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

Quantification of the neurochemical profile of the human putamen using STEAM MRS in a cohort of elderly subjects at 3 T and 7 T: Ruminations on the correction strategy for the tissue voxel composition

The aim of this work is to quantify the metabolic profile of the human putamen in vivo in a cohort of elderly subjects using single-voxel proton magnetic resonance spectroscopy. To obtain metabolite concentrations specific to the putamen, we investigated a correction method previously proposed to account for the tissue composition of the volume of interest. We compared the method with the conventional approach, which a priori assumes equal metabolite concentrations in GM and WM. Finally, we compared the concentrations acquired at 3 Tesla (T) and 7 T MRI scanners. Spectra were acquired from 15 subjects (age: 67.7 ± 8.3 years) at 3 T and 7 T, using an ultra-short echo time, stimulated echo acquisition mode sequence. To robustly estimate the WM-to-GM metabolite concentration ratio, five additional subjects were measured for whom the MRS voxel was deliberately shifted from the putamen in order to increase the covered amount of surrounding WM. The concentration and WM-to-GM concentration ratio for 16 metabolites were reliably estimated. These ratios ranged from ~0.3 for γ-aminobutyric acid to ~4 for N-acetylaspartylglutamate. The investigated correction method led to significant changes in concentrations compared to the conventional method, provided that the ratio significantly differed from unity. Finally, we demonstrated that differences in tissue voxel composition cannot fully account for the observed concentration difference between field strengths. We provide not only a fully comprehensive quantification of the neurochemical profile of the putamen in elderly subjects, but also a quantification of the WM-to-GM concentration ratio. This knowledge may serve as a basis for future studies with varying tissue voxel composition, either due to tissue atrophy, inconsistent voxel positioning or simply when pooling data from different voxel locations.

Harmonization of multi-scanner in vivo magnetic resonance spectroscopy: ENIGMA consortium task group considerations

Magnetic resonance spectroscopy is a powerful, non-invasive, quantitative imaging technique that allows for the measurement of brain metabolites that has demonstrated utility in diagnosing and characterizing a broad range of neurological diseases. Its impact, however, has been limited due to small sample sizes and methodological variability in addition to intrinsic limitations of the method itself such as its sensitivity to motion. The lack of standardization from a data acquisition and data processing perspective makes it difficult to pool multiple studies and/or conduct multisite studies that are necessary for supporting clinically relevant findings. Based on the experience of the ENIGMA MRS work group and a review of the literature, this manuscript provides an overview of the current state of MRS data harmonization. Key factors that need to be taken into consideration when conducting both retrospective and prospective studies are described. These include (1) MRS acquisition issues such as pulse sequence, RF and B0 calibrations, echo time, and SNR; (2) data processing issues such as pre-processing steps, modeling, and quantitation; and (3) biological factors such as voxel location, age, sex, and pathology. Various approaches to MRS data harmonization are then described including meta-analysis, mega-analysis, linear modeling, ComBat and artificial intelligence approaches. The goal is to provide both novice and experienced readers with the necessary knowledge for conducting MRS data harmonization studies.

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

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

Advanced magnetic resonance spectroscopic neuroimaging: Experts' consensus recommendations

Magnetic resonance spectroscopic imaging (MRSI) offers considerable promise for monitoring metabolic alterations associated with disease or injury; however, to date, these methods have not had a significant impact on clinical care, and their use remains largely confined to the research community and a limited number of clinical sites. The MRSI methods currently implemented on clinical MRI instruments have remained essentially unchanged for two decades, with only incremental improvements in sequence implementation. During this time, a number of technological developments have taken place that have already greatly benefited the quality of MRSI measurements within the research community and which promise to bring advanced MRSI studies to the point where the technique becomes a true imaging modality, while making the traditional review of individual spectra a secondary requirement. Furthermore, the increasing use of biomedical MR spectroscopy studies has indicated clinical areas where advanced MRSI methods can provide valuable information for clinical care. In light of this rapidly changing technological environment and growing understanding of the value of MRSI studies for biomedical studies, this article presents a consensus from a group of experts in the field that reviews the state-of-the-art for clinical proton MRSI studies of the human brain, recommends minimal standards for further development of vendor-provided MRSI implementations, and identifies areas which need further technical development.

