Short TE in vivo (1)H MR spectroscopic imaging at 1.5 T: acquisition and automated spectral analysis

Vespa: Integrated applications for RF pulse design, spectral simulation and MRS data analysis

PURPOSE

The Vespa package (Versatile Simulation, Pulses, and Analysis) is described and demonstrated. It provides workflows for developing and optimizing linear combination modeling (LCM) fitting for ¹ H MRS data using intuitive graphical user interface interfaces for RF pulse design, spectral simulation, and MRS data analysis. Command line interfaces for embedding workflows in MR manufacturer platforms and utilities for synthetic dataset creation are included. Complete provenance is maintained for all steps in workflows.

THEORY AND METHODS

Vespa is written in Python for compatibility across operating systems. It embeds the PyGAMMA spectral simulation library for spectral simulation. Multiprocessing methods accelerate processing and visualization. Applications use the Vespa database for results storage and cross-application access. Three projects demonstrate pulse, sequence, simulation, and data analysis workflows: (1) short TE semi-LASER single-voxel spectroscopy (SVS) LCM fitting, (2) optimizing MEGA-PRESS (MEscher-GArwood Point RESolved Spectroscopy) flip angle and LCM fitting, and (3) creating a synthetic short TE dataset.

RESULTS

The LCM workflows for in vivo basis set creation and spectral analysis showed reasonable results for both the short TE semi-LASER and MEGA-PRESS. Examples of pulses, simulations, and data fitting are shown in Vespa application interfaces for various steps to demonstrate the interactive workflow.

CONCLUSION

Vespa provides an efficient and extensible platform for characterizing RF pulses, pulse design, spectral simulation optimization, and automated LCM fitting via an interactive platform. Modular design and command line interface make it easy to embed in other platforms. As open source, it is free to the MRS community for use and extension. Vespa source code and documentation are available through GitHub.

Regional metabolite concentrations in the brain of healthy dogs measured by use of short echo time, single voxel proton magnetic resonance spectroscopy at 3.0 Tesla

OBJECTIVE

To investigate regional differences of relative metabolite concentrations in the brain of healthy dogs with short echo time, single voxel proton magnetic resonance spectroscopy ((1)H MRS) at 3.0 T.

ANIMALS

10 Beagles.

PROCEDURES

Short echo time, single voxel (1)H MRS was performed at the level of the right and left basal ganglia, right and left thalamus, right and left parietal lobes, occipital lobe, and cerebellum. Data were analyzed with an automated fitting method (linear combination model). Metabolite concentrations relative to water content were obtained, including N-acetyl aspartate, total choline, creatine, myoinositol, the sum of glutamine and glutamate (glutamine-glutamate complex), and glutathione. Metabolite ratios with creatine as the reference metabolite were calculated. Concentration differences between right and left hemispheres and sexes were evaluated with a Wilcoxon signed rank test and among various regions of the brain with an independent t test and 1-way ANOVA.

RESULTS

No significant differences were detected between sexes and right and left hemispheres. All metabolites, except the glutamine-glutamate complex and glutathione, had regional concentrations that differed significantly. The creatine concentration was highest in the basal ganglia and cerebellum and lowest in the parietal lobes. The N-acetyl aspartate concentration was highest in the parietal lobes and lowest in the cerebellum. Total choline concentration was highest in the basal ganglia and lowest in the occipital lobe.

CONCLUSIONS AND CLINICAL RELEVANCE

Metabolite concentrations differed among brain parenchymal regions in healthy dogs. This study may provide reference values for clinical and research studies involving (1)H MRS performed at 3.0 T.

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

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

The role of advanced MR imaging findings as biomarkers of traumatic brain injury

Treatment of traumatic brain injury (TBI) requires proper classification of the pathophysiology. Clinical classifiers and conventional neuroimaging are limited in TBI detection, outcome prediction, and treatment guidance. Advanced magnetic resonance imaging (MRI) techniques such as susceptibility weighted imaging, diffusion tensor imaging, and magnetic resonance spectroscopic imaging are sensitive to microhemorrhages, white matter injury, and abnormal metabolic activities, respectively, in brain injury. In this article, we reviewed these 3 advanced MRI methods and their applications in TBI and report some new findings from our research. These MRI techniques have already demonstrated their potential to improve TBI detection and outcome prediction. As such, they have demonstrated the capacity of serving as a set of biomarkers to reveal the heterogeneous and complex nature of brain injury in a regional and temporal manner. Further longitudinal studies using advanced MRI in a synergistic approach are expected to provide insight in understanding TBI and imaging implications for treatment.

