For absolute quantification of TSPO, up to 36 healthy controls (up to 12 each of three TSPO genotypes) will have brain PET imaging using [(11)C]ER176 and these subjects will have the arterial line and a brain MRI scan. In about half of those subjects from each genotype group, lungs will be scanned in the same session. For radiation dosimetry of [(11)C]ER176, up to 11 subjects will have whole-body PET imaging. These subjects will not have arterial line and MRI scans.