Background: We recently reported using echocardiography that end-systolic right ventricular eccentricity index(RVEI) shows a gradual escalation with an increment of tricuspid regurgitation(TR) severity. This study was performed to confirm this relationship with cardiac MRI. Methods and Results: We enrolled 43 patients; 23 patients with severe functional TR(22 women, 56짹11yrs, Gr1), 10 controls(5 women, 57짹12yrs, Gr2), 10 with severe pulmonary regurgitation(5 women, 32짹6yrs, Gr3). RVEI and RV sphericity index were defined as in Figure using cardiac MRI. Body mass index were not different among the groups(21짹2 kg/m2 for Gr1, 21짹2 kg/m2 for Gr2, and 24짹3 kg/m2 for Gr3). End-systolic and end-diastolic RV volumes and RV ejection fraction were comparable between Gr1 and Gr3. Although end-systolic RVSI and end-diastolic RVEI were similar, end-systolic RVEI was significantly increased in Gr1 and end-diastolic RVSI, in Gr3. Tricuspid annulus size was more lengthy at both end-systole and end-diastole in Gr1 vs. Gr3(Figure). Conclusions: In addition to increased systolic and diastolic tricuspid annulus sizes, severe functional TR was closely associated with unique RV geometry shape, with a significant increment in end-systolic RVEI although RV volumes showed no difference compared to the severe PR group, underscoring the importance of eccentric RV enlargement on top of tricuspid annular dilation in functional TR. These results suggest the need for development of more rational surgical approaches to functional TR beyond tricuspid valve annuloplasty.