Functional MRI

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 BOLD (blood oxygenation level-dependent) effect. fMRI techniques show these changes superimposed on anatomic information. fMRI is used to document regional neuronal activity during a specific activity: language (Fig. 8-1), memory, or motor function. It has been a very useful research tool in increasing our understanding of brain function during various tasks. Clinically, fMRI has been used in areas such as the evaluation of patients with seizures and the planning of surgical approaches to brain tumors.

FIGURE 8-1. Functional MRI. Right-handed 18-year-old with tuberous sclerosis and chronic seizures imaged for preoperative evaluation of hemispheric language dominance. Auditorily presented noun-covert verb generation. Areas of significant BOLD-related signal change (orange-yellow areas) during silent verb production with auditorily presented noun are noted to be predominately left-sided in the left inferior frontal lobe and left temporal parietal regions, consistent with left hemispheric language dominance. Multiple areas of signal increase are noted on the (FLAIR) sequence used as an anatomic overlay, consistent with cortical and subcortical tuber formation.

Images courtesy of James L. Leach, MD.

MR Spectroscopy

Proton (hydrogen) MR spectroscopy is a tool that is now routinely used to help 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. 8-2). Metabolic profiles depicted on MR spectroscopy have been much less specific than originally hoped. However, the information obtained does provide useful information that complements the information derived from MRI in many pediatric disorders, including neoplasms, ischemia, and white matter diseases.

FIGURE 8-2. Normal single voxel ¹H-MR spectroscopy. Left image shows single voxel placement. Right image shows spectra. Note the normal stair-step relationship of choline (Cho; 3.2 ppm); creatine (Cr; 3.0 ppm); and NAA (NAA; 2.0 ppm) peaks. A peak related to myoinositol is also noted (Ml; 3.6 ppm). Location of pathologic lactate peak (not present in this normal example of deep gray matter) is denoted by the arrow (1.3 ppm).

Images courtesy of James L. Leach, MD

The commonly evaluated brain metabolites include N-acetyl aspartate (NAA), creatine, choline, and lactate (see Fig. 8-2). NAA is a neuronal marker and is decreased in most disorders that destroy brain tissues, such as neoplasms (Fig. 8-3), infarcts, radiation injuries, and seizures. NAA is markedly elevated in Canavan disease. Choline compounds reflect the synthesis and degradation of cell membranes; therefore, increased choline is seen when there is high cellular turnover, as occurs with most tumors (see Fig. 8-3). Lactate is typically elevated in the setting of acute ischemia, infarction, and many high-grade tumors and severe infections (Fig. 8-4; and see Fig. 8-3). Creatine remains stable in the presence of many disorders but can be increased in hypometabolic states and decreased in hypermetabolic states.

FIGURE 8-3. Abnormal spectroscopy in a 5-year-old child with left brainstem glioblastoma multiforme. Note large choline (Cho) peak, diminished NAA, and markedly elevated lactate peak, typical of high-grade glial neoplasm.

Images courtesy of James L. Leach, MD

FIGURE 8-4. Abnormal spectroscopy. 12-year-old child with right parietal abscess. Note absence of normal brain metabolites and large lactate peak consistent with necrosis.

Diffusion-Weighted Imaging

Diffusion-weighted imaging (DWI) images are determined by the variability in the diffusivity of water in tissues. Molecules move randomly in fluid (Brownian motion). In tissues, there is variable restriction of the movement of water molecules relative to tissue structure (cell membranes, vascular structures, density of cells, axons). Pathologic states alter the diffusion characteristics of brain water and therefore affect the appearance of the image. DWI images are created by applying two additional magnetic field gradients. The first dephases the spins and the second rephases the spins, if no movement occurs. If there is free movement of molecules, such as in the free-moving water in the cerebrospinal fluid (CSF), the protons lose spin coherence and there is signal loss. If movement is restricted, the movement of molecules is less random and more signal is returned.

Images in DWI are influenced not only by diffusion but also by other tissue properties such as T2. In order to eliminate the influences on imaging appearance of factors unrelated to diffusion, the data are frequently processed, most commonly by a technique called an apparent diffusion coefficient (ADC) map. Therefore, for each DWI sequence, two sets of images are often created: the DWI images and the ADC map. What the bright and low areas mean on these two sets of images can be confusing, because they are the opposites of each other. In a pathologic process in which there is restricted diffusion, the involved area appears high in signal on the DWI images and low in signal on the ADC map. Therefore, for clarification: restricted diffusion = bright signal on DWI = dark signal on ADC map = diminished ADC. Conversely, in pathologic processes in which there is increased diffusion, there is a dark signal on DWI images and a high signal on the ADC map. Table 8-1 shows a glossary of terms related to DWI and diffusion tensor imaging (DTI).

