CT number definition

The accuracy of CT number plots has been found lacking in several medical applications. This is of concern since the ability to compare and evaluate results on a reproducible and standard basis is essential to long term development. Apart from the technical limitations arising from the CT scanner and the data treatment, there are fundamental issues with the definition of the Hounsfield number, namely the absence of a standard photon energy and the need to specify the attenuation mechanism for standard measurements. This paper presents calculations to demonstrate the shortcomings of the present definition with a brief discussion. The remedy is straightforward, but probably of long duration as it would require an international agreement.

► The dependence of the CT number definition on photon energy is examined.
► Graphical examples of the CT number variation with photon energy are given.
► The influence of absorption edges and scattering on CT numbers is discussed.
► A proposal is made for an international standard devoted to CT number evaluation.