I-124 PET quantitation in the presence of I-131 background activity during I-131 therapy: A phantom study

Objectives: In order to evaluate the radiopharmacokinetics of an administered therapeutic amount of I-131 activity such as to study the radiopharmacokinetics of radiation thyroiditis or the blocking effects of stable I-127 (cold iodine) on increasing the residence time of I-131, simultaneous I-124 PET imaging and quantitation may have utility. The objective of this phantom study was to investigate the effects of background activity of I-131, a non-positron emitter, on the PET imaging and quantitation of I-124.

Methods: A NEMA IEC body phantom containing six spheres of different diameters (10, 13, 17, 22, 28 and 37 mm) was used in this study. The initial PET/CT scan was acquired 2 hr post-injection of a tracer amount of I-124 in the spheres and the background, with a sphere to background ratio of 39:1. Subsequently, PET/CT scans were acquired in the presence of 4.4, 5.9, 7.4 and 14.8 mCi of I-131 in the background, at 2.3, 2.5, 2.7 and 2.9 hr post-injection of I-124, respectively. Imaging was performed using a Philips Gemini ToF PET/CT camera. The acquired images were analyzed by drawing spheres of the same size as the true size on the phantom spheres and on the background, on the CT images. The mean activity concentration (mAC) was measured in the drawn spheres and the recovery coefficient was studied as a function of the amount of I-131 activity in the background.

Results: The measured mAC in the hot spheres decreased with increasing I-131 activity in the background. This trend was especially evident for the two largest spheres (28 and 37 mm diameter). Measurement of the background activity in the different scans shows that the mAC in the background decreased with increasing I-131 activity in the background, with the ratio of mAC relative to that with no I-131 present decreasing from 0.91 to 0.71 with 4.4 and 14.8 mCi of I-131, respectively.

Conclusion: The PET scans acquired when only I-124 was present in the spheres and background and when different activity amounts of I-131 was added to the background suggests that the presence of I-131 activity in the background has an impact on PET imaging and quantitation with I-124. The amount of I-131 activity present leads not only to increased number of random coincidences, but may also increase deadtime due to an elevated rate of single events. In addition prompt coincidences from I-131 photons could result in a background that interferes with scatter tail fitting. Scanner design and acquisition settings need to be considered in determining the feasibility of measuring the pharmacokinetics of radioiodine using I-124 PET in the presence of I-131 activity in the background. Research Support: Grant from grateful patients at MedStar Washington Hospital Center.