A 5-Pulse Sequence for Harmonic and Sub-Harmonic Imaging W. G. Wilkening1, J. Lazenby2, H. Ermert1 1Department of Electrical Engineering, Ruhr-University, Bochum 2Siemens Medical Systems, Ultrasound Group, P.O. Box 7002, Issaquah WA 98027, USA

Outline Introduction 2-pulse sequence 3-pulse sequences 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

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

Multi-Pulse Sequences 3 Equidistant Phases Coherent summation  cancellation of 1st and 2nd harmonic 1 240 120 2nd 1st 3rd

Multi-Pulse Sequences 3 Non-Equidistant Phases Non-equidistant phase + weighted summation of echo signals  cancellation of the 1st harmonic Transmit pulses: s1, s2, s3 phases: 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)

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

Choosing Phases / Weights Multi-Pulse Sequences with 3 Non-Equidistant Phases Preferable weights: a2 = a3  1 Efficient detection of 2nd and 3rd harmonic Examples: 2 a2 2nd harm. 3rd harm. 60° -1 2 0.75 120° 1 72° –1.618 3.6 0.9 144° 0.618 1.38 0.345

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” 30 210 60 240 90 270 120 300 150 330 180

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

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

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

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 30 210 60 240 90 270 120 300 150 330 180 1

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

Experimental Results 7.2 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

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

Experimental Results, 3.6 MHz 1st harmonic suppressed cm 2 4 6 8 10 12 14 16 0.5 1 1.5 2.5 3 3.5 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)