonance (NMR) spectroscopy provides detailed information about dynamic processes through line-shape changes, which are traditionally limited to equilibrium conditions. However, a wealth of information is available by studying chemical reactions under off-equilibrium conditions-e.g., in states that arise upon mixing reactants that subsequently undergo chemical changes-and in monitoring the reactants and products in real time. Herein, we propose and demonstrate a time-resolved kinetic NMR experiment that combines rapid mixing techniques, continuous flow, and single-scan spectroscopic imaging methods, leading in unison to a 2D spectrotemporal NMR correlation that provides high-quality kinetic information of off-equilibrium chemical reactions. These kinetic 2D NMR spectra possess a high-resolution spectral dimension revealing the individual chemical sites, correlated with a time-independent, steady-state spatial axis that delivers information concerning temporal changes along the reaction coordinate. A comprehensive description of the kinetic, spectroscopic, and experimental features associated with these spectrotemporal NMR analyses is presented. Experimental demonstrations are carried out using an enzymatically catalyzed reaction leading to site- and time-resolved kinetic NMR data, that are in excellent agreement with control experiments and literature values.

Chemical exchange saturation transfer (CEST) enhances solution-state NMR signals of labile and otherwise invisible chemical sites, by indirectly detecting their signatures as a highly magnified saturation of an abundant resonance─for instance, the 1H resonance of water. Stimulated by this sensitivity magnification, this study presents PROgressive Saturation of the Proton Reservoir (PROSPR), a method for enhancing the NMR sensitivity of dilute heteronuclei in static solids. PROSPR aims at using these heteronuclei to progressively deplete the abundant 1H polarization found in most organic and several inorganic solids, and implements this 1H signal depletion in a manner that reflects the spectral intensities of the heteronuclei as a function of their chemical shifts or quadrupolar offsets. To achieve this, PROSPR uses a looped cross-polarization scheme that repeatedly depletes 1H–1H local dipolar order and then relays this saturation throughout the full 1H reservoir via spin-diffusion processes that act as analogues of chemical exchanges in the CEST experiment. Repeating this cross-polarization/spin-diffusion procedure multiple times results in an effective magnification of each heteronucleus’s response that, when repeated in a frequency-stepped fashion, indirectly maps their NMR spectrum as sizable attenuations of the abundant 1H NMR signal. Experimental PROSPR examples demonstrate that, in this fashion, faithful wideline NMR spectra can be obtained. These 1H-detected heteronuclear NMR spectra can have their sensitivity enhanced by orders of magnitude in comparison to optimized direct-detect experiments targeting unreceptive nuclei at low natural abundance, using modest hardware requirements and conventional NMR equipment at room temperature.

Spatiotemporal Encoding (SPEN) is an ultrafast imaging technique where the low-bandwidth axis is rasterized in a joint spatial/k-domain. SPEN benefits from increased robustness to field inhomogeneities, folding-free reconstruction of subsampled data, and an ability to combine multiple interleaved or signal averaged scans –yet its relatively high SAR complicates volumetric uses. Here we show how this can be alleviated by merging simultaneous multi-band excitation, with intra-slab multi-echo (ME) phase encoding, for the acquisition of high definition volumetric DWI/DTI data. A protocol involving phase-cycling of simultaneous multi-banded z-slab excitations in independently ky-interleaved scans, together with ME trains that kz-encoded positions within these slabs, was implemented. A reconstruction incorporating a CAIPIRINHA-like encoding of the multiple bands and exploiting SPEN's ability to deliver self-referenced, per-shot phase maps, then led to high-definition diffusivity acquisitions, with reduced SAR and acquisition times vis-à-vis non-optimized 3D counterparts. The new protocol was used to collect full brain 3 T DTI experiments at a variety of nominal voxel sizes, ranging from 1.95 to 2.54 mm3. In general, the new protocol yielded superior sensitivity and fewer distortions than what could be observed in comparably timed phase-encoded 3D SPEN, multi-slice 2D SPEN, or optimized EPI counterparts. A robust procedure for acquiring volumetric DWI/DTI data was developed and demonstrated.

Recent magnetic resonance studies in healthy and cancerous organs have concluded that deuterated metabolites possess highly desirable properties for mapping non-invasively and, as they happen, characterizing glycolysis and other biochemical processes in animals and humans. A promising avenue of this deuterium metabolic imaging (DMI) approach involves looking at the fate of externally administered 2H6,6′-glucose, as it is taken up and metabolized into different products as a function of time. This study employs deuterium magnetic resonance to follow the metabolism of wildtype and preeclamptic pregnant mice models, focusing on maternal and fetoplacental organs over ≈2 h post-injection. 2H6,6′-glucose uptake was observed in the placenta and in specific downstream organs such as the fetal heart and liver. Main metabolic products included 2H3,3′-lactate and 2H-water, which were produced in individual fetoplacental organs with distinct time traces. Glucose uptake in the organs of most preeclamptic animals appeared more elevated than in the control mice (p = 0.02); also higher was the production of 2H-water arising from this glucose. However, the most notable differences arose in the 2H3,3′-lactate concentration, which was ca. two-fold more abundant in the placenta (p = 0.005) and in the fetal (p = 0.01) organs of preeclamptic-like animals, than in control mice. This is consistent with literature reports about hypoxic conditions arising in preeclamptic and growth-restricted pregnancies, which could lead to an enhancement in anaerobic glycolysis. Overall, the present measurements suggest that DMI, a minimally invasive approach, may offer new ways of studying and characterizing health and disease in mammalian pregnancies, including humans.

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 2H’s short T1s, DMI’s 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’-glucose’s and 2H3,3’-lactate’s SNR were indeed higher for bSSFP than for CSI, three-fold for glucose (57 ± 30 vs. 19 ± 11, P

2D NOESY plays a central role in structural NMR. We have recently discussed methods that rely on solvent‐driven exchanges, to enhance NOE correlations between exchangeable and non‐exchangeable protons in nucleic acids. Such methods, however, fail when trying to establish connectivities within pools of labile protons. This study introduces an alternative that also enhances NOEs between such labile sites, based on encoding a priori selected peaks by selective saturations. The resulting selective magnetization transfer (SMT) experiment proves particularly useful for enhancing the imino‐imino cross‐peaks in RNAs, which is a first step in the NMR resolution of these structures. The origins of these enhancements are discussed, and their potential demonstrated on RNA fragments derived from the genome of SARS‐CoV‐2, recorded with better sensitivity and an order of magnitude faster than conventional 2D counterparts.

