Over the last decade we have been witness to a burgeoning literature on three-dimensional (3D) and four-dimensional (4D) ultrasound-based studies of the fetal cardiovascular system. Recent advances in the technology of 3D/4D ultrasound systems allow almost real-time 3D/4D fetal heart scans. It appears that 3D/4D ultrasound in fetal echocardiography may make a significant contribution to interdisciplinary management team consultation, health delivery systems, parental counseling, and professional training.
Three-dimensional (3D) and four-dimensional (4D) applications in fetal ultrasound scanning have made impressive strides in the past two decades. Today, many more centers have 3D/4D ultrasound capabilities at their command, and we are witness to a burgeoning literature of 3D/4D-based studies. Perhaps in no other organ system is this recent outstanding progress so evident as in the fetal cardiovascular system. Recent technological developments of motion-gated cardiac scanning allow almost real-time 3D/4D heart examination. It appears from this growing body of literature that 3D/4D ultrasonography can make a significant contribution to our understanding of the developing fetal heart in both normal and anomalous cases, to interdisciplinary management team consultation, to parental counseling, and to professional training. 3D/4D ultrasound may facilitate screening methods, and by dint of its offline networking capabilities may improve health-care delivery systems. These features may work to extend the benefits of prenatal cardiac screening to poorly-served areas. The introduction of ‘virtual planes’ to fetal cardiac scanning has helped sonographers obtain views of the fetal heart not generally accessible with a standard two-dimensional (2D) approach.
It is perhaps too early to evaluate whether 3D/4D cardiac scanning will improve the accuracy of fetal echocardiography programs. However, there is no doubt that 3D/4D ultrasonography gives us another look at the fetal heart.
The purpose of this review is to summarize the recent technological advances in 3D/4D fetal echocardiography, demonstrating their application through normal and anomalous case examples.
3D/4D Techniques and their Application to Fetal Cardiac Scanning
Spatio-temporal image correlation (STIC)
STIC acquisition is an indirect motion-gated offline scanning mode1–4. The automated volume acquisition is made possible by the array in the transducer performing a slow single sweep, recording a single 3D data set consisting of many 2D frames one behind the other. The volume of interest (VOI) is acquired over a period of about 7.5 to 30 s at a sweep angle of approximately 20–40° (depending on the size of the fetus) and frame rate of about 150 frames per second. A 10-second, 25° acquisition would contain 1500 B-mode images4.
Following acquisition the ultrasound system applies mathematical algorithms to process the volume data and detect systolic peaks, which are used to calculate the fetal heart rate. The B-mode images are arranged in order according to their spatial and temporal domain, correlated to the internal trigger, the systolic peaks that define the heart cycle4 (Figure 1). The resultant 40 consecutive volumes are a reconstructed complete heart cycle that displays in an endless loop. This cine-like file of a beating fetal heart can be manipulated to display any acquired scanning plane at any stage in the cardiac cycle (Figure 2).
While a complex process to describe, this reconstruction takes place directly following the scan in a matter of seconds; the STIC acquisition can be reviewed with the patient still present and repeated if necessary, and saved to the scanning machine or a network. Optimal STIC acquisition technique for examination of the fetal heart is thoroughly and succinctly described by Goncalves et al.5.
In post-processing, various methodologies have been proposed to optimize the acquisition to demonstrate the classic planes of fetal echocardiography6, 7 (Figure 3), as well as ‘virtual planes’ that are generally inaccessible in 2D cardiac scanning8–11. These views once obtained are likewise stored in the patient's file, in addition to the original volume, either as static images or 4D motion files. Any of the stored information can be shared for expert review, interdisciplinary consultation, parental counseling, or teaching.
STIC is an acquisition modality that can be combined with other applications by selecting the appropriate setting before acquisition (B-flow, color and power Doppler, tissue Doppler, high-definition flow Doppler) or with post-processing visualization modalities (3D volume rendering, inversion mode, tomographic ultrasound imaging).
