Cerebral lactate detected by regional proton magnetic resonance spectroscopy in a patient with cerebral infarction

In Vivo 1H Spectroscopy of the Human Brain at 1.5 Tesla; Preliminary Experience at a Clinical Installation

In vivo localized water suppressed proton spectroscopy of human brain was carried out on 15 healthy volunteers and 2 patients suffering from a brain tumour and an infarction, respectively. The measurements were performed on a whole body MR system, operating at 1.5 tesla using the stimulated echo technique. Our preliminary results indicate that it is possible to detect a number of metabolites in the brain within a total measurement time of one hour. The dominant peaks in the spectra from healthy volunteers are N-acetyl aspartate, choline and creatine/phosphocreatine. The spectra obtained from the brain tumour and the infarct, respectively, differed very much from those obtained in healthy brain tissue. Our preliminary results indicate that localized proton spectroscopy may contribute to non-invasive brain tumour classification and possibly also to the differentiation between tumours and infarcts in clinically doubtful cases.

¹H-magnetic resonance spectroscopy of the breast at 3.0-T: comparison of results obtained before and after administration of gadolinium-based contrast agent

PURPOSE

To assess the effects of gadolinium-based contrast agent (GBCA) on (1) H-magnetic resonance spectroscopy (MRS) of the breast at 3.0-T.

MATERIALS AND METHODS

Patients (n = 98) with breast cancer (98 lesions) underwent MRS (point-resolved spectroscopy sequence [PRESS]; TR/TE, 2000/100 msec; voxel size, 15 × 15 × 15 mm) before the administration of GBCA. In 52 of those patients, MRS was also performed after the administration of GBCA. The voxel-of-interest (VOI) was placed by referring to the noncontrast-enhanced MRI (diffusion-weighted images combined with fat-suppressed T2-weighted images). We reviewed and graded the appropriateness of VOI location compared to the correlating enhancement lesions. Integral values of the choline peak at a frequency of 3.2 ppm on MRS were compared before and after the administration of GBCA.

RESULTS

The VOI was placed correctly in 64 lesions (65%), although the VOI was placed outside the targeted lesion in 34 lesions (35%). The integral value of the choline peak on MRS decreased significantly after the administration of GBCA (P < 0.001).

CONCLUSION

Accumulation of GBCA in breast cancer could affect the choline peak on MRS. MRS of breast cancer at 3.0-T can be recommended to be acquired before contrast-enhanced study; however, some problems remain in VOI placement with reference to the noncontrast-enhanced study.

Magnetic resonance characterization of ischemic tissue metabolism

Magnetic resonance imaging (MRI) and spectroscopy (MRS) are versatile diagnostic techniques capable of characterizing the complex stroke pathophysiology, and hold great promise for guiding stroke treatment. Particularly, tissue viability and salvageability are closely associated with its metabolic status. Upon ischemia, ischemic tissue metabolism is disrupted including altered metabolism of glucose and oxygen, elevated lactate production/accumulation, tissue acidification and eventually, adenosine triphosphate (ATP) depletion and energy failure. Whereas metabolism impairment during ischemic stroke is complex, it may be monitored non-invasively with magnetic resonance (MR)-based techniques. Our current article provides a concise overview of stroke pathology, conventional and emerging imaging and spectroscopy techniques, and data analysis tools for characterizing ischemic tissue damage.

Magnetic resonance spectroscopic methods for the assessment of metabolic functions in the diseased brain

Magnetic resonance spectroscopy (MRS) is a non-invasive technique that can be used to detect and quantify multiple metabolites. This chapter will review some of the applications of MRS to the study of brain functions. Typically, (1)H-MRS can detect metabolites reflecting neuronal density and integrity, markers of energy metabolism or inflammation, as well as neurotransmitters. The complexity of the proton spectrum has however led to the development of other nuclei-based methods, such as (31)P- and (13)C-MRS, which offer a broader chemical shift range and therefore can provide more detailed information at the level of single metabolites. The versatility of MRS allows for a wide range of clinical applications, of which neurodegeneration is an interesting target for spectroscopy-based studies. In particular, MRS can identify patterns of altered brain chemistry in Alzheimer's patients and can help establish differential diagnosis in Alzheimer's and Parkinson's diseases. Using MRS to follow less abundant neurotransmitters is currently out of reach and will most likely depend on the development of methods such as hyperpolarization that can increase the sensitivity of detection. In particular, dynamic nuclear polarization has opened up a new and exciting area of medical research, with developments that could greatly impact on the real-time monitoring of in vivo metabolic processes in the brain.

Neuroprotective effect of the free radical scavenger MCI-186 in patients with cerebral infarction: clinical evaluation using magnetic resonance imaging and spectroscopy

A newly developed free radical scavenger, 3-methyl-1-phenyl-2-pyrazolin-5-one (MCI-186), holds promise for clinical application. We clinically evaluated the effect of MCI-186 on cerebral infarction by using magnetic resonance imaging (MRI) and proton MR spectroscopy (MRS). Six patients with large supratentorial infarction were evaluated with sequential MRI and proton MRS. These patients were also administered MCI-186 for 14 days after ischemic insult (MCI-186 group). The findings were compared with those for patients who had supratentorial infarctions equivalent in size to those in the MCI-186 group but who had received only conventional therapy. The course of change of the size of infarction was evaluated by MRI, and the metabolic changes following cerebral infarction were evaluated by proton MRS. As a result, there was no significant difference between the initial size of infarction in the conventionally treated group and that in the MCI-186 treated groups, nor did the groups show significant difference in the sequential changes depicted by MRI in the area of infarction, midline shift, or amount of edema. However, on MRS, the N-acetyl aspartate signal was significantly higher in the MCI-186 group than in the conventionally treated patients. In conclusion, MCI-186 has an effect of preservation of N-acetyl-aspartate, which is thought to be a neuronal marker, in cerebral infarction.

