1. Volume I Companion Presentation Frank R. Miele Pegasus Lectures, Inc.
Ultrasound Physics & Instrumentation 4th Edition
Volume I
Companion Presentation
Frank R. Miele
Pegasus Lectures, Inc.
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2. License Agreement Pegasus Lectures, Inc. All Copyright Laws Apply.
This presentation is the sole property of
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No part of this presentation may be copied or used for any purpose other than as part of the partnership program as described in the license agreement. Materials within this presentation may not be used in any part or form outside of the partnership program. Failure to follow the license agreement is a violation of Federal Copyright Law.
All Copyright Laws Apply.
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3. Volume I Outline Pegasus Lectures, Inc. Chapter 1: Mathematics
Chapter 2: Waves
Chapter 3: Attenuation
Chapter 4: Pulsed Wave
Chapter 5: Transducers
Level 1
Level 2
Chapter 6: System Operation
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4. Chapter 5: Transducers - Level 1
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5. Chapter 5: Transducers Pegasus Lectures, Inc.
A transducer is any device which converts one form of energy to another form of energy.
nerves
lights
speakers
heaters
etc.
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6. Piezoelectric Effect Pegasus Lectures, Inc.
Ultrasound transducers use the piezoelectric effect to convert electrical energy into mechanical energy and mechanical energy back into electrical energy.
PZT
Electrical to acoustic transformation
PZT
Acoustic to electrical transformation
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7. Piezoelectric Effect Pegasus Lectures, Inc.
Fig. 2a Expansion Fig. 2b Contraction Fig. 2c At Rest
(Pg 235)
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8. Block Analogy of Crystal Oscillation
This analogy is useful to illustrate the concept of the piezoelectric effect. A B C D E
A: At Rest
B: Stretched
C: Recoiled
D: Oscillate
E: At Rest
Fig. 3: (Pg 236)
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9. Crystal Thickness (t) and PW Operating Frequency
A thicker crystal “vibrates” at a lower frequency when driven in a pulsed mode. There is therefore an inverse relationship between crystal thickness and operating frequency in a pulsed mode operation.
Thickness (t)
Fig. 4: (Pg 238)
(Lower Frequency)
Longer Period
(Higher Frequency)
Shorter Period
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10. PW Operating Frequency (Animation)
(Pg 238)
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11. PW Operating Frequency Equation
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12. CW Operating Frequency
In a CW mode of operation, the frequency at which the crystal vibrates is related to the frequency of the electrical drive signal (as visualized in the animation of the next slide).
5 MHz Voltage MHz Acoustic
10 MHz Voltage MHz Acoustic
Fig. 5: (Pg 239)
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13. CW Frequency (Animation)
(Pg 239)
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14. CW Operating Frequency Equation
5 MHz Voltage
5 MHz Acoustic
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15. Pulse Response Pegasus Lectures, Inc.
Like a bell when rung, a single pulse produces multiple cycles of ringing (the “impulse response”). The following two figures represent the impulse response for a 2 MHz and a 4 MHz transducer design.
2 MHz
4 MHz
Fig. 6: (Pg 240)
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16. Single Crystal Dimensions
The diameter of a crystal affects the beamwidth and hence, the focus. The thickness of the crystal affects the operating frequency. These two parameters should not be confused.
Diameter (D)
PZT Crystal
thickness (t)
Fig. 7: (Pg 241)
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17. CW Beamshape Pegasus Lectures, Inc.
Since CW is continuously transmitting, the wave exists at all locations simultaneously producing a beam similar to that of a flashlight. (As visualized in the animation of the next slide)
Lateral
Elevation
Depth
PZT Crystal
Fig. 8: (Pg 241)
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18. CW Beamshape (Animation)
(Pg 241)
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19. PW Beamshape Pegasus Lectures, Inc. Fig. 9: (Pg 242)
Unlike CW mode, in PW, the transmit is turned on and off. The beamshape is therefore a “description” of the shape of the path the sound wave travels over time (as visualized in the animation of the next slide).
Lateral
Elevation
Depth
PZT CRYSTAL T1
Lateral
Elevation
Depth
PZT CRYSTAL T1
Lateral
Elevation
Depth
PZT CRYSTAL T1
Lateral
Elevation
Depth
PZT CRYSTAL T1
Lateral
Elevation
Depth
PZT CRYSTAL T1
Depth
PZT CRYSTAL
Lateral
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20. PW Beamshape (Animation)
(Pg 242)
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21. Basic Beam Characteristics
Although greatly simplified, the basic beamshape is helpful in roughly describing the beam parameters. Notice that the beam is approximately half as wide as the crystal diameter at the focus and the same width as the crystal diameter at the twice the focal depth.
Fresnel Zone
Fraunhoefer Zone
Natural Focus
PZT Crystal D1
D/2 D
NZL = D2 • f0 6
2 • Near Zone Length
Fig. 10: (Pg 243)
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22. Crystal Diameter and Focus
A larger crystal diameter results in a deeper focus for the same operating frequency.
Fig. 11: (Pg 244)
PZT Crystal
Deeper Focus
Shallower Focus D1 D2
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23. Aperture Effects on Beam Diameter
Notice that increasing the crystal aperture by a factor of 2 increases the focal depth by a factor of 4. D2 D1
NZL1
NZL2
D1/2
D2/2
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24. Quarter Wavelength Matching Layer
The ideal matching layer thickness is one fourth the wavelength (quarter wavelength). With quarter wavelength thickness, the energy that reflects back from the front surface is 180 degrees out of phase with the reflection from the front surface, resulting in destructive interference. This is beneficial since reflections from the matching layer would otherwise obscure the actual desired image from the patient.
Fig. 12: (Pg 246)
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25. Axial Resolution Pegasus Lectures, Inc.
The roundtrip effect helps separate by a factor of 2 the echoes returning in the time. Therefore, the resolution in the depth direction is better (less) than the spatial pulse length by a factor of 2.
Fig. 13: (Pg 247)
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26. Axial Resolution Further Defined
*Higher frequencies have shorter wavelengths improving axial resolution.
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27. Lateral Resolution and Beamwidth
The lateral resolution equals the lateral beamwidth dimension. If the beam is wider than the distance between two structures, the echo from both structures will overlap, making it impossible to distinguish between the two structures laterally.
Fig. 14: (Pg 248)
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28. Lateral Resolution Pegasus Lectures, Inc.
*Higher frequencies form narrower beams improving lateral resolution.
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29. Techniques for Changing Focus
There are four techniques which can be used to change the focus from the natural focus of a crystal design
Lenses
Curved elements
Electronic focusing
Mirrors
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30. Use of Lenses for Focusing
PZT Crystal
Fig. 15: (Pg 249)
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31. Curved Surface for Focusing
Concave Surface
PZT Crystal
Fig. 16: (Pg 250)
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32. Techniques for Changing Focus
Mirrors were rarely used and not used currently so will not be further discussed. Electronic focusing is discussed in level 2.
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33. Simple Transducer Block Diagram
The simple block diagram is useful since it illustrates the principal transducer components. You should be able to describe each components purpose and function.
Lens
Matching Layer
Piezoelectric Crystal
Backing Material
Wires _ +
Fig. 17: (Pg 250)
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34. Notes:
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