Imaging modalities are necessary to provide information on mitral valve anatomy complexity and obtain a detailed plan for transcatheter procedures. Although echocardiography has represented the gold standard for pre- and post-treatment grading, diagnosis, and monitoring, MDCT has become widespread for interventional planning in patients who are candidates for TMVI. It allows rapid image acquisition time and wide availability, excellent spatial resolution, allowing high-quality 2D and 3D reconstructions of the entire mitral apparatus at any moment of the cardiac cycle, high reproducibility and relative operator independence, visualization, quantification of calcifications, and a complete view of the heart in its entirety and the chest wall
[52]. CT provides relevant information on mitral leaflet pathology. Compared with echocardiography, CT provided the highest accuracy in identifying prolapse (sensitivity of 96%; specificity of 93%)
[53].
Furthermore, cardiac CT is an excellent technique for diagnosing annular disjunction, a remodeling process associated with mitral valve prolapse, since observation of the entire cardiac cycle allows
peopleus to evaluate not only the degree of disjunction but also the dynamics of the ring
[54]. In the context of secondary or functional MR, CT also provides essential information regarding the geometry of the valve and the remodeling process (including flap tenting heights and coaptation depth). Knowing that these data are essential for docking the device, CT provides details on the anatomy of the left ventricle, which is also crucial in assessing feasibility. A ventricular diameter greater than 70 mm is often considered an exclusion criterion for transcatheter procedures
[55].
Exact measurements of the mitral annulus are critical for choosing the correct device size and, consequently, for an effective TMVI procedure
[56]. Equally important is the understanding of the modifications of the annulus during the various phases of the cardiac cycle (modifications that vary according to the type of etiology of the insufficiency); based on this, the landing zone and the type of device are decided
[57]. It appears evident that the annulus is larger in patients with mitral regurgitation than in controls, with a greater remodeling in an anterior–posterior, rather than lateral, direction
[56]. Furthermore, in functional MR, the annulus reduces movement compared to primary MR
[58]. According to Palmisano et al., some annular geometry changes in patients with severe mitral regurgitation were detected using CT. They demonstrated that the mitral valve annulus was larger in patients with severe MR, especially in patients with prolapse (Carpentier type II). It is curious how, in these patients, the largest dimensions were found in the systolic phase, unlike controls in which the largest area is found in the early diastolic phase
[59].
Annulus Sizing
Measurement of the mitral annulus is of primary importance since inaccurate sizing has devastating consequences. Moreover, given the complex mitral annulus 3D shape, collecting annular sizing could be challenging
[60]. A direct calculation method involves tracing the edges of the annulus on conventional two-, three-, four-chamber, and short-axis views along the posterior mitral leaflet insertion and fibrous continuity. However, this method is time-consuming and can lead to an oversized device, increasing the risk of left ventricular outflow tract obstruction (LVOT). Blanke et al. developed a more straightforward method that assumes assimilating the mitral annulus to a planar D shape; given these premises, the two fibrous trigones are connected along a virtually straight line, effectively excluding the anterior horn
[61]. Since the dimensions of the mitral annulus change during the cardiac cycle, the calculations are performed in the cardiac cycle phase in which the annulus is larger. Area and perimeter are the most used measures. However, other parameters are developed, such as the trigone–trigone distance; the intercommissural distance, which corresponds to the maximum diameter of the annulus parallel to the trigone–trigone distance; the septum-lateral distance, which corresponds to the maximum diameter of the annulus perpendicular to the IC. It must be emphasized that each device refers to different parameters for sizing
[59].
Predicting LVOT Obstruction
The LVOT lies between the mitro-aortic continuity and the interventricular septum. The placement of a transcatheter mitral valve prosthesis could extend and narrow the LVOT (neo-LVOT) due to anterior deflection of the AML
[62]. Indeed, LVOT obstruction is a significant cause of mortality and morbidity following TMV replacement procedures (TMVR)
[63]. CT can predict the size of anticipated neo-LVOT, thus predicting potential obstruction. A recent study demonstrated that a CT neo-LVOT area measuring 1.7 cm
2 or smaller predicts LVOT obstruction with a sensitivity of 96% and a specificity of 92%
[64]. Notably, the neo-LVOT size is also dynamic; for this reason, it is usually measured throughout systole.
Notably, LVOT changes the cardiac cycle; therefore, this measurement is made in systole. Several device- and patient-specific factors could influence the size of the neo-LVOT. The former include stent protrusion, flare, and skirt size
[65]. A more ventricular position of the mitral prosthesis, defined as a ventriculo-atrial offset of 80:20 (i.e., 80% of the device is positioned on the ventricular side and the remaining 20% on the atrial side), increases the risk of neo-LVOT obstruction. On the other hand, patient-specific characteristics include aortomitral angle, basal septal thickness, and anterior mitral leaflet length. The aortomitral angle is defined as the internal angle between the central axis of the mitral and aortic annulus; in particular, the risk of obstruction increases when the measurement of this angle approaches or exceeds 90°. Another factor influencing the risk of obstruction is the thickness of the interventricular septum, particularly a value greater than 14 mm
[66]. Finally, longer anterior leaflet lengths (>30 mm) are associated with the possibility of neo-LVOT obstruction
[67]. All these parameters should be carefully evaluated before proceeding with percutaneous mitral valve replacement to reduce complications and improve procedure success.