Advanced neuroimaging in children with nonaccidental trauma

Physical abuse associated with nonaccidental trauma (NAT) affects approximately 144,000 children per year in the USA and, frequently, these injuries affect the developing brain. Most infants with suspected NAT are initially evaluated by skull X-rays and computed tomography to determine whether fractures are present, the severity of the acute injury and the need for urgent neurosurgical intervention. Increasingly, magnetic resonance imaging (MRI) is conducted as it provides additional diagnostic and prognostic information about the extent and nature of the injury. In this review, we examine 4 MRI techniques as they apply to children who present acutely after NAT. Susceptibility-weighted imaging is a 3-D high-resolution MRI technique that is more sensitive than conventional imaging in detecting hemorrhagic lesions that are often associated with diffuse axonal injury (DAI). Magnetic resonance spectroscopy acquires metabolite information reflecting neuronal integrity and function from multiple brain regions and provides a sensitive, noninvasive assessment of neurochemical alterations that offers early prognostic information regarding outcome. Diffusion-weighted imaging (DWI) is based on differences in the diffusion of water molecules within the brain and has been shown to be very sensitive in the early detection of ischemic injury. It is now being used to study the direct effects of traumatic injury as well as those due to secondary ischemia. Diffusion tensor imaging is a form of DWI and allows better evaluation of white matter fiber tracts by taking advantage of the intrinsic directionality (anisotropy) of water diffusion in the human brain. It has been shown to be useful in identifying white matter abnormalities after DAI when conventional imaging appears normal. Although these imaging methods have been studied primarily in adults and children with accidental traumatic brain injury, it is clear that they have the potential to provide additional value in the imaging and clinical evaluation of children with NAT.

MRS in presymptomatic MAPT mutation carriers: a potential biomarker for tau-mediated pathology

OBJECTIVE

To determine the proton magnetic resonance spectroscopy ((1)H MRS) changes in carriers of microtubule-associated protein (MAPT) mutations in a case-control study.

METHODS

Patients with MAPT mutations (N279K, V337M, R406W, IVS9-10G>T, P301L) from 5 different families (n = 24) underwent MRI and single voxel (1)H MRS from the posterior cingulate gyrus inferior precuneus at 3 T. Ten of the patients were symptomatic with median Clinical Dementia Rating sum of boxes score (CDR-SOB) of 6.5 and 14 patients were presymptomatic with CDR-SOB of 0. Age- and sex-matched controls (n = 24) were recruited.

RESULTS

Symptomatic MAPT mutation carriers were characterized by decreased N-acetylaspartate/creatine (NAA/Cr) ratio, an index of neuronal integrity, increased myoinositol (mI)/Cr ratio, a possible marker for glial activity, decreased NAA/mI, and hippocampal atrophy (p < 0.001). Whereas presymptomatic MAPT mutation carriers had elevated mI/Cr and decreased NAA/mI (p < 0.001), NAA/Cr levels and hippocampal volumes were not different from controls. Decrease in NAA/Cr (R(2) = 0. 22; p = 0.021) and hippocampal volumes (R(2) = 0.46; p < 0.001) were associated with proximity to the expected or actual age at symptom onset in MAPT mutation carriers.

CONCLUSION

(1)H MRS metabolite abnormalities characterized by an elevated mI/Cr and decreased NAA/mI are present several years before the onset of symptoms in MAPT mutation carriers. The data suggest an ordered sequencing of the (1)H MRS and MRI biomarkers. MI/Cr, a possible index of glial proliferation, precedes the decrease in neuronal integrity marker NAA/Cr and hippocampal atrophy. (1)H MRS may be a useful inclusion biomarker for preventive trials in presymptomatic carriers of MAPT mutations and possibly other proteinopathies.

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.

BMI and neuronal integrity in healthy, cognitively normal elderly: a proton magnetic resonance spectroscopy study

Recent studies associated excess body weight with brain structural alterations, poorer cognitive function, and lower prefrontal glucose metabolism. We found that higher BMI was related to lower concentrations of N-acetyl-aspartate (NAA, a marker of neuronal integrity) in a healthy middle-aged cohort, especially in frontal lobe. Here, we evaluated whether NAA was also associated with BMI in a healthy elderly cohort. We used 4 Tesla proton magnetic resonance spectroscopy ((1)H MRS) data from 23 healthy, cognitively normal elderly participants (69.4 +/- 6.9 years; 12 females) and measured concentrations of NAA, glutamate (Glu, involved in cellular metabolism), choline-containing compounds (Cho, involved in membrane metabolism), and creatine (Cr, involved in high-energy metabolism) in anterior (ACC) and posterior cingulate cortices (PCC). After adjustment for age, greater BMI was related to lower NAA/Cr and NAA/Cho ratios (beta < -0.56, P < 0.008) and lower Glu/Cr and Glu/Cho ratios (beta < -0.46, P < 0.02) in ACC. These associations were not significant in PCC (beta > -0.36, P > 0.09). The existence of an association between NAA and BMI in ACC but not in PCC is consistent with our previous study in healthy middle-aged individuals and with reports of lower frontal glucose metabolism in young healthy individuals with elevated BMI. Taken together, these results provide evidence that elevated BMI is associated with neuronal abnormalities mostly in frontal brain regions that subserve higher cognitive functions and impulse control. Future studies need to evaluate whether these metabolite abnormalities are involved in the development and maintenance of weight problems.

