Overview
This lecture explains the differences between continuous wave (CW) and pulse wave (PW) Doppler ultrasound imaging, covering fundamental principles, technical details, and clinical applications.
Doppler Ultrasound Principles
- Doppler shift is the difference between received and transmitted frequencies, calculated using the Doppler equation.
- Doppler shift increases with higher transmitted frequency, higher object velocity, and smaller Doppler angle (cosine θ).
- Main goal is often to calculate the velocity of moving objects (e.g., blood flow) using known variables.
Continuous Wave (CW) Doppler Ultrasound
- CW Doppler uses two transducer crystals: one continuously transmits, one continuously receives.
- Produces a pure, narrow-bandwidth wave, ideal for detecting frequency shifts but cannot determine depth or create an image.
- Samples all velocities along the wave's path, leading to possible signal overlap from multiple vessels.
- Can measure very high velocities but lacks spatial resolution.
Pulse Wave (PW) Doppler Ultrasound
- PW Doppler emits pulses and then listens for returning echoes, allowing depth localization.
- Utilizes dampened, broad-bandwidth pulses for imaging and velocity detection.
- Color Doppler overlays color on B-mode images to show direction and relative speed of blood flow, but does not give exact velocities.
- Spectral Doppler isolates a small region (the gate) to measure precise velocities and displays them over time in a spectral waveform.
- Pulse repetition frequency (PRF) limits the maximum measurable velocity (Nyquist limit); deeper targets decrease PRF and max velocity.
Key Technical Differences
- CW Doppler cannot localize depth or isolate single vessels but detects higher velocities without aliasing.
- PW Doppler provides depth resolution and precise vessel localization but is limited by Nyquist velocity and can exhibit aliasing.
Other Doppler Modes
- Color Doppler shows blood flow direction and magnitude using color coding on images.
- Spectral Doppler displays a graph of velocity over time for a selected region.
- Power Doppler measures only the magnitude of flow, not direction; useful for low or deep flow detection.
Key Terms & Definitions
- Doppler Shift — Change in frequency due to motion between source and receiver.
- Doppler Angle (θ) — Angle between ultrasound beam and blood flow direction, affecting shift magnitude.
- Pulse Repetition Frequency (PRF) — Number of pulse-echo cycles per second.
- Nyquist Limit — Maximum Doppler frequency shift measurable without aliasing; half the PRF.
- Gate — The region selected in PW Doppler to measure flow velocity.
- Spectral Waveform — Graph plotting velocity (y-axis) versus time (x-axis) in Doppler imaging.
Action Items / Next Steps
- Review the next lecture on spectral waveforms and aliasing artifacts in PW Doppler.
- Prepare for questions on comparing CW and PW Doppler, including advantages and limitations.
- Optional: Practice using the linked question bank if studying for ultrasound physics exams.