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.

Ultrasound waves are created at harmonics of the delivered frequency. Subharmonic imaging uses the harmonic oscillation of a system at a frequency that is a simple fraction of its fundamental frequency. The subharmonic response frequencies has half of the fundamental frequency. The second subharmonic has a half fundamental frequency of one half the frequency, and so on.

See also Harmonic Imaging and Superharmonic Imaging.

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.

Ultraharmonic is an oscillation at a frequency that is a rational multiple of that of its fundamental sinusoidal oscillation, for example 1.5 or 2.5 times the fundamental frequency. Ultraharmonic imaging is a method to eliminate tissue artifacts and therefore increase contrast to tissue ratio.
Also called Superharmonic Imaging.

See also Power Modulation.

(THI) Tissue harmonic imaging (also called native harmonic imaging) is a signal processing technique which addresses ultrasound limitations like penetration and resolution. Tissue harmonic imaging reduces noise and clutter by improving signal to noise ratio and resolution. The signal penetration in soft tissue increases as the transmit frequency is decreased, by simultaneous decreased image resolution. As an ultrasound wave propagates through the target media a change occurs in the shape and frequency of the transmitted signal. The change is due to the normal resistance of tissue to propagate sound energy. This resistance and the resulting signal change is called a harmonic oscillation.
For harmonic imaging the input frequency doubles the output frequency, for example a transmit frequency of 3.0 MHz. which would provide maximum penetration will return a harmonic frequency of 6.0 MHz. The returning higher frequency signal has to only travel one direction to the probe. The advantages of high frequency imaging and the one-way travel effect are decreased reverberation, beam aberration, and side lobes, as well as increased resolution and cystic clearing.