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General Imaging Approach to Brain Malformations

General Imaging Approach to Brain Malformations

  • ·         Whenever an infant or child is referred for imaging because of either seizures or delayed development, a brain malformation is a possible cause.
  • ·         If the child appears dysmorphic in any way (low-set ears, abnormal facies, hypotelorism), the likelihood of an underlying brain malformation is even higher. In all such cases, imaging should be geared toward showing a structural abnormality.
  • ·         The imaging sequences should maximize contrast between gray matter and white matter, have high spatial resolution, and should be acquired as volumetric data whenever possible so that images can be reformatted in any plane or as a surface rendering.
  • ·         The high resolution and ability to reformat will aid in the diagnosis of subtle abnormalities.
  • ·         High-resolution T1-weighted volumetric images are essential for this purpose.
  • ·         If possible, volumetric T2-weighted images can be acquired, but the images must have excellent spatial resolution and sharp contrast between gray matter and white matter, which is not currently easy to achieve with volumetric T2-weighted sequences.
  • ·         If contrast between gray and white matter is poor with volumetric acquisition, acquire two dimensional sequences (2D) in at least two planes and with relatively thin (3-4 mm) section size.
  • ·         FLAIR images are not particularly useful in looking for malformations, as the contrast between gray matter and white matter is often poor.
  • ·         Diffusion-weighted images are not currently of diagnostic utility, although the use of diffusion tensor imaging (DTI) to acquire color fractional anisotropy (FA) maps and perform tractography is useful to better understand the connectivity of the malformed brain and may become clinically useful in the near future.
  • ·         After acquisition of appropriate images, image analysis must take place in an orderly manner.
  • ·         The midline structures (including cerebral commissures, septum pellucidum, nose and rhinencephalon, pituitary gland, and hypothalamus), the cerebral cortex (cortical thickness, gyral pattern, and corticalwhite matter junction), cerebral white matter (myelination, presence of nodules or clefts), the basal ganglia, the ventricular system (are all ventricles completely present and normally shaped), the interhemispheric fissure, and the midbrain hindbrain structures (brainstem and cerebellum) should all be scrutinized in every patient.
  • ·         Evaluate the midline structures first, as many disease processes of children take place in the midline, including anomalies of the cerebral commissures (corpus callosum, anterior commissure, and hippocampal commissure), midline tumors (suprasellar, pineal, brainstem, and fourth ventricle), anomalies of the cerebellar vermis, and anomalies of the craniocervical junction.
  • ·         Anomalies of the cerebral commissures are the most common of brain malformations; more than 130 syndromes involving them have been described. Many of these are associated with anomalies of the hypothalamus, so remember to always look at the hypothalamus and pituitary gland to ensure that the posterior pituitary gland is in the sella turcica and not in the median eminence of the hypothalamus.
  • ·         The midline leptomeninges are important in commisural development, so make sure to look for other anomalies associated with abnormal midline leptomeninges, such as interhemispheric lipomas and interhemispheric cysts when the commissures are absent or dysmorphic.
  • ·         Remember that large cerebrospinal fluid (CSF) spaces in the posterior fossa (mega cisterna magna) are often associated with anomalies of the cerebellum. The reason for this has only recently been discovered. Several cerebellar growth factors derive from the overlying leptomeninges. Therefore, abnormalities of the cerebellar leptomeninges may result in anomalies of the cerebellum itself, as well as abnormalities of the surrounding CSF spaces. This is the basis of development of the Dandy-Walker malformation: It requires abnormal development of the cerebellum itself and of the overlying leptomeninges.
  • ·         Midline and head size : Looking at the midline image also gives an idea of the relative head size by assessing the craniofacial ratio. In the normal neonate, the ratio of the cranial vault to the face on midline images is 5:1 or 6:1. By the age of 2 years, it should be 2.5:1, and by age 10 years, it should be about 1.5:1.
  • ·         After looking at the midline, evaluate the brain from outside to inside. Start with the cerebral cortex. Is the thickness normal (2-3 mm)? If it is too thick, think of pachygyria or polymicrogyria. Is the cortical-white matter junction smooth or irregular? If it is irregular, think of polymicrogyria or the cobblestone cortex seen associated with congenital muscular dystrophies such as muscle-eye-brain disease.
  • ·         The location of these abnormalities is important as well.
  • ·         Pachygyria more severe in the parietal and occipital lobes suggests a mutation of TUBA1A, whereas pachygyria worst in the frontal lobes suggests a mutation of DCX. Similarly, there are many different polymicrogyria syndromes that depend upon the location of the polymicrogyria: Bilateral frontal polymicrogyria is a different entity than bilateral perisylvian polymicrogyria or bilateral parasagittal parietooccipital polymicrogyria; it is important to be specific in reporting the location of the abnormality.
  • ·         If the cortex is abnormally thin, one should think of a prenatal injury (infectious or ischemic), particularly if the thinning is focal or multifocal.
