|Year : 2020 | Volume
| Issue : 2 | Page : 55-61
The central role of computed tomography imaging in transcatheter aortic valve replacement planning
Yu-Xin He1, Hasan Jilaihawi2
1 Department of Cardiology, The Second Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, China
2 Heart Valve Center, Nyu Langone Health, New York, USA
|Date of Submission||01-Jun-2020|
|Date of Acceptance||24-Jun-2020|
|Date of Web Publication||30-Jun-2020|
Suite 9V, 530 1st Avenue, New York 10016, New York
Source of Support: None, Conflict of Interest: None
Transcatheter aortic valve replacement (TAVR) has developed rapidly in recent years as an alternative to surgical aortic valve replacement for patients with aortic valve diseases. As a minimally invasive approach, procedure success and good outcomes for patients largely depend on the procedure planning before TAVR. Moreover, imaging plays a vital role in contemporary TAVR planning. Among different types of imaging modalities, computed tomography (CT) is central. This article provides a systematic review of the role of CT in TAVR planning, including anatomic measurement and TAVR risk evaluation.
Keywords: Tomography, X-Ray Computed, heart valve prosthesis Implantation, transcatheter aortic valve replacement
|How to cite this article:|
He YX, Jilaihawi H. The central role of computed tomography imaging in transcatheter aortic valve replacement planning. Cardiol Plus 2020;5:55-61
| Introduction|| |
Huge advances in transcatheter aortic valve replacement (TAVR) have been made since the first TAVR procedure performed in 2002. TAVR is recommended as an alternative to surgical aortic valve replacement (SAVR) for patients with elevated surgical risk., Meanwhile, randomized clinical trials regarding low-risk patients also proved the safety and efficacy of TAVR compared with SAVR., Multiple types and generations of transcatheter heart valve (THV) have been developed, and the outcomes of patients undergoing TAVR have been improved.,
Imaging has always been one of the key roles in the era of TAVR. In the preliminary stage of TAVR, echocardiography was the first choice for annular sizing and to monitor periprocedural complications, and computed tomography (CT) was restricted to use for access approach evaluation. However, recently, CT has become the primary imaging multimodality for THV sizing and patient selection overall.
This article aims to provide an overview of the central role of CT imaging for TAVR planning [Figure 1].
|Figure 1: The aortic valvular complex and avoidance of procedural complications. (a) Summary of the measurements for aortic valvular complex; (b) TAVR risk assessment should include evaluation of anatomy, understanding of the technique and device; (c) Procedural complications related to TAVR. SE: Self-expanding, STJ: Sinotubular junction, BE: Balloon-expandable, SOV: Sinuses of Valsalva, LVOT: Left ventricular outflow tract, TAVR: Transcatheter aortic valve replacement, PVL: Paravalvular leakage|
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| Aortoiliofemoral Evaluation|| |
The aortic arch should be assessed regarding the calcification and atheroma for the evaluation of the risk of stroke. The ascending aorta should be measured at a plane with maximum diameter for the presence of aortopathy. The threshold value of the ascending aorta diameter as a contraindication of TAVR remains controversial. The choice between TAVR and surgery should be made by the heart team based on a comprehensive evaluation and is particularly relevant in the setting of aortopathy with bicuspid aortic valve disease.
The iliofemoral arteries should be carefully assessed for suitability because transfemoral access is the first choice. If a transfemoral approach is not suitable, alternative access approaches should be considered. Traditionally, these were thoracic approaches such as transapical and direct transaortic; however, these yielded worse outcomes than transfemoral TAVR and in some studies worse than open surgery., Recent momentum has grown on extrathoracic alternative approaches, including transcaval, transcarotid, and transsubclavian, suggesting favorable outcomes.,,, Alternatively, peripheral intervention such as atherectomy can be considered to facilitate a transfemoral approach.
The iliofemoral arteries can easily be evaluated on CT either manually with double oblique views or semi-automatically with dedicated software; CT allows not only an accurate assessment of dimension but also other factors such as calcification and tortuosity. The minimum luminal diameters should be measured for each segment from a potential access site in the femoral artery to the external iliac artery to the common iliac artery. Larger sheath/femoral artery diameter ratio (or sheath femoral rtery Ratio) is associated with higher vascular complications, and a threshold of 1.12 can be used for new generation devices.
In addition to minimal luminal diameter, calcium severity should be assessed as well. Circumferential or near-circumferential and protruding calcification should be described and reported, particularly in areas of bends and bifurcations, as these may be potential sites of trauma when the delivery system passes through.
