(TI) The definition of the thermal index is the ratio of the total acoustic power to that required raising a maximum temperature increase of 1 °C under defined assumptions. A thermal index of 1 indicates the acoustic power achieving a temperature increase of 1 °C. A thermal index of 2 has the doubled power but would not necessarily indicate a peak temperature rise of 2 °C. The temperature rise is dependent on tissue type and is particularly dependent on the presence of bone.
Classifications of thermal indices:
For fetal ultrasound, the highest temperature increase would be expected occurring at bone. Therefore, TIB gives the worst-case conditions. If the ultrasound system can exceed an index of 1, the mechanical index and thermal index must be displayed. The displayed indices are based on the manufacturer's data.

See also Cranial Bone Thermal Index, Bone Thermal Index, Soft Tissue Thermal Index.

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.

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.

The wavelength is a unit of relative distance equal to the length of a wave. This could be a light wave, a radio wave, or even a sound wave. For sound waves the formula is:
l=c/f (wavelength = propagation speed/frequency)
In ultrasound imaging is the wavelength the distance between the onset of peak compression or cycle to the next. The wave propagates as bands of compression and rarefaction. One wavelength is the distance between two bands of compression, or rarefaction. Maximum compression corresponds to maximum pressure. The wavelength (see also Angstrom) is important in image resolution.

See also Spectral Reflector.