A bursa is a thin, fluid-filled sac lined with synovial membrane that reduces friction between moving structures in the musculoskeletal system, such as between a tendon and bone or between skin and bone. Bursae are found throughout the body at points of mechanical stress, including the shoulder, elbow, hip, and knee. On ultrasound, a normal bursa may be barely visible or appear as a thin, fluid-containing structure, while an inflamed or infected bursa becomes distended with fluid and is easily seen. Musculoskeletal ultrasound is an excellent tool for evaluating bursitis because it provides real-time, dynamic imaging and can guide aspiration or injection of the affected bursa.
Sonography Term