Single-voxel proton MRS of the human brain at 1.5T and 3.0T

Magnetic resonance spectroscopic imaging of benign prostatic tissue: findings at 3.0 T compared to 1.5 T-initial experience

In a retrospective study of 71 voxels of benign peripheral zone tissue from 3 men who underwent endorectal magnetic resonance (MR) spectroscopic imaging of the prostate at both 1.5 and 3 T, 21 voxels that appeared more malignant at 3 T to either of two readers demonstrated significantly higher levels of choline and polyamines at 3 T compared to 1.5 T using a Wilcoxon ranked-sum test; awareness of this selective amplification of these metabolic signals at high field strength may help avoid overdiagnosis of prostate cancer.

Measuring T2 in vivo with J-difference editing: application to GABA at 3 Tesla

PURPOSE

To develop an experimental approach for determining in vivo transverse relaxation rates (T(2)) of metabolites that are detected by spectral editing without using simulations, and to demonstrate this approach to measure the T(2) of γ-aminobutyric acid (GABA).

MATERIALS AND METHODS

The proposed method first determines the TE-dependence of the edited signals using measurements in a pure phantom solution (10 mM γ-aminobutyric acid; GABA); the phantom T(2) is also determined. Once the editing echo time (TE)-modulation pattern is known, it can then be used to determine T(2) in vivo. The method was applied to measure GABA T(2) in the occipital lobe of five healthy adult subjects at 3T, using a J-difference editing method. Unwanted macromolecular contributions to the GABA signal were also measured.

RESULTS

The in vivo T(2) of edited GABA signal was 88 ± 12 ms; this preliminary result is somewhat shorter than other metabolite T(2) values in the literature at this field strength.

CONCLUSION

Spectral editing methods are now widely used to detect low concentration metabolites, such as GABA, but to date no edited acquisition methods have been proposed for the measurement of transverse relaxation times (T(2)). The method described has been successfully applied to measuring the T(2) of GABA.

Magnetic resonance spectroscopy study of the antioxidant defense system in schizophrenia

Accumulating evidence suggests that oxidative stress associated with impaired metabolism of the antioxidant glutathione (GSH) plays a key role in the pathophysiology of schizophrenia. Magnetic resonance spectroscopy (MRS) is one of the brain-imaging techniques that can quantitatively measure bioactive substances such as GSH in the intact human brain. Four different measurement sequences including double quantum coherence (DQC) filtering, MEscher-GArwood Point-RESolved Spectroscopy (MEGA-PRESS), Stimulated Echo Acquisition Mode (STEAM), and PRESS have been used to evaluate the (1)H-MRS measurement of GSH in the brains of patients with schizophrenia. Although the results of these studies were somewhat diverse, a negative correlation between brain GSH levels and the severity of negative symptoms in schizophrenia patients suggests that increasing the brain GSH levels might be beneficial for schizophrenia patients with negative symptoms. Moreover, a recent double-blind, placebo-controlled study demonstrated that add-on of N-acetyl-l-cysteine (NAC), a precursor of GSH, to antipsychotics improved the negative symptoms and reduced the side effects (akathisia) in patients with chronic schizophrenia. MRS study of the antioxidant defense system in schizophrenia still remains in the infantile stage; future studies are needed to examine the brain GSH level before and after NAC treatment, and thereby to provide direct evidence of the induced production of GSH in the living brain.

Advancements in MR imaging of the prostate: from diagnosis to interventions

Prostate cancer is the most frequently diagnosed cancer in males and the second leading cause of cancer-related death in men. Assessment of prostate cancer can be divided into detection, localization, and staging; accurate assessment is a prerequisite for optimal clinical management and therapy selection. Magnetic resonance (MR) imaging has been shown to be of particular help in localization and staging of prostate cancer. Traditional prostate MR imaging has been based on morphologic imaging with standard T1-weighted and T2-weighted sequences, which has limited accuracy. Recent advances include additional functional and physiologic MR imaging techniques (diffusion-weighted imaging, MR spectroscopy, and perfusion imaging), which allow extension of the obtainable information beyond anatomic assessment. Multiparametric MR imaging provides the highest accuracy in diagnosis and staging of prostate cancer. In addition, improvements in MR imaging hardware and software (3-T vs 1.5-T imaging) continue to improve spatial and temporal resolution and the signal-to-noise ratio of MR imaging examinations. Another recent advancement in the field is MR imaging guidance for targeted prostate biopsy, which is an alternative to the current standard of transrectal ultrasonography-guided systematic biopsy.

Cerebral magnetic resonance spectroscopy at 7 Tesla: standard values and regional differences

RATIONALE AND OBJECTIVES

At lower magnetic field strengths, regional differences of cerebral metabolite distributions have been described, but these data are controversial. Magnetic resonance spectroscopy at 7 T is expected to deliver high spectral resolution and good differentiation, but there are problems arising at high magnetic field strengths that may diminish spectral quality. Because there have been only a few studies in humans so far, there are no standard values for 7 T concerning regional metabolite distributions and concentrations.

MATERIALS AND METHODS

In the present study, the metabolites detectable with (1)H magnetic resonance spectroscopy, N-acetyl-aspartate, choline, and creatine (Cr), were evaluated with a single-voxel sequence. Five voxels were placed in the frontal and parietal white matter and the insular, thalamic, and occipital gray matter.

RESULTS

For N-acetyl-aspartate, the lowest values were found in frontal white matter and the highest in thalamic gray matter. Choline displayed the lowest values in frontal white matter and the highest in insular gray matter. Cr showed the lowest values in frontal white matter and the highest in thalamic gray matter. The highest ratio of choline to Cr was found in parietal white matter and the lowest in thalamic gray matter. The highest ratio of N-acetyl-aspartate to Cr was found in thalamic gray matter and the lowest in frontal white matter.

CONCLUSIONS

In the present study, regional cerebral metabolite differences were verified with high-field magnetic resonance spectroscopy. Quantitative values and metabolite ratios could be a basis for further clinical studies.

Applications of multi-nuclear magnetic resonance spectroscopy at 7T

AIM: To discuss the advantages of ultra-high field (7T) for ¹H and ¹³C magnetic resonance spectroscopy (MRS) studies of metabolism.

METHODS: Measurements of brain metabolites were made at both 3 and 7T using ¹H MRS. Measurements of glycogen and lipids in muscle were measured using ¹³C and ¹H MRS respectively.

RESULTS: In the brain, increased signal-to-noise ratio (SNR) and dispersion allows spectral separation of the amino-acids glutamate, glutamine and γ-aminobutyric acid (GABA), without the need for sophisticated editing sequences. Improved quantification of these metabolites is demonstrated at 7T relative to 3T. SNR was 36% higher, and measurement repeatability (% coefficients of variation) was 4%, 10% and 10% at 7T, vs 8%, 29% and 21% at 3T for glutamate, glutamine and GABA respectively. Measurements at 7T were used to compare metabolite levels in the anterior cingulate cortex (ACC) and insula. Creatine and glutamate levels were found to be significantly higher in the insula compared to the ACC (P < 0.05). In muscle, the increased SNR and spectral resolution at 7T enables interleaved studies of glycogen (¹³C) and intra-myocellular lipid (IMCL) and extra-myocellular lipid (EMCL) (¹H) following exercise and re-feeding. Glycogen levels were significantly decreased following exercise (-28% at 50% VO₂ max; -58% at 75% VO₂ max). Interestingly, levels of glycogen in the hamstrings followed those in the quadriceps, despite reduce exercise loading. No changes in IMCL and EMCL were found in the study.

CONCLUSION: The demonstrated improvements in brain and muscle MRS measurements at 7T will increase the potential for use in investigating human metabolism and changes due to pathologies.

A comparison of single-voxel clinical in vivo hepatic 31P MR spectra acquired at 1.5 and 3.0 Tesla in health and diseased states

With the increasing availability of human MR scanners at various field strengths, the optimal field strength for in vivo clinical MR studies of the liver has become a focus of attention. Comparison between results at 3.0 and 1.5 T is of particular clinical interest, especially for multicentre studies. For MRS studies, higher field strengths should be advantageous, because improved sensitivity and chemical shift dispersion are expected. We report a comparison between single-voxel hepatic proton-decoupled (31)P MRS performed at 1.5 and 3.0 T in the same subjects using similar methodologies. Twelve healthy volunteers and 15 patients with chronic liver disease were studied. Improved spectral resolution was achieved using proton decoupling, and there was an improvement (21%) in the signal-to-noise ratio (SNR) of the phosphomonoester (PME) resonance at 3.0 T relative to 1.5 T. There was no significant change in nuclear Overhauser effects for PME or phosphodiesters (PDEs) between the two field strengths. The T(1) value of PDE was significantly longer at 3 T, although there was no significant change in the T(1) value of PME. There was no significant difference in the mean PME/PDE ratios for either the control or patient groups at both 1.5 and 3.0 T, but there was a small positive mean difference in PME/PDE at 3.0 T on pairwise testing between field strengths (+ 0.05, p < 0.01). There were significant correlations between PME/PDE values at the two magnetic field strengths (r = 0.806, p < 0.001). The underlying broad resonance was reduced at 3.0 T relative to 1.5 T, related to line broadening of the phospholipid bilayer signal. In conclusion, there was an improvement in hepatic (31)P MR signal quality at 3.0 T relative to 1.5 T. Broadly similar hepatic (31)P MR parameters were obtained at 1.5 and 3.0 T. The modest difference noted in the PME/PDE ratio between field strengths for patients with chronic liver disease should inform multicentre study design involving these field strengths.

High resolution spectroscopic imaging of GABA at 3 Tesla

A spin echo-based MRSI sequence was developed to acquire edited spectra of γ-aminobutyric acid in an entire slice. Water and lipid signals were suppressed by a dual-band presaturation sequence, which included integrated outer volume suppression pulses for additional lipid suppression. Experiments in three normal volunteers were performed at 3 T using a 32-channel head coil. High signal-to-noise ratio spectra and metabolic images of γ-aminobutyric acid were acquired from nominal 4.5 cm3 voxels (estimated actual voxel size 7.0 cm3) in a scan time of 17 min. The sequence is also expected to co-edit homocarnosine and macromolecules, giving a composite γ-aminobutyric acid+ resonance. The γ-aminobutyric acid+ to water ratio was measured using a companion water MRSI scan and was found to correlate linearly with the % gray matter (GM) of each voxel (γ-aminobutyric acid+/water=(1.5×GM+3.2)×10(-5), R=0.27), with higher γ-aminobutyric acid+ levels in gray matter compared with white. In conclusion, high signal-to-noise ratio γ-aminobutyric acid-MRSI is possible at 3 T within clinically feasible scan times.

In vivo 1H NMR spectroscopy of the human brain at 9.4 T: initial results

In vivo proton NMR spectroscopy allows non-invasive detection and quantification of a wide range of biochemical compounds in the brain. Higher field strength is generally considered advantageous for spectroscopy due to increased signal-to-noise and increased spectral dispersion. So far (1)H NMR spectra have been reported in the human brain up to 7 T. In this study we show that excellent quality short echo time STEAM and LASER (1)H NMR spectra can be measured in the human brain at 9.4 T. The information content of the human brain spectra appears very similar to that measured in the past decade in rodent brains at the same field strength, in spite of broader linewidth in human brain. Compared to lower fields, the T(1) relaxation times of metabolites were slightly longer while T(2) relaxation values of metabolites were shorter (<100 ms) at 9.4 T. The linewidth of the total creatine (tCr) resonance at 3.03 ppm increased linearly with magnetic field (1.35 Hz/T from 1.5 T to 9.4 T), with a minimum achievable tCr linewidth of around 12.5 Hz at 9.4 T. At very high field, B(0) microsusceptibility effects are the main contributor to the minimum achievable linewidth.

Short echo time proton magnetic resonance spectroscopy in Alzheimer's disease: a longitudinal multiple time point study

Short echo time localized proton magnetic resonance spectroscopy provides quantification of brain metabolites, including N-acetyl-aspartate, myo-inositol, creatine/phosphocreatine and choline-containing compounds, which may be useful biomarkers for monitoring Alzheimer's disease. We aimed to quantify the rate of metabolite change in Alzheimer's disease, to assess factors influencing changes and to investigate the potential for serial magnetic resonance spectroscopy as an Alzheimer's disease trial biomarker. A total of 42 patients and 22 controls each had up to six magnetic resonance spectroscopy examinations over a 2-year period, using a midline posterior cingulate single-voxel point resolved spectroscopy sequence (1.5 T; time to repetition = 2000 ms; echo time = 30 ms; 192 averages). Metabolite ratios N-acetyl-aspartate:creatine/phosphocreatine, choline-containing compounds:creatine/phosphocreatine, and myo-inositol:creatine/phosphocreatine were measured using online software (PROBE-Q) and the N-acetyl-aspartate:myo-inositol ratio was derived. Baseline ratios were compared between patients and controls. A linear mixed model was used to quantify longitudinal changes and extended to assess the effect of age, disease severity and baseline use of acetylcholinesterase inhibitors. Patients and controls were matched for age (patients: 68.9 ± 7.2 years; controls: 69.1 ± 6.7 years); 71% of the patients were on acetylcholinesterase inhibitors at baseline; mean Mini-Mental State Examination for patients was 19.4 ± 4.1. A total of 307 spectra were acquired. In cross-sectional analyses, patients were significantly different from controls for N-acetyl-aspartate:creatine/phosphocreatine (11% lower, P < 0.001), N-acetyl-aspartate:myo-inositol (24% lower, P < 0.001), and myo-inositol:creatine/phosphocreatine (17% higher, P < 0.001). After adjustment for N-acetyl-aspartate:myo-inositol, none of the other variables differed significantly. In patients there was significant decline in N-acetyl-aspartate:creatine/phosphocreatine (mean: 2.2%/year; 95% confidence interval: 0.9-3.5) and N-acetyl-aspartate:myo-inositol (mean: 3.7%/year; 95% confidence interval: 1.7-5.7), with no evidence for influence by age, disease severity or acetylcholinesterase inhibitor use. There was significant excess decline in patients compared with controls only in N-acetyl-aspartate:myo-inositol (mean: 3.6%/year; 95% confidence interval: 0.8-6.4; P = 0.014). Between-subject standard deviation for N-acetyl-aspartate:myo-inositol was 0% for controls and 3.5%/year for patients; within-subject standard deviation for a 1 year, two-time-point study was 9.2%/year for both patients and controls. These results confirm that magnetic resonance spectroscopy can be used to quantify excess metabolite decline in Alzheimer's disease, which may provide a useful measure of disease progression. We found no evidence that age, disease severity or acetylcholinesterase inhibitor use influenced rate of decline, although numbers were small. The substantial variability in longitudinal measurements that drives sample size requirements is principally within-subject and technique related: technical developments to reduce this variability may make serial magnetic resonance spectroscopy a viable biomarker in clinical trials for Alzheimer's disease.

