How Does CT Fit Into the Evaluation and Management of Patients With AS?

Aortic stenosis (AS) etiology, in western country, is nowadays principally degenerative with calcium deposition in aortic valve leaflets. It has long been known that moderate/severe aortic valve calcification (AVC), as documented by Doppler echocardiography, is a predictor of worse outcome in patients with AS.1 However, echocardiography is based on semi-quantitative assessment and allows only an imprecise evaluation of AVC load.2 Although Doppler echocardiography is the first line for the hemodynamic evaluation of AS severity, computed tomography (CT) may provide important complementary information in the evaluation and management of patients with AS.

CT has been shown to provide accurate and reproducible measurement of calcification load in the aortic valve.2,3 AVC is measured on non-contrast, non-reconstructed CT scans during trained end-inspiration breath-hold (Figure 1), with the use of the Agatston method, described for coronary artery calcification evaluation.4 Nowadays, multidetector CT scans are generally used and parameterized with 30-80 mA and 120-130 kV, depending on prospective or retrospective gating at 70-80% of the R-R interval. Calcification is defined as 4 adjacent pixels with density >130 Hounsfield units and can be easily measured with commercially available software. The region of the aortic valve should be delimited with caution in order to avoid inclusion of calcifications in the wall of the aorta, the left ventricle outflow tract (LVOT), and/or the annulus of the mitral valve (Figure 1).

Figure 1: CT Images With and Without Highlighted Calcification

Figure 1
In natural CT images (2 top lines), calcium appears as white spots. After automatic highlighting of the calcium by the software, calcification appears as a yellow spot (2 bottom lines). AVC was manually selected, and color was changed from yellow to pink. The sum of the pink lesions is calculated by the software and represents the AVC load.

AVC load has been shown to be a predictor of death and adverse events in AS even after adjustment for hemodynamic severity of AS (i.e., aortic valve area [AVA], peak aortic jet velocity, and/or mean gradient).2,5 Mortality in patients with severe AVC was largely alleviated by aortic valve replacement, demonstrating that the link between AVC and death was largely dependent on AS.5 Moreover, some studies demonstrated that AVC measured by CT is a major predictor of anatomic and hemodynamic progression of AS.6-8 Hence, in patients with no indication for aortic valve replacement (i.e., hemodynamically non-severe and/or asymptomatic AS), AVC can be used to identify patients who are at higher risk for rapid AS progression and therefore adapt the time interval between follow-up visits. Patients with severe AVC will progress more rapidly and have higher risk for aortic valve replacement or death and should thus have closer follow-up than patients with non-severe calcification (Figure 2).

Figure 2: Integration of CT in Evaluation and Management of Patients With AS

Figure 2
Note: Patients with mean gradient ≥40 mm Hg and AVA >1 cm2 are generally young patients with a bicuspid valve, patients with concomitant AR, or measurement error of AVA.

