Step-by-Step NT Scanning Technique

NT Scanning Technique

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Step-1. Patient Preparation

Ask the mother to have a partially filled bladder (optional).
Position her comfortably in the supine or slightly tilted position.

Step-2. Transducer Placement

Use a transabdominal probe (3.5–5 MHz).
In early gestations or with unfavorable views, a transvaginal probe (5–9 MHz) may be used.

Step-3. Aligning the Fetus in a True Mid-Sagittal Plane

Introduction: Importance of accurate nuchal translucency (NT) & CRL measurement.
Role of mid-sagittal alignment in reducing errors and improving screening sensitivity.
Key anatomical landmarks:
a). Nasal bone
b). Tip of the nose
c). Rectangular palate
d). Diencephalon
e). Fourth ventricle
f). Nuchal membrane
i). Sonographic Criteria for Mid-Sagittal Alignment
The fetal head and thorax must be in a neutral position (neither hyperextended nor flexed).
The echogenic tip of the nose should be visible.
The rectangular shape of the palate must be clearly seen.
The translucent diencephalon and fourth ventricle (intracranial translucency) should be visible.
ii). Technique for Achieving Alignment
Maternal Preparation: Empty bladder (if transvaginal), partially full bladder (if transabdominal).
Probe Orientation: Align the transducer parallel to the fetal body axis.
Adjusting Fetal Position: Use gentle abdominal pressure or ask the patient to change position.
Zoom & Focus: Enlarge the image so that the fetal head and upper thorax occupy at least 75% of the screen.
iii). Common Errors and How to Avoid Them
Off-axis images: May mimic the midline but lack key structures (e.g., missing nasal bone).
Fetal movement: Wait for stillness or reattempt alignment.
Incorrect flexion/extension: Identify and correct before measuring NT.
iv). Visual Aids and Examples
Annotated ultrasound images showing:
a). Correct mid-sagittal plane
b). Incorrect off-midline planes
Diagrams highlighting critical landmarks
v). Clinical Implications of Incorrect Alignment
Risk of inaccurate NT measurement
False positives/negatives in screening for chromosomal abnormalities

Step-4. Optimize the Image

Zoom in until the fetal head and upper thorax fill at least 75% of the screen.
Use grayscale only (do not use Doppler).
Adjust gain and focus to enhance bony structures like the frontal bone.

Step-5. Differentiation of Structures

In NT screening, one of the most important steps is correctly identifying and distinguishing the NT space from adjacent anatomical structures.
Grayscale ultrasound imaging provides the necessary contrast to make these distinctions clearly and accurately.
Grayscale imaging helps differentiate the nuchal translucency from:
- Amnion (the thin membrane surrounding the fetus)
- Fetal skin
- Subcutaneous tissues
- Umbilical cord if nearby
Avoids misidentifying thickened amnion or skin folds as increased NT.
Why Structural Differentiation Matters:
Accurate NT measurement depends on visualizing only the fluid space behind the fetal neck.
Misidentification can lead to false-positive or false-negative risk assessments for chromosomal abnormalities.
Using grayscale imaging only for NT measurement:
- No Doppler
- No color overlays
- No power Doppler