Neuro

Diffusion-Weighted Imaging

Diffusion-weighted imaging (DWI) is determined by the variability in the diffusivity of water molecules in tissues. The random movement of water molecules is known as Brownian motion. In tissues, there is variable restriction of the movement of water molecules relative to tissue structure. Pathologic states may alter the diffusion characteristics of water molecules in the brain and therefore affect the appearance of the DWI images. Images on DWI are influenced not only by diffusion of water molecules but also by other tissue properties, such as T2. To eliminate the influences on imaging appearance of factors unrelated to diffusion of water molecules, the data are processed, most commonly by using a technique called an apparent diffusion coefficient (ADC) map . For each DWI sequence, at least two sets of images are created: the DWI images and the ADC map. In a pathologic process in which there is true restricted diffusion, the involved area appears high in signal on the DWI images and low in signal on the ADC map ( Fig. 8-1A and B ). Conversely, in pathologic processes in which there is facilitated diffusion, signal will be low on DWI images and high on the ADC map. Due to the influences of T2 tissue properties on DWI imaging, an area of increased signal on T2-weighted sequences (including T2 fluid-attenuated inversion recovery sequences) may show as increased DWI signal and not represent true diffusion restriction. In this case, signal will be normal or increased on ADC map, instead of decreased. This phenomenon is known as T2 shine-through ( Fig. 8-2A–C ) and should not be confused with restricted diffusion.

Restricted diffusion is seen in pathologic processes that present with cytotoxic edema including brain infarct and/or ischemia, nonhemorrhagic traumatic injury, seizure-related edema, and encephalitis. Other causes of diffusion restriction include purulent fluid collections ( Fig. 8-3A–C ), epidermoid cysts, subacute hematoma, and hypercellular tumors. Processes that show facilitated diffusion include vasogenic edema and cerebrospinal fluid (CSF) collections, such as an arachnoid cyst.

DWI has become an extremely important tool in neuroimaging. One of its major advantages is its high sensitivity for ischemia and infarction—often revealing findings not yet evident on conventional magnetic resonance (MR) sequences. It is also helpful in differentiating among posterior fossa tumors. Hypercellular medulloblastomas have restricted diffusion, whereas low-grade astrocytomas typically do not. Restricted diffusion is also rare in ependymomas. Table 8-1 shows a glossary of terms related to DWI and diffusion tensor imaging (DTI).

Diffusion Tensor Imaging and Tractography

Diffusion is anisotropic in white matter fibers, as axonal membranes and myelin sheaths act as barriers to the motion of water molecules in directions not parallel to their own orientation. By applying diffusion gradients in multiple directions during a diffusion MRI sequence, DTI technique enables the identification of the direction of maximum diffusivity in a voxel and therefore the identification of location, orientation, and directionality of the white matter tracts. DTI provides several capabilities not previously possible. (1) Large, individual white matter tracts can be depicted as discrete anatomic structures. The three-dimensional postprocessing technique used to create such images is tractography. (2) Metrics describing the microarchitecture of tissue can be calculated. They include fractional anisotropy, mean diffusivity, radial diffusivity, and axial diffusivity.

Tractography images are typically shown in color. By convention, white matter tracts oriented left to right are shown as red, cephalocaudad as blue, and anteroposterior as green ( Fig. 8-4 ).

DTI and tractography can be used to evaluate myelination. Increased myelination increases anisotropy. Therefore, in normal infants, anisotropy increases with age. Most disease states that affect white matter decrease anisotropy. DTI has been used to evaluate stroke, hydrocephalus, trauma, and demyelinating disorders. DTI and tractography have also been used to study and depict the white matter tract abnormalities associated with congenital brain anomalies and in the presurgical evaluation of brain tumors ( Fig. 8-5 ). One important limitation of DTI-based tractography is that it underestimates the white matter tracts in parenchymal locations where there are crossing fibers. This is because a single tensor can resolve only a single fiber direction within a voxel. Recent studies have described new diffusion-based nontensor tractography techniques that demonstrate fiber tracts more accurately, but these are not yet being used in the routine clinical practices.

