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Background: Measurement of left ventricular outflow tract area (LVOTa) for estimation of aortic valve area (AVA) and for transcatheter aortic valve implantation (TAVI) using transthoracic 2D echocardiography (2DE) and the continuity equation assumes a round shape for the LVOT. The aim of this study was to determine the effect of direct measurement of LVOTa using cardiac computed tomography angiography (CCTA) and 3D echocardiography (3DE) on LVOTa and AVA estimation.Methods: We prospectively studied 50 patients (age 68±12y, 24 males, 25 with aortic stenosis (AS) and 25 without AS). LVOTa and AVA were estimated using 2DE and the continuity equation (Pai x sqr(LVOTd/2) assuming a circular LVOT). LVOTa and diameters (D1 & D2) were measured using retrospective gated 128 slice CCTA and 3DE. AVA was also planimetered using CCTA in mid systole. LVOTa and AVA estimated by 2DE were correlated with 3DE and CCTA measurements.Results: LVOT was usually oval and not circular, with an eccentricity index (D2/D1) of 1.24±0.10 for CCTA and 1.18±0.1 for 3DE. There was good correlation between 2DE and CCTA for LVOTa (r=0.84) but 2DE systematically underestimated LVOTa (and therefore AVA) by 15±16% (AS vs. no AS p=0.8). The correlation between CCTA and 3DE for LVOTa was only moderate (r=0.72), due to inadequate 3DE image quality. AVA was 0.92±0.44 cm2 by 2DE and 1.15±0.72 cm2 by CCTA ( p=0.003). There was good correlation between 2DE and CCTA for AVA (r=0.94), but 2DE underestimated AVA by 13±19% compared to CCTA. After correcting 2DE determined AVA by a factor of 1.15 (accounting for the elliptic LVOTa), the difference was only -0.3±22%.Conclusions: 3D imaging revealed an oval LVOT in most patients, resulting in underestimation of LVOTa and AVA by 15% using 2DE. This accounted for the difference in AVA between 2DE and CCTA. Current 3D TTE image quality is inadequate to accurately determine LVOTa. These findings carry important implications for selecting AS patients for surgery or TAVI.