摘要:

AbstractRat brains were imaged after cortical electroshock pulse trains (1 ms pulses at 100 Hz) of varying durations (0.1–10 s), with diffusion‐weighted echo planar imaging sequences at 2.0 T. The apparent water diffusion coefficient (ADC) decreased after either single or repeat electroshock trains. ADC reductions were observed within 6 s after the first shock. The size of the affected area of the brain increased in subsequent images during the 1st min after a 10‐pulse (0.1 s) train, and also increased with the duration of electroshock trains. ADC reduction was reproducible in extent and time course after single 10‐shock trains and was reversible. In the affected pixels the mean ADC reduction was 4% for a single shock train (0.1 s), and 7–8% for trains repeated once a minute, independent of electroshock train duration. The results indicate that neuronal activity associated with electrostimulation may be monitored with water diffusion measurements, and they may be useful for measuring the severity of seizure activity in patients with medically intractable

ISSN:0740-3194    

DOI:10.1002/mrm.1910370102

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractGradient acoustic noise has been measured and characterized for an epoxy‐potted, shielded gradient assembly in a 1.5 T MRI system. Noise levels vary by 10 dB or more as a function of longitudinal position in the scanner and reflect the pattern of forces applied to the gradient assembly. The noise level increases slightly (1–3 dB) with a patient in the scanner. The spectrum of the noise is similar (but not identical) to the spectrum of the input signal. A gradient‐pulse‐to‐acoustic‐noise transfer function was obtained by using a white noise voltage input to the gradient system. The transfer function enabled us to accurately predict acoustic noise output for a pulse sequence consisting of a series of trapezoidal pulses on a single axis and for a clinical fast spin echo sequence with gradients present on all

ISSN:0740-3194    

DOI:10.1002/mrm.1910370103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThe structure and dynamics of silicone envelopes from virgin and explanted breast prostheses have been studied by1H,13C, and29Si high‐resolution solid‐state NMR spectroscopy. The results indicate that this combination of techniques can be used to unambiguously identify the presence of methyltri‐fluoropropylsiloxane units and diphenylsiloxane units at concentrations of about 2.0% and 3.5%, respectively, in some of the envelopes. The presence of about 10% silica also was detected and a trace of lipids was found in the explanted silicone envelopes. We have also measured the protonT2andT1relaxation times of the envelopes to characterize the dynamics of the silicone in the enve

ISSN:0740-3194    

DOI:10.1002/mrm.1910370104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractProton magnetic resonance spectroscopy (MRS) has demonstrated reduction ofN‐acetylaspartate (NAA) in the epileptogenic temporal lobe. However, the correlation of NAA reduction with cerebral metabolic abnormalities is unknown in temporal lobe epilepsy (TLE). Proton MRS and18F‐fluorodeoxyglucose positron emission tomography (FDG/PET) were used to study 12 unilateral TLE patients with medically intractable seizures and 26 age‐matched healthy volunteers. The epileptogenic temporal lobe of each patient was determined by both electroencephalography and FDG/PET. The NAA/choline‐plus‐creatine (NAA/(Cho+Cr)) ratio correlated significantly with the interictal glucose metabolism (r= 0.54,P0.01) in 12 TLE patients. The mean NAA/(Cho + Cr) ratio in the epileptogenic temporal lobe was significantly less than that in the contralateral side (P<0.01), and less than that in normal control temporal lobes (P<0.0001). These results suggest that quantitative MRS abnormalities reflect underlying metabolic patholo

ISSN:0740-3194    

DOI:10.1002/mrm.1910370105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractNAA has been described as a neuron‐specific compound. NAA levels as determined by magnetic resonance spectroscopic imaging (MRSI) have been used to determine degree of neuronal loss in several neurological diseases, but there has been limited work to document the accuracy and reliability of this technique. This study addresses this question quantitatively with histological analysis of cell viability and tissue shrinkage in quinolinic acid (QA)‐induced damage of the rat striatum compared with1H MRSI measurement ofN‐acetyl aspartate (NAA) as a noninvasive measure of neuronal loss. Both 'H MRSI and histology detect damage to the lesioned striatum; however, there are differences in the degree of damage as assessed by the two methods. Although partial‐volume effects and tissue shrinkage may decrease the sensitivity of MR to such damage, the sparing of axons by QA may be another important factor in the differences in assessment. These results indicate that further studies of NAA metabolism and its distribution within neurons are wa

ISSN:0740-3194    

DOI:10.1002/mrm.1910370106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractUsing a 32‐echo imaging pulse sequence, T2relaxation decay curves were acquired from five white‐ and six gray‐matter brain structures outlined in 12 normal volunteers. The water contents of white and gray matter were 0.71 (0.01) and 0.83 (0.03) g/ml, respectively. All white‐matter structures had significantly higher myelin water percentages (signal percentage with T2between 10 and 50 ms) than all gray‐matter structures. The range in geometric mean T2of the main peak for both white and gray matter was from 70 to 86 ms.T2distributions from the posterior internal capsules and splenium of the corpus callosum were significantly wider (width is related to water environment inhomogeneity) than those from any other white‐ or gray‐matter structures. Thus, quantitative measurement and analysis ofT2relaxation reveals differences in brain tissue water environments not discernible on conventional MR images. These differences may make shortT2components reliable markers for

