Transcript Chapter10_level_2

Ultrasound Physics & Instrumentation
4th Edition
Volume II
Companion Presentation
Frank Miele
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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):
 Z 2 -Z1 
Reflection % = 

 Z 2 +Z1 
2
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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Relative Signal Amplitudes
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).
Tissue
Bubbles
Amplitude (dB)
50
Blood
40
30
20
10
Fundamental
Frequency
Fig. 1: (Pg 660)
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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).
Fundamental Frequency  Transmit Frequency
2nd Harmonic Frequency  2  Transmit Frequency
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Tradeoffs Related to Frequency
High Frequency
(Inadequate Penetration)
Low Frequency
(Poor Resolution)
Fig. 2a: (Pg 661)
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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Broadband Transducers and Harmonics
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.
Sensitivity
XDCR BW
Transmit
BW
Receive
BW
Frequency
Fig. 4: (Pg 662)
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Generation of Harmonic Energy Through
Tissue
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.
Amplitude
Amplitude
Fundamental
N=2
N=3
Transmitted Frequency
Received Frequency
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Fig. 5: (Pg 663)
Relative Amplitudes
Tissue
Bubbles
Amplitude (dB)
50
Blood
40
30
20
10
Fundamental
Frequency
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.
Fig. 6: (Pg 663)
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Non-Linear Wave Propagation
Compression With compression and
Transmitted Wave
Compression
Nonlinear Response
Increased c
Decreased c
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.
Fig. 7: (Pg 664 )
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Relative Amplitude of the Harmonic
Series
0
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.
Amplitude (dB)
-10
-20
-30
-40
-50
-60
-70
F0
2F0
3F0
4F0
Frequency
Fig. 8: (Pg 665 )
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Harmonic Generation versus Depth
“Time View”
“Frequency View”
Nearfield
Midfield
Farfield
Fig. 9: (Pg 666)
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Weak Harmonics
Generated
Best Harmonic
Effects
Harmonic Frequency
Attenuated Faster
Than Produced
Harmonic versus Fundamental Beam
Fundamental Energy
Tissue
Harmonic Energy
Fig. 10: (Pg 666 )
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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.
Reduction in Grating Lobes
Fundamental
Harmonic
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 offaxis energy into the main
beam.
Fig. 11: (Pg 667)
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Reduction in “Clutter” from Harmonics
Typical Major “Clutter” Zone
f0
Amplitude
2f0
5
10
15
Depth (cm)
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 )
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Reduction in Reverberation Artifact
Transmitted
Fundamental
Reverberations
Here
f0
2f0
f0
2f0
Received
Harmonics
Transmitted Signal
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.
Receive Filters
Fig. 13: (Pg 668 )
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Conventional versus Harmonic Imaging
(from Animation CD)
Harmonic Imaging
Conventional Imaging
(Pg 669 A)
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Conventional versus Harmonic Imaging
As discussed in the previous slides, harmonics usually reduces the
clutter present in the relative nearfield.
Conventional Imaging of Right ICA
with Reverberation Artifact
Harmonic Imaging of Right ICA
Fig. 15: (Pg 669)
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Conventional versus Harmonics
(from Animation CD)
(Pg 669 B)
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Conventional versus Harmonics
(from Animation CD)
Images of a right kidney with multiple cysts.
(Pg 669 C)
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Degradation in Axial Resolution
More Clutter
Better Axial Resolution
Short Time
f0
2f0
BW Overlap
Less Clutter
Worse Axial Resolution
Long Time
f0
2f0
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.
Reduced BW Overlap
Fig. 16: (Pg 670)
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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.
(f0) at maximum and harmonic (2f0) at maximum
(f0) at maximum and harmonic (2f0) at minimum
Fig. 17: (Pg 671)
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Pulse or Phase Inversion
(f0) First Pulse (Phase = 0°)
(2f0) Harmonic (Phase = 0°)
(f0) First Pulse (Phase = 180°)
(2f0) Harmonic (Phase = 0°)
Cancellation of f0
Enhancement of 2f0
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)
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Conventional versus Pulse Inversion
Harmonic Imaging
Notice how dramatic the difference in ability to visual the thrombus
using pulse inversion in comparison with conventional imaging.
Fundamental Imaging
Pulse Inversion Harmonics
Fig. 19: (Pg 672)
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