Compton scattering spectroscopy for determining filtration of a clinical computed tomography system

Purpose: Compton spectroscopy is a widely accepted tool for measuring the primary X-ray spectra of a diagnostic computed tomography (CT) system. The aim of this work is to spectroscopically detect CT X-ray photons subjected to 90-degree Compton scattering for accurate modelling of CT beam-shaping filtration, which necessary for computation of radiation exposure and for other research applications in medical physics. Methods: For a tube potential of 120 kVp X-ray spectra have been measured under fan angles of 0° to 20° in steps of 2° and at 21° from the central ray. At each fan angle the value of the X-ray path length determining the relative filter transmission was iteratively varied in order to estimate the thickness of the aluminium filter and the filter geometry was implemented in a Monte Carlo (MC) simulation of the CT scanner. Results: Direct comparison of measured and simulated filtered X-ray spectra indicates that the spectra match very well, with a correlation of 1 for small fan angles (below 4°) and an average correlation of 0.996 for the whole field-of-view (FOV). Conclusion: The agreement between the simulated X-ray spectra and the measured spectra suggest that spectral measurements using Compton spectroscopy across the fan beam of a CT system are suitable for modelling the beam-shaping filtration currently used in commercially available CT systems.