What is attenuation?
Attenuation describes how bright or dark a tissue is on a CT image and is based on how much a tissue blocks (i.e., attenuates) the x-ray beam before reaching the x-ray detector. Attenuation of a tissue is predominantly determined by its atomic number (electron density) and physical density, although x-ray tube voltage also affects tissue attenuation. Every pixel (or voxel) on a CT image is assigned a gray scale value according to the mean attenuation of the tissue it corresponds to, which is called the CT number or relative attenuation coefficient. This is measured in Hounsfield units (HU), where pure water is the reference standard with an assigned CT number of 0 HU. Gas (−1000 HU) appears black, fat (−20 to −150 HU) appears dark gray, fluid (0 to 20 HU) appears gray, soft tissue (≈20 to 80 HU) appears bright gray, and bone, calcification, metal, and concentrated iodinated contrast material (≈150 to 1000 HU) appear white.