## 4 Discussion  
In this study, we illustrated the feasibility and accuracy of multiparametric analyses of the landing zone using 4D straightened segmentation generated from 4D C-CTA in sheep to measure and characterize the dynamic changes in the landing zone in pre-CTs and the implanted stent in post-CTs for TPVR at five selected planes. Furthermore, we used this methodology to obtain the 4D RV volume from all CTs, the landing zone volume from pre-CTs, the stent volume from post-CTs, and the ellipticity index at the selected planes, all of which could be used to estimate the right heart function, new heart valve selection, stent conditions following TPVR, and to help us gain a better understanding of the deformation of the landing zone at the native RVOT. Finally, we determined the  
Figure 7: Dynamic variation in the cross-sectional area and circumference throughout the cardiac cycle at five planes from post-operative CT. RVOT—right ventricular outflow tract, BPS—bottom plane of the stent, MPS—middle plane of the stent, TPS—top plane of the stent, PA—pulmonary artery. In each plane, one error bar indicates one frame (from 10% to 100%) in the cardiac cycle. differences in the cross-sectional area and perimeter on the selected pulmonary artery plane and the RVOT plane by comparing the corresponding pre- and post-CTs, thus reflecting the structural changes after TPVR. Compared with anatomical 4D segmentation, this straightened model not only has the comparable accuracy of relative measurement, but it also aids in better understanding the deformation of the landing zone at the native RVOT and the implanted stent, while supporting the development of new TPVR devices that are not only morphologically appropriate for the majority of patients scheduled to undergo TPVR, but also achieve better mechanical performance in the long term. Additionally, this straightened 4D segmentation aids in determining proper valve sizing by measuring the parameters in the appropriate planes. Methods involving 3D echocardiography, fluoroscopy imaging, and 3D C-CTA are usually employed to evaluate cross-sectional changes at the RVOT, which are based on 3D alignment and fixed-plane segmentation [16; 17; 18]. Currently, 4D C-CTA can measure and characterize the changes in heart morphology and size throughout the cardiac cycle [19]. However, the current method for 4D segmentation is based on anatomical right heart reconstruction, which can neither fully describe the features of the RVOT-PA nor reveal intuitive observations. Furthermore, there is no method to quantify the RVOT-PA using comprehensive 4D C-CTA analysis. Along with the 4D anatomical segmentation, the angle between the RVOT and the main pulmonary artery may prevent the measuring lines from perfectly aligning with the planes of the landing zone (such as the annular plane and the sinotubular junction plane, which are extremely important for TPVR), which could lead to inaccurate landing zone measurements. Additionally, as the landing zone for TPVR is not a straight conduit, this could increase the difficulty of placing the planes in the right position, as well as the measurements. In clinical practice, it is crucial to obtain the precise length of the TPVR landing zone to avoid right and/or left pulmonary artery occlusion when adapting a self-expandable stented heart valve. In addition, in the landing zone for TPVR, only the RVOT plane and main pulmonary artery plane are the \"real 2D flat planes.\" The other three planes, STJ, sinus, and the basal plane of the pulmonary valve, are defined by three points (STJ--the three commissures; sinus--the three peaks of the sinuses; and the basal plane of the pulmonary valve--the nadir of the pulmonary valve). These three planes are virtual planes, or at least not 2D flat planes; they are characterized by 3D features. Even multi-planar measurements are widely used in clinical practice to evaluate the RVOT prior to TPVR, but planes could be placed inaccurately in anatomical segmentations due to their 3D features and the angle of observation.