Ultrasound in various media and tissues

Ultrasound is a cyclic sound pressure wave with a frequency greater than 20kHz(the upper limit of the human hearing range). Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz. Ultrasound is used in many different fields. Ultrasonic devices are used to detect objects and measure distances. Ultrasonic imaging (sonography) is used in human and veterinary medicine. In non-destructive testing of products and structures, ultrasound is used to detect invisible flaws. Industrially, ultrasound is used for cleaning and for mixing, and to accelerate chemical processes. Organisms such as bats and porpoises use ultrasound for locating prey and obstacles.

In order to use ultrasound for diagnostic or therapeutic purposes, a beam of ultrasound must be directed into the tissues of the subject over the site of interest. The ultrasonic energy then interacts with the tissues along its path.

Acoustic Impedance is the measure of the resistance of the particles of a medium to mechanical vibrations. This resistance increases in proportion to the density of the medium and the velocity of ultrasound in the medium and is defined by the equation: Z = density x velocity

Acoustic Boundaries are positions within tissue where the acoustic impedance change. These boundaries are very important in ultrasound imaging.

Relflection of ultrasound occurs when a beam of ultrasound strikes an acoustic boundary. Part of the energy beam is transmitted across the boundary while some is reflected. There are two types of Reflection; Specular and non-specular