Quantitative MRI using STrategically Acquired Gradient Echo (STAGE): optimization for 1.5 T scanners and T1 relaxation map validation

OBJECTIVES

The strategically acquired gradient echo (STAGE) protocol, developed for 3T scanners, allows one to derive quantitative maps such as T1, T2*, proton density, and quantitative susceptibility mapping in about 5 min. Our aim was to adapt the STAGE sequences for 1.5T scanners which are still commonly used in clinical practice. Furthermore, the accuracy and repeatability of the STAGE-derived T1 estimate were tested.

METHODS

Flip angle (FA) optimization was performed using a theoretical simulation by maximizing signal-to-noise ratio, contrast-to-noise ratio, and T1 precision. The FA choice was further refined with the ISMRM/NIST phantom and in vivo acquisitions. The accuracy of the T1 estimate was assessed by comparing STAGE-derived T1 values with T1 maps obtained with an inversion recovery sequence. T1 accuracy was investigated for both the phantom and in vivo data. Finally, one subject was acquired 10 times once a week and a group of 27 subjects was scanned once. The T1 coefficient of variation (COV) was computed to assess scan-rescan and physiological variability, respectively.

RESULTS

The FA_1,2 = 7°,38° were identified as the optimal FA pair at 1.5T. The T1 estimate errors were below 3% and 5% for phantom and in vivo measurements, respectively. COV for different tissues ranged from 1.8 to 4.8% for physiological variability, and between 0.8 and 2% for scan-rescan repeatability.

CONCLUSION

The optimized STAGE protocol can provide accurate and repeatable T1 mapping along with other qualitative images and quantitative maps in about 7 min on 1.5T scanners. This study provides the groundwork to assess the role of STAGE in clinical settings.

KEY POINTS

• The STAGE imaging protocol was optimized for use on 1.5T field strength scanners. • A practical STAGE protocol makes it possible to derive quantitative maps (i.e., T1, T2*, PD, and QSM) in about 7 min at 1.5T. • The T1 estimate derived from the STAGE protocol showed good accuracy and repeatability.

Sleep quality is related to brain glutamate and symptom severity in schizophrenia

Up to 80% of patients with schizophrenia experience sleep disturbances, which negatively impact daytime functioning. Given that the glutamatergic system is involved in the pathophysiology of schizophrenia as well as normal sleep-wake neurobiology, the current project aimed to determine whether sleep quality was related to brain glutamate levels in schizophrenia. The Pittsburgh Sleep Quality Index (PSQI) was used to assess subjective sleep quality and proton magnetic resonance spectroscopy (MRS) was used to quantify glutamate in the bilateral anterior cingulate, left parietal cortex, and left hippocampus. Results indicate that global PSQI scores were negatively correlated with the anterior cingulate and parietal glutamate levels. In patients with schizophrenia, poorer sleep quality correlated with greater positive symptom severity. Our findings suggest that poor sleep quality is related to greater positive symptom severity and lower levels of anterior cingulate glutamate in individuals with schizophrenia. Interventions to enhance sleep quality may prove beneficial for patients. Future studies will examine whether glutamate relates to objective measures of sleep quality, and whether glutamate may mediate the relationship between sleep quality and symptom severity across the schizophrenia-spectrum.

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.

Reduction of variance in measurements of average metabolite concentration in anatomically-defined brain regions

Multiple methods have been proposed for using Magnetic Resonance Spectroscopy Imaging (MRSI) to measure representative metabolite concentrations of anatomically-defined brain regions. Generally these methods require spectral analysis, quantitation of the signal, and reconciliation with anatomical brain regions. However, to simplify processing pipelines, it is practical to only include those corrections that significantly improve data quality. Of particular importance for cross-sectional studies is knowledge about how much each correction lowers the inter-subject variance of the measurement, thereby increasing statistical power. Here we use a data set of 72 subjects to calculate the reduction in inter-subject variance produced by several corrections that are commonly used to process MRSI data. Our results demonstrate that significant reductions of variance can be achieved by performing water scaling, accounting for tissue type, and integrating MRSI data over anatomical regions rather than simply assigning MRSI voxels with anatomical region labels.

Use of quantitative brain water imaging as concentration reference for J-edited MR spectroscopy of GABA

PURPOSE

To compare two different methods of obtaining the water reference for determination of quantitative water-scaled in vivo concentration estimates of γ-aminobutyric acid (GABA).

METHODS

Water-scaled GABA estimates from localized J-difference edited MR spectroscopy experiments can be computed using standard values for tissue-specific water content and relaxation times. Water content and relaxation may, however, be altered in pathology. This work re-analyzed data from a recent study in healthy controls and patients with minimal (mHE) or grade I (HE 1) hepatic encephalopathy, a disease associated with slight elevation of brain water content. J-difference edited MR spectroscopy data were combined with quantitative brain water measures, which provided individual water density references and T1 relaxation times. Resulting GABA estimates were compared to concentration values obtained using standard tissue-specific water content and relaxation values.

