## 4 Computed Tomography and Tricuspid Valve
### CT Assessment of the Tricuspid Apparatus for Transcatheter Procedures  
The multiphasic, contrast-enhanced, retrospective cardiac-gated CT acquisition enables multiplanar reconstruction of the entire heart silhouette, including the proximal main vessels, allowing a comprehensive understanding of the right heart anatomy (Figure 11) [105]. The amounts of contrast and radiation required have been considerably reduced with the new CT scanners, as well as the duration of the acquisition phase. It is also possible to perform more precise reconstructions even in elevated heart rates, such as atrial fibrillation, a widespread pathology in patients with significant TR [106]. Notably, a dedicated protocol for tricuspid valve CT acquisition is necessary; it requires opacification of the right heart cavities by a monophasic or biphasic injection with a mixture of saline and contrast in different percentages and following bolus tracking in the right ventricle or the pulmonary artery [107]. The comprehensive assessment of the right heart and the adjacent anatomical structures, which the tricuspid defect could significantly remodel, provides an appropriate evaluation for TR severity, patients' clinical risk stratification, anatomical suitability for tricuspid interventions, and preprocedural planning in patients undergoing TV therapies [105]. Specifically, several fundamental parameters might be obtained by CT imaging in order to design the optimal procedure and achieve the highest success rate during transcatheter tricuspid valve interventions (TTVI), such as dimensions and morphology of the TV annulus, location of commissures, tethering parameters, anatomical regurgitant orifice area, right atrium (RA) and RV dimensions, surrounding structures, device landing zone, and vascular access routes [11]. Using multiplanar reconstruction, tricuspid cross-sectional area, perimeter, septalateral (SL), and anteroposterior (AP) diameters can be obtained by a short-axis view on the annular level. The SL diameter refers to the maximal distance in septal to the lateral direction (that corresponds to the annulus measurement in the four-chamber view at the transthoracic echocardiography), whereas the AP diameter is orthogonal to the previous one. Due to the complex saddle-shaped structure of the tricuspid annulus, a 2D approach does not precisely address the valvular anatomy. Therefore, 3D semiautomated software can help in this setting [108]. Moreover, dimensions are usually obtained both at end-systole and mid-diastole because of the dynamic variability in annular size. Interestingly, in a study by [PERSON] et al., there was a good correspondence between TEE and CT for tricuspid annulus sizing and valve area in patients with severe TR [109]. Finally, distances between commissures (anteroseptal, posterosptal, and anteroposterior) and distances between the centrum of the TV and commissures can also be measured [12]. Figure 11: CT assessment of the tricuspid annulus using a semiautomated software-based approach (3 mensio Structural Heart; Pie Medical Imaging, Maastricht, The Netherlands): (**A**) Transverse plane (short-axis) at level of tricuspid annulus that is delineated by the red line. The centroid of the tricuspid valve (Ce) and the three main commissures (SA: septal anterior, SP: septal posterior, AP: anteroposterior) are identified according to specific anatomical landmarks. (**B**) Coronal plane (long axis) at level of the anteroposterior diameter of the tricuspid annulus. (**C**) Fluoroscopic and angiographic simulation showing the three-dimensional position of the tricuspid annulus and commissures within the right heart chambers. (**D**) Three-dimensionalcontraction by the volume rendering of the right chambers showing the geometrical spatial relationship of the tricuspid annulus and surrounding structures. RA: right atrium; RV: right ventricle; MV: mitral valve; AV: aortic valve. The leaflet tethering is another important parameter to consider in TTVI planning. In a study by [PERSON] et al.