Mach Bands

Introduction

When viewing plain film images we tend to assume that what we see is a real and undistorted anatomical reality. Mach Bands are an optical illusion that leads us to perceive densities that do not exist in reality.

The Mach Band Illusion

"Mach bands are an <a class="external" href="http://en.wikipedia.org/wiki/Optical_illusion" rel="nofollow" target="_blank" title="Optical illusion">optical illusion</a> consisting of an image of two wide bands, one light and one dark, separated by a narrow strip with a light-to-dark gradient. The <a class="external" href="http://en.wikipedia.org/wiki/Human_eye" rel="nofollow" target="_blank" title="Human eye">human eye</a> perceives two narrow bands of different brightnesses either side of the gradient that are not present in the original image." (1) Even when this image is magnified, the illusion persists- the solid grey appears to change where it meets the graduated ctripes

	If you look closely at this grey scale it appears that each band of grey is not a consistent density. The left side of each band appears slightly darker than the right. This is another example of the Mach band optical illusion.
	These are the same grey blocks separated by a white strip. They now appear as solid shades of grey.

Mach Bands in Action

This is a supine chest X-ray image. The arrowed structure is the left paraspinal stripe. This stripe appears to be less dense along it's edge (arrowed)- this is not real, it is a Mach Band optical illusion. The left paraspinal stripe is approximately uniform density. The appearance of the band being less dense along it's edge is anatomically ilogical.

This is a section of the left paraspinal stripe (marked top and bottom)	When the image is cropped along a line between the triangles the mach band is no longer visible.	When the cropped image is duplicated, flipped horizontally, and juxtaposed, the Mach band is similarly not visible.