ISSN:1053-1807    

DOI:10.1002/jmri.1880010302

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe use of magnetic resonance (MR) imaging is investigated for noninvasively estimating the oxygen saturation of human blood (%HbO2) in vivo by means of relaxation characteristics identified in earlier MR spectrometry studies. To this end, a sequence is presented for determining the T2of vascular blood in regions in which motions of the body and of the blood itself present a major challenge. With use of this sequence on a commercial 1.5‐T whole‐body imager, the relationship between the T2and %HbO2of blood is calibrated in vitro for the conditions expected in vivo. T2varies predictably from about 30 to 250 msec as %HbO2varies from 30% to 96%. T2values measured in situ for vascular blood in the mediastinum of several healthy subjects qualitatively reflected the behavior observed in vitro. Estimates of %HbO2for these vessels obtained with the in vitro calibration appear reasonable, particularly for venous blood, although difficulties arise in selecting the appropriate calibration factors. These encouraging initial results support a more systematic study of potential sources of error and an examination of the accuracy of in vivo measurements by comparison with direct measurements of %HbO2in vess

ISSN:1053-1807    

DOI:10.1002/jmri.1880010303

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

年代:1991

数据来源: WILEY

ISSN:1053-1807    

DOI:10.1002/jmri.1880010304

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

年代:1991

数据来源: WILEY

摘要:

AbstractGradient‐refocused echo‐planar magnetic resonance (MR) images (TE = 18 msec) were acquired in rats during bolus injection of iron oxide particles, and the first pass of the contrast agent through the brain was monitored. In control rats, contrast agent (0.1 mmol/kg iron) produced significant signal‐intensity (SI) reduction over the right hemisphere and similar declines over the left. SI loss occurred first in the cortex and basal ganglia and later in the periventricular regions, along the midline, and in the thalamic zone. Sequential volume‐localized proton spectra acquired during transit of 0.02 mmol/kg iron showed substantial reduction in SI, slight asymmetric broadening, and no change in chemical shift of the water resonance. In rats with unilateral occlusion of the middle cerebral artery, peak reduction in ischemic brain SI was to 70% ± 9% of control, while normal brain SI was reduced to 18% ± 2% (P<.01), allowing distinction of the ischemic regions. The presence and location of injury were confirmed with diffusion‐weighted imaging and postmortem vital staining. These results demonstrate abnormal transit profiles in a rat model of regional brain ischemia. Evaluation of dynamic contrast delivery patterns may provide unique information in early bra

ISSN:1053-1807    

DOI:10.1002/jmri.1880010305

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

年代:1991

数据来源: WILEY

摘要:

AbstractA group of 70 professional divers and 47 healthy control subjects who had never dived were examined with magnetic resonance (MR) imaging to determine the prevalence of focal white matter changes in the brain. Spots of high signal intensity in white matter on proton density‐and/or T2‐weighted spin‐echo images were detected in 42% of the control subjects and in 34% of the divers. In the control subjects, the prevalence of more than three changes was related to smoking, use of alcohol, head trauma, age of more than 35 years, and a combination of several cerebrovas‐cular risk factors. This relationship was not present in the divers. The prevalence of changes in divers was inversely related to diving depth, amount of diving, participation in “unsafe diving,” and decompression sickness. The reasons for these results could not be ascertained. The results are compared with those of MR imaging studies of white matter changes recently presented by other rese