Evolution of the neurochemical profile after transient focal cerebral ischemia in the mouse brain

Evolution of the neurochemical profile consisting of 19 metabolites after 30 mins of middle cerebral artery occlusion was longitudinally assessed at 3, 8 and 24 h in 6 to 8 microL volumes in the striatum using localized 1H-magnetic resonance spectroscopy at 14.1 T. Profound changes were detected as early as 3 h after ischemia, which include elevated lactate levels in the presence of significant glucose concentrations, decreases in glutamate and a transient twofold glutamine increase, likely to be linked to the excitotoxic release of glutamate and conversion into glial glutamine. Interestingly, decreases in N-acetyl-aspartate (NAA), as well as in taurine, exceeded those in neuronal glutamate, suggesting that the putative neuronal marker NAA is rather a sensitive marker of neuronal viability. With further ischemia evolution, additional, more profound concentration decreases were detected, reflecting a disruption of cellular functions. We conclude that early changes in markers of energy metabolism, glutamate excitotoxicity and neuronal viability can be detected with high precision non-invasively in mice after stroke. Such investigations should lead to a better understanding and insight into the sequential early changes in the brain parenchyma after ischemia, which could be used for identifying new targets for neuroprotection.

Metabolite Profile in Pyramidal Tracts after Ischemic Brain Stroke Assessed by 1H MRS. A Multicenter Study

The magnitude of the motor deficit in patients with stroke depends not only on the size and location of the destroyed brain tissue, but also on axonal injury in the descending motor pathways which appears after stroke. After cerebral ischemia, there are no visible abnormalities in conventional MRI in the intact pyramidal tracts despite the process of neuronal destruction by Wallerian degeneration. Conventional MRI is not a sensitive test for Wallerian degeneration in the acute or subacute time period as it shows no changes within the first four weeks. Magnetic resonance spectroscopy (MRS) has been used for better quantification of the extent or severity of fibre damage by evaluating metabolite alterations in normal-appearing corticospinal and corticopontal tracts. This study assessed the role of 1H MRS in the detection of changes in cerebral metabolite levels in pyramidal tracts after cortical/ subcortical infarction and to compare metabolite alterations to clinical outcome (assessed by Barthel index, Scandinavian Stroke Scale). The study included 31 patients who had suffered an ischemic cortical/subcortical stroke involving the motor cortex or the descending fibers. Ratios of NAA/Cr, Cho/Cr, lip/Cr and Lac/Cr from internal capsules and cerebral peduncles were measured and compared with clinical status assessed by Barthel index and Scandinavian Stroke Scale (SSS). The ratio of NAA/Cr was significantly decreased (p<0.001) in the normal-appearing ipsilateral internal capsule in comparison with the control group. Cho/Cr and lac/Cr ratios were increased compared to the control group (p=0.019). Decrease of NAA/Cr ratio correlated with clinical status assessed by Barthel index and there was a correlation between clinical improvement (assessed by SSS) and lac/Cr ratio. Tissue metabolite concentrations distant from the infarcted region correlated with the clinical course and had predictive value. Proton MRS is very useful tool for evaluating major changes in metabolite levels in pyramidal tracts after brain stroke.

Detection and differentiation of lactate and lipids by single-voxel proton MR spectroscopy

The signals of lactate and lipids partially overlap in single-voxel proton MR spectroscopy (1HMRS), sometimes making them difficult to differentiate in clinical settings. Our aim in this study was to identify lactate and lipids by varying the echo time (TE). We expect that the accurate detection of lactate and lipids will have high diagnostic value in the diagnosis of brain tumors. Following our protocol, we obtained meaningful 1HMRS spectra from 213 patients, including 163 patients with brain tumors, between August 1999 and February 2004. 1HMRS was performed with a TE of 144 ms followed by a TE of 30 ms and/or a TE of 288 ms, if necessary. For the 213 patients, lactate level was "negative" in 47 patients, "positive" in 131 patients, and "strongly positive" in 35 patients. The lipid level was "negative" in 90 patients, "positive" in 56 patients, and "strongly positive" in 67 patients. Based on logistic discriminant analyses of neuro-epithelial tumor WHO grade and lactate and lipid levels, lactate and lipid levels were significant between WHO grades 2 and 3 (P=0.0239) and between grades 3 and 4 (P=0.0347). Lipids are a more significant factor for the discrimination between WHO grades 2 and 3 (P=0.0073) and between grades 3 and 4 (P=0.0048). With our method of varying the TE, it is possible accurately and efficiently to detect lactate and lipids in the brain. We found a significant correlation between lactate and lipid expression and WHO grade of neuro-epithelial tumors.

Proton magnetic resonance spectroscopic changes of the primary motor cortex and supplementary motor area in hemiparetic patients with corticospinal tract injury due to deep intracerebral hematoma

This study was conducted to investigate the metabolic changes in the motor and motor association cortices following axonal injury in the internal capsule that was caused by deep intracerebral hematoma. Using proton magnetic resonance spectroscopy (1H MRS), the authors studied the primary motor cortices (M-1) and supplementary motor areas (SMA) of 9 hemiparetic patients with documentable hemiparesis of varying severity, and we studied 10 normal volunteers as controls. To measure the M-1 and SMA biochemical changes, 4 separate single volumes of interest (VOIs) were located bilaterally in the affected and unaffected hemisphere (AH and UH). 1H MRS provided a neuronal and axonal viability index by measuring levels of N-acetylaspartate (NAA) and creatine/phosphocreatine (Cr). The M-1/SMA NAA/Cr ratios of the AH and UH in patients, and the AH and normal volunteers were compared. The NAA/Cr ratios of the M-1 and SMA in AH, and the SMA in UH were significantly lower than those of normal volunteers. These 1H MRS findings indicate that axonal injury in the descending motor pathway at the level of internal capsule could induce metabolic changes in the higher centers of the motor pathway.

