Echocardiography has become the modality of choice for the complete assessment of AVSDs [20]. This technique includes all of the standard views to optimize imaging of specific structures. The subcostal views allow to determine abdominal and atrial situs. The long-axis view will determine the cardiac position within the thorax. Long- and short-axis views show the systemic venous and some of the pulmonary venous return and the size and location of any atrial septal defects. Both the right and left ventricular outflow tracts can be evaluated from these projections. A slow sweep in the short-axis view toward the apex will reveal the size and location of ventricular level shunts, the cross-sectional anatomy of the AV valves, and the location and number of left ventricular papillary muscles. This view is also good for the evaluation of ventricular function. From the short-axis plane, the transducer is rotated counterclockwise approximately 45º to assess AV valve morphology and attachments to the ventricular septum and papillary muscle anatomy. It is the best view to evaluate if the valve tissue is balanced over the two ventricles. The atrial and ventricular septal defects are best seen from the apical four-chamber view. A slow sweep from the apical window offers good visualization of the leaflet anatomy, AV valve attachments, defect size and location, and some information on the mechanism of AV valve regurgitation. This view is also used to measure ventricular length. The left ventricular outflow tract is imaged from the apical two-chamber view. Careful imaging of mitral valve attachments to the ventricular septum, in addition to color and spectral Doppler interrogation, is used to evaluate the presence of, and potential for, left ventricular outflow tract obstruction. Parasternal long-axis imaging gives another excellent view of the anatomy of the left ventricular outflow tract.

High parasternal imaging will complete the assessment of the branch pulmonary arteries and pulmonary venous return and the presence of patent ductus arteriosus and a left superior vena cava. The aortic arch should be imaged either from a high parasternal window or from the suprasternal notch. Long-axis imaging is used to evaluate for aortic arch obstruction. Short-axis views are used to determine aortic arch sidedness and the branching pattern of the brachiocephalic arteries and to exclude a vascular ring [15, 20].

The echocardiography provides precise images of the anatomic findings of this congenital malformation with details of the AVSD, the common AV valve or two separated AV valves, the relationships between the leaflets of the valves in both forms, septal structures and their defects, disproportion between the left ventricular inflow and outflow tracts, unwedged aorta, and the shunts that determine the clinical presentation. This anatomic information is of enormous value for the clinician in planning surgical treatment.

Recent studies demonstrate the incremental value of three-dimensional transthoracic echocardiography (3DTTE) over two-dimensional transthoracic echocardiography (2DTTE) in the evaluation of the AVSDs. Because the various morphological features of AVSDs could be viewed in three dimensions, a more comprehensive and accurate assessment of the size and extent of the septal defects; size, number, and abnormalities of AV valve leaflets (Fig. 14.2) and their attachment sites; as well as the relation of the valvular structures to the great vessels is provided by 3DTTE. In some cases, 3DTTE served to verify the findings seen on 2DTTE and, therefore, increased the confidence level with which various morphological features or abnormalities were detected. In others, 3DTTE substantially changes the morphological diagnosis by providing additional details not apparent on 2DTTE. The three-dimensional datasets also provide en face views of defects in the AV valves such as commissures, resulting in more accurate assessment of their size and extent. Such views are especially helpful when planning for surgical repair.

Color Doppler is useful not only in evaluating the three-dimensional extent of valvular regurgitant jets in the atrial chambers but also in providing en face views of the valve orifices, facilitating assessment of its exact size. This has the potential of providing a more accurate quantitative assessment of regurgitant volumes as compared to 2DTTE methods [15, 21, 22].

Echocardiography is limited, however, in the evaluation of certain portions of the aorta (particularly the ascending aorta and the transverse arch), the distal pulmonary arteries, the right ventricle, and the pulmonary veins. These are relative “blind spots” for the echocardiographer. MRI, unhindered by “blind spots,” is excellent for the depiction of cardiac anatomy. In all cases of AVSD, the tricuspid and mitral valves originate at the same level. These findings are best seen on axial or four-chamber MR images. Cine MRI can demonstrate the presence of atrial and ventricular septal defects and valve regurgitation.

Cine MR sequences are used to obtain multiple images over the cardiac cycle at each anatomic location. Spin echo pulse sequences are usually used to produce images in which blood signal intensity is low and appears dark (“black blood” imaging). An ECG-gated velocity-encoded cine MRI (VEC MRI) sequence, a type of gradient echo sequence, can be used to measure blood flow velocity and quantify blood flow rate [23].

Multislice CT has important strengths in comparison with each of these imaging modalities, because it provides excellent anatomic information [24]. The primary disadvantage of CT is that it requires the use of ionizing radiation. Echocardiography, MRI, and CT are complementary in diagnosis of AVSD. Intracardiac anatomy is well depicted by all three modalities. Function and flow quantification are well assessed with Echocardiography and MRI. CT provides exquisite images of the great vessels and associated extracardiac anomalies [25].

Anatomo-imaging correlation between heart specimens with AVSD and echocardiogram, MRI, and CT studies of patients with equivalent findings has demonstrated a high degree of precision that can be achieved with noninvasive imaging diagnosis [1].

The AVSDs can be found in any situs of the heart, situs solitus and situs inversus (Fig. 14.3), or in the isomeric positions of dextroisomerism or levoisomerism (Fig. 14.4). In all four situs positions, AVSD can occur in its two forms of common valve (Fig. 14.4) or two separate valves (Fig. 14.5). It is the relationships among the leaflets within the common AV junction that determine the number of AV orifices, while the relationships between the leaflets and the septal structures dictate the possible shunts that can be established between the two sides of the heart [14–17, 26]. Other anatomic characteristics that can be determined by noninvasive imaging include the interatrial or interventricular septal defects. Failure of closure of the ostium primum results in a large atrial septal defect, although in some cases the ostium primum is absent and the interatrial septum is complete [27]. The integrity of the atrial septum can be examined with a 4-chamber image at the level of the AV junction and the inferior portion of the atrial septum (Fig. 14.6). If the septum is complete, its inferior portion reaches the AV junction and there is no ostium primum defect (Fig. 14.6b, c). When the atrial septum is deficient, its inferior portion is far above the plane of the AV junction and an ostium primum defect is present. Cases have been reported in the literature in which there are more than one atrial septal defect [14–16]. In those cases in which the atrial septum is fused with the leaflets of the common AV valve, a shunt would only be possible through a ventricular septal defect. Two cases are reported in the literature in which the septal defects were obliterated by fusion of the AV valve leaflets to the cardiac septa, and the patients had no septal defects [16, 17].