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.

This study explored the usefulness of multiple quantitative MRI approaches to detect pancreatic ductal adenocarcinomas in two murine models, PAN‐02 and KPC. Methods assayed included 1H T1 and T2 measurements, quantitative diffusivity mapping, magnetization transfer (MT) 1H MRI throughout the abdomen and hyperpolarized 13C spectroscopic imaging. The progress of the disease was followed as a function of its development; studies were also conducted for wildtype control mice and for mice with induced mild acute pancreatitis. Customized methods developed for scanning the motion‐ and artifact‐prone mice abdomens allowed us to obtain quality 1H images for these targeted regions. Contrasts between tumors and surrounding tissues, however, were significantly different. Anatomical images, T2 maps and MT did not yield significant contrast unless tumors were large. By contrast, tumors showed statistically lower diffusivities than their surroundings (≈8.3 ± 0.4 x 10−4 for PAN‐02 and ≈10.2 ± 0.6 x 10−4 for KPC vs 13 ± 1 x 10−3 mm2 s−1 for surroundings), longer T1 relaxation times (≈1.44 ± 0.05 for PAN‐02 and ≈1.45 ± 0.05 for KPC vs 0.95 ± 0.10 seconds for surroundings) and significantly higher lactate/pyruvate ratios by hyperpolarized 13C MR (0.53 ± 0.2 for PAN‐02 and 0.78 ± 0.2 for KPC vs 0.11 ± 0.04 for control and 0.31 ± 0.04 for pancreatitis‐bearing mice). Although the latter could also distinguish early‐stage tumors from healthy animal controls, their response was similar to that in our pancreatitis model. Still, this ambiguity could be lifted using the 1H‐based reporters. If confirmed for other kinds of pancreatic tumors this means that these approaches, combined, can provide a route to an early detection of pancreatic cancer.

Diffusion‐weighted imaging (DWI) can improve breast cancer characterizations, but often suffers from low image quality –particularly at informative b > 1000 s/mm2 values. The aim of this study was to evaluate multishot approaches characterizing Gaussian and non‐Gaussian diffusivities in breast cancer. This was a prospective study, in which 15 subjects, including 13 patients with biopsy‐confirmed breast cancers, were enrolled. DWI was acquired at 3 T using echo planar imaging (EPI) with and without zoomed excitations, readout‐segmented EPI (RESOLVE), and spatiotemporal encoding (SPEN); dynamic contrast‐enhanced (DCE) data were collected using three‐dimensional gradient‐echo T1 weighting; anatomies were evaluated with T2‐weighted two‐dimensional turbo spin‐echo. Congruence between malignancies delineated by DCE was assessed against high‐resolution DWI scans with b‐values in the 0–1800 s/mm2 range, as well as against apparent diffusion coefficient (ADC) and kurtosis maps. Data were evaluated by independent magnetic resonance scientists with 3–20 years of experience, and radiologists with 6 and 20 years 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 Sorensen–Dice 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 “size”, “shape” 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.

Multidimensional TOCSY and NOESY are central experiments in chemical and biophysical NMR. Limited efficiencies are an intrinsic downside of these methods, particularly when targeting labile sites. This study demonstrates that the decoherence imparted on these protons through solvent exchanges can, when suitably manipulated, lead to dramatic sensitivity gains per unit time in the acquisition of these experiments. To achieve this, a priori selected frequencies are encoded according to Hadamard recipes, while concurrently subject to looped selective inversion or selective saturation procedures. Suitable processing then leads to protein, oligosaccharide and nucleic acid cross-peak enhancements of ≈200–1000% per scan, in measurements that are ≈10-fold faster than conventional counterparts. The extent of these gains will depend on the solvent exchange and relaxation rates of the targeted sites; these gains also benefit considerably from the spectral resolution provided by ultrahigh fields, as corroborated by NMR experiments at 600 MHz and 1 GHz. The mechanisms underlying these experiments’ enhanced efficiencies are analyzed on the basis of three-way polarization transfer interplays between the water, labile and non-labile protons, and the experimental results are rationalized using both analytical and numerical derivations. Limitations as well as further extensions of the proposed methods, are also discussed.

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.

Purpose: To develop schemes that deliver faithful 2D slices near field heterogeneities of the kind arising from non-ferromagnetic metal implants, with reduced artifacts and shorter scan times. Methods: An excitation scheme relying on cross-term spatio-temporal encoding (xSPEN) was used as basis for developing the new inhomogeneity-insensitive, slice-selective pulse scheme. The resulting Fully refOCUSED cross-term SPatiotemporal ENcoding (FOCUSED-xSPEN) approach involved four adiabatic sweeps. The method was evaluated in silico, in vitro and in vivo using mice models, and compared against a number of existing and of novel alternatives based on both conventional and swept RF pulses, including an analogous method based on LASER's selectivity spatial selectivity. Results: Calculations and experiments confirmed that multi-sweep derivatives of xSPEN and LASER can deliver localized excitation profiles, centered at the intended positions and endowed with enhanced immunity to B
₀ and B
₁ distortions. This, however, is achieved at the expense of higher SAR than non-swept counterparts. Furthermore, single-shot FOCUSED-xSPEN and LASER profiles covered limited off-resonance ranges. This could be extended to bands covering arbitrary off-resonance values with uniform slice widths, by looping the experiments over a number of scans possessing suitable transmission and reception offsets. Conclusions: A series of novel approaches were introduced to select slices near metals, delivering robustness against B
_o and B
₁
⁺ field inhomogeneities.

