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T2 values were measured at 0.23 and 4.7 T for deoxygenated blood samples (43%-73% O2 saturation) with hematocrits of 18%-100%. An increase in the hematocrit produced a marked reduction in T2 at both field strengths. Cell lysis did not abolish the T2 effect at either field strength. The authors conclude that the increase in hemoglobin concentration caused by formation of a retracted clot is a cause of the hypointense appearance of acute hemorrhage compared with brain on T2-weighted clinical magnetic resonance images. This is particularly important on low-field-strength systems, which are not sensitive to the T2 shortening effects of paramagnetic intracellular deoxyhemoglobin.
Hayman et al. (Mon,) studied this question.