pegasus lectures, inc. volume ii companion presentation frank miele pegasus lectures, inc....
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Pegasus Lectures, Inc.
Volume II
Companion Presentation
Frank MielePegasus Lectures, Inc.
Ultrasound Physics & Instrumentation4th Edition
Pegasus Lectures, Inc.
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All Copyright Laws Apply.
License Agreement
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Volume II Outline
Chapter 7: Doppler
Chapter 8: Artifacts
Chapter 9: Bioeffects
Chapter 10: Contrast and Harmonics
Level 2
Chapter 11: Quality Assurance
Chapter 12: Fluid Dynamics
Chapter 13: Hemodynamics
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Contrast and the Acoustic Impedance
Recall that the amount of reflection that occurs is based on the acoustic impedance mismatch (as defined in Chapter 3):
2
2 1
2 1
Z -ZReflection % =
Z +Z
The use of a contrast agent increases the acoustic impedance mismatch within the blood as a result of the high compressibility and low density of the gas.
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Notice that the signal from blood is much weaker than the signal from tissue. The contrast signal (“bubbles”) increases the blood signal significantly (approximately 30 dB).
Relative Signal Amplitudes
Fig. 1: (Pg 660)
Tissue
Bubbles
Blood
50
40
30
20
10
Fundamental Frequency
Am
pli
tud
e (d
B)
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Fundamentals of Harmonics
The classic tradeoff in ultrasound is penetration versus resolution. The use of harmonics somewhat lessens the tradeoff by allowing for receiving at a higher frequency (the 2nd harmonic frequency) and transmitting at the lower frequency (the fundamental frequency).
2 2 nd
Fundamental Frequency Transmit Frequency
Harmonic Frequency Transmit Frequency
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Tradeoffs Related to Frequency
High Frequency (Inadequate Penetration)
Fig. 2a: (Pg 661)
Low Frequency (Poor Resolution)
Fig. 2b: (Pg 661)
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Harmonic Imaging
Harmonic Image produced by transmitting at 1.8 MHz and receiving at 3.6 MHz.
Fig. 3: (Pg 661)
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The ability to transmit at the fundamental frequency and receive at the higher frequency requires broadband transducer capabilities. As shown below, note how the transmit BW and the receive BW “fit” within the overall transducer BW.
Broadband Transducers and Harmonics
Fig. 4: (Pg 662)
XDCR BW
Transmit BW
Receive BW
Frequency
Sen
siti
vity
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The non-linear response of the tissue “distorts” the transmitted wave, producing harmonic energy. As pictured below, not just 2nd harmonic energy is produced, but an entire spectrum of harmonics (2nd, 3rd, 4th, etc.). Currently ultrasound uses only the 2nd harmonic.
Generation of Harmonic Energy Through Tissue
Fig. 5: (Pg 663)Transmitted Frequency Received Frequency
Fundamental
Am
pli
tud
e
Am
pli
tud
e
N=2
N=3
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Notice that the amplitude of the harmonic signal produced by tissue is very close to the amplitude of the harmonic signal produced by contrast agent. This fact implies that it is difficult to distinguish “blood” signals from tissue signals when using harmonic imaging with contrast.
Relative Amplitudes
Fig. 6: (Pg 663)
Tissue
Bubbles
Blood50
40
30
20
10
Fundamental Frequency
Am
pli
tud
e (d
B)
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With compression and rarefaction, the density of the medium changes, resulting in a change in propagation velocity. This change in propagation velocity is nonlinear and results in the generation of harmonic energy from the fundamental.
Non-Linear Wave Propagation
Fig. 7: (Pg 664 )
Compression
Compression
Increased c
Decreased c
Transmitted Wave
Nonlinear Response
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Relative Amplitude of the Harmonic Series
Notice that each successive harmonic signal is weaker than the preceding harmonic signal, and that the 2nd harmonic signal is weaker than the fundamental signal.
