220- Unit 1- 2

Fetal Head Anatomy and Transverse Plane Evaluation

• Transverse Scanning Plane Requirements:

  • Anatomical evaluation and biometric measurements of the fetal head are performed primarily in the transverse plane.
  • Initial assessment evaluates skull shape (ranging from round, to oval, to egg-shaped) and surface smoothness to rule out structural anomalies.

• Cephalic Level Landmarks:

  • The most cephalic scanning level is defined as the plane closest to the top of the fetal head.
  • Key structural landmark: The interhemispheric falx, also termed the falx cerebri or midline falx (all three terms are completely synonymous).
  • Function of the falx: Divides the brain tissue into two distinct cerebral hemispheres.
  • Technical positioning: For accurate biometric measurements and optimal visualization, the midline falx must be oriented horizontally across the ultrasound screen.

• Anatomical Visualization at $26\,\text{weeks}$:

  • At approximately $26\,\text{weeks}$ gestational age, key recognizable structures in the transverse head section include:
  • Interhemispheric falx (labeled IF), appearing as a bright echogenic line running longitudinally through the brain.
  • Normal brain parenchyma/tissue in both hemispheres.
  • Anterior choroid plexus within the lateral ventricles.
  • Cerebral peduncles.
  • Lateral ventricles.

Ventricular System, Fluid Dynamics, and Neural Tube Defects

• Anatomy of the Ventricular System:

  • Consists of four ventricles: two lateral ventricles, a third ventricle, and a fourth ventricle.
  • The two lateral ventricles are located superiorly within the cerebral hemispheres.
  • The third ventricle is situated medially between the paired thalami.
  • The fourth ventricle is located posteriorly, adjacent to the cerebellum in the posterior fossa.
  • Ventricles are filled with cerebrospinal fluid (CSF).

• Path of Cerebrospinal Fluid (CSF) Flow:

  1. CSF is produced by the choroid plexus inside the lateral ventricles.
  2. Travels from the lateral ventricles into the third ventricle via the Foramen of Monro.
  3. Flows from the third ventricle through the Aqueduct of Sylvius into the fourth ventricle.
  4. Exits the fourth ventricle into the cerebral subarachnoid spaces.

• Neural Tube Defects and Ventricular Pathology:

  • Ventriculomegaly and hydrocephalus are the most common neural tube defects identified on fetal ultrasound.
  • Mild ventriculomegaly is frequently associated with underlying congenital anomalies.
  • Aqueductal Stenosis:
  • Occurs at the level of the Aqueduct of Sylvius.
  • Represents the single most common cause of fetal hydrocephaly.
  • Results in an abnormal accumulation/excess of fluid localized within the lateral ventricles and the third ventricle.
  • Generalized Ventricular System Dilation:
  • Involves dilation across all ventricles (lateral ventricles, third ventricle, and fourth ventricle).
  • Typically associated with spinal defects (e.g., spina bifida).

Technical Protocol for Measuring the Dependent Lateral Ventricle

• Dependent Ventricle Principle:

  • Acoustic artifact and shadowing frequently obscure the anterior (near-field) lateral ventricle.
  • Consequently, measurements must always be performed on the posterior or most dependent lateral ventricle.

• Caliper Placement and Measurement Boundaries:

  • Caliper orientation: Placed strictly perpendicular to the ventricular borders (straight up and down).
  • Measurement method: Inner border to inner border (inner-to-inner).
  • Anatomical landmark for caliper placement: Located precisely at the edge/border of the choroid plexus body (also called the globulus or granules of the choroid plexus).
  • Anatomical location definition: The lateral ventricle atrium/body is defined as a cystic space filled with choroid plexus housed between the main ventricle body (MBB) and the lateral ventricle border (LVB).

• Gestational Variations & Diagnostic Thresholds:

  • First trimester: The choroid plexus is disproportionately large and occupies almost the entire lateral ventricle space.
  • Shape: Choroid plexus typically demonstrates a teardrop shape on axial/transverse sections.
  • Quantitative Ventricular Standards:
  • Normal lateral ventricle measurement: Less than $10\,\text{mm}$ ($< 10\,\text{mm}$).
  • Average normal measurement: Approximately $6.5\,\text{mm}$.
  • Abnormal threshold: Any measurement greater than $10\,\text{mm}$ ($> 10\,\text{mm}$) indicates ventriculomegaly or hydrocephalus.
  • Choroid Plexus Cysts:
  • The texture of the choroid plexus must be thoroughly evaluated for focal cystic lesions (choroid plexus cysts).

• Surveying Strategy for Head Contour and Cranial Anomalies:

  • Step 1: Examine the outline and contour of the cranial bone/calvarium ("cape bone").
  • Step 2: Perform a continuous sweep starting at the highest cephalic level (visualizing the midline falx and teardrop choroid plexus) down to the base of the skull.
  • Step 3: Screen for severe cranial defect progression:
  • Acrania: Complete absence of the cranial vault while brain tissue is present.
  • Anencephaly: Progression from acrania where exposed brain tissue deteriorates due to prolonged contact with chemical agents in the amniotic fluid.