Perfusion

Perfusion MRI evaluates cerebral microvascular parameters, including cerebral blood volume, mean transit time, and cerebral blood flow. It relies on the use of a tracer that may be exogenous (gadolinium) in dynamic susceptibility contrast-enhanced and dynamic contrast-enhanced techniques or endogenous as in noncontrast arterial spin labeling technique. The main applications of perfusion MRI are evaluation of vascular pathologies (ischemic strokes, vasculopathies, vasospasm) and tumors. In the evaluation of brain tumors, perfusion may help with preoperative tumor grading and biopsy guidance by identifying high perfusion areas suspicious to represent higher grade portions of the tumor. Perfusion also plays a role in the evaluation of tumor treatment response and in the attempt to answer the always difficult question of tumor recurrence versus radiation necrosis.

Magnetic Resonance Spectroscopy

Proton (hydrogen) MR spectroscopy (MRS) is a tool that is used to help to characterize a number of pediatric neurologic conditions. Most often, single-voxel spectroscopy is created using a 1-cubic centimeter sample area. The results of spectroscopy are typically depicted as a spectrum, with each metabolic peak characterized by resonance frequency, height, width, and area ( Fig. 8-6 ). Metabolic profiles depicted on MRS have been much less specific than originally hoped. However, the information obtained can provide useful information that complements the information derived from conventional MRI in many pediatric disorders, including neoplasms, hypoxic ischemic insult, and metabolic diseases.

The most commonly evaluated brain metabolites include N -acetylaspartate (NAA), creatine (Cr), choline (Cho), myoinositol (mI), and lactate (Lac) (see Fig. 8-6 ). NAA is a neuronal marker and is decreased in most disorders that decrease neuronal viability and density, such as neoplasms ( Fig. 8-7 ), infarcts, radiation injuries, and many white matter diseases. NAA is markedly elevated in Canavan disease and may also be elevated in Salla disease and Pelizaeus–Merzbacher disease. Cho compounds reflect the synthesis and degradation of cell membranes; therefore, increased Cho is seen when there is high cellular turnover, as occurs with most tumors (see Fig. 8-7 ). Lac is a product of anaerobic glycolysis, typically elevated in the setting of acute ischemia ( Fig. 8-8 ), infarction, abscess, many high-grade tumors, and some metabolic diseases, such as mitochondrial disorders. mI is considered a glial marker, elevated in gliosis and astrocytosis in pediatrics. Cr is relatively constant; therefore, it is used as an internal reference for calculation of metabolite ratios. Cr deficiency syndromes are rare causes of decreased Cr levels. Taurine (Tau) is not a commonly seen metabolite in clinical MRS; however, it has been shown to be elevated in medulloblastoma and therefore is an important metabolite to recognize in pediatric posterior fossa tumors (at 3.3 ppm, above baseline on short TE 30–35 and below baseline on TE 144).

There are differences in metabolite levels in the gray and white matter. The concentration of NAA in gray matter is higher than in white matter, and Cho concentration is typically higher in white matter. There are also differences in metabolite levels in the developing brain, especially during the first year of life. In the newborn period, NAA levels are low and mI and Cho levels are high ( Fig. 8-9 ). Small Lac peak can also be found in healthy newborns.

Functional Magnetic Resonance Imaging

Functional MRI (fMRI) is a noninvasive method of evaluating regional neuronal activity within the brain. Neuronal activity increases metabolic activity, which results in increased blood flow to that region and a relative increase in the ratio of oxygenated hemoglobin to deoxygenated hemoglobin. Deoxyhemoglobin is paramagnetic; therefore, a change in the ratio affects the magnetic state of the tissue and, as a result, changes the local MRI signal. This phenomenon is called the blood oxygenation level–dependent (BOLD) effect. fMRI techniques show these changes superimposed on anatomic information. fMRI is used to document regional neuronal activity during a specific task. The most validated use is for localization of different body representations in the primary motor and somatosensory cortex, as well as language lateralization ( Fig. 8-10A–F ). Clinically, fMRI has been used in the evaluation of patients with seizures and planning of surgical approaches to brain tumors. It also has been a very useful research tool in increasing our understanding of brain function. Current research is mostly focused on resting state techniques (R-fMRI), which measure BOLD fluctuations of spontaneous neuronal activity without the performance of a specific task.

