Impact of high 131I-activities on quantitative 124I-PET

Abstract

Peri-therapeutic I-PET/CT is of interest as guidance for radioiodine therapy. Unfortunately, image quality is complicated by dead time effects and increased random coincidence rates from high I-activities. A series of phantom experiments with clinically relevant I/I-activities were performed on a clinical PET/CT-system. Noise equivalent count rate (NECR) curves and quantitation accuracy were determined from repeated scans performed over several weeks on a decaying NEMA NU-2 1994 cylinder phantom initially filled with 25 MBq I and 1250 MBq I. Six spherical inserts with diameters 10–37 mm were filled with I (0.45 MBq ml) and I (22 MBq ml) and placed inside the background of the NEMA/IEC torso phantom. Contrast recovery, background variability and the accuracy of scatter and attenuation corrections were assessed at sphere-to-background activity ratios of 20, 10 and 5. Results were compared to pure I-acquisitions. The quality of I-PET images in the presence of high I-activities was good and image quantification unaffected except at very high count rates. Quantitation accuracy and contrast recovery were uninfluenced at I-activities below 1000 MBq, whereas image noise was slightly increased. The NECR peaked at 550 MBq of I, where it was 2.8 times lower than without I in the phantom. Quantitative peri-therapeutic I-PET is feasible.