In vivo MR evaluation of age-related increases in brain iron.

PURPOSE To assess the validity of an MR method of evaluating tissue iron. METHODS The difference between the transverse relaxation rate (R2) measured with a high-field MR instrument and the R2 measured with a lower field instrument defines a measure termed the field-dependent R2 increase (FDRI). Previous in vivo and in vitro studies indicated that FDRI is a specific measure of tissue iron stores (ferritin). T2 relaxation times were obtained using two clinical MR instruments operating at 0.5 T and 1.5 T. T2 relaxation times were measured in the frontal white matter, caudate nucleus, putamen, and globus pallidus of 20 healthy adult male volunteers with an age range of 20 to 81 years. R2 was calculated as the reciprocal of T2 relaxation time. These in vivo MR results were correlated with previously published postmortem data on age-related increases of nonheme iron levels. RESULTS The FDRI was very highly correlated with published brain iron levels for the four regions examined. In the age range examined, robust and highly significant age-related increases in FDRI were observed in the caudate and putamen. The correlations of age and FDRI in the globus pallidus and white matter were significantly lower and did not have statistical significance. CONCLUSIONS The data provide additional evidence that FDRI is a specific measure of tissue iron stores. The data also show that age-related increases in tissue iron stores can be quantified in vivo despite significant age-related processes that oppose the increase in R2 caused by iron. These results are relevant to the investigation of neurodegenerative processes in which iron may catalyze toxic free-radical reactions.