Figure 5a. Lipoma arborescens in a 50-year-old man with knee swelling. (a) Lateral radiograph of the knee reveals fullness of the suprapatellar bursa with radiolucent areas (arrow) suggestive of fat. (b) Sagittal proton density-weighted MR image shows a lobulated mass in the suprapatellar bursa (arrow) with signal intensity equivalent to that of fat. (c) Sagittal proton density-weighted MR image with fat saturation shows loss of signal of the synovial proliferation (arrow) surrounded by a large joint effusion extending into the popliteal bursa (*). (d) Long-axis ultrasound (US) image shows distention of the suprapatellar bursa by a mass with characteristic frondlike projections (arrows) surrounded by a joint effusion (*).