Diffusion fractional anisotropy-based transformation in skeletal muscle caused by pressure

We used diffusion tensor imaging (DTI) to evaluate anisotropic changes in skeletal muscle cells under external pressure.

In 6 healthy volunteers, we compared DTI of the tibialis anterior (TA) and soleus (SOL) muscles under pressure. We performed imaging using a 1.5-tesla magnetic resonance (MR) scanner and diffusion-weighted stimulated-echo echo-planar pulse sequences optimized for skeletal muscle. We calculated diffusion tensor eigenvalues (λ), apparent diffusion coefficients, and fractional anisotropy (FA) values in a series of axially acquired DTI and compared them between the pressurized and nonpressurized lower limbs. We also measured a cross-sectional area of skeletal muscle.

We observed clear differences in FA and λ₃ between pressurized muscles and the nonpressurized muscles we used as controls. The mean control FAs were 0.36±0.02 (TA) and 0.30±0.02 (SOL). The mean control λ₃s were 0.74±0.02 s/mm² (TA) and 0.85±0.03 s/mm² (SOL). FA values in the pressurized (200 mmHg) limbs increased to 0.39±0.02 (TA) and 0.35±0.04 (SOL) compared with those values in the nonpressurized controls. λ₃ values in the pressurized (200 mmHg) limbs decreased to 0.68±0.05 s/mm² (TA) and 0.77±0.06 s/mm² (SOL) compared with those in controls. Moreover, the mean value of cross-sectional area of skeletal muscle (control) was 907.3±140.1 mm² (TA) and 1522±201.0 mm² (SOL). The cross-sectional area in the pressurized (200 mmHg) limbs decreased to 590.3±68.1 mm² (TA) and 1131±112.6 mm² (SOL) compared with those in controls. One-way analysis of variance (ANOVA) and post hoc Tukey-Kramer tests showed significant differences.

Anisotropy changed markedly on pressurizing the lower limb based on the correlation of the cross-sectional area and λ₃ suggested marked changed in anisotropy following application of pressure to the lower limb. If compression of the cross-sectional area is assumed to represent compression of the cell, change in λ₃ reflected the change in the size of muscle cells.