High-resolution NMR spectroscopy of cerebral white matter in multiple sclerosis

Understanding aqueous and non-aqueous proton T1 relaxation in brain

A full picture of longitudinal relaxation in complex heterogeneous environments like white matter brain tissue remains elusive. In tissue, successive approximations, from the solvation layer model to the two pool model, have highlighted how longitudinal magnetization evolution depends on both inter-compartmental exchange and spin-lattice relaxation. In white matter, however, these models fail to capture the behaviour of the two distinct aqueous pools, myelin water and intra/extra-cellular water. A challenge with testing more comprehensive multi-pool models lies in directly observing all pools, both aqueous and non-aqueous. In this work, we advance these efforts by integrating three main experimental and analytical elements: direct observation of the longitudinal relaxation of both the aqueous and the non-aqueous protons in white matter, a wide range of different initial conditions, and application of an analysis pipeline which includes lineshape, CPMG, and fitting of a four pool model. An eigenvector interpretation of the four pool model highlights how longitudinal relaxation in white matter depends on initial conditions. We find that a single set of model parameters is able to describe the entire range of relaxation behaviour observed in all the separable aqueous and non-aqueous pools in experiments involving six different initial conditions. Understanding of the nature and connectedness of the tissue components is crucial in the design and interpretation of many MRI measurements, especially those based on magnetization transfer and longitudinal relaxation. In particular, the dependency of relaxation behaviour on initial conditions is likely the basis for understanding method-dependent discrepancies in in vivo T₁.

Quantifying the Metabolic Signature of Multiple Sclerosis by in vivo Proton Magnetic Resonance Spectroscopy: Current Challenges and Future Outlook in the Translation From Proton Signal to Diagnostic Biomarker

Proton magnetic resonance spectroscopy (¹H-MRS) offers a growing variety of methods for querying potential diagnostic biomarkers of multiple sclerosis in living central nervous system tissue. For the past three decades, ¹H-MRS has enabled the acquisition of a rich dataset suggestive of numerous metabolic alterations in lesions, normal-appearing white matter, gray matter, and spinal cord of individuals with multiple sclerosis, but this body of information is not free of seeming internal contradiction. The use of ¹H-MRS signals as diagnostic biomarkers depends on reproducible and generalizable sensitivity and specificity to disease state that can be confounded by a multitude of influences, including experiment group classification and demographics; acquisition sequence; spectral quality and quantifiability; the contribution of macromolecules and lipids to the spectroscopic baseline; spectral quantification pipeline; voxel tissue and lesion composition; T₁ and T₂ relaxation; B₁ field characteristics; and other features of study design, spectral acquisition and processing, and metabolite quantification about which the experimenter may possess imperfect or incomplete information. The direct comparison of ¹H-MRS data from individuals with and without multiple sclerosis poses a special challenge in this regard, as several lines of evidence suggest that experimental cohorts may differ significantly in some of these parameters. We review the existing findings of in vivo¹H-MRS on central nervous system metabolic abnormalities in multiple sclerosis and its subtypes within the context of study design, spectral acquisition and processing, and metabolite quantification and offer an outlook on technical considerations, including the growing use of machine learning, by future investigations into diagnostic biomarkers of multiple sclerosis measurable by ¹H-MRS.

Adenosine Triphosphate Metabolism Measured by Phosphorus Magnetic Resonance Spectroscopy: A Potential Biomarker for Multiple Sclerosis Severity

BACKGROUND/AIMS

Phosphorus magnetic resonance spectroscopy (31P-MRS) has previously shown abnormal changes in energy metabolites in the brain of multiple sclerosis (MS) patients. However, the relationship between these energy metabolites - particularly adenosine triphosphate (ATP) - and the disease severity remains unclear. The objective of this study was to determine whether measuring ATP metabolites can help to predict disease severity in MS patients.

METHODS

31P-MRS at 3 tesla was performed in 9 relapsing remitting (RRMS), 9 secondary progressive MS patients (SPMS), and 10 age-matched healthy controls. ATP metabolites (expressed as %) in normally appearing white matter of the centrum semiovale were compared between patients and healthy controls. The relationship between Expanded Disability Status Scale (EDSS) and ATP metabolites was evaluated.

