Overview
This lecture covers the physics of spectral Doppler ultrasound, focusing on how spectral waveforms are generated, interpreted, and applied to vascular assessment using key measurements and clinical examples.
Spectral Doppler Ultrasound Basics
- Spectral Doppler displays velocity changes over time within a region of interest.
- Doppler shift measures tissue movement and, with angle correction, gives velocity values.
- Both continuous wave and pulse wave Doppler modalities can generate spectral Doppler data.
Creation of a Spectral Waveform
- Start by obtaining a B-mode image to visualize anatomy.
- Place a Color Doppler zone to locate vessels of interest.
- Select a specific A-line and position the Doppler "gate" to set imaging depth and sample volume.
- Set Doppler angle parallel to blood flow for accurate velocity measurements.
- The resulting spectral waveform plots time (x-axis) against velocity (y-axis).
Spectral Waveform Interpretation
- Baseline shows zero velocity; it can be shifted for optimal display.
- Positive velocities = flow towards transducer; negative = away.
- Waveform thickness indicates velocity range—narrow for laminar, wide for turbulent flow.
- Key points: peak systolic velocity (highest), end diastolic velocity (lowest before next beat).
- The spectral window beneath waveform may fill in with turbulent flow or excessive gain.
Measurement Parameters & Calculations
- Systolic upstroke indicates blood acceleration; downstroke shows deceleration (not reversal).
- "Time to peak" is time from systole start to peak velocity; shorter with faster acceleration.
- Gate size affects spectral waveform thickness without altering peak/end velocities.
- Velocity scale (y-axis) depends on pulse repetition frequency (PRF); higher PRF allows detection of higher velocities.
- Adjusting PRF changes velocity display but not the actual blood flow.
Resistive Index and Clinical Application
- Resistive Index (RI) = (Peak systolic velocity – End diastolic velocity) / Peak systolic velocity.
- RI reflects downstream vascular resistance; high RI = high resistance, low RI = low resistance.
- Organs like brain, kidneys need continuous flow (low RI); muscles at rest have higher RI.
- Spectral changes in carotid arteries can indicate stenosis, turbulence, or compensatory mechanisms.
- Pathology alters waveform shape, peak velocities, and RI, aiding clinical interpretation.
Key Terms & Definitions
- Doppler shift — frequency change due to movement in tissues, used to calculate velocity.
- Spectral Doppler — graphical display of blood velocity over time.
- B-mode image — standard ultrasound image showing anatomy.
- Doppler gate — region within tissue sampled for velocity.
- Pulse Repetition Frequency (PRF) — number of ultrasound pulses per second.
- Resistive Index (RI) — ratio indicating vascular resistance.
Action Items / Next Steps
- Review how spectral waveforms are generated and how parameter changes affect them.
- Prepare for upcoming topic: aliasing in Doppler ultrasound and its solutions.
- (If available) Complete curated question bank on spectral Doppler and aliasing.