Dr. Paul Johns, Department of Physics

Research Areas

    1. X-ray scatter imaging

    My work focusses on novel applications of x-ray physics to medicine. Although x ray is the oldest modality used to image internal anatomy, it can still be used in new ways. Historically in medicine, x rays have been treated only as a beam of particles and used to make shadow picture images on a 2-d image receptor, or line projections for computed tomography (CT). This traditional and simple approach can now be augmented by techniques based on the wave behaviour of x rays. I am developing methods to use scattered x rays to obtain information, instead of using or in addition to using the attenuation of primary photons. Coherent single scatter, being forward directed, is a significant part of the image receptor signal. The differential coherent cross section varies with scattering angle θ and photon energy E in a material-specific manner, even for amorphous materials. The dependence on Z and chemical structure means that coherent scatter can provide large soft tissue contrast based on the biochemical differences between tissues. The overall goal is to obtain more information while minimizing patient dose. There are also possible applications of these techniques outside of medicine, to non-destructive testing, security screening, and elsewhere.

  • Collimation and acquisition for x-ray scatter imaging - Scatter imaging is practical in terms of acquisition time only if many scatter patterns are acquired simultaneously. At the sophisticated BioMedical Imaging & Therapy beamline at the Canadian Light Source (Saskatoon) we demonstrated scatter imaging with up to 5 beams with disentanglement of the overlapping scatter patterns and have been able to reduce the acquisition time to a few minutes, comparable to nuclear medicine [C. Dydula, G. Belev and P.C. Johns, "Development and assessment of a multi-beam continuous-phantom-motion x-ray scatter projection imaging system", Review of Scientific Instruments 90, 035104-1-13 (2019)]. Our images confirm model predictions that scatter-based images have much more soft-tissue contrast than do conventional primary images. We are now implementing these ideas with conventional x-ray equipment in our lab at Carleton to demonstrate that the concept can be applied widely.
    For an introduction see C. Dydula, G. Belev, and P. C. Johns, "Development of synchrotron-based x-ray scatter projection imaging", poster presented at the 2016 CAP Congress. Click here for pdf (1.2 MB).
  • X-ray scatter cross sections of tissues and phantom materials - A library of cross sections (form factors) has been needed for x-ray scatter imaging machine design optimization. Coherent scatter is a cooperative phenomenon involving elastic scatterings of the x-ray wave from several electrons which can be on different molecules, and thus it is virtually impossible to calculate cross sections from first principles. There are two basic measurement approaches: angle (θ) dispersive and energy (E) dispersive.
    We established that the θ-dispersive technique, while standard in crystallography, has difficulties with amorphous materials such as tissues and plastics. The chief obstacle is θ-dependent machine effects. In crystallography the scatter is mostly in confined peaks which allows background to be corrected for, but for amorphous materials there is scatter at all θ and background cannot be isolated. Additionally, all measurements are relative and must be scaled to a calculated “independent atom model” (IAM) value at high θ or E. [Johns & Wismayer, Measurement of coherent x-ray scatter form factors for amorphous materials using diffractometers, Phys. Med. Biol. 49, 5233-5250 (2004)].
    Therefore we devised an alternate high-accuracy laboratory technique. It is E-dispersive, works directly at medical beam energies, and generates absolute results without scaling to IAM values. The small θ range minimizes susceptibility to the θ-dependent machine effects that plague conventional diffractometers [King & Johns, “An energy- dispersive technique to measure x-ray coherent scattering form factors of amorphous materials”, Phys. Med. Biol. 55, 855-871 (2010)].
    Using this method we measured cross sections for five tissues, four plastics and H2O over a large momentum transfer range. [King, Landheer, & Johns, “X-Ray coherent scattering form factors of tissues, water and plastics using energy dispersion”, Phys. Med. Biol. 56, 4377-4397 (2011)]. The tissues, but not plastics or H2O, show an unexplained increase in scattering towards small θ or E.
  • System modelling of x-ray scatter imaging - Our older landmark paper [Leclair & Johns, “A semi-analytic model to investigate the potential applications of x-ray scatter imaging”, Med. Phys. 25, 1008-1020 (1998)] reported the first system model for scatter imaging relating signal, noise, and patient exposure. A series of experiments with plastic/H2O phantoms verified the model [Leclair & Johns, “X-ray forward-scatter imaging: Experimental validation of model”, Med. Phys. 28, 210-219 (2001)].
  • 2. Other activities

  • 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.
  • Focal spot pinhole radiography - the x-ray tube focal spot intensity distribution is a key determinant of image quality. Traditional pinhole radiography of the focal spot required an enormous x-ray tube thermal loading due to the use of non-screen film. Partially for this reason, the effect of the focal spot on system resolution was usually determined by imaging a line or edge, not a point. In today's world several types of digital imaging detector are available which make focal spot pinhole radiography much more practical.
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