RESULTS

RRMS and SPMS patients had higher phosphocreatine (PCr) and lower phosphodiesters than healthy controls. In addition, RRMS patients had higher β-ATP% than SPMS patients. β-ATP% was negatively correlated with EDSS in all patients.

CONCLUSION

Our findings suggest a defective PCr metabolism in both patient groups, and a higher state of energy production in RRMS that might reflect a compensatory mechanism in face of the increased needs. The correlation of β-ATP with EDSS makes it a candidate biomarker for assessing MS disease severity.

Impact of fluoxetine on the human brain in multiple sclerosis as quantified by proton magnetic resonance spectroscopy and diffusion tensor imaging

The antidepressant fluoxetine stimulates astrocytic glycogenolysis, which serves as an energy source for axons. In multiple sclerosis patients fluoxetine administration may improve energy supply in neuron cells and thus inhibit axonal degeneration. In a preliminary pilot study, 15 patients with multiple sclerosis (MS) were examined by diffusion tensor imaging (DTI) and (1)H magnetic resonance spectroscopy (MRS) in order to quantify the brain tissue diffusion properties (fractional anisotropy, apparent diffusion coefficient) and metabolite levels (choline, creatine and N-acetylaspartate) in cortical gray matter brain tissue, in normal appearing white matter and in white matter lesions. After oral administration of fluoxetine (20 mg/day) for 1 week, the DTI and MRS measurements were repeated and after treatment with a higher dose (40 mg/day) during the next week, a third series of DTI/MRS examinations was performed in order to assess any changes in diffusion properties and metabolism. One trend was observed in gray matter tissue, a decrease of choline measured at weeks 1 and 2 (significant in a subgroup of 11 relapsing remitting/secondary progressive MS patients). In white matter lesions, the apparent diffusion coefficient was increased at week 1 and N-acetylaspartate was increased at week 2 (both significant). These preliminary results provide evidence of a neuroprotective effect of fluoxetine in MS by the observed partial normalization of the structure-related MRS parameter N-acetylaspartate in white matter lesions.

Relationships between brain water content and diffusion tensor imaging parameters (apparent diffusion coefficient and fractional anisotropy) in multiple sclerosis

Fifteen multiple sclerosis patients were examined by diffusion tensor imaging (DTI) to determine fractional anisotropy (FA) and apparent diffusion coefficient (ADC) in a superventricular volume of interest of 8 x 8 x 2 cm(3) containing gray matter (GM) and white matter (WM) tissue. Point resolved spectroscopy 2D-chemical shift imaging of the same volume was performed without water suppression. The water contents and DTI parameters in 64 voxels of 2 cm(3) were compared. The water content was increased in patients compared with controls (GM: 244+/-21 vs. 194+/-10 a.u.; WM: 245+/-32 vs. 190+/-11 a.u.), FA decreased (GM: 0.226+/-0.038 vs. 0.270+/-0.020; WM: 0.337+/-0.044 vs. 0.402+/-0.011) and ADC increased [GM: 1134+/-203 vs. 899+/-28 (x10(-6) mm(2)/s); WM: 901+/-138 vs. 751+/-17 (x10(-6) mm(2)/s)]. Correlations of water content with FA and ADC in WM were strong (r=-0.68, P<0.02; r=0.75; P<0.01, respectively); those in GM were weaker (r=-0.50, P<0.05; r=0.45, P<0.1, respectively). Likewise, FA and ADC were more strongly correlated in WM (r=-0.88; P<0.00001) than in GM (r=-0.69, P<0.01). The demonstrated relationship between DTI parameters and water content in multiple sclerosis patients suggests a potential for therapy monitoring in normal-appearing brain tissue.