Spectroscopic data following stroke reveal tissue abnormality beyond the region of T2-weighted hyperintensity

Cerebral tissue with T2 magnetic resonance imaging (MRI) abnormalities following stroke is generally considered infarcted, while surrounding regions with normal MRI appearance are believed to be healthy. To assess whether these surrounding regions consist of normal tissue, we explored the distribution of N-acetylaspartate (NAA) and lactate within and around the hyperintense area on T2-weighted MRI using proton MR spectroscopy. The study was carried out in 25 patients with middle cerebral artery occlusion imaged between 1 and 42 days after stroke onset. NAA/choline (Cho) ratios were significantly reduced in both areas of T2 hyperintensity and in surrounding tissue. The reduction was greater in the region of T2 hyperintensity than in the surrounding region (-50% vs. -28%, respectively) and was unrelated to the delay after the ictus. Lactate/Cho ratios increased massively within the abnormal T2 area, but did not differ from control values beyond the margin of hyperintensity. Overall data indicate that T2 visible lesions on MRI do not infer the entire injured tissue.

Zero-quantum filter offering single-shot lipid suppression and simultaneous detection of lactate, choline, and creatine resonances

A zero-quantum (ZQ) filter offering single-shot lipid suppression and providing for simultaneous detection of the lactate methyl doublet (1.3 ppm) and nonoverlapping singlets including choline (Cho, 3.2 ppm) and creatine (Cr, 3.0 ppm) is described. Filtering is provided by soft mixing and reading pulses (RF(mix), RF(rd)) that are selective for the lactate methine quartet (4.1 ppm), Cho, and Cr resonances but exclude the 1.3 ppm lactate component and overlapping lipids. Surrounding RF(mix) and RF(rd) are magnetic field gradient pulses of equal magnitude but opposite signs to enable the rephasing of the zero-quantum lactate coherence and the creation of a stimulated echo for singlets within the pulse passbands. The sequence is designed to retain half the original lactate and singlet signal intensities. Theoretical predictions were confirmed experimentally at 1.5T using phantom acquisitions. The lipid suppression factor was measured to be over 10(3).

Application of magnetic resonance to animal models of cerebral ischemia

The present review has been compiled to highlight the role of magnetic resonance imaging (MRI) and MR spectroscopy (MRS) for the investigation of cerebral ischemia in the animal experimental field of basic research. We have focused on stroke investigations analyzing the pathomechanisms of the disease evolution and on new advances in both nuclear MR (NMR) methodology or genetic engineering of transgenic animals for the study of complex molecular relationships and causes of the disease. Furthermore, we have tried to include metabolic and genetic aspects, as well as the application of functional imaging, for the investigation of the disturbance or restitution of functional brain activation under pathological conditions as relates to controlled animal experiments.

Detection of an intense resonance at 2.4 ppm in 1H MR spectra of patients with severe late-delayed, radiation-induced brain injuries

Proton MRS and MRI were used to monitor the progression of severe cerebral radiation injuries in 10 patients over a period of 18 months. An unknown resonance (Px) in the 2.37-2.40 ppm region was consistently detected in the affected temporal lobes of four patients. The detection of Px was only confined to spectra with lactate (Lac) and in patients with the highest severity grading of radiation injury. The incidence of Px in Lac-positive spectra was 42.8% (15/35) and in lesions with highest injury grading was 46.8% (15/32). Lesions with Px had significantly higher Lac/creatine (Cr) ratios and more extensive mass effect changes when compared to lesions without Px. The probable identity of Px was examined in the context of anaerobic glycolysis producing pyruvate (2.37 ppm) and the model of metabolic changes in brain abscess formation implicating succinate (2.40 ppm).

Correlation between proton magnetic resonance spectroscopic lactate measurements and vascular reactivity in chronic occlusive cerebrovascular disease: a comparison with positron emission tomography

The purpose of this study is to investigate the correlation between lactate levels and cerebral vascular reactivity (VR) in regions outside an area of chronic cerebral infarction. Multivoxel proton magnetic resonance spectroscopy ((1)H-MRS) and positron emission tomography (PET) were performed in 11 patients who suffered chronic cerebral infarction. Of these 11 patients, 4 were examined before and after bypass surgery. Two regions-of-interests (ROIs) were placed outside the area of chronic infarction. One ROI was placed within a control region on the contralateral side. A lactate peak area was obtained in all ROIs. An N-acetyl aspartate (NAA) peak area was obtained in the ROI within the control region. The ratio of the lactate peak area and NAA peak area (Lct/NAA) was calculated for normalization of the lactate level, and was found to be 0.13 +/- 0. 10 (range, 0 to 0.43). The VR was recorded at 13.3 +/- 20.7% (range, - 44.3 to 68.9%), utilizing PET and administering acetazolamide. A significant negative correlation was observed between the Lct/NAA ratio and VR (r = - 0.709, p < 0.0001). These results suggest that lactate levels and VR are closely related in regions outside areas of chronic cerebral infarction.

Probability of metabolic tissue recovery after thrombolytic treatment of experimental stroke: a magnetic resonance spectroscopic imaging study in rat brain

The effect of thrombolytic therapy on metabolic changes was studied in rats submitted to thromboembolic stroke. Reperfusion was initiated at three different time points, 1.5, 3, and 4.5 hours after embolism (n = 3 each), by injection of recombinant tissue-type plasminogen activator (rt-PA). Recovery was observed during 5 hours of reperfusion using perfusion-weighted images and a two-dimensional 1H magnetic resonance spectroscopic imaging (MRSI) technique. Temporal evolution of the cerebral metabolites lactate and N-acetyl-aspartate (NAA) was determined. To analyze the chances of metabolic tissue recovery, the outcome of treatment, defined by a reversal of lactate concentration, was compared with the lactate intensity before treatment. In untreated animals (n = 4), clot embolism resulted in a drop of perfusion signal intensity in the occluded hemisphere followed by an increase of lactate concentration and a decrease of NAA that persisted throughout the observation period. Thrombolysis partially restored blood flow, but the mean lactate concentration decreased only slightly after successful lysis in animals treated 1.5 hours after embolism. If treatment was initiated later, no decline of lactate level was observed. Five hours after initiation of thrombolysis, the average tissue lactate amounted to 95 +/- 6, 111 +/- 17, and 139 +/- 60% of the early ischemic value (40 minutes after embolization) if treatment began 1.5, 3, and 4.5 hours after embolism, respectively. The NAA level declined slightly but never showed a recovery after rt-PA treatment. In individual pixels, the probability of metabolic tissue recovery clearly declined with increasing lactate concentration before thrombolysis. Interestingly, this probability was independent of treatment delay, but the number of pixels with low lactate declined with increasing ischemia time. Potential clinical applications of MRSI include monitoring of therapeutic intervention as well as support for prognosis of outcome after rt-PA treatment.

