Posterior Layering of Fluorine-18 Fluorodeoxyglucose in the Urinary Bladder.

An 89-year-old man undergoing fluorine-18 fluorodeoxyglucose positron emission tomography/ computed tomography (18F-FDG PET/CT) study posttherapy for hepatocellular carcinoma was noted to have 18F-FDG posterior layering of the urinary bladder. This phenomenon of posterior layering of 18F-FDG in the urinary bladder during PET/CT studies is physiological, and it is important to recognize to avoid misinterpretation and unnecessary investigations. The hypotheses for the cause of this phenomenon are highlighted in these interesting images including the reversed phenomenon where there is anterior layering of 18F-FDG.

An 89-year-old man with coronary artery disease, hypertension, and gout was diagnosed with hepatocellular carcinoma in February 2017. In fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) study in September 2017, he was noted to have posterior layering of 18F-FDG in the urinary bladder [Refer to Figures 1 and 2] in addition to the multiple metabolically active lesions in the liver and lungs.

Figure 1

The urine with 18F-FDG is seen layered to the urine without 18F-FDG in the urinary bladder (a:axial CT view; b: axial pet view; fused PET/CT view)

Figure 2

Other fused PET/CT images demonstrating 18F-FDG layering in the urinary bladder (a: sagittal view; b: coronal view)

The incidence of this physiological phenomenon is reported to be 4%. There are few hypotheses for this phenomenon. One hypothesis suggests the delayed urinary excretion of 18F-FDG and the delayed mixing of urine-containing 18F-FDG with nonradioactive urine. This layering is more likely to occur in patients with the distended bladder.[1] Another hypothesis suggests that the layering of 18F-FDG in the urinary bladder is due to sedimentation of metabolically active cellular components such as inflammatory cells, malignant cells, or bacteria.[23] In this case, there were neither symptoms nor clinical evidence that the patient had urinary tract infection. For 18F-FDG PET/CT studies with intravenous iodinated contrast, the reversal of layering occurs where 18F-FDG has layered anteriorly. This is due to the displacement of urine with 18F-FDG by the urine-containing iodinated contrast.[4] The commonly seen fusion error due to urinary bladder filling changes is easily distinguishable from posterior layering of 18F-FDG in the urinary bladder.[5] Knowledge of this phenomenon is important to avoid misinterpretation and unnecessary investigations. However, delayed imaging of the bladder would be the appropriate method for further clarification if bladder pathology is suspected.[6]

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The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.