Modulated (or coded) excitation signals can potentially improve the quality and increase the frame rate in medical ultrasound scanners. The aim of this dissertation is to investigate systematically the applicability of modulated signals in medical ultrasound imaging and to suggest appropriate methods for coded imaging, with the goal of making better anatomic and flow images and three-dimensional images. On the first stage, it investigates techniques for doing high-resolution coded imaging with improved signal-to-noise ratio compared to conventional imaging. Subsequently it investigates how coded excitation can be used for increasing the frame rate. The work includes both simulated results using Field II, and experimental results based on measurements on phantoms as well as clinical images. Initially a mathematical foundation of signal modulation is given. Pulse compression based on matched filtering is discussed. Correlation and compression properties of coded signals are shown to depend on a single parameter of the coded signals: the time-bandwidth product. It is shown that, due to attenuation in the tissues, the matched flter output is related to the ambiguity function of the excitation signal. Although a gain in signal-to-noise ratio of about 20 dB is theoretically possible for the time-bandwidth product available in ultrasound, it is shown that the effects of transducer weighting and tissue attenuation reduce the maximum gain at 10 dB for robust compression with low sidelobes. Frequency modulation and phase modulation are considered separately and their resolution, sidelobes, expected signal-to-noise gain and performance in tissue imaging are discussed in detail. A method to achieve low compression sidelobes by reducing the ripples of the amplitude spectrum of the FM signals is described. Application of coded excitation in array imaging is evaluated through simulations in Field II. The low degree of the orthogonality among coded signals for ultrasound systems is first discussed, and the effect of mismatched filtering in the cross-correlation properties of the signals is evaluated. In linear array imaging it is found that the frame rate can be doubled without any degradation in image quality, by using two coded sequences that have a cross-correlation of at least 11 dB. Other coding schemes that can increase the frame rate by nearly 5 times with a small compromise in resolution are discussed. Coded synthetic transmit aperture imaging with only 4 emissions is shown to yield the same signal-to-noise ratio as with conventional phased-array imaging which uses 51 emissions. Further frequency-division coding can make it possible to obtain images with acceptable resolution with only two emissions. Finally, a novel coding technique which uses pulse train excitation is presented.
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Jensen, Jørgen Arendt
Technical University of Denmark, Department of Electrical Engineering, 2001