Power Doppler is a technique that uses the amplitude of Doppler signal to detect moving matter. Power Doppler:
- is independent of velocity and direction of flow, so there is no possibility of signal aliasing
- is independent of angle, allowing detection of smaller velocities than colour Doppler, facilitating examinations in certain technically challenging clinical setting
- has higher sensitivity than colour Doppler, which makes a trade-off with flash artefacts
Ultrasound images are formed by reflected echoes. These waves have an amplitude (as those in A-, B- and M-mode) and a frequency, which is equal to the frequency of the emitted wave, if the tissue is static. Tissue movement (e.g. blood) promotes a frequency shift (Doppler shift) in the reflected echoes. This is the information used to form B-mode superimposed 2D colour Doppler images. The amplitude of frequency shifted echoes is used to form B-mode superimposed 2D Power Doppler images.
Spectral analysis of Doppler signal contains both frequency and amplitude information of a small tissue sample. The vertical axis represents frequency shift (related to colour Doppler), the brightness of the pixels represents the amplitude of the signal (related to Power Doppler) and the horizontal axis represents time.
Power Doppler is particularly useful when examining superficial structures, like thyroid, testis, renal grafts and subcutaneous lesions. It may be used to look for tumour vessels, to evaluate tiny low-flow vessels and detect subtle ischaemic areas.
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