Low MI contrast imaging techniques
Find out more about three low MI imaging techniques: pulse inversion, power modulation and coherent contrast imaging (CCI).
A pulse cancellation technique
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.
Linear reflector at low MI = 0.1
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.
Assuming linear oscillation of tissue at low MI, this subtraction results in zero tissue signal. However, due to non-linear oscillation of microbubbles, reflecting pulses from contrast differ in amplitude and shape.
Thus the subtraction still leaves a signal - purely from contrast rather than tissue – which is detected by the U/S machine.
Coherent Contrast Imaging (CCI)
The theoretical limitation of both power modulation & pulse inversion is that each scan line needs to be pulsed multiple times, which reduces frame rate and increases bubble destruction.
CCI is a non-linear technique that can cancel the fundamental frequency with a single pulse. It is a grey-scale rather than doppler-based technique utilising transmit pulse shaping and single pulse cancellation technologies.
Coherent imaging simply means that multiple beamformers are used to construct an image, rather than the usual discrete scan lines which are phase independent of each other.
Thus, in a coherent dataset, both amplitude AND phase information are present, so all acquired data is tied together (i.e. is coherent).
High MI contrast imaging techniques
Find out more about three high-MI contrast imaging techniques: Power Doppler Harmonic Imaging (PDHI), Ultraharmonics and Power Pulse Inversion.
Power Doppler Harmonic Imaging (PDHI)
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
Rest & stress PDHI images
Normal perfusion at rest but a clear dipyridamole-induced perfusion detect in the LAD territory (septum & anterior wall – white arrows).
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.
Power Pulse Inversion
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.