Difficulty Of Decomposition Research Articles

Volume Conservation Constrained Multi-Material Reconstruction for Inconsistent Spectral CT Imaging

Spectral computed tomography (spectral CT) is a promising medical imaging technology because of its ability to provide information on material characterization and quantification. However, the difficulty of decomposition also increases due to the nonlinearity nature of the measurements and ill condition of the problem, especially in the case of geometric inconsistency, which typically leads to low image qualities. Therefore, it is a crucial issue for inconsistent spectral CT imaging to improve the accuracy of material decomposition while suppressing the noise. This paper depends on a statistical reconstruction model with different priors to propose one-step multi-material algorithms. In these approaches, the gradient sparisty-based and convolutional neural network based methods are designed for the case of the consistent numbers of material and energies. And volume conservation constraint are further developed while the two numbers are not equal. An efficient Newton descent method is adopted based on the simple surrogate function. For simulation experiments with different noise levels, the largest peak signal-to-noise ratio (PSNR) obtained by the proposed method approximately increases by 20.924 dB and 18.283 dB compared with those of other algorithms. Magnified areas of real data also further demonstrated that the proposed methods has a better ability to suppress noise. Numerical experiments verify that the proposed method efficiently reconstructed the material maps, and reduced noise compared with the state-of-the-art methods.

Direct Multi-Material Reconstruction via Iterative Proximal Adaptive Descent for Spectral CT Imaging.

Spectral computed tomography (spectral CT) is a promising medical imaging technology because of its ability to provide information on material characterization and quantification. However, with an increasing number of basis materials, the nonlinearity of measurements causes difficulty in decomposition. In addition, noise amplification and beam hardening further reduce image quality. Thus, improving the accuracy of material decomposition while suppressing noise is pivotal for spectral CT imaging. This paper proposes a one-step multi-material reconstruction model as well as an iterative proximal adaptive decent method. In this approach, a proximal step and a descent step with adaptive step size are designed under the forward-backward splitting framework. The convergence analysis of the algorithm is further discussed according to the convexity of the optimization objective function. For simulation experiments with different noise levels, the peak signal-to-noise ratio (PSNR) obtained by the proposed method increases approximately 23 dB, 14 dB, and 4 dB compared to those of other algorithms. Magnified areas of thorax data further demonstrated that the proposed method has a better ability to preserve details in tissues, bones, and lungs. Numerical experiments verify that the proposed method efficiently reconstructed the material maps, and reduced noise and beam hardening artifacts compared with the state-of-the-art methods.

Enhancement of hydrocarbon recovery from CH4-C2H6-C3H8 mixed hydrates via gas sweep

Mixed hydrates have been found in some ocean and permafrost deposits, and the dissociation of mixed hydrates by depressurization alone has been reported to be difficult and inefficient. However, most studies in the field of mixed hydrate decomposition have mainly focused on dissociation behaviors at the crystal lattice and guest molecular levels, but there has been little discussion about recovery efficiency. The dissociation of CH4-C3H8 or CH4-C2H6-C3H8 mixed hydrates in sandy sediments using depressurization followed by the gas sweep method was investigated to determine the underlying mechanism of difficulty in decomposition and to enhance hydrocarbon recovery. It was found that CH4 and C2H6 were preferentially released over C3H8 from mixed hydrates in the depressurization stage. Particularly, the dissociation of mixed hydrates tended to stop despite substantial hydrates remaining above the water freezing point, which was similar to the self-preservation effect of CH4 hydrates below the ice point. Interestingly, when gas sweep was performed, mixed hydrates continued to decompose, and more C3H8 was recovered at the beginning of the gas sweep. Compositional analysis of the dissociated gas confirmed that the gradual formation of C3H8-rich hydrate shell led to the cessation of hydrate decomposition, which hinders the diffusing hydrocarbons in the internal hydrate to be released. Furthermore, continuous gas sweep resulted in continuous and rapid decomposition of mixed hydrates, but when it was stopped, the decomposition of mixed hydrates gradually slowed down and tended to stop, and the C3H8-rich hydrate shell was re-formed. This suggested that the formation and decomposition of the C3H8-rich hydrate shell may control the decomposition rate of mixed hydrates. These results are of significance for guiding successful and efficient hydrocarbon recovery from mixed hydrates.

History traces of HCHs and DDTs by groundwater dating and their behaviours and ecological risk in northeast China

Organochlorine pesticides legacies, such as hexachlorocyclohexane (HCH) and dichlorodiphenyltrichloroethane (DDT), remained in sediments or soils due to their difficulty in decomposition, especially in the agricultural areas where pesticides were widely used historically. Different from the little disturbed depositional environment of lake, it was difficult for rivers to explore the timing of DDT and HCH inputs through dating sediment cores as records. Based on groundwater dating, this study ascertained the historic pollution of DDT and HCH in Taizi River basin. HCH and DDT residues in groundwater were consistent with the historical production and usage, which increased from the 1950s to the 1980s and declined from the 1980s to the 1990s. Moreover, the partitioning behaviours of HCHs and DDTs in surface water and suspended particulate matter were discussed. It was revealed that β-HCH and o,p’-DDT were more likely to attach to suspended particulate matter than other isomers. Furthermore, species sensitivity distribution curves were generated using 54 toxicity data records to assess the risk of HCHs and DDTs in water and suspended particulate matter. These results indicated that p,p’-DDT in surface water posed a high risk to 95% of the aquatic life in the long run.