Terminology of three‐dimensional and four‐dimensional ultrasound imaging of the fetal heart and other moving body parts

With the recent advent of high spatial and temporal resolution three-dimensional (3D) ultrasound, fetal moving anatomy, especially the heart, will become a major interest of clinical research and application. This Opinion discusses terms (Table 1) used (or misused, in the author’s opinion) for describing various multi-dimensional imaging features, including ‘3D’, ‘four-dimensional (4D)’, ‘real time’, ‘cardiac gating’, ‘online’ and ‘offline’. Real-time imaging should only be used to indicate a system capable of displaying images (1) virtually as they are acquired, and (2) at about a cinematic rate (to ‘fool’ our visual perception). It should not be used (1) to describe systems only capable of displaying images with a certain delay after data acquisition, or (2) to imply whether or not the rate is sufficient to distinguish temporal events, such as rapid fetal cardiac phasic changes. I believe the terms ‘direct’ and ‘indirect’ volume scans should be introduced (Table 1) to describe whether a volume can or cannot be scanned within a time sufficiently short that movement is negligible, emphasizing the relativity in speed between 3D scanning and target motion. With conventional, slice-reconstruction 3D approaches, data are acquired using an imaging plane scanning over a volume of interest (VOI). The 3D scanning needs to cope with three situations in terms of VOI motion. First, if the VOI is an immobile target, such as a stationary fetal face, only spatial tracking of the imaging plane movement is necessary for correct reconstruction of the acquired slices into 3D images. Second, if the VOI is a target in regular motion (such as the heart), temporal tracking must also be done to allow the slices to be reconstructed not just correctly in 3D spatial dimensions but also correctly in the fourth, temporal dimension (cardiac cycle). In pediatric and adult studies, electrocardiography, by means of cardiac gating, has successfully served this purpose. Unfortunately, it cannot be reliably used in fetal studies due to maternal and other interference. Third, if the VOI shows irregular motion (such as a random fetal smile) within the time constraint for scanning, there is no way to synchronize the movements with the slice-reconstruction approaches. The resulting 3D images will be degraded by motion artifacts (Figure 1). Over the last several years, new methods and techniques have been developed in order to avoid image degradation caused by motion of the anatomy and to attain dynamic information arising from the motion. The most related developments are real-time 3D imaging1–7, sonographic motion gating8–15, and minimally compressive scanning16,17. Real-time 3D ultrasound makes it straightforward to comprehend some morphological dynamics, such as yawning, sucking, smiling, crying and blinking1–4. This offers a practical means for assessment of neurophysiological development, as well as for detection of anatomical pathology18–20. Ultrasonic cardiac gating can be performed offline8,9 or online. Online gating can be achieved by pre-3Dacquisition heart-rate setting10,11, by in-3D-acquisition (real-time) tracking12,13 or by post-3D-acquisition correlation14,15. Cardiac cyclical information is extracted by M-mode, spectral Doppler or similar techniques from the fetal heart or arteries, allowing the removal of motion artifacts and the creation of dynamic 3D (or 4D) images of the in-utero heart. Preliminary gated and non-gated studies have shown the potential of 3D for depiction of complex normal/abnormal cardiac structures (Figure 2)12,21,22 and intracardiac flow22,23, for exclusion (including tele-screening) of major congenital cardiac defects11,15,24–27, for estimation of overall heart volume28 or stroke volume29, and for detection of fetal arrhythmias30,31. With the use of real-time 3D ultrasound, 4D cardiac data can now be acquired more easily, sometimes without the need for cardiac gating5–7. The latest progress in matrix transducer technology will see all future 4D acquisitions being carried out with real-time 3D systems, and with real-time gating when necessary32,33. Based on our experience, together with a review of related (English) literature, this Opinion attempts to discuss and define the above terms. Although theoretical, this discussion may be of help for objective selection of competent 3D systems for specific scientific and clinical applications.