Brooks R A, Di Chiro G
Neuroimaging Section, NINCDS, Bethesda, Maryland 20892.
Med Phys. 1987 Nov-Dec;14(6):903-13. doi: 10.1118/1.595994.
The magnetic resonance imaging appearance of blood, as with other body tissues, is affected strongly by magnetic relaxation rates of the water protons. For blood containing only oxyhemoglobin, as for most tissues, the relaxation times are determined by diamagnetic effects related primarily to protein content. However blood containing either deoxyhemoglobin or methemoglobin exhibits additional paramagnetic relaxation effects, which have important consequences for magnetic resonance imaging of hematomas. First, the field inhomogeneity created by the concentration of paramagnetism in the red blood cells lowers the effective T2. This effect depends on field strength, and so is more striking at high fields, and is greater if gradient echoes are used. In fact, the observation of a difference in T2 with the two different echo methods provides an unequivocal indication of field inhomogeneity such as is produced by erythrocytes. A second paramagnetic relaxation effect is the direct interaction of protons with the electron spin of methemoglobin, which markedly lowers both T1 and T2. This effect is important in the imaging of hematomas that are at least several days old, after significant conversion of hemoglobin to the met form has taken place.
与其他身体组织一样，血液的磁共振成像表现受到水质子磁弛豫率的强烈影响。对于只含有氧合血红蛋白的血液，如同大多数组织一样，弛豫时间由主要与蛋白质含量相关的抗磁效应决定。然而，含有脱氧血红蛋白或高铁血红蛋白的血液会表现出额外的顺磁弛豫效应，这对血肿的磁共振成像有重要影响。首先，红细胞中顺磁性物质的浓度所产生的磁场不均匀性会降低有效T2。这种效应取决于场强，因此在高场时更为显著，并且如果使用梯度回波则会更大。事实上，通过两种不同回波方法观察到的T2差异明确表明了由红细胞产生的磁场不均匀性。第二个顺磁弛豫效应是质子与高铁血红蛋白的电子自旋的直接相互作用，这会显著降低T1和T2。在血红蛋白大量转化为高铁血红蛋白形式后，这种效应在至少几天大的血肿成像中很重要。
