HU is a measure of -

Ans. is 'a' i.e., CT Hounsfield's number (CT number)o Hounsfield's contribution is memorialized in the Hounsfield scale, which is used to measure the x-ray attenuation in CT scanning. Water is arbitrarily assigned a value of 0 Hounsfield units (HU).o The basic parameter in a CT image is CT number. The CT number is defined with respect to water X-ray absorption and is dependent on absorption/attenuation coefficiento When an X-ray beam (used in CT) passes through the body; there is a logarithmic attenuation of its intensity over distance. A measurement of the intensity of the exiting beam is known as the linear attenuation coefficient when the attenuation thickness is 1 cm. The linear attenuation coefficient is related to the absorption of the material. This depends upon the energy' of the X-ray beam and the type of material being irradiated. CT images are a map of the relative linear attenuation coefficient of tissues. The relative attenuation coefficient is normally expressed in Hounsfield units (HU) or CT number = Attenuation coefficient of matter.H2O = Attenuation coefficient of water.SubstanceHU unitsWater0Dense cortical bone+1000(+400 to+1000)Soft tissue+ 40 to + 80Fat-60 to -100Lung tissue- 400 to -600Air-1000