下腿の筋の特徴と脂肪分布を調べるため23名の男性(平均年齢49±18歳)のMR画像を解析した。下腿の近位1/3部でのT1強調像とT2強調像から前脛骨筋,ひらめ筋,腓腹筋のそれぞれについてMR緩和時間(T1値とT2値)を測定し,T1強調像からは筋,皮下組織,下腿総断面での脂肪面積と断面積を算出した。MR画像上で測定した下腿脂肪率(下腿の総断面積中の脂肪面積をパーセントで表す)は体脂肪率および筋の脂肪率(筋の断面積中の脂肪面積をパーセントで表す)と正の相関を示した。腓腹筋のT1値はひらめ筋よりも有意に長く,また腓腹筋のT2値は前脛骨筋よりも有意に長かった。また,いずれの筋においてもT1値,T2値ともに年齢および下腿脂肪率が上がるにつれて長くなる傾向にあった。筋の間でのMR特性値の違いは筋線維組成の他に脂肪や水分量の違いも反映していると考えられた。腓腹筋の筋脂肪率は前脛骨筋よりもすべての年代で高かった。筋,皮下組織,筋膜の間での脂肪分布は年齢ではなく下腿脂肪率と密接に関係していた。筋や皮下組織への脂肪蓄積は年齢よりも個人差にもとづくものと考えられた。

To evaluate the characteristics of muscle and fat distribution in human legs, we examined 23 men (average age 49±18 years) using magnetic resonance imaging (MRI). MR relaxation times (T1 and T2 values) in the gastrocnemius, soleus, and tibialis anterior were measured on T1-weighted and T2-weighted MR images obtained at the proximal 1/3 position of the leg. Fat area and cross-sectional area of muscles, subcutaneous tissue and whole leg were measured on T1-weighted MR images. The leg fat ratio (percentage of fat area in a total cross-sectional area of leg) measured on MR images was positively correlated to the percentage of body fat and also to the muscle fat ratio (percentage of fat area in a cross-sectional area of muscle). T1 values in the gastrocnemius were significantly elevated compared to the soleus, and T2 values in the gastrocnemius were significantly elevated compared to the tibialis anterior. Longer relaxation times in both T1 and T2 values of the muscles were associated with increase in age and percentage of leg fat ratio. There is a possibility that a difference in MR relaxation times in muscles may reflect a difference in muscle fiber composition and/or other factors like fat and water compositions. The muscle fat ratio in the gastrocnemius and soleus were higher than those in the tibialis anterior in all the ages examined. Fat distribution patterns in muscles, subcutaneous tissue, and peri- and epimysium were not affected by age but by the fat ratio. These results indicate that the accumulation of fat in muscles and subcutaneous tissue in legs has a tendency to depend on the individual variations and is independent of age.

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