Volume correction for edema in single-volume proton MR spectroscopy of contrast-enhancing multiple sclerosis lesions

Decreased Cerebral Creatine and N-Acetyl Aspartate Concentrations after Severe COVID-19 Infection: A Magnetic Resonance Spectroscopy Study

Background/Objectives: The aim of this study was to evaluate brain metabolism using MR spectroscopy (MRS) after recovery from Coronavirus disease (COVID-19) and to test the impact of disease severity on brain metabolites. Methods: We performed MRS on 81 individuals (45 males, 36 females, aged 40-60), who had normal MRI findings and had recovered from COVID-19, classifying them into mild (17), moderate (36), and severe (28) groups based on disease severity during the acute phase. The study employed two-dimensional spectroscopic imaging above the corpus callosum, focusing on choline (Cho), creatine (Cr), and N-acetylaspartate (NAA). We analyzed Cho/Cr and NAA/Cr ratios as well as absolute concentrations using water as an internal reference. Results: Results indicated that the Cho/Cr ratio was higher with increasing disease severity, while absolute Cho and NAA/Cr ratios showed no significant differences across the groups. Notably, absolute Cr and NAA levels were significantly lower in patients with severe disease. Conclusions: These findings suggest that the severity of COVID-19 during the acute phase is associated with significant changes in brain metabolism, marked by an increase in Cho/Cr ratios and a reduction in Cr and NAA levels, reflecting substantial metabolic alterations post-recovery.

Optimized multi-voxel TE-averaged PRESS for glutamate detection in the human brain at 3T

PURPOSE

To optimize possible combinations of echo times (TE) for multi-voxel TE-averaged Point RESolved Spectroscopy (PRESS) while reducing the total number of TEs required to separate glutamate (Glu) and glutamine (Gln) within a clinically feasible scan time.

METHODS

General Approach to Magnetic resonance Mathematical Analysis (GAMMA) was used to implement 2D J-resolved PRESS technique, and the spectra of 14 individual brain metabolites were simulated at 64 different TEs. Monte Carlo simulations were used for selecting the best TE combinations to separate Glu and Gln using TE-averaged PRESS with a total number of two, three, four and five TEs. Single-voxel 1H-MRS data were acquired using 64 different TEs from a healthy volunteer on a clinical 3T MR scanner to validate the echo time combinations selected with simulations. Additionally, 2D 1H-MRSI data of eight healthy volunteers were acquired on a clinical 3T MR scanner using four different TEs that were determined by Monte Carlo simulations. Optimized TE-averaged PRESS spectra were created by averaging the spectra acquired at selected TEs. LCModel was used for spectral quantification. A Wilcoxon signed-rank test was used to detect statistically significant differences in Glu/Gln ratios between 35 ms PRESS and optimized TE-averaged PRESS data.

RESULTS

Glu could be clearly separated from Gln at 2.35 ppm, using optimized TE-averaged PRESS with only four TEs (35, 37, 40, and 42 ms) that were selected through Monte Carlo simulations. Glu/Gln ratios were significantly higher in the optimized TE-averaged PRESS data of healthy volunteers than in the 35 ms PRESS data (P = 0.008).

CONCLUSION

Optimized multi-voxel TE-averaged PRESS enabled faster and unobstructed quantification of Glu at multiple voxels in the human brain in vivo at 3T.

In vivo cortical glutathione response to oral fumarate therapy in relapsing-remitting multiple sclerosis: A single-arm open-label phase IV trial using 7-Tesla 1H MRS

BACKGROUND

This is an open-label, single-arm, single-center pilot study using 7-Tesla in vivo proton magnetic resonance spectroscopy (1H MRS) to measure brain cortical glutathione concentration at baseline before and during the use of oral fumarates as a disease-modifying therapy for multiple sclerosis. The primary endpoint of this research was the change in prefrontal cortex glutathione concentration relative to a therapy-naïve baseline after one year of oral fumarate therapy.

METHODS

Brain glutathione concentrations were examined by 1H MRS in single prefrontal and occipital cortex cubic voxels (2.5 × 2.5 × 2.5 cm3) before and during initiation of oral fumarate therapy (120 mg b.i.d. for 7 days and 240 mg b.i.d. thereafter). Additional measurements of related metabolites glutamate, glutamine, myoinositol, total N-acetyl aspartate, and total choline were also acquired in voxels centered on the same regions. Seven relapsing-remitting multiple sclerosis patients (4 f / 3 m, age range 28-50 years, mean age 40 years) naïve to fumarate therapy were scanned at pre-therapy baseline and after 1, 3, 6 and 12 months of therapy. A group of 8 healthy volunteers (4 f / 4 m, age range 33-48 years, mean age 41 years) was also scanned at baseline and Month 6 to characterize 1H-MRS measurement reproducibility over a comparable time frame.

RESULTS

In the multiple sclerosis cohort, general linear models demonstrated a significant positive linear relationship between prefrontal glutathione and time either linearly across all time points (+0.05 ± 0.02 mM/month, t(27) = 2.6, p = 0.02) or specifically for factor variable Month 12 (+0.6 ± 0.3 mM/12 months, t(24) = 2.2, p = 0.04) relative to baseline. No such effects of time on glutathione concentration were demonstrated in the occipital cortex or in the healthy volunteer group. Changes in occipital total choline were further observed in the multiple sclerosis cohort as well as prefrontal total choline and occipital glutamine and myoinositol in the control cohort throughout the study duration.

CONCLUSIONS

While the open-label single-arm pilot study design and abbreviated control series cannot support firm conclusions about the influence of oral fumarate therapy independent of test-retest factors or normal biological variation in a state of either health or disease, these results do justify further investigation at a larger scale into the potential relationship between prefrontal cortex glutathione increases and oral fumarate therapy in relapsing-remitting multiple sclerosis.

An In-vivo 1H-MRS short-echo time technique at 7T: Quantification of metabolites in chronic multiple sclerosis and neuromyelitis optica brain lesions and normal appearing brain tissue

Magnetic Resonance Spectroscopy (MRS) allows for the non-invasive quantification of neurochemicals and has the potential to differentiate between the pathologically distinct diseases, multiple sclerosis (MS) and AQP4Ab-positive neuromyelitis optica spectrum disorder (AQP4Ab-NMOSD). In this study we characterised the metabolite profiles of brain lesions in 11 MS and 4 AQP4Ab-NMOSD patients using an optimised MRS methodology at ultra-high field strength (7T) incorporating correction for T2 water relaxation differences between lesioned and normal tissue. MS metabolite results were in keeping with the existing literature: total N-acetylaspartate (NAA) was lower in lesions compared to normal appearing brain white matter (NAWM) with reciprocal findings for myo-Inositol. An unexpected subtlety revealed by our technique was that total NAA differences were likely driven by NAA-glutamate (NAAG), a ubiquitous CNS molecule with functions quite distinct from NAA though commonly quantified together with NAA in MRS studies as total NAA. Surprisingly, AQP4Ab-NMOSD showed no significant differences for total NAA, NAA, NAAG or myo-Inositol between lesion and NAWM sites, nor were there any differences between MS and AQP4Ab-NMOSD for a priori hypotheses. Post-hoc testing revealed a significant correlation between NAWM Ins:NAA and disability (as measured by EDSS) for disease groups combined, driven by the AP4Ab-NMOSD group. Utilising an optimised MRS methodology, our study highlights some under-explored subtleties in MRS profiles, such as the absence of myo-Inositol concentration differences in AQP4Ab-NMOSD brain lesions versus NAWM and the potential influence of NAAG differences between lesions and normal appearing white matter in MS.

