Visual illusions in the interpretation of the radiographic image

The human body is a complex of tissues of different radiographic densities. Overlapping shadows, differences in background density, subjective contour formation, and the parallax effect often produce a variety of optical illusions that exist only in the "mind's eye" and may be mistaken for significant pathology. Most of these phenomena are produced by the Mach effect, a consequence of the physiological process of lateral inhibition. Subjective contour formation is a "psychophysiological" phenomenon in which complete "structures" appear from minimal outlines. A parallax effect occurs if real structures contribute to the radiographic image. This review discusses the basic physiology involved in image reception and the formation of false images. Clinical consequences of the presence of Mach bands, background density effect, subjective contours, and real or physical phenomena are illustrated.