Uncollimated gamma camera measurements at two metres to the patient provide a practical and statistically robust method for SeHCAT retention calculation

[75Se]tauroselcholic acid (SeHCAT) retention studies may be performed using a collimated or uncollimated (intrinsic) gamma camera, with trade-offs between counting sensitivity, collision risk, and stray source influence. In this work, we evaluate the uncertainty in SeHCAT studies performed in 'outer room' mode, where the patient sits or stands two metres away from the uncollimated camera face. 768 SeHCAT studies performed at the Royal Free Hospital were analysed retrospectively, and in all cases, the absolute uncertainty was found to be well within the clinically acceptable limit of ±1 percentage point at 15% retention. The mean counting sensitivity was found to be 1.64 cps/kBq, which is higher than collimated counting but lower than uncollimated counting with the patient on the couch. Seeing as the outer room method does not require patients to transfer onto the couch it may provide savings in setup time which may offset the necessarily longer acquisition time. Because the patient does not come into close proximity to the unprotected camera face the risk of collision is also reduced. Spectra from a SeHCAT capsule in a water-filled torso phantom were used to examine the potential impact of modifying the energy windows. We recommend adopting a wide central photopeak and downscatter window (160-296 keV) plus a lower photopeak window (111-159 keV), which may provide significant improvements in sensitivity. This improved sensitivity could be used to reduce the acquisition time, which is of particular interest in the context of increasing numbers of referrals for SeHCAT retention tests.