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

Quantification of magnetic resonance spectroscopy data using a combined reference: Application in typically developing infants

Quantification of proton magnetic resonance spectroscopy (¹ H-MRS) data is commonly performed by referencing the ratio of the signal from one metabolite, or metabolite group, to that of another, or to the water signal. Both approaches have drawbacks: ratios of two metabolites can be difficult to interpret because study effects may be driven by either metabolite, and water-referenced data must be corrected for partial volume and relaxation effects in the water signal. Here, we introduce combined reference (CRef) analysis, which compensates for both limitations. In this approach, metabolites are referenced to the combined signal of several reference metabolites or metabolite groups. The approach does not require the corrections necessary for water scaling and produces results that are less sensitive to the variation of any single reference signal, thereby aiding the interpretation of results. We demonstrate CRef analysis using 202 ¹ H-MRS acquisitions from the brains of 140 infants, scanned at approximately 1 and 3 months of age. We show that the combined signal of seven reference metabolites or metabolite groups is highly correlated with the water signal, corrected for partial volume and relaxation effects associated with cerebral spinal fluid. We also show that the combined reference signal is equally or more uniform across subjects than the reference signals from single metabolites or metabolite groups. We use CRef analysis to quantify metabolite concentration changes during the first several months of life in typically developing infants.

Osprey: Open-source processing, reconstruction & estimation of magnetic resonance spectroscopy data

BACKGROUND

Processing and quantitative analysis of magnetic resonance spectroscopy (MRS) data are far from standardized and require interfacing with third-party software. Here, we present Osprey, a fully integrated open-source data analysis pipeline for MRS data, with seamless integration of pre-processing, linear-combination modelling, quantification, and data visualization.

NEW METHOD

Osprey loads multiple common MRS data formats, performs phased-array coil combination, frequency-and phase-correction of individual transients, signal averaging and Fourier transformation. Linear combination modelling of the processed spectrum is carried out using simulated basis sets and a spline baseline. The MRS voxel is coregistered to an anatomical image, which is segmented for tissue correction and quantification is performed based upon modelling parameters and tissue segmentation. The results of each analysis step are visualized in the Osprey GUI. The analysis pipeline is demonstrated in 12 PRESS, 11 MEGA-PRESS, and 8 HERMES datasets acquired in healthy subjects.

RESULTS

Osprey successfully loads, processes, models, and quantifies MRS data acquired with a variety of conventional and spectral editing techniques.

COMPARISON WITH EXISTING METHOD(S)

Osprey is the first MRS software to combine uniform pre-processing, linear-combination modelling, tissue correction and quantification into a coherent ecosystem. Compared to existing compiled, often closed-source modelling software, Osprey's open-source code philosophy allows researchers to integrate state-of-the-art data processing and modelling routines, and potentially converge towards standardization of analysis.

CONCLUSIONS

Osprey combines robust, peer-reviewed data processing methods into a modular workflow that is easily augmented by community developers, allowing the rapid implementation of new methods.

Big GABA II: Water-referenced edited MR spectroscopy at 25 research sites

Accurate and reliable quantification of brain metabolites measured in vivo using ¹H magnetic resonance spectroscopy (MRS) is a topic of continued interest. Aside from differences in the basic approach to quantification, the quantification of metabolite data acquired at different sites and on different platforms poses an additional methodological challenge. In this study, spectrally edited γ-aminobutyric acid (GABA) MRS data were analyzed and GABA levels were quantified relative to an internal tissue water reference. Data from 284 volunteers scanned across 25 research sites were collected using GABA+ (GABA + co-edited macromolecules (MM)) and MM-suppressed GABA editing. The unsuppressed water signal from the volume of interest was acquired for concentration referencing. Whole-brain T₁-weighted structural images were acquired and segmented to determine gray matter, white matter and cerebrospinal fluid voxel tissue fractions. Water-referenced GABA measurements were fully corrected for tissue-dependent signal relaxation and water visibility effects. The cohort-wide coefficient of variation was 17% for the GABA + data and 29% for the MM-suppressed GABA data. The mean within-site coefficient of variation was 10% for the GABA + data and 19% for the MM-suppressed GABA data. Vendor differences contributed 53% to the total variance in the GABA + data, while the remaining variance was attributed to site- (11%) and participant-level (36%) effects. For the MM-suppressed data, 54% of the variance was attributed to site differences, while the remaining 46% was attributed to participant differences. Results from an exploratory analysis suggested that the vendor differences were related to the unsuppressed water signal acquisition. Discounting the observed vendor-specific effects, water-referenced GABA measurements exhibit similar levels of variance to creatine-referenced GABA measurements. It is concluded that quantification using internal tissue water referencing is a viable and reliable method for the quantification of in vivo GABA levels.

