Waves Experiment 3
Understanding Ultrasonic
Imaging
Prelab questions can be found on page W-18
Safety
Make sure that you have read the safety notes in the introductory section of this manual before
beginning any practical work.
Do not, under any circumstances, attempt to repair any of the equipment should you think it to be
faulty. Rather, turn off the apparatus at the power-point and consult your demonstrator.
Outline
In this exercise you will investigate aspects of ultrasound imaging, beginning by using the
relationship between frequency, wavelength and velocity to determine the velocity of ultrasound
pulses in air.
You will explore image construction using pulse echo timing. In addition, you will measure the
change of frequency of an ultrasound when it is reflected from a moving object. This technique is
used in the measurement of blood flow and foetal heart motion.
References
121/2:
Section 17.3.
141/2:
Section 18-8.
Introduction
Ultrasound imaging is an important medical diagnostic tool, not only in obstetrics where the low
risk allows prenatal imaging, but also more generally for soft tissue areas such as the kidneys,
where traditional X-ray imaging typically doesn’t have sufficient contrast for many applications.
Ultrasound imaging doesn’t use radiation in the normal sense of electromagnetic or nuclear
radiation, such as X-rays. Instead, it uses tiny vibrations, similar to sound but at frequencies
much higher than we (or even dogs) can hear. Ultrasound imaging is much safer than other
alternatives such as X-ray imaging, because the sound waves are non-ionising.
Physics 121/2 and 141/2 Laboratory Manual
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Background
Ultrasonic imaging relies on some of the properties of waves you have met in earlier lab exercises,
in particular the reflection of waves from a change in the medium. You will use the relationship
between speed, wavelength and frequency of waves to help you understand the behaviour of
ultrasonic waves..
Ultrasonic waves
For our experiments today the oscillations in the medium will be produced using an ultrasonic
transducer. This is rather like a loudspeaker: it has a mechanical element that oscillates back
and forth when supplied with an oscillating electrical signal.
Ultrasonic transducer
Ultrasonic wave output
Electronic oscillator
The frequency of the waves is what distinguishes ultrasonic waves from sound. While we might
drive the transducer at any frequency, they work best close to their resonant frequency, in this
case 215 kHz. This is more than 10 times the frequency that the ears of a typical first-year
student can detect.
Prelab questions
Question (a):
What is the period of the waves produced by the 215 kHz transducers?
Question (b):
Later you will measure the wavelength and velocity of ultrasound waves. For
the moment, assume that someone has measured the wavelength for the
215 kHz waves and found it to be 1.6 mm. What is the speed of sound in that
case?
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Physics 121/2 and 141/2 Laboratory Manual
Echo timing
Ultrasound imaging is not done with continuous sound waves. Instead bursts or pulses are used.
Such bursts travel at the same wave speed, and so by measuring the time taken for a burst to
travel to an object and back, we know the distance travelled.
Sound waves are reflected not only from a hard surface, but also from a change in medium, e.g.
from amniotic fluid to foetus. The relative strengths of the transmitted and reflected components
depend on the relative densities, and the velocities of sound, in the two different media. Thus the
strength of the reflected pulse depends on the tissue type.
An image is constructed from the measured strength of the reflected pulse, as a function of the
time taken:
Figure 1:
This figure shows ultrasound pulses emitted from a
transponder (not shown), outside the womb, at the top of the image. Some
fraction of the pulses is reflected at various tissue boundaries, for example the
front and back of the foetal skull.
Figure 1:
This figure shows ultrasound pulses emitted from a transponder (not
shown), outside the womb, at the top of the image. Some fraction of
the pulses is reflected at various tissue boundaries, for example the
front and back of the foetal skull.
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Section A:
Measuring the wavelength and calculating the velocity of ultrasound
Ultrasound imaging generally uses pulses of ultrasonic waves, typically at frequencies of between
2MHz and 7MHz, and sometimes even higher in order to improve resolution. Unfortunately, this
type of ultrasound equipment requires very expensive electronics, and so we will use a slightly
lower frequency.
In this experiment, you will generate an ultrasonic wave in air and measure the frequency and
wavelength, and then calculate the velocity.
The electronics is contained within a box, the ULTRASONIC GENERATOR. This generates an
oscillating voltage to drive the transmitting ultrasonic transducer (TX), either continuously or in
bursts.
It also amplifies the signal detected by a second ultrasonic transducer, the receiver (RX), with
amplified signal available at the MODES 1 & 2 output. The MODE 3 output is a beat signal
between the oscillator output and the detected input (see section 4). The electronics box looks like
the front panel shown below.
Meaning of controls:

f:
MODE 1:
MODE 2:
MODE 3:
frequency tuner (not Doppler shift!)
continuous
burst
Doppler
Lead connections
TX from Generator  Channel 1 of Oscilloscope
MODE 1 & 2 OUTPUT from Generator  Channel 2 of Oscilloscope
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Physics 121/2 and 141/2 Laboratory Manual
Settings for electronics:
Ultrasonic generator




Input Gain: max for undistorted output
Output Gain: max amplitude
Frequency: 215kHz
Continuous
Oscilloscope





5V/div on channel 1
2.5 s/div on timebase (sec/div)
(or hit AUTOSET)
5V/div on channel 2
Trigger on channel 1
1.
Adjust the transmitter and detector such that they are facing each other on the track,
separated by about 30cm. You should see a strong received signal on channel 2 of the
oscilloscope. Adjust the f knob on the ultrasonic generator to maximise the signal on
Channel 2 and at the same time keep a non-distorted trace on Channel 1. Do not adjust the
f knob after this initial adjustment for the rest of the experiments. You may need to adjust
the output gain to keep the wave on Channel 2 undistorted.
2.
Measure the frequency of the ultrasound. You can do this knowing the timebase setting on
the oscilloscope (the time per interval along the horizontal axis), or by hitting the
MEASURE button and following the menus.
3.
Slide the detector back and forth, and note how the detector wave (channel 2) shifts relative
to the source wave (channel 1). By moving the detector very slowly, you should be able to
find a position such that the peaks and troughs line up perfectly. It may help to use the
vertical positioning knob of Channel 2 to raise the trace till the two waveforms overlap.
4.
Then move the detector slowly such that the waves anti-align, and then align again. You
have then moved the detector exactly one wavelength! Repeat for, say, 10 or 20
wavelengths and measure the total distance you moved the detector, and calculate the
wavelength and its uncertainty.
5.
Knowing the frequency and wavelength, calculate the velocity of sound in air and its
uncertainty.
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Section B
Measuring the velocity using pulse timing
In this experiment you will produce pulses of ultrasound, and measure the time required for these
pulses to propagate known distances.
Lead Connections
As for experiment A
Settings for electronics:
Ultrasonic generator



Output Gain: max amplitude
Frequency: 215kHz
Burst
Oscilloscope





5V/div on channel 1
s /div on timebase (Sec/Div)
(or hit AUTOSET)
1V/div on channel 2
Trigger on channel 1
1.
Adjust the transmitter and detector such that they are facing each other on the track,
separated by about 30cm. You should see the source pulse on channel 1 and a clear
detector pulse on channel 2 of the oscilloscope. Move the detector closer and further from
the source and note the effect.
2.
You can use the CURSOR measuring facilities of the oscilloscope to measure the time
between source and detector pulses.
Press the CURSOR button.
Set TYPE to Time
Move the CURSOR 1 button to place the first cursor on the leading edge of the channel 1
waveform. Similarly adjust the position of CURSOR 2 to the peak amplitude position of the
pulse on Channel 2, this time by moving the CURSOR 2 button. The time between the
cursors may be read in the DELTA box under the cursor menu.
Measure the time between source and detector pulses for various measured separations and
plot your results.
3.
What is the velocity of sound and how does it compare with the result from your
measurement of f and ? What is its uncertainty?
4.
Velocity varies as the square root of the absolute temperature. Compare your measured
value today to the 0˚C value of 331.46 ms-1.
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Physics 121/2 and 141/2 Laboratory Manual
Section C
Mapping a two-dimensional image
using pulse echo time from oscilloscope
Ultrasound imaging relies on the detection of sound pulses reflected from objects. Using the same
equipment as above, try to detect sound pulses reflected from a hard surface. Now insert a piece
of tissue in front of the hard reflector. You should be able to see two reflected pulses, one from the
tissue directly, and one from the hard reflector.
Lead Connections
As for experiment A
Settings for electronics:
Ultrasonic generator

