The standard examination of the fetal brain includes the transventricular axial plane and the midsagittal one, allowing the evaluation of the main cerebral structures, and head biometric measurements.

Abstract: Prenatal detection of congenital malformations of the central nervous system typically begins with the first trimester ultrasound at 11 – 13 weeks of gestation. Advancements in prenatal diagnosis have led to a detection in the first trimester close to 100% for some brain anomalies resulting in significant distortion of the brain anatomy, such as alobar holoprosencephaly, acrania-exencephaly-anencephaly sequence, and cephalocele. The standard examination of the fetal brain includes the transventricular axial plane and the midsagittal one, allowing the evaluation of the main cerebral structures, and head biometric measurements. If a brain malformation is suspected during this examination, the patient should be referred for early neurosonography to confirm and characterize the anomaly, determine its prognosis, and establish an appropriate management plan. While the best approach for early neurosonography remains unknown, we recommend a multiplanar examination of the fetal brain, including additional axial (i.e., transthalamic and transcerebellar plane), coronal (i.e., transfrontal, transcaudate, transthalamic and occipital planes), and midsagittal planes (i.e., posterior midsagittal plane). This chapter will provide an overview of the fetal brain planes and normal anatomical structures that should be recognized during routine and advanced fetal brain ultrasound examinations at 11-13 weeks of pregnancy.

Authors: Nicola Volpe1, Ruben Ramirez Zegarra1, Tullio Ghi1

1. Department of Medicine and Surgery, Obstetrics and Gynecology Unit, University of Parma, Parma, Italy

Reviewers: Karen Fung-Kee-Fung

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Introduction

The combined screening for major chromosomal anomalies is usually performed in the first trimester of pregnancy and includes an ultrasound examination of the fetus between 11 and 13 completed weeks of gestation. This evaluation encompasses a thorough assessment of the fetal anatomy, including the measurement of the nuchal translucency (NT), a key parameter in the risk assessment of fetal chromosomal disorders (1–3). Despite the improved detection rates of major fetal abnormalities in first trimester ultrasound (4–14), there remains a lack of standardized ultrasound methodology for early fetal anatomy examinations (15). In an attempt to address this issue, various international societies have developed different protocols and guidelines for performing routine anatomy examinations at 11-13 weeks with the aim of achieving standardization (15–17). 


The detection rate of fetal central nervous system (CNS) anomalies leading to a gross distortion of brain anatomy, such as alobar holoprosencephaly, anencephaly and cephalocele, is close to 100% in first trimester ultrasound (4,14). Additionally, other major CNS defects, including cystic anomalies of the posterior fossa (18–20) or open spinal dysraphism, with Chiari II malformation (21–26) – traditionally considered visible only at midtrimester ultrasound – can be suspected as early as 11-13 weeks of gestation in the presence of subtle changes in some fetal brain structures. Achieving such high detection rates requires the inclusion of specific brain anatomical landmarks and adherence to a standardized protocol during routine first trimester ultrasound (4).


However, there is still an ongoing debate regarding the specific planes and structures of the fetal brain that should be incorporated into a routine sonographic anatomical survey during the first trimester; albeit, certain minimal requirements have been suggested (16,17). The routine assessment of the fetal brain should include axial (27) and midsagittal planes to conduct a basic examination of the brain anatomy, also including the measurement of NT (16,23). Identification of any brain abnormality should prompt referral for expert ultrasound – i.e., early neurosonography – to confirm and characterize the anomaly, determine its prognosis, and establish an appropriate management plan (27). This expert evaluation should encompass a multiplanar examination of the fetal brain anatomy, including additional axial, sagittal and coronal views (28).  


This chapter will provide an overview of the fetal brain planes and normal anatomical structures that should be recognized during a routine fetal brain ultrasound at 11-13 weeks of pregnancy, with a particular focus on two modalities: 
1) Routine examination of the fetal brain (recommended for low-risk pregnancies) 
2) Targeted early neurosonography (recommended for high-risk pregnancies or those with suspected brain anomalies). 

Screening examination of the fetal brain

Indication

The basic assessment of the fetal brain on axial planes should be offered to all pregnant women between 11+0 and 14+0 weeks of gestation as part of routine pregnancy care (17).

Routine ultrasound planes of the fetal brain at 11+0-14+0 weeks’ gestation

The routine evaluation of the fetal brain during the first trimester of pregnancy is usually performed using transabdominal ultrasound. This sonographic evaluation relies on the acquisition and assessment of standardized scanning planes, which are defined by specific anatomical landmarks. These include the following standard planes (16,17):

-    Transventricular axial plane
-    Midsagittal plane

The transventricular axial plane of the fetal head is obtained by positioning the ultrasound probe transversally to the fetus at the level of the vault of the calvarium and performing a caudal sweep of the probe until the lateral ventricles are visualized (Figure 1), just above the thalami. The midsagittal plane is obtained with the probe placed longitudinally over the anterior fontanelle, parallel to the midline of the fetal brain (Figure 2). Each plane is characterized by the following anatomical structures and landmarks:

Transventricular axial plane: 
o    cranial bone ossification and integrity
o    symmetric brain hemispheres
o    interhemispheric fissure
o    lateral ventricles filled with choroid plexuses 

In more detail, the transventricular axial plane (Figure 1) allows the anatomical evaluation of the following structures:

The fetal calvarium (skull) appears as a uniformly hyperechoic bony structure with an uninterrupted oval shape. Any bulging or protrusion of the intracranial structures or irregularities in the skull shape should raise suspicion of brain abnormalities, such as cephalocele, acrania-exencephaly-anencephaly sequence, or another neural tube defect (29,30). 

The interhemispheric fissure appears as a straight, uninterrupted hyperechoic midline echo dividing the fetal brain into two equal-size hemispheres. Disruption of the continuity of the interhemispheric line could reveal brain malformations such as alobar holoprosencephaly (31). 

The lateral ventricles are two large, oval-shaped, fluid-filled structures located on both sides of the midline echo. Either lateral ventricle contains the hyperechoic choroid plexus, which occupies about half or more of the ventricle length/area (25,32). The normal appearance of the choroid plexuses on the axial plane is commonly referred to as the “butterfly sign”. A smaller choroid plexus size may indicate an abnormally increased amount of ventricular fluid, as seen in ventriculomegaly (32,33); whereas a choroid plexus filling most of the ventricular area may suggest fluid reduction, as seen in open spinal dysraphism (25,34). 

The rudimentary cortex appears as a thin smooth layer surrounding the ventricles and the external brain surface with two small recesses on the lateral surface of each brain hemisphere, representing the future Sylvian fissures. No other sulci or gyri are visible at this stage. 

Midsagittal plane: 
o    fetal profile depicting the forehead, nose (bone, skin, and tip), and chin
o    rectangularly-shaped hard palate
o    diencephalon
o    Sylvian aqueduct
o    brainstem, fourth ventricle (with choroid plexus) and cisterna magna in the posterior fossa

The midsagittal plane (Figure 2A) serves as the reference plane for the measurement of the NT and allows the evaluation of the following structures:

The diencephalon appears as a hypoechoic, round structure in the middle of the fetal brain, of which the thalami and the third ventricle are recognizable at this stage. In approximately 50% of cases when using a transvaginal probe with high-resolution imaging, the cavum veli interpositi can be visualized on the roof of the diencephalon as an anechoic elongated structure (35) (Figure 2B). 

Posterior and caudal to the diencephalon, one can recognize three parallel anechoic spaces between the sphenoid bone and occipital bone (36). From anterior to posterior, these spaces represent the brainstem (including the mesencephalon, pons and medulla oblongata), the fourth ventricle (i.e., “intracranial translucency”), and the cisterna magna (Figure 2A). Posterior to mesencephalon and superior to the fourth ventricle, one can visualize the Sylvian aqueduct as an elongated fluid-filled cavity (Figures 2A, 2B). Under normal circumstances, the ratio between the brainstem thickness and the distance between the brain stem and the occipital bone – or also called BS/BSOB – ranges from 0.5 to 1.0 (21,22). 

