It is as simple as passing X-rays through the patient and obtaining information with a detector on the other side. The X-ray source and the detector are interconnected and rotated around the patient during scanning period. Digital computers then assemble the data that is obtained and integrate it to provide a cross sectional image (tomogram) that is displayed on a computer screen. The image can be photographed or stored for later retrieval and use as the case may be.
X-rays are electromagnetic waves. The main reason why X-rays is used in diagnosis is because all substances and tissues differ in their ability to absorb X-rays. Some substances are more permeable to X-rays while some others impermeable. Owing to this difference, different tissues seem different when the X-ray film is developed.
Dense tissues such as the bones appear white on a CT film while the soft tissues such as the brain or kidney appear gray. The cavities filled with air such as the lungs appear black.