Ultrasonics Kundt’s tube method. A long glass tube

Ultrasonics and Its Use in Medicine

 

Abstract

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Introduction

 

Ultrasonics is defined as sound waves with a frequency above
20kHz up to around 1GHz, above which is the hypersonic regime. (A) Ultrasonic
waves are above the audible range of hearing and their high frequencies and relatively
short wavelengths give them a number of properties that are useful both in
nature and everyday life. Ultrasonics has profoundly impacted technology giving
us many applications, as shown by figure 1:

 

 

 

 

 

 

 

 

 

 

 

Acoustics

 

Ultrasonic waves are mechanical and so like all sound waves
require a medium for propagation. As they are longitudinal the displacements of
the waves are parallel to the direction of travel. For a sinusoidal wave
propagating in the positive  direction the wave function is
given by equation 1: (L)

Equation 1

 

 

 

Where A is the displacement amplitude, k
is the wave number and w
is the angular frequency. The speed of a longitudinal wave travelling through a
fluid (n)
is given by equation 2:

Equation 2

 

 

 

Where B is the bulk modulus of the fluid
and r
is the density of the fluid. (L)

 

 

Ultrasonics in nature

 

Many animals are capable of ultrasonic communication,
including some mammals and birds. This sensory function is not only used for
communication but for navigation and many survival techniques such as detecting
prey. The importance of ultrasonics to the animal depends on factors such as
‘attenuation, scattering’ and ‘audible noise’. (A) However, this sensing
mechanism is often only used where normal mechanisms are less effective, for
instance when audible background noise is high. (A)

 

Detection

 

There are various methods of detection ultrasonic waves:

 

Ultrasonic waves with a wavelength of only a few millimetres
can be detected using the Kundt’s tube method. A long glass tube filled with
lycopodium powder is suspended horizontally; when ultrasonic waves pass through
the superposition of incident and reflected waves cause a stationary wave.
Heaps of the powder form at the nodes allowing the wavelength of the ultrasonic
wave to be calculated. (J) (K)

 

Ultrasonic waves can also be detected using thermal
detectors. If platinum wire detectors are placed in the region of ultrasonic
waved the wire vibrates rapidly. Stationary waves form and a cooling and
heating effect results from the pressure varying alternatively at the nodes of
the wave. The resistance changes accordingly and so the ultrasonic wave can be
detected. (J)(K)

 

Flames are also used as a method of detection. If a narrow
flame is moved along the medium in which the ultrasonic wave is travelling in
the nodes of the wave cause the flame to flicker. Determining the distance
between the nodes allows the wavelength, frequency and velocity of the
ultrasonic wave through the medium to be calculated. (J)

 

There are other ways in which ultrasonic waves can be
detected, some more appropriate than others depending on the situation. One of
the most common methods of detection in medicine uses the principles of the
piezoelectric effect, which is also used to artificially produce ultrasonic
waves.

 

Ultrasonics in
Medicine

 

Ultrasonics have provided many useful applications in
medicine that aid in both the diagnosis and treatment of various conditions. It
has allowed ‘affordable and effective imaging tools’ to be developed which,
unlike some diagnostic techniques, are safe and non-invasive. Research
continues to find other ways in which ultrasound can be used by clinicians to
further benefit the healthcare of patients. (F) The piezoelectric effect is an
integral to many of the uses of ultrasonics in medicine:

 

The Piezoelectric
Effect

 

Transducers contain piezoelectric materials which allow
ultrasonic waves to be both produced and detected. The piezoelectric effect
explains why this is possible.

 

Many simple transducers consist of a
piezoelectric ceramic connected to electrodes, often consisting of a thin metal
film, such as silver, which are then connected to electrical wires. Different
shaped ceramics are used, the most common being square and round. The two main
classifications of transducers are narrow-band and broad-band. Narrow-band
transducers are frequently used for high-intensity applications where low
frequencies of 20-100 kHz are used; whereas, broad-band transducers are
generally used for non-destructive testing and imaging with typical frequencies
being 0.5-50 MHz.  (D)

 

The piezoelectric material within a transducer has the
ability to produce an electric charge when a mechanical stress is applied; as well
as deform mechanically under the application of an electric field.  Piezoelectricity was first discovered in the
1880s when quartz crystal was found to have this property, enabling ‘a
transducer to transmit ultrasound and, reciprocally, to generate electrical
signals from received ultrasound waves.’ (B) The application of an electric
field to the piezoelectric material causes a variation in the shape of the dipoles
of the material, which causes a slight change in the materials dimensions, this
produces the ultrasonic waves. The reverse of this effect allows waves to be
detected; when ultrasonic waves reach the transducer they apply mechanical
stress to the piezoelectric material and so ‘the molecular dipole moments
re-orient themselves and thus cause a variation in surface charge density and
thus a voltage.’ (X) This effect is illustrated by figure 2:

 

 

Quartz is an example of piezoelectric single crystal. Other
examples of piezoelectric materials, more commonly used today, include lead
zirconate titanate, lead titanate and lead metaniobate (these materials are all
piezoceramics). Piezoelectric ceramics are widely used due to their high
coupling capability and low dielectric loss (V), compared to single crystals
they have a higher piezoelectric performance. (W) The properties of
piezoelectric materials vary and so different materials are used depending on
the intended application of the transducer. (c)

 

When deciding which piezoelectric material to use there are
many parameters to consider, the most important parameters being: the
electromechanical coupling constant (keff), the dielectric
permittivity (er),
and the acoustic impedance (Z). These factors all ‘determine the of ultrasonic
transducers.'(R) (R)(D) (S)

