How is the CT number for any tissue calculated?

The CT number, also known as Hounsfield unit (HU), provides a way to quantify the density of different tissues in a CT scan. It is calculated using the difference in linear attenuation coefficients (µ) of the tissue and water. Water is assigned a CT number of 0, and the CT number for any given tissue is derived from its linear attenuation coefficient compared to that of water.

The formula takes into account the attenuation coefficients of both the tissue and water. Specifically, it calculates the contrast (or difference) in how much the tissue attenuates X-rays compared to water. By subtracting the linear attenuation coefficient of water from that of the tissue (µ tissue - µ water), the formula quantifies how much more or less the tissue attenuates X-rays compared to water. This difference is then scaled by dividing it by the linear attenuation coefficient of water and multiplying by 1000 to provide a meaningful unit scale.

This approach ensures that the CT number reflects the relative density and composition of the tissue compared to a standard reference (water), which is essential in medical imaging for identifying different types of tissues and their properties in a scan.