Spectral, Color, and Tissue Doppler Modalities

The Doppler Equation and Angle Dependence

All Doppler echocardiography techniques rely on the Doppler shift — the change in frequency between the transmitted ultrasound and the ultrasound reflected from moving red blood cells (or, in tissue Doppler, moving myocardium). The relationship is expressed by the Doppler equation:

Δf = (2 × f₀ × v × cos θ) / c

where Δf is the measured Doppler shift, f₀ is the transmitted frequency, v is the velocity of the reflector, θ is the angle between the ultrasound beam and the direction of flow, and c is the speed of sound in soft tissue. Solving for velocity — the quantity actually needed clinically — gives:

v = (Δf × c) / (2 × f₀ × cos θ)

Because velocity is calculated using cos θ, Doppler accuracy is critically angle-dependent: the beam must be aligned as parallel as possible to the direction of blood flow.

Because error grows non-linearly and becomes unacceptable well before 90°, sonographers keep the intercept angle at or below roughly 20° whenever possible and do not routinely apply angle correction to intracardiac spectral Doppler — correcting for a large angle amplifies any small angle-estimation error into a large velocity error.

Pulsed-Wave (PW) versus Continuous-Wave (CW) Doppler

Spectral Doppler is acquired in two fundamentally different ways.

Pulsed-wave (PW) Doppler uses a single crystal that alternately transmits brief pulses and listens for returning echoes, timed so that only echoes returning from a specific, operator-placed depth (the sample volume) are analyzed. This gives PW Doppler true range resolution — it can isolate flow at one exact location, such as the LV outflow tract or mitral inflow at the leaflet tips. The trade-off is a hard ceiling on the maximum velocity it can measure without distortion: because PW Doppler must wait for each pulse's echo before sending the next, its pulse repetition frequency (PRF) is limited, and the Nyquist limit — the maximum unambiguously measurable velocity — equals PRF/2. Real flow velocities above the Nyquist limit are displayed as aliased: the waveform is "cut off" at the top (or bottom) of the display and wraps around, reappearing as if flowing in the opposite direction. Aliasing is a pulsed-Doppler phenomenon; it does not occur in continuous-wave Doppler.

Continuous-wave (CW) Doppler uses two crystals — one continuously transmitting, one continuously receiving — active along the entire length of the beam. Because it samples continuously rather than in timed pulses, CW Doppler has no Nyquist limit and cannot alias, so it can accurately record very high velocities such as the jet across a severely stenotic or regurgitant valve (for example, aortic stenosis or the tricuspid regurgitation jet used to estimate right ventricular systolic pressure). Its trade-off is the loss of range resolution: CW Doppler records the highest velocity found anywhere along the entire beam but cannot localize where along that path the signal originated, so the operator must already know from 2D and color imaging where the suspected high-velocity jet lies before positioning the CW cursor.

Color-Flow Doppler

Color-flow Doppler overlays a 2D, color-coded map of mean flow velocity and direction onto the standard grayscale image, using an autocorrelation technique applied to multiple sample volumes across the sector rather than a single site. By convention, flow toward the transducer is coded red and flow away is coded blue, with lighter shades or added green/yellow encoding higher velocity or turbulence/variance — commonly remembered by the mnemonic BART: Blue Away, Red Toward. Because color-flow Doppler is fundamentally a pulsed technique sampling many sites simultaneously, it is subject to the same Nyquist-limited aliasing as PW Doppler; on color imaging, aliasing appears as an abrupt color reversal or a bright "mosaic" pattern of mixed colors within a jet, which is itself frequently used as a visual clue to significant flow disturbance, such as outlining a regurgitant jet or a PISA hemisphere.

Tissue Doppler Imaging (TDI)

Tissue Doppler imaging applies Doppler processing to the myocardium itself rather than to blood. Whereas conventional spectral and color Doppler use filters that reject the low-frequency, high-amplitude signals returning from moving tissue in order to isolate the faint, high-frequency signals from moving blood, TDI inverts these filter settings to instead accept the low-velocity, high-amplitude myocardial signal and reject the blood-flow signal. TDI can be displayed as a color-coded overlay or, most commonly for quantification, as a pulsed spectral tracing sampling the mitral annulus, yielding systolic (s′) and diastolic (e′, a′) myocardial velocities that are combined with transmitral inflow velocities (E/e′) as core components of diastolic function grading, covered in Chapter 6.