Short echo time proton magnetic resonance spectroscopic imaging of macromolecule and metabolite signal intensities in the human brain

Hippocampal single-voxel MR spectroscopy with a long echo time at 3 T using semi-LASER sequence

The hippocampus is one of the most challenging brain regions for proton MR spectroscopy (MRS) applications. Moreover, quantification of J-coupled species such as myo-inositol (m-Ins) and glutamate + glutamine (Glx) is affected by the presence of macromolecular background. While long echo time (TE) MRS eliminates the macromolecules, it also decreases the m-Ins and Glx signal and, as a result, these metabolites are studied mainly with short TE. Here, we investigate the feasibility of reproducibly measuring their concentrations at a long TE of 120 ms, using a semi-adiabatic localization by adiabatic selective refocusing (sLASER) sequence, as this sequence was recently recommended as a standard for clinical MRS. Gradient offset-independent adiabatic refocusing pulses were implemented, and an optimal long TE for the detection of m-Ins and Glx was determined using the T2 relaxation times of macromolecules. Metabolite concentrations and their coefficients of variation (CVs) were obtained for a 3.4-mL voxel centered on the left hippocampus on 3-T MR systems at two different sites with three healthy subjects (aged 32.5 ± 10.2 years [mean ± standard deviation]) per site, with each subject scanned over two sessions, and with each session comprising three scans. Concentrations of m-Ins, choline, creatine, Glx and N-acetyl-aspartate were 5.4 ± 1.5, 1.7 ± 0.2, 5.8 ± 0.3, 11.6 ± 1.2 and 5.9 ± 0.4 mM (mean ± standard deviation), respectively. Their respective mean within-session CVs were 14.5% ± 5.9%, 6.5% ± 5.3%, 6.0% ± 3.4%, 10.6% ± 6.2% and 3.5% ± 1.4%, and their mean within-subject CVs were 17.8% ± 18.2%, 7.5% ± 6.3%, 7.4% ± 6.4%, 12.4% ± 5.3% and 4.8% ± 3.0%. The between-subject CVs were 25.0%, 12.3%, 5.3%, 10.7% and 6.4%, respectively. Hippocampal long-TE sLASER single voxel spectroscopy can provide macromolecule-independent assessment of all major metabolites including Glx and m-Ins.

Contribution of macromolecules to brain 1 H MR spectra: Experts' consensus recommendations

Proton MR spectra of the brain, especially those measured at short and intermediate echo times, contain signals from mobile macromolecules (MM). A description of the main MM is provided in this consensus paper. These broad peaks of MM underlie the narrower peaks of metabolites and often complicate their quantification but they also may have potential importance as biomarkers in specific diseases. Thus, separation of broad MM signals from low molecular weight metabolites enables accurate determination of metabolite concentrations and is of primary interest in many studies. Other studies attempt to understand the origin of the MM spectrum, to decompose it into individual spectral regions or peaks and to use the components of the MM spectrum as markers of various physiological or pathological conditions in biomedical research or clinical practice. The aim of this consensus paper is to provide an overview and some recommendations on how to handle the MM signals in different types of studies together with a list of open issues in the field, which are all summarized at the end of the paper.

MEGA-PRESS of GABA+: Influences of acquisition parameters

γ-aminobutyric acid (GABA) was the first molecule that was edited with MEGA-PRESS. GABA edited spectroscopy is challenged by limited selectivity of editing pulses. Coediting of resonances from macromolecules (MM) is the greatest single limitation of GABA edited spectroscopy. In this contribution, relative signal contributions from GABA, MM and homocarnosine to the total MEGA-PRESS edited signal at ~3 ppm, i.e., GABA+, are simulated at 3 tesla using several acquisition schemes. The base scheme is modeled after those currently supplied by vendors: it uses typical pulse shapes and lengths, it minimizes the first echo time (TE), and the delay between the editing pulses is kept at TE/2. Edited spectra are simulated for imperfect acquisition parameters such as incorrect frequency, larger chemical shift displacement, incorrect transmit B₁ -field calibration for localization and editing pulses, and longer TE. An alternative timing scheme and longer editing pulses are also considered. Additional simulations are performed for symmetric editing around the MM frequency to suppress the MM signal. The relative influences of these acquisition parameters on the constituents of GABA+ are examined from the perspective of modern experimental designs for investigating brain GABA concentration differences in healthy and diseased humans. Other factors that influence signal contributions, such as T₁ and T₂ relaxation times are also considered.

A novel method to measure T1 -relaxation times of macromolecules and quantification of the macromolecular resonances

PURPOSE

Macromolecular peaks underlying metabolite spectra influence the quantification of metabolites. Therefore, it is important to understand the extent of contribution from macromolecules (MMs) in metabolite quantification. However, to model MMs more accurately in spectral fitting, differences in T₁ relaxation times among individual MM peaks must be considered. Characterization of T₁ -relaxation times for all individual MM peaks using a single inversion recovery technique is difficult due to eventual contributions from metabolites. On the contrary, a double inversion recovery (DIR) technique provided flexibility to acquire MM spectra spanning a range of longitudinal magnetizations with minimal metabolite influence. Thus, a novel method to determine T₁ -relaxation times of individual MM peaks is reported in this work.

METHODS

Extensive Bloch simulations were performed to determine inversion time combinations for a DIR technique that yielded adequate MM signal with varying longitudinal magnetizations while minimizing metabolite contributions. MM spectra were acquired using DIR-metabolite-cycled semi-LASER sequence. LCModel concentrations were fitted to the DIR signal equation to calculate T₁ -relaxation times.

RESULTS

T₁ -relaxation times of MMs range from 204 to 510 ms and 253 to 564 ms in gray- and white-matter rich voxels respectively at 9.4T. Additionally, concentrations of 13 MM peaks are reported.

CONCLUSION

A novel DIR method is reported in this work to calculate T₁ -relaxation times of MMs in the human brain. T₁ -relaxation times and relaxation time corrected concentrations of individual MMs are reported in gray- and white-matter rich voxels for the first time at 9.4T.

