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Transcatheter Aortic Valve Replacement

Imaging Techniques for Aortic Root Sizing

Wichmann, Julian L. MD*,†; Varga-Szemes, Akos MD, PhD*; Suranyi, Pal MD, PhD*; Bayer, Richard R. II MD*,‡; Litwin, Sheldon E. MD*,‡; De Cecco, Carlo N. MD, PhD*,§; Mangold, Stefanie MD*,∥; Muscogiuri, Giuseppe MD*,¶; Fuller, Stephen R. BSc*; Vogl, Thomas J. MD; Steinberg, Daniel H. MD; Schoepf, U. Joseph MD*,‡

doi: 10.1097/RTI.0000000000000167
Symposium Review Articles
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Transcatheter aortic valve replacement (TAVR) is an increasingly used alternative to surgical aortic valve replacement in patients with severe aortic stenosis and prohibitive perioperative risk. Several studies have shown an improved clinical outcome and lower rate of complications with TAVR in this patient population. Furthermore, TAVR has shown promising results in patients at elevated risk from surgical aortic valve replacement. Because of the endovascular nature of this technique, comprehensive preprocedural assessment of the aortic root and vascular access path is crucial. Although echocardiography is still commonly performed to assess the aortic root, cross-sectional imaging modalities are increasingly used given their superior results in the diagnostic accuracy of TAVR measurements. In particular, computed tomography (CT) is gaining an increasing role in pre-TAVR imaging because of fast 3-dimensional assessment of aortic root anatomy and improvements in clinical outcome after TAVR when CT is used for pre-TAVR planning. However, different algorithms exist for matching valve size to the aortic root and left ventricular outflow tract, and these measurements may substantially impact valve prosthesis selection and postinterventional complication rates. Cardiac magnetic resonance may play a role especially in post-TAVR assessment, as it provides both anatomic information and blood flow dynamics. This article reviews multimodality imaging approaches to pre-TAVR aortic root size assessment, provides an overview of the impact on post-TAVR complications and clinical outcome, and describes recent techniques to reduce contrast material volume in TAVR assessment with CT.

*Department of Radiology and Radiological Science, Division of Cardiovascular Imaging

Department of Medicine, Division of Cardiology, Medical University of South Carolina, Charleston, SC

Department of Diagnostic and Interventional Radiology, University Hospital Frankfurt, Frankfurt

Department of Diagnostic and Interventional Radiology, University Hospital of Tübingen, Tübingen, Germany

§Department of Radiological Sciences, Oncology and Pathology, University of Rome “Sapienza” – Polo Pontino, Latina

Department of Medical-Surgical Sciences and Translational Medicine, University of Rome “Sapienza,” Rome, Italy

U. Joseph Schoepf is a consultant for and/or receives research support from Bayer, Bracco, GE Healthcare, Medrad, and Siemens Healthcare. Daniel H. Steinberg receives grants from Edwards Lifesciences and personal fees from Terumo Interventional Systems, Boston Scientific, St Jude Medical, and Astra Zeneca. The remaining authors declare no conflicts of interest.

Correspondence to: U. Joseph Schoepf, MD, Department of Radiology and Radiological Science, Division of Cardiovascular Imaging, Medical University of South Carolina, Ashley River Tower, 25 Courtenay Drive, Charleston, SC 29425-2260 (e-mail: schoepf@musc.edu).

Several recent large-scale multicenter studies have demonstrated that transcatheter aortic valve replacement (TAVR) is a beneficial therapeutic option in patients with severe, symptomatic aortic stenosis and that TAVR is especially useful in patients with a high risk for perioperative complications during traditional surgical aortic valve replacement (SAVR).1–4 The Placement of AoRTic TraNscathetER Valves (PARTNER) trials demonstrated equivalent mortality and improved quality of life in high-risk patients over 1 year compared with SAVR and a mortality benefit with TAVR compared with standard treatment in inoperable patients.1,2 In a recent meta-analysis of 62 studies, TAVR was reported to improve symptoms and physical function and increase disease-relevant quality of life.5

Although careful patient selection remains crucial in providing good clinical outcomes, a growing body of research has been dedicated to the optimization of preprocedural imaging for surgical planning and reduction of perioperative and postoperative complications in TAVR procedures. Selection of an inappropriately sized transcatheter heart valve (THV) prosthesis has been associated with valve dislodgement, paravalvular regurgitation, coronary occlusion, aortic annular rupture, and device embolization.6–9 Thus, reproducible and accurate sizing of the aortic root is essential to pre-TAVR planning but may be challenging in patients with severely diseased aortic valve structures with extensive calcifications.10

Current postprocessing software has helped automate the evaluation of critical measurements of the cardiovascular anatomy for proper preoperative assessment and selection of the appropriate THV.11–14 Furthermore, novel computer tomography (CT) technologies have been applied to reduce examination times, the volume of contrast material used, and radiation dose in preprocedural CT for TAVR planning.15,16 Cardiac magnetic resonance (CMR) may also play a role especially in the visualization of post-TAVR blood flow dynamics and complications. This article reviews the current state of multimodality imaging techniques for assessment of the aortic root during preinterventional and postinterventional diagnostic workup in patients undergoing TAVR.