Table 8-1. Glossary of Terms in Diffusion-Weighted and Diffusion Tensor Imaging

Pathologic processes that show restricted diffusion include acute infarction; cellular edema (such as that caused by acute ischemia or nonhemorrhagic traumatic injury); purulent fluid collections (Fig. 8-5); epidermoid cysts; and hypercellular tumors such as medulloblastoma. Processes that show increased diffusion include vasogenic edema and CSF collections such as those in an arachnoid cyst.

FIGURE 8-5. DWI. Brain abscess in a 12-year-old male. The arrow points to vasogenic edema. The arrowhead points to the abscess cavity. (A) Axial FLAIR sequence shows relatively low-signal area with surrounding vasogenic edema. (B), Contrast-enhanced axial T1-weighted image shows large ring enhancing the lesion identified in the right parietal lobe. (C) DWI. Abscess cavity has high signal probably related to increased T2 properties. Surrounding edema has increased diffusion and is high in signal. (D) ADC map. Abscess cavity is low in signal, consistent with restricted diffusion from purulent fluid. Surrounding vasogenic edema is high in signal, consistent with increased diffusion.

Images courtesy of James L. Leach, MD

DWI has become a commonly used tool in neuroimaging. One of its major advantages is its high sensitivity for ischemia and infarction; it often reveals them before such findings are seen in conventional MR sequences. It is also helpful in differentiating among posterior fossa tumors. The hypercellular medulloblastoma has restricted diffusion, whereas ependymoma and astrocytoma typically do not.

Diffusion Tensor Imaging

By applying diffusion gradients in at least six directions during a diffusion MRI sequence, DTI techniques enable the identification of the 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 called tractography. (2) Metrics describing the microarchitecture of tissue can be calculated. They include fractional anisotrophy and 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 anterioposterior as green (Fig. 8-6).

FIGURE 8-6. Eigenvalue map showing white matter tracts. Note that red denotes tracts oriented left to right; blue denotes cephalocaudad; and green denotes anteroposterior.

Images courtesy of James L. Leach, MD

DTI and tractography can be used to evaluate myelination. Increased myelination increases anisotrophy. Therefore, in normal infants, anisotrophy increases with age. Most disease states that affect white matter decrease anisotrophy. DTI has been used to evaluate stroke, brain tumor (Fig. 8-7), 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 (see Fig. 8-7).

FIGURE 8-7. DTI. Left brainstem glioblastoma multiforme in 5-year-old male extending into the brachium pontis. Tractography (A) and three-dimensional fiber representation of the left corticospinal tract (B, axial; C, sagittal; D, coronal) obtained for surgical planning. Left corticospinal tract (arrows) overlaid on anatomic images. Note the corticospinal tract (arrows) displaced medially by the large brainstem mass (arrowhead).

Images courtesy of James L. Leach, MD

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. The germinal matrix is highly vascular and is subject to hemorrhaging. The germinal matrix lies within the caudothalamic groove (the space between the caudate head and the thalamus). Germinal matrix hemorrhage most commonly occurs in premature infants during the first 3 days after birth. Potential complications include destruction of the precursor cells within the germinal matrix, hydrocephalus, and hemorrhagic infarction of the surrounding periventricular tissues.

On ultrasound, germinal matrix hemorrhage is seen as an ovoid echogenic mass within the caudothalamic groove (Figs. 8-9, 8-10A, B, 8-11). 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. 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 actually thought to be secondary to venous infarction rather than a direct extension of hemorrhage (see Fig. 8-11). Grades I and II hemorrhages tend to have good prognoses. In contrast, grades III and IV hemorrhages tend to have poor prognoses, including high incidences of neurologic impairment, hydrocephalus, and death.

FIGURE 8-9. Left, grade 1 germinal matrix hemorrhage in 29-week gestational age, premature infant. Off-midline sagittal ultrasound demonstrates echogenic mass (arrows) in left caudothalamic groove. The lateral ventricle is not dilatated. C, caudate nucleus; T, thalamus.

