Brain development in infants is characterized by growth and myelination. Myelin is a cell membrane devoid of MRI signal; the MRI images obtained at different stages of myelination result from changes in brain tissue water content, from the multiplication of glial cells which precedes myelination (the so-called myelination gliosis), and from the accumulation of lipid myelin precursors contained in cells. T1-weighted sequences are used for the "premyelination" process and T2-weighted sequences for myelination proper. The development of myelination in the white matter is sequential, precisely determined, identical in all individuals, and it has been well studied by histologists. In vivo, myelination in infants is shown at MRI as the same precise sequence but with some changes in time towards the end. The myelination process takes place at different times and different speeds in different brain regions, and for any given structure the speed of myelination varies in relation to time.