Landing Zone
CT provides information about the potential landing zone, defined as the site where the mitral prosthetic device should be deployed. It varies according to the device used and the specific mitral defect (functional MRI or mitral prolapse). As mentioned, a landing zone of 80/20 should be achieved. This means that, after deployment, about 80% of the device should be located on the ventricular side, whereas the remaining 20% on the atrial side. However, many factors may affect landing zone definition, such as the presence of mitral annulus (MAC) calcification, annular disjunction, and atrioventricular platform (abnormal atrial displacement of the mitral valve leaflet hinge point). In those devices that are anchored by friction and radial force, the MAC in the landing zone is essential
[60]. Hence, CT accurately estimates gravity, density, and magnitude. Furthermore, an atrioventricular shelf of the LV myocardium can often be found near the junction point of the posterior mitral leaflet in patients with basal myocardial remodeling
[68]. The presence of this shelf is more relevant for those mitral devices that use the inferolateral basal myocardium for anchoring. Finally, in these patients, the measurements must be performed in the physiological position
[60].
Mitral Anulus Calcification
MAC is crucial when considering TMVI; it has been demonstrated to be an independent predictor of an elevated mean diastolic gradient after the MitraClip procedure
[69]. It also contraindicates the use of some valve replacement devices. CT is the gold standard for its diagnosis and characterization, demonstrating a higher diagnostic performance than MRI and echocardiography. Degenerative annular mitral calcification (MAC) is a common finding in the elderly, with an incidence of about 6%
[67]. It is the consequence of a progressive calcification of the fibrous part of the mitral annulus. Although typically confined to the posterior border of the annulus, its extension may circumferentially involve the annulus. Another cause of MAC is caseous calcification of the mitral annulus (CCMA). CCMA has a different territory of distribution (typically the area close to the posterior mitral leaflet) and a different composition (with calcium disposition only in the periphery and a central zone of variable attenuation without contrast enhancement). Therefore, CT can accurately describe these formations and the exact distinction between MAC and CCMA
[70].
Vascular Structures
Preprocedural planning for TMVI includes careful evaluation of surrounding vascular structures using CT. Therefore, a fundamental part of the evaluation concerns the course and patency of the circumflex artery (Cx) due to its proximity to the posterior mitral annulus. When this distance is too short, some transcatheter procedures are contraindicated due to the risk of vessel closure during the fixation of the device. The distance between these two structures should be measured on multiple views to avoid the risk of dissection, closure, or perforation of the coronary sinus (CS) during or after the procedure
[71][72]. In particular, the relationship between the distance and angle between these two structures and the procedural success of patients undergoing transcatheter annuloplasty with the Carillon device has been demonstrated
[73]. Consistently, a distance of CS to Cx of <8.6 mm in the distal device landing zone has been shown to be predictive of complications on the circumflex artery
[74].
Other Potential Use of CT in Transcatheter Mitral Valve Interventions
Choosing the optimal approach for TVMI is also essential in procedural planning, and CT plays a crucial role in this context. Thus, in patients undergoing a transapical procedure, CT can be used to understand which intercostal space is close to the apex of the left ventricle and could allow the device to be deployed perpendicular to the mitral annulus, thus avoiding obstruction, perivalvular leaks, and canting
[60]. Although most mitral procedures require intraprocedural transesophageal echocardiography guidance, CT can determine the optimal site for the septal puncture in those with the transseptal approach. Although there are no specific guidelines, it is preferred to puncture the inferoposterior section to obtain good coaxiality to the mitral annulus
[75]. Moreover, it is very useful to calculate the distance from the puncture site to the mitral valve.
CT is also helpful in determining the optimal fluoroscopic viewing angle of the mitral annulus for device deployment, which should be perpendicular to the mitral annulus (coplanar angle)
[60]. Knowledge of this angle is fundamental for interventionists for accurate coaxial device positioning. Indeed, any change from this fluoroscopic angle can translate into inaccurate deployment, potentially increasing procedural complications.
In patients undergoing mitral valve-in-valve (VIV), CT can identify patients at higher risk of complications, providing information on the pre-existing prosthesis and thus predicting the risk of LVOT obstruction
[70].
Finally, CT has a role in the follow-up of patients undergoing TMVI, especially in assessing the presence of any subclinical thrombosis. Although the clinical significance of the latter is not yet known, CT can diagnose this complication by highlighting it as a thickening of the hypodense flap with the decreased movement of the prosthetic flaps
[76]. CT also has a role in predicting the risk of mitral stenosis after the MitraClip procedure. In a study by Kaewkes et al., annulus diameters (both anteroposterior and medial–lateral), annulus area, and mitral valve orifice area were inversely associated with transvalvular gradients
[77].