Proton magnetic resonance spectroscopy of late delayed radiation-induced injury of the brain

We prospectively evaluated metabolite changes in late delayed radiation-induced injury to the temporal lobes on proton ((1)H) magnetic resonance spectroscopy (MRS) in 34 patients. Morphologically more severe injury on imaging tended to have lower N-acetyl aspartate (NAA)/creatine (Cr) and NAA/choline (Cho) ratios. A significantly higher Cho/Cr ratio was found in the most severe grade of cerebral necrosis, in which lactate might be present. The progressive decrease in NAA with increasing severity reflected neuronal loss at different stages of late delayed radiation-induced brain injury. The absence of Cho elevation in mild and moderate lesions did not suggest demyelination or glial hyperplasia as an etiologic mechanism of late delayed radiation-induced brain injury. The association of severe morphologic lesions with elevated lactate suggests ischemia as the underlying mechanism for severe lesions. (1)H MRS may provide metabolite information conducive to the understanding of the pathophysiology of late radiation-induced brain injury. J. Magn. Reson. Imaging 1999;10:130-137.

Imaging of acute cerebral ischemia

Until recently, there was no efficacious treatment for acute cerebral ischemia. As a result, the role of neuroimaging and the radiologist was peripheral in the diagnosis and management of this disease. The demonstration of efficacy using thrombolysis has redefined this role, with the success of intervention becoming increasingly dependent on timely imaging and accurate interpretation. The potential benefits of intervention have only begun to be realized. In this State-of-the-Art review of imaging of acute stroke, the role of imaging in the current and future management of stroke is presented. The role of computed tomography is emphasized in that it is currently the most utilized technique, and its value has been demonstrated in prospective clinical trials. Magnetic resonance techniques are equally emphasized in that they have the potential to provide a single modality evaluation of tissue viability and vessel patency in an increasingly rapid evaluation.

In vivo observation of lactate methyl proton magnetization transfer in rat C6 glioma

Magnetic resonance spectroscopy (MRS) measurements of the lactate methyl proton in rat brain C6 glioma tissue acquired in the presence of an off-resonance irradiation field, analyzed using coupled Bloch equation formalism assuming two spin pools, demonstrated the occurrence of magnetization transfer. Quantitative analysis revealed that a very small fraction of lactate (f = 0.0012) is rotationally immobilized despite a large magnetization transfer effect. Off-resonance rotating frame spin-lattice relaxation studies demonstrated that deuterated lactate binds to bovine serum albumin and the proteins present in human plasma, thereby providing a possible physical basis for the observed magnetization transfer effect. These results demonstrate that partial or complete saturation of the motionally restricted lactate pool (as well as other metabolites) by the application of an off-resonance irradiation field, such as that used for water presaturation, can lead to a substantial decrease in resonance intensity by way of magnetization transfer effects, resulting in quantitation errors.

Feasibility study of lactate imaging of head and neck tumors

A proton spectroscopic imaging sequence was used to investigate the feasibility of lactate imaging in head and neck tumors. The sequence employs a two-shot lactate editing method with inversion recovery for additional lipid suppression, and a restricted field of view to suppress motion artifacts. Variations in acquisition parameters and two different receive coils were investigated on twelve patients. Elevated lactate was detected in three patients, no lactate was observed in seven patients, and two studies were inconclusive because of severe motion or inhomogeneity artifacts. Best results were obtained with an anterior/posterior neck coil at a 288 ms echo time (TE).

In vivo lactate editing with simultaneous detection of choline, creatine, NAA, and lipid singlets at 1.5 T using PRESS excitation with applications to the study of brain and head and neck tumors

Two T2-independent J-difference lactate editing schemes for the PRESS magnetic resonance spectroscopy localization sequence are introduced. The techniques, which allow for simultaneous acquisition of the lactate doublet (1.3 ppm) and edited singlets upfield of and including choline (3.2 ppm), exploit the dependence of the in-phase intensity of the methyl doublet upon the time interval separating two inversion (BASING) pulses applied to its coupling partner after initial excitation. Editing method 1, which allows for echo times TE = n/J (n = 1, 2, 3, . . . . ), alters the BASING carrier frequency for each of two cycles so that, for one cycle, the quartet is inverted, whereas, for the other cycle, the quartet is unaffected. Method 2, which also provides water suppression, allows for editing for TE > 1/J by alternating, between cycles, the time interval separating the inversion pulses. Experimental results were obtained at 1.5 T using a Shinnar Le-Roux-designed maximum phase inversion pulse with a filter transition bandwidth of 55 Hz. Spectra were acquired from phantoms and in vivo from the human brain and neck. In a neck muscle study, the lipid suppression factor, achieved partly through the use of a novel phase regularization algorithm, was measured to be over 10(3). Spectra acquired from a primary brain and a metastatic neck tumor demonstrated the presence of lactate and choline signals consistent with abnormal spectral patterns. The advantages and limitations of the methods are analyzed theoretically and experimentally, and significance of the results is discussed.

