摘要:

AbstractLocalized proton spectroscopy was used to monitor changes in metabolism and the biophysical status of tissue water in cat brain induced by occlusion of the middle cerebral artery. Changes in the intensity ofN‐acetyl‐aspartate (NAA), total creatine (tCr), and lactate (Lac) signals in localized volumes of interest in the ischemic hemisphere were quantified relative to the preischemic signal. Changes in the apparent diffusion coefficient (ADC), T1‐ and T2‐relaxation times of water in those volumes were also quantified. Lactate was shown to increase rapidly in the first 0.5−2.0 h of ischemia and stabilized afterwards. The ADC of water started to decrease from 0.64 × 10−9m2/s to 0.54 × 10−9m2/s in the first minutes following occlusion, as was shown in two cases where ADC was measured with high temporal resolution, and stabilized after approximately 3 h at 0.38 × 10−9m2/s (n= 6). NAA and tCr decreased by 35% (P<0.0001) and 30% (P<0.005), respectively, in the first 8 h of ischemia in comparison with the preischemic control levels. T1and T2gradually increased with 0.3 s (P<0.0001) and 5.2 ms (P<0.0001), respectively, during

ISSN:0740-3194    

DOI:10.1002/mrm.1910320602

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

年代:1994

数据来源: WILEY

摘要:

AbstractIn this high resolution magnetic resonance spectroscopic study of experimental allergic encephalomyelitis (EAE) and Semliki Forest virus (SFV) infection of the Bioui AB/H mouse, marked increases in the initially low levels ofN‐trimethyl compounds in the spinal cord were observed during probable demyelinating episodes. There was also a pronounced and reproducible modulation of the levels of taurine andmyo‐inositol during acute and again during chronic relapsing EAE. The ratio ofN‐acetyl‐aspartate to creatine in the spinal cord of mice infected with the mutant M9 strain of SFV decreased to approximately 70% of that seen in norm

ISSN:0740-3194    

DOI:10.1002/mrm.1910320603

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe fast spin echo sequence combines data from many echo signals in a Carr‐Purcell‐Meiboom‐Gill echo train to form a single image. Much of the signal in the second and later echoes results from the coherent addition of stimulated echo signal components back to the spin echo signal. Because stimulated echoes experience no dephasing effects during the time that they are stored as M, magnetization, they experience a different gradient first moment than does the spin echo. This leads to flow‐related phase differences between different echo components and results in flow voids and ghosting, even when the first moment is nulled for the spin echo signal. A method of gradient moment nulling that correctly compensates both spin echo and stimulated echo components has been developed. The simplest solution involves nulling the first gradient moment at least at the RF pulses and preferably at both the RF pulses and the echoes. Phantom and volunteer studies demonstrate good suppression of flowrelated ar

ISSN:0740-3194    

DOI:10.1002/mrm.1910320604

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

年代:1994

数据来源: WILEY

摘要:

Abstractq‐Space imaging (Callaghan, J. Magn. Reson. 88,493 (1990)) has been used to obtain mouse brain water displacement profiles. These profiles take the form of a unidirectional incoherent‐displacement probability density distribution. Two groups of mice were studied, a normal group and one in which surgery had been performed to reduce the supply of blood to the forebrain. In the normal group the incoherent displacement of water was reducedpostmortem. Four of the surgically treated mice yielded displacement profiles that resembled those obtained postmortem; the remaining two were near normal. This study demonstrates the feasibility ofin vivoq‐space imaging. The displacement profile changes that occur subsequent to an interruption of the forebrain blood supply are consistent with the hyperintensity changes seen in diffusion‐weighted

ISSN:0740-3194    

DOI:10.1002/mrm.1910320605

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

年代:1994

数据来源: WILEY

摘要:

AbstractWe propose a compartmental model to explain the signal enhancement curves following the bolus injection of Gd‐DTPA. The model incorporates vascular volume fraction contribution, and the possibility of having different transport constants between the plasma and extravascular components. A Walker 256 carcinoma grown in rat muscle was used to demonstrate the capability of this model. Several different types of tissues were included in the measurements: normal, quickly enhanced, slowly enhanced, and necrotic tissues. Blood volume and blood‐tissue permeability information can be derived from the dynamic contrast‐enhanced MRI study employing the proposed model. In the tissue contrast enhancement curve, the initial rising slope after injection is related to the blood volume (or, vascular volume fraction), the maximum enhancement ratio is related to the uptake of tissue, and the decay rate is related to the clearance of tracer from tissue. The measured permeability constant is not the conventional permeability; instead they are contrast agents uptake and clearance rates, which are limited by the blood perfusion. These parameters can be used to characterize different enhancement pat

ISSN:0740-3194    

DOI:10.1002/mrm.1910320606

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

年代:1994

数据来源: WILEY

摘要:

