Images produced by positron emission tomography (PET) typically suffer from poor statistics and appear noisy. To reduce image noise, radial filtering of emission projection data is routinely performed prior to back-projection. Angular smoothing is not typically done, due to the introduction of a nonuniform blur in the image. The purpose of this study is to characterize the effects of radial and angular smoothing on noise and spatial resolution in PET images. Angular smoothing was incorporated after radial filtering into reconstruction of emission data on the Scanditronix PC2048-15B brain PET scanner. In-plane spatial resolution and noise were measured for different amounts of radial and angular smoothing. For radial positions beyond the central 2 cm of the image, the expected noise reduction and degraded tangential resolution with no loss of radial resolution were seen. The results inside 2 cm were unexpected. Even with an angular filter that would have degraded the tangential resolution from 7 mm to 14 mm at 10 cm from the center, no resolution loss was observed in the center, but there was also no reduction in noise. These results suggest that the benefits of angular smoothing during reconstruction of emission PET data are negligible.The next phase of the study will be to confirm these results for transmission data, an area where angular smoothing is more commonly applied, and to study other spatially varying smoothing schemes that will reduce image noise while preserving spatial resolution uniformly across the image.
