On completion of this chapter, you should be able to:
Recognize normal neuroanatomy as it pertains to the ultrasound examination of the preterm and term neonate
Describe the coronal, sagittal, and mastoid view studies
Discuss the sonographic findings in neonatal brain pathology
Neurosonography is the primary imaging modality for high-risk and unstable premature infants because it is portable, nonionizing, and noninvasive and can be tolerated by the sickest infants, even immediately after birth. Furthermore, it is safe when adhering to the ALARA (as low as reasonably achievable) principle and is without contraindications, thus allowing for serial imaging of brain maturation and evolution of lesions. In the hands of a skilled sonographer or physician it is a reliable tool for the detection of most hemorrhage cystic and ischemic brain lesions, structural brain anomalies, and calcifications and cerebral infections. Although some conditions are not treatable, neurosonography allows for the assessment of neurologic prognosis, which aids parental counseling, as well as decisions on continuation of neonatal intensive care. Furthermore, it helps to optimize treatment of the infant and provides support to the family both during and after the neonatal period.
Neurologic impairment is one of the primary concerns about the health of premature infants. (See Table 27-1 for age/weight categories.) Intraventricular and subependymal hemorrhages occur in 40% to 70% of premature neonates under 34 weeks of gestation. Multifocal necrosis of the white matter, referred to as periventricular leukomalacia (PVL), may develop in 12% to 20% of infants weighing less than 2000 g. Additionally, any neonate who suffered a difficult delivery associated with hypoxia or asphyxia may be examined for PVL. These lesions are associated with increased mortality and an abnormal neurologic outcome.
|Preterm Definition||Gestational Age at Birth||Birth Weight categories and Median Weight at Gestational Age|
|Late preterm (or near-term)||Between 34 and 36 weeks (before 37 weeks)||Most premature births occur in this stage and carry the least amount of mortality and morbidity risks |
Low birth weight (LBW) defined as weighing less than 2500 g at birth = boys (35 weeks), girls (35.5 weeks)
|Moderately preterm||Between 32 and 34 weeks||2000 g at birth = boys (33 weeks), girls (33.5 weeks)|
|Very preterm||Less than 32 weeks||Very low birth weight (VLBW) defined as less than 1500 g at birth = boys (30.5 weeks), girls (31 weeks)|
|Extremely preterm||At or before 25 weeks||Extremely low birth weight (ELBW) defined as less than 1000 g at birth = boys (27.5 weeks), girls (28 weeks)|
This chapter aims to provide the reader with an introduction to the neonatal head examination and therefore the focus is on normal cranial anatomy, along with sonographic findings and protocols. Pathology in this chapter includes hydrocephalus, intracranial hemorrhage (ICH), hypoxic-ischemic lesions, congenital malformations, and infection in the neonate.
Normal anatomy and sonographic findings
Knowledge of the normal cranial anatomy is essential to performing the neonatal head examination. The cranial cavity contains the brain and its surrounding meninges and portions of the cranial nerves, arteries, veins, and venous sinuses. The following neonatal head structures and sonographic findings are provided to aid in performing neurosonology.
Fontanels are the spaces between the bones of the skull, which allow for compression at birth and rapid brain growth thereafter. They provide the sonographer with acoustic windows ( Figure 27-1 ) where the transducer is carefully placed to visualize and record brain structures. It is important to note their closure, which heavily hampers or completely impairs sonographic imaging. The anterior fontanel is the largest at birth and provides an optimal sonographic view of the brain until around 9 to 12 months of age, although the median age of closure is 13.8 months, with a range of 9 to 15 months. This fontanel may remain open longer than the normal range in cases of prematurity, hydrocephalus, hypothyroidism, and some bone disorders and chromosomal abnormalities, such as trisomy 13, 18, and 21. If hydrocephalus is present, it is felt to be bulging. Compressed or overlapping fontanels, on the other hand, due to oligohydramnios or a difficult delivery, may be difficult to palpate and provide a limited acoustic window to adequately image the structures of the brain.
There are three membranes called meninges that surround and form a protective covering for the brain: the dura mater, arachnoid, and pia mater membranes. The dura mater, or “soft mother,” lies against the delicate brain parenchyma; the arachnoid membrane is in the middle; and the dura mater, or “tough mother,” is a double-layered outer membrane that forms the strongest barrier ( Figure 27-2 ). The subarachnoid space is the interface between the vascular channels and the cerebrospinal fluid (CSF), playing an important role in the blood-brain barrier. It connects to the pia matter via the arachnoid trabeculae and to the dura matter via arachnoid granulations. The falx cerebri is a fibrous structure separating the two cerebral hemispheres. The tentorium cerebelli is an extension of the falx cerebri and separates the cerebrum and cerebellum. Structures above this V -shaped echogenic structure are said to be supratentorial, and structures below it are infratentorial.