 

The electromechanical coupling constant can be defined as,
‘the square root of the ratio of energy available in electrical (mechanical)
form under ideal conditions to the total energy stored from a mechanical
(electrical) source.’ This can be calculated using the equation 3:

Equation 3

 

 

 

Where fs is the frequency of the maximum
conductance and fp is the frequency of the maximum transducer. (S) The
efficiency of emitters and sensitivity of receivers are both dependent on this
‘in such a way that a high k factor is always desirable.'(D)

 

The dielectric permittivity is the ability of the material
to store charge. (T)

 

‘The normal acoustic impedance of an absorbing material is
the complex ratio of the sound pressure at the surface of the material to the
resulting volume current crossing the surface along a normal direction.’ (E)
For imaging acoustic impedance ,Z, is an important physical property of tissue
that depends on the density of the tissue, r,
and the speed of the wave in the medium, c, as shown by equation 4:

Equation 4

 

 

 

This is particularly important factor to consider when the
wave is passing from one tissue type to another. The acoustic impedance of the
different materials affects how much is transmitted between them and how much
is reflected back. If the difference in acoustic impedance between the tissues
is large then the reflection is high.

 

When waves are incident n the boundary between two media of
acoustic impedance Z1 and Z2 the ratio of reflected
intensity Ir and incident intensity Ii is given by
equation 5(M):

Equation 5

 

 

 

 

Lead Bases
Piezoelectric Ceramics

 

Lead based piezoelectric ceramics have been widely used for
many decades due to ‘remarkable properties and relatively low cost of
processing.’ (A1) However, it has become apparent more recently that they are
‘serious environmental concerns regarding the manufacture, use and disposal,’
(X) of them. Therefore, the development of lead-free ceramics with similar
properties has become necessary. In 2007 an article was published out-lining
research in the development of lead-free piezoelectric ceramics. The results of
which are shown in figures 3 and 4:

 

 

 

 

 

 

 

 

 

 

 

From figures 3 and 4 it can be seen that the dielectric
permittivity and piezoelectric coefficients of lead-free materials are less
than that of the PZTs materials. It was also found that for lead based
materials the electromechanical coupling factor was 50% higher and the ‘high
clamped permittivity  for electrical impedance matching of small
elements in high frequency arrays,’ was nearly three times larger than
lead-free materials.(B1) From these results it is clear that current lead-free
piezoelectric ceramics are not as effective and that further investigation is
needed.

 

Ultrasound Imaging

 

One of the most widely used applications of ultrasonics in
medicine is ultrasound imaging, which can assist in diagnostics. Ultrasound
scans, sonograms, are used for many reasons including: the monitoring of a
developing fetus, the studying of abdominal and pelvic organs to diagnose a
condition and to guide surgeons during some surgical procedures. Ultrasound
images are ‘visual representations of the interaction between sound waves and
the medium of wave propagation.'(F). Transducers are used to transmit acoustic
pulses; the incident waves travel into the tissue and when they reach the
boundary between different tissue types some of the energy is reflected back and
received by the transducer which then converts the image into signals that are amplified
and processed into an image. There are a few modes of ultrasound scanning which
all have different uses:

 

·     
Amplitude-mode
display (A-mode): a single transducer is fixed and sends signals along a
one dimensional line and the echoes can be plotted as a function of depth.

 

·     
Brightness-mode
display (B-mode): a linear array of transducers is moved to scan a plane
through the body allowing a two-dimensional image to be produced.

 

·     
Time-motion
mode (T.M.-mode/C-mode): A rapid display of successive B-mode images allows
the motion of internal organs to be see. This is because the reflections
produced by the boundaries of the organ move relative to the probe. (C1D1)

 

There is a large difference in the acoustic impedance of air
and skin and so the transmission of ultrasonic waves into the bodies tissues is
low. For successful imaging ‘liquid coupling agents are required to transmit
ultrasonic waves effectively from the transducer face to the tissues.'(G)

 

As ultrasound imaging requires contact between the
transducer and the skin of the patient. As the skin has some resistance, it may
be irritated by the current generated by the electrodes. Therefore, a cover is
required on the electrodes ‘particularly if the galvanic action is intended for
the deeper tissues,’ (J1) to reduce this irritation. This makes imaging more
comfortable for the patient and makes it possible to keep the transducer in
contact with the skin for a longer period of time, allowing the best possible
images to be taken.(J1)

 

Enhanced Imaging

 

Researchers are continually trying to find ways of improving
the quality of images produced by ultrasound scans. Improvements in image
quality allow more detail to be seen and increase the accuracy of diagnostics.

 

One way in which the quality of imaging has been improves is
the development of microbubble contrast agents, liquids containing microbubbles
of gas. These agents are injected into the blood stream with the aim of
enhancing ultrasonic images. The agents are ‘intense sound wave reflectors
because of the acoustic differences between the liquid and the gas
microbubbles,’ (U) and so this development allows blood flow to be
distinguished from surrounding tissue.

 

When an ultrasonic wave propagates through a microbubble, it
causes it to oscillate producing waves with a harmonic content. The harmonic
content can be increased by increasing the amplitude of the ultrasonic wave or
by reaching frequencies close to the harmonic frequency of the microbubble. (F)

 

Despite being strong scatterers at the fundamental frequency
it is difficult to separate the energy from the microbubbles with the energy
from surrounding tissues. (F) It is predicted that the intensity of ultrasound
backscatter can be increased with the size of the microbubble. The ultrasound
backscatter intensity ()
is given by equation 6(G1):

Equation 6