Conventional, Diffusion and Magnetization Transfer MRI in the Study of Myelination

Myelination of the brain starts at the second trimester of gestation and continues throughout life. Post mortem studies have demonstrated that myelination continues into the second to third decade of life. It follows a caudal to rostral and a posterior to anterior pattern and proceeds rapidly up to two years of age. Cranial nerves are the first to myelinate, while myelination of white matter tracts carrying sensory information precede to those carrying information relating to movement. The myelination process affects dramatically tissue contrast and changes in the MR appearance of the developing brain have been described initially in conventional sequences. These sequences offer anatomic information and assess macroscopic structural changes of the brain. With the larger application of Diffusion and Magnetization Transfer imaging, a semiquantitative evaluation of the myelination process has been performed and regional age related changes of the mean diffusivity (MD), the apparent diffusion coefficient (ADC), the fractional anisotropy (FA) and the magnetization transfer ratio (MTR) of developing brain have been described. These techniques assess microstructural changes of the brain.