Author: Alejandro Bertolet, Carlos Huesa-Berral, Victor V. Onecha π¨βπ¬
Affiliation: Massachusetts General Hospital and Harvard Medical School π
Purpose: Radioembolization for liver tumors using 90Y microspheres uses SPECT as confirmatory dosimetry, although 90Y is a pure Ξ²-emitter, and SPECT-imaging is only available through subsequent bremsstrahlung radiation, unsuited for current SPECT-imaging. There is a need for algorithms that can account for non-scattered bremsstrahlung photons besides the energy window typically centered around radionuclide-specific emission-peaks. To address this,we present a proof-of-concept GPU-based backward photon Monte Carlo propagator (BPMCP), propagating scattered photons from the detector to the source, which offers the potential to enhance imaging quality in 90Y SPECT.
Methods: We generated 12 projections of a spherical uniform source of 99mTc inside a water phantom in a generic gamma camera with the TOPAS MC toolkit. Reconstruction was performed with a simple backprojection method, which does not consider photon-attenuation, and the BPMCP. The BPMCP backtracked all detected photons from the detector to the source, reducing the photon statistical weight based on the material attenuation properties. Additionally, we implemented an inverse Compton database to account for scattering that determines at each step whether a photon is scattered. The backtracked photonβs new energy and direction are sampled according to their double-differential cross-sections. Photons reaching 140keV contribute to the final image reconstruction. Reconstructed objects were compared to the ground truth by calculating the Mean Square Error(MSE) across voxels, assuming activity equal to one in the entire sphere as the comparative metric.
Results: Attenuation- and scattering-corrected reconstruction using BPMCP recovers significantly better the shape of the spherical source compared to backprojection, with an MSE of 0.047 versus 0.19, which supports the adequacy of the inverse Compton database implemented.
Conclusion:This work shows a proof-of-concept for BPMCP as a fully physics-driven reconstruction algorithm to account for image-degrading factors in SPECT imaging.Future efforts will focus on applying this approach to non-monoenergetic sources such as 90Y, aiming to enhance the quantification of radioembolization treatments.