Increased Glutamate concentrations during prolonged motor activation as measured using functional Magnetic Resonance Spectroscopy at 3T(2020) NeuroImage. 223, 117338. Abstract
Recent implications of glutamatergic signaling in a wide range of psychiatric disorders has highlighted the need to study the dynamics of glutamate (Glu) in the brain outside of steady state. A promising modality for doing so is functional Magnetic Resonance Spectroscopy (fMRS). Recent human studies at high magnetic fields (7T) have reported small but consistent changes in metabolite concentrations, in particular a 2–4% increase in Glu during visual and motor stimulation. While the origins of these changes remain the topic of ongoing research, the ability of fMRS to observe metabolites directly associated with neurotransmission and brain energetics could potentially aid our understanding of brain pathophysiology and the interpretation of functional imaging experiments. For this to happen, the current ultrahigh field results must be reproduced at lower, widely available clinical field strengths, in response to a wide variety of stimuli classes. Our goal herein was to investigate metabolite changes during a hand-clenching motor task at 3T, and to investigate the effect of the stimulation's temporal characteristics on the magnitude of the fMRS changes; specifically, we compared two block-designed functional activation paradigms, using short- and long-cycled clenching designs. Small but statistically significant increases in Glx=Glutamate+Glutamine (3.8%) and Glu (4.0%) concentrations were detected during the long-cycled design, while no statistically significant changes were observed during the short-cycled design. Activation during the long-cycled tasks was correlated to the frequency of clenching. We have also shown that using subject-level analysis in combination with a linear mixed model increases the observed effect size, and could help analyzing the weak MRS signals. Our results are in good agreement with the previous reports acquired at higher field systems, and support the viability of fMRS as a research tool at clinical field strengths, while also emphasizing the importance of the functional paradigm itself.
(2020) NMR in Biomedicine. 4316. Abstract
B-0 field maps are used ubiquitously in neuroimaging, in disciplines ranging from magnetic resonance spectroscopy to temperature mapping and susceptibility-weighted imaging. Most B-0 maps are acquired using standard gradient-echo-based vendor-provided sequences, often comprised of two echoes spaced a few milliseconds apart. Herein, we analyze the optimal spacing of echo times, defined as those maximizing precision-minimizing the standard deviation-for a fixed total acquisition time. Field estimation is carried out using a weighted least squares estimator. The standard deviation is shown to be approximately inversely proportional to the total acquisition time, suggesting a law of diminishing returns, whereby substantial gains are obtained up to a certain point, with little improvement beyond that point. Validations are provided in a phantom and a group of volunteers. Multi-gradient echo sequences are readily available on all manufacturer platforms, making our recommendations straightforward to implement on any modern scanner.
Potential clinical impact of multiparametric quantitative MR spectroscopy in neurological disorders: A review and analysis(2020) Magnetic Resonance in Medicine. 83, 1, p. 22-44 Abstract
Purpose Unlike conventional MR spectroscopy (MRS), which only measures metabolite concentrations, multiparametric MRS also quantifies their longitudinal (T-1) and transverse (T-2) relaxation times, as well as the radiofrequency transmitter inhomogeneity (B1+). To test whether knowledge of these additional parameters can improve the clinical utility of brain MRS, we compare the conventional and multiparametric approaches in terms of expected classification accuracy in differentiating controls from patients with neurological disorders. Theory and Methods A literature review was conducted to compile metabolic concentrations and relaxation times in a wide range of neuropathologies and regions of interest. Simulations were performed to construct receiver operating characteristic curves and compute the associated areas (area under the curve) to examine the sensitivity and specificity of MRS for detecting each pathology in each region. Classification accuracy was assessed using metabolite concentrations corrected using population-averages for T-1, T-2, and B1+ (conventional MRS); using metabolite concentrations corrected using per-subject values (multiparametric MRS); and using an optimal linear multiparametric estimator comprised of the metabolites' concentrations and relaxation constants (multiparametric MRS). Additional simulations were conducted to find the minimal intra-subject precision needed for each parameter. Results Compared with conventional MRS, multiparametric approaches yielded area under the curve improvements for almost all neuropathologies and regions of interest. The median area under the curve increased by 0.14 over the entire dataset, and by 0.24 over the 10 instances with the largest individual increases. Conclusions Multiparametric MRS can substantially improve the clinical utility of MRS in diagnosing and assessing brain pathology, motivating the design and use of novel multiparametric sequences.
What is the optimal schedule for multiparametric MRS?: A magnetic resonance fingerprinting perspective(2019) NMR in Biomedicine. e4196. Abstract
Clinical magnetic resonance spectroscopy (MRS) mainly concerns itself with the quantification of metabolite concentrations. Metabolite relaxation values, which reflect the microscopic state of specific cellular and sub-cellular environments, could potentially hold additional valuable information, but are rarely acquired within clinical scan times. By varying the flip angle, repetition time and echo time in a preset way (termed a schedule), and matching the resulting signals to a pre-generated dictionary - an approach dubbed magnetic resonance fingerprinting - it is possible to encode the spins' relaxation times into the acquired signal, simultaneously quantifying multiple tissue parameters for each metabolite. Herein, we optimized the schedule to minimize the averaged root mean square error (RMSE) across all estimated parameters: concentrations, longitudinal and transverse relaxation time, and transmitter inhomogeneity. The optimal schedules were validated in phantoms and, subsequently, in a cohort of healthy volunteers, in a 4.5 mL parietal white matter single voxel and an acquisition time under 5 minutes. The average intra-subject, inter-scan coefficients of variation (CVs) for metabolite concentrations, T-1 and T-2 relaxation times were found to be 3.4%, 4.6% and 4.7% in-vivo, respectively, averaged over all major singlets. Coupled metabolites were quantified using the short echo time schedule entries and spectral fitting, and reliable estimates of glutamate+glutamine, glutathione and myo-inositol were obtained.
