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Ultrasonic Nonlinear Imaging- Tissue Harmonic Imaging

Ultrasonic Nonlinear Imaging- Tissue Harmonic Imaging. Conventional B-mode image (AP4CH). THI. Fundamental. THI. Fundamental. Tissue Nonlinear Imaging. Performance of ultrasound has been sub-optimal on technically difficult bodies.

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Ultrasonic Nonlinear Imaging- Tissue Harmonic Imaging

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  1. Ultrasonic Nonlinear Imaging-Tissue Harmonic Imaging Tissue Harmonic Imaging

  2. Conventional B-mode image (AP4CH)

  3. THI Fundamental

  4. THI Fundamental

  5. Tissue Harmonic Imaging

  6. Tissue Nonlinear Imaging • Performance of ultrasound has been sub-optimal on technically difficult bodies. • Most recent new developments have bigger impact on technically satisfactory bodies. • Poor image quality leads to uncertainty in diagnosis and costly repeat examinations. Tissue Harmonic Imaging

  7. Tissue Harmonic Imaging • Methods to improve image quality: • Different acoustic window. • Lower frequency. • Adaptive imaging. • Non-linear imaging (or harmonic imaging). Tissue Harmonic Imaging

  8. Origin of Tissue Non-linearity • Finite amplitude distortion: peaks of the waveform travels faster than the troughs. Tissue Harmonic Imaging

  9. Pressure Before distortion After distortion t Tissue Non-Linearity • Signal Source • Finite amplitude distortion generated tissue harmonics

  10. Non-Linear Propagation Tissue Harmonic Imaging

  11. 80 Fundamental 2nd Harmonic 60 Velocity(cm/sec) 40 20 0 0 20 40 60 80 Depth(mm) Axial Amplitude Tissue Harmonic Imaging

  12. THI Characteristics 4MHz Beam Patterns transducer transducer dB Lateral Position (mm) Tissue Non-Linearity

  13. Tissue Non-linearity • Tissue harmonics are virtually zero at the probe face.The intensity continues to increase until attenuation dominates. • The higher the intensity is, the more tissue harmonics are generated. • Such a mechanism automatically increase the difference between signal and acoustic noise. Tissue Harmonic Imaging

  14. Advantages of Tissue Harmonic Imaging • Low sidelobes. • Better spatial resolution compared to fundamental imaging at the original frequency. • Less affected by tissue inhomogeneities – better performance on technically difficult bodies. Tissue Harmonic Imaging

  15. Non-linear Parameter B/A • B/A defines non-linearity of the medium. The larger the B/A, the higher the non-linear response. Tissue Harmonic Imaging

  16. B/A Parameters: Measurements • Finite amplitude method: • B/A is related to the second harmonic generation. Thus, it can be found by relating the signal amplitude at the fundamental frequency to the second harmonic component. • Thermodynamic method: • The B/A value is determined by measuring the change of sound speed with pressure and temperature. Tissue Harmonic Imaging

  17. B/A Parameters: Typical Values • Typical values: • Water:5.5+/-0.3. • Liver: 7.23. • Fat: 10.9. • Muscle: 7.5. • Results from both methods have excellent agreement. • B/A imaging may be used for tissue characterization. Tissue Harmonic Imaging

  18. Image Analysis Issues • Low signal-to-noise ratio: coded excitation, simultaneous multiple transmit focusing. • Spectral leakage and image quality degradation. • Spatial covariance analysis for correlation-based processing. • Motion artifacts in pulse inversion imaging . Tissue Harmonic Imaging

  19. MHz MHz LPF HPF Filter Based Image Formation • Fundamental and Harmonic Imaging Fundamental Imaging Spectrum Spectrum Harmonic Imaging Transmit Signal Received Signal

  20. MHz MHz MHz MHz Effects of Harmonic Leakage • Motive : • Contrast resolution degradation 4MHz Beam Patterns dB Transmit Spectrum Received Spectrum Lateral Position (mm)

  21. Transducer High Voltage Amplifier & T/R Switch Waveform Generator Sources of Harmonic Leakage • Designed transmit waveform. • System nonlinearity. • Electromechanical conversion. Tissue Harmonic Imaging

