pegasus lectures, inc. volume ii companion presentation frank miele pegasus lectures, inc....

Pegasus Lectures, Inc.

Volume II

Companion Presentation

Frank MielePegasus Lectures, Inc.

Ultrasound Physics & Instrumentation4th Edition


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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


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.


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)


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


Tradeoffs Related to Frequency

High Frequency (Inadequate Penetration)

Fig. 2a: (Pg 661)

Low Frequency (Poor Resolution)

Fig. 2b: (Pg 661)


Harmonic Imaging

Harmonic Image produced by transmitting at 1.8 MHz and receiving at 3.6 MHz.

Fig. 3: (Pg 661)


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


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


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)


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


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)


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


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


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


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


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


Conventional versus Harmonic Imaging (from Animation CD)

(Pg 669 A)

Conventional Imaging Harmonic Imaging


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


Conventional versus Harmonics(from Animation CD)

(Pg 669 B)


Conventional versus Harmonics(from Animation CD)

(Pg 669 C)

Images of a right kidney with multiple cysts.


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


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


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


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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pegasus lectures, inc. volume ii companion presentation frank miele pegasus lectures, inc....

Documents

nd harmonic frequency

harmonic signal

bw frequency sensitivity

higher frequency

lower frequency

hemodynamics slide

nd harmonic energy

contrast signal bubbles