The ventricular system is filled with CSF, which surrounds and protects the brain and spinal cord from physical impact. Putatively, it also acts as a communication route for hormones and transmitters between areas of the central nervous system. The CSF-filled ventricular system includes the ventricles, their connecting foraminae, and subarachnoid space, which are all contiguous with the central spinal column.
The lateral ventricles, located on either side of the brain, are the largest of the CSF-filled cavities located within the cerebral hemispheres and appear anechoic sonographically. The lateral ventricles are divided into four segments and are generally named after the lobe of the brain into which they project: the frontal horns, central body, temporal horns, and occipital horns. The body is the central section, just posterior to the frontal horn. The atrium (or trigone ) of the lateral ventricle is the site where the body, occipital, and temporal horns join together ( Figure 27-3 ).
The sonographer should be aware that ventricular size varies with gestational age and that the premature infant will normally have larger-appearing ventricles than the term infant. Dilation of the lateral ventricles often begins in the occipital horns. Minor asymmetry of the lateral ventricles is not uncommon, occurring in 20% to 40% of infants. The left side is often larger than the right. Another more rare variant is coarctation of the ventricle ( Figure 27-4 ), which will appear as a cyst in coronal view at the superior and lateral ventricle(s), often at the level of the intraventricular foramen. In both of these variant cases, care should be taken to make certain the cause is not a sequela of intraventricular hemorrhage (IVH), either dilation or subependymal cyst, respectively.
The lateral ventricles communicate with the third ventricle through the intraventricular foramen, or foramen of Monroe. The third ventricle is a narrow, irregularly shaped opening, which sits inferior and midline between the lateral ventricles. The roof is formed by the corpus callosum. Often the third ventricle is not visualized beyond 32 weeks of gestation. The cerebral aqueduct, or aqueduct of Sylvius, connects the third and fourth ventricles and is the most narrow passage. It is also the most common site for intraventricular blockage of CSF in the neonate. The medulla oblongata forms the floor of the fourth ventricle. The roof is formed by the cerebellar vermis and posterior medullary vellum. On a coronal view, the fourth ventricle should not be larger than half of the size of the vermis. The lateral angles of the fourth ventricle form the foramen of Luschka. The inferior angle, the foramen of Magendie, is continuous with the central canal of the spinal cord.
Subarachnoid space and cisterns.
Like the ventricles, the subarachnoid space and cisterns also play a role in the flow of CSF. The narrow subarachnoid space surrounding the brain and spinal cord contains a small amount of CSF. The subarachnoidal cisterns are the spaces at the base of the brain where the arachnoid becomes widely separated from the pia, giving rise to large cavities. The cisterna magna is one of the largest of these subarachnoidal cisterns and is consistently seen with ultrasound, appearing anechoic; it is located in the posterior fossa between the medulla oblongata, cerebellar hemispheres, and occipital bone.
Cerebrospinal fluid flow.
CSF from the lateral ventricles passes through the foramen of Monro to the third ventricle. The CSF then passes through the aqueduct of Sylvius to the fourth ventricle. From that point, the CSF may leave through the central foramen of Magendie or the lateral foramen of Luschka into the cisterna magna and the basal subarachnoid cisterns. The anterior flow continues upward through the chiasmatic cisterns, sylvian fissure, and the pericallosal cisterns up over the hemispheres, where it is reabsorbed by the arachnoid granulations in the sagittal sinus. Posteriorly the CSF flow moves around the cerebelli, through the tentorial incisure, the quadrigeminal cistern, the posterior callosal cistern, and up over the hemispheres. A small amount flows down into the spinal subarachnoid space.
The majority of the CSF results from the production of fluid by the choroid plexuses ( Figure 27-5 ). The ventricular ependyma, the intracranial subarachnoid lining, and the spinal subarachnoid lining also produce it. The CSF production does not change appreciably with increased intracranial pressure; however, when the production is significantly increased, there will be a corresponding change in intracranial pressure.
