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Velocity-encoded cine (VEC) imaging 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 flow velocity measurements obtained with turbo-BRISK acquisitions were correlated with conventional magnetic resonance imaging measurements and achieved r values of 0.99 +/- 0.004 (mean +/- SD) with stroke volumes agreeing to within 4%. A potential limitation of BRISK is reduced accuracy for rapidly varying velocity profiles. 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 of 0.98 +/- 0.01.

Investigations of valvular regurgitation attempt to specify flow field characteristics and apply them to the proximal isovelocity surface area (PISA) method for quantifying regurgitant flow. Most investigators assume a hemispherical shape to these equivelocity shells proximal to an axisymmetric (circular) orifice. However, in vivo flow fields are viscous and regurgitant openings vary in shape and size. By using centerline profiles and isovelocity surfaces, this investigation describes the flow field proximal to circular and elliptical orifices. Steady, proximal flow fields are obtained with two- and three-dimensional computational fluid dynamic (CFD) simulations. These simulations are verified by in vitro, laser-Doppler velocimetry (LDV) experiments. The data show that a unique, normalized proximal flow field results for each orifice shape independent of orifice flow or size. The distinct differences in flow field characteristics with orifice shape may provide a mechanism for evaluating orifice characteristics and regurgitant flows. Instead of the hemispherical approximation technique, this study attempts to show the potential to define a universal flow evaluation method based on the details of the flowfield according to orifice shape. Preliminary results indicate that Magnetic Resonance (MR) and Color Doppler (CD) may reproduce these flow details and allow such a procedure in vivo.

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Hemodynamic studies of regurgitant lesions in the heart focus on identifying a reliable noninvasive method of volumetric flow calculation. In these studies the influence of blood viscosity to the flowfield under pulsatile flow conditions and constraining wall geometry has not been examined in detail. Pulsatile flow studies in straight tubes have shown that viscous effects significantly influence the periodic flowfield, especially near the wall. The purpose of this study is to investigate the significance of transient effects in the flowfield proximal to a lesion under constraining wall geometry. The proximal flowfield was analyzed with computational fluid dynamics (CFD) computer simulations and color flow Doppler mapping (CFM). Three different stroke volumes and regurgitant waveforms were investigated for upstream wall orientations that varied from -64 degrees to +64 degrees (measured from the orifice plane). Results showed that for each upstream wall orientation, a single instantaneously normalized centerline velocity distribution characterized the flowfield throughout the cycle. The centerline distributions were in phase with the pressure gradient and almost identical to the corresponding steady-state distributions. Minor deviations were observed near the wall, where viscous effects were predominant. Transient flow effects such as blunt profiles and pressure velocity phase shifts, which were observed in straight circular tubes, were not observed in regurgitant orifice flowfields. This is true even under severe confinement conditions.