NORMAL MYELINATION OF BRAIN

Dr.Chandra sekharDR.DUONG

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.

Composition of myelin

Myelin is composed of about 80% lipid fat and about 20% protein.

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.

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.

Axons with normal myelin Demyelinated axons: Nerve impulse conduction slows or stops completely

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.

•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 hemispheresOptic tract in occipital lobe 1st > parietal + frontal

MR imaging is the most sensitive

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.

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.

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.

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.

FLAIR:Follows the same pattern as T2 but somewhat lags behind PD WIUseful in distinguishing gliosis fromDWI:In acute setting more sensitive than T1 or T2

MR SpectroscopyIncreased Myo-Inostol and Choline in neonatesNAA increased with myelination (1st yr)

Normal Myelination

Birth (full term)

Brainstem Cerebellum Posterior limb of internal capsule

One month

Deep cerebellar white matter

Corticospinal tracts

Pre/ postcentral gyri

Optic nerves, tracts

3 Months

Brachium pontis, cerebellar follia Ventral brainstem Optic radiations Anterior limb of internal capsule Occipital subcortical U fibres Corpus callosum splenium

6 Months

Corpus callosum genu Paracentral subcortical U fibres Centrum semiovale (partial)

8 Months

Centrum semiovale (complete except for some fronto temporal areas)

Subcortical U fibres (complete except for most rostral frontal areas)

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”

18 Months

Progressive myelination in the IC, CC, forceps minor, forceps major and central and subcortical white matter

24 Months

Myelination milestones

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

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.

Terminal zones

Persistent signal intensity in lateral, superior, and posterior to the lateral ventricles, particularly in the region of trigones.

Perivascular spaces

Curvilinear periventricular areas that are iso intense to CSF on all imaging sequences.

Anatomic Region

T1WI T2WI

Superior cerebellar peduncle

28 gest wks 27 gest wks

Median longitudinal fasciculus

25 gest wks 29 gest wks

Medial lemnisci 27 gest wks 30 gest wks

Lateral lemnisci 26 gest wks 27 gets wks

Myelination Patterns on MRI

High signal Low signalAnatomic Region: Brain T1 T2Cerebellar peduncle, middle 0 0-2 months

Cerebral White matter 0-4 months 3-5 months

Posterior limb internal capsule Anterior Portion Posterior Portion

1 month0

4-7 months0-2 months

Anterior limb internal capsule 2-3 months 7-11 months

Corpus callosum, genu 4-6 months 5-8 months

Corpus callosum, splenium 3-4 months 4-6 months

Occipital white matter Central Peripheral

3-5 months4-7 months

9-14 months11-15 months

Frontal white matter Central Peripheral

3-6 months7-11 months

11-16 months14-18 months

Centrum semiovale 2-4 months 7-11 months

Myelination Patterns on MRI

Thank you