Core Curriculum, The: Ultrasound
1st Edition
© 2001 Lippincott Williams & Wilkins
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10
Neonatal Neurosonography
Sonography provides a cost-effective, readily available method to image the infant brain. It is especially valuable because it can be performed at the bedside of premature or critically ill infants, keeping the infant within the protective environment of the neonatal intensive care unit. US is particularly accurate in the detection and follow-up of intracranial hemorrhage and is used to screen for a wide variety of congenital brain anomalies and for complications of central nervous system infection [1].
Spinal sonography is used to demonstrate the anatomy of the spinal canal and the position of the conus medullaris in infants at risk for a tethered spinal cord.
Cranial Sonography
Imaging Technique
The anterior fontanelle provides an excellent sonographic window to image the infant brain. High-frequency 7.5-MHz sector or curved array transducers are utilized to examine the brain in angled sagittal and coronal planes. From the anterior fontanelle, the transducer is swept from the frontal to the occipital lobes in coronal orientation to obtain symmetrical images of the cerebral hemispheres and lateral ventricles. Care must be taken to orient the transducer properly to display the right side of the infant brain on the left side of the image. The transducer is turned 90 degrees for the sagittal plane. A midline sagittal view demonstrates the third ventricle, brain stem, and posterior fossa. The transducer is angled right and left from its midline sagittal position to examine each hemisphere and lateral ventricle in detail. The posterior fontanelle, the foramen magnum, and the thin squamous portion of the temporal bone may be utilized to provide supplementary images whenever indicated [2,3]. Lower-frequency transducers (3.5-5.0 MHz) may be needed to improve penetration in older and larger infants. Representative views of the cerebral hemispheres, lateral ventricles, third ventricle, choroid plexus, caudothalamic groove, corpus callosum, cavum septum pellucidum/vergae, fourth ventricle, and vermis of the cerebellum are documented [4]. US examination can routinely be performed up to approximately 12-14 months of age when the fontanelle becomes too small to serve as an effective window.
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Anatomy
Basic Infant Brain Anatomy
Several concepts of infant brain anatomy must be kept in mind when performing and interpreting sonography of the neonatal brain. The ventricles can be conceptualized as the easily identified “skeleton” of the brain. Their complex shape must be recognized in each plane of section. Surrounding structures are then identified with respect to their location relative to the ventricles. The lateral ventricles form the framework of each cerebral hemisphere. Each lateral ventricle has a frontal horn, body, atrium, occipital horn, and temporal horn. The lateral ventricles are commonly (in up to 70% of individuals) mildly asymmetric in size. Normal lateral ventricles may be tiny slits or form angulated or comma-shaped lucencies on coronal images [5]. The atrium or trigone is the junction of the temporal and occipital horns with the body of the lateral ventricle. The choroid plexus functions as the site of production of cerebrospinal fluid (CSF) and forms a prominent echogenic structure that lines the temporal horn, atrium, and body of each lateral ventricle. No choroid plexus is present in the frontal horns or occipital horns. This is most important to note because acute hemorrhage has the same echogenicity as the choroid plexus and is recognized primarily by location. Echogenic material in the anterior or occipital horns is hemorrhage, not choroid plexus. The choroid plexus is biggest in the atrium of each lateral ventricle. This prominent blob of choroid plexus in the atrium is called the glomus. The choroid plexus extends forward from the glomus to pass through the foramen of Monro and then reflects posteriorly along the roof of the third ventricle. The anterior-most portion of the choroid plexus in the roof of the third ventricle serves as a marker for the location of the foremen of Monro.
The germinal matrix is a group of loosely organized proliferating cells that give rise to the neurons of the cerebral cortex during embryologic development. The germinal matrix is exceptionally vascular with a network of thin fragile capillaries highly susceptible to injury by hypoxia. In early gestation, the germinal matrix lines the wall of the entire ventricular system, lying just beneath the ependyma, the thin membranous lining of the ventricular system. After 12 weeks gestation, the germinal matrix begins to regress. By 24 weeks, only the germinal matrix over the caudate nucleus persists. By full term at 40 weeks, the germinal matrix no longer exists. Thus hemorrhage of the germinal matrix is a disease of premature infants. It originates in the residual germinal matrix that overlies the caudate nucleus in the frontal horns of the lateral ventricles. The normal germinal matrix is not visualized by US.
A prominent fluid-filled structure is seen in the midline of the developing brain. This cystic structure is solitary and continuous, but for reasons unknown to me, it is given two unhandy names [6]. The anterior portion is called the cavum septum pellucidum whereas the posterior portion is called the cavum vergae. The foramen of Monro marks the divide between the two names. This structure involutes from back to front during fetal life and infancy. At full term, only the cavum septum pellucidum exists in most infants. This structure normally closes completely by 3-6 months of life. However, in approximately 5% of adults it persists as a residual fetal structure. The cavum septum pellucidum/vergae has been mistaken for hydrocephaly.
In premature infants, the surface of the cerebral hemispheres is smooth and flat [7]. With development between the equivalent of 24 and 40 weeks gestation, the sulci deepen, bend, and branch to form the prominent normal gyral pattern of full-term infants.
Although cisterns are by definition CSF-filled spaces around the brain, on US the cisterns are often echogenic rather than echolucent. The echogenicity is produced by reflection from folds of meninges that float in the fluid of the cisterns.
Coronal Plane Anatomy
In coronal plane, the most anterior section is obtained anterior to the frontal horns. This image documents the frontal lobes extending over the orbits (Fig. 10.1).
