Quantitative imaging characteristics of zirconium-89 on Gemini Time-Of-Flight PET/CT

Abstract

Purpose Interest in PET imaging using zirconium-89 (89 Zr) (t 1/2 =78.41 h)-labeled tracers for the tracking and quantification of monoclonal antibodies (mAbs) is growing, mainly because of its well-matched physical half-life with the biological half-life of intact mAbs. This study aims to evaluate the imaging characteristics of 89 Zr-PET in comparison with those obtained using fluorine-18 fluorodeoxyglucose (18 F-FDG) PET (gold standard tracer in PET imaging) using a Time-Of-Flight (TOF) PET/computed tomography (CT) scanner. Materials and methods The system's spatial resolution, sensitivity, scatter fraction (SF), image uniformity, and image quality were measured on a Gemini TOF PET/CT scanner according to the NEMA NU2-2001 protocols. The NEMA 2001 kit was used to carry out these measurements. Timing and energy resolutions were measured using 22 Na and 18 F-FDG point sources only. Results Spatial resolution in transverse and axial planes measured at 10 mm off access were 4.7 and 4.6 mm for 89 Zr and 18 F-FDG, respectively. At 100 mm, radial, tangential, and axial spatial resolution values were 5.2, 5.1, and 5.2 mm for 89 Zr and 5.1, 4.9, and 5.2 mm for 18 F-FDG, respectively. Sensitivity measured at the center of the field of view was 14.6 and 4.16 cps/kBq for 89 Zr and 18 F-FDG, respectively. SF was 32.6% for 89 Zr in comparison with 31.8% for 18 F-FDG. Image contrast for 89 Zr-PET images was 36.9 and 29.7% for 18 F-FDG for the smallest (10 mm)-sized sphere, and it was 70.6 and 72.8% for 89 Zr and 18 F-FDG, respectively, for the largest (37 mm)-sized sphere. Background variation was 10.3% for 89 Zr and 6.8% for 18 F-FDG for the smallest-sized sphere and 3.4 and 3.8% for 89 Zr and 18 F-FDG, respectively, for the largest-sized sphere. Conclusion In this study, we measured imaging characteristics of 89 Zr on a Gemini TOF PET/CT scanner. Our results show that 89 Zr has lower spatial resolution and noise-equivalent count rate with increased SF and background variation; however, it offered superior sensitivity and improved image contrast in comparison with 18 F-FDG. 89 Zr is an ideal radiotracer for immuno-PET imaging because of its physical half-life, which is well matched with mAbs, in addition to its affinity to be trapped inside the target cell after internalization of the mAbs.