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Find out more about three low MI imaging techniques: pulse inversion, power modulation and coherent contrast imaging (CCI).
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Alternate transmission of two identical pulses per image line but of opposing polarity.
Tissues behave linearly at low MI, so the returning tissue signal is the same as the emitted (fundamental frequency), so they cancel each other.
However, non-linear microbubble oscillation ensures this signal is not cancelled, and thus a pure contrast harmonic signal is obtained.
Pulse inversion method is remarkably sensitive to contrast even at low-MI settings.
U/S machine selectively detects backscatter signal from microbubbles whilst suppressing reflections from tissue
Transducer sends two pulses of identical shape along a scan line, the 2nd pulse being ½ the amplitude of the first. The smaller reflection (from 2nd pulse) is doubled and subtracted from 1st reflection.
Thus the subtraction still leaves a signal - purely from contrast rather than tissue – which is detected by the U/S machine.
Find out more about three high-MI contrast imaging techniques: Power Doppler Harmonic Imaging (PDHI), Ultraharmonics and Power Pulse Inversion.
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 (see figure below). Tissues return a very low signal at ultraharmonic frequencies so microbubbles are easily detected.
The figure beneath shows that the amplitude of microbubble signals between fundamental and 2nd harmonic are low (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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