Doppler and Hemodynamics
The Doppler equation
fd = (2 × f0 × v × cosθ) / c. The factor of 2 accounts for the round trip. Shift is greatest at 0° or 180° and vanishes at 90°, so keep the angle at or below 60°. Common calculation errors: using mean velocity when peak is requested, omitting the cosine term, or entering frequency in kHz.
Aliasing
Pulsed-wave and color Doppler alias when the shift exceeds the Nyquist limit (PRF/2). Remedies: raise PRF by imaging shallower, using fewer gates, or shrinking the color box; lower the baseline; steer toward a larger angle; transmit at a lower frequency; or switch to continuous wave. CW has no aliasing but no range resolution-it reports the highest velocity anywhere along the line.
Hemodynamics
Continuity: A1V1 = A2V2, so velocity increases through a stenosis. Poiseuille flow varies with the fourth power of radius, so small caliber changes cause large resistance changes. The simplified Bernoulli equation, ΔP = 4v², estimates the pressure drop across a narrowing. Laminar flow produces a narrow spectral band; turbulence broadens and may mirror the spectrum. A Reynolds number above roughly 2000 indicates turbulent flow. Resistive index = (PSV − EDV)/PSV; high-resistance beds such as resting limb arteries show little diastolic flow.
Traps
Confusing power Doppler (amplitude-based, no direction, no aliasing) with color velocity mapping. Remember spectral broadening also comes from a sample volume that is too large, not only from turbulence.
Worked example
3 MHz, 60°, peak velocity 100 cm/s: fd = 2 × 3,000,000 × 1.0 × 0.5 / 1540 ≈ 1950 Hz. Avoiding aliasing requires PRF above 2 × 1950 ≈ 3900 Hz.