Neonatal and Infant Head

Fontanels

The fontanels are unossified spaces between the bones of the infant skull, allowing for compression at birth and rapid brain growth thereafter. Colloquially termed “soft spots,” they provide the sonographer with acoustic windows, where the transducer is carefully placed to visualize and record brain structures ( Fig. 27.1 ). It is important to note their closure, which heavily hampers or completely impairs sonographic imaging. Generally, an acoustic window becomes limited at the beginning of the respective fontanel closure timeframe. Furthermore, these timeframes are relative to a term neonate, and preterm infants will be adjusted according to their gestational age at birth ( Box 27.1 ).

Neurosonography primarily utilizes the anterior and mastoid fontanels. The anterior fontanel is the largest at birth and provides an optimal sonographic view of the brain until 9 to 12 months of age, when it begins to close. 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 fontanelles, 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.

Meninges

There are three membranes called meninges that surround and form a protective covering for the brain: the dura mater, arachnoid mater, and pia mater. The pia 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 ( Fig. 27.2 ).

Lying between the delicate pia mater and arachnoid mater, the subarachnoid space contains cerebrospinal fluid (CSF) and branches of the arteries and veins of the brain. The intercommunication between these vascular channels and the CSF plays 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 composed of a double fold of dura mater, separating the right and left cerebral hemispheres. The tentorium cerebelli is an extension of the falx cerebri and separates the cerebrum from the cerebellum. Structures superior to this inverted echogenic V-shaped structure are supratentorial, and structures below it, infratentorial. The infratentorial structures of the brain are collectively referred to as the posterior fossa .

Ventricular System

The ventricular system is filled with CSF, which surrounds and protects the brain and spinal cord from physical impact. It also acts as a communication route for hormones and transmitters between areas of the central nervous system (CNS). The CSF-filled ventricular system includes the ventricles, their connecting foramina, and subarachnoid space, which are all contiguous with the central spinal column through the foramen magnum.

Lateral Ventricles

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 ( Fig. 27.3 ).

Ventricular Size and Variants

The sonographer should be aware that ventricular size varies with gestational age, and the premature infant will normally have larger-appearing ventricles than the term infant. Also, 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 rarer variant is coarctation of the ventricle ( Fig. 27.4 ), which will appear as a cyst in coronal view at the superior and lateral ventricle, often at the level of the intraventricular foramen. In both variant cases, care should be taken to make certain the cause is not a sequela, or sequential result, of an intraventricular hemorrhage (IVH) , such as ventricular dilation or a subependymal cyst.

Flow of Cerebrospinal Fluid

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 basal subarachnoid cisterns and cisterna magna. 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. The remaining small amount flows down into the subarachnoid space of the spinal canal, bathing the spinal cord ( Fig. 27.5 ).

Choroid Plexus

The choroid plexus is a mass of specialized cells located in the lateral, third, and fourth ventricles. The main and largest choroid is in the atria (atriums) of the lateral ventricles and is responsible for most of the CSF production. The choroid is prominent in preterm infants under 25 weeks of gestation and often develops a normal appearance by 30 weeks.

The atrial glomus is the largest part of the choroid plexus and tapers posteriorly. The glomus is a site for intraventricular bleeding in the term neonate compared with the much more common intraventricular hemorrhage in the caudothalamic groove in premature infants. The periventricular blush, or white matter, is the area of parenchyma surrounding the lateral ventricle containing the glomus. It is a major site for hypoxic-ischemic injury. The choroid plexus should always appear more echogenic than the surrounding brain tissue ( Fig. 27.6 ).

Other structures that produce CSF in the ventricular system include the intracranial and spinal subarachnoid lining. Additionally, the ventricles are lined with specialized ependyma that also produce CSF. These cells, along with the choroid plexus, regulate the normal intraventricular pressure by both secreting and absorbing CSF. However, in the setting of pathology, CSF production does not appreciably change, and absorption does not keep up, which may result in the ventricles enlarging, putting increased pressure on the brain parenchyma (intracranial pressure).

Cerebrum

Lobes of the Brain

The cortex is divided into four sections or lobes. These lobes correspond to the cerebral cortex located under the cranial bone of the same name and include the frontal, parietal, occipital, and temporal lobes ( Fig. 27.7 ).

Fissures

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 through which the middle cerebral artery courses ( Fig. 27.8 ). The quadrigeminal fissure is located posterior and inferior to 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, when prominent, and not a malformation of the vein of Galen.

Corpus Callosum

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 CSP ( Fig. 27.9 ). The development of the corpus callosum occurs between the 8th and 18th week 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 those portions that develop later will be absent. This is known as agenesis or dysgenesis of the corpus callosum, respectively.

Basal Ganglia

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 ( Fig. 27.10 ). It is further divided into the head, body, and tail.

The head of the caudate nucleus, at the caudothalamic groove or notch ( Fig. 27.11 ), is the most 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 a highly vascular bed of delicate blood vessels, especially in infants under 34 weeks of gestation.

