Non-water-suppressed proton MR spectroscopy improves spectral quality in the human spinal cord

MR Imaging in Ataxias: Consensus Recommendations by the Ataxia Global Initiative Working Group on MRI Biomarkers

With many viable strategies in the therapeutic pipeline, upcoming clinical trials in hereditary and sporadic degenerative ataxias will benefit from non-invasive MRI biomarkers for patient stratification and the evaluation of therapies. The MRI Biomarkers Working Group of the Ataxia Global Initiative therefore devised guidelines to facilitate harmonized MRI data acquisition in clinical research and trials in ataxias. Recommendations are provided for a basic structural MRI protocol that can be used for clinical care and for an advanced multi-modal MRI protocol relevant for research and trial settings. The advanced protocol consists of modalities with demonstrated utility for tracking brain changes in degenerative ataxias and includes structural MRI, magnetic resonance spectroscopy, diffusion MRI, quantitative susceptibility mapping, and resting-state functional MRI. Acceptable ranges of acquisition parameters are provided to accommodate diverse scanner hardware in research and clinical contexts while maintaining a minimum standard of data quality. Important technical considerations in setting up an advanced multi-modal protocol are outlined, including the order of pulse sequences, and example software packages commonly used for data analysis are provided. Outcome measures most relevant for ataxias are highlighted with use cases from recent ataxia literature. Finally, to facilitate access to the recommendations by the ataxia clinical and research community, examples of datasets collected with the recommended parameters are provided and platform-specific protocols are shared via the Open Science Framework.

Neurometabolic profile of the amygdala in smokers assessed with 1H-magnetic resonance spectroscopy

Tobacco smoking is one of the main causes of premature death worldwide and quitting success remains low, highlighting the need to understand the neurobiological mechanisms underlying relapse. Preclinical models have shown that the amygdala and glutamate play an important role in nicotine addiction. The aims of this study were to compare glutamate and other metabolites in the amygdala between smokers and controls, and between different smoking states. Furthermore, associations between amygdalar metabolite levels and smoking characteristics were explored. A novel non-water-suppressed proton magnetic resonance spectroscopy protocol was applied to quantify neurometabolites in 28 male smokers (≥15 cigarettes/day) and 21 non-smoking controls, matched in age, education, verbal IQ, and weekly alcohol consumption. Controls were measured once (baseline) and smokers were measured in a baseline state (1-3 h abstinence), during withdrawal (24 h abstinence) and in a satiation state (directly after smoking). Baseline spectroscopy data were compared between groups by independent t-tests or Mann-Whitney-U tests. Smoking state differences were investigated by repeated-measures analyses of variance (ANOVAs). Associations between spectroscopy data and smoking characteristics were explored using Spearman correlations. Good spectral quality, high anatomical specificity (98% mean gray matter) and reliable quantification of most metabolites of interest were achieved in the amygdala. Metabolite levels did not differ between groups, but smokers showed significantly higher glutamine levels at baseline than satiation. Glx levels were negatively associated with pack-years and smoking duration. In summary, this study provides first insights into the neurometabolic profile of the amygdala in smokers with high anatomical specificity. By applying proton magnetic resonance spectroscopy, neurometabolites in smokers during different smoking states and non-smoking controls were quantified reliably. A significant shift in glutamine levels between smoking states was detected, with lower concentrations in satiation than baseline. The negative association between Glx levels and smoking quantity and duration may imply altered glutamate homeostasis with more severe nicotine addiction.

Prospective motion correction for cervical spinal cord MRS

PURPOSE

To develop prospective motion correction for single-voxel MRS in the human cervical spinal cord.

METHODS

A motion MR navigator was implemented using reduced field-of-view 2D-selective RF excitation together with EPI readout. A short-echo semi-LASER sequence (TE  = 30 ms) was updated to incorporate this real-time image-based motion navigator, as well as real-time shim and frequency navigators. Five healthy participants were studied at 3 T with a 64-channel head-neck receive coil. Single-voxel MRS data were measured in a voxel located at the C3-5 vertebrae level. The motion navigator was used to correct for translations in the X-Y plane and was validated by assessing spectral quality with and without prospective correction in the presence of subject motion.

RESULTS

Without prospective correction, motion resulted in severe lipid contamination in the MR spectra. With prospective correction, the quality of spinal cord MR spectra in the presence of motion was similar to that obtained in the absence of motion, with comparable spectral signal-to-noise ratio and linewidth and no significant lipid contamination.

CONCLUSION

Prospective motion and B0 correction allow acquisition of good-quality MR spectra in the human cervical spinal cord in the presence of motion. This new technique should facilitate reliable acquisition of spinal cord MR spectra in both research and clinical settings.

Accumbal-thalamic connectivity and associated glutamate alterations in human cocaine craving: A state-dependent rs-fMRI and 1H-MRS study

Craving is a core symptom of cocaine use disorder and a major factor for relapse risk. To date, there is no pharmacological therapy to treat this disease or at least to alleviate cocaine craving as a core symptom. In animal models, impaired prefrontal-striatal signalling leading to altered glutamate release in the nucleus accumbens appear to be the prerequisite for cocaine-seeking. Thus, those network and metabolic changes may constitute the underlying mechanisms for cocaine craving and provide a potential treatment target. In humans, there is recent evidence for corresponding glutamatergic alterations in the nucleus accumbens, however, the underlying network disturbances that lead to this glutamate imbalance remain unknown. In this state-dependent randomized, placebo-controlled, double-blinded, cross-over multimodal study, resting state functional magnetic resonance imaging in combination with small-voxel proton magnetic resonance spectroscopy (voxel size: 9.4 × 18.8 × 8.4 mm3) was applied to assess network-level and associated neurometabolic changes during a non-craving and a craving state, induced by a custom-made cocaine-cue film, in 18 individuals with cocaine use disorder and 23 healthy individuals. Additionally, we assessed the potential impact of a short-term challenge of N-acetylcysteine, known to normalize disturbed glutamate homeostasis and to thereby reduce cocaine-seeking in animal models of addiction, compared to a placebo. We found increased functional connectivity between the nucleus accumbens and the dorsolateral prefrontal cortex during the cue-induced craving state. However, those changes were not linked to alterations in accumbal glutamate levels. Whereas we additionally found increased functional connectivity between the nucleus accumbens and a midline part of the thalamus during the cue-induced craving state. Furthermore, obsessive thinking about cocaine and the actual intensity of cocaine use were predictive of cue-induced functional connectivity changes between the nucleus accumbens and the thalamus. Finally, the increase in accumbal-thalamic connectivity was also coupled with craving-related glutamate rise in the nucleus accumbens. Yet, N-acetylcysteine had no impact on craving-related changes in functional connectivity. Together, these results suggest that connectivity changes within the fronto-accumbal-thalamic loop, in conjunction with impaired glutamatergic transmission, underlie cocaine craving and related clinical symptoms, pinpointing the thalamus as a crucial hub for cocaine craving in humans.

Metabolic profile of complete spinal cord injury in pons and cerebellum: A 3T 1H MRS study

The aim of this exploratory study was the assessment of the metabolic profiles of persons with complete spinal cord injury (SCI) in three region-of-interests (pons, cerebellar vermis, and cerebellar hemisphere), with magnetic resonance spectroscopy, and their correlations to clinical scores. Group differences and association between metabolic and clinical scores were examined. Fifteen people with chronic SCI (cSCI), five people with subacute SCI (sSCI) and fourteen healthy controls were included. Group comparison between cSCI and HC showed lower total N-acetyl-aspartate (tNAA) in the pons (p = 0.04) and higher glutathione (GSH) in the cerebellar vermis (p = 0.02). Choline levels in the cerebellar hemisphere were different between cSCI and HC (p = 0.02) and sSCI and HC (p = 0.02). A correlation was reported for choline containing compounds (tCho) to clinical scores in the pons (rho = - 0.55, p = 0.01). tNAA to total creatine (tNAA/tCr ratio) correlated to clinical scores in the cerebellar vermis (rho = 0.61, p = 0.004) and GSH correlated to the independence score in the cerebellar hemisphere (rho = 0.56, p = 0.01). The correlation of tNAA, tCr, tCho and GSH to clinical scores might be indicators on how well the CNS copes with the post-traumatic remodeling and might be further examined as outcome markers.

