TU‐A‐9A‐09: Proton Beam X‐Ray Fluorescence CT

Abstract

Purpose:To evaluate x‐ray fluorescence computed tomography induced with proton beams (pXFCT) for imaging of gold contrast agent.Methods:Proton‐induced x‐ray fluorescence was studied by means of Monte Carlo (MC) simulations using TOPAS, a MC code based on GEANT4. First, proton‐induced K‐shell and L‐shell fluorescence was studied as a function of proton beam energy and 1) depth in water and 2) size of contrast object. Second, pXFCT images of a 2‐cm diameter cylindrical phantom with four 5‐ mm diameter contrast vials and of a 20‐cm diameter phantom with 1‐cm diameter vials were simulated. Contrast vials were filled with water and water solutions with 1‐5% gold per weight. Proton beam energies were varied from 70‐250MeV. pXFCT sinograms were generated based on the net number of gold K‐shell or L‐shell x‐rays determined by interpolations from the neighboring 0.5keV energy bins of spectra collected with an idealized 4π detector. pXFCT images were reconstructed with filtered‐back projection, and no attenuation correction was applied.Results:Proton induced x‐ray fluorescence spectra showed very low background compared to x‐ray induced fluorescence. Proton induced L‐shell fluorescence had a higher cross‐section compared to K‐shell fluorescence. Excitation of L‐shell fluorescence was most efficient for low‐energy protons, i.e. at the Bragg peak. K‐shell fluorescence increased with increasing proton beam energy and object size. The 2% and 5% gold contrast vials were accurately reconstructed in K‐shell pXFCT images of both the 2‐cm and 20‐cm diameter phantoms. Small phantom L‐shell pXFCT image required attenuation correction and had a higher sensitivity for 70MeV protons compared to 250MeV protons. With attenuation correction, L‐shell pXFCT might be a feasible option for imaging of small size (∼2cm) objects. Imaging doses for all simulations were 5‐30cGy.Conclusion:Proton induced x‐ray fluorescence CT promises to be an alternative quantitative imaging technique to the commonly considered XFCT imaging with x‐ray beams.