The next plane posterior documents the frontal horns, which appear as paired slit-like or comma-shaped anechoic structures (Fig. 10.2). The caudate nuclei form the angled
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floors of the frontal horns and must be inspected in detail to detect small echogenic hemorrhages of the germinal matrix. The cavum septum pellucidum is the anechoic fluid-filled structure in the midline. The hypoechoic band extending between the cerebral hemispheres just above the cavum septum pellucidum is the corpus callosum [8]. The midline
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sagittal fissure extends to the level of the corpus callosum. Anterior cerebral arteries pulsate and are visible with color Doppler in the sagittal fissure. The tips of the temporal lobes are seen inferiorly and laterally.
Figure 10.1 Coronal Plane–Frontal Lobes. This anatomic plane is just anterior to the frontal horns of the lateral ventricles. Normal periventricular echogenicity is evident centrally in each frontal lobe (straight arrow). The curved arrow indicates the midline sagittal fissure. o, orbit.
Figure 10.2 Coronal Plane–Frontal Horns. Images demonstrate the full field of view (A) and a coned down view (B) of the frontal horns (shortest arrow) in two different infants. The cavum septum pellucidum is the cystic space (long thin arrow) between the frontal horns. The lateral wall (the septum pellucidum) of the cavum septum pellucidum is the medial wall of the frontal horn. The corpus callosum (open arrow) is the hypoechoic white matter band that connects the two hemispheres and provides the floor of the sagittal fissure. The caudate nucleus (c) serves as the angled floor of the frontal horn. The echogenic blob of the choroid plexus in the roof of the third ventricle is absent (large solid arrow) indicating that this anatomic plane is anterior to the foramen of Monro. Echogenic foci in the caudate nuclei in this plane are indicative of germinal matrix hemorrhage. t, temporal lobe.
Figure 10.3 Coronal Plane–Third Ventricle. The echogenic choroid plexus (large arrow) is visible lying along the roof of the third ventricle just inferior to the cavum septum pellucidum. This landmark indicates the plane of section is posterior to the foramen of Monro. Echogenic choroid plexus may be seen normally overlying the caudate nucleus in the lateral ventricles in this plane of section. The third ventricle is usually slit-like and is not discretely visualized in coronal plane. Also seen on this image are a portion of the brainstem (b), the cerebellar hemisphere (h), the cisterna magna (small arrow), and the temporal lobes (t).
Moving posteriorly to a direct coronal plane, the echogenic choroid plexus is visualized in the midline just below the cavum septum pellucidum (Fig. 10.3). The choroid plexus is normally present in both lateral ventricles in the same anatomic planes where the choroid is visualized in the roof of the third ventricle, but not in the frontal horns more anteriorly. The third ventricle is slit-like and may be seen as a linear lucency between the oval globes of the thalami. Commonly the normal third ventricle is not seen as a discrete structure. Portions of the hypoechoic brainstem are seen in the midline below the third ventricle and thalami.
A posteriorly angled coronal plane demonstrates the prominent echogenic glomus of the choroid plexus in atria of the lateral ventricles (Fig. 10.4). The glomus swings dependently toward the downside of the baby’s head, commonly revealing a gap between the wall of the lateral ventricle and the choroid plexus. The tentorium may be seen as an echogenic wall between the cerebrum and cerebellum.
Further posterior angulation of the transducer produces an image of the occipital lobes above the level of the occipital horns (Fig. 10.5). Vague symmetric echogenicity should be evident bilaterally. The echogenicity is produced by sound reflection from periventricular white matter tracts. The acute hemorrhagic stage of periventricular leukomalacia (PVL) produces asymmetric or increased echogenicity in this area. Cystic change is easily seen in this plane during the later cystic stage of PVL.
Sagittal Plane Anatomy
A direct midline sagittal image demonstrates the corpus callosum as a well-defined, curving hypoechoic band above the cystic cavum septum pellucidum/vergae (Fig. 10.6) [8]. The anterior genu and posterior splenium are normally slightly thicker than the trunk of the corpus callosum. The entire corpus callosum should be visualized to avoid missing partial agenesis. Immediately inferior to the cavum septum pellucidum is the curving echogenic band of the choroid plexus in the roof of the third ventricle. The third ventricle is normally slit-like and is not distinctly visualized. The midline cerebellar vermis is seen as a rounded echogenic mass in the posterior fossa. The fourth ventricle is visualized as a
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triangular lucency at the mid-base of the echogenic vermis [9]. Anterior to the vermis is the hypoechoic and poorly defined brainstem. The pons is more echogenic than the remainder of the brainstem. Special note should always be made of the presence or absence of the cisterna magna inferior to the vermis. Obliteration of the cisterna magna is seen with Chiari malformation.
Figure 10.4 Coronal Plane–Lateral Ventricles. A. Posteriorly angled coronal view shows the prominent glomus (arrow) of the choroid plexus in the atrium of the lateral ventricles. When this scan was obtained, the infant was lying with the left side of his head down. The choroid plexus lies dependently against the left lateral wall of the ventricle. B. Image angled further posteriorly shows the occipital horns of the lateral ventricles (small arrow). The size of the occipital horns is commonly asymmetric. Note the prominent but symmetric echogenicity of the periventricular white matter tracts (large arrow).