Multiplanar reconstruction (MPR), 3D rendering, and tomographic ultrasound imaging (TUI)
3D/4D volume sets contain a ‘block’ of information, which is generally a wedge-shaped chunk of the targeted area. In order to analyze this effectively, the operator displays 2D planes in either MPR mode (Figure 2), or in 3D volume rendering. In MPR the screen is divided into four frames, referred to as A (upper left), B and C; the fourth frame (lower right) will show either the volume model for reference, or the rendered image. Each of the three frames shows one of the three orthogonal planes of the volume. The reference dot guides the operator in navigating within the volume, as it is anchored at the point of intersection of all three planes. By moving the point the operator manipulates the volume to display any plane within the volume; if temporal information was acquired, the same plane can be displayed at any stage of the scanned cycle.
From a good STIC acquisition5 the operator can scroll through the acquired volume to obtain sequentially each of the classic five planes6 of fetal echocardiography, and any plane may be viewed at any time-point throughout the reconstructed cardiac cycle loop. The cycle can be run or stopped frame-by-frame to allow examination of all phases of the cardiac cycle, for example opening and closing of the atrioventricular valves.
By comparing the A- and B-frames of the MPR display, the operator can view complex cardiac anatomy in corresponding transverse and longitudinal planes simultaneously. So, for example, an anomalous vessel that might be disregarded in cross-section is confirmed in the longitudinal plane.
3D rendering is another analysis capability of an acquired volume. It is familiar from static 3D applications, such as imaging the fetal face in surface rendering mode. In fetal echocardiography it is readily applied to 4D scanning. The operator places a bounding box around the region of interest within the volume (after arriving at the desired plane and time) to show a slice of the volume whose depth reflects the thickness of the slice. For example, with the A-frame showing a good four-chamber view, the operator places the bounding box tightly around the interventricular septum. The rendered image in the D-frame will show an ‘en face’ view of the septum. The operator can determine whether the plane will be displayed from the left or right, i.e. the septum from within the left or the right ventricle; the thickness of the slice will determine the depth of the final image, for example to show texture of the trabeculations within the right ventricle (Figure 4).
TUI is a more recent application that extends the capabilities of MPR and rendering modes. This multislice analysis mode resembles a magnetic resonance imaging or computer-assisted tomography display. Nine parallel slices are displayed simultaneously from the plane of interest (the ‘zero’ plane), giving sequential views from − 4 to + 4. The thickness of the slices, i.e. the distance between one plane and the next, can be adjusted by the operator. The upper left frame of the display shows the position of each plane within the region of interest, relative to the reference plane. This application has the advantage of displaying sequential parallel planes simultaneously, giving a more complete picture of the fetal heart (Figure 5).
3D/4D with color Doppler, 3D power Doppler (3DPD) and 3D high definition power flow Doppler
Color and power Doppler have been extensively applied to fetal echocardiography; one could hardly imagine performing a complete fetal echo scan today without color Doppler. Color or power Doppler, and the most recent development, high-definition flow Doppler, can be combined with static 3D direct volume non-gated scanning to obtain a 3D volume file with color Doppler information or 3DPD (one-color) volume files.
Color Doppler can be used more effectively in 3D/4D ultrasonography when combined with STIC acquisition12 in fetal echocardiography, resulting in a volume file that reconstructs the cardiac cycle, as above, with color flow information. This joins the Doppler flow to cardiac events2 and provides all the advantages of analysis (MPR, rendering, TUI) with color. This combination of modalities is very sensitive for detecting intracardiac Doppler flow through the cardiac cycle, for example mild tricuspid regurgitation that occurs very early in systole or very briefly can be clearly seen13.
Extreme care must be taken when working with Doppler applications in post-processing, however, to avoid misinterpretation of flow direction as the volume is rotated.
3DPD is directionless, one-color Doppler that is most effectively joined with static 3D scanning2. 3DPD uses Doppler shift technology to reconstruct the blood vessels in the VOI, isolated from the rest of the volume. Using the ‘glass body’ mode in post-processing, surrounding tissue is eliminated and the vascular portion of the scan is available in its entirety for evaluation. The operator can scroll spatially to any plane in the volume (but not temporally: in this case, color Doppler with STIC is more effective, see above). 3DPD can reconstruct the vascular tree of the fetal abdomen and thorax14, 15, relieving the operator of the necessity of reconstructing a mental picture of the idiosyncratic course of an anomalous vessel from a series of 2D planes. This has been shown to aid our understanding of the normal and anomalous anatomy and pathophysiology of vascular lesions16 (Figure 6).
