The hypothesis that increased muscle T2 after exercise is caused by increased extracellular fluid volume was tested by comparing the effects of exercise versus external leg negative pressure on muscle T2 relaxation in normal human subjects. T2 in lower leg muscles was measured by echo-planar imaging at 63 echo times from 24 to 272 ms, and the relaxation spectrum was calculated by using a non-negative least squares algorithm. T2 relaxation in anterior leg muscle before exercise was characterized by a single component with mean T2 = 29.3 +/- 0.7 (SE, n = 5). After ankle dorsiflexion exercise, this single component broadened, and mean T2 increased to 38.3 +/- 0.7 ms. In contrast, after leg negative pressure, which increased the total leg muscle cross-sectional area by 21% (range 12-32% n = 6), there was a variable appearance of much slower-relaxing components (60-500 ms). The results suggest that increased extracellular fluid can account for only a minor portion of the increase in muscle T2 observed during exercise.
Echo-planar magnetic resonance imaging was used to study the effect of exercise rate and duration on magnetic resonance imaging signal intensity (SI) of anterior tibialis muscle in normal human subjects (mean age 35 yr, n = 6). Axial midcalf echo-planar images (repetition time/echo time = 6,000/45, acquisition time = 80 ms) were acquired every 6 s for 1 min before and during 15 min of dynamic ankle dorsiflexion exercise (peak force 36% of 1 repetition maximum) at 10, 20, and 30 contractions/min. At each rate, muscle SI rose along an approximately exponential time course (mean time constant 1.8 min) toward a plateau that was linearly dependent on force times contraction rate (r = 0.64, P < 0.01) but varied significantly among subjects. The results confirm previous reports that changes in muscle SI correlate with exercise intensity, but not with total work performed, over a submaximal range of exercise intensities.
By using a receiver operating characteristic approach and a standardized set of chest radiographs, observer accuracy and variability are easily quantified. This approach provides a basis for comparing the diagnostic performance of physicians. When value is measured as a diminution in uncertainty, board-certified radiologists contribute substantial value to the diagnostic imaging system.
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