Characterization of T2* heterogeneity in human brain white matter

Abstract

Recent in vivo MRI studies at 7.0 T have demonstrated extensive heterogeneity of T₂* relaxation in white matter of the human brain. In order to study the origin of this heterogeneity, we performed T₂* measurements at 1.5, 3.0, and 7.0 T in normal volunteers. Formalin‐fixed brain tissue specimens were also studied using T₂*‐weighted MRI, histologic staining, chemical analysis, and electron microscopy. We found that T₂* relaxation rate (R₂* = 1/T₂*) in white matter in living human brain is linearly dependent on the main magnetic field strength, and the T₂* heterogeneity in white matter observed at 7.0 T can also be detected, albeit more weakly, at 1.5 and 3.0 T. The T₂* heterogeneity exists also in white matter of the formalin‐fixed brain tissue specimens, with prominent differences between the major fiber bundles such as the cingulum (CG) and the superior corona radiata. The white matter specimen with substantial difference in T₂* has no significant difference in the total iron content, as determined by chemical analysis. On the other hand, evidence from histologic staining and electron microscopy demonstrates these tissue specimens have apparent difference in myelin content and microstructure. Magn Reson Med, 2009.