Brain metabolism and cognitive impairment in HIV infection: a 3-T magnetic resonance spectroscopy study

BACKGROUND AND PURPOSE

Human immunodeficiency virus (HIV)-associated dementia (HAD) has been extensively studied using magnetic resonance spectroscopy (MRS) at field strengths of 1.5 T. Higher magnetic field strengths (such as 3 T) allow for more reliable determination of certain compounds, such as glutamate (Glu) and glutamine (Gln). The current study was undertaken to investigate the utility of 3-T MRS for evaluating HIV+ patients with different levels of cognitive impairment with emphasis on the measurement of Glu and Glx (the sum of Glu and Gln).

METHODS

Eighty-six HIV+ subjects were evaluated at 3 T using quantitative short echo time single-voxel MRS of frontal white matter (FWM) and basal ganglia (BG). Subjects were divided into three groups according to the Memorial Sloan Kettering (MSK) HIV dementia stage: 21 had normal cognition (NC) (MSK 0), 31 had mild cognitive impairment (MCI) without dementia (clinical MSK stage=0.5), and 34 had dementia (HAD) (MSK≥1). HIV+ subjects had also undergone standardized cognitive testing covering the domains of executive function, verbal memory, attention, information processing speed and motor and psychomotor speed. Between-group differences in metabolite levels in FWM and BG were evaluated using ANOVA. Pearson correlation coefficients were used to explore the associations between the Glu and Glx metabolites and neurocognitive results.

RESULTS

FWM Glx was lower in HAD (8.1 ± 2.1 mM) compared to both the MCI (9.17 ± 2.1 mM) and NC groups (10.0 ± 1.6 mM) (P=.006). FWM myo-inositol (mI) was higher in HAD (4.15 ± 0.75 mM) compared to both MCI (3.86 ± 0.85 mM) and NC status (3.4 ± 0.67 mM) (P=.006). FWM Glx/creatine (Cr) was lower and FWM mI/Cr was significantly higher in the HAD compared to the MCI and NC groups (P=.01 and P=.004, respectively). BG N-acetyl aspartate (NAA) was lower in the HAD group (6.79 ± 1.53 mM), compared to the MCI (7.5 ± 1.06 mM) and NC (7.6 ± 1.01 mM) groups (P=.036). Significant negative correlations were observed between Glu, Glx and NAA concentrations with Trail-Making Test B (P=.006, P=.0001 and P=.007, respectively), and significant positive correlation was found with the Digit symbol test (P=.02, P=.002 and P=.008, respectively). FWM Glx and NAA concentrations showed negative correlation with Grooved Pegboard nondominant hand (P=.02 and P=.04, respectively).

CONCLUSION

Patients with HAD have lower levels of Glx concentrations and Glx/Cr ratio in FWM, which was associated with impaired performance in specific cognitive domains, including executive functioning, fine motor, attention and working memory performance. Three-Tesla MRS measurements of Glx may be a useful indicator of neuronal loss/dysfunction in patients with HIV infection.

Detection of Amide and Aromatic Proton Resonances of Human Brain Metabolites Using Localized Correlated Spectroscopy Combined with Two Different Water Suppression Schemes

The purpose of the study was to demonstrate the J-coupling connectivity network between the amide, aliphatic, and aromatic proton resonances of metabolites in human brain using two-dimensional (2D) localized correlated spectroscopy (L-COSY). Two different global water suppression techniques were combined with L-COSY, one before and another after localizing the volume of interest (VOI). Phantom solutions containing several cerebral metabolites at physiological concentrations were evaluated initially for sequence optimization. Nine healthy volunteers were scanned using a 3T whole body MRI scanner. The VOI for 2D L-COSY was placed in the right occipital white/gray matter region. The 2D cross and diagonal peak volumes were measured for several metabolites such as N-acetyl aspartate (NAA), creatine (Cr), free choline (Ch), glutamate/glutamine (Glx), aspartate (Asp), myo-inositol (mI), GABA, glutathione (GSH), phosphocholine (PCh), phosphoethanolamine (PE), tyrosine (Tyr), lactate (Lac), macromolecules (MM) and homocarnosine (Car). Using the pre-water suppression technique with L-COSY, the above mentioned metabolites were clearly identifiable and the relative ratios of metabolites were calculated. In addition to detecting multitude of aliphatic resonances in the high field region, we have demonstrated that the amide and aromatic resonances can also be detected using 2D L-COSY by pre water suppression more reliably than the post-water suppression.

Quantitative Comparison of Minimum Inductance and Minimum Power Algorithms for the Design of Shim Coils for Small Animal Imaging

High-performance shim coils are required for high-field magnetic resonance imaging and spectroscopy. Complete sets of high-power and high-performance shim coils were designed using two different methods: the minimum inductance and the minimum power target field methods. A quantitative comparison of shim performance in terms of merit of inductance (ML) and merit of resistance (MR) was made for shim coils designed using the minimum inductance and the minimum power design algorithms. In each design case, the difference in ML and the difference in MR given by the two design methods was <15%. Comparison of wire patterns obtained using the two design algorithms show that minimum inductance designs tend to feature oscillations within the current density; while minimum power designs tend to feature less rapidly varying current densities and lower power dissipation. Overall, the differences in coil performance obtained by the two methods are relatively small. For the specific case of shim systems customized for small animal imaging, the reduced power dissipation obtained when using the minimum power method is judged to be more significant than the improvements in switching speed obtained from the minimum inductance method.