Metabolite-cycled echo-planar spectroscopic imaging of the human heart

Purpose

Spectroscopic imaging could provide insights into regional cardiac triglyceride variations, but is hampered by relatively long scan times. It is proposed to synergistically combine echo‐planar spectroscopic imaging (EPSI) with motion‐adapted gating, weighted acquisition and metabolite cycling to reduce scan times to less than 10 min while preserving spatial‐spectral quality. The method is compared to single‐voxel measurements and to metabolite‐cycled EPSI with conventional acquisition for assessing triglyceride‐to‐water (TG/W) ratios in the human heart.

Methods

Measurements were performed on 10 healthy volunteers using a clinical 1.5T system. EPSI data was acquired both without and with motion‐adapted gating in combination with weighted acquisition to assess TG/W ratios and relative Cramér‐Rao lower bounds (CRLB) of TG. For comparison, single‐voxel (PRESS) spectra were acquired in the interventricular septum.

Results

Bland–Altman analyses did not show a significant bias in TG/W when comparing both metabolite‐cycled EPSI methods to PRESS for any of the cardiac segments. Scan time was 8.05 ± 2.06 min and 17.91 ± 3.93 min for metabolite‐cycled EPSI with and without motion‐adapted gating and weighted acquisition, respectively, while relative CRLB of TG did not differ significantly between the two methods for any of the cardiac segments.

Conclusions

Metabolite‐cycled EPSI with motion‐adapted gating and weighted acquisition allows detecting TG/W ratios in different regions of the in vivo human heart. Scan time is reduced by more than 2‐fold to less than 10 min as compared to conventional acquisition, while keeping the quality of TG fitting constant.

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Respiratory-triggered quantitative MR spectroscopy of the human cervical spinal cord at 7 T

PURPOSE

Ultra-high field 1 H MR spectroscopy (MRS) is of great interest to help characterizing human spinal cord pathologies. However, very few studies have been reported so far in this small size structure at these fields due to challenging experimental difficulties caused by static and radiofrequency field heterogeneities, as well as physiological motion. In this work, in line with the recent developments proposed to strengthen spinal cord MRS feasibility at 7 T, a respiratory-triggered acquisition approach was optimized to compensate for dynamic B 0 field heterogeneities and to provide robust cervical spinal cord MRS data.

METHODS

A semi-LASER sequence was purposely used, and a dedicated raw data processing algorithm was developed to enhance MR spectral quality by discarding corrupted scans. To legitimate the choices done during the optimization stage, additional tests were carried out to determine the impact of breathing, voluntary motion, body mass index, and fitting algorithm. An in-house quantification tool was concomitantly designed for accurate estimation of the metabolite concentration ratios for choline, N-acetyl-aspartate (NAA), myo-inositol and glutathione. The method was tested on a cohort of 14 healthy volunteers.

RESULTS

Average water linewidth and NAA signal-to-noise ratio reached 0.04 ppm and 11.01, respectively. The group-average metabolic ratios were in good agreement with previous studies and showed intersession reproducibility variations below 30%.

CONCLUSION

The developed approach allows a rise of the acquired MRS signal quality and of the quantification robustness as compared to previous studies hence offering strengthened possibilities to probe the metabolism of degenerative and traumatic spinal cord pathologies.

Neurometabolic alterations in the nucleus accumbens of smokers assessed with 1 H magnetic resonance spectroscopy: The role of glutamate and neuroinflammation

Tobacco use is one of the leading causes of premature death and morbidity worldwide. For smokers trying to quit, relapse rates are high, even after prolonged periods of abstinence. Recent findings in animal models highlight the role of alterations in glutamatergic projections from the prefrontal cortex onto the nucleus accumbens (NAc) in relapse vulnerability. Moreover, inflammatory responses in the NAc have been reported during withdrawal. A novel proton magnetic resonance spectroscopy (¹ H-MRS) protocol was applied in humans to measure molar concentrations for glutamate, its sum with glutamine (Glx), and myoinositol plus glycine (mI + Gly) in the NAc (19 smokers, 20 matched controls). Smokers were measured at baseline and during withdrawal and satiation. No difference between groups or smoking states was found for glutamate or Glx, but, in smokers, stronger craving and more severe nicotine dependence were associated with lower baseline glutamate and Glx levels, respectively. Interestingly, mI + Gly concentrations were higher during withdrawal than baseline and correlated negatively with nicotine dependence severity and pack years of smoking. The lack of glutamatergic changes between groups and smoking states may imply that glutamate homeostasis is not significantly altered in smokers or that changes are too small for detection by ¹ H-MRS. Moreover, the observed increase in mI + Gly may imply that neuroinflammatory processes occur in the NAc during nicotine withdrawal. These findings shed light on neurobiological relapse mechanisms in smokers and may provide the opportunity to develop more effective treatment options targeting the glutamate and neuroinflammation system.

Impaired glutamate homeostasis in the nucleus accumbens in human cocaine addiction

Cocaine addiction is characterized by overwhelming craving for the substance, which drives its escalating use despite adverse consequences. Animal models suggest a disrupted glutamate homeostasis in the nucleus accumbens to underlie addiction-like behavior. After chronic administration of cocaine, rodents show decreased levels of accumbal glutamate, whereas drug-seeking reinstatement is associated with enhanced glutamatergic transmission. However, due to technical obstacles, the role of disturbed glutamate homeostasis for cocaine addiction in humans remains only partially understood, and accordingly, no approved pharmacotherapy exists. Here, we applied a tailored proton magnetic resonance spectroscopy protocol that allows glutamate quantification within the human nucleus accumbens. We found significantly reduced basal glutamate concentrations in the nucleus accumbens in cocaine-addicted (N = 26) compared with healthy individuals (N = 30), and increased glutamate levels during cue-induced craving in cocaine-addicted individuals compared with baseline. These glutamatergic alterations, however, could not be significantly modulated by a short-term challenge of N-acetylcysteine (2400 mg/day on 2 days). Taken together, our findings reveal a disturbed accumbal glutamate homeostasis as a key neurometabolic feature of cocaine addiction also in humans. Therefore, we suggest the glutamatergic system as a promising target for the development of novel pharmacotherapies, and in addition, as a potential biomarker for a personalized medicine approach in addiction.

Improved prospective frequency correction for macromolecule-suppressed GABA editing with metabolite cycling at 3T

PURPOSE

To combine metabolite cycling with J-difference editing (MC MEGA) to allow for prospective frequency correction at each transient without additional acquisitions and compare it to water-suppressed MEGA-PRESS (WS MEGA) editing with intermittent prospective frequency correction.

METHODS

Macromolecule-suppressed gamma aminobutyric acid (GABA)-edited experiments were performed in a phantom and in the occipital lobe (OCC) (n = 12) and medial prefrontal cortex (mPFC) (n = 8) of the human brain. Water frequency consistency and average offset over acquisition time were compared. GABA multiplet patterns, signal intensities, and choline subtraction artifacts were evaluated. In vivo GABA concentrations were compared and related to frequency offset in the OCC.

