Ryan Nguyen

Introduction. Ischemic strokes make up 85% of strokes and can be caused by thrombi.¹ Tissue affected by ischemia can be categorized by probability of infarction. Penumbra is tissue likely to infarct but is still salvageable with timely intervention.^1,2 Standard treatment for stroke is reperfusion therapy via tissue plasminogen activator (tPA) and/or thrombectomy, the retrieval of the thrombus.^1,2 However, these interventions have strict indications due to the rising risk of intracerebral hemorrhage (ICH) over time. Reperfusion cannot be performed after 4.5 hours or 6 hours from stroke symptom onset for tPA or thrombectomy, respectively.^1,2 Due to these strict indications for reperfusion therapy, only 6% of ischemic stroke patients receive tPA.³ 14-27% of patients are not eligible for therapy due to unknown time of stroke onset.⁴ In addition, individuals have variable collateral circulation and could benefit from reperfusion past the indicated timeline.² The use of different imaging modalities is hypothesized to determine whether reperfusion could benefit patients by quantifying penumbral volume defined by hypo-perfusion without restricted diffusion associated with transmembrane ion gradient disruption, cytotoxic edema, or estimating ICH risk via presence of vascular macromolecule leakage, vasogenic edema.^4-7 Methods. Perfusion weighted (PWI), diffusion weighted (DWI) and/or fluid attenuated inversion recovery (FLAIR) imaging were used to evaluate ischemic changes (hypo-perfusion, cytotoxic edema, and vasogenic edema, respectively) in rat brains after large artery occlusion compared to controls. Lesion volume on imaging was compared with final infarct volume via histological analysis.^5,8-11 Results. PWI showed significant increase in the travel time of contrast agent to and from tissue affected by artery occlusion.⁵ In DWI, significantly restricted diffusion was observed in affected tissue.^5,10,11 FLAIR studies showed hyperintensity lesions starting at 3 hours.^5,9 DWI and FLAIR correlated well with areas of ischemia past 3.5 hours.⁹ DWI lesion volume grew to match PWI hypo-perfusion lesion volume over time, and DWI lesion volume decreased in size after reperfusion therapy.^5,10,11 Conclusions. Recent clinical trials have developed imaging criteria based on penumbral or vasogenic changes to determine if reperfusion could benefit patients past current indications. These trials have shown there is benefit with reperfusion for patients chosen with imaging criteria with unknown time of onset and extended-window therapy up to 9 hours and 16 hours for tPA or thrombectomy, respectively, without increased risk of ICH.^4,6,7 Therefore, perfusion, diffusion and/or FLAIR imaging may be important adjuncts in the evaluation of stroke in patients who could benefit from reperfusion but are excluded under current indications.

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