Characterization of middle cerebral artery occlusion infarct development in the rat using fast nuclear magnetic resonance proton spectroscopic imaging and diffusion-weighted imaging

A nuclear magnetic resonance study of the middle cerebral artery occlusion in the rat is presented. Experiments were performed on seven animals before and after occlusion, which occurred in situ. The emphasis in this study was on evaluating rapid proton spectroscopic imaging. Data were acquired with experimental durations of between 4 and 15 minutes for a 32 by 32 spatial matrix, with 64 spectroscopic data points per spatial element. The spectroscopic data were interleaved with diffusion-weighted nuclear magnetic resonance water images of the same slice. The study was terminated at about 6 hours after occlusion. The brains were then frozen in liquid nitrogen for biochemical imaging. The results showed that the signal from N-acetyl aspartate decreased and that of lactate increased within the infarcted region. The temporal course of these intensity changes varied between animals. Nineteen cortical spreading depressions (CSD) were observed by electrophysiologic monitoring during the experiments. Of these, 11 could be unambiguously detected in the lactate images, and a further 3 were on the threshold of detectability. As only a single slice could be examined, it is possible that the centers of depression for the remaining 6 CSD were outside the slice. To the authors' knowledge, this is the first report of the measurement of CSD using proton spectroscopic imaging. Thus, it is shown that this method is valuable not only in following the continuous evolution of proton metabolites with a good spatial and temporal resolution, but also in observing transient phenomena which are believed to play an important role in the expansion of the infarcted territory.

Proton magnetic resonance spectroscopy in deep cerebral venous thrombosis

A 63-year-old man developed a severe left frontal headache followed by an acute change of mentality 6 days later. Magnetic resonance imaging revealed bilateral thalamic ischemia. Angiography confirmed the occlusion of deep cerebral veins. Proton magnetic resonance spectroscopy (1H-MRS) of the thalami showed normal N-acetylaspartate (NAA) peak and the presence of lactate peak, indicating a relatively preserved neuronal viability. The patient improved during the follow-up period, and returned to work 45 days after the onset of the disease. With 1H-MRS, prognosis following venous infarctions may be feasible.

Prognostic significance of metabolic changes detected by proton magnetic resonance spectroscopy in ischaemic stroke

Proton magnetic resonance spectroscopy has proved to be useful for monitoring a number of metabolites in cerebral infarction. Combined magnetic resonance imaging and spectroscopy investigations were carried out in 14 patients with a recent ischaemic stroke (< 1 week); follow-up examinations were performed from day 28 to day 252 after stroke. The aim of this study was to assess the correlation between the changes of N-acetyl-aspartate, choline, creatine-phosphocreatine, lactate and clinical evolution measured by the Scandinavian Neurological Scale (SNS). Initially the lactate magnetic resonance signal was present in all patients and the other metabolite contents were significantly reduced (P < 0.001; unpaired t-test) as compared with those on the contralateral side. Spearman's rank correlation test showed a positive correlation between the initial reduction of N-acetyl-aspartate content and the SNS (P < 0.05), and between the final N-acetyl-aspartate content and the SNS (P < 0.001). Our results suggest that serial examination in stroke patients may provide further prognostic information and encourage controlled clinical studies.

Mapping of lactate and N-acetyl-L-aspartate predicts infarction during acute focal ischemia: in vivo 1H magnetic resonance spectroscopy in rats

The time course, anatomic distribution, and extent of changes in cerebral lactate, N-acetyl-L-aspartate (NAA), and other metabolite levels determined by three-dimensional in vivo 1H magnetic resonance spectroscopy and single-voxel spectral analysis after middle cerebral artery occlusion in rats. Increased lactate was detected in the central ischemic region within 1.3 hours after the onset of permanent occlusion (n = 22) or 0.5 hour after the onset of 1 hour of temporary occlusion and then reperfusion (n = 8). Permanent occlusion resulted in persistent lactate elevation and a 25.4 +/- 4.1% reduction in the NAA peak after 1.3 hours; NAA was almost completely depleted after 24 hours. Results also demonstrated delayed depletion of all other magnetic resonance spectroscopy-visible 1H metabolites, including creatine, choline, and glutamate, after permanent occlusion. After 1 hour of temporary focal ischemia, lactate returned to nearly normal levels within 0.4 hour after the onset of reperfusion; at 72 hours, a recurrent increase in lactate and a new decrease in NAA were observed, suggesting delayed tissue injury. Histological analysis, performed in 10 rats, demonstrated infarcts that corresponded in distribution to regions of NAA depletion at 72 hours. These findings indicate that lactate elevation is a sensitive early marker of ischemia; however, temporary recovery of lactate accumulation after reperfusion did not predict sustained metabolic recovery. In contrast, NAA depletion within 1.3 hours after the onset of ischemia identified central ischemic regions that were destined for infarction. Potential clinical applications include selection and monitoring of therapeutic intervention, as well as prediction of outcome, in patients with acute stroke.

H-1 MR spectroscopic imaging of white matter signal hyperintensities: Alzheimer disease and ischemic vascular dementia

PURPOSE

To investigate the association of white matter signal hyperintensities (WMSHs) with changes in hydrogen-1 metabolites.

MATERIALS AND METHODS

T2-weighted magnetic resonance (MR) imaging and H-1 MR spectroscopic imaging were performed in 21 elderly control subjects without or with minimal WMSHs, eight elderly subjects with substantial WMSHs, 11 probable Alzheimer disease patients with WMSHs, and eight ischemic vascular dementia (IVD) patients with WMSHs. N-acetylaspartate (NAA), choline-containing metabolites (Cho), and creatine-containing metabolites (Cr) were analyzed.

RESULTS

Differences in regional metabolite levels were found within the supraventricular brain of elderly control subjects. In Alzheimer disease patients, extensive WMSHs showed a lower percentage of NAA and a higher percentage of Cho compared with contralateral normal-appearing white matter (NAWM); in IVD patients, extensive and large WMSHs were associated with a higher percentage of Cho and a lower percentage of Cr compared with contralateral NAWM.

