During the first two years of life, there are great changes of myelination in the brain. Before myelination, white matter is hydrophilic and appears high in signal on T2 imaging and low signal on T1. With myelination, white matter becomes hydrophobic and becomes lower signal on T2 and higher on T1. The adult pattern of myelination is complete by 18 to 24 months and progresses in an expected pattern. It is interesting to note that brain maturation occurs at different rates and times on T1 and T2 weighted images. The exact reason is not understood. Myelination progresses from a caudal to cranial direction. Typically, before age 6 months, T1 weighted images are used to determine brain maturation. After 6 months of age, T2 weighted images are used.

Ages when changes on myelination appear on MRI