T 2 Distribution in the Forearm Muscles and the T 2 Threshold for Defining Activated Muscle.

Purpose: In order to detect exercised muscles by the increase in T ₂ , we have defined a Gaussian T ₂ distribution and reference values (T _2r and SD _r ) in resting state muscles, and a threshold for detecting exercised muscles.
Methods: The subjects were healthy adult volunteers (14 males and 12 females). Multiple-spin-echo (MSE) MR images were obtained with 10 TE values from 10 to 100 ms using a 0.2T MRI system. T ₂ values for 10 forearm muscles were obtained in the resting state and after isometric wrist flexion exercise with 5%, 15%, and 25% of the maximum voluntary contraction (MVC). Z values were obtained by (T _2e - T _2r )/SD _r , where T _2e was T ₂ after exercise. Based on sample size calculations, three thresholds (Z _T = 1.00, 2.56, and 3.07) were applied to agonist and antagonist muscles.
Results: A normal distribution of T ₂ was detected in resting muscles at 34 ± 3 ms (mean ± standard deviation [SD]) in 26 subjects using the Kolmogorov-Smirnov test, the Shapiro-Wilk test, and the Jarque-Bera test (P > 0.05). No gender differences were shown between the T ₂ or SD, and a similar result was obtained in 12 measurements on a single subject (P < 0.01). The T _2r and SD _r were used for reference values. The threshold Z _T = 1.00 showed the highest sensitivity (0.86) even with 5% MVC, but it showed a lower specificity (0.85) than the other thresholds. Z _T = 3.07 showed the highest specificity (1.0), but it showed a lower sensitivity (0.36) with the 5% MVC, compared with Z _T = 2.56 (0.50). The receiver operating characteristics analysis also supported these results.
Conclusion: We found that the T ₂ distribution in muscles was Gaussian, suggesting that a one-sample t-test can be applied, and that Z _T = 2.56 could cover low-intensity exercise with high specificity and a low false-positive rate.