Positron emission tomography imaging analysis. (A) [11C]PK11195 cortex to subcortical white-matter ratio indicated no differences in microglial activation between juvenile (n = 8) and aged (n = 7) animals before or after immunization. (B) Comparison between pre- and post-immunization scans showed a trend towards increased [11C]PK11195 retention over time in aged animals. (C) Representative transaxial parametric image of [11C]PiB standardized uptake value ratio (SUVR) of an aged animal showed no evidence of significant amyloid deposition. The PET image is superimposed over a corresponding MRI image to aid anatomic orientation. (D, E), Representative transaxial parametric images of [11C]PK11195 cortex to subcortical white-matter ratio (SWMR) revealed no significant microglial activation in (D) aged or (E) juvenile animals.