THREE-DIMENSIONAL (3D) ECHOCARDIOGRAM IN ATRIAL SEPTAL DEFECTS: CONCEPTS AND PROTOCOLS

The interatrial septum (IAS) is a complex, dynamic, three-dimensional anatomic structure A 2D echocardiogram cannot give a complete orientation of the atrial septal defects.¹ Two-dimensional imaging cannot align or interrogate the IAS because it does not exist in a true flat plane. Three-dimensional imaging with high frame rates and temporal resolution helps in defining the dynamic morphology of the defect, accurately determining atrial septal defect (ASD) size and shape and the relationship of ASD to the surrounding cardiac structures.²

At present, 3D matrix array transducers are composed of nearly 3000 individually connected and simultaneously active piezoelectric elements with frequencies ranging from 2 to 4 MHz and 5 to 7 MHz for transthoracic and transesophageal transducers, respectively. There are three different planes in which electronically controlled firing of elements in the matrix generates scan lines. They are x-axis or azimuthal direction; y-axis or axial direction and z-axis or vertical direction (elevation). And finally a volumetric pyramid of data is acquired. The image quality will be affected by point spread function of the system, which represents the imaging system response to a point input. The best images with less blurring will be obtained when using the axial dimension and more blurring while using the elevation dimension as the approximate spread in the axial (y) dimension is 0.5 mm and 3 mm in the elevation (z) dimension.³

In the parasternal approach, because the structures are primarily imaged in axial and lateral dimensions, good quality images are obtained. Poorer-quality images are obtained in the apical approach, which mostly uses the lateral and elevation dimensions.

The three different methods for 3D data set acquisition are (i) multiplane imaging, (ii) real-time live 3D imaging and (iii) multibeat electrocardiogram (ECG) gated imaging.⁴ In multiplane mode, at a high frame rate, multiple 2D views can be acquired using predefined plane orientations. This is useful in situations like atrial fibrillation or interventricular dyssynchrony, where assessment of multiple views from same cardiac cycle is useful. In real-time mode, image orientation and plane can be changed by rotating or tilting the probe. Multibeat acquisition requires sequential acquisitions of narrow smaller volumes acquired from several ECG-gated cardiac cycles. Cropping, slicing and rotation were the three main actions done by the operator to obtain the desired view from a 3D volumetric dataset. Cropping is the process of removing irrelevant neighbouring tissue. Volume rendering, surface rendering and tomographic slices were the three different display modalities for depth perception of 3D images.⁴

The 3D echocardiogram has advantages in assessing size of the ASD in that it is radiation free compared to sizing-balloon measurement of ASD diameter and is non-invasive compared to TEE, which usually requires general anaesthesia in the paediatric population and carries the risk of aspiration and esophageal perforation.⁵ Furthermore, 3D TTE has been shown to better approximate the dimensions and location and the anatomy of surrounding structures compared to 2D images. The American Society of Echocardiography currently recommends 2D TEE during percutaneous closure and repair of ASDs. But this is dependent on the observer’s mental ability to recreate these images in 3D space, which can be difficult. But 3D imaging circumvents this problem by providing real-time three-dimensional images, aiding the operator in better assessment of ASDs and transcatheter closure.

Three-dimensional imaging of atrial septal defect is a potentially helpful tool in assessing the ASD device and its points of contact or pressure. For transthoracic 3D images, the subcostal view is the preferred view because its projection is en face to the atrial septum. In a prospective study, comparing 2D TTE, 2D TEE and 3D TTE in 37 patients with OS-ASD who underwent transcatheter device closure, there was significant correlation between the ASD diameter measured by 3D TTE and that by TEE (r = 0.759, p − 0.001).⁶ Also, the rims of OS-ASD were assessed using 3D TEE, and atrioventricular rim (AV rim) was assessed in particular to avoid encroachment of the mitral valve and aorta. A cut-off value of 8.3 mm for AV rim was derived with an accuracy of 83.3% to avoid the encroachment on the aortico-mitral continuity plan than the previously published ratio (1.5 × ASD size). There was a direct relationship between the ratio of the left disc of the device to the total septum.⁵ A new formula was constructed for device choice: ½ the defect size + AV rim length (not ≤ 8 mm) = ½ the left disc size of the device.