Quantifying the Metabolic Signature of Multiple Sclerosis by in vivo Proton Magnetic Resonance Spectroscopy: Current Challenges and Future Outlook in the Translation From Proton Signal to Diagnostic Biomarker

Proton magnetic resonance spectroscopy (¹H-MRS) offers a growing variety of methods for querying potential diagnostic biomarkers of multiple sclerosis in living central nervous system tissue. For the past three decades, ¹H-MRS has enabled the acquisition of a rich dataset suggestive of numerous metabolic alterations in lesions, normal-appearing white matter, gray matter, and spinal cord of individuals with multiple sclerosis, but this body of information is not free of seeming internal contradiction. The use of ¹H-MRS signals as diagnostic biomarkers depends on reproducible and generalizable sensitivity and specificity to disease state that can be confounded by a multitude of influences, including experiment group classification and demographics; acquisition sequence; spectral quality and quantifiability; the contribution of macromolecules and lipids to the spectroscopic baseline; spectral quantification pipeline; voxel tissue and lesion composition; T₁ and T₂ relaxation; B₁ field characteristics; and other features of study design, spectral acquisition and processing, and metabolite quantification about which the experimenter may possess imperfect or incomplete information. The direct comparison of ¹H-MRS data from individuals with and without multiple sclerosis poses a special challenge in this regard, as several lines of evidence suggest that experimental cohorts may differ significantly in some of these parameters. We review the existing findings of in vivo¹H-MRS on central nervous system metabolic abnormalities in multiple sclerosis and its subtypes within the context of study design, spectral acquisition and processing, and metabolite quantification and offer an outlook on technical considerations, including the growing use of machine learning, by future investigations into diagnostic biomarkers of multiple sclerosis measurable by ¹H-MRS.

Normalizing data from GABA-edited MEGA-PRESS implementations at 3 Tesla

Standardization of results is an important milestone in the maturation of any truly quantitative methodology. For instance, a lack of measurement agreement across imaging platforms limits multisite studies, between-study comparisons based on the literature, and inferences based on and the generalizability of results. In GABA-edited MEGA-PRESS, two key sources of differences between implementations are: differences in editing efficiency of GABA and the degree of co-editing of macromolecules (MM). In this work, GABA editing efficiency κ and MM-co-editing μ constants are determined for three widely used MEGA-PRESS implementations (on the most common MRI platforms; GE, Philips, and Siemens) by phantom experiments. Implementation-specific κ,μ-corrections were then applied to two in vivo datasets, one consisted of 8 subject scanned on the three platforms and the other one subject scanned eight times on each platform. Manufacturer-specific κ and μ values were determined as: κ_GE=0.436, κ_Siemens=0.366 and κ_Philips=0.394 and μ_GE=0.83, μ_Siemens=0.625 and μ_Philips=0.75. Applying the κ,μ-correction on the Cr-referenced data decreased the coefficient of variation (CV) of the data for both in vivo data sets (multisubjects: uncorrected CV=13%, κ,μ-corrected CV=5%, single subject: uncorrected CV=23%, κ,μ-corrected CV=13%) but had no significant effect on mean GABA levels. For the water-referenced results, CV increased in the multisubject data (uncorrected CV=6.7%, κ,μ-corrected CV=14%) while it decreased in the single subject data (uncorrected CV=24%, κ,μ-corrected CV=21%) and manufacturer was a significant source of variance in the κ,μ-corrected data. Applying a correction for editing efficiency and macromolecule contamination decreases the variance between different manufacturers for creatine-referenced data, but other sources of variance remain.

Metabolic Patterns in Chronic Multiple Sclerosis Lesions and Normal-appearing White Matter: Intraindividual Comparison by Using 2D MR Spectroscopic Imaging

Purpose To perform a direct metabolic comparison of chronic lesions and diffusely injured normal-appearing white matter (NAWM) in multiple sclerosis (MS). Materials and Methods In this institutional review board-approved study, with the written informed consent of all patients, two-dimensional magnetic resonance spectroscopic imaging data in 46 patients with relapsing-remitting MS (median disease duration, 0.8 year) were analyzed by using the spectral quantification tool LCModel. Metabolic patterns were evaluated for non-gadolinium-enhancing chronic lesions and the corresponding contralateral NAWM. The sensitivity of the method was assessed by reproducing the known metabolic differences between cortical gray matter (GM) and NAWM. In addition to individual spectra, averaged spectra were calculated by accumulating free induction decays over all subjects to yield an increased signal-to-noise ratio (SNR), and in turn, to allow improved curve fitting as demonstrated by lower error bounds for low-concentration metabolites. Metabolite concentrations were statistically tested for intraindividual differences (paired t tests) to avoid effects resulting from variations in disease severity or treatment. Results Differences between the metabolite concentrations in the NAWM and the cortical GM were highly significant (P < .001), demonstrating the reliability of the spectral analysis used here. The spectral patterns of the individual and averaged spectra of chronic lesions and NAWM were qualitatively very similar at visual inspection. Furthermore, in the quantitative comparison, the estimated metabolite concentrations showed only slight differences (P > .07). Owing to increased SNRs in the averaged spectra compared with individual spectra (eg, for chronic lesions, 63 vs 28.4 ± 4.1), it was possible to reliably (Cramér-Rao lower bound [CRLB], <20%) estimate scyllo-inositol levels with a CRLB of 14%. Conclusion These findings revealed that NAWM exhibits the same metabolic changes as chronic white matter lesions, even very early in the disease course, further supporting the view that such lesions may not be as relevant as widely assumed. ^© RSNA, 2016.