CONCLUSION

Regional metabolite variation and the presence of WMSHs are important covariants that must be accounted for in analysis of MR spectroscopic data.

Re-establishment of cerebral metabolism after carotid endarterectomy

OBJECTIVES

The purpose of this study was to evaluate the metabolic changes that occur in the human brain in patients with a symptomatic carotid artery stenosis.

MATERIALS AND METHODS

N-acetyl-aspartate (NAA), choline, creatine and lactate were measured both before, and 4 days after, carotid endarterectomy, by magnetic resonance spectroscopic imaging (MRSI). Eight controls and 16 patients were examined. MRI and MRSI studies were performed on a Philips Gyroscan S15 whole body system operating at 1.5 Tesla. 1H spectra were selected from regions in the centrum semi-ovale outside areas showing white matter hyper intensities on MRI.

RESULTS

All patients showed a decrease of the NAA/choline and NAA/creatine ratio in the symptomatic hemisphere compared to the contralateral hemisphere and also compared to the controls. Lactate was present in some patients (5/16). After endarterectomy, the NAA/choline and NAA/creatine ratios increased significantly compared to the ratios preoperative. Lactate was absent or more than 50% reduced after the operation. MRSI showed metabolic changes in areas of the brain that did not show any abnormalities on MRI.

CONCLUSIONS

There are marked changes in brain metabolism in the symptomatic hemisphere of patients with a severe carotid artery stenosis. These metabolic changes normalise four days after endarterectomy.

Short echo time proton spectroscopy of the brain in healthy volunteers using an insert gradient head coil

An insert gradient head coil with built-in X, Y, and Z gradients was used for localized proton spectroscopy in the brain of healthy volunteers, using short echo time stimulated echo acquisition mode (STEAM) sequences. Volume of interest size was 3.4 ml, repetition time was 6.0 s, and echo times were 10 and 20 ms, respectively. Good quality proton spectra with practically no eddy current artefacts were acquired allowing observation of strongly coupled compounds, and compounds with short T2 relaxation times. The gradient head coil thus permits further studies of compounds such as glutamine/glutamate and myo-inositols. These compounds were more prominent within grey matter than within white matter. Rough estimations of metabolite concentrations using water as an internal standard were in good agreement with previous reports.

Proton magnetic resonance spectroscopy and gadolinium-DTPA perfusion imaging of asymptomatic MRI white matter lesions

In the elderly, asymptomatic white matter hyperintensities are common on T2-weighted magnetic resonance imaging (MRI). In symptomatic patients, such MRI appearances correlate with varied postmortem findings including demyelination or stroke. What structural correlates underlie the T2 hyperintensities in patients whose lesions are asymptomatic is controversial. Therefore, in order to investigate the underlying metabolism and perfusion in white matter lesions (exhibiting T2 hyperintensity), 13 patients underwent proton magnetic resonance spectroscopy and dynamic gadolinium-DTPA perfusion-weighted MR imaging. N-acetyl aspartate (NA) levels were reduced in the lesions compared with age-matched controls (P = 0.031), implying neuronal/axonal loss. Creatine levels were also reduced (P = 0.001). Choline levels were unchanged in the lesions. Lactate was identified in the lesions of 5 of the 13 patients. Although not statistically significant, perfusion studies exhibited a trend toward lower cerebral blood volumes in patients with high grade extracranial carotid stenosis and lactate-containing lesions. These findings suggest that neuronal/axonal loss underlies the majority of T2-weighted asymptomatic lesions in the older population, and in many cases these changes may be due to chronic ischemia.

Investigation of the 1H NMR visibility of lactate in different rat and human brain cells

In recent years, 1H MRS has been used in a number of studies to measure the lactate content of brain, and it is generally assumed that the methyl resonance at 1.3 ppm reflects the total amount of lactate present in the tissue. However, reduced NMR visibility of lactate has recently been reported for blood, heart and skeletal muscle as well as for bacteria. We have assessed the NMR visibility of lactate in cultures of human and rat brain cells, comparing the concentrations measured by NMR and by biochemical methods. Contributions of fatty acids have been eliminated using their different relaxation behavior. We found approximately 30% of the lactate to be undetectable by NMR in the studied cell cultures. While the mechanism partially masking lactate in 1H spectra is not yet understood, the potential invisibility of some pools of lactate to NMR may greatly affect the interpretation of brain spectra.

Proton magnetic resonance spectroscopy in suspected vascular ischemic parkinsonism

Up to now the existence of "vascular parkinsonism" has been doubtful because conclusive clinicopathologic studies are lacking. The objective of the present magnetic resonance spectroscopy (MRS) study is to detect metabolic signs as a reflect of ischemic lesions which could be responsible for the clinical features of vascular parkinsonism. Proton MRS of the brain was performed in 12 patients suspected of vascular parkinsonism on clinical grounds and ischemic score, and in a control group of 15 patients with idiopathic Parkinson's disease. The MR spectra were measured in the striatum and deep white matter. MRS did not demonstrate metabolic evidence for the existence of ischemia (elevated lactate) or cell loss (decreased N-acetyl-aspartate levels) in patients suspected of vascular parkinsonism. Several explanations for our findings are discussed.

Proton magnetic resonance spectroscopy in patients with ischemic stroke

Proton magnetic resonance spectroscopy (1H-MRS) is a non-invasive technique which has proved to be useful for monitoring various brain metabolites (N-acetyl-aspartate, choline, creatine-phosphocreatine, lactate). A total of 18 patients underwent a combined magnetic resonance imaging (MRI)/1H-MRS protocol in order to evaluate the distribution of the metabolites in the various stages of cerebral ischemia. Our results show a marked decrease of N-acetyl-aspartate and a large content of Lactate during the early phases, and a decrease in N-acetyl-aspartate, choline and creatine-phosphocreatine (Cr-PCr) during the chronic phase.

