Fluid-Fluid Level In The Urinary Bladder On PET-CT

Abstract

A 60-y-old male patient with left pelvic mass was referred for FDG PET/CT for metabolic characterization and also possible primary tumour/metastatic lesions. PET/CT images revealed high FDG uptake of the mass (fig 1b). PET images showed layering of excreted FDG in posterior region of the distended bladder lumen (fig 1a) which was in fluid density on CT slices whereas in anterior region of bladder lumen there was FDG-free urine.  “Fluid-fluid level” in the urinary bladder was occasionally observed on FDG PET/CT studies. The possible mechanism of this clear demarcation between the urine without FDG and FDG-loaded urine is the delayed mixing of the excreted FDG to the urine in patients with distended urinary bladder due to differences between the gravity of the fluids.