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(35-10A) Sagittal midline T1WI shows classic callosal agenesis. The anterior commissure is present, as is a tiny remnant of the genu .

(35-10B) Coronal T2WI shows Probst bundles indenting the lateral ventricles. Note vertically oriented hippocampi .

Imaging Approach to Brain Malformations

Technical Considerations

CT

Clinicians sometimes order NECT scans as an initial screening procedure in a patient with seizures or suspected brain malformation. Although parenchymal calcifications, ventricular size/configuration, and major abnormalities can be identified, subtle abnormalities such as cortical dysplasia are difficult to detect and easy to overlook.

Bone CT is helpful in depicting midline facial defects, synostoses, and anomalies of endochondral bone.

MR

MR is the procedure of choice. The two most important factors are graywhite matter differentiation and high spatial resolution. Many pediatric neuroradiologists recommend a sagittal T1 or T1 FLAIR sequence, volumetric T1 sequences (e.g., MP-RAGE), and sagittal and coronal heavily T2-weighted sequences with very long TR/TEs. 3D imaging acquisitions allow isotropic orthogonal reformations.

A T2* sequence (GRE, SWI) can be a helpful addition if abnormal mineralization or vascular anomaly is suspected. DTI tractography is valuable when commissural anomalies are identified on initial sequences.

Contrast-enhanced T1 and FLAIR sequences are generally optional, as they add little useful information in most congenital malformations. However, they can be very helpful in delineating associated vascular anomalies. DWI and MRS are useful in evaluating mass lesions and inborn errors of metabolism.

Image Analysis

The following approach to analyzing imaging studies is modified and adapted from A. James Barkovich's guidelines on imaging evaluation of the pediatric brain (see reference cited in Chapter 36).

(35-11) Axial T2WI shows "cobblestone" lissencephaly in both occipital poles . (Courtesy M. Warmuth-Metz, MD.)

Sagittal Images

Begin with the midline section, and examine the craniofacial proportion. At birth, the ratio of calvaria to face should be 5:1 or 6:1. At 2 years, it should be 2.5:1. In adults and children over the age of 10 years, it should be approximately 1.5:1. Assess myelination of midline structures such as the corpus callosum and brainstem.

The most common of all brain malformations are anomalies of the cerebral commissures (especially the corpus callosum), which can be readily identified on sagittal T1 scans (35-10A). Commissural anomalies are also the most common malformation associated with other anomalies and syndromes, so, if you see one, keep looking! Look for abnormalities of the pituitary gland and hypothalamus. Evaluate the size and shape of the third ventricle, especially its anterior recesses.

Look for other lesions such as lipomas and cysts. These are often midline or paramidline and can be readily identified. The midline sagittal scan also permits a very nice evaluation of the posterior fossa structures. Does the fourth ventricle appear normal? Can you find its dorsally pointing fastigium?

Evaluate the position of the tonsils and the craniovertebral junction for anomalies.

If the lateral and third ventricles are large and the fourth ventricle appears normal, look for a funnel-shaped aqueduct indicating aqueductal stenosis. If you see aqueductal stenosis, look at the quadrigeminal plate carefully to see whether the cause might be a low-grade tectal glioma.

Sagittal images are also especially useful in evaluating the cerebral cortex. Is the cortex too thick? Too thin? Irregular? "Lumpy-bumpy"? Anomalies of cortical development such as pachygyria and cortical dysplasia associated with brain clefting ("schizencephaly") are often most easily identified on sagittal images. Finally, note position and size of the vein of Galen, straight sinus, and torcular Herophili.

Coronal Images

Cortical dysplasias are often bilateral and most frequently cluster around the sylvian fissure. Coronal scans make side-to-side comparison relatively easy. Follow the interhemispheric fissure (IHF) all the way from front to back. If the hemispheres are in contiguity across the midline, holoprosencephaly is present. If the IHF appears irregular and the gyri "interdigitate" across the midline, the patient almost certainly has a Chiari 2 malformation.

Evaluate the size, shape, and position of the ventricles. If the third ventricle appears "high riding" and the frontal horns of the lateral ventricles look like a "Viking helmet," corpus callosum dysgenesis is present (35-10B).

If the frontal horns appear squared-off or box-like, look carefully for an absent cavum septi pellucidi. Absent cavum septi pellucidi is seen in septooptic dysplasia (SOD) and often occurs with callosal dysplasia or schizencephaly. If absent septi pellucidi is noted, look for fusion of the anterior columns of the fornix

Carefully evaluate the temporal horns and hippocampi to make sure that they are normally folded and oriented horizontally (not vertically, as often occurs with holoprosencephaly, lissencephaly, callosal anomalies, and malformations of cortical development).

