For both balloon expandable as well as self-expanding transcatheter heart valves (THV) accurate assessment of annular anatomy has become an essential aspect of the pre-procedural assessment of every patient and determines the selection of valve size.¹ Annular sizing as well as characterization of the peri-annular region (left ventricular outflow tract [LVOT] and proximal aortic root) may improve anticipation or avoidance of complications such as paravalvular regurgitation, left main occlusion, or annular rupture.^2-4 Multiple studies suggest that three-dimensional (3-D) transesophageal echocardiography (TEE) can accurately measure the annular aortic annulus^5-10 and improve prediction of complications.^11,12 This review will describe the two-dimensional (2-D) and 3-D echocardiographic methods for sizing the aortic annulus for TAVR.

It is the virtual aortic annular ring and, thus, the lowest hinge-points of the aortic cusps that define the annulus measured for THV sizing. Importantly, the aortic annulus is not circular but ovoid with the smallest dimension in the sagittal plane and the longest dimension in the coronal plane. Any single linear diameter measurement would necessarily fail to accurately characterize this ovoid shape. Measurement of orthogonal diameters, perimeter or area would better describe the annulus. Which measurement should be used has been debated in the setting of a dynamic annulus, the complex relationship between these measurements^13,14 depending on the geometry of the annulus and the changes in shape of the annulus with TAVR.  Complicating the debate about which measurement is most appropriate is the significant difference in final shape of the annulus and THV depending on the speicific valve implanted. Following deployment of the balloon expandable THV, 86% of valves are circular at the central coaptation point,¹⁵ whereas for the self-expanding valve, only 50% are circular.¹⁶

Two-Dimensional Echocardiography

Because of the anatomy of the aortic valve cusps, 2-D linear measurements of annular diameter are inherently limited. In a trileaflet aortic valve, the largest diameter of the annulus bisects a trigone on one side, and a cusp on the other side. Using the traditional long-axis (sagittal) imaging plane, imaging two cusps of the aortic valve centered in the aorta would actually be imaging a plane tangential to the largest diameter, causing significant underestimation of the true annular diameter. An on-axis image of the largest annular dimension can only typically image the right coronary cusp hinge-point; the corresponding annular measurement is in the fibrous trigone. When the cusps are open in systole measurement of the virtual annulus within the fibrous trigone is particularly difficult and care must be taken not to measure the calcium along the scalloped attachments in the aortic root. The most accurate method is to approximate the location of the annular plane by assuming the annulus is perpendicular to the long-axis of the aorta.

The nuances of imaging and measurement of the sagittal annular plane may explain studies showing an underestimation of annular dimensions on transthoracic (TTE) measurements compared to transesophageal (TEE)¹⁷ or multi-slice computed tomography (MSCT).¹⁸ Other studies have, however, shown that when comparing similar imaging planes, the correlation between modalities is nearly perfect.¹⁹ Biplane imaging of simultaneous long-axis and short-axis views utilizes 3-D technology and is available for both TTE and TEE imaging. The use of the biplane function of the 3-D probes allows accurate positioning of the longitudinal plane at the largest annular diameter (i.e., bisecting the short-axis plane). Although the sagittal annular diameter measurement is no longer used as the sole measure of annular size, it remains a useful rapid assessment of appropriateness of the chosen THV size; a 26 mm sagittal plane annular measurement would suggest a 29 mm balloon-expandable valve would be more appropriate than a 26 mm THV. Likewise, a 31 mm self-expanding valve would more appropriate than a 29 mm THV. 

Three-Dimensional Imaging

Similar to MSCT, 3-D reconstruction using 3-D echocardiography can be used to obtain accurate measurements of the annulus. The American Society of Echocardiography recommendations for 3-D acquisition and display²⁰ have created a uniform protocol for imaging all valvular structures. Optimizing the image on 2-D must first be performed either using a short-axis or long-axis view of aortic valve. A user-defined 3-D volume is then acquired using the narrowest possible depth with adjustment of lateral width and elevation width to obtain a volume containing the whole aortic root, the LVOT and part of the ascending aorta. Maximizing the volume rate (volumes per second) is important in order to sample peak systole for the measurement of annular size. 

