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We tested the impact of different values of the relaxation parameter lambda (λ) on contrast and noise in line-of-response row-action maximum likelihood algorithm (LOR-RAMLA) in 18F-fluorodeoxyglucose positron emission tomography (PET)/computed tomography (CT) imaging.Phantom studies were performed on a Gemini XL PET/CT scanner. The NEMA/IEC (National Electrical Manufacturers Association/International Electro technical Commission) torso phantom was used and acquisition data were reconstructed with λ values ranging from 0.025 to 0.1. Quality of the reconstructed images was evaluated by contrast recovery coefficients and background variability values according to the NEMA NU 2-2001 procedures.Contrast recovery coefficients and background variability increased significantly when λ increased. The best noise-versus-resolution trade-off was obtained with λ in the 0.04–0.06 range. For LOR-RAMLA reconstruction, the manufacturer allows a possible λ choice from 0.025 to 0.1. We would not advise too small (0.025) or too large (0.1) λ values which result in too smooth or too noisy images.We determined optimal λ values in LOR-RAMLA on a Gemini XL PET/CT scanner. Caution is needed when using λ values out of that range.