Pegasus Lectures, Inc. Volume II Companion Presentation Frank Miele Pegasus Lectures, Inc. Ultrasound Physics & Instrumentation 4 th Edition

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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): 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. 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 Fundamental Frequency Amplitude (dB)

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 2 nd harmonic frequency) and transmitting at the lower frequency (the fundamental frequency).

Pegasus Lectures, Inc. Tradeoffs Related to Frequency High Frequency (Inadequate Penetration) Fig. 2a: (Pg 661) Low Frequency (Poor Resolution) 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. 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 Sensitivity

Pegasus Lectures, Inc. The non-linear response of the tissue “distorts” the transmitted wave, producing harmonic energy. As pictured below, not just 2 nd harmonic energy is produced, but an entire spectrum of harmonics (2 nd, 3 rd, 4 th, etc.). Currently ultrasound uses only the 2 nd harmonic. Generation of Harmonic Energy Through Tissue Fig. 5: (Pg 663) Transmitted FrequencyReceived Frequency Fundamental Amplitude N=2 N=3

Pegasus Lectures, Inc. 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 Blood Fundamental Frequency Amplitude (dB)

Pegasus Lectures, Inc. 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 Increased c Decreased c Transmitted Wave Nonlinear Response

Pegasus Lectures, Inc. Relative Amplitude of the Harmonic Series Notice that each successive harmonic signal is weaker than the preceding harmonic signal, and that the 2 nd harmonic signal is weaker than the fundamental signal. Fig. 8: (Pg 665 ) Frequency F0F0 2F 0 3F 0 4F Amplitude (dB)

Pegasus Lectures, Inc. Harmonic Generation versus Depth Fig. 9: (Pg 666) “Time View”“Frequency View” Nearfield Weak Harmonics Generated Midfield Best Harmonic Effects Farfield Harmonic Frequency Attenuated Faster Than Produced

Pegasus Lectures, Inc. 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

Pegasus Lectures, Inc. 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) FundamentalHarmonic

Pegasus Lectures, Inc. 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 f0f0 2f Depth (cm) Amplitude

Pegasus Lectures, Inc. 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 Here Transmitted Fundamental Received Harmonics f0f0 2f 0 f0f0

Pegasus Lectures, Inc. Conventional versus Harmonic Imaging (from Animation CD) (Pg 669 A) Conventional Imaging Harmonic Imaging

Pegasus Lectures, Inc. 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

Pegasus Lectures, Inc. Conventional versus Harmonics (from Animation CD) (Pg 669 B)

Pegasus Lectures, Inc. Conventional versus Harmonics (from Animation CD) (Pg 669 C) Images of a right kidney with multiple cysts.

Pegasus Lectures, Inc. 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 f0f0 2f 0 More Clutter Better Axial Resolution Less Clutter Worse Axial Resolution Reduced BW Overlap f0f0 2f 0

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. Fig. 17: (Pg 671) ( f 0 ) at maximum and harmonic ( 2f 0 ) at maximum ( f 0 ) at maximum and harmonic ( 2f 0 ) at minimum

Pegasus Lectures, Inc. 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) ( f 0 ) First Pulse (Phase = 0°) ( f 0 ) First Pulse (Phase = 180°) ( 2f 0 ) Harmonic (Phase = 0°) Cancellation of f 0 Enhancement of 2f 0

Pegasus Lectures, Inc. 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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