Effective use of X rays in diagnostic radiology : Guidance for the optimisation of image quality and absorbed dose in the patient by use of a Monte Carlo computational model of the imaging chain

Abstract: The primary objective of this thesis is to give guidance for the reduction of absorbed dose in the patient and the improvement of image quality by the optimisation of the various parameters which control the performance of the radiological imaging system. The optimisation means that image quality (contrast in screen-film imaging and signal-to-noise ratio in digital imaging) is kept constant and combinations of operating parameters which yields the lowest mean absorbed dose in the patient is determined. The starting point, the definition of an acceptable image quality, is taken from procedures commonly considered to represent good radiological praxis.The methodology used includes a Monte Carlo computational model of the imaging chain. X-ray transport is simulated by tracing individual photons from the X-ray tube through a soft tissue phantom (patient), an anti-scatter grid and into the image receptor. Quantitative values for the image quality (contrast, SNR) and radia lion risk (mean absorbed dose) are derived, both of which are required for the optimisation.Investigation of the influence of X-ray photon energy spectrum (tube potential), contrasting detail and image receptor on the signal-to-noise ratio (SNR) and detective quantum efficiency (DQE) shows that by matching the atomic composition of the detail and the receptor, the DQE can be enhanced. The influence on the mean absorbed dose in the phantom of shaping the X-ray spectrum with different filters was investigated and it was found that added filters of copper are optimal. Guidance on appropriate anti-scatter grids is given by determining optimal anti-scatter grid designs in relation to the imaging situation. It is shown that fibre materials in the grid interspaces can provide a significantdose reduction compared to using aluminium. Optimal grid performancecan be achieved for a variety of strip densities and grid ratios provided that the grids are used with appropriate strip width and tube potential. Commercial grids are better suited for examinations of adult patients (large scattering volume) than for examinations of children (small scattering volume).