An X-ray is a type of electromagnetic radiation, like visible light. X-ray equipment sends individual X-ray particles through the body and records the images on a computer or on film. Bones, metal and other dense structures (in addition to any dye used to highlight body areas) block most of these particles and appear white on an X-ray image. Structures that contain air will appear black on the image, while fat, muscle and fluid will appear in shades of gray.