| Objective:One of the aim of this study was to compare the changes in relaxation value of the brain in rats after administration of pure oxygen. With BOLD-fMRI techniques, we used three oxygen-inhaling methods for different duration in order to detect the BOLD signal intensity. To clarify how the pure oxygen impact the relaxation value and the BOLD signal intensity of the brain. We again compared the changes in relaxation value of the brain and the BOLD signal intensity after administration of pure oxygen in a rat stroke model of middle cerebral artery occlusion(MCAO). Methods:Forty six male Sprague-Dawley rats weighting 200-250g examined with 7T MR scanner. T1, T2 and T2* value of the brain were determined in air, respectively. After the air changed to 100% oxygen, T1, T2 and T2* value were again determined. Percentage changes in all values were compared. Three oxygen-inhaling methods for different duration in order to detect the BOLD signal intensity were compared. 1h and 5h After MCAO, T1, T2 and T2* value and the BOLD signal intensity of the brain were determined, respectively. Result:Compared with room air, T1 values of the cortex, caudate and corpus callosum decreased obviously, whereas significant T2 and T2* prolongation of of the cortex, caudate and corpus callosum was demonstrated. Percentage changes in all values between the cortex, caudate and corpus callosumwere different (P<0.001) when exposed to 100% oxygen.-Percentage changes of T1 and T2* value is respectively biggest in the cortex, whereas that of T2 value is biggest in the corpus callosum. After inhaling short duration of pure oxygen, the change of the BOLD signal intensiy of were 0.731%±0.071% in cortex,1.034%±0.049% in caudate,0.635%±0.051% in hippocampus and 0.758%±0.089% in thalamus, respectively. After After inhaling long duration of pure oxygen, the change of the BOLD signal intensiy were 0.962%±0.064% in cortex,1.556%±0.082% in caudate,1.107%±0.060% in hippocampus and 1.235%±0.085% in thalamus, respectively. The different change of BOLD signal intensity between the short duration and long duration inhale of pure oxygen was significant (P<0.001). 1h After MCAO,the different of the T1 relaxation value between the ischemia side and the normal side in cortex, caudate and corpus callosum were significant (P<0.001). The T2* relaxation value between the ischemia side and the normal side in cortex,and corpus callosum were significant. The T1 relaxation value of the ischemia side in cortex, caudate and corpus callosum were prolonged, whereas significant T2 and T2* shortening of of the cortex, caudate and corpus callosum was demonstrated. 1h After MCAO, the different of the T2 relaxation value between the ischemia side and the normal side in cortex, caudate and corpus callosum were not significant.5h After MCAO, the different of the all relaxation values between the ischemia side and the normal side in cortex, caudate and corpus callosum were significant (P<0.01). The T1 relaxation value of the ischemia side in cortex, caudate and corpus callosum were prolonged, whereas significant T2 and T2* shortening of of the cortex, caudate and corpus callosum was demonstrated. 1h After MACO, the change of BOLD signal intensity of the ischemia side and the normal side were (0.677%±0.071% vs.0.981%±0.074%) in cortex, 1.236%±0.056% vs.1.508%0.102% in caudate,1.024%±0.014% vs.1.102%±0.035% in hippocampus and 1.159%±0.073% vs.1.209%±0.085% in thalamus。5h After MACO, the change of BOLD signal intensity of the ischemia side and the normal side were 0.192%±0.020% vs.0.964%±0.094% in cortex,0.162%±0.020% vs.1.585%±0.121% in caudate,1.054%±0.079% vs.1.130%±0.06% in hippocampus and 1.108%±0.041% vs.1.221%±0.081% in thalamus.1h and 5h After MCAO, the different of change of the BOLD signal intensity between the ischemia side and the normal side in cortex, caudate, hippocampus and thalamus were significant (P<0.001).The BOLD signal intenstiy and the rADC change of the cortex, caudate, hippocampus and thalamus is coherent.Conclusion: Using 7T MR scanner permits getting higher spatial resolution and more reliable experiment data. The shortening T1 was induced by the increased amount of paramagnetic free oxygen. The contribution of reduction of CBF was negligible in changes to T1 and T2* value. The prolonging T2 and T2* was caused by the increased fraction of oxyhaemoglobin. Inhaling oxygen for different time may cause different BOLD signal intensity change in brain tissue. BOLD signal intensity is corresponding with heamoglobin saturation for inhaling pure oxygen. Administered pure oxygen increase the BOLD signal intensity because of its oxygenation sensitive paramagnetic characteristics. The prolong of the T1 relaxation value after MACO mainly because of the cytotoxic edema in ischemia brain. The shortening of the T2 and T2* relaxation value in ischemia brain was caused by the paramagnetism characteristics of the deoxyhemoglobin. The failure of the oxygen-metablism in ischemia brain caused continuely decrease of the BOLD signal intensity. Administered pure oxygen was shown to be effective as a exogenous'contrast agent' on high field MRI system that can be used as a new method to study the oxygen-metablism in brain and the cerebrovascular responses.
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