<prompt> Maximal valve of HU Unit -WaterFatSoft tissueBone  <response>: Ans. is d i.e., Bone Hounsfields number CT numbero Hounsfields 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 ux-uH2O PHpx 100ux Attenuation coefficient of matter.uH2O Attenuation coefficient of water.SubstanceHL unitsWater0Dense cortical bone 1000 400 to - 1000Soft tissue 40 to 80Fat- 60 to -100Lung tissue- 400 to -600Air- 1000 