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Aims: We observed hitherto unreported layering of radioactivity in the bladder on PET/CT in prostate cancer (PC) when combined with contrast-enhanced CT (CECT). This effect facilitates assessment of the prostate bed in PC., Methods: Among 128 patients imaged with [ 18 F]PSMA-1007, we selected all 8 studies without and 28 studies with CECT. 20 patients also underwent PET/MR. As controls, we chose 20 and 16 males studied with [ 18 F]FDG for extrapelvic disease with and without CECT. Posterior anterior (PA) ratio was calculated as SUV post /SUV ant * 100 % based on maximal standard uptake values (SUV) in 20 mm spheres in the anterior and posterior bladder. Four nuclear physicians scored assessibility of the bladder base on a 3-point Likert scale (3 = optimal, 1 = poor). We acquired serial PET/CT over 4 hours of a flask with layering of 100 ml intravenous contrast agent and 100 ml physiological saline with 40 MBq of [ 18 F]PSMA-1007, while a control flask was shaken at the start of the experiment., Results: Layering of tracer was observed in all PET/CT studies with CE-CT, but not in studies without contrast. Median PA ratios were 44 % (interquartile range 33-62) for [ 18 F]PSMA-1007 and 73 % (52-67) for [ 18 F]FDG, respectively. Intravenous contrast improved assessibility scores in PET of the bladder base, but the effect only reached significance in the PET/MR data. In the in vitro data, radioactivity was retained in the aqueous supernatant over the entire experiment whereas there was no separation of phases in the control flask over time., Conclusion: When performing PET combined with CECT, sedimentation of contrast agent in the bladder leads to upward displacement of radioactivity, enhancing clarity of PET images in the posterior bladder and the prostate bed on both PET/CT and PET/MR., Competing Interests: The last author is consultant (unpaid) for Segami Corporation, Inc. The remaining authors have no conflicts of interest., (Thieme. All rights reserved.)