As for previous experiment
Oscilloscope




5V/div on channel 1
250 s/div on timebase (Sec/Div)
(or hit AUTOSET)
100mV/div on channel 2 (may need to be
adjusted)
Trigger on channel 1

Assemble the source and detector one above the other into the combined mount provided, on the
optical rail. Using the horizontal position knob on the oscilloscope, move the Channel 1 trace so
that the pulse lies at the extreme left of the screen. Position the optical bench so that the
transducers are 1 to 2 cm directly in front of the unknown object as shown below. Slide the
combination past the unknown object and note how the echo time changes. Plot the pulse echo
time as a function of source/detector position along the straight line. You may get a bit of direct
signal pickup between the transmitter and receiver. Ignore pulses that occur before 250 s.
1 to 2 cm
transmitter/
receiver
x
This figure shows an unknown object inside a box or a collection of known objects.
The transmitter/receiver pair should be moved on the track provided, along
direction x, and pulse reflection times measured as a function of x. Convert to
distance and check visually.
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Section D
Measuring Doppler shift
You’ve probably noticed that the sound of a train or a car changes when it passes you. As it
approaches, the frequency is higher; as it leaves, the frequency is lower. This is known as the
Doppler effect, and can be understood in terms of the relative velocity of source and detector.
First of all, imagine you are listening to a stationary source. The waves reach you at a certain
rate or frequency:
Now imagine that the source is moving towards you:
You will “hear” more waves per second, and therefore hear a higher frequency.
This new higher frequency depends on the relative velocity, let’s call it vDop, and is given by:
v Dop

f  f  f 1 
 v sound




and the extra number of waves per second, f is:
f  number of extra waves per second 
v Dop
v Dop
distance per second


f.
wavelength

v sound
Knowing the wavelength , a measurement of the shift in frequency, f, can be used to measure
movement. For example, reflecting ultrasound from blood cells can be used to measure the
velocity of blood flow; reflecting ultrasound from a foetal heart is often used to measure the
heartbeat.
Question (c):
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Calculate the Doppler shift for your ultrasound waves for the kind of speed you
might use today, e.g. 1 m/s and compare to the full frequency of the ultrasound.
Physics 121/2 and 141/2 Laboratory Manual
Making the measurements
The velocity of sound in air is quite large compared to most body motions, and hence the Doppler
shift tends to be very small (see Question (c) above). We can measure these frequency shifts by
adding the fundamental ultrasound signal to the frequency-shifted signal:
I  I 0  I Dop
The combined signal will appear to oscillate at the fundamental, but with amplitude that varies
with time:
The oscillation in amplitude is known as a beat signal, and the beat frequency is just the Doppler
shift. The beat signal alone can be obtained from the filtered product of I0 and IDop, as shown as
the bottom trace of the figure above. This signal is available from the ULTRASONIC
GENERATOR as the MODE 3 OUTPUT.
Lead Connections
Mode 3 Output from Generator to Channel 1 of Oscilloscope.
Settings for electronics:
Ultrasonic generator



Max amplitude
215kHz frequency
Continuous
Oscilloscope




50mV/div on channel 1
1ms/div on timebase
(or hit AUTOSET)
Trigger on channel 1
1. The output of MODE 3 is the beat signal, which has a frequency equal to the Doppler-shift.
Put the transmitter and detector into the combination mount and arrange them to detect a
reflection from your hand, at a distance of about 30cm. You should see a strong signal on the
oscilloscope, with amplitude and frequency that vary as you move your hand back and forth.
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2. Move your hand smoothly, and ask your prac partner to hit the STOP button on the
oscilloscope when you have a nice waveform. This will store the current image on the screen.
3. Using the oscilloscope’s in-built CURSORs, measure the frequency and relate to an estimate
of your hand velocity.
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