2. What is myelin ?
Myelin is an electrically insulating phospholipid
layer that surrounds the axons of many neurons.
Myelin is produced by specialized cells:
Oligodendrocytes in the central nervous system
Schwann cells in the peripheral nervous system.
5. Myelinisation
Myelin sheaths wrap themselves around
axons.
Each oligodendrocyte can myelinate several
axons (up to 40), so the destruction of even only
a few oligodendrocytes can have an extensive
demyelination effect.
6. Function of myelin layer
The main consequence of a myelin layer is an
increase in the speed at which impulses
propagate along the myelinated fiber.
Myelination also helps prevent the electrical
current from leaving the axon.
7. Axons with normal myelin Demyelinated axons: Nerve
impulse conduction slows or
stops completely
8. Normal Myelination pattern of the
pediatric brain
Myelination causes changes in the signal
intensity of the brain.
Myelination starts during the 5th fetal month with
myelination of the cranial nerves and continues
throughout life.
Myelination progress is from:
caudal to cephalad
dorsal to ventral
central to periphery.
9. •Starts in the 2nd trimester and continous into adulthood
•Evolves in predictable sequential fashion over the first 2 postnatal years
•Functional significance and psychomotor development of brain
•Beginning with PNS > Spinal cord > Brain stem > Supratentorial brain
Brainstem first > Cerebellum > Internal capsule > Basal ganglia > Corpus
callosum > Cerebral hemispheres
Optic tract in occipital lobe 1st > parietal + frontal
MR imaging is the most sensitive
10.
11. Imaging approaches
MR is the only imaging technique that assesses
myelination.
T1WI show myelination as increasing
hyperintensity.
T2WI show myelination as increasing
hypointensity.
Diffusion imaging shows myelnation as
decreasing diffusivity.
12. Assessment of myelination
Many ways to assess myelination by MR
Qualitative method: Assess milestones
when changes of myelination appear on
T1,T2 weighted images.
Quantitative methods: Assess changes
in diffusivity,FA,MT and compare with
values of age-matched patients.
13. Basic principles of myelination
on MRI
Myelinated WM appears hyper intense on
T1W and hypointense on T2W images.
Unmyelinated white matter appears
hypointense on T1W and hyper intense on
T2W images.
Increase in signal intensity on T1W images
precede the decrease in signal intensity on
T2W images.
14. T1WI:
Most sensitive in children less than 1 year of age 1
Hyper-intense
T2WI:
Most sensitive in children between the age of 1 and 2
Gradual shift from hyper- to hypo-intense ralative to GM
Because T2 of the immature brain have relatively poor
grey/white matter distinction due to high water content of
the unmyelinated matter.
15. FLAIR:
Follows the same pattern as T2 but somewhat lags behind
PD WI
Useful in distinguishing gliosis from
DWI:
In acute setting more sensitive than T1 or T2
MR Spectroscopy
Increased Myo-Inostol and Choline in neonates
NAA increased with myelination (1st yr)
28. 12 Months
Peripheral
extension into the
subcortical WM
begins at about 1
year and is
essentially
complete by 22-24
months except in
the “terminal
zones”
31. Term birth : Brainstem, cerebellum, posterior limb of the IC
1 months : Deep cerebellar WM
3 months : Anterior limb of the IC, splenium of the CC
6 months : Genu of the CC
8 months: Centrum semiovale
12 months: Peripheral extension into the subcortical WM
18-24 months: Like adult
32. Terminal zones
These are areas of known slow myelination
within the brain and should not be mistaken
for areas of ischemia.
They are seen from about age 16 months
until age 10 years.
33. Terminal zones
Persistent
signal intensity
in lateral,
superior, and
posterior to the
lateral
ventricles,
particularly in
the region of
trigones.
