Gradient of Tissue Injury after Stroke: Rethinking the Infarct versus Noninfarcted Dichotomy

Abstract

Aim: To evaluate the degree of variability in microstructural injury within and adjacent to regions identified as infarcted tissue using diffusion tensor imaging (DTI). Methods: In this prospective longitudinal study, 18 patients presenting within 12 h of anterior circulation acute ischemic stroke who underwent CT perfusion (CTP) at baseline followed by fluid-attenuated inversion recovery (FLAIR) and DTI 1-month were analyzed. Four regions of interest (ROI) corresponding to the severity of hypoperfusion on CTP within and beyond the radiological infarct lesion defined on FLAIR were segmented. Fractional anisotropy (FA) and mean diffusivity (MD) were quantified for each ROI and compared to a mirror homologue in the contralateral hemisphere. Ipsilateral to contralateral FA and MD ratios were compared across ROIs. Results: Lower FA and higher MD values were observed within both the infarct lesion and the peri-infarct tissue compared with their homologous contralateral brain regions (all comparisons p ≤ 0.01). No difference was observed in FA and MD between remote nonhypoperfused tissue and its contralateral homologous region (FA p = 0.42, MD p ≥ 0.99). The magnitude of asymmetry (ipsilateral/contralateral ratios) of FA and MD was greater with increasing severity of hypoperfusion in a dose-response pattern. Asymmetry greatest in the area of infarction with severe hypoperfusion, followed by infarction with moderate hypoperfusion, the peri-infarct hypoperfused tissue, and lastly the remote nonhypoperfused normal tissue (median on clustered quantile regression p ≤ 0.01). Conclusion: A gradient of microstructural injury corresponding to the severity of ischemic insult is present within and beyond conventionally defined infarct boundaries. The traditional dichotomized notion of infarcted versus noninfarcted tissue widely adopted in clinical research and in practice warrants reexamination.