ABSTRACT
Tomosynthesis addresses one of the primary weaknesses of conventional single-projection x-ray imaging, the superposition of objects in the image. This superposition may result in the obscuring of an object of interest and/or the production of pseudoobjects that mimic a disease (e.g., pseudomasses or a reduction in bone joint space). Tomosynthesis decreases superposition by generating slice images of the body from a series of projections taken at a variety of angles. In this sense, it is much like computed tomography (CT); however, instead of acquiring projections over 360° as in CT, the projections for tomosynthesis are acquired over a limited range of angles (e.g., 10° or 60°). The resulting tomosynthesis images have much better spatial resolution than CT within the slice, but poorer resolution in the depth direction, between the slices. The basic tomosynthesis principle is illustrated in Figure 1.1. A series of snapshot radiographs (films) are taken, each from a different viewing angle, as an x-ray tube translates across a patient. These snapshots are later combined by shifting the snapshots relative to one another and superimposing (adding) them to bring different planes within the patient in focus. Today, we use digital radiographs instead of films and more sophisticated reconstruction algorithms like those employed in CT instead of the “shift-and-add” technique.
