Conventional (velocity) doppler assesses the speed of motion of red cells. However, the intensity of the doppler signal reflects the number of scatterers within the ultrasound beam. The image display can be altered to show the amplitude or power of the doppler signal rather than velocity – this is the basis of PDHI. It is a multi-pulse technique (sends ultrasound pulses along a single scan line & detects the changes between pulses). The greater the change between pulses, the greater the intensity of the PDHI display. A colour is displayed if there has been a change between pulses, and the saturation of the colour reflects the amplitude of the echo that has changed.
Courtesy of Belcik et al J Am Soc Echo (2005); 18; 1083 - 1092
When the MI is deliberately increased beyond the point of oscillation, microbubbles produce a very transient, high-amplitude signal before destruction. This is an ultraharmonic frequency produced only by microbubbles and not tissue (after the 2nd but before the 3rd tissue harmonic). Tissues return a very low signal at ultraharmonic frequencies so microbubbles are easily detected.
The figure opposite shows that the microbubble signals beyond the 2nd harmonic are of low amplitude (y axis), so it is best to combine with a high-MI destructive technique to intensify resultant signals.
This is the same principle as pulse inversion, but combines the non-linear detection performance of pulse inversion with the motion discrimination of power doppler. A sequence of > 2 pulses of alternating phase are emitted. Echoes from successive pulses are then recombined to eliminate the effect of tissue motion. This allows suppression of moving tissue without needing to disrupt the bubbles and thus can be performed either as high MI or as real-time, low MI imaging.
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