Altered macromolecular pattern and content in the aging human brain

The resonances originating from proteins underlie those of metabolites in brain ¹ H nuclear magnetic resonance (NMR) spectra. These resonances have different physical properties from those of metabolites, such as shorter T₁ and T₂ relaxation time constants. The age dependence of the macromolecular pattern and content in the human brain was investigated with a focus on adults over 66 years of age using ultrahigh-field in vivo magnetic resonance spectroscopy. Eighteen young and 23 cognitively normal older adults were studied at 7 T. Metabolite spectra were acquired in the occipital cortex and the posterior cingulate cortex with single-voxel stimulated echo acquisition mode (STEAM) spectroscopy in 14 young and 20 older adults. Macromolecular spectra were acquired in the occipital cortex using an inversion recovery STEAM sequence in four young and three older adults. The macromolecular pattern was apparent over the 0.5-4.5-ppm range in the inversion recovery spectra and the 0.5-2-ppm range in the metabolite spectra. Macromolecular content was quantified from metabolite spectra using LCModel and from inversion recovery spectra using integration. Age-associated differences in the macromolecular pattern were apparent via both types of spectra, with the largest difference observed for the 1.7- and 2-ppm macromolecular resonances. A higher macromolecular content was observed in the older adults for both brain regions. Age-specific macromolecular spectra are needed when comparing metabolite spectra from subjects of differing ages because of age-associated differences in macromolecular pattern. Age-associated pattern and content differences may provide information about the aging process.

A method for longitudinal relaxation time measurement in inhomogeneous fields

The spin-lattice relaxation time (T₁) plays a crucial role in the study of spin dynamics, signal optimization and data quantification. However, the measurement of chemical shift-specific T₁ constants is hampered by the magnetic field inhomogeneity due to poorly shimmed external magnetic fields or intrinsic magnetic susceptibility heterogeneity in samples. In this study, we present a new protocol to determine chemical shift-specific T₁ constants in inhomogeneous fields. Based on intermolecular double-quantum coherences, the new method can resolve overlapped peaks in inhomogeneous fields. The measurement results are in consistent with the measurements in homogeneous fields using the conventional method. Since spatial encoding technique is involved, the experimental time for the new method is very close to that for the conventional method. With the aid of T₁ knowledge, some concealed information can be exploited by T₁ weighting experiments.

Simultaneous mapping of metabolites and individual macromolecular components via ultra-short acquisition delay 1 H MRSI in the brain at 7T

Purpose

Short‐echo‐time proton MR spectra at 7T feature nine to 10 distinct macromolecule (MM) resonances that overlap with the signals of metabolites. Typically, a metabolite‐nulled in vivo MM spectrum is included in the quantification`s prior knowledge to provide unbiased metabolite quantification. However, this MM model may fail if MMs are pathologically altered. In addition, information about the individual MM peaks is lost. In this study, we aimed to create an improved MM model by parameterization of the in vivo MM spectrum into individual components, and to use this new model to quantify free induction decay MR spectroscopic imaging (FID‐MRSI) data.

Methods

The measured in vivo MM spectrum was parameterized using advanced method for accurate, robust, and efficient spectral fitting (AMARES) and Hankel‐Lanczos singular value decomposition algorithms from which six different MM models were derived. Soft constraints were applied to avoid over‐parameterization. All MM models were combined with simulated metabolite spectra to form complete basis sets. FID‐MRSI data from 14 healthy volunteers were quantified via LCModel, and the results were compared between all basis sets.

Results

The MM model using nine individual AMARES‐parameterized MM components with additional soft constraints achieved the most reliable results. Nine MMs and seven metabolites were mapped simultaneously over the whole slice.

Conclusion

The proposed MM model may facilitate studies that involve patients with pathologically altered MMs. Magn Reson Med 79:1231–1240, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

The age dependence of T2 relaxation times of N-acetyl aspartate, creatine and choline in the human brain at 3 and 4T

Knowledge of the T2 age dependence is of importance for MRS clinical studies involving subject groups with a wide age range. A number of studies have focused on the age dependence of T2 values in the human brain, with rather conflicting results. The aim of this study was to analyze the age dependence of T2 values of N-acetyl aspartate (NAA), creatine (Cr) and choline (Cho) in the human brain using data acquired at 3T and 4T and to assess the influence of the macromolecule (MM) baseline handling on the obtained results. Two distinct groups of young and elderly controls have been measured at 3T (TE = 30-540 ms, 9 young and 11 elderly subjects) and 4T (TE = 10-180 ms, 18 young and 14 elderly subjects) using single-voxel spectroscopy. In addition, MM spectra were measured from two subjects using the inversion-recovery technique at 4T. All spectra were processed with LCModel using basis sets with different MM signals (measured or simulated) and also with MM signals included for a different TE range. Individual estimated T2 values were statistically analyzed using the R programming language for the age dependence of T2 values as well as the influence of the MM baseline handling. A significant decrease of T2 values of NAA and Cr in elderly subjects compared with young subjects was confirmed. The same trend was observed for Cho. Significantly higher T2 values calculated using the measured MM baseline for all studied metabolites at 4T were observed for both young and elderly subjects. To conclude, while the handling of MM and lipid signals may have a significant effect on estimated T2 values, we confirmed the age dependence of T2 values of NAA and Cr and the same trend for Cho in the human brain. Copyright © 2016 John Wiley & Sons, Ltd.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Measuring glucose concentrations in the rat brain using echo-time-averaged point resolved spectroscopy at 7 tesla

Glucose has multiple functions in the brain, and there is interest in estimating in vivo concentrations rather than merely the uptake determined by nuclear medicine. Glucose can be estimated using magnetic resonance spectroscopy, but measurement is difficult due to its multiple J-coupled proton signals overlapping with other metabolite signals. To minimize the effect of interfering signals, echo time (TE) values between 60 and 95 ms were averaged, and the loss in signal due to the T2 effect was corrected in both the estimation of glucose concentration and in creation of the basis files for fitting. The effectiveness of the TE-averaging method was evaluated by measuring the glucose concentration in fasted rats before and after feeding. The brain glucose in all rats increased after feeding with fasted and fed glucose-to-creatine ratios of 0.15 ± 0.03 and 0.24 ± 0.04, respectively. Data at a short TE of 13 ms measured ratios of 0.30 ± 0.16 and 0.36 ± 0.24 for the fasted and fed rats, respectively, demonstrating the difficulty in obtaining reliable glucose measurements at short TE. Overall, TE averaging minimizes the influence of macromolecular signals and nearby peaks to give precise, consistent estimates of glucose.

