Spatial resolution uniformity, isotropy, and the effect of depth of interaction information in a 1mm3 resolution, limited-angle PET system

Limited-angle tomography PET systems use configurations that result in incomplete and irregular sampling of projection data. In a two paneled, 1mm resolution PET system dedicated to breast imaging with 1mm depth-of-interaction (DOI) resolution, we measure both the uniformity and isotropy of the spatial resolution. Uniformity measures the variation in spatial resolution over the field of view (FOV), and isotropy measures the difference in resolution in the x, y, and z dimensions. Reconstructing data from point sources in air placed across the FOV with MLEM, we measure spatial resolution across the FOV in 3 dimensions. At the center of the FOV, the resolution was 0.775 mm, 1.296 mm, and 0.782mm in x, y, and z, respectively. We examine the impact of MLEM iterations on the measured uniformity and isotropy of spatial resolution. We compare MLEM resolutions to metrics in literature, see some correspondence, and discuss their limitations. We step a point source in 100 µm steps to show the ability of the system to resolve closely spaced points. The system separates point sources by more than one FWHM at 0.700mm spacing. DOI resolutions impact is studied by varying the binning resolution of interactions in crystals along the direction away from the region of interest. Resolution in all directions is substantially impacted by DOI rebinning. Resolution in the DOI direction (orthogonal to the panels) degrades from 1.296mm FWHM at 1mm DOI resolution to 5.908mm with 16mm DOI resolution showing the importance of DOI and sampling from oblique lines of response in limited-angle systems.