Evaluation of Acute Brain-Fat Embolism with Magnetic Resonace Imaging and Positron Emission Tomograpy in an Experimental Rat Model

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(Background) Fat embolism syndrome may occur after bone fracture. The neurological manifestations are thought to be the consequence of multiple lesions of the brain. Brain magnetic resonance (MR) imaging in the early stage of this syndrome shows signs of edema. The oil injection technique is commonly used to investigate the pathological changes in the brain in the experimental studies. However, we do not believe that this technique fully simulate clinical conditions. In this study, we designed a technique to more closely simulate the actual clinical state and reveal the relation between the MR images and the histologic findings. (Methods) We used eight-weeek old male rats. The rats were anesthetized with pentobarbital, and one femoral bone was fractured during a surgical procedure. We intravenously injected 0.15 ml/100g of heparinized blood taken from femoral bone marrow cavity into rats (FES group). There was no bone marrow debris in blood sample. Rats injected with heparinized saline … More , 0.15 ml/100g, were used as controls (control group). A) To examine characteristics of the model, we first measured the mean blood pressure, arterial blood gases, hemoglobin level and platelet count and the concentrations of triglyceride, lipase and free fatty acid in the FES and control groups. Then, we examined the brain MR images using the spin-echo technique, 1.5 tesla (repetition times = 350-2000 msec ; echo times = 25,80 msec) at 24 hours and one week after the injection. Finally, we stained brains, lungs, spleens and kidneys with fat and phoshotungstic acid hematoxylin stain to reveal fibrin emboli in the FES group. B) We compared the abnormal brain MR images with the histology of the brain. C) We observed the activity of both groups of rats and tested their learning and memory in a radical eight-arm maze at 24 hours after the injection. (Results) A) A decrease in the levels of trigyceride, lipase, platelet, hemoglobin and PaCO_2 was noted in the FES group. In 5 of 20 rats in the FES group, high signal intensity on T2-welghted images, which showed edema in the brains, were observed at 24 hours. However, this disappeared one week later. Fat droplets were found in the lungs and spleens with oil red O and Sudan III stains, but the same stains failed to show fat droplets in brains. We found fat droplets in the capillaries of the brains and lungs with osmium staining. Fibrin deposits were not found in the organs examined. B) A high signal intensity lesion on T2-weighted was seen in the ipsilateral cerebral cortex. Histologically, however, there were massive micro-fat emboli in the capillaries of the deep cerebral cortex and substantia nigra bilaterally in the brain. C : No difference between FES and control groups was noted. (Conclusion) There is a possible discrepancy between brain MR images and the histologic changes of the brain in the acute stage of fat embolism syndrome. Less