Cramér-Rao bound analysis of echo time selection for (1)H-MR spectroscopy

The choice of echo time (TE) is a complicated and controversial issue in proton MR spectroscopy, and represents a balancing act between signal-to-noise ratio and signal complexity. The TE values used in previous literature were selected either heuristically or based on limited empirical studies. In this work, we reconsider this problem from an estimation theoretic perspective. Specifically, we analyze the Crame r-Rao lower bound on estimated spectral parameters as a function of TE, which serves as a metric to quantify the reliability of the estimation procedure. This analysis reveals that a good choice of TE often depends on the particular metabolite of interest, and is a function of both the coupling properties of the metabolites and the general complexity of the spectrum.

Spectroscopic findings in attention-deficit/hyperactivity disorder: review and meta-analysis

OBJECTIVES

The last decade has seen an increasing interest in the method of magnet resonance spectroscopy (MRS) since this is the only research tool that allows a non-invasive in vivo assessment of neurochemical aspects of ADHD without employing ionising radiation. In this paper we review published MRS results with respect to childhood, adolescence and adult ADHD.

METHOD

We searched the Medline (Pub Med) database using the key words ADHD, attention-deficit/hyperactivity disorder, magnet resonance spectroscopy, MRS and spectroscopy. Citations of identified articles were also searched for relevant studies. Meta-analyses were performed for the measured metabolites and regions of assessment.

RESULTS

Sixteen studies could be identified that used MRS to investigate the neurobiology of ADHD. Two regions could be identified as the focus of spectroscopic investigations--the frontal lobe including anterior cingulate cortex and parts of prefrontal cortex and the basal ganglia, mostly striatum, alongside the fronto-striato-thalamo-frontal circuits. As for metabolites, in the majority of studies the ratios to creatine and not absolute concentrations of metabolites were estimated. Choline compounds, N-acetyl-aspartate and glutamate/glutamine (to creatine ratios) could be identified as being altered in several studies in ADHD. The meta-analysis showed increased choline compounds in several researched regions.

DISCUSSION

MRS is a promising tool for the non-invasive in vivo assessment of the cerebral neurochemistry in ADHD. More regions of interest (ROI) like amygdala, hippocampus, thalamus and cerebellum should be assessed in future studies. Further methodological improvements of MRS are desirable in order to assess the absolute metabolite concentration of several ROIs at the same time. Such developments will open novel perspectives in spectroscopic investigations of ADHD.

Quantification of muscle choline concentrations by proton MR spectroscopy at 3 T: technical feasibility

OBJECTIVE

The quantification of choline in musculoskeletal tissues has several potential uses, including characterizing malignancy, but has not been previously achievable. We present a method of measuring the absolute concentration of choline by proton MR spectroscopy (MRS) in skeletal muscle at 3 T.

MATERIALS AND METHODS

At 3 T, choline measurements were performed in phantoms and healthy volunteers using proton MRS (point-resolved spectroscopy sequence [PRESS]; TR/TE, 2,000/135). In vitro choline concentrations were measured in three phantom solutions (10, 5, 1.25 mmol). Choline T1 and T2 relaxation times were measured in the muscles of five healthy subjects. In vivo choline concentrations were measured using water as an internal reference and average T1 and T2 relaxation times in 20 muscle locations (quadriceps, hamstring, adductor) of seven healthy subjects (four men, three women). Descriptive statistics are reported.

RESULTS

In vitro, the average measured choline concentrations of the 10-, 5-, and 1.25-mmol solutions were 9.91, 5.03, and 1.22 mmol, respectively. In vivo, the average T1 and T2 relaxation times of choline were 1,372+/-57 (SD) and 134+/-11 milliseconds, respectively. The average choline concentrations in the quadriceps and hamstring muscles were 10.0+/-0.4 (SD) and 8.0+/-2.9 mmol/kg. Interindividual variation existed in the choline concentrations (quadriceps range, 6.7-13 mmol/kg), but there was little variation by patient sex.

CONCLUSION

In the musculoskeletal system, the measurement of choline concentration by proton MRS at 3 T is feasible using water as an internal reference. These data provide a quantitative basis for future investigations of metabolite concentrations in normal and diseased musculoskeletal tissues.

In vivo 1H NMR spectroscopy of the human brain at high magnetic fields: metabolite quantification at 4T vs. 7T

A comprehensive comparative study of metabolite quantification from the human brain was performed on the same 10 subjects at 4T and 7T using MR scanners with identical consoles, the same type of RF coils, and identical pulse sequences and data analysis. Signal-to-noise ratio (SNR) was increased by a factor of 2 at 7T relative to 4T in a volume of interest selected in the occipital cortex using half-volume quadrature radio frequency (RF) coils. Spectral linewidth was increased by 50% at 7T, which resulted in a 14% increase in spectral resolution at 7T relative to 4T. Seventeen brain metabolites were reliably quantified at both field strengths. Metabolite quantification at 7T was less sensitive to reduced SNR than at 4T. The precision of metabolite quantification and detectability of weakly represented metabolites were substantially increased at 7T relative to 4T. Because of the increased spectral resolution at 7T, only one-half of the SNR of a 4T spectrum was required to obtain the same quantification precision. The Cramér-Rao lower bounds (CRLB), a measure of quantification precision, of several metabolites were lower at both field strengths than the intersubject variation in metabolite concentrations, which resulted in a strong correlation between metabolite concentrations of individual subjects measured at 4T and 7T.

Improving SNR of RF coils using composite coil elements

A composite coil element consists of up to three independent orthogonal loops. It improves the flexibility in shaping the radio frequency (RF) field in its vicinity, compared with a single-loop coil element. Computer simulations were conducted to explore the potential advantages of this type of coil configuration for improving the signal-to- noise ratio (SNR), including the intrinsic SNR (ISNR) and the realistic SNR, when the effects of resistive loss of the coil were included. A 'half-space' model was considered, with a variable B(0) direction relative to the surface of a large conductive medium. The SNR performance of a square single-loop coil parallel to the surface of the medium was compared with that of a composite coil element where up to two additional orthogonal square loops of the same size were added to the single coil loop. The SNR performances of coil arrays consisting of single-loop elements and composite elements were also evaluated. The RF field was calculated using the finite-difference time-domain method. The results show that the composite coil element has a substantially better ISNR at all depths from the surface than that of a single-loop element covering the same surface area. Furthermore, the ISNR of a composite element is not sensitive to the surface orientation relative to the B(0) field. The computer simulation also revealed that at 128 MHz, the resistive loss from the copper coil loops standing upright on the surface at room temperature is substantial compared to the loss in the medium. Consequently, the realistic SNR is significantly lower than ISNR at 128 MHz for a composite coil element. The coil loading by the medium becomes more dominant at 170 and 298 MHz, and the differences between the realistic SNR and ISNR become smaller at these higher frequencies.

