@article {Feng989, author = {Ling Feng and Per Jensen and Gerda Thomsen and Agnete Dyssegaard and Claus Svarer and Lars V. Knudsen and Kirsten M{\o}ller and Carsten Thomsen and Jens D. Mikkelsen and Denis Guilloteau and Gitte M. Knudsen and Lars H. Pinborg}, title = {The Variability of Translocator Protein Signal in Brain and Blood of Genotyped Healthy Humans Using In Vivo 123I-CLINDE SPECT Imaging: A Test{\textendash}Retest Study}, volume = {58}, number = {6}, pages = {989--995}, year = {2017}, doi = {10.2967/jnumed.116.183202}, publisher = {Society of Nuclear Medicine}, abstract = {123I-CLINDE is a radiotracer developed for SPECT and targets the 18-kDa translocator protein (TSPO). TSPO is upregulated in glial cells and used as a measure of neuroinflammation in a variety of central nervous system diseases. The aim of this study was to examine the test{\textendash}retest variability of 123I-CLINDE binding in healthy subjects. Methods: SPECT scans were acquired over 90 min in 16 healthy controls (9 women, 8 mixed-affinity binders [MABs] and 8 high-affinity binders [HABs] twice with an interval of 35 {\textpm} 15 d). Arterial input functions were based on individual blood measurements in 8 subjects and a population-based approach in combination with individual whole-blood time{\textendash}activity curves in the other 8 subjects. Seven brain volumes of interest were extracted and quantified by SUVs and by 2-tissue-compartment modeling for calculation of distribution volumes (VT). Test{\textendash}retest variability was measured by percentage difference (PD), the absolute PD, intraclass correlation coefficient (ICC), and coefficient of variation. Results: The absolute PD of brain SUV and the VT had similar values. The ICC values were higher for VTs than for brain SUVs, which were both moderate to high; however, lower ICC values were observed when calculated separately for HABs and MABs. Test{\textendash}retest reproducibility was higher in subjects with immediate centrifugation of blood samples. The population-based method efficiently recovered data with delayed centrifugation. The VT of a 49-y-old male HAB was 7.5 {\textpm} 1.4 mL/cm3 compared with 4.6 {\textpm} 1.4 mL/cm3 of a sex- and age-matched MAB. The SUVs of a 49-y-old male HAB and MAB were 1.03 {\textpm} 0.14 and 0.88 {\textpm} 0.15 g/mL, respectively. Conclusion: The test{\textendash}retest reproducibility of 123I-CLINDE is comparable or better than that reported for commonly used PET TSPO tracers. Because of the binding of 123I-CLINDE to blood cells and peripheral tissues, SUV is not a sufficient surrogate of VT from 2-tissue-compartment modeling. The population-adjusted method has the potential to reduce the complexity of blood analyses of TSPO tracers.}, issn = {0161-5505}, URL = {https://jnm.snmjournals.org/content/58/6/989}, eprint = {https://jnm.snmjournals.org/content/58/6/989.full.pdf}, journal = {Journal of Nuclear Medicine} }