RESULTS

MC MEGA was more stable with 21% and 32% smaller water frequency SDs in the OCC and mPFC, respectively. MC MEGA also had 39% and 40% smaller average frequency offsets in the OCC and mPFC, respectively. Phantom GABA multiplet patterns and signal intensities were similar. In vivo GABA concentrations were smaller in MC MEGA than in WS MEGA, with median (interquartile range) of 2.52 (0.27) and 2.29 (0.19) institutional units (i.u.), respectively in the OCC scans without prior DTI, and 0.99 (0.3) and 1.72 (0.5), respectively in the mPFC. OCC WS MEGA GABA concentrations, but not MC MEGA GABA concentrations were moderately correlated with frequency offset. mPFC WS MEGA spectra contained significantly more subtraction artifacts than MC MEGA spectra.

CONCLUSION

MC MEGA is feasible and allows for prospective frequency correction at every transient. MC MEGA GABA concentrations were not biased by frequency offsets and contained less subtraction artifacts compared to WS MEGA.

Preprocessing, analysis and quantification in single-voxel magnetic resonance spectroscopy: experts' consensus recommendations

Once an MRS dataset has been acquired, several important steps must be taken to obtain the desired metabolite concentration measures. First, the data must be preprocessed to prepare them for analysis. Next, the intensity of the metabolite signal(s) of interest must be estimated. Finally, the measured metabolite signal intensities must be converted into scaled concentration units employing a quantitative reference signal to allow meaningful interpretation. In this paper, we review these three main steps in the post-acquisition workflow of a single-voxel MRS experiment (preprocessing, analysis and quantification) and provide recommendations for best practices at each step.

Water and lipid suppression techniques for advanced 1 H MRS and MRSI of the human brain: Experts' consensus recommendations

The neurochemical information provided by proton magnetic resonance spectroscopy (MRS) or MR spectroscopic imaging (MRSI) can be severely compromised if strong signals originating from brain water and extracranial lipids are not properly suppressed. The authors of this paper present an overview of advanced water/lipid-suppression techniques and describe their advantages and disadvantages. Moreover, they provide recommendations for choosing the most appropriate techniques for proper use. Methods of water signal handling are primarily focused on the VAPOR technique and on MRS without water suppression (metabolite cycling). The section on lipid-suppression methods in MRSI is divided into three parts. First, lipid-suppression techniques that can be implemented on most clinical MR scanners (volume preselection, outer-volume suppression, selective lipid suppression) are described. Second, lipid-suppression techniques utilizing the combination of k-space filtering, high spatial resolutions and lipid regularization are presented. Finally, three promising new lipid-suppression techniques, which require special hardware (a multi-channel transmit system for dynamic B₁⁺ shimming, a dedicated second-order gradient system or an outer volume crusher coil) are introduced.

Multiparametric in vivo analyses of the brain and spine identify structural and metabolic biomarkers in men with adrenomyeloneuropathy

OBJECTIVE

Progressive myelopathy causes severe handicap in men with adrenomyeloneuropathy (AMN), an X-linked disorder due to ABCD1 pathogenic variants. At present, treatments are symptomatic but disease-modifying therapies are under evaluation. Given the small effect size of clinical scales in AMN, biomarkers with higher effect size are needed. Here we used high-resolution magnetic resonance techniques to identify non-invasive in vivo biomarkers of the brain and spine with high effect sizes.

METHODS

We performed a multiparametric imaging and spectroscopy study in 23 male patients with AMN (age: 44 ± 11) and 23 male controls (age: 43 ± 11) of similar age and body-mass index. We combined (i) macrostructural analyses of the spine, using cross-sectional area (CSA) and magnetization transfer ratio (MTR), (ii) microstructural analyses of the spine and the brain, using diffusion tensor and the newly developed fixel-based analysis, and (iii) advanced metabolic analyses of the spine using metabolite cycling coupled to a semi-LASER sequences.

RESULTS

Macrostructural alterations (decrease in CSA and MTR) were observed in patients at all spinal cord levels studied (C1-T2 for CSA and C1-C5 for MTR) (p < 0.001). Microstructural alterations were observed in the spine and brain on diffusion tensor and fixel-based metrics though the latter showed higher effect sizes. Metabolic alterations were observed in patients as a decreased total N-acetylaspartate/myo-inositol ratio (p < 0.001). Overall, MTR showed the highest effect size.

CONCLUSION

This cross-sectional study supports the use of multiparametric techniques that elucidate the structural, microstructural and metabolic alterations in AMN. These outcome measures should be tested longitudinally and in clinical trials.

Terminology and concepts for the characterization of in vivo MR spectroscopy methods and MR spectra: Background and experts' consensus recommendations

With a 40-year history of use for in vivo studies, the terminology used to describe the methodology and results of magnetic resonance spectroscopy (MRS) has grown substantially and is not consistent in many aspects. Given the platform offered by this special issue on advanced MRS methodology, the authors decided to describe many of the implicated terms, to pinpoint differences in their meanings and to suggest specific uses or definitions. This work covers terms used to describe all aspects of MRS, starting from the description of the MR signal and its theoretical basis to acquisition methods, processing and to quantification procedures, as well as terms involved in describing results, for example, those used with regard to aspects of quality, reproducibility or indications of error. The descriptions of the meanings of such terms emerge from the descriptions of the basic concepts involved in MRS methods and examinations. This paper also includes specific suggestions for future use of terms where multiple conventions have emerged or coexisted in the past.

Metabolite cycled liver 1 H MRS on a 7 T parallel transmit system

INTRODUCTION

Single-voxel ¹ H MRS in body applications often suffers from respiratory and other motion induced phase and frequency shifts, which lead to incoherent averaging and hence to suboptimal results.

METHODS

Here we show the application of metabolite cycling (MC) for liver STEAM-localized ¹ H MRS on a 7 T parallel transmit system, using eight transmit-receive fractionated dipole antennas with 16 additional, integrated receive loops. MC-STEAM measurements were made in six healthy, lean subjects and compared with STEAM measurements using VAPOR water suppression. Measurements were performed during free breathing and during synchronized breathing, for which the subjects did breathe in between the MRS acquisitions. Both intra-session repeatability and inter-session reproducibility of liver lipid quantification with MC-STEAM and VAPOR-STEAM were determined.

RESULTS

The preserved water signal in MC-STEAM allowed for robust phase and frequency correction of individual acquisitions before averaging, which resulted in in vivo liver spectra that were of equal quality when measurements were made with free breathing or synchronized breathing. Intra-session repeatability and inter-session reproducibility of liver lipid quantification were better for MC-STEAM than for VAPOR-STEAM. This may also be explained by the more robust phase and frequency correction of the individual MC-STEAM acquisitions as compared with the VAPOR-STEAM acquisitions, for which the low-signal-to-noise ratio lipid signals had to be used for the corrections.

CONCLUSION

Non-water-suppressed MC-STEAM on a 7 T system with parallel transmit is a promising approach for ¹ H MRS applications in the body that are affected by motion, such as in the liver, and yields better repeatability and reproducibility compared with water-suppressed measurements.

Non-water-excitation MR spectroscopy techniques to explore exchanging protons in human brain at 3 T

PURPOSE

To develop localization sequences for in vivo MR spectroscopy (MRS) on clinical scanners of 3 T to record spectra that are not influenced by magnetization transfer from water.

METHODS

Image-selected in vivo spectroscopy (ISIS) localization and chemical-shift-selective excitation (termed I-CSE) was combined in two ways: first, full ISIS localization plus a frequency-selective spin-echo and second, two-dimensional (2D) ISIS plus a frequency-selective excitation and slice-selective refocusing. The techniques were evaluated at 3 T in phantoms and human subjects in comparison to standard techniques with water presaturation or metabolite-cycling. ISIS included gradient-modulated offset-independent adiabatic (GOIA)-type adiabatic inversion pulses; echo times were 8-10 ms.

