Zero HU units is of-

Hounsfield unit: CT scan is based on linear attenuation of X-ray Linear attenuation is measured in Hounsfield units Hypodense -0 or less than zero Hyperdense- More than zero NOTE: * The reference value for Hounsfield unit is 0 seen with CSF or Water * HU of (-50) - (-100) is seen with fat * HU of (-1000) is seen with air * HU of 60 - 70 Acute bleed * HU of 130 Contrast * HU of 200 to +250 Calcifications * HU of 400 to +1000 Coical bone