Pixel T2 distribution in functional magnetic resonance images of muscle

Increases in skeletal muscle (1)H-NMR transverse relaxation time (T2) observed by magnetic resonance imaging have been used to map whole muscle activity during exercise. Some studies further suggest that intramuscular variations in T2 after exercise can be used to map activity on a pixel-by-pixel basis by defining an active T2 threshold and counting pixels that exceed the threshold as "active muscle." This implies that motor units are nonrandomly distributed across the muscle and, therefore, that the distribution of pixel T2 values ought to be substantially broader after moderate exercise than at rest or after more intense exercise, since moderate-intensity exercise should recruit some motor units, and hence some pixels, but not others. This study examined the distribution of pixel T2 values in three muscles (quadriceps, 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 repetition maximum at 20/min to failure = Max), and at 1/2Max (25% 1 repetition maximum, same number of repetitions as Max). Although for each muscle there was a linear relationship between exercise intensity and mean pixel T2, there was no significant difference in the variance of pixel T2 between 1/2Max and Max exercise. There was a modest (10-43%) increase in variance of pixel T2 after both exercises compared with rest, but this was consistent with a Monte Carlo simulation of muscle activity that assumed a random distribution of motor unit territories across the muscle and a random distribution of muscle cells within each motor unit's territory. In addition, 40% of the pixel-to-pixel muscle T2 variations were shown to be due to imaging noise. The results indicate that magnetic resonance imaging T2 cannot reliably map active muscle on a pixel-by-pixel basis in normal subjects.