Ultrasmall superparamagnetic iron oxide-enhanced magnetic resonance imaging for monitoring neuroinflammation in rats following focal ischemia-reperfusion injury

Abstract

Objective To assess the feasibility of ultrasmall superparamagnetic iron oxide (USPIO)-enhanced magnetic resonance imaging (MRI) for monitoring the phagocytic activity in the brain tissue of rats following focal cerebral ischemia-reperfusion (IR) injury. Methods Forty male SD rats were randomized into 5 groups, namely 2, 3, 5-triphenyltetrazolium chloride (TTC) staining group (n=4), sham-operated group (n=6), and 3 cerebral IR injury groups with reperfusion time of 24, 48, and 72 h (n=10). USPIO was intravenously injected after focal cerebral IR injury, and MRI was performed at 24, 48, and 72 h after the reperfusion. The rats were sacrificed at 24, 48 and 72 h, and frozen sections of the local brain tissues were prepared to observe the cell death with HE staining, iron particle distribution with Prussian blue staining and the activity of the macrophages by CD68 immunohistochemical staining and immunofluorescent labeling. Results The ischemic lesions were identified as hyperintense area on T2-weighted images (T2WI) after middle cerebral artery occlusion (MCAO). The accumulation of USPIO appeared as hyperintense areas on T1WI and hypointense area on T2WI. The maximum signal change was observed at 24 h on T1WI (1.60±0.28) and at 48 h on T2WI (0.92±0.17) (P<0.05), and at each of the time points, the enhancement was significantly greater on T1WI than on T2WI (P<0.05). No obvious signal changes were found in the control group. Prussian blue staining detected iron oxide particles in both the peripherals of the ischcmic region and the necrotic area. A similar distribution pattern of the macrophagcs or activated microglia was found by CD68 immunohistochemistry and immunofluorescent labeling. Conclusion USPIO-cnhanced MRI allows dynamic monitoring of the inflammatory reaction in the local brain tissues aftcr focal cerebral IR injury. Key words: Ultrasmaii superparamagnetic iron oxide; Magnetic resonance imaging; Macrophages; Brain ischemia; Inflammatory response