Doppler principles

Clinical uses in anaesthesia/ICU

• Echocardiography (TTE/TOE): estimate blood flow velocity, pressure gradients (Bernoulli), valve stenosis/regurgitation, cardiac output (LVOT VTI).
  • Continuous-wave Doppler for high velocities (e.g., aortic stenosis); pulsed-wave Doppler for site-specific sampling (e.g., LVOT).
• Vascular ultrasound: carotid/venous/arterial assessment, stenosis screening (velocity criteria), DVT assessment (mainly compression; Doppler supportive).
  • Colour Doppler maps mean frequency shift; spectral Doppler provides velocity-time waveform.
• Foetal monitoring: Doppler fetal heart rate detection (handheld Doppler).
• Transcranial Doppler: cerebral blood flow velocity trends, vasospasm monitoring, emboli detection.
• Ultrasound-guided regional anaesthesia: Doppler to identify vessels and avoid intravascular injection.

How to optimise a Doppler measurement (practical)

• Maximise signal: align beam as parallel to flow as possible; use appropriate sample volume (PW) and gain; avoid excessive wall filter.
  • Doppler angle correction: use the angle between ultrasound beam and direction of flow; keep θ small (ideally ≤ 20°, acceptable up to ~60°).
• Choose modality: PW for localisation; CW for very high velocities; colour Doppler to find jet then spectral Doppler to quantify.
• Avoid aliasing: increase PRF/scale, lower transmitted frequency, shift baseline, use shallower depth, or switch to CW Doppler.

Core Doppler concept

• Doppler effect: change in observed frequency due to relative motion between source and observer. In medical ultrasound, moving red blood cells act as scatterers producing a frequency shift in the reflected wave.
• Backscattered Doppler shift depends on velocity component along the ultrasound beam (not the true velocity unless beam is parallel to flow).

Doppler equation (reflection from moving target)

• Frequency shift: Δf = (2 f0 v cosθ) / c
  • Δf = Doppler frequency shift (Hz).
  • f0 = transmitted (carrier) frequency (Hz).
  • v = target (blood) velocity (m·s⁻1).
  • θ = angle between ultrasound beam and direction of flow.
  • c = speed of sound in tissue (~1540 m·s⁻1).
  • Factor 2 because there is a Doppler shift on transmission to the moving target and again on reflection back to the transducer.
• Rearranged to estimate velocity: v = (Δf c) / (2 f0 cosθ).

Angle dependence (high-yield)

• Measured Doppler shift is proportional to cosθ.
• θ = 0° (beam parallel to flow): cosθ = 1 → maximum shift and best accuracy.
• θ = 90° (beam perpendicular): cosθ = 0 → no Doppler shift (cannot measure flow).
• Error increases rapidly as θ approaches 90°; small absolute angle errors at high θ cause large velocity errors (because cosθ changes steeply).

Pulsed-wave (PW) vs Continuous-wave (CW) Doppler

• PW Doppler: transducer alternates transmit/receive; allows range gating (depth-specific sampling). Limited by Nyquist (aliasing).
  • Range resolution depends on pulse length; sample volume defines region contributing to signal.
• CW Doppler: continuously transmits and receives (often separate crystals). No aliasing; can measure very high velocities. No range resolution (signals from entire beam path).
  • Used for high-velocity jets (e.g., valve stenosis/regurgitation).

Nyquist limit and aliasing (PW Doppler)

• Nyquist limit: maximum Doppler frequency that can be sampled without aliasing = PRF/2.
• Aliasing: when Δf exceeds Nyquist; spectral waveform wraps around baseline; colour Doppler shows abrupt colour reversal.
• PRF is limited by imaging depth (time-of-flight): deeper structures require lower PRF → more aliasing risk.
• Ways to reduce aliasing: increase PRF/scale, reduce depth, use lower f0, shift baseline, use high-PRF mode, or switch to CW Doppler.

Spectral Doppler display and derived measures

• Spectral Doppler plots velocity (or frequency shift) vs time; brightness indicates signal power (number of scatterers at that velocity).
• Spectral broadening: widened spectrum due to range of velocities (e.g., turbulence, stenosis, large sample volume, poor angle correction).
• VTI (velocity-time integral): area under the velocity curve per beat; used with cross-sectional area to estimate stroke volume and cardiac output.
  • Stroke volume ≈ CSA × VTI (e.g., LVOT CSA = π(D/2)^2).

Colour and power Doppler (overview)

• Colour Doppler: encodes mean Doppler shift (direction and magnitude) over a 2D image; typically red = towards, blue = away (machine convention).
  • Variance/“mosaic” may indicate turbulence or aliasing.
• Power Doppler: displays total Doppler signal power (sensitive to low flow, less angle dependent for detection), but does not provide direction or velocity.

Common Doppler-related equations used in echo

• Simplified Bernoulli: pressure gradient across a stenosis/valve ≈ 4 v^2 (v in m·s⁻1, gradient in mmHg).
  • Assumes negligible proximal velocity and viscous losses; best for high-velocity jets.
• Continuity principle (flow conservation): Q = A × v_mean; used to estimate valve area (e.g., aortic valve area from LVOT and aortic VTI).

Instrumentation concepts that appear in Doppler questions

• Wall filter: removes low-frequency shifts from vessel wall/tissue motion; too high a filter can remove true low-velocity blood flow.
• Gain: too high causes noise and blooming (colour bleed); too low misses flow.
• Sample volume (PW): larger volume increases signal but worsens range specificity and increases spectral broadening.
• Doppler frequency choice: higher f0 gives larger Δf (more sensitivity) but more attenuation; lower f0 helps penetration and reduces aliasing for a given velocity.