Ganglion

Ganglia are thin walled cysts filled with clear mucinous fluid arising from tendon sheaths or joints. They most commonly occur in the wrist, hands or feet.
Here a large (2 cm) ganglion is seen associated with the extensor carpi radialis brevis tendon. It is displacing this tendon laterally and its neighbouring tendons - the extensor digitorum and indicis tendons medially. It extends down to but does not involve the dorsal radiocarpal ligament. There is no bony involvement.

Axial T1, T2 and STIR images are shown: