This chapter should be cited as follows:
De Robertis V, Bosco M, et al, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.419303

The Continuous Textbook of Women’s Medicine Series – Obstetrics Module

Volume 18

Ultrasound in obstetrics

Volume Editors: Professor Caterina M (Katia) Bilardo, Amsterdam UMC, Amsterdam and University of Groningen, Groningen, The Netherlands
Dr Valentina Tsibizova, PREIS International School, Florence, Italy

Chapter

Ultrasound Evaluation of the Fetal Brain

First published: November 2024

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INTRODUCTION

Prenatal ultrasonographic evaluation of the fetal brain remains challenging due to the complexity of cerebral anatomy and the continuous changes in brain appearance throughout gestation. The widespread use of ultrasound screening for chromosomal anomalies in the first trimester,^1,2 along with the introduction of high-frequency transabdominal probes, has made it evident that several central nervous system (CNS) malformations can now be diagnosed at earlier stages of pregnancy.³ Some major brain structural defects (i.e. acrania, alobar holoprosencephaly, encephalocele) can be identified in virtually all cases during the first trimester.¹ As a result, thorough assessment of the fetal head and brain to identify major CNS anomalies is considered a minimum requirement for ultrasound examination at 11 + 0 to 14 + 0 weeks’ gestation.^4,5 However, since a significant proportion of fetal brain malformations is detectable mainly or only in the midtrimester (e.g. agenesis of the corpus callosum), the anomaly scan at 19–21 weeks’ gestation remains the cornerstone in screening for CNS abnormalities.^6,7,8,9,10 Although there is no unanimous agreement among scientific societies regarding the role of third-trimester ultrasound in screening for fetal anomalies, several structural anomalies, including some CNS anomalies, become evident only at this gestational age. This is largely due to neuronal development, particularly the migration process, which occurs later in gestation.¹¹ Additionally, a wide range of acquired conditions, such as brain hemorrhage/stroke, tumors and arachnoid cysts are more commonly identified in the third trimester.⁶ Therefore, when a third-trimester scan is performed for any indication, assessment of the fetal CNS is recommended.¹²

BASIC EXAMINATION OF THE FETAL BRAIN AT THE FIRST-TRIMESTER SCAN

Axial plane of the fetal head and brain

Obtaining the axial view of the head and brain (Figure 1A) is considered among the minimum requirements for ultrasound examination at 11 + 0 to 14 + 0 weeks’ gestation, as outlined in the latest first-trimester examination guidelines.^4,5 This view allows visualization of skull ossification and symmetry of the developing brain structures. Cranial bone ossification should be visible by 11 completed gestational weeks.⁵ In the axial section, the two lateral ventricles are visible, largely occupied by the hyperechoic choroid plexuses, forming the characteristic butterfly sign.¹³ The two hemispheres, similar in size, are separated by a straight, uninterrupted midline echo (interhemispheric fissure and falx). The cortex, which appears very thin, is best observed anteriorly, lining the large fluid-filled ventricles. Although dating of pregnancy is based on crown–rump length (CRL) measurement, the biparietal diameter (BPD) and the head circumference (HC) may also be measured at this stage of pregnancy in the axial view.⁴

Midsagittal plane of the face and brain

The midsagittal plane is that used for measurement of nuchal translucency and assessment of the nasal bone (Figure 1B). It can also be used to evaluate the posterior fossa and visualize the intracranial translucency (IT, fourth ventricle), as well as the brainstem, serving as a screening method for early diagnosis of open neural tube defects and cystic posterior fossa malformations.^{14,15,16,17,18} In the midsagittal view, the diencephalon appears as a round, anechoic structure in the middle of the fetal brain. More posteriorly, the structures of the posterior fossa, including the brainstem, fourth ventricle and cisterna magna, appear as three anechoic spaces, approximately equal in size (Figure 1B).

