Congenital heart disease patients require precise and reliable imaging to facilitate decision making with respect to surgery, catheter intervention or ongoing review. There is currently a plethora of imaging choices available, including echocardiography, CT and MRI. All of these modalities can be presented in a three-dimensional (3D) rendered format or interrogated by “slicing” images in user-defined imaging planes. Guidelines have been published for each of these modalities for the congenital heart disease patient ^[1][2][3][4]. The reader is referred to these publications for detailed review of each modality.

The use of a multiplanar approach permits detailed re-analysis of an imaging dataset in any desired plane, but does not present truly three-dimensional (3D) images. In contrast, 3D rendered images can be produced using ultrasound, CT and MRI data. However, the display of such images generally remains on a flat screen, with colour coding used to enhance depth perception. To overcome the limitations of this means of image display, 3D printing has gained traction to allow surgeons and interventionists to be able to have a static physical representation of the true cardiac anatomy. 

2. Three-Dimensional Echocardiography

Echocardiography is frequently the only imaging modality used in congenital cardiac patients, even when surgical repair is required. Conventional two-dimensional (2D) echocardiography necessitates mental reconstruction of the 3D image for each cardiac lesion. The way in which images are displayed in 2D is not intuitive to many surgeons and often the appearance does not resemble the anatomy as viewed intraoperatively. The limitations to 2D echocardiography become more evident in complex lesions, especially when compared to CT and MRI data. By using 3D echocardiography, cardiac anatomy is depicted in all dimensions; while retaining adjacent anatomic landmarks to assist with orientation. The major advantage of this is that it provides clinicians with the depth of field that is missing in conventional echocardiography. Furthermore, in contrast to 3D CT and MRI, dynamic motion of the heart, including heart valves, is retained. This provides clinicians with better perspective of cardiac structures and allows for a more accurate evaluation of morphology, relative position, area, circumference and dynamic variation during the cardiac cycle ^[5] (Figure 1). This technique also offers non-conventional views, not available on 2D imaging ^[6], including the so called ‘en face’ view which mirrors the intraoperative surgical view and allows for rapid understanding of the pathology with surgeons’ and cardiologists’ collaboration in postprocessing ^[7]. There is extensive literature reporting the value of 3D echocardiography in specific congenital cardiac defects. It has proven most helpful for the evaluation of the mitral valve where it provides physiologic, morphologic and functional assessment ^[8]. Three-dimensional echocardiography is a more sensitive modality compared with 2D in identifying the lesion in the mitral valve apparatus responsible for pathology ^[9]. It demonstrates the interaction of the valve leaflets, and the geometry of the sub-valvar support apparatus with superior delineation of chords and papillary muscles ^[10] (Figure 2). The addition of colour to 3D imaging can further facilitate identification of the regurgitant orifice and provides additional quantitative information. Similar advances have been noted in the management of AVSD spectrum defects, features of which are sometimes difficult to appreciate in 2D imaging (Figure 3). Three-dimensional echocardiography can demonstrate abnormal insertion of the anterolateral papillary muscle, abnormal chordal insertion to the superior bridging leaflet, imbalance of superior and inferior bridging leaflet size, and commissural abnormalities ^[6]. This information can facilitate surgical planning and identify whether the closure of the zone of apposition in the left AV valve is indicated to prevent AV valve failure ^[11]. There is also a clear role for 3D echocardiography in delineating anatomy and surgical planning in patients with double outlet right ventricle; when the ventricles are balanced and a biventricular repair remains the desired approach (Figure 4). Three-dimensional rendered images can demonstrate the size of interventricular communications and its relationship to the origin of the great arteries and also identify factors precluding biventricular repair, such as straddling chords. This approach assists in assessment of the anatomy to determine whether baffling the left ventricular output to the aorta would be feasible, whether an RV to PA conduit is required, or whether the anatomy is more suitable for an arterial switch operation ^[12]. The use of 3D echocardiography in patients with complex left ventricular outflow tract obstruction can provide additional details on the obstructive substrate (Figure 5). Volume rendered data from different perspectives including projection through the aortic valve combined with view from the ventricle and a long axis view of the left ventricle, provide an accurate demonstration of the obstructive lesion and relevant surrounding structures. Identification of the mechanism of obstruction is crucial before surgery, as surgical resection is frequently performed through the aortic valve which affords a restricted view and subsequently extensive resection may lead to significant aortic valve regurgitation postoperatively ^[6]. 3D TOE provides additional information during elective or pre-procedural imaging of selected patients and has led to advances in interventional procedures. There is an ever-growing group of TOE-guided procedures where 3D echocardiography facilitates both appropriate patient selection and also intra-procedural guidance and decision making. Real-time 3D imaging is recommended for visualization of catheters, delivery systems, and identifying device position, any residual shunt and the relationship of the device to adjacent structures ^[4]. Three-dimensional TOE has an established role in ASD device closure, since it can provide a precise delineation of the anatomic defect and investigate its suitability for device closure ^[13] (Figure 6). One study has demonstrated 54% reduction in total fluoroscopic time and 78% radiation reduction in ASD closures using 3D TOE guidance with no associated complications ^[14]. Similarly, 3D TOEs are used to guide percutaneous VSD closure as well as paravalvar and baffle leak closure, where an ‘en face’ view of the defect is feasible, making the location of the leak more easily identifiable ^[15] Another interventional procedure that has been made feasible through 3D imaging and especially 3D TOE guidance is the transcatheter closure of sinus venosus ASDs which are usually associated with partially anomalous pulmonary venous return of the right sided upper and or middle pulmonary veins (Figure 7). In a single centre study, >75% of patients referred for transcatheter correction by placing a covered stent into the SVC were successfully treated, rendering this technique an effective and reproducible alternative to surgical closure ^[16]. Transoesophageal 3D echocardiography is used to assess the defect and the relationship of the right pulmonary veins to the superior vena cava–right atrium junction during stent deployment, and to assess the position of the deployed stent as well as potential residual shunts ^[17].

