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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3. Volume I Outline Pegasus Lectures, Inc. Chapter 1: Mathematics
Chapter 2: Waves
Chapter 3: Attenuation
Chapter 4: Pulsed Wave
Level 1
Level 2
Chapter 5: Transducers
Chapter 6: System Operation
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4. Chapter 4: Pulsed Wave - Level 1
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5. Chapter 4: Pulsed Wave Pegasus Lectures, Inc.
The motivation for using pulsed wave is to generate some ability to resolve structures in depth (range resolution). Continuous wave offers no range specificity, whereas pulsed wave offers good range resolution.
The animations of the next pages demonstrate how continuous wave results in complete range ambiguity, and how the spatial pulse length (SPL) relates to the range resolution.
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6. Chapter 4: Pulsed Wave Pegasus Lectures, Inc.
The ability to resolve structures is paramount in diagnostic ultrasound.
In chapter 2 we discussed waves, with a tacit assumption that the wave is continuous.
Continuous waves offer no “depth” or “range” resolution.
The desire for range resolution is the motivation for pulsed wave imaging and Doppler.
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7. Continuous Wave Pegasus Lectures, Inc.
Imagine these wavefronts moving in time, with a continuous stream of new wavefronts traveling from left to right.
With a continuous transmit, notice how the echoes form all three mountains connect together and become indistinguishable. (no range resolution) 1 2 3
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8. Continuous Wave Animation
Fig. 1: (Pg 194)
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9. Pulsed Wave Pegasus Lectures, Inc.
Now imagine these short bursts of wavefronts moving in time traveling from left to right.
Imagine these wavefronts moving in time, with a continuous stream of new wavefronts traveling from left to right. 1 2 3
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10. Range Specificity and a Very Short Pulse
Fig. 2: (Pg 194)
Notice that the echoes from each of the three mountains return at distinct times such that each mountain is resolved (range resolution).
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11. Longer Pulse but Still Resolved
Fig. 3: (Pg 195)
Notice that in this case, the pulse length is longer than in the previous case but that the echoes from each of the three mountains are still separated in time. Therefore, the three mountains are still adequately resolved although there is less separation between the echoes.
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12. Range Ambiguity Pegasus Lectures, Inc. Fig. 4: (Pg 195)
In this case, the pulse length is so long that the echoes from mountain 1 and mountain 2 overlap, making it impossible to distinguish mountain 1 from mountain 2. The echo from mountain 3 is still distinct since the separation between mountain 2 and mountain 3 is greater than the separation between mountain 2 and mountain 1.
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13. Pulsed Wave Pegasus Lectures, Inc.
Four points made obvious by our mountain analogy:
Continuous wave did not have any range resolution
Pulsed wave did allow for range resolution
Shorter pulses result in better resolution
The roundtrip effect separates the echoes by a factor of twice the distance between the objects. This roundtrip affect improves our resolution by a factor of 2.
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14. Pulsed Wave Definitions
Many new definitions are required to specify the pulsed wave characteristics such as:
The time the pulse lasts (Pulse Duration)
How often the pulse repeats (Pulse Repetition Period)
The percentage of time the transmit is on versus off (Duty Factor)
The physical length of the pulse (Spatial Pulse Length)
The following slides define each of these parameters.
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15. Pulse Duration Pegasus Lectures, Inc. Fig. 5: (Pg 196)
The Pulse Duration (PD) is a time measure of how long the transmit pulse lasts.
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16. The PRP Pegasus Lectures, Inc. Fig. 6: (Pg 196)
The Pulse Repetition Period (PRP) is the time between repeating transmit pulses.
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17. The PRF Pegasus Lectures, Inc. Fig. 6: (Pg 196)
Notice that the PRP and the PRF are really the same information, just in reciprocal form. The Pulse Repetition Frequency (PRF) is simply calculated as the reciprocal of the PRP.
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18. Fig. 7: Work Week Duty Factor Example (Pg 197)
The Duty Factor
Fig. 7: Work Week Duty Factor Example (Pg 197)
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19. The Duty Factor and the PD
Fig. 8: Longer Pulse Duration (PD) Increases the Duty Factor (Pg 198)
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20. The Duty Factor and the PRP
Fig. 9: Shorter Pulse Repetition Period (PRP) Increases the Duty Factor
(Pg 198)
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21. Duty Factor (Example) Pegasus Lectures, Inc. Fig. 10: (Pg 199)
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22. Spatial Pulse Length (SPL)
Fig. 11: (Pg 199)
The Spatial Pulse Length refers to the physical dimension the pulse occupies in space. As was seen from the mountain analogy and animations, a longer spatial pulse length results in worse range resolution.
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23. The Meaning of Resolution
The word “resolution” signifies the ability or inability to distinguish between two structures or to visualize an activity.
There are many important points about resolution such as:
For resolution, smaller numbers are always better (it is better to able to resolve structures 1 mm apart than 7 mm apart)
Resolution is a general term which can refer to many different aspects of ultrasound.
The limiting factor which ultimately determines the resolution can be as a result of many factors such as:
Limits based on physics of the situation
Limits based on equipment
Limits based on display device (such as monitor)
Limits based on human perception (eyesight, hearing, etc.)
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24. SPL and Range Resolution
Fig. 12: (Pg 200)
This diagram depicts why the range resolution smaller than the SPL by a factor of 2. Notice that the echoes between mountains 2 and 3 are twice as far apart (2x) as the separation between mountains 2 and 3 (x).
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25. Range Resolution Animation
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26. Other Names for Range Resolution
There are at least three other names often used interchangeable in reference to the Range resolution.
These terms are:
Depth Resolution
Longitudinal Resolution
Axial Resolution
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27. Relating Wave Parameters and PW Parameters
Fig. 13: Calculating the Pulse Duration (Pg 201)
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28. Numerical Example Pegasus Lectures, Inc. Fig. 14: (Pg 202)
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29. Effect of Period on Duty Factor
Notice that although both transmit pulses have 2 cycles per pulse and the same PRP, the duty cycle is higher for the wave with the longer period. Since a longer period corresponds to a lower frequency, leaving all things equal except lowering the transmit frequency, increases the duty factor.
Fig. 15: (Pg 203)
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30. SPL and Wavelength Pegasus Lectures, Inc. Fig. 16: (Pg 203)
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31. Foundational Drawing For PW
Fig. 17: (Pg 204)
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