Cranial Midline Landmarks and Diagnostic Criteria

• Midline Scanning Levels:

  • Top Level: Visualizes the Cavum Septum Pellucidum (CSP), choroid plexus, and lateral ventricles.
  • Middle Level: Visualizes the CSP and the thalamus.
  • Inferior/Posterior Level: Visualizes the CSP, posterior fossa structures (cerebellum, cisterna magna, and nuchal fold).

• Cavum Septum Pellucidum (CSP):

  • Appearance: An anechoic, box-like / square structure located in the anterior portion of the fetal head, anterior to the thalamus.
  • Full Spelling: C-A-V-U-M S-E-P-T-U-M P-E-L-L-U-C-I-D-U-M.
  • Diagnostic Significance: Visualization of a normal CSP definitively excludes complete agenesis of the corpus callosum.

• Thalamus:

  • Appearance: Paired, symmetrical, heart-shaped hypoechoic structures situated on either side of the midline echo complex.
  • Orientation: The apex of the heart-shaped thalamus points directly toward the anterior face/front of the fetal head, assisting in orientation even if the CSP is absent.

• Third Ventricle:

  • Position: Located directly along the midline between the paired thalami.
  • Normal Appearance: Appears as a thin, narrow echogenic line formed by adjacent borders; visible cystic enlargement signals pathology.

• Corpus Callosum:

  • Structure: A band of neural tissue located superiorly between the frontal horns of the lateral ventricles.
  • Visualization: Best identified in a sagittal plane from the top of the head; often ill-defined or difficult to depict in standard transverse views.

• Sylvian Fissure and Middle Cerebral Artery (MCA):

  • Located near the lateral aspect of the skull.
  • Contains the Middle Cerebral Artery (MCA), which is routinely interrogated using cranial Doppler for specific high-risk fetal conditions.
  • Standard anatomical landmarks can be identified reliably from $15\,\text{weeks}$ gestation onwards, though formal anatomical surveys are routinely conducted at approximately $20\,\text{weeks}$.

Standard Biometric Measurement Protocol (BPD and HC)

• Anatomical Selection for BPD/HC Plane:

  • Biometric measurements must be taken at the widest transverse diameter of the skull.
  • Standard Required Landmarks:
  1. Continuous midline echo complex (interhemispheric fissure).
  2. Paired thalami.
  3. Third ventricle.
  4. Cavum Septum Pellucidum (CSP).

• Measurement Execution:

  • Biparietal Diameter (BPD):
  • Line of measurement must be drawn strictly perpendicular to the midline falx/echo complex.
  • Caliper placement: Leading edge to leading edge (outside skull edge to inside skull edge).
  • Head Circumference (HC):
  • Elliptical measurement traced around the outer perimeter of the cranial bone vault.
  • Caliper placement: Measured outer-to-outer around the skull bone, strictly excluding the superficial skin line.

• Avoiding Technical Errors and Artifacts:

  • Cerebral Peduncles vs. Thalamus:
  • Peduncles are located slightly inferior and posterior to the thalamus.
  • Peduncles are also heart-shaped but are distinctly smaller in size.
  • Posterior Fossa Exclusion:
  • Inferior tilting of the probe brings the posterior fossa into view, containing:
    • Cerebellum: Dumbbell-shaped or barbell-shaped structure in the posterior fossa behind the peduncles.
    • Cisterna Magna: Anechoic fluid space posterior to the cerebellum.
    • Nuchal Fold: Soft tissue thickness posterior to the occipital bone.
  • CRITICAL RULE: The posterior fossa structures (cerebellum, cisterna magna, nuchal fold) must NEVER be present in the cross-section used for BPD and HC measurements. Inclusion of the posterior fossa will produce an invalid, incorrect head measurement.

Questions, Discussion, and Study Assignments

• Classroom Dialogue and Clarifications:

  • Query on CSP Acronym: The instructor emphasized the exact spelling of Cavum Septum Pellucidum (C-A-V-U-M S-E-P-T-U-M P-E-L-L-U-C-I-D-U-M), noting that while the acronym "CSP" is standard clinical shorthand, knowing the full anatomical term is mandatory for examination evaluation.
  • Query on Visual Appearance of Thalamus: A student noted that the paired thalami combined with the third ventricle often resemble a wall or continuous block rather than a distinct heart shape. The instructor clarified that in many normal cases, it presents simply as an echogenic line without pathological dilation.
  • Query on Head Orientation: When determining front vs. back on tilted axial slices, students must track the apex of the thalami and the position of the box-like CSP to identify the anterior aspect of the fetal head accurately.

• Required Student Assignments:

  • Fetal Circulation: Comprehensive review of fetal cardiovascular circulation pathways prior to the next class meeting.
  • Textbook Reading: Read Chapters 51 through 53 in the assigned textbook.
  • Reference Books: Summarize and extract key anatomical landmarks, diagnostic thresholds, and measurement criteria from class into personal clinical reference books.