Diffusion-weighted imaging with apparent diffusion coefficient mapping and spectroscopy in prostate cancer

Prostate cancer is a major health problem, and the exploration of noninvasive imaging methods that have the potential to improve specificity while maintaining high sensitivity is still critically needed. Tissue changes induced by tumor growth can be visualized by magnetic resonance imaging (MRI) methods. Current MRI methods include conventional T2-weighted imaging, diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) mapping and magnetic resonance spectroscopy (MRS). Techniques such as DWI/ADC provide functional information about the behavior of water molecules in tissue; MRS can provide biochemical information about the presence or absence of certain metabolites, such as choline, creatine, and citrate. Finally, vascular parameters can be investigated using dynamic contrast-enhanced MRI. Moreover, with whole-body MRI and DWI, metastatic disease can be evaluated in 1 session and may provide a way to monitor treatment. Therefore, when combining these various methods, a multiparametric data set can be built to assist in the detection, localization, assessment of prostate cancer aggressiveness, and tumor staging. Such a comprehensive approach offers more power to evaluate prostate disease than any single measure alone. In this article, we focus on the role of DWI/ADC and MRS in the detection and characterization using both in vivo and ex vivo imaging of prostate pathology.

Relaxation times of skeletal muscle metabolites at 7T

PURPOSE

To demonstrate the feasibility of quantitatively evaluating and measuring T(1) and T(2) relaxation times of human tibialis anterior (TA) muscles metabolites in vivo at 7T and to compare these results with those of 3T.

MATERIALS AND METHODS

A model lipid phantom (corn oil) and healthy volunteers (n = 4, mean +/- SD age 35.6 +/- 5.6 years) were scanned on 3T and 7T whole-body MR scanners. A voxel of 10 x 10 x 10 mm(3) was positioned on the lipid phantom and right calf TA muscles using the single-voxel stimulated echo acquisition mode (STEAM) pulse sequence. All magnetic resonance spectroscopy (MRS) data were processed with Java-based Magnetic Resonance User Interface (JMRUI) using Hankel Lanczos Singular Value Decomposition (HLSVD) filtering to remove the residual water signal.

RESULTS

T(1) shows a steady increase while T(2) shows a slight decrease with B(0) and the spectra show larger spectral resolution at 7T than at 3T in the lipid phantom. T(1) values of all the metabolites are higher, while T(2) values are slightly lower at 7T than those of 3T compared to reported results in TA. The maximum percentage of increase in T(1) is about approximately 488%, the maximum percentage of decrease in T(2) is about approximately 65%.

CONCLUSION

The preliminary results can potentially be used for calculating relaxation correction factors required for absolute quantitation of skeletal muscle metabolite concentrations and for further protocol and sequence optimization.

Spectral resolution amelioration by deconvolution (SPREAD) in MR spectroscopic imaging

PURPOSE

To develop, implement, and evaluate a novel postprocessing method for enhancing the spectral resolution of in vivo MR spectroscopic imaging (MRSI) data.

MATERIALS AND METHODS

Magnetic field inhomogeneity across the imaging volume was determined by acquiring MRI datasets with two differing echo times. The lineshapes of the MRSI spectra were derived from these field maps by simulating an MRSI scan of a virtual sample whose resonance frequencies varied according to the observed variations in the magnetic field. By deconvolving the lineshapes from the measured MRSI spectra, the linebroadening effects of the field inhomogeneities were reduced significantly.

RESULTS

Both phantom and in vivo proton MRSI spectra exhibited significantly enhanced spectral resolutions and improved spectral lineshapes following application of our method. Quantitative studies on a phantom show that, on average, the full width at half maximum of water peaks was reduced 42%, the full width at tenth maximum was reduced 38%, and the asymmetries of the peaks were reduced 86%.

CONCLUSION

Our method reduces the linebroadening and lineshape distortions caused by magnetic field inhomogeneities. It substantially improves the spectral resolution and lineshape of MRSI data.

Age dependence of regional proton metabolites T2 relaxation times in the human brain at 3 T

Although recent studies indicate that use of a single global transverse relaxation time, T(2), per metabolite is sufficient for better than +/-10% quantification precision at intermediate and short echo-time spectroscopy in young adults, the age-dependence of this finding is unknown. Consequently, the age effect on regional brain choline (Cho), creatine (Cr), and N-acetylaspartate (NAA) T(2)s was examined in four age groups using 3D (four slices, 80 voxels 1 cm(3) each) proton MR spectroscopy in an optimized two-point protocol. Metabolite T(2)s were estimated in each voxel and in 10 gray and white matter (GM, WM) structures in 20 healthy subjects: four adolescents (13 +/- 1 years old), eight young adults (26 +/- 1); two middle-aged (51 +/- 6), and six elderly (74 +/- 3). The results reveal that T(2)s in GM (average +/- standard error of the mean) of adolescents (NAA: 301 +/- 30, Cr: 162 +/- 7, Cho: 263 +/- 7 ms), young adults (NAA: 269 +/- 7, Cr: 156 +/- 7, Cho: 226 +/- 9 ms), and elderly (NAA: 259 +/- 13, Cr: 154 +/- 8, Cho: 229 +/- 14 ms), were 30%, 16%, and 10% shorter than in WM, yielding mean global T(2)s of NAA: 343, Cr: 172, and Cho: 248 ms. The elderly NAA, Cr, and Cho T(2)s were 12%, 6%, and 10% shorter than the adolescents, a change of under 1 ms/year assuming a linear decline with age. Formulae for T(2) age-correction for higher quantification precision are provided.

Assessment of citrullinated myelin by 1H-MR spectroscopy in early-onset multiple sclerosis

BACKGROUND AND PURPOSE

Myelin instability and citrullinated myelin basic protein have been demonstrated in the brains of patients with chronic and fulminating forms of multiple sclerosis (MS). Our aim was to trace citrulline in the brains of patients with early-onset MS by using proton MR spectroscopy ((1)H-MR spectroscopy).

MATERIALS AND METHODS

A short-echo single-voxel (1)H-MR spectroscopy by using the point-resolved proton spectroscopy sequence was performed in 27 patients with MS and 23 healthy subjects. Voxels of interest were chronic demyelinating lesions (CDLs, n = 25) and normal-appearing white matter (NAWM, n = 25) on T2-weighted imaging, and when available in patients with MS, enhancing demyelinating lesions (EDLs, n = 8). Frontal white matter (WM) was studied in control subjects. N-acetylaspartate, choline, and myo-inositol (mIns)-creatine (Cr) ratios and the presence of a citrulline peak were noted.

RESULTS

Citrulline peaks were more frequently observed in patients with MS than in control subjects (P = .035), located in the NAWM in 8/25 (32%), in CDLs in 7/25 (28%), and in EDLs of 1/8 (12.5%) patients with MS. The presence of citrulline and measured metabolite/Cr ratios was not related to age at imaging, age at disease onset, duration of disease, or number of relapses. There was no significant metabolic difference between the NAWM of patients with MS and the WM of the control subjects. mIns/Cr was significantly greater in CDLs compared with the NAWM of patients with MS and the WM of healthy subjects.

