Chapter 44 CT for Pre-TAVR Evaluation

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1 Chapter 44 CT for Pre-TAVR Evaluation MONA BHATIA Aortic stenosis is one of the most prevalent cardiac valvular affections, and symptomatic untreated patients of severe aortic stenosis have a high morbidity and mortality 1, 2. Surgical valve replacement has been the definitive treatment for symptomatic patients of aortic stenosis; however, multiple comorbidities and high surgical risk often preclude the surgical option in as many as 30% of these patients 3, 4. Transcatheter aortic valve replacement (TAVR) or transcatheter aortic valve implantation (TAVI) is fast emerging as a potential alternative in the treatment of high-risk severe aortic stenosis 5 7. Multiple multicentre trials are ongoing to establish the safety, efficacy and noninferiority of the TAVR procedure when compared to the conservative and surgical options Prerequisites for the success of the TAVR procedure are an extremely accurate assessment of the aortic root anatomy, status of the coronary arteries and dedicated planning of the access route 12. Computed tomography (CT) today provides an extremely accurate and comprehensive assessment in pre-tavr workup of patients. The aim of the pre-tavr evaluation is to ensure appropriate patient and device selection with prosthesis sizing. This is ensured through detailed assessment of the aortic annulus anatomy, geometry, measurements; aortic root analysis, coronary status and planning of vascular access. These ensure procedural success and reduce postprocedural complications. The most frequently employed routes are the retrograde transarterial routes, typically transfemoral or trans-subclavian. If these routes are not appropriate, a direct ascending aorta approach or transapical approach via the cardiac apex may be utilized for prosthesis implantation. AORTIC ROOT ANATOMY The left ventricular outflow tract continues as the aortic root. The most basal attachment points of the aortic valve leaflets form a virtual plane termed, the annulus, which is in direct continuity to the left ventricle. The aortic root extends from the annulus through the sinus of Valsalva, which is the widest point, till it narrows again at the sinotubular junction where it continues as the ascending aorta 13, 14. Anatomy and geometry of the aortic root are critical to the success of the TAVR procedure with respect to valve selection and sizing. The aortic annulus is noncircular, often oval in shape. This is the reason two-dimensional (2D) imaging techniques as 2D echocardiography have not accurately assessed the annular dimensions in the past 15 17, and three- dimensional (3D) CT is today the method of choice for aortic root assessment. PRE-TAVR CT SCAN The CT scan acquisition technique and data collection play a crucial role in subsequent multiplanar reconstructions, maximum intensity projections and volume rendering. An ECG-gated multiphase imaging of the aortic root is acquired for motionfree imaging to enable 3D assessment of the same. The CT data set is carefully analysed to determine the best phase for aortic valve assessment given that geometric distortions occur through the cardiac cycle. The choice of phase is most often the systolic phase when the annulus has maximal dimensions 18. Suitable image manipulation is essential to achieve the true orthogonal plane of the aortic annulus, aortic root and coronary artery assessment. The procedure also entails acquisition and evaluation of the entire aorta including the proximal supra-aortic vessels and iliofemoral axis. Since patients of severe aortic stenosis often have multiple comorbidities and compromised renal functions, care must be taken to minimize contrast media volumes. 367

2 368 SECTION V Cardiac Imaging CT EVALUATION OF THE AORTIC ANNULUS The aim of 3D CT image manipulation is to achieve the true annular plane, which lies immediately below the hinge point of the aortic valve cusps in the systolic phase when the dimensions of the annulus are maximal. Measurements then include the longaxis (maximum) and the short-axis (minimum) diameter, with calculation of the mean diameter. Annulus planimetry enables measurement of the perimeter and area of the annulus, with calculation of the derived diameters basis formula for perimeter and area of a circle, respectively 19 ( Fig A and B). CT measurements have been shown to be highly reproducible 20, 21. On account of the noncircular configuration of the annulus, with an often oval shape, reliance on 2D transthoracic echocardiography underestimates the annular dimensions 22. Hence, 3D imaging tools such as CT, magnetic resonance (MR) imaging and 3D Transesophageal Echocardiography (TEE) are currently the preferred modalities for assessing the annulus and aortic root with CT being the method of choice 15, 23. CT provides the most comprehensive and accurate data for valve sizing and prediction of complications. While valve under sizing may result in paravalvular regurgitation 24, 25, 26, oversizing is associated with contained aortic root rupture in particular, in the presence of landing zone calcification 27, 28. Integration of CT into Transcatheter Heart Valve (THV) sizing has shown to reduce greater than mild paravalvular regurgitation from 12.8% to 5.3% 25. CT EVALUATION OF VALVULAR AND LANDING ZONE CALCIUM BURDEN CT can accurately assess the site, extent and quantification of calcium burden. Valvular calcifications ( Fig ) may be symmetric or asymmetric and can be assessed either visually or quantified as coronary calcium scoring. Asymmetric calcium distribution may be at leaflet edges, commissures or attachment sites of the cusps on the aortic valve and landing zone. The role of calcification is growing as a predictor of paravalvular regurgitation 29. Severe calcification may impair complete apposition of the valve to the native commissures, resulting in paravalvular regurgitation and this may require postdilation to mitigate the regurgitation. Calcification of commissures and of attachment sites may be a stronger predictor of paravalvular regurgitation than calcification of the leaflet edges 30. CT EVALUATION OF THE AORTIC ROOT A Additional data points integral to patient and valve selection include height of the coronary ostia ( Fig A and B) and sinus of Valsalva to the annulus plane ( Fig A C), leaflet length, width of the sinus of Valsalva ( Fig ), sinotubular junction ( Fig ), LVOT ( Fig ) and ascending aorta ( Fig ). B Figure (A and B) Annulus measurements by diameter and planimetry. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.) Figure Eccentric calcification of the aortic valve leaflet. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.)

3 Chapter 44 CT for Pre-TAVR Evaluation 369 Right Left A B Figure Measurement of the left (A) and right (B) coronary ostial height from the annular plane. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.) LCC A RCC B NCC C Figure Measurement of the sinus height for the left (A), right (B) and noncoronary sinus (C) from the annular plane. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.) Figure Dimensions of the coronary sinus of Valsalva. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.) Figure Sinotubular junction. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.)

4 370 SECTION V Cardiac Imaging Figure Left ventricular outflow tract dimensions. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.) Figure Maximum dimension of the ascending aorta. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.) These measurements are essential to valve sizing, selection and deployment to prevent potential complications as coronary occlusion and root rupture 31. In TAVR, the native leaflets and calcifications are displaced by the THV, with ensuing risk of coronary occlusion. A detailed assessment of the distance of the coronary ostia from the aortic annular plane on multiplanar reformations is thus essential. The risk of coronary occlusion is higher in low-origin coronary arteries with small sinus and long leaflets with bulky calcifications. Although there are no definite exclusion criteria, mm distance cut-off range between the coronary ostia and the leaflet insertion has been proposed 31. Other potential causes of coronary occlusion include length of the aortic valve cusps and extent of calcification. Heavily and diffusely calcified cusps are at higher risk than isolated commissural insertion calcification. The length of the left coronary cusp and its relationship to the height of the left main coronary ostium can also be assessed by CT. Depending on the valve selected and its size, THV manufacturer s may have specifications for minimum width and height of the sinus of Valsalva, and diameter of the proximal ascending aorta all of which can be derived from the CT data set. CT-BASED PREDICTION OF ANGIOGRAPHIC PROJECTION ANGLES FOR TAVR Inaccurate orientation and device positioning raise the risk of procedural complications such as stent embolization 31, 32. To ensure accurate device orientation during implantation with coaxial deployment along the centreline of the aorta, fluoroscopic guidance needs to be oriented orthogonal to the native valve plane. CT acquisition data can enable estimation of the desired angulation, either manually or by specifically designed software applications thus reducing procedural time, radiation doses and contrast medium volume. ACCESS ROUTE ASSESSMENT The iliofemoral route is the most frequent access route for device delivery in TAVR. CT is also a helpful adjunct for the evaluation of other access routes. CTbased 3D volume-rendered image display, curved multiplanar reformats ( Figs 44-9 and ) and maximum intensity projections are used for evaluation of the aorta and iliofemoral ( Figs and ) arteries. CT enables detailed evaluation of the aorta and iliofemoral arteries for possible risk factors ( Fig ) for minor and major vascular complications and potential contraindications. CT can depict atheroma or bulky calcifications in the aortic arch, which might add to the risk of stroke. Analysis includes detailed minimum true axial Figure Curved planar reformat of the thoracic aorta. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.)

5 Chapter 44 CT for Pre-TAVR Evaluation 371 Figure Curved planar reformat of the abdominal aorta. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.) Figure Curved planar reformat of the right iliofemoral artery. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.) Figure Curved planar reformat of the left iliofemoral artery. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.) Figure Maximum intensity projection and volume-rendered image of the aorta. (Source: Department of Radiology and Imaging, Fortis Escorts Heart Institute, New Delhi.)

6 372 SECTION V Cardiac Imaging luminal diameter measurement with evaluation of atherosclerosis, overall plaque burden, moderate to severe vascular calcification in particular circumferential or horseshoe type, and vessel tortuosity 5, 33, 34. Usage of CT in access route evaluation has reduced major vascular complications from 8% to 1% and of minor vascular complications from 24% to 8% 33. Based on these features, if applicable, the report may include a recommendation on the favourable side of iliofemoral access. While isolated tortuosity is not a contraindication to TAVR, small and borderline luminal diameter with circumferential or horseshoe calcifications may hamper arterial expandability increasing the risk of vascular complications such as dissection or perforation. In the event of contraindications to iliofemoral access, subclavian or transapical approach may be selected. Similar detailed anatomical evaluation of the subclavian artery is possible on CT, as is preprocedural localization of the left ventricular apex, for a transapical puncture. CONCOMITANT CARDIAC DISEASE Patients of severe aortic stenosis may have left ventricular dysfunction with left ventricular hypertrophy, often involving the basal septum. Septal bulge at the left ventricular outflow tract can be challenging for valve deployment. The angle between the aorta and the left ventricle is also important in planning the TAVR procedure as is pre-existing aortic regurgitation 35. CONCLUSION TAVR is fast evolving with growing numbers and data demonstrating its safety and outcomes. Given that patient selection, valve sizing and access route planning are critical components to the success of the procedure, CT provides a comprehensive, excellent and accurate assessment of all components of pre-tavr patient assessment including prediction of the fluoroscopic angle thus minimizing procedural time, radiation and contrast volume. Advanced CT imaging, hybrid cath labs and improvements in devices will continue to improve outcomes and reduce potential complications, thus enhancing the role of TAVR in symptomatic aortic stenosis. REFERENCES 1. Iung, B., Baron, G., Butchart, E. G., Delahaye, F., Gohlke- Bärwolf, C., Levang, O. W., et al. ( 2003 ). A prospective survey of patients with valvular heart disease in Europe: The Euro Heart Survey on Valvular Heart Disease. European Heart Journal, 24 (13), Nkomo, V. T., Gardin, J. M., Skelton, T. N., Gottdiener J. S., Scott, C. G., & Enriquez-Sarano, M. ( 2006 ). Burden of valvular heart diseases: A population-based study. 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