Metabolite proton T(2) mapping in the healthy rhesus macaque brain at 3 T

The structure and metabolism of the rhesus macaque brain, an advanced model for neurologic diseases and their treatment response, is often studied noninvasively with MRI and (1)H-MR spectroscopy. Due to the shorter transverse relaxation time (T(2)) at the higher magnetic fields these studies favor, the echo times used in (1)H-MR spectroscopy subject the metabolites to unknown T(2) weighting, decreasing the accuracy of quantification which is key for inter- and intra-animal comparisons. To establish the "baseline" (healthy animal) T(2) values, we mapped them for the three main metabolites' T(2)s at 3 T in four healthy rhesus macaques and tested the hypotheses that their mean values are similar (i) among animals; and (ii) to analogs regions in the human brain. This was done with three-dimensional multivoxel (1)H-MR spectroscopy at (0.6 x 0.6 x 0.5 cm)(3) = 180 microL spatial resolution over a 4.2 x 3.0 x 2.0 = 25 cm(3) ( approximately 30%) of the macaque brain in a two-point protocol that optimizes T(2) precision per unit time. The estimated T(2)s in several gray and white matter regions are all within 10% of those reported in the human brain (mean +/- standard error of the mean): N-acetylaspartate = 316 +/- 7, creatine = 177 +/- 3, and choline = 264 +/- 9 ms, with no statistically significant gray versus white matter differences.

1H magnetic resonance spectroscopy in dementia

Present data support the concept that (1)H magnetic resonance spectroscopy ((1)H MRS) may become an adjunct to clinical evaluation for differential diagnosis of dementia in the future. The value of (1)H MRS in monitoring the disease progression in dementia is expected to be in areas where group effects are sought such as monitoring effectiveness of therapies in drug trials. Elevation of myoinositol to creatine (mI/Cr) and choline to creatine (Cho/Cr) and reduction in the neuronal integrity marker N-acetylaspartate to creatine (NAA/Cr) levels in individuals with mild cognitive impairment and pre-symptomatic Alzheimer's disease suggests that (1)H MRS may also be valuable in predicting future development of dementia and monitoring early disease progression for preventive therapies. Investigations of in vivo (1)H MRS as a marker for differential diagnosis and progression of dementia, however, has been limited to clinically confirmed cohorts and remains to be validated by histopathology at autopsy. Overall, MRS is a promising investigational technique in ageing and dementia at this time. The potential clinical application of MRS in ageing and dementia, however, is growing with technical advances in the field.

(1)H MR spectroscopy of inflammation, infection and ischemia of the brain

Different pathologic patterns in multiple sclerosis (MS) are reflected by alterations of metabolites in (1)H MR spectroscopy of the brain. Elevated choline (Cho), lactate (Lac), lipids and macromolecules are reliable markers for acute demyelination regardless of the clinical entity (also in acute disseminated encephalomyelitis). N-acetyl-aspartate (NAA) is a suitable marker for neuronal integrity. It is reduced in acute MS lesions and in normal appearing white matter, even distant to acute and chronic-lesions. Recovery from reduced NAA levels to subnormal values during remyelination, and varying time courses of NAA in normal appearing white matter during relapsing remitting disease indicate the value of this spectroscopic marker for monitoring activity and recovery. Inositol (Ins) is increased in chronic MS lesions being a marker for astrocytic gliosis. In viral disease, Cho and Ins are always increased, whereas a reduction of NAA mostly reflects an advanced or a detoriated clinical state. In bacterial brain abscesses, numerous amino acids, lipids and Lac can be elevated. In ischemia, especially the Lac/NAA in comparison with perfusion and diffusion weighted imaging seems to be a new measure for areas of metabolic need, and may help to better characterise the penumbra of the stroke and the final infarct size.

Quantitative proton spectroscopic imaging of the neurochemical profile in rat brain with microliter resolution at ultra-short echo times

Proton spectroscopy allows the simultaneous quantification of a high number of metabolite concentrations termed the neurochemical profile. The spin echo full intensity acquired localization (SPECIAL) scheme with an echo time of 2.7 ms was used at 9.4T for excitation of a slab parallel to a home-built quadrature surface coil in conjunction with phase encoding in the two remaining spatial dimensions to yield an effective spatial resolution of 1.7 microL. The absolute concentrations of at least 10 metabolites were calculated from the spectra of individual voxels using LCModel analysis. The calculated concentrations were used for constructing quantitative metabolic maps of the neurochemical profile in normal and pathological rat brain. Summation of individual spectra was used to assess the neurochemical profile of unique brain regions, such as corpus callosum, in rat for the first time. Following focal ischemia in rat pups, imaging the neurochemical profile indicated increased choline groups in the ischemic core and increased glutamine in the penumbra, which is proposed to reflect glutamate excitotoxicity. We conclude that it is feasible to achieve a sensitivity that is sufficient for quantitative mapping of the neurochemical profile at microliter spatial resolution.

Improved model-based magnetic resonance spectroscopic imaging

Model-based techniques have the potential to reduce the artifacts and improve resolution in magnetic resonance spectroscopic imaging, without sacrificing the signal-to-noise ratio. However, the current approaches have a few drawbacks that limit their performance in practical applications. Specifically, the classical schemes use less flexible image models that lead to model misfit, thus resulting in artifacts. Moreover, the performance of the current approaches is negatively affected by the magnetic field inhomogeneity and spatial mismatch between the anatomical references and spectroscopic imaging data. In this paper, we propose efficient solutions to overcome these problems. We introduce a more flexible image model that represents the signal as a linear combination of compartmental and local basis functions. The former set represents the signal variations within the compartments, while the latter captures the local perturbations resulting from lesions or segmentation errors. Since the combined set is redundant, we obtain the reconstructions using sparsity penalized optimization. To compensate for the artifacts resulting from field inhomogeneity, we estimate the field map using alternate scans and use it in the reconstruction. We model the spatial mismatch as an affine transformation, whose parameters are estimated from the spectroscopy data.

GAVA: spectral simulation for in vivo MRS applications

An application that provides a flexible and easy to use interface to the GAMMA spectral simulation package is described that is targeted at investigations using in vivo MR spectroscopic methods. The program makes available a number of widely used spatially localized MRS pulse sequences and NMR parameters for commonly observed tissue metabolites, enabling spectra to be simulated for any pulse sequence parameter and viewed in an integrated display. The application is interfaced with a database for storage of all simulation parameters and results of the simulations. This application provides a convenient method for generating a priori spectral information used in parametric spectral analyses and for visual examination of the effects of difference pulse sequences and parameter settings.

Quantitative short echo-time 1H LASER-CSI in human brain at 4 T

A novel short echo-time (1)H chemical shift imaging (CSI) pulse sequence is presented that incorporates localization by adiabatic selective refocusing (LASER) for FOV-reduction, k-space weighted averaging and macromolecule subtraction, to obtain quantitative concentration measurements of N-acetyl-aspartate, glutamate, glucose, myo-inositol, creatine and choline using a nominal voxel size of 0.56 cm(3). A comparison of spectral quality and metabolite concentration measurements was made between LASER-CSI and LASER-single voxel spectroscopy (SVS) in a region of homogeneous parietal white matter (N = 8). No significant differences were found in linewidths, signal-to-noise ratios or the effectiveness of the macromolecule subtraction between SVS and CSI. Water suppression was 45% more effective in SVS than in CSI (p < 0.05). A linear regression of all paired metabolite measurements resulted in a slope = 1.01 +/- 0.03 (r(2) = 0.73). LASER-CSI concentration measurements of N-acetyl-aspartate, glutamate, glucose, myo-inositol, creatine and choline were in agreement (within standard deviations) with LASER-SVS measurements. LASER-CSI is, therefore, a viable and attractive option for future (1)H CSI investigations.

Single voxel proton magnetic resonance spectroscopy in post-stroke depression

Mood disorders are associated with structural, metabolic and spectroscopic changes in prefrontal regions. In the case of depression associated with stroke, there is little information about the biochemical profile of these regions, as assessed by proton magnetic resonance spectroscopy ((1)H-MRS). In a group of first-ever stroke patients, we studied the association between post-stroke depression and (1)H-MRS measurements in unaffected frontal lobes. Twenty-six patients with a first ischemic stroke located outside the frontal lobes were included in the study. Single voxel proton magnetic resonance spectroscopy ((1)H-MRS) was performed to assess N-acetylaspartate/creatine (NAA)/Cr, glutamate+glutamine (Glx)/Cr, choline (Cho)/Cr and myo-inositol (mI)/Cr ratios. Patients were assessed within the first 10 days after stroke and again four months later. The diagnosis of depression was made on the basis of clinical observation, interview and Hamilton Depression Rating Scale scores. In a group of 26 patients, eight (31%) met criteria for depression at the first assessment, and nine (35%) met criteria for depression at follow-up. Patients with depression in the immediate post-stroke phase had significantly higher Glx/Cr ratios in the contralesional hemisphere than non-depressive patients. No biochemical differences were found between the groups at 4-month follow-up. These findings suggest that post-stroke depression is accompanied by changes in frontal lobe glutamate/glutamine levels, perhaps reflecting abnormalities in glutamatergic transmission in the immediate post-stroke period.