  • ·         After the cortex, look at the cerebral white matter. Make sure myelination is appropriate for age (there are many sources of normal myelination charts, including journal articles and textbooks). Then, look for areas of abnormal myelination within the deep white matter.
  • ·         Diffuse layers of hypomyelination or amyelination associated with overlying polymicrogyria should raise suspicion for congenital cytomegalovirus infection.
  • ·         More localized foci of delayed or absent myelination are often seen in deep white matter of patients with congenital muscular dystrophy and in the subcortical white matter of those with focal cortical dysplasias (FCDs).
  • ·         With FCDs, the absent myelination may be localized to a gyrus or may extend centrally as a curvilinear cone-shaped abnormality coursing from the cortex to the superolateral margin of a lateral ventricle (this is known as the "transmantle" sign).
  • ·         Also, look for nodules of heterotopic gray matter in the periventricular or deep white matter.
  • ·         Subcortical heterotopia typically extend from the cortex all the way to the lateral ventricular wall, while periventricular nodular heterotopia are more localized to the immediate subependymal/periventricular region. Heterotopia might be difficult to differentiate from unmyelinated or injured white matter on T1-weighted images, so be sure to look at T2-weighted images or FLAIR images to ensure that the lesion is isointense to gray matter on all sequences.
  • ·         The basal ganglia are sometimes abnormal in disorders of neuronal migration, as they are formed from neurons generated in the medial and lateral ganglionic eminences, the same germinal zones that produce GABAergic neurons that migrate to the cerebral cortex. In particular, the basal ganglia tend to be dysmorphic in appearance in patients with subcortical heterotopia.
  • ·         In addition, the hippocampi are often abnormal in malformations of cortical development.
  • ·         In patients with lissencephaly, in particular, the hippocampi are incompletely folded. Sometimes, the only structural abnormalities in children with developmental delay arehippocampal; always look to make sure that they are fullyfolded and not too round.
  • ·         Always look at the entire interhemispheric fissure (IHF); if the cerebral hemispheres are continuous across the midline, holoprosencephaly should be diagnosed.
  • ·         In severe holoprosencephalies, the interhemispheric fissure is completely absent, whereas in milder forms of holoprosencephaly certain areas of the interhemispheric fissure will be absent (anterior IHF in semilobar holoprosencephaly, central IHF in syntelencephaly).
  • ·         Look at the septum pellucidum; absence of the septum is seen in corpus callosum dysgenesis/agenesis, septo-optic dysplasia, and in some cases of schizencephaly or bilateral polymicrogyria.
  • ·         While checking the septum, look at the lateral ventricles to ensure that they are normal in size and shape.
  • ·         Abnormally enlarged trigones and temporal horns are often associated with callosal anomalies and pachygyria.
  • ·         Enlarged frontal horns are often seen in bilateral frontal polymicrogyria.
  • ·         Don't forget to look carefully at the posterior fossa; anomalies of the brainstem and cerebellum are commonly overlooked.
  • ·         Make sure that the 4th ventricle and cerebellar vermis are normally sized.
  • ·         In newborns, the vermis should extend from the inferior colliculi to the obex, while infants and older children should have a vermis that extends from the intercollicular sulcus to the obex.
  • ·         Also, make sure you see normal vermian fissures. If the fissuration of the vermis looks abnormal, look at an axial or coronal image to make sure the vermis is present; if the cerebellar hemispheres are continuous without a vermis between them, make a diagnosis of rhombencephalosynapsis.
  • ·         If the 4th ventricle has an abnormal rectangular shape (with a horizontal superior margin) with a narrow isthmus and small vermis, think about a molar tooth malformation.To confirm this diagnosis, look for the molar tooth sign of the lower midbrain, consisting of large, horizontal superior cerebellar peduncles extending posteriorly toward the cerebellum, and a longitudinal cleft in the superior vermis.
  • ·         Make sure that the components of the brainstem are of normal size; in a child, the height of the pons should be double that of the midbrain on the midline sagittal image.
  • ·         An important clue can be provided by looking at the size of the pons compared to that of the cerebellar vermis.
  • ·         Since much of the anterior pons is composed of the decussation of the middle cerebellar peduncles, development hypoplasia of the cerebellum is nearly always associated with hypoplasia of the ventral pons.
  • ·         If the pons is normal in the setting of a small cerebellum, it is most likely that the cerebellum lost volume near the end of gestation or after birth.
  • ·         Remember that a small posterior fossa, intracranial hypotension, or intracranial hypertension can result in descent of the cerebellum below the foramen magnum.Look for causes of a small posterior fossa (clival anomaly, anomaly of the craniovertebral junction), intracranial hypertension (space-occupying mass, hydrocephalus), or evidence of intracranial hypotension (large dural venous sinuses, large pituitary gland, "slumping" brainstem) before making a diagnosis of Chiari 1 malformation.
  • ·         Finally, remember to look at the size of the CSF spaces in the posterior fossa, enlargement of which may be a sign of abnormal leptomeningeal development.



