<qa>  What is a highly selective ligand for the peripheral benzodiazepine bindingR-PK11195 1-2-chlorophenyl-N-methyl-N-11-methylpropyl-3 isoquinolinecarboxamide is a highly selective ligand for the peripheral benzodiazepine binding site. R-PK11195, labelled with the positron emitter carbon-11, can be used to monitor the peripheral type benzodiazepine receptors using Positron Emission Tomography PET. At the Vrije Universiteit Medical Centre R-11CPK11195 is used for studying microglia activation in-vivo in patients with traumatic brain damage, minimal cognitive impairment and Alzheimer disease. 