Figure 1 After selecting a user-defined, single beat capture volume, select 3DQ to enter the quantification package.

Figure 2 The algorithm for direct planimetry of the short-axis annulus is outlined in the steps below.

TEE is typically used to measure the annulus on 3-D volumes.  Currently, two manual methods can be used. The first uses commercially available software packages that manipulate the 3-D volumes in a multi-planar modality (Figure 1). The short-axis (transverse) annular plane is obtained using the orthogonal long-axis views (sagittal and coronal) as a guide. The following algorithm can be used:

1. Open the 3-D quantification package which then creates a quad-screen with three orthogonal 2-D images (transverse, sagittal, and coronal).Volumes may be acquired from either a short-axis or long-axis view. 
2. A frame in mid-systole is selected (red arrow). 
3. Magnifying the image (red arrow) and adjusting gain may help reduce noise and increase the accuracy of this technique.  Align the sagittal (classic long-axis view) and coronal (orthogonal) planes (blue arrow) to bisect the long axis of the aorta. 
4. In the sagittal and coronal planes, align the transverse plane (in this instance, the green plane) at the level of the annulus by dragging the plane down from the aorta (red arrow) towards the ventricle until the most caudal attachments of the aortic valve leaflets (hinge points) are reached.
5. Fine adjustment of the transverse plane can be made in the two orthogonal long-axis planes (red arrow).
6. Rotating the two orthogonal planes within the transverse (short-axis) plane (blue arrow) is used to confirm the transverse plane is at the level of the hinge points of the aortic valve leaflets. Using this double-oblique technique, when the hinge-point of the cusp is not in the transverse plane, then fine adjustments in the sagittal (or coronal plane) can be made. It is important to remember that the virtual annular plane is defined by the three points of the lowest cusp hinge points. Thus, defining the annulus by meticulously following this “turn around rule” is the best way to ensure an accurate annular measurement. 
7. Once the virtual annulus has been identified, select “area” measurement (red arrow).
8. Direct planimetry of the area of the annulus is performed in the transverse plane. 

In the setting of severely calcified and immobile cusps, side-lobe artifact or bulky calcium on the cusps may extend into the plane of the annulus in systole, making an accurate measurement from this transverse plane challenging. Indeed, these limitations may be the reason that early studies using this method showed the average cross-sectional diameter by computed tomography (CT) offered a slightly higher degree of discrimination for predicting paravalvular regurgitation (area under the curve = 0.82, 95% CI: 0.73 to 0.90, p <0.0001) compared to the mean cross-sectional diameter by 3-D TEE (area under the curve = 0.68, 95% CI: 0.54 to 0.81, p = 0.036). ²¹

Figure 3 After selecting a user-defined, single beat capture volume, select the mitral valve package to enter the quantification package.

Figure 4 The algorithm for off-label use of the mitral quantification package is outline in the steps below.

The second method of measuring the annulus avoids direct planimetry of the transverse plane, thus eliminating hand-tracing errors and allowing the use of anatomic structures in the long-axis planes to identify the annulus. This method uses vendor-specific software originally designed for the mitral valve (Figure 3) and “tricks” the program into measuring the aortic annulus instead of the mitral annulus.⁵ The following algorithm can be used (Figure 4):

