Parallel imaging with 3D TPI trajectory: SNR and acceleration benefits

Three-dimensional (3D) twisted projection imaging (TPI) trajectory has a unique advantage in sodium (²³Na) imaging on clinical MRI scanners at 1.5 or 3 T, generating a high signal-to-noise ratio (SNR) with a short acquisition time (∼10 min). Parallel imaging with an array of coil elements transits SNR benefits from small coil elements to acquisition efficiency by sampling partial k-space. This study investigates the feasibility of parallel sodium imaging with emphases on SNR and acceleration benefits provided by the 3D TPI trajectory. Computer simulations were used to find available acceleration factors and noise amplification. Human head studies were performed on clinical 1.5/3-T scanners with four-element coil arrays to verify simulation outcomes. In in vivo studies, proton (¹H) data, however, were acquired for concept–proof purpose. The sensitivity encoding (SENSE) method with the conjugate gradient algorithm was used to reconstruct images from accelerated TPI-SENSE data sets. Self-calibration was employed to estimate coil sensitivities. Noise amplification in TPI-SENSE was evaluated using multiple noise trials. It was found that the acceleration factor was as high as 5.53 (corresponding to acceleration number 2×3, ring-by-rotation), with a small image error of 6.9% when TPI projections were reduced in both polar (ring) and azimuthal (rotation) directions. The average noise amplification was as low as 98.7%, or 27% lower than Cartesian SENSE at that acceleration factor. The 3D nature of both TPI trajectory and coil sensitivities might be responsible for the high acceleration and low noise amplification. Consequently, TPI-SENSE may have potential advantages for parallel sodium imaging.     

非旋转性前进波Euler与Lagrange解间的转换

对等深水中非旋转性的前进重力波动场,以求得的Euler与Lagrange两种形式至第三阶的解,按照同一流体质点在相同时间与位置处其流速唯一与质量守恒性及在自由表面水位处Euler形式解与Lagrange形式解为同一值的特性,来推导二者可相互转换.由连续的Taylor级数展开,考虑波动场中各流体质点的运动轨迹与运动周期,将已知的Euler形式解转换成完全未知的Lagrange形式解,解决了以往成果中出现含时间的不合理的共振项,以及无法得到与Euler系统不同的Lagrange形式的流体质点运动频率与平均运动 关键词： 非旋转性前进波 Euler-Lagrange转换 质点运动轨迹 质点运动频率     

The Husimi Distribution of Circular Billiard with?an?Applied?Uniform Magnetic Field

We present a theoretical computation of the Husimi distribution function in phase-space for studying the semiclassical dynamics of the circular electron billiard subjected to a constant magnetic field in the perpendicular direction. The results reveal that with the increase of the applied magnetic field the peaks of Husimi function tend to the billiard boundaries, along with the movements a periodic splitting-recombining (alternative single-double) peak structure is arisen. This fact implies the localization of the eigenstates and coincides to the classical trajectory distribution what we obtained by use of representation on the billiard boundary. It becomes possible to compare the local properties of the quantum and classical distributions. Our analysis provides a new perspective to understand the quantum-classical correspondence.     

Monte Carlo studies of micromegas as a neutron detector and its track reconstruction

In this paper a two dimensional readout micromegas detector with a polyethylene foil as converter was simulated on GEANT4 toolkit and GARFIELD for fast neutron detection. A new track reconstruction method based on time coincidence technology was developed in the simulation to obtain the incident neutron position. The results showed that with this reconstruction method higher spatial resolution was achieved.     

