Although the DCD method does not assume either a flat flow profile or constant flow area, it does assume a circular shaped flow area and an axial symmetry of the spatial flow profile (10,12- 14,15). Noncircular geometry may be present in the RVOT or MPA in postoperative patients with TOF as in our model. Our study demonstrated that the DCD method using the average of two orthogonal planes could mitigate the limitations of assuming axial symmetry of flow area and velocity profile, providing accurate estimation of pulmonary RSVs and RFs. In our in vitro model, the DCD method using a single plane overestimated the actual FSVs and RSVs, and the paired orthogonal plane underestimated them. As a result, the DCD method that used the average of two orthogonal planes estimated more accurately the actual FSVs and RSVs than it did by using a single plane. In our animal model, the DCD method using a single plane corresponding to the oblique outflow view overestimated FSVs, and the paired orthogonal plane corresponding to the long axis view underestimated them. Both single planes (oblique outflow view and long axis view) methods overestimated the actual RSVs. As a result, the DCD method using the average of two orthogonal planes yielded a better estimate of the actual FSVs, RSV and RFs than it did using the single plane, although it slightly overestimated the actual FSVs and RSVs. In our in vitro model, the shape of the cross-sectional flow area of the MPA was ovoid, and the spatial velocity distribution of the regurgitant flow in the ovoid flow area was almost uniform with the regurgitant orifice located in the center of the tube mimicking the pulmonary annulus. In fact, although the diameters were different, the shape of velocity profiles of regurgitant flow was almost similar in two orthogonal planes in our in vitro model (Figure 2). In contrast, in our animal model, the cross-sectional flow area of the MPA was different from that in the in vitro model: near circular as opposed to ovoid. The spatial velocity distribution of the regurgitant flow, however, was different in the in vivo model both as a result of variable orifice position and MPA curvature. The diameters of the velocity profiles of the regurgitant flow were similar in the oblique outflow view and the long axis view at the same phase of diastole, but the shapes of the velocity profiles were different (Figure 3). In addition, although the shapes of the cross-sectional flow area of the RVOT and MPA were the same in our vitro model, they differed substantially in our animal model (Figure 3) and would probably differ substantially in patients. In the in vivo study, the reason for overestimation in RSVs obtained by either single plane, and in RSVs and FSVs obtained by the average of two orthogonal planes, may be related to the difficulty in obtaining exact orthogonal planes and, in particular, in not identifying the actual minor axis. Recently, in preliminary studies, we have demonstrated that the three-dimensional DCD method without geometric assumption so as to calculate the flow volumes should be more objective and less susceptible to error (21).