High-definition power flow Doppler, the newest development in color Doppler applications, uses high resolution and a small sample volume to produce images with two-color directional information with less ‘blooming’ of color for more realistic representation of vessel size. It depicts flow at a lower velocity than does color or power Doppler, and has the advantage of showing flow direction. It can be combined with static 3D or 4D gated acquisition (STIC) and the glass-body mode, to produce high-resolution images of the vascular tree with bidirectional color coding (Figure 7). This technique is particularly sensitive for imaging small vessels. High-resolution bidirectional power flow Doppler combines the flow information provided by color Doppler with the anatomic acuity associated with power Doppler. Owing to this modality's sensitivity systolic and diastolic flow are observed at the same time, for example, when used with STIC acquisition the ductus venosus is shown to remain filled both in systole and in diastole.
Inversion mode (IM)
IM is another post-processing visualization modality that can be combined with static 3D or STIC acquisition17, 18. IM analyzes the echogenicity of tissue (white) and fluid-filled areas (black) in a volume and inverts their presentation, i.e. fluid-filled spaces such as the cardiac chambers now appear white, while the myocardium has disappeared. In fetal echocardiography it can be applied to create ‘digital casts’ of the cardiac chambers and vessels19. It can also produce a reconstruction of the extracardiac vascular tree, similar to 3DPD. IM has the additional advantage of showing the stomach and gall bladder as white structures, which can aid the operator in navigating within a complex anomaly scan. Most recently, IM has been joined with STIC to quantify fetal cardiac ventricular volumes, which may prove useful in the evaluation of fetal heart function.
B-flow is an ‘old-new’ technology that images blood flow without relying on Doppler shift. B-flow is an outgrowth of B-mode imaging that, with the advent of faster frame rates and computer processing, allows the direct depiction of blood cell reflectors. It avoids some of the pitfalls of Doppler flow studies, such as aliasing and signal drop-out at orthogonal scanning angles. The resulting image is a live gray-scale depiction of blood flow and part of the surrounding lumen, creating sensitive ‘digital casts’ of blood vessels and cardiac chambers (Figure 8). This also makes B-flow more sensitive for volume measurement. When applied to 3D fetal echocardiography B-flow modality is a direct volume non-gated scanning method able to show blood flow in the heart and great vessels in real-time, without color Doppler flow information20.
Screening Examination of the Fetal Heart with 3D/4D Ultrasound
Guidelines for the performance of fetal heart examinations have been published by the Internatinal Society of Ultrasound in Obstetrics and Gynecology (ISUOG)21. These guidelines for ‘basic’ and ‘extended basic’ fetal cardiac scanning are amenable to 3D/4D applications, and 3D/4D can enhance both basic and extended basic fetal cardiac scans, as well as evaluation of congenital anomalies. Many research teams have applied 3D ultrasound and STIC acquisition to fetal echocardiography, and various techniques have been put forward to optimize the use of this modality.
A well-executed STIC acquisition5 contains all the necessary planes for evaluation of the five classic transverse planes of fetal echocardiography6, 7. The operator can examine the fetal upper abdomen and stomach, then scroll cephalad to obtain the familiar four-chamber view, the five-chamber view, the bifurcation of the pulmonary arteries, and finally the three-vessel and trachea view. Slight adjustment along the x- or y-axis may be necessary to optimize the images. Performed properly, this methodology will provide the examiner with all the necessary planes to conform with the guidelines. However, it must be remembered that STIC acquisition that has been degraded by maternal or fetal movements, including fetal breathing movements, will contain artifacts within the scan volume.
Among the most attractive facets of 3D/4D scanning is the potential for digital archiving and sharing of examination data over a network. These capabilities were applied by Vinals et al. to increase delivery of prenatal cardiac scanning to poorly-served areas. Local practitioners in distant areas acquired and stored 3D volume sets at their centers; they were subsequently sent over an internet link and analyzed by expert examiners in central locations22, 23. This can have important implications in increasing penetration of prenatal ultrasound services in poorly-served or outlying areas of many countries.
