Dr. Paul Johns

Biography

Dr. Paul Johns Professor, Medical Physics

Research Interest

Investigating means of obtaining diagnostic information using coherent and incoherent scatter. Although coherent scatter has a small cross section compared with Compton scattering, it is a forward directed process, so that coherently-scattered photons are likely to reach the image receptor. Furthermore, the differential coherent cross section varies with scattering angle and photon energy in a material-specific manner, even for amorphous materials; this is the diffraction signature of the material. This dependence on Z and chemical structure suggests that it can be used to obtain chemical information about tissues. There is growing interest in this area, and a number of workers have demonstrated use of both elastic and inelastic scatter. It had been difficult, however, to compare different scatter imaging schemes, either with each other or with conventional imaging. Therefore, we formulated a semi-analytic model of the process, and validated it experimentally. Our results predict that scatter imaging will indeed be advantageous compared to conventional primary imaging. Current experimental work in the X-Ray Lab at Carleton seeks to better establish the scattering properties of tissues and plastics and to develop collimation strategies for scatter imaging. Dual-energy radiography is a quantitative technique based on imaging the patient with two x-ray spectra. From the two images, data are generated which permit removal of unwanted contrast from obscuring structures and which can be used to generate CT scan images free of energy artefacts. The critical step is a nonlinear transformation from the measurements with the two polynergetic spectra to two basis values. Conventionally this transformation uses an empirical polynomial function with coefficients from calibration data. We are investigating analytic approaches based on the energy dependence of the attenuation coefficients, and evaluating their limitations.