High-resolution MRS in the presence of field inhomogeneity via intermolecular double-quantum coherences on a 3-T whole-body scanner

Precision of metabolite-selective MRS measurements of glutamate, GABA and glutathione: A review of human brain studies

Single-voxel proton magnetic resonance spectroscopy (SV 1 H-MRS) is an in vivo noninvasive imaging technique used to detect neurotransmitters and metabolites. It enables repeated measurements in living participants to build explanatory neurochemical models of psychiatric symptoms and testing of therapeutic approaches. Given the tight link among glutamate, gamma-amino butyric acid (GABA), glutathione and glutamine within the cellular machinery, MRS investigations of neurocognitive and psychiatric disorders must quantify a network of metabolites simultaneously to capture the pathophysiological states of interest. Metabolite-selective sequences typically provide improved metabolite isolation and spectral modelling simplification for a single metabolite at a time. Non-metabolite-selective sequences provide information on all detectable human brain metabolites, but feature many signal overlaps and require complicated spectral modelling. Although there are short-echo time (TE) MRS sequences that do not use spectral editing and are optimised to target either glutamate, GABA or glutathione, these approaches usually imply a precision tradeoff for the remaining two metabolites. Given the interest in assessing psychiatric and neurocognitive diseases that involve excitation-inhibition imbalances along with oxidative stress, there is a need to survey the literature on the quantification precision of current metabolite-selective MRS techniques. In this review, we locate and describe 17 studies that report on the quality of simultaneously acquired MRS metabolite data in the human brain. We note several factors that influence the data quality for single-shot acquisition of multiple metabolites of interest using metabolite-selective MRS: (1) internal in vivo references; (2) brain regions of interests; (3) field strength of scanner; and/or (4) optimised acquisition parameters. We also highlight the strengths and weaknesses of various SV spectroscopy techniques that were able to quantify in vivo glutamate, GABA and glutathione simultaneously. The insights from this review will assist in the development of new MRS pulse sequences for simultaneous, selective measurements of these metabolites and simplified spectral modelling.

Correlation distance dependence of the resonance frequency of intermolecular zero quantum coherences and its implication for MR thermometry

PURPOSE

Because the resonance frequency of water-fat intermolecular zero-quantum coherences (iZQCs) reflects the water-fat frequency separation at the microscopic scale, these frequencies have been proposed and used as a mean to obtain more accurate temperature information. The purpose of this work was to investigate the dependence of the water-fat iZQC resonance frequency on sample microstructure and on the specific choice of the correlation distance.

METHODS

The effect of water-fat susceptibility gradients on the water-methylene iZQC resonance frequency was first computed and then measured for different water-fat emulsions and for a mixture of porcine muscle and fat. Similar measurements were also performed for mixed heteronuclear spin systems.

RESULTS

A strong dependence of the iZQC resonance frequency on the sample microstructure and on the specific choice of the correlation distance was found for spin systems like water and fat that do not mix, but not for spin systems that mix at the molecular level.

CONCLUSIONS

Because water and fat spins do not mix at the molecular level, the water-fat iZQC resonance frequency and its temperature coefficient are not only affected by sample microstructure but also by the specific choice of the correlation distance. Magn Reson Med 79:1429-1438, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Remote detection of hyperpolarized 129Xe resonances via multiple distant dipolar field interactions with 1H

A remote detection scheme utilizing the distant dipolar field interaction between two different spin species was proposed by Granwehr et al. [J. Magn. Reson. 176(2), 125 (2005)]. In that sequence 1H spins were detected indirectly via their dipolar field interaction with 129Xe spins, which served as the sensing spins. Here we propose a modification of the proposed detection scheme that takes advantage of the longer T1 relaxation time of xenon to create a long lasting dipolar field with which the fast relaxing 1H spins are allowed to interact many times during a single acquisition. This new acquisition scheme improves detection sensitivity, but it also presents some challenges.