RESULTS

The novel 2D and 3D I-CSE methods yield upfield spectra that are comparable to those from standard MRS, except for shorter echo times and a limited frequency range. On the downfield/high-frequency side, they yield much more signal for exchangeable protons when compared to MRS with water presaturation or metabolite-cycling and longer echo times.

CONCLUSION

Novel non-water-excitation MRS sequences offer substantial benefits for the detection of metabolite signals that are otherwise suppressed by saturation transfer from water. Avoiding water saturation and using very short echo times allows direct observation of faster exchanging moieties than was previously possible at 3 T and additionally makes the methods less susceptible to fast T₂ relaxation.

Metabolites of neuroinflammation relate to neuropathic pain after spinal cord injury

OBJECTIVE

To determine whether cervical cord levels of metabolites are associated with pain sensation after spinal cord injury (SCI) by performing magnetic resonance spectroscopy in patients with SCI with and without neuropathic pain (NP).

METHODS

Cervical cord single-voxel spectroscopic data of 24 patients with SCI (14 with NP, 10 pain-free) and 21 healthy controls were acquired at C2/3 to investigate metabolite ratios associated with neuroinflammation (choline-containing compounds to myoinositol [tCho/mI]) and neurodegeneration (total N-acetylaspartate to myo-inositol [tNAA/mI]). NP levels were measured, and Spearman correlation tests assessed associations between metabolite levels, cord atrophy, and pinprick score.

RESULTS

In patients with NP, tCho/mI levels were increased (p = 0.024) compared to pain-free patients and negatively related to cord atrophy (p = 0.006, r = 0.714). Better pinprick score was associated with higher tCho/mI levels (p = 0.032, r = 0.574). In pain-free patients, tCho/mI levels were not related to cord atrophy (p = 0.881, r = 0.055) or pinprick score (p = 0.676, r = 0.152). tNAA/mI levels were similar in both patient groups (p = 0.396) and were not associated with pinprick score in patients with NP (p = 0.405, r = 0.242) and pain-free patients (p = 0.117, r = 0.527).

CONCLUSIONS

Neuroinflammatory metabolite levels (i.e., tCho/mI) were elevated in patients with NP, its magnitude being associated with less cord atrophy and greater pain sensation (e.g., pinprick score). This suggests that patients with NP have more residual spinal tissue and greater metabolite turnover than pain-free patients. Neurodegenerative metabolite levels (i.e., tNAA/mI) were associated with greater cord atrophy but unrelated to NP. Identifying the metabolic NP signature provides new NP treatment targets and could improve patient stratification in interventional trials.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that levels of magnetic resonance spectroscopy-identified metabolites of neuroinflammation were elevated in patients with SCI with NP compared to those without NP.

Noninvasive in situ proton MRS in muscle tissue and bone marrow as a novel approach to identify previous freezing in a completely thawed cadaver

The temporary or permanent storage of human bodies in freezers following a homicide is a documented method for criminal disposal of human corpses. In these cases, the detection of characteristics which indicate that a thawed cadaver or body part was previously frozen provides crucial information for forensic casework. Previous histological and radiological approaches to detect characteristics of previous freezing are based on the formation of bubble-like gas patterns, which are difficult to distinguish from common postmortem gas formation in the course of decomposition. The objective of this study was to detect changes in the muscle tissue and in the bone marrow after freezing and thawing by means of in situ proton magnetic resonance spectroscopy (¹ H-MRS) to provide a noninvasive approach to detect postfreezing alterations in human cadavers. In this experimental study, the hind legs of seven sheep were used as substitutes for human tissue. One hind leg underwent ¹ H-MRS before and daily after storage in a deep freezer (-20°C) and complete thawing at room temperature (study group: n = 7). The opposite hind leg was kept at room temperature and was measured daily (control group: n = 7). Spectra and relaxation times were measured using single voxel measurements in the muscle tissue and in the bone marrow. ¹ H-MRS revealed several changes in the muscle tissue and in the bone marrow after freezing and thawing. A strongly reduced peak area ratio (<20) between bulk methylene and olefinic and glycerol methine and a reduced T₂ relaxation time for bulk methylene (<45 ms) measured in the bone marrow were found to be indicators that a sheep leg was previously frozen and thawed independent of the postmortem interval. Noninvasive in situ ¹ H-MRS in the bone marrow potentially provides a new method for detecting previous freezing or extreme cooling in cadavers.

Dynamic Metabolic Changes in the Human Thalamus at the Transition From Waking to Sleep - Insights From Simultaneous Functional MR Spectroscopy and Polysomnography

An important contribution of the thalamus to the transition from wakefulness to sleep is a consistent finding in animal studies. In humans, only little is currently known about the specific role of the thalamus in regulating wake-sleep transitions. Although changes in thalamic blood flow and activity have been reported, the underlying molecular mechanisms have not been investigated. Knowledge about neurotransmitter changes at the wake-to-sleep transition would be indispensable for a better translation of basic animal research findings to humans. Here, we start to fill this important scientific gap. More specifically, we benefit from recent advances in magnetic resonance (MR) spectroscopy, which allow for the non-invasive, local-specific and high-quality detection of naturally occurring metabolite changes in the human brain. We demonstrate in nine young adults able to produce consolidated sleep in the MR spectroscopy scanner, a specific decrease in thalamic glutamate concentration from wakefulness to stage N2 sleep. The magnitude of this decrease was highly correlated with individual N2 sleep duration. When five participants of the original experiment were kept awake in a separate control condition, no decrease in thalamic glutamate levels occurred. The study highlights for the first time in humans that dynamic changes in distinct brain metabolites can be reliably detected at the transition from waking to sleep. The reported methodology to simultaneously acquire functional MR spectroscopy data and neurophysiological signals offers great potential for investigating the molecular mechanisms underlying the transition between and the maintenance of sleep and wake states in humans.

Navigator-free metabolite-cycled proton spectroscopy of the heart

PURPOSE

Respiratory gating in cardiac water-suppressed (WS) proton spectroscopy leads to long and unpredictable scan times. Metabolite cycling allows to perform frequency and phase correction on the water signal and, hence, offers an approach to navigator-free cardiac spectroscopy with fixed scan time. The objective of the present study was to develop and implement navigator-free metabolite-cycled cardiac proton spectroscopy (MC nonav) and compare it with standard navigator-gated WS (WS nav) and navigator-free WS (WS nonav) measurements for the assessment of triglyceride-to-water ratios (TG/W) and creatine-to-water ratios (CR/W) in the intraventricular septum of the in vivo heart.

METHODS

Navigator-free metabolite-cycled spectroscopy was implemented on a clinical 1.5T system. In vivo measurements were performed on 10 young and 5 older healthy volunteers to assess signal-to-noise ratio efficiency as well as TG/W and CR/W and the relative Cramér-Rao lower bounds for CR. The performance of the metabolite-cycled sequence was verified using simulations.

RESULTS

On average, scan times of MC nonav were 3.4 times shorter compared with WS nav, while no significant bias for TG/W was observed (coefficient of variation = 14.0%). signal-to-noise ratio efficiency of both TG and CR increased for MC nonav compared with WS nav. Relative Cramér-Rao lower bounds of CR decreased for MC nonav. Overall spectral quality was found comparable between MC nonav and WS nav, while it was inferior for WS nonav.

CONCLUSION

Navigator-free metabolite-cycled cardiac proton spectroscopy offers 3.4-fold accelerated assessment of TG/W and CR/W in the heart with preserved spectral quality when compared with navigator-gated WS scans.

