Quantitative analysis of 1H NMR detected proteins in the rat cerebral cortex in vivo and in vitro

In vivo macromolecule signals in rat brain 1 H-MR spectra at 9.4T: Parametrization, spline baseline estimation, and T2 relaxation times

PURPOSE

Reliable detection and fitting of macromolecules (MM) are crucial for accurate quantification of brain short-echo time (TE) ¹ H-MR spectra. An experimentally acquired single MM spectrum is commonly used. Higher spectral resolution at ultra-high field (UHF) led to increased interest in using a parametrized MM spectrum together with flexible spline baselines to address unpredicted spectroscopic components. Herein, we aimed to: (1) implement an advanced methodological approach for post-processing, fitting, and parametrization of 9.4T rat brain MM spectra; (2) assess the concomitant impact of the LCModel baseline and MM model (ie, single vs parametrized); and (3) estimate the apparent T₂ relaxation times for seven MM components.

METHODS

A single inversion recovery sequence combined with advanced AMARES prior knowledge was used to eliminate the metabolite residuals, fit, and parametrize 10 MM components directly from 9.4T rat brain in vivo ¹ H-MR spectra at different TEs. Monte Carlo simulations were also used to assess the concomitant influence of parametrized MM and DKNTMN parameter in LCModel.

RESULTS

A very stiff baseline (DKNTMN ≥ 1 ppm) in combination with a single MM spectrum led to deviations in metabolite concentrations. For some metabolites the parametrized MM showed deviations from the ground truth for all DKNTMN values. Adding prior knowledge on parametrized MM improved MM and metabolite quantification. The apparent T₂ ranged between 12 and 24 ms for seven MM peaks.

CONCLUSION

Moderate flexibility in the spline baseline was required for reliable quantification of real/experimental spectra based on in vivo and Monte Carlo data. Prior knowledge on parametrized MM improved MM and metabolite quantification.

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.

Magnetization Transfer Contrast and Chemical Exchange Saturation Transfer MRI. Features and analysis of the field-dependent saturation spectrum

Magnetization Transfer Contrast (MTC) and Chemical Exchange Saturation Transfer (CEST) experiments measure the transfer of magnetization from molecular protons to the solvent water protons, an effect that becomes apparent as an MRI signal loss ("saturation"). This allows molecular information to be accessed with the enhanced sensitivity of MRI. In analogy to Magnetic Resonance Spectroscopy (MRS), these saturation data are presented as a function of the chemical shift of participating proton groups, e.g. OH, NH, NH₂, which is called a Z-spectrum. In tissue, these Z-spectra contain the convolution of multiple saturation transfer effects, including nuclear Overhauser enhancements (NOEs) and chemical exchange contributions from protons in semi-solid and mobile macromolecules or tissue metabolites. As a consequence, their appearance depends on the magnetic field strength (B₀) and pulse sequence parameters such as B₁ strength, pulse shape and length, and interpulse delay, which presents a major problem for quantification and reproducibility of MTC and CEST effects. The use of higher B₀ can bring several advantages. In addition to higher detection sensitivity (signal-to-noise ratio, SNR), both MTC and CEST studies benefit from longer water T₁ allowing the saturation transferred to water to be retained longer. While MTC studies are non-specific at any field strength, CEST specificity is expected to increase at higher field because of a larger chemical shift dispersion of the resonances of interest (similar to MRS). In addition, shifting to a slower exchange regime at higher B₀ facilitates improved detection of the guanidinium protons of creatine and the inherently broad resonances of the amine protons in glutamate and the hydroxyl protons in myoinositol, glycogen, and glucosaminoglycans. Finally, due to the higher mobility of the contributing protons in CEST versus MTC, many new pulse sequences can be designed to more specifically edit for CEST signals and to remove MTC contributions.

NMR spectroscopy of macrophages loaded with native, oxidized or enzymatically degraded lipoproteins

Oxidized and enzymatically modified low-density lipoproteins (oxLDL and eLDL) play a key role in early stages of atherogenesis. Their uptake by recruited macrophages leads to endolysosomal phospholipidosis or foam cell formation, respectively, each of which is preceded by highly differential lipid restructuring processes. We applied ¹H-NMR spectroscopy (NMRS) to elucidate these structural rearrangements both in consequence of lipoprotein modifications and following phagocytosis. Being specifically sensitive to the mobile lipid subset, NMRS of oxLDL and eLDL revealed a partial and total immobilization of lipids, respectively. NMRS of intact macrophages showed a sixfold increase in mobile lipids in case of loading with eLDL but no significant changes for oxLDL or native LDL. This finding reflected the disparate lipid storage in lipid droplets and in multilamellar endolysosomal clusters when loaded with either eLDL or oxLDL, respectively. Moreover, a significant shift of the degree of saturation towards mainly polyunsaturated fatty acid chains was found for the mobile lipid pool in eLDL-loaded macrophages. Additional analyses of lipid extracts by NMRS and mass spectrometry (MS) reflected these changes in lipid content and in fatty acid composition only partially. In summary, in-cell NMRS represents a unique lipidomics tool to investigate structural changes within the mobile lipid pool following atherogenic triggers that can be not detected by the analysis of lipid extracts by MS or NMRS.

1H-nuclear magnetic resonance spectroscopy of glioblastoma cancer stem cells

The noninvasive tracking of glioblastoma cancer stem cells (CSCs) in vivo constitutes a prerequisite for the development of CSC-specific therapies. Therefore, as a pilot study to identify CSC biomarkers for clinical magnetic resonance spectroscopy, 10 CSC lines were investigated using high-resolution (1)H-nuclear magnetic resonance ((1)H-NMR) spectroscopy at 600 and 800 MHz (14.4 and 18.8 T) under reproducible in vitro conditions. The spectra were analyzed using principal component analysis (PCA), and spectral regions of high variability were evaluated regarding correlations to stem cell-related properties (clonogenic index and CD133 positivity) and cell death. PCA revealed that duplicates of CSC lines clustered together suggesting a characteristic (1)H-NMR pattern of each CSC line. PCA enabled discrimination between samples with high and low clonogenicity, that is, clustering according to one of the hallmarks of stemness in samples with high viability. High/moderate correlations to clonogenicity and CD133 were found in spectral regions with high variability. In contrast, the mobile lipid signal at 1.28 ppm correlated to cell death, but not to stemness, as published previously for neural progenitor cells. In conclusion, our exploratory study demonstrates the correlation of specific resonances within (1)H-NMR spectra with stem cell properties and advocates the use of the 1.28 ppm resonance as biomarker for cell death also in CSCs.

Short-term temperature effect on the HRMAS spectra of human brain tumor biopsies and their pattern recognition analysis

OBJECT

To investigate the effect of temperature (0 versus 37 degrees C) in the high-resolution magic angle spinning spectroscopy (HRMAS) pattern of human brain tumor biopsies and its influence in recognition-based tumor type prediction. This proof-of-principle study addressed the bilateral discrimination between meningioma (MM) and glioblastoma multiforme (GBM) cases.

MATERIALS AND METHODS

Forty-three tumor biopsy samples were collected (20 MM and 23 GBM), kept frozen and later analyzed at 0 degrees C and 37 degrees C by HRMAS. Post-HRMAS histopathology was used to validate the tumor type. Time-course experiments (100 min) at both temperatures were carried out to monitor HRMAS pattern changes. Principal component analysis and linear discriminant analysis were used for classifier development with a training set of 20 biopsies.

RESULTS

Temperature-dependent, spectral pattern changes mostly affected mobile lipids and choline-containing compounds resonances and were essentially reversible. Incubation of 3 MM and 3 GBM at 37 degrees C during 100 minutes produced irreversible pattern changes below 13% in a few resonances. Classification performance of an independent test set of 7 biopsies was 100% for the pulse-and-acquire, CPMG at echo times (TE) of 30 ms and 144 ms and Hahn Echo at TE 30 ms at 0 degrees C and 37 degrees C. The performance for Hahn Echo spectra at 136 ms was 83.3% at 0 degrees C and 100% at 37 degrees C.

CONCLUSION

The spectral pattern of mobile lipids changes reversibly with temperature. HRMAS demonstrated potential for automated brain tumor biopsy classification. No advantage was obtained when acquiring spectra at 37 degrees C with respect to 0 degrees C in most of the conditions used for the discrimination addressed.