Germinal Matrix Hemorrhage

The germinal matrix is a fetal structure that is a stem source for neuroblasts. It typically involutes by term but is still present in premature infants, mostly within the caudothalamic groove (the space between the caudate head and the thalamus). The germinal matrix is highly vascular and is subject to hemorrhaging with fluctuation in cerebral blood pressure. Germinal matrix hemorrhage most commonly occurs in premature infants during the first days after birth. Potential complications include destruction of the precursor cells within the germinal matrix, hydrocephalus, and hemorrhagic infarction of the surrounding periventricular tissues. Cerebellar hemorrhage can also be seen in very premature patients ( Fig. 8-13A and B ).

On ultrasound, germinal matrix hemorrhage is seen as an ovoid echogenic mass within the caudothalamic groove ( Figs. 8-14A and B ). For those not well acquainted with head ultrasound, there may be confusion in differentiating germinal matrix hemorrhage from the normally echogenic choroid plexus. In contrast to germinal matrix hemorrhage, normal choroid plexus should never extend as anterior as the caudothalamic groove on a parasagittal view ( Fig. 8-11B ). Hemorrhage may extend into the ventricular system and lead to hydrocephalus. Germinal matrix hemorrhage is categorized into one of four grades ( Table 8-2 ). Intraparenchymal hemorrhage (grade IV) is secondary to venous infarction of the deep medullary veins ( Fig. 8-15A and B and Fig. 8-16 ) rather than a direct extension of hemorrhage. Grades I and II hemorrhages tend to have good prognosis. In contrast, grades III and IV hemorrhages tend to have poor prognosis, including high incidences of neurologic impairment, hydrocephalus, and death.

Benign Enlargement of Extraaxial Spaces

Benign enlargement of extraaxial spaces (benign macrocrania) is a diagnosis of exclusion. The term refers to children with large heads (head circumference greater than 97% for age), prominent subarachnoid spaces, and normal development. Typically, such children present between 6 months and 2 years of age. After 2 years of age, the head size usually normalizes, and the children have no long-term consequences. The parents of such children often have large heads or had large heads as infants. On imaging, there is a prominent size of subarachnoid spaces and normal to borderline prominent ventricular system ( Fig. 8-17A–C ). Imaging is otherwise normal. Clinical follow-up is necessary to exclude loss of milestones or continued increase in head circumference that may indicate communicating hydrocephalus.

White Matter Injury of Prematurity and Hypoxic Ischemic Insult

Perinatal partial asphyxia in premature neonates can result in damage to the periventricular white matter, the watershed zone of the premature infant, and result in white matter injury of prematurity (periventricular leukomalacia). It most commonly affects the white matter adjacent to the atria, frontal horns, and corona radiata along the lateral ventricles ( Fig. 8-22 ). On ultrasound, increased heterogeneous echogenicity is seen within the periventricular white matter ( Fig. 8-23A and B ). It is associated with neurologic sequelae, such as movement disorders, seizures, and spasticity. In severe cases, there may be cystic necrosis and development of periventricular cysts during the subacute and chronic phases ( Fig. 8-24 ). With time, there is often volume loss of the involved white matter ( Fig. 8-25A and B ). Doppler interrogation of the circle of Willis may add value in the evaluation of acute ischemia and increase intracranial pressure. Premature and full-term infants exhibit a generally accepted resistive index range of approximately 0.6–0.9, in the absence of cardiac disease or peripheral vascular disease. Lower values may indicate acute hypoxia or ischemia, as a result of increased diastolic flow through cerebral vasodilation. Higher values may suggest cerebral swelling where intracranial pressures rise higher than systemic pressures, leading to decreased diastolic flow.