(2019) Magnetic Resonance in Medicine. 82, 3, p. 867-876 Abstract
Purpose: To design and implement a multislice MRSI method for fast spectroscopic imaging, using a modified version of echo planar spectroscopic imaging (EPSI) that offers higher spectral width and/or shorter scan time.Methods: Echo planar spectroscopic imaging suffers from inconsistencies between readout lines acquired with gradients of opposite signs, which has typically been addressed by reconstructing the "positive" and "negative" data sets separately and averaging the two. Nevertheless, consistency between the readout lines of each phase encode can be achieved by interposing the EPSI readouts with alternating "blipped" phase-encode gradients. This method exchanges inconsistencies along the temporal dimension with inconsistencies along the phase-encode dimension, which are straightforward to correct, as is conventionally done in various EPI reconstruction schemes. Such consistent k-t-space EPSI doubles the spectral width in comparison to EPSI, or, in an alternative realization, yields the same spectral width as EPSI, but at half the acquisition time. In this work, multiband CAIPIRINHA (controlled aliasing in parallel imaging results in higher acceleration) slice selection was integrated with consistent k-t-space EPSI to further accelerate the measurement 2-fold.Results: The feasibility of a consistent k-t-space EPSI was demonstrated in both phantoms and in vivo brain imaging at 3 T, and four pulse scheme variants were evaluated. It was demonstrated to be useful in optimizing the spectral width and scan acceleration, both of which are limiting factors in vivo. Dual-band implementation was shown to shorten the duration of the scan 4-fold.Conclusion: The consistent k-t-space EPSI can be used to accelerate MRSI or, alternatively, double its spectral width. Adding dual-band CAIPIRINHA further accelerates the acquisition by a factor of 2.
(2019) Magnetic Resonance in Medicine. 82, 1, p. 145-158 Abstract
Purpose: Multi-echo spin-echo (MESE) protocol is the most effective tool for mapping T-2 relaxation in vivo. Still, MESE extensive use of radiofrequency pulses causes magnetization transfer (MT)-related bias of the water signal, instigated by the presence of macromolecules (MMP). Here, we analyze the effects of MT on MESE signal, alongside their impact on quantitative T-2 measurements.Methods: Study used 3 models: in vitro urea phantom, ex vivo horse brain, and in vivo human brain. MT ratio (MTR) was measured between single-SE and MESE protocols under different scan settings including varying echo train lengths, number of slices, and inter-slice gap. MTR and T-2 values were extracted for each model and protocol.Results: MT interactions biased MESE signals, and in certain settings, the corresponding T-2 values. T-2 underestimation of up to 4.3% was found versus single-SE values in vitro and up to 13.8% ex vivo, correlating with the MMP content. T-2 bias originated from intra-slice saturation of the MMP, rather than from indirect saturation in multi-slice acquisitions. MT-related signal attenuation was caused by slice crosstalk and/or partial T-1 recovery, whereas smaller contribution was caused by MMP interactions. Inter-slice gap had a similar effect on in vivo MTR (21.2%), in comparison to increasing the number of slices (18.9%).Conclusions: MT influences MESE protocols either by uniformly attenuating the entire echo train or by cumulatively attenuating the signal along the train. Although both processes depend on scan settings and MMP content, only the latter will cause underestimation of T-2.
Quantitative multivoxel proton MR spectroscopy for the identification of white matter abnormalities in mild traumatic brain injury: Comparison between regional and global analysis(2019) Journal of Magnetic Resonance Imaging. 50, 5, p. 1424-1432 Abstract
Background 3D brain proton MR spectroscopic imaging (H-1 MRSI) facilitates simultaneous metabolic profiling of multiple loci, at higher, sub-1 cm(3), spatial resolution than single-voxel H-1 MRS with the ability to separate tissue-type partial volume contribution(s). Purpose To determine if: 1) white matter (WM) damage in mild traumatic brain injury (mTBI) is homogeneously diffuse, or if specific regions are more affected; 2) partial-volume-corrected, structure-specific H-1 MRSI voxel averaging is sensitive to regional WM metabolic abnormalities. Study Type Retrospective cross-sectional cohort study. Population Twenty-seven subjects: 15 symptomatic mTBI patients, 12 matched controls. Field Strength/Sequence 3T using 3D H-1 MRSI over a 360-cm(3) volume of interest (VOI) centered over the corpus callosum, partitioned into 480 voxels, each 0.75 cm(3). Assessment N-acetyl-aspartate (NAA), creatine, choline, and myo-inositol concentrations estimated in predominantly WM regions: body, genu, and splenium of the corpus callosum, corona radiata, frontal, and occipital WM. Statistical Tests Analysis of covariance (ANCOVA) to compare patients with controls in terms of regional concentrations. The effect sizes (Cohen's d) of the mean differences were compared across regions and with previously published global data obtained with linear regression of the WM over the entire VOI in the same dataset. Results Despite patients' global VOI WM NAA being significantly lower than the controls', no regional differences were observed for any metabolite. Regional NAA comparisons, however, were all unidirectional (patients' NAA concentrations <controls') within a narrow range: 0.3 ≤ Cohen's d ≤ 0.6.Data ConclusionSince the patient group was symptomatic and exhibiting global WM NAA deficits, these findings suggest: 1) diffuse axonal mTBI damage; that is 2) below the 1H MRSI detection threshold in small regions. Therefore, larger, ie, more sensitive, single‐voxel 1H MRS, placed anywhere in WM regions, may be well suited for mTBI 1H MRS studies, given that these results are confirmed in other cohorts.
Inhibitory and excitatory mechanisms in the human cingulate-cortex support reinforcement learning: A functional Proton Magnetic Resonance Spectroscopy study(2019) NeuroImage. 184, p. 25-35 Abstract
The dorsal anterior cingulate cortex (dACC) is crucial for motivation, reward- and error-guided decision-making, yet its excitatory and inhibitory mechanisms remain poorly explored in humans. In particular, the balance between excitation and inhibition (E/I), demonstrated to play a role in animal studies, is difficult to measure in behaving humans. Here, we used functional magnetic-resonance-spectroscopy (H-1-fMRS) to measure the brain's major inhibitory (GABA) and excitatory (Glutamate) neurotransmitters during reinforcement learning with three different conditions: high cognitive load (uncertainty); probabilistic discrimination learning; and a control null condition. Participants learned to prefer the gain option in the discrimination phase and had no preference in the other conditions. We found increased GABA levels during the uncertainty condition, potentially reflecting recruitment of inhibitory systems during high cognitive load when trying to learn. Further, higher GABA levels during the null (baseline) condition correlated with improved discrimination learning. Finally, glutamate and GABA levels were correlated during high cognitive load. These results suggest that availability of dACC inhibitory resources enables successful learning. Our approach helps elucidate the potential contribution of the balance between excitation and inhibition to learning and motivation in behaving humans.
(2018) NMR in Biomedicine. 31, 11, 4001. Abstract
Magnetic resonance fingerprinting has been proposed as a method for undersampling k-space while simultaneously yielding multiparametric tissue maps. In the context of single voxel spectroscopy, fingerprinting can provide a unified framework for parameter estimation. We demonstrate the utility of such a magnetic resonance spectroscopic fingerprinting (MRSF) framework for simultaneously quantifying metabolite concentrations, T-1 and T-2 relaxation times and transmit inhomogeneity for major singlets of N-acetylaspartate, creatine and choline. This is achieved by varying T-R, T-E and the flip angle of the first pulse in a PRESS sequence between successive excitations (i.e. successive T-R values). The need for multiparametric schemes such as MRSF for accurate medical diagnostics is demonstrated with the aid of realistic in vivo simulations; these show that certain schemes lead to substantial increases to the area under receiver operating characteristics of metabolite concentrations, when viewed as classifiers of pathologies. Numerical simulations and phantom and in vivo experiments using several different schedules of variable length demonstrate superior precision and accuracy for metabolite concentrations and longitudinal relaxation, and similar performance for the quantification of transverse relaxation.
(2018) Medical Physics. 45, 9, p. 4066-4084 Abstract
Purpose: Magnetic resonance fingerprinting (MRF) is a relatively new approach that provides quantitative MRI measures using randomized acquisition. Extraction of physical quantitative tissue parameters is performed offline, without the need of patient presence, based on acquisition with varying parameters and a dictionary generated according to the Bloch equationsimulations. MRF uses hundreds of radio frequency (RF) excitation pulses for acquisition, and therefore, a high undersampling ratio in the sampling domain (k-space) is required for reasonable scanning time. This undersampling causes spatial artifacts that hamper the ability to accurately estimate the tissue's quantitative values. In this work, we introduce a new approach for quantitative MRI using MRF, called magnetic resonance fingerprinting with low rank (FLOR).Methods: We exploit the low-rank property of the concatenated temporal imaging contrasts, on top of the fact that the MRF signal is sparsely represented in the generated dictionary domain. We present an iterative recovery scheme that consists of a gradient step followed by a low-rank projection using the singular value decomposition.Results: Experimental results consist of retrospective sampling that allows comparison to a well defined reference, and prospective sampling that shows the performance of FLOR for a real-data sampling scenario. Both experiments demonstrate improved parameter accuracy compared to other compressed-sensing and low-rank based methods for MRF at 5% and 9% sampling ratios for the retrospective and prospective experiments, respectively.Conclusions: We have shown through retrospective and prospective experiments that by exploiting the low-rank nature of the MRF signal, FLOR recovers the MRF temporal undersampled images and provides more accurate parameter maps compared to previous iterative approaches.
(2018) Magnetic Resonance in Medicine. 79, 5, p. 2481-2490 Abstract
PurposeApplication of phase rotation to the STRESS (=STEAM+PRESS) localization scheme, to shorten echo time, minimize J-coupling dephasing and estimate B1+ inhomogeneity. STRESS (=STEAM+PRESS) simultaneously refocuses and acquires the double spin echo (SE123) and stimulated echo (STE-) pathways, combining PRESS-like signal with lower chemical shift displacement as in STEAM. Phase rotation effectively separates coherence pathways, allows reduction of spoiling gradients moments leading to reduction in echo time. Implementing it in STRESS allows one to individually phase-correct SE123 and STE- prior to combination. Moreover, B1+ inhomogeneity can be assessed by comparing the measured ratio of resonance intensities of SE123 and STE- pathways to the simulated one.MethodsIn vivo spectra were acquired from a single voxel placed in the sensory-motor cortex of 10 healthy volunteers, using phase rotation-STRESS/PRESS/STEAM sequences at 3 T scanner. The phases of each slice-selective pulse were incremented by 1/2/3=22.5 degrees/-45 degrees/45 degrees.ResultsPhase rotation-STRESS showed quantification accuracy (% Cramer Rao lower bounds) and reproducibility (% coefficients of variation) comparable to PRESS and STEAM, in both phantoms and in vivo study. Minimal echo time achieved was 13 ms.ConclusionPhase rotation complements STRESS by reducing echo time, allowing processing of each pathway individually prior to addition and providing B1+ estimation in single voxel proton magnetic resonance spectroscopy. Magn Reson Med 79:2481-2490, 2018. (c) 2017 International Society for Magnetic Resonance in Medicine.
Proton MR spectroscopy of lesion evolution in multiple sclerosis: Steady-state metabolism and its relationship to conventional imaging(2017) Human Brain Mapping. 38, 8, p. 4047-4063 Abstract
Although MRI assessment of white matter lesions is essential for the clinical management of multiple sclerosis, the processes leading to the formation of lesions and underlying their subsequent MRI appearance are incompletely understood. We used proton MR spectroscopy to study the evolution of N-acetyl-aspartate (NAA), creatine (Cr), choline (Cho), and myo-inositol (mI) in pre-lesional tissue, persistent and transient new lesions, as well as in chronic lesions, and related the results to quantitative MRI measures of T1-hypointensity and T2-volume. Within 10 patients with relapsing-remitting course, there were 180 regions-of-interest consisting of up to seven semi-annual follow-ups of normal-appearing white matter (NAWM, n = 10), pre-lesional tissue giving rise to acute lesions which resolved (n = 3) or persisted (n = 3), and of moderately (n = 9) and severely hypointense (n = 6) chronic lesions. Compared with NAWM, pre-lesional tissue had higher Cr and Cho, while compared with lesions, pre-lesional tissue had higher NAA. Resolving acute lesions showed similar NAA levels pre- and post-formation, suggesting no long-term axonal damage. In chronic lesions, there was an increase in mI, suggesting accumulating astrogliosis. Lesion volume was a better predictor of axonal health than T1-hypointensity, with lesions larger than 1.5 cm(3) uniformly exhibiting very low (
(2017) NMR in Biomedicine. 30, 7, e3710. Abstract
Metabolite levels measured using magnetic resonance spectroscopy (MRS) are often expressed as ratios rather than absolute concentrations. However, the inter-subject variability of the denominator metabolite can introduce uncertainty into a metabolite ratio. In a clinical setting, there are no guidelines on whether ratios or absolute quantification should be used for a more accurate classification of normal versus abnormal results based on their statistical properties. In a research setting, the choice of one over the other can have significant implications on sample size, which must be factored in at the study design stage. Herein, we derive the probability distribution function for the ratio of two normally distributed random variables, and present analytical expressions for the comparison of ratios with absolute quantification in terms of both sample size and area under the receiver operator characteristic curve. The two approaches are compared for typical metabolite values found in the literature, and their respective merits are illustrated using previously acquired clinical MRS data in two pathologies: mild traumatic brain injury and multiple sclerosis. Our analysis shows that the decision between ratios and absolute quantification is non-trivial: in some cases, ratios might offer a reduction in sample size, whereas, in others, absolute quantification might prove more desirable for individual (i.e. clinical) use. The decision is straightforward and exact guidelines are provided in the text, given that population means and standard deviations of numerator and denominator can be reliably estimated.
