Evaluation of PET Image Quality Using Non-Conventional Isotopes

1516 Introduction: Recently many radiopharmaceuticals labelled using non-18F isotopes have been developed (e.g., 64Cu, 68Ga, 89Zr, 90Y and 124I), some of which have great potential in oncology. These non-conventional isotopes present various challenges to PET imaging including long positron range, prompt gamma contamination and low positron branching ratio. In this paper we evaluate the image quality of 124I and 90Y and compare with 18F using phantom. Methods: A NEMA image quality phantom (Data Spectrum Corporation, Durham, NC) is used for quantitative comparison of the isotopes. For 18F, the test was part of a comprehensive physics testing based on NEMA NU 2-2007. For 124I, the phantom was prepared such that the two largest spheres were filled with water and the four smaller spheres filled with 124I solution and the sphere-to-background ratio was 4.0. The background activity concentration was 12.0 kBq/cc at the beginning of the scan. The 124I phantom was scanned with a 3 bed, 10 min/bed protocol. For 90Y, the sphere-to-background ratio was 7.7, and all six spheres were filled with 90Y solution. The background activity concentration was 233.7 kBq/cc at the beginning of the scan and we used a single-bed, 2 hour protocol to scan the 90Y phantom. Images were reconstructed using a list-mode 3D TOF-OSEM algorithm without PSF modeling. For 18F and 124I, we used 2 iterations and 20 subsets. For 90Y, we used 3 iterations and 10 subsets. A Gaussian filter with 6mm in full-width-at-half-maximum was applied to all the images. We calculated contrast recovery of the hot and cold spheres and background variability and residual bias in the lung insert, according to NEMA NU 2-2007. Results: 90Y images are very noisy, with larger background variability of the 37mm sphere. The lung residual of 90Y is much higher than 18F and 124I, presumably caused by the bremsstrahlung photons. We note that similar lung residual results have been observed in other phantom studies. For the 10mm diameter sphere, 124I images have less contrast, due to the positron range effect; 90Y images have the least contrast. Conclusions: Results show that despite the challenges in imaging non-conventional isotopes including high positron energy (124I), prompt-gamma (124I), Bremsstrahlung photon (90Y), and very low positron branching ratio (90Y), it is possible to acquire quantitative PET images. Image quality depends on the properties of the isotope, which need to be taken into account when designing preclinical and clinical studies.