Three-dimensional (3D) and four-dimensional (4D) sonography have been proposed as adjunctive diagnostic imaging modalities for prenatal diagnosis of congenital heart disease1–24. Here we report 3D and 4D rendering of the aortic and ductal arches using a novel approach to 3D reconstruction of hollow structures (Inversion 3D mode using a Voluson 730 Expert (General Electric Medical Systems Kretztechnik, Zipf, Austria) ultrasound machine and 4DView 2000 version 2.1, General Electric Medical Systems Kretztechnik). This rendering algorithm transforms echolucent structures into solid voxels. Thus, anechoic structures such as the heart chambers, lumen of the great vessels, stomach and bladder appear echogenic on the rendered image, whereas structures that are normally echogenic prior to gray-scale inversion (e.g. bones) become anechoic.
Figure 1 shows volume-rendered images of the aortic and ductal arches of a fetus with normal cardiac anatomy examined at 22 + 2 weeks. The volume dataset was acquired with sagittal sweeps through the fetal chest and abdomen using the spatio-temporal image correlation (STIC) technique. 3D reconstruction was performed with a combination of ‘gradient light’ and inversion rendering algorithms. Low threshold and transparency levels were adjusted until the structures of interest were visualized. In this particular case, the transparency level was set to 57, and the low threshold level to 91 (both scales range from 0 to 250). Although we chose to use the gradient light algorithm for better visualization, similar images could be visualized using a combination of ‘X-ray’ and ‘surface smooth’ algorithms, or the surface smooth algorithm alone. This volume dataset was rendered with the direction of view set from the right to the left side of the body. As the inversion mode cannot distinguish between blood vessels and other hollow structures (e.g. stomach and gallbladder), the rendered image depicts all anechoic structures contained within the region of interest. This characteristic of the method allowed for simultaneous visualization of the esophagus, ascending aorta, aortic arch, descending aorta, pulmonary artery, ductus arteriosus and superior vena cava in the 3D rendered image. Since the diaphragm is normally visualized as a hypoechoic structure by two-dimensional ultrasound, it appeared as a thin echogenic rim separating the thorax from the abdomen after applying the inversion mode. Abdominal structures depicted in Figure 1 include the portal vein, stomach and a portion of the gallbladder. Four-dimensional rendering of this image can be downloaded from the Journal's website (Videoclip S1).
A case of transposition of the great arteries at 28 weeks is presented in Figure 2. In this image, the anterior vessel represents the aorta. The pulmonary artery courses parallel to the ascending aorta. Left and right branches of the pulmonary artery and the ductus arteriosus are clearly visualized. As this fetus was not swallowing at the time of acquisition, the esophagus was not visualized. The rendering technique used to obtain this image was identical to the one used for Figure 1, except that the transparency and low threshold levels were set to 49 and 106, respectively. Four-dimensional rendering of this image can be downloaded from the Journal's website (Videoclip S2).
In conclusion, we have described the application of 3D and 4D rendering of the outflow tracts and other hollow structures of the thorax and abdomen using the novel inversion mode. Since this technique does not use color or power Doppler sonography as a ‘digital contrast’ to highlight the blood vessels, it does not have the inherent limitations to image reconstruction related to the angle of insonation, temporal resolution, or intensity of the Doppler signal. In addition, the technique does not differentiate blood vessels from other hollow structures. Therefore, the relationships between the fetal heart and great vessels and other fluid-filled non-vascular structures such as the esophagus and stomach can be visualized in a single volume dataset. This feature may prove valuable in the prenatal evaluation of complex disorders requiring precise definition of visceral situs, such as heterotaxy syndromes. Among the limitations of inversion mode are the absence of information regarding the velocity or direction of blood flow, and that it may not work well in cases with poor tissue contrast between adjacent echolucent structures.