Axial Images

The combination of a true T1WI together with a long TR/TE T2WI is necessary in evaluating all cases of delayed development to assess myelin maturation. The thickness and configuration of the cortical mantle are well seen (35-11). The size, shape, and configuration of the ventricles is easily evaluated on these sequences.

After 6-8 months of age, FLAIR sequences are especially useful in evaluating abnormalities such as focal or Taylor cortical dysplasia and the flame-shaped subcortical WM hyperintensities seen in tuberous sclerosis complex (35-12).

Don't forget the posterior fossa! The fourth ventricle in axial plane is normally shaped like a kidney bean on its side. If the vermis is absent and the cerebellar hemispheres appear continuous from side to side, rhombencephalosynapsis is present (35-13). If the fourth ventricle and superior cerebellar peduncles resemble a molar tooth, then a molar tooth malformation is present (35-14).

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(35-12) FLAIR shows subependymal nodules and cortical tubers with subcortical hyperintensities of tuberous sclerosis.

(35-13) T2WI of rhombencephalosynapsis shows continuity of cerebellar hemispheres across the midline . (Courtesy M. Warmuth-Metz, MD.)

The cerebellum is one of the earliest cerebral structures to develop. Its development is also unusually protracted as cellular proliferation, migration, and maturation extend into the first few postnatal months. It is therefore particularly vulnerable to development mishaps.

Neural structures in the posterior fossa are derived from the embryonic hindbrain (rhombencephalon), whereas the mesencephalon gives rise to midbrain structures. Mesodermal elements give rise to the meninges and bone that surround and protect the neural structures. Developmental errors in either give rise to the spectrum of midbrain and hindbrain malformations that we will discuss in this chapter. A summary of the imaging findings of these malformations is presented at the end of this chapter (Table 36-1).

We begin our discussion of posterior fossa malformations with the anomalies known as Chiari malformations. Moving to the hindbrain, we consider the Dandy-Walker spectrum and a group of miscellaneous malformations.

We review the normal posterior fossa anatomy as the foundation for understanding these lesions. Some structures (e.g., the fourth ventricle, arteries, dural venous sinuses, cranial nerves) have been discussed in detail in previous chapters. Here, we summarize the major anatomic features specifically as they relate to the posterior fossa itself.

Posterior Fossa Anatomy

Gross Anatomy

The posterior fossa (PF) is the largest and deepest of all the cranial fossae. It is a bowl-shaped, relatively protected space that lies below the tentorium. The PF contains the hindbrain with the brainstem (pons, medulla) anteriorly, the vermis, and the cerebellar hemispheres posterolaterally.

The midbrain lies within the tentorial incisura. The midbrain represents the transition between the cerebral hemispheres above and the pons and cerebellar hemispheres below the tentorium.

PF CSF-containing spaces include part of the cerebral aqueduct, the fourth ventricle, and CSF cisterns that surround the brainstem and cerebellum.

Bone and Dura

(36-1) Graphic shows anterior PF bordered by clivus . Note rounded tonsil . Nucleus gracilis , junction between 4th ventricle obex, and central canal lie above foramen magnum. Primary fissure of vermis lies along tentorial surface.

the tent-shaped tentorium cerebelli covers the PF superiorly. The PF communicates superiorly with the supratentorial compartment through the U-shaped tentorial incisura and inferiorly with the cervical subarachnoid space through the ovoid foramen magnum.

A layer of dura with a loosely adherent arachnoid membrane lines the bony part of the PF. The cranial dura has two layers, an inner (meningeal) and an outer (endosteal) layer, that are fused together except where they separate to enclose the dural venous sinuses.

The meningeal layer of the dura covers the PF with two prominent crescentic infoldings, the leaves of the tentorium cerebelli, that separate the infrafrom the supratentorial compartments. A large U-shaped central opening, the tentorial incisura, contains the midbrain. Variable amounts of the upper cerebellar hemispheres and vermis project into the tentorial hiatus behind the midbrain.

The convex outer margins of the dura split posteriorly along the occipital squamae to contain the sinus confluence (torcular herophili) and transverse sinuses, attaching laterally to the temporal bones and posteriorly to the occipital bone. The falx cerebelli consists of one or more small crescentic folds of dura that project into the cisterna magna and attach superiorly to the undersurface of the tentorium.

