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

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.

Improved sensitivity to overlapping multiplet signals in in vivo proton spectroscopy using a multiecho volume selective (CPRESS) experiment

A method for volume selective proton spectroscopy is presented based on a multiecho sequence with short refocusing interval tcp. It is demonstrated, that by appropriate choice of tcp on the order of 4-6 ms, signals from overlapping multiplets like the glutamine and glutamate (Glu/Gln) resonances in spectra of the human brain are considerably increased compared with a conventional PRESS volume selection scheme. Thus proton spectra from J-coupled multiplet signals can be acquired with TE on the order of 20-30 ms avoiding the baseline problems arising at shorter echo times due to broad resonances. This allows to selectively acquire spectra from substances with longer T2 without the confounding effects from J-coupling occurring in conventional volume selection techniques.

Visual activation in alpha-chloralose-anaesthetized cats does not cause lactate accumulation in the visual cortex as detected by [1H]NMR difference spectroscopy

The hypothesis that neuronal activation results in lactate accumulation due to mismatch between glucose and oxygen consumption was tested in the cat model of visual activation by monitoring cerebral metabolism with localized 1H nuclear magnetic resonance spectroscopy (MRS). Adult cats were anaesthetized with alpha-chloralose, paralysed and mechanically ventilated. Visual evoked potentials measured over the occipital cortex showed maximal amplitude at 2 Hz stimulation, but the latencies of the early cortical potentials, N1 and P1, were independent of stimulation frequency. High signal-to-noise ratio, short echo time volume-selected [1H]MRS was used to monitor cerebral lactate with a temporal resolution of 70 s. Difference proton spectroscopy unambiguously showed no lactate peak in the visual cortex during visual activation at stimulation frequencies ranging from 1 to 16 Hz. Absence of change in lactate concentration during visual stimulation was confirmed by averaging all the spectra acquired during activation and subtracting them from reference spectra collected in darkness, a procedure that had a calculated lactate detection limit of 0.17 mM. We also reduced the O2 in the inspired air to 13%, which decreased pO2 from 94.5 +/- 8.9 to 47.0 +/- 6.8 mmHg, during visual stimulation at 2 or 4 Hz. At this low PO2 level, visual stimulation did not cause lactate accumulation in the visual cortex, however. The present data show that neuronal activation to this degree in the cat brain is not associated with aerobic lactate production to an extent that can be detected with 1H MRS.

Low brain GABA level is associated with poor seizure control

Low gamma-aminobutyric acid (GABA) concentrations in the cerebrospinal fluid are seen in a variety of epileptic syndromes. Low GABA levels outside of the epileptic focus may facilitate spread of discharges beyond the focus. In vivo measurements of GABA were made by 1H spectroscopy using a 2.1-T magnetic resonance imager-spectrometer and an 8-cm surface coil to measure a 14-cm3 volume in the occipital lobe. Patients with complex partial seizures had lower GABA levels (1.03 mmol/kg of brain; 95% confidence interval [CI], 0.95-1.12; n = 28; p < 0.02) than did subjects without epilepsy (1.18; 95% CI, 1.13-1.24; n = 19). There was a significant association between low GABA levels and recent seizures (correlation coefficient of 0.548, p < 0.01, df of 32). Conversely, patients with well-controlled seizures had higher brain GABA levels than did patients with recent seizures. Patients with seizures within a day of the measurement had lower GABA levels (0.92 mmol/kg; 95% CI, 0.78-1.06; n = 7) than did patients who were seizure free for 5 years or longer (1.28; 95% CI, 1.09-1.47; n = 4). Poor seizure control is associated with low brain GABA levels.

Lactate, N-acetylaspartate, choline and creatine concentrations, and spin-spin relaxation in thalamic and occipito-parietal regions of developing human brain

Previous studies of the brains of normal infants demonstrated lower lactate (Lac)/choline (Cho), Lac/creatine (Cr), and Lac/ N-acetylaspartate (Naa) peak-area ratios in the thalamic region (predominantly gray matter) compared with occipitoparietal (mainly unmyelinated white matter) values. In the present study, thalamic Cho, Cr, and Naa concentrations between 32-42 weeks' gestational plus postnatal age were greater than occipito-parietal: 4.6 +/- 0.8 (mean +/- SE), 10.5 +/- 2.0, and 9.0 +/- 0.7 versus 1.8 +/- 0.6, 5.8 +/- 1.5, and 3.4 +/- 1.1 mmol/kg wet weight, respectively: Lac concentrations were similar, 2.7 +/- 0.6 and 3.3 +/- 1.3 mmol/kg wet weight, respectively. In the thalamic region, Cho and Naa T2s increased, and Cho and Lac concentrations decreased, during development. Lower thalamic Lac peak-area ratios are principally due to higher thalamic concentrations of Cho, Cr, and Naa rather than less Lac. The high thalamic Cho concentration may relate to active myelination; the high thalamic Naa concentration may be due to advanced gray-matter development including active myelination. Lac concentration is higher in neonatal than in adult brain.

