Degenerative aortic stenosis is very common with a prevalence of 2–5% in the elderly population >65 years. The pathomechanisms are similar to those in coronary heart disease, involving a degenerative process leading to aortic valve calcification and developing slowly during a lifetime. Rheumatic disease can cause aortic stenosis as well. Rarely, subaortic stenosis, defined as the presence of a congenital subaortic annulus membrane leading to narrowing of left ventricular outflow tract, may be present. These thin membranes are often well visualized by cardiac CT, and appear as hypodense flap-like lesions below the annulus.

Transthoracic echocardiography is the clinical reference modality to establish the diagnosis of aortic stenosis, based on measurement of transvalvular pressure gradients and calculation of the aortic valve orifice area using the Doppler-velocity time integral (continuity equation). This formula has some limitations and its accuracy for hemodynamic estimation of the effective aortic valve orifice is limited, e.g., in the presence of low-flow, low-pressure gradient aortic stenosis, or in case of reduced cardiac output. In addition, other factors such as eccentricity of left ventricular outflow tract (LVOT) influence these measurements. In contrast, the anatomic (= geometric) aortic valve area obtained directly from cross-sectional imaging techniques such as cardiac CT (Fig. 16.2) or magnetic resonance imaging (MRI) is not affected by external factors and flow phenomena. Several studies, with more than 300 patients enrolled, showed a high correlation of CT with transthoracic echocardiography. The size of the aortic valve area is used to classify the severity of aortic stenosis as mild, moderate, and severe (Table 16.2). Images should be reconstructed at early/mid systole, which is the time point of maximal aortic valve opening, and depicted at 10–25% of the RR-interval pending on a patient’s individual heart rate. Importantly, the phase with the best image quality should be selected for final sizing of the AVA. The best phase is also somewhat variable among various scanner generations. CT scanners with highest temporal resolution are advantageous for optimal image quality without motion artifacts during systole. The views for evaluation of the aortic valve are shown in Fig. 16.3 and listed in Table 16.1. Multiplanar reformations should be applied, and left coronal oblique views, three-chamber views, and cross-sectional oblique axial views of the aortic valve should be reconstructed.

The following clinical applications for measurement of the aortic valve area are useful: first, in patients referred for coronary CT angiography, if valve calcification is present, the aortic valve area may be measure because these patients may have aortic stenosis or just non-stenotic valve calcification. Second, patients who require a second imaging modality for accurate sizing of the aortic valve area, e.g., in case of pending cardiac surgery, and/or if echocardiography has inherent limitations such as in low-flow low-gradient aortic stenosis. In these patients, CT can also yield information about the presence of coronary artery disease, left ventricular function, and the size of the aortic annulus. Left ventricular function is a valuable predictor of outcome in severe aortic stenosis. Accurate sizing of aortic root dimensions is required especially before minimally invasive aortic valve replacement or percutaneous transcatheter valve implantation.

CT can also identify bicuspid or tricuspid valve morphology (Fig. 16.4). The aortic valve can be congenitally bicuspid (without or with a fused raphe) or develop a secondary degenerative bicuspid shape due to calcification and adhesions. This information is of interest in clinical practice for cardiac surgeons to define the surgical approach (valve replacement versus possible surgical reconstruction) or to define further patient management. Congenital bicuspid valves are prone to developing dysfunction (regurgitation or stenosis), hence those patients may need to be followed up closer.

CT allows quantification of aortic valve calcifications by using the same commercially available software as for coronary artery calcium scores. The aortic valve calcium score provides independent prognostic information in patients with asymptomatic aortic stenosis. Moreover, an aortic valve calcium score cut-off of 1,100 Agatston units is associated with a high likelihood of aortic stenosis. Thus all patients referred for nonenhanced coronary calcium scoring, in whom aortic valve calcification is an incidental finding, should be referred for transthoracic echocardiography for further evaluation.

The onset of aortic regurgitation (AR) is either acute or chronic. Whilst acute aortic regurgitation is seen in fulminant infective endocarditis or ascending aortic dissection involving the valve, chronic aortic regurgitation can develop on the basis of several underlying diseases, the most common being aortic root dilatation and aneurysm formation (Fig. 16.5) as well as degenerative or rheumatic disease.

In patients with aortic regurgitation, cardiac CT allows visualization of the anatomic regurgitant orifice area. This area can be seen as central valvular leakage area, reflecting an incomplete co-adaption of cusps (Fig. 16.5

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