Selective detection of the microbubble contrast medium can be enhanced by Doppler processing that removes signals with zero Doppler frequency shifts. This will remove tissue harmonics. By detecting overlong bursts of inverted pulses and using Doppler detection methods, very high sensitivity to microbubbles can be achieved. The bubbles can be detected at sufficiently low incident power levels to avoid destroying them. Pulse inversion Doppler has demonstrated the first real-time images of myocardial perfusion using perfluorocarbon gas agents.

See also Pulse Inversion Imaging, Myocardial Contrast Echocardiography, and Perfluorochemicals.

Doppler ultrasound is a medical imaging technique for calculating the relative velocity between two points by measuring the frequency shift of a sound wave transmitted from one point to the other, based on the Doppler effect. Continuous or pulsed Doppler is frequently used to examine cardiovascular blood flow. The combination of routine 2D-mode and Doppler ultrasound allows a complete evaluation of the heart's anatomy and function (including the fetal heart). See also Doppler Fluximetry in Pregnancy.
Doppler ultrasound depends on the fact that if a moving object reflects the ultrasound waves, the echo frequencies are changed. A higher frequency is created if the object is moving toward the probe//transducer and a lower frequency if it is moving away from it. How much the frequency is changed depends upon how fast the object is moving. Doppler ultrasound shows the different rates of blood flow in different colors on a monitor in real time.
The major Doppler parameters are the peak systolic velocity and the end-diastolic velocity. The peak systolic velocity ratio compensates the variability between different patients and instrumentations.

Different Doppler and duplex techniques:

(PII) Pulse inversion imaging (also called phase inversion imaging) is a non-linear imaging method specifically made for enhanced detection of microbubble ultrasound contrast agents. In PII, two pulses are sent in rapid succession into the tissue; the second pulse is a mirror image of the first. The resulting echoes are added at reception. Linear scattering of the two pulses will give two echoes which are inverted copies of each other, and these echoes will therefore cancel out when added.
Linear scattering dominates in tissues. Echoes from linear scatterers such as tissue cancel, whereas those from gas microbubbles do not. Non-linear scattering of the two pulses will give two echoes which do not cancel out completely due to different bubble response to positive and negative pressures of equal magnitude. The harmonic components add, and the signal intensity difference between non-linear and linear scatterers is therefore increased. The resulting images show high sensitivity to bubbles at the resolution of a conventional image.
In harmonic imaging, the frequency range of the transmitted pulse and the received signal should not overlap, but this restriction is less in pulse inversion imaging since the transmit frequencies are not filtered out, but rather subtracted. Broader transmit and receive bandwidths are therefore allowed, giving shorter pulses and improved axial resolution, hence the alternative term wideband harmonic imaging. Many ultrasound machines offer some form of pulse inversion imaging.

See also Pulse Inversion Doppler, Narrow Bandwidth, Dead Zone, Ultrasound Phantom.

The degree that a transducer is finely tuned to specific narrow frequency range. For example: A low Q-value means wide bandwidth and high Q-value means narrow bandwidth.

See also Pulse Inversion Doppler, Narrow Bandwidth, Dead Zone, Ultrasound Phantom.

Real-time mode has been developed to present motion like a movie of the body's inner workings, showing this information at a high rate. The special real-time transducer uses a larger sound beam than for A, B or M-modes. A linear array transducer with multiple crystal elements displays real-time compound B-mode images with up to 100 images per second.
At each scan line, one sound pulse is transmitted and all echoes from the surface to the deepest range are received. Then the ultrasound beam moves on to the next scan line position where pulse transmission and echo recording are repeated.

See also Compound B-Mode, Pulse Inversion Doppler, and Frame Averaging.