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Title:Deep structure ultrasound imaging using fundamental and third harmonic coded excitation techniques
Author(s):Ridgway, William
Advisor(s):Oelze, Michael L.
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):coded excitation
Quantitative Ultrasound (QUS)
Golay codes
third harmonic
Abstract:A novel coded excitation method, combining Golay coding with fundamental and third harmonic 1, 3 chirps with resolution enhancement compression (REC) and frequency compounding, increases the axial resolution, lateral resolution, penetration depth and the contrast-to-noise ratio (CNR) for an ultrasonic imaging system. The thickness mode of piezoelectric transducers produces a signal that is resonant at a fundamental frequency and odd harmonics. This behavior is exploited using coded excitation techniques to increase the usable bandwidth of the source and create compounded images of different bands centered at the first and third harmonics. The images are already registered using a single transducer and the frame rate is not decreased because a single transmit per line is used, rather than transmit at each frequency. With 1, 3 REC compounding, several advantages were demonstrated: 1) the REC technique increased both the fundamental and third harmonic bandwidth which boosted axial resolution, 2) the third harmonic improved lateral resolution of ultrasonic images, and 3) frequency compounding of the fundamental and third harmonic signals increased CNR by averaging speckle patterns from uncorrelated frequency bands. Combining Golay codes with 1, 3 REC compounding improved the signal-to-noise ratio (SNR) before Wiener filtering the REC chirp. The improved SNR forced the Wiener filter to act more like an inverse filter during compression and resulted in improved axial resolution. To test improvement, four experiments were conducted using conventional pulsing, 1, 3 chirp compounding, 1, 3 REC compounding, and Golay codes with a 1, 3 REC compounding chip. The first experiment tested the ability to image small differences in positive and negative contrast targets. Golay coding with 1, 3 REC compounding improved the CNR of all positive and negative lesions that were imaged with notable improvements in the -3 dB target where the CNR was improved by a factor of three over conventional pulsing. The second experiment tested the ability to detect small anechoic targets (cysts) deep within a tissue-mimicking phantom. Golay coding with 1, 3 REC compounding was able to detect 3 mm and 2 mm diameter anechoic targets deep within the tissue-mimicking phantom while chirp 1, 3 compounding and conventional pulsing could only detect 4 mm diameter and larger anechoic targets deep within the phantom. The third experiment tested the ability to detect and resolve wire targets deep in a tissue-mimicking phantom. To quantify the axial resolution the full width half max (FWHM) criteria of the echo envelope and the modulation transfer function (MTF) were used. With Golay coding and 1, 3 REC, the axial resolutions of the fundamental and third harmonic components were calculated to be 0.91 mm and 0.7 mm, respectively. These values for axial resolution were 0.6 and 0.46 times the values obtained with conventional pulsing using the same transducer. Using the MTF for lateral resolution wavelength calculation, the lateral resolution of the third harmonic of the REC and linear chirp was estimated to be about 0.42 and 0.31 times the values obtained with their fundamental frequency counterparts using conventional pulsing. The fourth experiment tested 1, 3 chirp excitation to estimate scatterer diameters within a well characterized phantom. 1, 3 chirp compounding reduced the standard deviation of estimated scatterer diameters (ESD) from 39.1 with 2.25 MHz conventional pulsing to 8.0 with 2.25 MHz 1, 3 chirp compounding. The encouraging results from the four experiments demonstrate the capability of Golay coding with 1, 3 REC compounding to improve the visibility of low contrast targets, spatial resolution for deep tissue imaging, and estimation of scatterer diameters.
Issue Date:2012-05-22
Genre:Dissertation / Thesis
Rights Information:Copyright 2012 William Ridgway
Date Available in IDEALS:2012-05-22
Date Deposited:2012-05

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