CONCLUSIONS

Citrulline was more frequently identified in the brains of patients with early-onset MS than in healthy subjects by (1)H-MR spectroscopy, suggesting an association of increased citrullination of myelin proteins with demyelinating diseases.

[Modern magnetic resonance procedures for assessing tumor response]

Magnetic resonance imaging (MRI) is a mainstay in oncological imaging with respect to tumor detection, characterization and treatment monitoring. Besides quantifying metric changes of tumor tissue, a wide range of different other surrogate parameters of therapy response can be imaged and quantified by MRI. Early monitoring of treatment success is critical both for medical and economical reasons specifically with more expensive target-specific drugs entering the clinical arena. Dynamic contrast-enhanced (DCE) and steady state MRI can help to assess tumor perfusion and vessel permeability. The cellular state of tissue can be measured by diffusion-weighted imaging (DWI) and metabolic changes can be monitored by MR spectroscopy (MRS). New target-specific contrast agents potentially allow selective imaging of apoptotic events.This review aims to give a brief overview of new MR-based imaging approaches to assess tumor response to new target-specific therapy regimes, with special emphasis on anti-angiogenic and antivascular treatment effects.

3.0-T MR imaging of the abdomen: comparison with 1.5 T

Three-tesla magnetic resonance (MR) imaging offers substantially higher signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) than 1.5-T MR imaging does, which can be used to improve image resolution and shorten imaging time. Because of these increases in SNR and CNR, as well as changes in T1 and T2 relaxation times, an increase in magnetic susceptibility, and an increase in chemical shift effect, many abdominal applications can benefit from 3.0-T imaging. Increased CNR obtained with a gadolinium-based contrast agent improves lesion conspicuity, requires less intravenous contrast material, and improves MR angiography by increasing spatial and temporal resolution. Increased SNR improves fluid conspicuity and resolution for applications such as MR cholangiopancreatography. Increased chemical shift effect also improves spectral resolution for MR spectroscopy. Several potential problems remain for abdominal imaging at 3.0 T. Limitations on energy deposition may require compromises in pulse sequence timing and flip angles. These compromises result in prolonged imaging time and altered image contrast. Magnetic susceptibility and chemical shift artifacts are worsened, but they may be counteracted by shortening echo time, performing parallel imaging, and increasing bandwidth. Radiofrequency field inhomogeneity is also a major concern in imaging larger fields of view and often leads to standing wave effects and large local variations in signal intensity. Many issues related to MR device compatibility and safety have yet to be addressed at 3.0 T. A 3.0-T MR imaging system has a higher initial cost and a higher cost of upkeep than a 1.5-T system does.

(31)P magnetic resonance spectroscopy to measure in vivo cardiac energetics in normal myocardium and hypertrophic cardiomyopathy: Experiences at 3T

BACKGROUND

(31)P magnetic resonance spectroscopy (MRS) allows measurement of in vivo high-energy phosphate kinetics in the myocardium. While traditionally (31)P cardiac spectroscopy is performed at 1.5T, cardiac MRS at higher field strength can theoretically increase signal to noise ratio (SNR) and spectral resolution therefore improving sensitivity and specificity of the cardiac spectra. The reproducibility and feasibility of performing cardiac spectroscopy at 3T is presented here in this study in healthy volunteers and patients with hypertrophic cardiomyopathy.

METHODS

Cardiac spectroscopy was performed using a Phillips 3T Achieva scanner in 37 healthy volunteers and 26 patients with hypertrophic cardiomyopathy (HCM) to test the feasibility of the protocol. To test the reproducibility a single volunteer was scanned eight times on separate occasions. A single voxel (31)P MRS was performed using Image Selected In vivo Spectroscopy (ISIS) volume localization.

RESULTS

The mean phosphocreatine/adenosine triphosphate (PCr/ATP) ratio of the eight measurements performed on one individual was 2.11+/-0.25. Bland Altman plots showed a variance of 12% in the measurement of PCr/ATP ratios. The PCr/ATP ratio was significantly reduced in HCM patients compared to controls, 1.42+/-0.51 and 2.11+/-0.57, respectively, P<0.0001. (All results are expressed as mean+/-standard deviation).

CONCLUSIONS

Here we demonstrate that cardiac (31)P MRS at 3T is a reliable method of measuring in vivo high-energy phosphate kinetics in the myocardium for clinical studies and diagnostics. Based on our data an impairment of cardiac energetic state in patients with hypertrophic cardiomyopathy is indisputable.

Brain imaging in pediatric obsessive-compulsive disorder

OBJECTIVE

To review progress in understanding pediatric obsessive-compulsive disorder (OCD). The focus is on the frontal-striatal-thalamic model of OCD, neurobiological and genetic studies of the disorder, and their influence on recent advances in treatment.

METHOD

Computerized literature searches were conducted with the key words "obsessive-compulsive disorder" in conjunction with "pediatric," "genetics," and "imaging."

RESULTS

Neuroimaging studies find evidence to support the frontal-striatal-thalamic model. Genetic and neurochemical studies also implicate glutamate in the pathological finding of OCD. This has led to the application of glutamate-modulating agents to treat OCD.

CONCLUSIONS

Studies of pediatric OCD have led to a refined frontal-striatal-thalamic model of pathogenesis and are having an evidence-based impact on treatment. Despite this progress, fully explanatory models are still needed that would allow for accurate prognosis and the development of targeted and efficacious treatments.

3T MRI in paediatrics: challenges and clinical applications

3T MRI is being increasingly performed for clinical purposes in paediatrics, primarily because of the potential to improve spatial and temporal resolution - these can assist in overcoming the unique anatomic, physiologic and behavioural challenges of imaging children. The increased spatial resolution improves the capacity to image small patients; with particular reference to smaller structures such as the inner ear, brachial plexus, biliary system and the vascular system. The challenges inherent to imaging at high field strength remain pertinent especially, the altered T1 contrast, artefacts (susceptibility, chemical shift and B1 inhomogeneity) and safety issues, including specific absorption rate - several of these are circumvented due to software and hardware advances, or by trade off of some of the increased signal. The above mentioned challenges also create opportunities at 3T, with improvement in MR angiography, arterial spin labelling, functional MRI, susceptibility weighted imaging, and MR spectroscopy - all of which have distinctive applications in paediatrics. Whole body imaging also becomes more practical because of the capacity for faster scans. 3T MRI has the potential to image all the systems in paediatrics. However, neonatal brain and paediatric spine imaging have specific challenges at 3T. Several factors also limit cardiac imaging at present. Further improvements in coil technology and newer sequences may help overcome the challenges that remain.

Magnetic resonance spectroscopy of the brain: review of metabolites and clinical applications

Magnetic resonance imaging (MRI) provides anatomic images and morphometric characterization of disease, whereas magnetic resonance spectroscopy (MRS) provides metabolite/biochemical information about tissues non-invasively in vivo. MRS has been used clinically for more than two decades. The major applications of this advanced MRI tool are in the investigation of neurological and neurosurgical disorders. MRS has also been used in the evaluation of the prostate gland and muscle tissue, but these applications will not be addressed in this review. The aim of this review is to attempt to introduce the technique, review the metabolites and literature, as well as briefly describe our clinical experience.

