PET Image Quality in the Vicinity of the Bladder with Fluorine-18 and Gallium-68

Abstract

Gallium-68 labeled compounds have shown an important role in PET imaging of detecting neuroendocrine tumors and prostate tumors. As the prostate exists near the bladder, image quality of prostate tumors can be challenged by radioactivity concentration accumulated in the bladder. Previous studies have shown that activity measurements within 4 cm distal to the bladder were more affected by its higher 〖^18〗F concentration and improved with time of flight (TOF) technique. The aim of this research is to understand and compare the effects of 〖^68〗Ga and 〖^18〗F activity concentration on evaluation of structures closer to the bladder, and compare it for different scanners, acquisition modes and reconstruction techniques.Methods: A bladder insert was placed in the center of an oval phantom with radioactivity at three different bladder-to-background ratios: 1:1, 40:1 and 80:1. Twelve 1-cm spheres representing 8:1 small lesions were symmetrically located in two axial planes, all with 1.2 cm distance to the bladder surface. The whole phantom was scanned on both the GE Discovery 690 and GE Discovery IQ, filled with 〖^18〗F and 〖^68〗Ga separately. In addition to default reconstruction settings, images were also reconstructed with nTOF and TOF modes on D690, and OSEM and REG modes on DIQ. The same ROIs were applied to all these images.Results: Radioactivity in the bladder resulted in worse visualization and quantitation of the small spheres. The value difference among the 12 spheres became greater, and the lesion-to-background ratio were also affected by high bladder activity. The lesion value could be overestimated to be about three time as much as, or underestimated by 50% at most of its true measurement. The greatest difference was seen when the bladder was filled with 〖^68〗Ga. Both TOF and REG modes provided better lesion value variations compared with nTOF/OSEM reconstruction. Image contrast was significantly improved with more iterations.Conclusions: Radioactivity of 〖^68〗Ga in the bladder has a similar but more serious effect than 〖^18〗F in the value measurements of surrounding structures. Greater measurement variations occur with higher bladder activity concentration. Iterative reconstruction with TOF information or REG reconstruction can be used to improve image quality, otherwise more iterations are recommended.