Quantitative measurement of cerebral oxygen metabolism using asymmetric spin echo version of echo planar imaging sequence in an animal model of ischimia

Objective To assess whether asymmetric spin echo version of echo planar imaging (ASE EPI) sequence can reflect cerebral oxygen metabolism by means of the changes in oxygen extraction fraction values before and after surgical operations in the canine model of ischimia. Methods A total of 12 healthy crossbreed dogs were enrolled in this study. All canines underwent cerebral MR imaging including coronal T2WI, DWI, MRA, ASE EPI and 3D PCASL. The bilateral carotid arteries (CA) were separated by surgical operations. The bilateral CA were ligated in 6 dogs, and the other 6 dogs had embolization in ligated bilateral CA. The cerebral MR imaging with the above protocol was repeated at 1 hour after surgical operation for each dog. All dogs were executed when they finished the MR examinations. Then the whole brains of the dogs were taken out for cutting coronal sections and 2, 3, 5-triphenyltetrazolium chloride (TTC) staining. The ASE EPI data were processed by the Functool software in the workstation to generate OEF maps. For measuring the OEF values before and 1 hour after surgical operations, one and the same region of interest (ROI) was respectively placed in the left and right hemispheres on the OEF maps for each dog. 3D PCASL data were processed by the Functool software in the workstation to yield CBF maps. The method for measuring the CBF values before and 1 hour after surgical operations on CBF maps was same as that for measuring on OEF maps. The changes of measurements before and after surgical operations were compared between the dogs with ligated bilateral CA and the dogs with embolization in ligated bilateral CA by using an independent samples t test. The measurements before and after surgical operations were respectively compared in the dogs with ligated bilateral CA and the dogs with embolization in ligated bilateral CA by using a paired t-test. The cutting slices with TTC staining were analysed for determining ischimia. Results The CBF values of the dogs with ligated bilateral CA were (59.81±23.59) and (38.56±12.98) ml/(min·100 g) before and after surgical operations, respectively. The CBF values of the dogs with embolization in ligated bilateral CA were (58.94±18.35) and (28.01±11.41) ml/(min·100 g) before and after surgical operations, respectively. The CBF values were significantly lower before surgical operations than after surgical operations in the dogs with ligated bilateral CA and the dogs with embolization in ligated bilateral CA, respectively (t=8.92 and 13.42, respectively; P=0.00, respectively). The CBF values were decreased more significantly in the dogs with embolization in ligated bilateral CA than in the dogs with ligated bilateral CA (t=2.92, P=0.00). The OEF values of the dogs with ligated bilateral CA were 0.29±0.02 and 0.32±0.06 before and after surgical operations, respectively. The OEF values of the dogs with embolization in ligated bilateral CA were 0.29±0.02 and 0.35±0.06 before and after surgical operations, respectively. The OEF values were significantly higher before surgical operations than after surgical operations in the dogs with ligated bilateral CA and the dogs with embolization in ligated bilateral CA, respectively (t=-7.21 and -4.43, respectively; P=0.00, respectively). The OEF values were increased more significantly in the dogs with embolization in ligated bilateral CA than in the dogs with ligated bilateral CA (t=2.18, P=0.03). The pathological results showed that the dogs with embolization in ligated bilateral CA had swollen cortex with focal infarction which was matched with the hyperintensity areas on the OEF and DWI images. However, the dogs with ligated bilateral CA only had swollen cortex whereas didn't have focal infarction. Conclusions ASE EPI sequence can reliably reflect cerebral oxygen metabolism in ischemia. This sequence has an important role for assessing hemodynamic state in patients with cerebral vascular diseases. Key words: Brain ischemia; Magnetic resonance imaging; Animal experimentation