Improved initial value estimation for short echo time magnetic resonance spectroscopy spectral analysis using short T2 signal attenuation

Robust spectral analysis of magnetic resonance spectroscopy data frequently uses a spectral model with prior metabolite signal information within a nonlinear least squares optimization algorithm. Starting values for the spectral model greatly influence the final results. Short echo time magnetic resonance spectroscopy contains broad signals that overlap with metabolite signals, complicating the estimation of starting values. We describe a method for more robust initial value estimation using a filter to attenuate short T(2) signal contributions (e.g., macromolecules or residual lipids). The method attenuates signals by truncating early points in the data set. Metabolite peak estimation is simplified by the removal of broad, short T(2) signals, and corrections for metabolite signal truncation are described. Short echo time simulated Monte Carlo data and in vivo data were used to validate the method. Areas for metabolite signals in the Monte Carlo data with singlet (N-acetylaspartate, creatine, choline) and singlet-like (myo-inositol) resonances were estimated within 10% of actual value for various metabolite line widths, signal-to-noise ratios, and underlying broad signal contributions. Initial value estimates of in vivo magnetic resonance spectroscopy data were within 14% of metabolite area ratios relative to the creatine peak fitted using established magnetic resonance spectroscopy spectral analysis software.

J-refocused coherence transfer spectroscopic imaging at 7 T in human brain

Short echo spectroscopy is commonly used to minimize signal modulation due to J-evolution of the cerebral amino acids. However, short echo acquisitions suffer from high sensitivity to macromolecules which make accurate baseline determination difficult. In this report, we describe implementation at 7 T of a double echo J-refocused coherence transfer sequence at echo time (TE) of 34 msec to minimize J-modulation of amino acids while also decreasing interfering macromolecule signals. Simulation of the pulse sequence at 7 T shows excellent resolution of glutamate, glutamine, and N-acetyl aspartate. B(1) sufficiency at 7 T for the double echo acquisition is achieved using a transceiver array with radiofrequency (RF) shimming. Using an alternate RF distribution to minimize receiver phase cancellation in the transceiver, accurate phase determination for the coherence transfer is achieved with rapid single scan calibration. This method is demonstrated in spectroscopic imaging mode with n = 5 healthy volunteers resulting in metabolite values consistent with literature and in a patient with epilepsy.

Detection of cancer in cervical tissue biopsies using mobile lipid resonances measured with diffusion-weighted (1)H magnetic resonance spectroscopy

The purpose of this study was to implement a diffusion-weighted sequence for visualisation of mobile lipid resonances (MLR) using high resolution magic angle spinning (HR-MAS) (1)H MRS and to evaluate its use in establishing differences between tissues from patients with cervical carcinoma that contain cancer from those that do not. A stimulated echo sequence with bipolar gradients was modified to allow T(1) and T(2) measurements and optimised by recording signal loss in HR-MAS spectra as a function of gradient strength in model lipids and tissues. Diffusion coefficients, T(1) and apparent T(2) relaxation times were measured in model lipid systems. MLR profiles were characterised in relation to T(1) and apparent T(2) relaxation in human cervical cancer tissue samples. Diffusion-weighted (DW) spectra of cervical biopsies were quantified and peak areas analysed using linear discriminant analysis (LDA). The optimised sequence reduced spectral overlap by suppressing signals originating from low molecular weight metabolites and non-lipid contributions. Significantly improved MLR visualisation allowed visualisation of peaks at 0.9, 1.3, 1.6, 2.0, 2.3, 2.8, 4.3 and 5.3 ppm. MLR analysis of DW spectra showed at least six peaks arising from saturated and unsaturated lipids and those arising from triglycerides. Significant differences in samples containing histologically confirmed cancer were seen for peaks at 0.9 (p < 0.006), 1.3 (p < 0.04), 2.0 (p < 0.03), 2.8 (p < 0.003) and 4.3 ppm (p < 0.0002). LDA analysis of MLR peaks from DW spectra almost completely separated two clusters of cervical biopsies (cancer, 'no-cancer'), reflecting underlying differences in MLR composition. Generated Receiver Operating Characteristic (ROC) curves and calculated area under the curve (0.962) validated high sensitivity and specificity of the technique. Diffusion-weighting of HR-MAS spectroscopic sequences is a useful method for characterising MLR in cancer tissues and displays an accumulation of lipids arising during tumourigenesis and an increase in the unsaturated lipid and triglyceride peaks with respect to saturated MLR.

Brain metabolite proton T2 mapping at 3.0 T in relapsing-remitting multiple sclerosis

PURPOSE

To test the hypothesis that T2 signals in lesions and normal-appearing tissue are sufficiently similar that signal variations represent true variations in metabolite concentration.

MATERIALS AND METHODS

The T2 distributions of N-acetylaspartate (NAA), creatine (Cr), and choline (Cho) at 3.0 T were mapped in the brain of 10 relapsing-remitting (RR) MS patients of 0.3-12 years disease duration with multivoxel (four sections of 80 1-cm(3) voxels) point-resolved proton spectroscopy imaging in a two-point protocol. Institutional review board approval and written informed consent were obtained; the study was Health Insurance Portability and Accountability-compliant. Mixed-model analysis of variance was performed to compare brain regions and lesion types for each metabolite; a Wilcoxon test was performed to compare observed T2 values with age-based predictions.

RESULTS

The T2 histograms from 320 voxels in each patient were similar in peak position for mean values (+/- standard error) for NAA (250 msec +/- 9), Cr (166 msec +/- 3), and Cho (221 msec +/- 6); shape was characterized by full width at half maximum values of 174 msec +/- 11, 98 msec +/- 3, and 143 msec +/- 5, respectively. Regional T2 values in white matter (WM; 298 msec +/- 6, 162 msec +/- 1, and 222 msec +/- 4 for NAA, Cr, and Cho, respectively) were all significantly longer than in gray matter (GM; 221 msec +/- 7, 143 msec +/- 4, and 205 msec +/- 8, respectively) but not different from isointense (313 msec +/- 24, 188 msec +/- 12, and 238 msec +/- 17, respectively) or hypointense (296 msec +/- 27, 163 msec +/- 12, and 199 msec +/- 12, respectively) lesions, except for the Cho value for hypointense lesion, which was significantly lower. When compared with corresponding values in healthy contemporaries, these T2 values were shorter by 18%, 8%, and 14% in GM and by 21%, 12%, and 13% in WM for NAA, Cr, and Cho, respectively.

CONCLUSION

For the purpose of metabolic quantification at 3.0 T and echo times of less than 100 msec, an average T2 value per metabolite should suffice for any brain region and lesion regardless of disease duration, age, or disability in any RR MS patient and their controls. (c) RSNA, 2010.

