Download 122 uses ultrasound.p65

Physics Factsheet
www.curriculum-press.co.uk
Number 122
Uses of Ultrasound
Humans can hear sound across a wide range of frequencies, typically
from 20 to 20,000 Hertz (waves per second). Younger people can
usually hear even higher pitched sounds, possibly up to 22,000Hz.
Ultrasound is simply sound that is of too a high frequency for
humans to hear. Many animals can hear ultrasound and we can
detect it electronically.
Exam Hint: learn the definition of a transducer: a device that
can convert between forms of energy for a useful output, e.g. a
measurement.
Now we know how ultrasound is produced artificially, but how is
this signal used? Most uses of ultrasound involve sending a signal
which reflects from a distant object and this echo is detected. The
time difference between the emitted pulse and the received echo
tells us how far the sound has travelled, giving us a distance
measurement.
The only difference between sound and ultrasound is
frequency. The definition of ultrasound is based on the hearing
range of humans.
Ultrasound has a variety of medical and industrial uses and is
commonly used in the fishing industry. Several different types of
animals, including bats, whales, dolphins, porpoises and two species
of birds, use it for navigation and feeding.We will look at the
production and use of ultrasound by humans and animals.
Many ships use an echo sounder for sea-depth measurement. A
sound is emitted, reflects at the sea bed and the echo is detected.
The time difference, taken with the speed of sound in sea water,
tells us the distance to the sea bed. Why use ultrasound? A ship,
particularly the engine, is quite noisy. Echo-sounding uses a
50,000Hz frequency, far higher than ship noise which avoids any
confusion. The other benefit is that these very high frequency
waves are not diffracted much and they keep to a tight beam:
preventing energy loss and allowing a clear echo. Fishing ships
can also use ultrasound echo location to detect shoals of fish.
How do humans produce ultrasound? We must use artificial means
as we cannot produce such sounds naturally! We use a group of
materials that have piezoelectric properties. A voltage (potential
difference) can be generated across a material by squeezing or
stretching it (applying mechanical stress).
Piezoelectric
material
1) squeezing the material
produces a voltage
Piezoelectric
material
3) applying a voltage in one
direction, the material
contracts
Piezoelectric
material
2) stretching the material
produces the opposite
voltage
ultrasound transducer
reflected pulse
Piezoelectric
material
emitted pulse
seafloor
3) applying a voltage in the
opposite direction, the
material expands
Example calculation
A ship emits an ultrasound pulse and receives the echo 0.43 seconds
later. What is the depth of the sea at this point? Assume the speed
of sound in water is 1500m/s.
Distance = Speed × Time
Distance = 1500m/s × 0.43s
Distance = 645m or 0.65km
The reverse is also true; you can make the material change shape
by applying a potential difference across it. The material will expand
when a pd is applied in one direction and contract when the opposite
pd is applied. Introduce a rapidly alternating pd and the material
expands and contracts rapidly. Ideally, you need to cause vibrations
at the resonant frequency for the material. This gives large amplitude
vibrations which can pass through to any connecting material. To
produce ultrasound, shape your piezoelectric material so the
resonant frequency is well above 20,000Hz.
However, the ultrasound travels from the ship to the sea bed AND
BACK. The sea is actually 645m / 2 = 322.5m deep!
Exam Hint: Do not forget that ultrasound distance
measurement relies on ECHOES. The ultrasound pulse travels
TWICE as far the required distance measurement, so divide by
two.
A material which can produce mechanical vibrations (and therefore
sound) from an applied electrical voltage is known as a transducer:
it can convert between different forms of energy. Quartz is one
example of a piezoelectric material, often used in clocks and watches
due to the precise frequency produces when a voltage is applied.
More commonly for medical and industrial uses is lead zirconium
nitrate (PZT).
1
Physics Factsheet
122. Uses of Ultrasound
Defect detection
Medical uses
1. Physiotherapy
Ultrasound is also commonly used for detecting flaws inside
materials without having to cut them apart: non-destructive
evaluation. Once a metal bar has been cast, there could be an
internal flaw, a gap where the material is weaker. Depending on the
purpose, this could cause serious problems. An ultrasound
transducer is placed against the material with a jelly-like material to
allow the sound waves to pass easily into the bar. Only one signal
should be detected: the reflection from the end of the bar. Another
signal could indicate a flaw, reflecting some sound waves within
the bar.