Partial asphyxia in term neonates and older children will affect the border zones (watershed zones) between the anterior and middle and/or middle and posterior circulations (see Fig. 8-31A and B ).

Profound prolonged asphyxia affects the most metabolic active areas of the brain, especially the basal ganglia and perirolandic regions, in both preterm and term neonates. Findings on MRI include symmetric increase T1 and T2 signal, especially along the posterior lentiform nuclei (putamen and globus pallidi) and the ventrolateral thalami. Diffusion restriction is often present if the patient is imaged after 24 h and prior to 10–14 days after the insult. In term neonates (above 36 weeks of gestation), another early finding of profound hypoxic ischemic insult includes absent visualization of the T1 hyperintense signal of the myelinated posterior limb of the internal capsule ( Fig. 8-26A and B ). The recent use of hypothermia to treat neonates with hypoxic ischemic insult has led to a decreased number of positive MRI cases and, when positive, the resultant imaging findings may be delayed. A mixed pattern involving both the basal ganglia and white matter can also be present. Head ultrasound may identify early signs of hypoxic ischemic insult, such as accentuated gray–white matter differentiation, slit-like ventricles secondary to edema, and blurring of interhemispheric fissure ( Fig. 8-27 ). Chronic changes of profound hypoxic ischemic insult include signal abnormality and volume loss of the posterior lentiform nuclei and ventrolateral thalami ( Fig. 8-28 ) with or without associated white matter volume loss.

Imaging mimickers of hypoxic ischemic insult in encephalopathic neonates include neonatal hypoglycemia and metabolic diseases such as molybdenum cofactor deficiency ( Fig. 8-29A and B ), pyruvate dehydrogenase deficiency, nonketotic hyperglycinemia (will have elevated glycine peak 3.55 ppm), urea cycle disorders, and aminoacidurias. Neonatal hypoglycemia presents with diffusion restriction along the posterior parieto-occipital regions and can be confirmed clinically with glucose levels ( Fig. 8-29A ). Metabolic diseases mimicking hypoxic ischemic insult tend to present after a week or more of life, different from perinatal hypoxic ischemic insult, which usually presents during the first 24 h of life.

Porencephaly and Encephalomalacia

Before the end of the second trimester, parenchymal injury does not result in glial scar formation. During this early time period, focal injury results in the development of a fluid-filled space called a porencephalic cyst . On imaging, this appears as a thin-walled, CSF-containing cyst that may communicate with the ventricles ( Fig. 8-30 ). During the third trimester and neonatal period, parenchymal injury incites some glial scar formation, resulting in softening of the brain parenchyma (encephalomalacia) and tissue loss with or without reactive gliosis. Encephalomalacia appears as areas of high T2-weighted signal frequently with associated septations in the region of injury ( Fig. 8-31A and B ).

Hydranencephaly

Hydranencephaly is the result of significant injury to the majority of the cerebral hemispheres secondary to an in utero destructive process. The cause is thought to be vascular in origin with involvement of both internal carotid arteries. Infectious vasculitis has been identified as a potential cause. The supratentorial compartment is almost entirely filled with CSF. A few areas of preserved parenchyma may be seen along the convexities and thalami ( Fig. 8-32A–C ). The brainstem is usually hypoplastic due to decreased descending white matter tracts, and the cerebellum is typically intact. These structures remain because they are supplied by the vertebrobasilar arterial system. The presence of the falx cerebri and the separation of the thalami seen in hydranencephaly (see Fig. 8-32B ) help to differentiate this disorder from severe (alobar) holoprosencephaly. It is sometimes very difficult and impossible to differentiate hydranencephaly from severe hydrocephalus ( Fig. 8-33 ).

