Imaging in Transcatheter Aortic Valve Replacement (TAVR): role of the radiologist
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- Litmanovich, D.E., Ghersin, E., Burke, D.A. et al. Insights Imaging (2014) 5: 123. doi:10.1007/s13244-013-0301-5
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Transcatheter aortic valve replacement (TAVR) is a novel technique developed in the last decade to treat severe aortic stenosis in patients who are non-surgical candidates because of multiple comorbidities.
Since the technique is performed using a transvascular approach, pre-procedural assessment of the aortic valve apparatus, ascending aorta and vascular access is of paramount importance for both appropriate patient selection and correct device selection. This assessment is performed by a multi-disciplinary team with radiology being an integral and important part.
Among imaging modalities, there is growing scientific evidence supporting the crucial role of MDCT in the assessment of the aortic valve apparatus, suitability of the iliofemoral or alternative pathway, and determination of appropriate coaxial angles. MDCT also plays an important role in post-procedure imaging in the assessment of valve integrity and position.
This review outlines the principal aspects of TAVR, the multidisciplinary approach and utilisation of different imaging modalities, as well as a step-by-step approach to MDCT acquisition protocols, reconstruction techniques, pre-procedure measurements and post-procedure assessment.
• TAVR is a new technique to treat severe aortic stenosis in high-risk and nonsurgical candidates.
• MDCT assessment of the aortic annulus is important for appropriate patient and device selection.
• Multidisciplinary approach is required for patient selection, procedure planning and performance.
• MDCT is required for assessment of the aortic root, iliofemoral or alternative vascular pathway.
KeywordsTAVRAortic annulusMDCTECG-gatingMultiplanar reconstructions
Aortic stenosis is the most common valvular disorder in developed countries, affecting 2–5 % of the population over 75 years old [1, 2]. Once symptoms develop, the mortality increases rapidly to approximately 50 % within 2 years [3, 4]. The standard of care in treatment of patients with symptomatic severe aortic stenosis remains surgical aortic valve replacement (sAVR), which has low peri-operative mortality and improved clinical outcomes . However, high-risk patients often have significant comorbidities such as coronary heart disease, renal insufficiency, lung disease, cerebrovascular disease or frailty that limit their chance of survival [6, 7]. The most commonly used scoring tool to estimate 30-day mortality after cardiac surgery is the Society for Thoracic Surgery Predicted Risk of Mortality score (STS PROM) . For the patient population considered unsuitable or at high-risk for sAVR because of underlying comorbidities, the development of transcatheter aortic valve replacement (TAVR) has resulted in an alternative therapy for symptom relief and extension of life .
Aortic valve apparatus anatomy
Distally, the three semilunar leaflets are attached to the wall of the aortic sinus at the surgical annulus that corresponds to the sinotubular junction: the junction between the SOV and ascending aorta. Multiple manipulations of the raw imaging data are required to create an image that would exactly correspond to the aortic annulus (virtual basal ring). This virtual basal ring is often not orthogonal to the LVOT, and insertion of the right coronary cusp can often be inferior to the left and non-coronary cusps .
TAVR: review of the device and procedure
Comparison of the Edwards SAPIEN XT and Medtronic CoreValve Prostheses
Edwards SAPIEN XT
Ascending aorta fixation
Sheath internal diameter
Sheath external diameter
Minimal arterial diameter
Dilated ascending aorta
Yes, limited experience
Yes, limited experience
Longest published follow-up
3 %–8 %
14 %–40 %
SAPIEN transfemoral only
Randomised trial results
PARTNER A and B
Results anticipated 2013
Optimal aortic annulus, aortic root and peripheral vessel lumen dimensions for different types and sizes of THV
Aortic annulus diameter, mm
Distance aortic annulus to left main ostium, mm
Ascending aorta diameter, mm
Sinus of Valsalva height/width, mm
Peripheral vessels patent lumen diameter, mm
Valve height, mm
Edwards SAPIEN XT 23 mm
Edwards SAPIEN XT 26 mm
Edwards SAPIEN XT 29 mm
Medtronic CoreValve 23 mm
Medtronic CoreValve 26 mm
Medtronic CoreValve 29 mm
Medtronic CoreValve 31 mm
Available in 23-, 26-, 29- and 31-mm sizes, the Medtronic CoreValve device (Table 1) is a self-expanding prosthetic valve requiring an 18-French delivery catheter (a minimal iliofemoral luminal diameter of 6 mm). The longer CoreValve device frame requires sufficient ‘outflow’ room to be deployed but the device is available in a wider range of sizes allowing patients with particularly larger annular diameters to be treated. The radial force of the self-expanding frame holds the device in position at the level of the annulus. This same radial force is thought to increase the need for a permanent pacemaker because of compression on conduction tissues, an issue more prevalent with the CoreValve device.
