Three-dimensional reconstruction of color Doppler-imaged flow convergence region: limitations.

Doppler–Imaged Flow Convergence Region: Limitations To the Editor: I would like to congratulate Ishii et al1 for a very interesting study in the February 6, 2001, issue of Circulation on the use of 3D reconstruction of the color Doppler–imaged flow convergence region in ventricular septal defects. Their study, however, fails to mention 2 significant limitations of the flow convergence method. First, color Doppler isovelocity contours are distortions of the true isovelocity contours because there is loss of the velocity signal on color flow Doppler wherever flow is nonparallel to the Doppler beam. This usually affects the appearance of the color Doppler isovelocity surface closest to its outer edges near the tissue boundaries. Such distortions lead to underestimation of the area of the true isovelocity surface and thus underestimation of flow rates.2 The second limitation arises from the fact that the flow convergence method was derived from continuity and control volume principles. The flow rate (Q) is obtained by integrating the velocity component normal to the isovelocity surface over that surface3: Q V cos dS,