The effect of beam purity and scanner complexity on proton CT accuracy

Abstract

Purpose To determine the dependence of the accuracy in reconstruction of relative stopping power (RSP) with proton computerized tomography (pCT) scans on the purity of the proton beam and the technological complexity of the pCT scanner using standard phantoms and a digital representation of a pediatric patient. Methods The Monte Carlo method was applied to simulate the pCT scanner, using both a pure proton beam (uniform 200 MeV mono-energetic, parallel beam) and the Northwestern Medicine Chicago Proton Center (NMCPC) clinical beam in uniform scanning mode. The accuracy of the simulation was validated with measurements performed at NMCPC including reconstructed RSP images obtained with a preclinical prototype pCT scanner. The pCT scanner energy detector was then simulated in three configurations of increasing complexity: an ideal totally absorbing detector, a single stage detector and a multi-stage detector. A set of 15 cm diameter water cylinders containing either water alone or inserts of different material, size, and position were simulated at 90 projection angles (4° steps) for the pure and clinical proton beams and the three pCT configurations. A pCT image of the head of a detailed digital pediatric phantom was also reconstructed from the simulated pCT scan with the prototype detector. Results The RSP error increased for all configurations for insert sizes under 7.5 mm in radius, with a sharp increase below 5 mm in radius, attributed to a limit in spatial resolution. The highest accuracy achievable using the current pCT calibration step-phantom and reconstruction algorithm, calculated for the ideal case of a pure beam with totally absorbing energy detector, was 1.3% error in RSP for inserts of 5 mm radius or more, 0.7 mm in range for the 2.5 mm radius inserts, or better. When the highest complexity of the scanner geometry was introduced, some artifacts arose in the reconstructed images, particularly in the center of the phantom. Replacing the step-phantom used for calibration with a wedge phantom led to RSP accuracy close to the ideal case, with no significant dependence of RSP error on insert location or material. The accuracy with the multi-stage detector and NMCPC beam for the cylindrical phantoms was 2.2% in RSP error for inserts of 5 mm radius or more, 0.7 mm in range for the 2.5 mm radius inserts, or better. The pCT scan of the pediatric phantom resulted in mean RSP values within 1.3% of the reference RSP, with a range error under 1 mm, except in exceptional situations of parallel incidence on a boundary between low and high density. Conclusions The pCT imaging technique proved to be a precise and accurate imaging tool, rivalling the current x-rays based techniques, with the advantage of being directly sensitive to proton stopping power rather than photon interaction coefficients. Measured and simulated pCT images were obtained from a wobbled proton beam for the first time. Since the in-silico results are expected to accurately represent the prototype pCT, upcoming measurements using the wedge p...

Helium CT: Monte Carlo simulation results for an ideal source and detector with comparison to proton CT

“…For the idealized energy‐to‐WEPL calibration curve of HeCT, shown in Fig. , an average WEPL error of 1.06 mm was obtained, whereas for pCT in the same condition it was 2.08 mm, which is almost double. This is an additional proof of the smaller deviation of helium beams with respect to protons.…”

“…The WEPL calibration of the scanner is therefore critical to the accuracy of pCT and HeCT reconstruction. For the previous experimental pCT scans, a polystyrene wedge phantom was developed to speed up and simplify the experimental calibration procedure . For the TOPAS simulations, a polystyrene wedge phantom was used to match the experimental setup, with thickness increasing from 0 to 50.8 mm over a length of 254 mm.…”

“…The limitation to 90 projections was made since previous works demonstrated the equivalency of this setting to a continuous scan, regarding the RSP reconstruction accuracy . The image reconstruction method for HeCT was the same as used previously for pCT . The simulated images were reconstructed with six iterations of the DROP‐TVS algorithm using a slice thickness of 1.25 mm and starting from a FBP‐reconstructed image as the initial iterate; the algorithm used 40 blocks, and a relaxation parameter of 0.1 for the block iterative reconstruction.…”

“…On the other hand, computed tomography with charged particles (protons and ions) can provide a 3D map of RSP of the object under study, directly reconstructed from energy loss data without any conversion needed . Previously, the accuracy of proton CT (pCT) and its dependence on beam purity and technological complexity of a pCT scanner using particle tracking and energy loss or water equivalent path length (WEPL) measurements was evaluated and the maximum accuracy of the RSP reconstruction proved to be better than 1% in most cases . However, one disadvantage of pCT is its limited spatial resolution due to multiple Coulomb scattering (MCS), which causes the particle path inside the object to deviate from a straight line.…”

“…[9][10][11] Previously, the accuracy of proton CT (pCT) and its dependence on beam purity and technological complexity of a pCT scanner using particle tracking and energy loss or water equivalent path length (WEPL) measurements was evaluated and the maximum accuracy of the RSP reconstruction proved to be better than 1% in most cases. 12 However, one disadvantage of pCT is its limited spatial resolution due to multiple Coulomb scattering (MCS), which causes the particle path inside the object to deviate from a straight line. Ions heavier than protons are affected much less by MCS than protons due to the smaller charge-to-mass ratio.…”