The Neuro-Immune Pathophysiology of Central and Peripheral Fatigue in Systemic Immune-Inflammatory and Neuro-Immune Diseases

Many patients with systemic immune-inflammatory and neuro-inflammatory disorders, including depression, rheumatoid arthritis, systemic lupus erythematosus, Sjögren's disease, cancer, cardiovascular disorder, Parkinson's disease, multiple sclerosis, stroke, and chronic fatigue syndrome/myalgic encephalomyelitis, endure pathological levels of fatigue. The aim of this narrative review is to delineate the wide array of pathways that may underpin the incapacitating fatigue occurring in systemic and neuro-inflammatory disorders. A wide array of immune, inflammatory, oxidative and nitrosative stress (O&NS), bioenergetic, and neurophysiological abnormalities are involved in the etiopathology of these disease states and may underpin the incapacitating fatigue that accompanies these disorders. This range of abnormalities comprises: increased levels of pro-inflammatory cytokines, e.g., interleukin-1 (IL-1), IL-6, tumor necrosis factor (TNF) α and interferon (IFN) α; O&NS-induced muscle fatigue; activation of the Toll-Like Receptor Cycle through pathogen-associated (PAMPs) and damage-associated (DAMPs) molecular patterns, including heat shock proteins; altered glutaminergic and dopaminergic neurotransmission; mitochondrial dysfunctions; and O&NS-induced defects in the sodium-potassium pump. Fatigue is also associated with altered activities in specific brain regions and muscle pathology, such as reductions in maximum voluntary muscle force, downregulation of the mitochondrial biogenesis master gene peroxisome proliferator-activated receptor gamma coactivator 1-alpha, a shift to glycolysis and buildup of toxic metabolites within myocytes. As such, both mental and physical fatigue, which frequently accompany immune-inflammatory and neuro-inflammatory disorders, are the consequence of interactions between multiple systemic and central pathways.

Spectral-editing measurements of GABA in the human brain with and without macromolecule suppression

PURPOSE

The conventional spectral-editing experiment used to measure GABA in the human brain also contains a contribution from macromolecules (MM), and the combined GABA plus MM signal is often referred to as "GABA+". More recently, methods have been developed to estimate GABA free from MM contamination. In this study, the relationship between GABA acquired with MM suppression and conventional GABA+ measurements was examined.

METHODS

GABA-edited MEGA-PRESS experiments with and without MM suppression were performed in the sensorimotor and occipital cortex of 12 healthy subjects at 3 Tesla. The correlation between GABA+ and MM-suppressed GABA measures was then determined.

RESULTS

Across all data, a significant correlation between GABA+ and MM-suppressed GABA was found (r = 0.48; P = 0.02). Regionally, the sensorimotor voxel showed a trend toward a correlation of r = 0.53, P = 0.07 and the occipital voxel did not show a correlation, r = 0.058, P = 0.9.

CONCLUSION

GABA+ and MM-suppressed GABA are moderately correlated, but statistical power to reveal this relationship may vary regionally. The MM signal, while often assumed to be functionally irrelevant, appears to show inter-individual and inter-regional variance that impacts the correlation of GABA+ and MM-suppressed GABA.

Impact of frequency drift on gamma-aminobutyric acid-edited MR spectroscopy

PURPOSE

To investigate the quantitative impact of frequency drift on Gamma-Aminobutyric acid (GABA+)-edited MRS of the human brain at 3 Tesla (T).

METHODS

Three sequential GABA+-edited MEGA-PRESS acquisitions were acquired in fifteen sessions; in ten of these, MRS was preceded by functional MRI (fMRI) to induce frequency drift, which was estimated from the creatine resonance at 3.0 ppm. Simulations were performed to examine the effects of frequency drift on the editing efficiency of GABA and co-edited macromolecules (MM) and of subtraction artifacts on GABA+ quantification. The efficacy of postprocessing frequency correction was also investigated.

RESULTS

Gradient-induced frequency drifts affect GABA+ quantification for at least 30 min after imaging. Average frequency drift was low in control sessions and as high as -2 Hz/min after fMRI. Uncorrected frequency drift has an approximately linear effect on GABA+ measurements with a -10 Hz drift resulting in a 16% decrease in GABA+, primarily due to subtraction artifacts.

CONCLUSION

Imaging acquisitions with high gradient duty cycles can impact subsequent GABA+ measurements. Postprocessing can address subtraction artifacts, but not changes in editing efficiency or GABA:MM signal ratios; therefore, protocol design should avoid intensive gradient sequences before edited MRS Magn Reson Med 72:941-948, 2014. © 2013 Wiley Periodicals, Inc.

Neuroprotection with glatiramer acetate: evidence from the PreCISe trial

The phase III, multicenter, randomized, placebo-controlled PreCISe trial assessed glatiramer acetate (GA) effects in patients with clinically isolated syndromes (CIS) suggestive of multiple sclerosis (MS). To assess the neuroprotective effect of GA in a subset of patients in the PreCISe trial, we used proton magnetic resonance spectroscopy (MRS) to measure N-acetylaspartate (NAA), a marker of neuronal integrity, in a large central volume of brain. Thirty-four CIS patients randomized to GA 20 mg/day (n = 19) SC or placebo (n = 15) were included. Patients who relapsed (developed clinically definite MS [CDMS]) were removed from the substudy. NAA/creatine (NAA/Cr) ratios were compared between GA-treated and placebo-treated patients. Twenty patients with CIS had not converted to CDMS and were still in the double-blind phase of the trial at 12 months of follow-up. Paired changes in NAA/Cr differed significantly in patients treated with GA (+0.14, n = 11) compared with patients receiving placebo (-0.33, n = 9, p = 0.03) at 12 months, consistent with a neuroprotective effect of GA in vivo. Patients with CIS who received GA showed improvement in brain neuroaxonal integrity, as indicated by increased NAA/Cr, relative to comparable patients treated with placebo, who showed a decline in NAA/Cr consistent with findings from natural history studies.

Implementation of an absolute brain 1H-MRS quantification method to assess different tissue alterations in multiple sclerosis

Magnetic resonance spectroscopy has emerged as a sensitive modality to detect early and diffuse alterations in multiple sclerosis. Recently, the hypothesis of neurodegenerative pathogenesis has highlightened the interest for measurement of metabolites concentrations, to gain specificity, in a large brain volume encompassing different tissue alterations. Therefore, we proposed in this paper the implementation of an absolute quantification method based on localized spectroscopy at short (30 ms) and long (135 ms) echo time of a volume including normal appearing white matter, cortical gray matter, and lesions. First, methodological developments were implemented including external calibration, and corrections of phased-array coil sensitivity and cerebrospinal fluid volume contribution. Second, these improvements were validated and optimized using an original methodology based on simulations of brain images with lesions. Finally, metabolic alterations were assessed in 65 patients including 26 relapsing-remitting, 17 primary-progressive (PP), 22 secondary-progressive (SP) patients, and in 23 normal subjects. Results showed increases of choline, creatine, and myo-inositol concentrations in PP and SP patients compared to controls, whereas the concentration of N-acetyl compounds remained constant. The major finding of this study was the identification of Cho concentration and Cho/tNA ratio as putative markers of progressive onset, suggesting interesting perspectives in detection and followup of neurodegenerative processes.

