What is the CT number associated with water?

The CT number associated with water is defined as 0. This value is based on the Hounsfield scale, which is the standard reference for computed tomography (CT) imaging. The Hounsfield scale quantifies the attenuation of X-ray beams as they pass through different tissues, with water assigned a value of 0 because it serves as the baseline reference point.

In the context of CT imaging, different materials have specific CT numbers based on their density and atomic composition relative to water. For example, air has a much lower density and is assigned a value of approximately -1000, while materials denser than water, like bone, will have positive CT numbers. The designation of water as 0 helps in normalizing the scale so that radiologists and technicians can easily compare the attenuation characteristics of various tissues in the body effectively.