Resolution Enhancement Compression- Synthetic Aperture Focusing Techniques

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Resolution Enhancement Compression-Synthetic
Aperture Focusing Techniques

• Student
• Hans Bethe
• Advisor Dr. Jose R. Sanchez
• Bradley University
• Department of Electrical Engineering

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Motivation

• Ultrasound Imaging is important in medical
  diagnosis

Figure 1 Imaging fetus 1
Figure 2 Imaging pancreas 1
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Motivation

• Ultrasound imaging involves exciting transducer
  and forming ultrasound pulses to be fired at
  internal tissue
• Synthetic Aperture Focusing Techniques (SAFT)
  beam-forming techniques capable of enhancing
  lateral resolution
• Resolution Enhancement Compression (REC) coded
  excitation (wave shaping) technique employed to
  produce excitation signal capable of enhancing
  axial resolution
• Objectives
• a/ Investigate REC and SAFT techniques through
  literature research and simulation
• b/ Combine REC and SAFT

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Outline

• I. Ultrasound Imaging System
• II. Functional Requirements
• III. Progress

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I. Ultrasound Imaging System
Excitation (REC)
Image reconstruction system
Transducer
Beam-forming (SAFT)
Figure 3 Block diagram
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Transducer

• Converts signal or energy of one form to another
• In imaging, converts electrical signal to
  ultrasound signal

Transducer
Target
Ultrasound pulses
Echoes
Figure 4 Ultrasound emission and reflection
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Image Reconstruction System
excitation
Pre- amplifier
Matched filter
A
Delay Unit
Transducer
Echo
image
A
Apodization
S
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Image Reconstruction System
excitation
Pre- amplifier
Matched filter
A
Delay Unit
Transducer
Echo
image
A
Apodization
S
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Image Reconstruction System
excitation
Pre- amplifier
Matched filter
A
Delay Unit
Transducer
Echo
image
A
Apodization
S
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Image Reconstruction System
excitation
Pre- amplifier
Matched filter
A
Delay Unit
Transducer
Echo
image
A
Apodization
S
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Image Reconstruction System
excitation
Pre- amplifier
Matched filter
A
Delay Unit
Transducer
Echo
image
A
Apodization
S
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Image Reconstruction System
excitation
Pre- amplifier
Matched filter
A
Delay Unit
Transducer
Echo
image
A
Apodization
S
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III. Functional Requirements

• A/ SAFT
• Transducer shall be a linear array comprising 128
  elements
• SAFT shall be performed through MATLAB Field II
• SAFT mode excite all elements and receive with 1
  element person emission
• Delay and sum calculations shall be performed
  through a GPGPU
• Total synthetic aperture processing time shall be
  lt 1 second
• (Adjustment total processing time shall be about
  10-20 seconds)
• Signal-to-noise ratio (SNR) of the images shall
  be at least 50 dB

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III. Functional Requirements

• B/ REC
• Actual impulse response of system (denoted as
  h1(t)) shall have a center frequency f0 of 2 MHz.
  
• System bandwidth shall be about 83.
• Sampling frequency fs shall be 400 MHz.
• Desired impulse response of imaging system
  (denoted as h2(t) ) shall have a bandwidth about
  1.5 times the bandwidth of h1(t).
• The side lobes associated with compressed pulse
  shall be reduced below 40 dB.

