Endometriosis. Coloured 3-D ultrasound scan of an ovarian endometrioma (upper centre) caused by endometriosis. Endometriosis is a condition where tissue from the inner lining of the uterus (endometrium) is found elsewhere in the pelvic cavity. If the tissue forms a well-defined mass it is called an endometrioma. The ovaries are the most commonly affected site. In this case, the ovary (centre left to lower centre) also contains an ovarian cyst. Ultrasound scanning is a diagnostic technique that sends high- frequency sound waves into the body via a transducer. The returning echoes are recorded and used to build an image of an internal structure.