CMR is an established modality for anatomic and especially functional assessment of the aortic valve and aortic root. However, traditional CMR sequences have mostly been limited to 2-dimensional views suffering from similar limitations as TEE/TTE.35 Novel whole-heart 3-dimensional noncontrast CMR sequences with high spatial resolution and narrow section thickness were only recently developed.36,37
Several studies have evaluated CMR for preprocedural TAVR imaging. La Manna et al38 initially compared TTE and CMR in 49 elderly patients scheduled for TAVR and found a moderate correlation in terms of AA size (r2=0.48) and a lower correlation in terms of aortic valve area (r2=0.24). Jabbour et al39 compared CMR and CT in 202 patients undergoing TAVR assessment and reported highly reproducible measurements of the aortic root structures for both modalities, whereas CMR showed a lower intraobserver and interobserver variability. Furthermore, they demonstrated that the presence and severity of post-TAVR aortic regurgitation were associated with larger AA measurements by both CMR and CT but not TTE. Quail and colleagues performed CT and noncontrast CMR in 21 patients scheduled for valve-in-valve TAVR. Although they observed a good agreement between both modalities for measurements of aortic geometry, they observed extensive metal artifacts in patients with metal strut aortic valve constructions on CMR.40 Koos et al37 compared CMR and CT measurements of the aortic root in 58 pre-TAVR patients and observed an overall good correlation (r=0.86). They also reported that the TAVR strategy would have been modified based on CMR/CT measurements compared with traditional TEE measurements. Although these results are encouraging, and noncontrast CMR may be especially beneficial in multimorbid patients with renal dysfunction, CMR for TAVR assessment currently remains limited by extensive aortic calcifications and metal artifacts.40 Furthermore, sequences for simultaneous imaging of the pelvic vasculature commonly performed with CT are still in development for CMR. Nevertheless, the fact that aortic root measurements are systematically underestimated by echocardiography compared with CMR and CT has been demonstrated by multiple studies that have underlined the importance of cross-sectional imaging modalities. Furthermore, both CMR and CT allow for accurate assessment of left ventricular function, whereas CMR facilitates assessment of both aortic stenosis and aortic insufficiency.37,39,41
The functional imaging capabilities of CMR may play a more crucial role in predicting clinical outcome in pre-TAVR imaging and post-TAVR functional assessment. Barone-Rochette et al42 performed late gadolinium enhancement CMR in patients with severe aortic stenosis undergoing SAVR or TAVR. They found that focal late gadolinium enhancement indicative of focal fibrosis or unrecognized infarct was an independent predictor of mortality in both groups. Milano et al43 reported that the amount of myocardial fibrosis in patients with severe aortic stenosis had a significant effect on long-term survival after SAVR. Hartlage et al44 found that in symptomatic post-TAVR patients, CMR helped reclassify the paravalvular leak grade compared with TTE and showed a superior prognostic value. Merten et al45 observed a significant improvement of left ventricular function and volume in post-TAVR patients and described that mild to moderate aortic regurgitation was commonly seen. Ribeiro et al46 reported that the severity of aortic regurgitation after TAVR was consistently underestimated by TTE compared with CMR. These findings emphasize the increasingly important role of CMR in post-TAVR imaging, as it is a more accurate modality for assessment of aortic regurgitation compared with TEE/TTE (Fig. 5).44
Patients considered for TAVR typically have multiple comorbidities including advanced age and a coincidental high prevalence of chronic renal disease.1 Reduced glomerular perfusion due to aortic stenosis is a known cause for renal dysfunction and has even been considered a risk factor for acute kidney injury after contrast administration.47 Thus, current radiologic research has focused on new imaging technologies to substantially lower the required amount of contrast volume while providing diagnostic image quality. Interestingly, some of these techniques simultaneously result in reduced radiation exposure, although the latter aspect is of secondary importance in the patient population considered for TAVR.
A large area of the body has to be covered in pre-TAVR CT for comprehensive imaging of both the heart and the vascular access path including the pelvic vasculature. Traditionally, separate acquisitions of these areas were performed, requiring 2 contrast bolus administrations or a single larger bolus.48,49 A simple approach to lowering the total contrast volume using noncontrast imaging of the pelvis has been suggested.50 This would allow for basic assessment of the vessel course and tortuosity as well as detection of potentially stenotic calcifications. However, noncalcified plaques or thrombi will be missed with this approach.
The introduction of CT scanners with increased temporal resolution and acquisition speeds has allowed for image acquisition with a substantially increased pitch factor and ultimately shorter examination times. High-pitch acquisition remains a main principle in cardiac imaging and has been used to perform coronary CT angiography with resulting radiation doses of <0.1 mSv.51 For TAVR assessment, high-pitch acquisition can be used to perform CT angiography of the whole body including the pelvic vasculature using a single contrast material bolus.15,16,52,53 Figure 6 demonstrates a clinical case in which TAVR assessment was performed using high-pitch acquisition with a single 40 mL bolus of contrast. Recently, the feasibility of comprehensive pre-TAVR imaging with a single bolus of 20 mL has been demonstrated.54
Although high-pitch acquisition is beneficial for visualizing the vascular access path, imaging of the aortic root for TAVR assessment should be performed using retrospectively electrocardiography-gated protocols to allow for measurements during end-systole due to changes in the AA dimensions throughout the cardiac cycle.20 Although cardiac CT for TAVR assessment is focused on visualization of aortic root anatomy, it has recently shown a high diagnostic accuracy for the simultaneous detection of coronary artery stenosis, further emphasizing the beneficial role of comprehensive pre-TAVR imaging.55
Simultaneous image acquisition at 2 different x-ray spectra with dual-energy CT provides multiple postprocessing opportunities to enhance cardiac imaging.56,57 In vascular imaging, dual-energy CT is primarily used to calculate virtual monoenergetic images with keV levels closer to the k-edge of iodine to improve intravascular attenuation.58,59 Therefore, contrast volume can be substantially reduced while maintaining image contrast and diagnostic image quality.60,61 Dubourg et al62 demonstrated that monoenergetic reconstructions of dual-energy CT data allow for a reduction of iodine load in pre-TAVR imaging without compromising image quality. Initial studies have shown substantial advantages of a novel advanced image-based monoenergetic algorithm over the traditional monoenergetic algorithm especially at lower keV levels, which may be particularly beneficial for TAVR assessment to reduce iodine load and improve image quality (Fig. 7).63–65
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