ISSN:0740-3194    

DOI:10.1002/mrm.1910370107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractCells immobilized in gels experience potential metabolic restrictions in the form of reduced diffusion rates of metabolites and ions and their possible selective adsorption on the gel matrix. Diffusion and relaxation characteristics of common solutes in agarose and barium alginate gels were investigated at 37°C by using1H PFGSE and23Na TQF NMR spectroscopy. Glucose, glycine, alanine, lactate, sodium ions, and HDO were studied. There were no selective interactions between any of the metabolites and the gel materials but the diffusion coefficients were uniformly reduced. The effects of metabolite diffusion and utilization, in gel beads and threads containing cells, were simulated by using a reaction diffusion model incorporating the measured diffusion coefficients. Metabolism is expected to be very significantly limited by diffusion of solutes to and from the cells that are centrally located within gel threads or spheres of radius ∼2.0 mm, which is a cornmonly used si

ISSN:0740-3194    

DOI:10.1002/mrm.1910370108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThe effect of molecular weight, concentration, and structure on 1/T1ρ, the rotating frame relaxation rate, was investigated for several proteins using the on‐resonance spin‐lock technique, for locking fieldsB1<200 μT. The measured values of 1/T1ρ, were fitted to a simple theoretical model to obtain the dispersion curves 1/T1ρ(ω1) and the relaxation rate at zeroB1field, 1/T1ρ,(O). 1/T1ρ, was highly sensitive to the molecular weight, concentration, and structure of the protein. The amount of intra‐ and intermolecular hydrogen and disulfide bonds especially contributed to 1/T1ρ. In all samples, 1/T1ρ(O) was equal to 1/T2ρ measured at the main magnetic fieldBo= 0.1 T, but at higher locking fields the dispersion curves mono‐tonically decreased. The results of this work indicate that a model considering the effective correlation time of molecular motions as the main determinant for 1/T1ρ relaxation in protein solutions is not valid at very lowB1fields. The underlying mechanism for the relaxation rate 1/T1ρ at B1fields below

ISSN:0740-3194    

DOI:10.1002/mrm.1910370109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThe model used for calculating perfusion by MRI techniques that use endogenous water as a tracer assumes that arterial water is a freely diffusible tracer. Evidence shows that this assumption is not valid in the brain at high blood flow rates, at which movement of water into and out of the microvasculature becomes limited by diffusion across the blood‐brain barrier. In this work, the arterial spin‐labeling technique is used to show that fraction of arterial water that is dependent on blood flow rate remains in the vasculature and does not exchange with brain tissue water. By using perfusion measurements without and with magnetization transfer (MT) effects, one can distinguish arterial label that exchanges into tissue because blood has much smaller MT than brain tissue. Using this technique, the extraction fraction for water is measured in the rat brain at various cerebral blood flow rates. At high flow rates (‐5 ml/g/min), the extraction fraction for water is found to be about 45% in rat brain. Disruption of the blood‐brain barrier with D‐mannitol caused an increase in the extraction fraction for water. It was possible to form an image related to the extraction fraction for water. The ability to estimate the amount of vascular water exchanging with tissue water by MRI may represent a noninvasive approach to detect the integrity of the blood‐br

ISSN:0740-3194    

DOI:10.1002/mrm.1910370110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractIn this study2H double quantum‐filtered (DQF) NMR spectros‐copy of water molecules inside the blood vessel wall was used to monitor structural changes after the application of strain. This method was applied to a variety of bovine large blood vessels, including aorta, coronary artery, carotid artery, and vena cava. Inducing strain in coronary arteries by inflating them by air or water revealed dramatic changes in the2H DQF spectral lineshapes. Uniaxial extensions of longitudinal and circumferential strips led to distinct spectral responses for the different blood vessels. While the spectral lineshapes for the coronary and the carotid arteries were very sensitive to longitudinal strain, they were not affected by circumferential stretching. For the aorta there were no changes in the line‐shape upon strain. For the vena cava a large effect of strain was found that was independent of the choice of elongation axis. The effect of elongation on the spectra was assigned to the outer layer, the adventitia, whereas spectral lineshapes that originated from inner layers, media and intima, were almost insensitive to strain. Angular dependence experiments proved the presence of macroscopic order in the strained state superimposed on a broad distribution of the local directors. Our results demonstrate the sensitivity of2H DQF NMR as a probe for dynamic processes occurring on the microscopic scale in a molecular network. Key words: blood vessels; strain; double‐quantum

ISSN:0740-3194    

DOI:10.1002/mrm.1910370111

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY