Zero HU units is of -

Ans. is 'a' i.e., Water Hounsfield's number (CT number)o Hounsfleld'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 in the material. This depends upon the energy of the X-ray beam and the type of material being irradiated. CT images are map of the relative linear attenuation coefficient of tissues. The relative attenuation coefficient is normally expressed in Hounsfield units (HU) or CT number.CT number / Hounsfield unit of a matter =mx - mH2O------------mH2Ox 100= Attenuation coefficient of matter.mH2O = 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