Chiari

Chiari Type 1

Chiari type 1 is the presence of an abnormal inferior location of the cerebellar tonsils, at least 5 mm below the foramen magnum ( Fig. 8-34 ). The cerebellar tonsils are typically elongated in morphology rather than round, and there are different degrees of CSF effacement at the craniocervical junction. The medulla and fourth ventricle are commonly in normal positions although low position of the cervicomedullary junction may be seen in cases of complex Chiari 1 (also known as Chiari 1.5) which are often associated with a posteriorly tilted dens of C2 and sometimes with a hypoplastic clivus and basilar invagination. Complications of Chiari type 1 include hydrocephalus and hydromyelia (see Fig. 8-34 ). It may be suspected on CT when the foramen magnum appears “full” with soft tissue ( Fig. 8-35 ). It is best visualized on sagittal MR images. Phase-contrast CSF flow MR sequence may be used to show CSF space effacement and pistoning movement of the cerebellar tonsils. Chiari 1 may be acquired with increase intracranial pressure and may also resolve with child growth, therefore not always a true malformation.

Chiari Type 2

Chiari type 2 is a true malformation of hindbrain and is almost always associated with spinal myelomeningoceles. Conversely, almost all patients with myelomeningoceles have Chiari type 2 malformation. Patients with this abnormality have small posterior fossa and associated inferior displacement of the cerebellum, medulla, and fourth ventricle into the upper cervical canal ( Fig. 8-36 ). Associated imaging findings include a kinked appearance of the medulla, colpocephaly (disproportionate enlargement of the occipital horns of the lateral ventricles), fenestration of the falx associated with interdigitating gyri across the midline, enlargement of the massa intermedia, inferior pointing of the lateral ventricles, and tectal beaking (a pointed appearance of the quadrigeminal plate). Chiari type 2 malformations are usually associated with hydrocephalus, often diagnosed prenatally ( Fig. 8-37A and B ).

Prenatal correction of myelomeningocele can be performed in a subset of fetuses, following specific inclusion criteria. The aim is to decrease the degree of hindbrain herniation and the postnatal need of ventricular shunting, as well as to improve lower extremity function ( Fig. 8-38A and B ).

Chiari type 3 is very rare and includes an occipital encephalocele or upper cervical dysraphism instead of a lower spinal myelomeningocele.

Holoprosencephaly

Holoprosencephaly results from lack of complete cleavage of the brain into two hemispheres. Although there is a continuous spectrum of severity, holoprosencephaly is classically classified into one of three distinct groups: alobar, semilobar, or lobar. The severity of the brain abnormality is usually reflected in the severity of the associated midline facial abnormality.

Alobar holoprosencephaly ( Fig. 8-39A and B ) is the most severe form and is characterized by a monoventricle. The thalami are fused and there is no attempt at cleavage of the cerebral hemispheres. There is no falx cerebri or corpus callosum. There is a single anterior cerebral artery (azygous artery). These infants are commonly stillborn or have a very short life span.

With the intermediate form, semilobar holoprosencephaly ( Fig. 8-40A and B ), the cerebral hemispheres are partially cleaved from each other posteriorly. The temporal horns of the lateral ventricles may be formed, but there is a single ventricle anteriorly. There is at least partial separation of the thalami. Midline structures, such as the falx and corpus callosum, may be present posteriorly but not anteriorly.

Lobar holoprosencephaly is a milder form in which the occipital and temporal horns are well formed, but there is failure of cleavage of the frontal lobes. The septum pellucidum is absent, and the corpus callosum may be absent or dysplastic.

A less severe form of holoprosencephaly is syntelencephaly or middle interhemispheric variant, and may be seen as fusion only of the midline posterior frontal lobes and/or parietal lobes ( Fig. 8-41 ). Different degrees of septum pellucidum and corpus callosum agenesis/dysgenesis can be present.

Another milder form of lobar holoprosencephaly is septopreoptic holoprosencephaly. In this variant, the midline fusion is limited to the septal and preoptic regions. The fornices are commonly thick or fused and the anterior commissure may be hypoplastic or absent.

