Optimization and assessment of quantitative 124I imaging on a Philips Gemini dual GS PET/CT system

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Abstract
Quantitative 124I PET imaging is challenging as 124I has a complex decay scheme. In this study the performance of a Philips Gemini dual GS PET/CT system was optimized and assessed for 124I. The energy window giving the maximum noise equivalent count rate (NECR) and NEMA 2001-NU2 image quality were measured. The activity concentration (AC) accuracy of images calibrated using factors from 18F and 124I decaying source measurements were investigated. The energy window 455–588 keV gave the maximum NECR of 9.67 kcps for 233 MBq. 124I and 18F image quality was comparable, although 124I background variability was increased. The average underestimation in AC in 124I images was 17.9 ± 2.9% for nonuniform background and 14.7 ± 2.9% for single scatter simulation (SSS) subtraction scatter correction. At 224 MBq the underestimation was 10.8 ± 11.3%, which is comparable to 7.7 ± 5.3% for 18F, but increased with decreasing activity. The best 124I PET quantitative accuracy was achieved for the optimized energy window, using SSS scatter correction and calibration factors from decaying 124I source measurements. The quantitative accuracy for 124I was comparable to that for 18F at high activities of 224 MBq but diminishing with decreasing activity. Specific corrections for prompt γ-photons may further improve the quantitative accuracy.