A special tube inside the x-ray machine sends out a controlled beam of radiation. Tissues in the body absorb or block the radiation to varying degrees. Dense tissues such as bones block most radiation, but soft tissues, such as fat or muscle, block less. After passing through the body, the beam falls on a piece of film or a special detector. Tissues that block high amounts of radiation, such as bone, show up as white areas on a black background. Soft tissues block less radiation and show up in shades of gray, and organs that are mostly air (such as the lungs) normally look black. Tumors are usually denser than the tissue around them, so they often show up as lighter shades of gray.