What is myelin ? Myelin is an electrically insulating phospholipidlayer that surrounds the axons of many neurons. Myelin is produced by specialized cells:Oligodendrocytes in the central nervous systemSchwann cells in the peripheral nervous system.
Composition of myelin Myelin is composed of about 80% lipid fatand about 20% protein.
Myelinisation Myelin sheaths wrap themselves aroundaxons. Each oligodendrocyte can myelinate severalaxons (up to 40), so the destruction of even onlya few oligodendrocytes can have an extensivedemyelination effect.
Function of myelin layer The main consequence of a myelin layer is anincrease in the speed at which impulsespropagate along the myelinated fiber. Myelination also helps prevent the electricalcurrent from leaving the axon.
Axons with normal myelin Demyelinated axons: Nerveimpulse conduction slows orstops completely
Normal Myelination pattern of thepediatric brain Myelination causes changes in the signalintensity of the brain. Myelination starts during the 5th fetal month withmyelination of the cranial nerves and continuesthroughout life. Myelination progress is from:caudal to cephaladdorsal to ventralcentral 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 brainBrainstem first > Cerebellum > Internal capsule > Basal ganglia > Corpuscallosum > Cerebral hemispheresOptic tract in occipital lobe 1st > parietal + frontalMR imaging is the most sensitive
Imaging approachesMR is the only imaging technique that assessesmyelination. T1WI show myelination as increasinghyperintensity. T2WI show myelination as increasinghypointensity. Diffusion imaging shows myelnation asdecreasing diffusivity.
Assessment of myelinationMany ways to assess myelination by MR Qualitative method: Assess milestoneswhen changes of myelination appear onT1,T2 weighted images. Quantitative methods: Assess changesin diffusivity,FA,MT and compare withvalues of age-matched patients.
Basic principles of myelinationon MRI Myelinated WM appears hyper intense onT1W and hypointense on T2W images. Unmyelinated white matter appearshypointense on T1W and hyper intense onT2W images. Increase in signal intensity on T1W imagesprecede the decrease in signal intensity onT2W images.
T1WI:Most sensitive in children less than 1 year of age 1Hyper-intense T2WI:Most sensitive in children between the age of 1 and 2Gradual shift from hyper- to hypo-intense ralative to GM Because T2 of the immature brain have relatively poorgrey/white matter distinction due to high water content ofthe unmyelinated matter.
FLAIR:Follows the same pattern as T2 but somewhat lags behindPD WIUseful in distinguishing gliosis fromDWI:In acute setting more sensitive than T1 or T2MR SpectroscopyIncreased Myo-Inostol and Choline in neonatesNAA increased with myelination (1st yr)
Normal MyelinationBirth (full term) Brainstem Cerebellum Posterior limb of internal capsule
One month Deepcerebellarwhite matter Corticospinaltracts 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 Centrumsemiovale(complete exceptfor some frontotemporal areas) Subcortical Ufibres (completeexcept for mostrostral frontalareas)
12 Months Peripheralextension into thesubcortical WMbegins at about 1year and isessentiallycomplete by 22-24months except inthe “terminalzones”
Term birth : Brainstem, cerebellum, posterior limb of the IC1 months : Deep cerebellar WM3 months : Anterior limb of the IC, splenium of the CC6 months : Genu of the CC8 months: Centrum semiovale12 months: Peripheral extension into the subcortical WM18-24 months: Like adult
Terminal zones These are areas of known slow myelinationwithin the brain and should not be mistakenfor areas of ischemia. They are seen from about age 16 monthsuntil age 10 years.
Terminal zones Persistentsignal intensityin lateral,superior, andposterior to thelateralventricles,particularly inthe region oftrigones.
Perivascular spaces Curvilinearperiventricularareas that areiso intense toCSF on allimagingsequences.