OBJECTIVE: Case series with (11)C-PK11195 and positron emission tomography (PET) in stroke patients suggest that activated microglia may be detected in remote brain regions with fiber tract connections to the lesion site as an indicator of poststroke neuroinflammation. However, the specificity of these imaging findings remains to be demonstrated. METHODS: In a prospective controlled study, we measured microglia activity using (11)C-PK11195-PET along the pyramidal tract, as defined by diffusion tensor imaging, in 21 patients with first-time acute subcortical ischemia within 2 weeks of stroke. Uptake ratios (affected vs unaffected side) were determined for a set of standardized volumes of interest along the pyramidal tracts (PT). Uptake ratios from patients in whom the PT was affected were compared with those in whom the PT was not affected. Uptake ratios were related to motor deficit and lesion size according to correlation analyses. RESULTS: Increased uptake ratios were only found in patients in whom the PT was affected by stroke. In the affected hemisphere, uptake was increased at the level of pons, midbrain, and internal capsule, but not in the oval center. The extent of remote microglia activation was independent of infarct size or clinical measures of stroke severity. INTERPRETATION: A specific activation of microglia was only found in patients in whom the PT was affected by the stroke and only caudal (anterograde) to the lesion; no activation was found in the retrograde direction or in those patients in whom the PT was not affected. These findings were independent of infarct size and may represent changes secondary to early Wallerian degeneration.
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