Mach Line

 

A Mach line is an optical illusion usually appearing as a black line produced by the retina of the observer in response to a border of bone or soft tissue as it interfaces with another, less dense object on a radiograph; Mach lines usually are seen at the base of the dens created by the superimposition of the occiput (blue line below) or the lateral border of the heart at its interface with the air in the lung.