Correlations between changes in the hypothalamic-pituitary-adrenal axis and neurochemistry of the anterior cingulate gyrus in postpartum depression

BACKGROUND

This study aimed to investigate associations between indicators of hypothalamic-pituitary-adrenal axis (HPA) functioning and metabolite levels in the anterior cingulate gyrus (ACG) of women with postpartum depression (PPD).

METHODS

The sample (mean age = 28.5 ± 4.6 years) consisted of 20 women with PPD and 19 postpartum euthymic (PPE) women. Brain metabolites were quantified by proton magnetic resonance spectroscopy (¹H-MRS). Salivary cortisol samples were collected upon awakening and 30 min and 12 h later, at 20.6 ± 6.6 (PPD) and 23.0 ± 7.4 (PPE) weeks after childbirth.

RESULTS

There were no significant differences between groups in respect to metabolite levels in the ACG. Compared with PPE, PPD women had less diurnal variation (DVr%). In the PPD group, positive correlations were found between DVr% and myo-inositol (mI/Cr) levels, and between cortisol awakening response (CARi%) and glutamate + glutamine (Glx/Cr) levels. The correlation between CARi% and Glx/Cr remained significant even after controlling for the interval, in weeks, from birth and MR spectroscopy and to hormonal data collection, and the use of contraceptives.

LIMITATIONS

The limitations of the study include the small sample size and the use of oral contraceptives by around half of the sample.

CONCLUSIONS

In the remote postpartum period (mean 21.8 ± 6.9 weeks) and in the presence of depressive episodes, the decreased responsiveness of the HPA axis after awakening and a smaller decrease in cortisol levels over the day were associated with lower levels of metabolites in the ACG. These results may contribute to the development of biological models to explain the etiology of PPD.

Errors in 1 H-MRS estimates of brain metabolite concentrations caused by failing to take into account tissue-specific signal relaxation

Accurate measurement of brain metabolite concentrations with proton magnetic resonance spectroscopy (¹ H-MRS) can be problematic because of large voxels with mixed tissue composition, requiring adjustment for differing relaxation rates in each tissue if absolute concentration estimates are desired. Adjusting for tissue-specific metabolite signal relaxation, however, also requires a knowledge of the relative concentrations of the metabolite in gray (GM) and white (WM) matter, which are not known a priori. Expressions for the estimation of the molality and molarity of brain metabolites with ¹ H-MRS are extended to account for tissue-specific relaxation of the metabolite signals and examined under different assumptions with simulated and real data. Although the modified equations have two unknowns, and hence are unsolvable explicitly, they are nonetheless useful for the estimation of the effect of tissue-specific metabolite relaxation rates on concentration estimates under a range of assumptions and experimental parameters using simulated and real data. In simulated data using reported GM and WM T₁ and T₂ times for N-acetylaspartate (NAA) at 3 T and a hypothetical GM/WM NAA ratio, errors of 6.5-7.8% in concentrations resulted when TR = 1.5 s and TE = 0.144 s, but were reduced to less than 0.5% when TR = 6 s and TE = 0.006 s. In real data obtained at TR/TE = 1.5 s/0.04 s, the difference in the results (4%) was similar to that obtained with simulated data when assuming tissue-specific relaxation times rather than GM-WM-averaged times. Using the expressions introduced in this article, these results can be extrapolated to any metabolite or set of assumptions regarding tissue-specific relaxation. Furthermore, although serving to bound the problem, this work underscores the challenge of correcting for relaxation effects, given that relaxation times are generally not known and impractical to measure in most studies. To minimize such effects, the data should be acquired with pulse sequence parameters that minimize the effect of signal relaxation.