Nuclear magnetic resonance detection of increased cortical GABA in vigabatrin-treated rats in vivo

1H Nuclear magnetic resonance ([1H]NMR) spectroscopy was used to detect elevation of gamma-aminobutyric acid (GABA) in rat brain after administration of the antiepileptic drug vigabatrin (VGB). Rats were treated for 3 weeks with VGB added to their drinking water to deliver a dose of 250 mg/kg body weight per day. NMR spectroscopy was performed noninvasively in vivo, and a GABA concentration of 6.0 +/- 2.3 mmol/kg wet weight (mean +/- SD, n = 5) was measured. GABA could not be detected in control animals in vivo. Postmortem GABA levels of 1.3 +/- 0.5 and 4.5 +/- 1.0 mmol/kg (mean +/- SD, n = 5) were measured in perchloric acid extracts of frozen brain from control and treated animals, respectively. Noninvasive measurement of increased cerebral GABA should allow detailed studies of the pharmacology of GABA-increasing drugs in vivo. With future developments, these measurements may be feasible in human subjects.

Proton spectroscopy of human stroke: assessment of transverse relaxation times and partial volume effects in single volume steam MRS

Proton T2 relaxation times were measured in 13 stroke patients and 13 aged-matched normal subjects at 2.1 T. Spectra were acquired from an 8-cc volume using the STEAM sequence with echo times (TE) of 30.4 ms and 270.0 ms and repetition time of 2.8 s. Transverse relaxation times were estimated using two-point calculations. Percentage volume of infarct in the STEAM voxel was measured on spin-echo MRI encompassing the infarct and correlated with the peak amplitude of N-acetylated compounds (NA). T2 values of NA, creatine, and choline resonances showed no significant difference between patients and controls. T2 for lactate in patients was 780 +/- 257 ms, respectively (mean +/- SE, n = 7). In stroke patients, high inverse correlation was found between the absolute NA signal and partial volume of normal brain contributing to each spectrum (p < .001, r = 0.97). Together with unchanged T2, this suggests that NAA largely disappears from infarcted tissue within 24 hr postinfarct.

Nuclear magnetic resonance spectroscopy. Principles and applications in neuroophthalmology

Magnetic resonance spectroscopy is a valuable method for the non-invasive investigation of metabolic processes and can now be combined with conventional magnetic resonance imaging in patients. This article gives a brief introduction into the principles and physiological and clinical applications of in vivo proton magnetic resonance spectroscopy, surveys experiences in healthy volunteers and presents exemplary results in patients suffering from cortical blindness or visual field defects. The causes of visual loss include brain trauma, cerebral ischemia, and brain tumors. In traumatic, ischemic and neoplastic lesions, an important spectral finding is an elevated lactate resonance which has been explained by increased anaerobic glycolysis of ischemic brain tissue and macrophages invading necrotic tissue. In our investigations using a clinical spectroscopy protocol on a 1.5 T MR system, a significant lactate signal was absent in spectra obtained from the visual cortex of normal volunteers, even during photic stimulation with a stroboscope. Other spectral changes in the patients include a decreased N-acetyl-aspartate resonance which indicates a decreased number of viable neurons in the examined brain region.

Mapping of cerebral metabolites in rats by 1H magnetic resonance spectroscopic imaging. Distribution of metabolites in normal brain and postmortem changes

The goal of this study was to examine metabolic differences between cortex and basal ganglia in normal rat brain and to determine postmortem changes using in vivo 1H magnetic resonance spectroscopic imaging at 300 MHz. The resonances observed were: choline, creatine + phosphocreatine, N-acetyl aspartate (NAA), lactate (Lac), and three small resonances in the amino acid region which included resonances from aspartate + NAA (Asp), glutamine + NAA (Gln), and glutamate + GABA (Glu). A previously unassigned resonance was observed at 1.13 ppm in brain of rats anesthetized with pentobarbital. Spectroscopic images in normal brain demonstrated increased NAA and Gln and decreased Glu in cortex compared to basal ganglia. The major postmortem changes were an increase of Lac, Glu and Cho and a decrease of NAA and Asp. The rise in Lac was significantly higher in cortex than in basal ganglia.

Lactate imaging of the human brain at 1.5 T using a double-quantum filter

The use of a double-quantum filtered 1H NMR spectroscopic imaging technique is described to detect the spatial distribution of lactate in the human brain. In two patients the feasibility of this technique is shown and compared with existing single-quantum spectroscopic imaging and single voxel techniques. Single-slice double-quantum filtered lactate images were obtained showing the lactate distribution over the entire slice in the brain. The lipid signal suppression was sufficient for the unambiguous detection of lactate. The signal loss of the lactate signal due to the incorporation of the double-quantum filter was 50-70% relative to the single-quantum signal.

Proton spectroscopy of the neonatal brain following hypoxic-ischaemic injury

Proton magnetic resonance spectroscopy was used to examine, within the first month of life, the brains of 11 infants born at term--10 with signs of hypoxic-ischaemic encephalopathy (HIE) and one who was neurologically normal at birth. All the infants had peak resonances on their spectra which could be assigned to N-acetyl-aspartase (NAA), choline-containing compounds (Cho) and creatine plus phosphocreatine (Cr). When neurodevelopmental outcome at one year was correlated with initial spectroscopy findings, the NAA/Cho and NAA/Cr ratios reflected clinical outcome. This study suggests that proton spectroscopy not only provides new information about biochemical changes occurring in the brains of infants with HIE, but also may help to predict outcome within the first month of life.

