An accurate parametrisation of the x-ray attenuation coefficient

A brief note is presented concerning the derivation of an expression for the parametrisation of the linear x-ray attenuation coefficient, starting from the fundamental theory of photon-electron interactions. Accuracy is discussed for elements common in tissue and contrast media. Calculated atomic cross-sections for coherent nd incoherent scattering are compared with those of other workers (Hubbell, Schofield, et al l) and improved parametrisation for the photoelectric effect is also discussed. It is concluded that approximations in terms of atomic number, the number of atoms per unit volume and the photon energy have no sound theoretical foundation and have limited accuracy and validity, and that many quantities defined as energy independent parameters of the mixture (e.g. Compton and photoelectric parameters) are in fact dependent on energy over a wide range. (U.K.)