Alternative control volume geometries for measuring regurgitant flow through a valve

Control-volume (CV) methods applied to magnetic resonance velocity-encoded cine images of the convergent proximal flow field of a regurgitant valve have been shown to measure reverse blood flow volume accurately. Spatial and temporal averaging are known to affect accuracy, but the effects of slice thickness and orientation relative to the flow field have not been systematically studied, nor have CV configurations requiring fewer scans been explored. Further, surface area calculations at the intersection of CV walls are a previously unrecognized source of error. Using a computational fluid dynamics model of steady flow through an orifice, we evaluated five different CV configurations in terms of accuracy, time costs, and clinical potential. CVs incorporating a basal wall were affected by blurring of axial velocity gradients near the orifice, and voxel grid alignment relative to the orifice was the most significant source of inaccuracy. Errors in surface area calculations at plane intersections produced deviations of 7-20%, depending on configuration. A CV formed by slices parallel to the orifice plane was deemed clinically unusable, while a cylindrical CV yielded good accuracy in simulated tests and showed potential for practical implementation based on scan time, ease of view selection, and visualization of the flow field

Validation of rapid velocity encoded cine imaging of a dynamically complex flow field using turbo block regional interpolation scheme for k space

A study on block regional interpolation scheme for k space (BRISK) magnetic resonance imaging (MRI) velocity encode cine (VEC) demonstrates that these complex flow fields can be performed with minimal loss of information while remaining well within clinically acceptable scan durations. Compared to conventional MRI, Turbo BRISK represented the complex throughplane flow with excellent accuracy in as little as 6% of the conventional scan time. At conventional resolutions, Turbo BRISK was proven as a promising tool for the eventual clinical evaluation of complex intravascular flows of non-unidirectional character. In addition, it should make three-dimensional flow imaging in humans possible

Correction of temporal misregistration artifacts in jet flow by conventional phase-contrast MRI

Purpose: To show that accuracy of jet flow representation by magnetic resonance (MR) phase-contrast (PC) velocity-encoded (VE) cine imaging is dominated by error terms resulting from the temporal distribution of data, and to present a generally applicable data interpolation-based approach to correct for this phenomenon. Materials and Methods: Phase-contrast data were acquired in a stenotic orifice flow phantom using a physiologic pulsatile flow waveform. A temporally registered scan, acquired without data segmentation or interleaving was obtained (17 minutes) and taken as the reference (REF). Conventional PC data sets were acquired using segmentation and data interleaving. An enhanced temporal registration (ETR) algorithm was applied to the acquired data to temporally interpolate component sets and output data at matching time points, thereby reducing temporal dispersion. Results: Compared to the REF data, conventionally processed PC data consistently overestimated peak velocities in laminar jet flow regions (127% ± 28%) and exhibited relatively weak correlations (r = 0.67 ± 0.23). The ETR-processed data better represented peak velocities (101% ± 13%, P < 0.001) and correlated more closely with the REF data (r = 0.94 ± 0.05. P < 0.001). Conclusion: The temporal distribution of PC data impacts the accuracy of velocity representation in pulsatile jet flow. A temporal registration postprocessing algorithm can minimize loss of accuracy

Hemodynamic evaluation with turbo brisk - a rapid phase contrast angiography technique

Hemodynamic imaging by phase contrast angiography was significantly accelerated by selective interpolation and segmentation in k-space using TURBO BRISK. The method was tested in vitro on three independent flow fields, representative of human blood rheology: a straight tube simulating the descending aorta, a curved tube simulating the aortic arch and a two-chamber orifice flow model simulating valvular regurgitation. The results were compared to data obtained by Laser Doppler Velocimetry (LDV) and showed good agreement. For the straight tube, the flow velocity obtained by five TURBO BRISK methods with increasing segmentation factors and corresponding time savings showed good agreement with LDV. For the curved tube, the velocity showed good general agreement with some differences in the decelerating part of the cycle, and in the low-velocity secondary flow structures. The orifice flow evaluation, the most time consuming case, was performed by the control volume method. It showed good agreement with actual flows through the orifice. Data acquisitions for TURBO-4 BRISK could be performed in 20s for each velocity component. The method shows promise for breath-hold acquisitions in clinical applications, including calculation of blood flow volumes through diseased arteries, measurement of blood backflow volumes through dysfunctional heart valves to time valve replacement operations, and evaluation of arterial wall shear stress, an important factor in the genesis of atherosclerosis

A spheroidal control volume for the quantitative measurement of regurgitant flow by cardiac MRI

