基于DCPAN的低剂量能谱CT图像去噪方法

能谱式计算机断层扫描(CT)成像技术具备良好的能量分辨率,能够精确地鉴别人体组织成分,从而为后续诊断提供更准确的检测信息.随着辐射剂量的降低,能谱CT图像中噪声水平显著提高,对成像质量产生严重影响,进而降低了组织成分的解析精度.基于卷积神经网络(CNN)的去噪模型虽然可以显著降低图像中的噪声含量,但随着卷积层数的增加,深层神经网络通常会丢失高频信息.为了解决这一问题,并实现在低剂量条件下重建出高质量能谱CT图像,本文提出了一种结合通道注意力机制(CA)和持续自注意力机制(PSA)的密集连接持续注意力网络(DCPAN).两种注意力机制分别建立特征图像在通道和全局维度的联系以提高网络对图像高频分量的敏感程度,进而抑制高频细节信息的丢失.该模型所采用的密集连接结构通过特征复用的方式可以在前馈传播中保留高频信息,使用复合损失函数来监督网络的训练可以使该模型对边缘特征和组织细节信息更加敏感.实验结果表明,经该模型处理的腹部切片CT图像峰值信噪比、结构相似性指数和特征相似性指数分别达到了38.25 dB、0.9937和0.9732以上.相比于目前先进的CT噪声去除方法,该方法具有更好的噪声抑制能力,处理得到的重建图像组织结构清晰、噪声含量更低,为后续诊断和其他处理工作提供更精确的检测信息.

Vulnerability of Water Resources and Its Spatial Heterogeneity in Haihe River Basin, China

To manage water resources effectively, a multiscale assessment of the vulnerability of water resources on the basis of political boundaries and watersheds is necessary. This study addressed issues on the vulnerability of water resources and provided a multiscale comparison of spatial heterogeneity under a climate change background. Using improved quantitative evaluation methods of vulnerabil- ity, the Theil index and the Shannon-Weaver index, we evaluated the vulnerability of water resources and its spatial heterogeneity in the Haihe River Basin in four scales, namely, second-class water resource regions （Class II WRRs）, third-class water resource regions （Class III WRRs）, Province-Class II WRRs, and Province-Class III WRRs. Results show that vulnerability enhances from the north to south in the different scales, and shows obvious spatial heterogeneity instead of moving toward convergence in multiscale assessment results. Among the Class II WRRs, the Tuhai-Majia River is the most vulnerable area, and the vulnerability of the Luanhe River is lower than that of the north of the Haihe River Basin, which in turn is lower than that of the south of the Haihe River Basin. In the scales of Class III WRRs and Province-Class III WRRs, the vulnerability shows obvious spatial heterogeneity and diversity measured by the Theil index and the Shannon-Weaver index. Multiscale vulnerability assessment results based on political boundaries and the watersheds of the Haihe River Basin innovatively provided in this paper are important and useful to characterize the real spatial pattern of the vulnerability of water resources and improve water resource management.

A brief review on SWAT applications in the Great Lakes Watersheds

Soil and Water Assessment Tool(SWAT)is an efficient model to simulate hydrology and water quality in large watersheds. During the past decades, SWAT has been used as a decision support tool to evaluate environmental effects of land usage change, BMPs, and hydraulic structures in Great Lakes watersheds. In addition, it has been increasingly used to predict water resources under different climate change scenarios. This paper briefly reviewed SWAT applications in various watersheds draining into the Great Lakes and intended to provide readers with insights regarding water quality issues in the Great Lakes concerned by researchers and capability of SWAT in dealing with those problems. Future development of SWAT with respect to dealing with cold region climate and vegetation conditions was also discussed.

