A stable imaging functional for anisotropic periodic media in electromagnetic inverse scattering

The paper is concerned with the inverse scattering problem for Maxwell's equations in three dimensional anisotropic periodic media. We study a new imaging functional for fast and stable reconstruction of the shape of anisotropic periodic scatterers from boundary measurements of the scattered field for a number of incident fields. This imaging functional is simple to implement and very robust against noise in the data. Its implementation is non-iterative, computationally cheap, and does not involve solving any ill-posed problems. The resolution and stability analysis of the imaging functional is investigated. Our numerical study shows that this imaging functional is more stable than that of the factorization method and more efficient than that of the orthogonality sampling method in reconstructing periodic scatterers.Comment: 23 page

Low-Dose CT Image Denoising using Image Decomposition and Sparse Representation

X-ray computed tomography (CT) is now a widely used imaging modality for numerous medical purposes. The risk of high X-ray radiation may induce genetic, cancerous and other diseases, demanding the development of new image processing methods that are able to enhance the quality of low-dose CT images. However, lowering the radiation dose increases the noise in acquired images and hence affects important diagnostic information. This paper contributes an efficient denoising method for low-dose CT images. A noisy image is decomposed into three component images of low, medium and high frequency bands; noise is mainly presented in the medium and high component images. Then, by exploiting the fact that a small image patch of the noisy image can be approximated by a linear combination of several elements in a given dictionary of noise-free image patches generated from noise-free images taken at nearly the same position with the noisy image, noise in these medium and high component images are effectively eliminated.Specifically, we give new solutions for image decomposition to easily control the filter parameters, for dictionary construction to improve the effectiveness and reduce the running-time. Instead of using a large dataset of patches, only a structured small part of patches extracted from the raw data is used to form a dictionary, to be used in sparse coding. In addition, we illustrate the effectiveness of the proposed method in preserving image details which are subtle but clinically important. Experimental results conducted on both synthetic and real noise data demonstrate that the proposed method is competitive with the state-of-the-art methods

Information dissemination and corporate bankruptcies

I examine informed trading around Chapter 11 corporate bankruptcy filings. Using unique bankruptcy data and improved measures of high-frequency posterior probabilities of informed trading, I document a substantial increase in informed selling several days before bankruptcy announcements. This pre-announcement informed selling attenuates subsequent announcement returns, suggesting that part of the private information embedded in informed selling was already incorporated into stock prices before the announcement. This attenuation effect of informed selling on stock market reactions is weaker for firms that receive more media coverage or adverse news sentiment. I further show that the informed trading documented is most likely driven by private information and that post-announcement informed trading can predict subsequent bankruptcy outcomes. I also investigate the liquidity dynamics of unsecured creditor stocks around their debtors’ Chapter 11 bankruptcy filings. Using matched pair fixed effect panel regressions, I find that creditors experience a short-term reduction in stock liquidity after their debtors declare bankruptcy. This is evidenced by an increase in the pairwise differences in the relative effective spread, realised spread, and lambda as well as the drop in the bid depth differential between creditors and the matched firms. This short-term liquidity reduction effect is much stronger for creditors with high credit exposure to bankrupt debtors. In the longer term, debtors’ bankruptcy announcements do not affect spread measures, but increase market depth measures of creditor stocks.Thesis (Ph.D.) -- University of Adelaide, Business School, 202