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.

Non-linear scattering describes in physics the conversion of a photon from one wavelength to one or more other wavelengths. In ultrasound, non-linear scattering is the generation of an echo containing higher harmonics. In the case of a microbubble, the oscillation is asymmetric with time, producing echoes with even harmonics.

See also Cross-section Scattering.

Higher frequencies are attenuated by tissue more than lower frequencies. This means that the higher the frequency the lower the depth of penetration but the greater the resolution.
Harmonic imaging allows the use of a lower frequency pulse to be picked up and sampled at the second harmonic (higher frequency) where the low frequency allows greater penetration and high frequency provides better resolution.

See also Skinline.

From Siemens Medical Systems;
'This high-performance, multi-specialty system supports and improves your daily ultrasound routine. Embedded DICOM creates the integrated foundation for a complete connectivity solution while MultiHertz™ multiple frequency imaging and Tissue Harmonic Imaging (THI) expands the clinical versatility of the system. Advanced applications such as stress echo, SieScape™ panoramic imaging, and transesophageal imaging can be seamlessly integrated.'

From GE Healthcare.;
'GE is defining a new age of ultrasound. We call it Volume Ultrasound. GE's Voluson 730 Expert is a powerful system that enables real-time techniques for acquiring, navigating and analyzing volumetric images so that you can make clinical decisions with unprecedented confidence.'