This study introduces an MRI approach to map diffusion of water in vivo with high resolution under challenging conditions; the approach's potential is then used in diffusivity characterizations of embryos and fetoplacental units in pregnant mice, as well as of newborn mice in their initial postnatal period. The method relies on performing self-referenced spatiotemporal encoded MRI acquisitions, which can achieve the motional and susceptibility immunities needed to target challenging regions such as a mouse's abdominal cavity in a single shot. When suitably combined with zooming-in and novel interleaving procedures, these scans can overcome the inhomogeneity and sensitivity challenges arising upon targeting approximate to 100 mu m in-plane resolutions, and thereby enable longitudinal development studies of abdominal organs that have hitherto eluded in vivo diffusion-weighted imaging. This is employed here to follow processes related to embryonic implantation and placentation, including the final stages of mouse gastrulation, the development of white matter in fetal brains, the maturation of fetal spines, and the evolution of the different layers making up mouse hemochorial placentas. The protocol's ability to extract diffusivity information in challenging regions as a function of embryonic mouse development is thus demonstrated, and its usefulness as a tool for visualizing pregnancy-related developmental changes in rodents is discussed.

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 HyperW’s 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.

MRI leverages multiple modes of contrast to characterize stroke. High-magnetic-field systems enhance the performance of these MRI measurements. Previously, we have demonstrated that individually sodium and stem cell tracking metrics are enhanced at ultrahigh field in a rat model of stroke, and we have developed robust single-scan diffusion-weighted imaging approaches that utilize spatiotemporal encoding (SPEN) of the apparent diffusion coefficient (ADC) for these challenging field strengths. Here, we performed a multiparametric study of middle cerebral artery occlusion (MCAO) biomarker evolution focusing on comparison of these MRI biomarkers for stroke assessment during sub-acute recovery in rat MCAO models at 21.1 T. T
₂-weighted MRI was used as the benchmark for identification of the ischemic lesion over the course of the study. The number of MPIO-induced voids measured by gradient-recalled echo, the SPEN measurement of ADC, and
²³Na MRI values were determined in the ischemic area and contralateral hemisphere, and relative performances for stroke classification were compared by receiver operator characteristic analysis. These measurements were associated with unique time-dependent trajectories during stroke recovery that changed the sensitivity and specificity for stroke monitoring during its evolution. Advantages and limitations of these contrasts, and the use of ultrahigh field for multiparametric stroke assessment, are discussed.

Dynamic Nuclear Polarization (DNP) can increase the sensitivity of Nuclear Magnetic Resonance (NMR), but it is challenging in the liquid state at high magnetic fields. In this study we demonstrate significant enhancements of NMR signals (up to 70 on C-13) in the liquid state by scalar Overhauser DNP at 14.1 T, with high resolution (similar to 0.1 ppm) and relatively large sample volume (similar to 100 mu L).

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 urea’s 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.

Cross-term spatiotemporal encoding (xSPEN) is a recently introduced imaging approach delivering single-scan 2D NMR images with unprecedented resilience to field inhomogeneities. The method relies on performing a pre-acquisition encoding and a subsequent image read out while using the disturbing frequency inhomogeneities as part of the image formation processes, rather than as artifacts to be overwhelmed by the application of external gradients. This study introduces the use of this new single-shot MRI technique as a diffusion-monitoring tool, for accessing regions that have hitherto been unapproachable by diffusion-weighted imaging (DWI) methods. In order to achieve this, xSPEN MRI's intrinsic diffusion weighting effects are formulated using a customized, spatially-localized b-matrix analysis; with this, we devise a novel diffusion-weighting scheme that both exploits and overcomes xSPEN's strong intrinsic weighting effects. The ability to provide reliable and robust diffusion maps in challenging head and brain regions, including the eyes and the optic nerves, is thus demonstrated in humans at 3T. New avenues for imaging other body regions are also briefly discussed.

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.

A method to detect NMR spectra from heteronuclei through the modulation that they impose on a water resonance is exemplified. The approach exploits chemical exchange saturation transfers, which can magnify the signal of labile protons through their influence on a water peak. To impose a heteronuclear modulation on water, an HMQC-type sequence was combined with the FLEX approach. 1D ¹⁵N NMR spectra of exchanging sites could thus be detected, with about tenfold amplifications over the ¹⁵N modulations afforded by conventionally detected HMQC NMR spectroscopy. Extensions of this approach enable 2D heteronuclear acquisitions on directly bonded ¹H–¹⁵N spin pairs, also with significant signal amplification. Despite the interesting limits of detection that these signal enhancements could open in NMR spectroscopy, these gains are constrained by the rates of solvent exchange of the targeted heteronuclear pairs, as well as by spectrometer instabilities affecting the intense water resonances detected in these experiments.

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

Efficient acquisition of high-quality ultra-wideline (UW) solid-state NMR powder patterns in short experimental time frames is challenging. UW NMR powder patterns often possess inherently low signal-to-noise (S/N) and usually overlap for samples containing two or more magnetically distinct nuclides, which obscures spectral features and drastically lowers the spectral resolution. Currently, there is no reliable method for resolving overlapping powder patterns originating from unreceptive nuclei affected by large anisotropic NMR interactions. Herein, we discuss new methods for resolving individual UW NMR spectra associated with magnetically distinct nuclei by exploiting their different relaxation characteristics using 2D relaxation-assisted separation (RAS) experiments. These experiments use a non-negative Tikhonov fitting (NNTF) routine to process high-quality T₁ and T2^eff relaxation data sets to produce high-resolution, 2D spin-relaxation correlation spectra for both spin-1/2 and quadrupolar nuclei in organic and organometallic solids under static (i.e., stationary) conditions. It is found that (i) T2^eff RAS data sets can be acquired in a fraction of the time required for analogous T₁ RAS data sets, because a time-incremented 2D data set is not required for the former, and (ii) Tikhonov regularization is superior to conventional non-negative least-squares fitting, as it more reliably and robustly results in cleaner separation of patterns based on relaxation time constants.

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 F1–t2 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.

