Harmonics

A harmonic frequency is a frequency that is an integer multiple of the fundamental frequency.

For example, if the fundamental frequency is 3 MHz, then the second harmonic is 6 MHz (2 x 3 MHz), the third harmonic is 9 MHz (3 x 3 MHz), and the fourth harmonic is 12 MHz (4 x 3 MHz)

In diagnostic ultrasound harmonic frequencies can be generated in tissue or by microbubble contrast.

Tissue Harmonic Imaging (THI) makes use of the harmonics frequencies that are generated in tissue.

Contrast Harmonics makes use of the harmonic frequencies that are generated by microbubble contrast.

The following video explains harmonics and how  they are used in diagnostic ultrasound to improve image quality.