## 4 Discussion
In this study, we applied a 4D straightened segmentation with comparable accuracy and efficacy for five planar measurements from seven pre-CTs and seven post-CTs, which not only illustrates and visualizes the right heart's total deformation throughout the cardiac cycle but also quantitatively characterizes the RVOT as a whole. All measurements at the necessary planes can be easily checked using this 4D straightened segmentation. The cross-sectional area, circumference, and max-min diameters measured from the 4D straightened models show a strong linear correlation and strong agreement with the 4D anatomical models. With the 4D straightened models from the pre-CTs, the cross-sectional area and circumference were found to vary greatly at the RVOT and the BPV compared with the other three planes, which is consistent with anatomical measurements from a previous study [12]; however, only the values at the RVOT were found to vary greatly from those derived from the post-CTs. It is evident that single parameters alone are unable to reflect the features of the right heart clearly and comprehensively. Therefore, multiple parameters are needed to enable adequate pre- and post-TPVR evaluation of the right heart. Conventionally, cardiac function is assessed by 2D/3D echocardiography and cardiac magnetic resonance imaging (MRI) and is based mainly on the left ventricular ejection fraction (LVEF) and not the RVEF [20; 21]. Furthermore, the RV volume cannot be evaluated precisely by echocardiography during clinical application due to the patient's complex anatomy after open-heart surgery and the influence of breathing. In order to address this issue and improve the evaluation, RV volume should be estimated by multiple parameters. In this context, 4D RV volume measurements from 4D C-CTA could play an important role. We successfully segmented and calculated 4D RV volume from pre- and post-CT (Table S1) to measure the distance between the right ventricular inflow tract and the RVOT, which allowed for the innovation of the delivery system and the planning of the access route for TPVR according to the length of the delivery system from pre-CT, evaluating the RVEF perioperatively. In addition, for the pre- and post-TPVR evaluation, we defined volume among the five dynamic planes as our specific landing zone for our self-designed TPVR device and quantitatively measured the volume deformation throughout the entire cardiac cycle to obtain directly perceived motions, such as the dynamics of the landing zone volume, the implanted stent volume, and the native PV volume (Table S1). Two of the enrolled sheep (sheep F and J) were selected for a representative illustration of the 4D dynamic segmentation (Videos S1-S8). Additionally, calculating the 4D ellipticity index of the implanted stent allowed us to easily observe the stent deformations during the cardiac cycle and evaluate the stented valve size. The morphology of the implanted stent after full deployment is crucial for the new heart valve to function. Improper morphology of the implanted stent could give rise to a paravalvular leak or new heart valve regurgitation and could result in an inefficient opening area, which could lead to poor long-term performance [9; 11; 22]. In order to prevent the improper morphology of the implanted stent, a better understanding of the anatomy and device innovation are critical. We calculated the 4D ellipticity index of the landing zone from pre- and post-CT, which showed a lower ellipticity index at the RVOT and the BPV planes compared with the three other planes from pre-CT, while only the RVOT had a lower ellipticity index in post-CT because of the fixation of the implanted stent. The oval geometry of the BPV is an indicator of whether patients will benefit from TPVR because the myocardium at the BPV level would squeeze the stent into an undesirable geometry during the systolic phase, which could lead to an inefficient opening area in the new heart valve.