Figure 10.5 Coronal Plane–Periventricular White Matter. With the transducer held in coronal orientation at the anterior fontanelle, this image is obtained by angling the transducer far posteriorly approaching an axial plane. The prominent echogenic periventricular white matter (arrow) is demonstrated superior to the lateral ventricles. The normal echogenicity is symmetric. Asymmetry of the periventricular echogenicity suggests the early hemorrhagic stage of periventricular leukomalacia.
Figure 10.6 Sagittal Plane–Midline. Images of a premature (A) and a full-term (B) newborn infant demonstrate the important normal anatomic landmarks. The midline cystic cavum septum pellucidum (long arrow) and cavum vergae are prominent in the premature infant but are smaller and limited to only the cavum septum pellucidum in the full-term infant. Note the increased prominence of the gyri and sulci in the full-term infant compared to the relatively featureless brain of the premature. The corpus callosum (fat arrow) forms a curving hypoechoic band just above the cavum septum pellucidum. The choroid plexus (short curved arrow) in the roof of the third ventricle forms an echogenic band just below the cavum septum pellucidum. The vermis (v) of the cerebellum is a round echogenic mass in the posterior fossa. The fourth ventricle (tiny arrow) is seen as a triangular lucency indenting the vermis. Anterior to the vermis is the hypoechoic brainstem (b). The pons (p) is echogenic compared to the remainder of the brainstem. A normal fluid-filled cisterna magna (long curved arrow) should always be present. Absence of the cisterna magna is indicative of a Chiari defect.
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Approximately 10 degrees of lateral angulation to each side will demonstrate the lateral ventricles (Fig. 10.7). Note that the frontal horns are slightly closer together than the atria of the lateral ventricles. To show the long axis of the ventricles, the posterior aspect of the transducer is angled slightly more laterally than the anterior portion. The important landmarks here are the round shape of the thalamus and the tongue shape of the caudate nucleus. These two structures are nearly isoechoic but are separated by a slight V-shaped defect and echogenic line called the caudothalamic groove. The caudothalamic groove marks the location of the foramen of Monro and the anterior-most location of the choroid plexus [10]. Echogenicity in the caudate nucleus anterior to the caudothalamic groove indicates hemorrhage in the germinal matrix. The lateral ventricle enlarges posteriorly and contains the glomus of the choroid plexus. The glomus is commonly lobulated in contour. The occipital horns may be seen further posteriorly. Their size is quite variable and they are commonly asymmetric. Remember no choroid plexus is present in the occipital horns, so echogenicity in the occipital horns usually represents intraventricular blood clot.
Further lateral angulation will demonstrate the ill-defined normal echogenicity of the periventricular white matter tracts (Fig. 10.8). Asymmetric or increased echogenicity suggests the hemorrhagic stage of PVL. This laterally angled image plane is best for demonstrating the cystic change of PVL as well.
Figure 10.7 Sagittal Plane–Lateral Ventricle. Laterally angled, sagittally oriented image demonstrates the rounded mass of the thalamus (T) and the tongue-shaped structure of the caudate nucleus (c). The slight indentation between these two structures is the caudothalamic groove (straight arrow). The caudothalamic groove is the sagittal plane marker of the location of the foramen of Monro. Echogenic mass replacing the tip of the tongue of the caudate nucleus anterior to the caudothalamic groove is indicative of germinal matrix hemorrhage. The echogenic glomus of the choroid plexus (curved arrow) is seen in the atrium of the lateral ventricle. Echogenic choroid plexus extends anteriorly and inferiorly into the temporal horn.
Figure 10.8 Sagittal Plane–Periventricular White Matter. Sagittally oriented view angled further laterally demonstrates the echogenic periventricular white matter (arrow). This view is utilized to detect periventricular leukomalacia. The echogenicity of the right side should be compared to the echogenicity of the left side.
Figure 10.9 Axial Plane–Third Ventricle. An axial plane view obtained by imaging through the thin squamous portion of the temporal bone is optimal for demonstrating the third ventricle (straight arrow). The normal third ventricle is a slit-like structure between the two walnut-shaped thalami (T). The brain-cranium interface (curved arrow) should be noted to detect extra-axial fluid collections.
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Axial Plane Anatomy
Axial plane images are a useful addition to routine examination. The squamous portion of the temporal bone is thin enough in infants to allow adequate US penetration. Two image planes are most useful.
A mid-thalamic plane, similar to the plane used to measure biparietal diameter in the fetus, demonstrates the third ventricle and thalami (Fig. 10.9). The third ventricle is normally a narrow slit. In axial plane the sound beam is perpendicular to the lateral walls of the third ventricle and shows the size of the ventricle better than the coronal or sagittal plane. Axial plane views also demonstrate the lateral interface between brain and cranium and are useful for detection of extra-axial fluid collections and abnormalities of the cerebral cortex.
Just inferior to the plane of the third ventricle is the plane of the suprasellar cistern (Fig. 10.10). The cistern has the appearance of a 5-pointed echogenic star. The hypothalamus is seen as an oval structure centered in the cistern. The arteries of the circle of Willis are clearly visualized as pulsating vessels encircling the cistern on real-time US. This is an excellent plane for Doppler evaluation of these vessels. Posterior to the cistern is the heart-shaped structure of the cerebral peduncles. In the midline posterior aspect of the peduncles courses the cerebellar aqueduct, seen as an echogenic dot with or without a central lucency.
Hypoxic Brain Injury
Hypoxia is a common event in the sick neonate, especially in the premature infant with underdeveloped lungs. Hypoxia may result in two basic types of infant brain injuries: germinal matrix hemorrhage (GMH) and PVL [11]. The prevalence of GMH in infants born before 32 weeks gestation or with birth weight less than 1500 g is 30-55%. The prevalence of PVL in these infants is 10-40%.