Correlation of proton MR spectroscopy and diffusion tensor imaging

Proton magnetic resonance spectroscopy ((1)H-MRS) provides indices of neuronal damage. Diffusion tensor imaging (DTI) relates to water diffusivity and fiber tract orientation. A method to compare (1)H-MRS and DTI findings was developed, tested on phantom and applied on normal brain. Point-resolved spectroscopy (T(R)/T(E)=1500/135) was used for chemical shift imaging of a supraventricular volume of interest of 8 x 8 x 2 cm(3) (64 voxels). In DTI, a segmental spin-echo sequence (T(R)/T(E)=5500/91) was used and slices were stacked to reproduce the slab used in MRS. The spatial distributions of choline and N-acetylaspartate (NAA) correlated to mean fractional anisotropy and apparent diffusion coefficient (ADC) for the inner 6 x 6=36 voxels defined in MRS, most notably NAA and ADC value (r=-.70, P<.00001; correlation across four subjects, 144 data pairs). This is the first association of neuron metabolite contents in volunteers with structure as indicated by DTI.

A comparison of cell and tissue extraction techniques using high-resolution 1H-NMR spectroscopy

Analysis of brain metabolites by a wide range of analytical techniques is typically achieved using biochemical extraction methodologies that require either two separate samples or two separate extraction steps to prepare both aqueous and organic metabolite fractions. However there are a number of brain pathologies in which both aqueous metabolite and lipid changes occur so that a simultaneous extraction of both fractions would be valuable. The methanol-chloroform (M/C) technique enables extraction of both aqueous metabolites and lipids simultaneously. It is already well established for lipid extraction of cells and tissue but its efficiency and reproducibility for extraction of aqueous metabolites is unknown. Therefore, we compared the aqueous metabolite yield and the reproducibility of the M/C method to the commonly used perchloric acid (PCA) method, using 1H-NMR spectroscopy of adult rat brain and purified rat astrocyte culture extracts. The results indicate that M/C is a superior technique for aqueous metabolite extraction from both brain tissue and cells when compared to the PCA method. The M/C extraction technique enables the simultaneous extraction of both lipids and aqueous metabolites from a single sample using small solvent-volumes, making it well suited for NMR investigations of both tissues and cells.

Representation of strong baseline contributions in 1H MR spectra

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

Value of magnetization transfer contrast as a sensitive technique to reflect histopathological changes in the white matter adjacent to the frontal horns of lateral ventricles

The purpose of this study is to evaluate the usefulness of magnetization transfer contrast (MTC) as a technique to reflect histopathological changes in the white matter adjacent to the frontal horns of the lateral ventricles. Radiological-pathological correlation was performed in six patients who underwent Magnetic Resonance (MR) examination prior to death and in whom postmortem examinations of the brain were obtained. The extent and the severity of degeneration in the white matter adjacent to the frontal horns were evaluated histopathologically, and compared with those observed on the conventional proton density (PD) weighted MR images (Group 1). Changes in the white matter of another 35 patients were classified into three types according to the pattern of high signals adjacent to the frontal horns on conventional PD weighted MR images, and magnetization transfer ratio (MTR) in the white matter adjacent to the frontal horns was calculated from multi-slice and single-slice FSE images (Group 2). The relationship between signal intensities and MTR in the white matter adjacent to the frontal horns was evaluated. The extent of degeneration in the white matter adjacent to the frontal horns was classified into mild, moderate and severe types on the basis of stainin for myelins, axons and astrocytes. In Group 1, histopathological findings indicated a difference in severity of degeneration in the white matter adjacent to the frontal horns among the three types, while no significant differences were noted in the signals on PD weighted MR images. In Group 2, MTR showed significant differences in the signal intensities in the white matter adjacent to the frontal horns (p < 0.01) between the three types, while conventional PD weighted MR images failed to differentiate between them. In conclusion, MT imaging is a sensitive technique to evaluate the histopathological changes in the white matter adjacent to the frontal horns that cannot be detected by conventional MR imaging.