Disruption of the normal anatomy of the posterior fossa should raise suspicion of potential brain anomalies. For instance, in cases of open spinal dysraphism, the obliteration of the cisterna magna and posterior displacement of the brain structures may cause an increased BS/BSOB ratio (21,22). Conversely, in cases of a cystic anomaly of the posterior fossa, enlargement of the spaces behind the brainstem (i.e., fourth ventricle and cisterna magna) might lead to a decreased BS/BSOB ratio (18,20,24). 

Biometric measurements of the fetal brain

The primary objective of fetal biometry in the first trimester of pregnancy is to accurately estimate gestational age (17). For such purpose, the crown-rump length (CRL) is considered the most accurate parameter, especially when the CRL falls within 45 – 84 mm (37–39). Alternatively, the biparietal diameter (BPD) and head circumference (HC) (Figure 3), measured on the axial plane of the fetal head, can be used for estimating gestational age when the CRL exceeds 84 mm. However, the routine measurement of BPD (even with CRL < 84 mm) may be useful in early screening for open spinal dysraphism (40–43). Smaller values of BPD, especially below the fifth percentile, may indicate a cerebrospinal fluid leakage, a phenomenon observed in cases with open spinal dysraphism.

So far, there is no consensus on the preferred axial plane (transventricular or transthalamic) of the fetal head for measurement purposes (17,38,44). Recent guidelines suggest the transventricular axial plane as the standard for biometric measurements in the first trimester of pregnancy (17). The BPD can be measured using either an outer-to-outer (calipers on the external edges of the parietal bones) or an outer-to-inner technique (calipers on the leading edges of the parietal bones). The fetal HC can be measured directly with ellipse calipers placed around the calvarium or calculated using the ellipsoid formula, which combines the BPD and the fronto-occipital diameter. Regardless of the chosen methodology for measurements, it is important to refer to the corresponding nomograms.

Management of Abnormal Findings

If there is suspicion of a brain abnormality during the first trimester ultrasound, it is important to provide a detailed description and report of the findings. Subsequently, the patient should undergo a dedicated, multiplanar diagnostic fetal brain examination – i.e., early neurosonography. This advanced examination can be conducted by the same operators who performed the basic ultrasound assessment (provided they possess the necessary expertise) or in a specialized center for prenatal diagnosis. The goal of this expert examination is to obtain a final diagnosis and offer appropriate counseling regarding the management and prognosis of the fetal brain malformation (17).

Advanced evaluation of the fetal brain (early fetal neurosonography)

Indication

Suspected brain malformation identified during the first trimester ultrasound screening examination.

Ultrasound Planes for advanced evaluation of the fetal brain

The best approach for early fetal neurosonography is still unknown, but a multiplanar approach (similar to the second trimester neurosonography) (45) may be considered (28). This approach involves additional axial, coronal and sagittal planes of the fetal head for a thorough evaluation of fetal brain anatomy. In most cases, it can be performed using a transabdominal approach. However, under some circumstances (obesity or unfavorable fetal lie), the vaginal approach should be considered as it offers superior high-resolution imaging due to the probe characteristics (higher frequency) and the shorter distance from the ultrasound probe to the anatomical structures. 

In-depth knowledge of the fetal CNS anatomy and embryogenesis is essential for adequate assessment and interpretation of fetal brain findings at first trimester advanced ultrasound. The subsequent sections will provide a detailed description of each plane which is to be obtained additionally to those included in the first trimester routine ultrasound of the fetal brain with its corresponding anatomical landmarks.

The additional axial planes are:
-    Transthalamic plane
-    Transcerebellar plane

Transthalamic plane: includes from anterior to posterior
o    the frontal portion of the lateral ventricles, 
o    the interhemispheric fissure
o    the third ventricle
o    the thalami
o    the Sylvian aqueduct

The transthalamic plane (Figure 4) is obtained by slightly moving the ultrasound probe caudally from the transventricular plane. Only the anterior section of the interhemispheric fissure is visible, located between the frontal portion of the lateral ventricles – later developing into the frontal horns – filled with the corresponding choroid plexus. Subsequently, the interhemispheric fissure is interrupted by the third ventricle, which appears as a thin, anechoic space at the level of the thalami. The thalami present as two separate, symmetrical, anechoic ovoid structures on each side of the midline echo surrounding the third ventricle. Moving posteriorly, the Sylvian aqueduct manifests at the midline as an anechoic, rectangle-shaped cavity.