FIGURE 8-10. Bilateral grade III germinal matrix hemorrhage in 25-week gestational age, premature infant. A, Coronal ultrasound shows bilateral echogenic masses (arrows) in the region of the caudothalamic groove. There is dilatation of the lateral ventricles (V), making the lesion grade 3. Note the smooth-appearing brain with sulcation consistent with the 25-week gestational age. B, Right sagittal off-midline ultrasound shows echogenic mass (arrows) in the region of the caudothalamic groove. Note ventricular dilatation (V).

FIGURE 8-11. Right, grade IV germinal matrix hemorrhage. On coronal ultrasound, there is a large echogenic mass (M) in the region of the right germinal matrix and increased echogenicity extending into the adjacent white matter (arrows).

Table 8-2. Grades of Germinal Matrix Hemorrhage

Periventricular Leukomalacia

Perinatal partial asphyxia can result in damage to the periventricular white matter, the watershed zone of the premature infant. This is termed periventricular leukomalacia. It most commonly affects the white matter adjacent to the atria and the frontal horns of the lateral ventricles. It is associated with neurologic sequelae, such as movement disorders, seizures, and spasticity. On ultrasound, increased heterogeneous echogenicity is seen within the periventricular white matter. In severe cases, there may be cystic necrosis and development of periventricular cysts (Fig. 8-12A, B). With time, there is often volume loss of the involved white matter (Fig. 8-13).

FIGURE 8-12. Periventricular leukomalacia in a 29-week premature infant. A, Coronal ultrasound shows heterogeneous echogenicity in the frontal periventricular white matter (arrows). B, A repeat ultrasound 20 days later shows development of cystic necrosis in the bilateral frontal white matter (arrows), consistent with cystic periventricular leukomalacia.

FIGURE 8-13. Periventricular leukomalacia in a 2-year-old formerly premature infant. Axial FLAIR images show volume loss and increased signal (arrows) in the periventricular white matter.

Benign Macrocrania

Benign macrocrania is a diagnosis of exclusion. The term refers to children with large heads (head circumference greater than 97% for age). 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 prominent size of the lateral ventricles and extraaxial spaces (Fig. 8-14). Imaging is otherwise normal. Imaging findings alone cannot differentiate between benign macrocrania and communicating hydrocephalus.

FIGURE 8-14. Benign macrocrania in a 3-month-old with a large head. The image from coronal ultrasound shows prominent extraaxial spaces (E) and prominence of the lateral ventricles (V). In most infants, the lateral ventricles are slitlike.

Chiari Malformations

CHIARI MALFORMATION TYPE 1

Chiari malformation type 1 is the presence of an abnormally inferior location of the cerebellar tonsils at least 5 mm below the foramen magnum (Fig. 8-15). The medulla and fourth ventricle are in normal positions. Complications of Chiari type 1 malformation include hydrocephalus and hydrosyringomyelia (up to 25%; see Fig. 8-15). It may be suspected on CT when the foramen magnum appears “full” of soft tissue. It is best visualized on sagittal T1-weighted MR images. The cerebellar tonsils are seen positioned inferiorly and typically appear elongated rather than round. MR cine sequences are used to show movement of the tonsils into the foramen magnum.

FIGURE. 8-15 Chiari 1 malformation with associated hydrosyringomyelia. Sagittal T1-weighted MRI shows the cerebellar tonsils (T) to be pointed and extending inferiorly into the foramen magnum. There is a large associated hydrosyringomyelia (arrows).

CHIARI MALFORMATION TYPE 2

Chiari malformations type 2 are almost always associated with myelomeningoceles. Conversely, almost all patients with myelomeningoceles have Chiari type 2 malformations. Patients with these lesions have small posterior fossae and associated inferior displacement of the cerebellum, medulla, and fourth ventricle into the upper cervical canal (Figs. 8-16, 8-17). Associated imaging findings include a kinked appearance of the medulla, colpocephaly (disproportionate enlargement of the posterior body of the lateral ventricles), fenestration of the falx associated with interdigitation of 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.

