Limiting iterations vs. post smoothing for noise control in PET

We have performed physical phantom and simulation study with PET to compare the effects of limiting iterations and post-smoothing. The phantom was designed to mimic small hot lesions typical in FDG PET. The phantom was a body-sized 25 L phantom. Eight 1 cm hot spheres were filled with activity 8x the background. The phantom was scanned in 2D for 15 minutes on a GE Discovery STE. Data were framed into 1, 3, 5, and 15 min frames. Images were reconstructed with a variety of subset sizes and up to 60 iterations, with corrections for attenuation, scatter, randoms, and dead-time. All image sets were post-smoothed with 4, 6, 8, 10, and 12 mm FWHM Gaussian. ROI´s were placed on all hot spheres, and 40 additional background ROI´s were placed. Noise was measured as the variation in background ROI´s. Additionally, noise was measured on a pixel-by-pixel basis as the variation over the ensemble of realizations of each frame time. Noise vs. contrast (sphere-background) were plotted for each image. The background noise method showed better results for limited iterations (~20% lower noise at a given contrast level) while the ensemble-based noise method favored post-smoothing of high iterations (with a similar ~20% difference). Similar results were obtained for simulations.