In-vivo assessment of tissue metabolite levels using 1H MRS and the Electric REference To access In vivo Concentrations (ERETIC) method

Quantitative values of metabolite concentrations in (1)H magnetic resonance spectroscopy have been obtained using the Electric REference To access In vivo Concentrations (ERETIC) method, whereby a synthetic reference signal is injected during the acquisition of spectra. The method has been improved to enable quantification of metabolite concentrations in vivo. Optical signal transmission was used to eliminate random fluctuations in ERETIC signal coupling to the receiver coil due to changes in position of cables and highly dielectric human tissue. Stability and reliability of the signal were tested in vitro, achieving stability with a mean error of 2.83%. Scaling of the signal in variable loading conditions was demonstrated and in-vivo measurements of brain were acquired on a 3T Philips system using a transmit/receive coil. The quantitative brain water and metabolite concentration values are in good agreement with those in the literature.

Electronic reference for absolute quantification of brain metabolites by 1H-MRS on clinical whole-body imaging

BACKGROUND AND PURPOSE

The electronic reference to access in vivo concentrations (ERETIC) method is a promising technique for absolute concentration quantification by brain proton magnetic resonance spectroscopy ((1)H-MRS). However, in its usual form, the technique cannot be implemented in most clinical MR scanners. For this reason, we propose a new strategy for transmitting the ERETIC signal before localized spectroscopy acquisition, thereby allowing its use in clinical MR scanners.

METHODS

ERETIC signal acquisition, using a dedicated sequence, was carried out immediately before the MR sequence. This approach was evaluated on phantoms of known metabolite concentrations and in 10 healthy volunteers. The results were then compared with those obtained using the water signal as reference.

RESULTS

Measurements in vitro showed that the standard deviations measured by the ERETIC method were similar to those using the water-signal reference method. Also, values for metabolite concentrations in vivo were in good agreement with those found in the literature for normal white matter in human brains. Concentrations obtained by ERETIC showed good linear correlation compared with the values obtained by the water-signal reference method.

CONCLUSION

Our preliminary study shows that the ERETIC method appears to be a reliable technique that can overcome most of the drawbacks observed with other absolute quantification methods. However, further studies involving larger patient groups are needed to confirm these findings.

Quantitative spectroscopic imaging with in situ measurements of tissue water T1, T2, and density

The use of tissue water as a concentration standard in proton magnetic resonance spectroscopy ((1)H-MRS) of the brain requires that the water proton signal be adjusted for relaxation and partial volume effects. While single voxel (1)H-MRS studies have often included measurements of water proton T(1), T(2), and density based on additional (1)H-MRS acquisitions (e.g., at multiple echo or repetition times), this approach is not practical for (1)H-MRS imaging ((1)H-MRSI). In this report we demonstrate a method for using in situ measurements of water T(1), T(2), and density to calculate metabolite concentrations from (1)H-MRSI data. The relaxation and density data are coregistered with the (1)H-MRSI data and provide detailed information on the water signal appropriate to the individual subject and tissue region. We present data from both healthy subjects and a subject with brain lesions, underscoring the importance of water parameter measurements on a subject-by-subject and voxel-by-voxel basis.

Absolute metabolite concentrations calibrated using the total water signal in brain (1)H MRS

Magnetic resonance spectroscopy (MRS) has been coupled with a multi-echo imaging sequence to determine the relaxation corrected signal areas of the metabolites and the tissue water. Stimulated echo acquisition mode (STEAM) spectra (TE/TM/TR 30/13.7/5000 ms) acquired from gray and white matter voxels in 43 healthy volunteers were fit using LCModel. Corresponding water signals, measured using a multi-echo T(2) imaging sequence, were fit with a Non-Negative Least Squares algorithm. Using this approach the water area could be T(1) and T(2) corrected for all three water compartments: cerebrospinal fluid (CSF), intra- and extra-cellular water, and myelin water. The image-based water measurement is an improvement over spectroscopy methods because it can be more sensitive to water changes in diseased tissue. Metabolite areas were also corrected for relaxation losses. In occipital gray matter, the concentrations of Cho, Cr, and N-acetyl aspartate (NAA) were 1.27 (0.06), 8.9 (0.3), and 9.3 (0.3) mmol/L tissue, respectively and in parietal white matter they were 1.90 (0.05), 7.9 (0.2), and 9.8 (0.2) mmol/L tissue. The Cho and Cr concentrations were different in occipital gray compared to parietal white matter (p < 0.0001 and <0.005, respectively).

Detection of glutamate and glutamine (Glx) by turbo spectroscopic imaging

Turbo spectroscopic imaging (TSI) is a spin echo spectroscopic imaging technique in which two or more echoes are acquired per excitation to reduce the acquisition time. The application of TSI has primarily been limited to the detection of uncoupled spins because the signal from coupled spins is modulated as a function of echo time. In this work we demonstrate how the TSI sequence can be modified to observe spins like the C(2) protons of Glx (approximately 3.75 ppm) which are involved solely in weak-coupling interactions. The technique exploits the chemical shift displacement effect by employing TSI refocusing pulses that have bandwidths which are less than the chemical shift difference between the target spins and the spins to which they are weakly coupled. The modified TSI sequence rewinds the J-evolution of the target protons in the slice of interest independently of the echo time or echo spacing, thereby removing any signal variation between successive echoes (apart from T(2) relaxation effects). In this study we tailored the narrow-bandwidth TSI sequence for observation of the C(2) Glx protons. The echo time was experimentally optimized to minimize signal contamination from myo-inositol, and the efficacy of the method was verified on phantom solutions of Glx and on brain in vivo.

MRI in multiple sclerosis: current status and future prospects

Many promising MRI approaches for research or clinical management of multiple sclerosis (MS) have recently emerged, or are under development or refinement. Advanced MRI methods need to be assessed to determine whether they allow earlier diagnosis or better identification of phenotypes. Improved post-processing should allow more efficient and complete extraction of information from images. Magnetic resonance spectroscopy should improve in sensitivity and specificity with higher field strengths and should enable the detection of a wider array of metabolites. Diffusion imaging is moving closer to the goal of defining structural connectivity and, thereby, determining the functional significance of lesions at specific locations. Cell-specific imaging now seems feasible with new magnetic resonance contrast agents. The imaging of myelin water fraction brings the hope of providing a specific measure of myelin content. Ultra-high-field MRI increases sensitivity, but also presents new technical challenges. Here, we review these recent developments in MRI for MS, and also look forward to refinements in spinal-cord imaging, optic-nerve imaging, perfusion MRI, and functional MRI. Advances in MRI should improve our ability to diagnose, monitor, and understand the pathophysiology of MS.