  22. Designed Waveform (I) • Characteristics of transmit waveforms. Waveforms Normalized Amplitude s Spectra dB MHz Tissue Harmonic Imaging

  23. Spectrum of Transmit Signal Leakage MHz Fundamental band Harmonic band Designed Waveform (II) • Signal bandwidth. Tissue Harmonic Imaging

  24. Non-linear Propagation Wave at distance z angular spectrum method Linear propagation to z+Dz frequency domain solution to Burgers’ equation Nonlinear propagation at z+Dz Tissue Harmonic Imaging

  25. Nonlinear Simulation Model • Model the Nonlinear Propagation • Δf: fundamental frequency • un： Sin(2π(nΔf)t) • β：nonlinear parameter • c：sound velocity

  26. Results: Effect of Bandwidth • Gaussian at 25% and 50% • Contrast v.s. Spatial Harmonic Beam Patterns 0 BW=25% BW=50% -20 dB -40 -60 -10 -5 0 5 10 Lateral Position (mm)

  27. Results: Signal Type • Sine, square and Gaussian wave, BW=25% • Smooth envelope has better contrast Harmonic Beam Patterns 0 Gaussian Gated sine Gated square -20 dB -40 -60 -10 -5 0 5 10 Lateral Position (mm)

  28. Results: Signal Type • Gaussian, gated sine and gated square waves. • BW=50%. Harmonic Beam Patterns Integrated Harmonic Beam Patterns dB Gaussian Gated sine Gated square 4MHz linear Lateral Position (mm) Lateral Position (mm) Tissue Harmonic Imaging

  29. 30 2.5 (mm) 0 0 (ns) -2.5 -30 -5 0 10 -10 5 (mm) Effects of Harmonic Leakage • Tissue Inhomogeneities • Fat layer: 15mm thick, B/A=10. • Aberrating plane: max. time delay error=30ns, correlation length=5mm. Phase Aberration Pattern

  30. Results: Tissue Inhomogeneities Integrated Harmonic Beam Patterns Harmonic Beam Patterns • BW=50%. dB Gaussian Gated sine Gated square 4MHz linear Lateral Position (mm) Lateral Position (mm) Tissue Harmonic Imaging

  31. Effects of Harmonic Leakage • Drive Voltage • Magnitude => nonlinearity  Spectra Beam Patterns Harmonic(1 Volt) Fundamental(1 Volt) Harmonic(5 Volt) Fundamental(5 Volt) 1 Volt 5 Volt dB dB Frequency(MHz) Lateral Position(mm)

  32. Harmonic Beam Patterns BW=25% BW=50% dB Lateral Position(mm) Results: Bandwidth Spectra • Gaussian envelope, 1 Volt, 25% vs. 50%. BW=25% BW=50% dB Frequency(MHz) Tissue Harmonic Imaging

  33. Results: Drive Voltage Beam Patterns Spectra • 1Volt vs. 5Volt. Harmonic(1 Volt) Fundamental(1 Volt) Harmonic(5 Volt) Fundamental(5 Volt) 1 Volt 5 Volt dB dB Lateral Position(mm) Frequency(MHz) Tissue Harmonic Imaging

  34. Pulse Inversion Fundamental signal Positive driving pulse t f Harmonicsignal Nonlinear propagation t f Negative driving pulse t f Tissue Harmonic Imaging ONLY harmonic signal

  35. Pulse Inversion • Pulse inversion reduces sidelobe levels Fundamental Beam Harmonic Beam (Filtering) Harmonic Beam (Filtering) Harmonic Beam (Pulse inversion) Gaussian pulse Tissue Harmonic Imaging Sine pulse Sine pulse Gaussian pulse

  36. Pulse Inversion • harmonic leakage could be avoided • all linearly propagated components are cancelled Harmonic Beam Patterns at 50% Bandwidth dB Tissue Harmonic Imaging Lateral Position(mm)

