Detection and measurement in emission and transmission tomography and applications

The theory and application of computerised tomography in non-destructive testing, in emission and transmission mode, utilising X- and gamma-rays is the subject of this study. Detection characteristics pertinent to imaging applications were measured for a number of detectors (e.g. NaI, BGO, CsF, HPGe, CdTe). 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 were 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 was also developed and its successful application demonstrated. The results obtained from the scanning experiments illustrate the image degradation caused by scattered photons. The effects of scattered photons increase with energy and are more acute in emission mode. (author)