It is important to note that the arterial watershed areas are different in the infant brain than in the adult or older child. In the premature infant, the watershed areas most susceptible to hypoxic injury are in the regions of the periventricular white matter. Autoregulation of cerebral blood pressure is commonly lacking in the neonate, making the infant more susceptible to hypoxic injury and hemorrhage associated with hypotension or hypertension.
Germinal Matrix Hemorrhage
GMH is a disease confined to premature infants, because the germinal matrix resorbs and is no longer present at full term. GMH is also commonly called subependymal hemorrhage
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because of the subependymal location of the germinal matrix, or is called intraventricular hemorrhage because extension of hemorrhage into the ventricles is common. The Papile classification of GMH is commonly utilized (Table 10.1) [12]. More severe grades of hemorrhage are predictive of worse neurological prognosis. GMH may result in cerebral palsy, developmental delays, and learning disabilities (see Fig. 10.14) [13]. Hemorrhages occur initially in the subependymal caudate nucleus in the frontal horn and then extend elsewhere in the brain [14].
Figure 10.10 Axial Plane–Suprasellar Cistern. A. Axial plane view obtained more inferiorly demonstrates the suprasellar cistern as a 5-pointed-star-shaped echogenic structure (short arrow) surrounding the hypothalamus (long arrow). Posterior to the suprasellar cistern is the heart-shaped structure of the cerebral peduncles (p). The echogenic spot (curved arrow) at the apex of the “heart” is the cerebral aqueduct (of Sylvius). B. With color Doppler (shown here in gray scale), the circle of Willis is clearly visible in the suprasellar cistern.
  • GMH may be unilateral or bilateral.
  • Grade I GMH is seen as a small discrete focus of increased echogenicity in the region of the caudate nucleus (Fig. 10.11).
  • Grade II GMH extends into the ventricular system and appears as an echogenic cast of a portion of, or the entire, lateral ventricle. The ventricle itself may be obscured by the
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    clot. Floating clot may be seen in the frontal or occipital horns. Blood may form a fluid level with CSF. The size of the ventricle remains normal (Fig. 10.12).
  • Grade III GMH is intraventricular hemorrhage with ventricular enlargement. The ventricles are measurably larger. The contour of the ventricles becomes rounded. Clot is seen within the ventricular system (Fig. 10.13).
  • Grade IV GMH is intraparenchymal hemorrhage caused by venous infarction complicating GMH (Fig. 10.14). Large GMH compresses and may thrombose subependymal veins that course through the area of hemorrhage. This results in hemorrhagic infarction of
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    periventricular brain parenchyma in the frontal to parieto-occipital lobes. Brain necrosis leads to porencephaly and results in spastic hemiparesis by disruption of the corticospinal motor tracts that course through the area of infarction. US shows echogenic clot in the involved parenchyma (Fig. 10.15).
  • Blood clot shows characteristic evolution over time (Fig. 10.13) [15]. Following acute hemorrhage, clot is equal to or exceeds the echogenicity of choroid plexus. As the clot ages, it shrinks and becomes more echolucent centrally while remaining echogenic peripherally. As clot lyses, floating echogenic debris is seen within the ventricles. Clot fragments appear as larger floating masses. Clots may adhere to the walls of the ventricles and to the choroid plexus causing a “lumpy-bumpy” choroid (Fig. 10.15A).
  • Chemical ventriculitis resulting from hemorrhage causes thickening and increased echogenicity of the ependymal lining, “ependymitis.”
  • Hydrocephaly commonly results from intraventricular hemorrhage (Fig. 10.16, Box 10.1). Clots may obstruct the foramen of Monro, the cerebral aqueduct, the outflow
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    tracts of the fourth ventricle, or the arachnoid granulations. Shunting may be required if hydrocephaly is progressive, but most cases resolve spontaneously with lysis of the clots. Ventricular size can be followed by US measurement of the frontal horns.
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  • Subependymal and intraparenchymal cysts may occur in areas of necrosis caused by hemorrhage (Fig. 10.17). The cysts appear as the hemorrhage resolves usually 3-6 weeks after the acute event. These cystic areas do not communicate with the ventricles. The cysts may subsequently resolve completely or form a linear echogenic scar.
  • Porencephaly describes the resorption of an area of infarcted brain parenchyma resulting in an area of absent brain tissue that communicates with the ventricular system (Figs. 10.15, 10.18). Porencephaly results from large intraparenchymal hemorrhages and is commonly associated with spastic paresis.
Table 10.1: Grading of Germinal Matrix Hemorrhage
Grade Description US Findings
I Subependymal germinal matrix hemorrhage confined to region of caudate nucleus Hemorrhage is hyperechoic compared to brain parenchyma and equal in echogenicity to choroid plexus.
II Intraventricular extension of germinal matrix hemorrhage without ventriculomegaly Echogenic clot makes a cast of the normal size ventricle.
Clot is seen within the frontal or occipital horns of the lateral ventricle.
III Intraventricular hemorrhage with hydrocephaly Same as Grade II but with enlarged ventricles.
IV Intraparenchymal hemorrhage Echogenic clot within brain parenchyma.
Adapted from Papile L, Burstein J, Burstein R, et al. Incidence and evolution of subependymal and intraventricular hemorrhage: study of infants with birth weight less than 1500 grams. J Pediatr 1978;92:529–534, with permission.
