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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 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
Outline • Introduction • 2-pulse sequence • 3-pulse sequences • 5-pulse sequence • Harmonics, speckle • Experimental results • Conclusion and outlook
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
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
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
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)
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
Choosing Phases / WeightsMulti-Pulse Sequences with 3 Non-Equidistant Phases • Preferable weights: a2 = a3 1 • Efficient detection of 2nd and 3rd harmonic Examples:
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”
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
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
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
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
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
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
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.
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
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)