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 darker 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
Knowledge of the phenomenon may help prevent Mach bands being mistaken for a true abnormality such as pneumomediastinum.
Mach effect is also known to cause the paraspinal line (which are actually interfaces) to appear as two separate lines namely the right and left paraspinal lines.
History and etymology
Named after the Austrian physicist Ernst Waldfried Josef Wenzel Mach (1838-1916) who first described it in 1865 5.
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
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
5. Mach E. Über die Wirkung der räumlichen Verteilung des Lichtreizes auf die Netzhaut [On the effect of the spatial distribution of the light stimulus on the retina]. Sitzungsberichte der mathematischnaturwissenschaftlichen Classe der kaiserlichen Akademie der Wissenschaften. 1865;52:303–322.
6. Kishimoto K, Kishimoto WT, Kishimoto TY, Kishimoto. Pseudo-pneumothorax: skin fold is an excellent imitator. (2018) BMJ case reports. doi:10.1136/bcr-2018-226360 - Pubmed
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