There are numerous exciting new technologies beyond the basic physics of ultrasound previously discussed that hold promise for expanding diagnostic, interventional, and therapeutic ultrasound applications. This chapter will review the current state of some of the more popular of these technologies.
Three-dimensional ultrasound is most familiar to the lay public as the source of beautiful baby pictures in utero. An internet image search will yield billions of images of before (fetal) and after (infant) pictures, with remarkable results. The technology is proving itself also quite useful in nonobstetric applications, such as breast, abdominal, thyroid, etc., limited only by the imagination of the user.
Hands-on conventional two-dimensional ultrasound can be construed in and of itself as a three-dimensional technology if one considers the user’s manipulations of the beam and mental reconstruction of the image. Over 10 years ago computational speed and power, however, allowed for automatic reconstruction, display, and archiving of these images. In addition, 3D reconstruction allows for imaging awkward or otherwise inaccessible areas of the body. In certain applications, accurate volumetric quantification is essential for disease diagnosis and follow-up.
In the early days of 3D ultrasound, mechanical transducers were employed to sweep the volume of interest. The reconstruction was performed after the examination, at a PACS workstation. Alternatively a conventional 2D transducer was manually (“freehand”) swept with the movement either tracked or untracked to build the 3D image. The transducers currently used for 3D ultrasound are electronic probes that steer the beam in pyramid-like volumes to obtain multiplanar image data for the calculation of the 3D image. Now real-time displays are possible. The image may be displayed as a whole volume (volume-rendering), with the user given the ability to rotate the volume, or as individual planes, or as a crossed-plane view (multiplanar reformatting), which may be more intuitive for a user more comfortable with 2D imaging. One advantage is the ability to reconstruct the image “behind” obstructing or shadowing structures, which, in a conventional two-dimensional image, would remain elusive. The addition of time information results in a “4D” image, for use in echocardiography or vascular applications, for instance.
There is a convention for the multiplanar reconstruction. Three-dimensional ultrasound is described as three planes: the A plane, B plane, and C plane (Figure 3-1). The A plane is the plane parallel to the acquisition plane, the B plane is perpendicular to the A plane but still parallel to the ultrasound beam, and the C plane is often referred to as the coronal plane, or the two-dimensional slices at various depths from and parallel to the transducer face (and perpendicular to the ultrasound beam). Many scanners will display these three planes with these monikers in addition to a volumetric rendering (Figure 3-2).
Multiplanar image display schematic for 3D ultrasound.