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.

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.

(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.

Transducers can be divided in:
1.) Transducers where the sound wave is transmitted and received by different elements.
2.) Transducers where multiple elements part of the time transmit and part of the time receive sound energy.
The first type of ultrasound transducer is used in detection of blood flow (also called nonimaging transducers). For example, the continuous wave transducer (Pedoff transducer) has two separate elements, where one element is always transmitting while the other element is always receiving.
Probes of the second type are used to image cardiac structures and have the capability to use various Doppler techniques to detect blood flow (also called imaging transducers). For example, continuous wave, pulsed wave, high pulse repetition frequency, color flow, M-mode, and 2D-mode are the various modes that this type of transducer can perform.

Transducers can also be divided in mechanical and electronic or phased scan types.
Mechanical transducers use a combination of single element oscillation, multiple element rotation, or a single element and set of acoustic mirrors to generate the sweeping beam for 2D mode. Caused by the vibration (created as the mirrors rotate or oscillate inside the cover) is this type sometimes called the 'wobbler'. Mechanical transducers are cheaper than electronic transducers.
Different types of electronic or phased array probes can create a linear or rectangular shaped scan plane as well as a sector or pie shaped scan plane. Sector scanners are most useful for cardiac ultrasound examinations where the beam is directed between the ribs to image the heart. A linear array transducer is more useful in abdominal, OB/GYN, and small parts examinations. Electronic transducers are more expensive but they provide dynamic focusing and smaller probe.

See also Rectangular Array Transducer.