Septo-optic Dysplasia

Septo-optic dysplasia is another type of ventral induction malformation, analogous to mild holoprosencephaly. It was originally described as absence of the septum pellucidum and hypoplasia of the optic nerves (diagnosed most often by ophthalmological evaluation) ( Fig. 8-42A and B ). It was later discovered that there are often associated pituitary abnormalities and dysfunction (in approximately two thirds of patients). Currently, several authors believe that a combination of any two of the three findings is part of the disease spectrum. Pituitary abnormalities may include absent visualization of T1 bright posterior pituitary gland, ectopic posterior pituitary gland ( Fig. 8-43 ), interrupted or hypoplastic pituitary stalk, or hormonal abnormalities with no obvious imaging finding. Septo-optic dysplasia may also present with schizencephaly, heterotopia, and incomplete hippocampi inversion. In septo-optic dysplasia, the frontal horns of the lateral ventricles have a squared appearance on coronal MR images. The septum pellucidum may also be completely or partially absent in cases of congenital hydrocephalus due to increase ventricular pressure (see Figs. 8-53A and 8-121A ), an important differential diagnosis, as not all cases of absent septum pellucidum are related to septo-optic dysplasia.

Dysgenesis of Corpus Callosum

Dysgenesis of the corpus callosum includes complete absence (agenesis), partial absence, and changes in morphology and size. The corpus callosum is the largest brain commissure (white matter tract connecting the left and right cerebral hemispheres). It normally develops from two independent portions, anterior and posterior, and the anterior corpus callosum enlarges and becomes visible earlier. When a portion of the corpus callosum is absent or dysgenic, it is most commonly the posterior portion that is absent, although this is not always true ( Fig. 8-44A–D ). Absence of the corpus callosum can occur in isolation or in conjunction with many of the other developmental ( Fig. 8-45A–D ) and genetic abnormalities. On coronal MR images, the lateral ventricles are separated, and the third ventricle extends more superiorly than normal, positioned between the lateral ventricles. The white matter tracts, which normally would cross the midline via the corpus callosum, run along the medial surface of the lateral ventricles and form the bundles of Probst (see Fig. 8-44C ). Lateral ventricles assume a parallel configuration on axial views, and colpocephaly (see Fig. 8-44B ) is often present (dilatation of atrium and occipital horns of the lateral ventricles). Midline masses, such as lipomas and interhemipheric cysts (see Figs. 8-44B and 8-45A and B ), can also be associated.

Posterior Fossa Anomalies with Enlarged Cerebrospinal Fluid Spaces

Causes of enlarged posterior fossa fluid spaces include a spectrum of etiologies that vary from normal developmental variants (mega cisterna magna) to more significant malformations. They can be divided into five major groups: Dandy–Walker malformation, cerebellar vermian hypoplasia, Blake pouch cyst, mega cisterna magna, and arachnoid cyst.

Dandy–Walker malformation is characterized by complete or partial agenesis of the cerebellar vermis in conjunction with the presence of a large posterior fossa. The CSF spaces are enlarged, and the torcula is elevated ( Fig. 8-46 ). The falx cerebelli is absent. Often there are also supratentorial abnormalities that may include holoprosencephaly, agenesis of the corpus callosum, polymicrogyria, and heterotopias. Hydrocephalus is also not uncommon. The patient outcome depends on severity of vermian hypoplasia and associated supratentorial abnormalities.

Cerebellar vermian hypoplasia (formerly known as Dandy–Walker variant or continuum) is characterized by mild, moderate, or severe hypoplasia of the cerebellar vermis. As a result, there is enlargement of the normal communication between the fourth ventricle and posterior fossa extra axial spaces. The overall size of the posterior fossa is not enlarged, and there is a normally positioned torcula Fig. 8-47 .

Blake pouch cyst is more commonly seen in the prenatal scenario, when fetal MRI is performed to evaluate for Dandy–Walker malformation versus vermian hypoplasia. Blake pouch is a normal embryological CSF-filled structure. It is seen during the development of the posterior fossa as a CSF outpouching that extends posteriorly from the inferior aspect of the developing fourth ventricle. When there is absent, incomplete, or late fenestration of this pouch (future foramens of Luschka and Magendie), a persistent cystic structure causing enlarged communications of the fourth ventricle and retrocerebellar extraaxial spaces is seen and referred to as Blake pouch cyst. The cerebellar vermis is usually uplifted but normally formed.