RESULTS

Occipital GABA concentration values obtained from individual water and T1 maps were 1.64±0.35mM in controls, and significantly higher (P<0.01) than in mHE (1.15±0.28mM) and HE 1 patients (1.18±0.09mM). Results from the tissue-dependent approach (1.58±0.30mM (controls), 1.10±0.27mM (mHE) and 1.12±0.12mM (HE 1)) were slightly lower (P<0.05 in each group).

CONCLUSION

Water-scaled in vivo GABA estimates can be obtained with individual water density and T1 relaxation mapping. This approach may be useful for studying GABA levels in pathologies with substantial brain water content or relaxation changes.

Reduced GABAergic Action in the Autistic Brain

An imbalance between excitatory/inhibitory neurotransmission has been posited as a central characteristic of the neurobiology of autism [1], inspired in part by the striking prevalence of seizures among individuals with the disorder [2]. Evidence supporting this hypothesis has specifically implicated the signaling pathway of the inhibitory neurotransmitter, γ-aminobutyric acid (GABA), in this putative imbalance: GABA receptor genes have been associated with autism in linkage and copy number variation studies [3-7], fewer GABA receptor subunits have been observed in the post-mortem tissue of autistic individuals [8, 9], and GABAergic signaling is disrupted across heterogeneous mouse models of autism [10]. Yet, empirical evidence supporting this hypothesis in humans is lacking, leaving a gulf between animal and human studies of the condition. Here, we present a direct link between GABA signaling and autistic perceptual symptomatology. We first demonstrate a robust, replicated autistic deficit in binocular rivalry [11], a basic visual function that is thought to rely on the balance of excitation/inhibition in visual cortex [12-15]. Then, using magnetic resonance spectroscopy, we demonstrate a tight linkage between binocular rivalry dynamics in typical participants and both GABA and glutamate levels in the visual cortex. Finally, we show that the link between GABA and binocular rivalry dynamics is completely and specifically absent in autism. These results suggest a disruption in inhibitory signaling in the autistic brain and forge a translational path between animal and human models of the condition.

Responses of the Human Brain to Mild Dehydration and Rehydration Explored In Vivo by 1H-MR Imaging and Spectroscopy

BACKGROUND AND PURPOSE

As yet, there are no in vivo data on tissue water changes and associated morphometric changes involved in the osmo-adaptation of normal brains. Our aim was to evaluate osmoadaptive responses of the healthy human brain to osmotic challenges of de- and rehydration by serial measurements of brain volume, tissue fluid, and metabolites.

MATERIALS AND METHODS

Serial T1-weighted and (1)H-MR spectroscopy data were acquired in 15 healthy individuals at normohydration, on 12 hours of dehydration, and during 1 hour of oral rehydration. Osmotic challenges were monitored by serum measures, including osmolality and hematocrit. MR imaging data were analyzed by using FreeSurfer and LCModel.

RESULTS

On dehydration, serum osmolality increased by 0.67% and brain tissue fluid decreased by 1.63%, on average. MR imaging morphometry demonstrated corresponding decreases of cortical thickness and volumes of the whole brain, cortex, white matter, and hypothalamus/thalamus. These changes reversed during rehydration. Continuous fluid ingestion of 1 L of water for 1 hour within the scanner lowered serum osmolality by 0.96% and increased brain tissue fluid by 0.43%, on average. Concomitantly, cortical thickness and volumes of the whole brain, cortex, white matter, and hypothalamus/thalamus increased. Changes in brain tissue fluid were related to volume changes of the whole brain, the white matter, and hypothalamus/thalamus. Only volume changes of the hypothalamus/thalamus significantly correlated with serum osmolality.

CONCLUSIONS

This is the first study simultaneously evaluating changes in brain tissue fluid, metabolites, volume, and cortical thickness. Our results reflect cellular volume regulatory mechanisms at a macroscopic level and emphasize that it is essential to control for hydration levels in studies on brain morphometry and metabolism in order to avoid confounding the findings.

Reproducibility of phase rotation stimulated echo acquisition mode at 3T in schizophrenia: Emphasis on glutamine

PURPOSE

To determine the reproducibility and reliability of glutamine (Gln), measured with a very short echo time phase rotation stimulated echo acquisition mode (VTE-PR STEAM) sequence at 3T, in subjects with schizophrenia.

