Radiologic soft tissue shadows in the pelvis: another look.

In patients with adequate amounts of intramuscular fat, the normal contours of the muscles of the pelvic floor and side-walls may be visualized on plain radiographs. Knowledge of the relationships of these muscles to surrounding structures has been facilitated by computed tomography. Obliteration of the medial margin of the psoas muscle with an attendant soft tissue mass implies pathology in the region of the medial chain of external iliac lymph nodes; similarly, obliteration of the obturator internus shadow suggests internal iliac adenopathy. Nonnodal masses in these regions can cause similar radiographic abnormalities. The normal shadow of the piriformis muscle may simulate external iliac adenopathy. Pitfalls which may lead to overinterpretation are discussed.