Use of advanced neuroimaging techniques in the evaluation of pediatric traumatic brain injury

Advanced neuroimaging techniques are now used to expand our knowledge of traumatic brain injury, and increasingly, they are being applied to children. This review will examine four of these methods as they apply to children who present acutely after injury. (1) Susceptibility weighted imaging is a 3-dimensional high-resolution magnetic resonance imaging technique that is more sensitive than conventional imaging in detecting hemorrhagic lesions that are often associated with diffuse axonal injury. (2) Magnetic resonance spectroscopy acquires metabolite information reflecting neuronal integrity and function from multiple brain regions and provides sensitive, noninvasive assessment of neurochemical alterations that offers early prognostic information regarding the outcome. (3) Diffusion weighted imaging is based on differences in diffusion of water molecules within the brain and has been shown to be very sensitive in the early detection of ischemic injury. It is now being used to study the direct effects of traumatic injury as well as those due to secondary ischemia. (4) Diffusion tensor imaging is a form of diffusion weighted imaging and allows better evaluation of white matter fiber tracts by taking advantage of the intrinsic directionality (anisotropy) of water diffusion in human brain. It has been shown to be useful in identifying white matter abnormalities after diffuse axonal injury when conventional imaging appears normal. An important aspect of these advanced methods is that they demonstrate that 'normal-appearing' brain in many instances is not normal, i.e. there is evidence of significant undetected injury that may underlie a child's clinical status. Availability and integration of these advanced imaging methods will lead to better treatment and change the standard of care for use of neuroimaging to evaluate children with traumatic brain injury.

Effects of Alzheimer disease on fronto-parietal brain N-acetyl aspartate and myo-inositol using magnetic resonance spectroscopic imaging

Previous magnetic resonance (MR) spectroscopy studies of Alzheimer disease (AD) reporting reduced N-acetyl aspartate (NAA) and increased myo-Inositol (mI) used single voxel techniques, which have limited ability to assess the regional distribution of the metabolite abnormalities. The objective of this study was to determine the regional distribution of NAA and mI alterations in AD by using MR spectroscopic imaging. Fourteen patients with AD and 22 cognitively normal elderly were studied using structural MR imaging and MR spectroscopic imaging. Changes of NAA, mI, and various metabolite ratios were measured in frontal and parietal lobe gray matter (GM) and white matter. This study found: (1) when compared with cognitively normal subjects, AD patients had increased mI and mI/creatine (Cr) ratios primarily in parietal lobe GM, whereas frontal lobe GM and white matter were spared; (2) in the same region where mI was increased, AD patients had also decreased NAA and NAA/Cr ratios, replicating previous findings; (3) however, increased mI or mI/Cr ratios did not correlate with decreased NAA or NAA/Cr ratios; and (4) using mI/Cr and NAA/Cr together improved sensitivity and specificity to AD from control as compared with NAA/Cr alone. In conclusion, decreased NAA and increased mI in AD are primarily localized in parietal lobe GM regions. However, the NAA and mI changes are not correlated with each other, suggesting that they represent different processes that might help staging of AD.

Quantitative glutamate spectroscopic imaging of the human hippocampus

We have evaluated a three-dimensional localized spectroscopic imaging sequence that uses two pairs of adiabatic full-passage pulses, which optimizes the detection of glutamate resonances at moderate echo times. This sequence provides excellent volume localization while simultaneously reducing J-modulation losses of glutamate. We have simulated the performance of this sequence for glutamate and used it to quantitatively measure glutamate in the human hippocampus using a linear components model. Using tissue segmentation and regression analysis, we measured a glutamate concentration of 8.8 +/- 2.1 mM in hippocampal and temporal gray matter and 3.7 +/- 1.1 mM in temporal white matter (95% CI). We have used this approach in a small group of patients (n = 5) with unilateral hippocampal epilepsy.

Independent component analysis to proton spectroscopic imaging data of human brain tumours

In proton magnetic resonance spectroscopic imaging (1H MRSI), the recorded spectra are often linear combinations of spectra from different cell and tissue types within the voxel. This produces problems for data analysis and interpretation. A sophisticated approach is proposed here to handle the complexity of tissue heterogeneity in MRSI data. The independent component analysis (ICA) method was applied without prior knowledge to decompose the proton spectral components that relate to the heterogeneous cell populations with different proliferation and metabolism that are present in gliomas. The ability to classify brain tumours based on IC decomposite spectra was studied by grouping the components with histopathology. To this end, 10 controls and 34 patients with primary brain tumours were studied. The results indicate that ICA may reveal useful information from metabolic profiling for clinical purposes using long echo time MRSI of gliomas.

1H MR spectroscopy in common dementias

OBJECTIVE

To determine the 1H MR spectroscopic (MRS) findings and intergroup differences among common dementias: Alzheimer disease (AD), vascular dementia (VaD), dementia with Lewy bodies (DLB), and frontotemporal lobar degeneration (FTLD).

METHODS

The authors consecutively recruited 206 normal elderly subjects and 121 patients with AD, 41 with FTLD, 20 with DLB, and 8 with VaD. The 1H MRS metabolite ratio changes in common dementias were evaluated with respect to normal and also differences among the common dementias.

