Physics, Techniques and Procedures
(Sir Godfrey N. Hounsfield, born 1919, British electroengineer, developed the first clinically useful CT machine), a standardized and accepted unit for reporting and displaying reconstructed X-ray computed tomography CT values. The system of units represents an afine (line) transformation from the original linear attenuation coefficient measurements into one where water is assigned a value of zero and air is assigned a value of -1 000. If mw, ma, and m are the linear attenuation coefficients of water, air and a substance of interest, the CT number of the substance of interest is:
H = 1 000 (m - mw)/(mw - ma).
Thus, a change of one Hounsfield unit (HU) corresponds to 0.1% of the attenuation coefficient difference between water and air, or approximately 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero.
The use of this standardized scale facilitates the intercomparison of CT values obtained from different CT scanners and with different X-ray beam energy spectra, although the CT number of materials whose atomic composition is very different from that of water will be energy dependent. See attenuation number. HU is also used to designate the heat unit.