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. Direct volume: scanned in totality within a time in which movement is negligible with sufficient spatial resolution 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). Target speed, scanning speed, motion artifact and motion gating. Some Chinese opera actors are very skilled at changing faces—altering facial masks in a blink. Imagine an actor can change one face per second and cyclically makes four changes (upper panel). If a camera scans the actor's head from the reader's left to right, and the scanning speed is 1 s per head-width, it is possible (though not necessary) for every scan to capture an entire face (lower panel, left). If the scanning speed is 4 s per head-width, one scan can only capture one quarter of each of the four faces, e.g. the 1st, 2nd, 3rd and 4th quarters of the four faces, respectively (lower panel, middle), causing distortion (motion artifact) in the resulting photograph. However, a 4-s scan can be synchronized (gated) with the face changing, say, starting the first scan with the red-cheeked face through to the white-cheeked face, then moving the camera right for a quarter of a head-width and repeating the scan, and repeating this process twice more. Four non-distorted faces can then be reconstructed; shown here is only the red-cheeked face (lower panel, right) from the 1st, 2nd, 3rd and 4th quarters of the 1st, 2nd, 3rd and 4th scans, respectively. This gating principle for the actor changing faces applies similarly in cardiac gating as the heart changes phases. (Peking Opera face paintings by M.L. Zhao, courtesy of http://www.jingjuok.com). 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-3D-acquisition 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. Four-dimensional (4D) ultrasound imaging of a 21-week-old fetal heart in part and in totality. (a) Real-time three-dimensional (3D) surface display of the open heart revealed a ventricular septal defect during a ‘Live 3D’ scan (using Philips Sonos7500, Philips). (b) The whole chest and upper abdomen were also acquired by four gated imaging volumes. Three imaging planes, reformatted offline using 4D CardioView (TomTec, Munich, Germany), demonstrated simultaneously all three features required for confirmative diagnosis of fetal tetralogy of Fallot: the ventricular septal defect, the overriding aorta and the narrowed pulmonary trunk. The detailed visualization of the entire course of the pulmonary artery and branches (including the secondary branches) is very important for helping parental counseling and surgical planning. (Movies can be found under the Fetal Heart entry on http://www.medphys.ucl.ac.uk/mgi/jdeng). 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. When time is treated as a dimension, a two-dimensional (2D) display can mean either a display with one temporal and one spatial dimension (such as M-mode waveforms) or a display with two spatial dimensions (as seen in early, static cross-sectional ultrasound). Similarly, a 3D display can mean either a display with one temporal and two spatial dimensions (as seen in real-time cross-sectional ultrasound) or a display with three structural dimensions (as seen in static 3D ultrasound). To avoid such confusion, it is advisable to restrict the use of ‘dimension(s)’/‘dimensional’ to indicate only spatial dimension(s) while using ‘time’/‘temporal’, ‘motion’/‘moving’, or ‘dynamics’/‘dynamic’ to indicate the temporal dimension. An exception can be made in the case of 4D which can be unmistakably used to indicate the three spatial dimensions plus the temporal dimension. 4D imaging is a simple description of space and time which most of us can comprehend without any difficulty. To facilitate re-creation of a virtual functioning anatomy for detailed analysis, it is sometimes necessary to treat as additional dimensions some distinct biophysical or biochemical properties, such as the heart sounds, myocardial kinetics, intracardiac hemodynamics and oxygenated hemoglobin saturation. It may also be necessary to divide one primary dimension into several secondary dimensions. For example, the temporal dimension should be further split into maternal and fetal cardiac cyclical dimensions when the placental circulation is studied. Several terms have been used to describe high volume-rate scanning for 3D/4D imaging (note, the term ‘frame rate’ used in cross-sectional imaging should be replaced with ‘volume rate’ in 3D imaging). The most commonly used (and probably the most attractive) term is ‘real time’, such as in ‘real-time 3D’, ‘real-time 4D’, and ‘real-time volumetric’ imaging. These catchphrases are usually introduced by manufacturers for commercial promotion, but the actual volume rates to which they refer vary greatly (c. 8–24 Hz). According to the Oxford dictionary, ‘real time’ means the actual time during which a process or event occurs, especially one analyzed by a computer, in contrast to subsequent time when, for example, computer processing may be done or a recording may be replayed. In sonography, the term should be used to indicate a system's ability to display dynamic morphology or morphological dynamics as the anatomical data are being acquired, or with a negligible delay (a fraction of a second) between acquisition and visualization. It should also imply that 2D imaging frames or 3D imaging volumes can be updated within an interval equal to or shorter than the persistence of vision (or ‘impression’). It is generally stated that the human eye and brain retain a visual impression for about one 10th to one 30th of a second (equivalent to a frame or volume rate of 10–30 Hz). The