Metabolic Abnormalities in the Hippocampus of Patients with Schizophrenia: A 3D Multivoxel MR Spectroscopic Imaging Study at 3T(2016) American Journal of Neuroradiology. 37, 12, p. 2273-2279 Abstract
BACKGROUND AND PURPOSE: Schizophrenia is well-known to be associated with hippocampal structural abnormalities. We used(1)H-MR spectroscopy to test the hypothesis that these abnormalities are accompanied by NAA deficits, reflecting neuronal dysfunction, in patients compared with healthy controls. MATERIALS AND METHODS: Nineteen patients with schizophrenia (11 men; mean age, 40.6 +/- 10.1 years; mean disease duration, 19.5 +/- 10.5 years) and 11 matched healthy controls (5 men; mean age, 33.7 +/- 10.1 years) underwent MR imaging and multivoxel point-resolved spectroscopy (TE/TR, 35/1400 ms)H-1-MRS at 3T to obtain their hippocampal GM absolute NAA, Cr, Cho, and mins concentrations. Unequal variance t tests and ANCOVA were used to compare patients with controls. Bilateral volumes from manually outlined hippocampal masks were compared by using unequal variance t tests. RESULTS: Patients' average hippocampal GM Cr concentrations were 19% higher than that of controls, 8.7 +/- 2.2 versus 7.4 +/- 1.2 mmol/L (P.1). There was a positive correlation between mins and Cr in patients (r = 0.57, P=.05) but not in controls. The mean bilateral hippocampal volume was similar to 10% lower in patients: 7.5 +/- 0.9 versus 8.4 +/- 0.7 cm(3) (P
(2016) 2016 38th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC). p. 439-442 Abstract
Magnetic Resonance Fingerprinting (MRF) is a relatively new approach that provides quantitative MRI using randomized acquisition. Extraction of physical quantitative tissue values is preformed off-line, based on acquisition with varying parameters and a dictionary generated according to the Bloch equations. MRF uses hundreds of radio frequency (RF) excitation pulses for acquisition, and therefore high under-sampling ratio in the sampling domain (k-space) is required. This under-sampling causes spatial artifacts that hamper the ability to accurately estimate the quantitative tissue values. In this work, we introduce a new approach for quantitative MRI using MRF, called Low Rank MRF. We exploit the low rank property of the temporal domain, on top of the well-known sparsity of the MRF signal in the generated dictionary domain. We present an iterative scheme that consists of a gradient step followed by a low rank projection using the singular value decomposition. Experiments on real MRI data demonstrate superior results compared to conventional implementation of compressed sensing for MRF at 15% sampling ratio.
Hypo-metabolism of the rostral anterior cingulate cortex associated with working memory impairment in 18 cases of schizophrenia(2016) Brain Imaging and Behavior. 10, 1, p. 115-123 Abstract
Working memory (Work-Mem), the capacity to hold and manipulate information, activates the anterior cingulate cortex (ACC), especially its caudal subregion. Impaired Work-Mem and structural and functional abnormalities of the ACC are reported in schizophrenia. This study aims to elucidate the pathogenesis of Work-Mem dysfunction in schizophrenia by comparing metabolite concentrations across ACC subregions. This retrospective study of 18 schizophrenia cases and 10 matched controls used proton magnetic resonance spectroscopic imaging (H-1-MRSI, TR/TE = 1800/35 ms, 0.5 cm(3) spatial resolution) to test whether the Work-Mem Index of the Wechsler Adult Intelligence Scale, third edition is associated with differences in the rostral to caudal ACC ratios of N-acetylaspartate (NAA) and creatine (Cr). Higher caudal: rostral ACC Cr (but not NAA) concentrations were associated with decreased Work-Mem Index in cases (r=-0.6, p=0.02), with a similar trend in controls (r=-0.56, p=0.10), although caudal: rostral ACC Cr correlated with NAA in cases and controls (r=0.67 and 0.62, p
Early glial activation precedes neurodegeneration in the cerebral cortex after SIV infection: A 3D, multivoxel proton magnetic resonance spectroscopy study(2015) HIV Medicine. 16, 6, p. 381-387 Abstract
ObjectivesAs approximate to 40% of HIV-infected individuals experience neurocognitive decline, we investigated whether proton magnetic resonance spectroscopic imaging (H-1-MRSI) detects early metabolic abnormalities in the cerebral cortex of a simian immunodeficiency virus (SIV)-infected rhesus monkey model of neuroAIDS. MethodsThe brains of five rhesus monkeys before and 4 or 6 weeks after SIV infection (with CD8(+) T-cell depletion) were assessed with T-2-weighted quantitative magnetic resonance imaging (MRI) and 16x16x4 multivoxel H-1-MRSI (echo time/repetition time=33/1440ms). Grey matter and white matter masks were segmented from the animal MRIs and used to produce cortical masks co-registered to H-1-MRSI data to yield cortical metabolite concentrations of the glial markers myo-inositol (mI), creatine (Cr) and choline (Cho), and of the neuronal marker N-acetylaspartate (NAA). The cortex volume within the large, 28cm(3) (approximate to 35% of total monkey brain) volume of interest was also calculated for each animal pre- and post-infection. Mean metabolite concentrations and cortex volumes were compared pre- and post-infection using paired sample t-tests. ResultsThe mean (standard deviation) pre-infection concentrations of the glial markers mI, Cr and Cho were 5.8 +/- 0.9, 7.2 +/- 0.4 and 0.9 +/- 0.1mM, respectively; these concentrations increased 28% (p approximate to 0.06), 15% and 10% (both p
(2015) Magnetic Resonance in Medicine. 73, 1, p. 31-43 Abstract
PurposeTo design a proton MR spectroscopy (H-1-MRS) localization sequence that combines the signal-to-noise-ratio (SNR) benefits of point resolved spectroscopy (PRESS) with the high pulse bandwidths, low chemical shift displacements (CSD), low specific absorption rates (SAR), short echo times (TE), and superior radiofrequency transmit field (B1+) immunity of stimulated echo acquisition mode (STEAM), by simultaneously refocusing and acquiring both the double-spin and stimulated echo coherence pathways from the volume of interest.Theory and MethodsWe propose a family of H-1-MRS sequences comprising three orthogonal spatially selective pulses with flip angles 90 degreesResultsPhantom and in vivo brain experiments show that 83-100% of the PRESS SNR (metabolite-dependent) is achieved at under 75% of the SAR and 66% lower in-plane CSD.ConclusionThe advantages of STEAM can be augmented with the higher SNR of PRESS by combining the spin and stimulated echoes. Quantification, especially of J-coupled resonances and intermediate and long TEs, must be carefully considered. Magn Reson Med 73:31-43, 2015. (c) 2014 Wiley Periodicals, Inc.
