This review focuses on the maturation of brain white-matter, as revealed by magnetic resonance (MR) imaging carried out in healthy subjects. The review begins with a brief description of the nature of the MR signal and its possible biological underpinnings, and proceeds with a description of MR findings obtained in newborns, infants, children and adolescents. On MR images, a significant decrease in water content leads to a decrease of longitudinal relaxation times (T1) and transverse relaxation times (T2) and consequent "adult-like" appearance of T1-weighted and T2-weighted images becomes evident towards the end of the first year of life. Owing to the onset of myelination and the related increase of lipid content, MR images gradually acquire an exquisite grey-white matter contrast in a temporal sequence reflecting the time course of myelination. Albeit less pronounced, age-related changes in white matter continue during childhood and adolescence; white matter increases its overall volume and becomes more myelinated in a region-specific fashion. Detection of more subtle changes during this "late" phase of brain 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 behavioral test-batteries, with structural MR imaging, to study developmental changes in structure-function relationships. (C) 2001 Elsevier Science Inc.
