PET Image Quality in the Vicinity of the Bladder

Abstract

In positron emission tomography (PET), large concentrations of radioactivity sometimes present in the urinary bladder can be detrimental to surrounding image quality. This physical phantom study evaluates both the effect and the reduction of the effect with three PET/CT systems with different time-of-flight (TOF) resolutions: 590 ps, 380 ps and 185 ps. The phantom consists of a 25 liter oval tank, with a centrally positioned 100 mL sphere mimicking the bladder and eight 8 mm surrounding spheres, 20.5 mm from the interior bladder wall to the center of the surrounding spheres. The bladder was filled with three different radioactivity concentrations relative to the uniform background concentration: 1:1, 40:1 and 80:1. The three bladder concentrations were scanned on the three different scanners, and the images were reconstructed with TOF and without TOF information. In addition, the number of iterations was varied in the reconstruction parameters for additional comparison. Analysis metrics include small sphere contrast relative to background and the coefficient of variability (COV) for sphere intensity and surrounding background. Overall, the image quality surrounding the bladder gets dramatically worse as the bladder concentration increases, for images with no TOF information. TOF information and increasing the number of iterations improves the surrounding sphere contrast. Finally, as the TOF resolution of the scanner improves, the surrounding sphere image quality improves.