Perception and reality in a case of suspected cervical fracture.

The case of a 36-year-old female, who following a road accident, had a cervical whiplash, is discussed. A radiographic examination of the affected region was performed at the Emergency Department. It showed a transverse radiolucent line at the basal odontoid process of the axis. A definitive interpretation was not possible and fracture of the odontoid process of the axis was suggested as differential diagnosis. On coronal CT reconstruction the presence of this fracture was ruled out. The line seen on the film was actually the product of a psychophysiologic visual illusion known as "Mach band". The use of conventional radiography correlated with digital CT images shows that the practical application of Mach band theory allows a better differentiation between normal and pathologic anatomy.