Early detection of human glioma sphere xenografts in mouse brain using diffusion MRI at 14.1 T.

Glioma models have provided important insights into human brain cancers. Among the investigative tools, MRI has allowed their characterization and diagnosis. In this study, we investigated whether diffusion MRI might be a useful technique for early detection and characterization of slow-growing and diffuse infiltrative gliomas, such as the proposed new models, LN-2669GS and LN-2540GS glioma sphere xenografts. Tumours grown in these models are not visible in conventional T2 -weighted or contrast-enhanced T1 -weighted MRI at 14.1 T. Diffusion-weighted imaging and diffusion tensor imaging protocols were optimized for contrast by exploring long diffusion times sensitive for probing the microstructural alterations induced in the normal brain by the slow infiltration of glioma sphere cells. Compared with T2 -weighted images, tumours were properly identified in their early stage of growth using diffusion MRI, and confirmed by localized proton MR spectroscopy as well as immunohistochemistry. The first evidence of tumour presence was revealed for both glioma sphere xenograft models three months after tumour implantation, while no necrosis, oedema or haemorrhage were detected either by MRI or by histology. Moreover, different values of diffusion indices, such as mean diffusivity and fractional anisotropy, were obtained in tumours grown from LN-2669GS and LN-2540GS glioma sphere lines. These observations highlighted diverse tumour microstructures for both xenograft models, which were reflected in histology. This study demonstrates the ability of diffusion MRI techniques to identify and investigate early stages of slow-growing, invasive tumours in the mouse brain, thus providing a potential imaging biomarker for early detection of tumours in humans.

Morphological and compositional monitoring of a new cell-free cartilage repair hydrogel technology - GelrinC by MR using semi-quantitative MOCART scoring and quantitative T2 index and new zonal T2 index calculation.

OBJECTIVE: To evaluate cartilage repair tissue (RT) using MOCART scoring for morphological and T2 mapping for biochemical assessment following implantation of GelrinC, a biosynthetic, biodegradable hydrogel implant. DESIGN: MR imaging (1.5/3T) was performed on 21 patients at six sites. Standard protocols were used for MOCART evaluation at 1 week (baseline) 1, 3, 6, 12, 18 and 24 months. Multi-echo SE was used for T2 mapping. Global (T2 in RT divided by T2 in normal cartilage) and zonal T2 index (deep T2 divided by superficial T2) of RT were calculated. RESULTS: Average MOCART score was 71.8 (95% CI 62.2 to 81.3) at six, 75.2 (95% CI 62.8 to 87.5) at twelve, 71.8 (95% CI 55.4 to 88.2) at eighteen and 84.4 (95% CI 77.7 to 91.0) at twenty-four months. The global T2 index ranged between 0.8 and 1.2 (normal healthy cartilage) in 1/11 (9%) patients at baseline, 8/12 (67%) at 12 months, 11/13 (85%) at 18 months and 13/16 (81%) at 24 months. The zonal T2 index for RT was <20% difference to the zonal T2 index for normal cartilage in: 6/12 patients (50%) at 12 months, 7/13 (53.8%) at 18 months and 10/16 (63.5%) at 24 months. The standard deviation for T2 showed a significant decrease over the study. CONCLUSIONS: The increase of MOCART scores over follow-up indicates improving cartilage repair tissue. Global and zonal T2 repair values at 24 months reached normal cartilage in 81% and 63.5% of the patients respectively, reflecting collagen organization similar to hyaline cartilage.

An in vivo ultrahigh field 14.1 T (1) H-MRS study on 6-OHDA and α-synuclein-based rat models of Parkinson's disease: GABA as an early disease marker.

