A phantom is used to control the imaging performance of ultrasound transducers. The spatial resolution, dead zone, linear fidelity, depth of penetration and image uniformity is important for the image quality. For the axial and lateral resolution, the standard definition is the resolution of objects parallel and perpendicular to the path of the sound beam. Ultrasound pictures created by scans of specially designed ultrasound phantoms can quantify the imaging quality and transducer performance.
Phantoms contain one or more materials that simulate a tissue in its interaction with ultrasound. Several phantoms are available specifically for quality control. Transducer characterization consists of a standard pulse echo analysis and insertion loss measurement for each probe. The quality variation from the baseline should be tracked over a period.

Ultrasound physics is based on the fact that periodic motion emitted of a vibrating object causes pressure waves. Ultrasonic waves are made of high pressure and low pressure (rarefactional pressure) pulses traveling through a medium.

Properties of sound waves:

The speed of ultrasound depends on the mass and spacing of the tissue molecules and the attracting force between the particles of the medium. Ultrasonic waves travels faster in dense materials and slower in compressible materials. Ultrasound is reflected at interfaces between tissues of different acoustic impedance e.g., soft tissue - air, bone - air, or soft tissue - bone.
The sound waves are produced and received by the piezoelectric crystal of the transducer. The fast Fourier transformation converts the signal into a gray scale ultrasound picture.

The ultrasonic transmission and absorption is dependend on:
See also Sonographic Features, Doppler Effect and Thermal Effect.

Unlike regular sound, ultrasound can be directed into a single direction. The echoes received by a stationary probe will result in a single dimensional signal showing peaks for every major material change.
To generate a 2D picture, the probe is swiveled, either mechanically or through a phased array of ultrasound transducers. The data is analyzed by computer and used to construct the image. In a similar way, 3D pictures can be generated by computer using a specialized probe. In this way, a photo of an unborn baby may be made.
Some ultrasonography machines can produce color pictures, of sorts. Doppler ultrasonography is color coded onto a gray scale picture. From the amount of energy in each echo, the difference in acoustic impedance can be calculated and a color is then assigned accordingly.

See also Densitometry and 3D Ultrasound.

The traveling ultrasonic wave causes a low-level ultrasound radiation force when this energy is absorbed in tissues (absorbed dose). This force produces a pressure in the direction of the beam and away from the transducer. It should not be confused with the oscillatory pressure of the ultrasound wave itself. The pressure that results and the pressure gradient across the beam are very low, even for intensities at the higher end of the range of diagnostic ultrasound. Mechanical effects like radiation forces lead to stress at tissue interfaces. The effect of the force is manifest in volumes of fluid where streaming can occur with motion within the fluid. The fluid velocities which result are low and are unlikely to cause damage.
The effects of ultrasound radiation force (also called Bjerknes Forces) were first reported in 1906 by C. A. and V. F. K. Bjerknes, when they observed the attraction and repulsion of air bubbles in a sound field.
While incompressible objects do experience radiation forces, compressible objects driven at their resonant frequency experience far larger forces and can be observably displaced by low-amplitude ultrasound waves. A microbubble driven near its resonance frequency experiences a large net radiation force in the direction of ultrasound wave propagation. Ultrasound pulses of many cycles can deflect resonant microbubbles over distances on the order of millimeters.
In addition to primary radiation force, which acts in the direction of acoustic wave propagation, a secondary radiation force for which each individual bubble is a source and receptor causes the microspheres to attract or repel each other. The result of this secondary force is that a much larger concentration of microbubbles collects along a vessel wall than might otherwise occur.

See also Acoustically Active Lipospheres.

Regulations governing the output of diagnostic ultrasound have been largely set by the USA's Food and Drug Administration (FDA), although the International Electrotechnical Commission (IEC) is currently in the process of setting internationally agreed standards.
The relevant national societies for ultrasound users (e.g. American Institute of Ultrasound in Medicine (AIUM), British Medical Ultrasound Society (BMUS)) usually have safety committees who offer advice on the safe use of ultrasound. In 1992, the AIUM, in conjunction with the National Electrical Manufacturers Association (NEMA) developed the Output Display Standard (ODS), including the thermal index and mechanical index which have been incorporated in the FDA's new regulations.
Within Europe, the Federation of Societies of Ultrasound in Medicine and Biology (EFSUMB) also addresses safety and has produced safety guidelines (through the European Committee for Ultrasound Radiation Safety). The World Federation (WFUMB) held safety symposia in 1991 (on thermal issues) and 1996 (thermal and non-thermal issues), at which recommendations were proffered.
The FDA ultrasound safety regulations from 1993 combine an overall limit of spatial peak time averaged intensity (I-SPTA) of 720 mW/cm2 for all equipment. A system of output displays allows users to employ effective and judicious levels of ultrasound appropriate to the examination. The output display is based on two indices, the mechanical index (MI) and the thermal index (TI).

See also ALARA Principle, and Radiological Society of North America.