1. Open the MVQ analysis software, which then creates a quad-screen with three orthogonal 2-D images (transverse, sagittal, and coronal).
2. A frame in mid-systole is selected (red arrow). 
3. In the long-axis (sagittal) plane, the blue plane is then re-positioned (red arrow) to be parallel to the annular plane by rotating either counterclockwise (blue arrow).
4. In the MVQ package, re-positioning of the blue plane (which will now be the annular planes) automatically changes the top panels to the orthogonal long axis views.
5. Fine-tuning the position of the blue plane is then performed by using the orthogonal long-axis views. In step 5 of Figure 4, the blue plane is being moved up.
6. Steps 6 to 8 show the iterative positioning of the blue (transverse) plane at the virtual annulus. 
7. In order to confirm the correct position of the blue plane, the hinge-point of the cusps must be imaged. In the blue plane, the orthogonal red and green planes are rotated.
8. If the position of the blue plane does not meet the hinge-point of the cusp (blue arrow) in a long-axis view, it is re-positioned.
9. Once re-positioning of the long-axis plane is performed, further rotation (from the transverse plane) should be performed until the position of the annulus plane has been accurately determined.  
10. Once the level/location of the annulus has been confirmed, make sure the shape of the short-axis (blue, transverse plane) makes anatomic sense. 
11. Select the “end systolic” frame balloon (red arrow) and the “add/edit reference points” will now be highlighted. Select this task (blue arrow).
12. Select a pair of orthogonal points (two for each long-axis view) on the annulus; make sure each of these points not only is on the annulus, but also bisects the blue line. If the points do not respect the blue line, then non-planar annulus will be created. Note: the MVQ package automatically approximates the annulus using these four points. 
13. Using an eight-segment protocol, the annular plane is confirmed for each pair of orthogonal segments in the green and red planes. To move to the next pair of adjustable points, use the arrows in either long-axis view (red arrow). It is imperative that all points remain on the blue line; any deviation will significantly increase the perimeter (and less so, the area) measurement.
14. The annulus measurement must exclude ectopic calcifications, with the points measured outside the calcifications. In some cases, real-time imaging may help to distinguish side-lobe and reverberation artifacts from real structures. 
15. Using the transverse view to confirm the location of the annular points should always be performed; again, make sure the annular shape makes anatomic sense and is not an irregular polygon. 
16. Once all points have been confirmed (turning from red to green), the measurement of the maximum dimension (green box), minimum dimension (blue box), perimeter (red box) and area (orange box) will be generated.

Although this method involves more steps than the simple planimetry of the short-axis annulus, using the long-axis planes allows the use of anatomic structures such as the mitral leaflet, aortic root and septum, to help define the annular plane. This is particularly useful when the lowest hinge points of the cusps are not in the plane of view. Acoustic shadowing and acoustic noise (side-lobes) are easily managed, particularly if the short-axis annular plane is used as a reference, making sure to keep the annular points “harmonious” and consistent with their neighboring points. Because of these advantages, studies comparing MSCT and 3-D TEE using this method have shown that differences in receiver-operator curve analysis between MSCT and 3-D TEE perimeter and area cover indexes were not statistically significant (p = 0.15 and 0.35, respectively).

The use of the double-oblique technique using adjacent structures to precisely image the annular plane is similar to techniques used by MSCT. Echocardiography, however, offers a number of advantages over MSCT. Multiple cardiac cycles can be analyzed and the results averaged; this is not the case with MSCT, which typically is a composite image of multiple cardiac cycles. Real-time imaging of the hinge points and elimination of hand-tracing errors of direct planimetry are also significant advantages to the 3-D echo technique. Nonetheless, neither 3-D echo techniques described can overcome the limitations of ultrasound physics that creates blooming and side-lobe artifacts as well as acoustic drop-out. In addition, these techniques require expertise and practice. Advances in software packages are currently being developed that should automate many of the steps outlined above and interobserver variability of echocardiographic measurement of the aortic annulus.  

Conclusion

Echocardiography is an essential tool in the measurement of the aortic annulus for TAVR. 2-D techniques, while still useful, have been replaced by 3-D methods for quantifying this oval structure and have improved the accuracy of the sizing algorithms used for both the balloon-expandable and self-expanding valves.

References:

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Keywords: Algorithms, Aorta, Aortic Valve, Artifacts, Echocardiography, Three-Dimensional, Echocardiography, Transesophageal, Heart Valve Prosthesis, Heart Ventricles, Imaging, Three-Dimensional, Mitral Valve, Off-Label Use, Software, Systole, Tomography Scanners, X-Ray Computed, Transcatheter Aortic Valve Replacement

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