Glutamatergic and neural dysfunction in postpartum depression using magnetic resonance spectroscopy

Although postpartum depression (PPD) is a prevalent subtype of major depressive disorder, neuroimaging studies on PPD are rare, particularly those identifying neurochemical abnormalities obtained by proton magnetic resonance spectroscopy (¹H-MRS). The dorsolateral prefrontal (DLPF) and the anterior cingulate gyrus (ACG) are part of the neural pathways involved in executive functions and emotional processing, and both structures have been implicated in the neurobiology of depressive disorders. This study aimed to evaluate brain metabolites abnormalities in women with PPD compared with healthy postpartum (HP) women. Thirty-six PPD (34 without antidepressants) and 25 HP women underwent a ¹H-MRS acquired on a 3-T MRI system, with the volume of interest positioned in ACG and DLPF. An ANCOVA was conducted with age, postpartum time, and contraceptive type as covariates. PPD group presented significantly lower Glutamate+Glutamine (Glx, -0.95mM) and N-acetylaspartate+N-acetylaspartylglutamate (NAA, -0.60mM) values in DLPF. There were no significant differences between groups in ACG, but we found a significant increase of Glutamate (Glu, 2.18mM) and Glx (1.84mM) in participants using progestogen-only contraceptives. These findings suggest glutamatergic dysfunction and neuronal damage in the DLPF of PPD patients, similarly to other subtypes of depressive disorders. Progestogens seem to interfere in the neurochemistry of ACG.

Quantification of γ-aminobutyric acid (GABA) in 1 H MRS volumes composed heterogeneously of grey and white matter

The quantification of γ-aminobutyric acid (GABA) concentration using localised MRS suffers from partial volume effects related to differences in the intrinsic concentration of GABA in grey (GM) and white (WM) matter. These differences can be represented as a ratio between intrinsic GABA in GM and WM: r_M . Individual differences in GM tissue volume can therefore potentially drive apparent concentration differences. Here, a quantification method that corrects for these effects is formulated and empirically validated. Quantification using tissue water as an internal concentration reference has been described previously. Partial volume effects attributed to r_M can be accounted for by incorporating into this established method an additional multiplicative correction factor based on measured or literature values of r_M weighted by the proportion of GM and WM within tissue-segmented MRS volumes. Simulations were performed to test the sensitivity of this correction using different assumptions of r_M taken from previous studies. The tissue correction method was then validated by applying it to an independent dataset of in vivo GABA measurements using an empirically measured value of r_M . It was shown that incorrect assumptions of r_M can lead to overcorrection and inflation of GABA concentration measurements quantified in volumes composed predominantly of WM. For the independent dataset, GABA concentration was linearly related to GM tissue volume when only the water signal was corrected for partial volume effects. Performing a full correction that additionally accounts for partial volume effects ascribed to r_M successfully removed this dependence. With an appropriate assumption of the ratio of intrinsic GABA concentration in GM and WM, GABA measurements can be corrected for partial volume effects, potentially leading to a reduction in between-participant variance, increased power in statistical tests and better discriminability of true effects.

Single-voxel (1)H spectroscopy in the human hippocampus at 3 T using the LASER sequence: characterization of neurochemical profile and reproducibility

The hippocampus is crucial for long-term episodic memory and learning. It undergoes structural change in aging and is sensitive to neurodegenerative and psychiatric diseases. MRS studies have seldom been performed in the hippocampus due to technical challenges. The reproducibility of MRS in the hippocampus has not been evaluated at 3 T. The purpose of the present study was to quantify the concentration of metabolites in a small voxel placed in the hippocampus and evaluate the reproducibility of the quantification. Spectra were measured in a 2.4 mL voxel placed in the left hippocampus covering the body and most of the tail of the structure in 10 healthy subjects across three different sessions and quantified using LCModel. High-quality spectra were obtained, which allowed a reliable quantification of 10 metabolites including glutamate and glutamine. Reproducibility of MRS was evaluated with coefficient of variation, standard errors of measurement, and intraclass correlation coefficients. All of these measures showed improvement with increased number of averages. Changes of less than 5% in concentration of N-acetylaspartate, choline-containing compounds, and total creatine and of less than 10% in concentration of myo-inositol and the sum of glutamate and glutamine can be confidently detected between two measurements in a group of 20 subjects. A reliable and reproducible neurochemical profile of the human hippocampus was obtained using MRS at 3 T in a small hippocampal volume.