3. CT

CT provides extremely high-resolution cardiovascular imaging, including 3D rendered views, with the most recent generation of ultrafast scanners reducing both scan time and radiation dose. This facilitates high-resolution imaging in the context of un-cooperative or unstable patients, for example in an intensive care setting. However, CT still uses ionising radiation and paediatric patients are likely to need repeat imaging and their rapidly dividing cells make them more susceptible to radiation damage ^[18]. It is also known that CT alongside angiography are the main sources of radiation exposure during follow up of adult patients with congenital heart disease ^[19]. Due to the lifelong sequalae involved, CT scanners have incorporated radiation dose reduction technology and dose optimisation protocols to minimise associated risks ^[20]. CT acquisition parameters are adjusted to minimise exposure, and post-processing filters are applied to reduce noise and maintain image quality ^[21][22]. Dual-energy CT (DECT) is a new technology that is able to reduce the radiation exposure dose even further ^[23]. CT offers the option of both surface and volume rendering, with surface rendering using only part of the available data for 3D reconstruction (Figure 8). After the determination of surface representation, this can be a fast technique with good depth perception, but it relies on well-differentiated surfaces and thresholds ^[20]. The introduction of dynamic CT has allowed for the uninterrupted acquisition of cardiac structures, resulting in even shorter scanning times. The main advantage of CT is the imaging of complex vascular structures. In patients with Tetralogy of Fallot, 3D CT data demonstrates pulmonary arterial distribution, with the anatomy of the right ventricular outflow tract and any potential coronary artery anatomy variations precluding or complicating surgical or interventional procedures. Three-dimensional CT imaging provides a comprehensive delineation of the pulmonary blood supply in patients with pulmonary atresia and major aortopulmonary collaterals, allowing for the planning of unifocalisation and pulmonary artery reconstruction. Three-dimensional aortic arch reconstructions can accurately demonstrate the arch anatomy and length of the narrowed segment in complex cases of coarctation, as well as the relationship with head and neck vessels ^[20]. The increasing detection of vascular rings due to comprehensive foetal screening has resulted in the development of specific 3D CT protocols for these vascular structures, combined with an airway assessment via endoscopic bronchoscopy to allow for early recognition of any potential for compression ^[24]. In patients being considered for complex biventricular repair, such as unbalanced AVSD or double outlet right ventricle, 3D CT can depict complex intracardiac anatomy by identifying great artery relationships, ventricular septal defect size/location and some information on the atrioventricular valves (Figure 4). Some types of repairs involve baffling the blood flow from the left ventricle to the aorta, and CT can assist in the assessment of feasibility and an estimation of the post-repair size of the remaining ventricular cavity.

4. MRI

3D MRI acquired data can produce whole heart imaging and visualise the extracardiac vasculature and relationships to adjacent structures while remaining free of radiation. Contrast-enhanced angiography and non-contrast enhanced 3D whole heart imaging can be used as required to interrogate cardiac anatomy on a multiplanar reformat platform ^[20]. A new offline 3D planning method has been compared to conventional 2D MRI imaging ^[25]. The anatomical coverage was of equal imaging quality, using both methods. However, the 3D offline tool was more time efficient, which is particularly relevant in patients under general anaesthesia or sedation. The recent development of 4D flow MRI includes a phase contrast MRI study with flow encoding in all three spatial directions relative to all three dimensions of space and to the dimension of time throughout the cardiac cycle ^[26] (Figure 9). This allows for volume flow quantification with the added advantage of enabling the retrospective calculation of blood flow through any plane of interest within the 3D volume acquired. Simulations using computational fluid dynamics (CFD) derived from MRI data and patient-specific 3D models offer the possibility for modelling circulation in congenital heart defects, and can provide measures of flow and how these would be affected by interventions that change the geometry ^[27]. A clinical application of current methods remains limited, mainly due to long computation times and complexity. A newer lean steady state simulation technique has been recently described, with fast and reliable results in patients with physiologically univentricular hearts ^[27].