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How Does Ultrasound Imaging Work?
Stacy S. Klein, Ph.D.
AP Physics and Biomedical Physics Teacher
University School of Nashville
Nashville, TN
Research Assistant Professor of
Biomedical Engineering, Radiological Sciences, and Education
Vanderbilt University
Nashville, TN
[email protected]
615-322-6085
ASEE K12 Workshop
Chicago, IL
June 17, 2006
1
Presentation Outline
1. Overview of the High School Medical Imaging
curriculum
2. Challenge Question
3. Generate Ideas
4. Ultrasound Curriculum
a. Briefly review sound waves
b. Piezoelectrics*
c. Ultrasound tissue interactions*
d. Ultrasound modes
e. Doppler Effect**
5. Go Public
6. Summarize/Overview of Curriculum
7. How to participate in research study if desired
* = activity to do today
Illinois State Standards that will be met through this workshop and curriculum:
STATE GOAL 7: Estimate, make and use measurements of objects, quantities and
relationships and determine acceptable levels of accuracy. (specifically goal 7.A.4a,
7.A.4b, 7.A.5, 7.B.3, 7.C.5a
STATE GOAL 8: Use algebraic and analytical methods to identify and describe patterns
and relationships in data, solve problems and predict results. (specificially goals 8.B.4a,
8.B.5)
STATE GOAL 10: Collect, organize and analyze data using statistical methods; predict
results; and interpret uncertainty using concepts of probability. (specifically goal 10.A.4a,
10.A.4b, 10.A.4c, 10.B.5)
STATE GOAL 12: Understand the fundamental concepts, principles and
interconnections of the life, physical and earth/space sciences.(specifically goal 12.C.4a,
12.C.5b, 12.D.5a, 12.D.5b.
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The Challenge
Your mom, who has a history of high cholesterol, has been experiencing
lightheadedness recently. She goes to her doctor for a checkup. The doctor
recommends an ultrasound of her carotid artery. Watch the video recording.
What can you see in this image? How was this image made?
http://www.medical.philips.com/main/products/ultrasound/assets/images/image_library/hd11/vascular/hd11_
carotid_artery.jpg
=================
Generate Ideas: In class have students make journal entries to answer the
following three questions – (1) What are your initial ideas about how this question
can be accomplished? (2) What background knowledge is needed? (3) What
terminology do you know about images? (4) What do you know about waves?
3
What is a piezoelectric?
Introduction:
A piezoelectric crystal is a material that, when subjected to mechanical
pressure, creates a voltage potential. The reverse is also true. When a voltage is
applied to the crystal, it generates a mechanical deformation. This property is
key for ultrasound since the machine needs some way to make pressure (sound)
waves. The piezoelectric crystals in the transducer can convert pressure into
voltage, or voltage into pressure. Ultrasound transducers actually contain
hundreds of piezo elements that are used to send ultrasound waves into the
body and pick up their reflections. In this experiment you will be able to see how
pressure applied to the piezoelectric material can create a voltage, which will
light up an LED.
Initial Thoughts:
 What will happen when a piezoelectric crystal is squeezed?
 How could this be useful in creating ultrasound?
Materials:
Item
Qty
Picture
Price
Piezo buzzer
(Radio Shack
Part No. 273-060)
9 Volt Battery
1
4.29
2
3.29
9 Volt Snap Battery
Connector
(Radio Shack
Part No. 270-325)
2
1.99
Op-Amp LM741
(Radio Shack
Part No. 276-007)
1
0.99
220 Ohm resistor
(Radio Shack
Part No. 271-1111 )
Colors: red, red,
brown
1
0.99
4
1000 Ohm resistor
(Radio Shack
Part No. 271-1118)
Colors: brown,
black, red
Wire
(Radio Shack
Part No. 278-1215)
2
0.99
1 inch
1.00
LED
(Radio Shack
Part No. 276-041)
1
0.65
Breadboard
(Radio Shack
Part No. 276-175)
1
8.39
Method:
In order to see the voltage created by applying pressure to the piezo
buzzer, you will need to build a circuit. When pressure is applied the sensor, the
piezoelectric transducer will create a voltage, but it will be a very small voltage.
Therefore, in order to see this voltage, amplification is necessary, which is where
the op-amp and resistors are used. The amplified voltage will be sent to the
LED, which will light in response to the voltage. To create this circuit, follow the
diagrams in Figure 1, a diagram of how the circuit should look on the
Figure 1. Set up your circuit on the breadboard like this.
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breadboard.
Follow these steps to put together the circuit:
1. Place the black wire from the piezo element in box 1A.
2. Place the red wire from the piezo element in box 11A.
3. Place one side of the 220-Ohm resistor in box 1C. (It does not matter
which side this is, since resistors work both ways.)