In vivo assessment of neurotransmitters and modulators with magnetic resonance spectroscopy: application to schizophrenia

In vivo measurement of neurotransmitters and modulators is now feasible with advanced proton magnetic resonance spectroscopy ((1)H MRS) techniques. This review provides a basic tutorial of MRS, describes the methods available to measure brain glutamate, glutamine, γ-aminobutyric acid, glutathione, N-acetylaspartylglutamate, glycine, and serine at magnetic field strengths of 3T or higher, and summarizes the neurochemical findings in schizophrenia. Overall, (1)H MRS holds great promise for producing biomarkers that can serve as treatment targets, prediction of disease onset, or illness exacerbation in schizophrenia and other brain diseases.

Semi-LASER 1 H MR spectroscopy at 7 Tesla in human brain: Metabolite quantification incorporating subject-specific macromolecule removal

PURPOSE

To develop an in vivo ¹ H short-echo-time semi-LASER spectroscopy protocol at 7 Tesla (T) incorporating subject-specific macromolecule removal.

METHODS

T₁ constants of the major metabolites were measured with little macromolecule contribution in seven healthy volunteers and used to optimize double inversion metabolite nulling. Spectra were acquired from parietal-occipital cortex of five healthy volunteers. Metabolite-nulled macromolecule spectra were subtracted from the metabolite spectra before fitting in the time domain with prior-knowledge templates. Absolute metabolite concentrations were determined by referencing to the water signal, following partial volume and relaxation corrections.

RESULTS

The average signal to noise ratio, N-acetylaspartate peak height divided by the baseline noise standard deviation, was 48 ± 6. T₁ constants for N-acetylaspartate, glutamate, creatine, and choline were 1.71 ± 0.15 s, 1.68 ± 0.19 s, 1.63 ± 0.10 s, and 1.41 ± 0.09 s, respectively. The optimal double inversion times for metabolite suppression were TI₁  = 2.09 s and TI₂  = 0.52 s. The coefficient of variation was less than 10% for N-acetylaspartate, creatine, choline, and myo-inositol, and less than 20% for glutamate and glutamine.

CONCLUSION

Short echo-time ¹ H semi-LASER spectroscopy at 7T incorporating subject-specific macromolecule removal yielded reproducible brain metabolite concentrations ideal for applications in disease conditions where macromolecule contributions may deviate from the norm. Magn Reson Med 74:4-12, 2015. © 2014 Wiley Periodicals, Inc.

Moderate relationships between NAA and cognitive ability in healthy adults: implications for cognitive spectroscopy

BACKGROUND

Proton Magnetic Resonance Spectroscopy ((1)H-MRS) is a non-invasive imaging technique that enables quantification of neurochemistry in vivo and thereby facilitates investigation of the biochemical underpinnings of human cognitive variability. Studies in the field of cognitive spectroscopy have commonly focused on relationships between measures of N-acetyl aspartate (NAA), a surrogate marker of neuronal health and function, and broad measures of cognitive performance, such as IQ.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we used (1)H-MRS to interrogate single-voxels in occipitoparietal and frontal cortex, in parallel with assessments of psychometric intelligence, in a sample of 40 healthy adult participants. We found correlations between NAA and IQ that were within the range reported in previous studies. However, the magnitude of these effects was significantly modulated by the stringency of data screening and the extent to which outlying values contributed to statistical analyses.

CONCLUSIONS/SIGNIFICANCE

(1)H-MRS offers a sensitive tool for assessing neurochemistry non-invasively, yet the relationships between brain metabolites and broad aspects of human behavior such as IQ are subtle. We highlight the need to develop an increasingly rigorous analytical and interpretive framework for collecting and reporting data obtained from cognitive spectroscopy studies of this kind.

Frontal lobe γ-aminobutyric acid levels during adolescence: associations with impulsivity and response inhibition

BACKGROUND

The brain undergoes major remodeling during adolescence, resulting in improved cognitive control and decision-making and reduced impulsivity, components of behavior mediated in part by the maturing frontal lobe. γ-Aminobutyric acid (GABA), the main inhibitory neurotransmitter system, also matures during adolescence, with frontal lobe GABA receptors reaching adult levels late in adolescence. Thus, the objective of this study was to characterize in vivo developmental differences in brain GABA levels.

METHODS

Proton magnetic resonance spectroscopy was used at 4 T to acquire metabolite data from the anterior cingulate cortex (ACC) and the parieto-occipital cortex (POC) in adolescents (n=30) and emerging adults (n = 20).

