Shine-through artifact due to high-radioactivity bladder and bowel gas in ¹⁸F-FDG PET/CT: impact of time-of-flight algorithm and radioactivity concentration of urine in the bladder on the occurrence of the artifacts

Shine-though artifact can appear as regions with falsely increased uptake in the immediate vicinity of large hot sources in ¹⁸F-FDG PET/CT. This artifact may adversely affect the assessment of tumor involvement in the regions adjacent to the bladder. The purpose of this study was to evaluate the prevalence of shine-through artifacts in clinical ¹⁸F-FDG PET/CT examinations and the factors that can influence their occurrence and extent.

PET/CT images were acquired with Discovery PET/CT 690. One hundred six patients who underwent ¹⁸F-FDG PET/CT for clinical purposes were retrospectively reviewed. PET images were reconstructed using 3-dimensional ordered-subset expectation maximization with and without time-of-flight (TOF). The shine-through artifact was defined as an erroneous accumulation of ¹⁸F-FDG between the bladder and the air region in the intestine without attenuation correction (AC) errors. The maximum standardized uptake value (SUV_max) of the artifact was measured, and the effect of TOF on this artifact was evaluated. The SUV_max in the bladder was compared in patients with and without the artifacts. A phantom containing two spheres simulating bladder and rectal gas was imaged while changing the radioactivity of ¹⁸F-FDG solution in the bladder sphere. The relationship between the bladder sphere radioactivity and the SUV_max of the shine-through artifacts was evaluated.

The shine-through artifact was more frequently observed on PET images reconstructed without TOF (12/106, 11.3%) as compared to PET images with TOF (8/106, 7.5%, p = 0.046). The SUV_max of the shine-through artifacts was significantly decreased by TOF reconstruction compared to non-TOF reconstruction (4.7 ± 1.7 vs 7.6 ± 3.1, p = 0.0078). The mean SUV_max of urinary bladders in patients with the artifacts was significantly higher than those without the artifacts on non-TOF images (74.9 ± 61.1 vs 46.3 ± 35.2, p = 0.038). In the phantom study, the SUV_max of the shine-through artifact increased as the radioactivity in the bladder-simulating sphere increased.

Shine-through artifacts were observed in approximately 10% of clinical ¹⁸F-FDG PET/CT examinations. Their magnitude is significantly associated with the radioactivity in the bladder and can be reduced by employing TOF. Recognizing this artifact allows for a more accurate interpretation of ¹⁸F-FDG pelvic studies.