The detection of Parkinson's disease (PD) in its preclinical stages prior to outright neurodegeneration is essential to the development of neuroprotective therapies and could reduce the number of misdiagnosed patients. However, early diagnosis is currently hampered by lack of reliable biomarkers. (1) H magnetic resonance spectroscopy (MRS) offers a noninvasive measure of brain metabolite levels that allows the identification of such potential biomarkers. This study aimed at using MRS on an ultrahigh field 14.1 T magnet to explore the striatal metabolic changes occurring in two different rat models of the disease. Rats lesioned by the injection of 6-hydroxydopamine (6-OHDA) in the medial-forebrain bundle were used to model a complete nigrostriatal lesion while a genetic model based on the nigral injection of an adeno-associated viral (AAV) vector coding for the human α-synuclein was used to model a progressive neurodegeneration and dopaminergic neuron dysfunction, thereby replicating conditions closer to early pathological stages of PD. MRS measurements in the striatum of the 6-OHDA rats revealed significant decreases in glutamate and N-acetyl-aspartate levels and a significant increase in GABA level in the ipsilateral hemisphere compared with the contralateral one, while the αSyn overexpressing rats showed a significant increase in the GABA striatal level only. Therefore, we conclude that MRS measurements of striatal GABA levels could allow for the detection of early nigrostriatal defects prior to outright neurodegeneration and, as such, offers great potential as a sensitive biomarker of presymptomatic PD.

Diffusion-weighted spectroscopy: a novel approach to determine macromolecule resonances in short-echo time 1H-MRS.

Quantification of short-echo time proton magnetic resonance spectroscopy results in >18 metabolite concentrations (neurochemical profile). Their quantification accuracy depends on the assessment of the contribution of macromolecule (MM) resonances, previously experimentally achieved by exploiting the several fold difference in T(1). To minimize effects of heterogeneities in metabolites T(1), the aim of the study was to assess MM signal contributions by combining inversion recovery (IR) and diffusion-weighted proton spectroscopy at high-magnetic field (14.1 T) and short echo time (= 8 msec) in the rat brain. IR combined with diffusion weighting experiments (with δ/Δ = 1.5/200 msec and b-value = 11.8 msec/µm(2)) showed that the metabolite nulled spectrum (inversion time = 740 msec) was affected by residuals attributed to creatine, inositol, taurine, choline, N-acetylaspartate as well as glutamine and glutamate. While the metabolite residuals were significantly attenuated by 50%, the MM signals were almost not affected (< 8%). The combination of metabolite-nulled IR spectra with diffusion weighting allows a specific characterization of MM resonances with minimal metabolite signal contributions and is expected to lead to a more precise quantification of the neurochemical profile.

Metabolic changes in the normal ageing brain: consistent findings from short and long echo time proton spectroscopy.

OBJECTIVES: Sixty three healthy subjects were measured to assess dependence of brain metabolites on age using short- and long echo time spectroscopy in different brain regions. MATERIAL AND METHODS: Younger and elderly humans were measured with long echo time (TE=135ms) 3D-MR-spectroscopic imaging (MRSI) (10 subjects) and with ultra-short echo (TE=11ms) time 2D-MRSI (7 subjects). In addition, results from single voxel (1)H-spectroscopy (TE=20ms) of two cohorts of 46 healthy subjects were retrospectively correlated with age. RESULTS: 3D-MR SI revealed reduced NAA/Cr in the older group in the frontal lobe (-22%; p<0.01), parietal lobe (-28%; p<0.01) and semiovale (-9%; p<0.01) compared to the younger group. Cho/Cr was elevated in the semiovale (+35%; p<0.01) and in the n. lentiformis (+42%; p<0.01) in the older group. NAA/Cho was reduced in all regions measured, except the thalamus, in the older group compared to the younger group (from -21 to -49%; p<0.01). 2D-MRSI revealed decreased total NAA (-3.1% per decade; p<0.01) and NAA/Cr (-3.8% per decade; p<0.01), increased total Cho (+3.6% per decade; p<0.01) and Cho/Cr (+4.6% per decade; p<0.01) and increased total myo-Inositol (mI, +4.7% per decade; p<0.01) and mI/Cr (+5.4% per decade; p<0.01) and decreased NAA/Cho (-8% per decade; p<0.01) in semiovale WM. Results from single voxel spectroscopy revealed a significantly negative correlation of NAA/Cho in frontal (-13% per decade; p<0.01) and in temporal lobe (-7.4% per decade; p<0.01) as well as increased total Cr (10% per decade; p<0.01) in frontal lobe. Other results from single voxel measurements were not significant, but trends were comparable to that from multivoxel spectroscopy. CONCLUSION: Age-related changes measured with long echo time and short echo time 1H-MRS were comparable and cannot, therefore, be caused by different T2 relaxation times in young and old subjects, as suggested previously.

