Dr. Paul Johns

Professor, Medical Physics
2454 HP
(613) 520 - 2600 Ext. 4317

About Dr. Paul Johns

Research Summary

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.

Member of collaboration at Carleton investigating the use of gas microstrip detectors and gas electron multipliers for medical x-ray imaging. By operating in photon counting mode, the energy of each photon event can be measured, providing input for applications such as dual-energy radiography.

Studying iterative reconstruction techniques to reduce artefacts in computed tomography (CT) by accounting for the polyenergetic nature of the x-ray beam as well as scattered x rays.


  1. King BW, Johns PC. An energy-dispersive technique to measure x-ray coherent scattering form factors of amorphous materials. Phys Med Biol. 2010 Feb 7;55(3):855-71. Epub 2010 Jan 14.
  2. King BW, Johns PC. Measurement of coherent scattering form factors using an image plate. Phys Med Biol. 2008 Nov 7;53(21):5977-90. Epub 2008 Oct 3. Erratum in: Phys Med Biol. 2009 Oct 21;54(20):6437.
  3. B. King and P.C. Johns, "Measurement of Coherent Scattering Form Factors 
    using Polychromatic X-Ray Sources and Imaging Detectors", Proceedings of 
    52nd Annual Meeting of the Canadian Organization of Medical Physicists, 
    245-247 (June 2006) [Abstract: Medical Physics 33, 2673 (2006)].
  4. M. Nisar and P.C. Johns, "Coherent Scatter X-Ray Imaging of Plastic/Water 
    Phantoms", in Photonics North 2004: Applications in Astronomy, 
    Biomedicine, Imaging, Materials Processing, and Education, Proc. SPIE 
    5578, 445-453 (Ottawa, 29 September 2004).
  5. P.C. Johns and M.P. Wismayer, "Measurement of Coherent X-Xay Scatter Form Factors for Amorphous Materials using Diffractometers", Physics in Medicine and Biology 49, 5233-5250 (2004). Note: awarded the Sylvia Fedoruk Prize in Medical Physics of the Saskatchewan Cancer Agency at the July 2005 COMP/CCPM conference.
  6. M. Nisar and P.C. Johns, "Coherent Scatter X-Ray Imaging of Plastic/Water Phantoms", in Photonics North 2004:Applications in Astronomy, Biomedicine, Imaging, Materials Processing, and Education, Proc. SPIE 5578, 445-453 (Ottawa, 29 September 2004).

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