RESULTS

ACC GABA/creatine (Cr) levels were significantly lower in adolescents relative to emerging adults, whereas no age differences were observed in the POC. Lower ACC GABA/Cr levels were significantly associated with greater impulsivity and worse response inhibition, with relationships being most pronounced for ACC GABA/Cr and No-Go response inhibition in adolescent males.

CONCLUSIONS

These data provide the first human developmental in vivo evidence confirming frontal lobe GABA maturation, which was linked to impulsiveness and cognitive control. These findings suggest that reduced GABA may be an important neurobiological mechanism in the immature adolescent brain, contributing to the reduced yet rapidly developing ability to inhibit risky behaviors and to make suboptimal decisions, which could compromise adolescent health and safety.

Accuracy of noninvasive quantification of brain NAA concentrations using PRESS sequence: verification in a swine model with external standard

The metabolite ratios had been employed in the field of MR spectroscopy (MRS) for a long period. The main drawback of metabolite ratio is that ratio results are not comparable with absolute metabolite concentration in vivo. The purpose of this study was to examine the accuracy of noninvasive quantification of brain N-acetylaspartate (NAA) concentrations using previously reported MR external standard method. Eight swine were scanned on a GE 1.5 T scanner with a standard head coil. The external standard method was utilized with a sphere filled with NAA, GABA, glutamine, glutamate, creatine, choline chloride, and myo-inositol. The position resolved spectroscopy (PRESS) sequence was used with TE=135 msec, TR=1500 msec, and 128 scan averages. The analysis of MRS was done with SAGE/IDL program. In vivo NAA concentration was obtained using the equation S=N * e(-TE/T₂) * [1-e(-TR/T₁). In vitro NAA concentration was measured by high performance liquid chromatography (HPLC). In the MRS group, the mean concentration of NAA was 10.03 plusmn 0.74 mmol/kg. In the HPLC group, the mean concentration of NAA was 9.22 plusmn 0.55 mmol/kg. There was no significant difference between the two groups (p = 0.46). However, slightly higher value was observed in the MRS group (7/8 swine), compared with HPLC group. The range of differences was between 0.02~2.05 mmol/kg. MRS external reference method could be more accurate than internal reference method. ¹H MRS does not distinguish between N-acetyl resonance frequencies and other N-acetylated amino acids.

Quantitative proton magnetic resonance spectroscopy and spectroscopic imaging of the brain: a didactic review

This article presents background information related to methodology for estimating brain metabolite concentration from magnetic resonance spectroscopy (MRS) and magnetic resonance spectroscopic imaging measurements of living human brain tissue. It reviews progress related to this methodology, with emphasis placed on progress reported during the past 10 years. It is written for a target audience composed of radiologists and magnetic resonance imaging technologists. It describes in general terms the relationship between MRS signal amplitude and concentration. It then presents an overview of the many practical problems associated with deriving concentration solely from absolute measured signal amplitudes and demonstrates how a various signal calibration approaches can be successfully used. The concept of integrated signal amplitude is presented with examples that are helpful for qualitative reading of MRS data as well as for understanding the methodology used for quantitative measurements. The problems associated with the accurate measurement of individual signal amplitudes in brain spectra having overlapping signals from other metabolites and overlapping nuisance signals from water and lipid are presented. Current approaches to obtaining accurate amplitude estimates with least-squares fitting software are summarized.

Quantitative multivoxel proton MR spectroscopy study of brain metabolites in patients with amnestic mild cognitive impairment: a pilot study

INTRODUCTION

The purpose of this study is to investigate brain metabolic changes in patients with amnestic mild cognitive impairment (aMCI) using multivoxel proton MR spectroscopy ((1)H-MVS).

METHODS

Fourteen aMCI patients and fifteen healthy control subjects participated in this experiment. All MR measurements were acquired using a 1.5-T GE scanner. (1)H-MVS point resolved spectroscopy (2D PROBE-CSI PRESS) pulse sequence (TE = 35 ms; TR = 1,500 ms; phase × frequency, 18 × 18) was used for acquiring MRS data. All data were post-processed using Spectroscopy Analysis by General Electric software and linear combination of model (LCModel). The absolute concentrations of N-acetylaspartate (NAA), choline (Cho), myoinositol (MI), creatine (Cr), and the metabolite ratios of NAA/Cr, Cho/Cr, MI/Cr, and NAA/MI were measured bilaterally in the posterior cingulate gyrus (PCG), inferior precuneus (Pr), paratrigonal white matter (PWM), dorsal thalamus (DT), and lentiform nucleus (LN).

