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

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
2-Pulse SequencePhase Inversion, Pulse
Inversion

• Detects even order harmonics
• Commercially available

5
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, s3 phases ?1 0, ?2
  ?3 (symmetric)
• Echoes e1, e2, e3
• Weighted sum e a1e1 a2e2 a3e3
• Cancellation of 1st harmonica1 1, a2 a3
  f(?2)

7
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

?2 a2 2nd harm. 3rd harm.
60 -1 2 0.75
120 1 0 0.75
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

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 gt 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
12
Simulation
original echoes

• Suppression of 1st harmonic
• Reduced speckle ?unprocessed echoesSNRspeckle
  1.91after incoh. summationSNRspeckle 2.4

1st harmonicsuppressed
13
5-Pulse SequenceMeasurement 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

14
5-Pulse SequenceMeasurements with Levovist
Transducer

• 5-pulse sequence, 2 cycles, 3.6 MHz and 7.2 MHz
• 7.2 MHz linear array
• Tissue phantom with cylindrical hole

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
Spectrogram1st harmonic suppressed
B-Mode
Sub-Harm.
17
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)