Abstract
The Texas A&M University positron emission tomograph (TAMU PET) is an experimental, medical-imaging instrument designed to detect gamma rays produced by positron-electron annihilation. Each annihilation yields two coincident gamma rays that propagate away from each other, thus tracing a nearly straight line through the gamma-ray source. The TAMU PET is constructed using plastic scintillators, fiber optics, and photomultiplier tubes (PMTs). PMT signals encoded by the PET's electronics produce an address for each coincident gamma-ray pair hence a line through the gamma-ray source. Line addresses collected at various angles are used to reconstruct an image of the radiating source. An experimental subset of the TAMU PET was used to investigate gamma-ray scattering within the human head and to measure the point-spread function (PSF) of the gamma-ray point source (68Ge, a positron-emitting substance) . The human head was modeled by a solid plexiglas cylinder; plexiglas has a Compton scattering density similar to that of the human head. PSFs of the 1-m gamma-ray source were obtained with and without the plexiglas head, and th PSFs were fitted to the following Gaussian function: [ ] where a is the background value added to the Gaussian term with a peak coe displacement along the abscissa is p. The standard deviation of the PSF for the gamma-ray source located at th center of the plexiglas was identical to that of the PSF obtained without th plexiglas head. The peak amplitude of the PSF obtained with the plexiglas he in place is 1 smaller than that of the PSF obtained without the head. The findings agree with theory developed from the Klein-Nishina differential scatterin cross section and the attenuation factor evaluated for two mean-free-path length [ ] The PSF for locations throughout the head is invariant within 10%. Deco volving the measured gamma-ray source distribution with a fixed variation in the PSF was confirmed by the retical calculations.
Aguiar, James (1998). Study of scattering in positron emission tomography. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1998 -THESIS -A38.