基于HAMEN的稀疏投影能谱CT重建方法

能谱式计算机断层扫描(CT)可以提供不同能量下的衰减信息,有利于物质分解和组织分辨。稀疏投影可以有效降低辐射剂量,但会导致重建后的CT图像出现严重的伪影和噪声。基于卷积神经网络的深度学习重建方法虽在一定程度上改善了图像质量,却存在组织细节特征丢失严重等问题。提出一个基于混合注意力与多尺度特征融合相结合的边缘增强型网络(HAMEN)的能谱CT重建方法。首先利用边缘增强模块提取边缘特征并将其叠加到图像上,用于扩充输入图像信息;然后采用混合注意力模块分别生成通道注意力图和空间注意力图,以细化输入特征;并在网络的编码器处引入多尺度特征融合机制,增加跳跃连接以减少卷积层堆叠导致的特征丢失。实验结果表明,所提重建方法得到的CT图像的峰值信噪比可达37.64 dB,结构相似性指数达0.9935。此方法可在抑制稀疏投影导致的伪影和噪声的同时最大程度地保留组织细节信息,为后续的诊断等工作提供高质量图像。

Seasonal Variations of Hydrochemical Characteristics of Groundwater in Changping Plain, Beijing

Changping Plain, located in the northwest of Beijing, has become an important groundwater recharge area for the Beijing Plain and an important source for the urban water supply. In this study, groundwater samples were collected during the dry and wet seasons in 2015 from 24 monitoring wells distributed in Changping Plain. A Piper-Tri-linear diagram, a Schoeller diagram, a Gibbs diagram, and the isotope technique were used to investigate the temporal and spatial variations in the concentrations of groundwater hydrochemicals and the sources of groundwater recharge. The results indicated： 1） seasonal variations in the concentrations of HCO3^–, Ca^2＋, and Na^＋ were significant; the spatial variations of these ions were more dramatic in the dry season than in the wet season due to the dilution effect of precipitation; 2） Most groundwater samples had a HCO3-Ca-Mg based hydrochemical type and a few had a HCO3-Na-K based hydrochemical type; the hydrochemical type tended to evolve from HCO3-Ca-Mg based to HCO3-Na-K based in some monitoring wells that showed distinct seasonal variation; 3） the groundwater in the study area originated mainly from atmospheric precipitation, and it is affected by evaporation and concentration processes.

Zonal differences of runoff and sediment reduction effects for typical management small watersheds in China

In this particular study,99 typical managed small watersheds which representing five water erosion areas in China were selected to study zonality of Runoff Reduction Efficiency(RRE)and of Sediment Reduction Efficiency(SRE).The RRE is the ratio of Effect of Runoff Reduction(ERR)by soil and water conservation measure over management degree in a watershed.And The SRE is the ratio of Effect of Sediment Reduction(ESR)by soil and water conservation measure over management degree in a watershed.First of ah,statistical analysis was applied to test the zonal effects of RRE and SRE between different water erosion regions.The results showed that the mean RRE values in northern regions were significantly greater than those of southern regions;and the mean SRE values in northern regions were significantly greater than those in southern regions.Next,the variation of RRE with runoff depth(H)was studied in direction of both latitude and longitude across regions influenced by East Asian Monsoon.Meanwhile,the variation of SRE with specific sediment yield(Y)was studied in direction of both latitude and longitude across regions influenced by East Asian Monsoon.The results showed that RRE had the inverse variation trend as H in both latitude and longitude direction and SRE had the same variation trend as Y in both latitude and longitude direction.Furthermore,the variation of unit management area Runoff Reduction Rate(RRR)with H and RRE was studied in direction of both latitude and longitude.And the variation of unit management area Sediment Reduction Rate(SRR)with Y and SRE was studied in direction of both latitude and longitude.It was found that RRR had the similar variation trend as H in latitude direction and there was critical point around 37°N greater than which RRR began to be equal to H or even larger;RRR had the similar variation trend as H in longitude direction and there was a critical point around 109°E lees than which RRR began to equal to or greater than H;SRR had the similar variation trend as Y in latitude direction and there was critical point around 36°N greater than which SRR began to be equal to Y or even larger;SRR had the similar variation trend as Y in longitude direction and there was a critical point around 106°E lees than which SRR began to equal to or greater than Y.The zonality of RRE,RRR,SRE and SRR was determined by the combined influence of climate variation and special landform in regions controlled by East Asian Monsoon in China.