Magnetic Resonance Diffusion Tensor Imaging Of Peripheral Nerve Traction Injury And Correlation With Histology

Objective To make sciatic nerve traction injury models of rabbit, in order toprospectively evaluate possiblity and accuracy of diffusion tensor imaging(DTI)and diffusion tensor tracking (DTT) in sciatic nerve injures. To analysis therelation between FA-time curve,ADC-time curve, λ∥, λ⊥-time curve andpathology, limb fuction, so as to figure out the value of diffusion tensor parametersin diagnosis of periphrial nerve injuries and reveal the pathlogic basis ofabnormabilities of diffusion tensor parameters and its corelation with functionalrecovery.Materials and methods36New Zealand white rabbits were randomly divided intotraction injury group(32rabbits, right sciatic nerve as traction injury, left as the shamoperation side) and normal control group(4rabbits). All of the rabbits were observedat different time point as follows:before the operation, after operation,1day,3days,1week,2,4,6,8weeks.There were4rabbits in every time point. MRI scan wasperformed before operation and after operation at different time points as mentionedabove. MRI scan squences included T2weight imaging(T2WI),short time inversionrecovery sequence(STIR) and single shot spin-echo plannar imaging(SE-EPI). bvalue was selected to be0,1000s/mm2,diffusion gradient direction was32directions. MRIsignal and shape of nerve on DTI were observed. FA,ADC, λ∥,λ⊥of sciaticnerve at different time points were measured.At the same time, the limb functionwere evaluated(prolapse and toe reflection, Tarlov score exhibition).After the MRIscan, the rabbit was killed and removed the sciatic nerve to perform pathologic examination including light, electron microscopy and immunohistochemistry, then observedthe pathlogic changes of sciatic nerves at different time points.Result After traction injury3days in the sciatic nerve, signal on T2WI andSTIRwas increased, it is obviously edema in proximal and distal portions. At2weeks, thesignal decreased in traction portion, the signals returned to normal in proximal anddistal portions.At6-8weeks, the high signal was still exist at traction portion, thenerve was thicken.DTT revealed only the proximal portion after traction injury at1day, nerve of distal andtraction portion broke completely. AT3day, the nerve of the traction portions appeared partly butsmaller, shorter than sham-operated nerve, the distal portion was still undetected. At1week, thenerve of distal portion appeared thinner and shorter. At2-6week, nerve fibers of the distalportions expanded and thickened than before, it was as thick as the proximal and traction portionof sham-operated nerve. At8week, the distal nerve fibers had not restored to the level beforeinjury.Diffusion tensor parameters were obviously changed in traction and distalportions of sciatic nerve in traction injury, they were slighter changes in proximal portions.At3day, FA value was slightly decreased. λ⊥value was slightly increased and both ofthem returned to normal level at2week. FA,λ⊥value were significantly different betweentraction injury and sham-operated group, but ADC and λ∥value were no significantly differentbetween pre-operation and post-operation group. Diffusion tensor parameters did not haveobvious change in sham-operated group. Traction portion, FA value began to decline after1day. At3days it decreased to lowest level（FA0.334±0.011）. At1week, FA valuesgradually rose and restored to the maximum value (FA0.444±0.014) at8week. At1day, λ⊥bvalue slightly increased; At3day, it increased to the maximum (λ⊥1.520±0.034).At1weekλ⊥value gradually restored and returned to normal (λ⊥1.131±0.032) at4week. Changes ofdistal portion: At1day FA values began to decline. At3day, FA values decreased to the lowestlevel (FA0.367±0.009);At1week, FA values gradually increased and reached the maximum(FA0.467±0.121) at8week.. At1day, λ⊥value incresed and reached muximum at3day(λ⊥1.432±0.049); At1week,λ⊥value began to decline and returned to normal (λ⊥1.031±0.043) at4week. FA value-time curves of proximal, traction and distal portions of injuried nerve were fast-fall and slow-rise type.ADC value and λ⊥value were noobvious changes between pre-operation and post-operation.Limb functional changes: The score of the injuried limb was lowest at1day after sciaticnerve injury in toe-spreading relex(4points), and gradually recovered from3week to8weeks, the score of toe-spreading restored to the maximum (13points) at8week. Tarlov scaledecreased to minimum value (5points) at1day, gradually restored from3week to8week andreach the maximum (14points) at8weeks.Pathological changes: On light microscopy:1day after operation, myelin sheath of tractionportion was obviously swelling, part of the myelin sheath broke down and inflammatory cellinfiltrated; At3days, a large amount of myelin sheath broke down and axonal disintegrated;At2-6weeks, axon, myelin sheath degeneration and regeneration were coexisted at the same time,Schwann cells and myelin sheath proliferated and myelin sheath grew slowly to mature; At8weeks, nerve fibers arranged neatly, myelin sheath was basicly integritied, nerves wererestored to normal stucture.. Distal portion: At1day, the myelin sheath was mild swelling,congestion, and a little inflammatory cell infiltration. At3days, part of myelin sheathdiscomposed, the nerve fibers became vacuolar change. The other changes were similar to theseof the traction portion, but degree of injuries was lighter than the latter. On electronmicroscopy:3days after operation, myelin sheath of traction portion partly broke down.microtubule and microfilament disappeared; At2week, nerve degeneration and regenerationwere coexisted, myelin sheath began to regenerate. Distal portion: At3day, myelin sheath partlycollapsed, most of the myelin sheath swelled and became loose; At2week, myelin sheathbegan to regenerate and Schwann cell proliferated.Conclusion DTT can show abnormal change of the sciatic nerve with tractioninjury in rabbit clearly and earily. FA-time curve and λ⊥-time curve of nervetraction injury are consistent with the changes of pathology and limb function. DTTcan reflect early microscopic changes of the sciatic nerve, which can be used as asensitive and reliable methods to evaluate the degeneration and regeneration in nerveinjury.