[C-11](R)-PK11195 positron emission tomography imaging of activated microglia in vivo in Rasmussen's encephalitis

This study was designed to explore the feasibility of PET using [C-11](R)-P K11195 as an in vivo marker of activated microglia/brain macrophages for th e assessment of neuroinflammation in Rasmussen's encephalitis (RE). [C-11]( R)-PK11195 PET was carried out in four normal subjects, two patients with h istologically confirmed RE, and three patients with clinically stable hippo campal sclerosis and low seizure frequency. Binding potential maps showing specific binding of [C-11](R)-PR11195 were generated for each subject. Regi onal binding potential values were calculated for anatomically defined regi ons of interest after coregistration to and spatial transformation into the subjects' own MRI. In one patient with RE who underwent hemispherectomy, t he resected, paraffin-embedded brain tissue was stained with an antibody (C R3/43) that labels activated human microglia. Whereas specific binding of [ C-11](R)-PK11195 in clinically stable hippocampal sclerosis was similar to that in normal brain, patients with RE showed a focal and diffuse increase in binding throughout the affected hemisphere. in RE, [11C](R)-PK11195 PET can reveal in vivo the characteristic, unilateral pattern known from postmo rtem neuropathologic study. PET imaging of activated microglia/brain macrop hages offers a tool for investigation of a range of brain diseases where ne uroinflammation is a component and in which conventional MRI does not unequ ivocally indicate an inflammatory tissue reaction. [C-11](R)-PK11195 PET ma y help in the choice of appropriate biopsy sites and, further, may allow as sessment of the efficacy of antiinflammatory disease-modifying treatment.