Diagnostic Imaging - X-Ray

X-rays are the most common type of imaging. The images produced by X-rays are due to the different absorption rates of radiation from tissues. Calcium in bones absorbs X-rays the most, so bones look white on an X-ray film, also known as a radiograph. Muscle and other soft tissues absorb less radiation, and have more gray tones on the radiograph. Air absorbs the least, so lungs look black on a radiograph. The most familiar use of X-rays is checking for broken bones, but X-rays can also be used to check for cancer. For instance, chest radiographs are sometimes used to see if cancer has spread to the lungs or other areas in the chest.

Source: National Cancer Institute