Advanced image processing for accurate proton radiography with a scintillator screen coupled with a CCD camera: Geant4 simulations and experiments with a two-material phantom

Measurement of the proton water-equivalent path length (WEPL) and relative stopping power (RSP) of human tissues with proton radiography (pRG) can help improving the X-ray-CT-based RSP images of patients in proton-therapy treatment plans. This paper presents an extension to cylindrical symmetric two-material phantoms of a pRG method based on one single 2D-pixelated flat detector, previously applied by us to homogeneous materials. The experiments are simulated with Monte-Carlo simulations and performed with a scintillator screen coupled with a CCD camera. The spatial 2D distributions of the integral energy deposit and proton fluence in the detector after each phantom are determined. In the experiments, only the energy deposit is measured (by the intensity of the scintillation light from the screen), whereas the simulated fluence distribution is used as a proxy for the experimental counterpart. The ratio of the two distributions is calculated, which (in first instance) only depends on the stopping power of protons in the detector. This ratio is converted to an image of the residual range in water of protons after traversing the phantom with a calibration, which also involves correcting for the energy deposit from particles not determining the range values, e.g. protons having undergone hard interactions and secondary particles. The range image is then converted to a WEPL/RSP image, and the measured WEPLs/RSPs are compared with reference values to assess their accuracy. Simulations showed that 1 % accurate RSPs can be achieved for the homogeneous parts of the phantoms, whereas this is hampered near the border of different materials by range mixing due to proton multiple Coulomb scattering (MCS). The results from the experiments are overall consistent with the simulations, but 1 % accurate RSPs were not measured for some parts of the phantoms because of image artefacts related to the use of the simulated fluence distribution instead of the experimental one. A novel image-processing methodology for pRG with a scintillator screen was investigated. The WEPL/RSP of a target is determined from the ratio of the energy deposit and fluence of protons in the screen through a designated calibration. To test the WEPL/RSP accuracy of the method, the pRGs of two phantoms with simple geometry and made of two different homogeneous materials were simulated and measured. The simulations showed that the RSP can be determined with 1 % accuracy over all homogeneous parts, whereas this is hampered at the border between different materials by MCS-induced range mixing, which needs more complicated analysis to be disentangled. In the experiments, 1 % accurate RSPs could be measured only over a limited area of the image because of artefacts caused by using the simulated fluence distribution in place of the experimental one, which must then be measured in future experiments. • Proton radiography of two-material phantoms with a scintillator screen. • Novel image-processing method used to retrieve the RSP image. • In simulations, 1 % accurate RSPs except at the border between different materials. • Measuring the proton fluence experimentally is fundamental to avoid artefacts.