RESULTS

N-acetylaspartate (NAA)/creatine (Cr) was lower than normal in patients with AD, FTLD, and VaD. Myo-inositol (mI)/Cr was higher than normal in patients with AD and FTLD. Choline (Cho)/Cr was higher than normal in patients with AD, FTLD, and DLB. There were no metabolite differences between patients with AD and FTLD or between patients with DLB and VaD. NAA/Cr was lower in patients with AD and FTLD than DLB. MI/Cr was higher in patients with AD and FTLD than VaD. MI/Cr was also higher in patients with FTLD than DLB.

CONCLUSIONS

NAA/Cr levels are decreased in dementias that are characterized by neuron loss, such as AD, FTLD, and VaD. MI/Cr levels are elevated in dementias that are pathologically characterized by gliosis, such as AD and FTLD. Cho/Cr levels are elevated in dementias that are characterized by a profound cholinergic deficit, such as AD and DLB.

A Review of 1H MR Spectroscopy Findings in Alzheimer's Disease

Hydrogen-1 MR spectroscopy (MRS) studies demonstrate metabolic differences between patients who have Alzheimer's disease (AD) and cognitive normal age-matched controls. Clinical MRS also shows regional variations in metabolites between patients who have AD and those who have other dementias. Single-voxel and volumetric standard MRS techniques and automated data processing software are available for clinical MR scanners. Improvements in specificity and sensitivity of AD diagnosis, using MRS techniques as an adjunct to clinical imaging, are under evaluation. Multiparametric data analyses show, however, that metabolite changes correlate with in-vitro, postmortem, and metabolic changes and to changes in or predictions of cognitive scores.

Cigarette smoking exacerbates chronic alcohol-induced brain damage: a preliminary metabolite imaging study

BACKGROUND

Cigarette smoking is common among alcohol-dependent individuals. Nevertheless, previous research has typically not accounted for the potential independent or compounding effects of cigarette smoking on alcohol-induced brain injury and neurocognition.

METHODS

Twenty-four 1-week-abstinent recovering alcoholics (RAs; 14 smokers and 10 nonsmokers) in treatment and 26 light-drinking controls (7 smokers and 19 nonsmokers) were compared on measures of common brain metabolites in gray matter and white matter of the major lobes, basal ganglia, midbrain, and cerebellar vermis, obtained via multislice short-echo time proton magnetic resonance spectroscopic imaging. Smoking and nonsmoking RAs were also contrasted on measures of neurocognitive functioning, as well as laboratory markers of drinking severity and nutritional status.

RESULTS

Chronic alcohol dependence, independent of smoking, was associated with lower concentrations of frontal N-acetylaspartate (NAA) and frontal choline-containing compounds, as well as lower parietal and thalamic choline. Smoking RAs had lower NAA concentrations in frontal white matter and midbrain and lower midbrain choline than nonsmoking RAs. A four-group analysis of covariance also demonstrated that chronic cigarette smoking was associated with lower midbrain NAA and choline and with lower vermian choline. In smoking RAs, heavier drinking was associated with heavier smoking, which correlated with numerous subcortical metabolite abnormalities. The 1-week-abstinent smoking and nonsmoking RAs did not differ significantly on a brief neurocognitive battery. In smoking RAs, lower cerebellar vermis NAA was associated with poorer visuomotor scanning speed and incidental learning, and in nonsmoking RAs lower vermis NAA was related to poorer visuospatial learning and memory.

CONCLUSIONS

These human in vivo proton magnetic resonance spectroscopic imaging findings indicate that chronic cigarette smoking exacerbates chronic alcohol-induced neuronal injury and cell membrane damage in the frontal lobes of RAs and has independent adverse effects on neuronal viability and cell membranes in the midbrain and on cell membranes of the cerebellar vermis. Higher smoking levels are associated with metabolite concentrations in select subcortical structures. Greater consideration of the potential effects of comorbid cigarette smoking on alcohol-induced brain damage and other diseases affecting the central nervous system is warranted.

Magnetic resonance detects brainstem changes in chronic, active heavy drinkers

Neuropathological and neuroimaging studies show cortical and subcortical volume loss in alcohol-dependent individuals. Using quantitative magnetic resonance imaging (MRI) and proton magnetic resonance spectroscopic imaging ((1)H MRSI), we studied the size and potential cellular injury of the brainstem in untreated heavy alcohol drinkers. The brainstem is considered critical in the development and maintenance of drug and alcohol dependence. Two methods of brainstem size determination were compared: standard volumetry vs. midsagittal MR image area measurement. Heavy drinkers (n=12) and light drinkers (n=10) were compared with MRI; (1)H MRSI brainstem data were obtained from a subset of this cohort. Chronic heavy drinking was associated with significantly smaller midsagittal areas of the brainstem, midbrain, and pons, and with significantly smaller overall brainstem volume. Heavy drinking was also associated with significantly lower ratios of N-acetyl-aspartate and choline-containing metabolites compared with creatine-containing compounds in the brainstem, independent of brainstem atrophy. Additionally, brainstem volume and midsagittal brainstem area were correlated (r=0.78). These structural and metabolite findings are consistent with neuronal injury in the brainstem of untreated chronic heavy drinkers. The results also indicate that the midsagittal MRI brainstem area is an easily determined and reliable indicator of brainstem volume.

