Abstract
It is well recognized that the MR appearance of intracranial bleeding changes with the age of lesion. It is also well known that hemoglobin in stagnating blood undergoes oxidation to methemoglobin, a substance that lowers the relaxation times of surrounding water protons. To study these phenomena in a controlled way, about 3 ml of blood was injected into the right frontal lobe of two rhesus monkeys, and they were scanned sequentially for up to 2 months in a Picker NMR scanner (Bo = 0.25-0.5 T). The image intensity of the blood changed during the first week, consistent with the lowering of T1 and T2. On the inversion-recovery scans the initial appearance of the blood was less bright than was the contralateral white matter, reversing after 3-5 days. The opposite was true on spin-echo images. T1 and T2 values were calculated for all images. In parallel experiments, several milliliters of freshly drawn blood was placed in test tubes and relaxation times were measured in a bench-top analyzer at 0.25 T over a period of 10 days. The relaxation times dropped markedly, at a rate that depended on sterility, temperature, etc., closely approaching the expected result for complete conversion of hemoglobin to methemoglobin. Ten blood samples with different methemoglobin concentrations were prepared by adding varying doses of sodium nitrite. The change in 1/T1 was found to be roughly proportional to the methemoglobin concentration for values up to 40%, and the initial slope was consistent with published data.
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