CT numbers are expressed in what unit?

CT numbers, often referred to as Hounsfield Units (HU), are a standardized way to quantify the density of tissues in computed tomography (CT) imaging. This scale was developed to provide a reference that allows for the comparison of different tissues based on their x-ray attenuation properties. Water is assigned a CT number of 0 HU, while air, being less dense, is assigned a value of -1000 HU. Dense structures, such as bone, can have positive numbers exceeding 300 HU.

Using Hounsfield Units is crucial for distinguishing between different types of soft tissues, fluids, and pathological findings during imaging interpretation. This standardization enables clinicians to assess abnormalities and make informed decisions concerning diagnosis and treatment planning effectively. Other units like Gray, which measures radiation dose; Millisieverts, which measures radiation exposure; and Percent units, which express fractional values, do not apply to the context of CT imaging density measurement.