**4. 3D echocardiography**

3D echocardiography (3DE) allows imaging and analysis of cardiovascular structures as they move in time and space, which enables the creation of 4D datasets (3D in real-time). Real-time 3DE is a major innovation in the history of cardiovascular ultrasound [59].

3DE is increasingly used in patients with congenital heart disease, as it allows the visualization of lesions in three dimensions, with opportunities for increased appreciation of complex spatial relationships. Alternative imaging modalities such as CMR or computed tomography (CT) have, compared to 3DE, some disadvantages, such as expense, general anesthesia (CMR), or exposure to radiation (CT) [1].

Clinical benefits of 3DE are evident in three main areas: better visualization and understanding of the spatial relationships and 3D morphology of congenital heart defects; quantification of cardiac mass and volumes; planning and guiding therapeutic interventions, as seen in **Table 4** [1, 60].

However, 3DE scanning modalities that use the ECG for gating, and then collect the data acquired over several heartbeats are often problematic in younger patients, as there is a large potential for movement artifact, especially with higher respiratory rates. The main shortcomings of 3D imaging have been the lower spatial and temporal resolution compared with 2D imaging, and the requirement for offline analysis [1, 60].

Compared to cross-sectional imaging methods 3DE does not use the same geometric assumptions, which allows a more accurate assessment of the cardiac function. At the same time, with better visualization also comes a better understanding of the anatomy of CHD, such as atrial and ventricular septal defects, atrioventricular (AV) septal defects, and atrioventricular valve or outflow tract abnormalities. In addition, 3DE enables better volumetric assessment of cardiac chambers in patients with borderline sized ventricles, it can also work as an important diagnostic tool for cardiac mass. 3DE findings correlate well with surgical findings, and can significantly improve the planning of therapeutic interventions, sometimes may also reduce the operative time [1].


#### **Table 4.**

*Main clinical applications of 3D echocardiography.*

3DE offers the additional advantage to estimate the AV valve regurgitant volume, the mechanism, and the origin of regurgitation with clear visualization of the valves, which makes 3DE an ideal imaging modality to evaluate these structures and plan interventions [61–64]. 3DE is also a promising modality for 3D printing of AV valves, structures that are largely missing from cardiac models using exclusively MRI or CT data [64–66].

The 3DE and 4DE with spatiotemporal image correlation allow obtaining fetal cardiac volumes and their static and real-time analysis [67], and multiple two-dimensional images are stacked one behind the other to create a volume dataset [68–70]. 4DE is used mainly in the field of fetal echocardiography for the dynamic assessment of fetal cardiac structures and large vessels. The main challenge of fetal echocardiography is still a profound understanding of the spatial relationships and connections of the cardiac structures and great vessels, which 4DE overcomes with more accurate anatomic information. Although traditional 2D echocardiography is the basic modality for prenatal diagnosis of CHD, 3DE and 4DE should be considered as very useful additions [71].