The choroid plexus is a mass of specialized cells located in the lateral, third, and fourth ventricles. The main and largest choroid is located in the atrium of the lateral ventricles and is responsible for most of the CSF production. These cells regulate the intraventricular pressure by secretion or absorption of CSF. The choroid is prominent in preterm infants under 25 weeks of gestation and often develops a normal appearance by 30 weeks. The glomus ( Figure 27-6 ) is the largest part of the choroid plexus and tapers posteriorly. It is a major site for bleeding in the term neonate. The choroid appears hyperechoic and should be more echogenic than the surrounding brain tissue, where small hemorrhages and areas of infarct within the parenchyma will also appear hyperechoic.
Cavum septum pellucidum.
The cavum septum pellucidum is a thin triangular space filled with CSF, which lies between the anterior horn of the lateral ventricles and forms the floor of the corpus callosum. Thus the cavum septum pellucidum is anterior to the corpus callosum. The cavum vergae is the posterior extension of the cavum septum pellucidum and is often closed in term neonates. However, it may be seen in preterm infants, as it normally closes around 6 months of gestation. The cavum septum pellucidum is present at birth in 50% to 61% of normal neonates and often closes within 3 to 6 months of life. However, it may persist for life in some individuals.
There are two cerebral hemispheres connected by the corpus callosum. They extend from the frontal to the occipital bones above the anterior and middle cranial fossae. Posteriorly, they extend above the tentorium cerebelli. They are separated by a longitudinal fissure into which projects the falx cerebri. The cerebrum consists of the gray and white matter. The outermost portion of the cerebrum is the cerebral cortex (composed of gray matter). White matter is located at the innermost portion of the cerebrum. The largest and densest bundle of white matter is the corpus callosum.
Lobes of the brain.
The cortex is divided into four lobes: frontal, parietal, occipital, and temporal, which correspond to the cranial bones with the same names ( Figure 27-7 ).
Gyrus and sulcus.
The gyri are convolutions on the surface of the brain caused by infolding of the cortex. The sulcus is a groove or depression on the surface of the brain separating the gyri. The sulci further divide the hemispheres into frontal, parietal, occipital, and temporal lobes. Sulci and gyri development is heavily dependent on the age of an infant. Sulci develop first around 22 weeks of gestation, with most formed by 28 weeks. However, sonographically sulci are not detected until around 26 weeks, and these very premature brains have a smooth appearance. The cingulate sulcus forms fully between weeks 28 and 31. Gyral development occurs after sulci development and even at 32 weeks this may be seen as asymmetric, with the right side more advanced.
The interhemispheric fissure is the area in which the falx cerebri sits and separates the two cerebral hemispheres. The sylvian fissure is located along the lateral-most aspect of the brain and is the area where the middle cerebral artery is located ( Figure 27-8 ). The quadrigeminal fissure is located posterior and inferior from the cavum vergae. The vein of Galen is also posterior, so the sonographer must be aware that Doppler should be utilized to make sure it is a fissure and not an enlarged vein of Galen.
The corpus callosum forms broad bands of connecting fibers between the cerebral hemispheres. This structure forms the roof of the lateral ventricles. The corpus callosum sits superior to the cavum septum pellucidum ( Figure 27-9 ). The development of the corpus callosum occurs between the 8th and 18th weeks of gestation, beginning ventrally and extending dorsally. The genu of the corpus callosum develops first, followed by the body and splenium (the posterior element). The rostrum develops last. If a uterine insult occurs, development may be partially arrested or complete agenesis may occur. If partial, the genu will be preserved, whereas the later part(s) will be absent. This is known as agenesis or dysgenesis of the corpus callosum.
The basal ganglia are a collection of gray matter that includes the caudate nucleus, lentiform nucleus, claustrum, and thalamus. The caudate nucleus is the portion of the brain that forms the lateral borders of the frontal horns of the lateral ventricles and lies anterior to the thalamus ( Figure 27-10 ). It is further divided into the head, body, and tail. The head of the caudate nucleus, at the caudothalamic groove or notch ( Figure 27-11 ), is a common site for hemorrhage. The caudate nucleus and lentiform nucleus are the largest basal ganglia. They serve as relay stations between the thalamus and the cerebral cortex.
The thalamus consists of two ovoid, egg-shaped brain structures situated on either side of the third ventricle superior to the brainstem. The thalamus borders the third ventricle and connects through the middle of the third ventricle by the massa intermedia, which is composed of gray matter and exists in most neonatal brains. The hypothalamus forms the floor of the third ventricle. The pituitary gland is connected to the hypothalamus by the infundibulum.
