A prototype proton-cone-beam-computed-tomography (PCB-CT) system utilizing a proton radiatiotherapy beam has been developed. The system acquires CT data in the cone-beam geometry. The cone-beam is produced by scattering a 158.6 MeV narrow parallel proton beam on a range modifier in the form of a linear modulating wheel. The wheel is a PMMA propeller of variable thickness that rotates about its axis parallel to the beam line. The energy spectrum generated by the wheel is designed to result in a monotonically decreasing linear signal versus energy deposited in the detector system. Protons are detected by a system using an intensifying screen and CCD digital camera. The PCB-CT scanner measures relative stopping power of protons in 3D with equal resolution in each dimension. It operates at clinically relevant energies and geometries and in this way facilitates proton therapy planning techniques. The Feldkamp-Davis-Kress cone-beam reconstruction algorithm is applied to obtain the proton stopping powers.
Calibration of the proton CT projections is performed with the aid of a stack of PMMA plates positioned in front of the intensifying screen. Contrast and spatial resolution of the PCB-CT scan is evaluated from CT reconstructions of a contrast-resolution phantom. Artifacts in the reconstruction due to neutron noise in the detector system are corrected by a subtraction technique.
In addition, computer-simulations of proton CT projection data have been performed. For this purpose, a macroscopic proton transport algorithm has been developed. The algorithm derives from the Boltzmann equation. Energy loss is modeled by using experimental energy-range tables for specific materials, while energy deposition is modeled by using a measured dependence of dose on depth in water (the Bragg curve) and the concept of water equivalent thickness (WET). Nuclear collisions are accounted for by the inclusion of the experimental Bragg curve data in water. The small-angle-approximation is assumed in treating the multiple Coulomb scattering (MCS). Limitations of the PCB-CT in characterizing the relative proton stopping power due to the MCS phenomena are examined. A method of removing the MCS artifacts from the projection data is employed to obtain more accurate reconstructed proton stopping powers.
0610: Nuclear physics