Incorporation of a cascade gamma ray correction into the SRW iterative reconstruction for non-standard PET nuclides: Towards a unified correction weighted (UCW) scheme in the sensitivity image

We describe an extension of the scatter and randoms weighted (SRW) iterative PET reconstruction algorithm which includes a model-based cascade estimate in the trues fraction (TF) estimation for non-standard PET nuclides as an effort to unify the corrections for quantitative PET imaging into the sensitivity image. The unified correction weighted (UCW) scheme which includes the attenuation, normalization, and TF estimates was incorporated into the widely used 3D-OSEM reconstruction. A ⁷⁶Br contrast phantom with a ~54% background contamination (including the scatter, randoms, and cascade events) and a ⁸⁶Y contrast phantom with a ~83% background contamination were scanned, and the data were reconstructed using the standard and the UCW methods. The contrast vs noise and the reconstruction time comparisons were evaluated, and it was found that the UCW method outperforms the standard Ordinary Poisson (OP) approach in terms of convergence in hot and cold contrasts. A 50% overall time gain for the ⁷⁶Br study and an 80% time gain for the ⁸⁶Y study as compared to the OP method was achieved using the proposed method due to the better convergence rate (i.e. able to stop at earlier iteration) and faster reconstruction time (i.e. no need to store and process the scatter, randoms, and cascade sinograms in the forward model for every single iteration). These studies also demonstrated that the background correction weighted scheme in the sensitivity image works well with relatively high background fractions. It was observed that the higher the background contamination, the better the convergence rate for the UCW method as compared to the OP approach.