Methodological consensus on clinical proton MRS of the brain: Review and recommendations

Proton MRS (1 H MRS) provides noninvasive, quantitative metabolite profiles of tissue and has been shown to aid the clinical management of several brain diseases. Although most modern clinical MR scanners support MRS capabilities, routine use is largely restricted to specialized centers with good access to MR research support. Widespread adoption has been slow for several reasons, and technical challenges toward obtaining reliable good-quality results have been identified as a contributing factor. Considerable progress has been made by the research community to address many of these challenges, and in this paper a consensus is presented on deficiencies in widely available MRS methodology and validated improvements that are currently in routine use at several clinical research institutions. In particular, the localization error for the PRESS localization sequence was found to be unacceptably high at 3 T, and use of the semi-adiabatic localization by adiabatic selective refocusing sequence is a recommended solution. Incorporation of simulated metabolite basis sets into analysis routines is recommended for reliably capturing the full spectral detail available from short TE acquisitions. In addition, the importance of achieving a highly homogenous static magnetic field (B0 ) in the acquisition region is emphasized, and the limitations of current methods and hardware are discussed. Most recommendations require only software improvements, greatly enhancing the capabilities of clinical MRS on existing hardware. Implementation of these recommendations should strengthen current clinical applications and advance progress toward developing and validating new MRS biomarkers for clinical use.

Water removal in MR spectroscopic imaging with L2 regularization

PURPOSE

An L2-regularization based postprocessing method is proposed and tested for removal of residual or unsuppressed water signals in proton MR spectroscopic imaging (MRSI) data recorded from the human brain at 3T.

METHODS

Water signals are removed by implementation of the L2 regularization using a synthesized water-basis matrix that is orthogonal to metabolite signals of interest in the spectral dimension. Simulated spectra with variable water amplitude and in vivo brain MRSI datasets were used to demonstrate the proposed method. Results were compared with two commonly-used postprocessing methods for removing water signals.

RESULTS

The L2 method yielded metabolite signals that were close to true values for the simulated spectra. Residual/unsuppressed water signals in human brain short- and long-echo time MRSI datasets were efficiently removed by the proposed method allowing good quality metabolite maps to be reconstructed with minimized contamination from water signals. Significant differences of the creatine signal were observed between brain long-echo time MRSI without and with water saturation, attributable to the previously described magnetization transfer effect.

CONCLUSIONS

With usage of a synthesized water matrix generated based on reasonable prior knowledge about water and metabolite resonances, the L2 method is shown to be an effective way to remove water signals from MRSI of the human brain.

Retrospective phase-based gating for cardiac proton spectroscopy with fixed scan time

BACKGROUND

Respiratory motion is a major limiting factor for spectral quality and duration of in vivo proton MR spectroscopy of the heart. Prospective navigator gating is frequently applied to minimize the effects of respiratory motion, but scan durations are subject-dependent and hence difficult to predict.

PURPOSE

To implement cardiac proton MRS with fixed scan time by employing retrospective phase-based gating and to compare the proposed method to conventional navigator-gated MRS.

STUDY TYPE

Prospective.

SUBJECTS

Eighteen healthy volunteers (29.7 ± 7.8 years).

FIELD STRENGTH/SEQUENCE

1.5, navigator-gated (16 averages without, 96 with water suppression [WS]) data acquisition as reference and navigator-free data acquisition with a fixed scan time (48 without WS, 304 with WS), cardiac-triggered point-resolved spectroscopy (PRESS).

ASSESSMENT

Navigator-free data acquisition with retrospective phase-based gating was compared with prospective navigator-gating. Navigator-free acquisition was repeated in 10 subjects to assess reproducibility. Scan time was assessed for prospective and retrospective gating. Retrospective phase-based gating was performed using a threshold based on the standard deviation (SD) of individual water (W) and triglyceride (TG) phases.

STATISTICAL TESTS

T-tests and Bland-Altman analysis.

RESULTS

The duration of the prospective navigator-gated scans ranged from 6:09 minutes to 21:50 minutes (mean 10:05 ± 3:46 min, gating efficiency 40.4 ± 10.5%), while data acquisition for retrospective phase-based gating had a fixed scan time of 11:44 minutes. Retrospective phase-based gating using a threshold of 1 × SD yielded a gating efficiency of 72.7 ± 4.3% and a coefficient of variation (CoV) of triglyceride-to-water ratios of 9.8% compared with the navigated reference. The intrasubject reproducibility of retrospective gating revealed a CoV of 9.5%.

DATA CONCLUSION

Cardiac proton MRS employing retrospective phase-based gating is feasible and provides reproducible assessment of TG/W in a fixed scan time. Since scan time is independent of respiratory motion, retrospective phase-based gating offers an approach to motion compensation with predictable exam time for proton MRS of the heart.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1973-1981.

In situ postmortem ethanol quantification in the cerebrospinal fluid by non-water-suppressed proton MRS

Determination of the ethanol concentration in corpses with MRS would allow a reproducible forensic assessment by which evidence is collected in a noninvasive manner. However, although MRS has been successfully used to detect ethanol in vivo, it has not been applied to postmortem ethanol quantification in situ. The present study examined the feasibility of the noninvasive measurement of the ethanol concentration in human corpses with MRS. A total of 15 corpses with suspected alcohol consumption before demise underwent examination in a 3 T whole body scanner. To address the partial overlap of the ethanol and lactate signal in the postmortem spectrum, non-water-suppressed single voxel spectra were recorded in the cerebrospinal fluid (CSF) of the left lateral ventricle via the metabolite cycling technique. The ethanol signals were quantified using the internal water as reference standard, as well as based on a reference signal acquired in a phantom. The measured values were compared with biochemically determined concentrations in the blood (BAC) and CSF (CSFAC). In 8 of the 15 corpses a BAC above zero was determined (range 0.03-1.68 g/kg). In all of these 8 corpses, ethanol was measured in CSF with the proposed MRS protocol. The two applied MRS calibration strategies resulted in similar concentrations. However, the MRS measurements generally overestimated the ethanol concentration by 0.09 g/kg (4%) to 0.72 g/kg (45%) as compared with the CSFAC value. The presented MRS protocol allows the measurement of ethanol in the CSF in human corpses and provides an estimation of the ethanol concentration prior to autopsy. Observed deviations from biochemically determined concentrations are mainly explained by the approximate correction of the relaxation attenuation of the ethanol signal.

MR Spectroscopy of the Cervical Spinal Cord in Chronic Spinal Cord Injury

Purpose To investigate metabolic changes in chronic spinal cord injury (SCI) by applying MR spectroscopy in the cervical spinal cord. Materials and Methods Single-voxel short-echo spectroscopic data in study participants with chronic SCI and healthy control subjects were prospectively acquired in the cervical spinal cord at C2 above the level of injury between March 2016 and January 2017 and were compared between groups. Concentrations of total N-acetylaspartate (tNAA), myo-inositol (mI), total choline-containing compounds (tCho), creatine, and glutamine and glutamate complex were estimated from the acquired spectra. Participants were assessed with a comprehensive clinical evaluation investigating sensory and motor deficits. Correlation analysis was applied to investigate relationships between observed metabolic differences, lesion severity, and clinical outcome. Results There were 18 male study participants with chronic SCI (median age, 51 years; range, 30-68 years) and 11 male healthy control subjects (median age, 45 years; range, 30-67 years). At cervical level C2, tNAA/mI and tCho/mI ratios were lower in participants with SCI (tNAA/mI: -26%, P = .003; tCho/mI: -18%; P = .04) than in healthy control subjects. The magnitude of difference was greater with the severity of cord atrophy (tNAA/mI: R² = 0.44, P = .003; tCho/mI: R² = 0.166, P = .09). Smaller tissue bridges at the lesion site correlated with lower ratios of tNAA/mI (R² = 0.69, P = .006) and tCho/mI (R² = 0.51, P = .03) at the C2 level. Lower tNAA/mI and tCho/mI ratios were associated with worse sensory and motor outcomes (P < .05). Conclusion Supralesional metabolic alterations are observed in chronic spinal cord injury, likely reflecting neurodegeneration, demyelination, and astrocytic gliosis in the injured cervical cord. Lesion severity and greater clinical impairment are both linked to the biochemical changes in the atrophied cervical cord after spinal cord injury. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Lin in this issue.