4. Place the other side of the 220-Ohm resistor in box 6C.
5. Place the op amp (LM741 chip) on the circuit board so that its upper left
pin is in box 5E and its lower right pin is in box 8F. Make sure that the
notched side points up on the circuit board and the dot on the top is in the
upper left corner, as shown in Figure 1.
6. Take one of the 1000-Ohm resistors and place one side of it in box 6D.
7. Place the other side of this resistor in box 7G on the other side of the op
amp.
8. Place one side of the other 1000-Ohm resistor in box 7C.
9. Place the other side of this 1000-Ohm resistor in box 11C.
10. Take one of the battery clips and place the negative (black) side of it in
box 8A. (DO NOT connect the battery yet.)
11. Place the positive (red) side of this clip in box 11B.
12. Place one side of the 1-inch wire in 11E.
13. Place the other side of this wire on the opposite side of the board in box
11G.
14. Take the other battery clip and place the positive (red) side in box 6G.
(DO NOT connect the battery yet.)
15. Place the negative side of this battery clip in box 11I.
16. Take the LED and determine which one of its legs is longer than the
other. This is the positive side of the LED. Place it in box 11H.
17. Place the other end of the LED (the shorter leg) in box 7H.
18. Connect both of the batteries to each of the 9V battery connectors. They
should just snap in.
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When you're done, your entire circuit should resemble Figure 2.
Figure 2. The final circuit, including transducer and battery connections should look like this.
Now that your circuit is complete, all you have to do is send a pressure
wave through the piezo and you should see the LED light up as a result of an
applied voltage. To do this, tap the piezo transducer onto a hard surface, such
as the lab table. Watch the LED for changes and record your observations. Try
varying how hard you tap the piezo sensor and then record any differences that
you observe.
Conclusions:




What did you observe when you tapped the piezo transducer on the desk?
Why did this occur?
How does this represent what happens in ultrasound transduction?
What happened in terms of voltage when you varied the force?
How is ultrasound transduction similar or different from this observation?
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Slinky Experiment: Wave Interaction through different Tissue
Introduction:
Reflection occurs when sound waves encounter a different medium. This
new medium can be a wall or in the case of ultrasound, a different type of tissue.
A wall, for example, can produce an echo when someone’s voice travels to the
wall. bounces off. and is reflected back to be heard as an echo. Sound can also
be transmitted through the wall; another person on the other side of the wall can
hear the first person’s voice as it propagates through the air, into the wall, and
again back into the air. Just like audible sound waves, ultrasound can be
reflected and transmitted through new media. Just as light waves are transmitted
and reflected on a glass window, sound waves can be reflected and transmitted
through different media. Here, a Slinky® will represent the tissue as longitudinal
compression waves propagate into the body. Two Slinkys® will be used so that
they may be overlapped. By the coils of one Slinky® into the coils of another
Slinky®
Figure 1. Pure Reflection: The wave is completely reflected
due to the extreme difference in the media.
Figure 2 Reflection and Transmittance: The wave is partially
reflected due to the smaller difference in the media.
Transducer #2
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Initial Thoughts:
 What effects will the different tissues have on the propagating wave?
 How could this be useful in ultrasound imaging?
Materials:
Item
Slinky
Thin wire
Butcher paper
Lamp with Bulb
Quantity
2
1 foot
3 ft
1
Method:
Pure Reflection
Have two students, one at each end of the Slinky, hold the Slinky on the
ground, as in the Wave Properties activity. Create a single transverse wave by
moving one end of the Slinky once. Watch as the wave travels down the Slinky
from one student to the other, reflects at the second student, and moves back
towards the first student. Next, create a longitudinal wave. One can do this by
grabbing a section of Slinky, pulling it together at the end, and releasing it.
Observe how the longitudinal wave acts very much like the transverse wave,
reflecting off the stationary end.
Reflection and Transmittance
Assemble the Slinky apparatus by crossing the two individual Slinkys one over
the other so that there ends up being twice the thickness Slinky in the middle, as
shown in Figure 3. Next, secure them together with 1 inch pieces of the thin
wire, like in Figure 4.
Figure 4. Wire is used to secure the two
Slinkys together.
Figure 3. Overlap the Slinky coils.
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Now have two students hold up the
butcher paper vertically in front of the Slinky
apparatus. Next turn on the lamp and illuminate
the Slinky on the paper (Figure 5). Two other
students should hold the Slinky apparatus at
each side. Finally one of the people should drive
a longitudinal wave down the Slinky apparatus.
Watch and record what is seen.
Figure 5. Back light setup.
Conclusions:



How is the Slinky representative of an ultrasound wave in the body?
How is it not representative?
What do you think would happen if the Slinky section’s stiffness was
doubled? What would this represent in the body?