Nuclear magnetic resonance imaging and spectroscopy of human brain function

The techniques of in vivo magnetic resonance (MR) imaging and spectroscopy have been established over the past two decades. Recent applications of these methods to study human brain function have become a rapidly growing area of research. The development of methods using standard MR contrast agents within the cerebral vasculature has allowed measurements of regional cerebral blood volume (rCBV), which are activity dependent. Subsequent investigations linked the MR relaxation properties of brain tissue to blood oxygenation levels which are also modulated by consumption and blood flow (rCBF). These methods have allowed mapping of brain activity in human visual and motor cortex as well as in areas of the frontal lobe involved in language. The methods have high enough spatial and temporal sensitivity to be used in individual subjects. MR spectroscopy of proton and carbon-13 nuclei has been used to measure rates of glucose transport and metabolism in the human brain. The steady-state measurements of brain glucose concentrations can be used to monitor the glycolytic flux, whereas subsequent glucose metabolism--i.e., the flux into the cerebral glutamate pool--can be used to measure tricarboxylic acid cycle flux. Under visual stimulation the concentration of lactate in the visual cortex has been shown to increase by MR spectroscopy. This increase is compatible with an increase of anaerobic glycolysis under these conditions as earlier proposed from positron emission tomography studies. It is shown how MR spectroscopy can extend this understanding of brain metabolism.

Applications of magnetic resonance spectroscopy and diffusion-weighted imaging to the study of brain biochemistry and pathology

The first practical demonstration that nuclear magnetic resonance (NMR) spectroscopy could be applied to the study of brain biochemistry in vivo came in 1980, with the studies of the rat brain using a surface coil. Since then the technique has been rapidly and extensively developed into a versatile, non-invasive tool for the investigation of various aspects of brain biochemistry, physiology and disease. NMR is non-destructive and can be used to examine a wide variety of samples, ranging from localized regions within the whole brain in humans or animals, through tissue preparations (perfused organ, tissue slices and homogenates), to isolated cells and aqueous solutions, such as tissue extracts. 31P and 1H NMR spectra deriving from endogenous compounds of the brain in situ allow assessment of tissue metabolites and provide information about high-energy phosphates, lactate, certain amino acids, intracellular pH and ionic concentrations. Exogenous substrates or probes labelled with stable isotopes can also be introduced into the brain and used to monitor metabolism. Animal models of brain diseases have given some impetus to rapid progress in clinical NMR spectroscopy and also magnetic imaging techniques. The purpose of this article is to highlight the type of information available from these NMR techniques, and to present this in a neuroscience context, emphasizing the biochemical, physiological and pathological information that can be obtained using these methods.

In situ changes in purine nucleotide and N-acetyl concentrations upon inducing global ischemia in cat brain

Spectral changes upon death (global ischemia) were recorded in situ in cat cerebral cortex. A rapid signal loss of 10 +/- 4% in the N-acetyl resonance was observed (five cases), after which it remained constant for 1 h. For the first time signal changes are reported for low-field resonances, which are assigned to adenosine/inosine and hypoxanthine.

Cerebral metabolic studies in vivo by combined 1H/31P and 1H/13C NMR spectroscopic methods

Intracellular pH and ammonium ion concentration are potent modulators of cerebral amino acid metabolism. Furthermore, intracellular acidosis and hyperammonemia accompany conditions such as ischemic encephalopathy and seizures and may contribute to the pathological sequelae observed. In vivo NMR spectroscopy permits multiple, non-destructive measurements of important cerebral metabolic intermediates in the same animal. We describe here the use of 1H, and 31P NMR spectroscopy to investigate the effects of acute changes in intracellular pH and ammonium ions on cerebral glutamate, glutamine, and lactate levels in vivo. We then show how 1H NMR can be used to indirectly follow the flow of 13C label from [1-13C] glucose into the cerebral glutamate pool, allowing us to measure cerebral TCA activity in normal and chronically hyperammonemic rats. Male Sprague-Dawley rats (160-210 gm), fasted 24-hours, were tracheotomized, paralyzed and ventilated on 30% O2/70% N2O. NMR spectroscopy was performed at a field strength of 8.4 Tesla using a Bruker AM-360 wide bore spectrometer. An elliptical surface-coil (8 x 12 mm) was double-tuned to either the 1H and 31P or 1H and 13C frequencies. After retraction of extracranial tissues, the coil was positioned over the skull 2 mm posterior to the bregma. Tail arteries and veins were cannulated allowing periodic measurements of PO2, pCO2, pH and glucose in arterial blood and intravenous infusions. Respiratory acidosis was induced in rats by the addition of CO2 to the ventilation gas mixture. Arterial pCO2 increased within 5 min from a pre-hypercarbic value of 36.4 +/- 6.1 mm Hg to 200-220 mm Hg and was maintained at this level for over 1 hour. Hypercarbia led to rapid cerebral acidification. Intracellular pH decreased from 7.18 +/- 0.08 (pre-hypercarbic period) to 6.68 +/- 0.06 (n = 4) at 10 min and remained stable throughout the NMR observation period. Glutamate decreased to 53 +/- 4% of control after 60 min of hypercarbia, while glutamine increased to 126 +/- 7% of control. Acute hyperammonemia was produced by a programmed intravenous infusion of 250 mM ammonium acetate, which rapidly raised and maintained the concentration of ammonium ions in the blood at approximately 500 microM. Shortly after the start of the infusion (10-20 min), the levels of glutamine and lactate rose continuously throughout the experiment, reaching levels of 170 +/- 25% and 260 +/- 60% of control, respectively (n = 12) after 50 min. Glutamate decreased during the same time interval to 80 +/- 4% of control (n = 12).(ABSTRACT TRUNCATED AT 400 WORDS)

A robust algorithm for reduction of truncation artifact in chemical shift images

A new iterative technique to reduce the ringing artifacts in chemical shift images due to the truncation of the high spatial frequency is presented. In this approach the authors extrapolate the high spatial frequency data guided by the edge information obtained from a high resolution anatomic image of the region of interest. The fact that the edge information obtained from the anatomic image can be off by a few pixels (due to factors such as chemical shift artifact, error in edge detection or misregistration) is taken into account by assuming a confidence interval of several pixels around the anatomic edges. The algorithm is validated on simulated and in vivo data, and excellent results were obtained.