Using Monte-Carlo Simulations to implement corrections for I-124 as a non-pure positron emitter in small animal and human PET imaging

Using I-124 for PET imaging applications implies some difficulties concerning the image quality: The resolution is degraded by the large maximum positron energy of 2.1 MeV and the resultant long positron range. In addition I-124 is a non-pure PET isotope exhibiting additional gamma ray emissions with high contributions to the total decay scheme: 602.72 keV with 63% and 722.78 keV with 10.35%. These fractions cannot be quantified exactly in PET measurements. Therefore in our work we utilized GATE 6.1 [1] to investigate the effects of these supplementary “false” coincidences on a spectral, sinogram- and image-based basis. Three PET systems were modeled with GATE and confirming measurements were accomplished on them: Two small animal PET scanners (ClearPET and MicroPET) and one human scanner (Gemini TF). Derived from the simulated energy spectra, we propose narrower energy window configurations for the ClearPET and the Gemini TF in order to minimize the amount of false coincidences. Separating the simulated sinograms for true and false coincidences revealed that a two-component correction for I-124 has to be implemented. A homogenous background subtraction has to be amended by a part which considers an additional portion within the phantom borders.
[1] S. Jan et al.: GATE V6: a major enhancement of the GATE simulation platform enabling modelling of CT and radiotherapy. Phys. Med. Biol. (56) (2011) 881-901