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.
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