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Normal myelination

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 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:

  1. central to peripheral
  2. caudal to rostral
  3. dorsal to ventral
  4. sensory then motor
Myelination milestones
  • term birth : brainstem, cerebellum, posterior limb of the internal capsule, optic tract, perirolandic region
  • 2 months : anterior limb of the internal capsule
  • 3 months : splenium of the corpus callosum
  • 6 months : genu of the corpus callosum

Radiographic features

CT

Unmyelinated white matter is hypodense compared to normal white matter and grey matter.

MRI
  • T1 - T1 weighted images is the most sensitive in children less than 1 year of age 1.
  • T2 - 
    • T2 weighted images is the most sensitive 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.
  • FLAIR - 
    • 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 myo-Inositol and Choline in neonates. NAA increases with myelination (in the first year of life), Choline and myo-Inositol peaks gradually decline. 

See also

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