Ultrasound In Radiology

Ultrasound Machine.

Sound is propagated through a medium (e.g. air) as a mechanical vibration of the particles of that medium and in simple terms may be categorised by its loudness and pitch or frequency. “Ultra” means beyond, ultrasound is sound with a frequency beyond that of human perception (i.e. >20 kHz), and has the same physical properties as “audio” sound. Most clinical diagnostic applications of ultrasound employ frequencies in the range 2 - 10 MHz.

  

Ultrasonic energy travels through a medium in the form of a wave. Although a number of different wave modes are possible, in almost all diagnostic applications, ultrasound propagates in the form of a longitudinal wave, where the particles of the medium oscillate in the direction of propagation of the sound. Energy is transferred through the medium in a direction parallel to that of the oscillations of the particles. The particles themselves do not move through the medium. They simply vibrate to and fro about their mean position.

It is often useful to think of the source of ultrasound, the transducer, as a vibrating piston. As it moves it displaces the adjacent particles of the adjoining medium. These in turn displace more particles throughout the medium. Since the particles are not rigidly fixed to each other, they do not all move together. There is a delay between the movement of adjacent particles (analogous to a series of balls connected by springs). 

At a particular time there will be some regions where the particles are closer together and the pressure and density of the medium is increased (regions of compression) and areas where the particles are further apart and the pressure and density of the medium is decreased (regions of rarefaction). These regions of compression or rarefaction move through the medium as a wave.