PT - JOURNAL ARTICLE AU - Anja S. Gabler AU - Christian Kühnel AU - Thomas Winkens AU - Martin Freesmeyer TI - Assessment of Minimum <sup>124</sup>I Activity Required in Uptake Measurements Before Radioiodine Therapy for Benign Thyroid Diseases AID - 10.2967/jnumed.115.171066 DP - 2016 Aug 01 TA - Journal of Nuclear Medicine PG - 1201--1206 VI - 57 IP - 8 4099 - http://jnm.snmjournals.org/content/57/8/1201.short 4100 - http://jnm.snmjournals.org/content/57/8/1201.full SO - J Nucl Med2016 Aug 01; 57 AB - This study aimed to assess a hypothetical minimum administered activity of 124I required to achieve comparability between pretherapeutic radioiodine uptake (RAIU) measurements by 124I PET/CT and by 131I RAIU probe, the clinical standard. In addition, the impact of different reconstruction algorithms on 124I RAIU and the evaluation of pixel noise as a parameter for image quality were investigated. Methods: Different scan durations were simulated by different reconstruction intervals of 600-s list-mode PET datasets (including 15 intervals up to 600 s and 5 different reconstruction algorithms: filtered-backprojection and 4 iterative techniques) acquired 30 h after administration of 1 MBq of 124I. The Bland–Altman method was used to compare mean 124I RAIU levels versus mean 3-MBq 131I RAIU levels (clinical standard). The data of 37 patients with benign thyroid diseases were assessed. The impact of different reconstruction lengths on pixel noise was investigated for all 5 of the 124I PET reconstruction algorithms. A hypothetical minimum activity was sought by means of a proportion equation, considering that the length of a reconstruction interval equates to a hypothetical activity. Results: Mean 124I RAIU and 131I RAIU already showed high levels of agreement for reconstruction intervals of as short as 10 s, corresponding to a hypothetical minimum activity of 0.017 MBq of 124I. The iterative algorithms proved generally superior to the filtered-backprojection algorithm. 124I RAIU showed a trend toward higher levels than 131I RAIU if the influence of retrosternal tissue was not considered, which was proven to be the cause of a slight overestimation by 124I RAIU measurement. A hypothetical minimum activity of 0.5 MBq of 124I obtained with iterative reconstruction appeared sufficient both visually and with regard to pixel noise. Conclusion: This study confirms the potential of 124I RAIU measurement as an alternative method for 131I RAIU measurement in benign thyroid disease and suggests that reducing the administered activity is an option. CT information is particularly important in cases of retrosternal expansion. The results are relevant because 124I PET/CT allows additional diagnostic means, that is, the possibility of performing fusion imaging with ultrasound. 124I PET/CT might be an alternative, especially when hybrid 123I SPECT/CT is not available.