In vivo proton MRS to quantify anesthetic effects of pentobarbital on cerebral metabolism and brain activity in rat

To quantitatively investigate the effects of pentobarbital anesthesia on brain activity, brain metabolite concentrations and cerebral metabolic rate of glucose, in vivo proton MR spectra, and electroencephalography were measured in the rat brain with various doses of pentobarbital. The results show that (1) the resonances attributed to propylene glycol, a solvent in pentobarbital injection solution, can be robustly detected and quantified in the brain; (2) the concentration of most brain metabolites remained constant under the isoelectric state (silent electroencephalography) with a high dose of pentobarbital compared to mild isoflurane anesthesia condition, except for a reduction of 61% in the brain glucose level, which was associated with a 37% decrease in cerebral metabolic rate of glucose, suggesting a significant amount of "housekeeping" energy for maintaining brain cellular integrity under the isoelectric state; and (3) electroencephalography and cerebral metabolic activities were tightly coupled to the pentobarbital anesthesia depth and they can be indirectly quantified by the propylene glycol resonance signal at 1.13 ppm. This study indicates that in vivo proton MR spectroscopy can be used to measure changes in cerebral metabolite concentrations and cerebral metabolic rate of glucose under varied pentobarbital anesthesia states; moreover, the propylene glycol signal provides a sensitive biomarker for quantitatively monitoring these changes and anesthesia depth noninvasively.

Measurement sequences for single voxel proton MR spectroscopy

Proton spectroscopy is easy to perform at clinically used whole body MR scanners, since the necessary hardware is identical to that of MR imaging. Only specific measurements sequences for the acquisition of volume selected spectral information and appropriate evaluation software have to be installed. Several techniques for a volume-selective spectroscopy have been proposed, but only two are widely used in clinical examinations: the double spin-echo sequence (point resolved spectroscopy sequence (PRESS)) and the stimulated echo sequence (stimulated echo acquisition mode (STEAM)). The properties of these sequences are described and additional techniques for artifact reduction and the extraction of selected signals are explained.

(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.

Proton echo-planar spectroscopic imaging of J-coupled resonances in human brain at 3 and 4 Tesla

In this multicenter study, 2D spatial mapping of J-coupled resonances at 3T and 4T was performed using short-TE (15 ms) proton echo-planar spectroscopic imaging (PEPSI). Water-suppressed (WS) data were acquired in 8.5 min with 1-cm(3) spatial resolution from a supraventricular axial slice. Optimized outer volume suppression (OVS) enabled mapping in close proximity to peripheral scalp regions. Constrained spectral fitting in reference to a non-WS (NWS) scan was performed with LCModel using correction for relaxation attenuation and partial-volume effects. The concentrations of total choline (tCho), creatine + phosphocreatine (Cr+PCr), glutamate (Glu), glutamate + glutamine (Glu+Gln), myo-inositol (Ins), NAA, NAA+NAAG, and two macromolecular resonances at 0.9 and 2.0 ppm were mapped with mean Cramer-Rao lower bounds (CRLBs) between 6% and 18% and approximately 150-cm(3) sensitive volumes. Aspartate, GABA, glutamine (Gln), glutathione (GSH), phosphoethanolamine (PE), and macromolecules (MMs) at 1.2 ppm were also mapped, although with larger mean CRLBs between 30% and 44%. The CRLBs at 4T were 19% lower on average as compared to 3T, consistent with a higher signal-to-noise ratio (SNR) and increased spectral resolution. Metabolite concentrations were in the ranges reported in previous studies. Glu concentration was significantly higher in gray matter (GM) compared to white matter (WM), as anticipated. The short acquisition time makes this methodology suitable for clinical studies.

Application of MRS to mouse models of neurodegenerative illness

The rapid development of transgenic mouse models of neurodegenerative diseases, in parallel with the rapidly expanding growth of MR techniques for assessing in vivo, non-invasive, neurochemistry, offers the potential to develop novel markers of disease progression and therapy. In this review we discuss the interpretation and utility of MRS for the study of these transgenic mouse and rodent models of neurodegenerative diseases such as Alzheimer's (AD), Huntington's (HD) and Parkinson's disease (PD). MRS studies can provide a wealth of information on various facets of in vivo neurochemistry, including neuronal health, gliosis, osmoregulation, energy metabolism, neuronal-glial cycling, and molecular synthesis rates. These data provide information on the etiology, natural history and therapy of these diseases. Mouse models enable longitudinal studies with useful time frames for evaluation of neuroprotection and therapeutic interventions using many of the potential MRS markers. In addition, the ability to manipulate the genome in these models allows better mechanistic understanding of the roles of the observable neurochemicals, such as N-acetylaspartate, in the brain. The argument is made that use of MRS, combined with correlative histology and other MRI techniques, will enable objective markers with which potential therapies can be followed in a quantitative fashion.

Concentrations and magnetization transfer ratios of metabolites in gray and white matter

The concentrations and magnetization transfer ratios (MTRs) in gray matter (GM) and white matter (WM) of N-acetyl aspartate (NAA), creatine (Cr), choline (Cho), myo-inositol (Ins), and glutamate plus glutamine (Glx) were investigated using magnetic resonance spectroscopic imaging (MRSI). The macromolecule (MM) baseline was studied separately using a metabolite-nulling inversion. Three data sets were collected from a point-resolved spectroscopy (PRESS)-selected volume (TE/TR = 30/3000 ms) of human frontal lobe in vivo: one with MT pulses applied, one with an inversion pulse to null small metabolites, and one with no inversion or MT pulses. The MM signal, which was analyzed by integrating the metabolite-nulled spectrum between 0 and 3 ppm, was estimated to be 38% higher in GM than in WM. MM subtraction decreased the signal-to-noise ratio (SNR) and also decreased the reliability of LCModel quantification of most metabolites, but may have improved the accuracy of quantification of Glx. Glx and Cr were both found to correlate strongly with the GM volume fraction of the voxels. Cr showed the highest MTR, but the other metabolites also showed some attenuation of signal when the MT pulses were applied. The MTRs did not correlate with the GM volume fraction, which implies that the local environment of metabolites does not differ markedly between GM and WM.

Human brain-structure resolvedT2 relaxation times of proton metabolites at 3 tesla

The transverse relaxation times, T(2), of N-acetylaspartate (NAA), total choline (Cho), and creatine (Cr) obtained at 3T in several human brain regions of eight healthy volunteers are reported. They were obtained simultaneously in 320 voxels with three-dimensional (3D) proton MR spectroscopy ((1)H-MRS) at 1 cm(3) spatial resolution. A two-point protocol, optimized for the least error per given time by adjusting both the echo delay (TE(i)) and number of averages, N(i), at each point, was used. Eight healthy subjects (four males and four females, age = 26 +/- 2 years) underwent the hour-long procedure of four 15-min, 3D acquisitions (TE(1) = 35 ms, N(1) = 1; and TE(2) = 285 ms, N(2) = 3). The results reveal that across all subjects the NAA and Cr T(2)s in gray matter (GM) structures (226 +/- 17 and 137 +/- 12 ms, respectively) were 13-17% shorter than the corresponding T(2)s in white matter (WM; 264 +/- 10 and 155 +/- 7 ms, respectively). The T(2)s of Cho did not differ between GM and WM (207 +/- 17 and 202 +/- 8, respectively). For the purpose of metabolic quantification, these values justify to within +/-10% the previous use of one T(2) per metabolite for 1) the entire brain and 2) all subjects. These T(2) values (which to our knowledge were obtained for the first time at this field, spatial resolution, coverage, and precision) are essential for reliable absolute metabolic quantification.

