GENERAL IMAGING APPROACH

The most widely used modality for evaluation of the IAS is TTE, which is also the preferred initial diagnostic modality for the detection and diagnosis of patent foramen ovale (PFO), ASD and atrial septal aneurysm.¹ TTE is especially useful in small children in whom the ultrasound image quality will typically permit a full diagnostic study.² It can also be used for patient selection and real-time transcatheter ASD or PFO closure procedural guidance in paediatric patients.⁴

TTE can be used for the initial evaluation of ASD and PFO in adults; however, TEE is required to further characterize the atrial septal abnormalities, because the TTE image quality might not always permit a comprehensive evaluation of the IAS. TEE is not invariably required for assessment of a PFO if transcatheter closure is not being considered.³ Also, 2D and 3D TEE offer significant incremental anatomic information compared with TTE and should be performed in all adult patients being evaluated for percutaneous transcatheter closure or surgical therapy.³ In adults, TEE can identify the margins or rims of the ASD and assess the surrounding structures (e.g., aorta, cavae, pulmonary veins, AV valves and coronary sinus).

ICE is also used extensively to guide percutaneous ASD/PFO closure procedures and provides comparable but not identical imaging to TEE.⁴

Contrast echocardiography with agitated saline plays an important role in the evaluation of PFO and assessing residual shunts after transcatheter closure and has a more limited role in the diagnosis of ASD.⁵

Table 4.1 summarizes the recommended general imaging approach for atrial septal abnormalities stratified by the patient characteristics, imaging modality and intended application (e.g., diagnosis, procedure selection or guidance, follow-up).

THREE-DIMENSIONAL IMAGING OF THE INTERATRIAL SEPTUM

Three-dimensional TEE is described to improve the visualization of PFO and ASD, their surrounding tissue rims and surrounding structures and can also be used for guidance during percutaneous transcatheter closure.³ IAS is a complex, dynamic and 3D anatomic structure; limitations exist in its evaluation using any single form of 2D echocardiography. The interatrial atrial septum and its associated abnormalities (such as ASD or PFO) do not exist in a true flat plane that can be easily aligned and interrogated using 2D imaging.³ Both ASD and PFO exist in a wide variety of heterogeneous sizes, shapes and configurations (Figures 4.1 and 4.2). Three-dimensional imaging provides unique views of the IAS and, in particular, allows for en face viewing of the ASD and surrounding fossa, allowing for accurate determination of the ASD size and shape.⁶ Furthermore, 3D imaging offers the potential to clearly and comprehensively define the dynamic morphology of the defect, which has been shown to change during the cardiac cycle. Also, 3D imaging delineates the relationship of the ASD to the surrounding cardiac structures and the rims of tissue surrounding it² (Figure 4.3).

Two-dimensional biplane (or triplane) imaging, a feature of currently commercially available 3D imaging systems, is a unique modality that takes advantage of 3D technology. The advantages of biplane imaging include the display of simultaneous additional echocardiographic views, with high frame rates and excellent temporal resolution. Complementary simultaneously displayed orthogonal plane imaging provides incremental information compared with that from a single plane, and this imaging modality is uniquely suited to transcatheter procedure guidance. Numerous reports of the advantages of 3D TEE in guiding catheter interventions have been published and include the use of biplane imaging. Figure 4.4 illustrates the use of biplane imaging during percutaneous transcatheter closure of ASD before deployment of the device.

Also, 3D imaging allows for multiple acquisition modes, including narrow-angle, zoomed and wide-angle gated acquisition of multiple volumes. Once 3D volumes are acquired, post-processing using commercially available 3D software packages or 4D Cardio-View is performed to align the plane of the IAS with multiple 3D plane slices. This approach facilitates an assessment of the shape of an ASD and allows for measurement of the en face diameters in multiple orthogonal views, without the potential for bias due to malalignment of the ultrasound planes. The three-dimensional images should be reviewed in both systole and diastole to assess the dynamic changes in size which can occur. This 3D en face display can also aid in the recognition and quantification of rim deficiencies, because the extent of the deficiency relative to the surrounding structures such as the aorta can be easily demonstrated and quantified. The distance between the defect and the aorta can be easily measured, just as can the area of the defect and length of rim deficiency when present.

ROLE OF ECHOCARDIOGRAPHY IN PERCUTANEOUS TRANSCATHETER DEVICE CLOSURE

The role of TTE, TEE and ICE during the assessment and transcatheter management of ASD/PFO is essential for appropriate patient selection, real-time procedure guidance, assessment of device efficacy and complications and long-term follow-up.⁶ TTE provides information about the type of defect, its hemodynamic significance and any associated anomalies and can be used comprehensively in smaller paediatric patients for the diagnosis of ASD and PFO and even for patient selection and procedural guidance.⁶ TTE has the advantage of offering unlimited multiple planes to evaluate the atrial septum, but it has limited ability to interrogate the lower rim of atrial septal tissue above the IVC after device placement, the device shadowing has an impact and interferes with imaging in almost all planes. In addition, because the septum is relatively far from the transducer, the image quality is often suboptimal in larger paediatric and adult patients. If percutaneous closure is clinically indicated, a detailed assessment of the IAS anatomy and surrounding structures using TEE is typically required for patient selection and procedure guidance or ICE for procedure guidance in such patients. Transesophageal echocardiography provides real-time, highly detailed imaging of the IAS, surrounding structures, catheters and closure device during transcatheter closure. It requires either conscious sedation, with the attendant aspiration risk in a supine patient, or general anaesthesia, with an endotracheal tube placed to minimize aspiration risk. This approach also requires a dedicated echocardiographer to perform the TEE, while the interventionalist performs the transcatheter closure procedure. The advent of 3D TEE has enhanced the evaluation of ASD and PFO by clearly defining the IAS anatomy and enables an en face view of the defect and its surrounding structures.⁷ Multiplanar reconstruction of the 3D data set allows accurate measurement of the minimum and maximum dimensions of the defect or defects, facilitating selection of the optimal size and type of closure device.⁶ Moreover, intraprocedural real-time 3D TEE provides superior visualization of wires, catheters and devices and their relationships to neighbouring structures in a format that is generally more intuitively comprehended by the interventional cardiologist⁸ (Figure 4.5).

