Scattering and Absorption of Photons in Emission Computed Tomography
Egbert, Stephen David
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https://hdl.handle.net/2142/67787
Description
Title
Scattering and Absorption of Photons in Emission Computed Tomography
Author(s)
Egbert, Stephen David
Issue Date
1980
Department of Study
Nuclear Engineering
Discipline
Nuclear Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Nuclear
Energy
Language
eng
Abstract
This thesis reports the effects that the scattering and absorption of photons have on the ability to produce tomographic images of an internal gamma-ray source distribution. Computer simulations of the photon-transport process and the collimator-detector system are used in this study so that these scattering effects can be examined independently from other imaging factors. As a result of this investigation, two techniques have been developed for improving image quality in emission computed tomography (ECT).
A systematic method is devised for the elimination of Compton-scatter distortions in multiview gamma-ray ECT. The approach is based upon the collision-expansion formulism which arises from the integral-transport equation and is applicable to a wide variety of object and detector configurations. Approximate correlations for point-kernels are presented which allow considerable savings in computer time and storage requirements. Results for simulated source distributions and tissue shapes show that this approach is effective for suppressing scatter background and distortion from sources both inside and outside of the tomographic slice of interest.
A second tomographic technique is developed which estimates the depth of a gamma-emitting isotope by observing the energy spectrum of the scattered photons emerging from the body. These spectra are dependent upon the depth and location of the internal source. Monte Carlo methods are used to compute surface energy spectra for Tc-99('m) point sources at different depths. From these noise-free results, a discrete response matrix is constructed to relate the input source depth to the corresponding output spectrum. Observed spectra with noise are then simulated independently by Monte Carlo calculations assuming various measurement times and source strengths. The source distribution is determined by solving the unfolding problem by the Method of Regularization. The effective depth resolution is seen to depend strongly upon measurement time. These results indicate that limited depth resolution can be obtained from a single view of the object.
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