Abstract WP550: Pathophysiology of Acute, Subacute and Chronic Lesions in Stroke: A Radiological-Pathological Analysis

Introduction: The imaging characteristics of ischemic lesions seen on MRI aid in the interpretation of the lesion pathogenesis. Early ischemic lesions are seen on diffusion weighted imaging (DWI) with low apparent diffusion coefficient (ADC), later evolve on T2-FLAIR, develop elevated ADC, then disappear on DWI. Chronic white matter lesions (WML) seen on T2-FLAIR but not DWI may or may not be of ischemic origin. Here we present an MRI targeted pathology approach to better describe in-vivo MRI based on cellular changes. Methods: The brain of a patient was obtained for pathological evaluation early after stroke recurrence. As part of clinical care and study procedures, 3T MRI was obtained at baseline (within 2-hours), multiple follow-up visits, and during second admission. Radiological-pathological methods previously validated in trauma was employed. Whole brain 7T MRI was performed after formalin fixation, and MRI targeted pathology was performed, co-localizing three lesions of different ages based on history and presumed imaging characteristics; i) acute, ii) subacute, and iii) chronic vs WML. Homologous contralateral tissue was used as control. Results: Lesions seen in-vivo were observed in post mortem MRI, and reconfirmed in histology. An example of in-vivo and post-mortem MRI, along with corresponding tissue sections and H&E are shown in figure. Qualitative delineation of the boundaries of the chronic lesion on H&E was challenging. A large number of small vessels with enlarged perivascular spaces dominated. The subacute lesion, was characterized by loss of white mater, infiltration of macrophages, and astrocytosis. The acute lesion had a text-book appearance. Conclusion: Dramatic differences are seen in the volume fraction of free water, which likely a major contributor to the appearance on DWI/ADC and T2 FLAIR. Work continues with immunohistochemistry to better delineate the boundaries of the lesions and cellular morphology relevant to in-vivo MRI.