摘要:

AbstractA method for imaging the rapid temporal‐spatial evolution of myocardial deformations in the paced heart is proposed. High time resolution‐tagged MR images were obtained after stimulation of the myocardium with an MR‐compatible pacing system. The images were analyzed to reconstruct dynamic models of local 3D strains over the entire left ventricle during systole. Normal canine hearts were studiedin vivowith pacing sites on the right atrium, left ventricular free wall and right ventricular apex. This method clearly resolved local variations in myocardial contraction patterns caused by ventricular pacing. Potential applications are noninvasive determination of electrical conduction abnormalities and the evaluation of new pacing ther

ISSN:0740-3194    

DOI:10.1002/mrm.1910390402

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

年代:1998

数据来源: WILEY

摘要:

AbstractAn interleaved gradient‐echo echo‐planar imaging (IGEPI) sequence was modified for and applied to dynamic contrast‐enhanced imaging of the heart. Using IGEPI, images with 3.0 × 3.9 mm nominal in‐plane resolution are acquired in 100 ms, enabling eight slices per heartbeat for a heart rate of 60 beats/min. The acquisition speed and use of saturation pre‐pulses allows acquisition of short‐ and long‐axis images during the same contrast bolus. IGEPI maintains the acquisition characteristics required for performing a quantitative first‐pass perfusion analysis as well as providing improved coverage compared with conventional fa

ISSN:0740-3194    

DOI:10.1002/mrm.1910390403

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe blood oxygenation level dependent (BOLD) response during sustained visual stimulation has been studied by several groups using fMRI with controversial conclusions. This issue was investigated for the human brain at high (4 Tesla) magnetic field strength using a flashing goggle at 8 Hz. The results demonstrate that the overall BOLD response in the primary visual cortex has an initial overshoot after the onset of visual stimulation and an undershoot after the termination of visual stimulation. A significant and positive BOLD response, however, remains constant between the initial and terminal transient responses. The temporal BOLD responses in the primary visual cortex were spatially dependent. The regions identified as draining veins in images displayed proportionately larger initial and terminal transient responses, whereas regions devoid of such vessels and associated mainly with parenchyma exhibited a more time‐independent BOLD response. These results reveal that the BOLD effect and, presumably, the uncoupling between cerebral blood flow and cerebral metabolic rate of oxygen consumption, are maintained in the primary visual cortex during sustained visual stimulation, and the temporal characteristics of the BOLD effect are spatially dependen

ISSN:0740-3194    

DOI:10.1002/mrm.1910390404

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

年代:1998

数据来源: WILEY

摘要:

AbstractA multiple‐echo MR spectroscopic imaging (MRSI) method is presented that enables improved metabolite imaging in the presence of local field inhomogeneities and measurement of transverse relaxation parameters. Short echo spacing is used to maximize signal energy from inhomogeneously line‐broadened resonances, and time domain parametric spectral analysis of the entire echo train is used to obtain sufficient spectral resolution from the shortened sampling periods. Optimal sequence parameters for1H MRSI are determined by computer simulation, and performance is compared with conventional single‐echo acquisition using phantom studies at a field strength of 4.7 T. A preliminary example for use at 1.5 T is also presented using phantom and human brain MRSI studies. This technique is shown to offer improved performance relative to single‐echo MRSI for imaging of metabolites with shortened T2* values due to the presence of local field inhomogeneities. Additional advantages are the intrinsic measurement of metabolite T2values and determination of metabolite integrals without T2weighting, thereby facilitating quantitative metabolite

ISSN:0740-3194    

DOI:10.1002/mrm.1910390405

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

年代:1998

数据来源: WILEY

摘要:

AbstractWhen supporting plates of plastic material are subjected to alternating transverse Lorentz forces while in a strong magnetic field normal to the plate surface, compressional waves within the solid produce a modulation of the plate surface that launches an acoustic wave in air along the magnetic field axis. We have extended our previous theory describing this process to include a detailed description of the formation of an acoustic interference pattern in air described by Fraunhofer diffraction at a distance from the plate surface. The extended theory predicts that the observed acoustic signal midpoint and normal to the plate surface gives a variation with frequency in approximate agreement with our previous measurements. The acoustic output off axis shows acoustic blazing that produces two main diffraction peaks with a splitting inversely proportional to the velocity of sound in the plate material. The new results could have important ramifications for the minimization of sound output in gradient coil design for MRI. A new arrangement of coils is proposed to ameliorate the acoustic output problem centrally and normal to the plate by extending the frequency response of the supporting plates to much higher frequencies. Also presented are estimates of the compressional wave velocities deduced from frequency response data recorded at the center‐point of a number of different plate

ISSN:0740-3194    

DOI:10.1002/mrm.1910390406

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

年代:1998

数据来源: WILEY

摘要:

Abstract6‐Fluoropyridoxol (6‐FPOL) was evaluated as a simultaneous indicator of intracellular and extracellular pH and, hence, pH gradient in perfused rat hearts. After infusion,19F NMR spectra rapidly showed two well‐resolved peaks assigned to the intracellular and extracellular compartments, and pH was calculated on the basis of chemical shift with respect toasodium trifluoroacetate standard. To demonstrate use of this molecule, dynamic changes in myocardial pH were assessed with a time resolution of 2 min during respiratory and metabolic alkalosis or acidosis and ischemia. For a typical heart, intracellular pH (pHi) = 7.14 ± 0.01 and extracellular pH (pHe) = 7.52 ± 0.02. In response to metabolic alkalosis, pHi remained relatively constant and the pH gradient increased. In contrast, respiratory challenge caused a significant increase in pHi. Independent measurements using pH electrodes and31P NMR confirmed validity of the19F NMR

ISSN:0740-3194    

DOI:10.1002/mrm.1910390407

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe feasibility of monitoring intracellular sodium changes using Na triple quantum filtered NMR without a chemical shift reagent (SR) was investigated in an isolated rat heart during a variety of interventions for Ma, loading. Perfusion with 1 mMouabain or without K+present in the perfusate for 30 min produced a rise of the Na TQF signal with a plateau of ‐190% and ˜228% relative to the preintervention level, respectively. Stop‐flow ischemia for 30 min resulted in a TQF signal growth of ˜147%. The maximal Na TQF signal increase of 460% was achieved by perfusion without K+/Ca2+, corresponding to an elimination of the Na transmembrane gradient. The observed values of Na NMR TQF growth in the physiological and pathological ranges are in agreement with reported data by other methods and have a linear correlation with intracellular sodium content as determined in this study by Co‐EDTA method and by sucrose‐histidine washout of the extracellular space. Our data indicate that the increase in Na TQF NMR signal is determined by the growth of Nai, and the extracellular Na contribution to the total TQF signal is unchanged at ˜64%. In conclusion, Na TQF NMR without using SR offers a unique and noninvasive opportunity to monitor alterations of intracellular sodium. It may provide valuable insights for developing car‐dioprotective strategies and for observing the effects of pharmaceutical treatments on sodium

ISSN:0740-3194    

DOI:10.1002/mrm.1910390408

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

年代:1998

数据来源: WILEY

摘要:

AbstractFlow‐sensitive alternating inversion recovery (FAIR) is a recently introduced MRI technique for assessment of perfusion that uses blood water as an endogenous contrast agent. To characterize the FAIR signal dependency on spin tagging time (inversion time (Tl)) and to validate FAIR for cerebral blood flow (CBF) quantification, studies were conducted on the rat brain at 9.4 T using a conventional gradient‐recalled echo sequence. The 71, of cerebral cortex and blood was found to be 1.9 and 2.2 s, respectively, and was used for CBF calculations. At short Tls (1.5 s, the CBF calculated from FAIR is independent of the spin tagging time, suggesting that the observed FAIR signal originates predominantly from tissue/capillary components. CBF values measured by FAIR with Tl of 2.0 s were found to be in good agreement with those measured by the iodoantipyrine technique with autoradiogra‐phy in rats under the same conditions of anesthesia and arterial pCO2. The measured pCO2index on the parietal cortex using the FAIR technique was 6.07 ml/100 g/min per mmHg, which compares well with the pCO2index measured by other techniques. The FAIR technique was also able to detect the regional reduction in CBF produced by middle cerebral artery occlusion in

ISSN:0740-3194    

DOI:10.1002/mrm.1910390409

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

年代:1998

数据来源: WILEY

摘要:

AbstractA method for rapidly producing velocity images is presented. This sequence combines a modified bipolar gradient pulse to magnitude encode the velocity with the rotating ultra‐fast imaging sequence (RUFIS) to image the encoded spins. Velocity encoding is done in 3 msec, and RUFIS acquires 32 projections in 8 msec. The method is applied to turbulent jets associated with a 75% stenosis in a 15‐mm inner diameter glass pipe. Data is acquired upstream and downstream from the stenosis for Reynolds numbers from 560 to 3750. In addition, a robust method of reconstructing the unobserved short time region of a free induction decay is presented and incorporated into the image process

ISSN:0740-3194    

DOI:10.1002/mrm.1910390410

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

年代:1998

数据来源: WILEY

摘要:

AbstractThis paper describes a technique for characterizing the gradient subsystem of a magnetic resonance (MR) system. The technique uses a Fourier‐transform analysis to directly measure thek‐space trajectory produced by an arbitrary gradient waveform. In addition, the method can be easily extended to multiple dimensions and can be adapted to measuring residual gradient effects such as eddy currents. Several examples of gradient waveform and eddy‐current measurements are presented. Also, it is demonstrated how the eddy‐current measurements can be parameterized with an impulse‐response formalism for later use in system tuning. When compared to a peak‐fitting analysis, this technique provides a more direct extraction of thek‐space measurements, which reduces the possibility of analysis error. This approach also has several advantages as compared to the conventional eddy‐current measurement technique, including the ability to measure very short time c

ISSN:0740-3194    

DOI:10.1002/mrm.1910390411

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

年代:1998

数据来源: WILEY