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

Interest in PET imaging using zirconium-89 (Zr) (t1/2=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 Zr-PET in comparison with those obtained using fluorine-18 fluorodeoxyglucose (F-FDG) PET (gold standard tracer in PET imaging) using a Time-Of-Flight (TOF) PET/computed tomography (CT) scanner.

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 Na and F-FDG point sources only.

Spatial resolution in transverse and axial planes measured at 10 mm off access were 4.7 and 4.6 mm for Zr and F-FDG, respectively. At 100 mm, radial, tangential, and axial spatial resolution values were 5.2, 5.1, and 5.2 mm for Zr and 5.1, 4.9, and 5.2 mm for F-FDG, respectively. Sensitivity measured at the center of the field of view was 14.6 and 4.16 cps/kBq for Zr and F-FDG, respectively. SF was 32.6% for Zr in comparison with 31.8% for F-FDG. Image contrast for Zr-PET images was 36.9 and 29.7% for F-FDG for the smallest (10 mm)-sized sphere, and it was 70.6 and 72.8% for Zr and F-FDG, respectively, for the largest (37 mm)-sized sphere. Background variation was 10.3% for Zr and 6.8% for F-FDG for the smallest-sized sphere and 3.4 and 3.8% for Zr and F-FDG, respectively, for the largest-sized sphere.

In this study, we measured imaging characteristics of Zr on a Gemini TOF PET/CT scanner. Our results show that 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 F-FDG. 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.