exact time depends on the brightness of the image and perhaps individual variation, but most people would probably agree that a rate less than 10 Hz will certainly produce ‘jerky’ movies. Strictly speaking, therefore, a 3D ultrasound machine incapable of a volume rate at or above 10 Hz should not qualify as a real-time imaging system. Another point is that the threshold values for visual persistence are based on human tests (although traditional explanations for this phenomenon and the preciseness of the term itself have been challenged34). Consider an eagle flying high in the sky and at high speed, chasing a rabbit running rapidly on the ground; the predator must have much shorter (finer) persistence to be able to catch its prey. Hence, it is conceivable that persistence is species-dependent. When talking about real-time imaging, we are actually talking about whether an imaging system is capable of achieving the visual continuity to satisfy the human eye/brain. Therefore, ‘real time’ is an observer-dependent, subjective concept; it does not necessarily reflect whether a frame or volume rate is adequate for visualizing the dynamics of an object for specific scientific or clinical purposes (Figure 3). Schematic estimation of sufficiency of various temporal resolutions for distinguishing dynamic events of different time scales. At a 25-Hz volume rate, each of the four long (but not the two short, isovolumetric) cardiac phases can be fully sampled (darker blocks on each color column) at least once and most likely twice (A1, A2), making systole and diastole distinguishable during imaging. At 50 Hz, the short phases may (B2 during isovolumetric contraction) or may not (B1, and B2 during isovolumetric relaxation) be fully sampled once, resulting in the phases not always being recognizable in the images. At 100 Hz, these short phases can be fully sampled at least once and maximally twice, allowing their existence to be identified during a cardiac cycle. However, the accuracy for measuring the isovolumetric contraction time will only be about 50% at this rate. Note the time delay between the atrial and ventricular systole and diastole in the bars across the top, indicating potential pitfalls of using systolic peaks for gating the whole heart. In medical imaging, what matters is the ability to distinguish and analyze an anatomy's dynamic events, rather than to see it moving continuously in our mind. Hence, there is a need for introducing objective terms to describe whether a scanning system is capable of achieving this. A ‘direct volume scan’ refers to any volume scan in which a VOI is scanned (1) in totality, (2) in an instant, and (3) with sufficient spatial resolution. An instant is defined here as a time so short that during this period the spatial movement of interest of the target and/or its components is negligible. If any of the three conditions is not met, a volume scan is considered an ‘indirect volume scan’. In contrast to real time, the two new terms are object-dependent concepts. The differentiation between direct and indirect volume scanning depends on the relativity between the speed of volume scanning and that of target motion (Table 2), or, by analogy, the relativity between the camera shutter speed and the face changing speed (Figure 1). The importance of establishing whether a volume scan is indirect or direct is to help the operator determine whether or not spatial and temporal tracking are necessary when performing the scan. There are several aspects to be considered. As illustrated in Table 2, a direct volume scan can be completed even with a slice-reconstruction 3D approach, if a VOI stays immobile during the entire acquisition. However, it is hard to find consistently stationary VOIs in-utero due to unwanted patient movements (e.g. fetal activity, maternal respiration) and/or environmental movements (e.g. probe-movement-induced abdominal deformation when using conventional approaches). Real-time 3D systems can acquire a volume dataset without manual movement of the transducer. This is achieved by using a built-in mechanism for registering spatial and temporal information during a volume scan. Example systems include Philips Live 3D (based on matrix-array transducer technology33, 35, 36, and Kretz Combison 730 (based on rapidly cross-sectional transducer 15, and only one or more of the are also met, a real-time 3D scan can be as a direct scan, and no additional spatial and temporal tracking are In terms of cardiac 3D imaging, using a slice-reconstruction is certainly indirect volume scanning, but using a real-time may be either indirect or direct volume scanning (Table For of dynamic morphology of the fetal heart, we using a volume rate of about Hz (equivalent to a of for each imaging volume as the point between direct and indirect volume This is for isovolumetric other cardiac phases each last than are used in this discussion for and can be fully sampled at least once in one imaging volume per cardiac (Figure 3). However, the point between direct and indirect volume scanning will be by the different temporal resolutions for example, the of assessment is the myocardial then the time of interest will be to the isovolumetric of only It is for a 25-Hz volume rate to distinguish this from (Figure more in time point is shorter than a cardiac the volume scanning is In other real-time imaging does not necessarily mean that imaging of all dynamic is A scan is only capable of the maximally in one imaging volume per with no accuracy in measuring the To accuracy for the a volume rate may be This is the of any of the real-time 3D systems, which are to a volume rate. of the systems can be as being capable of direct volume scanning for this important 2D Doppler imaging of Hz may be able to the of a much The cardiac structures can be into major and scales. 