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Figure 5 Illustration of convolution equivalence
principle
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REC Mechanism
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REC Mechanism
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REC Mechanism
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REC Mechanism
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Figure 15 Illustration of convolution
equivalence principle
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K0(REC)
K0(CP)
Figure 16 Axial resolution between CP and REC
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QUESTIONS ?
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References
1 Ultrasound images gallery http//www.ultrasoun
d-images.com/pancreas.htm 2 http//sell.bizrice
.com/selling-leads/48391/Digital-Portable-Color-Do
ppler-Ultrasound-System.html 3 J. R. Sanchez
et al., "A Novel Coded Excitation Scheme to
Improve Spatial and Contrast Resolution of
Quantitative Ultrasound Imaging" IEEE Trans
Ultrasonics, Ferroelectrics, and Frequency
Control, vol. 56, no. 10, pp. 2111-2123, October
2009. 4 S. I. Nikolov, Synthetic Aperture
Tissue and Flow Ultrasound Imaging 5 T.
Misaridis and J. A. Jensen, Use of Modulated
Excitation Signals in Medical Ultrasound IEEE
Trans. Ultrason. Ferroelectr. Freq. Control, vol.
52, no. 2, February 2005. 6 M. L. Oelze,
Bandwidth and Resolution Enhancement Through
Pulse Compression, IEEE Trans. Ultrasonics,
Ferroelectrics, and Frequency Control, vol. 54,
no. 4, April 2007.
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References
7 J. R. Sanchez and M. L. Oelze, An Ultrasonic
Imaging Speckle-Suppression and
Contrast-Enhancement Technique by Means of
Frequency Compounding and Coded Excitation, IEEE
Trans. Ultrasonics, Ferroelectrics, and Frequency
Control, vol. 56, no. 7, Julyl 2009. 8 M.
Oelze, Improved Axial Resolution Using
Pre-enhanced Chirps and Pulse Compression, 2006
IEEE Ultrasonics Symposium 9 Tadeusz
Stepinski, An Implementation of Synthetic
Aperture Focusing Technique in Frequency Domain,
IEEE transactions on Ultrasonics, Ferroelectrics,
and Frequency control, vol. 54, no. 7, July
2007 10 J. A. Zagzebski, Essentials of
Ultrasound Physics
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Apodization

1. Process of varying signal strengths in
   transmission and reception across transducer
2. Reduces side lobes
3. Signal strength will become progressively weaker
   with increasing distance from the center
4. Control beam width gt improve or degrade lateral
   resolution

Center
Figure 5 Illustration of apodization
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Beam width and lateral resolution

• Lateral resolution capability of imaging
  system to distinguish 2 closely spaced objects
  positioned perpendicular to the axis of
  ultrasound beam
• Larger beam width gt greater likelihood of
  pulses covering objects gt echoes from reflectors
  more likely to merge gt degrade lateral resolution

beam axis
transducer
beam
objects
1
2
3
Figure 6 Illustration of the effect beam width
has on lateral resolution
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II. Theoretical Background
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SAFT

• In synthetic aperture focusing techniques
  (SAFT), a single transducer element is used both,
  in transmit and receive modes
• Each element in the transducer emits pulses one
  by one

1 2 3
Pulse
Echo
target
Figure 7 Illustration of SAF
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The essence of SAFT is delay-and-sum (DAS)
operation
Transducer
L6
L3
L1
L9
pulses
Target
Figure 8 Illustration of DAS
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The essence of SAFT is delay-and-sum (DAS)
operation
Transducer
L6
L3
L1
L9
echoes
pulses
Target
Figure 8 Illustration of DAS
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The essence of SAFT is delay-and-sum (DAS)
operation
Transducer
L6
L3
L1
L9
echoes
pulses
Target
Figure 8 Illustration of DAS
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The essence of SAFT is delay-and-sum (DAS)
operation
Delay unit
Transducer
Transducer
L6
L3
L1
L9
echoes
pulses
Target
Figure 8 Illustration of DAS
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The essence of SAFT is delay-and-sum (DAS)
operation
Delay unit
Sum
Transducer
Transducer
L6
L3
L1
L9
echoes
pulses
Target
Figure 8 Illustration of DAS
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Figure 9 Illustration of delay-and-sum 4
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REC

• Before REC, conventional pulsing (CP) was used
• CP proved ineffective in term of image resolution

Figure 10 Resolution Comparison 3
Figure 11 Background-target separation 3
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WHY REC?

• To enhance image resolution by CP, increase
  excitation voltage gt produces excessive heating
  gt hazardous to patients gt a better excitation
  technique is needed gt gave rise to the
  investigation of REC
• Advantages of REC
• a/ Improves axial resolution without increasing
  acoustic peak power
• b/ Offers the capability to obtain the optimal FM
  chirp to increase the bandwidth of imaging system

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• REC a coded excitation technique (wave shaping)
• Employs Convolution Equivalence Principle to
  generate pre-enhanced chirp excitation signal
• Excitation by pre-enhanced chirp increases
  bandwidth of imaging system gt produce
  shorter-duration pulses gt increases axial
  resolution
• (axial resolution ability of imaging system to
  distinguish objects closely spaced along
• the axis of the beam)

objects
transducer
beam
beam axis
Figure 12 Illustration of axial resolution
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objects
echoes
Figure 13 Effect pulse duration has on axial
resolution
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Figure 16 Comparison between CP and REC