Pixel T2 distribution in functional magnetic resonance images of muscle

Increases in skeletal muscle H-1-NMR transverse relaxation time (T2) observ ed by magnetic resonance imaging have been used to map whole muscle activit y during exercise. Some studies further suggest that intramuscular variatio ns in T2 after exercise can be used to map activity on a pixel-by-pixel bas is by defining an active T2 threshold and counting pixels that exceed the t hreshold as "active muscle." This implies that motor units are nonrandomly distributed across the muscle and, therefore, that the distribution of pixe l T2 values ought to be substantially broader after moderate exercise than at rest or after more intense exercise, since moderate-intensity exercise s hould recruit some motor units, and hence some pixels, but not others. This study examined the distribution of pixel T2 values in three muscles (quadr iceps, anterior tibialis, and biceps/brachialis) of healthy subjects (5 men and 2 women, 18-46 yr old) at rest, after exercise to fatigue (50% 1 repet ition maximum at 20/min to failure = Max), and at 1/2 Max (25% 1 repetition maximum, same number of repetitions as Max). Although for each muscle ther e was a linear relationship between exercise intensity and mean pixel T2, t here was no significant difference in the variance of pixel T2 between 1/2 Max and Max exercise. There was a modest (10-43%) increase in variance of p ixel T2 after both exercises compared with rest, but this was consistent wi th a Monte Carlo simulation of muscle activity that assumed a random distri bution of motor unit territories across the muscle and a random distributio n of muscle cells within each motor unit's territory. In addition, 40% of t he pixel-to-pixel muscle T2 variations were shown to be due to imaging nois e. The results indicate that magnetic resonance imaging T2 cannot reliably map active muscle on a pixel-by-pixel basis in normal subjects.