INTRODUCTION
Since last about half a century, ultrasound (US) has surfaced as the most informative, patient friendly, and essential investigation for the patients presenting to obstetricians and gynecologists. It has earned a credit of being a single noninvasive investigation giving maximum information. But still, it is a two-dimensional (2D) image of the three-dimensional (3D) structure. Therefore, to get the actual idea of the structure, shape, and morphology, ultrasound scan is done scrolling across the structure, multiple 2D images seen are virtually compiled together, and the details of the said organ/lesion/structure are “imagined”. But, when the 3D technology is actually added to the routine 2D US, the entire reconstruction is done by computer (Video 1) and therefore, it is far more accurate than the imaginary reconstruction.
Basic 3D ultrasound: 3D US is a compilation of a series of 2D US images, which is then reconstructed to form a 3D information set (Video 2). In the earliest days of development of 3D US, this set of 2D images was achieved by manual movement of the probe across the organ/lesion of interest. But, the currently available volume probes are motorized and, therefore, when 3D is initiated, the transducer head inside the probe takes an automatic sweep of a certain angle, capturing several 2D images, from which the 3D image is reconstructed (Video 3). These probes are held steady when the 3D volume is acquired.
The reconstructed 3D US image works like a tissue block that has been scooped out from the body (Fig. 1). The perception of the depth is additional to the routine 2D scan. This scooped out part can be seen from any direction, depending on the “render direction” selected (Fig. 2). It can be optimized to display the soft tissues within it better or bones within it (Figs. 3A and B). This scooped out volume can be cut into several slices in x-, y-, or z-axis similar to what can be done for tomography, computed tomography (CT), or magnetic resonance imaging (MRI). This is called tomographic ultrasound imaging (TUI) (Fig. 4). Individual slice can be rotated over x-, y-, or z-axis to achieve the desired plane (Video 4), no matter in whichever plane the 2D US scan was done. It, therefore, becomes possible to study even those planes that are not easily accessible by 2D US such as coronal section of the uterus, surgical view of the fetal heart, mid-sagittal plane of fetal head, etc. (Figs. 5A and B).
Advanced softwares in 3D ultrasound: The additional software, to adjust the light source from any side of the rendered image, is a useful tool to highlight particular parts of the volume and thus helping better demonstration of the same (Figs. 6A and B). More than one light sources can also be used to create specific effects and to highlight specific structures. This is similar to what is done in photo studio. This is named as “HD live studio” (Fig. 7).
MagiCut or electronic scalpel is a tool that also helps in highlighting certain parts of the body, by removing or scalpelling the unwanted shadows (Figs. 8A and B).
The acquired 3D data can be used to calculate exact volumes, for differential assessment of solid and cystic structures, and for subjective and objective evaluation of the global vascularity.4
Fig. 1: 3D ultrasound-acquired image of the fetal head, with fetal head seen in the rendered image, acts as a tissue block scooped out from maternal abdomen.
Figs. 3A and B: (A) 3D ultrasound-rendered image of the fetus showing surface on surface rendering; (B) 3D ultrasound rendered image of the fetus showing bones on transparent maximum mode rendering.
Fig. 4: 3D ultrasound-acquired volume of fetal head displayed as tomographic ultrasound imaging (TUI).
Figs. 5A and B: (A) 3D ultrasound-rendered image of the surgical view of the heart; (B) 3D ultrasound rendered image of corpus callosum in midsagittal plane of fetal head. (RV: right ventricle; LV: left ventricle; A: aortic valve; P: pulmonary valve)
Figs. 6A and B: 3D ultrasound-rendered image in HD-live mode of 8–9-week-old fetus with light source from two different directions.
6The volumes that have been reconstructed and modified by MagiCut or adjusting the light source and can also be walked in and out (Video 5) or can be rotated across 360° to observe and study it from all angles (Video 6). Automatic calculations for nuchal translucency, intracranial translucency, etc. have made these calculations much more accurate than ever before.
4D ultrasound: Over and above static 3D, real-time 3D scans can also be done and is called 4D US, time, being the 4th dimension. It is because of the 4D US that it has become possible to study fetal movements and facial expressions, that are reflection of the functional status and maturity of the fetal brain (Video 7).
4D cardiac evaluation: Spatiotemporal imaging correlation (STIC) has opened new horizons in fetal cardiac assessment. It is an offline 4D of the fetal heart (Video 8). STIC has allowed visualization and assessment of the cardiac sections and planes that were not achievable by 2D US. STIC has made assessment of the heart easy for the beginners. In case of abnormalities suspected, it allows the luxury of at ease and detailed assessment of the moving heart offline. It can be used with color Doppler also for better understanding of the flow across the valves and in major vessels. This can be combined with volume computer-aided diagnosis (VCAD) heart. This is an addition to TUI. VCAD allows achieving all the required and essential planes (four-chamber heart, left-ventricular outflow tract, right-ventricular outflow tract, three-vessel view, aortic arch view, ductal arch view, etc.) of the heart on one touch (Fig. 9). VCAD also can be used with color Doppler. This has significantly reduced the duration of scanning and also increased the accuracy of diagnosis. Even arrhythmias may be better studied, since M-mode can also be studied on this offline 4D US (Video 9).
Figs. 8A and B: (A) 3D ultrasound-rendered image of the fetal lower limbs; (B) 3D ultrasound-rendered image of the fetal lower limbs after MagiCut.
Fig. 9: Spatiotemporal imaging correlation (STIC)-acquired volume of fetal heart with VCAD applied to it.
Fig. 10: 3D ultrasound-acquired image of the fetal head with Sono-CNS showing different axial planes.
Softwares for obstetric applications: VCAD software is also available for its use during second stage of labor to evaluate the progress of fetal head and for prediction of obstructed labor.
Similar to VCAD for fetal heart, Sono-CNS (central nervous system) is an autotomo-graphy tool for detailed neurosonography. For the detailed assessment of the fetal brain, several planes of the fetal head need to be evaluated and it becomes a time-consuming process. This can be done on a button touch by Sono-CNS and transthalamic, transcerebellar, and transventricular all the planes can be achieved (Fig. 10).
Softwares useful in infertility patients: Sono-AVC (automated volume calculation) is a tool based on inversion mode.8
Fig. 11: 3D ultrasound-acquired image of ovary with Sono-AVC follicle showing color coding of follicles.
Fig. 12: 3D ultrasound-acquired image of gestational sac with fetus with Sono-AVC general used to calculate gestational sac volume.
Inversion mode renders cystic areas as solid areas. Sono-AVC adds color coding and volume assessment also to that. It is an excellent, highly accurate, and time-saving tool for follicular assessment (Fig. 11). Sono-AVC can also be used for the assessment of any other cystic lesions, not just the follicles. The software to assess follicles is named as Sono-AVC follicle and its counterpart that is used for volume calculation for other fluid-filled structures is Sono-AVC general (Fig. 12).9
Advantages of data storage: Since all the data can be stored and easily transferred, 3D US is a very useful tool for learning and teaching. The data can also be transferred to colleagues or experts for opinion. It is also possible to transfer this data directly from the scanner by emails or the data can now also be stored in the cloud and can be retrieved from anywhere.
Since 3D and 4D US data is stored in three dimensions that make volumes instead of planes, collectively these can also be named as volume ultrasound.
There is much more to discuss about each function and their applications, thus making volume ultrasound a specialty by itself.