High magnetic field water and metabolite proton T1 and T2 relaxation in rat brain in vivo

Comprehensive and quantitative measurements of T1 and T2 relaxation times of water, metabolites, and macromolecules in rat brain under similar experimental conditions at three high magnetic field strengths (4.0 T, 9.4 T, and 11.7 T) are presented. Water relaxation showed a highly significant increase (T1) and decrease (T2) with increasing field strength for all nine analyzed brain structures. Similar but less pronounced effects were observed for all metabolites. Macromolecules displayed field-independent T2 relaxation and a strong increase of T1 with field strength. Among other features, these data show that while spectral resolution continues to increase with field strength, the absolute signal-to-noise ratio (SNR) in T1/T2-based anatomical MRI quickly levels off beyond approximately 7 T and may actually decrease at higher magnetic fields.

MR spectroscopy: its potential role for drug development for the treatment of psychiatric diseases

Magnetic resonance spectroscopy (MRS) is likely in the near future to play a key role in the process of drug discovery and evaluation. As the pharmaceutical industry seeks biochemical markers of drug delivery, efficacy and toxicity, this non-invasive technique offers numerous ways to study adults and children repeatedly and without ionizing radiation. In this article, we survey an array of the information that MRS offers about neurochemistry in general and psychiatric disorders and their treatment in particular. We also present growing evidence of glial abnormalities in neuropsychiatric disorders and discuss what MRS is contributing to that line of investigation. The third major direction of this article is the discussion of where MRS techniques are headed and how those new techniques can contribute to studies of mechanisms of psychiatric disease and drug discovery.

In vivo localized1H MR spectroscopy of rat testes: Stimulated echo acquisition mode (STEAM) combined with short TI inversion recovery (STIR) improves the detection of metabolite signals

A noninvasive NMR technique for evaluating testicular function was explored in this study. Localized in vivo 1H NMR spectroscopy was performed on rat testes using a stimulated echo acquisition mode (STEAM) sequence with a short echo time (TE). In the 1H spectra, large lipid signals dominated the chemical shift range of 0.89-2.78 ppm, which prevented the observation of metabolite signals in this region. To suppress these lipid signals, short inversion time (TI) inversion recovery (STIR) was combined with STEAM (STIR-STEAM). The optimal TI was typically 320 ms. STIR-STEAM with a TE of 15 ms allowed successful suppression of the lipid signals and the sensitive detection of several new metabolite signals. In normal testes, choline, creatine, glutamate, and glycine signals were identified. In addition to these metabolites, a lactate signal was observed in ischemic testes. To our knowledge, the signals of glutamate, glycine, and lactate have not been previously assigned in 1H MR spectra of testes in vivo. Lipid suppression by STIR aided in the detection of these metabolites, which would otherwise have been masked by the lipid signals.

Lipid signal extraction by SLIM: Application to1H MR spectroscopic imaging of human calf muscles

The measurement of intramyocellular lipid (IMCL) using in vivo (1)H MRS is important for better understanding muscle physiology. However, the accurate measurement of IMCL in muscle adjacent to subcutaneous fat (SF) and bone marrow (BM) is often hampered by contaminations from the fat. In this article a new postacquisition processing method is proposed that selectively removes unwanted lipid signals based on the spectral localization by imaging (SLIM) technique, which can localize spectra from arbitrarily shaped regions. The effectiveness of this lipid extraction method is demonstrated by both computer simulation and in vivo experiments in the human calf. The advantage of this method is that unwanted lipid signal, such as SF signal, can be selectively and completely removed. After the contaminating fat signals are removed, the quality of muscle spectra adjacent to SF improves such that it becomes comparable to that in uncontaminated muscle regions in (1)H MRSI of the calf.

Characterization of brain tumors by MRS, DWI and Ki-67 labeling index

With the advent of fast imaging hardware and specialized software, additional non-invasive magnetic resonance characterization of tumors has become available through proton magnetic resonance spectroscopy (MRS), hemodynamic imaging and diffusion-weighted imaging (DWI). Thus, patterns could be discerned to discriminate different types of tumors and even to infer their possible evolution in time. The purpose of this study was to investigate the correlation between MRS, DWI, histopathology and Ki-67 labeling index in a large number of brain tumors. Localized proton spectra were obtained in 47 patients with brain tumors who subsequently underwent surgery (biopsy or tumor removal). We performed MRS with short echo-time (30 ms) and metabolic values in spectra were measured using an external software with 25 peaks. In all patients who had DWI, we measured apparent diffusion coefficients (ADC) in the same region of interest (ROI) where the voxel in MRS was located. In most tumors the histological diagnosis and Ki-67 labeling index had been determined on our original surgical specimen. Cho/Cr, (Lip+Mm)/Cr, NAA/(Cho+Cr) and Glx/Cr indexes in MRS allowed discriminating between low- and high-grade gliomas and metastases (MTs). Likewise, absolute ADC values differentiated low- from high-grade gliomas expressed by Ki-67 labeling index. A novel finding was that high Glx/Cr in vivo MRS index (similar to other known indexes) was a good predictor of tumor grading.

High-field localized 1H NMR spectroscopy in the anesthetized and in the awake monkey

Localized cerebral in vivo 1H NMR spectroscopy (MRS) was performed in the anesthetized as well as the awake monkey using a novel vertical 7 T/60 cm MR system. The increased sensitivity and spectral dispersion gained at high field enabled the quantification of up to 16 metabolites in 0.1- to 1-ml volumes. Quantification was accomplished by using simulations of 18 metabolite spectra and a macromolecule (MM) background spectrum consisting of 12 components. Major cerebral metabolites (concentrations >3 mM) such as glutamate (Glu), N-acetylaspartate (NAA), creatine (Cr)/phosphocreatine (PCr) and myo-inositol (Ins) were identified with an error below 3%; most other metabolites were quantified with errors in the order of 10%. Metabolite ratios were 1.39:1 for total NAA, 1.38:1 for glutamate (Glu)/glutamine (Gln) and 0.09:1 for cholines (Cho) relative to total Cr. Taurine (Tau) was detectable at concentrations lower than 1 mM, while lactate (Lac) remained below the detection limit. The spectral dispersion was sufficient to separate metabolites of similar spectral patterns, such as Gln and Glu, N-acetylaspartylglutamate (NAAG) and NAA, and PCr-Cr. MRS in the awake monkey required the development and refinement of acquisition and correction strategies to minimize magnetic susceptibility artifacts induced by respiration and movement of the mouth or body. Periods with major motion artifacts were rejected, while a frequency/phase correction was performed on the remaining single spectra before averaging. In resting periods, both spectral amplitude and line width, that is, the voxel shim, were unaffected permitting reliable measurements. The corrected spectra obtained from the awake monkey afforded the reliable detection of 6-10 cerebral metabolites of 1-ml volumes.

