Background <p>This study aimed to evaluate the impact of F-fluorodeoxyglucose (¹⁸F-FDG) uptake outside the field of view on image quality of a silicon photomultiplier tube (SiPM)–based dedicated breast (db) positron emission tomography scanner using a cylindrical breast phantom and simulated myocardium.</p> Methods <p>The spheres, arranged in a planar circle inside the phantom, had 3, 5, 7.5, and 10&#xa0;mm diameters. The cylinder and four spheres were filled with an ¹⁸F-FDG solution to maintain a sphere-to-background (BG) radioactivity concentration ratio of 8:1. The cylindrical breast phantom was positioned so that the centre position of the detector was aligned with the centre of the hot sphere. To simulate ¹⁸F-FDG accumulation in the myocardium, a cylindrical plastic container filled with ¹⁸F-FDG was placed on the cylindrical breast phantom. Scans were performed without and with varying levels of myocardial radioactivity concentration. The myocardial radioactivity concentration was set at ratios of 1, 5, 10, and 15 relative to the BG portion of the cylindrical breast phantom. The coefficient of variation of the BG (COV_BG), contrast recovery coefficient (CRC), and detectability index (DI) were assessed.</p> Results <p>The COV_BG tended to increase with higher myocardial radioactivity concentrations, but this trend was not entirely consistent. CRC showed a decreasing tendency for smaller hot spheres as myocardial radioactivity concentrations increased, with greater variability observed. DI tended to decrease as myocardial radioactivity concentrations increased.</p> Conclusions <p>Our findings suggested that the presence of radioactivity outside the field of view increases reduces small-lesion contrast at the centre of the detector ring of the SiPM-based db positron emission tomography scanner.</p>