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Key Terms
Three-dimensional ultrasonography (3DUS, or volume ultrasound): an imaging method that fundamentally converts the acquired digital 2D picture elements (pixels) into a 3D volume.
Four-dimensional ultrasonography: an imaging method that adds the dimension of time via the fast, and continuous, acquisition and display of 3DUS volumes, especially useful for imaging moving targets (ie, fetal heart, moving limbs, etc).
Multiplanar display: the interactive display of three perpendicular views (ie, sagittal, transverse, and coronal views) around a reference point, otherwise known as orthogonal display.
Voxel: short for volume pixel. The smallest unit of a three-dimensional volume, equivalent of a pixel in a 2D image.
Rendering: the display of voxels using postprocessing software highlighting either surface features (surface rendering) or internal anatomic structures (volume rendering).
Reformatting: the process of volume exploration, at times using postprocessing algorithms, where particular image planes are obtained in a multiplanar, rendered display, or both.
STIC (spatio-temporal image correlation): a volume analysis technique specifically designed to acquire and display moving volumes of the fetal heart.
Matrix 2D array (or electronic 4D probes): a transducer with a large number of piezoelectric elements (thousands) in rows, allowing electronic beam steering, and focusing, in all directions, avoiding the limitations of mechanically swept probes. They make possible rapid real-time acquisition of moving objects such as limbs or fetal heart.
Volume contrast imaging: a 3D image projection that provides excellent contrast resolution by combining several layers of voxels (ie, derived from multiple image slices in a rather thin volume acquisition), as compared to a single layer of pixels used in 2D ultrasonography.
Biplane imaging: provides simultaneous display of high resolution, high frame rate images in two perpendicular planes. A matrix 2D array can steer scan planes in perpendicular orientations allowing for acquisition, and true real-time display of two orthogonal planes.
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Ultrasound imaging in obstetrics and gynecology is commonly used for numerous indications. The most commonly used technology in everyday practice is the conventional high-resolution two-dimensional ultrasound (2DUS), and is widely considered clinically useful. Using 2DUS imaging technology, the operator scans the anatomy with a flat 2D ultrasound beam to obtain cross-sectional planes. These 2D imaging planes are typically quite thin, and the corresponding images obtained represent thin cross-sectional anatomical information along the path of the 2D beam. As they scan through the anatomy, ultrasound professionals typically orient themselves by mentally developing a spatial three-dimensional (3D) concept of the region of interest, accounting for depth, and perspective. Depending on fetal position, using 2DUS at times some image planes can be very difficult, or even inherently impossible to be obtained with conventional 2DUS techniques, especially if they are perpendicular (or close to) to the axis of the beam.
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Research work on 3D volume acquisition and display methods started in 1970.1 Sophisticated early prototypes had static arms, later equipped with position sensors to more reliably register the acquired 2D data in time and ...