Fig. 8: (Pg 665 )
Frequency
F0 2F0 3F0 4F0
-10
0
-30
-20
-40
-50
-60
-70
Am
plitu
de (
dB)
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Harmonic Generation versus Depth
Fig. 9: (Pg 666)
“Time View” “Frequency View”
NearfieldWeak Harmonics
Generated
Midfield Best Harmonic Effects
Farfield Harmonic Frequency Attenuated Faster
Than Produced
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Harmonic versus Fundamental Beam
Notice how much narrower the harmonic beam is relative to the fundamental beam, improving the lateral resolution. Also notice that the beam intensity is much weaker in the nearfield which reduces the amount of artifacts generated in the nearfield.
Fig. 10: (Pg 666 )
Fundamental Energy
Harmonic Energy
Tissue
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Reduction in Grating Lobes
Since harmonic energy produced is dependent on incident pressure, the lower energy grating lobes produce much weaker harmonic signals, reducing the energy in the grating lobes. Weaker grating lobes result in improved lateral resolution and less lateral translation of off-axis energy into the main beam.
Fig. 11: (Pg 667)
Fundamental Harmonic
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Reduction in “Clutter” from Harmonics
Notice that in the nearfield, the source of most imaging artifacts, the harmonic signal is significantly weaker than the fundamental signal. The result is a significant reduction in the strong signals responsible for most imaging artifacts. This clutter reduction is one of the greatest advantages to harmonic imaging.
Fig. 12: (Pg 668 )
Typical Major “Clutter” Zone f0
2f0
5 10 15Depth (cm)
Am
pli
tud
e
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Reduction in Reverberation Artifact
Notice how the weaker and narrower harmonic beam in the nearfield results in less reverberation artifact than occurs with the fundamental beam. Again, some of the greatest advantages to harmonic imaging is the reduction of “clutter” signals which result from beam interactions in the nearfield.
Fig. 13: (Pg 668 )
Transmitted Signal
Receive Filters
Reverberations HereT
ran
smit
ted
Fu
nd
amen
tal
Rec
eive
dH
arm
on
ics
f0 2f0
f0 2f0
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Conventional versus Harmonic Imaging (from Animation CD)
(Pg 669 A)
Conventional Imaging Harmonic Imaging
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Conventional versus Harmonic Imaging
As discussed in the previous slides, harmonics usually reduces the clutter present in the relative nearfield.
Fig. 15: (Pg 669)
Conventional Imaging of Right ICA with Reverberation Artifact
Harmonic Imaging of Right ICA
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Conventional versus Harmonics(from Animation CD)
(Pg 669 B)
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Conventional versus Harmonics(from Animation CD)
(Pg 669 C)
Images of a right kidney with multiple cysts.
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Degradation in Axial Resolution
With harmonic imaging, a longer transmit pulse duration (PD) is usually used to reduce the bandwidth of the transmit signal. By reducing the transmit BW, there is less overlap between the transmit and receive bandwidth decreasing the clutter in the image. However, the increase in PD also results in an increase in the SPL, decreasing the axial resolution.
Fig. 16: (Pg 670)
Short Time
Long Time
BW Overlap
f0 2f0
More Clutter Better Axial Resolution
Less Clutter Worse Axial Resolution
Reduced BW Overlap
f0 2f0
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Pulse or Phase Inversion
The following diagram demonstrates the foundational principle used for pulse (or phase) inversion harmonic imaging. Notice that the peak of the harmonic wave occurs at the same time as both the peak and the minima of the fundamental wave.
Fig. 17: (Pg 671)
(f0) at maximum and harmonic (2f0) at maximum
(f0) at maximum and harmonic (2f0) at minimum
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Pulse or Phase Inversion
By transmitting multiple lines with different phases and then adding the resulting lines together, the fundamental energy adds destructively while the harmonic data adds constructively. As a result the harmonic energy gets stronger and there is no need to degrade the axial resolution to help eliminate the fundamental energy.
Fig. 18: (Pg 672)
(f0) First Pulse (Phase = 0°) (f0) First Pulse (Phase = 180°)
(2f0) Harmonic (Phase = 0°) (2f0) Harmonic (Phase = 0°)
Cancellation of f0
Enhancement of 2f0
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Conventional versus Pulse Inversion Harmonic Imaging
Fig. 19: (Pg 672)
Fundamental Imaging Pulse Inversion Harmonics
Notice how dramatic the difference in ability to visual the thrombus using pulse inversion in comparison with conventional imaging.
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