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A 5-Pulse Sequence for Harmonic and Sub-Harmonic Imaging

A 5-Pulse Sequence for Harmonic and Sub-Harmonic Imaging. W. G. Wilkening 1 , J. Lazenby 2 , H. Ermert 1 1 Department of Electrical Engineering, Ruhr-University, Bochum 2 Siemens Medical Systems, Ultrasound Group, P.O. Box 7002, Issaquah WA 98027, USA. Outline. Introduction 2-pulse sequence

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A 5-Pulse Sequence for Harmonic and Sub-Harmonic Imaging

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  1. A 5-Pulse Sequence for Harmonic and Sub-Harmonic Imaging W. G. Wilkening1, J. Lazenby2, H. Ermert1 1Department of Electrical Engineering, Ruhr-University, Bochum2Siemens Medical Systems, Ultrasound Group, P.O. Box 7002, Issaquah WA 98027, USA

  2. Outline • Introduction • 2-pulse sequence • 3-pulse sequences • 5-pulse sequence • Harmonics, speckle • Experimental results • Conclusion and outlook

  3. Introduction • Pulse sequences enable non-linear imaging without a loss in spatial resolution • Multi-pulse sequences can increase the SNR • Advantages for contrast imaging • low acoustic power  increases blood / tissue contrast, less destruction of microbubbles • Advantages for tissue harmonic imaging • increased imaging depth • Disadvantages • increased sensitivity to motion

  4. Echo 1Echo 2Sum linear scatterer amplitude  time  Echo 1Echo 2Sum nonlinear scatterer amplitude  time  2-Pulse Sequence“Phase Inversion”, “Pulse Inversion” • Detects even order harmonics • Commercially available

  5. 120 120 120 1 1 1 1st 2nd 3rd 0 0 0 240 240 240 Multi-Pulse Sequences3 Equidistant Phases • 3-pulse sequence: 0°, 120°, 240° • Coherent summation  cancellation of 1st and 2nd harmonic

  6. Multi-Pulse Sequences3 Non-Equidistant Phases • Non-equidistant phase + weighted summation of echo signals cancellation of the 1st harmonic • Transmit pulses: s1, s2, s3phases: 1 = 0,2 = –3 (symmetric) • Echoes: e1, e2, e3 • Weighted sum: e = a1e1 + a2e2 + a3e3 • Cancellation of 1st harmonic:a1 = 1, a2 = a3 = f(2)

  7. 3 2nd harmonic 0° 2 a2 =a3 1 s1 0 3rd harmonic 2 -1 3 -2 s2 s3 -3 0 20 40 60 80 100 120 140 160 180 2,degrees Phases and WeightsMulti-Pulse Sequences with 3 Non-Equidistant Phases

  8. Choosing Phases / WeightsMulti-Pulse Sequences with 3 Non-Equidistant Phases • Preferable weights: a2 = a3  1 • Efficient detection of 2nd and 3rd harmonic Examples:

  9. 90 120 60 150 30 180 0 210 330 240 300 270 Subsets in a Sequence of 5 Equidistant Pulses • 5-pulse sequence • 5 subsets “type A” of 3 pulses, 2 = 72° • 5 subsets “type B” of 3 pulses, 2 = 144° • Weighted summation for all 10 subsets “subset echoes” • Demodulation of sums • Summation of demod. “subset echoes”

  10. The 0th Harmonic • For CW signals, a 2nd order non-linearity causes a DC component and a 2nd harmonic • For broadband signals, the DC component broadens  “0th harmonic”, propagation possible (f > 0 Hz) • Phase of the transmitted pulse has no influence on the phase of the 0th harmonic  phases of 2nd and 3rd harmonic in subset echoes vary, phase of the 0th harmonic remains constant speckle reduction

  11. 0 0 squared gaussian shaped pulse,1st harmonic at 7.2 MHz squared gaussian shaped pulse,0°, 72°, 144°, 216°, 288° -2 -200 -4 -400 -6 -600 -8 degrees normalized amplitude, [dB] -800 -10 -1000 -12 -1200 -14 -1400 -16 0th harmonic 2nd harmonic 0th harmonic 2nd harmonic -1600 -18 0 0.5 1 1.5 2 0 0.5 1 1.5 2 7 7 Hz Hz x 10 x 10 Spectrum and Phase of the 0th Harmonic Magnitude Spectrum of a Squared Gaussian Shaped Pulse Phase Spectrum of Squared Gaussian Shaped Pulses

  12. Suppression of 1st harmonic Reduced speckle unprocessed echoes:SNRspeckle = 1.91after incoh. summation:SNRspeckle = 2.4 1 40 40 0.5 20 20 0 0 amplitude, [a. u.] amplitude, [a. u.] normalized amplitude 0 -20 -20 -40 -40 -0.5 -1 0 0.1 0.2 0.3 0.4 lin. lin. +non-lin. lin. µs 0 1 2 3 4 5 cm Simulation original echoes 1st harmonicsuppressed

  13. Pulse sequence implemented on a Siemens Sonoline® Elegra Measurements from a string phantom Center frequency: 7.2 MHz Weights optimized for measured amplitudes and phases 90 120 60 1 150 30 180 0 210 330 240 300 270 5-Pulse SequenceMeasurement: String Target

  14. 5-pulse sequence, 2 cycles, 3.6 MHz and 7.2 MHz 7.2 MHz linear array Tissue phantom with cylindrical hole 1 0.5 normalized amplitude 0 -0.5 -1 0 0.2 0.4 0.6 0.8 1 µs 5-Pulse SequenceMeasurements with Levovist Transducer ROI 1.1 cm x 4.2 cm 3.6 MHz String Target Levovist Tissue

  15. Experimental Results7.2 MHz • B-mode • Contrast –4 dB • SNRspeckle= 1.8(0.5 – 1 cm) • Harmonic(all) • Contrast +14 dB • SNRspeckle 3(inc. w. depth) • Sub-Harmonic • Contrast +18 dB +50 dB

  16. 0 0.5 1 1.5 2 cm 2.5 3 3.5 4 0 2 4 6 8 10 12 14 16 MHz Spectrogram1st harmonic suppressed B-Mode Sub-Harm.

  17. 0 0.5 1 1.5 2 cm 2.5 3 3.5 4 0 2 4 6 8 10 12 14 16 MHz Experimental Results, 3.6 MHz1st harmonic suppressed • broadband pulses • transmit spectrum dominated by trans-ducer characteristics • phase errors increase with frequency • excitation above resonance frequency of microbubbles

  18. Conclusion and Outlook • 5-pulse sequences • enable 0th, 2nd and 3rd harmonic imaging • may be combined with flow imaging (data not shown) • can be optimized for non-ideal transmit waveforms • can be implemented on commercial systems • show the potential to improve SNR and to reduce speckle • Future work • real-time acquisitions in vitro and in vivo • symmetrical 3-pulse sequence for sub- and ultra-harmonic imaging (0.5f0, 1.5f0, 2.5f0)

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