After normal myelination in utero, myelination of the neonatal brain is far from complete. The first myelination is seen as early as the 16th week of gestation, in the column of Burdach, but only really takes off from the 24th week1. It does not reach maturity until 2 years or so. It correlates very closely to developmental milestones 3. The progression of myelination is predictable and abides by a few simple general rules; myelination progresses from:
- central to peripheral
- caudal to rostral
- dorsal to ventral
- sensory then motor
T1 Myelination milestones
- term birth: brainstem, cerebellum, posterior limb of the internal capsule, optic tract, perirolandic region
- 2-3 months: anterior limb of the internal capsule
- 3-4 months: splenium of the corpus callosum
- 4-6 months: genu of the corpus callosum
T2 Myelination milestones
- term birth: brainstem, cerebellum, posterior limb of the internal capsule (40 weeks gestation), optic tract, perirolandic region
- 7-11 months: anterior limb of the internal capsule
- 4-6 months: splenium of the corpus callosum
- 5-8 months: genu of the corpus callosum
Unmyelinated white matter is hypodense compared to normal white matter and grey matter.
- T1: most sensitive sequence in children < 1 year of age 1
- most sensitive sequence in children between the age of 1 and 2 demonstrating gradual shift from hyper- to hypo-intense relative to grey matter
- the only area to remain hyperintense after the age of 2 years, and often for quite some time, is the peritrigonal region 4 which is called terminal zones of myelination
- differentiation between terminal zones and PVL requires detection of normally myelinated white matter between the patchy hyperintense T2 signal and the lateral ventricle
- unsurprisingly follows the same pattern as T2 but lags behind somewhat. The exception is deep cerebral white matter, which begin as heterogenously hypointense during the first few months of life.
- this area then joins the remainder of white matter as hypointense before finally once more becoming hypointense in the second year of life 2
- proton density: PD weighted images are useful in distinguishing gliosis from hypomylination
In the acute setting DWI is more sensitive than either T1 or T2.
Fractional anisotropy (FA) increases with brain maturation (diffusion is restriced perpendicular to the direction of axons, thus the increase in DWI signal in large axon bundles running through the slice, e.g posterior limb of interal capsule)
MR spectrocopy demonstrates elevated myoinositol and choline in neonates which gradually declines. Myoinositol decreases to adult values by end of first year and choline by 2-3 years. NAA increases with myelination (in the first year of life).
- 1. Knaap MS, Valk J, Barkhof F. Magnetic resonance of myelination and myelin disorders. Springer Verlag. (2005) ISBN:3540222863. Read it at Google Books - Find it at Amazon
- 2. Murakami JW, Weinberger E, Shaw DW. Normal myelination of the pediatric brain imaged with fluid-attenuated inversion-recovery (FLAIR) MR imaging. AJNR Am J Neuroradiol. 1999;20 (8): 1406-11. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 3. Dietrich RB, Bradley WG, Zaragoza EJ et-al. MR evaluation of early myelination patterns in normal and developmentally delayed infants. AJR Am J Roentgenol. 1988;150 (4): 889-96. AJR Am J Roentgenol (abstract) - Pubmed citation
- 4. Ketonen L, Hiwatashi A, Sidhu R. Pediatric brain and spine, an atlas of MRI and spectroscopy. Springer Verlag. (2005) ISBN:3540213406. Read it at Google Books - Find it at Amazon
- 5. Barkovich AJ, Kjos BO, Jackson DE et-al. Normal maturation of the neonatal and infant brain: MR imaging at 1.5 T. Radiology. 1988;166 (1 Pt 1): 173-80. Pubmed citation