Synthetic signal injection using inductive coupling

Conversion of MR signals into units of metabolite concentration requires a very high level of diligence to account for the numerous parameters and transformations that affect the proportionality between the quantity of excited nuclei in the acquisition volume and the integrated area of the corresponding peak in the spectrum. We describe a method that eases this burden with respect to the transformations that occur during and following data acquisition. The conceptual approach is similar to the ERETIC method, which uses a pre-calibrated, artificial reference signal as a calibration factor to accomplish the conversion. The distinguishing feature of our method is that the artificial signal is introduced strictly via induction, rather than radiation. We tested a prototype probe that includes a second RF coil rigidly positioned close to the receive coil so that there was constant mutual inductance between them. The artificial signal was transmitted through the second RF coil and acquired by the receive coil in parallel with the real signal. Our results demonstrate that the calibration factor is immune to changes in sample resistance. This is a key advantage because it removes the cumbersome requirement that coil loading conditions be the same for the calibration sample as for experimental samples. The method should be adaptable to human studies and could allow more practical and accurate quantification of metabolite content.

Exploiting the chemical shift displacement effect in the detection of glutamate and glutamine (Glx) with PRESS

A PRESS (Point RESolved Spectroscopy) sequence for the improved detection of the C2 protons of Glx (glutamate and glutamine) at approximately 3.75ppm is presented in this work. It is shown that for spins like the C2 protons of Glx which are involved solely in weak coupling interactions, the chemical shift displacement effect can be turned to advantage by exploiting PRESS refocusing pulses with bandwidths less than the chemical shift difference between the target spins and the spins to which they are weakly coupled. The narrow-bandwidth PRESS sequence allows refocusing of the J-coupling evolution of the target protons in the voxel of interest independently of echo time yielding signal equivalent to that which can be obtained with a one-pulse acquire sequence (assuming ideal pulses and ignoring T2 relaxation). The total echo time of PRESS was set long enough for the decay of macromolecule signal and the two echo times were empirically optimized so that the Glx signal at 3.75ppm suffered minimal contamination from myo-inositol. The efficacy of the method was verified on phantom solutions of Glx and on brain in vivo.

MR evidence of long T2 water in pathological white matter

PURPOSE

To describe what, if any, specific long T(2)-related abnormalities occur in the white matter of subjects with either phenylketonuria (PKU) or multiple sclerosis (MS).

MATERIALS AND METHODS

The 48-echo T(2) relaxation data (maximum TE = 1.12 sec) were acquired from 15 PKU subjects, 20 MS subjects, and 15 healthy volunteers. Regions of interest were drawn in diffuse white matter hyperintensities (DiffWM), lesions, normal-appearing white matter (NAWM), and normal white matter. Long T(2) maps (200 msec < T(2) < 800 msec) were created for each subject.

RESULTS

A new water reservoir with a markedly prolonged T(2) peak was identified in DiffWM and NAWM in 12 out of 15 subjects with PKU and a long T(2) signal was also seen in 23/97 lesions in 50% of subjects with MS. Additionally, a long T(2) component was observed in the corticospinal tracts of 10 healthy volunteers. The characteristics of the long T(2) signal were unique for each subject group. Potential sources of this signal include vacuolation and increases in extracellular water.

CONCLUSION

This study supports the usefulness of increasing the data acquisition window of the multiecho T(2) relaxation sequence to better characterize the T(2) decay from pathological brain.

Long T2 water in multiple sclerosis: what else can we learn from multi-echo T2 relaxation?

Multi-echo T(2) measurements are invaluable in studying brain pathology in multiple sclerosis (MS). In addition to information about myelin water and total water content, the T(2) distribution has the potential to detect additional water reservoirs arising from other sources such as inflammation or edema. The purpose of this study was to better define the T(2) distribution in MS lesions and normal appearing white matter (NAWM) with particular emphasis on the characterisation of longer T(2) components. Magnetisation transfer (MT), T(1) and 48-echo T(2) relaxation data were acquired in 20 MS subjects and regions of interest were drawn in lesions and NAWM. Twenty-seven out of 107 lesions exhibited signal with a markedly prolonged T(2) (200-800 ms). Lesions with a Long-T(2) signal also exhibited a longer geometric mean T(2) (GMT(2)), increased water content (WC), higher T(1), reduced magnetisation transfer ratio (MTR) and decreased myelin water fraction (MWF) than lesions without a Long-T(2) signal. Those subjects with Long-T(2) lesions had a significantly longer disease duration than subjects without this lesion subtype. A strong correlation was observed between T(1) and Long-T(2) fraction, while a slightly weaker relationship was found for GMT(2), MTR and MWF with Long-T(2) fraction. A potential source of the Long-T(2) signal is an increase in extracellular water. This study supports the usefulness of increasing the data acquisition window of the multi-echo T(2) relaxation sequence to better characterise the T(2) decay in MS.

Recent advances in magnetic resonance neurospectroscopy

Over the past two decades, proton magnetic resonance spectroscopy (proton MRS) of the brain has made the transition from research tool to a clinically useful modality. In this review, we first describe the localization methods currently used in MRS studies of the brain and discuss the technical and practical factors that determine the applicability of the methods to particular clinical studies. We also describe each of the resonances detected by localized solvent-suppressed proton MRS of the brain and discuss the metabolic and biochemical information that can be derived from an analysis of their concentrations. We discuss spectral quantitation and summarize the reproducibility of both single-voxel and multivoxel methods at 1.5 and 3-4 T. We have selected three clinical neurologic applications in which there has been a consensus as to the diagnostic value of MRS and summarize the information relevant to clinical applications. Finally, we speculate about some of the potential technical developments, either in progress or in the future, that may lead to improvements in the performance of proton MRS.

1H MR spectroscopy of the brain: absolute quantification of metabolites

Hydrogen 1 (1H) magnetic resonance (MR) spectroscopy enables noninvasive in vivo quantification of metabolite concentrations in the brain. Currently, metabolite concentrations are most often presented as ratios (eg, relative to creatine) rather than as absolute concentrations. Despite the success of this approach, it has recently been suggested that relative quantification may introduce substantial errors and can lead to misinterpretation of spectral data and to erroneous metabolite values. The present review discusses relevant methods to obtain absolute metabolite concentrations with a clinical MR system by using single-voxel spectroscopy or chemical shift imaging. Important methodological aspects in an absolute quantification strategy are addressed, including radiofrequency coil properties, calibration procedures, spectral fitting methods, cerebrospinal fluid content correction, macromolecule suppression, and spectral editing. Techniques to obtain absolute concentrations are now available and can be successfully applied in clinical practice. Although the present review is focused on 1H MR spectroscopy of the brain, a large part of the methodology described can be applied to other tissues as well.

