The separation of Gln and Glu in STEAM: a comparison study using short and long TEs/TMs at 3 and 7 T

Multinuclear Magnetic Resonance Spectroscopy at Ultra-High-Field: Assessing Human Cerebral Metabolism in Healthy and Diseased States

The brain is a highly energetic organ. Although the brain can consume metabolic substrates, such as lactate, glycogen, and ketone bodies, the energy metabolism in a healthy adult brain mainly relies on glucose provided via blood. The cerebral metabolism of glucose produces energy and a wide variety of intermediate metabolites. Since cerebral metabolic alterations have been repeatedly implicated in several brain disorders, understanding changes in metabolite levels and corresponding cell-specific neurotransmitter fluxes through different substrate utilization may highlight the underlying mechanisms that can be exploited to diagnose or treat various brain disorders. Magnetic resonance spectroscopy (MRS) is a noninvasive tool to measure tissue metabolism in vivo. ¹H-MRS is widely applied in research at clinical field strengths (≤3T) to measure mostly high abundant metabolites. In addition, X-nuclei MRS including, ¹³C, ²H, ¹⁷O, and ³¹P, are also very promising. Exploiting the higher sensitivity at ultra-high-field (>4T; UHF) strengths enables obtaining unique insights into different aspects of the substrate metabolism towards measuring cell-specific metabolic fluxes in vivo. This review provides an overview about the potential role of multinuclear MRS (¹H, ¹³C, ²H, ¹⁷O, and ³¹P) at UHF to assess the cerebral metabolism and the metabolic insights obtained by applying these techniques in both healthy and diseased states.

Cross-sectional Exploration of the Relationship Between Glutamate Abnormalities and Tic Disorder Severity Using Proton Magnetic Resonance Spectroscopy

Glutamate (Glu) has been reported to be closely related to the pathophysiology of Tic Disorders (TD). By using proton magnetic resonance spectroscopy (1H-MRS), we aimed to investigate the relationship between in vivo Glu levels and the severity of TD. We performed a cross-sectional study in medication-free patients with TD and healthy controls aged between 5 and 13 years using 1H-MRS at 3 T. First, we measured the Glu levels in both patients and controls and observed the difference in subgroups, including mild TD patients and moderate TD patients. We then examined the correlations between the Glu levels and clinical features of the patients. Finally, we assessed the diagnostic value of 1H-MRS and the influencing factors. Our results show that the Glu levels in the striatum of all patients with TD were not significantly different from those of the healthy controls. Subgroup analysis revealed that the Glu levels in the moderate TD group were higher than those in the mild TD group and healthy controls. The correlation analysis showed that Glu levels are strongly positive correlated with TD severity. The optimal cutoff value of Glu levels to differentiate mild tics from moderate tics was 1.244, with a sensitivity of 88.2% and a specificity of 94.7%. Multiple linear regression models revealed that the severity of TD is one of the important factors that affect Glu levels. We conclude that Glu levels are mainly associated with the severity of tics, thus it could serve as a key biomarker for TD classification.

Quantifying the excitatory-inhibitory balance: A comparison of SemiLASER and MEGA-SemiLASER for simultaneously measuring GABA and glutamate at 7T

The importance of the excitatory-inhibitory (E/I) balance in a wide range of cognitive and behavioral processes has prompted a commensurate interest in methods for reliably quantifying it. Proton Magnetic Resonance Spectroscopy (¹H-MRS) remains the only method capable of safely and non-invasively measuring the concentrations of the brain's major excitatory (glutamate) and inhibitory (γ-aminobutyric-acid, GABA) neurotransmitters in-vivo. MRS relies on spectral Mescher-Garwood (MEGA) editing techniques at 3T to distinguish GABA from its overlapping resonances. However, with the increased spectral resolution at ultrahigh field strengths of 7T and above, non-edited spectroscopic techniques become potential viable alternatives to MEGA based approaches, and also address some of their shortcomings, such as signal loss, sensitivity to transmitter inhomogeneities and temporal resolution. We present a comprehensive comparison of both edited and non-edited strategies at 7T for simultaneously quantifying glutamate and GABA from the dorsal anterior cingulate cortex (dACC), and evaluate their reproducibility and relative bias. The combined root-mean-square test-retest reproducibility of Glu and GABA (CV_E/I) was as low as 13.3% for unedited MRS at TE=80 ms using SemiLASER localization, while edited MRS at TE=80 ms yielded CV_E/I=20% and 21% for asymmetric and symmetric MEGA editing, respectively. An unedited SemiLASER acquisition using a shorter echo time of TE=42 ms yielded CV_E/I as low as 24.9%. Our results show that non-edited sequences at an echo time of 80 ms provide better reproducibility than either edited sequences at the same TE, or non-edited sequences at a shorter TE of 42 ms. This is supported by numerical simulations and is driven in part by a pseudo-singlet appearance of the GABA multiplets at TE=80 ms, and the excellent spectral resolution at 7T. Our results uphold a transition to non-edited MRS for monitoring the E/I balance at ultrahigh fields, and stress the importance of using a properly-optimized echo time.

