The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement in one in which the radiodensity of distilled water at standard pressure and temperature (STP) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU.> For a material X with linear attenuation coefficient ?X, the corresponding HU value is therefore given by where is the linear attenuation coefficient of water.> Thus, a change of one Hounsfield unit (HU) represents a change of 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero.> It is the definition for CT scanners that are calibrated with reference to water.# Tissue CT number (HU) Bone 1000 Liver 40 - 60 White matter -20-30 HU Grey matter -37-45 HU Blood 40 Muscle 10-40 Kidney 30 Cerebrospinal fluid 15 Water 0 Fat-50--100 Air-1000> rad is a unit of absorbed radiation dose. It was restated in SI units in 1970 as the dose causing 0.01 joule of energy to be absorbed per kilogram of matter.> Megahertz is a frequency measurement unit used for Ultrasound crystals.">