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Doppler Effect: The Doppler Ball
Introduction :
The Doppler effect occurs when a sound source is moving, the sound
observer is moving, or both the source and observer are moving. The Doppler
effect causes a sound to be heard at some frequency other than what it really is.
(Example: The frequency of an approaching police siren seems to be higher than
the frequency of a receding one.) In general, if the observer and sound source
are moving toward each other, the frequency (pitch) of the sound increases. If
the observer and source are moving away from each other, the frequency
decreases. This change in frequency is known as a Doppler shift.
U
U
Consider first the case of a still source and a still observer. A sound wave
given by v = λf emits a compression every T seconds (T = 1/f). If the source of
the sound now begins to move toward the observer, the source “catches up” with
the previous compressions as it moves. This catching up causes the wavearrival rate at the observer to increase. An increased arrival rate is the same as
an increased frequency and an increased pitch. Because the frequency of the
arriving sound has changed, so has the wavelength (speed stays constant).
Wavelength decreases as frequency increases.
Sound wave fronts
Stationary
Observer
Moving source
Wave direction
(Increased frequency)
If the source had been moving away from the observer, the source would
be “running away” from the compressions and decreasing the arrival rate of the
compressions. A decrease in arrival rate implies a decrease in observed
frequency/pitch and an increase in wavelength.
If an observer approaches a stationary source, the observer gets both the
stationary number of compressions he/she would normally receive plus the ones
he/she runs into. The observed frequency increases. Had the observer been
moving away from the stationary source, he/she would be outrunning the
compressions, thus decreasing the frequency.
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In this activity, students will observe the Doppler effect by hearing a
buzzer’s changing frequency as is moves in a circular path through the air. The
person spinning the ball should hear a constant tone inside the circle while
observers outside of the circle will hear varying tones (the Doppler shifted
frequencies) as the ball approaches and recedes from them.
Note: The teacher should assemble the Doppler ball prior to class. One ball can
be used as a whole-class demonstration or multiple balls can be constructed for
student use. If students are to assemble the balls, supervise use of knives or
scissors as the tennis ball is quite difficult to cut through. Check for obstacles
before whirling the ball above your head. Make sure that all students stay out of
the ball’s path. Should the device hit something, the tennis ball should help
guard against any injury or damage.
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Initial Thoughts:


U
U
What do you expect to hear if a sound source is moving toward you?
What are some ways in which you see the Doppler effect in your everyday
life?
Materials:
Item
Piezoelectric Buzzer
(Radio Shack, 273-060 or
similar)
9-V Battery Connector
(Radio Shack, 270-325 or
similar)
Tennis Ball
9-V Battery
String
Rubber Bands
Electrical tape
Knife
Toggle Switch
Wire Stripper (optional)
Quantity
1
Cost
$4.30
1
$0.40
1
1
~ 8 feet
2
~ 6 inches
1
1
1
Total
Estimated Cost
$0.65
$1.25
$1.90
$0.05
$1.20
$3.00
$2.00
$14.75
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Assembly Instructions
U
slit
1.) Use a sharp knife to cut a 3-4” slit in a tennis ball
(about halfway around the ball).
2.) On the side of the ball opposite the slit, make a
small hole and thread a string through it. (Start from
the outside and thread the string into the inside).
Pull the string through the ball until you have about
6” of string extending out of the slit. Leave about 68 ft. of string on the other side of ball.
6' string
slit
3.) To counteract the muffling effect of the tennis ball on
the buzzer, cut a small hole in the side of the ball where
the buzzer will be located.
hole
s
buzzer
4.) Use a wire stripper or similar device (i.e. scissors,
knife, wire cutters) to strip the ends of both the
piezo buzzer and the battery connector (Be
careful! The wires may be stranded. Use minimal
force so you don’t inadvertently sever some of the
thin stranded wires in the stripping process.)
Twist the ends of the buzzer and battery
connector together (black to black and red to red). When
securely twisted, wrap the exposed ends
with electrical tape.
battery
connector
5.) Use rubber bands to attach the battery to the back
of the buzzer. Tie the buzzer to the 6” string
coming out of the slit. Make sure this is a secure
knot! Otherwise your ball could fly into an
innocent bystander!
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6.) Insert the entire battery/buzzer device into tennis
ball. Make sure the battery terminals are facing the
slit so the battery connector can easily be snapped
into place.
7.) To use the Doppler ball, snap the battery connector
onto the battery to activate the buzzer.
