Flow Convergence Flow Rates from 3-Dimensional Reconstruction of Color Doppler Flow Maps for Computing Transvalvular Regurgitant Flows Without Geometric Assumptions: An In Vitro Quantitative Flow Study,☆☆

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Abstract

Objective: This study was designed to develop and test a 3-dimensional method for direct measurement of flow convergence (FC) region surface area and for quantitating regurgitant flows with an in vitro flow system. Background: Quantitative methods for characterizing regurgitant flow events such as flow convergence with 2-dimensional color flow Doppler imaging systems have yielded variable results and may not be accurate enough to characterize those more complex spatial events. Method: Four differently shaped regurgitant orifices were studied: 3 flat orifices (circular, rectangular, triangular) and a nonflat one mimicking mitral valve prolapse (all 4 orifice areas = 0.24 cm2) in a pulsatile flow model at 8 to 9 different regurgitant flow rates (10 to 50 mL/beat). An ultrasonic flow probe and meter were connected to the flow model to provide reference flow data. Video composite data from the color Doppler flow images of the FC were reconstructed after computer-controlled 180° rotational acquisition was performed. FC surface area (S cm2) was calculated directly without any geometric assumptions by measuring parallel sliced flow convergence arc lengths through the FC volume and multiplying each by the slice thickness (2.5 to 3.2 mm) over 5 to 8 slices and then adding them together. Peak regurgitant flow rate (milliliters per second) was calculated as the product of 3-dimensional determined S (cm2) multiplied by the aliasing velocity (centimeters per second) used for color Doppler imaging. Results: For all of the 4 shaped orifices, there was an excellent relationship between actual peak flow rates and 3-dimensional FC-calculated flow rates with the direct measurement of the surface area of FC (r = 0.99, mean difference = –7.2 to –0.81 mL/s, % difference = –5% to 0%), whereas a hemielliptic method implemented with 3 axial measurements of the flow convergence zone from 2-dimensional planes underestimated actual flow rate by mean difference = –39.8 to –18.2 mL/s, % difference = –32% to –17% for any given orifice. Conclusions: Three-dimensional reconstruction of flow based on 2-dimensional color Doppler may add quantitative spatial information, especially for complex flow events. Direct measurement of 3-dimensional flow convergence surface areas may improve accuracy for estimation of the severity of valvular regurgitation. (J Am Soc Echocardiogr 1999;12:1035-44.)

Section snippets

Pulsatile Flow Model

A cylindric 2-chamber transparent acrylic pulsatile-flow model characterized by a proximal chamber and a distal chamber (both have 8 cm height, 24 cm length) was used in this study (Figure 1).

. The flow phantom used in this in vitro study consisted of 2 chambers (inflow and outflow) separated by a flat disk (12 mm thick), at the center of which a model orifice was inserted. At the top of the model, above the mounted orifice, the echocardiographic transducer could be rotated 180° on a round flat

RESULTS

During this study we transferred all the 2D flow convergence video composite data for the 4 orifices (total 34 hemodynamic conditions) into the TomTec computer. After 3D reconstruction was performed, the entire cardiac cycle of each hemodynamic condition was divided into image loops that enabled us to select the correct frame to measure the peak flow convergence surface area. High-resolution 3D reconstructions usually required 15 minutes including the first rough low-resolution 3D display.

Previous Flow Convergence Studies and Limitations

With advances in ultrasound technology, laminar acceleration phenomena for flows toward the regurgitant orifice can be detected more reproducibly by color Doppler flow mapping methods even in difficult patients. This acceleration field has been used experimentally and clinically to quantify regurgitant flow volume and flow rate in aortic and mitral regurgitation, assuming a simple or modified hemispherical or hemielliptic shape of the isovelocity surface with 2D color Doppler imaging.5, 6, 7, 8

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    Reprint requests: David J. Sahn, MD, Oregon Health Sciences University, The Clinical Care Center for Congenital Heart Disease - UHN60, 3181 SW Sam Jackson Park Rd, Portland, OR 97201 (e-mail: [email protected]) .

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