Robust retrospective frequency and phase correction for single-voxel MR spectroscopy

PURPOSE

Subject motion and static field (B₀ ) drift are known to reduce the quality of single voxel MR spectroscopy data due to incoherent averaging. Retrospective correction has previously been shown to improve data quality by adjusting the phase and frequency offset of each average to match a reference spectrum. In this work, a new method (RATS) is developed to be tolerant to large frequency shifts (>7 Hz) and baseline instability resulting from inconsistent water suppression.

METHODS

In contrast to previous approaches, the variable-projection method and baseline fitting is incorporated into the correction procedure to improve robustness to fluctuating baseline signals and optimization instability. RATS is compared to an alternative method, based on time-domain spectral registration (TDSR), using simulated data to model frequency, phase, and baseline instability. In addition, a J-difference edited glutathione in-vivo dataset is processed using both approaches and compared.

RESULTS

RATS offers improved accuracy and stability for large frequency shifts and unstable baselines. Reduced subtraction artifacts are demonstrated for glutathione edited MRS when using RATS, compared with uncorrected or TDSR corrected spectra.

CONCLUSIONS

The RATS algorithm has been shown to provide accurate retrospective correction of SVS MRS data in the presence of large frequency shifts and baseline instability. The method is rapid, generic and therefore readily incorporated into MRS processing pipelines to improve lineshape, SNR, and aid quality assessment.

Metabolite-cycled density-weighted concentric rings k-space trajectory (DW-CRT) enables high-resolution 1 H magnetic resonance spectroscopic imaging at 3-Tesla

Magnetic resonance spectroscopic imaging (MRSI) is a promising technique in both experimental and clinical settings. However, to date, MRSI has been hampered by prohibitively long acquisition times and artifacts caused by subject motion and hardware-related frequency drift. In the present study, we demonstrate that density weighted concentric ring trajectory (DW-CRT) k-space sampling in combination with semi-LASER excitation and metabolite-cycling enables high-resolution MRSI data to be rapidly acquired at 3 Tesla. Single-slice full-intensity MRSI data (short echo time (TE) semi-LASER TE = 32 ms) were acquired from 6 healthy volunteers with an in-plane resolution of 5 × 5 mm in 13 min 30 sec using this approach. Using LCModel analysis, we found that the acquired spectra allowed for the mapping of total N-acetylaspartate (median Cramer-Rao Lower Bound [CRLB] = 3%), glutamate+glutamine (8%), and glutathione (13%). In addition, we demonstrate potential clinical utility of this technique by optimizing the TE to detect 2-hydroxyglutarate (long TE semi-LASER, TE = 110 ms), to produce relevant high-resolution metabolite maps of grade III IDH-mutant oligodendroglioma in a single patient. This study demonstrates the potential utility of MRSI in the clinical setting at 3 Tesla.

Quantitative magnetic resonance spectroscopy at 3T based on the principle of reciprocity

Quantification of magnetic resonance spectroscopy signals using the phantom replacement method requires an adequate correction of differences between the acquisition of the reference signal in the phantom and the measurement in vivo. Applying the principle of reciprocity, sensitivity differences can be corrected at low field strength by measuring the RF transmitter gain needed to obtain a certain flip angle in the measured volume. However, at higher field strength the transmit sensitivity may vary from the reception sensitivity, which leads to wrongly estimated concentrations. To address this issue, a quantification approach based on the principle of reciprocity for use at 3T is proposed and validated thoroughly. In this approach, the RF transmitter gain is determined automatically using a volume-selective power optimization and complemented with information from relative reception sensitivity maps derived from contrast-minimized images to correct differences in transmission and reception sensitivity. In this way, a reliable measure of the local sensitivity was obtained. The proposed method is used to derive in vivo concentrations of brain metabolites and tissue water in two studies with different coil sets in a total of 40 healthy volunteers. Resulting molar concentrations are compared with results using internal water referencing (IWR) and Electric REference To access In vivo Concentrations (ERETIC). With the proposed method, changes in coil loading and regional sensitivity due to B₁ inhomogeneities are successfully corrected, as demonstrated in phantom and in vivo measurements. For the tissue water content, coefficients of variation between 2% and 3.5% were obtained (0.6-1.4% in a single subject). The coefficients of variation of the three major metabolites ranged from 3.4-14.5%. In general, the derived concentrations agree well with values estimated with IWR. Hence, the presented method is a valuable alternative for IWR, without the need for additional hardware such as ERETIC and with potential advantages in diseased tissue.

Spinal cord MRI at 7T

Magnetic resonance imaging (MRI) of the human spinal cord at 7T has been demonstrated by a handful of research sites worldwide, and the spinal cord remains one of the areas in which higher fields and resolution could have high impact. The small diameter of the cord (∼1 cm) necessitates high spatial resolution to minimize partial volume effects between gray and white matter, and so MRI of the cord can greatly benefit from increased signal-to-noise ratio and contrasts at ultra-high field (UHF). Herein we review the current state of UHF spinal cord imaging. Technical challenges to successful UHF spinal cord MRI include radiofrequency (B1) nonuniformities and a general lack of optimized radiofrequency coils, amplified physiological noise, and an absence of methods for robust B0 shimming along the cord to mitigate image distortions and signal losses. Numerous solutions to address these challenges have been and are continuing to be explored, and include novel approaches for signal excitation and acquisition, dynamic shimming and specialized shim coils, and acquisitions with increased coverage or optimal slice angulations.

Non-Water-Suppressed 1H MR Spectroscopy with Orientational Prior Knowledge Shows Potential for Separating Intra- and Extramyocellular Lipid Signals in Human Myocardium

Conditions such as type II diabetes are linked with elevated lipid levels in the heart, and significantly increased risk of heart failure; however, metabolic processes underlying the development of cardiac disease in type II diabetes are not fully understood. Here we present a non-invasive method for in vivo investigation of cardiac lipid metabolism: namely, IVS-McPRESS. This technique uses metabolite-cycled, non-water suppressed 1H cardiac magnetic resonance spectroscopy with prospective and retrospective motion correction. High-quality IVS-McPRESS data acquired from healthy volunteers allowed us to investigate the frequency shift of extramyocellular lipid signals, which depends on the myocardial fibre orientation. Assuming consistent voxel positioning relative to myofibres, the myofibre angle with the magnetic field was derived from the voxel orientation. For separation and individual analysis of intra- and extramyocellular lipid signals, the angle myocardial fibres in the spectroscopy voxel take with the magnetic field should be within ±24.5°. Metabolite and lipid concentrations were analysed with respect to BMI. Significant correlations between BMI and unsaturated fatty acids in intramyocellular lipids, and methylene groups in extramyocellular lipids were found. The proposed IVS-McPRESS technique enables non-invasive investigation of cardiac lipid metabolism and may thus be a useful tool to study healthy and pathological conditions.

Fully navigated 3 T proton magnetic resonance spectroscopy of liver metastases with inner-volume saturation

PURPOSE

To demonstrate that fully navigated magnetic resonance spectroscopy (MRS) with inner-volume saturation (IVS) at 3 T results in high-quality spectra that permit evaluating metabolic changes in hepatic metastases without the need for patient compliance.

