Application of the limited-memory quasi-Newton algorithm for multi-dimensional,large flip-angle RF pulses at 7T

Objective

Ultrahigh field MRI provides great opportunities for medical diagnostics and research. However, ultrahigh field MRI also brings challenges, such as larger magnetic susceptibility induced field changes. Parallel-transmit radio-frequency pulses can ameliorate these complications while performing advanced tasks in routine applications. To address one class of such pulses, we propose an optimal-control algorithm as a tool for designing advanced multi-dimensional, large flip-angle, radio-frequency pulses. We contrast initial conditions, constraints, and field correction abilities against increasing pulse trajectory acceleration factors.

Materials and methods

On an 8-channel 7T system, we demonstrate the quasi-Newton algorithm with pulse designs for reduced field-of-view imaging with an oil phantom and in vivo with scans of the human brain stem. We used echo-planar imaging with 2D spatial-selective pulses. Pulses are computed sufficiently rapid for routine applications.

Results

Our dataset was quantitatively analyzed with the conventional mean-square-error metric and the structural-similarity index from image processing. Analysis of both full and reduced field-of-view scans benefit from utilizing both complementary measures.

Conclusion

We obtained excellent outer-volume suppression with our proposed method, thus enabling reduced field-of-view imaging using pulse trajectory acceleration factors up to 4.

     

Raman,X-ray and electrical properties of MOD PZT/PLZT thin films

Abstract

Ferroelectric PZT/PLZT thin films have been fabricated using the metallo-organic precursor compounds. The structural development, spectroscopic and dielectric properties of these films have been investigated using atomic force microscopy (AFM), X-ray diffraction, Raman scattering and dielectric measurements. Experimental results show that Raman spectroscopy is an effective tool of monitoring the structural development of the small sized PZT films in the tetragonal phase field. Dielectric characteristics have been improved by the rapid thermal processing approach. A rosette growth model is proposed to explain the observation of the tri-intersection of the perovskite phase in PZT films.     

利用改进Rayleigh商逆迭代法进行大系统特定模式降阶

对大系统特定模式进行降阶计算.依据特征值灵敏度对系统状态矩阵A重新排序,仅保留与特定模式强相关的发电机组,使矩阵维数得以降低.由于降阶后矩阵一般为复数形式,把矩阵虚部作为实部的修正量.以实部的特定特征值作为初值,用改进Rayleigh商逆迭代格式准确计算.算例分析验证了该方法的有效性.     

Intrinsic hardness of crystalline solids

The current status of various theoretical approaches to the “prediction” of material hardness has been reviewed. It is shown that the simple empirical correlation with the shear moduli generally provide very good estimates of the Vickers hardness. Semi-empirical models based solely on the strength of the chemical bonds, although performed as well, are theoretically incomplete. First-principles calculations of the ideal stress and shear strength is perhaps the most reliable and theoretically sound approach available to compare theoretical predictions with experiment.     

Pressure Sensitive Adhesives Based on VectorR SIS Polymers I. Rheological Model and Adhesive Design Pathways

Dexco Polymers (a Dow/Exxon partnership) has been manufacturing Vector^R SIS polymers since 1990.¹ This paper describes experiments carried out to study Vector SIS polymers and model pressure sensitive adhesive (PSA) formulations based on Exxon Chemical's Escorez^R 1310LC tackifier. The adhesive behavior of tackified polymers was quantitatively analyzed by applying the rheological principle of time-temperature superposition² and the mapping approach,^3,4 and the pressure sensitive rheological model⁵ developed earlier. This model⁵ was developed by expanding and modifying an equation [adhesive fracture strength = (intrinsic adhesion) × (bulk energy dissipation)] proposed by Gent et al.^6,7 and Andrews et al.^8,9 This study delivers two key results. The first is that the fracture strength of the PSA/steel bond is the multiplication of three terms: the intrinsic (or interfacial) adhesion, the bonding and the debonding terms (Fig. 1). The debonding term is correlated with the logarithm of the loss modulus at the PSA debonding frequency or with the logarithm of the monomeric friction coefficient of the block copolymer/tackifier system. Both the loss modulus and the monomeric friction coefficient measure the energy dissipation in the bulk adhesive. The second is that PSA design pathways can be established by a mapping approach in the rheological space of the plateau modulus versus the loss modulus peak position in the frequency scale (Fig. 2). Plateau modulus is the bonding parameter because it measures the wetting capability of the adhesive with the substrate surface. The loss modulus peak position is the debonding parameter because it corresponds approximately to the time scale (or the frequency scale) in which one deforms the adhesive to maximize energy dissipation. Therefore, the tackifier and oil combination lowers the plateau modulus, but increases the T_g of the polyisoprene phase of the SIS polymer. This increase in T_g is equivalent to the lowering of the rate of local rearrangement (frequency of segment jumps) of the polyisoprene chains of the block copolymer. An optimal “tackification pathway” in this rheological space is achieved by tailoring the tackifier type and T_g, and the amount of oil used in the PSA.

