Overview
This lecture explains aliasing in Doppler ultrasound, the underlying concept of the Nyquist limit, and practical strategies to reduce or manage aliasing artifacts during spectral Doppler imaging.
Aliasing in Doppler Imaging
- Aliasing occurs when measured Doppler velocities exceed the display scale, causing incorrect velocity representation.
- It happens if blood velocity increases or if sample depth increases without adjusting machine settings.
- Aliasing prevents accurate measurement of peak systolic velocity.
The Nyquist Limit & Sampling
- The Nyquist limit is reached when the Doppler shift frequency exceeds half the pulse repetition frequency (PRF).
- Accurate velocity measurement requires the sampling rate (PRF) to be at least twice the Doppler shift frequency.
- If sampling is less than twice per cycle, direction and frequency become ambiguous or incorrect.
Factors Affecting and Reducing Aliasing
- Increasing sample depth increases pulse repetition period, reducing PRF and narrowing measurable velocity range.
- To reduce aliasing:
- Move the transducer to a shallower depth to increase PRF.
- Increase the scale on the Doppler machine by increasing PRF.
- Lower the transducer frequency to reduce the detected Doppler shift.
- Increase the Doppler angle (with caution for angle correction errors).
- Adjust the baseline of the spectral waveform to fit the waveform within the scale.
- Switch to continuous wave Doppler ultrasound if aliasing persists and resolution of specific location is less important.
Formulas and Relationships
- Maximum measurable Doppler shift = ½ × PRF (Nyquist limit).
- PRF is inversely proportional to pulse repetition period, which is determined by sample depth.
- Doppler shift depends on transducer frequency, blood velocity, and Doppler angle.
Practical Steps and Troubleshooting
- Try sequentially: reduce sample depth, increase PRF/scale, lower transducer frequency, adjust Doppler angle, move baseline, or use continuous wave Doppler.
- Continuous wave Doppler removes aliasing but sacrifices spatial specificity.
Key Terms & Definitions
- Aliasing — Artifact when detected velocity exceeds Nyquist limit, causing velocity wraparound on Doppler display.
- Pulse Repetition Frequency (PRF) — Number of ultrasound pulses emitted per second.
- Nyquist Limit — The maximum frequency (velocity) that can be accurately sampled, equal to half the PRF.
- Doppler Shift — Change in frequency of returning echoes, proportional to blood flow velocity.
- Continuous Wave Doppler — Doppler mode that continuously emits and receives signals, allowing measurement of high velocities.
Action Items / Next Steps
- Review Doppler imaging formulas and practice adjusting machine settings to minimize aliasing.
- Prepare for next lecture on harmonic imaging in ultrasound.
- Read about common artifacts and safety in ultrasound imaging.