Composite pulses for RF phase encoded MRI: A simulation study

BackgroundIn B₁ encoded MRI, a realistic non-linear phase RF encoding coil will generate an inhomogeneous B₁ field that leads to spatially dependent flip angles. The non-linearity of the B₁ phase gradient can be compensated for in the reconstruction, but B₁ inhomogeneity remains a problem. The effect of B₁ inhomogeneity on tip angles for conventional, B₀ encoded MRI, may be minimized using composite pulses. The objective of this study was to explore the feasibility of using composite pulses with non-linear RF phase encoding coils and to identify the most appropriate composite pulse scheme.MethodsRF encoded signals were simulated via the Bloch equation for various symmetric, asymmetric and antisymmetric composite pulses. The simulated signals were reconstructed using a constrained least squares method.ResultsRoot mean square reconstruction errors varied from 6% (for an asymmetric composite pulse) to 9.7% (for an antisymmetric composite pulse).ConclusionAn asymmetric composite pulse scheme created images with fewer artifacts than other composite pulse schemes in inhomogeneous B₀ and B₁ fields making it the best choice for decreasing the effects of spatially varying flip angles. This is contrary to the conclusion that antisymmetric composite pulses are the best ones to use for spin echo sequences in conventional, B₀ encoded, MRI.     

B 1 inhomogeneity effects on spin density andT 1 contrast in MRI

Summary An analysis of the effect ofB ₁ inhomogeneity on spin density andT ₁ contrasted images is here given. Distortions both in saturation recovery and inversion recovery sequences are found. The impact of these distortions onin vivo spectroscopy, where quantitative data are required, is also discussed. In general, these aspects are found to be reduced at shortertime parameters of the pulse sequences. Work partially supported by Progetto Finalizzato Tecnologie Biomediche of CNR and by Esacontrol, Genova.     

A study of fermions coupled to gauge and gravitational fields on a cylinder

Fermions on a cylinder coupled to background gravitation and gauge fields are examined by studying the geometric action associated with the symmetries of such a system. We are able to show that the gauge coupling constant is constrained to a value of 1/N where N is an integer. Furthermore, in direct analogy with a Yang-Mills theory a new gravitational theory is introduced which couples to the fermions by promoting the coadjoint vector of the diffeomorphism sector to a dynamical variable. The classical dynamics of this theory are examined by displaying its symplectic structure and showing that it is equivalent to a one-dimensional system.     

氩气感应耦合等离子体速度与温度数值模拟

为了获得用于研究再入飞行器热防护系统的感应耦合等离子体风洞流场数据,基于流场、电磁场和化学场的多场耦合建立了非平衡态感应耦合等离子体数值模型。利用该模型对不同入口质量流率和不同工作压力下的感应耦合等离子体进行了数值模拟,得到了相应工作参数下感应耦合等离子体温度与速度的分布特性。计算结果表明:等离子体中心线上的速度随着入口质量流率的增大而增大,而随着工作压力的增大而减小;同时,等离子体中心线上的温度随着入口质量流率的增大而减小,而随着压力的增大先减小后增大。这些结果可为感应耦合等离子体风洞优化设计及其工业应用提供理论指导。     

TriTone: a radiofrequency field (B1)-insensitive T1 estimator for MRI at high magnetic fields

Fast, high-resolution, longitudinal relaxation time (T1) mapping is invaluable in clinical and research applications. It has been shown that two spoiled gradient recalled echo (SPGR) images acquired in steady state with variable flip angles is an attractive alternative to the multi-image sets previously acquired with inversion or saturation recovery. The known sensitivity of the two-point method to transmit radiofrequency field (B1) inhomogeneity exacerbated at 3 T and above, however, mandates its combination with an additional, time-consuming and possibly specific-absorption-rate-intensive B1 measurement, preventing direct migration of the method to these fields. To address this, we introduce a method designed to be free of systematic errors caused by B1 inhomogeneity in which the value of T1 is extracted from three SPGR images acquired with echo planar imaging (EPI) readout. The precision of the T1 maps produced is found to be comparable to the two-point method, while the accuracy is greatly improved in the same time and spatial resolution. A welcome byproduct of the method is a map of B1 that can be used to correct other acquisitions in the same session. Tables of the optimal acquisition protocols are provided for several total imaging times.     

