While WT mice exhibited a high level of P2Y12R-positive microglia in cortical sections, Iba1 and P2Y12R were almost completely absent in PLX mice (Figure , Figure and). High levels of Iba1 and P2Y12R were also detectable in Trem2 -/-mice, but reflecting the results of the PET studies, P2Y12R levels were not elevated compared to WT mice. Cortical SUV levels, derived from PET imaging in the most significant time frame (25-60 min p.i.) (Figure ), were correlated with Iba1 or P2Y12R expression density. A strong association between Iba1 signal and tracer uptake (R = 0.6589, p = 0.0055, Figure ) and between the P2Y12R signal and tracer uptake (R = 0.754, p = 0.0007, Figure ) was observed. The P2Y12 signal was well associated with Iba1 (R = 0.6, p = 0.014, Figure ) indicating predominant microglial receptor visualization. The data provide strong evidence that P2Y12 PET signal reflects microglial P2Y12R binding and that the tracer is a potent PET imaging probe to assess P2Y12R expression on microglia in the CNS.