Dissolution dynamic nuclear polarization (dDNP) is used to enhance the sensitivity of nuclear magnetic resonance (NMR), enabling monitoring of metabolism and specific enzymatic reactions in vivo. dDNP involves rapid sample dissolution and transfer to 4 spectrometer/scanner for subsequent signal detection. So far, most biologically oriented dDNP studies have relied on hyperpolarizing long-lived nuclear spin species such as C-13 in small molecules. While advantages could also arise from observing hyperpolarized H-1, short relaxation times limit the utility of prepolarizing this sensitive but fast relaxing nucleus. Recently, it has been reported that H-1 NMR peaks in solution-phase experiments could be hyperpolarized by spontaneous magnetization transfers from bound C-13 nuclei following dDNP. Thia work demonstrates the potential of this sensitivity-enhancing approach to probe the enzymatic process that could not be suitably resolved by C-13 dDNP MR Here we measured, in microorganism, the action of pyruvate decarboxylase (PDC) and pyruvate formate lyase (PFL)- enzymes that catalyze the decarboxylation of pyruvate to form acetaldehyde and formate, respectively. While C-13 NMR did not possess the resolution to distinguish the starting pyruvate precursor from the carbonyl resonances in the resulting products, these processes could be monitored by H-1 NMR. at 50 MHz. These observations were possible in both yeast and bacteria in minute-long measurements where the hyperpolarized C-13 enhanced, via C-13 -> H-1 cross-relaxation, the signals of protons binding to the C-13 over the course of enzymatic reactions. In addition to these spontaneous heteronuclear enhancement experiments, single-shot acquisitions based on J-driven C-13 -> H-1 polarization transfers were also carried out. These resulted in higher signal enhancements of the H-1 resonances but were not suitable for multishot kinetic studies. The Potential of these H-1-based approaches for measurements in vivo is brie

Purpose: Evaluate the usefulness of diffusion-weighted spatiotemporally encoded (SPEN) methods to obtain apparent diffusion coefficient (ADC) maps of fibroglandular human breast tissue, in the presence of silicone implants.Methods: Seven healthy volunteers with breast augmentation were scanned at 3 Tesla (T) using customized SPEN sequences yielding separate silicone and water H-1 images in one scan, together with their corresponding diffusion-weightings.Results: SPEN's ability to deliver multiple spectrally resolved images in a single scan, coupled to the method's substantial robustness to magnetic field heterogeneities, served to acquire ADC maps that could be freed from contributions that did not belong to fibroglandular tissue.Conclusion: SPEN-based sequences incorporating spectral discrimination and diffusion-weighting enable the acquisition of reliable ADC maps despite the presence of dominant signals from silicone implants, thereby opening new screening possibilities for the identification of malignancies in breast augmented patients. 

Two-dimensional (2D) correlations between bonded heteroatoms, lie at the cornerstone of many uses given to contemporary nuclear magnetic resonance (NMR). Improving the efficiency with which these correlations are established is an important topic in modern NMR, with potential applications in rapid chemical analysis and dynamic biophysical studies. Alternatives have been developed over the last decade to speed up these experiments, based among others on reducing the number of data points that need to be sampled, and/or shortening the inter-scan delays. Approaches have also been proposed to forfeit multi-scan schemes altogether, and complete full 2D correlations in a single shot. Here we explore and discuss a new alternative enabling the collection of such very fast - in principle, single-scan - acquisitions of 2D heteronuclear correlations among bonded species, which operates on the basis of a partial reintroduction of J couplings. Similar approaches had been proposed in the past based on collecting coupled spectra for arrays of off-resonance decoupling values; the proposal that is here introduced operates on the basis of suitably incorporating frequency-swept pulses, into spin-echo sequences. Thanks to the offset-dependent amplitude modulations of the in- and anti-phase components that such sequences impart, chemical shifts of coupled but otherwise unobserved nuclear species, can be extracted from the relative intensities and phases of J-coupled multiplets observed in one-dimensional acquisitions. A description of the steps needed to implement this rapid acquisition approach in a quantitative fashion, as well as applications of the ensuing sequences, are presented.

A recent study explored the use of hyperpolarized water, to enhance the sensitivity of nuclei in biomolecules thanks to rapid proton exchanges with labile amide backbone and sidechain groups. Further optimizations of this approach have now allowed us to achieve proton polarizations approaching 25% in the water transferred into the NMR spectrometer, effective water T₁ times approaching 40 s, and a reduction in the dilution demanded for the cryogenic dissolution process. Further hardware developments have allowed us to perform these experiments, repeatedly and reliably, in 5 mm NMR tubes. All these ingredients - particularly the ≥3000× ¹H polarization enhancements over 11.7 T thermal counterparts, long T₁ times and a compatibility with high-resolution biomolecular NMR setups - augur well for hyperpolarized 2D NMR studies of peptides, unfolded proteins and intrinsically disordered systems undergoing fast exchanges of their protons with the solvent. This hypothesis is here explored by detailing the provisions that lead to these significant improvements over previous reports, and demonstrating 1D coherence transfer experiments and 2D biomolecular HMQC acquisitions delivering NMR spectral enhancements of 100-500× over their optimized, thermally-polarized, counterparts.

Natural abundance ¹³C NMR spectra of biological extracts are recorded in a single scan provided that the samples are hyperpolarized by dissolution dynamic nuclear polarization combined with cross polarization. Heteronuclear 2D correlation spectra of hyperpolarized breast cancer cell extracts can also be obtained in a single scan. Hyperpolarized NMR of extracts opens many perspectives for metabolomics.

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.

This manuscript examines the origins and nature of the function-derived activation detected by magnetic resonance imaging at ultrahigh fields using different encoding methods. A series of preclinical high field (7. T) and ultra-high field (17.2. T) fMRI experiments were performed using gradient echo EPI, spin echo EPI and spatio-temporally encoded (SPEN) strategies. The dependencies of the fMRI signal change on the strength of the magnetic field and on different acquisition and sequence parameters were investigated. Artifact-free rat brain images with good resolution in all areas, as well as significant localized activation maps upon forepaw stimulation, were obtained in a single scan using fully refocused SPEN sequences devoid of T2* effects. Our results showed that, besides the normal T2-weighted BOLD contribution that arises in spin-echo sequences, fMRI SPEN signals contain a strong component caused by apparent T1-related effects, demonstrating the potential of such technique for exploring functional activation in rodents and on humans at ultrahigh fields.

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.

