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  1. Three-dimensional ultrasonography (3DUS or volume sonography): an imaging method that fundamentally converts digital 2D picture elements (pixels) into 3D voxels.

  2. Four-dimensional ultrasonography: an imaging method that adds the dimension of time to volume data sets acquired using 3DUS that is especially useful for the fetal heart and moving limbs.

  3. Multiplanar display: the instantaneous and interactive display of 3 perpendicular views (ie, sagittal, transverse, and coronal views).

  4. Voxel: short for volume pixel. It is the smallest unit of a three-dimensional volume equivalent of a pixel in a 2D image.

  5. Rendering: the process of displaying groups of voxels with post-processing software to display either surface features (surface rendering) or internal anatomic structures (volume rendering).

  6. Reformatting: the process of volume exploration where particular image planes are obtained either in a multiplanar or rendered display.

  7. STIC (spatio-temporal image correlation): a volume analysis technique that is designed to acquire and display moving volumes of the fetal heart.

  8. Matrix 2D array: a transducer with a large number of piezoelectric elements that allow electronic steering and rapid real-time acquisition of moving objects such as the fetal heart.

  9. Volume contrast imaging: a 3D image projection that is variably defined by several layers of voxels (i.e. "thick slice") rather than from a thin layer of pixels used for 2D ultrasonography.

Conventional high-resolution two-dimensional ultrasound (2DUS) is commonly used for numerous indications in obstetrics and gynecology and is widely considered a clinically useful technique. This imaging technology requires the operator to scan through the patient's anatomy with a 2D ultrasound beam to obtain cross-sectional planes. The 2D imaging planes are typically thin and only display cross–sectional information. The ultrasound operators typically put in a lot of effort and time to mentally develop a three-dimensional (3D) concept of the region of interest with depth and perspective. In addition, several image planes are either very difficult or inherently simply impossible to be obtained with the conventional 2DUS techniques.

Some of the early research works on 3D volume acquisition and display methods started in 1970.1 Early prototypes had sophisticated setups for that time, with static arms, which were later equipped with position sensors to more reliably register the acquired 2D data in time and space. Only in the early to mid-1990s did the technology really start to leave the industry's research and development labs and become a more appealing clinical tool.2,3,4,5, and 6 Significant developments in transducer technologies, and advances in computer signal processing and image display finally made possible reliable techniques to acquire and visualize ultrasound volumes.7

Three-dimensional ultrasonography (3DUS) can depict the anatomy of the region of interest as static or even in motion. The displays have become fully interactive environments that allow the operator to have full control over the scan planes and evaluate the anatomy in user-selectable uniplanar, biplanar, orthogonal, and multiplanar displays, as well as rendered 3D views. Volume ...

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