High intensity pulses of ultrasound can be used for physiotherapy.
The heat generated in tissue can help alleviate muscle aches. For
medical imaging, much lower intensity ultrasound radiation is used.
There are several advantages for using ultrasound in medical
diagnoses. During pregnancy, the foetus is very sensitive to
ionising radiation like x-rays and the risk of harm to the unborn
baby is high. This is basically because the foetus is undergoing a
rapid cell division and growth which could be disturbed by x-rays,
causing foetal abnormalities. Another major advantage of ultrasound
is that ANY density change causes some reflection. So quite subtle
differences between tissue types can be detected via ultrasound
whereas an x-ray would be insensitive to the differences. This
means that ultrasound has a wide range of uses for pregnancy
alone: finding the size of the foetus and therefore time of pregnancy,
detecting a foetal heartbeat, the sex of the baby, physical
abnormalities and more.
1) Pulses of high frequency sound waves are produced by the
piezoelectric material and pass into metal bar
flaw
ultrasound emitter and detector
piezoelectric material
Newly produced metal bar
2. Foetal scan
When ultrasound is used to examine a
foetus, the ultrasound transducer is usually
placed against the woman’s abdomen. The
method is essentially similar to the distance
ranging methods discussed so far but in
practice is far more complex. The transducer
emits ultrasound pulses. These pass
through to the abdomen of the pregnant
woman. A jelly-like liquid is used to ensure
Ultrasound techniques
most of the wave passes into the patient.
have improved: 3D
Without this, most of the wave would reflect
imaging
from the skin due to the large density
difference between tissue and air.
2) Some reflection occurs when the ultrasound passes through
the flaw
flaw
ultrasound emitter and detector
piezoelectric material
Newly produced metal bar
3) An echo is detected by the ultrasound transducer. The
remaining ultrasound pulse has now reflected from the end of
the metal bar
flaw
ultrasound emitter and detector
Reflections occur at each tissue density change throughout the
body and foetus so getting a good angle is essential. The reflected
pulses are received by the piezoelectric ultrasound transducer. This
signal is amplified and passed to a computer where significant
processing is required to present an image on the screen for
diagnosis
piezoelectric material
Newly produced metal bar
4) The ultrasound transducer detects two signals, one from the
flaw and one from the end of the material
echo from flaw
3. Blood flow
echo from end of
material
Doppler ultrasound is also an essential medical tool. The Doppler
effect is noticeable when a fire engine drives past. As the vehicle
drives towards you, the siren sounds high pitched. As it passes
you and moves away, it sounds lower pitched. In the same way,
ultrasound is affected by the fluid flow in, for example, blood vessels.
If the blood is moving away from the transducer, the reflected signal
is lower frequency than it should have been. If the blood is moving
towards the transducer, the reflected signal is higher frequency
than it would otherwise have been. The shift in frequency tells us
how fast the blood is flowing and is useful for any cardiovascular
diagnosis. No Doppler shift indicates little or no blood flow, which
would require immediate treatment.
Example calculation:
Non-destructive testing is carried out on a steel bar. The speed of
sound in steel is approximately 5100m/s. Two ultrasound echoes
are detected, after 0.0021 and 0.0034 seconds. How long is the bar
and where is the flaw?
It must take longer for the ultrasound to reach the end of the bar, so
0.0034s is the time taken to travel the length of the whole bar AND
BACK.
Distance = speed × time
Distance = 5100m/s × 0.0034s
Distance = 17.34m
9,999.3 kHZ
The bar must be 17.34m / 2 = 8.67m long
Received ultrasound
signal: blood flowing
away from transducer
The time for the ultrasound to reach the flaw and travel BACK is
0.0021s.
Distance = speed × time = 5100m/s × 0.0021s = 10.71m
The distance to the flaw must be 10.71m / 2 = 5.36m from the sensor.
