Multimodality Imaging for Transcatheter Aortic Valve Replacement
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The far right is a 3D volume-rendered image of the aortic-iliofemoral vasculature with bone removed. These images display the presence of significant arterial stenosis, areas of minimal luminal diameter, the presence and extent of arterial calcification and degree of vessel tortuosity. Presence of extensive calcified plaque, small arterial size, and significant tortuosity represent contraindication for transfemoral approach for TAVR.
The patient underwent an ultrasound-guided direct thrombin injection into the pseudoaneurysm with a good result and she subsequently had a successful TAVI through the left femoral artery. CT allows assessment of not only the lumen diameter but also the arterial wall with regard to plaque composition and severity of calcification. The 3D-rendered images display vessel tortuosity, information that assists the operator in safely advancing the device. In patients with either extensive lower-extremity peripheral vascular or aortic atherosclerotic disease Figure 8 , CT can measure the diameter of both subclavian arteries to determine whether they are suitable for deploying the device.
Curved multiplanar CT reformation and reconstruction using a centerline approach of the peripheral iliofemoral arteries and the entire aorta. Centerline image processing of the distal aorta and iliofemoral arteries allows measurement of the maximum and minimum arterial diameters at variouis levels as well as assessment of vascular dimensions, arterial calcification, and tortuosity of the vessels from the site of femoral puncture up to the aortic annulus.
This figure shows an example of a year-old male patient who was accepted for a transfemoral TAVI approach since he had adequate arterial sizes, paucity of calcification, and minimal vessel tortuosity. Coronal oblique CT projection of distal aortic arch and aneurysmal descending thoracic aorta in an year-old female.
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- Multimodality Noninvasive Imaging for Transcatheter Aortic Valve Implantation: A primer?
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CT provides a detailed evaluation of the aortic valve, the annulus, and its relationship to the coronary arteries. In this regard, AS severity can be evaluated by planimetry of the aortic valve orifice during systole Figure 9. Previous studies have shown a good correlation between anatomic assessments of AS by CT as compared to functional assessments by echocardiography.
This is important since recent studies suggest a higher rate of post-procedure balloon dilatation is required in patients with heavily calcified valves. These measurements are critical for choosing the proper size prosthesis. An undersized prosthesis can lead to significant aortic insufficiency as well as device migration.
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The smallest aortic valve area is identified by scrolling up and down through the dataset. The AVA is then measured by planimetry.
In this patient, planimetry reveals critical aortic stenosis with only a slit-like opening. The extent of leaflet calcification can also be quantified on either the contrast above or non-contrast CT study. The ascending thoracic aorta and its relation to the LVOT i. Root orientation is critical for precise positioning of the device along the centerline of the aorta and perpendicular to the valve plane. In addition, measurement of the aortic root, sinotubular junction, and sinus of Valsalva height are critical for proper positioning of the device and ensuring there is no infringement on the coronary ostia Figure As is routine for CT, the thoracic aorta can be assessed for aneurysmal dilatation, and the aortic arch and the great vessels can be visualized Figure In this regard, CT is a critical imaging tool when choosing the proper patient for a TAVI procedure, selecting the safest device delivery route, and properly sizing the device to the individual patient.
Measurement of LVOT-ARA is performed using a coronal oblique projection and is defined as the angle between the axis of the first portion of the ascending aorta 1 corresponding to the upper part of the bioprosthesis, and the LVOT axis 2 corresponding the distal portion or landing zone of the prosthesis. This measurement is critical for determining whether the prosthesis will properly sit within the aorta. Standard diameter measurements in the aortic root are made in systole at the level of the annulus 1 , sinus of Valsalva 2 , and sinotubular junction STJ 3.
Additional aortic root measurements include the maximum ascending thoracic aorta diameter 4 and the sinus Valsalva height defined as the distance between the annulus plane and the STJ 5. A Assessment of ascending thoracic aorta, proximal aortic arch, and great vessel origins using a coronal oblique projection. Note the marked thickening and calcification of the aortic valves and relative absence of significant aortic atherosclerotic plaque.