Investigating the regional effect of the chemical shift displacement artefact on the J-modulated lactate signal at ultra high-field

The present work aims to show the applicability of an analytical model for the optimisation of the STEAM sequence timing parameters for lactate detection at ultra high-field. The effects of the chemical shift displacement artefact on the J-modulated signal for a weakly-coupled spin system were considered in the three applied directions of field gradients and the product operator formalism was used to obtain expressions for the signal modulation in each compartment of the excited volume. The validity of this model was demonstrated experimentally at 7 T in a phantom and acquisitions with optimised parameters were performed on a healthy volunteer. The spectra acquired with TE = 144 ms with the optimised mixing time and TE = 288 ms showed easily detectable lactate peaks in the normal human brain. Additionally, the acquisition with the longer TE resulted in a spectrum with less lipid/macromolecular contamination. The simulations shown here demonstrated that the proposed analytical model is suitable for correctly predicting the resulting lactate signal. With the optimised parameters, it was possible to use a simple sequence with sufficient signal-to-noise ratio to reliably distinguish lactate from overlapping resonances in a healthy brain.

Rostral Anterior Cingulate Glutamine/Glutamate Disbalance in Major Depressive Disorder Depends on Symptom Severity

BACKGROUND

Patients with major depressive disorder (MDD) show glutamatergic deficits in the ventral anterior cingulate cortex. The glutamine/glutamate (Gln/Glu) ratio was proposed to be connected to glutamatergic cycling, which is hypothesized to be dysregulated in MDD. As an indicator of regional metabolite status, this ratio might be a robust state marker sensitive to clinical heterogeneity.

METHODS

Thirty-two MDD patients (mean age 40.88 ± 13.66 years, 19 women) and control subjects (mean age 33.09 ± 8.24 years, 19 women) were compared for pregenual anterior cingulate cortex levels of Gln/Glu, Gln/total creatine (tCr), Glu/tCr, and gamma-aminobutyric acid/tCr as determined by high-field magnetic resonance spectroscopy. We tested if symptom severity (Hamilton Depression Rating Scale) and anhedonia (Snaith-Hamilton Pleasure Scale) influence the relation of metabolites to clinical symptoms.

RESULTS

MDD patients showed higher Gln/Glu. This was driven by marginally higher Gln/tCr and nonsignificantly lower Glu/tCr. Groups defined by severity moderated relationship between Gln/Glu and the Hamilton Depression Rating Scale. Moreover, severe cases differed from both control subjects and moderate cases. Groups defined by the Snaith-Hamilton Pleasure Scale also displayed differential relationship between Gln/Glu and levels of anhedonia, predominantly driven by Gln/tCr.

CONCLUSIONS

We elaborate previous accounts of metabolite deficits in the anterior cingulate cortex toward increased Gln/Glu. There is a moderated relationship between severity and the ratio, which suggests consideration of different mechanisms or disease state for the respective subgroups in future studies.

Investigating the metabolic alterations in a depressive-like rat model of chronic forced swim stress: An in vivo proton magnetic resonance spectroscopy study at 7T

Although recent investigations of major depressive disorder (MDD) have focused on the monoaminergic system, accumulating evidences suggest that alternative pathophysiological models of MDD and treatment options for patients with MDD are needed. Animals subjected to chronic forced swim stress (CFSS) develop behavioral despair. The purpose of this study was to investigate the in vivo effects of CFSS on systems other than the monoamine system in the rat prefrontal cortex (PFC) with 7T and short-echo-time (16.3 ms) proton magnetic resonance spectroscopy (¹H MRS). Ten male Wistar rats underwent 14 days of CFSS, and in vivo¹H MRS and forced swim tests were performed before and after CFSS. Point-resolved spectroscopy was used to quantify metabolite levels in the rat PFC. To investigate spectral overlap in glutamate and glutamine, spectral analyses in the spectra obtained in the in vivo¹H MRS, parametrically matched spectral simulation, and in vitro experiments were performed. The results of the spectral analyses showed that the glutamate/glutamine spectral overlap was not critical, which suggested that in vivo¹H MRS can be used to reliably assess the glutamate system. The rats showed significantly increased immobility times and decreased climbing times in the FST after CFSS, which suggested that the rats developed behavioral despair. The pre-CFSS and post-CFSS glutamate and glutamine levels did not significantly differ (p > 0.050). The levels of myo-inositol, total choline, and N-acetylaspartate, myo-inositol/creatine, and total choline/creatine increased significantly (p < 0.050). Similar findings have been reported in patients with MDD. Taken together, these results suggest that the CFSS-induced metabolic alterations were similar to those found in patients and that high-field and short-echo-time in vivo¹H MRS can be used to investigate depression-induced metabolic alterations. Such investigations might provide alternative insights into the nonmonoaminergic pathophysiology and treatment of depression.