The simulation of XYZ-spin chain in coupled cavities

We simulated anisotropic XYZ-spin chain with an array of coupled cavities, each of which contains one 4-level atom. The anisotropic parameters of the effective Hamilton can be tuned by controlling the external lasers. The validity of our approximations is confirmed via a numerical simulation.     

A lagrangian formulation of high-spin fields coupled to gravity

A coupling of high (half-integer) spin fields to gravity is discussed in terms of symmetric tensor-valued spinors for which restrictive integrability constraints are avoided. The theory is generated from a spin-invariant action which provides a means of computing the associated stress tensor. Explicit no-ghost solutions are presented and a covariance of the system under a local conformai isometry group of space-time is pointed out.We are grateful to G. R. Allcock for helpful correspondence.     

A class of static coupled zero-mass and electromagnetic fields

A class of solutions, which may be called the Majumdar-Papapetrou (MP) class, has been derived for static coupled zero-mass and source-free electromagnetic fields. This has been done in two stages. First, it has been shown that the well-known MP relation between theg ₄₄ component of the metric tensor and the electrostatic potential, in certain physical situations, continues to remain the same without being affected by the presence of the zeromass field. Second, using the method of Majumdar, we have developed a theorem that enables one to generate solutions for the coupled field from Einstein's vacuum equations and the solution of a differential equation which, due to its resemblance, may be called a generalized Laplace equation.     

容性耦合射频氩等离子体放电诊断研究及仿真模拟

针对中等气压、中等功率下射频容性耦合(CCRF)等离子体的放电特性,采用基于流体模型的COMSOL软件仿真,建立一维等离子体放电模型,以Ar为工作气体,研究同一气压时不同射频输入功率下等离子体电子温度和电子密度的分布规律。同时依据仿真模型设计制作相同尺寸的密闭玻璃腔体和平板电极,实验测量了不同射频输入功率时放电等离子体的有效电流电压及发射光谱,进而计算等离子体的电子温度及电子密度;利用玻耳兹曼双线测温法,得到光谱法下等离子体的电子温度及电子密度。结果表明：当气体压强为250 Pa、输入功率为100~450 W时,等离子体电压电流呈线性关系,电子密度随功率的增大而增大,而电子温度并未随功率的变化而有明显变化,其与功率无关。运用仿真模拟验证了实验的准确性,通过比较,三种方法所得的结果相近。通过结合等效回路法、光谱法和数值模拟仿真法初步诊断出中等气压下等离子体的放电参数,提出了结合三种方法作为实验研究的方法,使实验结果更具说服力,证明其方法的可靠性,也为进一步的等离子体特性研究提供依据。     

Computer simulation of topological effects in lattice theories: A model study

The quantum pendulum as a model is put on a lattice, producing a straightforward definition of topological charge. It is used to single out non-perturbative effects in a Monte Carlo calculation and compare them with a semiclassical picture. The method of subtracting perturbative contributions from numerical results recently used for the gluon condensate is tested.     

The sources of the A and B degenerate static vacuum fields

We study some known static vacuum solutions of Einstein's equations, namely, the A and B metrics of Ehlers and Kundt. All of them have axial symmetry, so they can be transformed to the Weyl form. In Weyl coordinates log –g ₄₄ obeys a Laplace equation, and from this a source, calledthe Newtonian image source, can be identified. We use the image sources to give a possible interpretation to the metrics. The procedure is successful in some cases. In others it fails because the Weyl transform does not have reasonable properties at infinity.     

NMR spectroscopy in inhomogeneous B0 and B1 fields with non-linear correlation

Resolved NMR spectra from samples in inhomogeneous B0 and B1 fields can be obtained with the so-called "ex situ" methodology, employing a train of composite or adiabatic z-rotation RF pulses to periodically refocus the inhomogeneous broadening during the detection of the time-domain signal. Earlier schemes relied on a linear correlation between the inhomogeneous B0 and B1 fields. Here the pulse length, bandwidth, and amplitude of the adiabatic pulses of the hyperbolic secant type are adjusted to improve the refocusing for a setup with non-linear correlation. The field correlation is measured using a two-dimensional nutation experiment augmented with a third dimension with varying RF carrier frequency accounting for off-resonance effects. The pulse optimization is performed with a computer algorithm using the experimentally determined field correlation and a standard adiabatic z-rotation pulse as a starting point for the iterative optimization procedure. The shape of the z-rotation RF pulse is manipulated to provide refocusing for the conditions given by the sample-, magnet-, and RF-coil geometry.     