Brain Macromolecules: In Vivo Measurement by Proton Magnetic Resonance Spectroscopy

New single- and multiple-volume in vivo proton magnetic resonance spectroscopy techniques detect signals from brain macromolecules and can separate them from overlapping small molecule resonances. In vitro and animal studies have identified these resonances as arising from cytosolic proteins and mobile lipids. Increased macromolecule signals from lipids have been detected in both subacute stroke and in active multiple sclerosis plaques that reflect tissue breakdown and, in conjunction with elevated lactate, can be used to monitor phagocytic cell activity. The ability to follow changes in brain lipids and proteins should help to elucidate biochemical abnormalities accompanying a variety of pathological conditions.

Metabolite concentrations and relaxation in perinatal cerebral hypoxic-ischemic injury

Regional cerebral metabolite concentrations, principally of choline-containing compounds (Cho), total creatine (Cr), N-acetylaspartate (Naa), and lactate (Lac), can be quantified by in vivo proton magnetic resonance spectroscopy. In order to estimate a metabolite concentration, it is often necessary to measure the transverse relaxation time (T2). Metabolite T2s depend on cytosolic viscosity: as [adenosine triphosphate] falls leading to Na+/K+ pump failure, cytosolic water increases and T2s lengthen. In central grey-matter in human infants, Naa may be almost exclusively neuronal: Naa T2 may index neuronal edema and energy generation. In this preliminary report, metabolite concentrations and T2s have been measured in central grey matter in human infants suspected of perinatal hypoxic-ischemic cerebral injury. In infants who developed serious cerebral injury or died, [Cho] and [Naa] were low (the latter suggesting neuronal loss), [Lac] and all metabolite T2s were increased: the Naa T2 increase possibly reflected neuronal edema following failure of energy generation in a fraction of remaining neurons.

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 magnetic resonance spectroscopy of the brain in normal preterm and term infants, and early changes after perinatal hypoxia-ischemia

The aims of this study were 1) to define normal perinatal maturational changes in proton metabolite peak-area ratios in two regions of the neonatal brain, the thalamic and occipitoparietal regions, and 2) to investigate abnormalities of these ratios after perinatal hypoxia-ischemia. Fifty-four infants were studied: 35 normal control infants at 31-42 wk of gestational plus postnatal age, and 19 "asphyxiated" infants suspected of cerebral hypoxic-ischemic injury. Proton spectra were collected at 2.4 tesla from (2 cm)3 voxels using the point-resolved spectroscopy technique with a 270-ms echo time. Lactate was detected in all infants studied. In the normal infants, lactate relative to N-acetylaspartate (NAA), choline and creatine was significantly greater in the occipitoparietal region than in the thalamus, and fell with increasing maturity in both regions, whereas NAA/ choline increased. The 19 asphyxiated infants were studied on a total of 34 occasions during the 1st wk of life (median age 1.8 d), at gestational plus postnatal ages of 27-41 wk. Maximum lactate/NAA was above 95% confidence limits for the control data in one or both regions in 11 of the 19 infants. Minimum NAA/choline was below 95% confidence limits in only one asphyxiated infants, who was later found to have congenital hypothyroidism. SD scores for lactate, relative to NAA, choline, and creatine, were higher in both regions in the asphyxiated infants compared with the normal infants, particularly in the thalamus. Early results of 1-y follow-up examinations indicate that raised lactate/NAA carries a poor long-term prognosis.

Observation of resolved glucose signals in 1H NMR spectra of the human brain at 4 Tesla

Measurement of the resonances of glucose between 3.2 and 3.9 ppm in 1H NMR spectra from the human brain is difficult due to spectral overlap with peaks from more concentrated metabolites. The H1 resonance of alpha-D-glucose at 5.23 ppm is resolved from other metabolite peaks, but potentially overlaps with the intense water signal at 4.72 ppm. This paper demonstrates that the increased resolution at 4 Tesla permits to suppress the water signal sufficiently to reliably detect glucose directly at 5.23 ppm by 1H MRS and the estimated peak intensity is consistent with previous 13C NMR quantification.

