Purpose: Proton computed tomography (pCT) facilitates direct determination of the electron density distribution within a patient, enabling accurate proton therapy treatment planning. However, pCT has never been implemented clinically due to its low spatial resolution, largely an effect of multiple Coulomb scattering. Currently, this effect is accounted for by extrapolating each proton’s most-likely-path and algebraically reconstructing the image. This method is an approximation and requires significant computing power. As an alternative, we have developed a simpler method in which detected protons scattered by an angle greater than a cutoff value θc are rejected, and standard cone-beam reconstruction algorithms are employed to generate images.
Methods: Using GEANT4, a 250 MeV proton cone-beam was placed at a distance of 100 cm from the target object. Two silicon strip detectors were modeled to determine each proton's entry and exit positions with respect to the object for calculating scatter angle. Protons with scatter angle exceeding θc were rejected. A QA phantom was designed to determine reconstruction accuracy and spatial resolution, and an anthropomorphic head phantom (cropped from a full-body extended cardiac-torso, XCAT phantom) was used to evaluate dose. Scans were performed in 1° steps with 2×10⁸ primary protons generated at each step. Images were reconstructed for θc=0.1°, 0.5°, 1.0°, and unrestricted.
Results: Spatial resolution and reconstruction accuracy generally improved with decreasing θc. At θc=0.1°, resolution was 8 lp/cm for bone-water and 9 lp/cm for air-water. Reconstruction accuracy for bone and water was within 1% of the expected electron density. The absorbed dose in the head phantom was 43.85 mGy, which is below the median dose for head X-ray CT.
Conclusion: Proton CT images exceeding clinical standards for spatial resolution, reconstruction accuracy, noise, and dose are achievable using a method in which detected protons scattered by an angle greater than some cutoff value are rejected.
Funding Support, Disclosures, and Conflict of Interest: This abstract is authored by UT-Battelle, LLC under Contract DE-AC05-00OR22725 with the U.S. Department of Energy. The Department of Energy will provide public access to these results in accordance with the Department of Energy Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).