Image-space 3D scatter correction following list mode acquisition with a large-area positron camera

Positron volume images will be produced by the new large-area positron camera PETRRA directly from list mode data acquisition. The present method of image reconstruction is based on the backprojection then filter algorithm and no projection data are produced. A 2D method of correcting images in back-projection space has been developed using measurements made with the existing MUP-PET camera. A relationship between the attenuation and scatter fractions has been derived, for a limited axial field-of-view, for a range of cylindrical and elliptical phantoms. Image-space scatter distributions produced using this relationship are converted to backprojection-space by convolution with an experimentally-derived scatter response function (SRF). These scatter distributions are subtracted from the acquired back-projected image using the integrated scatter fraction for normalization. The resulting data are corrected using a multiplicative backprojection space attenuation matrix and deconvolved with an in-air response function. The method has been applied to a Monte Carlo simulated phantom, an experimental phantom and a patient study. The predicted scatter distribution agrees with the simulation to ~±10% and the correction method improves image contrast for all three data sets studied. However the use of a spatially invariant SRF leads to over-subtraction of scatter in the center of the phantom. In order for the method to be extended to PETRRA using the whole axial extent (40 cm) of the camera, simulations have been used to generate scatter-attenuation relationships for several phantoms over the whole axial FoV. The scatter response function is fitted to a Gaussian with the center displaced from the point of emission. Improved linearity and contrast were obtained with simulated phantoms. The method could be applied to any large-area positron camera acquiring data in list mode