Figure 11b. Popliteal cyst with ossified loose bodies in a 64-year-old woman. (a) Sagittal T2-weighted MR image of the knee shows an apparent mass with small internal foci of low signal intensity (arrowhead) posterior to the medial femoral condyle. (b) Axial intermediate-weighted fat-suppressed MR image shows round foci of low signal intensity (arrowhead) in the popliteal cyst. The "neck" of the cyst extends between the tendons of the semi-membranosus muscle (small arrow) and the medial head of the gastrocnemius muscle (large arrow). (c) Lateral radiograph shows ossified loose bodies (arrow) that correspond to the round low-signal-intensity foci seen on MR images. This case emphasizes the importance of knowing the bursal anatomy and correlating MR images with radiographs.