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← Section I · Physical Principles, Instrumentation, Examination Principles
I.E

Principles of Doppler Flow Measurement

26 cards

Notes

The Doppler equation

v=c⋅Δf2⋅fT⋅cos⁡θv = \frac{c \cdot \Delta f}{2 \cdot f_T \cdot \cos\theta}

  • v = blood velocity, c = speed of sound in tissue (1540 m/s), Δf = Doppler shift, f_T = transmitted (transducer) frequency, θ = angle between beam and flow.
  • The 2 in the denominator reflects the double Doppler shift (source → RBC, then RBC → receiver).

Doppler shift

  • Shift = received freq − transmitted freq.
  • Positive (received > transmitted) when the reflector moves toward the transducer.
  • Negative when reflector moves away.
  • Intracardiac Doppler shifts fall in the audible range (~20 Hz–20 kHz), even though transducers operate at 2–10 MHz.
  • Doppler measures velocity (magnitude + direction), not speed.

Intercept angle

  • cos 0° = 1, cos 30° = 0.87, cos 60° = 0.5, cos 90° = 0.
  • Beam parallel to flow (0° or 180°) → true velocity is measured.
  • Beam perpendicular (90°) → measured velocity = 0.
  • Nonparallel angles always underestimate velocity.

CW vs PW Doppler

CWPW
Crystals≥ 2 (one always transmits, one receives)1 (alternates)
Depth resolutionNone (range ambiguity)Yes (sample-volume specific)
Max velocityUnlimitedLimited by aliasing
UseHigh velocities (valvular stenosis, regurgitation, TR jet)Low velocities at a specific site (LVOT, mitral inflow)
  • Maximum unambiguous PW velocity ≈ 1 m/s at ~6 cm depth (varies with depth).
  • Simultaneous imaging + Doppler = duplex ultrasound.

Aliasing and the Nyquist limit

  • Nyquist limit = ½ × PRF.
  • Aliasing appears when the Doppler shift exceeds the Nyquist limit - the top of the signal "wraps" to the opposite side of the baseline.
  • Aliasing can NEVER occur with CW (no PRF constraint).
  • Ways to reduce/eliminate PW aliasing:
    1. Switch to CW.
    2. Use a lower-frequency transducer (reduces Doppler shift for a given velocity).
    3. Move to a shallower sample volume (raises PRF/Nyquist).
    4. Increase the velocity scale.
    5. Baseline shift (appearance only - doesn't raise the actual limit).
  • High-PRF Doppler: deliberately places multiple sample gates so signals from twice (or more) the primary depth are recorded simultaneously; extends velocity range at the cost of range ambiguity.

Sample-volume behavior

  • Small sample volume → clean spectral window.
  • Large sample volume → spectral broadening (fill-in).
  • Sample-volume depth is set by the transmit-receive time; sample-volume length by the receive-cycle duration.

Color flow Doppler

  • Multi-gate PW Doppler with autocorrelation to estimate mean velocity at each location (spectral Doppler reports peak).
  • Same PRF / aliasing constraints as PW.
  • Standard color map: red = toward transducer, blue = away; brightness ∝ velocity up to the Nyquist limit.
  • Variance (usually green) marks flow disturbance or aliased high-velocity flow.
  • Typical burst length (packet) = ~8 pulses per scan line - trade-off between velocity accuracy and frame rate.
  • To reduce aliasing on color, shift the baseline (allows display up to ~2× the original Nyquist limit).

Tissue Doppler (TDI)

  • Same PW/color hardware, but tuned for the low-velocity, high-amplitude motion of myocardium (not RBCs).
  • Power output and gain kept low; velocity range small.

Harmonic imaging

  • Second harmonic imaging uses reflections at 2× the transmitted frequency.
  • Improves lateral resolution by 20–50%, but degrades axial resolution by 40–100%.
  • Makes tissue appear more white; useful in technically difficult studies.

Mechanical / thermal index

  • MI = quantifies acoustic pressure (cavitation risk). Lowering MI increases bubble resonance / harmonics (relevant for contrast).
  • TI = quantifies tissue-heating potential. Target < 1.5 °C tissue heating.