(2014) NMR in Biomedicine. 27, 11, p. 1275-1284 Abstract
Concentration of the neuronal marker, N-acetylaspartate (NAA), a quantitative metric for the health and density of neurons, is currently obtained by integration of the manually defined peak in whole-head proton (H-1)-MRS. Our goal was to develop a full spectral modeling approach for the automatic estimation of the whole-brain NAA concentration (WBNAA) and to compare the performance of this approach with a manual frequency-range peak integration approach previously employed. MRI and whole-head H-1-MRS from 18 healthy young adults were examined. Non-localized, whole-head H-1-MRS obtained at 3T yielded the NAA peak area through both manually defined frequency-range integration and the new, full spectral simulation. The NAA peak area was converted into an absolute amount with phantom replacement and normalized for brain volume (segmented from T-1-weighted MRI) to yield WBNAA. A paired-sample t test was used to compare the means of the WBNAA paradigms and a likelihood ratio test used to compare their coefficients of variation. While the between-subject WBNAA means were nearly identical (12.8 +/- 2.5mm for integration, 12.8 +/- 1.4mm for spectral modeling), the latter's standard deviation was significantly smaller (by similar to 50%, p=0.026). The within-subject variability was 11.7% (+/- 1.3mm) for integration versus 7.0% (+/- 0.8mm) for spectral modeling, i.e., a 40% improvement. The (quantifiable) quality of the modeling approach was high, as reflected by Cramer-Rao lower bounds below 0.1% and vanishingly small (experimental - fitted) residuals. Modeling of the whole-head H-1-MRS increases WBNAA quantification reliability by reducing its variability, its susceptibility to operator bias and baseline roll, and by providing quality-control feedback. Together, these enhance the usefulness of the technique for monitoring the diffuse progression and treatment response of neurological disorders. Copyright (c) 2014 John Wiley & Sons, Ltd.
Three-Dimensional Hadamard-Encoded Proton Spectroscopic Imaging in the Human Brain Using Time-Cascaded Pulses at 3 Tesla(2014) Magnetic Resonance in Medicine. 72, 4, p. 923-933 Abstract
PurposeTo reduce the specific-absorption-rate (SAR) and chemical shift displacement (CSD) of three-dimensional (3D) Hadamard spectroscopic imaging (HSI) and maintain its point spread function (PSF) benefits.MethodsA 3D hybrid of 2D longitudinal, 1D transverse HSI (L-HSI, T-HSI) sequence is introduced and demonstrated in a phantom and the human brain at 3 Tesla (T). Instead of superimposing each of the selective Hadamard radiofrequency (RF) pulses with its N single-slice components, they are cascaded in time, allowing N-fold stronger gradients, reducing the CSD. A spatially refocusing 180 degrees RF pulse following the T-HSI encoding block provides variable, arbitrary echo time (TE) to eliminate undesirable short T-2 species' signals, e.g., lipids.ResultsThe sequence yields 10-15% better signal-to-noise ratio (SNR) and 8-16% less signal bleed than 3D chemical shift imaging of equal repetition time, spatial resolution and grid size. The 13 6, 22 +/- 7, 24 +/- 8, and 31 +/- 14 in vivo SNRs for myo-inositol, choline, creatine, and N-acetylaspartate were obtained in 21 min from 1 cm(3) voxels at TE approximate to 20 ms. Maximum CSD was 0.3 mm/ppm in each direction.ConclusionThe new hybrid HSI sequence offers a better localized PSF at reduced CSD and SAR at 3T. The short and variable TE permits acquisition of short T-2 and J-coupled metabolites with higher SNR. Magn Reson Med 72:923-933, 2014. (c) 2013 Wiley Periodicals, Inc.
Structure-specific glial response in a macaque model of neuroAIDS: multivoxel proton magnetic resonance spectroscopic imaging at 3 Tesla(2013) AIDS. 27, 16, p. 2519-2528 Abstract
Objective:As approximate to 40% of persons with HIV also suffer neurocognitive decline, we sought to assess metabolic dysfunction in the brains of simian immunodeficiency virus (SIV)-infected rhesus macaques, an advanced animal model, in structures involved in cognitive function. We test the hypothesis that SIV-infection produces proton-magnetic resonance spectroscopic imaging (H-1-MRSI)-observed decline in the neuronal marker, N-acetylaspartate (NAA), and elevations in the glial marker, myo-inositol (mI), and associated creatine (Cr) and choline (Cho) in these structures.Design:Pre- and 4-6 weeks post-SIV infection (with CD8(+) T-lymphocyte depletion) was monitored with T-2-weighted quantitative MRI and 16x16x4 multivoxel H-1-MRSI (TE/TR=33/1400ms) in the brains of five rhesus macaques.Methods:Exploiting the high-resolution H-1-MRSI grid, we obtained absolute, cerebrospinal fluid partial volume-corrected NAA, Cr, Cho and mI concentrations from centrum semiovale, caudate nucleus, putamen, thalamus and hippocampus regions.Results:Pre- to post-infection mean Cr increased in the thalamus: 7.20.4 to 8.0 +/- 0.8mmol/l (+11%, P
Localization Errors in MR Spectroscopic Imaging Due to the Drift of the Main Magnetic Field and Their Correction(2013) Magnetic Resonance in Medicine. 70, 4, p. 895-904 Abstract
Purpose: To analyze the effect of B-0 field drift on multivoxel MR spectroscopic imaging and to propose an approach for its correction.Theory and Methods: It is shown, both theoretically and in a phantom, that for similar to 30 min acquisitions a linear B-0 drift (similar to 0.1 ppm/h) will cause localization errors that can reach several voxels (centimeters) in the slower varying phase encoding directions. An efficient and unbiased estimator is proposed for tracking the drift by interleaving short (similar to T-2*), nonlocalized acquisitions on the nonsuppressed water each pulse repetition time, as shown in 10 volunteers at 1.5 and 3 T.Results: The drift is shown to be predominantly linear in both the phantom and volunteers at both fields. The localization errors are observed and quantified in both phantom and volunteers. The unbiased estimator is shown to reliably track the instantaneous frequency in vivo despite only using a small portion of the FID.Conclusion: Contrary to single-voxel MR spectroscopy, where it leads to line broadening, field drift can lead to localization errors in the longer chemical shift imaging experiments. Fortunately, this drift can be obtained at a negligible cost to sequence timing, and corrected for in post processing.