METHODS

Nine patients with untreated, biopsy-proven large hepatic metastases (minimum diameter of 3 cm) were included. In each patient, localized proton MRS was performed in the metastatic lesion and in uninvolved liver parenchyma. To improve quality and consistency of proton MRS, navigator gating was thereby performed not only during acquisition of the spectroscopic data but also during localization imaging and throughout the preparation phases. IVS was utilized to reduce chemical shift displacement between different metabolites and to diminish flow artifacts. Metabolite quantities were normalized relative to the unsuppressed water peak and choline-containing compounds (CCC) to lipid ratios were determined. Wilcoxon signed-rank tests were used to assess differences in the amounts of lipids and CCC as well as the CCC-to-lipid ratios between liver metastases and normal-appearing liver parenchyma.

RESULTS

Fully navigated point-resolved spectroscopy with IVS resulted in high-quality spectra in all patients. Navigator gating during localization imaging and spectroscopic acquisition thereby ensured a precise localization of the spectroscopic voxel. Decreased quantities of lipid and CCC were observed in metastatic tissue compared with uninvolved liver parenchyma. However, the latter trend fell short of statistical significance. Moreover, elevated levels of the CCC-to-lipid ratios were detected in metastatic tissue relative to normal-appearing liver parenchyma.

CONCLUSIONS

The present study demonstrates that fully navigated MRS of the liver with IVS at 3 T allows for a precise localization of the spectroscopic voxel and results in high-quality spectra that permit evaluating liver metabolism without the need for patient compliance.

Accurate determination of brain metabolite concentrations using ERETIC as external reference

Magnetic Resonance Spectroscopy (MRS) can provide in vivo metabolite concentrations in standard concentration units if a reliable reference signal is available. For ¹ H MRS in the human brain, typically the signal from the tissue water is used as the (internal) reference signal. However, a concentration determination based on the tissue water signal most often requires a reliable estimate of the water concentration present in the investigated tissue. Especially in clinically interesting cases, this estimation might be difficult. To avoid assumptions about the water in the investigated tissue, the Electric REference To access In vivo Concentrations (ERETIC) method has been proposed. In this approach, the metabolite signal is compared with a reference signal acquired in a phantom and potential coil-loading differences are corrected using a synthetic reference signal. The aim of this study, conducted with a transceiver quadrature head coil, was to increase the accuracy of the ERETIC method by correcting the influence of spatial B₁ inhomogeneities and to simplify the quantification with ERETIC by incorporating an automatic phase correction for the ERETIC signal. Transmit field ( B1+) differences are minimized with a volume-selective power optimization, whereas reception sensitivity changes are corrected using contrast-minimized images of the brain and by adapting the voxel location in the phantom measurement closely to the position measured in vivo. By applying the proposed B₁ correction scheme, the mean metabolite concentrations determined with ERETIC in 21 healthy subjects at three different positions agree with concentrations derived with the tissue water signal as reference. In addition, brain water concentrations determined with ERETIC were in agreement with estimations derived using tissue segmentation and literature values for relative water densities. Based on the results, the ERETIC method presented here is a valid tool to derive in vivo metabolite concentration, with potential advantages compared with internal water referencing in diseased tissue.

Prior-knowledge Fitting of Accelerated Five-dimensional Echo Planar J-resolved Spectroscopic Imaging: Effect of Nonlinear Reconstruction on Quantitation

¹H Magnetic Resonance Spectroscopic imaging (SI) is a powerful tool capable of investigating metabolism in vivo from mul- tiple regions. However, SI techniques are time consuming, and are therefore difficult to implement clinically. By applying non-uniform sampling (NUS) and compressed sensing (CS) reconstruction, it is possible to accelerate these scans while re- taining key spectral information. One recently developed method that utilizes this type of acceleration is the five-dimensional echo planar J-resolved spectroscopic imaging (5D EP-JRESI) sequence, which is capable of obtaining two-dimensional (2D) spectra from three spatial dimensions. The prior-knowledge fitting (ProFit) algorithm is typically used to quantify 2D spectra in vivo, however the effects of NUS and CS reconstruction on the quantitation results are unknown. This study utilized a simulated brain phantom to investigate the errors introduced through the acceleration methods. Errors (normalized root mean square error >15%) were found between metabolite concentrations after twelve-fold acceleration for several low concentra- tion (<2 mM) metabolites. The Cramér Rao lower bound% (CRLB%) values, which are typically used for quality control, were not reflective of the increased quantitation error arising from acceleration. Finally, occipital white (OWM) and gray (OGM) human brain matter were quantified in vivo using the 5D EP-JRESI sequence with eight-fold acceleration.

Proton and multinuclear magnetic resonance spectroscopy in the human brain at ultra-high field strength: A review

Magnetic Resonance Spectroscopy (MRS) allows for a non-invasive and non-ionizing determination of in vivo tissue concentrations and metabolic turn-over rates of more than 20 metabolites and compounds in the central nervous system of humans. The aim of this review is to give a comprehensive overview about the advantages, challenges and advances of ultra-high field MRS with regard to methodological development, discoveries and applications from its beginnings around 15 years ago up to the current state. The review is limited to human brain and spinal cord application at field strength of 7T and 9.4T and includes all relevant nuclei (¹H, ³¹P, ¹³C).

Non-water-suppressed short-echo-time magnetic resonance spectroscopic imaging using a concentric ring k-space trajectory

Water-suppressed MRS acquisition techniques have been the standard MRS approach used in research and for clinical scanning to date. The acquisition of a non-water-suppressed MRS spectrum is used for artefact correction, reconstruction of phased-array coil data and metabolite quantification. Here, a two-scan metabolite-cycling magnetic resonance spectroscopic imaging (MRSI) scheme that does not use water suppression is demonstrated and evaluated. Specifically, the feasibility of acquiring and quantifying short-echo (TE  = 14 ms), two-dimensional stimulated echo acquisition mode (STEAM) MRSI spectra in the motor cortex is demonstrated on a 3 T MRI system. The increase in measurement time from the metabolite-cycling is counterbalanced by a time-efficient concentric ring k-space trajectory. To validate the technique, water-suppressed MRSI acquisitions were also performed for comparison. The proposed non-water-suppressed metabolite-cycling MRSI technique was tested for detection and correction of resonance frequency drifts due to subject motion and/or hardware instability, and the feasibility of high-resolution metabolic mapping over a whole brain slice was assessed. Our results show that the metabolite spectra and estimated concentrations are in agreement between non-water-suppressed and water-suppressed techniques. The achieved spectral quality, signal-to-noise ratio (SNR) > 20 and linewidth <7 Hz allowed reliable metabolic mapping of five major brain metabolites in the motor cortex with an in-plane resolution of 10 × 10 mm2 in 8 min and with a Cramér-Rao lower bound of less than 20% using LCModel analysis. In addition, the high SNR of the water peak of the non-water-suppressed technique enabled voxel-wise single-scan frequency, phase and eddy current correction. These findings demonstrate that our non-water-suppressed metabolite-cycling MRSI technique can perform robustly on 3 T MRI systems and within a clinically feasible acquisition time.

Proton and phosphorus magnetic resonance spectroscopy of the healthy human breast at 7 T

In vivo water- and fat-suppressed ¹ H magnetic resonance spectroscopy (MRS) and ³¹ P magnetic resonance adiabatic multi-echo spectroscopic imaging were performed at 7 T in duplicate in healthy fibroglandular breast tissue of a group of eight volunteers. The transverse relaxation times of ³¹ P metabolites were determined, and the reproducibility of ¹ H and ³¹ P MRS was investigated. The transverse relaxation times for phosphoethanolamine (PE) and phosphocholine (PC) were fitted bi-exponentially, with an added short T₂ component of 20 ms for adenosine monophosphate, resulting in values of 199 ± 8 and 239 ± 14 ms, respectively. The transverse relaxation time for glycerophosphocholine (GPC) was also fitted bi-exponentially, with an added short T₂ component of 20 ms for glycerophosphatidylethanolamine, which resonates at a similar frequency, resulting in a value of 177 ± 6 ms. Transverse relaxation times for inorganic phosphate, γ-ATP and glycerophosphatidylcholine mobile phospholipid were fitted mono-exponentially, resulting in values of 180 ± 4, 19 ± 3 and 20 ± 4 ms, respectively. Coefficients of variation for the duplicate determinations of ¹ H total choline (tChol) and the ³¹ P metabolites were calculated for the group of volunteers. The reproducibility of inorganic phosphate, the sum of phosphomonoesters and the sum of phosphodiesters with ³¹ P MRS imaging was superior to the reproducibility of ¹ H MRS for tChol. ¹ H and ³¹ P data were combined to calculate estimates of the absolute concentrations of PC, GPC and PE in healthy fibroglandular tissue, resulting in upper limits of 0.1, 0.1 and 0.2 mmol/kg of tissue, respectively.