- (1)The transapical approach is the second most common access route for TAVR using the Edwards SAPIEN valve (Fig. 6). It essentially affords a direct path to the aortic valve via surgical thoracotomy. While refinements in apical-purse string suturing techniques have reduced bleeding complications, this remains an invasive approach in elderly frail patients.
- (2)The direct aortic approach was originally developed for use with the CoreValve device as the larger stent frame precluded the transapical route. Although this still requires a partial sternotomy or right anterior thoracotomy, this is arguably less invasive than the incision required for the transapical route and is gaining favour (Fig. 7) .
The subclavian/axillary approach was originally developed for use with the CoreValve device; it can be used for the Edwards SAPIEN prosthesis. With CoreValve device this approach has been losing ground to direct aortic access in recent years. Disruption and dissection are more common with catheter manipulation in the subclavian artery, and caution is required in patients with previous coronary artery bypass grafting in which the left internal mammary artery was used [16, 17]. There are reports describing brachiocephalic access as an alternative approach .
Patient selection for TAVR suitability
Inclusion and exclusion criteria for TAVR
Critical aortic stenosis (with mean aortic valve area of <0.8 cm2)
Multiple comorbidities with 1-year mortality rate exceeding 20 %
Poor outcome with medical management
Non-surgical candidates with TAVR representing the only suitable alternative
Native aortic annular size appropriate for currently available THV size criteria
Exclusion criteria: unsuitable native anatomy
Lack of appropriate access to implant the device
Sinuses of Valsalva unable to accommodate prosthetic valve
Native aortic annular size inappropriate for currently available THV size criteria
Annular size not corresponding to any of the available THV devices would be a major exclusion criterion, followed by the lack of an appropriate vascular access route; for the CoreValve device, this criterion would be sinus of Valsalva dimensions too small or too large to accommodate the upper frame when positioned (Table 3).
Pre-TAVR assessment of aortic root anatomy with echocardiography and MDCT
Echocardiography is used to confirm severe aortic stenosis (aortic valve area of <0.8 cm2, peak velocity across valve >4 m/s and mean gradient >40 mmHg). It is also used to assess the degree of aortic incompetence and other valvular diseases, left ventricular systolic ejection fraction and diastolic dysfunction, as well as to estimate right-sided and pulmonary pressures. For TTE, the para-sternal long-axis view is used, and for TEE, the mid-esophageal long-axis view is used. The annulus is measured during early systole, with the valve leaflets open, from the hinge point of the right coronary leaflet to the hinge point of the non-coronary leaflet. THree-dimensional TEE may provide more accurate assessments of the aortic annulus compared to 2D-TEE, which may impact prosthesis size selection, although more research in this area is needed [24, 25]. Despite promising results and crucial information, 3D TEE is not yet a standard practice in pre-TAVR aortic annulus measurement assessment. TEE allows for real-time imaging during the TAVR procedure while assisting in device placement and positioning as well as assessing the degree of aortic regurgitation . Other complications such as pericardial tamponade, severe mitral regurgitation, aortic dissection, LV damage and embolisation of the implanted valve can also be diagnosed intra-procedurally with TEE. .
