Comparison of step-and-shoot and continuous-bed-motion PET modes of acquisition for limited-view organ scans

Continuous-bed-motion (CBM) acquisition mode has been made commercially available in PET/CT scanners. CBM mode is designed for whole-body imaging, with a long scan length (multiple axial fields of view [aFOVs]) and short acquisition duration (2-3 min/aFOV). PET/CT has recently been used after ⁹⁰Y-microsphere therapy to quantify ⁹⁰Y activity distribution in the liver. Here we compared counting efficiencies along the bed-motion direction (z-axis) between CBM and step-and-shoot (SS) acquisition modes for limited-view organ scans, such as ⁹⁰Y PET/CT liver studies, that have short scan lengths (≤2 aFOVs) and long acquisition durations (10-30 min/aFOV). Methods: The counting efficiencies, that is, analytic sensitivities, in SS mode (single-aFOV and multiple-aFOV scans) and CBM mode were theoretically derived and experimentally validated using a cylindric ⁶⁸Ge phantom. The sensitivities along the z-axis were compared between the SS and CBMmodes. Results: The analytic and experimental count profiles were in good agreement, validating the analytic models. For fixed scan durations, the overall coincidence counting efficiency in CBM mode was lower (~60%) than those in SS modes, and the maximum sensitivity in CBM mode was 50% or less of that in 1-aFOV SS mode and 100% or less of that in 2-aFOV SS mode. Conclusion: The ability of CBM mode to tailor-fit the PET/CT scan length and local scan duration is not realized in studies with a short scan length (≤30 cm) and long scan duration (20 min/aFOV for the scanner). SS acquisition mode is preferable to CBM mode for limitedview organ and count-starved scans, such as ⁹⁰Y PET/CT liver scans, because of the higher counting efficiency of SS mode, which leads to better image quality and quantification precision.