Essential knowledge about pitfalls in tomography.

Abstract

Tomographic cut is not synonymous to a roentgenogram of an anatomic cut. Blurring is inherent in the tomogram, and the edges do not appear as sharp as in the conventional roentgenogram. Part of the contrast is lost in tomography. Therefore, when tissue contrast is low, or when we intend to see an entire structure in one cut, zoneography is preferred. On the other hand, when many details are crowded in a small area and/or when the tissue contrast is high, a thin section tomography is indicated. Proper knowledge of the level of area of interest (focal plane) and proper spacing of the cuts decrease radiation and expenses. The Law of Tangent, namely, "The edge of a structure should be momentarily parallel to the beam at the focal plane, in order to show on the film," is an essential part of every tomographic study. The Law of Tangent applies equally to spheres and other curved surfaces. The effective tomographic angle is the angle described by the tube during exposure. The thickness of the cut depends on the effective tomographic angle. Prior viewing of conventional roentgenogram and proper knowledge of the mechanism of parasitic and phantom images help us to avoid mistaking these images as real structures.