The distance between the aortic annulus and the left main coronary artery arrow can be accurately measured by CT so as to ensure that the prosthesis does not impede coronary blood flow once inserted. The great vessels arise normally off the aortic arch.
A practical guide to multimodality imaging of transcatheter aortic valve replacement.
Note the absence of significant aortic atherosclerotic plaque. While the full potential of cardiac magnetic resonance CMR in the preoperative and postoperative evaluation of TAVI is still being realized, there are a number of areas where it is showing great promise and in some cases is considered a mandatory imaging modality necessary for a structural heart program. Specifically, in individuals with technically limited echocardiographic images, CMR offers the ability to obtain an independent measure of peak aortic valve velocity using a phase contrast technique.
CMR is a superb technique that can directly quantify mitral insufficiency severity without concern of acoustic window limitations from body habitus or coexisting lung disease. In addition to identifying the severity of valvular disease, CMR is the optimal modality to identify how the valve lesion s affects LV performance.
Cine CMR techniques can be used to obtained highly reliable measures of LV regional function, dimensions, volumes, mass, and ejection fraction Figure CMR can be used to obtain highly reproducible measures of LV size, volumes, and function. Cine angiography remains the primary imaging modality during a TAVI procedure. However, TEE plays a very important role. It can provide the baseline evaluation of aortic valve function and aortic annular and annular root anatomy, and it can evaluate for significant concomitant valve dysfunction MR or AR.
Multimodality Noninvasive Imaging for Transcatheter Aortic Valve Implantation: A primer
TEE also provides a baseline assessment of LV segmental wall motion and pericardial fluid collections that may be important should complications arise. Deep transgastric views are often employed to optimize the Doppler assessment of LVOT stroke volume and aortic valve gradient. A midesophageal probe position, typically at to multi-plane degrees, permits a long-axis view of the left ventricle, left atrium, and LVOT i.
At this view, the aortic annulus can be accurately measured as described above for transthoracic imaging. In addition, the proximal ascending aorta can usually be well delineated and the coronary sinuses easily identified. Often, a TEE within the operating room or hybrid OR-cath lab can be used to provide the required measurements of sinotubular junction diameter, sinotubular height from aortic annulus, and sinus of Valsalva diameter.
The distance from the aortic annulus to the sinotubular junction is an important measure, particularly with the Edwards SAPIEN valve, to ensure that the length of the longest aortic cusp is less than the distance from the aortic annulus to the sinotubular junction of the ascending aorta.
This initial view is also very helpful to assess the degree of aortic cusp and annular calcification as well as to delineate whether the calcification is symmetrically distributed throughout the valve circumference. Following this study of baseline morphology and function, the emphasis then turns to an evaluation of catheter and device positioning.
As the interventional team places a pig-tail catheter into the noncoronary cusp anatomically, the lowest cusp , they may ask the echocardiographer to confirm this catheter positioning. This can be accomplished by a 2D multi-planer assessment of the aortic valve in short-axis and long-axis, but it can be more quickly assessed using the x-plane bi-plane imaging feature now available when using a Matrix 3D TEE probe.
When the aortic valve cusps and at least a portion of the fluid-filled catheter can be identified in simultaneous short- and long-axis views, then catheter position can be determined with confidence Figure Transesophageal echocardiogram TEE can be used to assist with localization of catheters and devices. In this example, a pigtail catheter is clearly depicted blue arrow within the noncoronary cusp NCC of the aortic root. This form of biplane imaging can easily be created using the 3D matrix TEE probe. Currently, all patients undergo balloon dilatation of the native aortic valve immediately prior to TAVI.
This critical step is not dependent on TEE guidance; however, an assessment of aortic valve regurgitation severity is often required immediately following balloon dilatation Figure The critical step of deploying the stent-valve is often performed without real-time TEE imaging. In fact, in a minority of cases the TEE probe must be partially withdrawn because it may obstruct the cine angiographic views of the aortic valve.
