Parametric imaging of the NET radioligand [C-11]MRB in humans: A test-retest study

544 Objectives: In vivo quantification of the density of nor-epinephrine transporters (NET) has been hampered by the lack of suitable radioligands. [C-11]MRB is a new radioligand with suitable properties in monkeys. As part of [C-11]MRB evaluation in humans, the present study aimed to investigate the feasibility of computing parametric images of [C-11]MRB binding. Methods: Five volunteers were included in a test-retest protocol. Each subject was injected twice, 150 minutes apart, followed by a 90-min dynamic acquisition on the HR+ scanner. Parametric images were computed using the simplified reference tissue model (SRTM) and the 2-parameter version (SRTM2) where the efflux rate constant for the reference region (k29) is estimated by the median of SRTM estimates from throughout the brain. The occipital cortex was used as the reference region. The main outcome parameter was the relative distribution volume (DVR). Mean parameter values were computed for eight regions: four cortical lobes, caudate, putamen, cerebellum and thalamus. Reproducibility of the measures was estimated by computing the standard deviation (σTR) of the relative difference of the parameter estimates between test and retest scans. Results: DVR values were reproducible with SRTM2 (σTR 100%) when SRTM2 was applied to regional TACs, possibly due to difficulties in estimating k29 from a limited (8) number of curves, even though regional TACs are much less noisy than voxel TACs. DVR estimates with SRTM2 ranged from 1.02±0.02 (caudate) to 1.46±0.13 (thalamus). k29 was estimated as 0.027±0.014 /min. Conclusions: Reproducible DVR estimates can be obtained for [C-11]MRB with SRTM2 if a reliable k29 value can be estimated by averaging all voxel-by-voxel estimates.