On the voxel size and magnetic field strength dependence of spectral resolution in magnetic resonance spectroscopy

While the inherent low sensitivity of in vivo MR spectroscopy motivated a trend towards higher magnetic fields, B(0), it has since become apparent that this increase does not seem to translate into the anticipated improvement in spectral resolution. This is attributed to the decrease of the transverse relaxation time, T(2)*, in vivo due to macro- and mesoscopic tissue susceptibility. Using spectral contrast-to-noise ratio (SCNR) arguments, we show that if in biological systems the linewidth (on the frequency scale) increases linearly with the field, the spectral resolution (in parts per million) improves approximately as the fifth-root of B(0) for chemically shifted lines and decreases as about B(0)(4/5) (in hertz) for a structure of J-coupled multiplets. It is also shown that for any given B(0) there is a unique voxel size that is optimal in spectral resolution, linking the spectral and spatial resolutions. Since in practical applications the spatial resolution may be dictated by the target anatomy, nomograms to determine the B(0) required to achieve the desired spectral resolution at that voxel size are presented. More generally, the scaling of the nomograms to determine the achievable spectral and spatial resolutions at any given field is described.

Regional apparent metabolite concentrations in young adult brain measured by (1)H MR spectroscopy at 3 Tesla

PURPOSE

To quantify and examine the distribution of brain metabolites in normal young adults using single voxel MR spectroscopy at 3 Tesla (T).

MATERIALS AND METHODS

Short-echo time single-voxel PRESS technique was used to measure the apparent concentration of five metabolites at nine locations in the brains of young adults. Concentrations were estimated by means of an automated fitting method (LCModel) with reference to an unsuppressed water signal and were corrected for T(1) relaxation, T(2) relaxation, and cerebrospinal fluid partial volume. Analysis of variance with Tukey post hoc test was used to evaluate regional variations.

RESULTS

Statistically significant differences in regional concentrations were detected for each of the metabolites. The number of significant differences was greatest for total choline, whereas myo-inositol and the sum of glutamine and glutamate had the fewest. Magnitude of variation was greatest for total choline and least for the sum of N-acetyl aspartate and N-acetylaspartylglutamate.

CONCLUSION

In agreement with previous studies at other field strengths, we found heterogeneous distribution of the major spectroscopically measurable brain metabolites. Although the most distinct differences are between tissue types, there is appreciable variation within a tissue type at different locations. The spectra and metabolite concentrations presented should provide a useful reference for both clinical and research MR spectroscopy studies performed at 3T.

Short-TE localised 1H MRS of the human brain at 3 T: quantification of the metabolite signals using two approaches to account for macromolecular signal contributions

The goal of this study was to validate metabolite quantification at short TE, with particular focus on how to best account for the macromolecular signal contribution. A robust, short-TE PRESS protocol is presented, which allows reliable quantification, in vivo, of metabolite signals at 3 T in human brain. Water suppression was adapted to the experimental conditions at 3 T. Metabolite signal from the parietal white matter was quantified in the time domain using QUEST (jMRUI). The increased macromolecular signal contribution at short TE was dealt with by two approaches, based on either metabolite nulling or initial signal truncation. A detailed comparison of the two approaches was made. The first used a metabolite-nulled signal, measured either individually or averaged over different subjects. The second used the total signal, metabolites and macromolecules, from a single scan. The two approaches gave similar quantification results in terms of metabolite concentrations, but differed in their precision and the number of metabolites quantified reliably. With an average metabolite-nulled baseline, a set of seven metabolites could be reliably quantified in parietal white matter under these experimental conditions: N-acetylaspartate, myo-inositol, glucose, glutamate, glutathione, creatine and choline. When initial signal truncation was used, glucose was removed from this set. The short TE (10-11 ms) facilitated quantification of glutamate. The reliable quantification of N-acetylaspartyl glutamate at 3 T proved very difficult.

Hepatic fat and adenosine triphosphate measurement in overweight and obese adults using 1H and 31P magnetic resonance spectroscopy

BACKGROUND/AIMS

Magnetic resonance spectroscopy (MRS) measures hepatic fat and adenosine triphosphate (ATP), but magnetic resonance studies are challenging in obese subjects. We aimed to evaluate the inter- and intrarater reliability and stability of hepatic fat and ATP measurements in a cohort of overweight and obese adults.

METHODS

We measured hepatic fat and ATP using proton MRS ((1)H MRS) and phosphorus MRS ((31)P MRS) at baseline in adults enrolled in the Action for Health in Diabetes (Look AHEAD) clinical trial at one site. Using logistic regression, we determined factors associated with successful MRS data acquisition. We calculated the intra- and inter-rater reliability for hepatic fat and ATP based on 20 scans analysed twice by two readers. We also calculated the stability of these measures three times on five healthy volunteers.

RESULTS

Of 244 participants recruited into our ancillary study, 185 agreed to MRS. We obtained usable hepatic fat data from 151 (82%) and ATP data from 105 (58%). Obesity was the strongest predictor of failed data acquisition; every unit increase in the body mass index reduced the likelihood of successful fat data by 11% and ATP data by 14%. The inter- and intrarater reliability were excellent for fat (intraclass correlation coefficient=0.99), but substantially more variable for ATP. Fat measures appeared relatively stable, but this was less true for ATP.

CONCLUSIONS

Obesity can hinder (1)H and (31)P MRS data acquisition and subsequent analysis. This impact was greater for hepatic ATP than hepatic fat.

Liver fat content and T2*: simultaneous measurement by using breath-hold multiecho MR imaging at 3.0 T--feasibility

Research ethics committee approval was obtained for this study, and written informed consent was obtained from all participants. The purpose was to prospectively evaluate the feasibility of breath-hold multiecho in- and out-of-phase magnetic resonance (MR) imaging for simultaneous lipid quantification and T2* measurement. A spoiled gradient-echo sequence with seven echo times alternately in phase and out of phase was used at 3.0 T. Imaging was performed in a lipid phantom, in five healthy volunteers (all men; mean age, 37 years), and in five obese individuals with hyperlipidemia or diabetes (four men, one woman; mean age, 53 years). A biexponential curve-fitting model was used to derive the relative signal contributions from fat and water, and these results were compared with results of liver proton MR spectroscopy, the reference standard. There was a significant correlation between multiecho and spectroscopic measurements of hepatic lipid concentration (r(2) = 0.99, P < .001). In vivo, the T2* of water was consistently longer than that of fat and reliably enabled the signal components to be correctly assigned. In the lipid phantom, the multiecho method could be used to determine the fat-to-water ratio and the T2* values of fat and water throughout the entire range of fat concentrations. Multiecho imaging shows promise as a method of simultaneous fat and T2* quantification.

