The use of emission-transmission computed tomography for improved quantification in SPECT
The attenuation of photons within the body has been recognised as the major limiting factor hindering the ability of single photon emission computed tomography (SPECT) as a quantitative technique. This thesis investigates several aspects of an emission-transmission SPECT system using the Monte Carlo method and experimental techniques. The system was based on a rotating gamma camera fitted with a parallel hole collimator. The simulation of a transmission study was performed using a simple non-uniform mathematical phantom with two different external sources, a collimated line source and a flood source. The results showed that the attenuation maps were highly dependent on the geometry and photon energy of the source. The collimated line source produced improved image quality with lower statistical noise than the flood source. The results showed that, when high atomic number elements are present in the tissue composition, the attenuation coefficients at different energies are related through a second order polynomial transformation. If the object under study is formed of soft tissue equivalent materials, a linear transformation holds. The attenuation maps generated in the transmission study were used to correct for non-uniform attenuation compensation of an emission phantom. The results showed that non-uniform attenuation compensation improved image quality and reduced noise when compared to data without attenuation compensation. The presence of scattered photons in the emission data reduced the quality of the images and precluded accurate quantification. Absolute quantification was performed using the percent air sensitivity criterion. The largest difference between the theoretical and the Monte Carlo simulated images was approximately 8%. An emission-transmission myocardial perfusion study was simulated using an anthropomorphic phantom. Two photon energies of clinical interest were used, 75 keV and 140 keV, corresponding to the main photon emission energies of 201Tl and 99mTC. The results showed that 99mTc provided better image quality than 201Tl. Non-uniform attenuation compensation produced a very good agreement between the theoretical prediction and the simulation when scatter-free data were considered. The results presented in this thesis indicate that it is not possible to accomplish accurate attenuation compensation in general situations if scatter correction is not applied.