Sub-pixel shifted acquisitions for super-resolution proton magnetic resonance spectroscopy (1H MRS) mapping

PURPOSE

(1)H MRS typical studies lead to low-resolution maps (voxels size>1cm(3)). The low-resolution maps may involve the presence of two or more different tissues in a single voxel which gives partial volume effects. Such insufficient spatial resolution remains an obstacle for most routine clinical examinations and restricts the ability in the detection of small lesions.

MATERIALS AND METHODS

The proposed algorithm is based on the combination of three ideas: controlled sub-pixel shift, averaging, and initial pixel size reduction until the spectrum extinction because initial pixel size will determine the final spatial resolution. Frequency range limits are studied to determine the optimal window of spectral peak (WSP). This method is validated in-vitro and in-vivo.

RESULTS

The results in-vitro allowed us to consolidate the theoretical one. Results in-vivo are compared with the clinical protocol to demonstrate the increase in spatial resolution. The proposed method allowed us to reduce the pixel size to1/16 of its initial pixel size in total scan time (TS) of 17min 10s. We demonstrated that the optimal WSP is 80% of its initial value and the interpolation "Bilinear" has the minimum error.

CONCLUSION

A novel approach is established for processing images and reconstructing high-resolution map from low-resolution maps. We have presented a robust algorithm for high spatial resolution in-vivo (1)H MRS, and have achieved a super spatial resolution up to 0.034cm(3) to overcome the major technical challenges.

Relative metabolite concentrations and ratios determined by use of 3-T region-specific proton magnetic resonance spectroscopy of the brain of healthy Beagles

OBJECTIVE

To determine relative concentrations of selected major brain tissue metabolites and their ratios and lobar variations by use of 3-T proton (hydrogen 1 [(1)H]) magnetic resonance spectroscopy (MRS) of the brain of healthy dogs.

ANIMALS

10 healthy Beagles.

PROCEDURES

3-T (1)H MRS at echo times of 144 and 35 milliseconds was performed on 5 transverse slices and 1 sagittal slice of representative brain lobe regions. Intravoxel parenchyma was classified as white matter, gray matter, or mixed (gray and white) and analyzed for relative concentrations (in arbitrary units) of N-acetylaspartate (NAA), choline, and creatine (ie, height at position of peak on MRS graph) as well as their ratios (NAA-to-choline, NAA-to-creatine, and choline-to-creatine ratios). Peak heights for metabolites were compared between echo times. Peak heights for metabolites and their ratios were correlated and evaluated among matter types. Yield was calculated as interpretable voxels divided by available lobar voxels.

RESULTS

Reference ranges of the metabolite concentration ratios were determined at an echo time of 35 milliseconds (NAA-to-choline ratio, 1.055 to 2.224; NAA-to-creatine ratio, 1.103 to 2.161; choline-to-creatine ratio, 0.759 to 1.332) and 144 milliseconds (NAA-to-choline ratio, 0.687 to 1.788; NAA-to-creatine ratio, 0.984 to 2.044; choline-to-creatine ratio, 0.828 to 1.853). Metabolite concentration ratios were greater in white matter than in gray matter. Voxel yields ranged from 43% for the temporal lobe to 100% for the thalamus.

CONCLUSIONS AND CLINICAL RELEVANCE

Metabolite concentrations and concentration ratios determined with 3-T (1)H MRS were not identical to those in humans and were determined for clinical and research investigations of canine brain disease.

In vivo spatially localized high resolution 1H MRS via intermolecular single-quantum coherence of rat brain at 7 T

PURPOSE

To compare the conventional localized point-resolved spectroscopy (PRESS) with localized 2D intermolecular single-quantum coherence (iSQC) magnetic resonance spectroscopy (MRS) and obtain in vivo MRS spectrum of rat brain using the latter technique.

MATERIALS AND METHODS

A brain phantom, an intact pig brain tissue, and mature Sprague-Dawley rat were studied by PRESS, Nano magic-angle spinning spectroscopy, and iSQC MRS.