Quantitative description of proton exchange processes between water and endogenous and exogenous agents for WEX, CEST, and APT experiments

The proton exchange processes between water and solutes containing exchangeable protons have recently become of interest for monitoring pH effects, detecting cellular mobile proteins and peptides, and enhancing the detection sensitivity of various low-concentration endogenous and exogenous species. In this work, the analytic expressions for water exchange (WEX) filter spectroscopy, chemical exchange-dependent saturation transfer (CEST), and amide proton transfer (APT) experiments are derived by the use of Bloch equations with exchange terms. The effects of the initial states for the system, the difference between a steady state and a saturation state, and the relative contributions of the forward and backward exchange processes are discussed. The theory, in combination with numerical calculations, provides a useful tool for designing experimental schemes and assessing magnetization transfer (MT) processes between water protons and solvent-exchangeable protons. As an example, the case of endogenous amide proton exchange in the rat brain at 4.7 T is analyzed in detail.

Pimonidazole binding in C6 rat brain glioma: relation with lipid droplet detection

In C6 rat brain glioma, we have investigated the relation between hypoxia and the presence of lipid droplets in the cytoplasm of viable cells adjacent to necrosis. For this purpose, rats were stereotaxically implanted with C6 cells. Experiments were carried out by the end of the tumour development. A multifluorescence staining protocol combined with digital image analysis was used to quantitatively study the spatial distribution of hypoxic cells (pimonidazole), blood perfusion (Hoechst 33342), total vascular bed (collagen type IV) and lipid droplets (Red Oil) in single frozen sections. All tumours (n=6) showed necrosis, pimonidazole binding and lipid droplets. Pimonidazole binding occurred at a mean distance of 114 microm from perfused vessels mainly around necrosis. Lipid droplets were principally located in the necrotic tissue. Some smaller droplets were also observed in part of the pimonidazole-binding cells surrounding necrosis. Hence, lipid droplets appeared only in hypoxic cells adjacent to necrosis, at an approximate distance of 181 microm from perfused vessels. In conclusion, our results show that severe hypoxic cells accumulated small lipid droplets. However, a 100% colocalisation of hypoxia and lipid droplets does not exist. Thus, lipid droplets cannot be considered as a surrogate marker of hypoxia, but rather of severe, prenecrotic hypoxia.

Quantitative proton short-echo-time LASER spectroscopy of normal human white matter and hippocampus at 4 Tesla incorporating macromolecule subtraction

Accurate quantification of in vivo short-echo-time (TE) (1)H spectra must account for contributions from both mobile metabolites and less mobile macromolecules, which can fluctuate in disease. The purpose of this study was to develop an approach for the acquisition and processing of macromolecule information to optimize metabolite quantification accuracy and precision. Human parietal white matter (8-cm(3) voxel) and posterior hippocampus (1.7-cm(3) voxel) metabolite levels were quantified, following manomolecule subtraction, from short-echo-time spectra (TE = 46 ms) acquired at 4.0 Tesla with localization by adiabatic selective refocusing (LASER). Nineteen metabolites were fit using a time domain Levenberg-Marquardt minimization that incorporated prior knowledge of metabolite lineshapes. The macromolecule contribution to the spectrum was reduced by 87% (P < 0.05) when the acquisition of single averages of the full spectrum and macromolecule spectrum were interleaved to reduce subtraction errors due to motion. Subtracting the Hankel Lanczos singular value decomposition (HLSVD) fit of the macromolecule spectrum, which contained no random noise, did not alter quantified metabolite levels but did not increase metabolite quantification precision. Several metabolites had higher concentrations in the posterior hippocampus compared to parietal white matter, which emphasizes the need to carefully control for partial volume contamination in hippocampal spectroscopy studies.

Combined MR measurements of magnetization transfer, tissue diffusion and proton spectroscopy. A feasibility study with neurological cases

Magnetic resonance imaging (MRI) of diffusion and magnetization transfer was combined with 1H-spectroscopic imaging (CSI) to evaluate the clinical potential of in-vivo profiles of various brain pathologies. Ten patients (multiple sclerosis, cerebrovascular disease, leukodystrophy, Alzheimer dementia) and five healthy volunteers were investigated with diffusion-weighted MRI, magnetization transfer imaging, and CSI. Proton spectra were analyzed as ratios of NAA/Cr and Cho/Cr calculated from the peak areas of N-acetylaspartate (NAA), (phospho)-creatine (Cr) and choline (Cho). The apparent diffusion coefficient (ADC) and the magnetization transfer ratio (MTR) were determined in identical voxels to ensure identical partial volume effects compared to CSI. Compared to MTR and ADC assessments, the lower spatial resolution of CSI clearly indicates a hindrance at 1.5 T. In most demyelinating lesions, NAA/Cr reduction paralleled attenuated MTRs and elevated ADCs. By contrast, in acute stroke and some acute MS lesions the ADC was reduced, while MTR and NAA/Cr were also decreased. In Alzheimer's dementia, ADC was increased, MTR unchanged and Cho/Cr increased. In a case of leukodystrophy, ADC was pronouncedly increased, MTR and NAA/Cr both reduced, and Cho/Cr normal. Combined measurements of ADC, MTR and CSI are feasible and provide differential in-vivo information on various brain pathologies.

Parameterized evaluation of macromolecules and lipids in proton MR spectroscopy of brain diseases

Short echo time (TE) proton MR spectra of the brain include signals of several metabolites as well as macromolecules. In various pathologies, such as brain tumors and multiple sclerosis (MS), the presence of mobile lipids or pathologically altered macromolecules may provide useful additional diagnostic information. A reliable quantitation of these resonances, however, is often not possible due to the lack of adequate prior knowledge. Furthermore, even if advanced fitting procedures are used, a reliable evaluation of metabolites in the presence of pathological lipids or macromolecules often fails if the latter are omitted in the spectral evaluation. In this study, a method is presented for the simultaneous evaluation of all visible components, including metabolites, lipids, and macromolecules, by the use of the fitting procedure LCModel. A standard basis set of brain metabolites was extended by inclusion of parameterized components for macromolecules and lipids that were derived from metabolite-nulled in vivo spectra of normal brain and high-grade gliomas, respectively. The improved spectral quantitation is demonstrated in glial brain tumors and MS lesions as well as in normal brain. It is pointed out that both macromolecules and lipids must be included to provide a proper spectral evaluation.

