The X-ray attenuation coefficient and its application to transmission computed tomography

A simple yet accurate formula for the X-ray attenuation coefficient is derived, by reference to fundamental theory, for elements with atomic numbers up to 53 and photon energies from 1.02 MeV down to 30 keV. It is shown that the attenuation coefficient of tissue may be parametrised in terms of the concentrations of only two reference materials. The relationship between the attenuation coefficient of tissue and the Hounsfield Unit in Computed Tomography (CT) is derived and a new method of tissue analysis by dual energy CT presented, whereby the concentrations of the 2 reference materials, which characterise the tissue, are deduced. These concentrations may be related to gross tissue composition. The minimum detectable changes in fat content, bone mineralisation or iodine concentration is expected to be 0.12 kg/kg, 0.004 kg/kg or 2.0 x 10-4 kg/kg on an EMI CT5005 CT scanner in clinical practice. Technical improvements will improve these figures. A few clinical examples are presented. Also included are analyses of the propagation of projection noise to the reconstructed image, the effect of spatial averaging and the assessment of errors in clinical practice in CT. The effect of beam hardening is calculated and predictions are verified on the EMI CT5005. (author)