The value of high-field MRI (3T) in the assessment of sellar lesions.

The aim of this study was the evaluation of the normal sellar anatomy in vitro and in vivo with high-field MRI and its application in the diagnosis of sellar pathologies in comparison to standard MRI. All high-field MR images were obtained using a 3T Bruker Medspec 30/80 Scanner with a head birdcage transmit/receive coil and an actively shielded gradient system with a maximum gradient strength of 45 mT/m. Firstly an in vitro study of the sella turcica was performed to depict normal pituitary and sellar anatomy at high field. After a pilot-study this sequence-protocol was established: A RARE sequence (TR/TE = 7790/19 ms; matrix size, 512 x 512; RARE factor = 8, FOV, 200 mm) was used for T2-weighted coronal, axial and sagittal images. A 3D gradient echo sequence with magnetization-preparation (MP-RAGE, TR/TE/TI = 33.5/7.6/800 ms, matrix size, 512 x 512; FOV, 200 mm, effective slice thickness, 1.88 mm; 3 averages) was used for acquisition of T1-weighted pre- and post-contrast images. Between January 2002 and March 200458 patients were enrolled in this study. Seven patients were examined for suspected microadenoma and in 51 patients 3T MRI was used to obtain additional information about the sellar lesion already known to be present from standard MRI. In 21 cases the accuracy of the imaging findings was assessed afterwards by comparison with intraoperative findings. The infiltration of the medial cavernous sinus wall was suspected on standard MRI on 15 sides (47%), on high-field MRI on 9 sides (28%) and could be verified by intraoperative findings on 6 sides (19%). Accordingly, sensitivity to infiltration was 83% for 3T and 67% for standard MRI. Specificity was 84% for 3T and 58% for standard MRI. Moreover, high-field MRI revealed microadenomas in 7 patients with a median diameter of 4mm (range 2-9 mm). The segments of the cranial nerves were seen as mean 4 hypointense spots (range 2-5 spots) on high-field MRI in contrast to 3 spots (range 0-4 spots) on standard MRI. This difference was considerably significant (P < 0.001, Wilcoxon rank sum test). The histopathological results revealed pituitary adenoma in 16 patients and non-adenomatous sellar pathologies such as Rathke's cleft cyst, sarcoidosis, meningeoma and metastasis in 5 patients. High-field MRI is superior to standard MRI for the prediction of invasion of adjacent structures in patients with pituitary adenomas and improves surgical planning of sellar lesion.

Impact of high field (3.0 T) magnetic resonance imaging on diagnosis of osteochondral defects in the ankle joint.

OBJECTIVE: To evaluate high field magnetic resonance (MR) imaging for imaging of osteochondral defects. MATERIALS AND METHODS: Nine osteochondral defects were simulated in three cadaveric talus specimens using a diamond drill. All specimens were examined on a 1.0 T MR unit and a 3.0 T MR unit. A T2-weighted turbo spin-echo (TSE) sequence with a 2 mm slice thickness and a 256 x 256 matrix size was used on both scanners. The visibility of the osteochondral separation and the presence of susceptibility artifacts at the drilling bores were scored on all images. RESULTS: Compared to the 1.0 T MR unit, the protocol on the 3.0 T MR unit allowed a better delineation of the disruption of the articular cartilage and a better demarcation of the subchondral defect. Differences regarding the visualization of the subchondral defect were found to be statistically significant (P<0.05). Differences with regard to susceptibility artifacts at the drilling bores were not statistically significant (P>0.05). The average SNR was higher using 3.0 T MRI (SNR=12), compared to 1.0 T MRI (SNR=7). CONCLUSION: High field MRI enables the acquisition of images with sufficient resolution and higher SNR and has therefore the potential to improve the staging of osteochondral defects.

In vitro study of astrocytic tumour metabolism by proton magnetic resonance spectroscopy.