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ANATOMIC CONSIDERATIONS

One reason for the increasing application of cross-sectional imaging modalities in TAVR planning is the ability to perform full-volume imaging with reconstruction in unlimited planes, thus overcoming some of the challenges inherent in visualizing the complex anatomy of the aortic root and the valve itself. Although the term “annulus” implies a circle, the aortic annulus (AA) is a complex 3-dimensional structure that can be best described as a noncircular, 3-pronged coronet bounded at its nadir by the attachment of the 3 aortic leaflets.17 The semilunar aortic cusp attachments extend from the left ventricle throughout the aortic root to the sinotubular junction.17,18 The smallest diameter in the ventriculo-aortic blood path may be mistaken for the AA, as it may be more visible on 2-dimensional transthoracic echocardiography (TTE) or transesophageal echocardiography (TEE). Accordingly, these modalities may underestimate measurements of the AA when compared with invasive perioperative sizing.19 Cross-sectional imaging modalities such as CT or CMR can overcome this limitation and allow for more accurate valve sizing to prevent THV misplacement.13,20 Finally, differences in aortic root anatomy during the cardiac cycle need to be taken into account, as the AA shows its largest diameter during systole.21

Another important consideration is the relationship of the AA to the coronary ostia. Anatomic variations in the locations of the coronary ostia may increase the risk for periprocedural or postprocedural coronary obstruction, as the native valve leaflets are pushed into the aortic wall by the implanted valve.18,22 The left main ostium is more at risk for obstruction of a displaced native valve leaflet.17 Besides design characteristics of the implanted THV, other previously described risk factors for this complication include a low origin of the coronary ostium, a shallow sinus of Valsalva, a bulky native valve, and a prolonged left coronary leaflet.23,24

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RELEVANT PROSTHESIS MODELS

Edwards SAPIEN Valve

The first percutaneous valve prototype implanted in humans was the Cribier-Edwards THV (Edwards Lifesciences, Irvine, CA), which comprised a balloon-expandable trileaflet tissue valve of equine pericardium mounted in a stainless-steel frame.25 Subsequent improvements in the valve and delivery system technology resulted in the second-generation Edwards SAPIEN THV, which consists of a tubular slotted stainless-steel stent frame with a unidirectional trileaflet tissue valve consisting of pretreated bovine pericardium.26 This valve was redesigned to improve sealing and decrease paravalvular regurgitation and to increase durability through the use of bovine pericardium.27 It was available in 2 sizes: a 23-mm diameter prosthesis for aortic annular diameters of 18 to 22 mm, and a 26 mm prosthesis for annular diameters of 21 to 25 mm.

The third-generation SAPIEN XT THV is a trileaflet bovine pericardial valve mounted on a cobalt chromium frame. Several design modifications were performed to improve leaflet coaptation, improve durability, and decrease the sheath delivery size from 22 to 24 Fr to 18 Fr.26,28,29 The SAPIEN XT THV is available in 23, 26, and 29 mm sizes.

The SAPIEN 3 (S3) THV was designed to allow for crimping to a reduced profile and smaller sheath introducer sets (14 to 16 Fr) compared with the predicate SAPIEN THV generations.30 In addition, it incorporates an additional outer skirt to minimize paravalvular leak. The S3 is available in 23, 26, and 29 mm sizes in Europe and is currently under investigation in the US clinical trials.

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Medtronic CoreValve System

The Medtronic CoreValve THV (Medtronic Inc., Minneapolis, MN) is an asymmetrical self-expanding valve with a nitinol frame and in the first generation was preloaded with bovine pericardium. Nitinol shows superelastic properties at low temperatures and a very high radial strength at warmer temperatures, including body temperature.31 In the current generation, porcine pericardial tissue is sewn into the frame with diameters of 23, 26, 29, and 31 mm for treatment of an aortic annular perimeter ranging from 63.5 to 87.3 mm. The CoreValve THV systems can be loaded onto a catheter delivery system without a balloon and can therefore be gradually deployed in stages and collapsed again to correct valve placement.

