Ultrasound computer tomography using synthetic aperture focusing technique is sensitive to phase aberration errors. We propose a signal preprocessing method for coherent imaging, which can take the system specific phase aberration into account. The method detects local maxima in the envelope of a matched filtered signal and convolutes the result with a truncated difference of sinc-functions, where the main lobe is scaled to represent the phase aberration. For our system we could show that reflectivity images reconstructed such pre-processed signals are approximately within a factor two of the ideal resolution, while increasing the contrast by a factor of 30.
At Forschungszentrum Karlsruhe an ultrasound computer tomograph is developed for breast imaging in 3D. The aim of this project is the support of early diagnosis of breast cancer. Our current setup consists of a cylindrical tank assembled with 384 emitter and 1536 receiver elements which are grouped to 48 rectangular transducer array systems (TAS). It has been shown in previous work that the cylindrical aperture has major drawbacks in the vertical resolution. Given a new square TAS pattern with 4 emitters and 9 receivers, a new aperture for 160 TAS is derived which optimizes the image quality regarding three criteria. The optimization criteria regard the resolution of this 3D imaging system, the image contrast and utilization of the available ultrasound energy.An optimization with equal weighting of these criteria resulted in an 40% improvement of the resolution regarding the size and isotropy compared to the current cylindrical aperture. The resolution could be improved by 40% regarding the size and isotropy. Yet the contrast is degraded by 13%. Further work will be to find a reasonable weighting of the the three criteria. It could be shown that the presented method for aperture optimization makes this high-dimensional optimization problem feasible. The most important physical parameters like tissue attenuation, beam profile and scatterer profile, could be included to increase the authenticity of the results.
Abstract-In seismic, radar, and sonar imaging the exact determination of the reflectivity distribution is usually intractable so that approximations have to be applied. A method called synthetic aperture focusing technique (SAFT) is typically used for such applications as it provides a fast and simple method to reconstruct (3D) images. Nevertheless, this approach has several drawbacks such as causing image artifacts as well as offering no possibility to model system-specific uncertainties.In this paper, a statistical approach is derived, which models the region of interest as a probability density function (PDF) representing spatial reflectivity occurrences. To process the nonlinear measurements, the exact PDF is approximated by well-placed Extended Kalman Filters allowing for efficient and robust data processing.The performance of the proposed method is demonstrated for a 3D ultrasound computer tomograph and comparisons are carried out with the SAFT image reconstruction.
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