CT scans measure tissue density by using water as a baseline value of zero.

The Hounsfield Scale was named after Sir Godfrey Hounsfield, who received the Nobel Prize in Physiology or Medicine in 1979 for his work on Computed Tomography. This scale provides a quantitative basis for describing radiodensity, which is the ability of a material to block X-rays. By setting distilled water at 0 Hounsfield Units (HU) and air at -1,000 HU at standard pressure and temperature, the system creates a consistent universal reference for medical imaging.When an X-ray beam passes through the body, it is attenuated or weakened by the tissues it encounters. Detectors measure this attenuation and convert it into a gray-scale image where each pixel, or 'voxel' in 3D, represents a specific density value. For example, fat typically measures between -100 and -50 HU, while muscle falls between +10 and +40 HU. This level of precision allows radiologists to detect subtle differences, such as distinguishing a simple fluid-filled cyst from a solid tumor.Modern CT scanners can differentiate over 2,000 shades of gray based on these units, though the human eye can only perceive about 30 to 90 shades. To solve this, doctors use 'windowing' to focus on specific HU ranges, such as a 'bone window' or a 'lung window.' This technology has revolutionized diagnostics since the first commercial CT scan was performed at Atkinson Morley Hospital in London in 1971. Today, the Hounsfield Scale remains the global standard for ensuring that a density reading in one hospital matches the results in another.