Figure 10.11 Germinal Matrix Hemorrhage–Grade I. A. Coronal view. B. Sagittal view. Focal echogenicity (arrows) overlying the right caudate nucleus seen in two anatomic planes is indicative of germinal matrix hemorrhage, Grade I. The right ventricle is slightly larger than the left ventricle, but no evidence of blood is seen within the ventricular system.
Figure 10.12 Germinal Matrix Hemorrhage–Grade II. A. Coronal image reveals bilateral germinal matrix hemorrhages that extend into the normal sized ventricle system forming an echogenic cast of the lateral (fat arrow) and third (small arrow) ventricles. Clot is also evident in the temporal horn (curved arrow) of the left lateral ventricle. B. Sagittal view of the right lateral ventricle shows the ventricular cast (arrow) formed by clot.
Figure 10.13 Germinal Matrix Hemorrhage–Grade III–Evolution of Clot. A. Initial sagittal image shows echogenic blood clot (arrow) filling and enlarging the lateral ventricle. Note the lucency surrounding the clot that confirms enlargement of the ventricle. B. Image of the same ventricle obtained 16 days later shows further enlargement of the ventricle and typical evolution of blood clot. The clot (arrow) is reduced in size and is decreased in echogenicity centrally while maintaining an echogenic rim.
Figure 10.14 Anatomy of the Corticospinal Tracts and Medullary Veins. Line drawing of the left hemisphere in coronal plane orientation demonstrates how germinal matrix hemorrhage and periventricular leukomalacia can injure the corticospinal tracts and cause cerebral palsy. Germinal matrix hemorrhage can cause thrombosis of the nearby medullary veins that drain the motor cortex, resulting in hemorrhagic infarction in the parietal lobe.
Figure 10.15 Germinal Matrix Hemorrhage–Grade IV. A. Coronal view shows a large germinal matrix hemorrhage (short arrow) that extends to involve the medullary veins, resulting in hemorrhagic parenchymal infarction (large arrow). Compare to the drawing in Figure 10.14. An area of increased echogenicity at the angle of the left frontal horn (curved arrow) represents coexistent hemorrhagic periventricular leukomalacia. B. Image of the same patient obtained approximately 3 weeks later shows a large defect of porencephaly (arrow) in the area of brain involved by the hemorrhagic infarction.
Figure 10.16 Hydrocephaly Complicates Germinal Matrix Hemorrhage. A. Coronal view demonstrates marked ventriculomegaly involving both lateral ventricles (L) and the third ventricle (3). The temporal horns (t) are markedly dilated. Resorbing clot (arrow) is seen in the lateral ventricles. B. Midline sagittal view shows the dilated third (small arrow) and fourth (large arrow) ventricles. The circular band (long skinny arrow) crossing the third ventricle is the massa intermedia. The dilated lateral ventricles are seen as a fluid-filled structures (curved arrow) above the third ventricle.
Periventricular Leukomalacia
PVL is infarction and necrosis of the periventricular white matter [16]. It results from severe hypoxia usually caused by episodes of hypotension. PVL occurs commonly in premature infants but also occurs in compromised full-term neonates. Long-term sequelae of PVL include spastic diplegia or quadriplegia, visual defects, developmental delays, and intellectual deficits [14].
  • PVL is most commonly bilateral and symmetric. However, it may be unilateral (Fig. 10.19).
  • Initial US examination is commonly normal because the injury is ischemic and without associated hemorrhage.
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  • Hemorrhage, which occurs with reperfusion of the injured area, causes increased echogenicity of periventricular white matter. This finding is commonly equivocal because the periventricular white matter is normally echogenic. Echogenicity that exceeds that of the choroid plexus is highly indicative of PVL (Fig. 10.19).
  • Cysts occur in periventricular white matter at 2-3 weeks following acute injury (Fig. 10.20). These cysts reflect necrosis and loss of white matter tracts. Cysts are usually multiple and cause a fenestrated appearance to the brain parenchyma. Cysts vary in size up to 1-2 cm.
  • Cysts resolve within several weeks to several months, resulting in a glial scar that is shown with high sensitivity by MR but not by US.
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  • Because US examination may be normal in the acute phase of PVL, high-risk infants should be screened at approximately 3 weeks of age to detect the cystic change of PVL for which US is highly sensitive.
Figure 10.17 Subependymal Cyst. A. Coronal image. B. Sagittal image. Tiny subependymal cyst (arrow) is the only residual US finding of a Grade I germinal matrix hemorrhage.
Figure 10.18 Porencephaly. Coronal image shows a large cystic area (arrow) of infarcted parietal lobe that communicates openly with the right lateral ventricle. Hemorrhage resulted in infarction, necrosis, and resorption of brain parenchyma.
Figure 10.19 Periventricular Leukomalacia–Early Hemorrhagic Stage. Coronal plane image demonstrates asymmetric increased echogenicity in the right periventricular white matter (straight arrow). The echogenicity exceeds that of the choroid plexus (curved arrow).
Figure 10.20 Periventricular Leukomalacia. Four examples of periventricular leukomalacia (arrows) in different patients are shown. Note the fenestrated cystic appearance and location of periventricular leukomalacia. Image D shows the value of the far lateral sagittal plane image. A. Coronal plane. B. Sagittal plane. C. Posterior coronal plane. D. Far lateral sagittal plane.