Global existence of coupled Yang-Mills and scalar fields in 2 + 1 dimensional space-time

We present results on the Cauchy problem for coupled classical Yang-Mills and scalar fields in n + 1 dimensional space-time both in the temporal and in the Lorentz gauge. We prove the existence of local solutions for any n, and the existence of global solutions for n = 1, 2 in the temporal gauge and for n = 1 in the Lorentz gauge. The last result also holds for massive Yang-Mill fields.     

Singularities of coupled fields in warm stratified plasmas

This paper deals with waves, described by Maxwell's equations coupled to single-fluid hydrodynamic equations, in a warm continuously stratified electron plasma. The behaviour of the fields in a coupling region, excited by an incident electron-acoustic wave, is investigated.     

Size effects on characteristic magnetic fields of a spin-valve multilayer by computer simulation

The change in characteristic magnetic fields of a spin-valve multilayer is investigated as a function of the size by computer simulation. The spin-valve modeled in this work is IrMn (9 nm)/CoFe (4 nm)/Cu (2.6 nm)/CoFe (2 nm)/NiFe (6 nm). The spin-valve dimensions are varied widely from 20 mm×10 mm to 0.5 μm×0.25 μm, but the aspect ratio defined by the ratio of the length to the width is fixed at 2.0. The magnetostatic interactions begin to affect the magnetic properties substantially at a spin-valve length of 5 μm, and, at a length of 1 μm, they become even more dominant. The main consequences of the magnetostatic interactions are a significant increase of the coercivity and a very large shift of the bias field in both the pinned and free layers. It is shown that these changes can be explained by two separate contributions to the total magnetostatic interactions: the coercivity change by the self-demagnetizing field and the change of the bias field by the interlayer magnetostatic interaction field.     

气液双热源耦合热泵空调性能模拟研究

建立了气液双热源耦合换热器的耦合换热模型,并对其在耦合热源热泵空调中应用时的制热工况进行了性能模拟研究。模拟结果显示,使用该气液双热源耦合式换热器的耦合热源热泵空调系统,气液双热源模式与单空气热源模式相比,制热量和COP均有明显提高,低温时性能提高更为显著。当室外温度为-15℃时,双热源热泵的制热量较单一空气源热泵提高比例进一步增大,制热量提高近40%,COP提高近30%。     

A simple study of the correlation effects in the superposition of waves of electric fields: The emergence of extreme events

In this paper, we study the effects of correlated random phases in the intensity of a superposition of N wavefields. Our results suggest that regardless of whether the phase distribution is continuous or discrete if they are random correlated variables, we will observe a denser tail distribution and the emergence of extreme events (amplitudes 30-40 times larger than their average) as the phases correlation increase. Recent results in the literature discuss the role of phase correlations on the emergence of rogue waves both in linear and nonlinear systems, but the mechanisms to generate them are not always straightforward. We show here a simple way to correlate the wavefield that makes it clear that rogue waves or denser tails appear mainly due to wave correlations instead of any particular system property.     

High-resolution MR spectroscopy via intermolecular double-quantum coherences in inhomogeneous B0 and B1 fields

Inhomogeneity in static field B₀ and/or RF field B₁ is inevitable under some circumstances. In this work, a method based on intermolecular double-quantum coherences is employed for high-resolution 1D MR spectroscopy via 2D acquisition under such a condition. High-resolution information on chemical shifts, multiplet patterns, J coupling constants and relative peak areas can be retained in the resulting 1D projected spectra, as shown with results from a narrow-bore NMR spectrometer and a whole-body clinical scanner.     

Deconfinement transition in three-dimensional compact U(1) gauge theories coupled to matter fields

It is shown that permanent confinement in three-dimensional compact U(1) gauge theory can be destroyed by matter fields in a deconfinement transition. This follows from a nontrivial infrared fixed point caused by matter, and an anomalous scaling dimension of the gauge field. This leads to a logarithmic interaction between the defects of the gauge fields, which form a gas of magnetic monopoles. For logarithmic interactions, the original electric charges are unconfined. The confined phase, which is permanent in the absence of matter fields, is reached at a critical electric charge, where the interaction between magnetic charges is screened by a pair-unbinding in a Kosterlitz-Thouless-like phase transition.