The added value of 3D/4D ultrasound imaging in fetal cardiology: has the promise been fulfilled?

As sonography matures, with its constant technological advances and ever-growing base of experienced examiners, it is reasonable to expect that its sensitivity and specificity in the diagnosis of fetal anomalies will increase. Perhaps the most dramatic advance has been the introduction of three-dimensional (3D) and real-time 3D (4D) ultrasound imaging1-7, which have become common adjuncts to fetal examinations in many centers. In fetal echocardiography, the 4D ultrasound modality applied most widely to the assessment of structure and function is spatiotemporal image correlation (STIC). Some 50 papers have been published on STIC since its first appearance in 2003 in this Journal8, 9 and two more papers examining the reproducibility and reliability of STIC appear in this issue. The study by Bennasar et al.10 examines the reliability of qualitative evaluation and quantitative measurements of STIC-derived images. The study by Uittenbogaard et al.11 uses an in-vitro balloon model to examine the reproducibility of STIC-derived ventricular volume measures. While these studies add to the literature on STIC technology itself, they do not address another practical aspect of our use of STIC: does it improve rates of diagnosis or diagnostic accuracy in fetal echocardiography? Briefly, following STIC acquisition, rendered 3D ultrasound and multiplanar images of the heart are created from the volume dataset. The different views are available for review as still or moving images. During acquisition, STIC can be combined with B-flow, color or power Doppler, high-definition power Doppler and tissue Doppler. During post-processing, visualization modalities such as 3D volume rendering, inversion mode and tomographic ultrasound imaging (TUI) can be applied12-20. The saved volume can be manipulated offline, minimizing patient scanning time. STIC appears to offer promise of improvement in four main areas: increased diagnostic accuracy by virtue of the ability to view the heart from planes other than the acquisition plane and the depth perspective available on rendered images; evaluation of unique functional parameters; data availability for expert review offline, even from a remote computer; and improved patient counseling and interdisciplinary consultation through more comprehensible images, as well as creation of a library of cardiac anomalies that would be invaluable teaching materials for professional education. The images produced by this technology9, 21 are impressive, and occasionally allow potentially complicated diagnoses to be made at a glance. But has 3D/4D ultrasound imaging in general, and STIC in particular, enhanced our diagnostic capabilities in fetal cardiology? Has it resulted in improved screening results and diagnostic accuracy? Has the promise been fulfilled? The STIC technique is readily acquired in most cases22, and integrating STIC does not increase scanning time materially. This has also been our experience. By transmitting STIC datasets on the internet, remote diagnoses of anomalies have been achieved as early as 11 weeks' gestation16. We and others have applied STIC technology in teaching fetal cardiology, and find it to be an effective training tool for both normal fetal echocardiography and elucidating fetal cardiovascular anomalies. 3D ultrasound23 and STIC24-27 also facilitate the evaluation of cardiac function: only by using 3D/4D ultrasound can cardiac volumes be measured reliably and easily27, 28 and derivate indices such as the ejection fraction23 and 4D myocardial performance index25 be calculated, opening up new areas of research and clinical application. Two-dimensional (2D) fetal echocardiography (with color Doppler) under optimal conditions may achieve detection rates of up to 92%29-32 for congenital heart disease. It must be remembered that some cases of CHD will inevitably be missed at a mid-trimester scan, because of the developmental nature of the lesions29. In addition, not all other missed diagnoses are technology-dependent: operator error will affect anomaly detection regardless of technology29, 33. Our experience, in a tertiary referral center, of the added value of 3D/4D ultrasound to our examination protocols is only moderately encouraging. For example, in only 11/155 of our cases with confirmed anomalies did 3D/4D ultrasound add to the diagnostic accuracy: four ventricular septal defects diagnosed with the aid of virtual planes; two cases of total anomalous pulmonary venous return diagnosed with multiplanar imaging; one right ventricle aneurysm diagnosed with B-flow; one transposition of the great arteries with pulmonary atresia, and one segmental interrupted aortic arch diagnosed with TUI; and two portosystemic shunts to the coronary sinus diagnosed by multiplanar imaging and B-flow. In all these cases other anomalies observed on 2D ultrasound examination prompted in-depth cardiovascular evaluation34. It is perhaps not surprising that in the environment of a tertiary referral center, where the detection rate of CHD is > 90%, the margin of improvement with the introduction of new technology is modest. To discover whether 3D/4D ultrasound improves detection rates in a screening center, where the potential margin of added value of the technology may be greater, we must investigate its use under such conditions. All the information available on the 3D/4D scan is present on the 2D scan; the allure of 3D ultrasound is its novel information display. STIC acquisition, performed properly, will contain all the planes necessary to perform fetal echocardiography according to ISUOG (International Society of Ultrasound in Obstetrics and Gynecology) guidelines35. An experienced operator takes the information from multiple 2D planes36, 37 and reconstructs them in his mind. The 3D/4D modalities take this reconstructed information and display a more complete—and therefore more comprehensible—picture, so those examiners least experienced in this mental reconstruction stand to gain most from the technology. 3D/4D ultrasound also facilitates the display of standardized images of the heart13, 38-41, moving towards automated retrieval of fetal cardiac ultrasound planes from a volume dataset39, 40, 42. How close we are to standardization and automation of ultrasound diagnostic imaging, and whether these could confer any advantage in anomaly detection rates, remains to be seen. 3D/4D ultrasound modalities in fetal cardiology are here to stay. We expect that their proven value in research, in studying cardiac function, in sharing information between professionals, in teaching and in patient counseling will lead to their further penetration into our field. Their ability to increase detection rates in screening programs has not been shown. However, as the technology matures we may see corresponding improvement in diagnostic accuracy.