When enlarged posterior fossa CSF spaces are present with a fully developed and normally positioned cerebellar vermis, there are two possibilities. If the CSF spaces exhibit no mass effect on the cerebellum and the falx cerebelli is present, a mega cisterna magna is considered to be present. A mega cisterna magna may be in the spectrum of a persistent Blake's pouch cyst, as proposed by recent articles, and is considered a developmental variant. If the CSF spaces appear cystic and exhibit mass effect on the cerebellum, an arachnoid cyst is considered more likely ( Fig. 8-48 ). The differentiation between a mega cisterna magna and an arachnoid cyst may be difficult. However, regardless of the name chosen, if there is minimal or no mass effect on the cerebellum, the finding is considered incidental and of no clinical significance.

Molar Tooth Malformation

Molar tooth malformation is the main intracranial imaging finding in patients with Joubert syndrome and related disorders. Joubert syndrome and related disorders are a group of genetic disorders resulting in dysfunction of the neuronal progenitor cells cilia (ciliopathy). Imaging findings include elongated horizontally oriented superior cerebellar peduncles and deep interpeduncular fossa resembling a molar tooth on axial imaging ( Fig. 8-49A ). Additionally, a marked hypoplastic cerebellar vermis is characteristic ( Fig. 8-49B ). This may be confused with Dandy–Walker malformation on prenatal ultrasound.

Rhombencephalosynapsis

Rhombencephalosynapsis is a malformation of the rhombencephalon with complete or partial absence of the cerebellar vermis and midline fusion of the cerebellar hemispheres ( Fig. 8-50A and B ). Additional imaging findings include a small transverse cerebellar diameter (see Fig.8-121C ) and convex posterior cerebellar margins at the expected level of the vermis on axial plane as well as a rounded fastigial point and absent primary fissure on sagittal plane (see Fig. 8-45B ). This malformation is frequently associated with aqueduct stenosis diagnosed prenatally (see Fig. 8-121B-C ). Additional malformations/syndromes associated with rhombencephalosynapsis include VACTERL, holoprosencephaly, and Gomez–Lopez–Hernandez syndrome (rhombencephalosynapsis, focal alopecia, craniosynostosis, and trigeminal nerve abnormalities).

Microlissencephaly

Microlissencephaly is the result of an arrest in cell proliferation resulting in both microcephaly and a simplified pattern of gyration ( Fig. 8-51 ).

Lissencephaly (Agyria-Pachygyria)

Lissencephaly is the result of migrational abnormality resulting in a smooth appearance of the brain secondary to absent sulcation and gyration (agyria) ( Fig. 8-52 ) or markedly decreased sulcation and gyration (oligogyria). Pachygyria is another overlapping term defined as abnormally broad and flat gyri with shallow sulci. Findings are best visualized with MRI, usually with patchy areas of both agyria and oligogyria/pachygyria. These abnormalities are commonly associated with other migrational abnormalities. They also occur as part of a number of genetic syndromes. Two malformations often associated with agyria and pachygyria are classic lissencephaly and cobblestone malformations. Classic lissencephaly is due to arrest of neuronal migration and is formally known as lissencephaly type I. MRI demonstrates a smooth brain with a thickened cortex and a band of hyperintense T2 signal within the cortex (sparse cell zone). The brain may demonstrate the classic “Figure eight” morphology (see Fig. 8.52 ). Cobblestone malformations are due to overmigration of neurons into the subarachnoid space via defects in the pial limitans membrane. These were formally known as cobblestone lissencephalies or lissencephalies type II. They are commonly associated with congenital muscular dystrophy. On imaging, there is markedly decreased sulcation with a pebbled appearance of the cortex ( Fig. 8-53A ). Other findings may include vermian hypoplasia, cerebellar dysplasia ( Fig. 8-53C ), brainstem hypoplasia, kinked pontomesencephalic junction, pontine cleft, ocular anomalies, and aqueduct stenosis leading to ventriculomegaly ( Fig. 8-53A and B ).