The germinal matrix includes periventricular tissue and the caudate nucleus. It is located 1 cm above the caudate nucleus in the floor of the lateral ventricle, at the caudothalamic groove. It sweeps from the frontal horn posteriorly into the temporal horn. It is indicated in 90% of premature brain bleeds and is made up of highly vascular delicate blood vessels, especially in infants under 34 weeks of gestation.
The brainstem is the part of the brain connecting the forebrain (cerebral hemispheres, thalamus, and hypothalamus) and the spinal cord. It consists of the midbrain, pons, and medulla oblongata.
The midbrain portion of the brain is narrow and connects the forebrain to the hindbrain. It consists of two halves called the cerebral peduncles, the cerebral aqueduct, the tectum, and tegmentum.
Pons and medulla oblongata.
The pons and medulla oblongata are part of the brainstem and hindbrain. The pons is found on the anterior surface of the cerebellum below the midbrain and above the medulla oblongata. The medulla oblongata extends from the pons to the foramen magnum where it continues as the spinal cord. This structure contains the fiber tracts between the brain and the spinal cord, and the vital centers that regulate important internal activities of the body (heart rate, respiration, and blood pressure).
The cerebellum, part of the hindbrain, is composed of two hemispheres that have a cauliflower-like appearance. The cerebellum lies in the posterior cranial fossa under the tentorium cerebelli. The two hemispheres are connected by the vermis. Figure 27-12 summarizes the major midline anatomy.
The cerebrovascular system consists of the internal cerebral arteries, vertebral arteries, and circle of Willis. When clinically indicated, the circle of Willis and its major branches are evaluated with pulsed-wave and color Doppler ultrasound from a transtemporal approach in determining cerebral blood flow patterns. The cerebrovascular system is discussed in more detail in Chapters 38 and 60 .
For the premature or sick infant, most neurosonography examinations are performed portably in the neonatal intensive care unit (NICU). Only well, term or older infants may be examined in the ultrasound laboratory, either on the examination table, in a car seat, or on the parent’s lap. For inpatients, the sonographer must be aware of the infant’s condition before the examination, and therefore should never begin an examination before first contacting the infant’s nurse. Two key concerns are keeping premature infants both safe and warm. They can lose a potentially dangerous amount of body heat quickly, so a small amount of warm gel (ideally from an individual packet) should be used, while scanning through the isolette portholes. If a large amount of cold gel is applied to the fontanel, or if the isolette is open for an extended period of time, the infant’s temperature may drop, which will often set off the infant’s thermal regulation. Likewise, if too much pressure is applied while performing the examination, it may bring an onset of bradycardia or thermal regulation. Additionally, the sonographer should be acutely aware of any lifesaving wires, tubes, and/or monitors (e.g., endotracheal tubes), limit head and neck movement, and practice good infection control techniques to minimize the spread of disease in this vulnerable population.
Neurosonology is performed primarily through the anterior fontanel, or easily felt “soft spot” on top of the head. A curved-array or sector high-frequency transducer of 7 to 10 MHz is best for imaging most preterm infants and neonates, whereas lower frequencies of 5 to 7 MHz may be needed for older infants with closing fontanels or infants with thick hair. A 3- to 5-MHz transducer may also be necessary to visualize deeper structures. More than one transducer may be needed. Often a dedicated neonatal head probe will provide the much-desired small footprint and 110- to 130-degree angle needed to obtain excellent contact and brain visualization. Likewise, specialized settings with low gray-scale contrast and multiple focal zones throughout the depth of the image help to optimize the lateral resolution of brain parenchyma to detect any subtle changes.
Neonatal head examination
Overview of the standard evaluation
Sonography of the neonatal brain is initiated through the anterior fontanel in coronal and sagittal views to study the supratentorial and infratentorial compartments ( Box 27-1 ). Although the structures in the infratentorial compartment are located relatively far from the transducer, the alteration of a deep focal placement or lowering the transducer frequency is recommended, particularly in older infants. The posterior cranial bones should always be present in the standard images to ensure the entire brain is being visualized.
The mastoid fontanel is used to better visualize the cerebellum and infratentorial compartment, or posterior fossa, in young infants. Cine clips should be used when possible to show the coronal and sagittal sweeps through the neonatal head. Additional magnified views are also encouraged to provide better resolution of any abnormal or high-risk areas (e.g., the caudothalamic groove in a preterm infant). A color Doppler (and pulsed-wave Doppler when indicated) evaluation of the pericallosal artery from the midsagittal view is also encouraged.