Transcerebellar plane: includes from anterior to posterior
o    the anterior part of the lateral ventricles, 
o    the interhemispheric fissure
o    the third ventricle
o    the thalami and the midbrain tegmentum
o    the rudimentary cerebellum
o    the cisterna magna

The transcerebellar plane (Figure 5) is obtained by gently tilting the probe posterior towards the posterior fossa (oblique plane). In this plane, the main supratentorial brain structures (frontal portion of the lateral ventricles, interhemispheric fissure, third ventricle, and thalami) are visualized similarly to the transthalamic plane. Posterior to the thalami, the midbrain tegmentum – a region of the midbrain – can be depicted just below the Sylvian aqueduct, at the level of the midbrain curvature. During the first trimester of pregnancy, the fetal cerebellar hemispheres and vermis are still developing and are not clearly visible. At this stage the cerebellum may appear behind the thalami and midbrain as an arch-shaped structure. Posterior to the cerebellum, the cisterna magna is observed as a thin, fluid-filled space, with a central area sometimes appearing darker, representing the lower and posterior part of the fourth ventricle, known as Blake’s pouch.

The main coronal planes are: 
-    Transfrontal coronal plane
-    Transcaudate coronal plane
-    Transthalamic coronal plane
-    Occipital coronal planes

These planes are sequentially obtained by aligning the ultrasound beam perpendicularly to the sagittal suture and sweeping the probe from the frontal to the occipital pole of the fetal head (Figure 6). Each coronal plane is characterized by specific anatomic landmarks, as follows:

Transfrontal coronal plane
o    interhemispheric fissure
o    frontal portions of the lateral ventricles with choroid plexus
o    orbital process of the frontal bone
o    orbits and lenses

The transfrontal coronal plane (Figure 7) is the initial one among the coronal planes, obtained through the anterior fontanelle. It displays the frontal portions of the lateral ventricles filled with their corresponding choroid plexuses. These should appear symmetrical and located on each side of the uninterrupted, straight midline echo representing the interhemispheric fissure. Additionally, it allows visualization of the orbital processes – formed by the frontal bone – along with the fetal orbits and lenses below.

Transcaudate coronal plane:
o    frontal portion of the lateral ventricles with choroid plexuses
o    interhemispheric fissure interrupted by the foramina of Monro
o    third ventricle
o    ganglionic eminences
o    basal ganglia

The transcaudate coronal plane (Figure 8) is slightly posterior to the previous plane and is also obtained through the anterior fontanelle. The lateral ventricles with their corresponding choroid plexuses are seen converging towards the upper part of the midline echo. At this point, the interhemispheric fissure is interrupted by the foramina of Monro, also known as interventricular foramina. Inferiorly, the frontal portion of the third ventricle appears on the midline as a fluid- filled cavity. The ganglionic eminences are situated laterally and inferiorly to the frontal portion of each lateral ventricle, in an area referred to as the subventricular zone, appearing as a hypoechoic thickening. The basal ganglia, including the caudate nucleus, are visible below the ganglionic eminences at the level of the third ventricle.

Transthalamic coronal plane:
o    interhemispheric fissure 
o    temporal portions of the lateral ventricles with choroid plexus
o    thalami
o    third ventricle

The transthalamic coronal plane (Figure 9) is acquired by gently sweeping the probe further posteriorly. The interhemispheric fissure appears as hyperechoic midline echo, interrupted at the level of the thalami, which present as hypoechoic, round and symmetric structures. The temporal portions of the lateral ventricles with their corresponding choroid plexuses are visualized on either side of thalami. Between the thalami, the caudal portion of the third ventricle can be observed. 

Occipital coronal planes:
o    occipital portions of the lateral ventricles 
o    brainstem (midbrain, pons and medulla oblongata)
o    Sylvian aqueduct
o    cerebellar hemispheres 
o    fourth ventricle with choroid plexus

The occipital coronal planes (Figure 10) consist of three distinct planes defined by the position of the ultrasound beam in relation to the fourth ventricle: OC1, located anterior to the fourth ventricle; OC2, including the fourth ventricle; and OC3 posterior to the fourth ventricle. 

