Resolution Enhancement Compression Synthetic Aperture Focusing Techniques Student

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

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

Motivation n Ultrasound imaging involves exciting transducer and forming ultrasound pulses to be fired at internal tissue n Synthetic Aperture Focusing Techniques (SAFT): beam-forming techniques capable of enhancing lateral resolution n 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 n 3

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

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

Transducer Converts signal or energy of one form to another n In imaging, converts electrical signal to ultrasound signal n Transducer Target Ultrasound pulses Echoes Figure 4: Ultrasound emission and reflection 6

Image Reconstruction System excitation Transducer A Echo Preamplifier Apodization Matched filter Σ Delay Unit A image 7

Image Reconstruction System excitation Transducer A Echo Preamplifier Apodization Matched filter Σ Delay Unit A image 8

Image Reconstruction System excitation Transducer A Echo Preamplifier Apodization Matched filter Σ Delay Unit A image 9

Image Reconstruction System excitation Transducer A Echo Preamplifier Apodization Matched filter Σ Delay Unit A image 10

Image Reconstruction System excitation Transducer A Echo Preamplifier Apodization Matched filter Σ Delay Unit A image 11

Image Reconstruction System excitation Transducer A Echo Preamplifier Apodization Matched filter Σ Delay Unit A image 12

III. Functional Requirements A/ SAFT Transducer shall be a linear array comprising 128 elements n SAFT shall be performed through MATLAB Field II n SAFT mode: excite all elements and receive with 1 element person emission n Delay and sum calculations shall be performed through a GPGPU n Total synthetic aperture processing time shall be < 1 second (Adjustment: total processing time shall be about 10 -20 seconds) n Signal-to-noise ratio (SNR) of the images shall be at least 50 d. B n 13

III. Functional Requirements B/ REC n n n Actual impulse response of system (denoted as h 1(t)) shall have a center frequency f 0 of 2 MHz. System bandwidth shall be about 83%. Sampling frequency fs shall be 400 MHz. Desired impulse response of imaging system (denoted as h 2(t) ) shall have a bandwidth about 1. 5 times the bandwidth of h 1(t). The side lobes associated with compressed pulse shall be reduced below 40 d. B. 14

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Figure 5: Illustration of convolution equivalence principle 17

REC Mechanism 18

REC Mechanism 19

REC Mechanism 20

REC Mechanism 21

Figure 15: Illustration of convolution equivalence principle 22

K 0(REC) K 0(CP) Figure 16: Axial resolution between CP and REC 23

QUESTIONS ? 24

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. 25

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’ 26

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 Center 4. Control beam width => improve or degrade lateral resolution Figure 5: Illustration of apodization 27

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 axis transducer • Larger beam width => greater likelihood of pulses covering objects => echoes from reflectors more likely to merge => degrade lateral resolution beam objects 1 2 3 Figure 6: Illustration of the effect beam width has on lateral resolution 28

II. Theoretical Background 29

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 30

The essence of SAFT is delay-and-sum (DAS) operation Transducer L 1 L 3 L 6 L 9 pulses Target Figure 8: Illustration of DAS 31

The essence of SAFT is delay-and-sum (DAS) operation Transducer L 1 L 3 L 6 L 9 echoes pulses Target Figure 8: Illustration of DAS 32

The essence of SAFT is delay-and-sum (DAS) operation Transducer L 1 L 3 L 6 L 9 echoes pulses Target Figure 8: Illustration of DAS 33

The essence of SAFT is delay-and-sum (DAS) operation Delay unit Transducer L 1 L 3 L 6 L 9 echoes pulses Target Figure 8: Illustration of DAS 34

The essence of SAFT is delay-and-sum (DAS) operation Delay unit Sum Transducer L 1 L 3 L 6 L 9 echoes pulses Target Figure 8: Illustration of DAS 35

Figure 9: Illustration of delay-and-sum [4] 36

REC Before REC, conventional pulsing (CP) was used n CP proved ineffective in term of image resolution n Figure 10: Resolution Comparison [3] Figure 11: Background-target separation [3] 37

WHY REC? n 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 n 38

REC: a coded excitation technique (wave shaping) n Employs Convolution Equivalence Principle to generate pre-enhanced chirp excitation signal n 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) n objects transducer beam axis Figure 12: Illustration of axial resolution 39

echoes objects Figure 13: Effect pulse duration has on axial resolution 40

Figure 16: Comparison between CP and REC 41