Experiments in the application of positrons to Medical Physics

Three aspects of the use of positrons in relation to medical science are investigated. In order to gain a better, more complete understanding of the interaction of positrons and electrons with biological systems, a series of scattering experiments has been undertaken at the Australian National University. The current research extends the existing knowledge base by scattering positrons and electrons from thymine, which is a DNA nucleobase, and pyridine, which is a root molecule for a number of biologically relevant molecules. Cross-sections thus obtained contribute further to the development of a model for positron transport within the human body. Liquid water forms approximately 70 percent of the human body by mass. Being able to make measurements of positron interactions with liquid water also contributes to a more accurate model of positron transport in the human body. Until recently, positron beams were limited to high vacuum environments, however it is now possible to extract positrons into air and hence a liquid water target has become a viable option. The current investigation proposes and demonstrates the possible construction for such a target. One of the limiting factors which contributes to the relatively low resolution of Positron Emission Tomography (PET) images is the distance from the source isotope that a positron travels prior to annihilating with an electron. The current research investigates the possibility of reducing this distance by using heavy metals surrounding the positron source. Initial experiments relied on a positron producer being sandwiched between layers of platinum. This configuration did not result in a significant change in annihilation distance. Subsequent experiments were conducted using a colloid where the positron producer is bound in a lead compound. Analysis of the images thus obtained appeared to show a small reduction in apparent target size, however the factor which limited the observable improvement was the PET machine camera resolution. Further research is warranted, however possibly using a small animal PET scanner