BASIC EXAMINATION OF THE BRAIN AT THE MIDTRIMESTER ANOMALY SCAN

In the midtrimester, the basic ultrasound examination of the fetal brain is traditionally performed using a transabdominal probe.^8,9,10 The standard evaluation relies on the visualization of three axial planes (i.e. transthalamic, transventricular and transcerebellar). However, a significant limitation of these planes is the inadequate visualization of the hemisphere proximal to the transducer and the difficulty in visualizing midline brain structures, particularly the corpus callosum and the cerebellar vermis, due to their anatomical location and orientation. To address this issue, recent guidelines⁹ recommend supplementing the traditional axial planes with the median/midsagittal view of the fetal brain. This plane allows for direct assessment of the normal anatomy of the corpus callosum and cerebellar vermis. In most cases, standard transabdominal ultrasound is sufficient to demonstrate fetal brain structures in the midtrimester. However, in cases in which the fetus is with head down or in vertex presentation, a transvaginal scan can provide a higher-resolution evaluation. Any suspicion of an anomaly, either at the first-trimester scan or routine midtrimester screening ultrasound, is an indication for referral for targeted fetal neurosonography as a diagnostic examination.⁶ This examination requires a high level of expertise in both the transabdominal and transvaginal approaches, as well as the use of high-resolution transducers. Recently, a consensus was published on the description and interpretation of abnormal ultrasound findings in each trimester suggestive of the most common CNS anomalies.¹⁹ This effort to standardize the reporting of suspicious findings and the adoption of a standardized approach has the potential to influence referral management and also impact obstetrician training and contribute to reducing the prevalence of false-positive cases at the routine examination.

TARGETED FETAL NEUROSONOGRAPHY

Targeted fetal neurosonography is a dedicated, multiplanar, diagnostic examination indicated for fetuses at high risk or with suspicion of CNS or spinal malformation.⁶ Common indications for referral are listed in Table 1. Targeted fetal neurosonography is recognized as having a much greater diagnostic potential than does the basic screening examination and it is particularly helpful in the evaluation of complex malformations.^6,9 Nonetheless, proficiency in this technique requires a high level of expertise, encompassing not only dexterity in conducting the scan, but also a comprehensive understanding of cerebral developmental and acquired lesions as well as genetics.

Since CNS anomalies can be detected at various gestational ages depending on the specific anomaly, fetal neurosonography should be tailored to the gestational age period during which the anomaly is suspected.

The primary characteristics distinguishing fetal neurosonography from the screening examination are:

• The transvaginal approach should always be attempted as it is the preferred method to perform an adequate high-resolution targeted neurosonographic examination. When this is not technically feasible (e.g. breech presentation, twin pregnancy), the use of high-frequency transducers is encouraged, as these provide higher resolution than conventional convex probes.⁶
• The basis of the neurosonographic examination of the fetal brain and spine is the multiplanar approach with a combination of the axial, coronal and sagittal planes.
• The use of the three-dimensional (3D) multiplanar image correlation is highly recommended when performing fetal neurosonography.

CEREBRAL ANOMALIES BY SCANNING VIEW

Here we provide a comprehensive description of the planes obtained during targeted fetal neurosonography, detailing the structures that can be imaged in each of these planes and the possible associated malformations. The description specifically focuses on structures that can be visualized from the midtrimester onwards.

Axial planes

Axial transventricular plane and related malformations

This is the most cephalad axial plane of the fetal head demonstrating the lateral ventricles (Figure 2A, Videoclip 1), with the anterior (or frontal) horns and the continuum between the atria and the occipital horns, posteriorly. This plane allows for the examination of the normal shape, ossification and mineralization of the fetal skull. Additionally, it is used to assess the symmetry and proper separation of the cerebral hemispheres. Under normal conditions, the cerebral hemispheres are completely separated by a hyperechoic straight line representing the interhemispheric fissure and the falx. The only interruption of this line is where the cavum septi pellucidi (CSP) is located.¹⁷ The frontal horns appear as two comma-shaped, fluid-filled structures with well-defined lateral walls, and they are separated centrally by the CSP. The CSP is a box-like fluid-filled cavity between two thin membranes. The formation of the CSP begins as early as 12 weeks’ gestation, it is usually fully formed by 15–16 gestational weeks’ gestation, and it starts to close at approximately at approximately 24 weeks’ gestation. Therefore, it should always be demonstrable between 18 and 37 weeks, while failure to detect the CSP prior to 15–16 weeks or later than 37 weeks is a normal finding.²⁰ The frontal horns of the lateral ventricles and the CSP are main components of the anterior complex (Figure 2B, Videoclip 1),²¹ a key anatomical landmark observed in the routine transventricular plane. The importance of visualizing this complex is because its abnormal appearance is associated with midline cerebral anomalies. In the anterior complex a cross-section of the genu of the corpus callosum is also visible (Figure 2B).

From 16 weeks of gestation the occipital horns become more evident. Each atrium is characterized by the presence of the glomus of the choroid plexus, which is highly echogenic, while the occipital horn is filled with cerebrospinal fluid. Especially during the second trimester of gestation, both medial and lateral walls of each ventricle run parallel to the midline, making them clearly visible on this plane as two regular echogenic lines (Figure 3A). Under normal circumstances, the glomus of the choroid plexus fills the cavity of the ventricle at the level of the atrium, coming into contact with both the medial and lateral walls. However, in some physiological situations, a small rim of fluid may be visible between the medial wall and the choroid plexus.²² To exclude the presence of ventriculomegaly (VM), the width of the atrium distal to the transducer is measured at the level of the glomus of the choroid plexus. The measurement is made perpendicularly to the ventricular axis by positioning the calipers on the inner sides of the echogenic ventricular walls (inner-to-inner borders)^4,5 (Figure 3B). The width should be less than 10 mm, irrespective of gestational age.²³ The measurement site at the level of the glomus is chosen because, regardless of etiology, dilatation of the lateral ventricle typically begins at the caudal portion (atrium and posterior horn). It is important to note that, due to artifacts in the near field of the image caused by shadowing from the proximal parietal bone, the standard transventricular plane typically allows clear visualization only of the hemisphere and the lateral ventricle distal to the transducer (Figure 2A). The limited visualization of the lateral ventricle proximal to the transducer might restrict the detection rate of unilateral VM.

Detecting unilateral VM affecting the proximal ventricle often relies on a qualitative assessment (Videoclip 2), as the measurement of this ventricle is generally suboptimal. When the proximal ventricle seems subjectively significantly larger than the distal one, it is advisable to refer the patient for fetal neurosonography.²⁴ For the same reasons, on this plane, the evaluation of the symmetry of the hemispheres might be challenging. However, in the absence of major distortion of the falx, symmetry of the brain is generally assumed, at least in the context of screening examinations. On the transventricular view, Sylvian fissure opercularization can also be easily assessed.²⁵ This fissure is always visible during pregnancy but its appearance changes throughout gestation. At 20 weeks, the Sylvian fissure is smooth without any angularity, whereas, after 24 weeks’ gestation, Sylvian fissure opercularization typically becomes more evident. The CSP, lateral ventricles and midline should be evaluated carefully on the transventricular view. In fact, a large group of mild and severe malformations can be suspected in this key plane of the fetal head (Figure 4, Videoclip 3).

Axial transcerebellar plane and related malformations

The axial transcerebellar plane is caudal to the transventricular one and it is obtained by slightly tilting the transducer from the transthalamic axial plane towards the posterior fossa. The transcerebellar plane is used to visualize the posterior fossa with its content: the cerebellar hemispheres, the cerebellar vermis and the cisterna magna. On this plane, the cerebellum appears as a butterfly-shaped (or dumbbell-shaped) structure formed by the round cerebellar hemispheres joined in the middle by the slightly more echogenic cerebellar vermis (Figure 5A). The cerebellar hemispheres should be homogeneous and symmetrically round, with smooth borders. The transcerebellar diameter should be measured in this section and the distance between its lateral edges (cerebellar diameter) corresponds in millimeters to the gestational age in weeks (Figure 5B). If the probe is moved slightly downwards, the fourth ventricle is also visible as a tiny anechoic space between the vermis and the brainstem (Figure 5C). While the normal appearance of the cerebellar hemispheres and fourth ventricle can be assessed in this axial plane, a thorough evaluation of the vermis requires serial axial planes to demonstrate the different portions of the cerebellar vermis.⁴ Posterior to the cerebellum, the cisterna magna appears as a fluid-filled space which usually contains thin septations which are thought to represent the remnants of Blake’s pouch, that physiologically ruptures around 13 weeks’ gestation to form the Magendie’s foramen.²⁶ The anteroposterior diameter of the cisterna magna may be measured in this section as the distance between the vermis and the inner border of the occipital bone, and it should not exceed 10 mm (Figure 5B). While assessing the posterior fossa, particularly before 19 weeks of gestation, it should be taken into account that the rotation of the vermis may still be incomplete; in this case, the fourth ventricle may be visible between the two cerebellar hemispheres, creating a misleading impression of a vermian defect. As a rule of thumb, from 19 gestational weeks onwards, there should be no midline fluid-filled space between the two cerebellar hemispheres. Care should be taken to avoid overtilting of the probe, since this increases the likelihood of false-positive diagnosis of a vermian anomaly.

