MRI of mycetoma of the foot: two cases demonstrating the dot-in-circle sign.

Radiological and histological findings of two patients with fungal mycetoma of the foot are presented. MRI revealed multiple 2-5 mm lesions of high signal intensity interspersed within a low-intensity matrix. Within many of the lesions a minute low-intensity focus was identified. Ultrasound showed distinct hyperechoic foci within a hypoechoic mass. We speculate that the low-signal matrix represents fibrous tissue, the high-intensity lesions correspond to granulomata and the central low-signal focus to the characteristic organised fungal elements (grains) present in this condition. This "dot-in-circle sign" on MRI reflects the unique pathological features of mycetoma and is likely to be a highly specific sign for this lesion.