Comparison between the reconstructed 0.511 MeV images with different methods. The images are in the unit of ph/cm²/s/sr. The top left shows the injected thin model, while the reconstructed images with the proposed method, the conventional RL, and the RL using a Gaussian filter are shown on the top right, bottom left, and bottom right panels, respectively. Following the same discussion in Section 5.2.1, each pixel suffers from the Poisson fluctuation of ∼1000 background events (= 2.09 × 10⁶/3072) with a typical exposure of 3.5 × 10⁵ cm² s sr on a pixel, which corresponds to a noise level of ∼10⁻⁴ ph cm⁻² s⁻¹ sr⁻¹ in the reconstructed image.