In patients with an inconclusive echocardiographic exam, such as discordance between gradient and AVA, measurement of AVC by CT may be useful to corroborate AS severity. Indeed, AVC is well-correlated with hemodynamic severity of AS. However, this relationship is different between men and women.3 Women reach a more severe AS for similar AVC load, even after adjustment for body surface area or aortic annulus area.3,9 Thus cut-point values defining severe AS have to be different in men and women (Table 1-Figure 2). These cut-points provide a specificity to detect hemodynamically severe AS higher than 80% and a sensitivity higher than 80% for non-indexed AVC and higher than 90% for AVC indexed to aortic annulus area.9 AVC measurement may be useful to identify severe AS, especially in patients with low-flow, low-gradient AS with or without decreased left ventricular ejection fraction (LVEF). In patients with low-flow, low-gradient AS and reduced LVEF, dobutamine stress echocardiography is recommended to assess AS severity and flow reserve.10 However, in patients without flow reserve, AS severity cannot be confirmed by dobutamine stress echocardiography or catheterization.11,12 In these patients, quantitation of AVC by CT is the most reliable assessment of AS severity. In patients with low-flow, low-gradient AS and preserved LVEF, stress echocardiography has been proposed to assess AS severity,13 and exercise stress echocardiography is useful in asymptomatic patients or in patients with equivocal symptoms to confirm symptomatic status. However, in symptomatic patients, exercise stress echocardiography is contraindicated, and dobutamine stress echocardiography should not be performed in patients with restrictive left ventricle physiology pattern. Thus, AVC could be the method of choice to assess AS severity in this context. Finally, in patients with low-gradient, normal-flow AS with small AVA and normal LVEF, the use of stress echocardiography to increase the flow would probably not help assess AS severity given that patients already have a normal flow. Although these patients with normal-flow, low-gradient AS are considered as patients with progressive AS in the 2014 ACC/AHA guidelines,10 several studies and a recent meta-analysis demonstrated that about half of these patients have severe AS on the basis of AVC and that these patients may benefit from aortic valve replacement.9,14 Thus, the use of AVC in symptomatic patients with low gradient and small AVA may help in clinical decision-making. Aortic valve replacement should probably be considered in these patients if severe valve calcification is present (Figure 2).

Table 1: Cut-Points and Accuracy of AVC and AVC Divided by Aortic Annulus Area to Identify Severe AS9



Area Under the Curve


Sensitivity (%)

Specificity (%)




1,274 arbitrary unit (AU)





2,065 AUt



AVC Density*



292 AU/cm2





476 AU/cm2



* AVC indexed to aortic annulus area

In these patients with low gradient despite a tight AVA, the issue of non-circular LVOT has often been suspected as the reason for gradient-valve area discrepancy, especially when velocity ratio at echocardiography does not indicate severe AS (i.e., velocity time integral in LVOT divided by velocity time integral in the aorta >0.25). Numerous studies demonstrated that when assessed by three-dimensional-modality imaging (three-dimensional echocardiography or CT), LVOT area was always larger than when assessed by two-dimensional echocardiography, thus indicating an underestimation of AVA as calculated with the use of two-dimensional echocardiography. However, when compared with direct operative measurement of LVOT, CT has been shown to overestimate LVOT area.15 Moreover, AVA calculated with the use of LVOT measured by CT did not predict survival better, and the threshold to define severe AS was larger (i.e., 1.2 cm2 vs. 1.0 cm2 when AVA was calculated by two-dimensional echocardiography).16 Furthermore, measurement of LVOT by CT requires the use of contrast, which may be deleterious for patients with renal failure. Thus, the use of CT for LVOT measurement should not be used for routine assessment of AS severity and be reserved for pre-transcatheter aortic valve implantation evaluation.

In conclusion, in the evaluation and management of patients with AS, CT has an important role for the quantitation of AVC load in order to assess AS severity in patients with inconclusive echocardiographic findings, especially those with discordance between a low gradient and a tight AVA, with or without low flow/ejection fraction. In analyzing AVC load, sex-specific thresholds have to be used given that women reach a more hemodynamically severe AS than men for the same amount of valve calcification. AVC load evaluation may also be useful to assess the risk of rapid AS progression and, therefore, adjust the timing of follow-up for patients without an indication for surgery.


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Clinical Topics: Cardiac Surgery, Invasive Cardiovascular Angiography and Intervention, Noninvasive Imaging, Valvular Heart Disease, Aortic Surgery, Cardiac Surgery and VHD, Interventions and Imaging, Interventions and Structural Heart Disease, Computed Tomography, Echocardiography/Ultrasound, Nuclear Imaging

Keywords: Aorta, Aortic Valve, Aortic Valve Stenosis, Body Surface Area, Calcinosis, Catheterization, Coronary Vessels, Echocardiography, Echocardiography, Doppler, Echocardiography, Stress, Echocardiography, Three-Dimensional, Heart Ventricles, Mitral Valve, Renal Insufficiency, Stroke Volume, Tomography, Tomography, X-Ray Computed, Transcatheter Aortic Valve Replacement

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