USE OF TC-99M-BICISATE IN ACTIVATION STUDIES BY SPLIT-DOSE TECHNIQUE

The properties of the brain single photon emission computed tomography (SPECT) tracer Tc-99m-bicisate (ethyl cysteinate dimer) were examined in regard to its use in test-retest studies with short-interval, spli t dose studies. The stimulus applied in these tests was a visual activ ation by a ''flickering chessboard'' pattern. We used a fast dynamic S PECT (Tomomatic 232) recording eight slices. Twelve volunteers were as signed to two groups for reproducibility control and stimulation exper iments, respectively. All were examined on 2 days. On the first day, o nly one injection was given in the resting state (t = 0), while two re cordings were performed (t = 30 and t = 100), simulating the course an d timing of a full split-dose study. The average washout of 8.3 +/- 1. 0% was used as a correction on the second-day study. On that day the f irst injection was given during rest exactly as on day 1, allowing an estimate of day-to-day reproducibility. The second injection was made either in a resting state identical to the first or during visual stim ulation. All images were normalized to mean brain count value. Results were analyzed in predefined anatomical regions of interest (ROIs). Th e day-to-day reproducibility was fair, with only one ROI (inferior fro ntal cortex) showing a significant 2% decrease. The studies of day 1 r evealed a regional difference in observed washout ranging in the appli ed cortical ROIs from 7.9 to 11.5%. This error was seen to propagate i nto the split-dose rest-rest result of day 2 with a number of ROIs sho wing significant changes and with a decrease on the order of 2-3% (p < 0.005) in primary visual cortex. The stimulation experiment showed si gnificant increases from 2 to 7% (p < 0.0001) in the mean values of RO Is comprising primary visual cortex. The values are of the same magnit ude as those obtained with Tc-99m-exametazime (nonlinearized), but con siderably lower than corresponding results obtained with Xe-133 inhala tion. Due to the washout problems, Tc-99m-bicisate will have only limi ted use in split-dose protocols.