Ruhr-University Bochum RF Engineering Institute
Phase-Coded Pulse Sequences
A 5-Pulse Sequence for Harmonic A 5-Pulse Sequence for Harmonic and Sub-Harmonic Imagingand 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
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute2/18
OutlineOutline
• Introduction• 2-pulse sequence• 3-pulse sequences• 5-pulse sequence• Harmonics, speckle• Experimental results• Conclusion and outlook
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute3/18
IntroductionIntroduction
• 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
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute4/18
2-Pulse Sequence2-Pulse Sequence“Phase Inversion”, “Pulse Inversion”“Phase Inversion”, “Pulse Inversion”
• Detects even order harmonics
• Commercially available
Echo 1Echo 2Sum
Echo 1Echo 2Sum
time
time
ampl
itud
e
ampl
itud
e
line
ar s
catt
erer
nonl
inea
r sc
atte
rer
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute5/18
Multi-Pulse SequencesMulti-Pulse Sequences3 Equidistant Phases3 Equidistant Phases
• 3-pulse sequence: 0°, 120°, 240°
• Coherent summation cancellation of
1st and 2nd harmonic 0 .5 1 30210
60
240
90
270
120
300
150330180 0
0 .5 1 30210
60
240
90
270
120
300
150330180 0
0 .5 1 30210
60
240
90
270
120
300
150330180 0
1
240
120
0
1
240
120
0
1
240
120
02nd1st 3rd
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute6/18
Multi-Pulse SequencesMulti-Pulse Sequences3 Non-Equidistant Phases3 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 = 1e1 + 2e2 + 3e3
• Cancellation of 1st harmonic:1 = 1, 2 = 3 = f(2)
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute7/18
Phases and WeightsPhases and WeightsMulti-Pulse Sequences with 3 Non-Equidistant PhasesMulti-Pulse Sequences with 3 Non-Equidistant Phases
0 20 40 60 80 100 120 140 160 180-3
-2
-1
0
1
2
3
2, degrees
3rd harmonic
2nd harmonic
2 3
0°
2
3
s1
s2 s3
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute8/18
Choosing Phases / WeightsChoosing Phases / WeightsMulti-Pulse Sequences with 3 Non-Equidistant PhasesMulti-Pulse Sequences with 3 Non-Equidistant Phases
• Preferable weights: 2 = 3 1
• Efficient detection of 2nd and 3rd harmonicExamples:
2 2 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
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute9/18
Subsets in a Sequence of Subsets in a Sequence of 5 Equidistant Pulses5 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 0
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute10/18
The 0The 0thth Harmonic 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
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute11/18
Spectrum and Phase Spectrum and Phase of the 0of the 0thth Harmonic Harmonic
0 0.5 1 1.5 2
x 107
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
Hz
norm
aliz
ed a
mpl
itud
e, [
dB]
2nd harmonic0th harmonic
squared gaussian shaped pulse,1st harmonic at 7.2 MHz
0 0.5 1 1.5 2
x 107Hz
degr
ees
2nd harmonic0th harmonic
squared gaussian shaped pulse,0°, 72°, 144°, 216°, 288°
-1600
-1400
-1200
-1000
-800
-600
-400
-200
0
Magnitude Spectrum of a Squared Gaussian Shaped Pulse
Phase Spectrum of Squared Gaussian Shaped Pulses
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute12/18
SimulationSimulation
• Suppression of 1st harmonic
• Reduced speckle unprocessed echoes:SNRspeckle = 1.91after incoh. summation:SNRspeckle = 2.4
0 0.1 0.2 0.3 0.4-1
-0.5
0
0.5
1
µs
norm
aliz
ed a
mpl
itud
e
ori
gin
al ech
oes
1st h
arm
onic
sup
pre
ssed
-40
-20
0
20
40
ampl
itud
e, [
a. u
.]
0 1 2 3 4 5cm
-40
-20
0
20
40
ampl
itud
e, [
a. u
.]
lin. +non-lin.
lin.lin.
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute13/18
5-Pulse Sequence5-Pulse SequenceMeasurement: String Target 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
130
210
60
240
90
270
120
300
150
330
180 0
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute14/18
5-Pulse Sequence5-Pulse SequenceMeasurements with LevovistMeasurements with Levovist
• 5-pulse sequence, 2 cycles, 3.6 MHz and 7.2 MHz
• 7.2 MHz linear array• Tissue phantom with
cylindrical hole
String Target
Levovist
Transducer
Tissue
ROI1.1 cm x 4.2 cm
0 0.2 0.4 0.6 0.8 1
-1
-0.5
0
0.5
1
µs
norm
aliz
ed a
mplit
ude
3.6 MHz
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute15/18
Experimental ResultsExperimental Results7.2 MHz7.2 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
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute16/18
SpectrogramSpectrogram11stst harmonic suppressed harmonic suppressed
MHz
cm
0 2 4 6 8 10 12 14 16
0
0.5
1
1.5
2
2.5
3
3.5
4
B-Mode Sub-Harm.
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute17/18
Experimental Results, 3.6 MHzExperimental Results, 3.6 MHz11stst harmonic suppressed harmonic suppressed
MHz
cm
0 2 4 6 8 10 12 14 16
0
0.5
1
1.5
2
2.5
3
3.5
4
• broadband pulses• transmit spectrum
dominated by trans-ducer characteristics
• phase errors increase with frequency
• excitation above resonance frequency of microbubbles
Ruhr-University Bochum
Phase-Coded Pulse SequencesRF Engineering Institute18/18
Conclusion and OutlookConclusion 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)