METHODS

Seven subjects with schizophrenia were scanned twice with VTE-PR STEAM in a Siemens 3T TIM Trio scanner with a 32-channel head coil. Spectroscopic data were collected from two voxels in gray matter, one in the dorsal anterior cingulate and the other in the medial occipital cortex. Reproducibility was assessed using coefficients of variation (CVs) and reliability with standard error of measurement and intraclass correlations (ICCs). Phantoms containing increasing concentrations of Gln in a physiologic solution of other neurometabolites with overlapping resonances were scanned to assess the validity of spectral Gln measurement.

RESULTS

Very good reliability and reproducibility for Gln in both regions of interest were supported by CVs of ≤10.0% and ICCs of ≥0.6, respectively. Phantom studies documented a robust correspondence between known Gln concentrations and VTE-PR STEAM measurements of this metabolite (R(2)  = 0.988).

CONCLUSION

The VTE-PR STEAM approach at 3T permits the longitudinal assessment of Gln and other (1) H MR spectroscopy neurometabolites in a clinically plausible setting.

Whole-brain quantitative mapping of metabolites using short echo three-dimensional proton MRSI

BACKGROUND

To improve the extent over which whole brain quantitative three-dimensional (3D) magnetic resonance spectroscopic imaging (MRSI) maps can be obtained and be used to explore brain metabolism in a population of healthy volunteers.

METHODS

Two short echo time (20 ms) acquisitions of 3D echo planar spectroscopic imaging at two orientations, one in the anterior commissure-posterior commissure (AC-PC) plane and the second tilted in the AC-PC +15° plane were obtained at 3 Tesla in a group of 10 healthy volunteers. B1 (+) , B1 (-) , and B0 correction procedures and normalization of metabolite signals with quantitative water proton density measurements were performed. A combination of the two spatially normalized 3D-MRSI, using a weighted mean based on the pixel wise standard deviation metabolic maps of each orientation obtained from the whole group, provided metabolite maps for each subject allowing regional metabolic profiles of all parcels of the automated anatomical labeling (AAL) atlas to be obtained.

RESULTS

The combined metabolite maps derived from the two acquisitions reduced the regional intersubject variance. The numbers of AAL regions showing N-acetyl aspartate (NAA) SD/Mean ratios lower than 30% increased from 17 in the AC-PC orientation and 41 in the AC-PC+15° orientation, to a value of 76 regions of 116 for the combined NAA maps. Quantitatively, regional differences in absolute metabolite concentrations (mM) over the whole brain were depicted such as in the GM of frontal lobes (cNAA = 10.03 + 1.71; cCho = 1.78 ± 0.55; cCr = 7.29 ± 1.69; cmIns = 5.30 ± 2.67) and in cerebellum (cNAA = 5.28 ± 1.77; cCho = 1.60 ± 0.41; cCr = 6.95 ± 2.15; cmIns = 3.60 ± 0.74).

CONCLUSION

A double-angulation acquisition enables improved metabolic characterization over a wide volume of the brain.

Multivendor implementation and comparison of volumetric whole-brain echo-planar MR spectroscopic imaging

PURPOSE

To assess volumetric proton MR spectroscopic imaging (MRSI) of the human brain on multivendor MRI instruments.

METHODS

Echo-planar spectroscopic imaging was developed on instruments from three manufacturers, with matched specifications and acquisition protocols that accounted for differences in sampling performance, radiofrequency (RF) power, and data formats. Intersite reproducibility was evaluated for signal-normalized maps of N-acetylaspartate (NAA), creatine (Cre), and choline using phantom and human subject measurements. Comparative analyses included metrics for spectral quality, spatial coverage, and mean values in atlas-registered brain regions.

RESULTS

Intersite differences for phantom measurements were less than 1.7% for individual metabolites and less than 0.2% for ratio measurements. Spatial uniformity ranged from 79% to 91%. The human studies found differences of mean values in the temporal lobe, but good agreement in other white matter regions, with maximum differences relative to their mean of under 3.2%. For NAA/Cre, the maximum difference was 1.8%. In gray matter, a significant difference was observed for frontal lobe NAA. Primary causes of intersite differences were attributed to shim quality, B0 drift, and accuracy of RF excitation. Correlation coefficients for measurements at each site were over 0.60, indicating good reliability.

CONCLUSION

A volumetric intensity-normalized MRSI acquisition can be implemented in a comparable manner across multivendor MR instruments.

