Principles of myelination imaging
Myelination is the process of wrapping axons in myelin, which is produced by oligodendrocytes. On MRI, this manifests as a shortening of relaxation times. Due to increasing lipid concentration and water loss, the white matter signal changes.
The process has a clear vector: it spreads centrifugally, from caudal to cranial regions and from posterior to anterior (posterior-to-anterior).
The choice of pulse sequence depends directly on the patient's age:
- First 6 months: the process is best tracked on T1-weighted images (T1WI). Myelinated tracts appear hyperintense (bright).
- From 6 to 24 months: the T1-WI pattern becomes unreliable. T2-weighted images (T2-WI) become the main focus, where myelin produces a hypointense (dark) signal.
Maturation chronology according to MRI data
Knowledge of normal developmental milestones allows for the timely detection of developmental delay or pathology:
- Term newborn: Only the primary sensory pathways, brainstem, posterior limb of the internal capsule (posterior limb of the internal capsule), and cerebellar peduncles are myelinated.
- 3–4 months: On T1WI, signal shortening is observed in the anterior limb of the internal capsule. On T2WI, the splenium of the corpus callosum darkens, and mature white matter appears near the calcarine sulcus.
- 6 months: The corpus callosum is fully myelinated. The MRI appearance of the corpus callosum approaches the size seen in older children.
- 10–12 months: On T1WI, the brain appears fully mature (myelinated).
- 18–24 months: The process reaches the most anterior parts of the frontal and temporal lobes. By two years of age, the adult pattern is formed on T2WI.
Zones of terminal myelination: normal variants and pathology
Sometimes a hyperintense signal persists on T2WI in the peritrigonal areas (peritrigonal areas) or frontotemporal subcortical white matter even after 2 years of age. These are the so-called zones of terminal myelination, which can persist into adulthood and are considered a normal variant.
It is critically important to distinguish them from pathologies, such as periventricular leukomalacia (PVL). The main sign of normality is the presence of a thin dark rim (rim) of normally myelinated white matter between the bright terminal zone and the ventricular wall. In PVL, this rim is absent (the bright signal extends all the way to the ventricular margin), and the ventricles themselves are enlarged and have irregular contours due to tissue volume loss.
Development of the cortex and other structures
The process of gyration (sulcation and convolution formation) begins early. By the 16th gestational week, the interhemispheric and Sylvian fissures are visible, by the 22nd week the parieto-occipital sulcus appears, and by the 27th week — the central sulcus. Cortical folds form first in the visual and sensorimotor areas, while the frontal and temporal poles mature last.
The signal from the basal ganglia also changes during development. By 6 months, they darken on T2WI compared to the cortex due to myelination. By 9–10 years, the globus pallidus (globus pallidus) and substantia nigra (substantia nigra) acquire marked T2 hypointensity due to iron accumulation.
Briefly: answers to topical questions
What stages does cerebral cortical gyration go through during embryogenesis?
Cerebral cortical gyration during embryogenesis proceeds in a strict sequence, starting from a smooth hemispheric surface.
- 16 weeks — the interhemispheric fissure (interhemispheric fissure) and Sylvian fissures (Sylvian fissures) are formed.
- 22 weeks — the callosal sulcus (callosal sulcus) and parieto-occipital fissure (parieto-occipital fissure) are recognized.
- Subsequently — the cingulate (cingular) and calcarine (calcarine) sulci are formed.
- 27 weeks — the central sulcus (central sulcus) is visualized.
Convolutions in the sensorimotor areas and visual pathways form first, with the frontal and temporal poles being the slowest regions. By birth, the gyral pattern (gyral pattern) is almost identical to that of an adult.
What are the MRI signs of hypoxic-ischemic brain injury in premature newborns?
MRI signs of hypoxic-ischemic injury in premature newborns may include periventricular leukomalacia (PVL): periventricular hyperintense signal extending all the way to the ventricular margin, absence of the normal dark rim, enlarged posterior horns of the lateral ventricles, and irregular (scalloped) contours corresponding to white matter volume loss. Ischemic injury in premature infants can also cause cerebellar hypoplasia.
What is the normal thickness of the corpus callosum in children of various ages according to MRI?
Exact numerical values for corpus callosum thickness are not specified in the literature, but the chronology of its progressive thickening on sagittal T1WI is well-described.
- Up to 4–6 months — thickening of the splenium (splenium) occurs as visual pathways mature.
- By 6 months — the structure reaches dimensions approximately corresponding to normal sizes in older children.
- By 8–10 months — full thickness is attained, and the MRI appearance corresponds to that of an adult.
Normally, the corpus callosum continues to grow in thickness throughout the postnatal period, with the splenium being somewhat larger than the genu (genu).
Class and exam questions
Why are T1-weighted images specifically used to assess myelination in the first 6 months of life?
During this period, T1WI clearly demonstrates signal shortening (becoming bright) in areas of emerging myelin. T2WI signal changes require the accumulation of a larger volume of myelin, making this sequence insufficiently sensitive in the first few months.
In what direction does white matter maturation occur?
Myelination always proceeds centrifugally: from the posterior parts of the brain to the anterior, as well as from caudal structures (brainstem) to cranial and from the center to the periphery.
How to differentiate a normal terminal myelination zone from PVL on MRI?
Normally, a rim of dark (myelinated) white matter is always present between the bright terminal zone (on T2WI) and the lateral ventricular wall, and the ventricles themselves are not dilated. In PVL, the hyperintense signal merges with the ventricular margin, and its walls become scalloped.
What else to study on this topic
- Posterior fossa anomalies and cerebellar hypoplasia
- Infantile neuroaxonal dystrophy (INAD) and iron accumulation
- Septo-optic dysplasia (HESX1 gene mutation)
- Oculocerebrocutaneous syndrome and cortical dysplasia
- Differential diagnosis of adrenoleukodystrophy and PVL on MRI
Master the entire topic in the "Diagnostic Radiology" course
This is a summary. The course covers everything tested in classes and exams.
- Comprehensive study guide on "Normal Pediatric Brain Development" and 2,199 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 10 video lessons on the subject
Other Topics in "Pediatric Diagnostic Radiology"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.