DeVore et al. presented the ‘spin’ technique8 combining MPR and STIC acquisition to analyze acquired volumes and simplify demonstration of the ventricular outflow tracts. Using this technique the operator acquires a VOI from a transverse sweep of the fetal mediastinum that includes the sequential planes of fetal echocardiography. In post-processing the outflow tracts view is imaged in the A-plane, and outflow tracts and adjacent vessels are then examined by placing the reference point over each vessel and rotating the image along the x- and y-axes until the full length of each vessel has been identified8.
Abuhamad proposed an automated approach to extracting the required planes from an acquired volume, coining the term ‘automated multiplanar imaging’ or AMI9. Based on the idea that the scanned 3D volume contains all possible planes of the scanned organ, it should be possible to define the geometric planes within that volume that would be required to display each of the diagnostic planes of a given organ, for example the sequential scanning planes of fetal echocardiography. Beginning with the four-chamber view, all the other planes are in constant anatomic relationship to this plane, and a computer-automated program could present those planes once the appropriate volume block had been acquired9.
Most recently, Espinoza et al. introduced a novel algorithm combining STIC and TUI10 to image the diagnostic planes of the fetal heart simultaneously, and facilitate visualization of the long-axis view of the aortic arch. However, it must be stressed again that for any post-processing technique, if the original volume was sub-optimal, subsequent analysis will be prone to lower image quality and the introduction of artifacts.
Nuchal translucency screening programs will refer approximately 3–5% of patients who are deemed to be high risk for fetal echocardiography24, 25, increasing demand for early targeted fetal heart scans. STIC acquisition is amenable to younger gestational ages, as the smaller fetal heart can be scanned in a shorter acquisition time, thus reducing the chance of acquisition degradation from fetal movements.
Functional evaluation of the fetal heart: ventricular volumetry
The evaluation of fetal heart functional parameters has long challenged fetal echocardiographers. While duplex and color Doppler flow nomograms have been quantified and are long-established in 2D fetal echocardiography, many of the pediatric and adult measures are based on end-systolic and end-diastolic ventricular volumes: stroke volume, ejection fraction, and cardiac output. Without electrical trace or clinically applicable segmentation methods to determine the ventricular volume, these parameters have eluded practical prenatal quantification. 3D ultrasound opens new avenues for exploration into ventricular volumetry26, 27 and mass measurement.
Bhat et al. used non-gated static 3D acquisition and STIC to obtain mid-diastolic scans of fetal hearts and applied virtual organ computer-aided (VOCAL) analysis to determine cavity volume. The result was multiplied by myocardial density (1.050 g/cm3) to obtain the mass28, 29.
We recently presented30 a methodology that combines STIC acquisition with IM to determine the end-systolic and end-diastolic stages in the cardiac cycle, then applied inversion mode to isolate the fluid-filled ventricular volume, which was measured using VOCAL analysis (Figure 9). The resulting volumes allowed quantification of stroke volume and ejection fraction30. It was found that both the inversion mode and VOCAL analysis were highly dependent on operator-determined threshold parameters, which affect the intensity of signal to be colored and included in the volumetry. A similar study of cardiac mass is under way.
3D/4D Ultrasound in the Diagnosis of Congenital Heart Disease
One of the great advantages of 3D/4D systems is digital storage capabilities, which allow the operator to store examination volumes for later analysis, away from the patient and time constraints of a busy clinic. Nowhere is this advantage so appreciated as in cases of congenital heart disease (CHD). Other professionals can be invited to view the examination; they might be anywhere where an internet link is available. This allows the first examiner the chance to consult with the attending physician, cardiologist, surgical or other management teams, genetic counselors and parents. Complex malformations can be elucidated for interdisciplinary discussion and for laymen. In addition, stored data from cases of CHD are invaluable teaching materials for professional education.
Many teams have applied 3D/4D ultrasound capabilities to the diagnosis of congenital cardiovascular malformations. Each of the modalities and applications described above lends itself to different facets of this complex endeavor.
