What is Power Doppler?

The history of Doppler in the world of medical ultrasound began with spectral Doppler, but its use did not really take off until the introduction of colour Doppler in the early 1980s. Whilst a qualitative modality (arguably semi-quantitative with more recent publications in echocardiography on estimating regurgitant jet size as a percentage of the receiving chamber, for example), colour Doppler gives an instant overview of the patterns of flow within vessels and chambers, which makes the placing of pulsed wave gates or continuous wave Doppler lines much quicker and, often (with the exception of eccentric jets), more accurately.

 

One major limitation of colour Doppler, however, is its poor sensitivity to deep vessels and low-flow. This is for a number of reasons, including:

• Unlike B-mode which technically needs only one pulse per scan line, the ensemble length for colour Doppler must typically be between 2-20 pulses given that it is a phase-domain technique*. This means that it calculates regions of blood flow by comparing the phase shifts between pulses (autocorrelation), and the more pulses which are sent, the greater the sensitivity to lower velocity flow (clearly, blood cells moving very slowly would show no change in phase between one pulse and the next, but over several pulses, a change may be detected). However, when compared with the 80-100 pulses used for spectral Doppler which only needs to interrogate a single scan line (Hoskins et al., 2010), it is clear to see that the sensitivity of colour Doppler is much reduced and sensitivity to low flow therefore limited. Given the need for real-time imaging with colour Doppler (i.e. the need for fast frame rates despite the requirement for multiple scan lines to be interrogated), there has to be this limitation to the number of pulses which can be sent per line, and thus a clear limitation on sensitivity to low flow (Evans, 2009).

* For less common time-domain methods, the principle is similar; multiple pulses are needed to detect subtle changes in echo return times.

 

• Due to the requirements of time and distance resolution, colour Doppler is a pulsed wave technique as mentioned above. In order to detect smaller frequency shifts, a longer spatial pulse length could be used. However, this would severely degrade the axial resolution, and would also result in reduced frame rates unless the number of pulses per line were reduced even further.

 

• Colour Doppler’s dependence on angle of insonation makes imaging of tortuous vessels – where the angle between the Doppler beam and blood flow is continuously changing – particularly challenging. Where the angle between the blood flow and the transducer approaches 90 degrees, sensitivity is severely reduced and may even be coded as an area of no flow.

 

In order to overcome these limitations and offer an alternative modality for imaging deep vessels and small Doppler shifts, Power Doppler was introduced, described in its original form by Rubin et al. (1994).

 

Power Doppler does not display velocity information. Instead, it simply displays the amplitude of the returning Doppler shifted echoes. The degree of Doppler shift is irrelevant, so in this sense, Power Doppler is less dependent upon angle of insonation. As its purpose is to display regions of blood flow and not changing velocities over time, many more pulses can be sent per scan line, yielding a much greater sensitivity to lower velocity flow. In addition, there can be a greater degree of frame averaging (persistence) in order to improve the signal-to-noise ratio for imaging deeper vessels.

 

The enhanced sensitivity of Power Doppler makes it ideal for imaging deeper structures, such as blood flow within the kidneys, and tortuous superficial vessels such as those inside the testes (Babcock, 1996). Its application within echocardiography is limited due to the high frame rates required for cardiac work; however, it has had more recent application in myocardial perfusion imaging (MCE), where blood perfusion is much slower. It can be combined with contrast in a relatively new technique called ‘Harmonic Power Doppler imaging’ whereby a high MI pulse is emitted in order to destroy the microbubbles. This has shown greater sensitivity over standard B-mode harmonic imaging, but at the expense of frame rate (Masugata et al., 2000; Senior et al., 2000). In human imaging, this is most often used within the stress echo environment (though not used clinically in the USA due to lack of FDA approval for the use of contrast in myocardial perfusion imaging).

 

Power Doppler requires only configurational adjustments to pre-existing colour Doppler technology. It is an available option in many advanced colour systems, such as a the SonoScape S9. Some systems also now offer directional Power Doppler, where the direction of blood flow is indicated by colour coding in either red or below, depending on whether the Doppler shift is positive or negative.

 

References

Evans, D (2009). Colour Flow and Motion Imaging. Proceedings of the Institution of Mechanical Engineers.

Hoskins, P., Martin, K., & Trush, A. (2010). Diagnostic Ultrasound Physics and Equipment, 2nd Edition. Cambridge University Press.

Masugata, H., Cotter, B., Peters, B. et al. (2000). Assessment of Coronary Stenosis Severity and Transmural Perfusion Gradient by Myocardial Contrast Echocardiography. Circulation 102:1427-1433.

Rubin, J., Bude, R., Carson, P. et al. (1994). Power Doppler US: a potentially useful alternative to mean frequency based color Doppler US. Radiology, 190, 853-6.

Senior, R., Kaul, S., Soman, P., et al. Power Doppler harmonic imaging: a feasibility study of a new technique for the assessment of myocardial perfusion. Am Heart J;139:245–51.