The Dot-in-Circle Sign

The dot-in-circle sign refers to the appearance of mycetoma on T2-weighted and post-contrast images. It consists of a tiny hypointense focus within a high-intensity spherical lesion. Numerous lesions are typically seen, separated by a low-signal intensity matrix. It is thought that the small central hypointense focus represents the fungal ball or granule and the surrounding hyperintense area represents the inflammatory granuloma. The intervening tissue of low signal intensity represents the fibrous matrix.

The mycetomas can be found in the soft tissues or bone and can represent infection by fungi (eumycetoma) or aerobic actinomycetes (actinomycetomas). Recall from medical school that actinomycetes have microscopic characteristics similar to those of fungi, and were formerly classified as fungi.

Patients present with a history of a firm, painless nodule that follows an indolent but progressive course. The initial infection may communicate with the skin with discharge of fungal granules. Alternatively, the infection may fester internally and even lead to osteomyelitis and significant destruction and deformity. Definitive diagnosis may be provided by biopsy, or be elusive in cases of fastidious organisms.

The case above is a STIR image of the foot, revealing multiple round hyperintense lesions with central hypointense foci along the dorsum of the foot. The patient had a 10-year history a slowly growing foot mass following trauma and presented to us after biopsy (unknown) and unspecified treatment and at an outside facility. Biopsy at our facility revealed no evidence of fungal infection but was positive for bacteria. This complicated picture was suspected to be due to actinomycetoma.

Geography trivia: Mycetomas are more commonly found in the dry topics. The entity of Madura foot (mycetoma of the foot) was first described in the Madura district of Southern India in 1846. The infection usually starts with penetrating injury to the foot (e.g., thorn prick) with inoculation of organisms that are normal inhabitants of the soil.

References

• Cherian RS, Betty M, Manipadam MT, Cherian VM, Poonnoose PM, Oommen AT, Cherian RA. The "dot-in-circle" sign -- a characteristic MRI finding in mycetoma foot: a report of three cases. Br J Radiol. 2009 Aug;82(980):662-5.
• Kumar J, Kumar A, Sethy P, Gupta S. The dot-in-circle sign of mycetoma on MRI. Diagn Interv Radiol. 2007 Dec;13(4):193-5.
• Sarris I, Berendt AR, Athanasous N, Ostlere SJ; OSIRIS collaborative study group. MRI of mycetoma of the foot: two cases demonstrating the dot-in-circle sign. Skeletal Radiol. 2003 Mar;32(3):179-83.