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Title:Applying synthetic aperture, coded excitation, and tissue harmonic imaging techniques to allow ultrasound imaging with a virtual source
Author(s):Umeki, Robyn T.
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):ultrasound imaging
synthetic aperture
coded excitation
tissue harmonic imaging
Abstract:A focused transducer will concentrate its signal energy at its focus, producing a clear image of objects at that depth. However, spatial resolution and signal strength deteriorate away from the transducer’s focus. Therefore, having only one focal length places a great restriction on ultrasound imaging, especially with a single-element transducer. To extend the region of focus, three techniques were examined. The first was the synthetic aperture focusing technique (SAFT), a well-studied method that allows focus at every distance instead of just one distance. Using this method, lateral resolution is restored and signal-to-noise ratio (SNR) is increased due to the compounding of several scan lines. SAFT was combined with a virtual source technique to further extend the region of focus. The second technique examined was coded excitation, a means of improving SNR while maintaining the axial resolution. The third technique was tissue harmonic imaging, which produces a narrower beamwidth and reduced sidelobes, and therefore can improve spatial resolution and contrast of images. Each technique is individually known to improve ultrasound image qualities, with its own strengths and drawbacks. This study combines all three. In the final stage of this study using the virtual source technique, harmonic imaging was implemented using coded excitation beyond the transducer focus. Because of the low signal strength received under the synthetic aperture technique, harmonic imaging has never been attempted using SAFT and a virtual source. There has been no previous research to determine whether the techniques will build on each other or detract from each other. This study included simulations and experiments with the techniques applied to a single scatterer, as well as experiments with a tissue-mimicking phantom. Implementing coded excitation with SAFT led to a final SNR higher than was observed when applying SAFT alone. Although coding helped to increase SNR, the sidelobes became much more visible. Additionally, it was found that synthetic aperture was not a linear process and could cause distortion when the transmitted pulse is very long. This problem was eliminated when time compression was applied before SAFT. When applying tissue harmonic imaging, the sidelobes were less prominent than before. However, the speckle increased and the SNR decreased, likely due to the decreased signal strength of the transducer at the harmonic frequencies. iii This study has shown that synthetic aperture with a virtual source, coded excitation, and tissue harmonic imaging can be combined to image beyond the focal length of a transducer. However, there was no research on the maximum depth at which the technique can be practical. Additionally, this study did not test the effectiveness of the techniques when applied to real tissue.
Issue Date:2011-08-25
Rights Information:Copyright 2011 Robyn T. Umeki
Date Available in IDEALS:2011-08-25
Date Deposited:2011-08

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