Welcome to Day 25!
Today, we transition from B-mode artifacts to Doppler-specific artifacts — essential for understanding flow patterns, avoiding misinterpretation, and acing the SPI exam. Let’s focus on three core Doppler artifacts: Aliasing, Doppler Mirror Image, and Spectral Broadening.
🔄 1. Aliasing Artifact
Definition: A Doppler phenomenon where the flow appears to “wrap around” the spectral display or color scale, misrepresenting the direction or speed of flow.
Occurs in:
- Pulsed wave Doppler (PW)
- Color Doppler imaging
Cause:
When the Doppler shift exceeds half the PRF (called the Nyquist limit).
📌 Nyquist Limit = PRF ÷ 2
Appearance:
- On spectral Doppler: The waveform “wraps” from top to bottom.
- On color Doppler: A reversal of color (e.g., red turns to blue abruptly).
SPI Tip:
Aliasing does not occur in continuous wave (CW) Doppler because CW has no Nyquist limit.
✅ To reduce aliasing:
- Increase PRF
- Use a lower frequency transducer
- Shift baseline
- Use CW Doppler
🪞 2. Doppler Mirror Image Artifact
Definition: A duplicated spectral waveform appearing on the opposite side of the baseline.
Cause:
Occurs when the Doppler angle is close to 90°, leading to ambiguous directional detection.
Appearance:
An identical but inverted copy of the spectral waveform is seen below (or above) the baseline.
Example:
Observed in fetal vessels, hepatic veins, or near highly reflective structures.
SPI Tip:
To reduce this artifact, optimize the angle of insonation (ideally 45–60°).
🌫️ 3. Spectral Broadening
Definition: A widening or thickening of the spectral Doppler waveform.
Cause:
- Turbulent flow
- Wide sample volume
- Multiple velocities detected simultaneously
- Doppler angle misalignment
Appearance:
Fills in the spectral window; instead of a crisp line, the waveform looks “thickened” or noisy.
Clinical vs Artifact:
- Physiologic in turbulent flow (e.g., stenosis)
- Artifactual in poor settings or incorrect angle
SPI Tip:
Narrowing the sample gate and optimizing the angle can reduce unnecessary broadening.
🧠 Quick Recap Table
| Artifact | Cause | Appearance | Prevention Tip |
| Aliasing | Doppler shift > Nyquist | Wrap-around on display | Increase PRF or use CW Doppler |
| Doppler Mirror Image | Insonation near 90° angle | Duplicate waveform | Adjust beam angle |
| Spectral Broadening | Turbulent flow or wide gate | Thickened spectral trace | Narrow gate and align beam |
📘 SPI Exam Essentials
✅ Aliasing = high velocities + PW only
✅ Mirror artifact = beam too perpendicular
✅ Spectral broadening = multiple velocity signals; know real vs artifact
📝 Flashcard
Q: Which Doppler artifact appears as a wrap-around of spectral wave due to velocity exceeding Nyquist limit?
A: Aliasing
🧭 Conclusion
Mastering Doppler artifacts gives you an edge both in the exam and in clinical scanning. Stay tuned for Day 26, where we’ll explore Color Doppler artifacts and their interpretation.