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Line Scan Diffusion Spectrum of the Rat Skeletal Muscle of Neuropathic Model
Koji Abe, MD; Toshiyasu Nakamura, MD, PhD; Eiko Yamabe, MD, PhD; Yoshiaki Toyama, MD, PhD
Keio University, Tokyo, Japan

Introduction: Edema is one of the symptoms of complex regional pain syndrome (CRPS). Measuring the degree of edema can be useful to diagnose CRPS. Our hypothesis is diffusion MR spectroscopy (MRS) can detect early edematous changes in skeletal muscle of the rat neuropathic pain model. For this purpose, we sequentially measured the apparent diffusion coefficient (ADC) and T2 values in the lower extremity skeletal muscle to identify time course of the ADC and T2s for early detection of edema in rat models.
Materials and Methods: Five female adult Wistar rats weighing 180-200g were used in this study. Under sodium pentobarbital anesthesia (40mg/kg, i.p.), we removed the left L6 transverse process and ligated the ipsilateral L5 nerve tightly with 4-0 silk suture. Mechanical allodynia and heat hyperalgesia were analyzed with the dynamic plantar aesthiometer and hot plate test (set at 50 degrees), respectively. At one, three, and five days , and one to six consecutive weeks after the surgery, we measured the T2s and ADC in the ipsilateral gastrocnemius muscle using a line-scan diffusion spectrum on a 1.5 T clinical MR imager. A round surface coil 3 inches in diameter was used. All data were presented as mean values SD. Changes in ratios of T2 values and ADC of gastrocnemius muscle with respect to the contralateral muscle were compared statistically using one-way analysis of variance with the Turkey post hoc test.
Results: Mechanical allodynia was evident at one day after the surgery and lasted during experimental periods. Heat hyperalgesia was obvious at five days and also lasted during experimental periods. T2 values significantly increased gradually over two weeks to reach the peak ratios of 1.0610.016 (p=0.023) compared to the contralateral gastrocnemius muscle. The increase of ADC values was observed one day after the surgery with the significant ratios of 1.2130.018 (p=0.001) and began to decrease at one week.
Conclusions: Chung model is the most reliable animal models inducing neuropathic pain in rats, which resembles human CRPS. We could detect the extremely early edematous changes in the affected skeletal muscle in the neuropathic pain animal model using diffusion MRI. T2 ratios increased gradually with a peak at three weeks. ADC ratios jumped up at one day after the surgery and began to decrease at one week. Diffusion MRI can be a useful tool to detect the early edematous change of CRPS affected limbs.


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