Several anomalies of the cerebellum and the posterior fossa can be suspected or diagnosed through the transcerebellar view (Figure 6). However, only some diagnoses can be confirmed in this plane, such as Chiari II malformation, associated with open spinal dysraphisms (Figure 6A), and mega cisterna magna (Figure 6B). Conversely, when the cerebellar hemispheres are splayed and the vermis is not visible (Figure 6C, Videoclip 4), a cystic abnormality of the posterior fossa should be suspected. However, in this scenario, only the midsagittal view can effectively distinguish among various cystic posterior fossa anomalies, including Dandy–Walker malformation (DWM), Blake’s pouch cyst (BPC) and vermian hypoplasia.

Axial transthalamic plane and related malformations

The axial transthalamic plane is obtained parallel but caudal to the transventricular plane. The anatomic landmarks include, from anterior to posterior, the frontal horns of the lateral ventricles, the CSP, the thalami and the hippocampal gyri (Figure 7A). It is commonly referred to as the BPD plane as this plane is used for measuring the BPD to assess fetal size. This is because this plane has been shown to be easy to obtain in late gestation and, therefore, it allows more reproducible measurements throughout pregnancy than, for instance, the transventricular plane.²⁷ Apart from its use in the setting of fetal biometry, the transthalamic view allows the detection of several malformations of the skull and brain (Figure 7B).

Sagittal planes

Midsagittal (or median) plane

The midsagittal (or median) plane should be acquired through the anterior and posterior fontanelle (Figure 8, Videoclip 5). The selection of the fontanelle depends on the specific area of interest or pathology. If the examination focuses on the posterior part of the brain (i.e. posterior fossa), the imaging plane should be acquired through the posterior fontanelle. Conversely, for a comprehensive assessment of the midline anatomical structures, the midsagittal plane should be obtained through the anterior fontanelle or through the unossified sagittal suture (Figure 8A). The midsagittal view is the reference plane for assessing all major midline structures such as the corpus callosum and its components (rostrum, genu, body and splenium), and its close relationship with the CSP and cavum vergae, when present. Below the CSP, the third ventricle can be identified as a hypoechoic structure, while its cranial portion appears hyperechogenic due to the presence of the tela choroidea (choroid plexus of the third ventricle). As illustrated in Figure 8A, the anterior approach also allows access to the infratentorial area. However, if the focus is on this particular area, a posterior approach through the posterior fontanelle is also recommended (Figure 8B). In the latter scenario, care should be taken to avoid shadowing from the occipital bone, as this artifact can significantly limit anatomical assessment of the hindbrain.

The posterior midsagittal view enables a thorough assessment of all midline anatomical structures of the posterior fossa, such as the vermis (with the primary fissure and, later in gestation, the secondary fissure), the choroid plexus of the fourth ventricle (4^thV-CP), the fourth ventricle, the tentorium, the cisterna magna and the brainstem. After 18–19 weeks’ gestation, the vermis is formed and it closes the fourth ventricle; therefore, demonstration of an opening may be indicative of a cystic posterior fossa anomaly. Recent findings have shown that the 4^thV-CP has been mistakenly interpreted as the lower portion of the vermis. It is now understood that closure of the fourth ventricle is determined by both the 4^thV-CP and the vermis and not only by the vermis.²⁶ The medial portion of the 4^thV-CP can be visualized as a separate structure attached to the inferior part of the cerebellar vermis using high-frequency transabdominal and high- or standard-frequency transvaginal transducers (Figure 8B). Additionally, the position of the 4^thV-CP could inform the differential diagnosis of fetal cystic posterior fossa abnormalities, characterized by an open fourth ventricle, such as DWM and the BPC, which have a significantly different prognosis.^26,28,29,30 Accurate diagnosis of several forebrain and hindbrain anomalies requires the midsagittal plane (Figure 9, Videoclip 6).

Parasagittal (left and right) planes

The parasagittal planes are obtained by slowly tilting the transducer laterally from the midsagittal plane. On these planes, the following anatomic structures can be visualized: the lateral ventricles with the choroid plexuses, the periventricular brain parenchyma and, especially in the third trimester, the gyri of the cortex (on the convex surface of the brain) as well as a variable portion of the insulae/Sylvian fissures (Figure 10A). If the transducer is tilted more laterally, the temporal horns of the ventricles and the insulae will become visible. Abnormalities of the cortex can be detected in this view (Figure 10B).

