Abstract
The application of computerised tomography in non-destructive testing, in emission and transmission mode, utilising X- and gamma-rays is the subject of this study. Measurements for tomography using X- and gamma-rays and the operation of radiation detectors are based on photon interactions with matter and the theory governing these interactions is-discussed. The theory of the mathematical reconstruction of a two dimensional distribution from its projections is shown and reconstruction techniques and applications reviewed. The principles of radiation detection and measurement are presented in order that the physical significance of the data recorded can be assessed. Detection characteristics pertinent to imaging applications were measured for a number of detectors (e.g. NaI, BGO, CsF, HPGe, CdTe). The importance of these characteristics in determining the suitability of detectors for imaging applications are discussed. The relative merits of each detector type are compared and their relative suitability predicted. A prototype scanning rig was designed and used to carry out tomography experiments with phantoms in emission and transmission mode and contrast and spatial resolution under various conditions were determined. The usefulness of line scans and contrast measurements in quantitative image analysis is shown. The effects of scattered photons on image quality are studied by means of the selection of different discriminator energy windows. In each scan the counts within the full width at half and tenth maximum of the photopeak were used as the data for the reconstruction. A method for subtracting the scattered photon contribution from the peak window is also developed and its successful application demonstrated. The results obtained from the scanning experiments illustrate the image degradation caused by scattered photons. The effect of scattered photons increase with energy and are more acute in emission mode.