ISSN:1053-1807    

DOI:10.1002/jmri.1880010306

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

年代:1991

数据来源: WILEY

ISSN:1053-1807    

DOI:10.1002/jmri.1880010307

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe T2 of brain tissue is known to be field dependent, decreasing as B0increases. Previous studies have attributed reduced T2 in the structures of the extrapyramidal motor system (EPMS) to high iron concentrations. The present study was designed to manipulate physiologic iron concentrations and study the effects on T2 and on the field dependence of T2 at 7.0 T in whole formalin‐fixed brains. A rat model was devised in which iron concentrations in the structures of interest were altered by diet manipulation. Cerebral structures with different iron content were imaged and T2 measured with MR microscopy at both 2.0 and 7.0 T. T2 of all tissues was shorter by 40%–60% at 7.0 T. Although some dependence of T2 on iron concentration was evident, it was less than expected. The strongest correlation was in the substantia nigra. The highest‐resolution studies, at 30 × 30 × 50 μm, show the myelin bundles in many of the EPMS structures but not in the substantia nigra. From these data, it appears that T2 at greater field strengths depends more on susceptibility‐induced spin dephasing imposed by diffusion through the tissue micro‐structure than on the pres

ISSN:1053-1807    

DOI:10.1002/jmri.1880010308

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

年代:1991

数据来源: WILEY

摘要:

AbstractAn iterative algorithm that involves image filtering and data replacement (as suggested by Constable and Henkelman) is investigated for reducing the Gibbs artifact in magnetic resonance imaging. The image is processed with an edge‐preserving filter to estimate the height and location of a set of model elements (ó functions or boxes) for generating the missing high‐frequency information. Filtering was performed in the complex image domain to account for discontinuities in phase as well as magnitude. The process is repeated after each data replacement to handle varying degrees of contrast. The convergence and signal‐to‐noise characteristics of the algorithm are investigated by means of simulated and clinical examples. The results indicate that the algorithm performs reasonably well in reducing ringing artifacts due to nearby edge contrast seen in most of the homogeneous, isointense regions. Nevertheless, it may generate some spurious thickening of structures that do not match the assumed step‐e

ISSN:1053-1807    

DOI:10.1002/jmri.1880010309

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe FAISE (fast‐acquisition interleaved spin‐echo) technique consists of a hybrid rapid‐acquisition relaxation‐enhanced (RARE) sequence combined with a specific phase‐encode reordering method. Implemented on a 1.5‐T unit, this multisection, high‐resolution technique permits convenient contrast manipulation similar to that of spin‐echo imaging, with selection of a pseudo‐echo‐time parameter and a TR interval. With a TR of 2 seconds, eight 256 × 256 images are obtained in 34 seconds with either T2 or proton‐density weighting. A direct comparison between FAISE and spin echo for obtaining T2‐weighted head images in healthy subjects indicates that FAISE and spin‐echo images are qualitatively and quantitatively similar. Image artifacts are more pronounced on “proton‐density” FAISE images than on the T2‐weighted FAISE images. T1 contrast can be obtained with inversion recovery and short TR FAISE images. Preliminary temperature measurements in saline phantoms do not indicate excessive temperature increases with extended FAISE acquisitions. However, extensive studies of radio‐frequency power deposition effects should be performed if the FAIS

ISSN:1053-1807    

DOI:10.1002/jmri.1880010310

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

年代:1991

数据来源: WILEY

摘要:

AbstractFactors affecting blood vessel lumen definition for two‐dimensional and three‐dimensional inflow magnetic resonance (MR) imaging methods are considered. Vessel definition is affected (a) by the amount of dephasing of the blood in the vessels, both for uncompensated and velocity‐compensated gradients; (b) by the image reconstruction technique (normal Fourier reconstruction when asymmetric echoes are collected or a maximum‐intensity projection technique in postprocessing); (c) by loss of signal due to T2 de‐phasing; (d) by misregistration; (e) by vessel wall motion; and (f) by partial‐volume effects. The first two factors were found to dominate for resolution on the order of 1 mm3. To overcome these dephasing problems, the authors developed asymmetric echo, velocity‐compensated sequences with TEs as short as 4.8 msec. The data were then reconstructed with an iterative partial Fourier algorithm, enabling improved lumen definition to be obtained in phantom

ISSN:1053-1807    

DOI:10.1002/jmri.1880010311

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

年代:1991

数据来源: WILEY