Value of magnetic resonance imaging in assessing efficacy in clinical trials of multiple sclerosis therapies

Magnetic resonance imaging (MRI) has become an important technique for monitoring the effectiveness of putative treatments for multiple sclerosis (MS) because of its high sensitivity, objectivity, and noninvasive nature. Its importance as a surrogate measure of disease, however, is an issue that is more difficult to validate than might seem to be the case. In this review, we describe the role of MRI in the assessment of putative therapies for MS. New magnetic resonance techniques and methods of image analyses aimed at better demonstrating the nature and extent of disease are discussed, and the role of MRI in published MS therapeutic trials is examined. MRI is a frequently used secondary outcome measure for putative treatment strategies for MS. Although it is sensitive to changes in the inflammatory component of the MS disease process, poor correlation has been noted between MRI findings and long-term patient outcome. There is a widespread expectation that new magnetic resonance techniques--such as fluid-attenuated inversion recovery, magnetization transfer imaging, and magnetic resonance spectroscopy--will ultimately be useful for characterization of pathologic changes within the MS lesion and more generally of the MS disease process. Whether magnetic resonance changes seen in experimental therapies predict the long-term clinical course of the disease remains to be determined.

Finger printing of Mycobacterium tuberculosis in patients with intracranial tuberculomas by using in vivo, ex vivo, and in vitro magnetic resonance spectroscopy

In vivo, ex vivo, and in vitro proton magnetic resonance spectroscopy was performed in 12 patients with intracranial tuberculomas with an aim of detecting the biochemical constituents of Mycobacterium tuberculosis in a granuloma. One dimensional (1D) single pulse and spin-echo sequences and 2D correlative spectroscopy were used for the ex vivo study to confirm the resonances seen on in vivo study. Spectroscopic studies of the perchloric acid and lipid extract of granuloma and M. tuberculosis were performed to look for similarity of resonance. In vivo study showed the presence of lipids at 0.9, 1.3, 2.0, 2.8 ppm, and phosphoserine at 3.7 ppm. All these resonances were confirmed on ex vivo study. In addition, distinct resonances of serine and phenolic lipids were seen on ex vivo and in vitro study of tuberculous granuloma, which have not been observed in other intracranial tumors. Lipid extract of granuloma and M. tuberculosis showed phenolic lipids at 7.1 and 7.4 ppm, a constituent of the cell wall of the bacteria in a tuberculoma. It appears that it may be possible to finger print the biochemicals of the cell wall of M. tuberculosis in a tuberculous granuloma and thus may help in detection and diagnosis of such lesions.

Macroscopic and microscopic assessments of disease burden by MRI in multiple sclerosis: relationship to clinical parameters

We have evaluated macroscopic white matter abnormalities (visible lesions) together with microscopic abnormalities in the normal appearing white matter (NAWM) of patients with multiple sclerosis (MS) to determine their relative contributions to the development of disability. The total visible lesion volume (TLV) was computed as a measure macroscopic changes, whereas both texture analysis and T2 were used as possible indicators of diffuse disease in the NAWM. Dual echo T2-weighted SE images were obtained from 41 patients with definite MS: 10 primary progressive (PP), 11 secondary progressive (SP), 10 benign (BE), 10 early relapsing remitting (ERR), as well as from 10 healthy controls. Calculation of T2 and texture parameters were performed in a region of frontal NAWM of patients and controls. The TLV of each patient was measured using a semiautomated lesion detection program. No significant differences were found between the controls and the patients for all texture parameters examined. However, NAWM T2 was longer in the patients than in the controls (P = .02). Mean TLV was highest for SP and lowest for BE and ERR patients. A significant correlation was found between TLV and EDSS (P < .01) but not between NAWM T2 or texture and expanded disability status score (EDSS). Our study suggest that: (a) diffuse changes are present in NAWM, (b) texture analysis is unable to detect any subtle structure in the NAWM abnormalities, possibly because of the limited image resolution; (c) in the development of disability in MS, macroscopic lesions are more important than microscopic abnormalities in the NAWM.

Proton MRS in Pott's spine--a case report

Proton Magnetic Resonance Spectroscopy was carried out at 1.5 T on a patient with histologically proven Pott's spine affecting D11 vertebral body. In addition to the significantly reduced signals from the lipids in the region between 1 and 2 ppm, a prominent resonance at 5.1 ppm is seen. The spectrum is very different from that recorded on the one hand for a normal spine and, on the other, for a tumor arising from the vertebral body of a patient.

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.