Spectroscopic imaging of human brain glutamate by water-suppressed J-refocused coherence transfer at 4.1 T

The authors reported the development and implementation of a water-suppressed J-refocused coherence transfer sequence to observe glutamate in human brain at 4.1 T. The sequence is modeled for I2S2 and I2S2M spin systems analytically and plotted for a range of echo times. In this sequence, water suppression and refocusing of J-coupled resonances are achieved through a brief multiple quantum step without significant loss of signal. Phantom data are shown. Human brain spectroscopic imaging of glutamate, acquired with a total echo of 36 ms, demonstrates the application of the sequence to observe gray and white matter differences in glutamate content.

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

A novel approach is presented for imaging macromolecule and metabolite signals in brain by proton magnetic resonance spectroscopic imaging. The method differentiates between metabolites and macromolecules by T1 weighting using an inversion pulse followed by a variable inversion recovery time before localization and spectroscopic imaging. In healthy subjects, the major macromolecule resonances at 2.05 and 0.9 ppm were mapped at a nominal spatial resolution of 1 x 1 x 1.5 cm3 and were demonstrated to be highly reproducible between subjects. In subacute stroke patients, a highly elevated macromolecule resonance at 1.3 ppm was mapped to infarcted brain regions, suggesting potential applications for studying pathological conditions.

Accurate, noninvasive diagnosis of human brain tumors by using proton magnetic resonance spectroscopy

Although conventional proton magnetic resonance imaging has increased our ability to detect brain tumors, it has not enhanced to nearly the same degree our ability to diagnose tumor type. Proton magnetic resonance spectroscopy is a safe, noninvasive means of performing biochemical analysis in vivo. Using this technique, we characterized and classified tissue from normal brains, as well as tissue from the five most common types of adult supratentorial brain tumors. These six tissue types differed in their pattern across the six metabolites measured. 'Leaving-one-out' linear discriminant analyses based on these resonance profiles correctly classified 104 of 105 spectra, and, whereas conventional preoperative clinical diagnosis misclassified 20 of 91 tumors, the linear discriminant analysis approach missed only 1. Thus, we have found that a pattern-recognition analysis of the biochemical information obtained from proton magnetic resonance spectroscopy can enable accurate, noninvasive diagnosis of the most prevalent types of supratentorial brain tumors.

The effect of gabapentin on brain gamma-aminobutyric acid in patients with epilepsy

Gabapentin has come into clinical use as adjunctive therapy in the treatment of epilepsy. Designed to mimic gamma-aminobutyric acid (GABA), its mechanism of action remains elusive. In vivo measurements of GABA in human brain were made using 1H magnetic resonance spectroscopy. We used a 2.1-T magnetic resonance imager-spectrometer and an 8-cm surface coil to measure a 13.5-cm3 volume in the occipital cortex. GABA levels were measured in 14 patients enrolled in an open-lbel trial of gabapentin. GABA was elevated in patients taking gabapentin compared with 14 complex partial epilepsy patients, matched for antiepileptic drug treatment. Brain GABA levels appeared to be higher in patients taking high-dose gabapentin (3,300-3,600 mg/day) than in those taking standard doses (1,200-2,400 mg/day). Gabapentin appears to increase human brain GABA levels.

Homonuclear J-refocused spectral editing technique for quantification of glutamine and glutamate by 1H NMR spectroscopy

Quantification of the excitatory amino acid of glutamine and glutamate is extremely important partly because, when in excessive amounts, they can produce neuronal damage. However, quantification of such metabolites seems to be not straightforward owing to a large number of metabolic signals, particularly between the 2.0- and 3.0-ppm chemical shift range. The authors investigated a quantification method by using a homonuclear J-refocused spectral editing technique, namely the J-refocused PRESS (JPRESS), using both 1.5- and 4-T whole body NMR systems. At first, the J-coupling modulation of each metabolite at different TE values using the PRESS and JPRESS pulse sequences was compared. The suppression of J modulations with the JPRESS is well demonstrated on condition that TE values are less than 1/4J. Then, the accuracy of glutamate quantification after subtracting the two spectra obtained at two different echo times and integrating the difference spectral area at different concentrations was evaluated. The results exhibit an excellent linear relationship between the actual concentration versus the difference spectral peak area at both 1.5 and 4 T. The achievable minimum voxel size was also considered and a 1-ml voxel size seems to be possible at the typical glutamate concentration in vivo at a field strength of 4 T.

