摘要:

AbstractAn MR imaging method for measuring intravascular pressure variations is introduced. The technique is based on estimates of vascular compliance and vessel distension, which are obtained from a correlation of spatial and temporal velocity derivatives and measurements of the velocity gradient in the direction of flow, respectively. The accuracy of the technique was determined in vitro through a comparison of MR and transducer pressure measurements obtained in distensible vessel phantoms undergoing pulsatile flow. Results indicated that a root‐mean‐square error of 4–12% can be expected in phantoms covering a physiological range of compliance. In vivo feasibility was demonstrated by thoracic aorta pressure measurements, which produced pressure waveforms with an expected shape and magn

ISSN:1053-1807    

DOI:10.1002/jmri.1880050602

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

年代:1995

数据来源: WILEY

摘要:

AbstractA method for estimating vascular compliance using MR velocity imaging is presented. The technique combines an analysis of pulse propagation, based on spatially averaged equations of continuity and momentum, together with phase‐contrast velocity measurements to estimate the compliance from a correlation of second‐order spatial and temporal velocity derivatives. The technique can be applied in the presence of reflected flow waves and uses velocity data acquired throughout the entire cardiac cycle. The accuracy of the technique was assessed in distensible vessel phantoms spanning a physiological range of compliance through a comparison with compliance estimates obtained using high‐resolution MR imaging and pressure transducers. The mean error of all measurements was found to be 0.04 ±0.02% per mm Hg, with the relative errors ranging from 1.2% to 46%. Error was found to decrease as the temporal sampling rate and/or image plane separation were increased. This suggests that an accurate hemodynamic evaluation of a vessel's elastic properties is feasible with MR velocity imaging tech

ISSN:1053-1807    

DOI:10.1002/jmri.1880050603

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

年代:1995

数据来源: WILEY

摘要:

AbstractThe reproducibility of MR imaging for the measurement of aortic compliance was studied in 47 healthy volunteers. Long and short term reproducibility and iatraobserver variability were tested. The method was modified to improve image quality and short term reproducibility and intraobserver variability retested. For comparison, spin echo imaging was compared with cine gradient echo imaging. Initial long term reproducibility showed a mean difference (± SE) of 3% (± 7%) with 95% confidence interval (CI) for limits of agreement of ± 69%. Short term reproducibility (7% ± 6%, 95% CI ±46%) and intraobserver variability (1% ± 2%, 95% CI ± 31%) were better. After modification of the technique and optimization of image quality, both short term reproducibility and intraobserver variability improved (0% ± 3%. 95% CI ± 17% and 5% ± 2%, 95% CI ± 16% respectively). Aortic compliance can be measured using spin echo MR imaging with good reproducibility provided care is taken to obtain good quality Images with high spatial

ISSN:1053-1807    

DOI:10.1002/jmri.1880050604

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

年代:1995

数据来源: WILEY

摘要:

AbstractMR phase velocity mapping was used to calculate wall shear stress (WSS) in the suprarenal and infrarenal abdominal aorta, two sites with very different proclivities for development of atherosclerosis. For the eight subjects studied, the average value of the mean (tune averaged over the cardiac cycle) WSS in the suprarenal aorta was 10.4 dynes/cm2at the posterior wall and 8.6 at the anterior wall. In the infrarenal aorta, WSS values were 4.7 at the posterior wall and 6.1 at the anterior wall. Peak WSS over the cardiac cycle was 48 and 54 at the anterior and posterior walls of the suprarenal aorta, respectively, and 33 and 30 at the anterior and posterior walls of the infrarenal aorta, respectively. Wide variation was found in both mean and peak WSS values among subjects. However, for 28 of 32 locations examined, mean and peak WSS were ingher in the suprarenal aorta than in the infrarenal aorta. Because atherosclerosis is more likely to form in the infrarenal aorta than in the suprarenal aorta, tins study supports the hypothesis that low WSS is a localizing factor for atherosclerosis, and ingh WSS may act as a deterrent against formation of atherosclerosis.

ISSN:1053-1807    

DOI:10.1002/jmri.1880050605

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

年代:1995

数据来源: WILEY

摘要:

AbstractAlthough the clinical manifestations of Valsalva's maneuver are well known, the associated hemodynamic changes in the great vessels have not been extensively studied and documented. In each of six healthy subjects, we evaluated three “quasi‐steady‐state” phases of Valsalva's maneuver: (1) during normal respiration, (2) during late strain, and (3) 4 seconds after strain release. Continuous flow, velocity, and cross‐sectional area measurements were obtained in the superior vena cava, pulmonary trunk, and thoracic aorta with single‐shot echo‐planar MR imaging (EPI) with velocity‐encoded gradients, which provided 256 Images in 5 seconds, yielding 26 velocity‐encoded images per second. In the superior vena cava, Valsalva's maneuver induced an 11% decrease in average flow volume, a 102% increase in peak flow velocity, a 156% increase in the time velocity integral, and a 37% decrease in cross‐sectional area. MR velocity measurements agreed with echo‐cardiographic data and supplied additional information on flow and morphology. EPI showed a reduction in venous return during Valsalva's maneuver by simultaneously assessing flow, velocity, and vessel morphology; this technique appears to be useful in the analysis of flow dynamics

ISSN:1053-1807    

DOI:10.1002/jmri.1880050606

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

年代:1995

数据来源: WILEY

摘要:

AbstractWe present a multishot echo‐planar imaging (EPI) phase‐contrast implementation for flow quantitation. The measurement accuracy of this technique was evaluated in vitro and in vivo. A gated eight‐shot EPI phase‐contrast sequence (TR/TE = 16/7.4, 45° flip angle), with a flow‐phase interval of 32 msec and an in‐plane resolution of 2× 2 mm was initially evaluated in a pulsatile flow phantom. Subsequently, EPI phase‐contrast flow measurements of the ascending and descending aorta, obtained in 10 volunteers, were compared with flow volume data acquired with a conventional cine phase‐contrast sequence (TR/TE = 24/7, 45° flip angle, 48‐msec flow‐phase interval, 2 × 1 mm in‐plane resolution). Comparisons between flow measurements were made using data obtained with the flow probe and cine phase contrast as the standard of reference for in vitro and in vivo measurements, respectively. EPI phase‐contrast sequences reduced data acquisition times tenfold compared with cine phase‐contrast sequences. EPI phase‐contrast flow measurements correlated were with phantom flow (r = 0.98, slope = 1.1) as well as with aortic cine phase‐contrast flow volume determinations (r = 0.98). A 95% confidence interval of measurement differences between echo‐planar and cine phase‐contrast imaging, ranging from 2.0 to ‐ 1058 mL/min was computed. Ultrafast phase‐contrast flow measurements are possible. Multishot EPI phase‐contrast imaging provides high measurement accuracy in pulsatile vessels while keeping the image acquisition interval short enough to b

ISSN:1053-1807    

DOI:10.1002/jmri.1880050607

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

年代:1995

数据来源: WILEY

摘要:

AbstractA flow phantom was used to study MR volume flow measurements for monophasic and triphasic waveforms over the flow range expected in peripheral arteries at rest and with exercise (2–24 mL/sec,n= 50). The improvement in accuracy with phase‐correction image processing to eliminate errors caused by eddy currents was measured. Volume flow estimates with Doppler sonography were also measured. MR volume flow measurements correlated with volume collection withr= 0.996 and mean error = 4.6%. Phase–correction processing decreased mean error from 12.6% to 4.6% (P<.001, pairedt‐test). Doppler sonography had a higher mean error of 10.3% (P<.001, unpaired t‐test). Cine phase‐contrast MR imaging provides accurate estimates of volume blood flow for waveforms and flow ranges expected in peripher

ISSN:1053-1807    

DOI:10.1002/jmri.1880050608

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

年代:1995

数据来源: WILEY

摘要:

AbstractAn MR phase imaging sequence with a very short echo time was used to assess blood velocity and flow at the renal artery bifurcation. Cardiac‐gated MR imaging data were obtained in six healthy subjects in sagittal planes adjacent to the abdominal aorta and transverse planes above and below the renal artery bifurcation. Average renal artery flow rate was 23.8 ±9 mL/sec. A strong individual variability was found for the velocity profiles in the abdominal aorta during end‐systolic regurgitation. Flow rate was also determined in three patients with reduced renal artery blood flow. Two patients received therapy with percutaneous transluminal angioplasty. The successful outcome was documented with MR imaging. A reliable assessment of renal artery flow with MR phase imaging is feasible. Measurement of the velocity profiles yields valuable insights in the complicated flow regime at the renal artery bifurca

ISSN:1053-1807    

DOI:10.1002/jmri.1880050609

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

年代:1995

数据来源: WILEY

摘要:

AbstractThe signal loss that occurs in regions of disturbed flow significantly decreases the clinical usefulness of MR angiography in the imaging of diseased arteries. This signal loss is most often attributed to turbulent flow; but on a typical MR angiogram, the signal is lost in the nonturbulent upstream region of the stenosis as well as in the turbulent downstream region. In the current study we used a flow phantom with a forward‐facing step geometry to model the upstream region. The flow upstream of the step was convergent, which created high levels of convective acceleration. This region of the flow field contributes to signal loss at the constriction, leading to overestimation of the area of stenosis reduction. A computer program was designed to simulate the image artifacts that would be caused by this geometry in two‐dimensional time‐of‐flight MR angiography. Simulated images were compared with actual phantom images and the flow artifacts were highly correlated. The computer simulation was then used to test the effects of different orders of motion compensation and of fewer pixels per diameter, as would be present in MR angiograms of small arteries. The results indicated that the computational simulation of flow artifacts upstream of the stenosis provides an important tool in the design of optimal imaging sequences for the reduction of sign

ISSN:1053-1807    

DOI:10.1002/jmri.1880050610

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

年代:1995

数据来源: WILEY

摘要:

AbstractWe compared the cardiac image quality of multishot echo‐planar imaging (EPI), segmented K‐space, and conventional cine acquisitions. Three techniques were used to obtain gated multiphase acquisitions of an axial section traversing both ventricles in 10 volunteers: two‐shot EPI acquired nonsequentially over two heart beats breath‐held segmented K‐space cine with eight K‐space lines acquired per cardiac trigger over 16 R‐R intervals, also breath‐held and 24 cine phases obtained over 256 R‐R intervals. Intraventricular SNRs with two‐shot EPI were superior to segmented K‐space cine acquisitions (P<.005) and not statistically different from conventional cine acquisitions (P<. 1). Intraventricular signal was most homogeneous on conventional cine images (P<.05). Coronary artery visualization and myocardial delineation were better on the EPI image set than on segmented K‐space cine images (P<.05). Two‐shot EPI provides high‐quality gated cardiac images with an acquisi

ISSN:1053-1807    

DOI:10.1002/jmri.1880050611

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

年代:1995

数据来源: WILEY