The dura divides into two distinct layers as it passes inferiorly through the foramen magnum into the upper cervical canal. The endosteal layer becomes the periosteum of the vertebral canal, and the meningeal layer becomes the dura of the thecal sac. In the spine, the two layers are separated by fat, the epidural venous plexus, and loose connective tissue.

(36-2) Graphic shows that normal tegmento-vermian angle (yellow) should be ≤ 18°. The fastigium-declive line (blue) extends from the fastigium of the 4th ventricle to the declive. Approximately 50% of the vermis should lie below this line.

Brainstem

The brainstem has three anatomic divisions: the midbrain, pons, and medulla. The midbrain (mesencephalon) lies partly above and partly below the tentorium. It courses through the tentorial incisura, connecting the pons and cerebellum with the basal forebrain structures and cerebral hemispheres.

The bulb-shaped pons nestles into the gentle curve of the clivus (36-1). Its ventral aspect contains both transverse pontine fibers and the large descending white matter (WM) tracts that are continuous with the cerebral peduncles superiorly and the medullary pyramids inferiorly. Its dorsal part—the tegmentum—is common to all three brainstem structures (midbrain, pons, medulla) and contains the reticular formation and multiple cranial nerve nuclei.

The medulla is the most caudal brainstem segment and represents the transition from the brain to the spinal cord. Its ventral (anterior) segment contains the olives and pyramidal tracts. An important imaging landmark is the prominent "bump" along the dorsal medulla created by the nucleus gracilis. This demarcates the junction between the fourth ventricle (obex) and central canal of the spinal cord. The nucleus gracilis normally lies above the foramen magnum.

Cerebellum

The cerebellum is a bilobed structure located posterior to the brainstem and fourth ventricle. It consists of two hemispheres and the midline vermis.

Each cerebellar hemisphere has three surfaces: superior (tentorial), inferior (suboccipital), and anterior (petrosal). The superior surface abuts the undersurface of the tentorium. The

(36-3) Sagittal T2WI shows normal PF imaging landmarks: nucleus gracilis with junction of the 4th ventricle and central canal of the spinal cord , fastigium of the 4th ventricle , and primary fissure of the vermis .

inferior surface is mostly bordered by the occipital squamae, and the anterior surface lies along the posterior wall of the petrous temporal bone.

Fissures divide the cerebellum into lobes and lobules. The most prominent is the large horizontal fissure. This deep cleft wraps around the cerebellum and separates its superior from the inferior surfaces. The obliquely oriented primary fissure divides the cerebellum into anterior and posterior lobes. Smaller fissures subdivide the lobes into lobules.

Prominent superficial landmarks of the cerebellar hemispheres include the cerebellar tonsils, which extend inferomedially from the biventral lobules (36-1). A small nubbin of tissue, the flocculus, lies below each middle cerebellar peduncle and projects anteriorly into the cerebellopontine angle cistern.

Three paired peduncles attach the cerebellar hemispheres to the brainstem. The superior cerebellar peduncles (brachium conjunctivum) connect the cerebellum to the cerebral hemispheres via the midbrain. The superior cerebellar peduncles contain efferents to the red nucleus and thalamus.

The middle cerebellar peduncles (brachium pontis) connect the cerebellum to the pons and represent the continuation of the corticopontine tracts. The inferior cerebellar peduncles (also known as the restiform bodies) connect the cerebellum to the medulla and contain spinocerebellar tracts and tracts to the vestibular nuclei.

The vermis lies between both cerebellar hemispheres, behind the fourth ventricle. Its lobules are (moving clockwise from the fourth ventricle roof) the lingula, central lobule, culmen, declive, folium, tuber, pyramid, uvula, and nodulus. The prominent primary fissure continues across the vermis from

(36-4) Slightly more lateral scan reveals the horizontally oriented folia , rounded bottom of the tonsil , and the horizontal fissure .

the cerebellar hemispheres and separates the culmen from the declive. With the exception of the lingula, each vermian lobule is also in direct contiguity with an adjoining lobule of the cerebellar hemisphere.

The cerebellar cortex is a continuous sheet of tissue that is folded in accordion-like fashion to form a series of prominent ridges. The cortex has three main layers. The molecular layer is the most superficial and is a relatively neuron-sparse layer. The Purkinje cell layer primarily contains Purkinje cells, which are arranged in a single row between the more superficial molecular layer and the deeper granular layer. The granular layer is the most complex and most cellular, containing the bodies and axons of granular neurons.

