Ultrasound Physics & Instrumentation 4th Edition Volume II Companion Presentation Frank Miele Pegasus Lectures, Inc. Pegasus Lectures, Inc. License Agreement This presentation is the sole property of Pegasus Lectures, Inc. 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. Pegasus Lectures, Inc. 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 Pegasus Lectures, Inc. 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. Pegasus Lectures, Inc. 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) Pegasus Lectures, Inc. 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 Pegasus Lectures, Inc. Tradeoffs Related to Frequency High Frequency (Inadequate Penetration) Low Frequency (Poor Resolution) Fig. 2a: (Pg 661) Fig. 2b: (Pg 661) Pegasus Lectures, Inc. Harmonic Imaging Harmonic Image produced by transmitting at 1.8 MHz and receiving at 3.6 MHz. Fig. 3: (Pg 661) Pegasus Lectures, Inc. 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) Pegasus Lectures, Inc. 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 Pegasus Lectures, Inc. 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) Pegasus Lectures, Inc. 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 ) Pegasus Lectures, Inc. 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 ) Pegasus Lectures, Inc. Harmonic Generation versus Depth “Time View” “Frequency View” Nearfield Midfield Farfield Fig. 9: (Pg 666) Pegasus Lectures, Inc. 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 ) Pegasus Lectures, Inc. 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) Pegasus Lectures, Inc. 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 ) Pegasus Lectures, Inc. 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 ) Pegasus Lectures, Inc. Conventional versus Harmonic Imaging (from Animation CD) Harmonic Imaging Conventional Imaging (Pg 669 A) Pegasus Lectures, Inc. 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) Pegasus Lectures, Inc. Conventional versus Harmonics (from Animation CD) (Pg 669 B) Pegasus Lectures, Inc. Conventional versus Harmonics (from Animation CD) Images of a right kidney with multiple cysts. (Pg 669 C) Pegasus Lectures, Inc. 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) Pegasus Lectures, Inc. 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) Pegasus Lectures, Inc. 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) Pegasus Lectures, Inc. 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) Pegasus Lectures, Inc. 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