Intracardiac echocardiography has been used extensively to guide percutaneous ASD/PFO closure procedures and is the imaging modality of choice in many centres in the cardiac catheterization laboratory. The advantages of ICE include an image quality that is similar but not identical to that of TEE, facilitating a comprehensive assessment of the IAS, location and size of the defects, the adequacy of the rims and location of the pulmonary veins. It also retains an advantage compared with TEE in imaging the inferior and posterior portions of the IAS. Finally, the use of ICE eliminates the need for general anaesthesia and endotracheal intubation and can be performed with the patient under conscious sedation. An interventionalist can perform intracardiac echocardiography without the need for additional support personnel. However, the potential disadvantages of ICE include a limited far-field view, catheter instability, the expense of single-use ICE catheters, the need for additional training, the risk of provocation of atrial arrhythmias and increased technical difficulty for a single operator. Table 4.2 provides a summary of the advantages and disadvantages of TTE, TEE and ICE in percutaneous transcatheter guidance of PFO and ASD.

SUBXIPHOID FRONTAL (FOUR-CHAMBER) TTE VIEW

The subxiphoid four-chamber view allows imaging of the atrial septum along its anterior–posterior axis from the SVC to the AV valves. This is the preferred view for imaging the atrial septum, because the atrial septum runs near perpendicularly to the ultrasound beam, providing the highest axial resolution and permitting measurement of the defect diameter along its long axis. Because the septum is thin (especially in its midportion), placing the septum perpendicular to the ultrasound beam helps distinguish a true defect from dropout resulting from an artefact. Aneurysms of the atrial septum primum composed of tissue attached to the edges of the ASD are also well visualized from the subcostal frontal view. ASAs could be fenestrated (Figure 4.6) but also can be intact with no resultant atrial level shunt. Colour Doppler interrogation and contrast studies should be used to detect shunting. The surrounding rim from the defect to the right pulmonary veins can be measured in this view. Sinus venosus defects are difficult to visualize because the vena cava is not viewed longitudinally in this view.

SUBXIPHOID SAGITTAL TTE VIEW

The subxiphoid sagittal transthoracic view is acquired by turning the transducer 90 degrees clockwise from the frontal view. The view is ideal for imaging of the atrial septum along its superior–inferior axis in a plane orthogonal to the subxiphoid frontal four-chamber view. Sweeping the transducer from right to left in this axis allows determination of the orthogonal dimensions of the ASD (Figures 4.7 and 4.8). These dimensions can be compared with the dimensions measured in the subxiphoid frontal view to help determine the shape (circular or oval) of the defect. This view can be used to measure the rim from the defect to the SVC and IVC and is an excellent window to image a sinus venosus type defect (Figure 4.7 and 4.9).

LEFT ANTERIOR OBLIQUE TTE VIEW

The left anterior oblique TTE view is acquired by turning the transducer approximately 45° counter clockwise from the frontal (four-chamber) view. This view allows imaging of the length of the atrial septum and is therefore ideal to identify ostium primum ASDs and for assessment of coronary sinus dilation (Figures 4.8b and 4.10b). In addition, it allows evaluation of the relation of the SVC to the defect. Furthermore, this view can be used to evaluate the entrance of the right-sided pulmonary veins into the heart.

APICAL FOUR-CHAMBER TTE VIEW

In the apical four-chamber TTE view, the diagnosis and measurement of ASD should be avoided because the atrial septum is aligned parallel to the ultrasound beam. Thus, artefactual dropout is frequent in this view, which results in overestimation of the defect size. This view is better to assess the hemodynamic consequences of ASD, such as RA and RV dilation, and to estimate the RV pressure using the tricuspid valve regurgitant jet velocity. This view is also used to evaluate for right-to-left shunting with agitated saline⁵ (Figure 4.11).

MODIFIED APICAL FOUR-CHAMBER VIEW (HALF WAY BETWEEN APICAL FOUR-CHAMBER AND PARASTERNAL SHORT-AXIS TTE VIEW)

The modified apical four-chamber TTE view is obtained by sliding the transducer medially from the apical four-chamber view to the sternal border. This view highlights the atrial septum from an improved incidence angle to the sound beam (30°–45°). In the patients in whom the subcostal views are difficult to obtain, the modified apical four-chamber view is an alternative method for imaging the atrial septum in the direction of the axial resolution of the equipment.

PARASTERNAL SHORT-AXIS TTE VIEW

In the parasternal short-axis TTE view at the base of the heart, the atrial septum is visualized posterior to the aortic root running in an anterior–posterior orientation (Figure 4.12). This view is ideal to identify the aortic rim of the defect (Figures 4.13 and 4.14). It also highlights the posterior rim (or lack thereof) in sinus venosus and postero-inferior secundum defects. The size of the defect itself should not be measured in this view, because the beam orientation is parallel to the septum, and dropout resulting from artefact can occur.