The Society of Cardiac Computed Tomography (SCCT) expert consensus document on MDCT imaging before TAVR suggests using at least a 64-detector scanner, with an obvious advantage of 128, 256 and 320 slice scanners with their shortened acquisition time and decreased contrast volume [11, 26] and high-pitch spiral dual-source CT angiography protocol . Imaging should be performed in the supine position and during suspended respiration . Since precision in the range of 1 mm is desirable, spatial resolution must be high with an acquisition protocol that obtains a reconstructed slice width of ≤1.0 mm throughout the entire imaging volume, particularly of the aortic valve, aortic root and ilio-femoral arteries.
Aortic annulus apparatus imaging
For precise aortic valve annular sizing and root evaluation, ECG-gated CT angiograms of the ascending aorta and heart are obtained with either prospective or retrospective ECG triggering. Aortic valve and root assessment during systole has been shown to be preferable to during diastole because of the larger annular size noted in systole as well as dynamic changes [28, 29]. Thus, with both prospective and retrospective gating, the data should be acquired during systole (usually 20–50 % phase of the cardiac cycle) with no radiation during the rest of the cycle in prospective ECG triggering and very aggressive dose modulation with retrospective gating, allowing substantial dose savings. No routine administration of β-blockers is used for scanning purposes because of concerns with underlying severe aortic stenosis. A high incidence of arrhythmia in TAVR candidates precludes routine use of prospective gating on a routine basis. As a result, the estimated radiation dose may be relatively high, but acceptable given the advanced age of the vast majority of TAVR candidates and the amount of information acquired. For younger patients, with stable and low heart rates, prospective axial acquisition during the systolic phase is suggested . Tube potential of 100 kV is suggested for patients weighing less than 90 kg or with a body mass index (BMI) less than 30, whereas a tube potential of 120 kV is usually indicated for patients weighing more than 90 kg (BMI >30). The lowest setting possible should be selected in keeping with acceptable image noise .
Suggested IV contrast injection regiments
Gated cardiac CTA
Gated CTA chest
Bolus injection, 20 ml IV contrast
ROI: proximal descending aorta
ROI: proximal descending aorta
Peak enhancement selected
Threshold of +200 HU used
Main injection: 4 ml/s, 70–80 ml
Rate: 4 ml/s, 120–140 ml
Non-gated CTA abdomen, pelvis
Non-gated CTA abdomen, pelvis
Additional 50 ml is injected
Done immediately after chest CTA
Injection timing based on bolus results
No additional IV contrast required
Aortic valve apparatus/ascending aorta measurements pertinent for TAVR
Aortic annulus (AA) (virtual basal ring)
• AA maximal diameter
• AA perpendicular minimal diameter
• AA average diameter
• AA cross-sectional area (CSA)
• AA circumference
• Comissure calcifications
• Aortic annulus calcifications
• Severely calcified cusp that might compromise coronary the artery ostia: yes/no
• Width at 40 mm from the annulus
• Position relative to the sternum
Sinuses of Valsalva
• Maximum diameter
• Sinotubular junction maximum diameter
• Distance from the aortic annular plane to the coronary artery ostia
Crucial factors for assessing the aortic and peripheral vascular access
Minimal patent arterial luminal short axis diameters
• Common iliac arteries
• External iliac arteries
• Common femoral arteries
• Subclavian arteries
• Innominate artery
• Mild, moderate, severe
• Kinking (tortuosity >90º)
• Mild, moderate, severe
• At bifurcations
• Arterial dissection
Arterial complex atheromas
The diameter of the ascending aorta: For CoreValue devices, orientation of the direction of the device flow occurs when the frame contacts the inner and outer curvature of the aorta. Ascending aortic dilatation of more than 43 mm (when measured 4 cm above the basal aortic annulus plane) precludes the use of the 29- and 31-mm CoreValve device. Dilatation of more than 40 mm precludes the use of the 23- and 26-mm CoreValve device. Size is of less concern for valves that are short in length and confined to the aortic annulus and sinus of Valsalva such as the Edwards SAPIEN valve .
