Chemistry Reference
In-Depth Information
Because of this property, such waves can be used in diagnostic applications where they will be reflected by small objects.
Yet it is also for this reason that US does not penetrate very deeply; this is a serious limitation for an imaging tool. Some
general properties of sound waves will be mentioned briefly before discussing the use of US in imaging [44]. Wavelength λ
is inversely related to the frequency
f
by the sound velocity
c
: Thus the velocity equals the wavelength times the number of
oscillations per second (Eq. 1.6).
λ
=⇒=
c
/
f
c
λ
f
(1.6)
Therefore, at a given temperature in a given material/medium, sound velocity is constant. Sound velocity varies due to
the medium/material through which it is transmitting, and it is this property that is utilised in US imaging. This also
means that simple gaseous media are problematic, because the sound waves cannot propagate easily through the medium.
Therefore, ultrasound is unsuitable for imaging certain parts of the body, for example, the bowel, which is filled with air
and organs that are obscured by the bowel. In general ultrasound imaging, only amplitude information is used in the
reflected signal generated by an alternating current applied across piezoelectric crystals. These crystals are used in ultra-
sound probes to generate echoing signals to produce vibrations by compression and decompression. They also act as
receiver of the reflected ultrasound. In ultrasound imaging, millions of pulses and echoes are transmitted and received
every second. For each pulse emitted, the reflected signal is sampled multiple times. Different tissue structures reflect
different amounts of emitted energy to produce signals with different amplitudes caused by the different depths of the
structures. There are two different types of amplitudes: those from the transmitting pulses and those of the incoming
pulses or signals that are a result of reflections produced from the sound waves hitting a surface structure. The energy
of the amplitude of the reflected signals, as well as the incident, is known as the reflection coefficient, whereas the
energy of the amplitude of the incident pulse and the transmitted pulse are called the transmission coefficient. These
signals are affected by the difference in acoustic impedance of the different materials they are travelling through. The
acoustic impedance of a medium is given by the equation Z =
c
×
ρ
and is defined as the speed of sound in a material ×
the density [45].
When ultrasound signals hit a surface, not all the signals are reflected directly back to the transmitter; often many are lost
due to scattering via the nature of the reflecting surface. When ultrasound is scattered in multiple directions, the reflecting
surfaces are known as scatterers. There are two types of scatterers: irregular and regular. These are dependent on the types
of surfaces that the sound waves are subjected to (Figure 1.24). An irregular scatterer reflects only a small portion of the
incoming sound wave back to the detecting probe.A regular scatterer reflects a larger portion of the sound waves back and
is caused by reflecting surfaces that are perpendicular to the ultrasound beam. In general, the reflecting surface affects the
(a)
(b)
(c)
(d)
FIgure 1.24
(a) An ideal surface, where most of the energy is reflected back to the transducer (high amplitude echo). (b) An ideal
surface but at an angle of 45
o
, which will reflect most of the energy away from the surface (very low amplitude echo). (c) A curved surface
that is a scatterer because it spreads out energy in all directions (low amplitude signal). (d) A curved surface that is perpendicular to the
US beam; it is also a scatterer but more energy is reflected back to the beam.