Fast water concentration mapping to normalize (1)H MR spectroscopic imaging

OBJECT

To propose a fast and robust acquisition and post-processing pipeline that is time-compatible with clinical explorations to obtain a proton density (ρ) map used as a reference for metabolic map normalization. This allows inter-subject and inter-group comparisons of magnetic resonance spectroscopic imaging (MRSI) data and longitudinal follow-up for single subjects.

MATERIALS AND METHODS

A multi-echo T 2 (*) mapping sequence, the XEP sequence for B 1 (+) -mapping and Driven Equilibrium Single Pulse Observation of T 1-an optimized variable flip angle method for T 1 mapping used for both B 1 (-) -mapping and M 0 calculation-were used to determine correction factors leading to quantitative water proton density maps at 3T. Normalized metabolite maps were obtained on a phantom and nine healthy volunteers. To show the potential use of this technique at the individual level, we also explored one patient with low-grade glioma.

RESULTS

Accurate ρ maps were obtained both on phantom and volunteers. After signal normalization with the generated ρ maps, metabolic concentrations determined by the present method differed from theory by <7 % in the phantom and were in agreement with data from the literature for the healthy controls. Using these normalized metabolic values, it was possible to demonstrate in the patient with brain glioma, metabolic abnormalities in normalized N-acetyl aspartate, choline and creatine levels; illustrating the potential for direct use of this technique in clinical studies.

CONCLUSION

The proposed combination of sequences provides a robust ρ map that can be used to normalize metabolic maps in clinical MRSI studies.

Preliminary study of the association of white-matter metabolite concentrations with disease severity in patients with Huntington's disease

Proton magnetic resonance spectroscopy is used to measure several metabolites in cortical gray and white matter in patients with Huntington's disease. The preliminary results show that CAG-repeat length correlates with white-matter N-acetylaspartate concentrations, and disease severity correlates with several metabolites.

Fast and high-resolution quantitative mapping of tissue water content with full brain coverage for clinically-driven studies

An efficient method for obtaining longitudinal relaxation time (T1) maps is based on acquiring two spoiled gradient recalled echo (SPGR) images in steady states with different flip angles, which has also been extended, with additional acquisitions, to obtain a tissue water content (M0) map. Several factors, including inhomogeneities of the radio-frequency (RF) fields and low signal-to-noise ratios may negatively affect the accuracy of this method and produce systematic errors in T1 and M0 estimations. Thus far, these limitations have been addressed by using additional measurements and applying suitable corrections; however, the concomitant increase in scan time is undesirable for clinical studies. In this note, a modified dual-acquisition SPGR method based on an optimization of the sequence formulism is presented for good and reliable M0 mapping with an isotropic spatial resolution of 1×1×1mm(3) that covers the entire human brain in 6:30min. A combined RF transmit/receive map is estimated from one of the SPGR scans and the optimal flip angles for M0 map are found analytically. The method was successfully evaluated in eight healthy subjects producing mean M0 values of 69.8% (in white matter) and 80.1% (in gray matter) that are in good agreement with those found in the literature and with high reproducibility. The mean value of the resultant voxel-based coefficients-of-variation was 3.6%.

Optimization of scan parameters for T₁-FLAIR imaging at 1.5 and 3T using computer simulation

PURPOSE

We attempted to optimize scan parameters for T₁-weighted fluid-attenuated inversion recovery (T₁-FLAIR) sequence at 3 and 1.5 tesla (T) using computer simulation.

METHODS

We measured the T₁ and T₂ relaxation time values (T₁v and T₂v) of gray (GM) and white matter (WM) at 3 and 1.5T, generated computer-simulated T₁-FLAIR (CS-T₁-FLAIR) images using those values, and compared the simulated and actual T₁-FLAIR images to verify the contrast reliability of our computer simulation. We mathematically and visually evaluated CS-T₁-FLAIR images at various repetition times (TR) and echo times (TE).

RESULTS

At 3T, the measured relaxation values for GM were T₁v, 1524 ms, and T₂v, 85 ms, and for WM, T₁v, 750 ms, and T₂v, 65 ms. At 1.5T, the measured relaxation values for GM were T₁v, 1251 ms, and T₂v, 99 ms, and for WM, T₁v, 623 ms, and T₂v, 75 ms. Contrast of CS-T₁-FLAIR and actual T₁-FLAIR images was identical. An optimal TR of 3140 ms was determined for T₁-FLAIR at 3T and 2440 ms at 1.5T based on mathematical evaluation. The optimal TR ranges were 2400 to 3900 ms at 3T and 1800 to 3200 ms at 1.5T based on visual assessment of CS-T₁-FLAIR. A shorter TE provided better T₁ contrast.