On the OC1 (Figure 10A), the upper part of the image displays the occipital portions of the lateral ventricles. Below them, the Sylvian aqueduct and the brainstem with its main components are visualized. In relation the Sylvian aqueduct, the midbrain tectum is located laterally and superiorly, while the midbrain tegmentum is slightly below. Moving caudally, then pons and medulla oblongata are presented. On either side of the pons, the rudimentary cerebellar hemispheres can be imaged. 

On the OC2 (Figure 10B), the occipital portions of the lateral ventricles, the Sylvian aqueduct and the midbrain tectum are located similarly to OC1. The Sylvian aqueduct is followed caudally by the isthmus, connecting it to the fourth ventricle, which appears with its characteristic rhombic shape. Within the fourth ventricle, the two lateral portions of the choroid plexus appear as hyperechoic tissue on the two lateral walls of the cavity. The rudimentary cerebellar hemispheres are visualized laterally and slightly superiorly to the choroid plexuses. Below the fourth ventricle, the most inferior segment of the brainstem – i.e., the medulla oblongata – can be visualized. 

On the OC3 (Figure 10C), the most posterior part of the Sylvian aqueduct along with the adjacent midbrain tectum can be seen. Below these structures lies the rudimentary cerebellum and the choroid plexus of the fourth ventricle, appearing as a hyperechoic thick line on the lower surface of the cerebellum. Slightly inferior to the choroid plexus, the lowest posterior part of the fourth ventricle – Blake pouch's pouch – appears as an anechoic, round-shaped structure along the midline.

The additional sagittal plane is:
-    posterior midsagittal plane

Posterior midsagittal plane:
o    brainstem (midbrain, pons and medulla oblongata)
o    mesencephalic and pontine flexure
o    Sylvian aqueduct
o    fourth ventricle and choroid plexus
o    anterior and posterior membranous area
o    cerebellar vermis
o    cisterna magna

The posterior midsagittal plane (Figure 11) is obtained through the posterior fontanelle, which allows a more detailed visualization of the infratentorial region of the brain, also known as the posterior fossa (46–48).

The most anterior part of the posterior fossa is the brainstem, consisting of the midbrain, pons and medulla oblongata. This structure exhibits a characteristic ‘S’ shape, attributed to the mesencephalic and pontine flexures. Positioned dorsal to the brainstem, the Sylvian aqueduct and the fourth ventricle are visualized as two anechoic fluid-filled spaces interconnected by a thin isthmus at the level of the mesencephalic flexure. By the end of the first trimester, the Sylvian aqueduct is generally similar in size to the fourth ventricle. The posterior wall of the Sylvian aqueduct is covered by the midbrain tectum.

The fourth ventricle is situated posterior to the brainstem, along the pontine flexure. In the first trimester, the cerebellar vermis is positioned cranially to the fourth ventricle. The roof of the fourth ventricle consists of a medullary velum, divided into anterior and posterior membranous areas by the protruding choroid plexus. The anterior membranous area gradually decreases in size as the cerebellar vermis develops caudally towards the choroid plexus. The posterior membranous area protrudes into the developing cisterna magna, forming a finger-shaped structure known as Blake’s pouch (48,49).

CNS malformations, such as open spinal dysraphism and cystic anomalies of the posterior fossa, lead to specific alterations in the anatomy of this region. Thus, understanding of the normal anatomy of the posterior fossa might aid in the identification and classification of each condition. Moreover, changes in the position or size of some posterior fossa structures – such as the anterior membranous area, choroid plexus of the fourth ventricle and cerebellar vermis – can serve as potential sonographic markers for the differential diagnosis of cystic anomalies (19,49,50).

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This article should be cited as: Volpe N, Ramirez Zegarra R, Ghi T: First Trimester Ultrasound of Fetal Brain, Visual Encyclopedia of Ultrasound in Obstetrics and Gynecology, www.isuog.org, March 2024. 


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