A Temporal MRI Assessment of Neuropathology after Transient Middle Cerebral Artery Occlusion in the Rat: Correlations with Behavior

Abstract

The purpose of this study was to evaluate the temporal and spatial pathological alterations within ischemic tissue using serial magnetic resonance imaging (MRI) and to determine the extent and duration of functional impairment using objective behavioral tests after transient middle cerebral artery occlusion (tMCAO) in the rat. MRI signatures derived from specific anatomical regions of interest (ROI) were then appropriately correlated to the behavioral measures over the time course of the study (up to 28 days post-tMCAO). Sprague-Dawley rats (n = 12) were initially trained on the following behavioral tasks before surgery: bilateral sticky label test (for contralateral neglect); beam walking (for hindlimb coordination); staircase test (for skilled forelimb paw-reaching). Rats were then randomly assigned to receive either tMCAO (90 minutes, n = 6), by means of the intraluminal thread technique, or sham-control surgery (n = 6). Proton density, T2- and T2-diffusion-weighted MR images were acquired at 1, 7, 14, and 28 days post-tMCAO that were then smoothed into respective proton density, T2 relaxation, and apparent diffusion coefficient (ADC) maps. Apparent percent total lesion volume was assessed using T2W imaging. MR signatures were evaluated using the tissue maps by defining ROI for MCAO and sham-control groups, which corresponded to the caudate-putamen, forelimb, hindlimb, and lower parietal cortices both ipsilateral and contralateral to the occlusion site. Behavioral tests were undertaken daily from 1 to 28 days post-tMCAO. Results demonstrate that apparent percent lesion volume reduced from 1 to 7 days ( P < 0.05) but then remained constant up to 28 days for the MCAO group. Pathological changes in the temporal profile of T2 and ADC tissue signatures were significantly altered in specific ROI across the time course of the study ( P < 0.05 to < 0.001), reflecting the progression of edema to necrosis and cavitation. Both T2 and ADC measures of ischemic pathology correlated with parameters defined by each of the functional tests ( r ≥0.5, P < 0.05) across the time course. The staircase test revealed bilateral impairments for the MCAO group ( P < 0.001), which were best predicted by damage to the ipsilateral lower parietal cortex by means of hierarchical multiple regression analyses ( R2 changes ≥0.21, P < 0.03). Behavioral recovery was apparent on the beam walking test at 14 to 28 days post-MCAO, which was mirrored by MRI signatures within the hindlimb cortex returning to sham-control levels. This long-term study is the first of its kind in tracing the dynamic pathologic and functional consequences of tMCAO in the rat. Both serial MRI and objective behavioral assessment provide highly suitable outcome measures that can be effectively used to evaluate promising new antiischemic agents targeted for the clinic.