Feasibility of Ultra-Low-Activity18F-FDG PET/CT Imaging Using a Long–Axial-Field-of-View PET/CT System

18F-FDG PET/CT imaging is widely used in oncology. Long-axial-field-of-view (LAFOV) PET/CT systems exhibit ultrahigh sensitivity and signal-to-noise ratios, enabling significant reductions in tracer activity and shorter acquisition times. This study aimed to evaluate the feasibility of using ultralow activities for semiquantitative measurements in 18F-FDG PET/CT imaging performed on an LAFOV PET/CT system through a high-low activity test-retest study. Methods: Eleven oncology patients underwent 2 18F-FDG PET/CT scans, the first with standard activity (3.0 MBq/kg) and, within 7 d, 1 with ultralow activity (0.3 MBq/kg). List-mode data of both scans were resampled to simulate activities of 0.3 and 0.03 MBq/kg. Semiquantitative measurements (SUVmean, SUVpeak, and SUVmax) and their repeatability in healthy organs and lesions were compared across different activities and reconstruction protocols. Results: SUVmean remained stable across activity reductions and reconstruction protocols, whereas SUVpeak showed stability except when simulating 1% of the standard 18F-FDG activity. SUVmax significantly increased at lower 18F-FDG activities, particularly with clinically preferred scans. Repeatability analysis showed that SUVmean and SUVpeak maintained variability within acceptable limits (<15%) even at 0.03 MBq/kg, whereas SUVmax variability often exceeded these limits. Lesion detection was reliable at 0.3 MBq/kg, but several lesions could not be delineated at 0.03 MBq/kg, indicating compromised image quality. Conclusion: The use of ultralow 18F-FDG activity (0.3 MBq/kg) is feasible for PET/CT imaging on an LAFOV PET/CT system, as shown by semiquantitative measurements, repeatability analyses, lesion detectability, and semiautomated delineation methods. These findings support imaging protocols that keep radiation exposure levels as low as reasonably or practically achievable, which are particularly relevant for radiation-sensitive patients (e.g., children, pregnant women).