The study demonstrates that T2 relaxation has a quadratic field dependence in deoxygenated blood but a linear dependence in brain tissue and ferritin solutions, which has implications for MRI contrast mechanisms.
T2 was measured in samples of human blood and monkey brain over a field range of 0.02-1.5 Tesla, with variable interecho times, and was compared with previous data on ferritin solutions (taken with the same apparatus). 1/T2 in deoxygenated blood increased quadratically with field strength, as noted previously, but in brain gray matter the increase was linear, as also was the case in ferritin solution. In both deoxygenated blood and gray matter, 1/T2 increased with interecho time, but appeared to level off at times around 50 msec, as expected from the theory of diffusion through magnetic gradients. Diffusion times estimated by using the chemical exchange approximation were 3.4 msec for deoxygenated blood and 5.7 msec for the globus pallidus. The quadratic field dependence in blood is consistent with this same theory, but the linear dependence in brain tissue and in ferritin solutions remains unexplained.
Brooks et al. (Sat,) studied this question.