X-ray Fluorescence Computed Tomography Induced by Photon, Electron, and Proton Beams.

X-ray fluorescence CT (XFCT) has shown promise for molecular imaging of gold nanoparticles. To date, XFCT has been induced by kilovoltage photon beams due to the high photoelectric interaction probability. We compare K-shell and L-shell XFCT induced by photon, electron, and proton beams for two phantom sizes. A 2.5cm and 5.0cm-diameter phantom with four 5mm and 10mm vials, respectively, with gold-solutions of 0.1%-2% by weight were built in TOPAS, a GEANT4-based Monte Carlo simulation tool. The 2.5cm-phantom was imaged with XFCT induced by beams of 7.45 × 104 81keV-and 5MeV-photons, 220kVp-and 6MV-photons, 10MeV-and 100MeV-electrons, and 100MeV-and 250MeV-protons. The doses between each phantom size was equal. First-generation CT geometry with 0.5mm × 0.5mm pencil beams with 0.5mm-translation and 2 -rotation steps over each phantom was modeled. Scattered x-rays were detected on an idealized spherical detector from which K-shell and L-shell fluorescent x-rays were extracted in 0.5keV and 0.2keV bins. XFCT images were generated using iterative reconstruction algorithms. The highest gold sensitivity was seen in the 81keV-photon K-shell and L-shell images (0.004% and 0.007%) of the 5.0cm-phantom at 30mGy. For the 2.5cm-phantom, the detection limits were 0.006%, 0.62%, and 0.28% for 81keV-photon K-shell, 100MeV-electron K-shell, and 100MeV-proton L-shell images, respectively. The mean imaging dose was approximately 2-3 orders of magnitude higher in electron-and proton-XFCT compared to 81keV-photon XFCT. Our MC study demonstrates that small-object XFCT imaging achieves the best performance when induced with kilovoltage-photon beams. Due to high imaging doses, electron-and proton-induced XFCT might be feasible for guiding nanoparticle-enhanced charged-particle radiotherapy.