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 diamagneticeffects related primarily to protein content. However blood containing either deoxyhemoglobin or methemoglobin exhibits additional paramagneticrelaxationeffects, 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 T 2. 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 T 2 with the two different echo methods provides an unequivocal indication of field inhomogeneity such as is produced by erythrocytes. A second paramagneticrelaxationeffect is the direct interaction of protons with the electron spin of methemoglobin, which markedly lowers both T 1 and T 2. 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.