Unit 1


  • Normal Fetal Head Shape and Surface Characteristics:

    • The normal fetal head exhibits an oval or egg-shaped contour rather than a perfectly circular shape.

    • The outer surface of the calvarium and skull bone should appear smooth and continuous.

  • Falx Cerebri (Interhemispheric Falx / Midline Falx):

    • The terms falx cerebri, interhemispheric falx (IF), and midline falx are synonymous and describe the exact same anatomical structure.

    • The falx is best visualized at the most cephalic cross-sectional level (closest to the top of the fetal head).

    • Functionally, the falx cerebri is a linear membrane that separates the fetal brain into left and right hemispheres.

    • On ultrasound, it presents as a distinct, bright, echogenic linear structure running through the center of the brain.

    • To ensure optimal anatomical visualization and precise biometric measurements, the falx should be aligned horizontally across the ultrasound display screen.

  • Dependent vs. Non-Dependent Brain Evaluation:

    • The dependent side of the brain refers to the posterior side (the side positioned furthest from the ultrasound transducer).

    • The non-dependent (anterior) side often suffers from acoustic shadowing and near-field reverberation artifacts produced by the proximal skull bone, obscuring clear detail of the anterior brain tissue and lateral ventricular walls.

    • Although both sides of the brain are surveyed, quantitative evaluations and caliper measurements—specifically of the lateral ventricles—are strictly performed on the dependent (posterior) portion of the brain.

  • Key Anatomical Features in High Cephalic Cross-Sections:

    • High-level axial views demonstrate normal brain parenchyma, the anterior choroid / choroid plexus, the cerebral peduncles, the lateral ventricles, and the midline falx (e.g., routinely visualized around 26 weeks26\,\text{weeks} gestation).

Fetal Ventricular System, Cerebrospinal Fluid Dynamics, and Neural Tube Anomalies

  • Anatomical Structure of the Ventricular System:

    • Lateral Ventricles: Paired fluid-filled structures located within the cerebral hemispheres; evaluated on both sides, with standard biometric measurements taken from the posterior/dependent lateral ventricle.

    • Third Ventricle: Located centrally, directly between the paired thalami.

    • Fourth Ventricle: Located posteriorly, adjacent to the cerebellum within the posterior fossa.

  • Cerebrospinal Fluid (CSF) Circulation Pathway:

    • Cerebrospinal fluid is contained within and produced by the choroid plexus inside the lateral ventricles.

    • Fluid travels from the lateral ventricles into the third ventricle through the foramen of Monro (foramen of Monroe).

    • Fluid travels from the third ventricle into the fourth ventricle via the aqueduct of Sylvius (aqueductal sulcus / active duct of Sylvius).

    • From the fourth ventricle, CSF exits into the cerebral subarachnoid spaces.

  • Ventricular Pathologies and Associated Defect Patterns:

    • Ventriculomegaly and hydrocephalus represent the most common neural tube and cranial defects identified on prenatal ultrasound scans.

    • Mild ventriculomegaly is frequently associated with broader underlying congenital anomalies.

    • Aqueductal Stenosis:

      • Occurs due to obstruction at the level of the aqueduct of Sylvius.

      • Serves as the most common cause of fetal hydrocephaly.

      • Pathology leads specifically to excess fluid accumulation and abnormal dilation isolated to the lateral ventricles and third ventricle.

    • Generalized Ventricular Dilation:

      • Dilation encompassing the entire ventricular system—including the lateral ventricles, third ventricle, and fourth ventricle—is typically associated with coexisting spinal defects (e.g., spina bifida).

Lateral Ventricle Measurement Mechanics and Cranial Surveying

  • Cranial-to-Caudal Ventricular Progression:

    • Sweeping the transducer from the top of the fetal head downward encounters structures in sequence: lateral ventricles →\rightarrow third ventricle →\rightarrow fourth ventricle.

    • Ventriculomegaly manifests as abnormal fluid dilation starting within the lateral ventricles at the highest scanning levels.

  • Choroid Plexus Anatomy and Landmark Identification:

    • The choroid plexus consists of echogenic, teardrop-shaped tissue located inside the lateral ventricles.

    • In the first trimester, the choroid plexus is disproportionately large and fills nearly the entire lateral ventricle space.

    • The glomerulus (or globulus) represents the thickest posterior body of the choroid plexus and serves as the precise anatomical landmark for measuring the lateral ventricle.

  • Standardized Protocol for Measuring the Lateral Ventricle:

    • Measurements must be oriented perpendicular (straight up and down) to the ventricular border.

    • Calipers must be placed strictly inner-to-inner border (inner-to-inner\text{inner-to-inner}).

    • The measurement caliper is positioned across the atrium/cystic space at the exact level of the choroid plexus glomerulus.

    • The atrium of the lateral ventricle is defined as the fluid-filled space containing the choroid plexus, housed between the medial ventricle border (MVB/MPB) and the lateral ventricle border (LVB).

  • Cranial Contour Survey and Severe Structural Anomalies:

    • The initial step in evaluating the fetal head is verifying the integrity and continuous contour of the skull bone (calvarium) from the highest cranial level down to the base of the skull.

    • Acrania: A condition characterized by the complete absence of cranial bone development despite the initial presence/development of cerebral brain tissue.

    • Anencephaly: Progression of acrania wherein unprotected brain tissue degenerates over time due to direct exposure to toxic chemical components within the amniotic fluid, ultimately yielding an absent or severely underdeveloped brain and cranium.