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.

Biexponential transverse relaxation (T(2)) of the proton MRS creatine resonance in human brain

Differences in proton MRS T(2) values for phosphocreatine (PCr) and creatine (Cr) methyl groups (3.0 ppm) were investigated in studies of phantoms and human brain. Results from phantom studies revealed that T(2) of PCr in solution is significantly shorter than T(2) of Cr. Curve-fitting results indicated that the amplitude-TE curves of the total Cr resonance at 3.0 ppm in human brain (N = 26) fit a biexponential decay model significantly better than a monoexponential decay model (P < 0.006), yielding mean T(2) values of 117 +/- 21 ms and 309 +/- 21 ms. Using a localized, long-TE (272 ms) point-resolved spectroscopy (PRESS) proton MRS during 2 min of photic stimulation (PS), an increase of 12.1% +/- 3.5% in the mean intensity of the total Cr resonance in primary visual cortex (VI) was observed at the end of stimulation (P < 0.021). This increase is consistent with the conversion of 26% of PCr in VI to Cr, which is concordant with (31)P MRS findings reported by other investigators. These results suggest a significantly shorter T(2) for PCr than for Cr in vivo. This difference possibly could be exploited to quantify regional activation in functional spectroscopy studies, and could also lead to inaccuracies in some circumstances when the Cr resonance is used as an internal standard for (1)H MRS studies in vivo.

In-vivo proton magnetic resonance spectroscopy in adnexal lesions

OBJECTIVE

To explore the in-vivo 1H- MR spectral features of adnexal lesions and to characterize the spectral patterns of various pathologic entities.

MATERIALS AND METHODS

Thirty-one patients with surgically and histopathologically confirmed adnexal lesions underwent short echo-time STEAM (stimulated echo acquisition method) 1H- MR spectroscopy, and the results obtained were analysed.

RESULTS

The methylene present in fatty acid chains gave rise to a lipid peak of 1.3 ppm in the 1H- MR spectra of most malignant tumors and benign teratomas. This same peak was not observed, however, in the spectra of benign ovarian epithelial tumors: in a number of these, a peak of 5.2 ppm, due to the presence of the olefine group (-CH=CH-) was noted. The ratios of lipid peak at 1.3 ppm to water peak (lipid/water ratios) varied between disease groups, and in some benign teratomas was characteristically high.

CONCLUSION

An intense lipid peak at 1.3 ppm is observed in malignant ovarian tumors but not in benign epithelial tumors. 1H- MRS may therefore be helpful in the differential diagnosis of adnexal lesions.

Spectroscopic assessment of alterations in macromolecule and small-molecule metabolites in human brain after stroke

BACKGROUND AND PURPOSE

We sought to measure the temporal evolution and spatial distribution of lesion macromolecules and small molecules (lactate, N-acetyl compounds, creatine, and choline) in stroke patients by using short echo time in vivo proton MR spectroscopy.

METHODS

Single-voxel spectra with TE=22 ms were obtained with and without inversion recovery suppression of small-molecule resonances from 30 examinations of 24 patients 3 to 214 days after stroke. Subtraction of the suppressed from the unsuppressed spectra yielded metabolite spectra without overlap from macromolecules. Two-dimensional spectroscopic images were acquired with macromolecule and small-molecule suppression from 5 additional patients.

RESULTS

Macromolecule signals were elevated in lesions relative to normal brain and tended to increase in the subacute period, even as lactate peaks declined. Regions of increased lactate, increased macromolecule signal at 1.3 ppm, and decreased N-acetyl compounds were closely correlated in the 2D spectroscopic images.

CONCLUSIONS

Short echo time spectra can be acquired in vivo in a manner that improves signal-to-noise ratio over long echo experiments and resolves overlapping macromolecule and small-molecule signals. The prominent macromolecule signals seen in the subacute period in association with persistently elevated lactate may represent mobile lipids in macrophages or other cells.

Characterization of the macromolecule baseline in localized (1)H-MR spectra of human brain

Short-echo-time magnetic resonance spectra of human brain contain broad contributions from macromolecules. As they are a priori of unknown shape and intensity, they pose a problem if one wants to quantitate the overlying spectral features from low-molecular-weight metabolites. On the other hand, the macromolecular contributions may provide relevant clinical information themselves, if properly evaluated. Several methods, based on T(1), T(2), or spectral shape, have previously been suggested to suppress or edit the macromolecule contributions. Here, a method is presented based on a series of saturation recovery scans and that allows for simultaneous recording of the macromolecular baseline and the fully relaxed metabolite spectrum. In comparison to an inversion recovery technique aimed at nulling signals from long-T(1) components, the saturation recovery method is less susceptible to T(1) differences inherent in signals from different metabolites or introduced by pathology. The saturation recovery method was used to quantitate the macromolecular baseline in white and/or gray matter locations of the human brain in 40 subjects. It was found that the content and composition of MR visible macromolecules depends on cerebral location, as well as the age of the investigated subject, while no gender dependence could be found.

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.

Proton T (1) and T (2) relaxation times of human brain metabolites at 3 Tesla

Longitudinal and transverse relaxation times were measured for proton MRS signals from human brain metabolites at 3 T using a short-echo STEAM protocol and a surface coil as a transmitter/receiver. Volumes of interest containing mostly grey or mostly white matter were selected in occipital lobes of healthy subjects and relaxation times for the following resonances were obtained: N-acetylaspartate at 2.01 ppm (T(1) and T(2)), glutamate at 2.35 ppm (T(1)), creatine at 3.03 and 3.92 ppm (T(1) and T(2)), choline-containing substances at 3.22 ppm (T(1) and T(2)), myo-inositol at 3.57 and 3.65 ppm (T(1)) and the overlapping signals of glutamate and glutamine at 3.75 ppm (T(1)). The T(1) relaxation times obtained range from 0.97 to 1.47 s for grey matter and from 0.87 to 1.35 s for white matter. On the other hand, T(2) relaxation times range from 116 to 247 ms and from 141 to 295 ms in grey and white matter, respectively. Generally, the T(1) values measured at 3 T are close to the previously published data found at 1.5, 2 and 4.1 T. Also, the T(2) values confirm the previously observed decrease in transverse relaxation times with increasing static magnetic field. The proton relaxation times obtained will allow improved sequence design and spectra quantitation at 3 T, currently tested for enhanced clinical applications.

