What is the CT number for air?

The CT number for air is typically represented as -1000 Hounsfield Units (HU). This measurement is based on the attenuation properties of different tissues relative to water, which is designated as 0 HU. Air, having very low density and negligible ability to attenuate X-rays, results in a significantly negative value on the Hounsfield scale.

In a computed tomography (CT) scan, a higher density value corresponds to higher HU, while lower density materials like fat and air will yield negative numerical values. For example, water is the baseline reference at 0 HU, and as the density decreases past the point of water, the HU values become more negative.

This understanding of the Hounsfield Units serves a critical role in medical imaging by allowing radiologists and medical professionals to differentiate between various substances and tissues within the body based on their respective CT numbers.