A standard definition is proposed for the Hounsfield number. Any CT number can be converted to the Hounsfield scale after performing a simple calibration using air and water. The energy dependence of the Hounsfield number H is given by the expression H = (Hc + Hp Q)/(1 + Q), where Hc and Hp are the Compton and photoelectric coefficients of the material being measured, expressed in Hounsfield units, and Q is the “quality factor” of the scanner. Q can be measured by performing a scan of a single calibrating material, such as a KI solution.
By applying this analysis to dual-energy scans, the Compton and photoelectric coefficients of an unknown substance may easily be obtained. This can lead to a limited degree of chemical identification.