Factors affecting the appearance  
 

DENSITY

On the right is a radiograph taken of three cups containing substances of different densities. Which is the most dense?

The cup on the far right contains air, which is very low in density and therefore appears black on x-ray. Low density structures are also called "radiolucent".

The center cup contains water. Fluid is denser than air, and thus it will attenuate more of the beam and appear whiter than air.

Do you know what is in the cup on the left?  It is barium, a very dense liquid which attenuates much of the beam and so appears white on radiographs.

For some radiology exams, patients drink barium to opacify the gastrointestinal tract and make it visible on x-rays. Click here to see barium in the stomach.