Tortuosity can be assessed using a volume-rendered display. Tortuous segments may be more challenging, particularly in the setting of calcification. Moreover, aneurysm, dissection, stent, and occlusion should also be reported to the operators. The access sites should be carefully assessed and selected. High bifurcations and anterior calcium of the femoral artery, which may impact the efficacy of percutaneous closure, should be noted.
| Annular Assessment And Root Dimensions|| |
The aortic root consists of the aortic annulus, sinuses of Valsalva (SOV), valvular leaflets, coronary artery ostia, and sinotubular junction (STJ).
The aortic annulus described by surgeons is a crown-like ring, which is defined by the hinges of the leaflets. However, the concept of the annulus in the TAVR era is actually a virtual ring formed by joining the basal attachment points of the leaflets. In other words, the annulus-assessed pre-TAVR is the basal plane of surgical aortic annulus. It is critical to fully understand the definition of annulus and its adjacent structures before performing measurements.
The maximum diameter, minimum diameter, perimeter, and area are all essential measurements of annulus for annulus sizing. The measurement can be performed manually or by semi-automatically dedicated software. During the measurement, the contour created should be smooth. When annular calcification is presented, the contour of the calcified part should be harmonious with the rest. The feasible method is to draw the contour through the calcium as the calcification is absent. When the calcium is protruding, the distance from the inner side of the calcium to the contralateral point of the annulus should be measured to understand the space available for the device to diminish the risk of annular injury.
The aortic annulus is known to have dynamic changes during the cardiac cycle. In general, the aortic annulus is more circular in systole; area and perimeter are significantly larger in systole compared with diastole. As such, systolic measurements are mostly used in valve sizing algorithms, because diastolic measurements may result in undersizing of the vale. Ideally, the whole cardiac cycle should be reviewed to make sure that is the case.
SOV diameter should be measured cusp to commissure in parallel to the annular plane. Meanwhile, the height of coronary artery ostium should be measured perpendicularly from the annular plane to the lower edge of the coronary artery ostium. Coronary occlusion is a rare but highly fatal complication of TAVR, and coronary ostial height <12 mm and SOV mean diameter <30 mm are reported to be the risk factors of coronary occlusion. That is why the SOV and coronary artery height is crucial during the aortic root assessment. However, there is no absolute threshold of SOV and coronary artery height, at which the TAVR should be considered contraindicated. All aortic root dimensions should be combined to evaluate the risk of coronary occlusion, as well as device type and size.
Depending on aortic root dimensions, the STJ may come into contact with the implanted device. Both STJ diameter and height should be measured to evaluate the risk of STJ injury and occlusion of blood flow into SOV, which could happen in the case of small STJ with low STJ height. The STJ height should be measured perpendicularly from the annular plane to the lowest edge of the STJ. At least the maximum and minimum diameter of STJ should be measured in a cross-sectional plane.
| Aortic Valve Morphological Assessment And Bicuspid Aortic Valve Disease|| |
Bicuspid aortic valve is one of the most common congenital heart diseases, which has an estimated prevalence between 0.5% and 2%. However, most of the randomized controlled trials of TAVR have excluded bicuspid aortic valve disease. Accumulated data from observational studies have shown favorable results in this setting, especially with newer generation devices., With the trend of indication of TAVR expanding to low risk and younger population, more cases of bicuspid aortic stenosis (AS) are expected to present for TAVR.
Valve morphology should always be evaluated. Although a certain amount of aortic valve morphology classifications were reported, the most widely adopted one is the Sievers classification. The Sievers classification is based on the number of raphes and divides bicuspid valve morphology into type 0, type 1, and type 2. However, this classification was primarily created from a surgical perspective. In the TAVR era, a novel bicuspid classification has been proposed. in this paradigm, bicuspid aortic valve is divided into three types: tricommissural bicuspid, classical bicommissural raphe type, and bicommissural without raphe. It should be acknowledged that there is a spectrum of morphologies and hence some similarities and overlap of characteristics between one category and another. Indeed, tricommissural may appear “tricuspid like,” which has led some to label such cases as “acquired” or “functional.” It is more likely that there is a spectrum of commissural formation and leaflet separation, and the “tricommissural” category represents the presence of a raphe with a more “tricuspid-” like appearance. Although such distinctions may be debated, in practice, the “tricommissural” or tricuspid-like bicuspid morphology results in favorable outcomes with TAVR.