FIGURE 8-16. Chiari 2 malformation shown on ultrasound in a newborn infant with myelomeningocele. A, Midline sagittal ultrasound shows inferior displacement of the cerebellum (C). The fourth ventricle is effaced and not visualized. There is a prominent massa intermedia (M). The third ventricle (3^rd) is dilatated secondary to hydrocephalus. B, Coronal image shows inferior pointing of the lateral ventricles (arrows). There is obstructive hydrocephalus, with dilatation of the lateral ventricles. C, A more posterior coronal image shows interdigitation of the cerebral gyri, secondary to fenestration of the falx cerebri.

FIGURE 8-17. Chiari 2 malformation. Sagittal T2-weighted image shows downward displacement of cerebellar tonsils (T) and medulla (arrowhead). The fourth ventricle is compressed. The third ventricle (3^rd) is dilatated. There is a prominent massa intermedia (arrow).

Chiari type 3 and type 4 malformations are very rare.

Holoprosencephaly

Holoprosencephaly results from lack of 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 reflected in the severity of the midline facial abnormality.

Alobar holoprosencephaly 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. These infants are stillborn or die soon after birth.

With the intermediate form, semilobar holoprosencephaly (Fig. 8-18A-D), the cerebral hemispheres are partially cleaved from each other posteriorly. The temporal horns may be formed but there is a single ventricle anteriorly. There is partial separation of the thalami. Midline structures such as the falx and corpus callosum may be present posteriorly but not anteriorly.

FIGURE 8-18. Semilobar holoprosencephaly. A, Axial T2-weighted image shows separation of the thalami (T) and separated occipital lobes but fusion of the frontal lobes. There is a single monoventricle and absence of the septum pellucidum. B, Coronal T2-weighted image shows monoventricle frontally (arrowheads). Note the single anterior cerebral artery (arrow). C, Sagittal T1-weighted image shows absence of the corpus callosum (arrows). There is brachycephaly. D, Eigenvalue map shows decreased left-to-right tracts (absence of red tracts).

Lobar holoprosencephaly is the least severe form. 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.

Septooptic Dysplasia

Septooptic dysplasia is another type of ventral induction malformation, analogous to mild holoprosencephaly. It is characterized by absence of the septum pellucidum and hypoplasia of the optic nerves. It is often associated with schizencephaly, heterotopia, and hypothalamic and pituitary dysfunction. In septooptic dysplasia, the frontal horns of the lateral ventricles have a squared appearance and point inferiorly on coronal MR images.

Posterior Fossa Cystic Malformations

Posterior fossa cystic malformations include a spectrum of abnormalities. They can be divided into four groups: Dandy-Walker malformation, Dandy-Walker variant, mega cisterna magna, and arachnoid cyst.

Dandy-Walker malformation is complete or partial agenesis of the cerebellar vermis in conjunction with the presence of a posterior fossa cyst that communicates with the fourth ventricle (Fig. 8-19A, B). The posterior fossa is enlarged such that the torcular is elevated above the lambda (see Fig. 8-19). The falx cerebelli is absent. Often there are also supratentorial abnormalities, including holoprosencephaly, agenesis of the corpus callosum, polymicrogyria, and heterotopias. Hydrocephalus is common.

FIGURE 8-19. Dandy-Walker malformation in a newborn infant. A, Sagittal, T1-weighted MR image shows large posterior fossa cyst (DW) enlarging the posterior fossa. The cerebellar vermis is absent. The torcular (arrows) is elevated. B, Axial, proton-density-weighted MR image shows large posterior fossa cyst (DW) that communicates with the fourth ventricle (4^th).

Dandy-Walker variant is considered present when some but not all of the criteria for the classic malformation are present. Most commonly, the cerebellar vermis is hypoplastic but present, and there is a posterior fossa cyst, but the posterior fossa is not enlarged (Fig. 8-20). When an enlarged posterior fossa CSF cyst is present in the presence of a fully developed cerebellar vermis, there are two possibilities. If the cyst exhibits no mass effect on the cerebellum, a mega cisterna magna is considered to be present. If the cyst does exhibit mass effect on the cerebellum, an arachnoid cyst is considered to be present (Fig. 8-21).

FIGURE 8-20. Dandy-Walker variant. Sagittal, T1-weighted MR image shows a posterior fossa cyst (C) that communicates with the fourth ventricle (arrow). The vermis is hypoplastic but present. The posterior fossa is not enlarged.