Midline sagittal T1WI MR shows classic findings of Dandy Walker spectrum with a large posterior fossa cyst , high torcular  and a small, upwardly rotated vermis . There is also a significant commissural anomaly with only a small corpus callosum remnant present . The rostrum and splenium are absent. The anterior commissure is present and appears normal.

T2WI in the previous case shows the 4th ventricle is open dorsally , contiguous with the huge posterior fossa cyst.

Sagittal T1WI shows a hypoplastic callosum rostrum and splenium, plus a small interhemispheric lipoma .

Sagittal T2WI shows a very small posterior fossa with a low-lying torcular and an elongated 4th ventricle that lacks a fastigium. This patient has a classic Chiari 2 malformation.
Midline  sagittal T2WI shows a normal sized posterior fossa. The cerebellar tonsils are pointed  and displaced inferiorly 1 cm below the foramen magnum. Note cord hyperintensity , suggesting a "pre-syrinx" state in this case of Chiari 1 malformation.

Axial T2WI in the previous case shows the mass-like thickening of the right medial parietal gray matter and distorted sulcal-gyral pattern of cortical dysplasia.

Axial NECT scan in an 18-year-old male with seizures shows a unilateral schizencephalic cleft extending from the pial surface of the brain to the ventricle. Note the characteristic CSF "nipple" at the ventricular margin. The cleft is lined by thickened, dysplastic gray matter .

 Axial T2WI shows bilateral schizencephalic clefts  lined by dysplastic gray matter . Note the abnormal cortical veins associated with the clefts.

 Axial T2WI MR allows analysis of midline and shows absent interhemispheric fissure in frontal lobes (white matter continuous across midline ). This finding, plus the absence of frontal horns, gives the diagnosis of holoprosencephaly.

 coronal T2WI shows a squared-off appearance to the lateral ventricles with inferiorly pointed frontal horns , absent septum pellucidum , and hypoplastic optic chiasm  characteristic of septooptic dysplasia.

Midline MRI of posterior fossa structures shows an upwardly convex superior 4th ventricle and a dysplastic-appearing vermis.

Axial T2WI in the previous case shows the elongated 4th ventricle , cleft vermis , and thickened, horizontally oriented superior cerebellar peduncles forming the classic molar tooth sign of Joubert syndrome.

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