Maturation of white matter in the human brain: A review of magnetic resonance studies

This review focuses on the maturation of brain white-matter, as revealed by magnetic resonance (MR) imaging carried out in healthy subjects. The revie w begins with a brief description of the nature of the MR signal and its po ssible biological underpinnings, and proceeds with a description of MR find ings obtained in newborns, infants, children and adolescents. On MR images, a significant decrease in water content leads to a decrease of longitudina l relaxation times (T1) and transverse relaxation times (T2) and consequent "adult-like" appearance of T1-weighted and T2-weighted images becomes evid ent towards the end of the first year of life. Owing to the onset of myelin ation and the related increase of lipid content, MR images gradually acquir e an exquisite grey-white matter contrast in a temporal sequence reflecting the time course of myelination. Albeit less pronounced, age-related change s in white matter continue during childhood and adolescence; white matter i ncreases its overall volume and becomes more myelinated in a region-specifi c fashion. Detection of more subtle changes during this "late" phase of bra in development is greatly aided by computational analyses of MR images. The review also briefly outlines future directions, including the use of novel MR techniques such as diffusion tenser imaging and magnetization transfer, as well as the suggestion for the concurrent use of experimental behaviora l test-batteries, with structural MR imaging, to study developmental change s in structure-function relationships. (C) 2001 Elsevier Science Inc.