CONCLUSION

We optimized T₁-FLAIR by focusing on its most important scan parameters using computer simulations and determined that a longer TR was suitable at 3T than at 1.5T. Our computer simulation was useful for determining the optimal scan parameters.

Comparison of human brain metabolite levels using 1H MRS at 1.5T and 3.0T

Proton magnetic resonance spectroscopy (MRS) of the human brain has proven to be a useful technique in several neurological and psychiatric disorders and benefits from higher field scanners as signal intensity and spectral resolution are proportional to the magnetic field strength.

Computerized MRS voxel registration and partial volume effects in single voxel 1H-MRS

Partial volume effects in proton magnetic resonance spectroscopy in the brain have been studied previously in terms of proper water concentration calculations, but there is a lack of disclosure in terms of voxel placement techniques that would affect the calculations. The purpose of this study is to facilitate a fully automated MRS voxel registration method which is time efficient, accurate, and can be extended to all imaging modalities. A total of thirteen healthy adults underwent single voxel 1H-MRS scans in 3.0T MRI scanners. Transposition of a MRS voxel onto an anatomical scan is derived along with a full calculation of water concentration with a correction term to account for the partial volume effects. Five metabolites (tNAA, Glx, tCr, mI, and tCho) known to yield high reliability are studied. Pearson's correlation analyses between tissue volume fractions and metabolite concentrations were statistically significant in parietal (tCr, Glx, and tNAA) lobe and occipital lobe (tNAA). MRS voxel overlaps quantified by dice metric over repeated visits yielded 60%~70% and coefficients of variance in metabolites concentration were 4%~10%. These findings reiterate an importance of considering the partial volume effects when tissue water is used as an internal concentration reference so as to avoid misinterpreting a morphometric difference as a metabolic difference.

Quantitative proton density mapping: correcting the receiver sensitivity bias via pseudo proton densities

Most methods for mapping proton densities (PD) in brain tissue are based on measuring all parameters influencing the signal intensity with subsequent elimination of any weighting not related to PD. This requires knowledge of the receiver coil sensitivity profile (RP), the measurement of which can be problematic. Recently, a method for compensating the influence of RP non-uniformities on PD data at a field strength of 3T was proposed, based on bias field correction of spoiled gradient echo image data to remove the low spatial frequency bias imposed by RP variations from uncorrected PD maps. The purpose of the current study was to present and test an independent method, based on the well-known linear relationship between the longitudinal relaxation rate R1 and 1/PD in brain tissue. For healthy subjects, RP maps obtained with this method and the resulting PD maps are very similar to maps based on bias field correction, and quantitative PD values acquired with the new independent method are in very good agreement with literature values. Furthermore, both methods for PD mapping are compared in the presence of several pathologies (multiple sclerosis, stroke, meningioma, recurrent glioblastoma).

Test-retest reliability and reproducibility of short-echo-time spectroscopic imaging of human brain at 3T

A 1H magnetic resonance spectroscopic imaging study at 3T and short echo time was conducted to evaluate both the reproducibility, as measured by the interscan coefficient of variation (CV), and test-retest reliability, as measured by the intraclass correlation coefficient (ICC), of measurements of glutamate (Glu), combined glutamate and glutamine (Glx), myo-inositol (mI), N-acetylaspartate, creatine, and choline in 21 healthy subjects. The effect of partial volume correction on these measures and the relationship of reproducibility and reliability to data quality were also examined. A 1H magnetic resonance spectroscopic imaging slice was prescribed above the lateral ventricles and single repeat scans were performed within 30 min to minimize physiologic variability. Interscan CVs based on all the voxels varied from 0.05 to 0.07 for N-acetylaspartate, creatine, and choline to 0.10-0.13 for mI, Glu, and Glx. Findings on the reproducibility of gray and white matter estimates of N-acetylaspartate, creatine, and choline are consistent with previous studies using longer echo times, with CVs in the range of 0.02-0.04 and ICC in the range of 0.65-0.90. CVs for Glu, Glx, and mI are much lower than reported in previous studies at 1.5 T, while white matter mI (CV=0.04, ICC=0.93) and gray matter Glx (CV=0.04, ICC=0.68) demonstrated both high reproducibility and test-retest reliability.