1H MR spectroscopic imaging of phospholipase-mediated membrane lipid release in apoptotic rat glioma in vivo

The purpose of the current study was to determine regional spatiotemporal differences and to gain insight on the mechanisms responsible for lipid accumulation during apoptotic cell death using in vivo MR spectroscopic imaging in combination with histology and biochemical membrane lipid analyses. Rats bearing BT4C gliomas were treated with ganciclovir (GCV) for 14 days, and combined in vivo quantitative MR spectroscopic imaging (MRSI) of gliomas with histology and a biochemical analysis of major cell membrane constituents. By using 1H MRSI in vivo in combination with histology, we were able to demonstrate previously unattainable regional lipid concentration differences in tumors during GCV-induced apoptosis, with 5-microL tissue volume resolution. Our results also show that, during treatment, phospholipase A2 (PLA2) expression is significantly elevated by 37+/-13% (P<0.05) and tumor cell membranes loose a significant proportion of unsaturated fatty acyl moieties (56+/-6 mmol/kg, P<0.05). These changes are reflected in both histology and significant MR-visible lipid accumulation, demonstrating that phospholipid hydrolysis in tissue undergoing apoptosis can be imaged with MRSI. Our work demonstrates the versatility of 1H MRSI in studying apoptosis in vivo, which is likely to pave way for the use of MRSI in both experimental and clinical anticancer trials.

Tumour metabolomics in animal models of human cancer

Multinuclear Nuclear Magnetic Resonance Spectroscopy (MRS) and mass spectrometry (MS) are the key analytical techniques used in an increasing manner to explore tumor metabolite profiles. Recent work has revealed that metabolite profiles in various tumor preparations (i.e., cultured cells, tissue specimens, and tumors in vivo) show strong correlations with tumor type, proliferation, metabolic activity, and cell death. These data are regarded as highly promising for tumor diagnosis as well as assessment of prognosis and treatment response in a clinical setting. In this pursuit, animal models of human cancer have played a central role. In this short account, we review the potentials of MRS and MS techniques for animal tumor metabolomic work, as well as highlight some interesting applications of these techniques for various animal tumor types.

Assignment of the 2.03 ppm resonance in in vivo 1H MRS of human brain tumour cystic fluid: contribution of macromolecules

MRI and MRS are established techniques for the evaluation of intracranial mass lesions and cysts. The 2.03 ppm signal recorded in their (1)H-MRS spectra is often assigned to NAA from outer volume contamination, although it has also been detected in non-infiltrating tumours and large cysts. We have investigated the molecular origin of this resonance in ten samples of cystic fluids from human brain tumours. The NMR detected content of the 2.03 ppm resonance in 136 ms echo time spectra, assuming an N- CH(3) origin, was 3.19 +/- 1.01 mM. Only one third (34 +/- 12%) of the N-acetyl containing compound (NAC) signal could be extracted by perchloric acid (PCA) indicating that most of it originated in a macromolecular PCA-insoluble component. Chemical analysis of the cyst fluids showed that sialic acid bound to macromolecules would account for 64.3% and hexuronic containing compounds for 29.2% of the NMR-detectable ex vivo signal, 93.4% of the signal at TE 136 ms. Lactate content measured by NMR (6.4 +/- 4.4 mM) and the predominance of NAC originating in sialic acid point to a major origin from tumour rather than from plasma for this 2.03 ppm resonance.

In vivo GABA detection with improved selectivity and sensitivity by localized double quantum filter technique at 4.1T

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter for the normal function of mammal and human brain. It is difficult to detect GABA signal with the conventional single quantum technique due to its relatively low concentration and overlapping with other signals from creatine (Cr), glutathione (GSH), as well as macromolecules. Using a high-selective read pulse, DANTE, and at the facility of increased sensitivity and chemical shift resolution at high-field 4.1T, GABA editing by double quantum filter (DQF) with robust suppression of Cr and GSH was achieved. Our editing efficiency of 40-50% was achievable on a GABA phantom (50 mM GABA and 61 mM choline). Furthermore, GABA editing spectra were acquired with echo time TE = 77 ms, and any possible macromolecular contamination to GABA editing spectra was found to be negligible. This high-field DQF setup was applied to 11 healthy volunteers, and the mean GABA level was measured to be 1.12 +/- 0.15 mM in the occipital lobe in reference to 7.1 mM Cr concentration.

