6.2 Spectral Doppler: Pulsed Wave vs Continuous Wave

The Doppler Effect and the Doppler Equation

When ultrasound reflects off moving red blood cells, the returned frequency differs from the transmitted frequency — this is the Doppler effect, and the difference is the Doppler shift. Blood moving toward the transducer compresses the wave and returns a higher frequency (positive shift); blood moving away returns a lower frequency (negative shift). The relationship is quantified by the Doppler equation:

Doppler shift = (2 x f0 x v x cos theta) / c

where f0 is the transmitted frequency, v is blood-flow velocity, theta is the Doppler angle between the beam and the direction of flow, and c is 1540 m/s. The factor of 2 appears because the shift happens twice — once as the moving cells receive the wave and again as they retransmit it. A worked example: with a 5 MHz transducer, flow of 1.0 m/s, and a 0-degree angle, the shift is (2 x 5,000,000 x 1.0 x 1.0) / 1540, about 6,494 Hz or roughly 6.5 kHz — comfortably in the audible range, which is why Doppler shifts are also played through speakers.

Angle Dependence and the cos 90 Trap

The cosine term makes the measurement exquisitely angle-dependent. At 0 degrees the cosine is 1 and the full shift is detected; at 90 degrees the cosine is 0 and no Doppler shift is detected at all, no matter how fast the blood is moving — the classic exam trap and the reason flow in a vessel perpendicular to the beam can falsely appear absent. Clinically, velocity measurements should be made at 60 degrees or less, because cosine error accelerates beyond that point:

The angle-correct cursor recovers the true velocity from that reduced shift, so a properly corrected 60-degree measurement is not half the real value. The 60-degree ceiling exists because the correction divides by cos(theta): as the angle steepens, a small cursor misalignment is magnified into a large velocity error.

Pulsed-Wave Doppler: Depth Discrimination with a Speed Limit

Pulsed-wave (PW) Doppler uses one crystal that alternates between transmitting short pulses and listening. Because the machine knows when each pulse left, it can listen only for echoes returning from a chosen depth — a technique called range gating. The operator positions a sample volume (gate) precisely inside a vessel, which makes PW ideal when you must know exactly where the velocity came from, such as sampling the mitral inflow or a specific point in a stenotic carotid.

The price of pulsing is the Nyquist limit, equal to PRF / 2 (half the pulse repetition frequency). If the true Doppler shift exceeds this limit, the system samples too slowly to track it and the display aliases: the systolic peak is chopped off and wrapped around to the opposite side of the baseline. For example, at a PRF of 10 kHz the Nyquist limit is 5 kHz, and an 8 kHz shift will wrap. Deeper vessels make matters worse, because greater depth forces a lower PRF and therefore a lower Nyquist limit. Aliasing remedies include increasing the scale (PRF), shifting the baseline, using a lower-frequency transducer, increasing the Doppler angle to reduce the measured shift, or switching to continuous wave.

Continuous-Wave Doppler: No Aliasing, No Address

Continuous-wave (CW) Doppler dedicates two crystals in the same housing: one transmits constantly while the other receives constantly. With no waiting for go-return times, there is no PRF and therefore no Nyquist limit and no aliasing — CW can faithfully measure the highest velocities the body produces, such as the 4 to 5 m/s jets of aortic stenosis or a high-velocity tricuspid regurgitation jet. The cost is range ambiguity: the machine samples everything along the entire beam path simultaneously, so it cannot tell you which depth produced the peak velocity. CW is also more sensitive to low-flow states and requires no gate placement.

Reading the Spectral Trace

A spectral Doppler display plots velocity (or shift) on the vertical axis against time on the horizontal axis, with the baseline marking zero flow. Flow above the baseline moves toward the transducer; flow below moves away. Key components to recognize:

• Systolic upstroke and peak: the rapid acceleration and maximum velocity during systole; a dicrotic notch may mark aortic valve closure in peripheral arteries.
• Diastolic flow: the amount of forward (or reversed) flow in diastole reflects downstream resistance — high-resistance beds show little diastolic flow, low-resistance beds (internal carotid, renal, hepatic arteries) show continuous forward diastolic flow.
• Spectral window: the clear black area beneath the systolic peak in normal laminar flow, where nearly all red cells travel at similar velocities.
• Spectral broadening: filling-in of that window, indicating a wide spread of simultaneous velocities — produced by turbulent flow at or beyond a stenosis, by an oversized sample volume, or by excessive Doppler gain.

Choose PW when you need to know where flow is and velocities are modest; choose CW when velocities are high enough to alias on PW and the exact depth is either known from imaging or less important than capturing the true peak.