Overview
This lecture covers the clinical application of Doppler ultrasound, with a focus on interpreting spectral and color Doppler, optimizing controls, recognizing artifacts, and correcting aliasing.
Spectral Tracing and Waveform Analysis
- Spectral tracing graphs echo returns from blood cells within the Doppler gate.
- Position and size of the gate affect spectral tracing thickness and clarity.
- The angle correct must be set parallel to blood flow for accurate velocity measurements.
- Key waveform features: baseline (zero velocity), y-axis (velocity), x-axis (time), peak systolic velocity (PSV), end diastolic velocity (EDV), and spectral window.
- Laminar flow shows an open spectral window; turbulent flow or incorrect settings fill the window.
Doppler Controls and Optimization
- Baseline can be adjusted up or down to fit waveform display needs.
- Spectral gain amplifies all returning echoes; too much overestimates velocities, too little underestimates them.
- Pulse repetition frequency (PRF/scale) determines max measurable velocity; increasing PRF increases scale, decreasing it enlarges waveform.
- Wall filter removes slow velocities near baseline; overuse can erase important diagnostic info.
Aliasing and Correction Methods
- Aliasing is a misrepresentation of high velocities when Doppler shift exceeds the Nyquist limit (PRF/2).
- Increasing gate depth decreases PRF and Nyquist limit, raising aliasing risk.
- To fix aliasing: increase scale/PRF, decrease sample depth, use lower transducer frequency, move the baseline, or switch to continuous wave Doppler.
Doppler Artifacts
- Clutter (thump artifact) appears as low-level noise near baseline, often removed with wall filter.
- Crosstalk is mirror image artifact from high gain or 90° Doppler angle, seen as duplicated flow on both sides of baseline.
Color Doppler Display and Optimization
- Color Doppler maps flow direction: top color is toward, bottom color is away from the transducer.
- Color box size, depth, and steering affect sensitivity and frame rate.
- Avoid 90° angles between scan lines and flow for optimal filling.
- Color gain must be set for wall-to-wall vessel fill; too much causes confetti, too little mimics pathology.
- Color PRF (scale) adjustments balance sensitivity and aliasing.
- Wall filter removes slow flow or ghosting outside vessels.
Color Doppler Artifacts and Troubleshooting
- Aliasing appears as abrupt transitions between fastest colors on either side of the map.
- True flow reversal is indicated by a black bar (no shift) between opposing colors.
- Ghosting artifact is color “bleeding” outside vessels, reduced by wall filter and proper gain/PRF.
Key Terms & Definitions
- Spectral tracing — Doppler display of velocity vs. time from a sample volume.
- Baseline — Zero-velocity line separating flow toward and away from the transducer.
- PRF (Pulse Repetition Frequency) — Number of pulses per second; determines max velocity displayable.
- Nyquist limit — Maximum unaliased Doppler shift (PRF/2).
- Aliasing — Artifact when velocity exceeds the Nyquist limit, causing wraparound on the display.
- Wall filter — Removes low-velocity signals/artifacts from Doppler data.
- Crosstalk — Mirror image artifact creating artificial bidirectional flow appearance.
- Ghosting — Color artifact where flow appears outside vessel boundaries.
Action Items / Next Steps
- Review workbook activities and nerd check questions on Doppler concepts.
- Practice identifying and correcting aliasing and other artifacts.
- Memorize Doppler control effects and artifact correction techniques.
- Understand when to use pulse wave versus continuous wave Doppler.