## 5 CT and Left Atrial Appendage
### CT Assessment in LAA Transcatheter Closure  
Multimodality imaging is essential for LAAC. MDCT allows a great resolution of LAA due to its minimal motion [126]. For patients in sinus rhythm, prospective electrocardiographic (ECG) gating is necessary by identifying either atrial systole/ventricular diastole or atrial diastole/ventricular systole. For patients in AF, retrospective ECG gating may be required, and the optimal reconstruction phase depends on the ventricular rate [127]. Computed tomography is particularly important in three phases regarding patients undergoing LAAC: the preprocedural exclusion of LA thrombosis, the preprocedural planning, taking into account LAA morphology and dimensions, and the surveillance after the procedure [126]. CT can easily exclude thrombi in the LAA, even if transesophageal echocardiography (TEE) remains the gold standard. A major strength of computed tomography is the high capability to exclude LAA thrombosis (96-100%) [128]. Delayed imaging allows the differentiation between the slow-flow sluggish and the thrombi; thus, the filling defect that persists after one minute of contrast injection is more suggestive of a real thrombus [129]. In the preprocedural planning of LAAC, the MDCT helps acquire information about LAA shape, size, and surrounding structures. Few studies compared computed tomography, transesophageal echocardiography, and angiography for LAA assessment. Most of them are retrospective, single-center studies, reporting larger dimensions with CT than 2D/3D-TEE and angiography. [PERSON] et al. found that in 73 Watchman implantations, 2D-TEE and MDCT maximum diameter agreed on device size only in 25.4% of cases, mainly due to significative eccentricity of the orifice [130]. Notably, the LAAC device size choice depends on these measurements [126]. Computed tomography allows for determining LAA ostia and landing zones (the LAA zone where the LAAO device will ideally seat) and providing the minimum LAA diameter, maximum diameter, perimeter, and area. These measurements should be assessed at maximal LAA diastole or mid to late left ventricular systole, corresponding to maximum LAA end-diastolic filling. In this phase, the LAA is largest, thus reducing the risk of device undersizing and subsequent peri-device leaks. In a study by [PERSON] et al., the CT-detected diameter of the LAA ostium calculated from the perimeter resulted in the best parameter for sizing the LAA occluder device [131]. It is essential to point out that different devices require different modalities of LAA ostium measurement [126]. For the Watchman, the landing zone is measured from the circumflex artery inferiorly to a superior point that is established at 1-2 cm within the left upper superior pulmonary vein ridge. For the Amplatzer Cardiac Plug (ACP) or the Amulet (second-generation ACP), the landing zone is usually placed 10 mm and 12-15 mm inward of the LAA orifice, respectively [126]. Once the LAA orifice diameters have been established, the LAA's depth must also be determined in the CT sagittal and coronal views. It is measured from the central point of the orifice to the apex of the main lobe, and the chosen device should have the same diameter as the LAA depth [126]. From a technical view, the assessment of the LAA shape is a valuable aid in predicting the level of difficulty of device implantation. The \"chicken wing\" shape is usually considered to be the most complicated, particularly in case of a proximal and sharp bend. \"Cactus\" and \"cauliflower\" shapes may be challenging compared with the \"wind-sock\" shape (Figure 12) [126]. Finally, MDCT has an important role in post-procedural follow-up, which can be performed 1-6 months after the device implantation. The primary role of computed tomography in this context is the exclusion of device-associated thrombus, post-implant device orientation, pericardial effusion, and peri-device leaks. [PERSON] et al. found that CT detected leaks in 62% of cases in patients implanted with ACP, whereas TEE revealed only 36% [132].