Transesophageal echocardiogram with color Doppler is used to compare severity of aortic regurgitation before A and after B balloon valvuloplasty of the aortic valve. The potential complications that can be encountered during a TAVI procedure have been well described.
Background
Transgastric TEE view clearly depicts a large collection of blood within the pericardial space. In this case, intraprocedural pericardial tamponade was quickly recognized allowing prompt surgical intervention. The color Doppler jet of a paravalvular leak is often best appreciated from the deep transgastric TEE view Figure This view usually permits the best axial alignment for the quantitative Doppler measure of regurgitant pressure half-time.
The midesophageal views provide the cross-sectional and long-axis views to facilitate identification of the site and extent of paravalvular leak. The vena contracta diameter and area can be assessed using 2D and 3D color Doppler application, respectively Figure Doppler echocardiography is used to quantify paravalvular regurgitation severity. A deep transgastric TEE view with color Doppler is used to identify the site of regurgitation relative to the prosthetic aortic leaflets identified in red.
Spectral Doppler exam permits quantitative and quantitative assessment of the regurgitant jet shown with perimeter traced for pressure half-time determination.
Three-dimensional 3D TEE imaging for the characterization of paravalular flow. The vena contracta narrowest portion the regurgitant jet is shown in simultaneous short left and long axis center views. The extent of paravalular flow is appreciated using volume-rendered 3D imaging right. Although rarely required, CT can be used to gauge the correct anatomic position of a catheter-deployed valve. Because of its excellent special resolution, CT can accurately indicate the depth of implant within the LVOT and can be used to assess stent shape.
The latter may be particularly useful when there is concern about complete or partial stent deployment Figure Coronal oblique CT views of aortic root left and three-dimensional 3D volume-rendered image right of the ascending aorta in an male patient 2 weeks after transcatheter aortic valve replacement TAVR with a Medtronic CoreValve. The CT was performed due to the suggestion of suspected paravalvular aortic regurgitation seen on a surveillance echo. Note the adequate deployment and positioning of the prosthesis, the absence of in-stent stenosis, and the integrity of the valve stent struts.
Care must be taken to utilize optimized imaging sequences to minimize metallic artifact from the implanted Core valve. In general, standard steady-state free precession SSFP cine sequences will yield greater artifact, but use of gradient recalled echo GRE based sequences with short echo times can help to reduce image artifact Figure Additionally, GRE-based phase contrast CMR techniques allow quantification of postprocedural aortic insufficiency volume by measuring aortic forward and reverse flow in the aortic root.
While this requires further study, CMR may become the optimal method for assessing postprocedural aortic insufficiency as it is independent of jet morphology, unlike most echocardiographic techniques. Cardiac magnetic resonance uses phase contrast techniques to derive regurgitant flow by comparing total forward flow and reverse flow within the ascending aorta. An important consideration is the performance of CMR in patients with implanted cardiac devices.
A practical guide to multimodality imaging of transcatheter aortic valve replacement.
Since a significant proportion of patients post-TAVI will require permanent pacemaker placement, the use of CMR would seem to be limited in this population. As TAVI technology continues to evolve, so do the imaging modalities that support its use. Today the use of echocardiography TTE and TEE is firmly established for both patient selection and live intraprocedural imaging guidance Figure CT has also become a critical imaging component for patient selection. Not only is CT required to assess the peripheral arterial vasculature for catheter access and navigation, but CT has become increasingly important to assess the aortic root geometry and orientation to the LVOT — relationships that often cannot be assessed by echocardiography.
Although CMR has a more limited role today, it has already been shown to accurately assess AS severity, aortic root geometry, and peripheral vascular anatomy and function. It is likely that CMR will emerge as an increasingly important modality for the functional assessment of catheter-deployed valves over the next few years.