Monte-Carlo based system matrix generation for the Biograph mMR

Objectives The recently released Biograph mMR is the first integrated MR/PET system for clinical applications. The goal of this study is to generate a 3D system matrix based on Monte-Carlo simulations. Besides standard PET geometry and resolution-limiting effects, the system matrix also includes the effect of the MR body coil and 3T static field.

Methods The PET system and MR body coil were modeled with GATE according to the system specifications and the standard operation settings of the mMR. A first set of simulations were performed to account for geometry and inter-crystal scattering and penetration. A uniform activity distribution was simulated, followed by a post-processing step to sort the simulated coincidences according to the originating event’s position. Positron range under the effect of the 3T magnetic field was simulated separately, in a water medium. The resulting positron range kernel was applied to the images prior to forward projection during reconstruction.

Results Simulated point sources were used to evaluate the accuracy of the generated system matrix. The data were reconstructed with a 3D-MLEM algorithm. An inspection of the sinograms generated by forward-projecting with the Monte-Carlo system matrix correctly showed the blurring effect due to non-collinearity, inter-crystal effects, and scatter in the body coil. The scatter in the body coil covered around 4% of all counts. The reconstructed point spread functions were compared to those obtained from an analytically-derived system matrix. An average resolution (FWHM) improvement of 16% was observed in the transaxial direction and 10% in the axial direction.

Conclusions The evaluated results show that the Monte-Carlo generated system matrix effectively models the mMR's behavior, leading to reconstructed images with better spatial resolution than those obtained with analytical models of the system geometry. This improvement is expected to be more perceptible for larger FOV reconstructions, currently restricted by the simulation time. Work is ongoing to generate a system matrix encompassing the entire scanner bore