As described above, a properly executed STIC acquisition results in a volume ‘block’, reconstructed to reflect a complete cardiac cycle. When this block of spatial and temporal image data is analyzed in post-processing, the operator can access and display any plane at any time-point in the cardiac cycle. Many of these planes are not readily accessible in 2D ultrasound; the term ‘virtual planes’ was coined to refer to these rendered scanning planes. The interventricular and interatrial septa (IVS, IAS) planes, and the coronal atrioventricular (CAV) plane of the cardiac valves' annuli, have been investigated and applied to the evaluation of CHD11. They were shown to have added value in the diagnosis of ventricular septal defect, restrictive foramen ovale, alignment of the ventricles and great vessels, and evaluation of the AV valves.
The segmental approach to CHD has helped to standardize the description of cardiac lesions. In addition, it has contributed to an understanding of the pathophysiology of the malformed developing fetal heart, and subsequently to our conceptualization and diagnostic imaging. The sequential segmental approach essentially divides the heart into three basic segments: the atria, the ventricles, and the great arteries. These are divided and joined at the level of the atrioventricular valves, and at the ventriculo-arterial junctions. The segmental approach to the diagnosis of CHD is comprehensively and concisely described elsewhere31; we will follow this sequence in describing the application and added value of 3D/4D in the diagnosis of CHD, through index cases of anomalies diagnosed in our center.
Veins and atria: total anomalous pulmonary venous connection and interrupted inferior vena cava with azygos continuation
Total anomalous pulmonary venous connection (TAPVC) is a many-faceted group of malformations affecting the pulmonary veins; the variations and classification are described in detail elsewhere32. Essentially, in these anomalies the pulmonary veins do not drain into the left atrium but rather to various other locations: the right atrium, great veins or abdominal veins. We describe a case of intradiaphragmatic TAPVC with drainage of the pulmonary veins to the portal vein. Figure 10a shows the use of MPR with the reference point to navigate this complex lesion. Placement of the reference point in the suspected anomalous blood vessel in cross-section (A-frame) showed the vessel in longitudinal plane in the B-frame. This confirmed that the finding was not an artifact, rather the characteristic vertical vein. 3D power flow Doppler displayed the idiosyncratic vascular tree and absence of the pulmonary veins (Figure 10b); rotation of the image in post-processing allowed overall examination of the lesion through 360°.
Interrupted inferior vena cava (IVC) with azygos continuation is shown in Figure 11. This cardinal vein anomaly results from primary failure of the right subcardinal vein to connect to the hepatic segment of the IVC32. Blood is shunted directly into the right supracardinal vein (which will become the superior vena cava (SVC)); blood from the lower body flows through the azygos vein to the SVC. In this instance, B-flow acquisition provided real-time representation of the anomalous course of the IVC and connection to the fetal heart. It showed the azygos vein draining into the SVC, as well as the aorta, in one three-dimensional image that would be impossible to obtain with 2D color Doppler scanning. B-flow scanning provided superior imaging of the slower blood flow in the azygos vein than was demonstrated with 3DPD.
AVSD is characterized by incomplete atrial and ventricular septation, forming a common atrioventricular junction. AVSD has many forms, all of which involve an abnormality of the AV valves. Figure 12 shows the use of 3D rendering of a STIC volume acquired with color Doppler to demonstrate the anomalous intracardiac flow resulting from the AVSD.
Another group of AV valve lesions is mitral or tricuspid valve atresia, dysplasia, or stenosis. Figure 13 shows the CAV plane in a case of tricuspid stenosis. This ‘virtual plane’ is obtained from a STIC volume with color Doppler, by placing the bounding box with the superior side active tightly around the level of the AV connection in the four-chamber view (Frame A); the plane is slightly adjusted along the x- and y-axes and the rendered image (Frame D) shows the AV valves with anomalous anatomy (compare normal CAV plane, inset). This virtual plane provides a three-dimensional look at the AV and semilunar valves' annuli, resembling the surgical plane seen when the heart is opened in surgery.