Purpose: We sought to show that a spheroidally shaped control volume (CV), formed from a minimal MRI data set, can be used to measure regurgitant flow through a defective cardiac valve consistently and accurately under a variety of flow conditions. Materials and Methods: Using a pulsatile flow pump and phantoms simulating severe valvular regurgitation, we acquired 31 scans of two or three radially oriented slices, using a variety of flow waveforms and regurgitant volumes of 12 to 55 ml. Data sets included high- and low-resolution scans, and variable-rate sparse sampling was also applied to reduce the scan time. An oblate spheroid was placed in the pump chamber opposite the jet and fit as tightly as possible to isomagnitude velocity contours at 25% of the velocity encoding limit. Results: Normalized regurgitant volumes (NRVs) expressed as a percentage of the pump setting were obtained from the product of the spheroid surface area with the velocities normal to it. Mean ± SD NRV values were 96.8 ± 6.6% for all scans. Imaging times in the breath-hold range were obtained using reduced resolution and variable-rate sparse sampling approaches without significant degradation in accuracy. Conclusion: In our preliminary findings, the spheroidal CV method showed clear potential for the development of a robust, clinically feasible technique for the measurement of regurgitant volume

A spheroidal control volume for the quantitative measurement of regurgitant flow by cardiac MRI

Purpose: We sought to show that a spheroidally shaped control volume (CV), formed from a minimal MRI data set, can be used to measure regurgitant flow through a defective cardiac valve consistently and accurately under a variety of flow conditions. Materials and Methods: Using a pulsatile flow pump and phantoms simulating severe valvular regurgitation, we acquired 31 scans of two or three radially oriented slices, using a variety of flow waveforms and regurgitant volumes of 12 to 55 ml. Data sets included high- and low-resolution scans, and variable-rate sparse sampling was also applied to reduce the scan time. An oblate spheroid was placed in the pump chamber opposite the jet and fit as tightly as possible to isomagnitude velocity contours at 25% of the velocity encoding limit. Results: Normalized regurgitant volumes (NRVs) expressed as a percentage of the pump setting were obtained from the product of the spheroid surface area with the velocities normal to it. Mean ± SD NRV values were 96.8 ± 6.6% for all scans. Imaging times in the breath-hold range were obtained using reduced resolution and variable-rate sparse sampling approaches without significant degradation in accuracy. Conclusion: In our preliminary findings, the spheroidal CV method showed clear potential for the development of a robust, clinically feasible technique for the measurement of regurgitant volume

Journal of Cardiovascular Magnetic Resonance, 1(2), 185- 193 (1999) Recent Progress in Radiofrequency-Tagged Left Ventricular Function Studies

Magnetic resonance imaging (MRI) is the optimal tool for the evaluation of ventricular function due to improved contrast, spatial resolution, and signal-to-noise performanc

Rapid Velocity-Encoded Cine Imaging with Turbo-BRISK

Velocity-encoded cine (VEC) imging is potentially an important clinical diagnostic technique for cardiovascular diseases. Advances in gradient technology combined with segmentation approaches have made possible breathhold VEC imaging, allowing data to be obtained free of respiratory artifacts. However, when using conventional segmentation approaches, spatial and temporal resolutions are typically compromised to accommodate short breathhold times. Here we apply a sparse sampling technique, turbo-BRISK (i.e., segmented block regional interpolation scheme for k-space) to VEC imaging, allowing increased spatial and temporal resolution to be obtained in a short breathhold period. BRISK is a sparse sampling technique with interpolation used to generate unsampled data. BRISK was implemented to reduce the scan time by 70 % compared with a conventional scan. Further, turbo-BRISK scans, using segmentation factors up to 5, reduce the scan time by up to 94%. Phantom and in vivo results are presented that demonstrate the accuracy of turbo-BRISK VEC imaging. In vitro validation is performed using conventional magnetic resonance VEC. Pulsatile centerline Jrow velocity measurements obtained with turbo-BRISK acquisitions were correlated with conventional magnetic resonance imaging measurements and achieved r values of 0.99 rt 0.004 (mean 2 SD) with stroke volumes agreeing to within 4%. A potential limitation of BRISK is reduced accuracy for rapidly varying velocity projles. We present low- and high-resolution data sets to illustrate the resolution dependence of this phenomenon and demonstrate that at conventional resolutions, turbo-BRISK can accurately represent rapid velocity changes. In vivo results indicate that centerline velocity waveforms in the descending aorta correlate well with conventional measurements with an average r value o