RESULTS

Patients with aMCI displayed significantly lower NAA levels in the bilateral PCG (p < 0.01), PWM (p < 0.05), and left inferior Pr (p < 0.05). The metabolite ratio of NAA/MI was decreased in the bilateral PCG (p < 0.01) and PWM (p < 0.05) and in the left DT (p < 0.01). NAA/Cr was decreased in the left PCG (p < 0.01), DT (p < 0.05), right PWM (p < 0.05), and LN (p < 0.05). However, MI/Cr was elevated in the right PCG (p < 0.01) and left PWM (p < 0.05). Significantly increased Cho level was also evident in the left PWM (p < 0.05).

CONCLUSIONS

Our observations of decreased NAA, NAA/Cr, and NAA/MI, in parallel with increased Cho and MI/Cr might be characteristic of aMCI patients.

Preliminary evidence for white matter metabolite differences in marijuana-dependent young men using 2D J-resolved magnetic resonance spectroscopic imaging at 4 Tesla

Chronic marijuana (MRJ) use is associated with altered cognition and mood state, altered brain metabolites, and functional and structural brain changes. The objective of this study was to apply proton magnetic resonance spectroscopic imaging (MRSI) to compare proton metabolite levels in 15 young men with MRJ dependence and 11 healthy non-using (NU) young men. Spectra were acquired at 4.0 Tesla using 2D J-resolved MRSI to resolve coupled resonances in J-space and to quantify the entire J-coupled spectral surface of metabolites from voxels containing basal ganglia and thalamus, temporal and parietal lobes, and occipital white and gray matter. This method permitted investigation of high-quality spectra for regression analyses to examine metabolites relative to tissue type. Distribution of myo-inositol (mI)/creatine (Cr) was altered in the MRJ group whereas the NU group exhibited higher mI/Cr in WM than GM, this pattern was not observed in MRJ subjects. Significant relationships observed between global mI/Cr and distribution in WM, and self-reported impulsivity and mood symptoms were also unique between MRJ and NU groups. These preliminary findings suggest that mI, and distribution of this glial metabolite in WM, is altered by MRJ use and is associated with behavioral and affective features reported by young MRJ-dependent men.

Pitfalls and advantages of different strategies for the absolute quantification of N-acetyl aspartate, creatine and choline in white and grey matter by 1H-MRS

This study extensively investigates different strategies for the absolute quantitation of N-acetyl aspartate, creatine and choline in white and grey matter by (1)H-MRS at 1.5 T. The main focus of this study was to reliably estimate metabolite concentrations while reducing the scan time, which remains as one of the main problems in clinical MRS. Absolute quantitation was based on the water-unsuppressed concentration as the internal standard. We compared strategies based on various experimental protocols and post-processing strategies. Data were obtained from 30 control subjects using a PRESS sequence at several TE to estimate the transverse relaxation time, T(2), of the metabolites. Quantitation was performed with the algorithm QUEST using two different metabolite signal basis sets: a whole-metabolite basis set (WhoM) and a basis set in which the singlet signals were split from the coupled signals (MSM). The basis sets were simulated in vivo for each TE used. Metabolites' T(2)s were then determined by fitting the estimated signal amplitudes of the metabolites obtained at different TEs. Then the absolute concentrations (mM) of the metabolites were assessed for each subject using the estimated signal amplitudes and either the mean estimated relaxation times of all subjects (mean protocol, MP) or the T(2) estimated from the spectra derived from the same subject (individual protocol, IP). Results showed that MP represents a less time-consuming alternative to IP in the quantitation of brain metabolites by (1)H-MRS in both grey and white matter, with a comparable accuracy when performed by MSM. It was also shown that the acquisition time might be further reduced by using a variant of MP, although with reduced accuracy. In this variant, only one water-suppressed and one water-unsuppressed spectra were acquired, drastically reducing the duration of the entire MRS examination. However, statistical analysis highlights the reduced accuracy of MP when performed using WhoM, particularly at longer echo times.

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.

Mapping of brain metabolite distributions by volumetric proton MR spectroscopic imaging (MRSI)

Distributions of proton MR-detected metabolites have been mapped throughout the brain in a group of normal subjects using a volumetric MR spectroscopic imaging (MRSI) acquisition with an interleaved water reference. Data were processed with intensity and spatial normalization to enable voxel-based analysis methods to be applied across a group of subjects. Results demonstrate significant regional, tissue, and gender-dependent variations of brain metabolite concentrations, and variations of these distributions with normal aging. The greatest alteration of metabolites with age was observed for white-matter choline and creatine. An example of the utility of the normative metabolic reference information is then demonstrated for analysis of data acquired from a subject who suffered a traumatic brain injury. This study demonstrates the ability to obtain proton spectra from a wide region of the brain and to apply fully automated processing methods. The resultant data provide a normative reference for subsequent utilization for studies of brain injury and disease.

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).

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.

Limits on activation-induced temperature and metabolic changes in the human primary visual cortex

Changes in cerebral blood flow (CBF) and metabolism are now widely used to map and quantify neural activity, although the underlying mechanism for these changes is still incompletely understood. Magnetic resonance spectroscopy (MRS) at 3T, synchronized with a 32-s block design visual stimulation paradigm, was employed to investigate activation-induced changes in temperature and metabolism in the human primary visual cortex. A marginally significant increase in the local temperature of the visual cortex was found (0.1 degrees C, P = 0.09), excluding the possibility of a temperature decrease (95% confidence interval (CI) = 0.0-0.2 degrees C), which was previously suggested. A comparison with models of thermal equilibrium in the presence of blood flow suggests that an increase in heat production during activation, greater than or at least equal to that produced by the complete oxidative metabolism of the elevated glucose (Glc) utilization accompanying activation, would be required to offset the cooling effects of the increased blood flow. The total pools of glutamate (Glu), glutamine (Gln), myo-Inositol (mI), N-acetylaspartate (NAA), choline (Cho), and lactate (Lac) were not significantly affected by activation. Limits on Lac concentration changes were too weak to constrain theories of the metabolic use of elevated Glc consumption during stimulation, and emphasize the challenges of measuring even large Lac changes accompanying stimulation.

Use of tissue water as a concentration reference for proton spectroscopic imaging

A strategy for using tissue water as a concentration standard in (1)H magnetic resonance spectroscopic imaging studies on the brain is presented, and the potential errors that may arise when the method is used are examined. The sensitivity of the method to errors in estimates of the different water compartment relaxation times is shown to be small at short echo times (TEs). Using data from healthy human subjects, it is shown that different image segmentation approaches that are commonly used to account for partial volume effects (SPM2, FSL's FAST, and K-means) lead to different estimates of metabolite levels, particularly in gray matter (GM), owing primarily to variability in the estimates of the cerebrospinal fluid (CSF) fraction. While consistency does not necessarily validate a method, a multispectral segmentation approach using FAST yielded the lowest intersubject variability in the estimates of GM metabolites. The mean GM and white matter (WM) levels of N-acetyl groups (NAc, primarily N-acetylaspartate), choline (Ch), and creatine (Cr) obtained in these subjects using the described method with FAST multispectral segmentation are reported: GM [NAc] = 17.16 +/- 1.19 mM; WM [NAc] = 14.26 +/- 1.38 mM; GM [Ch] = 3.27 +/- 0.47 mM; WM [Ch] = 2.65 +/- 0.25 mM; GM [Cr] = 13.98 +/- 1.20 mM; and WM [Cr] = 7.10 +/- 0.67 mM.

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.

Single-voxel 1H PRESS at 4.0 T: precision and variability of measurements in anterior cingulate and hippocampus

The precision [coefficient of variation or CV (%) = 100SD/X] of single-voxel point resolved spectroscopic data was characterized bilaterally, in anterior cingulate and in hippocampus, at 4.0 T in a healthy subject. Data acquisition was replicated 10 times after voxel repositioning and readjusting higher order shims. Precision measurements show that the scan-to-scan precision is better in anterior cingulate than in hippocampus, with an average CV of 9.2% (for total NAA, tCho and Cr) in anterior cingulate and 13.9% in hippocampus. Variability measurements made by the same method in 24 healthy subjects and in 29 schizophrenia patients showed that there is substantial biological variability in metabolite levels, even in healthy subjects. Simple calculations suggest that more than 200 subjects would be needed to detect a 5% difference in NAA between patients and controls.

Increased glucose concentration in the hippocampus in early Alzheimer's disease following oral glucose ingestion

Glucose is the primary source of energy for brain cells. Because energy storage in the brain is limited, an uninterrupted supply of glucose and its rapid metabolism are essential for normal cognitive function. This study utilized an oral glucose load to examine hippocampal glucose metabolism in early Alzheimer's disease (AD) - a disease characterized by progressive deterioration of cognitive function and glucose hypometabolism. Short echo time 1H MR spectra (20 ms) from the right hippocampus of 8 patients with probable AD, 14 healthy elderly and 14 healthy young adults were compared pre- and post-glucose loading. In contrast to the healthy adults, the AD patients exhibited significantly elevated hippocampal glucose concentrations post-glucose ingestion relative to baseline (P < .01). These results suggest that cerebral glucose hypometabolism in AD leads to an increased steady-state concentration of cerebral glucose. This research demonstrates the feasibility of studying cerebral glucose metabolism in AD with 1H MR spectroscopy.

A phase rotation scheme for achieving very short echo times with localized stimulated echo spectroscopy

In a single-voxel stimulated echo localization sequence in magnetic resonance spectroscopy, magnetic field gradients are inserted within the echo time (TE) to filter signals generated through coherence pathways other than that leading to the stimulated echo. There is a significant penalty for these gradients as they increase the minimum TE, thereby leading to significant signal loss from spin-spin relaxation and phase distortions in coupled spin systems. Here, an RF phase rotation technique is described for a stimulated echo localization sequence that allows removal of the gradients in the TE intervals and, subsequently, reduction of the minimum TE to only 6 ms. Experiments carried out on six healthy volunteers on a 1.5-T whole-body MR system show a significant signal increase in the metabolite concentrations when measured with a 6-ms TE (N-acetyl-aspartate, 12%, P=.002; creatine, 15%, P=.04; and glutamate+glutamine, 92%, P=.02) compared to concentrations measured with data collected at TEs of 15 and 20 ms.

Increasing the speed of relaxometry-based compartmental analysis experiments in STEAM spectroscopy

In this work we present a method for improving the speed of spin-spin relaxation time (T2) measurements for compartmental analysis in stimulated echo localized magnetic resonance spectroscopy without reducing the sampling density. The technique uses a progressive repetition time (TR) to compensate for echo time (TE) dependent variations in saturation effects that would otherwise modulate the received signal at short TRs. The method was validated in T2 studies on 10 young healthy subjects in spectroscopic voxels localized along either the right or left Sylvian fissure (2 x 2 x 1.5 cm3, 10 ms mixing time (TM), 2048 data points, 819.2 ms acquisition time). The TR was automatically adjusted so that TR-TM-TE/2 was kept constant as the TE was incremented. Compared to long TR T2 experiments, the progressive TR technique consistently replicated the T2 relaxation times and reference signals of the tissue water compartment while reducing the data acquisition time by more than 50%. The percent error was on average less than 2% for estimates of T2 and S(0) for the tissue water, an indication that the progressive TR technique is a useful method for determining the tissue water signal for internal referencing.

Chemical shift magnetic resonance spectroscopy of cingulate grey matter in patients with minimal hepatic encephalopathy

Minimal hepatic encephalopathy (MHE) is frequently diagnosed in patients with liver cirrhosis who do not show overt clinical cirrhosis-associated neurological deficits. This condition manifests primarily with visuo-motor and attention deficits. We studied the association between visuo-motor deficits and magnetic resonance spectroscopic parameters in cingulate grey matter and white matter of centrum semiovale in patients with liver cirrhosis. The data revealed an increase in the glutamate-glutamine/creatine ratio and a decrease in choline/creatine and inositol/creatine ratios in patients with liver cirrhosis. The analysis of the data showed that cirrhosis-associated deterioration of the visuo-motor function significantly correlates with a decrease in the choline/creatine ratio and an increase in N-acetylaspartate/choline in cingulate grey matter but not in the neighbouring white matter. Furthermore, the increase in the glutamate-glutamine/creatine ratio correlated significantly with the increase in the N-acetylaspartate/creatine ratio. These data suggest an association between altered choline, glutamate-glutamine and NAA metabolism in cingulate grey matter and symptoms of MHE, and underline the importance of differentiation between grey and white matter in magnetic resonance spectroscopic studies on patients with cirrhosis-associated brain dysfunction.