What does CT number represent in a CT scan?

The CT number, also known as Hounsfield unit (HU), represents the relative density of tissues in a CT scan. It quantifies the attenuation of X-ray beams as they pass through different tissues in the body. The values are calibrated so that water has a CT number of 0, air has a value of approximately -1000, and dense bone can range from +300 to +1000. This allows for the differentiation of various tissues based on their density, which is critical for diagnosing a range of medical conditions. The accurate measurement of tissue density is essential for interpreting CT images and guiding clinical decisions.

Other options do not describe the function of CT numbers. The position of the patient is important for image acquisition but is not defined by the CT number. The speed of imaging refers to the performance characteristics of the CT scanner but does not relate to the density measurements. Lastly, the temperature of the scanner is a technical parameter that affects equipment performance but also has no direct relevance to what the CT number signifies.