UCSF

Pair Production Tomography (P2T) is an emerging imaging modality that detects pair production events from high-energy photon beams, providing unique atomic-number-based contrast. A fundamental challenge in P2T image reconstruction is positron range — the finite distance positrons travel before annihilating — which causes spatial blurring between the true pair production distribution and the reconstructed annihilation image. Unlike in Positron Emission Tomography (PET), the directed photon beam in P2T breaks the isotropy of positron emission, requiring a beam-direction-dependent correction approach. No positron range correction method has previously been developed for P2T.In this work, two positron range correction methods are developed and evaluated for P2T imaging: Richardson-Lucy deconvolution and a total variation (TV)-regularized optimization framework solved using the Fast Iterative Shrinkage-Thresholding Algorithm (FISTA). Beamlet-specific positron range kernels were constructed from Geant4 Monte Carlo simulations using a 10MV bremsstrahlung source, capturing the joint distribution of transverse positron displacement and emission angle relative to the beam direction. Kernels were constructed at three resolutions — 2mm, 1mm, and 0.5mm — to evaluate the effect of kernel resolution on correction performance.Corrections were applied to all 46 beamlets of a simulated computed tomography (CT)-based phantom containing three high-Z inserts. Both methods consistently improved beam profile spatial accuracy across all 46 beamlets, reducing average beam full width at half maximum (FWHM) from 8.5mm to approximately 6mm. At the full image level, pseudo-high resolution reconstruction at 1mm followed by rebinning to the intrinsic 2mm detector resolution was found to be the optimal pipeline. Both Richardson-Lucy and FISTA reduced insert FWHM from 18mm to 16mm with the 1mm resolution kernel correction. Direct 2mm reconstruction and correction showed no improvement in insert FWHM, demonstrating the importance of kernel resolution for accurate correction.These results demonstrate that positron range correction is feasible for P2T imaging and can meaningfully improve spatial resolution. The current pipeline represents a practical and effective approach, providing a foundation for future development of spatially varying kernels and expansion to full-view phantom and patient data.