In vivo quantitation of metabolite concentrations in the brain by means of proton MRS

MRS offers unique possibilities for non-invasive studies of biochemistry in the human brain in vivo. A growing body of evidence suggests that proton MRS may contribute to the clinical evaluation of a number of pathologies including ischaemia, tumours, epilepsy, metabolic and neuropaediatric disorders. In most cases results are expressed as ratios between metabolite signals obtained at certain experimental conditions. Presenting the results as metabolite signal ratios may lead to misinterpretation because such alterations can be due to changes in the content of either one of the metabolites or both, or may simply be due to changes in relaxation behaviour. Absolute quantitation of metabolite concentrations is therefore warranted. A number of studies using single volume proton MRS indicate that absolute quantitation of metabolite concentration is possible with respect to N-acetyl aspartate (NAA), total creatine, choline containing compounds, (Cho) and inositols (Ins). Internal standards (unsaturated water signal) as well as external standards have been used for signal calibration. Quality control with respect to signal linearity with concentration or with size of selected volume, selection efficiency, outer volume depression and signal contamination is essential for validation of the measurements. Furthermore, corrections for the influence of relaxation behavior are necessary. The results published so far indicate that the concentrations of NAA, total creatine, Cho and Ins in mmoles (kg wet weight)-1 range between 8.2 and 17.2 (mean 10.2), 5.9 and 11.6 (mean 7.2), 1.1 and 2.0 (mean 1.5) and 3.9 and 8.1 (mean 6.1), respectively. So far only a limited number of clinical studies has been published including studies of acute stroke, multiple sclerosis and Alzheimer's disease. The results are promising and encourage further exploitation of the utility of quantitative proton MRS in clinical practise.

Preliminary results of proton magnetic resonance spectroscopy in motor neurone disease (amytrophic lateral sclerosis)

Possible changes in brain metabolites in motor neurone disease/amytrophic lateral sclerosis (MND/ALS) were investigated using 1H magnetic resonance spectroscopy (MRS). A series of normal, healthy volunteer controls and MND patients have been studied using a spin echo (SE) 135 ms sequence, acquiring spectra from the region of the motor cortex. A further limited series of studies have been made for similar groups of volunteers and MND patients using a STEAM 20 ms sequence (stimulated echo). Analysis of the SE 135 ms spectra indicates there are statistically significant differences in the ratios of N-acetyl-aspartate to creatine and N-acetyl-aspartate to choline between controls and MND patients. Furthermore, metabolites identified using the STEAM 20 ms may be of great importance in the investigations of free radical mediated mechanisms, which have been postulated as being important contributors to the disease process. Preliminary results indicate that 1H MRS may provide important data to help understand the disease processes in MND and it could form a useful method for monitoring the effects of future trial treatment regimens.

1H MR visible lipids in colon tissue from normal and carcinogen-treated rats

Metabolic characteristics of colon mucosa, submucosa, muscularis and tumour specimens from four control (n = 105) and nine carcinogen (azoxymethane)-treated (n = 91) Sprague-Dawley rats were investigated by ex vivo 1H MRS. Ninety-seven per cent of pure mucosa samples (n = 59) yielded spectra with narrow lipid resonances (chemical shift delta of -(CH2)n-, 1.3 ppm; linewidth at half-height v1/2, 30-50 Hz). Eighty-two per cent of control mucosa samples with histologically proven submucosa contamination (n = 11) and 46% of control cross-sections (containing mucosa, submucosa and muscularis; n = 57) yielded spectra with broad lipid resonances (delta-(CH2)n-, 1.5 ppm; v1/2, 80-100 Hz) identical to those of adipose tissue surrounding rat colon. Thirty per cent of tumour samples (n = 10) yielded spectra with narrow lipid resonances while 70% contained no significant amount of MR visible lipids. We conclude that (i) lipids giving rise to broad resonances are in the heterogeneously distributed adipocytes of submucosa, (ii) lipids giving rise to narrow resonances are within the mucosa in an unknown structural environment, and (iii) the type and distribution of lipids in human and rat colon are similar. Tumours contained significantly more taurine than pure control mucosa (n = 15; p < 0.004) and pure mucosa containing aberrant crypt foci (putative preneoplasm, n = 36; p < 0.002). Our results suggest that the rat colon is a good model for 1H MR investigations of human colon carcinogenesis.

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.

Correlation of spectroscopy and magnetization transfer imaging in the evaluation of demyelinating lesions and normal appearing white matter in multiple sclerosis

Magnetization transfer imaging (MT) and localized proton spectroscopy (1H-MRS) were utilized in the evaluation of lesions (high signal abnormalities on T2-weighted images) and normal-appearing white matter (NAWM) in multiple sclerosis (MS). Eleven patients with a clinical diagnosis of MS were independently evaluated with both 1H-MRS and MT. The magnetization transfer ratio (MTR) of lesions was compared with the relative concentration of N-acetyl-aspartate (NAA) and a composite peak at 2.1 to 2.6 ppm termed "marker peaks." The MTR of white matter lesions in the MS patients was markedly decreased (6-34%; normal approximately 42%), and correlated well with increase in the marker peaks region (0.94-3.89). There was no correlation between the relative concentration of NAA and MTR. Increased resonance peaks in the 2.1 to 2.6 ppm range and marked decreases in MTR may be a relatively specific indicators of demyelination.

1H MRS of high grade astrocytomas: mobile lipid accumulation in necrotic tissue

Sixty-four samples from six grade 4 astrocytomas were investigated ex vivo by 1H MRS at 360 MHz and subsequently by histopathology to obtain percentages of viable and necrotic tumour and grey and white matter. MR-visible lipids were detected in 87% of tumour samples. Necrotic foci were < 3 x 3 x 6 mm3. The means of the intensities/unit weight tissue of the lipid resonances at 5.33, 2.80, 1.29 and 0.89 ppm were significantly higher (p < 0.05) for three sets of comparisons: samples with 85-100% vs 50-75%; with 50-75% vs 10-40% and with 10-40% vs 0-5% necrosis. For the lipid resonance at 2.04 ppm the difference in the means was significant only for samples with 50-75% compared to those with 85-100% necrosis, because for samples with < 50% necrosis resonances from glutamine and possibly small amounts of glutamate, gamma-aminobutyrate and N-acetylaspartate anions contribute significantly to the spectral area at 2.0 ppm. We conclude that necrotic foci below MRI resolution yield the resonances at 1.3 and 0.9 ppm, and contribute to the intense resonance at 2.0 ppm observed in in vivo 1H spectra of some high grade astrocytomas.

Benign and secondary progressive multiple sclerosis: a preliminary quantitative MRI study

In a preliminary study, we compared by means of quantitative magnetic resonance imaging (MRI) methods (1) the T2 values and the decay characteristics of chronic brain lesions, (2) the T2 values of normal-appearing frontal white matter (NAWM) and (3) brain lesion volumes in patients with benign and secondary progressive multiple sclerosis (MS) in order to evaluate the mechanisms underlying the development of disability. Eleven clinically definite MS patients with either benign MS (n = 5) or secondary progressive MS (n = 6) were studied. Fifty-two chronic lesions (identified by comparison with MRI scans obtained at least 12 months previously) were identified. The mean T2 of large lesions (cross-sectional area greater than 41 mm2) and of the NAWM was similar in both clinical groups. However, small lesions had higher mean T2 values (P < 0.01) in the benign group, probably at least in part because of partial volume effects. Analysis of large lesions revealed biexponential T2 relaxation in 6 of 8 "secondary progressive" and in 2 of 16 "benign" lesions, perhaps indicating a greater degree of axonal loss in large lesions of patients with secondary progressive MS. Patients with secondary progressive MS had higher (although not significant) total and infratentorial lesion loads than those of the benign group. These preliminary findings suggest, but do not establish, that variations in the extent, site and pathological nature of lesions may all contribute to different patterns of disease evolution in MS.

Magnetic resonance relaxation time mapping in multiple sclerosis: normal appearing white matter and the "invisible" lesion load

Prolonged T1 and/or T2 relaxation times (RT) in the normal appearing white matter (NAWM) of patients with multiple sclerosis (MS) have been attributed either to a diffuse abnormality, or to "small lesions" undetected by visual inspection of conventional MR images. In a comparison of brain slices from five MS patients and five healthy control subjects, we have confirmed that the average T1 and T2 RTs obtained from NAWM in patients with MS are significantly prolonged (p < .04). Quantitative pixel-by-pixel mapping shows that this overall prolongation is due to the averaging of RTs from two subfractions of NAWM. In all patients a proportion (average 54% for T1 and 63% for T2) of the total white matter pixel sample from each MR brain slice had RT values indistinguishable from those found in the white matter of matched healthy control subjects (i.e., "normal normal appearing white matter," NNAWM). Scattered throughout the NAWM were multiple small areas, often of only one or two pixels, with abnormal RT values. These lesions, which were revealed only by pixel-by-pixel mapping of RT, made up a significant proportion (average 47% for T1 or 57% for T2 estimates) of the total (visible plus "invisible") lesion load per slice, and of the NAWM (average 36% for T1, 27% for T2), with wide interpatient variability. Further studies of these minute lesions are required to determine their total volume in the brain, their precise nature, evolution and relevance to the functional deficit in MS.

Characterization of macromolecule resonances in the 1H NMR spectrum of rat brain

The 1H NMR spectrum of the macromolecule fraction of rat brain cytosol was investigated following centrifugation and dialysis to remove low molecular weight metabolites and peptides (< 3500 daltons). At least seven well resolved resonances were detected between 0.9 and 3.0 ppm in the 1H NMR spectrum of rat brain cytosol after dialysis, several of which cannot be observed in vivo due to overlap with N-acetylaspartate, glutamate, glutamine, creatine, and gamma-aminobutyric acid. Several cross-peaks detected in 2D COSY spectra of the cytosolic macromolecule fraction coincided with those measured in a previous study of rat brain tissue in vitro and in situ (K. L. Behar, T. Ogino, Magn. Reson. Med. 17, 285 (1991)). Treatment of the cytosolic macromolecule fraction with a nonspecific protease permitted partial assignments of resonances in the 1H NMR spectrum to specific amino acids. Fractionation of the dialyzed cytosol of rat brain by gel filtration yielded qualitatively similar 1H NMR spectra for elution volumes corresponding to molecular masses from 12.5 kDa to over 100 kDa. The results indicate that many of the background nonmetabolite resonances observed in the 1H NMR spectrum of normal brain tissue arise from cytosolic proteins.

Resolution of phospholipid molecular species by 31P NMR

Phosphorus-31 NMR chemical shifts of phospholipids (PLs) solubilized in bile salts were studied with respect to variations in the structure of the acyl substituents. The presence of double bonds in the acyl chains of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), and phosphatidic acid (PA) induced small but consistent upfield shifts relative to the corresponding disaturated acyl PL signals. The magnitudes of the unsaturation-related upfield shifts were approximately twice as large in sodium cholate as in sodium deoxycholate. Chemical shift separations of PC, PE, and PG dipalmitoyl-dioleoyl species pairs increased slightly at lower temperatures. Resolution of the PC species pair was maximized and nearly independent of cholate-to-PL ratio at values greater than about 100 in 2% cholate. Only the PA species resolution varied significantly over the pH range 6.5 to 9.5, in the vicinity of its pK2. Shift differences for a homologous series of disaturated acyl PCs showed a logarithmic dependence on chain length. Spectra for a variety of PC standards were used to interpret the composite PC signals from egg yolk extract, soybean extract and whole human amniotic fluid. The 31P NMR analyses were consistent with published chromatographic studies. Interpretation of composite PL signals in the cholate system is simplified for mixtures having restricted acyl chain length and degree of unsaturation.

Experimental allergic encephalomyelitis raises betaine levels in the spinal cord of strain 13 guinea-pigs

Chronic relapsing experimental allergic encephalomyelitis, an animal model of multiple sclerosis, was induced in Strain 13 guinea-pigs by subcutaneous injection of spinal cord homogenate and Freund's incomplete adjuvant supplemented with Mycobacterium tuberculosis. High resolution 1H NMR spectra of CNS tissue extracts indicated that the levels of choline metabolites, particularly betaine, were elevated in the spinal cord tissue, the principal site of lesion formation in this guinea-pig strain. The spectra also show that N-acetylated compounds are slightly depleted in the disease. The results are discussed in relation to the biochemical interpretation of NMR spectra obtained in vivo from patients with multiple sclerosis.

