| Backgrounds and Objective:Cerebral ischemia is a frequent cerebrovascular disease, which has a high disability. Therefore, it is important to study the early pathological changes in vivo for diagnosis ,therapy and the development of novel drugs.A variety of studies have confirmed that the inflammatory responses in ischemic stroke lesions is dominated by macrophages derived from both resident microglia and circulating monocytes. Activated macrophages can release a wide spectrum of cytotoxic substances and may thereby exacerbate the ischemic tissue damage and cause delayed expansion of the infarctions . It is provided that inflammatory mechanisms might play an important role for the delayed increase of infarct volume. In the past, visualization of immune competent cells was only possible by histological analysis on post-mortem tissue, and need a lot of animals , furthermore ,it is impossible to have continuous and dynamic observation of the pathological change in each animal in this way. New therapeutic strategies targeting inflammatory responses to focal ischemia require non-invasive methods to assess activation and infiltration of inflammatory cell. Ultrasmall superparamagnetic iron oxide (USPIO) particles have recently been introduced as a cell-specific MRI contrast agent taken up by mononuclear phagocytotic system(MPS ).This technique could be used to monitor neuroinflammation in focal ischemia in living animals, There was no report on the ischemia reperfusion injury so far. The aim of the present study is to verify the inflammatory response in experimental cerebral ischemia reperfusion injury model by means of in vivo USPIO-enhanced MRI. Materials and Methods :36 male Sprague-Dawley rats were divided into two groups in random: Sham operated group(n=6)and model group(n=30).The model group was subdivided into five groups:â‘ TTC (2,3,5-triphenyltetrazolium chloride )staining group(n=6);â‘¡Twenty-four, forty-eight and seventy-two hours after IV USPIO injection group(n=6 in each group);â‘¢control group(received an intravenous injection of 0.9% sodium chloride instead of USPIO) (n=6). MR imaging was performed after ischemia-reperfusion, then USPIO was intravenously administered and monitored by MRI at hour 24,48,72 respectively. Imaging data were correlated with histopathology. If there were animals died during experiment, additional animals would be supplied at once.All data were analyzed by SPSS 10.0 statistical software package. Test of normality and one-way ANOVA were performed among groups. Difference with P<0.05 was considered as significant.Results:In model group, the ischemic lesion appeared as hyperintense signal on T2-weighted images after occlusion of the MCA, while T1WI is not sensitive to the lesion. The infarcted tissues were dull yellow in color in TTC staining contrast to the reddish brown viable tissues. The accumulation of USPIO appeared as positive enhancement on T1-weighted imaging which appeared as hyperintensity and negative enhancement on T2-weighted imaging which appeared as low signal. The maximum T1 enhancement was observed at hour 24 (p<0.002), while T2 enhancement was displayed at hour 48 (p<0.001). No signal change was found in control and sham operated group. The comparison between USPIO-enhanced MRI and histochemistry indicated that a similar distribution of activated microglia or macrophages which appeared as yellow on CD68 immunochemistry staining. Prussian blue staining showed iron-positive cells at the sites corresponding to area enhanced on MRI, but iron particles was also found in the necrotic area. Conclusions : 1.This study proved that It is feassible to label microglia or macrophagocytes with USPIO-enhanced magnetic resonance imaging further. 2.Neuroinflammation is active in cerebral ischemia reperfusion injury and can be monitored by USPIO-enhanced MR imaging in vivo.3.T1- weighted enhanced MR imaging should be the first choice to be used in monitoring the inflammatory reaction after cerebral ischemia reperfusion injury because it is more sensitive than T2WI.
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