Reliable detection of macromolecules in single-volume 1H NMR spectra of the human brain

In short echo time proton MR spectra of the brain, resonances from macromolecules are visible. The macromolecular resonances in the 0.5-2.0 ppm region can be affected by lipid contamination arising from fat-containing regions outside the selected volume of interest (VOI). This study demonstrates that considerable lipid contamination may remain in stimulated echo acquisition mode (STEAM) spectra even if the spoiling of unwanted coherences is sufficient and the VOI is placed 2 cm or more away from fat-containing regions. The observed contamination was attributed to residual remote out-of-volume excitation, although only very small out-of-slice ripples of less than 0.2% of the in-slice excitation were found in the calculated excitation profile of the RF pulses. Spatial presaturation of fat-containing regions led to a sufficient suppression of the contamination and enabled the detection of highly reproducible macromolecular resonances. Thus, in single-volume spectroscopy as well as in spectroscopic imaging (SI or CSI), the combination of volume selection and outer volume presaturation, each in three dimensions, is highly recommended to ensure accurate detection and reliable evaluation of even small pathological alterations in macromolecules, e.g., proteins or lipids, or other resonances in the 0.5-2.0 ppm region.

Magnetic resonance lipid signals in rat brain after experimental stroke correlate with neutral lipid accumulation

Proton magnetic resonance spectroscopy (MRS) signals from lipids in brain have been observed to increase after ischemic brain injury. However, neither the chemical identity nor the cellular location of these lipids has been established. The aim of the present study was to identify the origin of MRS lipid signals in rat brain after temporary (90 min) middle cerebral artery occlusion (MCAO). Fatty acyl proton signals were detected by short-echo one and two dimensional (1)H MRS in superfused brain slices from the infarcted hemisphere 1-5 days after MCAO. The intensities of these signals were strongly correlated with the amount of triacylglyceride and cholesterol ester in lipid extracts from the samples (r(2)=0.96, P<0.05) and were not correlated with the amount of free fatty acids in the tissue. Histological staining of tissue revealed the presence of neutral lipid droplets in infarcted regions. Dual labeling by immunohistochemistry demonstrated that these droplets were localized to microglia/macrophage (OX-42-labeled cells). These results strongly suggest that (1)H MRS lipid signals from brain after stroke arise from microglia/macrophage phagocytosis of cellular membranes.

An integrated strategy for evaluation of metabolic and oxidative defects in neurodegenerative illness using magnetic resonance techniques

The number of physiologic and metabolic phenomena amenable to analysis using magnetic resonance (MR) techniques is increasing every year. MR techniques can now evaluate tissue parameters relevant to TCA cyclemetabolism, anerobic glycolysis, ATP levels, blood-brain barrier permeability, macrophage infiltration, cytotoxic edema, spreading depression, cerebral blood flow and volume, and neurotransmitter function. The paramagnetic nature of certain oxidation states of iron leads to the ability to map out brain function using deoxyhemoglobin as an endogenous contrast agent, and also allows for mapping of local tissue iron concentrations. In addition to these metabolic parameters, the number of ways to generate anatomic contrast using MR is also expanding; and in addition to conventional anatomic scans, mapping of axonal fiber tracts can also be performed using the anisotropy of water diffusion. A strategy for integration of these multifarious parameters in a comprehensive neurofunctional exam in neurodegenerative illness is outlined in this paper. The goals of the integrated exam, as applied to a given neurodegenerative illness, can be subdivided into three categories: etiology, natural history, and therapeutic end points. The consequences of oxidative stress and/or mitochondrial dysfunction are explored in the context of the various parameters that can be measured using the integrated MR exam.

Toward an in vivo neurochemical profile: quantification of 18 metabolites in short-echo-time (1)H NMR spectra of the rat brain

Localized in vivo (1)H NMR spectroscopy was performed with 2-ms echo time in the rat brain at 9.4 T. Frequency domain analysis with LCModel showed that the in vivo spectra can be explained by 18 metabolite model solution spectra and a highly structured background, which was attributed to resonances with fivefold shorter in vivo T(1) than metabolites. The high spectral resolution (full width at half maximum approximately 0.025 ppm) and sensitivity (signal-to-noise ratio approximately 45 from a 63-microL volume, 512 scans) was used for the simultaneous measurement of the concentrations of metabolites previously difficult to quantify in (1)H spectra. The strongly represented signals of N-acetylaspartate, glutamate, taurine, myo-inositol, creatine, phosphocreatine, glutamine, and lactate were quantified with Cramér-Rao lower bounds below 4%. Choline groups, phosphorylethanolamine, glucose, glutathione, gamma-aminobutyric acid, N-acetylaspartylglutamate, and alanine were below 13%, whereas aspartate and scyllo-inositol were below 22%. Intra-assay variation was assessed from a time series of 3-min spectra, and the coefficient of variation was similar to the calculated Cramér-Rao lower bounds. Interassay variation was determined from 31 pooled spectra, and the coefficient of variation for total creatine was 7%. Tissue concentrations were found to be in very good agreement with neurochemical data from the literature.

Elimination of residual lipid contamination in single volume proton MR spectra of human brain

Short echo time 1H NMR spectra of the human brain reveal signals from various metabolites. In addition, resonances from macromolecules are present that may provide further useful information in several brain diseases. The detection of all these signals is possible if excellent volume selection is obtained; even small lipid contamination from surrounding fat tissue leads to strong spectral contamination. It affects the macromolecule resonances in the 0.5 to 2.0 ppm region and some adjacent metabolite signals and jeopardizes their quantitative analysis. This paper demonstrates how spatial contamination from insufficiently dephased signals can be recognized, analysed, and removed in localized STEAM spectroscopy of the brain.

Mobile lipids in rat brain slices observed by gradient-enhanced NMR

Short-echo gradient-enhanced nuclear magnetic resonance (NMR) spectroscopy was utilized to identify mobile lipids in perfused neonate and juvenile rat brain slices. Lipid signals were present at low levels within 1 hr of tissue preparation and increased with time under standard perfusion conditions and in the presence of high phosphocreatine and low lactate levels. Both one- and two-dimensional NMR spectra demonstrate peaks consistent with the generation of free fatty acids or neutral lipids following tissue trauma. The present work demonstrates that injury-induced mobile lipids may make appreciable contributions to regions of brain tissue spectra that have recently been assigned to lactate or polypeptides alone.