Hyperpolarized metabolic imaging is a growing field that has provided a new tool for analyzing metabolism, particularly in cancer. Given the short life times of the hyperpolarized signal, fast and effective spectroscopic imaging methods compatible with dynamic metabolic characterizations are necessary. Several approaches have been customized for hyperpolarized ¹³C MRI, including CSI with a center-out k-space encoding, EPSI, and spectrally selective pulses in combination with spiral EPI acquisitions. Recent studies have described the potential of single-shot alternatives based on spatiotemporal encoding (SPEN) principles, to derive chemical-shift images within a sub-second period. By contrast to EPSI, SPEN does not require oscillating acquisition gradients to deliver chemical-shift information: its signal encodes both spatial as well as chemical shift information, at no extra cost in experimental complexity. SPEN MRI sequences with slice-selection and arbitrary excitation pulses can also be devised, endowing SPEN with the potential to deliver single-shot multi-slice chemical shift images, with a temporal resolution required for hyperpolarized dynamic metabolic imaging. The present work demonstrates this with initial in vivo results obtained from SPEN-based imaging of pyruvate and its metabolic products, after injection of hyperpolarized [1-¹³C]pyruvate. Multi-slice chemical-shift images of healthy rats were obtained at 4.7 T in the region of the kidney, and 4D (2D spatial, 1D spectral, 1D temporal) data sets were obtained at 7 T from a murine lymphoma tumor model.

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.

Purpose Single-scan multislice acquisition schemes play key roles in magnetic resonance imaging. Central among these "ultrafast" experiments stands echo-planar imaging, a technique that although of optimal sampling is challenged by T₂* artifacts. Recent studies described alternatives based on spatiotemporal encoding (SPEN), which are particularly robust if implemented in a "full-refocusing" mode. This work extends this modality from the single-slice acquisitions in which it has hitherto been implemented, by introducing a variety of multislice schemes scanning 3D volumes. Methods Multislice SPEN employing either inversion or stimulated echo pulses and timed to fulfill the demands of full refocusing, are demonstrated. The performance of the ensuing methods was examined in "Hybrid" modalities encoding data in k- and direct-space, in low-specific absorption rate stimulated-echo approaches, and in direct-space SPEN approaches. Results When applied in phantoms and in in vivo systems, the ensuing single-shot sequences evidenced similar robustness, sensitivity, and resolution qualities as previously discussed 2D single-slice schemes, while enabling a rapid sampling of the third dimension via multislicing. Conclusion The unique benefits deriving from fully refocused, multislice, single-scan SPEN sequences were corroborated by phantom tests, as well as by in vivo scans at 3 and 7 T. Low specific absorption rate multislice SPEN variants compatible with human studies were demonstrated.

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.

Two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy is widely used in chemical and biochemical analyses. Multidimensional NMR is also witnessing increased use in quantitative and metabolic screening applications. Conventional 2D NMR experiments, however, are affected by inherently long acquisition durations, arising from their need to sample the frequencies involved along their indirect domains in an incremented, scan-by-scan nature. A decade ago, a so-called ultrafast (UF) approach was proposed, capable of delivering arbitrary 2D NMR spectra involving any kind of homo-or heteronuclear correlation, in a single scan. During the intervening years, the performance of this subsecond 2D NMR methodology has been greatly improved, and UF 2D NMR is rapidly becoming a powerful analytical tool experiencing an expanded scope of applications. This review summarizes the principles and main developments that have contributed to the success of this approach and focuses on applications that have been recently demonstrated in various areas of analytical chemistry-from the real-time monitoring of chemical and biochemical processes, to extensions in hyphenated techniques and in quantitative applications.

Nitrogen is an element of utmost importance in chemistry, biology and materials science. Of its two NMR-active isotopes, N-14 and N-15, solid-state NMR (SSNMR) experiments are rarely conducted upon the former, due to its low gyromagnetic ratio () and broad powder patterns arising from first-order quadrupolar interactions. In this work, we propose a methodology for the rapid acquisition of high quality N-14 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 N-14 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., H-1). 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 T-2-driven S/N enhancement, and frequency-stepped acquisitions. The feasibility of utilizing the BRAIN-CP/WURST-CPMG sequence is tested for N-14, 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 N-14 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.

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.

Bulk C13 polarization can be strongly enhanced in diamond at room temperature based on the optical pumping of nitrogen-vacancy color centers. This effect was confirmed by irradiating single crystals at a ∼50 mT field promoting anticrossings between electronic excited-state levels, followed by shuttling of the sample into an NMR setup and by subsequent C13 detection. A nuclear polarization of ∼0.5% - equivalent to the C13 polarization achievable by thermal polarization at room temperature at fields of ∼2000 T - was measured, and its bulk nature determined based on line shape and relaxation measurements. Positive and negative enhanced polarizations were obtained, with a generally complex but predictable dependence on the magnetic field during optical pumping. Owing to its simplicity, this C13 room temperature polarizing strategy provides a promising new addition to existing nuclear hyperpolarization techniques.

Faster than ultrafast: A new sequence combining "ultrafast" single-shot 2DNMR and parallel receiving technologies is presented. The potential of the resulting parallel ultrafast 2D spectroscopy (PUFSY) NMR experiments is shown by simultaneously collecting homo- and heteronuclear correlation information for ¹H-¹⁹F systems (see picture) and a ¹H-³¹P system.

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.

Recent years have witnessed efforts geared at increasing the sensitivity of NMR experiments, by relying on the suitable tailoring and exploitation of relaxation phenomena. These efforts have included the use of paramagnetic agents, enhanced ¹H-¹H incoherent and coherent transfers processes in 2D liquid state spectroscopy, and homonuclear ¹³C- ¹³C spin diffusion effects in labeled solids. The present study examines some of the opportunities that could open when exploiting spontaneous ¹H-¹H spin-diffusion processes, to enhance relaxation and to improve the sensitivity of dilute nuclei in solid state NMR measurements. It is shown that polarization transfer experiments executed under sufficiently fast magic-angle-spinning conditions, enable a selective polarization of the dilute low- spins by their immediate neighboring protons. Repolarization of the latter can then occur during the time involved in monitoring the signal emitted by the low- nuclei. The basic features involved in the resulting approach, and its potential to improve the effective sensitivity of solid state NMR measurements on dilute nuclei, are analyzed. Experimental tests witness the advantages that could reside from utilizing this kind of approach over conventional cross-polarization processes. These measurements also highlight a number of limitations that will have to be overcome for transforming selective polarization transfers of this kind into analytical methods of choice.