Brainstem

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.

Cerebellum

The cerebellum , part of the hindbrain, is composed of two hemispheres that have a cauliflower-like appearance. The central cerebellar white matter is referred to as the arbor vitae (“tree of life”) due to its appearance and function of bringing motor and sensory information to and from the cerebellum. The cerebellum lies in the posterior cranial fossa under the tentorium cerebelli. The two hemispheres are connected by the vermis.

Fig. 27.12 summarizes the major anatomy of the midline sagittal plane. Fig. 27.13 summarizes the major sonographic anatomy of the midsagittal view for both a term infant and an extremely premature infant. (Notice the development of the gyri and cingulate sulcus and lack of a cavum vergae pellucidum in the term neonate. Also, the structures are better visualized in the premature infant due to a wider anterior fontanel and more prominent ventricular system.)

Cerebrovascular System

The cerebrovascular system consists of the internal cerebral arteries, vertebral arteries, the circle of Willis, and its major branches. The anterior cerebral artery (ACA) is one such major branch of the circle of Willis and can be visualized in a midsagittal plane. Branches of the ACA are routinely evaluated and include the pericallosal artery and callosomarginal artery. The pericallosal artery is frequently used to evaluate blood flow to the infant brain, and, as the name suggests, it flows up and around the corpus callosum. The cerebrovascular system is discussed in more detail in Chapter 38, Chapter 60 .

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 27.2 ). Since the structures in the infratentorial compartment are located relatively far from the transducer, the alteration of a deep focal placement and lowering the transducer frequency are recommended. In older infants, two transducers may be needed, one for the supratentorial structures and another for the infratentorial structures. 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 younger 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). Color Doppler evaluation (and pulsed wave Doppler, when indicated) of the pericallosal artery in the midsagittal view is also encouraged.

Knowing the gestational age of the premature infant when performing a neonatal sonogram of the head is essential. Table 27.2 summarizes the general sonographic findings in the term versus preterm neonate.

Additional/Modified Evaluation

While the posterior fontanel is often not useful beyond the neonatal period, or first four 4 weeks of life, it can provide more up-close sagittal and coronal views of the occipital horns, periventricular blush, and cerebellum in premature infants and neonates. It may be used routinely in this population or when pathology is suspected. It may also be used if imaging is restricted due to overlapping bones in the area of the anterior fontanel. The posterior fontanel approach is most helpful for any critical neonate on extracorporeal membrane oxygenation (ECMO) , where the mastoid view may be unattainable.

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. Clot in the occipital horns is best evaluated from a posterior approach (mastoid or posterior fontanel), and the foramen magnum is useful to evaluate the proximal end of the spinal canal, often in the setting of known or suspected Chiari malformation (type I or II). Additionally, any open suture, burr hole, or craniotomy defect can also serve as an acoustic window.

High-frequency linear array transducers used in the near-field can identify superior sagittal sinus thrombosis, cerebral edema, and subdural hematomas or subdural abscess secondary to meningitis. It is also used in evaluating for possible craniosynostosis, holding it perpendicular to the anticipated course of the coronal, sagittal, lambdoid, and metopic sutures.

Finally, although standard protocols are provided, additional oblique or axial views are very helpful and encouraged. This is particularly true in patients with ventricular shunt tubes, in which oblique angles help to identify the shunt and tip. 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.

Advanced Techniques in Neurosonography

Three-dimensional (3D) sonography is showing promise as an emerging method in imaging the neonatal head. This technique can reduce inter-operator variability and significantly reduce the time required to perform a neurosonology examination (about 15 minutes down to 1 to 2 minutes.) Research shows this is achievable 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. The 3D assessment of fluid volume in the setting of ventriculomegaly may be more accurate.

Additionally, ultrafast Doppler is a newer Doppler technique capable of quickly mapping cerebral vascular resistivity in neonates. It has superior temporal resolution, surpassing functional magnetic resonance imaging (MRI), sending out greater than 100,000 frames per second. For comparison, conventional color Doppler sends out around 50 frames per second. 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 (HIIs) in the future, as well as opening up knowledge about cerebrovascular regulation in infants in general.

Contrast-enhanced ultrasound allows for detailed perfusion studies of the infant head in the setting of post-cardiac arrest. Finally, shear-wave and strain elastography are also emerging in providing more information about hypoxic-ischemic events and the periventricular gray-white matter softening that can follow. All the above techniques provide clinicians a portable, relatively low-cost, and quick evaluation without sedation, providing valuable information, which may be used in serial monitoring of pathology.

Coronal Study

To obtain the coronal views, the transducer is placed with the notch to the infant’s right side on the anterior fontanel with the scanning plane following the coronal suture. 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.

Symmetric images must 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 skull to completely visualize the lateral and third ventricles, the deep subcortical white matter, the basal ganglia, as well as the peripheral brain parenchyma. The coronal view orientation and angles are summarized in Fig. 27.14 , with corresponding protocol images and annotations found in Fig. 27.15 and Table 27.3 , respectively.