In vivo magnetic resonance spectroscopy (MRS) studies of glial brain tumours reported that higher grade of astrocytoma is associated with increased level of choline-containing compounds (Cho) and decreased levels of N-acetylaspartate (NAA) and creatine and phosphocreatine (Cr). In this work, we studied the metabolism of glioma tumours by in vitro proton magnetic resonance spectroscopy (1H-MRS). 1H-MR spectra were recorded in vitro from perchloric acid extracts of astrocytoma (WHO II) and glioblastoma multiforme (WHO IV) samples. We observed differences between astrocytoma and glioblastoma multiforme in the levels of Cho, alanine, lactate, NAA, and glutamate/glutamine. In astrocytoma samples, we found higher MR signal of NAA and lower signal of Cho and alanine. MR spectra of glioblastoma samples reported significantly higher levels of lactate and glutamate/glutamine. In contrast, levels of Cr were the same in both tumour types. We also determined NAA/Cr and Cho/Cr ratios in the tumour samples. The NAA/Cr ratio was higher in astrocytomas than in glioblastomas multiforme. Conversely, the Cho/Cr ratio was higher in glioblastoma multiforme. The results indicate that MRS is a promising method for distinguishing pathologies in human brain and for pre-surgical grading of brain tumours.

Free fatty acids inhibit the glucose-stimulated increase of intramuscular glucose-6-phosphate concentration in humans.

To test Randle's hypothesis we examined whether free fatty acids (FFAs) affect glucose-stimulated glucose transport/phosphorylation and allosteric mediators of muscle glucose metabolism under conditions of fasting peripheral insulinemia. Seven healthy men were studied during somatostatin-glucose-insulin clamp tests [plasma insulin, 50 pmol/L; plasma glucose, 5 mmol/L (0-180 min), 10 mmol/L (180-300 min)] in the presence of low (0.05 mmol/L) and increased (2.6 mmol/L) plasma FFA concentrations. (31)P and (1)H nuclear magnetic resonance spectroscopy was used to determine intracellular concentrations of glucose-6-phosphate (G6P), inorganic phosphate, phosphocreatine, ADP, pH, and intramyocellular lipids. Rates of glucose turnover were measured using D-[6,6-(2)H(2)]glucose. Plasma FFA elevation reduced rates of glucose uptake at the end of the euglycemic period (R(d 150-180 min): 8.6 +/- 0.5 vs. 12.6 +/- 1.6 micromol/kg.min, P < 0.05) and during hyperglycemia (R(d 270-300 min): 9.9 +/- 0.6 vs. 22.3 +/- 1.7 micromol/kg.min, P < 0.01). Similarly, intramuscular G6P was lower at the end of both euglycemic (G6P(167-180 min): -22 +/- 7 vs. +24 +/- 7 micromol/L, P < 0.05) and hyperglycemic periods (G6P(287-300 min): -7 +/- 9 vs. +28 +/- 7 micromol/L, P < 0.05). Changes in intracellular inorganic phosphate exhibited a similar pattern, whereas FFA did not affect phosphocreatine, ADP, pH, and intramyocellular lipid contents. In conclusion, the lack of an increase in muscular G6P along with reduction of whole body glucose clearance indicates that FFA might directly inhibit glucose transport/phosphorylation in skeletal muscle.

Physicochemical properties of normal articular cartilage and its MR appearance.

Basic physical and physicochemical properties of articular cartilage are correlated with the MR parameters of this tissue. From these parameters, the typical appearance of cartilage in MR images is deduced. Some practical implications for clinical utilization of MRI of articular cartilage are summarized.

Magnetic resonance imaging of articular cartilage and evaluation of cartilage disease.

Clinical magnetic resonance imaging of articular cartilage is possible by using techniques that offer high contrast between articular cartilage and adjacent structures in reasonable examination times. The fat-suppressed, three-dimensional, spoiled gradient-echo sequence has been reported to be accurate and reliable, and the addition of this sequence to a routine examination does not significantly compromise patient throughput. Fast spin-echo imaging also shows promise in the clinical evaluation of articular cartilage, because the newer, stronger-gradient systems allow thinner slice acquisition with two-dimensional sequences. Together, these sequences allow the evaluation of intrachondral lesions and surface defects. Furthermore, quantitative measurements of cartilage volume for follow-up studies are possible with the use of the fat-suppressed, three-dimensional, spoiled gradient-echo sequence.

Transverse relaxation mechanisms in articular cartilage.