Neurochemical profile of the human cervical spinal cord determined by MRS

MRS enables insight into the chemical composition of central nervous system tissue. However, technical challenges degrade the data quality when applied to the human spinal cord. Therefore, to date detection of only the most prominent metabolite resonances has been reported in the healthy human spinal cord. The aim of this investigation is to provide an extended metabolic profile including neurotransmitters and antioxidants in addition to metabolites involved in the energy and membrane metabolism of the human cervical spinal cord in vivo. To achieve this, data quality was improved by using a custom-made, cervical detector array together with constructive averaging of a high number of echo signals, which is enabled by the metabolite cycling technique at 3T. In addition, the improved spinal cord spectra were extensively cross-validated, in vivo, post-mortem in situ and ex vivo. Reliable identification of up to nine metabolites was achieved in group analyses for the first time. Distinct features of the spinal cord neurochemical profile, in comparison with the brain neurotransmission system, include decreased concentrations of the sum of glutamate and glutamate and increased concentrations of aspartate, γ-amino-butyric acid, scyllo-inositol and the sum of myo-inositol and glycine.

(1) H MRS in the human spinal cord at 7 T using a dielectric waveguide transmitter, RF shimming and a high density receive array

Multimodal MRI is the state of the art method for clinical diagnostics and therapy monitoring of the spinal cord, with MRS being an emerging modality that has the potential to detect relevant changes of the spinal cord tissue at an earlier stage and to enhance specificity. Methodological challenges related to the small dimensions and deep location of the human spinal cord inside the human body, field fluctuations due to respiratory motion, susceptibility differences to adjacent tissue such as vertebras and pulsatile flow of the cerebrospinal fluid hinder the clinical application of (1) H MRS to the human spinal cord. Complementary to previous studies that partly addressed these problems, this work aims at enhancing the signal-to-noise ratio (SNR) of (1) H MRS in the human spinal cord. To this end a flexible tight fit high density receiver array and ultra-high field strength (7 T) were combined. A dielectric waveguide and dipole antenna transmission coil allowed for dual channel RF shimming, focusing the RF field in the spinal cord, and an inner-volume saturated semi-LASER sequence was used for robust localization in the presence of B1 (+) inhomogeneity. Herein we report the first 7 T spinal cord (1) H MR spectra, which were obtained in seven independent measurements of 128 averages each in three healthy volunteers. The spectra exhibit high quality (full width at half maximum 0.09 ppm, SNR 7.6) and absence of artifacts and allow for reliable quantification of N-acetyl aspartate (NAA) (NAA/Cr (creatine) 1.31 ± 0.20; Cramér-Rao lower bound (CRLB) 5), total choline containing compounds (Cho) (Cho/Cr 0.32 ± 0.07; CRLB 7), Cr (CRLB 5) and myo-inositol (mI) (mI/Cr 1.08 ± 0.22; CRLB 6) in 7.5 min in the human cervical spinal cord. Thus metabolic information from the spinal cord can be obtained in clinically feasible scan times at 7 T, and its benefit for clinical decision making in spinal cord disorders will be investigated in the future using the presented methodology. Copyright © 2016 John Wiley & Sons, Ltd.

Reproducibility of Neurochemical Profile Quantification in Pregenual Cingulate, Anterior Midcingulate, and Bilateral Posterior Insular Subdivisions Measured at 3 Tesla

The current report assessed measurement reproducibility of proton magnetic resonance spectroscopy at 3 Tesla in the left and right posterior insular, pregenual anterior cingulate, and anterior midcingulate cortices. Ten healthy male volunteers aged 21–30 years were tested at four different days, of which nine were included in the data analysis. Intra- and inter-subject variability of myo-inositol, creatine, glutamate, total-choline, total-N-acetylaspartate, and combined glutamine–glutamate were calculated considering the influence of movement parameters, age, daytime of measurements, and tissue composition. Overall mean intra-/inter-subject variability for all neurochemicals combined revealed small mean coefficients of variation across the four regions: 5.3/9.05% in anterior midcingulate, 6.6/8.84% in pregenual anterior cingulate, 7.3/10.00% in left posterior and 8.2/10.55% in right posterior insula. Head movement, tissue composition and day time revealed no significant explanatory variance contribution suggesting a negligible influence on the data. A strong correlation between Cramer–Rao Lower Bounds (a measure of fitting errors) and the mean intra-subject coefficients of variation (r = 0.799, p < 0.001) outlined the importance of low fitting errors in order to obtain robust and finally meaningful measurements. The present findings confirm proton magnetic resonance spectroscopy as a reliable tool to measure brain neurochemistry in small subregions of the human brain.

Motion correction and frequency stabilization for MRS of the human spinal cord

Subject motion is challenging for MRS, because it can falsify results. For spinal cord MRS in particular, subject movement is critical, since even a small movement > 1 mm) can lead to a voxel shift out of the desired measurement region. Therefore, the identification of motion corrupted MRS scans is essential. In this investigation, MR navigators acquired simultaneously with the MRS data are used to identify a displacement of the spinal cord due to subject motion. It is shown that navigators are able to recognize substantial subject motion (>1 mm) without impairing the MRS measurement. In addition, navigators are easy to apply to the measurement, because no additional hardware and just a minor additional user effort are needed. Moreover, no additional scan time is required, because navigators can be applied in the deadtime of the MRS sequence. Furthermore, in this work, retrospective motion correction combined with frequency stabilization is presented by combining navigators with non-water-suppressed (1)H-MRS, resulting in an improved spectral quality of the spinal cord measurements.

Mechanisms of SNR and line shape improvement by B0 correction in overdiscrete MRSI reconstruction

PURPOSE

Inhomogeneities of the main magnetic field cause line broadening and location-dependent frequency shifts in brain MRSI. These are often visible despite advanced B₀ shimming. The purpose of this work is to propose an advanced B₀ correction method that can easily be applied during postprocessing.

METHODS

A target-driven overdiscrete reconstruction method previously introduced for MRSI is modified by dividing it into two steps. In a first step, an intermediate spectroscopic image with arbitrarily high resolution is generated, on which B₀ correction is performed as an additional processing step based on an additionally acquired B₀ map. This frequency-aligns metabolite peaks and destroys noise correlations between neighboring subvoxels. Second, the voxel is shaped by application of the spatial response target. The method was tested with simulated spectroscopic imaging data as well as in a series of MRSI data sets obtained from four healthy volunteers at 7T.

RESULTS

A systematic gain in spectral signal-to-noise ratio is achieved, due to spatial averaging now occurring over peak aligned and noise decorrelated subvoxel spectra. At the same time, metabolite peak line widths are reduced.

CONCLUSION

In the presence of B₀ inhomogeneities across the field of view, the proposed method offers the potential to improve spectral quality with only a minimal additional effort during acquisition. Magn Reson Med 77:44-56, 2017. © 2015 Wiley Periodicals, Inc.