Proton MR Spectroscopy Correlates Diffuse Axonal Abnormalities with Post-Concussive Symptoms in Mild Traumatic Brain Injury(2013) Journal of Neurotrauma. 30, 13, p. 1200-1204 Abstract
There are no established biomarkers for mild traumatic brain injury (mTBI), in part because post-concussive symptoms (PCS) are subjective and conventional imaging is typically unremarkable. To test whether diffuse axonal abnormalities quantified with three-dimensional (3D) proton magnetic resonance spectroscopic imaging (H-1-MRSI) correlated with patients' PCS, we retrospectively studied 26 mTBI patients (mean Glasgow Coma Scale [GCS] score of 14.7), 18- to 56-year-olds and 13 controls three to 55 days post-injury. All were scanned at 3 Tesla with T1- and T2-weighted MRI and 3D H-1-MRSI (480 voxels over 360 cm(3), similar to 30% of the brain). On scan day, patients completed a symptom questionnaire, and those who indicated at least one of the most common subacute mTBI symptoms (headache, dizziness, sleep disturbance, memory deficits, blurred vision) were grouped as PCS-positive. Global gray matter and white matter (GM/WM) absolute concentrations of N-acetylaspartate (NAA), choline (Cho), creatine (Cr) and myo-inositol (mI) in PCS-positive and PCS-negative patients were compared to age-and gender-matched controls using two-way analysis of variance. The results showed that the PCS-negative group (n = 11) and controls (n = 8) did not differ in any GM or WM metabolite level. The PCS-positive patients (n = 15) had lower WM NAA than the controls (n = 12; 7.0 +/- 0.6 versus 7.9 +/- 0.5mM; p = 0.0007). Global WM NAA, therefore, showed sensitivity to the TBI sequelae associated with common PCS in patients with mostly normal neuroimaging, as well as GCS scores. This suggests a potential biomarker role in a patient population in which objective measures of injury and symptomatology are currently lacking.
(2013) Magnetic Resonance in Medicine. 70, 1, p. 7-15 Abstract
A non-spin-echo multivoxel proton MR localization method based on three-dimensional transverse Hadamard spectroscopic imaging is introduced and demonstrated in a phantom and the human brain. Spatial encoding is achieved with three selective 90 degrees radiofrequency pulses along perpendicular axes: The first two create a longitudinal +/- MZ Hadamard order in the volume of interest. The third pulse spatially Hadamard-encodes the +/- MZs in the volume of interest in the third direction while bringing them to the transverse plane to be acquired immediately. The approaching-ideal point spread function of Hadamard encoding and very short acquisition delay yield signal-to-noise-ratios of 20 +/- 8, 23 +/- 9, and 31 +/- 10 for choline, creatine, and N-acetylaspartate in the human brain at 1.5 T from 1 cm3 voxels in 21 min. The advantages of transverse Hadamard spectroscopic imaging are that unlike gradient (Fourier) phase-encoding: (i) the volume of interest does not need to be smaller than the field of view to prevent aliasing; (ii) the number of partitions in each direction can be small, 8, 4, or even 2 at no cost in point spread function; (iii) the volume of interest does not have to be contiguous; and (iv) the voxel profile depends on the available B1 and pulse synthesis paradigm and can, therefore, at least theoretically, approach ideal 1 inside and 0 elsewhere. Magn Reson Med, 2013. (c) 2012 Wiley Periodicals, Inc.
(2013) Epilepsy and Behavior. 27, 2, p. 319-325 Abstract
Since approximately 5-10% of the similar to 50,000 tuberous sclerosis complex (TSC) patients in the US are "MRI-negative," our goal was to test the hypothesis that they nevertheless exhibit metabolic abnormalities. To test this, we used proton MR spectroscopy to obtain and compare gray and white matter (GM and WM) levels of the neuronal marker, N-acetylaspartate (NAA), the glial marker, myo-inositol (mI), and its associated creatine (Cr), and choline (Cho) between two " MRI-negative" female TSC patients (ages 5 and 43 years) and their matched controls. The NAA, Cr, Cho and mI concentrations, 9.8, 6.3, 1.4, and 5.7 mM, in the pediatric control were similar to those of the patients, whereas the adult patient revealed a 17% WM NAA decrease and 16% WM Cho increase from their published means for healthy adults - both outside their respective 90% prediction intervals. These findings suggest that longer disease duration and/or TSC2 gene mutation may cause axonal dysfunction and demyelination. (C) 2013 Elsevier Inc. All rights reserved.