Fourth Ventricle and Cisterns

Anatomy of the fourth ventricle is delineated in more detail in Chapter 34.

The fourth ventricle is a complex diamond-shaped space that runs along the dorsal pons and upper medulla. Important anatomic landmarks are the dorsally pointed fastigium, the midline foramen of Magendie (the outlet from the fourth ventricle into the cisterna magna), and the paired lateral recesses that empty into the cerebellopontine angle (CPA) cisterns via the foramina of Luschka. Choroid plexus extends from the 4th ventricle through its lateral recesses, protruding into the adjacent CPA cisterns. Bulbous tufts of choroid plexus in the CPA cistern are eponymously named "Bochdalek's flower basket" and are a normal finding on imaging studies.

The posterior superior recesses are thin, blind-ending, earlike outpouchings that curve over the tops of the cerebellar tonsils. The obex is the inferior extension of the fourth

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ventricle and communicates directly with the central canal of the cervical spinal cord.

The major PF cisterns are the prepontine cistern, the cerebellopontine angle cistern, and the variably sized cisterna magna. Part of the cisterna magna (the vallecula) extends superiorly between the two cerebellar tonsils and is connected to the fourth ventricle via the foramen of Magendie.

Arteries, Veins, and Dural Sinuses

The arteries of the PF are detailed in Chapter 8; the veins and dural venous sinuses are discussed in Chapter 9.

Cranial Nerves

The cranial nerves—together with the cisterns through which they course and the bony foramina through which they enter

(36-5) Axial T2WI shows normal superior cerebellar peduncles , vermis , and horizontal fissures of the cerebellum. (36-6) Axial scan through the body of the fourth ventricle shows CSF-filled posterior superior recesses capping the tops of the cerebellar tonsils . Dentate nucleiare mineralized and hypointense.

(36-7) More inferior scan through the bottom of the fourth ventricle shows the midline foramen of Magendie , lateral recesses , tonsils , and floccular lobes of the cerebellum projecting into cerebellopontine angle cisterns. (36-8) T2WI through the foramen magnum shows the medulla , cerebellar tonsils , and vallecula lying between the tonsils at the bottom of the cisterna magna .

or leave the cranial cavity—are discussed in detail in Chapter 23.

Imaging Anatomy

Sagittal Plane

Midline images show the smooth floor of the fourth ventricle extending from the cerebral aqueduct above to the obex below. The junction of the obex and central canal is marked by a dorsal bump, the nucleus gracilis (36-3). The nucleus gracilis normally lies above a line drawn between the tip of the clivus anteriorly and the rim of the foramen magnum posteriorly ("basion-opisthion line").

The sharply pointed fastigium forms a triangle of CSF whose apex points toward the vermis. The primary fissure of the vermis faces the tentorium, dividing the culmen from the declive. Approximately 50% of the vermis should lie below a line from the fastigium to the declive (36-2).

Just slightly lateral to the midline, the cerebellar tonsils can be identified as ovoid structures lying between the vermis and inferior fourth ventricle. Normal tonsils display horizontally oriented folia and a gently rounded bottom (36-4). More lateral sections through the hemispheres show the dentate nuclei, the brachium pontis, and the primary fissure of the cerebellum.

Axial Plane

Images through the upper PF show the vermis and superior surfaces of the cerebellar hemispheres lying behind the pons and midbrain, just inside the tentorial incisura. Slightly farther down, the superior cerebellar peduncles are seen as thin white matter bands lying along either side of the upper fourth ventricle (36-5).

At the level of the middle cerebellar peduncles, the body of the fourth ventricle resembles a kidney bean on its side. The two bumps along its anterior aspect are the facial colliculi, and

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the midline posterior bump is the nodulus of the vermis. Sometimes, the thin posterior superior recesses can be seen capping the tops of the cerebellar tonsils (36-6). Anterolaterally, a flocculus projects from each hemisphere into the cerebellopontine angle cistern.

Moving inferiorly, the lateral recesses of the fourth ventricle pass anterolaterally under the middle cerebellar peduncles (36-7). Tufts of choroid plexus pass through the lateral recesses and the foramina of Luschka into the inferior cerebellopontine angle cisterns just medial to the flocculi.

The cerebellar tonsils can be identified just above or at the foramen magnum. The vallecula—part of the cisterna magna that receives the midline foramen of Magendie—is the CSF space that lies between the tonsils (36-8). The medulla lies just in front of (and medial to) the cerebellar tonsils. The anterior medulla is marked by paired "bumps" of tissue: the pyramids and the olives.