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The ability to estimate a surface from a set of cross sections allows calculation of the enclosed volume and the display of the surface in three-dimensions. This process has increasingly been used to derive useful information from medical data. However, extracting the cross sections (segmenting) can be very difficult, and automatic segmentation methods are not sufficiently robust to handle all situations. Hence, it is an advantage if the surface reconstruction algorithm can work effectively on a small number of cross sections. In addition, cross sections of medical data are often quite complex. Shape-based interpolation is a simple and elegant solution to this problem, although it has known limitations when handling complex shapes. In this paper, the shape-based interpolation paradigm is extended to interpolate a surface through sparse, complex cross sections, providing a significant improvement over our previously published maximal disc-guided interpolation. The performance of this algorithm is demonstrated on various types of medical data (X-ray computed tomography, magnetic resonance imaging and three-dimensional ultrasound). Although the correspondence problem in general remains unsolved, it is demonstrated that correct surfaces can be estimated from a limited amount of real data, through the use of region rather than object correspondence. 相似文献
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We show that a hierarchical Bayesian modeling approach allows us to perform regularization in sequential learning. We identify three inference levels within this hierarchy: model selection, parameter estimation, and noise estimation. In environments where data arrive sequentially, techniques such as cross validation to achieve regularization or model selection are not possible. The Bayesian approach, with extended Kalman filtering at the parameter estimation level, allows for regularization within a minimum variance framework. A multilayer perceptron is used to generate the extended Kalman filter nonlinear measurements mapping. We describe several algorithms at the noise estimation level that allow us to implement on-line regularization. We also show the theoretical links between adaptive noise estimation in extended Kalman filtering, multiple adaptive learning rates, and multiple smoothing regularization coefficients. 相似文献
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Field capacity has been defined as the water content at which the drainage flux from a soil ceases, or becomes negligible. Field capacity is most commonly estimated as the water content at a pressure head of 1∕3 bar, which ignores its flux-based nature. A number of authors have proposed estimating field capacity as the water content at a given (negligible) flux, assuming unit gradient conditions. This flux-based method of estimating field capacity is discussed and the differences between this method and the more common pressure-based method are illustrated. The drainage flux considered to be negligible will depend on the particular application; values between 10?6 and 10?8 cm∕s were considered here. The observed differences in field capacity resulting from the negligible flux range considered produce significant differences in the available water capacity. These differences directly impact the calculated drainage. The effect of available water capacity on the water budget for waste disposal facility covers at an arid and humid site was illustrated using the HELP code. At the arid site, drainage decreased by a factor of 4 (from 25 to 6 mm∕year) as the available water capacity increased from 0.101 to 0.199. At the humid site, drainage decreased from 190 to 173 mm∕year over the same range of available water capacity. Finally, a simple procedure for the determination of an appropriate value for the negligible flux is discussed. Analysis of data from a lysimeter at Coshocton, Ohio, indicated that field capacity for the lysimeter soils was associated with a negligible drainage flux of about 3 × 10?7 cm∕s. 相似文献
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Abstract The effect of air gap curvature on the Carter coefficient and magnetic field is studied in this article. In the analysis, a finite‐difference successive relaxation technique is employed to solve a dimensionless Maxwell equation. Constant fluxes are applied on a simplified model and the calculated Carter coefficients in the curved air gap are compared with the data in the existing literature. Flux plots, dimensionless radial flux densities in the air gap, and the Carter coefficient with normalized air gap G=0.1 and 0.2, slot numbers N=18, 24 and 36, rotor curvature K=0.24, 0.17 and 0.1, slot depths D=0.4 and 2.0, and half slot width W=0.2 to 0.8 are presented. 相似文献
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A powder sizing test developed for use on WC powders has been extended for use on iron and iron-neodymium-boron powders. In this test the particle size is derived from the rate of oxidation, because finer powders oxidize quicker. The rate of oxidation is monitored in a thermogravimetric analyser, where the powders are subjected to a controlled heating rate from room temperature to 1100 °C. If the constants from the Arrhenius law are known the powder size can be determined by comparing experimental oxidation plots with theoretical curves. For the sizing of a commercial spherical iron powder, the oxidation technique compared favourably with direct sizing using scanning electron microscopy and image analysis. The values for the activation energy of 125 kJ mol–1 determined in this study agree with previous studies. Validation of the sizing technique on a hydrogen-decrepitated stoichiometric Nd2Fe14B powder proved difficult because it was not possible to determine a definitive size distribution independently. Metallography of partially oxidized samples showed that the process is two-stage, at low temperatures the neodymium oxidizes, and above 400 °C the powder behaves as pure iron. Theoretical curves based on one oxidation process with an activation energy of 100 kJ mol–1 gave the best fit to the experimental curves. 相似文献
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钢筋与钢纤维混凝土的黏结滑移性能及其关系模型 总被引:1,自引:0,他引:1
基于内贴应变片的钢筋与钢纤维混凝土局部黏结试件的拉拔试验,研究钢筋与不同强度钢纤维混凝土的黏结性能。通过对实测钢筋应变的分析,建立了以三次多项式表达的黏结应力分布函数,得到了各级荷载作用下钢筋与钢纤维混凝土黏结应力和相对滑移沿黏结区段的分布,进一步分析了钢纤维和混凝土强度对黏结性能的影响。结果表明,随钢纤维的体积率和混凝土强度的增大,黏结试件加载端附近的黏结应力提高,黏结应力极值总体向加载端靠拢;同时,加载端与自由端的滑移减小。最后,提出了能够较好反映钢筋与钢纤维混凝土受力过程的黏结-滑移关系模型。 相似文献