Normal Myelination Pattern in MRI

In fully myelinated brain, the white matter is hyperintense to the grey matter (cortex) in T1-weighted images and hypointense to the grey matter in T2-weighted images.

In neonatal brain, unmyelinated white matter appears T1 hypointense and T2 hyperintense. During the first year of life myelination is better assessed with T1 WI and in the 2nd year with T2 WI.

The first indication of myelination is increased T1 signal intensity. This increase in T1 signal is supposed to be due to increased cholesterol and galactocerebroside content within the cell membrane of oligodendrocytes.

Although the exact mechanisms are not fully understood, it is known that the signal changes on T1-weighted MR parallel increases in certain lipids that occur during the formation of myelin from oligodendrocytes. The signal changes on T2-weighted MR have been presumed to be correlated with the period of maturation of the myelin sheath seen histologically as thickening and tightening of the spiral of myelin around the axon.

The temporal changes of the MR signal are believed to be the end result of the combined effects of the :

1. Interaction of extracellular water with hydrophilic lipids (galactocerebrosides, myelin-bound cholesterol) on the outer myelin layer
2. Progressive dehydration of maturing white matter &
3. Packing of long-chain fatty acids in the bilaminar membrane.

Progression of Myelination

• Proximal pathways before distal (e.g., brainstem before supratentorial brain)
• Sensory (visual and auditory) before motor
• Central white matter before peripheral
• Posterior before anterior 

Myelinated Regions at Birth (or Shortly After Birth) 

1. Dorsal brainstem
2. Inferior, superior cerebellar peduncles
3. Perirolandic region
4. Corticospinal tract
5. Central portion of centrum semiovale
6. Posterior limb of internal capsule to cerebral peduncle
7. Ventrolateral thalamus
8. Optic nerve, chiasm, tract 

ASSESSING BRAIN MATURATION: USING MYELINATION MILESTONES

Dietrich et al. approached the subject of normal brain maturation by dividing the appearance of the brain on T2-weighted spin echo images into three patterns: 

1. infantile (birth to 6 months); - hyperintense
2. isointense (8 to 12 months); and 
3. early adult (10 months onward) - hypointense. 

In the infantile pattern, the cerebral white matter is hyperintense relative to gray matter, whereas the adult pattern shows hypointense white matter. The appearance of the isointense and early adult patterns is delayed in patients with developmental delay. 

Bird et al. determined that the gray and white matter should be isointense by age 4 months on T1-weighted images and by 9 to 10 months on T2-weighted images. 

The age at which gray and white matter are isointense as a critical factor in evaluating patients for developmental delay. 

Any of the methods described above are easy to use and reliable.

 Ref : 

1. Magnetic Resonance Imaging of the Brain and Spine, Scott.W.Atlas.
2. Pediatric Neuroimaging - Barkovich, 4th edition.

To be contd...