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

Ruhr-University Bochum RF Engineering InstitutePhase-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 BochumPhase-Coded Pulse Sequences

RF Engineering Institute2/18

OutlineOutline

• Introduction• 2-pulse sequence• 3-pulse sequences• 5-pulse sequence• Harmonics, speckle• Experimental results• Conclusion and outlook



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



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

itude

am

plitu

de

linea

r sca

ttere

rno

nlin

ear s

catte

rer



Multi-Pulse SequencesMulti-Pulse Sequences3 Equidistant Phases3 Equidistant Phases

• 3-pulse sequence: 0°, 120°, 240°

• Coherent summation cancellation of

1st and 2nd harmonic 1

240

120

0

1

240

120

0

1

240

120

02nd1st 3rd



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, s3phases: 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)



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°

23

s1

s2 s3



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 0.75120° 1 0 0.7572° –1.618 3.6 0.9144° 0.618 1.38 0.345



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



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



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

itude

, [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



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

itude

orig

inal

ech

oes

1st h

arm

onic

supp

ress

ed

-40

-20

0

20

40

ampl

itude

, [a.

u.]

0 1 2 3 4 5cm

-40

-20

0

20

40

ampl

itude

, [a.

u.]

lin. +non-lin.

lin.lin.



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



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 TargetLevovist

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

alize

d am

plitu

de 3.6 MHz



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



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.



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



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)

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

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