Heterogeneous spatial-temporal evolution of DWI/PWI in human cerebral ischemia
Purpose: To characterize on a voxel-by-voxel basis salvageable from irreversibly-damaged ischemic tissue in human ischemic stroke using serial diffusion (DWI) and perfusion (PWI) weighted imaging. We sought to correlate tissue outcome with heterogeneity in the evolution of DWI and PWI. Methods: Patients with strokes involving the MCA territory who had been enrolled in a serial DWI and PWI study recording the natural history of human cerebral ischemia were retrospectively analyzed (n=6). Patients had received follow-up studies at predefined intervals. For each patient, maps of the apparent diffusion coefficient (ADC), fractional anisotropy (FA), cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT) and follow-up T2 FSE (F/U) were spatially coregistered (using AIR 3.08). Time to reach minimum/maximum values, time-to-normalization (or pseudonormalization), and minimum/maximum values were calculated for the DWI/PWI parameters. The ischemic core was operationally defined as the initial DWI abnormality, rim tissue as the mismatch between F/U and initial DWI, and salvaged-at-risk tissue as the mismatch between initial PWI and F/U. Values in core, rim and salvaged tissue were compared using standard two-sample t-tests. All significant findings were at p-values <0.05. Results: Pooling the six patients’ data, the depth of reduction at the acute stage of ADC, CBF and CBV values were found to be significantly greater in core than rim. MTT was found to be significantly elevated. Acutely, CBF was found to be significantly higher in salvaged tissue compared to rim; however no significant difference was found for MTT values (p=0.8). The time for ADC to reach its minimum value is faster in the core compared to rim as well as pseudonormalizing sooner. Elevated FA was found to occur earlier in core tissue than in the rim followed by a more rapid decrease. Pseudonormalization of FA occurred sooner than of ADC values. CBF and MTT recovered earlier in salvaged tissue versus rim tissue. Conclusion: Intralesional heterogeneity in the evolution of DWI/PWI may assist in the identification of salvageable tissue.