1. Volume I Companion Presentation Frank R. Miele Pegasus Lectures, Inc.
Ultrasound Physics & Instrumentation 4th Edition
Volume I
Companion Presentation
Frank R. Miele
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2. License Agreement 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 Chapter 1: Mathematics Chapter 2: Waves
Chapter 3: Attenuation
Level 1
Level 2
Chapter 4: Pulsed Wave
Chapter 5: Transducers
Chapter 6: System Operation
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4. Chapter 3: Attenuation - Level 1
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5. Chapter 3: Attenuation
Attenuation is a measure of how the medium affects the wave.
Attenuation will be divided into three subtopics:
Absorption
Reflection
Refraction
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6. Absorption Absorption is the conversion of sound energy into heat.
There are two very important points about absorption:
Absorption is the dominant form of attenuation in soft tissue
Absorption increases exponentially with increasing frequency
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7. Reflection
Reflection is the mechanism which make diagnostic ultrasound work.
We will divide reflection into two categories:
Geometric aspects and effects
Acoustic properties and mismatches of materials
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8. Reflection – Geometric Aspects
Reflection will vary depending on the surface of the reflecting surface.
Large, smooth and flat relative to the wavelength
Large but rough relative to the wavelength
Small relative to the wavelength
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9. Defining Angles and Terminology
Before discussing the types of reflection, it is important to understand the terminology used and references for angle measurements.
Fig. 1: Measuring the Incident Angle (Pg 145)
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10. Wave Direction and Wavefront
Notice that the wave direction is always perpendicular to the wavefront.
Fig. 2: (Pg 145)
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11. The Incident Angle Fig. 3: (Pg 146)
The incident angle is measured between the incident wave direction and the line normal, as shown below.
Fig. 3: (Pg 146)
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12. Normal Incidence Fig. 4: (Pg 146)
For “normal” incidence, the incident angle is 0 degrees and the wave direction is parallel to the normal line as shown in the figure below.
Fig. 4: (Pg 146)
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13. Visualizing Incident Angles
Fig. 5: (Pg 147)
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14. Visualizing Incident Angles (Animation)
(Pg 147)
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15. Angle of Reflection Fig. 6: (Pg 147)
For specular reflections, the angle of incidence equals the angle of reflection.
Fig. 6: (Pg 147)
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16. Angle of Transmission Fig. 7: (Pg 148)
The transmission angle, like the incident angle, is measured relative to the line normal as shown below.
Fig. 7: (Pg 148)
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17. Specular Reflection Specular Reflection
Specular reflections are mirror-like reflections which occur from structures which are large, smooth and flat relative to the wavelength. Note that specular reflections are highly angular dependent.
Specular Reflection
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18. Specular Reflection Animation
(Pg 148)
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19. Scattering Scattering
Scattering occurs when the surface is rough relative to the wavelength.
Scattering
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20. Scattering Animation (Pg 149 A) Incident Scattered
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21. Rayleigh Scattering
Rayleigh scattering occurs when the reflecting structures are small in comparison to the wavelength.
Note that Rayleigh Scattering is a very weak reflective mode.
Rayleigh Scattering
The diameter of red blood cells is small in comparison to the wavelength of most diagnostic ultrasound.
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22. Rayleigh Scattering Animation
(Pg 149 B)
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23. Reflection – Acoustic Aspects
The type of reflection that occurs is determined primarily by the geometric aspects as just discussed. The percentage of reflection and the percentage transmission at a boundary between two structures depends on the acoustic properties of the media at the interface.
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24. Acoustic Impedance The acoustic impedance is a property of the medium.
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25. Reflection and Transmission
Fig. 11: (Pg 152)
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26. Reflection – Acoustic Impedance
The amount of reflection from a boundary is determined by the acoustic impedance mismatch. What this equation says is: the greater the acoustic impedance mismatch between mediums, the greater the amount of reflection. How much reflection would there be if z2 = z1?
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27. Transmission
All of the energy at an interface between media must be conserved. Excluding absorption, this means that whatever energy does not reflect must transmit (continue on through the patient). Can you imagine why it is so important to not have too much reflection from any one interface?
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28. Refraction No Refraction Refraction
The portion of the beam that does not reflect or absorb at an interface between two media, transmits through. Sometimes the beam does not continue straight but instead is bent. This bending is called refraction.
No Refraction
Refraction ci ct
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29. Refraction ci = ct No Refraction ci ct Beam Perpendicular ( qi = 0º )
Whether or not refraction occurs depends on two parameters, if the angle of incidence is other than perpendicular, and if there is a change in propagation speeds at the boundary.
No Refraction ci ct
Beam Perpendicular
( qi = 0º )
No Refraction ci ct
ci = ct
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30. Refraction and Snell’s Law
θincident
θtransmitted
θreflected
cincident
ctransmitted
Medium 2
Medium 1
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31. What Causes Refraction
This edge of wave front is in a faster medium so it travels farther c1 c2
Medium 2
Medium 1
This edge of wave front is still in a slower medium so it travels a shorter distance t1 t2 t3 t5 t6 t7
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32. Refraction (Animation)
(Pg 154)
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33. Fig. 13: No Refraction if No Change in Propagation Velocity (Pg 155)
No Refraction Case
At time t4, the left edge of the
Wavefront travels at the same
Rate as the right edge of the
Wavefront since no change in
Propagation velocity.
Fig. 13: No Refraction if No Change in Propagation Velocity (Pg 155)
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34. Visualizing Refraction
Fig. 15: 60 Degree Incidence (Pg 155)
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35. No Refraction With Normal Incidence
Fig. 16: (Pg 156)
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36. Variables In Snell’s LawCi Ct
Fig. 17: (Pg 157)
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37. Determining Degree of Refraction
In this case, the transmitted angle is equal to the incident angle. When the transmitted and incident angles are equal, there is no refraction, as pictured in this figure.
Fig. 18: No Refraction (Pg 157)
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38. Determining Degrees of Refraction
In this case, the transmitted angle does not equal to the incident angle, so refraction exists. The amount of refraction is the difference between the incident and transmitted angles. In this case, there is 20 degrees of refraction (80°- 60°).
Fig. 19: 20 Degrees of Refraction (Pg 157)
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39. The Critical Angle: Total Internal Reflection
It is possible to have so much refraction that no energy crosses the boundary between two media such that all the energy is totally internally reflected.
The incident angle at which “Total Internal Reflection” occurs is called the “Critical Angle”.
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40. The Critical Angle Fig. 20b: (Pg 158) Pegasus Lectures, Inc.
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41. Total Internal Reflection
Fig. 20c: (Pg 158)
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42. Notes:
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