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Resolution Enhancement Compression- Synthetic Aperture Focusing Techniques

Resolution Enhancement Compression- Synthetic Aperture Focusing Techniques. Student: Hans Bethe Advisor: Dr. Jose R. Sanchez Bradley University Department of Electrical Engineering. Motivation. Ultrasound Imaging is important in medical diagnosis. Figure 1: Imaging fetus [1].

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Resolution Enhancement Compression- Synthetic Aperture Focusing Techniques

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  1. Resolution Enhancement Compression-Synthetic Aperture Focusing Techniques Student: Hans Bethe Advisor: Dr. Jose R. Sanchez Bradley University Department of Electrical Engineering

  2. Motivation Ultrasound Imaging is important in medical diagnosis Figure 1: Imaging fetus [1] Figure 2: Imaging pancreas [1]

  3. 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

  4. Outline I. Ultrasound Imaging System II. Functional Requirements III. Progress

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

  6. 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

  7. Image Reconstruction System excitation Pre- amplifier Matched filter Delay Unit A Transducer Echo image A Apodization Σ

  8. Image Reconstruction System excitation Pre- amplifier Matched filter Delay Unit A Transducer Echo image A Apodization Σ

  9. Image Reconstruction System excitation Pre- amplifier Matched filter Delay Unit A Transducer Echo image A Apodization Σ

  10. Image Reconstruction System excitation Pre- amplifier Matched filter Delay Unit A Transducer Echo image A Apodization Σ

  11. Image Reconstruction System excitation Pre- amplifier Matched filter Delay Unit A Transducer Echo image A Apodization Σ

  12. Image Reconstruction System excitation Pre- amplifier Matched filter Delay Unit A Transducer Echo image A Apodization Σ

  13. 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 < 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 III. Functional Requirements

  14. 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.

  15. Figure 5: Illustration of convolution equivalence principle

  16. REC Mechanism

  17. REC Mechanism

  18. REC Mechanism

  19. REC Mechanism

  20. Figure 15: Illustration of convolution equivalence principle

  21. K0(REC) K0(CP) Figure 16: Axial resolution between CP and REC

  22. QUESTIONS ?

  23. References [1] Ultrasound images gallery http://www.ultrasound-images.com/pancreas.htm [2] http://sell.bizrice.com/selling-leads/48391/Digital-Portable-Color-Doppler-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.

  24. 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’

  25. Apodization • Process of varying signal strengths in transmission and reception across transducer • Reduces side lobes • Signal strength will become progressively weaker with increasing distance from the center • Control beam width => improve or degrade lateral resolution Center Figure 5: Illustration of apodization

  26. 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 => greater likelihood of pulses covering objects => echoes from reflectors more likely to merge => degrade lateral resolution beam axis transducer beam objects 1 2 3 Figure 6: Illustration of the effect beam width has on lateral resolution

  27. II. Theoretical Background

  28. 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

  29. The essence of SAFT is delay-and-sum (DAS) operation Transducer L6 L3 L1 L9 pulses Target Figure 8: Illustration of DAS

  30. The essence of SAFT is delay-and-sum (DAS) operation Transducer L6 L3 L1 L9 echoes pulses Target Figure 8: Illustration of DAS

  31. The essence of SAFT is delay-and-sum (DAS) operation Transducer L6 L3 L1 L9 echoes pulses Target Figure 8: Illustration of DAS

  32. 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

  33. 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

  34. Figure 9: Illustration of delay-and-sum [4]

  35. 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]

  36. To enhance image resolution by CP, increase excitation voltage => produces excessive heating => hazardous to patients => a better excitation technique is needed => 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 WHY REC?

  37. 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 => produce shorter-duration pulses => 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

  38. objects echoes Figure 13: Effect pulse duration has on axial resolution

  39. Figure 16: Comparison between CP and REC

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