2
10,000 kHZ
Transmitted
Ultrasound
signal
10,006.7 kHZ
received
ultrasound signal:
blood flowing towards
transducer
Physics Factsheet
122. Uses of Ultrasound
4. Other medical uses
Several organisations encourage swimming with dolphins for a range
of illnesses and there is some evidence of depression being alleviated
by swimming with dolphins. It is not yet known whether this is
because of some special ability of dolphins or the well recognised
benefit of being around any animal generally. It is also believed
that dolphin and porpoises may use large amplitude pulses of
ultrasound to physically stun prey, although this is still inconclusive.
Medical uses of ultrasound also include echo-cardiography, where
the ultrasound is used to study the beating heart, essential for
determining the correct function of each chamber, blood vessel or
artery.
Exam Hint: Be prepared to discuss the advantages and
disadvantages of different medical techniques, e.g. ultrasound
and x-rays for diagnosing foetal abnormalities or heart disease.
Practice Questions
1. Give your definition of ultrasound.
Natural ultrasound
2. Define a transducer.
A variety of animals use ultrasound for finding their way and catching
prey. Ultrasound is used as an alternative to sight in low light
conditions: either dark or murky. Some species of bat emit ultrasound
in the larynx, just as humans ourselves produce audible sounds.
These sounds are emitted, usually through the mouth. Different
species of bat produce different frequency sounds, ranging from
14,000 to 100,000 Hz. Therefore, a small fraction is audible to humans
as a high-pitched click. Bats usually emit about 10-20 clicks per
second, using ultrasound pulses to identify objects as well as their
position and distance. Bats have TWO detectors: their ears. There
is a time difference between for the reflected pulse to arrive at each
ear which helps identify location as well as distance.
3. Describe, in detail, how ultrasound is artificially produced using
piezoelectric materials.
4. State two industrial uses of ultrasound and explain the relevant
advantages.
5. Describe two medical uses of ultrasound.
6. What is the advantage of bats having two ultrasound receivers
(ears)?
7 Sketch a diagram and explain how dolphins produce and receive
ultrasound for echolocation.
8. A fishing ship receives ultrasound echoes 0.24s and 0.38s after
transmission. How deep is the shoal of fish and the sea-bed?
Emitted signal
Received signal
Reflected signal
reaches one ear before the
other, indicating direction
9. An ultrasound transducer receives an echo after 8 × 10-5s and
1.6 × 10-4s during a pregnancy scan. Approximating the speed
of sound in all tissue as 2500m/s and assuming that the signal is
reflecting only from the head of the foetus, how large is its
head?
10. Calculate the wavelength of the following ultrasound signals in
air: 22,000Hz, 50,000Hz, 100kHz, 10MHz. The speed of sound in
air is around 340m/s.
Whales, dolphins and porpoises have probably the most
sophisticated form of ultrasound echolocation. These are all
mammals and have evolved from land based mammals around 50
million years ago. The shape of dolphins and porpoises have
evolved to make these creatures very effective at emitting and
receiving ultrasound. Dolphins produce all sound in a region just
behind the bulge at the front of their head, the equivalent of our
larynx and passes into the melon. The melon is the fatty bulge of
the forehead of the dolphin and allows dolphins to focus and direct
their clicks of ultrasound. They receive the echo vibrations from
objects through their long jaw bones and these vibrations are passed
backwards to the inner ear of the dolphin.
11. Calculate the wavelength of a 20MHz ultrasound signal in air
and water. The speed of sound in water is 1500m/s.
Numerical answers
8. 180m and 285m
9. 10cm
10. 1.5 × 10-2m, 6.8 × 10-3m, 3.4 × 10-3m, 3.4 × 10-5m
11. 1.7 × 10-5m, 7.5 × 10-5m
One area of interest is the ability of
dolphins to detect and treat human illness
or disabilities. There are some anecdotes
about dolphins apparently locating
tumours in swimmers, but whilst their
highly efficient ultrasound could possibly
function in this way, there is little evidence
to support it.
Acknowledgements:
This Physics Factsheet was researched and written by Jeremy Carter
The Curriculum Press,Bank House, 105 King Street,Wellington, Shropshire, TF1 1NU
Physics Factsheets may be copied free of charge by teaching staff or students, provided that
their school is a registered subscriber.
No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted,
in any other form or by any other means, without the prior permission of the publisher.
ISSN 1351-5136
3