Investigation of the effect of air gap size on the spatial resolution in proton- and helium radio- and tomography.

Abstract Purpose Proton computed (transmission) tomography (pCT) refers to the process of imaging an object by letting protons pass through it, while measuring their energy after, and their position and (optionally) direction both before and after their traversal through that object. The so far experimental technique has potential to improve treatment planning of proton therapy by enabling the direct acquisition of a proton stopping power map of tissue, thus removing the need to obtain it by converting X-ray CT attenuation data and thereby eliminating uncertainties which arise in the mentioned conversion process. The image reconstruction in pCT requires accurate estimates of the proton trajectories. In experimental pCT detector setups where the direction of the protons is not measured, the air gap between the detector planes and the imaged object worsens the spatial resolution of the image obtained. In this work we determined the mean proton paths and the corresponding spatial uncertainty, taking into account the presence of the air gap. Methods We used Monte Carlo simulations of radiation transport to systematically investigate the effect of the air gap size between detector and patient on the spatial resolution of proton (ion) computed tomography for protons with an energy of 200 MeV and 250 MeV as well as for helium ions (He-4) with an energy of 798 MeV. For the simulations we used TOPAS which itself is based on Geant4. Results For all particles, which are detected at the same entrance and exit coordinate, the average ion path and the corresponding standard deviation was computed. From this information, the dependence of the spatial resolution on the air gap size and the angular confusion of the particle beam was inferred. Conclusion The presence of the airgap does not pose a problem for perfect fan beams. In realistic scenarios, where the initial angular confusion is around 5 mrad and for typical air gap sizes up to 10 cm, using an energy of 200 MeV a spatial resolution of about 1.6 mm can be achieved. Using protons with E = 250 MeV a spatial resolution of about 1.1 mm and using helium ions (He-4) with E = 798 MeV even a spatial resolution below 0.7 mm respectively is attainable.