The recently introduced CoreValve Evolut R has a delivery profile reduced to 14 Fr to improve access and reduce the risk of major vascular complications. Additional design modifications were performed to improve self-centering of the valve within the annulus, allow for recapture and reposition, and achieve superior sealing by means of an extended skirt to further reduce aortic regurgitation. The Evolut R is implanted in a supra-annular position to improve hemodynamics and is available in the same valve sizes as the CoreValve THV.

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IMAGING-BASED SIZING ALGORITHMS

CT

Various algorithms for optimal CT-based measurement of the AA have been described and assessed in clinical-outcome studies.32 An established manual method to visualize the AA is based on a double oblique multiplanar reconstruction with 2 orthogonal planes representing the short and long axis of the virtual basal ring (Figs. 1, 2).10,33 Measurements should be taken in reconstructions depicting the systolic phase (20% to 40% R-R interval).21,33

FIGURE 1

FIGURE 1

FIGURE 2

FIGURE 2

Schuhbaeck et al10 evaluated a systematic manual approach for CT-based measurement of the dimensions of the AA by identifying the caudal insertion points of the 3 aortic cusps and sequentially aligning them in a double oblique plane. They found high interobserver and intraobserver agreement in measurements independent of the influence of aortic root calcification. In a similar study, Schmidkonz et al32 described reproducible measurements of the AA and aortic root geometry with close agreement between 3 observers. Although experience in manual assessment of the annulus remains crucial, novel semiautomated techniques are increasingly used to reduce the time required for these measurements and to increase reproducibility and standardization of both values and subsequent THV selection.11,13,34

Jilaihawi et al34 demonstrated the superiority of 3-dimensional CT-based assessment of the AA for TAVR compared with conventional 2-dimensional echocardiography and observed a modality-based influence on prosthesis sizing, patient selection, and post-TAVR paravalvular aortic regurgitation. They found that measurements of maximal and average dimension were more accurate with CT compared with TEE or TTE and reported that echocardiography underestimated annular dimensions. In support of this conclusion, they found a higher rate of post-TAVR paravalvular aortic regurgitation when TEE or TTE was used for presurgical assessment.

Binder et al13 prospectively investigated the clinical impact of a CT-based annular area sizing algorithm on TAVR outcomes in 133 patients compared with a control group of 133 patients wherein a conventional sizing approach had been used. With this algorithm, they found a decrease in mild post-TAVR paravalvular regurgitation from 12.8% (13/133) down to 5.3% (7/133) and a reduction in a composite measurement of in-hospital mortality, AA rupture, and severe paravalvular regurgitation from 11.3% (15/133) to 3.8% (5/133).

Blanke et al11 investigated a semiautomated on-site algorithm in 50 patients for 3-dimensional visualization of the aortic root with automated model-based identification of the major anatomic landmarks, delineation of the AA, and measurement of the distance to the coronary ostia (Figs. 3, 4). They found significantly lower time required for model-based measurements compared with manual assessment for both experienced (26 vs. 98 s) and inexperienced (34 vs. 123 s) observers. Although differences in measurements were not significant among experienced observers, there was substantially lower agreement among inexperienced observers between manual and algorithm-based measurements for virtual THV sizing (72%, κ=0.54). This indicates that such semiautomated algorithms may not only save time in clinical routine but also increase standardization of AA measurements across observers.

FIGURE 3

FIGURE 3

FIGURE 4

FIGURE 4

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CMR

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

FIGURE 5

FIGURE 5

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IMAGING WITH REDUCED CONTRAST VOLUME

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.

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Noncontrast Imaging of Pelvic Vessels

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.

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High-pitch Acquisition

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

FIGURE 6

FIGURE 6

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

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Dual-energy CT

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

FIGURE 7

FIGURE 7

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CONCLUSIONS

TAVR is an increasingly used alternative to SAVR in patients with severe aortic stenosis with a high risk for perioperative complications. Given present trends, improvements in the TAVR technique and valve prosthesis material are likely to expand the spectrum of patients eligible for TAVR, with a predicted “indication creep” to soon also include younger, healthier patients who now undergo SAVR. Preprocedural imaging provides crucial parameters for the procedure and has shown a beneficial effect on clinical outcome. CT has been established as the modality of choice because of its high resolution, widespread availability, fast examination times, and suitability for multimorbid patients. Standardized measurement techniques for accurate aortic root sizing have been described and should be incorporated into clinical routine to facilitate correct valve prosthesis selection. In addition, postprocessing algorithms for semiautomated TAVR assessment allow for a significant reduction in analysis time required, improve interobserver uniformity of measurements, and may thus improve clinical workflows. Multimodality imaging is simultaneously becoming increasingly important, and CMR has especially shown helpful applications in assessment of post-TAVR status and potential complications.

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Keywords:

transcatheter aortic valve replacement; transcatheter aortic valve implantation; aortic stenosis; computed tomography; magnetic resonance imaging

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