Differences in Aortic Valve Area Measured with CT Planimetry and Echocardiography (Continuity Equation) Are Related to Divergent Estimates of Left Ventricular Outflow Tract Area

OBJECTIVE. The purpose of this study was to compare measurements of aortic valve area (AVA) obtained with coronary CT angiography (CCTA) and transthoracic echocardiography (TTE) and to determine whether differences in these estimates are related to underestimation of the area of the left ventricular outflow tract (LVOT) measured with echocardiography. MATERIALS AND METHODS. A retrospective database review yielded the cases of 41 patients who had undergone CCTA and TTE within a 60-day period. AVA was measured with direct planimetry on CCTA images and was computed with the continuity equation at TTE. To ascertain the effect of LVOT measurements on the continuity equation, AVA was recomputed with substitution of the LVOT area and diameter measured on CCTA images for the dimensions obtained at TTE. RESULTS. TTE estimates of AVA varied from 0.6 to 7.0 cm(2) and included 10 patients with moderate to severe aortic stenosis (AVA <= 1.5 cm(2)). AVA obtained with CT planimetry was greater than that computed with TTE measurements (mean difference, 0.6 cm(2); p = 0.0037). There was little difference between CT and TTE measurements of LVOT diameter ( mean difference, 0.05 cm; p = 0.37), but measurements of LVOT area were significantly greater on CT planimetric images (mean difference, 0.6 cm(2); p = 0.0002). When CT measurements of LVOT area were substituted into the continuity equation in place of LVOT diameter, correlation between the CT planimetric and continuity equation values of AVA improved from r = 0.65 to r = 0.88, whereas the mean difference in AVA between the CT planimetric and continuity equation values decreased to 0.17 cm(2) (p = 0.36). CONCLUSION. AVA measured with CT planimetry is significantly greater than AVA calculated with the continuity equation. This difference is at least partially related to differences in LVOT area based on LVOT diameter versus direct planimetry of the LVOT area.