  37. Harmonic Leakage • Smooth envelopes provide lower sidelobes, but also require a more sophisticated transmitter. • Large bandwidths improve axial resolution, but also increase sidelobes. • Sidelobe differences decrease in the presence of tissue inhomogeneities. • Spectral leakage must be suppressed without affecting fundamental beams. • Pulse inversion technique is the most effective. Tissue Harmonic Imaging

  38. v1 v2 Array Transducer Sound Velocity Inhomogeneities Tissue Harmonic Imaging

  39. Spatial Covariance Analysis • Sound velocity inhomogeneities are conventionally corrected by correlation-based methods. • The covariance of signals received at different positions is critical to correlation-based correction techniques (the van Cittert-Zernike theorem). • Is it possible to further improve the image by combining tissue harmonic imaging and phase aberration correction? • Optimal frequency selection for imaging and time delay estimation. Tissue Harmonic Imaging

  40. Progress: Simulations • Transmit • beam formation by FDSBE • Receive • time-domain signal for each channel Tissue Harmonic Imaging a,b: length and width of channel

  41. channel time Progress: Simulations Correlation coefficient channel Tissue Harmonic Imaging

  42. Spatial Covariance: Experiments Spatial Covariance: Simulations 1 1 3.5MHz Fundamental 2MHz Fundamental 7MHz Second Harmonic 4MHz Second Harmonic 0.8 0.8 0.6 0.6 Correlation Coefficient 0.4 0.4 0.2 0.2 0 0.5 1 0 0.5 1 Progress: Results • Harmonic covariance is generally similar to or lower than fundamental covariance Tissue Harmonic Imaging Normalized Distance

  43. Spatial Covariance: Simulations Spatial Covariance : Experiments 1 1 2MHz Fundamental 3.5MHz Fundamental 4MHz Second Harmonic 7MHz Harmonic 0.8 0.8 0.6 0.6 Correlation Coefficient 0.4 0.4 0.2 0.2 0 0.5 1 0 0.5 1 Normalized Distance Progress: Results • With sound velocity inhomogeneities Tissue Harmonic Imaging

  44. Fundamental Spatial Covariance: Experiments Second Harmonic Spatial Covariance: Experiments 1 1 High SNR High SNR Low SNR Low SNR 0.8 0.8 0.6 0.6 Correlation Coefficient 0.4 0.4 0.2 0.2 0 0.5 1 0 0.5 1 Normalized Distance Progress: Results • Effects of SNR Tissue Harmonic Imaging

  45. Spatial Covariance • When adequate SNR is available • Whether the sound velocity variations are present or not, the harmonic covariance is generally similar to or lower than fundamental covariance. • When SNR is low • Harmonic covariance is significantly affected. • Imaging at the second harmonic frequency, correlation-based correction at the fundamental frequency. Tissue Harmonic Imaging

  46. Harmonic Interference • In contrast imaging, in which the tissue harmonic signals are un-desirable, the amplitude of the propagating wave needs to minimized. • Large apertures (smaller f-numbers) may be used. • It was reported that tissue harmonic signal can be reduced by 3dB by doubling the aperture size. Tissue Harmonic Imaging

  47. Harmonic Interference • Harmonic cancellation system: non-linear propagation is reduced by using a new signal at the harmonic frequency. • Phase and magnitude of the signal may be pre-calculated, but on-line adjustment is necessary. • Due to attenuation, optimal effects may only be achieved locally. Tissue Harmonic Imaging

  48. Ultrasonic Nonlinear Imaging- Contrast Harmonic Imaging Tissue Harmonic Imaging

  49. Contrast Harmonic Imaging • Contrast agents are used to provide higher contrast. The three commonly seen contrast agents are backscatter, attenuation and sound velocity. • Contrast agents could be solid particles, emulsion, gas bubbles, encapsulated gas, or liquid. Tissue Harmonic Imaging

  50. Contrast Harmonic Imaging • Primary clinical benefits: • Enhanced contrast resolution between normal and diseased tissues. • Outline of vessels or heart chambers. • Tissue characterization by using tissue specific agents. • Increasing blood flow signals. • Dynamic study using washout curve. Tissue Harmonic Imaging

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