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121.
Haze degrades visual information of remotely sensed images. Therefore, haze removal is a demanding and significant task for visual multispectral information improvement. The existing haze removal techniques utilize different restrictions and before restoring hazy images in an efficient manner. The review of existing haze removal methods demonstrates that the haze-free images suffer from colour distortion and halo artefacts problems. To solve these issues, an improved restoration model based dark channel prior is proposed in this paper. The proposed technique has redefined transmission map, with the aim to reduce the colour distortion problem. The modified joint trilateral filter is also utilized to improve the coarse estimated atmospheric veil. The experimental results reveal that the proposed approach provides visually significant haze-free images and also preserves the significant detail. 相似文献
122.
Maruti Mudunuru 《先进材料力学与结构力学》2017,24(7):556-590
This article concerns mesh restrictions that are needed to satisfy several important mathematical properties—maximum principles, comparison principles, and the nonnegative constraint—for a general linear second-order elliptic partial differential equation. We critically review some recent developments in the field of discrete maximum principles, derive new results, and discuss some possible future research directions in this area. In particular, we derive restrictions for a three-node triangular (T3) element and a four-node quadrilateral (Q4) element to satisfy comparison principles, maximum principles, and the nonnegative constraint under the standard single-field Galerkin formulation. Analysis is restricted to uniformly elliptic linear differential operators in divergence form with Dirichlet boundary conditions specified on the entire boundary of the domain. Various versions of maximum principles and comparison principles are discussed in both continuous and discrete settings. In the literature, it is well-known that an acute-angled triangle is sufficient to satisfy the discrete weak maximum principle for pure isotropic diffusion. Herein, we show that this condition can be either too restrictive or not sufficient to satisfy various discrete principles when one considers anisotropic diffusivity, advection velocity field, or linear reaction coefficient. Subsequently, we derive appropriate restrictions on the mesh for simplicial (e.g., T3 element) and nonsimplicial (e.g., Q4 element) elements. Based on these conditions, an iterative algorithm is developed to construct simplicial meshes that preserve discrete maximum principles using existing open source mesh generators. Various numerical examples based on different types of triangulations are presented to show the pros and cons of placing restrictions on a computational mesh. We also quantify local and global mass conservation errors using representative numerical examples and illustrate the performance of metric. 相似文献
123.
124.
Unsupervised tissue classification of brain MR images for voxel‐based morphometry analysis
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Luca Agnello Albert Comelli Edoardo Ardizzone Salvatore Vitabile 《International journal of imaging systems and technology》2016,26(2):136-150
In this article, a fully unsupervised method for brain tissue segmentation of T1‐weighted MRI 3D volumes is proposed. The method uses the Fuzzy C‐Means (FCM) clustering algorithm and a Fully Connected Cascade Neural Network (FCCNN) classifier. Traditional manual segmentation methods require neuro‐radiological expertise and significant time while semiautomatic methods depend on parameter's setup and trial‐and‐error methodologies that may lead to high intraoperator/interoperator variability. The proposed method selects the most useful MRI data according to FCM fuzziness values and trains the FCCNN to learn to classify brain’ tissues into White Matter, Gray Matter, and Cerebro‐Spinal Fluid in an unsupervised way. The method has been tested on the IBSR dataset, on the BrainWeb Phantom, on the BrainWeb SBD dataset, and on the real dataset “University of Palermo Policlinico Hospital” (UPPH), Italy. Sensitivity, Specificity, Dice and F‐Factor scores have been calculated on the IBSR and BrainWeb datasets segmented using the proposed method, the FCM algorithm, and two state‐of‐the‐art brain segmentation software packages (FSL and SPM) to prove the effectiveness of the proposed approach. A qualitative evaluation involving a group of five expert radiologists has been performed segmenting the real dataset using the proposed approach and the comparison algorithms. Finally, a usability analysis on the proposed method and reference methods has been carried out from the same group of expert radiologists. The achieved results show that the segmentations of the proposed method are comparable or better than the reference methods with a better usability and degree of acceptance. 相似文献
125.
126.
