The propagation of high amplitude ultrasound waves is inadequate described by a linear wave equation. Non-linear propagation is to expect if the power levels are high enough to make non-linear effects significant. A non-linear propagation results in the distortion of the transmitted waveforms, resulting in the generation of harmonics of the initial frequency components transmitted by the transducer.
In the near field of ultrasound probes, the occurring diffraction and focusing effects make this process complex. The distortion of a wavefront propagating in a medium in which the compressional phase moves slightly faster than the rarefactional phase, results is the conversion of some wave energy into higher harmonics of the fundamental frequency. The effect increases strongly with increasing wave amplitude.

Harmonic B-mode imaging takes advantage of the non-linear oscillation of microbubbles. During harmonic imaging, the sound signal is transmitted at a frequency of around 1.5 to 2.0 MHz and received at twice this frequency. The microbubbles also reflect waves with wavelengths different from the transmitted one, the detectors can be set to receive only the latter ones and create only images of the contrast agent.
Using bandpass filters the transmitted frequency is separated from the received signal to get improved visualization of vessels containing ultrasound contrast agents (USCAs). The signal to noise ratio during the presence of microbubbles in tissue is four- to fivefold higher at the harmonic compared with the basic frequency.
Using harmonic B-mode imaging, harmonic frequencies produced by the ultrasound propagation through tissue have to be taken into account. The tissue reflection produces only a small amount harmonic energy compared to USCAs, but has to be removed by background subtraction for quantitative evaluation of myocardial perfusion.

See also Non-linear Propagation.

(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.

Superharmonic imaging uses higher harmonics like third and fourth harmonic to increase the contrast to tissue ratio compared to a second harmonic imaging mode. Second harmonic imaging is better than fundamental imaging, but has limited capabilities to discriminate between tissue and microbubbles, caused by the non-linear propagation of ultrasound.