The relatively long times that may be involved in high-resolution two-dimensional nuclear magnetic resonance (2D NMR) have stimulated the search for alternative schemes to collect these data. Particularly onerous situations arise when both high-resolution and large spectral widths are sought along the indirect domain. Strategies proposed for dealing with such cases include folding-over procedures, Hadamard encoding, and nonlinear data sampling. This communication discusses an alternative strategy, which exploits a partial prior knowledge regarding the position of the NMR resonances along the indirect domain together with customized excitations for every particular t₁ increment, to achieve an optimal sampling in terms of resolution and bandwidth. On the basis of such optimized encoding of the indirect-domain evolution, which can easily be coped with by modern spectrometers, it becomes possible to maximize the resolution of fine structures without compromising on the spectral bandwidths. The processing of the resulting data along the indirect domain is based on the use of two serially applied discrete Fourier transforms; one to distinguish the main bands in the spectrum and the other to resolve the latter's fine features. A number of simple heteronuclear correlation experiments illustrating the significant acquisition time savings and simultaneous improvements in resolution that can be achieved with the resulting double-Fourier encoding procedure are illustrated.

A topic of active investigation in 2D NMR relates to the minimum number of scans required for acquiring this kind of spectra, particularly when these are dictated by sampling rather than by sensitivity considerations. Reductions in this minimum number of scans have been achieved by departing from the regular sampling used to monitor the indirect domain, and relying instead on non-uniform sampling and iterative reconstruction algorithms. Alternatively, so-called "ultrafast" methods can compress the minimum number of scans involved in 2D NMR all the way to a minimum number of one, by spatially encoding the indirect domain information and subsequently recovering it via oscillating field gradients. Given ultrafast NMR's simultaneous recording of the indirect- and direct-domain data, this experiment couples the spectral constraints of these orthogonal domains - often calling for the use of strong acquisition gradients and large filter widths to fulfill the desired bandwidth and resolution demands along all spectral dimensions. This study discusses a way to alleviate these demands, and thereby enhance the method's performance and applicability, by combining spatial encoding with iterative reconstruction approaches. Examples of these new principles are given based on the compressed-sensed reconstruction of biomolecular 2D HSQC ultrafast NMR data, an approach that we show enables a decrease of the gradient strengths demanded in this type of experiments by up to 80%.

Spatial encoding, as encompassing the monitoring of spin evolutions on the basis of selective frequency-swept pulses and of magnetic field gradients, provides a new way for measuring NMR spectra or MRI images. In contrast to time-domain schemes or to continuous-wave approaches, these new RF/gradient combinations can act together to create interaction-dependent spatial patterns of spin magnetizations or coherences extending throughout a sample. These patterns can then be read-out with the aid of a second set of gradients while digitizing the data, to endow NMR/MRI acquisitions with hitherto unavailable capabilities. The present Review described various facets of these new approaches to monitor NMR spectra and MRI images. We began with a thorough introduction on how to visualize the effects of swept RF pulses - be them of an excitation or refocusing nature - applied in the presence of linear field gradients. It was then discussed how, in a spectroscopic setting, the idea of spatial encoding can be exploited to compress an nD spectroscopic NMR experiment into a single-scan. Numerous acquisition schemes capable of retrieving this kind of results for a variety of 2D experiments were presented, and their relative merits and limitations were surveyed. Similar ideas were shown to have applications in other spectroscopic aradigms involving multi-scan experiments, such as Hadamard spectroscopy. It was once again shown that by partitioning the sample and exciting different patterns for each site, one could produce a single-scan, sub-second version of a complex experiment. Similar versions of multi-scan phase cycling have also been demonstrated [71]. The final part extended these spatially-selective encoding concepts in what we believe are novel imaging sequences, though related to decades-old developments in this field. By suitable excitation protocols, the spins in the sample can, in these MRI settings, be made to interfere destructively - except within a particular voxel which can be chosen at will. This voxel can be shifted along a predefined trajectory, set by shaping the acquisition gradients, yielding a signal proportional to the spin density along that path. Therefore, spatially encoded imaging differs from conventional Fourier imaging by acquiring images in real rather than in k-space. The point-by-point nature of the ensuing approach can then address a number of challenging measurements, including the single-scan acquisition of images arising from different chemical sites, or in the presence of field inhomogeneities. Overall, it is hoped that as the technical details underlying these new methods become clearer and as their user-base expands, further improvements will materialize and new, unforeseen applications of spatial-encoding will emerge - both in the spectroscopy and imaging realms. (Figure presented).

We present a method that implements directional, perfect state transfers within a branched spin network by exploiting quantum interferences in the time domain. This method provides a tool for isolating subsystems from a large and complex one. Directionality is achieved by interrupting the spin-spin coupled evolution with periods of free Zeeman evolutions, whose timing is tuned to be commensurate with the relative phases accrued by specific spin pairs. This leads to a resonant transfer between the chosen qubits and to a detuning of all remaining pathways in the network, using only global manipulations. Since the transfer is perfect when the selected pathway is mediated by two or three spins, distant state transfers over complex networks can be achieved by successive recouplings among specific pairs or triads of spins. These effects are illustrated with a quantum simulator involving C13 NMR on leucine's backbone; a six-spin network.

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 C-13 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 H-1 signals by about 600, which is significantly less than in optimized low-gamma liquid-state counterparts, the non-linear dependence of iMQC-derived signals on polarization can yield very large enhancements approaching 10(6), Cleary no practical amount of data averaging can match this kind of sensitivity gains. The fact that DNP endows iMQC-based H-1 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. (C) 2009 Elsevier Inc. All rights reserved.

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.

Two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy provides the means to extract diverse physical, chemical, and biological information at an atomic level. Conventional sampling schemes, however, may result in relatively long 2D experiments; this has stimulated the search for alternative, rapid acquisition schemes. Among the strategies that have been recently proposed for achieving this counts an "ultrafast" approach, relying on the spatial encoding of the indirect domain evolution to provide arbitrary spectra within a single scan. A common feature of all spatial encoding schemes hitherto described is their uniform encoding of a continuous bandwidth of indirect-domain frequencies, regardless of the chemical sites' spectral distribution within it. These very general conditions, however, are often associated with a number of tradeoffs and compromises in the spectral widths and resolutions that can be achieved for both the direct and indirect domains. This paper proposes a different strategy for single-scan acquisition of 2D spectra, which performs an optimal encoding by employing a priori information regarding the positions of NMR resonances along the indirect domain. We denote this as "spatial/ spectral encoding"; the underlying principles of this new approach, together with experimental results based on uni- and multidimensional rf pulse schemes, are presented.

An important development in the field of NMR spectroscopy has been the advent of hyperpolarization approaches, capable of yielding nuclear spin states whose value exceeds by orders-of-magnitude what even the highest-field spectrometers can afford under Boltzmann equilibrium. Included among these methods is an ex situ dynamic nuclear polarization (DNP) approach, which yields liquid-phase samples possessing spin polarizations of up to 50 %. Although capable of providing an NMR sensitivity equivalent to the averaging of about 1 000 000 scans, this methodology is constrained to extract its "superspectrum" within a single - or at most a few - transients. This makes it a poor starting point for conventional 2D NMR acquisition experiments, which require a large number of scans that are identical to one another except for the increment of a certain t1 delay. It has been recently suggested that by merging this ex situ DNP approach with spatially encoded "ultrafast" methods, a suitable starting point could arise for the acquisition of 2D spectra on hyperpolarized liquids. Herein, we describe the experimental principles, potential features, and current limitations of such integration between the two methodologies. For a variety of small molecules, these new hyperpolarized ultrafast experiments can, for equivalent overall durations, provide heteronuclear correlation spectra at significantly lower concentrations than those currently achievable by conventional 2D NMR acquisitions. A variety of challenges still remain to be solved before bringing the full potential of this new integrated 2D NMR approach to fruition; these outstanding issues are discussed.

Multidimensional spectroscopy plays a central role in contemporary magnetic resonance. A general feature of multidimensional NMR is its inherent multiscan nature, stemming from the methodology's reliance on a series of independent acquisitions to sample the spins' evolutions throughout the indirect time domains. Contrasting this traditional feature, an acquisition scheme has recently been reported that enables the collection of complete of multidimensional NMR data sets within one single scan. Provided that the signals to be observed are sufficiently strong, this new "ultrafast" protocol can thus shorten the acquisition times of multidimensional NMR experiments by several orders of magnitude. This new methodology operates by departing from temporal encoding principles used since the advent of Fourier-transform NMR, replacing them with a spatial encoding of the spin interactions. Spatial encoding operates in turn on the basis of novel radiofrequency irradiation and magnetic field gradient waveform manipulations, designed so as to impart on the sample a coherent spin magnetization pattern that reflects the internal interactions to be measured. Given the central role played by this new kind of spectroscopic-oriented manipulations in ultrafast NMR, we devote this review to surveying different variants that have hitherto been proposed for their implementation. These include both discrete and continuous versions, real- and constant-time implementations, as well as amplitude- and phase-modulated alternatives. The principles underlying these various spatial encoding approaches are treated, their operation is graphically illustrated as well as formally derived within suitable theoretical frameworks, and an in-depth comparison of their line shape characteristics is discussed. (c) 2008 American Institute of Physics.

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.

Recent years have witnessed increased efforts toward the accelerated acquisition of multidimensional nuclear magnetic resonance (nD NMR) spectra. Among the methods proposed to speed up these NMR experiments is "projection reconstruction," a scheme based on the acquisition of a reduced number of two-dimensional (2D) NMR data sets constituting cross sections of the nD time domain being sought. Another proposition involves "ultrafast" spectroscopy, capable of completing nD NMR acquisitions within a single scan. Potential limitations of these approaches include the need for a relatively slow 2D-type serial data collection procedure in the former case, and a need for at least n high-performance, linearly independent gradients and a sufficiently high sensitivity in the latter. The present study introduces a new scheme that comes to address these limitations, by combining the basic features of the projection reconstruction and the ultrafast approaches into a single, unified nD NMR experiment. In the resulting method each member within the series of 2D cross sections required by projection reconstruction to deliver the nD NMR spectrum being sought, is acquired within a single scan with the aid of the 2D ultrafast protocol. Full nD NMR spectra can thus become available by backprojecting a small number of 2D sets, collected using a minimum number of scans. Principles, opportunities, and limitations of the resulting approach, together with demonstrations of its practical advantages, are here discussed and illustrated with a series of three-dimensional homo- and heteronuclear NMR correlation experiments.

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 spectroscopy plays a number of essential roles in contemporary magnetic resonance. It brings a resolution enhancement without which numerous NMR applications in organic and inorganic chemistry would be unattainable, it serves as a basic tool in the assignment and structural elucidation of complex biological structures, and it is an integral part of the image formation protocol in MRI. The present review describes a recent scheme which we have developed, enabling the acquisition of complete 2D NMR data sets within a single continuous acquisition. Provided that an analyte's signal is sufficiently strong, the acquisition time of multidimensional NMR experiments can thus be shortened by several orders of magnitude. The new methodology is compatible with existing multidimensional pulse sequences (COSY, TOCSY, HSQC, MRI) and can be implemented using conventional hardware. The spatial encoding of the NMR interactions - which is the new principle underlying this ultrafast NMR protocol - is discussed, and the protocol's performance is exemplified with a variety of homonuclear and heteronuclear 2D NMR and MRI acquisitions.

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.

Measuring the nuclear magnetic resonance spectra of low-gamma heteronuclei such as N-15 constitutes an important analytical tool for the characterization of molecular structure and dynamics. The reduced resonance frequencies and magnetic moments of these heteronuclei, however, make the sensitivity of this kind of spectroscopy inherently lower than that of comparable H-1 NMR observations. A well-known solution to this sensitivity problem is indirect detection: a 2D NMR technique capable of enhancing the sensitivity of heteronuclear NMR by porting the actual data acquisition from the low-gamma nucleus to neighboring protons. This has become the standard method of observation in biomolecular NMR, where the resolution introduced by 2D spectroscopy is always a sought-after commodity. Indirect detection, however, has not gained a wide appeal in organic chemistry or in in vivo investigations, where one-dimensional heteronuclear NMR information usually suffices. The present study explores the possibility of retaining certain advantages derived from indirect detection while not giving up on the simple one-dimensional nature of heteronuclear NMR, by relying on the spatial-encoding scheme we have recently demonstrated for implementing single-scan multi-dimensional NMR spectroscopy. Preliminary results based on a 1D N-15 NMR can be enhanced significantly in this manner; the relevance of this experiment given the advent of dedicated H-1-observing cryogenic probeheads with very high sensitivities is briefly discussed.

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.