Global gray and white matter metabolic changes after simian immunodeficiency virus infection in CD8-depleted rhesus macaques: proton MRS imaging at 3 T(2013) NMR in Biomedicine. 26, 4, p. 480-488 Abstract
To test the hypotheses that global decreased neuro-axonal integrity reflected by decreased N-acetylaspartate (NAA) and increased glial activation reflected by an elevation in its marker, the myo-inositol (mI), present in a CD8-depleted rhesus macaque model of HIV-associated neurocognitive disorders. To this end, we performed quantitative MRI and 16x16x4 multivoxel proton MRS imaging (TE/TR=33/1400ms) in five macaques pre- and 46weeks post-simian immunodeficiency virus infection. Absolute NAA, creatine, choline (Cho), and mI concentrations, gray and white matter (GM and WM) and cerebrospinal fluid fractions were obtained. Global GM and WM concentrations were estimated from 224 voxels (at 0.125cm3 spatial resolution over similar to 35% of the brain) using linear regression. Pre- to post-infection global WM NAA declined 8%: 6.6 +/- 0.4 to 6.0 +/- 0.5mM (p=0.05); GM Cho declined 20%: 1.3 +/- 0.2 to 1.0 +/- 0.1mM (p
In Vivo Free Induction Decay Based 3D Multivoxel Longitudinal Hadamard Spectroscopic Imaging in the Human Brain at 3 T(2013) Magnetic Resonance in Medicine. 69, 4, p. 903-911 Abstract
We propose and demonstrate a full 3D longitudinal Hadamard spectroscopic imaging scheme for obtaining chemical shift maps, using adiabatic inversion pulses to encode the spins' positions. The approach offers several advantages over conventional Fourier-based encoding methods, including a localized point spread function; no aliasing, allowing for volumes of interest smaller than the object being imaged; an option for acquiring noncontiguous voxels; and inherent outer volume rejection. The latter allows for doing away with conventional outer volume suppression schemes, such as point resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM), and acquiring non-spin-echo spectra with short acquisition delay times, limited only by the excitation pulse's duration. This, in turn, minimizes T-2 decay, maximizes the signalto-noise ratio, and reduces J-coupling induced signal decay. Results are presented for both a phantom and an in vivo healthy volunteer at 3 T. Magn Reson Med 69:903-911, 2013. (C) 2012 Wiley Periodicals, Inc.
(2013) Neurology. 80, 1, p. 39-46 Abstract
Objective: To characterize and follow the diffuse gray and white matter (GM/WM) metabolic abnormalities in early relapsing-remitting multiple sclerosis using proton magnetic resonance spectroscopic imaging (H-1-MRSI).Methods: Eighteen recently diagnosed, mildly disabled patients (mean baseline time from diagnosis 32 months, mean Expanded Disability Status Scale [EDSS] score 1.3), all on immunomodulatory medication, were scanned semiannually for 3 years with T1-weighted and T2-weighted MRI and 3D 1H-MRSI at 3 T. Ten sex- and age-matched controls were followed annually. Global absolute concentrations of N-acetylaspartate (NAA), choline (Cho), creatine (Cr), and myo-inositol (mI) were obtained for all GM and WM in the 360 cm(3) H-1-MRSI volume of interest.Results: Patients' average WM Cr, Cho, and mI concentrations (over all time points), 5.3 +/- 0.4, 1.6 +/- 0.1, and 5.1 +/- 0.7 mM, were 8%, 12%, and 11% higher than controls' (pConclusions: Diffuse WM glial abnormalities were larger in magnitude than the axonal abnormalities and increased over time independently of conventional clinical or imaging metrics and despite immunomodulatory treatment. In contrast, the axonal abnormalities showed partial recovery, suggesting that patients' lower WM NAA levels represented a dysfunction, which may abate with treatment. Absence of detectable diffuse changes in GM suggests that injury there is minimal, focal, or heterogeneous between cortex and deep GM nuclei. Neurology (R) 2013; 80: 39-46
(2013) Journal of Neurology. 260, 1, p. 242-252 Abstract
Since mild traumatic brain injury (mTBI) often leads to neurological symptoms even without clinical MRI findings, our goal was to test whether diffuse axonal injury is quantifiable with multivoxel proton MR spectroscopic imaging (H-1-MRSI). T1- and T2-weighted MRI images and three-dimensional H-1-MRSI (480 voxels over 360 cm(3), about 30 % of the brain) were acquired at 3 T from 26 mTBI patients (mean Glasgow Coma Scale score 14.7, 18-56 years old, 3-55 days after injury) and 13 healthy matched contemporaries as controls. The N-acetylaspartate (NAA), choline (Cho), creatine (Cr) and myo-inositol (mI) concentrations and gray-matter/white-matter (GM/WM) and cerebrospinal fluid fractions were obtained in each voxel. Global GM and WM absolute metabolic concentrations were estimated using linear regression, and patients were compared with controls using two-way analysis of variance. In patients, mean NAA, Cr, Cho and mI concentrations in GM (8.4 +/- A 0.7, 6.9 +/- A 0.6, 1.3 +/- A 0.2, 5.5 +/- A 0.6 mM) and Cr, Cho and mI in WM (4.8 +/- A 0.5, 1.4 +/- A 0.2, 4.6 +/- A 0.7 mM) were not different from the values in controls. The NAA concentrations in WM, however, were significantly lower in patients than in controls (7.2 +/- A 0.8 vs. 7.7 +/- A 0.6 mM, p = 0.0125). The Cho and Cr levels in WM of patients were positively correlated with time since mTBI. This H-1-MRSI approach allowed us to ascertain that early mTBI sequelae are (1) diffuse (not merely local), (2) neuronal (not glial), and (3) in the global WM (not GM). These findings support the hypothesis that, similar to more severe head trauma, mTBI also results in diffuse axonal injury, but that dysfunction rather than cell death dominates shortly after injury.
(2012) NMR in Biomedicine. 25, 12, p. 1392-1400 Abstract
Since the brain's gray matter (GM) and white matter (WM) metabolite concentrations differ, their partial volumes can vary the voxel's 1H MR spectroscopy (1H-MRS) signal, reducing sensitivity to changes. While single-voxel 1H-MRS cannot differentiate between WM and GM signals, partial volume correction is feasible by MR spectroscopic imaging (MRSI) using segmentation of the MRI acquired for VOI placement. To determine the magnitude of this effect on metabolic quantification, we segmented a 1-mm3 resolution MRI into GM, WM and CSF masks that were co-registered with the MRSI grid to yield their partial volumes in approximately every 1 cm3 spectroscopic voxel. Each voxel then provided one equation with two unknowns: its i- metabolite's GM and WM concentrations CiGM, CiWM. With the voxels' GM and WM volumes as independent coefficients, the over-determined system of equations was solved for the global averaged CiGM and CiWM. Trading off local concentration differences offers three advantages: (i) higher sensitivity due to combined data from many voxels; (ii) improved specificity to WM versus GM changes; and (iii) reduced susceptibility to partial volume effects. These improvements made no additional demands on the protocol, measurement time or hardware. Applying this approach to 18 volunteered 3D MRSI sets of 480 voxels each yielded N-acetylaspartate, creatine, choline and myo-inositol CiGM concentrations of 8.5?+/-?0.7, 6.9?+/-?0.6, 1.2?+/-?0.2, 5.3?+/-?0.6mM, respectively, and CiWM concentrations of 7.7?+/-?0.6, 4.9?+/-?0.5, 1.4?+/-?0.1 and 4.4?+/-?0.6mM, respectively. We showed that unaccounted voxel WM or GM partial volume can vary absolute quantification by 510% (more for ratios), which can often double the sample size required to establish statistical significance. Copyright (c) 2012 John Wiley & Sons, Ltd.
