Penumbra oxygen metabolism and acute neuroinflammation in ischemic stroke : MRI and PET imaging of a M2 occlusion model in rat

Sammanfattning: Acute ischemic stroke (AIS) is caused by the sudden occlusion of a major artery of the brain, and results in infarction and severe ischemia within affected brain regions. If ischemic regions are rapidly revascularized, the neurological disability resulting from AIS can be significantly reduced. Accordingly, endovascular thrombectomy aims to restore blood flow to ischemic tissue-at-risk, penumbra, that remains viable in the short-term by virtue of collateral blood flow and an increased extraction of oxygen (OEF) from the blood. Diagnostic imaging is an essential component in the identification of patients who are suitable for revasculatory treatment. The aim of this thesis was to investigate AIS pathophysiology, with a special focus on oxygen metabolism and neuroinflammation, and subsequently to develop and improve methods for the identification of penumbra tissue through imaging with magnetic resonance imaging (MRI) and positron emission tomography (PET). To achieve this, we used a middle cerebral artery M2-segment occlusion model in rat that had previously been designed to increase the translational potential of experimental AIS research (M2CAO). In paper I we employed the PET tracer [11C]PBR28 to longitudinally profile the neuroinflammatory response during the first 14 days following transient M2CAO. We complemented PET examinations with ex-vivo immunohistochemistry (IHC). Results validated [11C]PBR28 and revealed early microglial activation and glial scar formation following M2CAO. In paper II, we used diffusion- and perfusion weighted MRI (DWI, PWI) to outline the emergence and expansion of ischemic injury at the expense of the penumbra during AIS. We used the established DWI/ PWI mismatch concept as a surrogate of the penumbra. Results showed that although the initial spread of ischemic injury is rapid, not all tissue contained in the DWI/ PWI mismatch is at risk of infarction. In paper III, we assembled a blood oxygen level dependent MRI protocol which was combined with PWI. The protocol was used to approximate regional tissue OEF during M2CAO and after blood flow had been restored. When combined with DWI, oxygen metabolism MRI achieved an improved penumbra specificity when compared to the DWI/PWI mismatch protocol used in paper II. In paper IV, we compared PET tracers [18F]FMISO and [64Cu]CuATSM in identifying tissue hypoxia resulting from M2CAO, and investigated the effects of hypoxia on ischemic tissue using IHC analysis. [18F]FMISO was superior to [64Cu]CuATSM in identifying tissue hypoxia. In conclusion, the imaging methodologies investigated in this study have high diagnostic potential in AIS as well as in cases of chronic cerebral hypoperfusion.