Abstract WP32: Heterogeneity of Oxygen Extraction Fraction Weighted MRI in Acute Ischemic Stroke Patients
Background: Increases in oxygen extraction fraction (OEF) are postulated to be a marker of tissue at risk of infarction in acute ischemic stroke patients. R2’, a parameter that can be measured rapidly using an asymmetric spin echo MRI sequence (ASE), can be considered an OEF-weighted MRI metric, being a combination of both OEF and venous cerebral blood volume. We investigated the feasibility and utility of OEF-weighted MRI in acute stroke.
Methods: Adult ischemic stroke patients were prospectively enrolled in a 3T MRI study of patients imaged within 48 hours of stroke onset. Only the subset of patients whose MRI included ASE and whose DWI was positive for acute infarction was analyzed. Acquisition time for ASE was 1.5 min. R2’ maps was calculated from ASE data on a voxel-wise basis after motion correction and smoothing. DWI and follow-up images (FU), when available, were co-registered to the ASE. Mean R2’ values and coefficient of variance (mean/SD) within the lesion, ipsilateral DWI-normal (IPS) and contralesional hemisphere (CNL) were calculated and compared.
Results: Fifteen subjects met the inclusion criteria with mean±SD age 64±14 years, median admission NIHSS 12 [IQR 7-15] and time from onset to MRI 39±30 h. Eleven patients were imaged after tPA therapy. Both elevated and decreased R2’ were observed within DWI and FU lesions. Mean R2’ values in the DWI lesion (6.0±1.5 Hz) were significantly lower than CNL (7.2±0.6 Hz, P=0.02) and IPS (7.2±0.7 Hz, P=0.01). Coefficient of variance in R2’ values in the DWI lesion were also significantly lower than IPS (P=0.03) or CNL (P=0.05). For the 12 subjects with follow-up imaging, mean R2’ in tissue that eventually infarcted (5.7±1.8 Hz) was significantly lower (P=0.02) than in ipsilateral tissue surrounding the eventually infarcted lesion (7.3±0.6 Hz) but not significantly lower than DWI lesion values (6.1±1.6 Hz).
Discussion: In this study using ASE, decreased OEF-weighted MRI values can potentially reflect tissue that is destined for infarction since OEF is proportional to cerebral oxygen metabolic rate. Increased OEF-weighted MRI values may be a sign of tissue that is at risk of infarction. Studies including measures of CBF and CBV will allow for better discrimination of tissue status.
- © 2012 by American Heart Association, Inc.