Diffuse Brain Edema
Severe hypoxia may cause diffuse brain swelling with or without acute hemorrhage in premature or full-term infants. This injury often occurs with asphyxia at birth.
  • The brain is mildly and diffusely increased in echogenicity with poor definition of anatomic structures.
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  • The ventricles are slit-like and the gyri appear compressed.
  • If infarction occurs, brain atrophy becomes evident approximately 2 weeks after the acute event. Extra-axial fluid spaces become enlarged. Gyri and sulci appear prominent. Ventricles enlarge due to loss of brain parenchyma. Diffuse cystic encephalomalacia or porencephaly may be evident. Dystrophic calcifications may be seen in areas of destroyed brain parenchyma.
  • This injury may be superimposed on findings of GMH and PVL.
Congenital Anomalies
US is used to screen for and to classify major congenital brain malformations [17]. However, MR is commonly needed to confirm the classification and to detect subtle associated anomalies such as neuronal migration anomalies not evident on US examination.
Holoprosencephaly
Holoprosencephaly is a spectrum of malformations caused by failure of complete division of midline structures.
  • Septo-optic dysplasia is the mildest form, with absence of the septum pellucidum and hypoplasia of the optic nerves. The frontal horns are flattened and the ventricles are characteristically downward pointing (Fig. 10.21).
  • Lobar holoprosencephaly has fused frontal horns with absent septum pellucidum but has separated occipital horns. The frontal horns have a box-shaped appearance. The third ventricle is present.
  • Semilobar holoprosencephaly consists of a single ventricle with some separation of the temporal and occipital horns. Incomplete falx and interhemispheric fissure are usually present. The thalami are partially separated and the third ventricle is rudimentary. The splenium of the corpus callosum is present but the remainder of the corpus callosum is absent.
  • Alobar holoprosencephaly is the most severe and usually fatal form (see Fig. 7.45). Severe facial anomalies including cyclopia, cebocephaly, and midline clefts are usually present. A single, large, horseshoe-shaped ventricle is present with the surrounding cortex being thin and poorly developed. The cerebral hemispheres and thalami are fused. The corpus callosum, falx, interhemispheric fissure, and third ventricle are absent. The posterior fossa structures are often normal.
Figure 10.21 Septo-Optic Dysplasia. The frontal horns are fused and downward pointing on this coronal image. The corpus callosum and septum pellucidum are absent.
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Hydranencephaly
Hydranencephaly is near complete destruction of the cerebral cortex caused by in utero occlusion of both internal carotid arteries or by other severely destructive processes.
  • The calvarium is filled with CSF, but no peripheral brain tissue is evident (see Fig. 7.46).
  • The falx is usually identifiable but often is incomplete. Visualization of even part of the falx helps differentiate this condition from alobar holoprosencephaly.
  • The thalamus and posterior fossa structures are normal.
  • Doppler shows no flow in the carotid arteries.
Congenital Hydrocephaly
Ventriculomegaly may be caused by obstruction to CSF flow, overproduction of CSF (choroid plexus tumor) or by loss of brain parenchyma (ex vacuo). Hemorrhage or infection may cause hydrocephaly by fibrosis of the foramen of Monro, the aqueduct, basal cisterns, or arachnoid granulations. Tumors or congenital malformations may obstruct the ventricles at any level.
  • The ventricles are enlarged but maintain their normal shape.
  • Peripheral cortex and midline structures are present.
  • Dilatation of all ventricles suggests an extraventricular cause.
  • Dilatation of the lateral and third ventricle with normal size of the fourth ventricle suggests aqueductal stenosis (Fig. 10.22). Aqueductal stenosis may be inherited as an X-linked recessive trait.
Dandy-Walker Malformation
Cystic anomalies of the posterior fossa are classified as Dandy-Walker malformations and variants [18].
  • Classic Dandy-Walker malformation consists of dilatation of the fourth ventricle with direct communication with the cisterna magna (see Fig. 7.47). The posterior fossa is enlarged and the tentorium is elevated. The cerebellar vermis is absent or hypoplastic. The brainstem is compressed. Most cases have associated hydrocephalus (80%). The corpus callosum is usually absent (70%). Additional anomalies of brain development are commonly coexistent.
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  • Dandy-Walker variant is a lesser degree of abnormality with hypoplasia of the vermis and lesser enlargement of the fourth ventricle. The posterior fossa is normal in size. No hydrocephalus is present.
  • Arachnoid cyst of the posterior fossa does not show communication of fourth ventricle and cisterna magna, although the fourth ventricle and brainstem may be displaced. A cyst wall may be visualized.
  • Mega cisterna magna is a normal variant with a normal fourth ventricle, brainstem, and cerebellum.
Figure 10.22 Congenital Hydrocephalus–Aqueductal Stenosis. Coronal image shows marked symmetric dilatation of the lateral (L) and third (3) ventricles and the foramina of Monro (curved arrow). The temporal horns (t) are prominently dilated. The fourth ventricle was normal in size, indicating congenital aqueductal stenosis as the most likely diagnosis. When the lateral ventricles are enlarged, the cavum septum pellucidum (straight arrow) is often compressed. Cursors (+) measure the biventricular diameter.
Chiari Malformations
Chiari malformations are a group of probably unrelated developmental anomalies that involve the posterior fossa.
  • Chiari I malformation is downward displacement of the cerebellar tonsils with or without displacement of the fourth ventricle or medulla. The cisterna magna is obliterated.
  • Chiari II malformations are the most common and are nearly always present in association with meningomyelocele (see Fig. 7.44). The posterior fossa is small and the tentorium is displaced downward. The cerebellar tonsils and vermis herniate into the spinal canal through an enlarged foramen magnum obliterating the cisterna magna. The fourth ventricle is elongated. Obstruction of the cerebellar aqueduct results in enlargement of the lateral and third ventricles. The septum pellucidum is usually partially or completely absent.
  • Chiari III is an encephalomeningocele in the high cervical region that contains the cerebellum, medulla, and fourth ventricle.
  • Chiari IV is severe hypoplasia of the cerebellum without displacement.
Agenesis of the Corpus Callosum
Agenesis of the corpus callosum may be partial or complete. It is commonly associated with other anomalies, but when isolated may have a normal prognosis [8].
Figure 10.23 Agenesis of the Corpus Callosum. A. Coronal image shows absence of the hypoechoic band of the corpus callosum that crosses between the two hemispheres (arrow). The midline sagittal sinus extends all the way to the top of the fused frontal horns. The septum pellucidum is absent. B. Midline sagittal image confirms absence of the corpus callosum. The gyral pattern is somewhat disorganized in appearance. The massa intermedia (arrow) is seen in the third ventricle.
Figure 10.24 Vein of Galen Aneurysm. Posterior coronal image shows large cystic structure (arrow) that extends asymmetrically to the right. Doppler confirmed pulsatile turbulent blood flow.
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  • The well-defined hypoechoic band of the corpus callosum is partially or completely absent or is thinned (Fig. 10.23).
  • The lateral ventricles are abnormally far apart.
  • In coronal plane images, the frontal horns and bodies of the lateral ventricles have sharply angulated lateral peaks.
  • The third ventricle is dilated and elevated. Its roof extends between the lateral ventricles and may extend to the interhemispheric fissure.
  • Lipoma of the corpus callosum causes a highly echogenic mass that extends into both hemispheres. Dysgenesis of the corpus callosum is always present.
Vein of Galen Malformation
The vein of Galen malformation is the most common intracranial vascular anomaly presenting in the neonatal period. Multiple abnormal feeding vessels or an arteriovenous fistula with few feeding vessels drain into the vein of Galen, which becomes massively enlarged.
  • The vein of Galen is greatly dilated, resulting in a large cystic mass posterior to the third ventricle (Fig. 10.24).
  • Spectral or color flow Doppler reveals turbulent flow within the mass. Flow is pulsatile reflecting the arterial feeders.
  • Blood flow in the more peripheral areas of the brain is decreased because of a vascular steal phenomenon shunting flow to the low resistance vascular malformation. Shunting of blood can result in brain atrophy and cortical calcifications.
Infection
Congenital Brain Infections
Congenital and perinatal brain infections are most commonly caused by Toxoplasma gondii, cytomegalovirus, rubella, and herpes simplex virus type II. All infections cause destructive lesions. Prenatal infections may cause developmental brain defects.
  • Brain parenchymal calcifications are a major clue to the presence of congenital brain infection (Fig. 10.25). Calcifications may or may not cause acoustic shadowing. Effective treatment may result in resolution of the calcifications [19].
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  • Toxoplasmosis results in periventricular and cortical calcifications, microcephaly, hydrocephaly, porencephaly, and diffuse brain atrophy.
  • Cytomegalovirus causes periventricular calcifications, absent or small gyri, and hypoplasia of the cerebellum [20].
  • Rubella causes calcifications in the periventricular white matter and basal ganglia, hydrocephaly, and microcephaly.
  • Herpes simplex virus causes punctate or diffuse gyral calcifications, diffuse cerebral atrophy, and multicystic encephalomalacia.
Figure 10.25 Congenital Brain Infection. Coronal image shows punctate periventricular calcifications (arrows), indicating high likelihood of congenital brain infection. Serum titers for cytomegalovirus were elevated.
Perinatal Meningitis
Meningitis is a serious illness in the neonate that can cause permanent neurological injury even if treated early in its course. Causative organisms include group B streptococcus, Escherichia coli, Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitides. Infections are usually hematogenously disseminated and cause ventriculitis and diffuse meningoencephalitis.
  • US may be normal with early meningitis.
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  • Meningitis manifests as echogenic thickening of the meninges covering the gyri and sulci. Extraaxial fluid collections are usually present. Brain parenchyma echogenicity is focally or diffusely increased. Hydrocephalus may occur.
  • Ventricles are slit-like with early brain edema but onset of the ventriculitis causes ventricular enlargement with thickening and increased echogenicity of the lining ependyma. Echogenic fluid layers within the ventricles and avascular strands may cross the ventricular lumen.
  • Diffuse cerebritis appears as irregular patchy areas of increased and decreased parenchymal echogenicity (Fig. 10.26).
  • Brain abscess appears as a well-defined hypoechoic lesion containing echogenic, layering fluid. The wall is thick and echogenic. Mass effect displacing ventricles and brain structures is evident.
Figure 10.26 Diffuse Cerebritis. Candida septicemia in a premature infant caused diffuse cerebritis manifest as diffuse patchy areas of increased echogenicity (arrows).
Spine Sonography
The primary indication for sonography of the infant spine is to detect a tethered spinal cord in infants who are clinically suspected of being at risk. US offers an easy, quick, readily available, and accurate method of examination [21]. The posterior elements of the infant spine are cartilage and can easily be penetrated by US up to age 12 weeks. Older infants should undergo MR imaging because posterior element ossification will obscure sonographic detail.