Mechanism of magnetization transfer during on-resonance water saturation. A new approach to detect mobile proteins, peptides, and lipids

The mechanism of magnetization transfer (MT) between water and components of the proton spectrum was studied ex vivo in a perfused cell system and in vivo in the rat brain (n = 5). Water was selectively labeled and spectral buildup consequential to transfer of longitudinal magnetization was followed as a function of time. At short mixing time (T(m)), nitrogen-bound solvent-exchangeable protons were observed, predominantly assigned to amide groups of proteins and peptides. At longer T(m), intramolecular nuclear Overhauser enhancement (NOE) was observed in the aliphatic proton region, leading to a mobile-macromolecule-weighted spectrum that resembles typical protein spectra described in the literature. This effect on the proton spectrum is distinct from that of classical off-resonance MT, which has been shown to be due to the immobile solid-like proton pool. When studying a solution of major brain metabolites under physiological concentrations and conditions (pH), no transfer effects were observed, in line with expectations based on reduced NOE effects in rapidly tumbling molecules and the fast proton exchange rates of amino, amine, SH, and OH groups. The spectral intensities of the amide protons may serve as indicators for pH and cellular levels of mobile proteins and peptides, while the aliphatic components are representative of several types of mobile macromolecules, including proteins, peptides, and lipids.

Apoptosis is associated with triacylglycerol accumulation in Jurkat T-cells

Magnetic resonance spectroscopy is increasingly used as a non-invasive method to investigate apoptosis. Apoptosis was induced in Jurkat T-cells by Fas mAb. (1)H magnetic resonance spectra of live cells showed an increase in methylene signal as well as methylene/methyl ratio of fatty acid side chains at 5 and 24 h following induction of apoptosis. To explain this observation, (1)H magnetic resonance spectra of cell extracts were investigated. These demonstrated a 70.0+/-7.0%, 114.0+/-8.0% and 90.0+/-5.0% increase in the concentration of triacylglycerols following 3, 5 and 7 h of Fas mAb treatment (P<0.05). Confocal microscopy images of cells stained with the lipophilic dye Nile Red demonstrated the presence of lipid droplets in the cell cytoplasm. Quantification of the stained lipids by flow cytometry showed a good correlation with the magnetic resonance results (P > or =0.05 at 3, 5 and 7 h). (31)P magnetic resonance spectra showed a drop in phosphatidylcholine content of apoptosing cells, indicating that alteration in phosphatidylcholine metabolism could be the source of triacylglycerol accumulation during apoptosis. In summary, apoptosis is associated with an early accumulation of mobile triacylglycerols mostly in the form of cytoplasmic lipid droplets. This is reflected in an increase in the methylene/methyl ratio which could be detected by magnetic resonance spectroscopy.

Metabolic changes in the brain of patients with anorexia and bulimia nervosa as detected by proton magnetic resonance spectroscopy

OBJECTIVE

To investigate the brain of patients with anorexia and bulimia nervosa by localized proton magnetic resonance spectroscopy (1H-MRS) and to look for metabolic alterations.

METHOD

Twenty patients with anorexia and bulimia nervosa were investigated by magnetic resonance imaging (MRI) and 1H-MRS in three regions of the brain. Age and sex-matched healthy subjects were investigated as controls.

RESULTS

1H-MRS revealed metabolic changes, such as a significant decrease of both myo-inositol and lipid compounds within the frontal white matter. The concentration of these compounds was further reduced with decreasing body mass index. Reduced lipid signals were also found in the occipital gray matter. In the cerebellum, the concentration of all metabolites including water, except lipids, was increased.

DISCUSSION

The metabolic changes found in this study seem to be a consequence of nutritional deficiency. It has to be further investigated whether these findings have any relevance for brain function. 1H-MRS might serve as a valuable investigative tool to observe eating disorders as anorexia and bulimia nervosa and to follow the success of therapy.

MRS of the brain in patients with anorexia or bulimia nervosa

Twenty patients with anorexia or bulimia nervosa were prospectively investigated by magnetic resonance spectroscopy (MRS) of the brain. Compared to healthy controls, MRS of those with eating disorders revealed metabolic changes, which seem to be a consequence of their nutritional deficiency.