Relaxation rates in the rotating frame (R1rho) and spin-spin relaxation rates (R2) were measured in articular cartilage at various orientations of cartilage layer to the static magnetic field (B0), at various spin locking field strengths and at two different static magnetic field strengths. It was found that R1rho in the deep radial zone depended on the orientation of specimens in the magnet and decreased with increasing the spin locking field strength. In contrast, R1rho values in the transitional zone were nearly independent of the specimen orientation and the spin locking field strength. Measurements of the same specimens at 2.95 and 7.05 T showed an increase of R1rho and most R2 values with increasing B0. The inverse B0 dependence of some R2 values was probably due to a multicomponent character of the transverse magnetization decay. The experiments revealed that the dominant T1rho and T2 relaxation mechanism at B0 < or = 3 T is a dipolar interaction due to slow anisotropic motion of water molecules in the collagen matrix. On average, the contribution of scalar relaxation due to rapid proton exchange in femoral head cartilage at 2.95 T is about 6% or less of the total R1rho at the spin locking field of 1000 Hz.

Low-power water suppression by hyperbolic secant pulses with controlled offsets and delays (WASHCODE).

A class of chemical-shift-selective (CHESS) water suppression (WS) schemes is presented in which the characteristic frequency-domain excitation profiles of "adiabatic" full-passage (AFP) RF pulses are utilized for frequency-selective excitation of the water resonance. In the proposed WS schemes, dubbed WASHCODE, hyperbolic secant (HS) pulses were used as the AFP pulses. Besides the high immunity of WS efficiency toward B(1) inhomogeneity, these sequences also exhibit extraordinary insensitivity to the dispersion of the water T(1) relaxation times. The actual performance of the proposed WS schemes was achieved in particular by optimizing the frequency offsets of WS HS pulses and the time intervals between them. To reduce the RF power requirements of these WS sequences for in vivo applications, HS pulses with the minimum possible frequency bandwidths were employed, which also substantially reduced the adverse effects on the observed proton MR spectra. The proposed WS schemes were evaluated by simulations based on the Bloch equations. Several WS sequences which looked particularly promising were verified experimentally on the human brain on a 3 T MR scanner using very short echo-time STEAM for volume selection and a standard single-loop surface coil for both signal transmission and reception. Routinely, water-suppression factors ranging from 2000 to 4000 were achieved in vivo without additional adjustment of parameters for individual subjects and without violating legal safety limits.

Multivoxel 3D proton spectroscopy in the brain at 1.5 versus 3.0 T: signal-to-noise ratio and resolution comparison.

BACKGROUND AND PURPOSE: The new 3.0-T imagers theoretically yield double the signal-to-noise ratio (SNR) and spectral resolution of 1.5-T instruments. To assess the possible improvements for multivoxel 3D proton MR spectroscopy (1H-MRS) in the human brain, we compared the SNR and spectral resolution performance with both field strengths. METHODS: Three-dimensional 1H-MRS was performed in four 21-29-year-old subjects at 1.5 and 3.0 T. In each, a volume of interest of 9 x 9 x 3 cm was obtained within a field of view of 16 x 16 x 3 cm that was partitioned into four (0.75-cm-thick) 16 x 16-voxel sections, yielding 324 (0.75-cm3) signal voxels per examination. RESULTS: In an acquisition protocol of approximately 27 min, average voxel SNRs increased 23-46% at 3.0 versus 1.5 T in the same brain regions of the same subjects. SNRs for N-acetylaspartate, creatine, and choline, respectively, were as follows: 15.3 +/- 4, 8.2 +/- 2.2, and 8.0 +/- 2.0 at 1.5 T and 22.4 +/- 7.0, 10.1 +/- 3.5, and 10.1 +/- 3.6 at 3.0 T. Spectral resolution (metabolite linewidths) were 3.5 +/- 0.5 Hz at 1.5 T versus 6.1 +/- 1.5 Hz at 3.0 T in approximately 900 voxels. Spectral baselines were noticeably flatter at 3.0 T. CONCLUSION: Expected gains in SNR and spectral resolution were not fully realized in a realistic experiment because of intrinsic and controllable factors. However, the 23-46% improvements obtained enable more reliable peak-area estimation and an 1H-MRS acquisition approximately 50% shorter at 3.0 versus 1.5 T.

