Mach bands or the Mach effect refers to an optical phenomenon from edge enhancement due to lateral inhibition of the retina 2. This is an inbuilt edge enhancement mechanism 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:
- a dark halo around a dense breast lesion in mammography 3
- apparent pneumomediastinum - the so-called pseudopneumomediastinum
- mimic a fracture
Knowledge of the phenomenon may lead to a false interpretation of a true radiological sign as Mach bands.
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