Insights into brain microstructure from the T2 distribution

T2 weighting is particularly sensitive, but notoriously unspecific, to a wide range of brain pathologies. However, careful measurement and analysis of the T2 decay curve from brain tissue promise to provide much improved pathological specificity. In vivo T2 measurement requires accurate 180 pulses and appropriate manipulation of stimulated echoes; the most common approach is to acquire multiple echoes from a single slice. The T2 distribution, a plot of component amplitude as a function of T2, can be estimated using an algorithm capable of fitting a multi-exponential T2 decay with no a priori assumptions about the number of exponential components. T2 distributions from normal brain show peaks from myelin water, intra/extracellular water and cerebral spinal fluid; they can be used to provide estimates of total water content (total area under the T2 distribution) and myelin water fraction (MWF, fractional area under the myelin water peak), a measure believed to be related to myelin content. Experiments on bovine brain suggest that magnetization exchange between water pools plays a minor role in the T2 distribution. Different white matter structures have different MWFs. In normal white matter (NWM), MWF is not correlated with the magnetization transfer ratio (MTR) or the diffusion tensor fractional anisotropy (FA); hence it provides unique information about brain microstructure. Normal-appearing white matter (NAWM) in multiple sclerosis (MS) brain possesses a higher water content and lower MWF than controls, consistent with histopathological findings. Multiple sclerosis lesions demonstrate great heterogeneity in MWF, presumably due to varying myelin contents of these focal regions of pathology. Subjects with schizophrenia were found to have significantly reduced MWF in the minor forceps and genu of the corpus callosum when compared to controls, suggesting that reduced frontal lobe myelination plays a role in schizophrenia. In normal controls, frontal lobe myelination was positively correlated with both age and education; this result was not observed in subjects with schizophrenia. A strong correlation between MWF and the optical density from the luxol fast blue histological stain for myelin was observed in formalin-fixed brain, supporting the use of the MWF as an in vivo myelin marker.

Metabolite changes in early relapsing-remitting multiple sclerosis. A two year follow-up study

Previous in vivo proton magnetic resonance spectroscopic imaging ((1)H-MRSI) studies have found reduced levels of N-acetyl-aspartate (NAA) in multiple sclerosis (MS) lesions, the surrounding normal-appearing white matter (NAWM) and cortical grey matter (CGM), suggesting neuronal and axonal dysfunction and loss. Other metabolites, such as myoinositol (Ins), creatine (Cr), choline (Cho), and glutamate plus glutamine (Glx), can also be quantified by (1)H-MRSI, and studies have indicated that concentrations of these metabolites may also be altered in MS. Relatively little is known about the time course of such metabolite changes. This preliminary study aimed to characterise changes in total NAA (tNAA, the sum of NAA and N-acetyl-aspartyl-glutamate), Cr, Cho, Ins and Glx concentrations in NAWM and in CGM, and their relationship with clinical outcome, in subjects with clinically early relapsing-remitting MS (RRMS). Twenty RRMS subjects and 10 healthy control subjects underwent (1)H-MRSI examinations yearly for two years. Using the LCModel, tNAA, Cr, Cho, Ins and Glx concentrations were estimated both in NAWM and CGM. At baseline, the concentration of tNAA was significantly reduced in the NAWM of the MS patients compared to the control group (-7%, p = 0.003), as well as in the CGM (-8.7%, p = 0.009). NAWM tNAA concentrations tended to recover from baseline, but otherwise tissue metabolite profiles did not significantly change in the MS subjects, or relatively between MS and healthy control subjects. While neuronal and axonal damage is apparent from the early clinical stages of MS, this study suggests that initially it may be partly reversible. Compared with other MR imaging measures, serial (1)H-MRSI may be relatively less sensitive to progressive pathological tissue changes in early RRMS.

1H-MRS quantification of tNA and tCr in patients with multiple sclerosis: a meta-analytic review

Meta-analysis was performed on the results of 75 comparisons from the 30 peer-reviewed publications that used proton magnetic resonance spectroscopy (1H-MRS) or spectroscopic imaging to (i) quantify the mean concentrations of total creatine (tCr, found in neurons, astrocytes and oligodendrocytes), and/or total N-acetyl groups (tNA, found only in neurons), in the lesional and/or non-lesional white matter (WM) and/or the grey matter (GM) of patients with multiple sclerosis (MS) and (ii) compare these values with those in the homologous tissues of normal controls (NC). For mean [tNA] values, there was (i) a large-effect-sized overall decrease in patients' lesional WM relative to NC WM (25 comparisons), (ii) a medium-effect-sized overall decrease in patients' non-lesional WM relative to NC WM (36 comparisons) and (iii) a medium-effect-sized overall decrease in patients' GM relative to NC GM (14 comparisons). Patients' mean [tNA] values were sometimes statistically normal but were never statistically increased. For mean [tCr] values, there was (i) no statistically significant overall change in the patients' lesional WM relative to NC WM (24 comparisons), although statistically significant increases and decreases were sometimes found, (ii) a medium-effect-sized overall increase in patients' non-lesional WM relative to NC WM (33 comparisons) and (iii) no statistically significant overall change in patients' GM relative to NC GM (12 comparisons), although a significant decrease was found in one comparison. Of 41 comparisons with statistically significant changes, 38 combined in a way that would probably result in decreased mean [tNA]/[tCr] ratios such that (i) 66% had statistically decreased mean [tNA] and statistically unchanged mean [tCr] values, (ii) 13% had statistically decreased mean [tNA] and statistically increased mean [tCr] values and (iii) 21% had statistically unchanged mean [tNA] values and statistically increased mean [tCr] values. Of the 25 comparisons that came from studies that also analysed [tNA]/[tCr] ratios, the direction of change in mean [tNA] values and mean [tNA]/[tCr] ratios was concordant in 84%. In comparisons that quantified both [tNA] and [tCr], there was a similar amount of variability in both measures in each of the different tissue types studied, both in patients and NCs. Together, these results suggest that within-voxel tNA/tCr ratios can be interpreted as valid and accurate surrogate measures of 'cerebral tissue integrity'-with decreased tNA/tCr ratios indicating some combination of neuroaxonal disturbance, oligodendroglial disturbance, and astrocytic proliferation. These results also suggest that, although within-voxel tNA/tCr ratios are not perfect indicators of [tNA] content, they do represent a practical compromise to acquiring surrogate measures of within-voxel neuroaxonal integrity.