Soft soil track interaction modeling in single rigid body tracked vehicle models

Single rigid body models are often used for fast simulation of tracked vehicle dynamics on soft soils. Modeling of soil-track interaction forces is the key modeling aspect here. Accuracy of the soil-track interaction model depends on calculation of soil deformation in track contact patch and modeling of soil resistive response to this deformation. An algorithmic method to calculate soft soil deformation at points in track contact patch, during spatial motion simulation using single body models of tracked vehicles, is discussed here. Improved calculations of shear displacement distribution in the track contact patch compared to existing methods, and realistically modeling plastically deformable nature of soil in the sinkage direction in single body modeling of tracked vehicle, are the novel contributions of this paper. Results of spatial motion simulation from a single body model using the proposed method and from a higher degree of freedom multibody model are compared for motion over flat and uneven terrains. Single body modeling of tracked vehicle using the proposed method affords quicker results with sufficient accuracy when compared to those obtained from the multibody model.     

Dynamics of the Smooth Positons of the Wadati-Konno-Ichikawa Equation

We discuss a modified Wadati-Konno-Ichikawa(m WKI) equation,which is equivalent to the WKI equation by a hodograph transformation.The explicit formula of degenerated solution of m WKI equation is provided by using degenerate Darboux transformation with respect to the eigenvalues,which yields two kinds of smooth solutions possessing the vanishing and nonvanishing boundary conditions respectively.In particular,a method for the decomposition of modulus square is operated to the positon solution,and the approximate orbits before and after collision of positon solutions are displayed explicitly.     

Promoting reinvention of a multiplication-of-fractions algorithm: A study of the Learning Through Activity research program

Whereas proficiency in performing the canonic multiplication-of-fractions algorithm is common, understanding of the algorithm is much less so. We conducted a teaching experiment with a fifth-grade student, based on an initial hypothetical learning trajectory (HLT), to promote reinvention of the multiplication-of-fractions algorithm. The instructional intervention built on two concepts, recursive partitioning and distributive partitioning. As a study of the Learning Through Activity research program, our goal was to promote particular activity on the part of the student through which she could abstract the necessary concepts. The results of the teaching experiment were analyzed and, based on conclusions from the research, a revised HLT was generated. Recursive partitioning and distributive partitioning proved to be a strong foundation for construction of the algorithm.     

Empirically-based hypothetical learning trajectories for fraction concepts: Products of the Learning Through Activity research program

The Measurement Approach to Rational Number (MARN) Project, a project of the ongoing Learning Through Activity (LTA) research program, produced eleven hypothetical learning trajectories (HLTs) for promoting fraction concepts. Four of these HLTs are the subject of research reports. In this article, we present the other seven HLTs We judged that the data and analyses of these seven would not separately make sufficient contributions to merit individual research reports. However, presenting these seven HLTs together was intended to meet the following goals:1. To give a broad set of examples of HLTs developed based on the LTA theoretical framework.2. To complete a set of HLTs that provide a comprehensive example of HLTs built on prior HLTs.3. To make available for future research and development the full set of HLTs generated by the MARN Project.LTA researchers have focused on how learners abstract a concept through their mathematical activity and how the abstractions can be promoted. The MARN Project continued this inquiry with rigorous single-subject teaching experiments.     

Promoting a concept of fraction-as-measure: A study of the Learning Through Activity research program

Promoting deep understanding of fraction concepts continues to be a challenge for mathematics education. Research has demonstrated that students whose concept of fractions is limited to part-whole have difficulty with advanced fraction concepts. We conducted teaching experiments to study how students can develop a measurement concept of fractions and how task sequences can be developed to promote the necessary abstractions. Building particularly on the work of Steffe and colleagues and aspects of the Elkonin-Davydov curriculum, we focused on fostering student reinvention of a measurement concept of fractions. As a study of the Learning Through Activity research program, our goal was to promote particular activity on the part of the students through which they could abstract the necessary concepts.     

Magnetic trajectories in three-dimensional Lie groups

We study magnetic trajectories in Lie groups equipped with bi-invariant Riemannian metric. We define the Lorentz force of a magnetic field in a Lie group G, and then, we give the Lorentz force equation for the associated magnetic trajectories that are curves in G. When the manifold is a Lie group G equipped with bi-invariant Riemannian metric, we derive differential equation system that characterizes magnetic flow associated with the Killing magnetic curves with regard to the Lie reduction of the curve γ in G.