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Computed tomography and magnetic resonance imaging assessment of aortic valve stenosis: an update

Computed tomography (CT) and magnetic resonance imaging (MRI) are being increasingly utilized for the evaluation of cardiac valves. Although echocardiography is the first-line imaging modality in the evaluation of aortic stenosis (AS), CT and MRI can be adopted as adjunct imaging modalities for assessing the severity of AS. First, CT has established its role in the evaluation of co-existing coronary artery disease in AS patients, while stress MRI can be employed to exclude myocardial perfusion abnormalities. For the preand post-procedural evaluation of transcatheter aortic valve implantation or replacement, CT plays a very important role in determining the size of prosthetic valves and evaluating post-procedural complications. CT also helps assess the prosthetic valve sizes with 3-dimensional printing. Late gadolinium enhancement, T1 mapping, and feature tracking of left ventricular myocardium enable assessment of left ventricular function and myocardial fibrosis in patients with AS. Four-dimensional flow analysis gives new insights on flow patterns, kinetic energy, and wall shear stress in the ascending aorta in AS patients. In summary, CT and MRI are playing increasingly important roles in the evaluation of aortic valve disease.


CORONARY ARTERY EVALUATION IN AS PATIENTS
Coronary computed tomography angiography (CCTA) may play a clinical role as an alternative to invasive coronary angiography (ICA) before cardiac valve surgery or transcatheter aortic valve implantation or replacement (TAVI/TAVR). With the high prevalence of coronary artery disease in patients with severe AS, percutaneous coronary intervention will become increasingly necessary in patients undergoing TAVI [22]. According to a meta-analysis of 13 studies, in 1,498 patients (mean age, 74 years; 47% men; 76% TAVI procedures) undergoing surgical aortic valve surgery or TAVI, the pooled prevalence of significant stenosis ( > 50%) determined by ICA was 43% [23]. CCTA showed the pooled sensitivity, specificity, and positive-likelihood and negative-likelihood ratios in detecting significant coronary stenosis, with ICA as the gold standard, of 95%, 79%, 4.48, and 0.06, respectively [23].

PRE-AND POST-TAVI EVALUATION WITH CT
Recent CT techniques enable comprehensive evaluation before and after TAVI/TAVR in one breath hold with a variable or high helical pitch [7,9,13,14,16]. The CT area showed the highest correlation (0.932) among CT and TEE estimates, as compared with surgical annulus measurements [26].
The use of CCTA in addition to echocardiographic data in selecting the transcatheter heart valve (THV) size reduced the long-term occurrence of cardiovascular events [27]. Imaging strategy with CCTA was found to optimize and improve the outcome of TAVI. According to Casset et al. [27], multidetec- perimeter oversizing and of 0.6% for ≥ 2.5% perimeter oversizing. With perimeter undersizing down to -2.5%, the rate of moderate or high PAR increased to 6.8% and then further increased to 9.4% for pronounced undersizing of below -2.5%.
The optimal cutoff for predicting moderate or high PAR was 0.05% for area oversizing and -1.24% for perimeter oversizing.  Three-dimensional (3D)-printing based on CT data is a unique patient-specific method, allowing a tailored and individualized approach for aortic valve-sparing root reconstruction surgery [29]. With optimization, 3D models may help predict PAR in specific patients [30]. According to Ripley et al. [30], aortic root 3D models were highly accurate, with excellent agreement between annulus measurements made by 3D models and those made by corresponding 2D data (mean difference, −0.34 mm; 95% limits of agreement, ± 1.3 mm).
Examination of the fit of valves within aortic root 3D models correctly predicted PAR in six of nine patients (six true positives, three false negatives) and absence of PAR in five of seven patients (five true negatives, two false positives).

