A 5 Pulse Sequence for Harmonic and SubHarmonic

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 Phase-Coded Pulse Sequences Ruhr-University Bochum RF Engineering Institute

Outline • • Introduction 2 -pulse sequence 3 -pulse sequences 5 -pulse sequence Harmonics, speckle Experimental results Conclusion and outlook Phase-Coded Pulse Sequences Ruhr-University Bochum 2/18 RF Engineering Institute

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 Phase-Coded Pulse Sequences Ruhr-University Bochum 3/18 RF Engineering Institute

2 -Pulse Sequence linear scatterer • Detects even order harmonics • Commercially available amplitude “Phase Inversion”, “Pulse Inversion” Echo 1 Echo 2 Sum amplitude nonlinear scatterer time Echo 1 Echo 2 Sum time Phase-Coded Pulse Sequences Ruhr-University Bochum 4/18 RF Engineering Institute

Multi-Pulse Sequences 3 Equidistant Phases • 3 -pulse sequence: 0°, 120°, 240° • Coherent summation cancellation of 1 st and 2 nd harmonic 120 120 1 1 st 2 nd 3 rd 240 0 0 240 Phase-Coded Pulse Sequences Ruhr-University Bochum 5/18 RF Engineering Institute

Multi-Pulse Sequences 3 Non-Equidistant Phases • Non-equidistant phase + weighted summation of echo signals cancellation of the 1 st harmonic • Transmitpulses: s 1, s 2, s 3 phases: 1 = 0, 2 = – 3 (symmetric) • Echoes: e 1, e 2, e 3 • Weighted sum: e = a 1 e 1 + a 2 e 2 + a 3 e 3 • Cancellation of 1 st harmonic: a 1 = 1, a 2 = a 3 = f( 2) Phase-Coded Pulse Sequences Ruhr-University Bochum 6/18 RF Engineering Institute

Phases and Weights Multi-Pulse Sequences with 3 Non-Equidistant Phases 3 0° 2 a 2 =a 3 1 s 1 0 2 3 s 2 2 nd harmonic 3 rd harmonic -1 s 3 -2 -3 0 20 40 60 80 100 120 140 160 180 2, degrees Phase-Coded Pulse Sequences Ruhr-University Bochum 7/18 RF Engineering Institute

Choosing Phases / Weights Multi-Pulse Sequences with 3 Non-Equidistant Phases • Preferable weights: a 2 = a 3 1 • Efficient detection of 2 nd and 3 rd harmonic Examples: 2 a 2 60° -1 2 0. 75 120° 1 0 0. 75 72° – 1. 618 3. 6 0. 9 144° 0. 618 1. 38 0. 345 2 nd harm. 3 rd harm. Phase-Coded Pulse Sequences Ruhr-University Bochum 8/18 RF Engineering Institute

Subsets in a Sequence of 5 Equidistant Pulses • 5 -pulse sequence – 5 3 90 subsets “type A” of pulses, 2 = 72° subsets “type B” of pulses, 2 = 144° 120 60 150 • Weighted summation 180 for all 10 subsets “subset echoes” 210 • Demodulation of sums • Summation of demod. “subset echoes” 30 0 330 240 300 270 Phase-Coded Pulse Sequences Ruhr-University Bochum 9/18 RF Engineering Institute

The 0 th Harmonic • For CW signals, a 2 nd order non-linearity causes a DC component and a 2 nd harmonic • For broadband signals, the DC component broadens “ 0 th harmonic”, propagation possible (f > 0 Hz) • Phase of the transmitted pulse has no influence on the phase of the 0 th harmonic phases of 2 nd and 3 rd harmonic in subset echoes vary, phase of the 0 th harmonic remains constant speckle reduction Phase-Coded Pulse Sequences Ruhr-University Bochum 10/18 RF Engineering Institute

Spectrum and Phase of the 0 th Harmonic Magnitude Spectrum of a Squared Gaussian Shaped Pulse Phase Spectrum of Squared Gaussian Shaped Pulses 0 squared gaussian shaped pulse, 1 st harmonic at 7. 2 MHz -2 -4 -400 -6 -8 -10 -12 -600 -800 -1000 -1200 -1400 -16 -18 squared gaussian shaped pulse, 0°, 72°, 144°, 216°, 288° -200 degrees normalized amplitude, [d. B] 0 0 th harmonic 0 0. 5 2 nd harmonic 1 1. 5 Hz 2 x 10 -1600 0 th harmonic 0 0. 5 7 2 nd harmonic 1 1. 5 Hz Phase-Coded Pulse Sequences Ruhr-University Bochum 11/18 RF Engineering Institute 2 x 10 7

Simulation -0. 5 -1 0 0. 1 0. 2 µs 0. 3 0. 4 • Suppression of 1 st harmonic • Reduced speckle unprocessed echoes: SNRspeckle = 1. 91 after incoh. summation: SNRspeckle = 2. 4 original echoes 0 40 20 0 -20 -40 lin. + non-lin. 1 st harmonic suppressed amplitude, [a. u. ] 0. 5 amplitude, [a. u. ] normalized amplitude 1 20 0 -20 -40 0 1 2 cm 3 4 5 Phase-Coded Pulse Sequences Ruhr-University Bochum 12/18 RF Engineering Institute

5 -Pulse Sequence Measurement: String Target • 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 Phase-Coded Pulse Sequences Ruhr-University Bochum 13/18 RF Engineering Institute

5 -Pulse Sequence Measurements with Levovist normalized amplitude • 5 -pulse sequence, 2 cycles, 3. 6 MHz and 7. 2 MHz • 7. 2 MHz linear array • Tissue phantom with ROI cylindrical hole 1. 1 cm x 4. 2 cm 3. 6 MHz 1 Transducer 0. 5 0 String Target -0. 5 Levovist -1 0 0. 2 0. 4 0. 6 µs 0. 8 Tissue 1 Phase-Coded Pulse Sequences Ruhr-University Bochum 14/18 RF Engineering Institute

Experimental Results 7. 2 MHz • B-mode • Contrast – 4 d. B • SNRspeckle = 1. 8 (0. 5 – 1 cm) • Harmonic (all) • Contrast +14 d. B • SNRspeckle 3 (inc. w. depth) • Sub. Harmonic • Contrast +18 d. B +50 d. B Phase-Coded Pulse Sequences Ruhr-University Bochum 15/18 RF Engineering Institute

Spectrogram 1 st harmonic suppressed 0 0. 5 1 cm 1. 5 2 2. 5 3 3. 5 4 0 2 4 6 8 10 MHz 12 14 16 B-Mode Sub-Harm. Phase-Coded Pulse Sequences Ruhr-University Bochum 16/18 RF Engineering Institute

Experimental Results, 3. 6 MHz 1 st harmonic suppressed 0 • broadband pulses • transmit spectrum dominated by transducer characteristics • phase errors increase with frequency • excitation above resonance frequency of microbubbles 0. 5 1 cm 1. 5 2 2. 5 3 3. 5 4 0 2 4 6 8 10 MHz 12 14 16 Phase-Coded Pulse Sequences Ruhr-University Bochum 17/18 RF Engineering Institute

Conclusion and Outlook • 5 -pulse sequences – enable 0 th, 2 nd and 3 rd 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 ultraharmonic imaging (0. 5 f 0, 1. 5 f 0, 2. 5 f 0) Phase-Coded Pulse Sequences Ruhr-University Bochum 18/18 RF Engineering Institute