Extension of the List-Mode MLEM Algorithm for Poly-Energetic Imaging with a Compton Camera

Proposed in the mid-seventies as a tool to retrieve the direction of gamma rays emerging from tumors marked by a radio-tracer, the Compton Camera (CC) was also proposed more recently to localize radioactivity for nuclear dismantling operations. For the TEMPORAL project, CeBr ₃ scintillation crystals coupled with SiPM arrays will be used both for the scatterer and the absorber. The sensitivity of the CC, as well as its angular resolution, depend on the distance between the scatterer and the absorber, on their sizes and on the energies of the detected gamma rays. Its angular resolution also depends on the scatterer and the absorber spatial and energy resolutions. Monte Carlo simulations have shown that CeBr ₃ is suitable for detection of energies from 200 keV to 2 MeV with a scatterer thickness of 1.5 cm and a distance between the scatterer and absorber front faces of 7.5 cm. To reconstruct source positions and activities, the most commonly used algorithm is the list-mode Maximum Likelihood Expectation Maximization (MLEM), which is limited to image reconstruction of mono-energetic gamma rays. We also propose an extension of the list-mode MLEM algorithm in order to allow poly-energetic imaging of radioactive sources (PE-LM-MLEM).