A newly-recognized peribladder artifact induced by the presence of bowel gas in the vicinity of high-activity bladder on ¹⁸F-FDG PET/CT: a phantom-based verification study

Objectives: High-activity bladder is known as a potential source of artifacts on ¹⁸F-FDG PET/CT. Recently we noticed an erroneous uptake extending from bladder to nearby air region in the bowel, such as rectum. This peribladder artifact has not been previously reported, as far as we aware. Our preliminary clinical study demonstrated that the peribladder artifacts were observed in 12 (11%) of 106 patients who underwent ¹⁸F-FDG PET/CT. Awareness of this artifact is important for precise interpretation in pelvis on ¹⁸F-FDG PET/CT. This phantom study was conducted to better understand the peribladder artifact and to investigate the factors affecting this erroneous uptake. Materials and Methods: PET/CT imaging was performed with GE Discovery 690 and a cylindrical water phantom. The phantom contained two 2-cm-diameter cylinders simulating rectal gas and bladder. The rectal gas cylinder was filled with air, and was placed posterior to a bladder cylinder sealing ¹⁸F-FDG solution. The experiments were performed by changing 2 factors: the ¹⁸F-FDG solution radioactivity in the bladder cylinder, and the distance between rectal gas and bladder cylinders. Phantom condition 1: The rectal gas cylinder was fixed 4 cm posterior to the bladder cylinder (Figure 1). Five different radioactivity level of the ¹⁸F-FDG solution (0, 50, 100, 200, and 300 kBq/ml (bladder to background ratio of 0, 20, 40, 80, and 120)) in the bladder cylinder was used. Phantom condition 2: Radioactivity concentration in the bladder cylinder was fixed to 200 kBq/ml. The position of the rectal gas cylinder was changed, ranging from 0 to 8 cm posterior to the bladder cylinder (Figure 2). PET data was acquired in the 3-dimensional (3D) mode for 20 min, and CT data for attenuation correction was acquired with 120 kVp and 300 mA. The PET images were reconstructed using the 3D ordered-subsets expectation maximization (3D-OSEM) with and without Time-of-Flight (TOF). The maximum standardized uptake value (SUVmax) in the peribladder artifacts region was measured on PET images.

Results: Figure 3 shows the PET images acquired in Phantom condition 1. Erroneous high-intensity region surrounding decreased uptake area extending from the high-activity bladder cylinder to the gas cylinder was observed. Higher radioactivity concentration in the bladder cylinder was associated with increased SUVmax of the artifact. SUVmax of the artifact was 1.2, 1.5, 1.7, 2.1, and 2.4 at bladder radioactivity of 0, 50, 100, 200, and 300 kBq/ml, respectively, on PET images without TOF reconstruction. Using TOF reconstruction, SUVmax decreased to 1.1, 1.1, 1.2, 1.5, and 1.6, respectively. Figure 4 shows the PET images acquired in Phantom condition 2. Erroneous uptake was observed in the conditions of 2-8 cm inter-cylinder distances. SUVmax of the artifact was almost unchanged regardless of the inter-cylinder distance. Using TOF reconstruction, the magnitude of the artifact was substantially decreased. SUVmax of the artifact was 2.1, 2.1, 2.1, and 2.1 without TOF, and 1.8, 1.5, 1.3, and 1.3 with TOF, at the inter-cylinder distance of 2, 4, 6, and 8 cm, respectively.

Conclusions: Erroneous uptake is observed in the area between high-activity bladder and gas region on ¹⁸F-FDG PET/CT. The magnitude of the artifact is associated with radioactivity concentration in the bladder. TOF reconstruction substantially mitigates this peribladder artifacts.

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