A CUDA-based reverse gridding algorithm for MR reconstruction

MR raw data collected using non-Cartesian method can be transformed on Cartesian grids by traditional gridding algorithm (GA) and reconstructed by Fourier transform. However, its runtime complexity is O(K× N²), where resolution of raw data is N× N and size of convolution window (CW) is K. And it involves a large number of matrix calculation including modulus, addition, multiplication and convolution. Therefore, a Compute Unified Device Architecture (CUDA)-based algorithm is proposed to improve the reconstruction efficiency of PROPELLER (a globally recognized non-Cartesian sampling method). Experiment shows a write–write conflict among multiple CUDA threads. This induces an inconsistent result when synchronously convoluting multiple k-space data onto the same grid. To overcome this problem, a reverse gridding algorithm (RGA) was developed. Different from the method of generating a grid window for each trajectory as in traditional GA, RGA calculates a trajectory window for each grid. This is what “reverse” means. For each k-space point in the CW, contribution is cumulated to this grid. Although this algorithm can be easily extended to reconstruct other non-Cartesian sampled raw data, we only implement it based on PROPELLER. Experiment illustrates that this CUDA-based RGA has successfully solved the write–write conflict and its reconstruction speed is 7.5 times higher than that of traditional GA.     

Quasiclassical calculation of the chemical reaction Ba+CH3I

This paper reports the quasiclassical trajectory (QCT) calculations on extended London–Eyring–Polanyi–Sato (LEPS) potential energy surface (PES) for the reaction of Ba atom with CH₃I. In the calculations, the Sato parameters needed in construction of the reaction PES are obtained from simulation of experimental results of BaI product vibrational distributions. The parameters obtained are further tested by comparison of the calculated BaI vibrational distributions with another research group's experimental results at different collision energy. In addition, the reaction cross-section versus collision energy, product BaI angular distribution and alignment are obtained by QCT calculations on the PES. The possible reaction configurations are also discussed upon the calculations.     

强磁场下He2++H(1s)的碰撞电离微分截面及电离机理研究

应用经典径迹蒙特卡罗方法详细研究了强磁场环境下He²⁺+H (1s)的重粒子碰撞电离过程. 首先得到了在不同强度的纵向和横向磁场下电离电子的一阶能量微分截面, 发现它们不仅在数值上比未加磁场下的结果有明显的增加, 而且随电离电子的能量变化不再是单调下降, 而是呈现一定的峰值结构. 通过对电离事例的分析, 解释了这些变化形成的原因. 其次由于强磁场会导致碰撞电离的物理机理有很大变化, 因此也分析并解释了磁场强度、方向及入射粒子能量等因素对电离机理的影响行为.     

Understanding rigid geometric transformations: Jeff's learning path for translation

This article describes the development of knowledge and understanding of translations of Jeff, a prospective elementary teacher, during a teaching experiment that also included other rigid transformations. His initial conceptions of translations and other rigid transformations were characterized as undefined motions of a single object. He conceived of transformations as movement and showed no indication about what defines a transformation. The results of the study indicate that the development of his thinking about translations and other rigid transformations followed an order of (1) transformations as undefined motions of a single object, (2) transformations as defined motions of a single object, and (3) transformations as defined motions of all points on the plane. The case of Jeff is part of a bigger study that included four prospective teachers and analyzed their development in understanding of rigid transformations. The other participants also showed a similar evolution.     

A hybrid multiagent approach for global trajectory optimization

In this paper we consider a global optimization method for space trajectory design problems. The method, which actually aims at finding not only the global minimizer but a whole set of low-lying local minimizers (corresponding to a set of different design options), is based on a domain decomposition technique where each subdomain is evaluated through a procedure based on the evolution of a population of agents. The method is applied to two space trajectory design problems and compared with existing deterministic and stochastic global optimization methods.     

Annealing of 54 132 Xe-ion tracks in Lexan and Makrofol plastic track detectors

Samples of Lexan and Makrofol-E polycarbonate plastic track detectors were exposed to 1·1 MeV/N ₅₄ ¹³² Xe-ions to investigate the thermal track fading properties of these plastics. The experimental results show that there is no effect of annealing on the bulk etch rate while the track etch rate decreases with annealing. The track diameter decreases with increase in annealing time and temperature. It is also observed that the track density is reduced as a result of annealing. The experiments reveal that the track lengths are, in general, decreased by the application of heat and that the oblique tracks are less stable than the vertical tracks. The decrease in diameter of ₅₄ ¹³² Xe-ion tracks in Makrofol-E due to heat treatment is faster than that of ₅₄ ¹³² Xe-ion tracks in Lexan.     

The “Duck Survival” Problem in Three-Dimensional Singularly Perturbed Systems with Two Slow Variables

We consider the system of ordinary differential equations x = f(x,y), y = g(x,y), where x ², y , 0 < 1, and f,g C. It is assumed that the equation g = 0 determines two different smooth surfaces y = (x) and y = (x) intersecting generically along a curve l. It is further assumed that the trajectories of the corresponding degenerate system lying on the surface y = (x) are ducks, i.e., as time increases, they intersect the curve l generically and pass from the stable part {y =(x),g'_y < 0} of this surface to the unstable part {y =(x),g'_y > 0}. We seek a solution of the so-called duck survival problem, i.e., give an answer to the following question: what trajectories from the one-parameter family of duck trajectories for = 0 are the limits as 0 of some trajectories of the original system.     

Poisson differential inclusion and suboptimality results for convex variational problems

This note deals with a nonsmooth convex problem of calculus of variations in which the cost term has the general form

     

Self diffusion in liquid Na-K alloy

Self diffusion coefficients,D _i, in liquid Na-K alloy at 373 K have been computed in the linear trajectory approximation of Helfand, with square well as an attractive tail. From the computedD _Na andD _K, the mutual diffusion coefficient,D _NaK has also been determined.D _Na,D _K andD _NaK all increase with increase of concentration of potassium, while the ratio,D _Na/D _K remains constant (1.45 ± 0.01) over the entire concentration range.