| Background and PurposeStroke is the most common cause of permanent disability in adults worldwide.The vast majority of strokes are caused by a decrease in cerebral blood supply in acute brain tissue,ie,ischemic stroke.The ischemic penumbra refers to the impaired function of the nerve tissue after ischemia but the morphology remains normal.After reperfusion for a certain period of time,the ischemic tissue can undergo reversible changes.For patients with acute ischemic stroke,thrombolytic therapy is currently the most effective treatment.The choice of patients with ischemic penumbra but no risk of bleeding is the key to successful outcomes of thrombolytic therapy,so pre-treatment assessment of ischemic penumbra is important.However,there is currently no exact imaging method to determine the extent of the ischemic penumbra.Oxygen challenge MRI（OC MRI）imaging,which uses a transient 100%oxygen challenge combined with T1,T2/T2*quantitative MRI scan imaging,analyzes the change of relaxation time,is a relatively novel Magnetic resonance scanning method.The paramagnetic oxygenated blood brought by OC replaces the high concentration of deoxyhemoglobin,changes the lateral and longitudinal relaxation time in the tissue,shortens the tissue T1 value,and increases the T2/T2*value.OC MRI uses oxygen as a contrast agent,which can be used to reflect the function of lung tissue,identify the hypoxic zone of the tumor,and assess the degree of liver fibrosis.It plays an important role in the treatment and evaluation of the disease.However,there are not many applications in cerebrovascular disease.Based on the middle cerebral artery occlusion（MCAO）model,the value of oxygen-enhanced magnetic resonance imaging in the identification of ischemic penumbra is explored.The T1 and T2 relaxation times of different brain regions before and after OC are explored.Based on the middle cerebral artery occlusion（MCAO）model,the value of oxygen-enhanced magnetic resonance imaging in the identification of ischemic penumbra is investigated.We explored the changes in T1and T2 relaxation times in different brain regions before and after OC.And we also analyzed the expression of RACK1 protein after cerebral ischemia and its correlation with relaxation time.Materials and Methods1.82 healthy male Sprague-Dawley rats（body weight 280g-350g）were prepared for MCAO model and the rats were subjected to modified neurological severity score（mNSS）and we performed OC MRI acquisition and histopathological examination in MCAO rats..2.Image acquisition process:We used the US GE Discovery MR 750 3.0T machine and Shanghai Chenguang 8-channel rat special coil to scan 10 MCAO rats.During the scan,we fixed the head of the rat.The scanning center is subject to the visual cross,the head is advanced,and the supine position.The scanning sequence is as follows:T1WI,T2WI,DWI,DKI,T1-mapping,T2-mapping.After the above scanning is completed,the T1-mapping and T2-mapping sequences of the OC are scanned,and the 100%oxygen is continuously opened for 6 minutes before the scanning,and the oxygen is continuously opened during the scanning,the oxygen flow rate is 7 L/min.The MK map and the MD map obtained by scanning the DKI image are paired,and the brain tissue was divided into three brain regions:the MD/MK map ischemic region（ie,the ischemic core region）,the MD/MK mismatch region,and the normal brain area on the opposite side of ischemia.The region of interest was selected in three different brain regions,and the T1 value,T2 value,and rate of change of the region of interest before and after 100%oxygen inhalation were measured.The data of non-oxygen was recorded as air group,and the data of oxygen inhalation was recorded as an oxygen group..3.36 MCAO rats were not subjected to oxygen challenge,recorded as air group,and the brain tissue samples of the ischemic area were taken directly after 1h,3h,6h,12h,and 3 rats were randomly selected for HE Staining,3 rats were used for immunohistochemistry,and 3 rats were used for Western blot detection.Another 36MCAO rats were taken for oxygen challenge treatment at 1h,3h,6h,and 12h,and were recorded as oxygen group.3 rats were randomly selected for HE staining,3 rats for immunohistochemistry,and 3 rats for Western blot detection.4.Data analysis was performed using statistical software SPSS 22.0,and mean data was recorded using mean±standard deviation.When the normal distribution and the homogeneity of the variance are met,the independent sample T test is used for the comparison between the two groups of samples;if the normal distribution and the homogeneity of the variance are not met,the comparison between the two samples is performed by the rank sum test;Multiple sets of samples were compared using repeated measures analysis of variance;One-way analysis of variance was used for comparison between different samples in the group,and the LSD test was used for comparison between the two groups;the correlation analysis was performed by Spearmen correlation analysis;the test level wasα=0.05,P<0.05.Result1.Before the inhalation of 100%oxygen,the T1 values of the same ROI were significantly different at the 1h,3h,6h,and 12h time points,and the difference was statistically significant;At the same time point,the T1 values of different ROIs were different,among which the T1 value of the MD/MK mismatched area was the highest,and the T1 value of the ischemic core area was the lowest,ie the ischemic core area<the contralateral normal brain area<MD/MK mismatched area,the difference was statistically significant,P<0.05.The T1 value after 100%oxygen inhalation changed the same as before.2.Before the inhalation of 100%oxygen,the T2 values of the same ROI were significantly different at the 1h,3h,6h,and 12h time points,and the difference was statistically significant;At the same time point,the T2 values of different ROIs were different,among which the T2 value of the MD/MK mismatched area was the highest,and the T2 value of the ischemic core area was the lowest,ie the ischemic core area<the contralateral normal brain area<MD/MK mismatched area,the difference was statistically significant,P<0.05.The T2 value after 100%oxygen inhalation changed the same as before.3.The rates of T1 and T2 change before and after 100%oxygen inhalation in the ischemic core region,the mismatched region,and the contralateral normal region were significantly different.The core region<contralateral normal region<MD/MK mismatched region,the difference was statistically significant,P<0.05.4.Before and after inhalation of 100%oxygen,the expression of RACK1protein in the cerebral ischemic area decreased at different time points,and the expression of RACK1 was higher in rats after oxygen inhalation than before,the difference was statistically significant（P<0.05）.There is a positive correlation between RACK1 protein and T1 value,and a negative correlation with T2 value.Conclusion1.At different time points,the relaxation values of different ROI of MCAO rats before and after 100%oxygen challenge were changed according to a certain trend.OC MRI could determine the oxygen metabolism of brain tissue after infarction.2.The change of T1 value and T2 value may identify the ischemic penumbra of MCAO rats.100%oxygen inhalation may delay the development of brain tissue ischemia and provide more time for the identification of ischemic penumbra. |
