Imaging description

When multiple structures overlap or abut on a radiograph, an optical illusion known as the “Mach effect” may simulate light and dark lines. This enhances edges and in some cases makes the structures easier to differentiate. However, the Mach effect may also simulate a fracture. The Mach effect is a normal phenomenon that results from the physiologic process called “lateral inhibition” in the radiologist’s retina [1]. As shown on Figure 79.1, each gray rectangle appears shaded, light at the top and darker at the bottom. Now use a sheet of paper to cover all but one of the gray rectangles – the illusion of a gradient of gray within the remaining rectangle disappears, revealing that each rectangle is, in fact, homogeneous in color.

When the Mach effect is being considered, careful inspection is necessary to avoid misdiagnosis. Additional views may remove the superimposition of structures creating the effect. In other cases, a CT may be necessary to exclude underlying pathology.