Medical Applications of Attenuation Numbers in Computerised Tomography.

A computerised tomographic scanner is used to obtain sectional images of biological structures. The success of any research program in CT scanning depends on an understanding of the derivation of CT numbers and what physical changes cause them to vary. An analysis of attenuation data gave an empirical function which was used to calculate photon attenuation cross sections. The CT numbers for biological materials were calculated and compared with measurements made by RaO, Gregg and Phelps. Spectral changes along ray paths through biological materials were studied, spectrum mean energies and total photon fluence were calculated for several phantom sizes and for the human body and head. The monochromatic equivalent energy was shown by theory and experiment to be similar to the scan averaged spectrum mean energy. Error sources were considered and the ability of the software to minimise spectral errors was studied using a constant size phantom in different fields. CT numbers increased with increasing field size and it was shown that dual wedge-energy measurements allowed correction for spectral change. Experiments with corpses proved that such a correction technique could be applied and that the reproducibility of CT numbers was +/- 0. 25 EU for liver tissue and trabecular bone. In vivo and in vitro measurements of the attenuation coefficient for trabecular bone were made using the EMI and ISOTOM CT scanners. Spectral hardening v/as shown not to be significant. The stability of the air scale factor was tested using patients and shown to be better than +/- 1 %. Air-water scale factors were measured and tested using glucose solutions. Measurements with haemochromatosis (iron) and fatty liver patients showed good correlation between measured liver CT numbers and liver biopsy estimates. Trabecular bone measurements in the distal radius, using the ISOTOM scanner, correlated well with EMI CT scanner vertebral bone CT numbers