MRI FOR TAVI EVALUATION
CMR is useful for TAVI evaluation before the procedure [31][32][33]. CMR can provide incremental information, including advanced tissue characterization with late gadolinium enhancement (LGE) and T1 mapping in patients with AS. In addition, left ventricle (LV) strain analysis with feature tracking and assessment of myocardial perfusion can be performed.
CMR is now ready for implementation in clinical practice [20].
In the study of Cannao et al. [34], a non-contrast MRA protocol combining self-navigated 3D (SN3D) radial whole-heart and quiescent-interval single-shot (QISS) pulse sequences for the assessment of cardiac and vascular access route anat-  occurring entity (5% to 20%) [38]. Projected AVA at normal flow rate (250 mL/sec) calculated with stress echocardiography using low-dose dobutamine is superior to traditional severity criteria (AVA < 1.0 cm 2 and mean gradient ≥ 40 mm Hg) in unmasking severe AS and predicting outcomes in patients with reduced LVEF. However, AVA proj is not reliable if the increase in transvalvular flow rate is below 15% and dobutamine-stress echocardiography may not be able to induce a significant increase in flow rate [38].

CT measurement of aortic valve area and calcium
According to the meta-analysis of nine studies (175 women CT and MRI of aortic valve stenosis and 262 men), MDCT is an efficient method for obtaining accurate AVA measurements in patients with AS [39]. The mean AVA as measured by CT was 1.0 ± 0.1 cm 2 and the mean AVA measured by TTE was 0.9 ± 0.1 cm 2 [39]. The correlation be- Yeon Hyeon Choe, et al. [45] assessed the accuracy of the Bernoulli principle in estimating the peak pressure drop at aortic valve using 3D CMR flow data in 32 subjects. Analysis of the pressure components confirmed that the spatial acceleration of the blood jet through the valve is the most significant aspect (accounting for 99% of the total drop in stenotic subjects). However, the Bernoulli formulation showed a consistent overestimation of the transvalvular pressure (average of 54%; range, 5% to 136%) resulting from the use of a single peak velocity value, which neglects the velocity distribution across the aortic valve plane. This assumption was the source of uncontrolled variability. A corrected formulation that accounts for the cross-sectional profile of the blood flow was proposed by the authors and adapted to both CMR and echocardiographic data [45].

MYOCARDIAL FIBROSIS
Myocardial fibrosis (MF) is regarded as an early and objective marker of left ventricular decompensation, particularly in asymptomatic AS patients [46]. Replacement MF or diffuse MF can be evaluated by LGE and T1 mapping [5,[47][48][49][50]. Treibel et al. [51] investigated MF in 133 patients with severe, symptomatic AS using invasive biopsy and non-invasive imaging.
Three patterns of MF were identified: (1) thickened endocardium with a fibrotic layer, (2) microscopic scars, with a subendomyocardial predominance, and (3) diffuse interstitial fibrosis. Collagen volume fraction (CVF) was elevated (P < 0.001), as compared with controls, and higher (P < 0.001) in endocardium-containing samples, with a decreasing CVF gradient from the subendocardium (P = 0.001). LGE in grams, by the 3-standard deviation method, correlated with CVF (P < 0.001) but not ECV. Both LGE and extracellular volume (ECV) correlated independently (P < 0.001) with N-terminal pro-brain natriuretic peptide and high-sensitivity troponin T levels.
Combined high ECV and LGE better identified patients with adverse LV remodeling, blood biomarkers, and histological parameters, and functional capacity than each parameter alone. A combined, multi-parametric approach with ECV and LGE allows efficient stratification of AS patients according to the response of the myocardial collagen matrix [51].
CMR techniques may be more sensitive than the conventional measures (LVEF or LV dimensions) to detect structural and functional changes in patients with severe left-sided valvular heart disease. The presence of MF has been associated with little improvement in clinical symptoms and recovery of LV systolic function [50]. On the contrary, Everett et al. [47] suggested that cellular hypertrophy and diffuse fibrosis (mid-wall LGE) progress in a rapid and balanced manner but are reversible after AVR in patients with AS. When mid-wall LGE is first identified, prompt AVR may improve clinical outcomes.
According to a prospective trial by Everett et al. [47], in 61 as- According to Hwang et al. [55],

CONFLICTS OF INTEREST
No potential conflict of interest relevant to this article was reported.