Metabolism in single rat brain slices measured by magnetic resonance spectroscopy

Nuclear magnetic resonance spectroscopy (MRS) has been used to study brain biochemistry in superfused brain slice preparations for over a decade. However, unlike techniques that monitor electrical activity, ion fluxes, or the release of radio-labeled compounds in single brain slices, MRS studies have required samples composed of several slices and inherently poor anatomical specificity in order to achieve adequate signal-to-noise levels, spectral resolution, or, in the case of 1H MRS, a high degree of artifact-free water signal suppression. We report that gradient-enhanced 1H MRS techniques combined with a simple slice positioning and perfusion technique yield high-quality spectra from single 400 microns rat forebrain or neocortical-hippocampal slices within 15 min of data acquisition time. Spectra of comparable quality were obtained from samples with three neocortical or three hippocampal slices within the same time frame. The assessment of anaerobic energy metabolism in single slices by 1H MRS is also demonstrated. In addition to greater anatomical resolution in studies on brain slice biochemistry, single slice MRS also presents the possibility of correlating, within the same slice, 1H MRS-detectable metabolite levels with other physiological measurements commonly performed on single brain slices.

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.

Proton NMR spectroscopy of solvent-saturable resonances: a new approach to study pH effects in situ

It is shown that the effect of pH changes can be measured in proton NMR spectra through the pH sensitivity of the signal intensities of metabolite protons exchanging with water. To observe this phenomenon, pulse sequences must be used that can sensitively observe these exchangeable protons under physiological conditions, which is achieved by avoiding magnetization transfer signal losses due to water saturation for solvent suppression purposes. These methods provide an order-of-magnitude enhancement of many signals between 5 and 10 ppm, containing both N-bound protons as well as aromatic C-H protons coupled to them, the intensity of which is influenced by exchange-relayed saturation. As a first application, the effects of pH change on these resonances are studied ex vivo (perfused cells) and in vivo (cat brain).

Regional metabolite concentrations in human brain as determined by quantitative localized proton MRS

The regional distribution of brain metabolites was studied in several cortical white and gray matter areas, cerebellum, and thalamus of young adults with use of quantitative single-voxel proton MRS at 2.0 T. Whereas the neuronal compound N-acetylaspartate is distributed homogeneously throughout the brain, N-acetylaspartylglutamate increases caudally and exhibits higher concentrations in white matter than in gray matter. Creatine, myo-inositol, glutamate, and glutamine are less concentrated in cortical white matter than in gray matter. The highest creatine levels are found in cerebellum, parallel to the distribution of creatine kinase and energy-requiring processes in the brain. Also myo-inositol has highest concentrations in the cerebellum. Choline-containing compounds exhibit a marked regional variability with again highest concentrations in cerebellum and lowest levels and a strong caudally decreasing gradient in gray matter. The present findings neither support a metabolic gender difference (except for a 1.3-fold higher myo-inositol level in parietal white matter of female subjects) nor a metabolic hemispheric asymmetry.

Assignment of glial brain tumors in humans by in vivo 1H-magnetic resonance spectroscopy and multidimensional metabolic classification

This study presents a simple approach for the noninvasive assignment of glial brain tumors according to malignancy by single-voxel proton magnetic resonance spectroscopy at short echo times (TE < or = 50 milliseconds). Based on peak area ratios, a five-dimensional data set was obtained for each investigated subject. This vector was then projected along metabolic coordinates in a two-dimensional metabolic space. These coordinates had been determined in a previous study (Hagberg G et al., 1995, Magn Reson Med 34: 242-252). Tumor assignment was done without any knowledge of histology by comparing the location of the new cases to the features of the previous study. All 11 investigated glioblastomas multiforme, as well as 4 of 5 astrocytomas grade II, could easily be assigned to the groups of high- and low-grade tumors, respectively. Classification was more difficult in the case of a cystic astrocytoma grade II and one astrocytoma grade III. Two spectra measured in normal-appearing matter of glioblastoma patients were not classified as healthy. Using single-voxel proton magnetic resonance spectroscopy at short echo times with the knowledge of a base study, a straightforward, fast, and noninvasive differential diagnosis of glial brain tumors is possible.

