摘要:

AbstractChanges in cerebral blood oxygenation and flow during prolonged activation of human visual cortex (6‐min video projection) were monitored using high‐resolution T2*‐ and T1‐weighted gradient‐echo MRI in identical sessions. Oxygenation‐sensitive recordings displayed an initial signal increase (oxygenation “overshoot”), a subsequent signal decrease extending over 4–5 min (relative deoxygenation), and a signal drop after the end of stimulation that mirrored the initial response (oxygenation “undershoot”). How‐senstive MRI demonstrated that the inflow effect remained elevated during the entire period of stimulation. The observation of gradually decreasing cerebral blood oxygenation, despite persisting elevation of blood flow, may be understood to be an accumulation of deoxyhemoglobin due to the progressive up‐regulation of oxidative phosphorylation. The present findings support a concept in which transitions between functional states lead to an uncoupling of perfusion (oxygen delivery) from oxidative metabolism (oxygen consumption) whereas steady‐state activft

ISSN:0740-3194    

DOI:10.1002/mrm.1910350602

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

年代:1996

数据来源: WILEY

摘要:

AbstractMagnetic resonance imaging contrast agents that are sensitive to pressure would be useful for evaluating cardiovascular function. One such potential contrast agent conslsts of gas‐filtod liposome microbubbles. The magnetic susceptibility of the microbubbles locally perturb the statte magnetic field, which influences the transverse‐relaxation properties of the surrounding medium. Changes in the pressure atter the bubble dimensions, which affects the magnetic field perturbations and, hence, the transverserelaxation. The effect of these microbubbles on the T2relaxation times of a water‐based medium was measured for liposomes filled with different gases—nitrogen, argon, air, oxygen, xenon, neon, perflu‐oropentane, perfluorobutane, and sulfur hexafluoride. The air‐filled, perfluoropentane‐fllled and the oxygen‐filted liposomes demonstrated the largest effect on transverse‐relaxation. The influence of pressure on both gradient‐echo and spin‐echo signal intenstties for air‐filled microbubbles was also evaluated. Pressure‐induced changes in signal intensity were consistently observed for both the spin‐echo and gra

ISSN:0740-3194    

DOI:10.1002/mrm.1910350603

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

年代:1996

数据来源: WILEY

摘要:

AbstractA method is proposed for processing time‐course fMRI data taken wKh successlve single‐shot echo‐planar imaging. The proposed method uses a two‐dimensional version of the multiple signal classification (MUSIC) algorithm and the technique called covariance averaging, both of which were developed in the field of sensor‐array processing. The proposed method consists of four steps: calculate the averaged data covariance matrix, determine the number of activities using this covariance matrix, estimate the locations of the activities, and estimate their time evolution curves. Computer simulation resutts showed that a nearly fourfold improvement in the spatial resolution can be attained due to the method's super‐resolution

ISSN:0740-3194    

DOI:10.1002/mrm.1910350604

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

年代:1996

数据来源: WILEY

摘要:

AbstractA technique is presented for rapidly and noninvasively determining aortic distensibiltty, by NMR measurement of pulse‐wave velocrty in the aorta. A cylinder of magnetization is excited along the aorta, wtth Fourier‐veloctty encoding and readout gradients applied along the cylinder axis. Cardiac gating and data interleaving improve the effective time resolution to as high as 3 ms. Wave velocities are determined from the position of the f oot of the flow wave in the velocity profiles. Evidence of helical flow distal to the aortic arch can be seen in normal subjects, while disturbed flow patterns are visible in patients with aneurysms and dissecti

ISSN:0740-3194    

DOI:10.1002/mrm.1910350605

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

年代:1996

数据来源: WILEY

摘要:

AbstractSeveral substrates are availablein vivofor oxidation by the myocardium. Although substrate selection has been studied extensively in normoxic myocardium, relatively little is known about substrate preference very early during reperfusion after ischemia. Carbon‐13 isotopomer analysis was used to study substrate usage by nonischemic and reperfused‐ischemic myocardium in a working heart that was subjected to 15 min of regional ischemia and reperfused for 5 min. Compared with nonischemic myocardium, the contribution of acetoacetate to acetyl coenzyme A was increased in the reperfused‐ischemic region, and the contribution of exogenous lactate was decreased. Free fatty acid oxidation, however, was not different in the two regions. These results indicate that (1) early during reperfusion, ketone body oxidation may be more significant than has been emphasized, (2) the relative contribution of fatty acids to acetyl coenzyme A is not sensitive to ischemia followed by reperfusion, and (3) Carbon‐13 magnetic resonance spectroscopy methods may be used for analysis of spatial heterogeneity of metabolism in th

ISSN:0740-3194    

DOI:10.1002/mrm.1910350606

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

年代:1996

数据来源: WILEY

摘要:

AbstractA novel noninvasive method of measuring local myocardial oxygen tension (pO2) In the perfused rat heart using19F MRI is demonstrated. Tissue pO2was determined on the basis of the19F spin‐lattice relaxation rate (R1) of perflubron (perfluorooctyl bromide) sequestered in the heart after IV infusion of an emulsion. Spectroscopic measurement of R1 was previously used to measure a global weighted average of oxygen status.19F MRI now provides 3D spatial resolution indicating local cardiac pO2under normally perfused, globally ischemic, and regionally ischemic condition

ISSN:0740-3194    

DOI:10.1002/mrm.1910350607

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

年代:1996

数据来源: WILEY

摘要:

AbstractIf magnetic resonance spectroscopy (MRS) is to become a useful tool in clinical medicine, it will be necessary to find reliable methods for analyzing and classifying MRS data. Automated methods are desirable because they can remove user bias and can deal with large amounts of data, allowing the use of all the available information. In this study, techniques for automatically extracting features for the classification of MRSin vivodata are investigated. Among the techniques used were wavelets, principal component analysis, and linear discriminant function analysis. These techniques were tested on a set of 75in vivo13C spectra of human adipose tissue from subjects from three different dietary groups (vegan, vegetarian, and omnivore). It was found that it was possible to assign automatically 94% of the vegans and omnivores to their correct dietary groups, without the need for explicit identification or measurement of peaks.

ISSN:0740-3194    

DOI:10.1002/mrm.1910350608

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

年代:1996

数据来源: WILEY

摘要:

AbstractSimultaneous multivoxel31P and19F 3D localized NMR spec‐troscopy is demonstrated on a phantom and in the liver of patients undergoing bolus‐infusion 5‐fluorouracil chemotherapy. The19F and31P spectra were localized with 8 × 8 × 8 3D chemical‐shift imaging, with both nuclei sharing the same field of view and voxel size (27 and 64 ml in phantom and liver, respectively) using a 1.5‐Tesla clinical imager with two RF channels and a dual‐tuned surface coil. The repetition time (TR= 0.26 s) and Ernst nutation angles (θE= 32° for19F, 28° for31P) were chosen to optimize the signal‐to‐noise ratio (SNR) per‐untt time for the 0.5‐ to 2‐s T1range of the19F and31P metabolites of interest The overall examination time, including tuning, imaging, shimming and dual‐nuclear spectroscopy, was under 90 min. Simultaneous acquisition of31P and19F spectra will permit the study of the influence of hepatic and/or tumor metabolism on the uptake and catabolism of fluoropyrimidine drugs wi

ISSN:0740-3194    

DOI:10.1002/mrm.1910350609

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

年代:1996

数据来源: WILEY

摘要:

AbstractRecently, it has been reported that hypochlorous acid (HOCl), a special product of neutrophil myeloperoxidase, degrades N‐acetyl groups of N‐acetylglucosamine, chondroitin sulfate, hyaluronic acid, and minced articular cartilage via a transient product to acetate. This work concerns1H NMR investigations of synovia! fluids of patients with rheumatoid arthritis (RA). Synovial fluids of patients with severe forms of this disease are characterized by enhanced1H NMR signals for N‐acetyl groups (˜2.0 ppm) and acetate (1.90 ppm) and the appearance of a broad but less intense signal at 2.35 ppm. It is likely that this signal corresponds to the transient, chlorinated product of degradation of N‐acetyl groups by hypochlorous acid. Moreover,1H NMR signal intensities of N‐acetyl groups and acetate strongly correlate with the myeloperoxidase activities in synovial fluids from patients with rheumatoid arthritis. These results have been confirmed by treatment of native sheep synovial fluid with sodium hypochlorite, resulting in the formation of the same resonances as observed in pathologically changed synovial fluids from humans. Thus, it is concluded that HOCI plays an important role for the cartilage degradation during rheumatoid

ISSN:0740-3194    

DOI:10.1002/mrm.1910350610

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

年代:1996

数据来源: WILEY

摘要:

AbstractUsing a keyhole technique, it is shown that the data acquisition rate of gradient‐echo imaging for functional MRI (fMRI) studies can be increased substantially. The resulting enhancement of the temporal resolution of fMRIs was accomplished without modifying the hardware of a conventional MRI system. High spatial resolution fMRI images were first collected with conventional fullk‐space acquisition and image reconstruction. Using the same data set, simulation reconstruction using the keyhole principle and zero‐padding were performed for comparison with the fullk‐space reconstruction. No significant changes were found for fMRI images generated from the keyhole technique with a data sharing profile of 50% of thek‐space. Ask‐space data sharing profiles increased to 75 and 87.5%, the keyhole fMRI images began to show only modest changes in activation intensity and area compared with the standard images. In contrast, zero‐padding fMRI images produced a significant disparity both in activation intensity and area relative to the truly high‐resolution fMRI images. The keyhole technique's ability to retain the intensity and area of fMRI information, while substantially reducing acquisition time, makes it a promising method f

ISSN:0740-3194    

DOI:10.1002/mrm.1910350611

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

年代:1996

数据来源: WILEY