Principles of imaging


Principles of imaging


The evaluation of the abnormalities of interatrial septum (IAS) and its associated conditions and abnormalities require a standardized and systematic approach with echocardiographic and Doppler finding and characteristics, including transthoracic echocardiographic (TTE), transesophageal echocardiographic (TEE), intracardiac echocardiographic (ICE) ultrasound, three-dimensional (3D) imaging, Doppler and transcranial Doppler (TCD) modalities as and when indicated.

A thorough echocardiographic evaluation includes the detection and quantification of the size and shape of the septal defects, the rims of tissue surrounding the defect, the degree and direction of shunting and the remodelling and changes in size and function of the cardiac chambers and pulmonary circulation. The advent of 3D visualization, especially coupled with the transesophageal echocardiographic-based characterization, has contributed decisive incremental information in the evaluation of the IAS.


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 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).

Figure 4.1

Figure 4.1 Three-dimensional transesophageal images of various shapes and sizes of ostium secundum ASD. Representative examples of round, small (a); round, large (b); oval, small (c); and oval, large (d) secundum ASD. (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)

Figure 4.2

Figure 4.2 Three-dimensional TEE images of a PFO. Excessive movement of the septum primum (fossa ovalis) in a patient with an ASA and a PFO; red arrow white white arrow head specifies a PFO opened fully under influence of pressure difference between RA and LA (a–c). PFO ‘tunnel’ as viewed from the LA perspective; red arrow specifies the PFO exit into the LA (d). PFO tunnel exiting into LA (white arrow) (e). (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)

Figure 4.3

Figure 4.3 Three-dimensional ASD assessment allows for delineation of an ASD (blue arrow) and its relationship between adjacent structures. The aortic valve is seen, and the entire aortic rim (white arrow) is visualized en face. (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)

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.

Figure 4.4

Figure 4.4 Biplane imaging performed during percutaneous transcatheter closure imaging of multiple planes simultaneously. The aortic rim and superior rim are seen (left arrow) and the device interaction with the aorta (left arrow) and atrial roof (right arrow) can be assessed simultaneously. (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)

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.


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).

Figure 4.5

Figure 4.5 Intraprocedural RT3D TEE provides superior visualization of wires, catheters and devices and their relationship to neighbouring structures in a format that is generally better comprehended by the interventional cardiologist than 2D echocardiography. An ostium secundum ASD has been closed with an Amplatzer device under RT3D TEE guidance. All views are shown from the LA perspective. The LA disc of the device opening in the LA (a). View showing continued opening of the device (b). An undersized device with a residual defect (c). A larger closure device was used instead (d). (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)

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.

Transthoracic echocardiography protocol for imaging the interatrial septum3

The atrial septum can be evaluated fully using TTE. Multiple views should be used to evaluate the size, shape and location of an atrial communication and the relationship of the defect to its surrounding structures. In particular, special attention must be paid to determine the relationship of the defect to the venae cavae, pulmonary veins, mitral and tricuspid valves and coronary sinus. Assessment of the amount of the surrounding rims of tissue present is crucial. A deficiency of rim tissue between the defect and pulmonary veins, AV valve or IVC will preclude transcatheter closure, and a deficiency of aortic rim can increase the risk of device erosion in certain circumstances.

Additional views of other structures, such as the ventricles and great arteries, are necessary to assess for secondary findings related to the hemodynamic consequences of an ASD such as RA, right ventricular (RV) and pulmonary artery (PA) dilation. In the paediatric population, the subxiphoid window typically allows the best visualization of the atrial septum and its related structures. In adolescence and adulthood, the subxiphoid window is often inadequate because of the distance from the probe to the atrial septum. Thus, other views such as the parasternal windows should be used to assess the atrial septum. In some cases, a full assessment of the atrial septum might not be possible with transthoracic echocardiogram, thus, transesophageal echo could be required.


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.

Figure 4.6

Figure 4.6 Subxiphoid TTE demonstrating multi fenestrated IAS without and with colour Doppler flow from left to right in a paediatric patient. (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)


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).

Figure 4.7

Figure 4.7 Transthoracic echocardiogram of a SVC type venosus ASD in subxiphoid sagittal view without and with colour in a paediatric patient. The yellow arrow represents the right superior pulmonary vein, and the white arrow the defect entering the atrium. (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)

Figure 4.8

Figure 4.8 Two-dimensional TTE (left) and with colour Doppler (right) demonstrating unroofed 
coronary sinus interatrial communication in four-chamber view; dilated coronary sinus is seen (a). 
Two-dimensional TTE (left) and with colour Doppler (right) demonstrate unroofed coronary sinus with interatrial communication in subcostal left anterior oblique view (b). (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)

Figure 4.9

Figure 4.9 Representative example of 2D TTE (left) and with colour Doppler (right) of an SVC type sinus venosus ASD from the high right parasternal view (a). Representative example of 2D TTE (left) and with colour Doppler (right) of an SVC type sinus venosus ASD from the subcostal sagittal view; RPA, right pulmonary artery (b). (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)


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.

Figure 4.10

Figure 4.10 Primum ASD by 2D TTE in apical four-chamber view (a). Primum ASD by 2D TTE in ­subcostal left anterior oblique view; CAVV, common AV valve (b).


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).

Figure 4.11

Figure 4.11 TTE of an apical four-chamber view during saline contrast injection. First images demonstrate prominent artefact over mitral valve (a). Opacification of the RA and RV (b). Delayed entry of contrast into the LA and LV, consistent with a pulmonary arteriovenous malformation; if the bubbles cross within the first three cardiac cycles, an intracardiac shunt is present (c). Subsequent cardiac cycles demonstrate continued opacification of the LA and LV consistent with intrapulmonary shunting (d and e). (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)


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.


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.

Figure 4.12

Figure 4.12 Two-dimensional TTE of ostium secundum ASD from parasternal short-axis view (a). 
Two-dimensional TTE (left) and with colour Doppler (right) of an ostium secundum ASD from the parasternal short-axis view with measurement of the diameter in the anterior–posterior orientation and left-to-right flow by colour Doppler. Ao, aortic root (b). (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)

Figure 4.13

Figure 4.13 TTE of a secundum type ASD in the parasternal short-axis view without and with colour Doppler in a paediatric patient. (Adapted from ASE guidelines for echocardiographic assessment of ASD and PFO, August 2015.)

Figure 4.14

Figure 4.14 Examples of ostium secundum by 2D TTE (left) and with colour Doppler (right) in the subcostal left anterior oblique view (a and b). Measurement of the ASD diameter (left) and left-to-right colour Doppler flow (right). Sagittal subcostal view in a patient (c)

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Aug 27, 2021 | Posted by in CARDIOLOGY | Comments Off on Principles of imaging
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