It is very important to note the approximate gestational age of the premature infant when performing a neonatal sonogram of the head. Table 27-2 summarizes the general sonographic findings in the term versus preterm neonate.
|Term Neonate (after 37 weeks)||Preterm Neonate (before 37 weeks)|
|Sulci and gyri are well developed.||Sulci and gyri are still developing or may be nearly absent in very preterm infants, also called the “smooth” brain appearance.|
|Slitlike ventricles or width increasing from 2 mm anteroposterior (AP) at frontal horns to 3–6 mm AP maximum at trigone on coronal study.||Preterm infants in general will have larger appearing ventricles overall.|
|Height of the bodies of the lateral ventricles are normally less than 7 mm AP at the level of midthalamus on a parasagittal study.|
|Third ventricle not usually seen on coronal study and may only visualize an echogenic formation immediately below the septum pellucidum.||Third ventricle is prominent and easily seen in premature infants less than 32 weeks.|
|Cavum septum pellucidum (CSP) is seen in 50%–61% of term neonates.||CSP nearly always visualized in preterm infants.|
|Cavum vergae is not visualized.||Cavum vergae may be present in very preterm infants, seen posterior to CSP.|
|Choroid plexus between 2–3 mm at the body of the lateral ventricles and 4–5 mm at the atria (or glomus * ) between 30–40 weeks.||Choroid plexus is very large in extremely preterm infants (25 weeks or less), should not be mistaken for intraventricular hemorrhages. Appearance similar to term neonate after 30 weeks.|
|Caudate nuclei show isoechoic or low areas of echogenicity compared with surrounding brain parenchyma.||Caudate nuclei (basal ganglia and thalami † ) may have a higher echogenicity than the surrounding brain parenchyma in preterm infants less than 32 weeks.|
|Periventricular white matter has low echogenicity, with thin echogenic streaks corresponding to small vessels. Should be less echogenic than the choroid.||‡ Periventricular area may show a slight increased echogenicity due to anisotropy in preterm infants. Scanning from the posterior fontanel this “blush” or “halo” should disappear.|
If imaging is restricted due to overlapping bones in the area of the anterior fontanel, or if pathology is suspected in the choroid plexus and lateral ventricles, the posterior fontanel may be used as an alternative window. The posterior fontanel approach may also be useful for any critical neonate on extracorporeal membrane oxygenation (ECMO), where the mastoid view is unattainable. The posterior fossa axial images, with the anterior portion of the transducer angled slightly cephalad, will demonstrate the infratentorial structures.
Pathology should always be evaluated in multiple planes, and with color and/or power and spectral Doppler. Additional views of pathology may be obtained from the posterior or mastoid fontanel, the foramen magnum, or thin areas over the temporal and parietal bones. A clot in the occipital horns is best evaluated from the posterior approach, and the foramen magnum is useful to evaluate the proximal end of the spinal canal. Additionally, any open suture, burr hole, or craniotomy defect can also serve as an acoustic window.
Small linear array, high-frequency transducers (10 to 15 MHz), with or without a standoff pad, may also be used to image near-field pathology. This transducer is used for thrombosis in the superior sagittal sinus, cerebral edema, and subdural hematomas or subdural abscess secondary to meningitis.
Finally, although protocols are provided, additional oblique or axial views may be very helpful and are encouraged for evaluating the neonatal brain. This is true in patients with ventricular shunt tubes, in which oblique angles help to identify the shunt and tip.
Three-dimensional (3D) sonography is showing promise as an emerging method in imaging the neonatal head. This technique can reduce interoperator variability and significantly reduce the time required to perform a neurosonology examination. On average a neonatal head examination will take 10 to 15 minutes, compared with 1 to 2 minutes with the 3D technique. Research now shows this is done without compromising diagnostic quality in comparison to the standard two-dimensional approach. It also allows for later 3D reconstructions and may be useful in better evaluating brain anomalies.
In addition to gray-scale 3D neurosonography, ultrafast Doppler is a new 3D Doppler technique capable of quickly mapping cerebral resistive index (RI) in neonates and infants. It has been called ultrasound angiography. It offers the evaluation of blood flow speed at a subcardiac cycle time scale, which other angiographic modalities are unable to assess. This may be useful for monitoring hypoxic-ischemic injuries in the future, as well as opening up knowledge about cerebrovascular regulation in infants in general.
