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

Quantitative I-124 PET imaging is challenging as I-124 has a complex decay scheme. In this study the performance of a Philips Gemini dual GS PET/CT system was optimized and assessed for I-124. 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 F-18 and I-124 decaying source measurements were investigated. The energy window 455-588 keV gave the maximum NECR of 9.67 kcps for 233 MBq. I-124 and F-18 image quality was comparable, although I-124 background variability was increased. The average underestimation in AC in I-124 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 F-18, but increased with decreasing activity. The best I-124 PET quantitative accuracy was achieved for the optimized energy window, using SSS scatter correction and calibration factors from decaying I-124 source measurements. The quantitative accuracy for I-124 was comparable to that for F-18 at high activities of 224 MBq but diminishing with decreasing activity. Specific corrections for prompt gamma-photons may further improve the quantitative accuracy.