Diagnostic Thresholds, Anatomical Levels, and Cavum Septum Pellucidum (CSP)

  • Quantitative Thresholds for the Atrium of the Lateral Ventricle:

    • Normal Threshold: An atrial measurement of less than 10 mm10\,\text{mm} (< 10\,\text{mm}).

    • Abnormal Threshold: An atrial measurement greater than 10 mm10\,\text{mm} (> 10\,\text{mm}), which strongly warrants suspicion of ventriculomegaly or hydrocephalus.

    • Average Measurement: Typically measures around 6.5 mm6.5\,\text{mm}.

  • Evaluation of Choroid Plexus Texture:

    • The tissue texture of the choroid plexus must be systematically inspected for choroid plexus cysts.

    • Choroid plexus cysts are fluid-filled structures that frequently undergo spontaneous resolution; however, they require documentation as soft markers for chromosomal abnormalities.

  • Three Key Anatomical Scanning Levels of the Fetal Head:

    • 1. High Ventricular Level:

      • Visualizes the Cavum Septum Pellucidum (CSP) anteriorly, the midline interhemispheric falx, and the dependent lateral ventricle housing the echogenic choroid plexus.

    • 2. Mid-Thalamic / BPD Level:

      • Visualizes the CSP anteriorly, the paired thalami (heart-shaped midline structure), and the narrow, slit-like third ventricle along the midline.

    • 3. Low Posterior Fossa Level:

      • Visualizes the CSP anteriorly while angling slightly inferiorly/posteriorly to capture the cerebellum (dumbbell/barbell-shaped structure), the cisterna magna, and the nuchal fold.

  • Cavum Septum Pellucidum (CSP):

    • Spelling & Terminology: Cavum Septum Pellucidum (abbreviated CSP).

    • Sonographic Appearance: Appears as a distinct, fluid-filled (anechoic), box-like or square-like structure located in the anterior midline of the fetal brain.

    • Diagnostic Importance: Clear visualization of an intact CSP is a critical diagnostic landmark that unequivocally excludes complete agenesis of the corpus callosum.

Midline Echo Complex and Biometric Measurements (BPD and HC)

  • Midline Echo Complex & Standard Biometry Plane:

    • Moving the transducer caudally from the high ventricular view reveals the midline echo complex at the widest transverse diameter of the fetal skull.

    • This exact transverse cross-section serves as the required anatomical plane for measuring the Biparietal Diameter (BPD) and Head Circumference (HC).

  • Required Anatomical Landmarks for Valid BPD and HC Biometry:

    1. Continuous, uninterrupted midline echo complex (interhemispheric falx).

    2. Anterior box-like Cavum Septum Pellucidum (CSP).

    3. Symmetrical paired thalami.

    4. Third ventricle positioned centrally between the thalami.

  • Biometric Caliper Placement Rules:

    • Biparietal Diameter (BPD): Measured perpendicular to the midline falx at the widest diameter of the skull, placing calipers from the outer edge of the proximal skull wall to the inner edge of the distal skull wall (outer-to-inner\text{outer-to-inner}).

    • Head Circumference (HC): Placed along the outer perimeter of the cranial bone ellipse, strictly excluding the outer soft tissue/skin line.

  • Anatomy of Adjacent Midline and Supratentorial Structures:

    • Paired Thalami: Heart-shaped midline structures positioned symmetrically on both sides of the midline. The apex of the heart shape points anteriorly toward the fetal face.

    • Third Ventricle: Positioned directly between the two halves of the paired thalami; normally appears as a single echogenic line or collapsed slit. Pathological widening or anechoic cystic appearance indicates ventricular system obstruction.

    • Corpus Callosum: A thick band of neural fibers situated between the frontal horns of the lateral ventricles.

    • Frontal Horns of Lateral Ventricles: Situated on either side of the CSP and corpus callosum; often ill-defined in standard axial views and best evaluated in sagittal projections.

    • Sylvian Fissure & Middle Cerebral Artery (MCA):

      • Located near the lateral aspect of the skull; routinely identifiable starting at 15 weeks15\,\text{weeks} gestation or greater (typically evaluated around 20 weeks20\,\text{weeks} during an anatomy survey).

      • Cranial Doppler assessment of the MCA provides diagnostic evaluation of fetal anemia and hemodynamic conditions.

Diagnostic Differentiation: Thalamic Level vs. Peduncular Level

  • Determining Fetal Head Orientation (Anterior vs. Posterior):

    • The narrowing apex of the heart-shaped thalamus points directly toward the front (anterior/face) of the fetal head.

    • The presence of the anteriorly located CSP confirms the anterior orientation.

    • The lateral ventricles and posterior fossa structures are positioned in the broader posterior segment of the fetal head.

  • Distinguishing Thalami from Cerebral Peduncles:

    • Thalami: Situated superiorly at the proper BPD/HC measurement level; larger; heart-shaped; apex points anteriorly.

    • Cerebral Peduncles: Situated slightly inferior (caudal) to the thalami; smaller in size; share a similar heart-shaped appearance but are located immediately adjacent/anterior to the structures of the posterior fossa.

    • Diagnostic Pitfall Warning: Inadvertently including the cerebral peduncles or the posterior fossa (cerebellum, cisterna magna, nuchal fold) in the head biometry plane produces an inaccurate, invalid BPD and HC measurement.