Chronic bursitis presenting as a mass in the pes anserine bursa: MR diagnosis

A case of chronic, nonspecific synovial inflammation presenting as a mass in the pes anserine bursa is demonstrated by MRI. The lesion was well demarcated and surrounded by a low intensity rim. On spin echo images it was homogeneous and of intermediate signal intensity, whereas on T2-weighted images it showed scattered areas of high signal producing a heterogeneous pattern. It is contrasted with a typical example of acute pes anserine bursitis, presenting as simple fluid within the bursa. The differential considerations vary accordingly between the acute and chronic forms of pes anserine bursitis, with the latter requiring differentiation from other forms of synovitis, synovial hemangioma, and synovial sarcoma. The MR features of these entities are reviewed as an aid in differential diagnosis.