Quantitative determination of glutamate turnover by 1H-observed, 13C-edited nuclear magnetic resonance spectroscopy in the cerebral cortex ex vivo: interrelationships with oxygen consumption

The kinetics of glutamate 13C-4 label appearance from D-[1-13C]-glucose and 13C-4 label disappearance from steady state following D-12C-glucose incubation were quantified with 1H-observed, 13C-edited nuclear magnetic resonance (NMR) spectroscopy in the superfused brain slices under largely varying oxygen consumption. Label incorporation to and from glutamate C-4 were fitted into mono- or bicompartmental models in order to determine the respective rate constants and to assess the presence of plausible multiple pools. At a steady-state oxygen consumption of approximately 4 mumol/min/g dry weight, glutamate labelling could be fitted into a biexponential equation, suggesting that there were two compartments with a large difference in their rates (respective rate constants of 0.022 and 0.149) and pool sizes (relative contributions of 91.2 and 8.8%, respectively). Stimulation of oxygen consumption in the brain slice preparations with either 40 mM KCl by 59.5 +/- 10.3% or 5 microM carbonyl cyanide m-fluorophenyl hydrazone by 61.4 +/- 8.4% increased glutamate C-4 labelling rate constants to 0.058 +/- 0.009 and 0.054 +/- 0.006, respectively. In the stimulated slice preparation, glutamate labelling could only be fitted into a monoexponential equation. 13C-4 label disappearance, independent of oxygen uptake, could also only be fitted into a monoexponential equation. There was a close match between the rate constants of label disappearance and appearance in non-stimulated and carbonyl cyanide m-fluorophenyl hydrazone-stimulated slices. In the presence of 40 mM KCl label disappearance did not, however, increase. These data show that glutamate C-4 turnover from exogenous D-[1-13C]-glucose can be used as an index of oxidative metabolism in situ under steady-state conditions as well as when oxygen metabolism is strongly stimulated. The results are discussed with respect to the use of NMR spectroscopy as a means of mapping brain oxidative metabolism.

Proton spectroscopy in asymptomatic HIV-infected adults: initial results in a prospective cohort study

The purpose of our study was to determine whether proton magnetic resonance spectroscopy (MRS) could detect early brain involvement by human immunodeficiency virus (HIV). We recruited 19 asymptomatic HIV-infected patients, 9 with and 10 without a history of intravenous drug use (IDU), as well as 10 control subjects. All subjects had to have normal MR imaging to be enrolled. We identified the following peaks on proton MRS: n-acetyl aspartate, creatine, choline, and a conglomerate amino acid peak between 2.1 and 2.6 parts per million that we call the marker peaks. Proton MRS was able to demonstrate a statistically significant difference between HIV-infected subjects and controls. The marker/Cr was the best ratio to separate patients from controls, with controls having a mean ratio of 0.50 +/- 0.51 and patients having a mean ratio of 1.8 +/- 0.85 (p = 0.001). There was no difference between HIV-infected subjects with and without a history of IDU. No significant relationship was found between either neuropsychological test scores or CD4 count and metabolite ratios. In brief, MRS seems more sensitive than magnetic resonance imaging (MRI), being able to detect abnormalities in HIV-infected patients when imaging is normal.

Increased macromolecular resonances in the rat cerebral cortex during severe energy failure as detected by 1H nuclear magnetic resonance spectroscopy

Changes in cerebral macromolecular 1H nuclear magnetic resonance (NMR) spectrum were studied in cortical brain slices in vitro. Aglycaemic hypoxia irreversibly increased various short T2 spectral components at 1.8-0.8 ppm in concordance with energy loss and independent of T1 and T2 relaxation effects. Removal of external calcium (Ca2+e) slightly attenuated the effect. The results suggest NMR-visible reorganisation of intracellular proteins due to hypoxic insult, and show that it may be possible to monitor early cytoplasmic changes due to brain energy depletion by NMR spectroscopy.

Proton MRS of gadolinium-enhancing MS plaques and metabolic changes in normal-appearing white matter

Localized short echo time (TE = 20 ms), stimulated echo acquisition mode, and double spin echo (TE = 135 ms) proton spectroscopy were performed in clearly defined, acute Gadolinium (Gd)-enhancing multiple sclerosis (MS) plaques of 22 patients with clinically definite MS. The resonances of N-acetylated metabolites (NA), creatine/phosphocreatine (Cr), choline-containing compounds (Cho), glycine/myo-inositol (Ino), and lactate were evaluated. The ratios of NA/Cr and NA/Cho were significantly decreased, Cho/Cr increased, and Ino/Cr remained unchanged. No marker peaks or elevated lactate levels were found. The measured metabolic changes were practically independent of the relative plaque size within the volume of interest (8 ml). Thus, the spectral changes measured with 1H MRS in acute Gd-enhancing MS plaques originate not only from the lesion as depicted by MRI, but also from the surrounding normal-appearing white matter.

