New approach for evaluating muscle function by using MR imaging

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1) RATIONALE AND OBJECTIVES. To determine whether the T2 relaxation time of skeletal muscle is affected by aging and to compare the effects of aging between fast- and slow-twitch muscles. METHODS. T2 relaxation times of the soleus and gastrocnemius muscles were examined in 59 healthy human subjects 22 to 76 years of age, using a clinical magnetic resonance imaging. In mice, T2 relaxation times, fat ratios, and extracellular space ratios of the psoas muscles were examined (old group of 7 mice, 24 to 26 months; young group of 7 mice, 8 to 10 weeks). RESULTS. T2 relaxation time of the gastrocnemius muscle increased significantly with aging (r=0.53, p<0.01). While that of the soleus muscle did not. T2 relaxation time of the psoas muscle was significantly linger (p<0.05) and extracellular space ratio of the psoas muscle was significantly wider (P<0.01) in old than in young mice. No significant difference in fat ratio was observed between old and young mice. A significant positive correlation was seen between extracellular space ratio and T2 relaxation time (r=0.84, p<0.01). CONCLUSION. T2 relaxation time of fast-twitch muscle increases with aging, due mainly to increased extracellular space reflecting age-related type II fiber atrophy.2) To determine the T2 relaxation time of muscle was affected by steroid treatment (steroid myopathy), T2 relaxation time of the soleus, gastrocnemius and tibialis anterior muscles were measured before and after steroid injection in rabbit. The T2 relaxation time of the gastrocnemius and tibialis anterior muscles (fast-twitch) increased significantly but not in the soleus muscle (slow-twitch). Extracellular space ratio of the gastrocnemius and tibialis anterior muscles was significantly wider (p<0.01) in steroid-treated than in control. T2 relaxation time is a potential indicator for steroid-induced myopathy.