A study of creatine kinase reaction in rat brain under chronic pathological conditions-chronic ischemia and ethanol intoxication.

Creatine kinase reaction rates were measured by the magnetisation transfer technique in brains of healthy adult and aged rats and in rats with chronic cerebral ischemia and chronic ethanol intoxication. These measurements indicated that the rate constant of the creatine kinase reaction is significantly reduced in the case of severe chronic cerebral ischemia in aged rats. In the adult rats, during chronic ethanol intoxication after 3 weeks of administration of 3 ml of 30% ethanol once a day via a gastric tube, a significant decrease in the pseudo first-order rate constant k(for) of the creatine kinase reaction was also found. In contrast, mild chronic cerebral ischemia in adult rats produced an increase in the reaction rate 4 weeks after occlusion. At the same time, corresponding conventional phosphorus magnetic resonance spectra showed negligible changes in signal intensities.

Investigation of laminar appearance of articular cartilage by means of magnetic resonance microscopy.

Magnetic resonance (MR) images and relaxation and diffusion maps of articular cartilage were obtained to explain discrepancies in its MR appearance. Porcine specimens were studied only by MR microscopy. For human specimens a combination of MR microscopy and large-scale MR imaging was used. Common features in the laminar structures of human and porcine samples are described. It was found that the decay of transverse magnetization was nonexponential with a rapidly decaying component which prevented construction of reliable proton-density maps. Dependence of T2 values on the orientation of specimens in the magnetic field as well as magnetization transfer experiments supported the previous suggestions about a significant role of dipolar interaction with protons of collagen in the laminar appearance of articular cartilage. The loss of the laminar structure induced by rotation of the human cartilage specimen around the axis normal to its surface demonstrated nonuniform angular distribution of the collagen fibers within the layer.

MRI visualization of proteoglycan depletion in articular cartilage via intravenous administration of Gd-DTPA.

The effect of intravenous administration of gadolinium diethylenetriamine-pentaacetic acid (Gd-DTPA) on MR images was studied in vitro, using pathologic osteochondral specimens removed during surgery for total endoprosthesis, and in vivo, on a group of volunteers. In ex vivo specimens, lesions of different shape having lower T1 were detected which corresponded to areas with depleted proteoglycans found histologically. In vivo experiments on young volunteers showed that the time course of cartilage enhancement was different for different anatomies. The time for maximum enhancement ranged from 45 min for the ventral femoral condyle to 270 min for patellar cartilage.

Bilaminar pattern of tibial condyle cartilage layer on the fat-suppressed 3D gradient echo images: artifact or structural and biochemical difference in composition of cartilage?

The purpose of this study was to examine if an unusual bilaminar pattern of lateral tibial condyle cartilage layer on the fat-suppressed three-dimensional (3D) spoiled gradient echo sequence is artifactual or correlates with structural and/or biochemical composition of cartilage. The laminar appearance of the lateral tibial condyle cartilage layer was studied on fat-suppressed 3D spoiled gradient echo MR images of the knee joint in 67 patients (mean age: 28y) performed at 1.0 Tesla. After i.v. administration of gadopentetate dimeglumine, diffusion of the contrast media into cartilage layer was qualitatively analysed over time on inversion recovery spin echo images of knee joints of five asymptomatic volunteers (mean age: 25y). In a patient with osteosarcoma and total knee replacement, MR examination of cartilage layer of lateral tibial plateau was compared with histologic specimens stained with Safranin-O, demonstrating proteoglycan distribution in cartilage. The retrospective analysis of 67 knee joints revealed a bilaminar appearance of lateral tibial condyle cartilage layer in the gradient echo images in the majority of cases (81%) with a statistically significant tendency to a trilaminar pattern in patients older than 20 years. With i.v. contrast administration, the contrast enhancement was only observed in the superficial zone of tibial cartilage layer. Histologic specimens in one patient demonstrated a good correlation between thickness of proteoglycan-free and proteoglycan-rich laminae of lateral tibial condyle on Safranin-O staining with hyperintense and hypointense zones, respectively, on corresponding fat-suppressed 3D spoiled gradient echo images (correlation coefficient of 0.87). Bilaminar pattern of tibial condyle cartilage layer on fat-suppressed 3D spoiled gradient echo images in younger subjects is not an artifact or an intrachondral lesion, but it may represent a regional difference in composition of extracellular cartilage matrix possibly produced by a highly-oriented collagen fiber structure associated with a high concentration of proteoglycans in the middle and deep portion of the cartilage layer.

