A Baker’s cyst, also known as a popliteal cyst, is a benign swelling of the semimembranous or more rarely some other bursa found behind the knee joint.
It is named after the surgeon who first described it, William Morrant Baker (1838–1896). This is not a “true” cyst, as an open communication with the synovial sac is often maintained.
Baker’s cyst on axial MRI with communicating channel between the semimebranosus muscle and the medial head of the gastrocnemius muscle.
Baker’s cyst on MRI, sagittal image. Baker’s cyst on MRI, sagittal image.