Basic Working Principle Of CT Scan
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.