Multivoxel Proton MR Spectroscopy Used to Distinguish Anterior Cingulate Metabolic Abnormalities in Patients with Schizophrenia(2011) Radiology. 261, 2, p. 542-550 Abstract
Purpose: To test the hypothesis that anterior cingulate cortex (ACC) subregions in patients with schizophrenia are metabolically different from those in healthy control subjects.Materials and Methods: This institutional review board-approved study was HIPAA compliant, and all participants provided written informed consent. Twenty-two patients with schizophrenia (13 male, nine female; 39.4 years +/- 10.6 [standard deviation]) and 11 age-and sex-matched control subjects (seven male, four female; 35.5 years +/- 10.7) underwent magnetic resonance (MR) imaging and three-dimensional 3-T voxel proton MR spectroscopy to measure absolute rostral and caudal ACC N-acetylaspartate (NAA), creatine (Cr), and choline (Cho) concentrations. Exact Mann-Whitney test was used to compare patient data with control data, paired-sample Wilcoxon signed rank test was used to compare subregions within groups, and receiver operating characteristic curve analysis was used to assess sensitivity and specificity in diagnosis of schizophrenia.Results: There were no significant metabolic differences between patients and control subjects or between ACC subregions in control subjects. In patients, rostral ACC NAA and Cr concentrations were significantly lower than those in caudal ACC (6.2 mM +/- 1.3 vs 7.1 mM +/- 1.3, P <.01; 5.7 mmol/L +/- 1.4 vs 6.3 mmol/L +/- 1.6, P <.01; respectively); however, this did not hold true for Cho concentrations (1.7 mmol/L +/- 0.5 vs 1.8 mmol/L +/- 0.5). For individual differences between caudal and rostral measurements, only NAA in patients was different from that in control subjects (0.9 mmol/L +/- 1.3 vs 2 0.1 mmol/L +/- 0.5, P <.01), enabling prediction of schizophrenia with 68% sensitivity and 91% specificity, for a difference of more than 0.4.Conclusion: Significant differences between caudal and rostral NAA concentration are found in ACC of patients with schizophrenia but not in ACC of healthy control subjects, indicating that neuronal density or integrity differences between ACC subregions may be characteristic of the disease. (C) RSNA, 2011
(2009) Magnetic Resonance in Chemistry. 47, 5, p. 415-422 Abstract
Single-scan 2D NMR relies on a spatial axis for encoding the indirect-domain internal spin interactions. Various strategies have been demonstrated for fulfilling the needs underlying this procedure. All such schemes use gradient-echoed sequences that leave at their conclusion solely the effects of the internal interactions along the indirect domain; they also include a real-time scheme that though simple, yields in general mixed-phase line shapes. The present paper introduces two new proposals geared up for easing the spatial encoding underlying single-scan 2D NMR methodologies. One of these is capable of delivering dispersive-free peaks along the indirect domain, and thereby purely-absorptive 2D line shapes, in amplitude-encoded experiments. The other demonstrates for the first time, the possibility to obtain single-scan 213 spectra without echoing the effects of the encoding gradient-simply by applying a single-pulse frequency sweep to encode the interactions. Both of these modes are compatible with homo- and heteronuclear correlations, and exhibit a number of complementary features vis-a-vis encoding alternatives that have so far been presented. The overall principles underlying these new spatially encoding protocols are derived, and their performance demonstrated with single-scan 2D NMR TOCSY and HSQC experiments on model compounds. Copyright (C) 2009 John Wiley & Sons, Ltd.
(2007) Journal of Magnetic Resonance. 189, 1, p. 46-58 Abstract
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. (C) 2007 Elsevier Inc. All rights reserved.
Spatial encoding and the single-scan acquisition of high definition MR images in inhomogeneous fields(2006) Journal of Magnetic Resonance. 182, 2, p. 179-194 Abstract
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 B-0 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 B-0 compensation method are reviewed for one- and high er-dimension al cases, and experimentally demonstrated on a series of 1D and 2D single-scan MRI experiments on simple phantoms. (c) 2006 Elsevier Inc. All rights reserved.
Translational entanglement and teleportation of matter wavepackets by collisions and half-collisions(2005) International Journal of Modern Physics B. 19, 26, p. 3897-3921 Abstract
To date, the translationally-entangled state originally proposed by Einstein, Podolsky and Rosen (EPR) in 1935 has not been experimentally realized for massive particles. Opatrny and Kurizki [Phys. Rev. Lett. 86, 3180 (2000)] have suggested the creation of a position- and momentum-correlated, i.e., translationally-entangled, pair of particles approximating the EPR state by dissociation of cold diatomic molecules, and further manipulation of the EPR pair effecting matter-wave teleportation. Here we aim at setting the principles of and quantifying translational entanglement by collisions and half-collisions. In collisions, the resonance width s and the initial phase-space distributions are shown to determine the degree of post-collisional momentum entanglement. Half-collisions (dissociation) are shown to yield different types of approximate EPR states. We analyse a feasible realization of translational EPR entanglement and teleportation via cold-molecule Raman dissociation and subsequent collisions, resolving both practical and conceptual difficulties it has faced so far: How to avoid entanglement loss due to the wavepacket spreading of the dissociation fragments? How to measure both position and momentum correlations of the dissociation fragments with sufficient accuracy to verify their EPR correlations? How to reliably perform two-particle (Bell) position and momentum measurements on one of the fragments and the wavepacket to be teleported?
(2005) Journal of Magnetic Resonance. 176, 1, p. 107-114 Abstract
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 ultra-fast 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. (c) 2005 Elsevier Inc. All rights reserved.
(2005) Physical Review Letters. 94, 16, Abstract
We study collisions mediated by finite-range potentials as a tool for generating translational entanglement between unbound particles or multipartite systems. The general analysis is applied to one-dimensional scattering, where resonances and the initial phase-space distribution are shown to determine the degree of postcollisional entanglement.