RESULTS

Using PRESS, high-resolution MRS can be obtained from the brain phantom and pig brain tissue with a small voxel in a relatively homogeneous field. When a large voxel is selected, the field homogeneity is distinctly reduced. No useful information is obtained from the PRESS spectra. However, using the iSQC MRS, high-resolution spectra can be obtained from the two samples with a relatively large voxel. In the same way, an iSQC MRS spectrum can be obtained from a relatively large voxel of in vivo rat brain with a comparable resolution to the PRESS spectrum with a small voxel.

CONCLUSION

Compared to PRESS, the iSQC MRS may be more feasible and promising for detection of strongly structured tissues with relatively large voxels.

Detection of iron-labeled single cells by MR imaging based on intermolecular double quantum coherences at 14 T

To evaluate the efficiency and feasibility of intermolecular multiple quantum coherence (iMQC) magnetic resonance (MR) imaging for single cell detection, we obtained intermolecular double quantum coherence (iDQC) and conventional gradient echo (GE) images of macrophage cells labeled by contrast agents in gel. The iDQC images obtained with echo-planar readout visualized the labeled cells effectively and with a higher contrast than seen in conventional GE images, especially at low planar resolutions and with thick slices. This implies that iDQC imaging with contrast agents could be a good alternative to conventional MR imaging for detecting labeled single cells or cell tracking under favorable conditions.

Signal-to-noise ratio enhancement of intermolecular double-quantum coherence MR spectroscopy in inhomogeneous fields with phased array coils on a 3 Tesla whole-body scanner

PURPOSE

To improve signal-to-noise ratio (SNR) of intermolecular double-quantum coherence (iDQC) MRS on a 3 Tesla (T) whole-body scanner.

MATERIALS AND METHODS

A 32-channel phased array coil was used to acquire iDQC signal of a MRS phantom in the presence of large field inhomogeneity. The obtained individual spectra from the array elements were combined together in the time domain using a multichannel nonparametric singular value decomposition algorithm. The results were compared quantitatively with those acquired with a circularly polarized (CP) head coil.

RESULTS

The achieved gain in SNR ranges from 1.63 to 2.10 relative to the CP coil, mainly depending on the relative position between the surface of the phased array coil and the voxel of acquisition.

CONCLUSION

SNR enhancement of iDQC MRS in inhomogeneous fields on a 3T whole-body scanner is feasible with phased array coils. This can facilitate iDQC applications of high-resolution in vivo spectroscopy in the presence of field inhomogeneity for potential disease diagnosis in humans.

High-resolution MR spectroscopy via intermolecular double-quantum coherences in inhomogeneous B0 and B1 fields

Inhomogeneity in static field B0 and/or RF field B1 is inevitable under some circumstances. In this work, a method based on intermolecular double-quantum coherences is employed for high-resolution 1D MR spectroscopy via 2D acquisition under such a condition. High-resolution information on chemical shifts, multiplet patterns, J coupling constants and relative peak areas can be retained in the resulting 1D projected spectra, as shown with results from a narrow-bore NMR spectrometer and a whole-body clinical scanner.

High-resolution 2D NMR spectra in inhomogeneous fields based on intermolecular multiple-quantum coherences with efficient acquisition schemes

High-resolution 2D NMR spectra in inhomogeneous fields can be achieved by the use of intermolecular multiple-quantum coherences and shearing reconstruction of 3D data. However, the long acquisition time of 3D spectral data is generally unbearable for in vivo applications. To overcome this problem, two pulse sequences dubbed as iDH-COSY and iDH-JRES were proposed in this paper. Although 3D acquisition is still required for the new sequences, the high-resolution 2D spectra can be obtained with a relatively short scanning time utilizing the manipulation of indirect evolution period and sparse sampling. The intermolecular multiple-quantum coherence treatment combined with the raising and lowering operators was applied to derive analytical signal expressions for the new sequences. And the experimental observations agree with the theoretical predictions. Our results show that the new sequences possess bright perspective in the applications on in vivo localized NMR spectroscopy.