Ventricles: ventricular septal defects (VSDs)
Ventricular septal defects are perhaps the most common—and most commonly missed—congenital heart defect. The natural history and in-utero development of these lesions have been described elsewhere33. Several groups have proposed methods for evaluating the interventricular septum34, 35. By using MPR, with the reference point placed on the septum with the four chamber view in the A-frame, the B-frame will show the septum and defect ‘en face’ (Figure 14). We recommend however the use of the bounding box in 3D rendering from STIC acquisition with color Doppler. This method has the advantage of allowing the operator to place the ‘active’ side of the box to the right or left (i.e. from within the left or right ventricle) and of giving the resulting image (in the D-frame) depth, for a more detailed examination of the size and nature (and number) of the VSD(s). The addition of color Doppler will demonstrate blood flow across the lesion and show at what stage in the cardiac cycle and to what degree the shunting occurs.
Ventriculo-arterial junctions (conotruncal anomalies): transposition of the great arteries and tetralogy of Fallot
Transposition (or malposition or malalignment) of the great arteries (TGA) is the general name for a complex group of anomalies with widely varying anatomic and clinical presentations. When the sequential segmental approach is applied to systematic diagnosis of CHD31, the morphology of each successive anatomic segment is assessed in turn. The morphologic right and left atria and ventricles are established; now the examiner addresses the ventriculo-arterial junction and the accordance or discordance of the great arteries and ventricles.
3D rendering with color Doppler has been applied to the evaluation of suspected malalignment of the great vessels, by examining the CAV (‘surgical plane’) at the level of the AV and semilunar valves' annuli.
We applied B-flow scanning to the evaluation of TGA and found that it was more effective than 3DPD or inversion mode in visualizing the great vessels' structure and relationships. Figure 15 shows a case of complete d-transposition of the great arteries. The B-flow scan clearly showed blood flow into the ventricles and out through the malaligned vessels, demonstrating the anatomic variant of the anomaly and assisting our consultations with the parents and their attending physician.
Arterial trunks: pulmonary stenosis and right aortic arch
The use of 3D rendering of a STIC acquisition with or without color Doppler to obtain virtual planes is discussed above. The CAV plane is an excellent tool for the evaluation of the semilunar valves. Once the CAV plane has been obtained, the 4D-cine option can be initiated to evaluate blood flow across the valves through the cardiac cycle. Figure 16 shows a case of critical pulmonary stenosis with retrograde flow in the main pulmonary artery (MPA).
Right aortic arch is a defect resulting from persistence of the right dorsal aorta and involution of the distal part of the left dorsal aorta. There are two main types, with or without a retroesophageal component36. Figure 17 shows a case of right aortic arch diagnosed with B-flow imaging; this modality showed the idiosyncratic course of the aortic arch to the right of the trachea.
Functional evaluation: ventricular volumes
We recently presented30 a novel methodology that combined STIC acquisition with post-processing application of the inversion mode to quantify end-systolic and end-diastolic ventricular volumes. We examined 100 fetuses of 20–40 weeks' gestation, and created nomograms of right and left ventricle end-systolic and end-diastolic volumes. The resulting measurements correlated strongly with gestational age and estimated fetal weight. From these volumes we were able to create nomograms for fetal stroke volume and cardiac ejection fraction.
During the study period we applied this methodology to saved STIC volumes of cases presenting with cardiac anomaly or dysfunction that showed changes in ventricular volume, stroke volume, or ejection fraction. These included critical pulmonary stenosis, twin-to-twin transfusion syndrome with secondary pulmonary stenosis, aortic valve stenosis with hypoplastic aortic arch, Ebstein's anomaly, supraventricular tachycardia (SVT), and vein of Galen aneurysm.
Our normal cases showed the effectiveness of fetal heart ventricular volumetry in cardiac evaluation and quantification; such volumetry is not readily available in 2D echocardiography. The pathological cases showed the potential added value of this methodology. In the case of critical pulmonary stenosis, for example, the diagnosis was more serious than suspected by 2D echocardiography. Ventricular volumetry also provided insight into the pathophysiology of lesions such as SVT and vein of Galen aneurysm, among others30.
Potential Pitfalls of 3D/4D Echocardiography
3D/4D fetal echocardiography scanning is prone to artifacts similar to those encountered in 2D ultrasonography, and some that are specific to 3D/4D acquisition and post-processing.