Imaging Technique
Available radiographs of the infant spine should be reviewed prior to examination. Note any evidence of segmentation anomalies or spinal dysraphism. A 7.0-MHz linear array transducer provides an optimal examination. The infant is placed in prone position with the upper body elevated slightly to distend the lower thecal sac with CSF. The spine is scanned in longitudinal and transverse planes. Vertebrae from T12 to S2 are identified and numbered. The location of any skin abnormality should be documented. The location of the tip of the conus medullaris is determined and documented. Any abnormalities of spine development or the surrounding soft tissues are shown on transverse and longitudinal images. A standoff pad is useful for examining subcutaneous abnormalities.
Anatomy
The spinal cord is a hypoechoic rod that contains a central linear bright echo (Fig. 10.27). This central bright echo is not the central canal as originally believed but is caused by an acoustic interface between the myelinated ventral white commissure and the central portion of the anterior median fissure [22]. Nonetheless, this bright echo is adjacent to the central canal and serves as a useful marker of its location and as a landmark for the spinal cord itself. The spinal cord tapers to the conus medullaris, the tip of which is normally never lower than the L2-3 disc space [23]. The spinal cord ends as the filum terminale, a string-like structure that attaches to the first segment of the coccyx. The cauda equina is the group of nerve roots that surround and extend below the conus medullaris until they exit the spinal canal at their designated levels. The normal filum terminale is usually indistinguishable from the cauda equina. The spinal cord and cauda equina are surrounded by CSF and encased within the thecal sac. The thecal sac terminates at the S2 level. The anterior spinal artery may be seen pulsating in the anterior median fissure of the spinal cord. Smaller posterior spinal arteries pulsate on the dorsolateral aspect of the cord.
The key to US examination is determining the correct spinal levels by counting vertebrae. Landmarks used for counting vertebrae are listed in Box 10.2. Five sacral vertebrae make a curving arc that extends dorsally (Fig. 10.28). The vertebrae of the coccyx are not
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ossified but may be seen as hypoechoic structures [24]. Identify S5, then count vertebrae proceeding cranially. The lumbosacral junction (L5-S1) is at the angle between the relatively straight alignment of the lumbar vertebrae and the curving alignment of the sacral vertebrae. The top of the iliac crest marks the L4-5 disc space. The tip of the last rib is at L2.
Figure 10.27 Normal Spinal Cord. A. Longitudinal image shows the normal distal spinal cord (large arrow) tapering to the conus medullaris (long skinny arrow). Note the echogenic central linear echo (small arrow) characteristic of the cord. The cauda equina is seen as linear echogenic strands around and below the conus medullaris. The cauda equina is formed by nerve roots that arise from the more superior cord and course within the thecal sac to their exit foramina. The thecal sac (between black arrows) is distended with cerebrospinal fluid. Posterior vertebral elements cast acoustic shadows across the thecal sac. This cord terminates normally at the top of L2. B. Transverse image shows the full diameter of the spinal cord (white arrow) centered within the thecal sac (between black arrows). C. Transverse image near the tip of the conus medullaris (long skinny arrow) shows the nerve roots of the cauda equina (curved arrow) and anechoic cerebrospinal fluid within the thecal sac (between black arrows). D. Transverse image below the conus medullaris shows the normal clumping of the nerve roots of the cauda equina (curved arrow). The thecal sac (between black arrows) is well distended because the baby’s torso has been elevated to cause cerebrospinal fluid to flow to the lower sac to improve visualization.
Figure 10.28 Counting Vertebrae. The sacral vertebrae are easily recognized in longitudinal plane by the dorsally directed, curving arc they make. Sacral vertebrae are ossified and echogenic while coccygeal vertebrae are cartilaginous and hypoechoic. The lumbosacral junction of L5-S1 is recognized by the angle (arrow) formed by the alignment of the lumbar and sacral vertebrae.
Tethered Cord
An abnormally low and fixed position of the spinal cord is associated with bowel, bladder, and lower limb dysfunction in childhood. The tethered cord suffers progressive injury caused by ischemia, stretching, and compression during normal activities. Muscle weakness, gait disturbances, back or perineal pain, and radiculopathy may not present clinically until age 5 to 15 years. Early diagnosis allows for surgical repair before progressive cord damage occurs. Clinical findings associated with tethered cord include low lumbar dimple or cleft; hairy patch, mass, or discolored skin in lumbosacral area; vertebral anomalies of the lower spine; abnormal neurological examination of the lower extremities; and abnormal rectum (imperforate anus, rectal stenosis, cloacal abnormality).
  • Tip of the conus medullaris below the mid-portion of L2 is suspicious for tethered cord [23].
  • Tip of the conus medullaris below the L2-3 interspace level is unequivocally a tethered cord.
  • The tethered spinal cord is fixed dorsally in the spinal canal. The normal spinal cord is centered in the spinal canal and can move with changes in infant position.
  • The conus medullaris is atypical in shape. It may be wedged or blunted.
  • The filum terminale is thickened (>2 mm) and echogenic because of fat infiltration. The filum terminale is short providing abnormal fixation of the cord.
  • Pulsations of the spinal cord are absent because of fixation.
  • Intradural lipomas cause a focal echogenic mass enclosed within the dural sac. The bony spinal canal may be normal or focally enlarged when a lipoma is present.
  • A cleft spinal cord (diastematomyelia) is split in the sagittal plane by a fibrous, cartilaginous, or bony septum. Transverse images show the two rounded, usually equal size, portions of the cord.
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