Radiologic sign

A radiologic sign is an objective indication of some medical fact (that is, a medical sign) that is detected by a physician during radiologic examination with medical imaging[1] (for example, via an X-ray, CT scan, MRI scan, or sonographic scan).

An X-ray image of the intestines, showing coiled loops that are darker on the left and right sides and lighter in the middle
The dark areas on both sides of the intestines indicate that air is present in both sides. This is called "Rigler's sign".