Color Doppler velocity accuracy proximal to regurgitant orifices: influence of orifice aspect ratio.

Many noninvasive methodologies used for the accurate evaluation of valvular regurgitation require precise velocity measurements from ultrasound instruments. Previous studies have indicated that velocity measurements from color Doppler (CD) instruments are susceptible to errors due to the interaction of the ultrasound beam and the proximal orifice flow field. This study examined the influence of high aspect ratio (AR) orifices on the CD velocity error. Center line velocity error distributions for orifices ranging from 7.07 to 78.5 mm2, varying in shape from circular to an AR = 8 ellipse, were evaluated using a numerical model of the ultrasound beam and the simulated regurgitant flow field. An in vitro study was also performed and confirmed the findings of the numerical model. The study showed that increasing AR does not significantly change the error characteristics. The study confirmed that orifice size is the dominant factor in the error distribution, and that corrections speculated for circular orifices can be extended to elliptical orifices without significant errors.

Rapid velocity-encoded cine imaging with turbo-BRISK.

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.

Evaluation of the proximal flow field to circular and noncircular orifices of different aspect ratios.

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.

An improved flow evaluation scheme in orifices of different aspect ratios.

An accurate and reliable method of regurgitant flow calculation is currently unavailable. The goal of this study was to define a new general method of flow calculation for orifices of different aspect ratios. The success of the method relies on matching the imaged flow field distribution obtained by color flow mapping (CFM) to a three-dimensional (3D) numerical flow field distribution of known geometry. The flow field in three orifices of identical cross-sectional area with aspect ratios of 1 (circular), 2 and 4 (elliptical) was evaluated by: (a) CFM, (b) 3D echocardiographic imaging, and (c) 3D finite element modeling (FEM). The orifice shape and size were accurately estimated by 3D echocardiographic imaging. FEM showed that the normalized centerline velocity profile of the flow field depends on the orifice aspect ratio. CFM provided a good description of the centerline profile for each case. For a given distance from the orifice center, the equivelocity contour surface area increases with increasing aspect ratio. A simple flow calculation scheme was developed to calculate regurgitant flow independent of orifice shape. This improved method showed better results than previous studies and may prove to be advantageous when analyzing in vivo flow fields with complex geometries.