Stability of α-[11C] methyl-L-tryptophan brain trapping in healthy male volunteers

Purpose: The purpose of this study was to assess the reproducibility in healthy volunteers of alpha-[C-11] methyl-L-tryptophan (alpha[C-11] MT) brain trapping imaging with positron emission tomography (PET), using volumes of interest (VOIs) and voxel-based image analysis. Methods: Six right-handed healthy male volunteers (34.3 +/- 10.9 years) with a negative family history for psychiatric disorders were scanned twice in the resting condition, 22 +/- 17 days apart. An unbiased semiautomatic segmentation of the brain was used to define VOIs. The trapping constant K* (ml g(-1) min(-1)) for a[C-11] MT was calculated for the whole brain and seven brain regions using the graphical method for irreversible tracers. In addition, parametric maps of K* were obtained from dynamic scans using the same method. Comparison of test and retest K* functional images was performed using SPM99. Student's paired t statistic was applied for comparisons of a[C-11] MT brain trapping in a priori selected VOIs. Results: a[C-11] MT brain trapping in VOIs showed a mean variability 2.6 +/- 1.8% (0.3 - 5%) for absolute and 1.5 +/- 2.1% (1.4 - 4.1%) for normalized K*. Intraclass correlations between test and retest conditions were 0.61 +/- 0.34 for absolute K* values and 0.73 +/- 0.20 for K* values normalized by global mean. SPM99 analysis using a height threshold of p= 0.05 ( two tailed) and an extent threshold of 100 voxels showed no significant differences between scans. Conclusion: Rest measurements in healthy male volunteers of the trapping constant for a[C-11] MT, using PET, appeared to be stable during an average interval of 3 weeks.