Besides valve morphology, other associated findings should also be reported, such as valve calcification, raphe calcification and position, aortopathy, and coronary artery variants. Valve calcification should be quantified and graded. Symmetricity and position of the calcification should also be reported. Raphe should be evaluated for this position, calcification, and its impact on device deformation, aortic root injury, and paravalvular leakage. Aortic dilation, aneurysms, and coarctation should be carefully evaluated for ascending aorta, arch, and descending aorta. Coronary artery variants can be seen in bicuspid cases and should be reported.
| Annular Sizing|| |
CT is generally recognized as the gold standard for annular sizing and device selection. Due to the different features of devices, device size selection is mainly based on the annular area for balloon-expandable devices and annular perimeter for self-expanding valve. Usually, a device larger than the annulus will be deployed for anchoring and sealing, which is known as oversizing. Oversizing percentage can be calculated as: (Device size/annular size − 1) × 100%. It should be aware that the calculated oversizing percentage is different when using the area or perimeter to do the calculation. Furthermore, the optimal oversizing percentage is various among different types of devices with different geometrical and mechanical properties. Too much oversizing may increase the risk of annular rupture, coronary ostial obstruction, pacemaker implantation, suboptimal stent expansion, and impaired leaflet functioning, while too little oversizing may result in more paravalvular leakage and valve malpositioning. Specific sizing recommendations are available from the manufacturers, but the final sizing decision should be made by the heart team for every individual. Notably for the balloon expandable Sapien 3 device (Edwards Lifesciences, Irvine, CA, USA), relative undersizing may be performed with addition of volume to the balloon. In patients with borderline annulus dimensions from CT, multimodality of imaging may be considered, with additional techniques such as aortography with balloon sizing.,
Device size selection is still challenging for bicuspid AS patients. Excessive or asymmetric leaflet or raphe calcification may limit device expansion or apposition and contribute to complications such as paravalvular leak or aortic root injury. A variety of methodologies have been proposed to address this problem, such as intercommissural distance (ICD) measurement, CT-based supra-annular measurement, and balloon-based supra-annular assessment. ICD was measured at 4 mm above the aortic annulus, which was reported helpful for sizing for bicuspid cases with taper landing zone (annular dimension larger than ICD). However, this algorithm remains debatable. Xiong et al. found that major interference between the implanted prosthesis and landing zone occurred at a level above the annulus in bicuspid cases, which may explain the feasibility of implanting a smaller valve than that suggested by annular sizing, but CT-based supra-annular measurement is still in development. Balloon-based supra-annular assessment, also known as “Hangzhou solution,” uses a sequence of balloon aortic valvuloplasty to assess supra-annular structure, which is presented by the “waist sign” on the balloon. No new permanent pacemaker (PPM) implantation and no moderate or severe paravalvular leakage occurred using this method in 12 bicuspid cases. Further studies are needed to test these methodologies. Despite these novel approaches, questions remain about the optimal sizing strategy for bicuspid AS. Computer simulation based on CT imaging has potential advantages in solving this problem.
| Angle Of Intraprocedural Fluoroscopic Projection|| |
TAVR should be performed under the fluoroscopic angulations which make the fluoroscopic detector parallel to the annular plane. CT is useful for optimal C-arm angulation planning when the patient is positioned similarly during TAVR procedure and CT scan. This technique can reduce radiation exposure, contrast usage, and procedural time. Optimal C-arm angulations consist of LAO or RAO degrees and the corresponding cranial or caudal degrees. Many operators employ a projection with the right coronary cusp sitting in the middle of the left and non-coronary cusp, while others prefer the non-coronary cusp to be isolated at the left of the screen; some of these approaches are device specific and are in the process of refinement and optimization.
During valve deployment, one technique, typically employed for self-expanding TAVR, the “double S curve” identifies the intersection of computer-calculated S-curves: (i) annulus S curve from pre-procedural CT and (ii) device delivery system S-curve from 2 orthogonal intraprocedural fluoroscopic projections where the catheter is seen to be coaxial; this intersection provides a specific coplanar view of both the aortic annulus and the device. Recently, an angulation left and right cusps overlapping (cusp-overlap technique), which isolates the non-coronary cusp, has been proposed for repositionable self-expanding TAVR. Both of these approaches may improve optimization of device depth for self-expanding TAVR. However, identified projections resulting in cranial or caudal angulation >25° may be difficult to achieve, given the physical restraints of the C-arms.
| Difficult Scenarios|| |
Severe valve/left ventricular outflow tract calcification
The majority of the TAVR patients are with degenerative calcified AS. The severity of valve calcification can be qualitatively graded as no calcification, mildly calcified, moderately calcified, and severely calcified. The degree of calcification can be defined with a non-contrast CTA with the Agatston score and expressed in Agatston Units (AU)., It can also be quantitatively measured from a contrast scan, such as a cutoff for detection of 850 Hounsfield Unit (HU). In low-gradient AS, the quantification of valve calcification may contribute to the assessment of AS severity., Severe valve calcification is a significant predictor of increased risk of paravalvular leakage., The left ventricular outflow tract (LVOT) is also one of the components of the landing zone. LVOT calcification is commonly seen in AS patients. LVOT calcification can be qualitatively graded as none, mild, moderate, and severe based on the extent of distribution, depth into the LVOT and protrusion into the chamber. Severe LVOT calcification is reported to be associated with increased risk of paravalvular leakage, annular rupture, and new pacemaker implantation.,,, Under these circumstances, overaggressive oversizing should be avoided, and devices that are more compatible with the adjacent structure may be preferred, such as self-expanding or mechanically expanding valves with an outer skirt.
A valve-in-valve (VIV) procedure is an alternative to reoperation for failed bioprosthetic aortic valves., THVs can be implanted into stented surgical valves, stentless surgical valves, and THVs. The procedural success rate and clinical outcomes of VIV are encouraging.,
CT still plays an important role in VIV procedure planning. For patients with certain medical records of bioprosthetic aortic valves, we can follow the recommendation of the VIV Aortic app to choose the THVs' size. If the information of the failed bioprosthetic aortic valve is not clear, inner valve area and area-derived diameter should be measured and be used to select the THV size. Vascular access should be evaluated as for TAVR in native aortic valves.
The VIVID registry reported that coronary obstruction following aortic VIV procedures occurred more frequently and is a life-threatening complication. Hence, another main goal of CT imaging pre-VIV is to evaluate the risk of coronary obstruction. Besides the measurement of coronary height and diameter of STJ, virtual THV to coronary (VTC) distance should be reported for VIV in stented bioprostheses. Using dedicated software, such as 3mensio, a virtual THV with defined diameter and height can be implanted into the failed bioprosthesis and should be carefully aligned with the failed bioprosthesis. Then, the distance measured between the edge of the simulated THV and the coronary orifice is VTC. A VTC distance <4 mm is identified as a high risk factor of coronary obstruction. For VIV in stentless bioprostheses (such as the Toronto valve) or stented bioprostheses that have the leaflets on the outside (such as the Mitroflow valve), assessing VTC is especially important as these designs confer additional risk. Novel techniques, such as BASILICA, which cuts bioprosthetic leaflets with electrocautery, may be beneficial in anatomies at high risk of coronary obstruction.
| Reducing Pacemaker With Information from Cta|| |
New PPM implantation is one of the most common complications of TAVR. Even though TAVR technology improved dramatically in recent years, the incidence of PPM implantation has remained relatively high. The pooled median PPM rate is 28% for the Medtronic CoreValve System and 6% for the Edwards SAPIEN valve. Higher risk of PPM is associated with deep implant depth, presence of preexistent right bundle branch block, shorter length of the membranous septum (MS), calcium distribution patterns, relatively larger device size, and male sex.,,, With the application of repositionable devices, a novel approach called MInimizing Depth According to the MS (MIDAS) can help to decrease the rate of PPM implantation. Before the TAVR procedure, the length of the MS is measured at the point between the annular level and vertex of the muscular septum perpendicularly to the annular plane. During the procedure, operators aim to implant the device at a depth of < MS length whenever possible. This approach decreases the rate of PPM implantation significantly from 9.7% to 3%. A broader application of the approach will be seen in daily clinical practice.
| Summary|| |
CT has become one of the most important parts of TAVR procedural planning and risk stratification. A comprehensive assessment of CT images consists of aortic root and vascular access measurements and evaluation of specific risk factors of the TAVR procedure. Advances may be made in future using artificial intelligence and machine learning, and to identify ways, the huge amount of information contained in this enormously detailed imaging technique may be used to, even further, improve outcomes for TAVR.
Financial support and sponsorship
Conflicts of interest
Dr. Jilaihawi is a consultant to Boston Scientific and Medtronic and has received grant/research support from Edwards Lifesciences, Medtronic, Abbott Vascular and HLT. Yuxin He does not have any conflicts of interest.
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