Metabolic effects of light deprivation in the prefrontal cortex of rats with depression-like behavior: In vivo proton magnetic resonance spectroscopy at 7T

Recent evidence suggests that the glutamate system plays an important role in the pathogenesis of major depressive disorder (MDD). The aim of this study was to investigate the effects of light deprivation (LD) in the prefrontal cortex (PFC) of animals with depression-like behavior, targeting the glutamate system, using in vivo proton magnetic resonance spectroscopy (¹H MRS). Male Sprague-Dawley rats were housed in constant darkness for six weeks (n = 12; LD group), while controls (n = 8) were housed under normal light cycles. The animals were assessed with forced swim tests. Point-resolved spectroscopy was used to quantify metabolite levels in the PFC. To substantiate the validity of the use of in vivo¹H MRS in this study, the spectra obtained in the in vivo¹H MRS, parametrically matched spectral simulation, and in vitro experiments were analyzed. The results of the spectral analyses showed that the quantification of glutamate and glutamine was not significantly affected by spectral overlaps. Thus, these results suggested that in vivo¹H MRS can be used to reliably investigate the glutamate system. The results of the forced swim test showed LD-induced behavioral despairs in the animals. The levels of glutamate, myo-inositol, phosphocreatine, and total creatine were found significantly (p < 0.010) increased in the PFC of the LD animals compared with the controls. These results suggested that the LD-induced metabolic changes were consistent with the previous findings in patients with MDD and that short-echo-time in vivo¹H MRS can be used to effectively measure depression-induced alterations in glutamate systems.

A spectroscopic approach toward depression diagnosis: local metabolism meets functional connectivity

Abnormal anterior insula (AI) response and functional connectivity (FC) is associated with depression. In addition to clinical features, such as severity, AI FC and its metabolism further predicted therapeutic response. Abnormal FC between anterior cingulate and AI covaried with reduced glutamate level within cingulate cortex. Recently, deficient glial glutamate conversion was found in AI in major depression disorder (MDD). We therefore postulate a local glutamatergic mechanism in insula cortex of depressive patients, which is correlated with symptoms severity and itself influences AI's network connectivity in MDD. Twenty-five MDD patients and 25 healthy controls (HC) matched on age and sex underwent resting state functional magnetic resonance imaging and magnetic resonance spectroscopy scans. To determine the role of local glutamate-glutamine complex (Glx) ratio on whole brain AI FC, we conducted regression analysis with Glx relative to creatine (Cr) ratio as factor of interest and age, sex, and voxel tissue composition as nuisance factors. We found that in MDD, but not in HC, AI Glx/Cr ratio correlated positively with AI FC to right supramarginal gyrus and negatively with AI FC toward left occipital cortex (p < 0.05 family wise error). AI Glx/Cr level was negatively correlated with HAMD score (p < 0.05) in MDD patients. We showed that the local AI ratio of glutamatergic-creatine metabolism is an underlying candidate subserving functional network disintegration of insula toward low level and supramodal integration areas, in MDD. While causality cannot directly be inferred from such correlation, our finding helps to define a multilevel network of response-predicting regions based on local metabolism and connectivity strength.

Effects of repeated dizocilpine treatment on glutamatergic activity in the prefrontal cortex in an animal model of schizophrenia: An in vivo proton magnetic resonance spectroscopy study at 9.4T

Repeated exposure to dizocilpine (MK-801) can be used as a model of schizophrenia that incorporates disease progression. Proton magnetic resonance spectroscopy (¹H MRS) has been widely used to investigate schizophrenia-related alterations in glutamate (Glu). The purpose of this study was to investigate metabolic alterations in the prefrontal cortex (PFC) in an animal model of schizophrenia by using in vivo ¹H MRS. Because of the spectral overlap of Glu and glutamine (Gln), high-field ¹H MRS with short echo time (TE) was used. A point-resolved spectroscopy sequence was used to measure the levels of Glu and Gln, and the brain metabolites in a volume of interest (22.5μL) located in the PFC region of rats (n=13) before and after 6days of MK-801 (0.5mg/kg) treatment. Analysis of the spectra showed that the cross-contamination of Glu and Gln can be considered to comparably low. No metabolic parameters were altered (p>0.05). However, differences in Glu and N-acetylaspartate (NAA) levels between two times were significantly correlated (p<0.01). The results showed both decreased (in 6 of the 13 rats) and increased (7 of the 13 rats) levels of Glu and NAA, which suggested that these opposite metabolic alterations reflect two stage of disease progression. The results suggest that high-field and short TE in vivo ¹H MRS can quantify Glu and Gln with reliably low level of cross-contamination and that repeated exposure to MK-801 induces the progressive development of schizophrenia.