1. 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

2. Outline Introduction 2-pulse sequence 3-pulse sequences
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. 2-Pulse Sequence “Phase Inversion”, “Pulse Inversion”
Detects even order harmonics
Commercially available
Echo 1 Echo 2 Sum
time 
amplitude 
linear scatterer
nonlinear scatterer

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

6. 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)

7. Phases and Weights Multi-Pulse Sequences with 3 Non-Equidistant Phases20 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

8. 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

9. 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

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. 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

12. 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

13. 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

14. 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

15. 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

16. 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.

17. Experimental Results, 3.6 MHz 1st harmonic suppressedcm 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

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)