Idea taken from http://www.seed.slb.com/en/scictr/lab/doppler/instruction.htm
HHTUT
(Optional):
A switch is a convenient way to turn the
Doppler ball on and off without having to
clip/unclip the battery each time. To install an
on/off toggle switch, twist the two red wires
(buzzer and battery snap) together as before
and insulate with electrical tape. Do not
connect the two black wires. Instead, attach
the black buzzer wire to the middle post on the
switch. Connect the black battery snap wire to the
outer post. Make secure connections by twisting
the wires through and around the post. Snap the battery
connector onto the battery. The buzzer should turn on
and off with a flip of the switch. Insert the entire device
into the tennis ball, making sure to leave the switch near
the opening for easy access.
UTTHH
toggle
switch
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Operating Instructions
U
1.) Turn the Doppler ball on and let students observe the frequency (pitch) of
the stationary ball.
2.) Gather the end of the string in your hand, leaving about 3-4’ of string
extending from the ball.
3.) Stand in an area free of obstacles and twirl the ball above your head . Ask
students to note any frequency changes while the ball is in motion.
A variation of this activity could involve omitting the string (or tucking it
inside) and simply throwing the ball back and forth to each other.
U
Questions/Applications



How does the frequency of the stationary ball compare to the frequency of
the moving ball?
When the ball is in motion, are there any points where the frequency
seems higher or lower than the stationary frequency? Where are these
locations?
Turn the buzzer on and leave the ball in one place. What happens to the
buzzer’s frequency when you run quickly toward it? Away from it? In a
circle around it?
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The Doppler Effect: Ultrasound Extension
The Doppler effect is used extensively in ultrasound imaging. Ultrasound
machines can be used to measure blood flow rates by utilizing the fact that
waves reflect off moving targets differently than stationary ones. Blood flow rates
are calculated by placing the transducer above a blood vessel. Because the
target (the blood cell) is moving, a Doppler frequency shift is created. The
reflected ultrasound pulses will be closer together (more compressed up/higher
frequency) when the blood is moving toward the transducer and farther apart
(less scrunched/lower frequency) when blood is moving away from the
transducer. This means that an individual blood cell will be responsible for two
Doppler shifts. First, the cell acts as a moving receiver, then a moving
transmitter. The amount of Doppler shifting depends on the magnitude of the
component of the blood’s velocity that is parallel to the beam path (v parallel ).
Ultrasound machines can measure this Doppler-shifted frequency and use this
information to calculate the direction and speed of blood flow.
BB
v parallel
BB
BB
BB
v perpendicular
BB
θ
v blood
BB
v parallel 
vblood
cos 
The following equation describes the change in observed frequency as a result of
a moving blood cell. f is the original frequency, f' is the frequency that is
observed at the transceiver, s is the speed of sound in the medium, and v is the
velocity of the moving object.
f
s

f ' ( s  v)
The blood flow rates are often represented on a color scale (blue for one
direction and red for the other), so that the user knows both the speed and
direction of the blood flow.
16
This activity will show that targets moving toward the wave sources
reflect things at a faster rate than targets moving away from the wave
source. You can reinforce the biomedical connection by placing an
image or drawing of red blood cells on the moving target.
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Materials
U
Item
Styrofoam or wax cup
Aluminum foil pan
Small nail or sharp object
Piece of construction paper
Markers
U
Quantity
1
1
1
1
1 pack
Total Estimated Cost
Cost
Method
1.) Starting from the inside of the cup, poke a small hole in the bottom. The
hole should be just big enough so that water drips out of the cup at a rate
of several drips per second. (This simulates the pulsed nature of waves
used in ultrasound imaging.) You may have to experiment with a few cups
to find the right size hole.
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2.) Attach the picture of the red blood cells to the pan. Make sure the cells
are clearly labeled so everyone will know what the pan represents.
3.) Fill the cup with water and put your finger over the hole. Hold the cup as
high as you can. You may want to stand on a chair or table to make the
demo more effective.
4.) Have a helper hold the aluminum pan about halfway between the cup and
the floor.
5.) Remove your finger from the bottom of the cup and allow the water to drip.
At the same time, the helper needs to raise and lower the pan*. When the
pan is being raised, the frequency of the water drops hitting the pan
increases. When the pan is lowered, the frequency decreases.
* In order to offset the effect of height-dependent loudness of the drop
hitting the pan, the distance that the helper raises and lowers the pan
should be small relative to the overall height of the cup above the floor.
For example: If the cup is 8 feet above the ground, the pan should
oscillate between 4 feet above the floor and 2 feet above the floor. If it
17
oscillates between 8 feet and 0 feet, the change in loudness will be much
more noticeable than the change in frequency.
6.) Based on the sounds of the drops hitting the pan, ask a blinded observer
to guess if the pan is stationary, moving up or moving down.
Questions
 What does the cup represent? The water drops? The pan?.
 What happens to the frequency of the drops hitting the pan as the pan is
moved toward the cup and away from the cup?
 If the speed of the moving pan increases, what happens?
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