FIGURE 8-21. Posterior fossa arachnoid cyst. Sagittal T1-weighted MR image shows posterior fossa cyst (A) that does not communicate with the fourth ventricle (arrow). There is mass effect on the cerebellum. The third ventricle (3^rd) is markedly dilatated.

Gray Matter Heterotopias

Heterotopias are abnormalities of neuronal migration characterized by arrest in migration of the neurons from the subependymal area to the cortex. Typically, heterotopias are associated with other migrational disorders, such as schizencephaly, lissencephaly, or polymicrogyria. When a heterotopia occurs as an isolated abnormality, it typically presents with focal seizures. On CT and MRI, the lesions appear as nodular (Fig. 8-22A, B) or linear (Fig. 8-23) areas within the white matter, most typically in the subcortical or subependymal regions. Heterotopias tend to be isointense, with gray matter on all pulse sequences.

FIGURE 8-22. Heterotopic gray matter in two different patients. A, CT shows nodular areas (arrows) adjacent to the lateral ventricles. The nodules have the same attenuation as gray matter. B, Axial T2-weighted MRI shows nodular areas (arrows) lining the occipital horns of the lateral ventricles. Nodules have the same signal as gray matter, consistent with heterotopic gray matter. There is associated colpocephaly (enlargement of the occipital horns of the lateral ventricles).

FIGURE 8-23. Band heterotopia. Axial, T2-weighted MR image shows heterotopias as linear structures (arrows) within the frontal white matter that are isointense to gray matter.

Schizencephaly

Schizencephaly is another migrational disorder. The term refers to a cleft in the cerebral hemisphere lined with gray matter (Fig. 8-24A, B). The cleft typically extends from the lateral ventricle to the surface of the brain. Schizencephaly is often characterized as being open-lipped or close-lipped. However, this has little clinical relevance. The lesion can be unilateral or bilateral and can occur anywhere in the cerebral hemispheres. In most cases, there is associated agenesis of the corpus callosum.

FIGURE 8-24. Schizencephaly. A, Coronal T1-weighted image shows large fluid-filled space that is in communication with the right lateral ventricle. The communication is lined by gray matter (arrows), consistent with schizencephaly. B, Axial T2-weighted image again shows large fluid-filled space that is in communication with the right lateral ventricle. The communication is lined with gray matter (arrows).

The presence of gray matter lining the entire cleft is the diagnostic feature that separates schizencephaly from other causes of clefts, such as porencephaly (see later material). In cases of porencephaly, the cleft is lined by (extends through) both gray and white matter.

Lissencephaly

Lissencephaly refers to arrest of migration of neurons, resulting in either total failure of development of sulci and gyri (agyria; Fig. 8-25) or development of abnormally broad and flat gyri with abnormally shallow sulci (pachygyria) (Fig. 8-26A, B). Agyria and pachygyria are best visualized with MR imaging. There is often thickening of the associated cortex. Neither lesion typically appears in isolation; there are usually patchy areas of both agyria and pachygyria. These abnormalities are commonly associated with other migrational abnormalities and also occur as part of a number of rare syndromes.

FIGURE 8-25. Agyria (lissencephaly) in a 10-year-old girl with seizures and developmental delay. Coronal T2-weighted image shows the smooth surface of the bilateral frontal lobes (arrows), without formation of sulci and gyri.

FIGURE 8-26. Pachygyria in two patients. A, Axial T1-weighted MRI shows poorly defined, thickened gyri (arrows) within the left frontal lobe. B, Coronal T1-weighted MRI shows all gyri to be abnormal. The most strikingly thickened, broad gyri are denoted (P).

Polymicrogyria is similar disorder that results in small, disorganized gyri. It too is usually associated with other migrational abnormalities. There is controversy as to whether polymicrogyria is a disorder of migration or is a cortical dysplasia.

Dysgenesis of Corpus Callosum

Dysgenesis of the corpus callosum includes both complete and partial absence. The corpus callosum normally develops from anterior to posterior. As a result, with partial absence, it is the more anterior part of the corpus callosum that is present. Absence of the corpus callosum can occur as an isolated lesion or in conjunction with many of the other developmental lesions of the brain already described in this chapter (see Fig. 8-18). 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 that normally cross the midline via the corpus callosum run along the medial surface of the lateral ventricles and form the bundles of Probst, which can be seen at imaging. Colpocephaly is often present. Midline masses, such as lipoma and arachnoid cyst, can also be associated.

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