MORPHOLOGY OF ASD

OS-ASD will not be circular as assumed most of the time. It can be circular or oval or multiple fenestrations in the atrial septum maybe present. A 3D echocardiogram can be used to assess the various morphologies of atrial septal defects, such as oval or circular. A defect is considered oval when the ratio of the shortest diameter to the longest diameter ≤ 0.75, when measured using computed tomography.⁷

Also, the shape of the defect varies dynamically with different phases of cardiac cycle, which can be recorded with live 3D echocardiographic imaging. The maximum size of an ASD is seen in late ventricular systole and minimum size during late left ventricular diastole.⁸

3D TEE

In a study of patients undergoing 3D TEE, a protocol was developed for properly orienting the interatrial septum and ASDs.⁹ The horizontal axis runs from right to left edge of the screen, the vertical axis from top to bottom of the screen and the z-axis is perpendicular to the computer monitor. The lateral (azimuth) direction is encoded in red, the elevation direction in green and the depth direction in blue by convention. Three-dimensional TEE images were obtained in the following modalities: (i) biplane imaging (a side-by-side display of a pair of 2D TEE images that are 90 degrees apart), (ii) full-volume imaging, (iii) narrow-angle live 3D imaging and (iv) wide-angle 3D zoom imaging. The image acquisitions at 0 degree and 90 degrees are performed first.

IMAGE ACQUISITION AT 0 DEGREE (TUPLE MANOEUVRE)

The 3D zoom mode is selected after getting a good midesophageal view of the interatrial septum at 0 degree in 2D mode. In the initial pair of biplane images appearing on the screen, the left image shows the lateral (azimuth, or red) plane and the right image shows the elevation (green) plane. There is a region-of-interest selection box in each image, and by moving it up and down the screen, the user can determine which portion of the depth (blue) plane will be displayed in the subsequent 3D view. After selecting the region of interest, a 3D zoom view is obtained and the interatrial septum appears in its long axis view, which is referred to as the ‘opening scene’. This view is then manipulated to get an en face view of the interatrial septum, which is the view as a surgeon would view it. In the tilt-up-then-left (TUPLE) manoeuvre, an initial 3D image of IAS is first tilted up along its horizontal axis to reveal en face view from right atrial perspective. The superior vena cava (SVC) and aorta are the most important structures in this view. The SVC is at the top of the screen and aortic valve and ascending aorta are on the right of the screen. In the next step, the image is tilted left around the vertical axis by 180 degrees to obtain the en face view of IAS from the left atrial perspective. Here the superior rim is at the top of the screen, the aortic rim on left side and the ostia of pulmonary veins on the right side of the monitor (Figure 13.1).

IMAGE ACQUISITION (TUPLE PLUS ROTATE LEFT IN Z-AXIS [ROLZ] MANOEUVRE)

As obtained previously, the opening scene 3D zoom image is obtained at 90 degrees. This has the same orientation as the bicaval view in 2D TEE. The image is tilted up to reveal the right atrial side of the septum in the first step, and the SVC now appears on the right side of the screen. Then, to obtain view of the IAS from left atrial side, it has to be rotated by 90 degrees in counterclockwise direction in the z-axis.

Three-dimensional TEE helps in assessing the rims of the ASD in a well-delineated manner (Figure 13.2). Three-dimensional TEE is a valuable tool in visualizing an OS ASD and its anatomical orientation to its neighbouring vital structures. The OS-ASD seen through the right atrial as well as left atrial side is depicted in Figure 13.3. Also, rare types of atrial septal defects with multiple fenestrations which are difficult to visualize in 2D TEE can be identified using a 3D TEE (Figure 13.4

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