Imaging of multiple sclerosis: role in neurotherapeutics

Magnetic resonance imaging (MRI) plays an ever-expanding role in the evaluation of multiple sclerosis (MS). This includes its sensitivity for the diagnosis of the disease and its role in identifying patients at high risk for conversion to MS after a first presentation with selected clinically isolated syndromes. In addition, MRI is a key tool in providing primary therapeutic outcome measures for phase I/II trials and secondary outcome measures in phase III trials. The utility of MRI stems from its sensitivity to longitudinal changes including those in overt lesions and, with advanced MRI techniques, in areas affected by diffuse occult disease (the so-called normal-appearing brain tissue). However, all current MRI methodology suffers from limited specificity for the underlying histopathology. Conventional MRI techniques, including lesion detection and measurement of atrophy from T1- or T2-weighted images, have been the mainstay for monitoring disease activity in clinical trials, in which the use of gadolinium with T1-weighted images adds additional sensitivity and specificity for areas of acute inflammation. Advanced imaging methods including magnetization transfer, fluid attenuated inversion recovery, diffusion, magnetic resonance spectroscopy, functional MRI, and nuclear imaging techniques have added to our understanding of the pathogenesis of MS and may provide methods to monitor therapies more sensitively in the future. However, these advanced methods are limited by their cost, availability, complexity, and lack of validation. In this article, we review the role of conventional and advanced imaging techniques with an emphasis on neurotherapeutics.

Evidence of elevated glutamate in multiple sclerosis using magnetic resonance spectroscopy at 3 T

Histopathological reports of multiple sclerosis and its animal models have shown evidence of a link between axonal injury in active lesions and impaired glutamate metabolism. Mature oligodendrocytes play a role in glutamate uptake to maintain glutamate homeostasis but in multiple sclerosis white matter the loss of expression of glutamate transporters in the lesion vicinity results in ineffective glutamate removal. Using a magnetic resonance spectroscopy technique that isolates the glutamate resonance at 3 T, we compared glutamate levels between normal subjects and multiple sclerosis patients in different brain areas. Metabolite concentrations (glutamate, glutamine, N-acetyl-aspartate, myo-inositol, choline, creatine) were derived from LCmodel and corrected for T1 relaxation time. Glutamate concentrations were found to be elevated in acute lesions (P = 0.02) and normal-appearing white matter (P = 0.03), with no significant elevation in chronic lesions (P = 0.77). The N-acetyl-aspartate level in chronic lesions was significantly lower (P < 0.001) than in acute lesions and normal-appearing white matter. The choline level in acute lesions was significantly higher (P < 0.001) than in chronic lesions. Evidence was also found for increased glial activity in multiple sclerosis, with significantly higher (P < 0.001) myo-inositol levels in acute lesions compared with control white matter. These in vivo results support the hypothesis that altered glutamate metabolism is present in brains of multiple sclerosis patients.

Relapsing-Remitting Multiple Sclerosis: Metabolic Abnormality in Nonenhancing Lesions and Normal-appearing White Matter at MR Imaging: Initial Experience

PURPOSE

To quantify, with three-dimensional proton magnetic resonance (MR) spectroscopy, metabolic characteristics of normal-appearing white matter and nonenhancing lesions in patients with relapsing-remitting multiple sclerosis (MS).

MATERIALS AND METHODS

Institutional review board approval and informed patient consent were obtained. Nine patients with relapsing-remitting MS (six women, three men) and nine age-matched control subjects (seven women, two men) were studied with T1- and T2-weighted MR imaging and three-dimensional proton MR spectroscopy at spatial resolution less than a cubic centimeter. Absolute N-acetylaspartate (NAA), creatine (Cr), and choline (Cho) levels were obtained from 171 voxels: 66 from lesions on T2-weighted MR images (43 hypointense and 23 isointense on T1-weighted MR images), 31 from normal-appearing white matter, and 74 from analogous normal white matter regions on images in control subjects.

RESULTS

Mean NAA level in hypointense lesions (5.30 mmol/L +/- 2.27 [standard deviation]) was significantly lower (P < or = .05) than that in isointense lesions (7.82 mmol/L +/- 2.28), normal-appearing white matter (7.37 mmol/L +/- 1.71), and normal white matter in control subjects (8.89 mmol/L +/- 1.54). Cho (1.79 mmol/L +/- 0.65) and Cr (5.64 mmol/L +/- 1.50) levels in isointense lesions were indistinguishable from those in normal-appearing white matter (1.74 mmol/L +/- 0.46 and 4.99 mmol/L +/- 0.97, respectively) but were significantly higher (Cho, 20%; Cr, 24%) than those in normal white matter in control subjects (1.44 mmol/L +/- 0.40 and 4.30 mmol/L +/- 1.32, respectively). NAA, Cho, and Cr levels in normal-appearing white matter were significantly different than those in normal white matter in control subjects (NAA, 20% lower; Cho, 14% higher; and Cr, 17% higher).

CONCLUSION

Abnormal metabolic activity persists in all MS tissue types. Increased Cr and Cho levels suggest (a) ongoing gliosis and attempted remyelination in isointense lesions on T1-weighted MR images and (b) membrane turnover (de- and remyelination), in addition to increased cellularity (gliosis, inflammation) in normal-appearing white matter.

Magnetic resonance techniques for the in vivo assessment of multiple sclerosis pathology: Consensus report of the white matter study group

On October 9-11, 2003, the third meeting of the White Matter Study Group of the International Society for Magnetic Resonance in Medicine was held in Venice, Italy. This article is the report of the meeting on how to use MRI in the diagnostic workup of multiple sclerosis (MS) and allied white matter disorders, and to define the nature and the extent of MS pathology in vivo. Both of these steps are central to the design of future treatment strategies aimed at limiting the functional consequences of the most disabling aspects of this disease.

Measurement of brain glutamate using TE-averaged PRESS at 3T

A method is introduced that provides improved in vivo spectroscopic measurements of glutamate (Glu), glutamine (Gln), choline (Cho), creatine (Cre), N-acetyl compounds (NAtot, NAA + NAAG), and the inositols (mI and sI). It was found that at 3T, TE averaging, the f1 = 0 slice of a 2D J-resolved spectrum, yielded unobstructed signals for Glu, Glu + Gln (Glx), mI, NA(tot), Cre, and Cho. The C4 protons of Glu at 2.35 ppm, and the C2 protons of Glx at 3.75 ppm were well resolved and yielded reliable measures of Glu/Gln stasis. Apparent T1/T2 values were obtained from the raw data, and metabolite tissue levels were determined relative to a readily available standard. A repeatibility error of <5%, and a coefficient of variation (CV) of <10% were observed for brain Glu levels in a study of six normal volunteers.

