<qa>  What is the PET ligand to peripheral type benzodiazepine receptors?PK11195 labeled with carbon-11 is a PET ligand to peripheral type benzodiazepine receptors which has already been used in patients with stroke Ramsay et al., 1992, Rasmussens encephalitis Banati et al., 1999, multiple sclerosis Banati et al., 1997 and facial nerve lesions Myers et al., 1999. However, no tracer kinetic model for quantification has been fully validated forR-11CPK11195. In order to use R-11CPK11195 for PET-imaging of microglia activation and to use it in the longitudinal monitoring of disease progression, baseline levels of ligand uptake in a healthy control population are required. This study aims to measure R-11CPK11195 uptake in normal brain in different age groups and to develop methods for quantification of specific binding of R-11CPK11195. Because R-11CPK11195 uptake depends on regional bloodflow, each R-11CPK11195 scan will be preceded by a cerebral bloodflow scan with H215O. 