Power doppler sonography

a Doppler ultrasound technique exploiting the total power in the Doppler signal to produce colour-coded real-time images of blood flow. The technique is also named amplitude Doppler sonography, colour Doppler energy (CDE), colour amplitude imaging (CAI), and ultrasound angiography. Power Doppler sonography is an option in colour Doppler sonography instruments, and just as in colour Doppler, the Doppler signal is sampled at multiple locations along each scan line. The technique differs from conventional colour Doppler in the way the Doppler signals are processed; instead of estimating mean frequency and variance through autocorrelation, the integral of the power spectrum is estimated and colour coded. The colours in the power Doppler image indicate only that blood flow is present; they contain no information on flow velocity.

The principle of power Doppler sonography is illustrated in Fig.1. In the left section of the figure, two vessels, A and B, are interrogated by an ultrasound beam. Both vessels have approximately the same flow velocity, but the Doppler angle is small in A and nearly 90 in B. The pulsed Doppler ultrasound time velocity spectral display of vessel A and B, respectively, is schematically shown (middle section, top and bottom). Due to the small Doppler angle, the relative velocity measured in A is high (close to the true velocity). In a colour Doppler display, the vessel would show up with a bright colour. The relative velocity measured in B, is close to zero; the diastolic velocity is hardly visible above the cut-off of the high pass filter (shaded area above and below the baseline). This vessel would be difficult to see in colour Doppler sonography. The power spectra of these Doppler signals are schematically shown (right). The total power is the area under the power (P) versus frequency curves. Since the acoustic power of the Doppler signal from blood is proportional to the total number of scatterers, i.e to the amount of blood at the particular location (see Rayleigh Tyndall scattering), the power of the Doppler signals from vessel A and B will be equal, provided the sample volume in vessel A contains the same number of red blood cells as that in vessel B. In vessel A, the centre frequency of the Doppler signal is relatively far from the centre transmit frequency of the Doppler transducer (f₀), and in addition the Doppler signal has a broad spectrum of frequencies, reflecting the broad spectrum of Doppler frequency shifts (or velocities) seen in the time - velocity spectral display. (The Doppler frequency shifts are actually the differences between the frequencies in the Doppler signal and the transmit frequency, f₀.) In vessel B, the centre frequency of the Doppler signal is very close to the centre transmit frequency, and the frequency spectrum is narrow. However, the areas under the power versus frequency curves, i.e. the integrals of the power spectra, are the same for the two vessels which are therefore equally well shown in a power Doppler image. Note that echoes from stationary tissue will have the same frequency as the transmit frequency, and therefore no Doppler signal.

As can be seen from Fig. 1, the total power of the Doppler signal from blood is independent of blood-flow velocity and Doppler angle, provided the Doppler frequency shift is different from zero. Further disturbing multicoloured background. The random noise has a fairly uniform low power, however, and is therefore displayed with a uniform dark colour (e.g. dark blue) in the power Doppler image, clearly separated from the high-power Doppler signals from blood flow (displayed in yellow to red). High gain settings are therefore possible with power Doppler. Motion is a severe problem, however. Echoes from moving body organs, for example, may have high power levels, and give bright flash artefacts. A combination of gas microbubble contrast media (see ultrasound contrast medium) and harmonic imaging, may solve the problem, however.

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Fig.1

Power doppler sonography, Fig.1		Power doppler sonography, Fig.2 (a)		Power doppler sonography, Fig.2 (b)