We have recently demonstrated that magnetic field gradients in combination with frequency selective pulses, can be employed to collect a complete multi-dimensional NMR spectrum within a single scan. Following similar guidelines, field gradients could also be exploited to parallelize other types of NMR experiments where the final results arise from the collection and analysis of a series of time-incremented spectra. The present Communication exemplifies this concept by showing how a combination of gradients can be employed to monitor within a single continuous acquisition, a slow dynamic process which is in turn followed by systematic increments in the duration of a magnetization transfer time. Further, since 2D exchange NMR spectra can nowadays be themselves collected within one scan, the acquisition of a complete set of mixing-incremented 2D exchange patterns could be achieved within a single experiment entailing a total time of ≈1s.

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.

Multinuclear solid-state nuclear magnetic resonance studies (^185/187Re, ⁵⁵Mn, ⁷⁵As, and ¹H NMR) were undertaken on a series of polycrystalline inorganic salts incorporating diamagnetic XO₄^- groups, X being a half-integer quadrupolar nucleus. Exploiting data acquisition protocols that were recently developed for observing undistorted half-integer quadrupole central transitions, some of the largest quadrupole coupling constants reported to date by high field NMR were characterized (e²qQ/h ≈ 300 MHz). On repeating such measurements as a function of temperature, certain samples displayed reversible changes that could not be rationalized in terms of the usual temperature dependencies of the nuclear quadrupolar couplings. Instead, dynamic exchange processes between chemically or magnetically inequivalent sites had to be invoked. To quantitatively analyze these processes, the semiclassical Bloch-McConnell formalism for chemical exchange was extended to account for second-order quadrupole effects. Insight into the potential nature of the chemical dynamics was also obtained from quantum chemical calculations of the coupling parameters on model systems.

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 order and dynamics of two lyotropic aromatic polyamides were studied by variable-director nuclear magnetic resonance (NMR). The spectra showed peaks shifting as well as broadening as a function of the director's orientation which are accounted on the basis of an exchange model involving molecular reorientations of the polymer chains that are happening in the intermediate NMR time scale. The description of the macromolecular order and dynamics in these fluids was extracted from the experimental line shapes.

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.

The present work discusses a new 2D NMR method for characterizing the principal values and relative orientations of the electric field gradient and the chemical shift tensors of half-integer quadrupolar sites. The technique exploits the different contributions that quadrupolar and shielding interactions impart on the evolution of multiple-quantum and of single-quantum coherences, in order to obtain 2D powder lineshapes that are highly sensitive to these nuclear spin coupling parameters. Different spinning variants of this experiment were assayed, but it was concluded that a static version can yield the highest sensitivity to the values of the principal components and to the relative geometries of the local coupfing tensors. It was found that correlating the central transition evolution with the highest available order of the spin coherence was also helpful for maximizing this spectral information. Good agreement between data obtained on ⁸⁷Rb (S = 3/2) and ⁵⁹Co (S = 7/2) samples and ideal theoretical lineshape predictions of this experiment was obtained, provided that heterogeneities in the multiple-quantum excitation and conversion processes were suitably accounted by procedures similar to those described in the spin-j multiple-quantum NMR literature,

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.

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.

Nuclear magnetic resonance (NMR) spatial imaging data may be acquired, processed, and interpreted in ways that provide information directly analogous to diffraction experiments, with length scales determined by gradient strengths rather than radiation wavelengths. This approach, originally considered by Mansfield nearly two decades ago, provides access to autocorrelations of sample density that statistically characterize small-scale density variations. These NMR "Patterson functions" can be acquired orders of magnitude more rapidly than comparably resolved NMR images and are suitable for spatial characterization of small features in bulk samples, such as morphology in structural materials. Unlike hindered diffusion approaches, neither mobility, penetrants, nor transport time are required for examining granularity and porosity.

¹H pulsed NMR techniques were used to investigate the solid-state dynamics of porphine molecules in which the central protons have been replaced by deuterons. Changes observed in the line width of the ¹H NMR signal suggest that, as happened to be the case with the fully protonated derivative, molecules of deuterated porphine rotate in the crystal. Variable-temperature measurements of the proton relaxation times in the rotating frame provided the activation parameters for this motion, which were found to be slightly higher than those observed for the nondeuterated compound. This effect can be related to a combined motion that has been proposed for reconciling the CPMAS NMR and the X-ray results available for porphine, according to which the solid-state tautomerism of the central hydrogens might produce a reorientation of the molecules in the crystals about their main molecular axes. Upon replacement of the central protons by deuterons this model predicts that kinetic isotope effects known to characterize the N-H tautomerism of porphyrins will slow the rates of macrocycle reorientations in the crystal, a dynamic behavior which is reflected in the experimental results.

An approach is introduced that allows the evaluation of the effects of magic‐angle spinning (MAS) in the NMR spectra of inhomogeneously broadened spin‐1/2 nuclei. The method, which consists in replacing the time‐dependent MAS Hamiltonian by a set of three time‐independent Hamiltonians, is able to predict one of the main characteristics of the MAS experiment, namely the appearance of rotor echoes. Spectra were calculated using this approach and compared with ¹³C NMR spectra of model compounds, showing good agreement in the range of spinning rates normally used. A simple extension of the method also makes it useful for simulating some cases of variable‐angle spinning NMR. Although the approach is suitable for the evaluation of normal MAS NMR spectra, its main applications may be found in cases where the spin Hamiltonian is no longer self‐commuting. As an example, the method was employed for analysing the case of an exchange process between two equally populated chemically equivalent sites. Calculated spectra obtained in this way allowed the correct reproduction of the changes observed in the MAS NMR spectra of two compounds which are known to undergo reorientations in the solid state. Further possible applications of the approach are discussed.

Solid‐state ¹³C NMR spectra of ¹⁴N‐containing compounds obtained under CPMAS conditions often show asymmetric doublets arising from the unaveraged ¹³C,¹⁴N residual dipolar coupling. A similar result has recently been noticed in deuteriated samples, whose ¹³C resonances showed the combined effect of ¹³C,²H dipolar and scalar couplings. A simple approach, easily adaptable to a desk microcomputer, is described for simulating ¹³C line shapes for arbitrary values of quadrupole parameters, CX (X = ¹⁴N, ²H) distances, external field and orientation of the internuclear vector in the axis system where the electric field gradient tensor is diagonal.