Coronal planes

Coronal transfrontal plane

The coronal transfrontal plane (Figure 11A) represents the most anterior coronal plane included in the neurosonographic examination, acquired via the anterior fontanelle. It displays the interhemispheric fissure and the frontal lobes of the brain. The skull base, with the sphenoid bone and, occasionally the orbits, are visible. This plane constitutes the key plane for excluding fusion of the frontal lobes, especially when suspecting lobar holoprosencephaly.

Coronal transcaudate plane

The coronal transcaudate plane (Figure 11B, Videoclip 7) is also acquired through the anterior fontanelle, slightly tilting posteriorly the transducer from the transfrontal plane. It likely represents one of the most important neurosonographic planes, alongside the midsagittal ones. On this plane, the following anatomic structures can be demonstrated: the frontal horns of the lateral ventricles, the CSP (appearing as a triangular/trapezoid structure below the corpus callosum and between the two frontal horns), the cross-section of the anterior part of the body of the corpus callosum (appearing as a mildly hypoechoic area on top of the CSP and between the frontal horns), the cerebral falx, the ganglionic eminence and the caudate nuclei. This view allows identification of several CNS malformations (Figure 12).

Coronal transthalamic plane

The coronal transthalamic plane (Figure 11C) is positioned just posterior to the transcaudate one. To achieve an optimal view, the transducer should be tilted slightly more posteriorly or, alternatively, slid along the as yet unossified sagittal suture. The midline marker of this plane is the slit-like third ventricle with the two Monro foramina. Close to the skull base, the basal cistern contains the blood vessels of the circle of Willis and the optic chiasm. Of note, the Sylvian fissure is best displayed in this view. However, to visualize effectively this structure, the tip of the transducer should firmly indent the fontanelle; otherwise, the parietal bone will cast a shadow that hinders visualization of the insulae and the Sylvian fissures.

Coronal transcerebellar plane

The coronal transcerebellar plane (Figure 11D, Videoclip 8) represents the only coronal plane that needs to be obtained through the posterior fontanelle. In this plane, the posterior sections of the hemispheres, as well as the infratentorial structures, are visualized. Specifically, the following anatomic landmarks are identifiable: the occipital horns of the lateral ventricles, the interhemispheric fissure, the calcarine, and, at a deeper level, the parieto-occipital fissures. Beneath the tentorium, the cerebellar hemispheres and relatively hyperechoic vermis are seen in cross-section.

BASIC EXAMINATION OF THE FETAL BRAIN AT THE THIRD-TRIMESTER SCAN

Some fetal abnormalities may go undetected during the routine midtrimester scan. Broadly, this can occur for two main reasons: first, the abnormality was present at the time of the anomaly scan but was missed; second, the fetal abnormality developed or became apparent only after the second trimester.¹² A recent systematic review of 13 studies, involving over 140 000 women, reported a prevalence of 3.7 per 1000 women with fetal anomalies diagnosed in the third trimester, with CNS anomalies accounting for 18% of all cases.³¹ According to recently published ISUOG guidelines,¹² a third-trimester structural examination should include:

• Assessment of head size and shape to rule out evolving conditions such as microcephaly and craniosynostosis.
• Evaluation of hemispheric symmetry, lateral ventricle width and the cerebral cortex. In some cases, transabdominal imaging may not provide a clear view of the brain's anatomy, requiring a transvaginal approach for better visualization.

To achieve these objectives, the transthalamic and transventricular views should be included in the protocol for the routine third-trimester scan.

THREE-DIMENSIONAL ULTRASOUND OF CNS ANATOMY

The use of 3D ultrasound is recommended in targeted neurosonography.⁶ This technology offers several advantages in evaluating the fetal brain, enhancing both the diagnostic process and image quality. Firstly, the ability to perform multiplanar image correlation allows for the acquisition of precisely aligned views across the three orthogonal planes (Figure 13).

Unlike 2D imaging, which provides a limited view, 3D ultrasound facilitates a more detailed and comprehensive assessment of the brain, capturing structures and planes that may otherwise be difficult to evaluate. Additionally, 3D imaging improves the reproducibility of results. By capturing volumetric data, images can be stored and revisited for further analysis, thereby reducing variability between examiners and increasing the consistency of findings. Finally, the option to display thicker ‘slices’ of the brain improves the signal-to-background noise ratio across all three planes, significantly enhancing image quality. These advantages underscore the recommendation to adopt a 3D approach in neurosonography.

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