STIC acquisition quality
The quality of a STIC acquisition may be adversely affected by fetal body or ‘breathing’ movements; quality is improved by scanning with the fetus in a quiet state, and using the shortest scan time possible. When reviewing a STIC acquisition, the B-frame will reveal artifacts introduced by fetal breathing movements (Figure 18). If the B-frame appears sound, the volume is usually acceptable, and can be used for further investigation. It must be stressed again that the quality of the original acquisition will affect all further stages of post-processing and evaluation.
Original angle of insonation
The original angle at which a scan was performed will impact on the quality of all the planes acquired. It is important to achieve an optimal beginning 2D plane before starting 3D or 4D acquisition.
Shadowing artifacts pose a particular problem for 3D/4D ultrasound. When commencing scanning from the 2D plane, acoustic shadows may not be apparent. However, they may be present within the acquired volume block. It is imperative to review suspected defects with repeated 2D and 3D scanning to confirm their presence in additional scanning planes.
3D rendering creates virtual images. It must be remembered that application of some algorithms designed to smooth the image can lead to loss of data from the original scan. 3D rendering should always be used in conjunction with the A-frame 2D image for comparison.
An acquired volume containing Doppler flow information is available for manipulation and may be sliced and rotated around the x-, y-, and z-axes for analysis. However, rotation of the volume with Doppler directional flow information can mislead the operator: if the directions are reversed, flow data can be misinterpreted. The operator must confirm any suspected pathological flow patterns by confirming the original direction of scanning, whether flow was toward or away from the transducer during the acquisition scan.
Several studies have compared imaging yield between 2D and 3D/4D fetal echocardiography, others have examined the feasibility of 3D/4D and STIC in screening programs, while others have described the application of various 3D/4D modalities to the diagnosis or evaluation of fetal cardiovascular anomalies. However, no large study has examined the contribution of 3D/4D ultrasonography to the accuracy of fetal echocardiography screening programs.
Levental et al. compared 2D and non-gated 3D ultrasound to obtain standard cardiac views37. Meyer-Wittkopf et al.38 evaluated 2D and Doppler-gated 3D ultrasound in obtaining standard echocardiography scanning planes in normal hearts. They found that 3D ultrasound provided additional structural depth and allowed a dynamic 3D perspective of valvar morphology and ventricular wall motion38.
In evaluating CHD, Meyer-Wittkopf et al.39 evaluated gated 3D volume sets of 2D-diagnosed cardiac lesions, and compared key views of the heart in both modalities. They determined that 3D had added value in a small proportion of lesions39. Wang et al.40 compared 3D and 2D scanning of fetuses in the spine-anterior position. This group found that only in the pulmonary outflow tract was 3D ultrasound superior to 2D.
Espinoza et al.18 examined the added value of IM in the evaluation of anomalous venous connections. The investigators found that IM improved visualization of cases of dilated azygos or hemiazygos veins and their spatial relationships with the surrounding vascular structures.
Most recently, Benacerraf et al.41 compared acquisition and analysis times for 2D and 3D fetal anatomy scanning at 17–21 weeks' gestation. 3D ultrasound compared favorably with 2D in mean scanning time and accuracy of fetal biometry.
The data archiving and networking capabilities of 3D/4D fetal echocardiography with STIC acquisition open up new avenues for disseminating fetal echocardiography programs to distant or poorly served areas. This can have important public health implications in these populations. Michailidis et al.42 and Vinals et al.22, 23 have shown the feasibility and success of programs based on 3D/4D exam volumes acquired in one center, and reviewed by experts in a center connected by internet link.
In coming years, studies will direct 3D/4D capabilities to the evaluation of fetal cardiac functional parameters. This may provide insights into the physiological effects of fetal structural or functional cardiac defects, or maternal diseases such as diabetes, on the developing fetus.
To the best of our knowledge, no large study has been performed to date to examine whether the addition of 3D/4D methods to fetal echocardiography screening programs increases the detection rate of cardiac defects. This technology has reached the stage when its reproducibility and added value in screening accuracy should be tested in large prospective studies, not only by teams or in centers that have made 3D/4D their specialty, but among the generality of professionals performing fetal echocardiography.