Filtered back-projection reconstruction for attenuation proton CT along most likely paths

Area of Science:

• Medical Physics
• Imaging Science
• Particle Physics

Background:

• Proton computed tomography (pCT) is an imaging technique that uses protons to map material properties.
• Attenuation pCT, which measures linear attenuation coefficients, faces challenges with spatial resolution due to proton multiple Coulomb scattering (MCS).
• Conventional energy-loss pCT also suffers from spatial resolution limitations caused by MCS.

Purpose of the Study:

• To adapt a filtered back-projection algorithm along the most likely path (MLP) for attenuation pCT.
• To evaluate the performance of the adapted MLP algorithm in attenuation pCT using Monte Carlo simulations.
• To investigate the impact of energy dependence of inelastic cross-sections and statistical limitations on attenuation pCT.

Main Methods:

• Monte Carlo simulations using Geant4 (via Gate) were performed for pCT acquisitions of density and spatial resolution phantoms.
• A filtered back-projection algorithm along the most likely path (MLP) was adapted for attenuation pCT.
• Analytical methods were used to determine statistical limitations and noise levels.

Main Results:

• The adapted MLP algorithm improved the spatial resolution of attenuation pCT compared to conventional straight-line path binning.
• Attenuation pCT images exhibited significantly higher noise levels (411x and 278x at 200 MeV and 300 MeV, respectively) compared to energy-loss pCT.
• A capping artifact was observed in attenuation pCT when the residual proton energy was below 100 MeV due to energy-dependent inelastic cross-sections.

Conclusions:

• Adapting the MLP algorithm enhances spatial resolution in attenuation pCT, offering potential benefits despite increased noise.
• The West-Sherwood effect, an interplay of MCS and attenuation, contributes to improved spatial resolution in certain attenuation pCT scenarios.
• Further research is needed to mitigate noise and artifacts for clinical applicability of attenuation pCT.