To perform the coronal study, the transducer is placed on the anterior fontanel with the scanning plane following the coronal suture ( Figure 27-13 ). When looking at the coronal sections, the vertex of the skull is at the top and the left side of the brain is to the right of the image, and should be annotated as such. The middle of the transducer must be centered in the coronal suture to reduce bone interference and to procure the most extensive image of the brain. It is critical that symmetric images be obtained; this is accomplished by using the skull bones and the middle cerebral arteries at the sylvian fissure as landmarks. The skull bones and the arteries should be the same size bilaterally. In the coronal plane, the transducer is angled from the anterior to the posterior of the skull to completely visualize the lateral and third ventricles, the deep subcortical white matter, and the basal ganglia ( Box 27-2 ).
Frontal lobes, anterior interhemispheric fissure, and orbits.
Frontal horns of the lateral ventricles, head of the caudate nucleus, and fluid-filled cavum septum pellucidum (CSP) between the anterior horns. Corpus callosum is seen anterior to the CSP.
Level of the third ventricle. Echogenic choroid plexus seen in the floor of the lateral ventricles, as well as in the middle, in the roof of the third ventricle (may visualize third ventricle in preterm infants as a hypoechoic structure inferior to middle choroid). CSP seen here if present, between the bodies of the lateral ventricles. Brainstem is visualized in the posterior section.
Thalami seen anterior to the quadrigeminal cistern (echogenic star shape). Tentorium is also visualized, with echogenic vermis and cerebellum underneath. Cisterna magna is anechoic.
Choroid plexus glomi within the trigones of the lateral ventricles and periventricular white matter.
Periventricular white matter (periventricular blush), gyri and sulci of occipital lobe, and posterior interhemispheric fissure.
When the transducer is angled anteriorly, the frontal horns of the lateral ventricles appear as slitlike hypoechoic or cystic formations ( Figure 27-14 , B, 5). As the transducer is angled posteriorly, the ventricles acquire a comma-like shape and their width increases from the frontal horns to the atria or trigone ( Figure 27-14 , E, 28 ). The choroid plexus ( Figure 27-14 , 15 ) is an echogenic structure inside the ventricular cavities surrounding the thalamic nuclei. It lies along the floor of the lateral ventricles, extending from the temporal horn into the atrium and body of the lateral ventricles. It should not appear extending anterior into the frontal horns or posterior into the occipital horns. Increased hyperechoic areas in the floor of the ventricles, anterior to the third ventricle, would indicate hemorrhage at the caudothalamic groove ( Figure 27-14 , B, arrows ). At the intraventricular foramen, the choroid plexus enters into the third ventricle ( Figure 27-14 , C-1 and C-2, asterisk ). The choroid plexus becomes enlarged at the level of the atria (glomus of the choroid plexus) and can almost entirely fill the ventricular cavity ( Figure 27-14 , E, 15 ).
The third ventricle is not well visualized in the normal coronal study in the term neonate ( Figure 27-14 , C-1 , 13). An off-axis approach and high-frequency transducer may be helpful to identify it. Occasionally, a thin and very echogenic formation can be seen in the midline immediately below the septum pellucidum. This echogenic image corresponds to the choroid plexus extending into the roof of the third ventricle. The septum pellucidum appears as a midline hypoechoic to cystic structure separating the bodies and frontal horns of the lateral ventricles ( Figure 27-14 , B, 10 ). The cavum septum pellucidum is posterior to the corpus callosum.
Coronal and modified coronal views also visualize the basal ganglia and the white matter. The white matter has low echogenicity, with thin echogenic streaks that correspond to small vessels. In premature infants this area is known as a “watershed zone,” which are terminal ends of the vessel bed and may be used to describe the periventricular white matter. This area should be evaluated carefully. The echogenicity should never be greater than that of the choroid plexus; if it is, hemorrhage or infarction should be suspected and follow-up studies are required.
The cerebellar vermis is a very echogenic structure in the midline ( Figure 27-14 , D, 24 ). The fourth ventricle appears in the midline as a small anechoic space located anteriorly to the vermis and is not always seen in the coronal view. The cerebellar hemispheres ( Figure 27-14 , D, 25 ) are contiguous with the vermis. The cisterna magna ( Figure 27-14 , D, 26 ) corresponds to a nonechogenic space between the vermis, the cerebellar hemispheres, and the occipital bone.
The sagittal study is made by rotating the coronal plane approximately 90 degrees, positioned over the anterior fontanel, and aligning with the sagittal suture ( Figure 27-15 ). These sections are viewed with the anterior brain to the left and the occipital portion of the brain to the right. Sagittal studies provide the most extensive visualization of the brain.