AbstractWater proton spin‐lattice relaxation rates are reported as a function of the magnetic field strength for cross‐linked bovine Serum albumin samples. The relaxation dispersion profile is analyzed using a relaxation model where the solid components have the magnetic field dependence proportional tov−0.5which may result from a defect diffusion model with two degrees of freedom. If the cross‐linking agent concentration is not sufficiently high, the relaxation dispersion curve may have significant contributions from freely rotating protein. The magnetic field dependence of the relaxation rates studied as a function of the proton mole fraction in the sample show that approximately 30% of the magnetization transfer rate is directly proportional to the proton mole fraction. This contribution is identified with the magnetization transfer from exchange of whole water molecules with buried binding sites on the protein. The second order magnetization transfer rate constant is 388 s−1assuming unit water spin concentration. The solid component relaxation obeys an Arrhenius activation law, but the overall temperature dependence of the crossrelaxation is complicated by chemical exchange processes which enter with oppo

ISSN:0740-3194    

DOI:10.1002/mrm.1910320607

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

年代:1994

数据来源: WILEY

摘要:

AbstractEcho planar MR imaging (EPI) has been developed to completely eliminate motion artifacts and is currently being prepared for implementation into clinical MR systems. Thus, the purpose of this study was to evaluate the clinical utility of EPI in the detection of focal liver lesions and to compare EPI with contrast‐enhanced CT. EPI studies were performed on an experimental 1.0 Tesla whole body system using fat‐suppressed single‐shot spin echo (SE) and inversion recovery (IR) pulse sequences. A total of 26 liver tumors in 12 patients scheduled for liver resection were prospectively examined and correlated with intraoperative ultrasound, surgery, and pathology as the gold standard. Quantitative analysis of EPI was performed by means of liver signal‐to‐noise and tumorliver contrast‐to‐noise calculations. Diagnostic performance compared with contrast‐enhanced CT was assessed by means of ROC analysis. Lesion‐liver contrast was highest with EPI SE at aTE‐time of 70 ms and this technique showed best lesion detectability as measured by area under curve (AUC) values. Among EPI techniques, the IR sequence with an inversion time of 300 ms to null the liver signal showed high lesion‐liver contrast but all four reviewers reported problems assessing liver anatomy. Improved EPI techniques may prove useful for screening o

ISSN:0740-3194    

DOI:10.1002/mrm.1910320608

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

年代:1994

数据来源: WILEY

摘要:

AbstractA new adaptive algorithm based on wavelet‐encoded MRI is presented for application in dynamic imaging. This algorithm is adaptive because the strategy for updating image data in the dynamic series of images is determined by the processing of the most recently acquired data. The spatially selective multi‐resolution properties of the wavelet transform are exploited to selectively update only those regions of the field of view where change is actually occurring. A theoretical imaging model is presented to motivate use of the adaptive algorithm, and simulation results using both artificial and experimental wavelet‐encoded data are pres

ISSN:0740-3194    

DOI:10.1002/mrm.1910320609

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

年代:1994

数据来源: WILEY

摘要:

AbstractThis paper is devoted to a theory of the NMR signal behavior in biological tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR signal in the static dephasing regime where diffusion phenomena may be ignored. This approach has been applied to evaluate the NMR signal in the presence of a blood vessel network (with an application to functional imaging), bone marrow (for two specific trabecular structures, asymmetrical and columnar) and a ferrite contrast agent. All investigated systems have some common behavior. If the echo timeTEis less than a known characteristic timetcfor a given system, then the signal decays exponentially with an argument which depends quadratically onTE. This is equivalent to an R2* relaxation rate which is a linear function ofTE. In the opposite case, whenTEis greater thantc, the NMR signal follows a simple exponential decay and the relaxation rate does not depend on the echo time. For this time interval, R2* is a linear function of a) volume fraction ζ occupied by the field‐creating objects, b) magnetic field B0or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference Δχ between the objects and the me

ISSN:0740-3194    

DOI:10.1002/mrm.1910320610

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

年代:1994

数据来源: WILEY

摘要:

AbstractCerebralin vivoproton magnetic resonance spectroscopy of 13 newborn infants displaying seizures and receiving phenobarbitone, in one case supplemented by phenytoin, showed signals from propan‐1,2‐diol (the injection vehicle for both these anticonvulsants). Subsequentin vitrospectroscopy of cerebro spinal fluid (CSF) from one of these infants also showed signals from this substance. The estimatedin vivopropan‐1,2‐diol concentration (∼3 mM) was less than that measured in the CSF sample (14.4 mM). These observations suggest that propan‐1,2‐diol may accumulate in cerebral tissue and misidentification of its signals in bothin vivoandin vitroproton spectra may confuse diagnoses of metabolic or ot

ISSN:0740-3194    

DOI:10.1002/mrm.1910320611

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

年代:1994

数据来源: WILEY