The structure and function of central nervous system myelin

Multiple sclerosis (MS) is characterized by the active degradation of central nervous system myelin, a multilamellar membrane system that insulates nerve axons. MS arises from complex interactions between genetic, immunological, infective, and biochemical mechanisms. Although the circumstances of MS etiology remain hypothetical, one persistent theme involves immune system recognition of myelin-specific antigens derived from myelin basic protein, the most abundant extrinsic myelin membrane protein, and/or another equally suitable myelin protein or lipid. Knowledge of the biochemical and physical-chemical properties of myelin proteins, and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to understanding how and why these antigens become selected during the development of MS. This article focuses on the current understanding of the molecular basis of MS as it may relate to the protein and lipid components of myelin, which dictate myelin morphology on the basis of protein-lipid and lipid-lipid interactions, and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

Intracranial tuberculomas: MRI signal intensity correlation with histopathology and localised proton spectroscopy

In seven cases of intracranial tuberculomas showing different signal intensities on MRI (five characteristic and two nonspecific), detailed histopathological examination was performed to look for number of macrophages, fibrosis, gliosis, degree of inflammatory cellular infiltrate, and type of caseation. The granulomas showing more macrophages, fibrosis and gliosis appeared hypointense on T2-weighted images. Tuberculomas showing minimal macrophages, marked cellular infiltration, and minimal fibrosis appeared hyperintense on T2-weighted images. Lesions showing similar signal intensity on T2-weighted images showed variation in the amount of macrophages, cellular infiltrates, maturity and fibrosis. Trace element estimation was done (iron, copper, and magnesium) in two of these lesions appearing hypointense on T2 and two normal brain samples; these were significantly lower in tuberculoma compared to normal brain tissue. Localised proton spectroscopy was performed in two hypointense lesions which showed marked increase in peaks in the region of mobile lipids (1.28 ppm) compared to normal brain parenchyma. It is concluded that the signal intensity of the lesions is dependent on the number of macrophages, fibrosis and cellular infiltrates. In addition increased lipid contents in the tuberculoma also contribute to the hypointensity on T2-weighted images.

Quantitative MR in the diagnosis of multiple sclerosis

In patients with multiple sclerosis (MS), the apparently uninvolved cerebral white matter between demyelinated plaques may have biochemical abnormalities. To what degree the changes in the white matter contribute to symptomatology in MS is unknown. In 39 patients with multiple sclerosis, and in 39 age-matched nondiseased volunteers, T1 and T2 were calculated from spin-echo images in four regions of apparently uninvolved white matter. In three of four white matter areas, the average T1 and T2 were significantly longer in the patients than in the controls. The T1 correlated with the disability, measured by the Kurtzke Extended Disability Status Scale, although the correlation was marginally significant. The results suggest that in patients with MS, white matter disease that is not visualized in MR as distinct foci of abnormal signal intensity may contribute to disease burden and disability.

1H and 31P magnetic resonance spectroscopy of the brain in degenerative cerebral disorders

Proton and phosphorus magnetic resonance spectroscopy of the brain was performed in 35 patients with degenerative cerebral disorders: 24 patients had demyelinating (white matter) disorders and 11 patients had neuronal (gray matter) disorders. Four grades of demyelination and three grades of cerebral atrophy were distinguished by magnetic resonance imaging criteria. The spectroscopic data were compared with normal values previously obtained. With increasing degrees of demyelination, lower ratios of phosphodiesters to beta-ATP were found. This trend was statistically significant. Decreased phosphodiester-beta-ATP ratios occurred simultaneously with imaging abnormalities. The decrease in phosphodiester-beta-ATP ratio in demyelinated areas is attributed to white matter rarefaction. Increasing cerebral atrophy was accompanied by lower ratios of N-acetyl aspartate to creatine. This trend was statistically significant. The decrease in the N-acetyl aspartate-creatine ratio was demonstrated before the magnetic resonance images showed signs of cerebral atrophy in patients with neuronal disorders. As N-acetyl aspartate is located exclusively in neurons and their branches, a decrease of the N-acetyl aspartate-creatine ratio can be attributed to neuronal and axonal damage and loss.