In brief, the PSA rheological model and mapping approach described in this work for Vector SIS polymers give a comprehensive understanding and adhesive design pathways. This concept and approach not only allow raw material suppliers to improve and design better tackifier and polymer products, but also provide PSA formulators a quantitative tool to achieve PSA end property results.     

Flame synthesis of graphene films in open environments

Few-layer graphene is grown on copper and nickel substrates at high rates using a novel flame synthesis method in open-atmosphere environments. Transmittance and resistance properties of the transferred films are similar to those grown by other methods, but the concentration of oxygen, as assessed by X-ray photoelectron spectroscopy, is actually less than that for graphene grown by chemical vapor deposition under near vacuum conditions. The method involves utilizing a multi-element inverse-diffusion-flame burner, where post-flame species and temperatures are radially-uniform upon deposition at a substrate. Advantages of the specific flame synthesis method are scalability for large-area surface coverage, increased growth rates, high purity and yield, continuous processing, and reduced costs due to efficient use of fuel as both heat source and reagent. Additionally, by adjusting local growth conditions, other carbon nanostructures (i.e. nanotubes) are readily synthesized.     

Generalization of Waveform Communications: The Fourier Analyzer Approach

In digital communications, fixed, chaotic, and random waveforms are used. Although the theory of fixed waveform communications is well established, its generalization to include chaotic and random carriers has not yet been carried out. The lack of an exact model prevents the optimization of chaotic and random communication systems. Introducing the Fourier analyzer concepts, this paper develops a unified model which is equally valid for communications with fixed, chaotic and random carriers. It gives an exact measure for the a priori information which is exploited at the receiver to optimize the noise performance. The unified model may be used to determine the performance bounds of different waveform communication systems and offers a systematic approach for the development of optimum detector configurations. Using the model developed, a new classification of digital communication systems is proposed.     

Design of complex-valued variable FIR digital filters and its application to the realization of arbitrary sampling rate conversion for complex signals

This brief studies the design of complex-valued variable digital filters (CVDFs) and their applications to the efficient arbitrary sample rate conversion for complex signals. The design of CVDFs using either the minimax or least-squares criteria is formulated as a convex optimization problem and solved using the second-order cone programming (SOCP). In addition, linear and convex quadratic inequality constraints can be readily incorporated. Design examples are given to demonstrate the effectiveness of the proposed approach.     

Manipulating the Mixed‐Perovskite Crystallization Pathway Unveiled by In Situ GIWAXS

Mixed perovskites have achieved substantial successes in boosting solar cell efficiency, but the complicated perovskite crystal formation pathway remains mysterious. Here, the detailed crystallization process of mixed perovskites (FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃) during spin‐coating is revealed by in situ grazing‐incidence wide‐angle X‐ray scattering measurements, and three phase‐formation stages are identified: I) precursor solution; II) hexagonal δ‐phase (2H); and III) complex phases including hexagonal polytypes (4H, 6H), MAI–PbI₂–DMSO intermediate phases, and perovskite α‐phase. The correlated device performance and ex situ characterizations suggest the existence of an “annealing window” covering the duration of stage II. The spin‐coated film should be annealed within the annealing window to avoid the formation of hexagonal polytypes during the perovskite crystallization process, thus achieving a good device performance. Remarkably, the crystallization pathway can be manipulated by incorporating Cs⁺ ions in mixed perovskites. Combined with density functional theory calculations, the perovskite system with sufficient Cs⁺ will bypass the formation of secondary phases in stage III by promoting the formation of α‐phase both kinetically and thermodynamically, thereby significantly extending the annealing window. This study provides underlying reasons of the time sensitivity of fabricating mixed‐perovskite devices and insightful guidelines for manipulating the perovskite crystallization pathways toward higher performance.