THE MACH PHENOMENON

Mach bands are optical illusions produced by an extensive lateral inhibition network within the retina. They are seen on every radiograph. Both positive and negative Mach bands can be seen at a boundary, but only one type is usually seen at boundaries created by biological structures because of their shape. Convex structures are associated with a negative Mach line and concave structures are associated with a positive Mach line. Using this principle, the shape of unknown structures can be deduced and structures of known shape can be more accurately identified. Sometimes, Mach lines can be very prominent and be perceived as real structures. Ways to make the distinction between Mach lines and real opacities are described. Understanding the Mach phenomenon improves accuracy of radiographic interpretation. Copyright © 1990, Wiley Blackwell. All rights reserved