Aperture Domain Processing and Its Applications in Ultrasound Imaging
Date Issued
2009
Date
2009
Author(s)
Wang, Shun-Li
Abstract
The purpose of this dissertation is to investigate various processing techniques for ultrasound image formation and signal processing based on aperture domain data for ultrasonic system using arrays. Conventionally, an array system utilizes the widely adopted delay-and-sum method to focus acoustic beams electrically at specific depths with arbitrary steering and shifting by the delay and weighting of each array element. This method can adjust lateral resolution and beam-shapes and therefore provides dynamic focusing throughout the scan depth. The data recorded from individual array channels prior to beam summation are referred to aperture domain data and are often discarded after beam summation due to a large data size. However, the delay-and-sum method only preserves the spatial information along the beam direction and therefore limits the clinical applications. In this thesis, two specific tasks of aperture domain data processing including vector velocity estimation and phase-aberration (i.e., focusing errors resulting from sound-velocity inhomogeneities) correction are investigated. he first topic in this dissertation is the vector flow estimation. A conventional scanner can only estimate the flow velocity parallel to the beam axis. The proposed flow estimation technique uses aperture domain data for 2D flow-velocity estimation. A time-shift profile along the array direction is constructed and approximated by a first-order polynomial to determine the axial and lateral velocity components. The efficacy of the vector velocity estimation method is verified by simulations and experiments. The results demonstrate that the accuracy of the proposed method is comparable to existing vector velocity estimation method and real-time two-dimensional velocity vector estimation is feasible.or phase-aberration correction, a sidelobe-reduction technique based on the coherence of the receive aperture domain data is tested with clinical breast data. The performance in lesion detection using B-mode ultrasound is often limited by poor contrast resolution. Experimental results demonstrate that the proposed weighting method is feasible in breast imaging and rivals the conventional correlation-based method with significant image quality improvement. n the third part of the dissertation, the coherence-based sidelobe-reduction technique is also extended to high-frame-rate adaptive imaging with a high accuracy Capon estimator to estimate the coherent energy. The high frame rate image is formed using plane-wave excitation and a synthetic transmit aperture method using only 8 firings. Significant improvement in contrast and lesion definition is demonstrated through the simulations and breast imaging experiments. The results demonstrate that these coherence-based methods are feasible to improve lesion detection in clinics since these techniques can effectively reduce sidelobe contributions without any assumption regarding the source of focusing errors. In summary, advanced imaging techniques were developed in this thesis to improve velocity and contrast resolution and thus increase diagnostic confidence in clinics. Potential extended application of these methods will also be described.
Subjects
Aperture domain data
Vector flow estimationl
Ultrasonic adaptive imaging
Coherence factor
High frame rate imaging
Type
thesis
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