Evaluation of the Fetal Heart by Four-dimensional Echocardiography 49 Evaluation of the Fetal Heart by Four-dimensional Echocardiography Paolo Volpe, Valentina De Robertis, Gianluca Campobasso, Nicola Volpe Department of Obstetrics and Gynecology, Hospital Di Venere, A.S.L. Bari, Bari, Italy Correspondence: Paolo Volpe, Divisionre di Ostetricia e Ginecologia, Ospedale Di Venere, 70125, Bari, Italy e-mail: paolo-volpe@libero.it INTRODUCTION Four-dimensional ultrasonography is a technology that adds the temporal dimension (motion) to 3-dimensional imaging. 1 Four-dimensional visualization of the fetal heart became a practical reality with the incorporation of spatiotemporal image correlation (STIC) algorithms into commercially available equipments. 2,3 STIC is a technique that allows acquisition of fetal heart volume and visualization of cardiac structures as a 4D cine sequence. 2,3 In this technique, the array inside the transducer housing performs a single slow sweep, recording a single 3D dataset. STIC derives the heart rate from the periodicity of the movements of the cardiac structures, and eventually a single virtual cardiac cycle, each frame of which is the result of the overlay of many acquired frames, is reconstructed. This volume consists of a high number of 2D frames. The acquisition of high-quality volume datasets is crucial for the successful examination of the fetal heart by 4D US. 4,5 Until recently, only gray scale volumes could be acquired with STIC. A new technological advance has added the possibility of incorporating color and power Doppler, B-flow and high-definition flow Doppler information into the volume dataset. Color and power Doppler have a slower frame rate compared with gray scale during scanning, which leads unavoidably to a reduction in image quality using color STIC when compared with gray scale STIC. PLANE OF ACQUISITION Once acquired, the volume is opened and can be used offline for multiplanar navigation and/or four-dimensional image renderings using the various modes described below. It must be kept in mind that in case of fetal arrhythmias this technique cannot be used since the anomalous fetal heart rate would compromise the adequacy of the acquired volume. The quality of the heart volume dataset depends on the frame rate of the two-dimensional (2D) image, the angle sweep and the acquisition time. 4 If the operator is interested in the evaluation of the 4- chamber, 5-chamber and 3-vessels and trachea views, the best approach to acquire an adequate volume is to start the acquisition procedure while displaying an apical 4-chamber view. Instead, if the examiner wants to review the aortic and ductal arches, or the venous return to the heart, high-quality volume datasets are best acquired using sagittal sweeps through the fetal thorax. 4 Acquisition Angle The volume of interest is acquired with a sweep angle of approximately 20-35° (depending on the size of the fetus ) that are usually sufficient to include the stomach, the heart, and its vascular connections in the volume dataset. Acquisition Time The acquisition time can be selected by the user and usually ranges from 7.5 to 12.5 sec. Using the longest possible acquisition time, the highest spatial resolution of the volume dataset is obtained. However, the longer the sweep takes, the higher the likelihood that artifacts related to fetal motion may be present in the volume dataset. 4,5 Multiplanar Display Volume datasets are displayed using multiplanar slicing and the original plane of acquisition is displayed in plane A (upper left panel) of the screen and the 2 orthogonal planes to the reference plane in the right upper panel (plane B) and left lower panel (plane C), respectively (Fig. 1). The basic approach to the examination of a volume dataset of the fetal heart is to simply scroll through the volume from the top to the bottom along the original plane of acquisition. In this way all echocardiographic planes can be obtained offline with the multiplanar mode. The reference dot guides the operator in navigating within the volume, as it is anchored at the point of intersection of the three planes (Fig. 1). It has to be underlined that the lightness with which a beginner as well as an experienced operator can navigate the volume of the heart and great vessels contributes significantly to a better understanding of the three- dimensionality of the central cardiovascular structures. Tomographic Ultrasound Imaging (TUI) This modality allows for the simultaneous display of up to 8 parallel planes, whose distance can be adjusted for a better Donald School Journal of Ultrasound in Obstetrics and Gynecology, July-Sept. 2007;1(3):49-53