Hounsfield number for water is-

Ans. is 'a' i.e., 0 * Water is arbitrarily assigned a value of zero (0) Hounsfield units (HU).Hounsfield's number (CT number)* Hounsfield's contribution is memorialized in the Hounsfield scale, which is used to measure the X-ray attenuation in CT scanning.* 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 coefficient.* 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/mH2O x 100* mx = Attenuation coefficient of matter.* mH2O = Attenuation coefficient of waterSubstanceHU units* Water* Dense cortical bone* Soft tissue* Fat* Lung tissue* Air* 0* +1000 (+400 to+1000)* +40 to +80* -60 to-100* -400 to -600* -1000