Harmonic imaging relies on detection of harmonics of the transmitted frequency produced by bubble oscillation. This method is widely available on ultrasound scanners and uses the same array transducers as conventional imaging. A major limitation of the use of ultrasound contrast agents is the problem that signals from the microbubbles are mixed with those from tissue. Echoes from solid tissue and red blood cells are suppressed by harmonic imaging.
In harmonic mode, the system transmits at one frequency, but is tuned to receive echoes preferentially at double that frequency, and the second harmonic echoes from the place of the bubble. Typically, the transmit frequency lies between 1.5 and 3 MHz and the receive frequency is selected by means of a bandpass filter whose center frequency lies between 3 and 6 MHz.
Color Doppler and real-time harmonic spectral Doppler modes have also been implemented and show a level of tissue motion suppression not available in conventional modes.

See also Harmonic B-Mode Imaging, and Harmonic Power Doppler.

(HPD) Harmonic power Doppler is currently one of the most sensitive techniques for detecting ultrasound contrast agents. HPD works by transmitting multiple pulses toward the object to be imaged and detecting the pulse-to-pulse changes in the received echo signals.
Second harmonic bandbass filtering is applied to the received signals to exploit the non-linear behavior of scattering from bubbles (clutter). Harmonic power Doppler operates best at high output levels because of increased contrast destruction, and pulse amplitudes close to the maximum allowed are used much of the time.
With a high mechanical index, non-linear propagation of the sound will cause significant harmonic components from tissue, and the contrast agent to tissue ratio will decrease.
Also called Harmonic Power Angio. See also Multiple Frame Trigger.

Biological flows vary throughout the cardiac cycle. The analog output can be filtered to give a pulsatile flow or mean flow signal.

(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 resolution is the ability to distinguish two adjacent objects or in Doppler measurements the smallest detectable change in flow. Probe resolutions are generally specified at 0.1Hz filtering.

See also Matrix Size.