A Mach band refers to an optical phenomenon from edge enhancement due to lateral inhibition of the retina 2. This is inbuilt edge enhancement mechanism of the of the retina, where the edges of darker objects next to lighter objects will appear lighter and vice versa; creating a false shadow 4.
This has been attributed as a source of diagnostic error in radiology. An example is an appearance of a dark halo around a dense breast lesion in mammography 3.
Another commonly encountered Mach band can cause an apparent pneumomediastinum, the so-called pseudopneumomediastinum, due to the contrasting densities of the mediastinal shadow over the lungs.
Another shortcoming of the Mach band is the knowledge of the phenomenon, which can lead to a misinterpretation of a true radiological sign as a Mach band.
Named after the Austrian physicist Ernst Waldfried Josef Wenzel Mach (1838-1916) who first described it in 1865.
- 1. Daffner RH. Visual illusions in computed tomography: phenomena related to Mach effect. AJR Am J Roentgenol. 1980;134 (2): 261-4. AJR Am J Roentgenol (abstract) - Pubmed citation
- 2. Sugita R, Sugimura E, Itoh M et-al. Pseudolesion of the bile duct caused by flow effect: a diagnostic pitfall of MR cholangiopancreatography. AJR Am J Roentgenol. 2003;180 (2): 467-71. AJR Am J Roentgenol (full text) - Pubmed citation
- 3. Gordenne WH, Malchair FL. Mach bands in mammography. Radiology. 1988;169 (1): 55-8. Radiology (abstract) - Pubmed citation
- 4. Panikkath R, Panikkath D. Mach band sign: an optical illusion. Proc (Bayl Univ Med Cent). 2014;27 (4): 364-5. Free text at pubmed - Pubmed citation