Infrapatellar (Hoffa's fat pad) ganglion cyst

Axial proton density-weighted image with fat saturation demonstrates a large, lobulated fluid signal focus (arrows) within Hoffa's fat pad centrally and extending laterally. This arises from the distal anterior cruciate ligament fibers. Thin internal septa are also present.

Sagittal T1-weighted view demonstrates the well demarcated intermediate signal intensity lesion (arrow) relative to the surrounding high signal intensity fat.

MR Evaluation

Soft tissue ganglia appear as simple, single lobule to multi-septated and/or multi-lobulated fluid signal foci, usually contiguous with a joint capsule or tendon sheath (E, F, G). Intraosseous ganglia present as fluid signal foci, with or without surrounding marrow edema, usually in close proximity to an articular surface. Ganglia are usually of low to intermediate signal on T1-weighted and of increased signal on T2-weighted sequences, similar to pure fluid. However, due to hemorrhage or high proteinaceous content, they can be of increased signal on T1-weighted sequences. Mild peripheral or septal enhancement may be seen on contrast enhanced imaging.

A sagittal T2 image of the proximal calf demonstrating a large, lobulated ganglion (arrows) arising from the proximal tibiofibular joint extending intramuscular into the peroneus longus muscle, presenting as a palpable mass.

A coronal STIR sequence of the left hand demonstrating an unusually large, lobulated ganglion (arrows) of the volar aspect of the small finger, superficial to the flexor tendons.

An axial T2-weighted image in a patient with a palpable abnormality reveals a lobulated, septated, multifocal ganglion (arrows) with components superficial to the peroneal tendons (arrowheads) at the tip of the fibula. The lesion lies lateral to the sinus tarsi (asterisk), likely arising from the cervical ligament.

Mucinous degeneration of the proximal medial collateral ligament with associated intraosseous ganglion (arrow) and moderate surrounding reactive marrow edema on this fat-suppressed proton density-weighted coronal image.

Differential Diagnosis

The differential diagnosis of these cystic lesions includes synovial myxoma ¹², meniscal or parameniscal cyst, synovial cysts, pigmented villonodular synovitis, synovial hemangioma, aneurysm (O,P), and synovial or other sarcoma (Q,R,S). In general, ganglion cysts are more well defined and in particular more homogeneous than the other diagnostic considerations. It is important to recognize that lesions that are not of typical fluid signal intensity on T1 or T2-weighted images often do not represent a ganglion.