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A direct volume scan must also be able to reliably the and This for real-time 3D systems which only major fetal cardiac but the latest real-time 3D systems can now much spatial resolution and structures (Figure For a volume scan to be as being point is whether an VOI can be acquired within a time constraint such as a cardiac without moving the transducer. we are to the if certain cannot be achieved without additional spatial and temporal as an example, the measuring the changes of the whole ventricular between and If a real-time imaging volume is to the in their the acquisition should not be as direct volume In order to the entire the transducer has to be However, as as the transducer is during a 4D scan, spatial tracking and cardiac gating become This is of whether a cross-sectional or a transducer is It needs to be out if the objective of a is not to stroke volume but to a cardiac the left it is possible to it in several imaging volumes. conditions in the three are met, each scan can be treated as a direct the need for gating in this direct scanning an operator to 4D image during acquisition imaging and The imaging will on the between the of a VOI and that of an imaging In other the the of additional In studies, the include fetal and and the VOI from the and its with the imaging of the fully chest (and 4D acquisitions of the heart have to (and be made by through different imaging to avoid all real-time 3D systems an imaging volume with a of and a of (Figure 36, the VOI in the of will the of the whole most generally more with the heart, and the can be making most 3D scans using real-time systems when at into the of direct volume scan. It is usually to any additional acquired data can be in 3D (using reformatted and/or or 3D some has to be carried time on computer terms ‘online’ and are used to and visualization performed on the acquisition system that the and performed on a system after data means that necessary for example, motion gating, can be carried out rapidly (but not necessarily in real so that resulting 3D images can be in real time or after an acquisition, usually the system. that necessary are carried or resulting 3D images are several or even after an acquisition, usually system. As data acquisition, processing and visualization are now the between and offline have become and the should no this The new here on a system (or a of is capable of clinical information in to a patient scan. Online such a short time not more than a or maximally a that the patient can be on the scanning if further acquisitions are by the they can be performed without the such a long time several or even that the patient may have to be after a scan. If further acquisitions are the patient has to be This depends on the If a real-time imaging system is to scan an entire VOI with required temporal and spatial resolution in and indirect volume above and Table 2), the scanning is and temporal tracking is this depends on the If a 3D scan is only gated at one time point and one is acquired per only one static 3D image can be imaging is but not necessarily imaging. This is not when imaging a of the heart. Some gating methods the use of M-mode (such as the to determine the time (such as of each However, to allow such to one of two related must be the heart rate must be during a slice-reconstruction scan, which usually about 10–30 Unfortunately, fetal heart rate is even under various structures must in a similar during phasic changes to allow of time This is for the whole heart. For example, the of most ventricular systolic but the only during ventricular diastole (Figure similar In even different of the not contraction and/or at the This is not always Some gating methods the use of an spectral Doppler transducer or to (such as the in to a real-time 2D or 3D transducer for structural volume the are generally synchronized with cardiac they can be used for real-time gating. However, a cardiac time to to an a time point on the then than the point on the If the heart rate and other vary from one to during a 4D scan, the time delay may no be and may become a of Real-time gating must be gating may be real-time gating, but not Online real time but need not be real This is some gating methods only temporal information simultaneously with structural data acquisition but without direct on when the data should be is only performed on the acquired data after acquisition or even offline the to this This depends on whether the gating are and either based on the cardiac an actual 4D acquisition or have been used for gating. The is between the time and time, the fetal heart rate may change making the on from the system during the time of structural data acquisition, would be more whether they are used for (including or offline gating. In this some 3D terms have been and for and and to further it is for these terms to be (and even for their in scientific should be as as In research and clinical it is the of rather than that matters more. Hence, whether a 3D is very much depends on whether it is able to anatomical and information from a With to 4D system it may be advisable to imaging volume rate, spatial resolution and and from a system the time structural of a VOI and required by research and clinical for discussion with and of for clinical help from and of Cardiac and of Fetal and for morphological from of of all at The multi-dimensional imaging is by an in with an on