The role of relaxation time corrections for the evaluation of long and short echo time 1H MR spectra of the hippocampus by NUMARIS and LCModel techniques

1H MR spectroscopy is routinely used for lateralization of epileptogenic lesions. The present study deals with the role of relaxation time corrections for the quantitative evaluation of long (TE=135 ms) and short echo time (TE=10 ms) 1H MR spectra of the hippocampus using two methods (operator-guided NUMARIS and LCModel programs). Spectra of left and right hippocampi of 14 volunteers and 14 patients with epilepsy were obtained by PRESS (TR/TE=5000/135 ms) and STEAM (TR/TE=5000/10 ms) sequences with a 1.5-T imager. Evaluation was carried out using Siemens NUMARIS software and the results were compared with data from LCModel processing software. No significant differences between the two methods of processing spectra with TE=135 ms were found. The range of relaxation corrections was determined. Metabolite concentrations in hippocampi calculated from spectra with TE=135 ms and 10 ms after application of correction coefficients did not differ in the range of errors and agreed with published data (135 ms/10 ms: NAA=10.2+/-0.6/10.4+/-1.3 mM, Cho=2.4+/-0.1/2.7+/-0.3 mM, Cr=12.2+/-1.3/11.3+/-1.3 mM). When relaxation time corrections were applied, quantitative results from short and long echo time evaluation with LCModel were in agreement. Signal intensity ratios obtained from long echo time spectra by NUMARIS operator-guided processing also agreed with the LCModel results.

Molality as a unit of measure for expressing 1H MRS brain metabolite concentrations in vivo

Absolute concentrations of cerebral metabolite in in vivo 1H magnetic resonance spectroscopy studies (1H-MRS) are widely reported in molar units as moles per liter of tissue, or in molal units as moles per kilogram of tissue. Such measurements require external referencing or assumptions as to local water content. To reduce the scan time, avoid assumptions that may be invalid under specific pathologies, and provide a universally accessible referencing procedure, we suggest that metabolite concentrations from 1H-MRS measurements in vivo be reported in molal units as moles per kilogram of tissue water. Using internal water referencing, a two-compartment water model, a simulated brain spectrum for peak identification, and a spectroscopic bi-exponential spin-spin relaxation segmentation technique, we measured the absolute concentrations for the four common 1H brain metabolites: choline (Cho), myo-inositol (mIno), phosphocreatine + creatine (Cr), and N-acetyl-aspartate (NAA), in the hippocampal region (n = 26) and along the Sylvian fissure (n = 61) of 35 healthy adults. A stimulated echo localization method (20 ms echo time, 10 ms mixing time, 4 s repetition time) yielded metabolite concentrations, uncorrected for metabolite relaxation or contributions from macromolecule resonances, that were expectantly higher than with molar literature values. Along the Sylvian fissure the average concentrations (coefficient of variation (CV)) in mmoles/kg of tissue water were 17.6 (12%) for NAA, 14.2 (9%) for Cr, 3.6 (13%) for Cho, and 13.2 (15%) for mIno. Respective values for the hippocampal region were 15.7 (20%), 14.7 (16%), 4.6 (19%), and 17.7 (26%). The concentrations of the two regions were significantly different (p </= 0.001) for NAA, mIno, and Cho, a trend in agreement with previous studies. All gray matter Sylvian fissure CV values, except for NAA, were also in agreement with previous 1H-MRS gray matter studies. The reduced precision of the NAA concentration was attributed to overlapping signal contributions from glutamate and glutamine (Glx), suggesting that a detailed Glx model is critical for accurate quantitation of the NAA 2.02 ppm resonance. The reduced precision of the measurements in the hippocampal region was attributed to poor spectral resolution.

Constrained modeling for spectroscopic measurement of bi-exponential spin-lattice relaxation of water in vivo

The (1)H NMR water signal from spectroscopic voxels localized in gray matter contains contributions from tissue and cerebral spinal fluid (CSF). A typically weak CSF signal at short echo times makes separating the tissue and CSF spin-lattice relaxation times (T(1)) difficult, often yielding poor precision in a bi-exponential relaxation model. Simulations show that reducing the variables in the T(1) model by using known signal intensity values significantly improves the precision of the T(1) measurement. The method was validated on studies on eight healthy subjects (four males and four females, mean age 21 +/- 2 years) through a total of twenty-four spectroscopic relaxation studies. Each study included both T(1) and spin-spin relaxation (T(2)) experiments. All volumes were localized along the Sylvian fissure using a stimulated echo localization technique with a mixing time of 10 ms. The T(2) experiment consisted of 16 stimulated echo acquisitions ranging from a minimum echo time (TE) of 20 ms to a maximum of 1000 ms, with a repetition time of 12 s. All T(1) experiments consisted of 16 stimulated echo acquisition, using a homospoil saturation recovery technique with a minimum recovery time of 50 ms and a maximum 12 s. The results of the T(2) measurements provided the signal intensity values used in the bi-exponential T(1) model. The mean T(1) values when the signal intensities were constrained by the T(2) results were 1055.4 ms +/- 7.4% for tissue and 5393.5 ms +/- 59% for CSF. When the signal intensities remained free variables in the model, the mean T(1) values were 1085 ms +/- 19.4% and 5038.8 ms +/- 113.0% for tissue and CSF, respectively. The resulting improvement in precision allows the water tissue T(1) value to be included in the spectroscopic characterization of brain tissue.

Magnetic resonance spectroscopy in multiple sclerosis: window into the diseased brain

Magnetic resonance spectroscopy provides noninvasive insight into the regional and global biochemical alterations that are concomitants of the dynamic processes that underlie the evolution of fundamental pathologic changes in multiple sclerosis. These include now well-recognized alterations of neuronal biochemical markers that accompany tissue destruction readily visualized by magnetic resonance imaging, but also dynamic changes in several metabolites that indicate pathological processes that precede the magnetic resonance imaging-defined lesion, or completely escape current high-resolution imaging.

Magnetization transfer of water T(2) relaxation components in human brain: implications for T(2)-based segmentation of spectroscopic volumes

Biexponential T(2) relaxation of the localized water signal can be used for segmentation of spectroscopic volumes. To assess the specificity of the components an iterative relaxation measurement of the localized water signal (STEAM, 12 echo times, geometric spacing from 30 ms to 2000 ms) was combined with magnetization transfer (MT) saturation (40 single lobe pulses, 12 ms duration, 1440 degrees nominal flip angle, 1 kHz offset, repeated every 30 ms). Voxels including CSF were examined in parietal cortex and periventricular parietal white matter (10 each), as well as 13 voxels in central white matter and 16 T(1)-hypointense non-enhancing multiple sclerosis lesions without CSF inclusion. Biexponential models (excluding myelin water) were fitted to the relaxation data. In periventricular VOIs the component of long T(2) (1736 +/- 168 ms) that is attributed to CSF was not affected by MT. In cortical VOIs this component had markedly shorter T(2)'s (961 +/- 239 ms) and showed both attenuation and prolongation with MT, indicating contributions from tissue. MS lesions and central WM showed a second tissue component of intermediate T(2) (160-410 ms). In white matter similar MT attenuation indicated strong exchange between the two tissue components, prohibiting segmentation. In MS lesions, however, markedly less MT of the intermediate component was found, which is consistent with decreased cellularity and exchange in a region that is large compared to diffusion motion.