Symbiosis between in vivo and in vitro NMR spectroscopy: the creatine, N-acetylaspartate, glutamate, and GABA content of the epileptic human brain

High resolution 1H NMR spectroscopy was used to analyze temporal lobe biopsies obtained from patients with epilepsy. Heat-stabilized cerebrum, dialyzed cytosolic macromolecules, and perchloric acid extracts were studied using one- and two-dimensional spectroscopy. Anterior temporal lobe neocortex was enriched in GABA, glutamate, alanine, N-acetylaspartate, and creatine. Subjacent white matter was enriched in aspartate, glutamine, and inositol. The N-acetylaspartate/creatine mole ratio was lower in anterior temporal neocortex with mesial (0.66) than neocortical (0.80) temporal lobe epilepsy. Human brain biopsy samples were separated into crude and refined synaptosomes, neuronal cell bodies, and glia using density gradient centrifugation. Neuronal fractions were enriched in glutamate and N-acetylaspartate. Glial cell fractions were enriched in lactate, glutamine, and inositol. The creatine content was the same in biopsied epileptic cortex (8.8-8.9 mmol/kg) and normal in vivo occipital lobe (8.9 mmol/kg). Glutamate content was higher in epileptic cortex at biopsy (10.1-10.5 mmol/kg) than normal in vivo occipital lobe (8.8 mmol/kg). GABA content was higher in biopsies of epileptic cortex (2.3-2.2 mmol/kg) than in normal in vivo occipital lobe (1.2 mmol/kg). N-acetylaspartate content was lower in biopsied epileptic temporal cortex (5.8-6.8 mmol/kg) than normal in vivo occipital lobe (8.9 mmol/kg). Paired in vivo and ex vivo measurements are critical for a firm understanding of the changes seen in the 1H-spectra from patients with epilepsy.

Analysis of macromolecule resonances in 1H NMR spectra of human brain

Macromolecule resonances underlying metabolites in 1H NMR spectra were investigated in temporal lobe biopsy tissue from epilepsy patients and from localized 1H spectra of the brains of healthy volunteers. The 1H NMR spectrum of brain tissue was compared with that of cytosol and dialyzed cytosol after removal of low molecular weight molecules (< 3500 daltons) at 8.4 and 2.1 Tesla. The assignment of specific resonances to macromolecules in 2.1 Tesla, short-TE, localized human brain 1H NMR spectra in vivo was made on the basis of a J-editing method using the spectral parameters (delta, J) and connectivities determined from 2D experiments in vitro. Two prominent connectivities associated with macromolecules in vitro (0.93-2.05 delta and 1.6-3.00 delta) were also detected in vivo by the J-editing method. Advantage was taken of the large difference in measured T1 relaxation times between macromolecule and metabolite resonances in the brain spectrum to acquire 'metabolite-nulled' macromolecule spectra. These spectra appear identical to the spectra of macromolecules isolated in vitro.

What might be the impact on neurology of the analysis of brain metabolism by in vivo magnetic resonance spectroscopy?

In vivo nuclear magnetic resonance spectroscopy (MRS) of the human brain is a recently developed technique which allows to assay noninvasively in vivo key molecules of brain metabolism. After a review of the origin of the signals detected by phosphorus and proton MRS of human brain, the impact of MRS on clinical neurology is examined. MRS of the brain does not purport to be a metabolic "biopsy", but unique applications for brain MRS are (1) quantitating the oxidative state of the brain and defining neuronal death, (2) assessing and mapping neuron damage, (3) evaluating membrane alterations, and (4) characterizing encephalopathies. In the near future brain MRS will be performed routinely after conventional MRI, as a valuable metabolic (and functional) complement to the anatomical evaluation of cerebral pathologies, particularly the toxic, metabolic and infectious encephalopathies.