Technical Factors:
Axial STIR (1), coronal T1 (2), sagittal T1 (3) and sagittal STIR (4)

Findings: (Images 1 - 4)
Ovoid mass at the anterolateral aspect of the ankle, lying within the subcutaneous superficial soft tissues just anterior to the lateral malleolus. Central signal hypointensity (long arrow) likely represents flow void, with surrounding signal hyperintensity likely representing mural thrombus (short arrow).

Discussion:
Aneurysms, although unusual in the foot and ankle, appear as round or ovoid structures in an arterial distribution, usually demonstrating hypointense flow void but sometimes demonstrating mixed T1 and T2 signal characteristics, depending on the presence or absence of thrombus and the degree of turbulent flow. A varix or thrombosed varix can also present as a mass, usually subcutaneous, with its tubular or tortuous appearance frequently leading to the correct diagnosis. Tracking the course of the vascular and tendinous structures along the anterolateral aspect of the ankle should lead to the correct diagnosis.

Sources:
Rolfes Richard et al. MRI of the Foot & Ankle, Pearls, Pitfalls & Pathology. Cincinnati. MRI-EFI Publications. 2002.