Absolute metabolite quantification by in vivo NMR spectroscopy: II. A multicentre trial of protocols for in vivo localised proton studies of human brain.

We have performed a multicentre trial to assess the performance of three techniques for absolute quantification of cerebral metabolites using in vivo proton nuclear magnetic resonance (NMR). The techniques included were 1) an internal water standard method, 2) an external standard method based on phantom replacement, and 3) a more sophisticated method incorporating elements of both the internal and external standard approaches, together with compartmental analysis of brain water. Only the internal water standard technique could be readily implemented at all participating sites and gave acceptable precision and interlaboratory reproducibility. This method was insensitive to many of the experimental factors affecting the performance of the alternative techniques, including effects related to loading, standing waves and B1 inhomogeneities; and practical issues of phantom positioning, user expertise and examination duration. However, the internal water standard method assumes a value for the concentration of NMR-visible water within the spectroscopic volume of interest. In general, it is necessary to modify this assumed concentration on the basis of the grey matter, white matter and cerebrospinal fluid (CSF) content of the volume, and the NMR-visible water content of the grey and white matter fractions. Combining data from 11 sites, the concentrations of the principal NMR-visible metabolites in the brains of healthy subjects (age range 20-35 years) determined using the internal water standard method were (mean+/-SD): [NAA]=10.0+/-3.4 mM (n=53), [tCho]=1.9+/-1.0 mM (n=51), [Cr + PCr]=6.5+/-3.7 mM (n=51). Evidence of system instability and other sources of error at some participating sites reinforces the need for rigorous quality assurance in quantitative spectroscopy.

Cerebral glutamate metabolism during hypoglycaemia in healthy and type 1 diabetic humans.

BACKGROUND: The mechanisms responsible for the progressive failure of hypoglycaemia counterregulation in long-standing type 1 diabetes are poorly understood. Increased brain glucose uptake during hypoglycaemia or alterations of brain energy metabolism could effect glucose sensing by the brain and thus contribute to hypoglycaemia-associated autonomic failure. MATERIALS AND METHODS: Type 1 diabetic patients (T1DM) and healthy volunteers (CON) were studied before, during and after a hypoglycaemic (50 mg dL(-1)) hyperinsulinaemic (1.5 mU kg(-1) min(-1)) clamp test. The (1)H magnetic resonance spectroscopy of the occipital lobe of the brain was performed employing the STEAM localization technique. The water signal was suppressed by the modified SWAMP method. All spectra were acquired on a 3 Tesla scanner (80 cm MEDSPEC-DBX, Bruker Medical, Ettlingen, Germany) using a 10-cm diameter surface coil. RESULTS: During hypoglycaemia, T1DM showed blunted endocrine counterregulation. At baseline the brain tissue glucose : creatine ratio was lower in CON than in T1DM (CON 0.13 +/- 0.05 vs. T1DM 0.19 0.11; P < 0.01). During hypoglycaemia glucose : creatine ratios decreased in both groups (CON 0.07 +/- 0.08, P < 0.05; T1DM 0.03 +/- 0.03, P < 0.001). A significant drop in the glutamate : creatine ratio could only be found in CON during hypoglycaemia (CON 1.36 +/- 0.08 vs. 1.26 +/- 0.11; P < 0.01; T1DM 1.32 +/- 0.13 vs. 1.28 +/- 0.15; P = NS). The ratios of glutamine, N-acetylaspartate, choline and myo-inositol : creatine were not different between both groups and did not change throughout the experiment. CONCLUSIONS: Only in CON does moderate hypoglycaemia reduce intracerebral glutamate concentrations, possibly owing to a slower substrate flux through the tricarboxylic acid cycle in neurones. The maintenance of normal energy metabolism in T1DM during hypoglycaemia might effect glucose sensing in the brain and contribute to hypoglycaemia-associated autonomic failure.