Time course of NAA T2 and ADCw in ischaemic stroke patients: 1H MRS imaging and diffusion-weighted MRI

BACKGROUND AND PURPOSE

Proton spectroscopy and quantitative diffusion-weighted imaging (DWI) were used to investigate the pertinence of N-acetyl aspartate (NAA) as a reliable marker of neuronal density in human stroke.

METHODS

The time courses of tissue water apparent diffusion coefficient (ADC(w)) and metabolite T2 were investigated on a plane corresponding to the largest area of cerebral infarction, within and outside the site of infarction in 71 patients with a large cortical middle cerebral artery territory infarction.

RESULTS

Significant reductions are seen in NAA T2 deep within the infarction during the period comprised between 5 and 20 days postinfarction; the relaxation times appearing to normalise several months after stroke. After an acute reduction in ADC(w), the pseudonormalisation of ADC(w) occurs at 8-12 days after the ischaemic insult. The minimum in N-acetyl aspartate T2 relaxation times and the pseudonormalisation of ADC(w) appear to coincide.

CONCLUSIONS

The data suggest that modifications in the behaviour of the observed proton metabolites occur during the period when the vasogenic oedema is formed and cell membrane integrity is lost. For this reason, NAA may not be a reliable marker of neuronal density during this period.

Effects of heavy drinking, binge drinking, and family history of alcoholism on regional brain metabolites

BACKGROUND

The main goals are to investigate the effects of chronic active heavy drinking on N-acetylaspartate (NAA) and other metabolites throughout the brain and to determine whether they are affected by family history (FH) of alcoholism and long-term drinking pattern.

METHODS

Forty-six chronic heavy drinkers (HD) and 52 light drinkers (LD) were recruited from the community and compared on measures of regional brain structure using magnetic resonance imaging and measures of common brain metabolites in gray matter (GM) and white matter (WM) of the major lobes, subcortical nuclei, brainstem, and cerebellum using short-echo time magnetic resonance spectroscopic imaging. Regional atrophy-corrected levels of NAA, myoinositol (mI), creatine, and choline-containing metabolites were compared as a function of group, FH of alcoholism, and bingeing.

RESULTS

Frontal WM NAA was lower in FH-negative HD than FH-positive HD and tended to be lower in women than men. Creatine-containing metabolites in parietal GM were higher in HD than LD. FH-negative compared with FH-positive HD also had more mI in the brainstem and tended to have lower NAA and more mI in frontal GM. Although parietal GM NAA was not significantly lower in HD than LD, it was lower in non-binge drinkers than bingers. Frontal WM NAA was lower in HD than LD, with the difference driven by a small number of women, FH-negative HD, and older age. Lower frontal WM NAA in HD was associated with lower executive and working memory functions and with lower P3b amplitudes at frontal electrodes.

CONCLUSIONS

Community-dwelling HD who are not in alcoholism treatment have brain metabolite changes that are associated with lower brain function and are likely of behavioral significance. Age, FH, and binge drinking modulate brain metabolite abnormalities. Metabolite changes in active HD are less pronounced and present with a different spatial and metabolite pattern than reported in abstinent alcoholics.

Comparison of inversion recovery preparation schemes for lipid suppression in 1H MRSI of human brain

To reduce contamination from subcutaneous lipid regions in MR spectroscopic imaging (MRSI) of whole brain, lipid signals are often suppressed using T(1) nulling methods. If a range of lipid T(1) values is present, the suppression efficiency will be improved using multiple inversion recovery (MIR) preparation. This study compared single IR (SIR) and double IR (DIR) applied with a volumetric MRSI sequence at 1.5 T based on experimental measurement of lipid T(1) and T(2) relaxation rates. At short and medium echo times (TEs), an approximately 28-47% improvement in lipid suppression was achieved with DIR compared to SIR. However, it also led to a loss of 37-43% in signal-to-noise ratio (SNR) for metabolites. Thus, SIR appears to be the better choice for suppressing lipid signals and maintaining metabolite sensitivity.

Improved spectral quality for 3D MR spectroscopic imaging using a high spatial resolution acquisition strategy

Spectral quality in (1)H MR spectroscopic imaging (MRSI) of the brain is often significantly degraded in regions subject to local magnetic susceptibility variations, which results in broadened and distorted spectral lineshapes. In this report, a modified acquisition strategy for volumetric echo-planar spectroscopic imaging (3D EPSI) is presented that extends the region of the brain that can be observed. The data are sampled at higher spatial resolution, then corrected for local B(0) shifts and reconstructed such that the final spatial resolution matches that of 3D EPSI data acquired with the conventional lower spatial resolution. Comparison of in vivo data obtained at 1.5 T with these two acquisition schemes shows that the high spatial resolution acquisition provides considerable reduction of spectral linewidths in many problematic brain regions, though with a reduction in signal-to-noise ratio by a factor of approximately 1.4 to 1.6 for the matrix sizes used in this study. However, the effect of the increased noise was largely offset by the improved spectral quality, leading to an overall improvement of the metabolite image quality obtained using automated spectral analysis.