Cards

  • basicI.E-001
    State the Doppler equation for blood velocity.
    → v = (c × Δf) / (2 × f_T × cos θ). Where c = speed of sound (1540 m/s), Δf = Doppler shift, f_T = transmitted frequency, θ = angle between beam and flow.
  • basicI.E-002
    Why is there a factor of 2 in the Doppler equation denominator?
    → There is a double Doppler shift: once when sound strikes the moving RBC, and again when the RBC reflects sound back to the transducer.
  • basicI.E-003
    When is the Doppler shift positive vs negative?
    → Positive (received > transmitted freq) when the reflector moves toward the transducer. Negative when it moves away.
  • clozeI.E-004
    Intracardiac Doppler shifts fall in the audible range of ~20 Hz to 20 kHz, even though transducer frequencies are 2–10 MHz.
  • basicI.E-005
    Does Doppler measure speed or velocity? Why does it matter?
    → Velocity (magnitude AND direction). Doppler shift is signed — its sign tells us flow direction relative to the transducer.
  • clozeI.E-006
    At an intercept angle of 0° or 180°, cos θ = 1 and the measured velocity equals the true velocity.
  • basicI.E-007
    What happens to the measured velocity when the beam is perpendicular (90°) to flow?
    → It is measured as zero. cos 90° = 0, so the Doppler shift is nulled.
  • basicI.E-008
    Does a non-parallel Doppler angle over- or under-estimate the true velocity?
    → Underestimates it. Since |cos θ| < 1 for any angle other than 0° or 180°, the measured velocity is always less than the true velocity.
  • basicI.E-009
    By convention, when performing Doppler how large an intercept angle is generally accepted before you correct or search a better window?
    → Angles up to ~20° introduce < 6% error (cos 20° ≈ 0.94); at 30° error is ~13%. Best practice is to keep the angle < 20°.
  • basicI.E-010
    Which mode — spectral or color — reports peak velocity, and which reports mean?
    → Spectral Doppler (PW and CW) reports peak velocity. Color Doppler reports mean velocity (via autocorrelation of a packet of pulses).
  • basicI.E-011
    What is 'duplex' ultrasound?
    → Simultaneous B-mode imaging and Doppler using a single transducer.
  • basicI.E-012
    Which mathematical technique extracts frequencies from a spectral (PW/CW) Doppler signal?
    → Fast Fourier Transform (FFT).
  • basicI.E-013
    Which mathematical technique is used by color Doppler to estimate mean velocity?
    → Autocorrelation. It is less accurate than FFT but much faster, making real-time color flow imaging feasible.
  • basicI.E-014
    Why is a lower-frequency transducer preferred for pulsed Doppler when aliasing threatens?
    → For a given velocity, a lower f_T produces a smaller Doppler shift (v ∝ 1/f_T in reverse... shift Δf ∝ f_T). Smaller shifts are less likely to exceed the Nyquist limit.
  • clozeI.E-015
    In color Doppler, red = flow toward the transducer, blue = flow away from the transducer.
  • basicI.E-016
    What does a green (variance) overlay on color Doppler indicate?
    → Variance in the mean-velocity estimate across the packet — flow disturbance, turbulence, or aliasing of high-velocity flow.
  • basicI.E-017
    What is the mechanical index (MI) and why is it lowered during contrast studies?
    → MI quantifies the acoustic pressure (cavitation potential) of the beam. Lowering MI preserves microbubbles by reducing their destruction and enhancing harmonic resonance.
  • basicI.E-018
    What is the thermal index (TI)? What tissue-heating threshold is used?
    → TI quantifies potential tissue heating. General guideline: keep tissue heating < 1.5 °C.
  • basicI.E-019
    For an aortic stenosis jet, what maximal angle deviation from parallel is acceptable to avoid > 5% velocity underestimation?
    → An angle up to 20° gives cos 20° ≈ 0.94 — about 6% error in velocity. Beyond ~20° the error becomes clinically significant. Always seek the highest velocity across multiple windows (apical, right parasternal Pedoff).
  • basicI.E-020
    What determines the Nyquist limit and how do you raise it?
    → Nyquist = ½ × PRF. To raise Nyquist: (1) reduce imaging depth (raises PRF), (2) shift baseline in direction of flow, or (3) use lower-frequency transducer (reduces the Doppler shift for a given velocity).
  • basicI.E-021
    How does a lower-frequency transducer help with aliasing?
    → Doppler shift is proportional to the transmitted frequency (Δf ∝ f_T × v). Lowering f_T reduces the Doppler shift for a given velocity, making it less likely to exceed the Nyquist limit.
  • basicI.E-022
    On a PW spectral trace, what is 'spectral broadening' and what causes it?
    → Filling in of the spectral envelope with lower-velocity signals adjacent to peak velocity. Caused by turbulent flow, large sample volume, or high Doppler gain. A narrow, clean spectral window suggests laminar flow.
  • basicI.E-023
    What does the term 'range specificity' mean in PW Doppler?
    → The ability to sample velocity from a specific range (depth) along the beam. PW Doppler gates the receive time to sample only signals returning after a specific delay (proportional to depth). CW lacks range specificity.
  • basicI.E-024
    How does color-flow variance mapping differ from standard velocity-only color mapping?
    → Standard color: red/blue represent flow toward/away with brightness proportional to velocity. Variance mapping ADDS green to indicate variability in the mean velocity estimate at each pixel — usually turbulence or aliasing.
  • basicI.E-025
    What is the relationship between Doppler shift frequency and blood velocity?
    → Δf = (2 × v × cos θ × f_T) / c. So Doppler shift is directly proportional to blood velocity, transmitter frequency, and cos θ; inversely proportional to speed of sound in medium.
  • basicI.E-026
    Why can you never accurately estimate a stenosis velocity through a color Doppler window?
    → Color Doppler measures MEAN velocity via autocorrelation. Peak velocity across a stenosis (the value used in Bernoulli's equation) must be obtained with SPECTRAL (PW or CW) Doppler.