Jürgen R. Reichenbach Marco Essig E. Mark Haacke Benjamin C. Lee Christian Przetak Werner A. Kaiser Lothar R. Schad 《Magma (New York, N.Y.)》1998,6(1):62-69
The purpose of this study was to evaluate a non-flow related magnetic resonance imaging method to visualize small veins independent of arteries in the human brain. A long TE, high-resolution 3D gradient echo MR acquisition was used to highlight venous information. The method is based on the paramagnetic property of deoxyhemoglobin and the resulting phase difference between veins and brain parenchyma at long echo times. The MR magnitude images were masked with a phase mask filter to enhance small structure visibility.. Venous information down to sub-pixel vessel diameters of several hundred microns is visible. Venous data are displayed in an angiographic manner using a minimum intensity projection algorithm. Both superficial veins and deep white matter veins are visible. The method has been successfully applied in volunteers. Preliminary results in patients with cerebral arteriovenous malformations indicate its potential in clinical applications. The proposed method is easy to implement and does not require administration of a contrast agent or application of specially designed rf pulses to highlight the veins. Rather it exploits the intrinsic magnetic properties (BOLD-effect) and the prolonged T 2* of venous blood. The method may be of diagnostic potential in the assessment of arteriovenous malformations or other vascular venous lesions. © 1998 Elsevier Science B.V. All rights reserved. 相似文献
127.
Cavallaro A Fellner F Matzel KE Stadelmaier U Rupprecht T Böwing B Hohenberger W Bautz W 《Magma (New York, N.Y.)》1998,7(3):179-183
The aim of this study was to determine whether low-field magnetic resonance (MR) imaging can safely and accurately depict
inflammatory changes in patients with anal dynamic graciloplasty, in whom high-field MR imaging is contraindicated and ultrasonography
and computed tomography are inadequate. A 0.2-T field-strength MR examination was performed in six patients with anal dynamic
graciloplasty malfunction in whom reoperation was contemplated. The following sequences were applied:T
2-weighted turbo spinecho with fat saturation,T
1-weighted conventional spin-echo, and contrastenhancedT
1-weighted conventional spin-echo with fat saturation. Results indicated that none of the patients experienced relevant discomfort,
pacemaker malfunction, or electrode dislocation with low-field MR imaging. Inflammatory pelvic changes were visualized in
four patients and atrophy of the transposed gracilis muscle in another. Surgery was thus avoided in the four, who underwent
conservative treatment for their pelvic inflammation. It was concluded that these prelininary results demonstrate the feasibility
of MR imaging with a low field strength in patients with anal dynamic graciloplasty. In such patients, in whom diagnostic
imaging had been problematic, the potential for safe and accurate visualization will be a boon to treatment planning. 相似文献
128.
Viehweg P Heinig A Lampe D Buchmann J Heywang-Köbrunner SH 《Magma (New York, N.Y.)》1998,7(3):141-152
Purpose The aim of this study was to assess the value of contrast-enhanced (c.-e.) MRI in the follow-up of patients with conservatively
treated breast cancer since detection and exclusion of malignancy may interfere significantly with posttherapeutic changes
within the treated breast.
Material and methods A total of 207 patients with a history of limited surgery and radiation therapy underwent MR imaging, 40 patients were examined
0–12 months and 167 patients were examined later than 12 months after radiotherapy. Suspicious or indeterminate findings were
suggested by clinical examination or conventional imaging in 80 studies. In 127 women, MRI was performed within breast tissue
that was difficult to assess due to scarring or dense breast tissue.
Results Recurrent carcinoma was confirmed in 27 patients by surgical biopsy. All 27 carcinomas, except for one with a slow signal
increase, demonstrated early rise of signal intensity on dynamic T1-weighted contrast enhanced images. During the first year
after therapy, the diagnostic accuracy could not be improved by additional use of c.-e. MRI. Differentiation between posttherapeutic
changes and recurrent carcinoma was frequently not possible because of strong and sometimes early and ill-circumscribed enhancement.
Later than 12 months after therapy enhancement decreased significantly, thus the false positive calls could be reduced from
49 (conventional imaging) to 12 (conventional imaging plus MRI). A total of 12 of 26 recurrences and multifocality in 4/5
cases were diagnosed by MR imaging alone at this time interval.
Conclusion In the first year after therapy, c.-e. MRI is only indicated in selected cases. The results later than 12 months emphasize
that c.-e. MRI may contribute significant additional information. It allows better distinction of posttherapeutic fibrosis
from recurrent carcinoma and proved to be able to detect recurrent disease more sensitive and at an earlier stage. 相似文献
129.
Norbert Wilke 《Magma (New York, N.Y.)》1998,6(2-3):147-147
130.
车辆图像的预处理效果的好坏直接影响着车牌定位的准确率和运算速度;为了去除复杂背景区域的影响,消除图像的噪声,提出了一种针对视频序列改进的预处理流程;基于混合高斯背景建模结合小波包多尺度去噪的思想,去除大量的背景信息,缩小了车牌定位的范围再继续后面的预处理工作;上述方法得到的图片,噪声干扰降低且含有较少的不规则连通区域,分别以文献[6]和文献[7]提供的车牌定位方法为例进行测试,效果良好。 相似文献