Peripheral vascular pathway, aorta, chest wall and left heart imaging
Imaging of peripheral access can be obtained with non-gated spiral acquisition to minimise radiation. Peripheral access includes the iliofemoral, transsubclavian, transapical and direct aortic access routes. Transapical and direct aortic access (ascending aorta) routes are assessed with the ECG-gated portion of the MDCT. The common femoral artery should be included in the field of view when assessing femoral access. Assessment of the axillary and subclavian arteries should be obtained with the patient’s arms placed along his/her body to exclude pseudo narrowing of the imaged vasculature.
Comparative analysis of echocardiography vs. MDCT for aortic valve apparatus assessment
MDCT can provide better estimates of both the long- and short-axis diameter of the aortic annulus, surface area and perimeter measurements. Extensive work has been done to establish the role of MDCT in aortic annular sizing with Edward Sapiens transcatheter heart valves (THV) [11, 26, 46]. While exaggerated oversizing of the THV and excessive calcification of the native aortic valve can result in aortic annular rupture, some THV oversizing is required for both types of THV to prevent PAR. For the CoreValve device the recommended device/annulus oversizing is 15 % of the aortic basal ring perimeter. For the Edwards Sapiens valve, the recommended device/annulus oversizing is 15–25 % of the area and 7–12 % of the mean diameter. This degree of oversizing of the THV appears to provide the best risk-benefit ratio in terms of PAR reduction and conduction disorders [57, 58]. Undersizing of the THV can lead to increased PAR and greater likelihood of valve ‘pop-out’ or migration [10, 59].
Post-implantation imaging can be divided into immediately post-procedure and long-term follow-up. For the immediate assessment of valve position and haemodynamic status, including the gradients and effective valve area, the modality of choice is TEE, which can be done in the hybrid procedure room. Paravalvular and transvalvular regurgitation can also be estimated in real time, allowing appropriate steps to be taken to minimise complications and optimise device positioning [16, 54].
For long-term follow-up of TAVR ECG-gated MDCT is useful in diagnosing prosthesis misplacement by careful inspection of the exact positioning of the device in relation to the aortic annulus plane .
Significantly low implantation may result in severe paravalvular regurgitation, residual aortic valve stenosis, mitral valve insufficiency, conduction abnormalities and, in extreme cases, device drop into the left ventricular cavity. Considerably high implantation may result in paravalvular regurgitation, coronary flow obstruction and device embolisation into the thoracic aorta [10, 60].
Challenges and perspectives
TAVR is a novel procedure that provides unique opportunities for multimodality research comparing different imaging strategies and different THV devices. Given the exponential rise in the number of procedures being performed with varying local expertise, there is a need to standardise imaging protocols as part of pre- and post-procedure assessment of TAVR patients. Implementation of standardised protocols is crucial not only for precise assessment of the aortic valve apparatus and vascular access with different scanners in different institutions, but also for creating opportunities for further multicentre research. The SCCT expert consensus document devoted to this topic suggests potential algorithms and techniques for imaging before TAVR. Although there is growing evidence of the importance of MDCT in the pre-procedure assessment, one of the current challenges is to find the precise role of MDCT in valve assessment and sizing. Thus, developing a rational algorithm for imaging use both before and after implant should become a priority to avoid substantial redundancy in imaging. Another potential role for imaging is the assessment of outcome prediction, based on the diagnosis of both cardiac and non-cardiac comorbidities. If successful, this should allow for optimised patient selection for the currently costly procedure. Overall, current trends strongly suggest that imaging, in particular MDCT, will play an increasingly important role in all aspects related to the TAVR procedure.
During the last decade transcatheter aortic valve replacement has become widely used in many centres across the world with good clinical outcomes. The planning and performance of the procedure are based on a multidisciplinary team approach, with imaging proving crucial for pre-procedure planning. MDCT plays an important role in assessing the aortic valve, aortic root and ascending aorta, as well as the access root for the procedure (i.e., ilio-femoral, direct aortic or subclavian) utilising the combination of multiplanar reconstruction and 3D imaging. TAVR technology and equipment are rapidly advancing, with increased utilisation of advanced MDCT imaging contributing to continuing outcome improvement and broadening of procedure applications.
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