Continuing ischemic damage after acute middle cerebral artery infarction in humans demonstrated by short-echo proton spectroscopy

BACKGROUND AND PURPOSE

Proton MR spectroscopy is a noninvasive method of monitoring in vivo metabolite concentration changes over time. The aim of this work was to study the ischemic penumbra in humans by measuring the metabolic changes that occur after a middle cerebral artery territory infarction.

METHODS

Diagnostic MRI and short-echo time MR spectroscopy were performed on a 1.5-T system. Localized proton MR spectroscopy was performed within the area of cerebral infarction and in a homologous area of the contralateral hemisphere. The residual water resonance in the spectra was removed with the use of the Hankel Lanczos singular value decomposition method, after which peak area estimates were obtained by means of the variable projection time domain fitting analysis. The unsuppressed water signal was used as an internal concentration standard. Ten patients with acute middle cerebral artery infarction were studied within 28 hours of stroke onset and followed up for a period of up to 3 months.

RESULTS

Significant changes were seen in the initial spectra from the infarct compared with the contralateral spectra. Lactate, a marker of anaerobic metabolism, was present within the infarct but not detected in the contralateral hemisphere. N-Acetyl aspartate, a neuronal marker, and total creatine were significantly reduced. The initial choline signal, arising from choline-containing compounds within the cell and cell membrane, remained unchanged in the infarct core compared with the contralateral hemisphere. Further reductions in N-acetyl aspartate and total creatine concentrations occurred within the first week. A fall in the lactate concentration was seen within the infarct core during the first 7 to 10 days. Similar reductions in the choline concentration were observed during this period.

CONCLUSIONS

The demonstration of the continuing loss of cerebral metabolites within an infarct region suggests that further cell loss occurs up to 10 days after infarction. The continuing loss of neurons may represent continued ischemic damage after middle cerebral artery infarction.

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.

Initial observations on effect of vigabatrin on in vivo 1H spectroscopic measurements of gamma-aminobutyric acid, glutamate, and glutamine in human brain

Recent developments involving 1H nuclear magnetic resonance (NMR) spectroscopic editing techniques have allowed noninvasive measurements of gamma-aminobutyric acid (GABA) in human cerebrum. The additional information gained from GABA and macromolecule measurements permitted more precise glutamate (Glu) and glutamine (Gln) measurements. Occipital lobe GABA in 10 nonepileptic, healthy subjects was 1.0 mumol/g brain [95% confidence interval (CI) 0.9-1.1]. Vigabatrin (VGB) is a safe and effective antiepileptic drug (AED) that irreversibly inhibits neuronal and glial GABA-transaminase. GABA levels were increased in all patients treated with VGB. With a standard dose of 3-6 g/day, GABA levels were 2.6 mumol/g (95% CI 2.3-2.8). Mean occipital GABA level measured in epileptic patients not receiving VGB was 0.9 mumol/g (95% CI 0.7-1.1). Gln was increased by 1.9 mumol/g and Glu was decreased by 0.8 mumol/g in patients receiving VGB as compared with patients receiving standard medications alone.

Image directed proton spectroscopy of gerbil brain at 7 tesla

Image directed localized 1H NMR spectra were obtained at 7 T (300 MHz) from cubic volumes of less than 40 microL in the gerbil brain. Signals from cerebral metabolites commonly detected in other rodent species were easily assigned, and high resolution spectroscopy (360 MHz) of aqueous brain extracts assisted the tentative identification of partially overlapping resonances from lower concentration compounds like alanine, lysine, gamma-aminobutyrate, valine, leucine and isoleucine. Weak coupling at 7 T was manifest in the resolution of signals from the gamma-CH2 groups of glutamine and glutamate. Down-field of water, signals assigned to purine nucleotides were conspicuous in the extract spectra, but localized spectra acquired routinely in vivo, using selective excitation and gradient crushing (SUBMERGE) for water suppression, exhibited little or no signal from purines. When localized in vivo spectra were acquired without water suppression, however, or using a low power binomial excitation sequence rather than SUBMERGE, a broad signal appeared at the resonant frequency of purine aromatic protons. NMR experiments on the nucleotide adenosine 5'-monophosphate (AMP) in 90% glycerol/10% D2O solution demonstrated that pre-irradiation of the water signal even for less than 100 ms attenuated the nucleotide signal appreciably. This implies that the soft pulses required for selective excitation of water in sequences such as SUBMERGE induce spin-diffusion which eliminates or diminishes the signal from nucleotides in vivo.

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.