Assessment of in vivo 1H magnetic resonance spectroscopy in the liver: a review

With increased availability of magnetic resonance (MR) systems at ultra-high field strength for clinical studies, other organs besides the brain have received renewed consideration for MR spectroscopy (MRS). Because signal-to-noise ratio and chemical shift increase proportional to the static magnetic field, a concomitant increase in signal intensity and spectral resolution of metabolite resonances can be exploited. Improved resolution of adjacent metabolite peaks would not only provide for more accuracy of metabolite identification but also metabolite quantification. While the superiority of high-field imaging and spectroscopy has already been demonstrated clearly in the brain, this article reviewed issues around 1H MRS of the liver. These include optimization strategies such as coil technology, minimizing of motion artefacts using breath-holding and postprocessing of the spectra. Moreover, we reviewed the pertinent experience hitherto reported in the literature on potential clinical issues where liver MRS may be useful. These included determination and characterization of liver fat content, liver tumours and focal lesions. While these applications have been used experimentally, liver MRS does not yet have a clearly defined role in the clinical management of any disease state. Accordingly, it remains primarily a research modality to date.

Towards 1H-MRSI of the human brain at 7T with slice-selective adiabatic refocusing pulses

OBJECTIVE

To explore the possibilities of proton spectroscopic imaging (1H-MRSI) of the human brain at 7 Tesla with adiabatic refocusing pulses.

MATERIALS AND METHODS

A combination of conventional slice selective excitation and two pairs of slice selective adiabatic refocusing pulses (semi-LASER) results in the formation of an echo from a localized volume. Depending on the used radio frequency (rf) coil efficiency and available rf power, the duration of the adiabatic full passage pulses (AFPs) is adapted to enable echo times down to 50 ms (head coil) or 30 ms (local surface coil).

RESULTS

An AFP duration of 5 ms with a corresponding bandwidth of 5.1 kHz resulted in a chemical shift displacement error of 23% over 3.8 ppm at 7T. Using a local surface coil and an echo time down to 30 ms, we detected not only the three main metabolites (NAA, Cr and Cho), but also coupled signals from myo-inositol and glutamate/glutamine in spectra from 0.14 cc voxels with linewidths down to 10 Hz in 10 min measurement time.

CONCLUSIONS

The semi-LASER pulse sequence enables 1H-MRSI of the human brain at 7T for larger parts of the brain as well as small localized areas with both a high spectral and spatial resolution.

A review of MR physics: 3T versus 1.5T

This article illustrates changes in the underlying physics concepts related to increasing the main magnetic field from 1.5T to 3T. The effects of these changes on tissue constants and practical hardware limitations is discussed as they affect scan time, quality, and contrast. Changes in susceptibility artifacts, chemical shift artifacts, and dielectric effects as a result of the increased field strength are also illustrated. Based on these fundamental considerations, an overall understanding of the benefits and constraints of signal-to-noise ratio and contrast-to-noise ratio changes between 1.5T and 3T MR systems is developed.

3D 1H MRSI of brain tumors at 3.0 Tesla using an eight-channel phased-array head coil

PURPOSE

To implement proton magnetic resonance spectroscopic imaging (1H MRSI) at 3 Tesla (3T) using an eight-channel phased-array head coil in a population of brain-tumor patients.

MATERIALS AND METHODS

A total of 49 MRI/MRSI examinations were performed on seven volunteers and 34 patients on a 3T GE Signa EXCITE scanner using body coil excitation and reception with an eight-channel phased-array head coil. 1H MRSI was acquired using point-resolved spectroscopy (PRESS) volume selection and three-dimensional (3D) phase encoding using a 144-msec echo time (TE).

RESULTS

The mean choline to N-acetyl aspartate ratio (Cho/NAA) was similar within regions of normal-appearing white matter (NAWM) in volunteers (0.5 +/- 0.04) and patients (0.6 +/- 0.1, P = 0.15). This ratio was significantly higher in regions of T2-hyperintensity lesion (T2L) relative to NAWM for patients (1.4 +/- 0.7, P = 0.001). The differences between metabolite intensities in lesions and NAWM were similar, but there was an increase in SNR of 1.95 when an eight-channel head coil was used at 3T vs. previous results at 1.5T.

CONCLUSION

The realized increase in SNR means that clinically relevant data can be obtained in five to 10 minutes at 3T and used to predict the spatial extent of tumor in a manner similar to that previously used to acquire 1.5T data in 17 minutes.

Regional variations and the effects of age and gender on glutamate concentrations in the human brain

Proton magnetic resonance spectroscopy was performed at 3 T using the echo time-averaged point-resolved spectroscopy method to determine the effects of age, gender and brain region on glutamate (Glu) concentrations in the healthy human brain. Thirty healthy men and 20 healthy women aged between 21 and 71 years were studied. Significant regional variations of Glu concentrations were observed. Glu concentration in the gray matter (GM) was approximately 25% higher than that in the white matter. Significant age-dependent decreases in Glu concentrations were observed in the basal ganglia (r=-0.75, P<.001) and parietal GM (r=-0.66, P<.001) of men but not those of women. Our findings demonstrate regional variations of Glu concentrations and suggest that the male brain may be more vulnerable to aging than the female brain. Our results also highlight the importance of brain region, age and gender matching in clinical studies.

Metabolic profiling of human brain metastases using in vivo proton MR spectroscopy at 3T

Background

Metastases to the central nervous system from different primary cancers are an oncologic challenge as the overall prognosis for these patients is generally poor. The incidence of brain metastases varies with type of primary cancer and is probably increasing due to improved therapies of extracranial metastases prolonging patient's overall survival and thereby time for brain metastases to develop. In addition, the greater access to improved neuroimaging techniques can provide earlier diagnosis. The aim of this study was to investigate the feasibility of using proton magnetic resonance spectroscopy (MRS) and multivariate analyses to characterize brain metastases originating from different primary cancers, to assess changes in spectra during radiation treatment and to correlate the spectra to clinical outcome after treatment.

Methods

Patients (n = 26) with brain metastases were examined using single voxel MRS at a 3T clinical MR system. Five patients were excluded due to poor spectral quality. The spectra were obtained before start (n = 21 patients), immediately after (n = 6 patients) and two months after end of treatment (n = 4 patients). Principal component analysis (PCA) and partial least square regression analysis (PLS) were applied in order to identify clustering of spectra due to origin of metastases and to relate clinical outcome (survival) of the patients to spectral data from the first MR examination.

Results

The PCA results indicated that brain metastases from primary lung and breast cancer were separated into two clusters, while the metastases from malignant melanomas showed no uniformity. The PLS analysis showed a significant correlation between MR spectral data and survival five months after MRS before start of treatment.

Conclusion

MRS determined metabolic profiles analysed by PCA and PLS might give valuable clinical information when planning and evaluating the treatment of brain metastases, and also when deciding to terminate further therapies.

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.