Introduction Prenatal evaluation of the fetal heart is one of the most challenging components of the obstetrical ultrasound examination. Sonographers must conduct a comprehensive examination, which includes a detailed assessment of the four-chamber view (Table 13.1),1,2 the connections of the great vessels to the ventricular chambers, the venous return to the heart, and cardiac rhythm. Besides the limited amount of time that is usually available for a comprehensive ultrasound examination in clinical practice, other challenges that are of particular relevance to the examination of the fetal heart include the following, which may negatively impact image quality: (1) maternal obesity, abdominal scars, anterior placentas, and oligohydramnios; (2) frequent movement or breathing during the examination; (3) less than ideal fetal position that cannot be controlled by the operator; and (4) faster heart rates than adult and pediatric patients.3-6 Thus, extensive training is required to develop the skills necessary to effectively examine the fetal heart. Operator skill is considered one of the most important factors affecting prenatal diagnosis of congenital heart disease.7-12 Failure to diagnose a life-threatening cardiac disorder in utero may negatively impact survival, since there is evidence that prompt intervention after delivery is associated with improved outcomes for disorders such as transposition of the great arteries, hypoplastic left heart syndrome, and coarctation of the aorta.13-17
Four-dimensional ultrasonography (4DUS) with spatiotemporal image correlation (STIC) allows examiners to acquire volume datasets of the fetal heart using grayscale imaging only or with the addition of blood flow information from color Doppler, power Doppler, or B-flow imaging.5,18-25 Once acquired, volume datasets can be examined using the standard planes of section of two-dimensional ultrasonography (2DUS), as well as novel planes that are only possible by volumetric imaging. These volume datasets can be thought of as “digital specimens” of the fetal heart, akin to actual heart specimens that are examined by pathologists during a necropsy. The “digital heart” can be oriented on the screen to be displayed in a standardized position, after which standard
planes of section are obtained with the use of “digital scalpel” tools that allow visualization of one or more slices of the “digital heart.” Moreover, sophisticated three-dimensional (3D) rendering techniques can be applied to display “digital casts” of cardiac chambers and great vessels that look similar to postmortem casts obtained by injecting silicone rubber into cardiovascular structures.22,26-28 Advantages of the “digital specimen” when compared to the “actual specimen” include the following: (1) functional information is preserved since heartbeats are included in the volume dataset; (2) the direction of blood flow can be analyzed in volume datasets acquired with color or power Doppler; and (3) if the examiner accidentally makes a mistake during the review of the volume dataset, the “digital specimen” is not damaged forever, and all that it takes to begin the examination all over again is to reset it to its original state with the click of a button.