Computed Tomography Scan in Mitral Valve Disease and Its Treatments

INTRODUCTION

Anatomic and functional evaluation prior to transcatheter mitral valve (TMV) repair and TMV replacement (TMVR) by multidetector computed tomography angiography (CTA) has emerged as an essential part of pre-procedural planning, complimenting the significant amount of information obtained from transesophageal echocardiogram (TEE). The role of CTA has evolved, mimicking its use in TAVR with a focus on reproducible annular measurement, evaluation of landing zone, neo-left ventricular outflow tract assessment, and pattern of mitral annular calcification. ^, Computer tomography (CT) provides three-dimensional (3D) sets of data of cardiac morphology with a larger field of view compared to echocardiography, excellent image quality owing to millimetric spatial resolution, isotropic image quality throughout the data set, and good temporal resolution. Multiplanar reconstruction capabilities and functional data sets are additional benefits of the modality that can help provide information on the morphology and function of the mitral valve. ^,

CT ANATOMY OF THE MITRAL VALVE

The mitral valve structure is complex; it comprises non-planar non-circular annulus, leaflets, and subvalvular apparatus. The adjacent anatomical structures, notably the left ventricular outflow tract (LVOT) and circumflex artery, contribute to its complexity.

The Leaflets

CT anatomy of mitral valve leaflets and segmentation according to Carpentier’s nomenclature is depicted by Fig. 9.1A and Fig. 9.1C . The most lateral scallop (labeled as P1) is adjacent to the anterolateral commissure, P2 (middle) is centrally located, and the most medial (P3 scallop) is adjacent to posteromedial commissure. Similarly, the anterior scallops are labeled as A1, A2, and A3. The leaflets converge at the anterolateral and posteromedial commissures. The coaptation zone, or line, is a semilunar arc conformation with the leaflets converging at the commissures. Despite better visualization of the mitral valve scallops by echocardiography, which is proven to have better temporal resolution than CT, post-processing techniques such as minimum intensity projection (MIP) may improve visualization of the thin low-attenuation leaflets.

Mitral Annulus

Mitral annulus (MA) (from Latin “small ring”) gives attachment to the mitral valve formed by a junction of the left atrium, left ventricle (LV), and the leaflets. It is a non-rigid fibromuscular structure changing shape throughout the cardiac cycle and incorporating several structures along the hinge points. The MA 3D configuration is saddle-shaped with anterior and posterior peaks or horns. The anterior horn, more fibrous and less prone to dilatation, extends between the trigones to the aortic root (“aortomitral continuity” or “intervalvular fibrosa”) ( Fig. 9.1B and Fig. 9.1D ). ^, The continuity between the aortic and mitral valve is a cardiac CT anatomical landmark for identifying the LV. Its relationship with LVOT is crucial for consideration of suitability for TMVR due to potential risk of LVOT obstruction (refer to further discussion in the Left Ventricular Outflow Tract Obstruction Assessment section).

In healthy individuals, the posterior horn is located at the junction of the atrial and ventricular myocardium and formed by insertion of the posterior mitral leaflet (PML), and the nadir of the annulus is located close to the fibrous trigones. The fibrous trigones represent the anatomic junction between the anterior and posterior annulus and can be identified on CT scan as focal areas of triangular thickening of the medial and later ends of the fibrous annulus ( Fig. 9.1E ). This part of the annulus is mainly muscular and, therefore, susceptible to dilatation (seen in mitral regurgitation [MR]) and calcification.

TECHNIQUE OF ACQUISITION, CONTRAST INJECTION PROTOCOL

ECG-gated contrast-enhanced multiphasic CTA covering the entire cardiac cycle is required for mitral valve and annulus assessment.

Data acquisition is either synchronized by retrospective or prospective electrocardiography (ECG)—gating, tube voltage, and current set according to the patient’s body habitus. For example, tube voltage varies from 100 to 120 kV in our institution and current from 400 to 600 mA to ensure optimal image quality. Gantry rotation time is one of the adjustable parameters and may be set to 0.24 to 0.33 second. However, some of the aforementioned settings are vendor-specific. Tube current modulation is avoided to ensure high image quality throughout the cardiac cycle.

Image acquisition initiates when the region of interest attenuation reaches the 150 HU threshold within the LV. The acquisition starts at the level of carina and covers the heart to its base. In the case of transapical access, the acquisition should cover the entire thoracic cage to better assess the intercostal space.

Contrast material injection protocol requires a single breath-hold following biphasic injection of 60 to 70 mL of intravenous contrast media (320 mg I/mL) at the rate of 4.5 to 6.5 mL/s and 50 mL saline chaser via an 18-gauge needle in an antecubital vein. Other recommended parameters include slice thickness of 0.5 mm, increment 0.5, and iterative reconstruction for noise reduction.