A 42-year-old man enrolled in a research study to measure microglial activation using [O-methyl-11C] N-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine ([11C]PBR28), an improved positron emission tomography (PET) radioligand. His history included human immunodeficiency virus infection, diabetes, hypertension, hypercholesterolemia, smoking (12 pack-years), and remote use of cocaine and heroin. Medical and neurological examination results were normal except for elevated blood pressure of 157/100 mm Hg. Brain magnetic resonance imaging (MRI) showed occasional foci of mild chronic white matter ischemia but was otherwise unremarkable (Figure, A).