Understanding…



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

stacy.s.klein@vanderbilt.edu

615-322-6085

ASEE K12 Workshop

Chicago, IL

June 17, 2006

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.

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?



=================

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?

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 |1 | |4.29 |

|(Radio Shack | | | |

|Part No. 273-060) | | | |

|9 Volt Battery |2 | |3.29 |

|9 Volt Snap Battery Connector |2 | |1.99 |

|(Radio Shack | | | |

|Part No. 270-325) | | | |

|Op-Amp LM741 |1 | |0.99 |

|(Radio Shack | | | |

|Part No. 276-007) | | | |

|220 Ohm resistor |1 | |0.99 |

|(Radio Shack | | | |

|Part No. 271-1111[pic][pic] ) | | | |

|Colors: red, red, brown | | | |

|1000 Ohm resistor |2 | |0.99 |

|(Radio Shack | | | |

|Part No. 271-1118) | | | |

|Colors: brown, black, red | | | |

|Wire |1 inch | |1.00 |

|(Radio Shack | | | |

|Part No. 278-1215) | | | |

|LED |1 | |0.65 |

|(Radio Shack | | | |

|Part No. 276-041) | | | |

|Breadboard |1 | |8.39 |

|(Radio Shack | | | |

|Part No. 276-175) | | | |

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 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.

When you're done, your entire circuit should resemble Figure 2.

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?

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®

Initial Thoughts:

• What effects will the different tissues have on the propagating wave?

• How could this be useful in ultrasound imaging?

Materials:

|Item |Quantity |

|Slinky |2 |

|Thin wire |1 foot |

|Butcher paper |3 ft |

|Lamp with Bulb |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.

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.

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?

Doppler Effect: The Doppler Ball

UIntroductionU:

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.

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 wave-arrival 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

Moving source

[pic]

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.

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.

UInitial Thoughts: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?

UMaterials:

|Item |Quantity |Cost |

|Piezoelectric Buzzer |1 |$4.30 |

|(Radio Shack, 273-060 or similar) | | |

|9-V Battery Connector |1 |$0.40 |

|(Radio Shack, 270-325 or similar) | | |

|Tennis Ball |1 |$0.65 |

|9-V Battery |1 |$1.25 |

|String |~ 8 feet |$1.90 |

|Rubber Bands |2 |$0.05 |

|Electrical tape |~ 6 inches |$1.20 |

|Knife |1 |$3.00 |

|Toggle Switch |1 |$2.00 |

|Wire Stripper (optional) |1 | |

| |Total Estimated Cost |$14.75 |

UAssembly InstructionsU 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 6-8 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 battery

with electrical tape. 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!

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 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 toggle

connector onto the battery. The buzzer should turn on switch

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.

UOperating InstructionsU

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.

UQuestions/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?

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 (vBBparallelBB). Ultrasound machines can measure this Doppler-shifted frequency and use this information to calculate the direction and speed of blood flow.

vBBparallelBB vBBperpendicular

θ

vBBblood

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.

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.

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.

UMaterialsU

|Item |Quantity |Cost |

|Styrofoam or wax cup |1 | |

|Aluminum foil pan |1 | |

|Small nail or sharp object |1 | |

|Piece of construction paper |1 | |

|Markers |1 pack | |

| |Total Estimated Cost | |

UMethodU

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.

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 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?

-----------------------

Figure 2. The final circuit, including transducer and battery connections should look like this.

[pic]

Figure 1. Set up your circuit on the breadboard like this.

[pic]

Figure 5. Back light setup.

Figure 4. Wire is used to secure the two Slinkys together.

Figure 3[pic]$%=ÈË I J K g h u v w x ‰ Š ž £ ¤ » Ë æ ñ ò øðèÝðÒÊÒð»°ž»‘»°‰?ðynyfnð[?[S

h |e_CJ$aJ$hØ&. Overlap the Slinky coils.

Figure 2 Reflection and Transmittance: The wave is partially reflected due to the smaller difference in the media.

Figure 1. Pure Reflection: The wave is completely reflected due to the extreme difference in the media.

Transducer #2

Wave direction

(Increased frequency)

Stationary Observer

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