Proton magnetic resonance spectroscopy with metabolite nulling reveals regional differences of macromolecules in normal human brain

PURPOSE

To quantify the macromolecular content in different anatomic brain regions and to evaluate an age dependency of the macromolecular concentrations.

MATERIAL AND METHODS

A short echo time Stimulated Echo Acquisition Mode (STEAM) sequence was used without and with inversion recovery metabolite nulling in 8-12 healthy volunteers. Quantitation was achieved by an extended LCModel, and macromolecular resonances at 0.9, 1.4, 2.1, and 3.0 ppm were evaluated.

RESULTS

In the cerebellum, the 1.4, 2.1, and 3.0 ppm resonances were highest compared to all other regions (P < 0.02); the 0.9 ppm resonance was significantly higher than that of pons (P < 0.01). In the motor cortex, the 0.9, 1.4, and 2.1 ppm resonances were higher than those of white matter and pons (P < 0.02). Pons and white matter did not differ significantly from each other. A significant correlation of the macromolecular concentrations with the age could not be found.

CONCLUSION

There were higher macromolecular concentrations in the cerebellum and motor cortex than in pons or white matter. These were probably due to the higher portions of gray matter in these volumes of interest (VOIs) than in the other regions.

Assessment of 3D proton MR echo-planar spectroscopic imaging using automated spectral analysis

For many clinical applications of proton MR spectroscopic imaging (MRSI) of the brain, diagnostic assessment is limited by insufficient coverage provided by single- or multislice acquisition methods as well as by the use of volume preselection methods. Additionally, traditional spectral analysis methods may limit the operator to detailed analysis of only a few selected brain regions. It is therefore highly desirable to use a fully 3D approach, combined with spectral analysis procedures that enable automated assessment of 3D metabolite distributions over the whole brain. In this study, a 3D echo-planar MRSI technique has been implemented without volume preselection to provide sufficient spatial resolution with maximum coverage of the brain. Using MRSI acquisitions in normal subjects at 1.5T and a fully automated spectral analysis procedure, an assessment of the resultant spectral quality and the extent of viable data in human brain was carried out. The analysis found that 69% of brain voxels were obtained with acceptable spectral quality at TE = 135 ms, and 52% at TE = 25 ms. Most of the rejected voxels were located near the sinuses or temporal bones and demonstrated poor B0 homogeneity and additional regions were affected by stronger lipid contamination at TE = 25 ms.

Short echo time multislice proton magnetic resonance spectroscopic imaging in human brain: metabolite distributions and reliability

Multislice proton magnetic resonance spectroscopic imaging (1H MRSI) at 25 ms echo time was used to measure concentrations of myo-inositol (mI), N-acetylaspartate (NAA), and creatine (Cr) and choline (Cho) in ten normal subjects between 22 and 84 years of age (mean age 44 +/- 18 years). By co-analysis with MRI based tissue segmentation results, metabolite distributions were analyzed for each tissue type and for different brain regions. Measurement reliability was evaluated using intraclass correlation coefficients (ICC). Significant differences in metabolite distributions were found for all metabolites. mI of frontal gray matter was 84% of parietal gray matter and 87% of white matter. NAA of frontal gray matter was 86% of parietal gray matter and 85% of white matter. Cho of frontal gray matter was 125% of parietal gray matter and 59% of white matter and Cho of parietal gray matter was 47% of white matter. Cr of parietal gray matter was 113% of white matter. Reliability was relatively high (ICC from.70 to.93) for all metabolites in white matter and for NAA and Cr in gray matter, though limited (ICC less than.63) for mI and Cho in gray matter. These findings indicate that voxel gray/white matter contributions, regional variations in metabolite concentrations, and reliability limitations must be considered when interpreting 1H MR spectra of the brain.

Representation of strong baseline contributions in 1H MR spectra

A comparison is made between two optimization procedures and two data models for automated analysis of in vivo proton MR spectra of brain, typical of that obtained using MR spectroscopic imaging at 1.5 Tesla. First, a shift invariant wavelet filter is presented that provides improved performance over a conventional wavelet filter method for characterizing smoothly varying baseline signals. Next, two spectral fitting methods are described: an iterative spectral analysis method that alternates between optimizing a parametric description of metabolite signals and nonparametric characterization of baseline contributions, and a single-pass method that optimizes a complete spectral and baseline model. Both methods are evaluated using wavelet and spline models of the baseline function. Results are shown for Monte Carlo simulations of data representative of both long and short TE, in vivo 1H acquisitions.