Model-free arterial spin labelling for cerebral blood flow quantification: introduction of regional arterial input functions identified by factor analysis

PURPOSE: To identify regional arterial input functions (AIFs) using factor analysis of dynamic studies (FADS) when quantification of perfusion is performed using model-free arterial spin labelling. MATERIAL AND METHODS: Five healthy volunteers and one patient were examined on a 3-T Philips unit using quantitative STAR labelling of arterial regions (QUASAR). Two sets of images were retrieved, one where the arterial signal had been crushed and another where it was retained. FADS was applied to the arterial signal curves to acquire the AIFs. Perfusion maps were obtained using block-circulant SVD deconvolution and regional AIFs obtained by FADS. In the volunteers, the ASL experiment was repeated within 24 h. The patient was also examined using dynamic susceptibility contrast MRI. RESULTS: In the healthy volunteers, CBF was 64+/-10 ml/[min 100 g] (mean+/-S.D.) in GM and 24+/-4 ml/[min 100 g] in WM, while the mean aBV was 0.94% in GM and 0.25% in WM. DISCUSSION: Good CBF image quality and reasonable quantitative CBF values were obtained using the combined QUASAR/FADS technique. We conclude that FADS may be a useful supplement in the evaluation of ASL data using QUASAR.     

尖楔前体飞行器FADS系统的径向基网络建模及验证

文章研究了针对一种用于尖楔外形的嵌入式大气数据传感（flush air data sensing，FADS）系统的解算模型及精度.首先基于飞行包络及CFD数据建立了FADS系统的测压孔选取标准；然后基于径向基函数（radial basis function，RBF）的人工神经网络建模技术构建了FADS系统的网络解算模型；最后给出了模型的测试误差，分析了气动延时效应、位置误差等误差源模型对算法精度的影响，并给出了网络模型的预测精度.结果表明，针对尖楔外形测压孔配置特征，基于RBF的人工神经网络算法解算精度较好，攻角、侧滑角、Mach数及静压的网络输出预测值与真实值吻合较好，输出的测试误差（绝对值）分别小于0.25°，0.5°，0.05及250 Pa.结果同时表明神经网络建模技术在尖楔前体飞行器FADS系统中的有效性.      

Closed-form analysis for indirect learning digital pre-distorter for wideband wireless systems

Indirect learning architecture (ILA) for digital pre-distortion (DPD) is commonly used to linearize power amplifiers (PA). To the author’s best knowledge, most of the DPD results in the literature obtain the matrix form of the least-square solution in order to get the DPD coefficients numerically. There exists no explicit closed-form for these coefficients that can be used as plug-and-play in simulations, or used for further closed-form analysis of important measures such as signal-to-noise ratio (SNR) and mean square error (MSE), bit-error rate (BER), …etc. In this paper, we analyze the ILA-DPD system for general memory-polynomial PA models. We provide a closed-form solution for the DPD coefficients. We first present the analytical methodology for deriving the mathematical expressions for each DPD coefficient and then introduce an open-access code that generates the DPD coefficients in symbolic form that is used to mathematically model the DPD. We consider case studies for PA and show that the analytical DPD solution matches the Monte Carlo simulations. Moreover, we also provide a closed-form solution for the iterative adaptive ILA-DPD. Our analysis shows that in the case of a large training block length the non-iterative DPD achieves approximately the same performance as an iterative DPD with a shorter training block length. System impairments are also considered, e.g. the thermal noise and the quantization noise in analog–digital conversion (ADC). We derive the normalized mean square error (NMSE) for the transmit chain in the presence of these impairments. The NMSE expression is verified through numerical simulations.     

Analysis of the Look-Locker T(1) mapping sequence in dynamic contrast uptake studies: simulation and in vivo validation

An alternative to the pulse sequences at present used in dynamic contrast uptake MRI is the dynamic LL-EPI T(1) mapping method. This method generates T(1) estimates in a few seconds, thereby allowing dynamic studies. A particular advantage of the LL-EPI technique is that it provides the opportunity to generate spatial and temporal information about the paramagnetic contrast agent concentration independently of the inflow rate. This paper illustrates, by computer simulations, the accuracy of the estimated 1/T(1) value when using the LL-EPI technique in situations that are not supported by the model. The simulated situations not supported by the model are those in which the longitudinal and transversal relaxation rates change during the T(1) mapping. The most critical moment occurs during a bolus passage of contrast agent when the concentration gradient is large. The computer simulations of the LL-EPI T(1) mapping method in non-supported situations show that in normal perfused capillary tissue the error in the estimated 1/T(1) value is within the absolute error of 0.1 s(-1) in most simulated situations, although in a typical vessel the simulations do indicate that the stated absolute error tolerance of 0.5 s(-1) is exceeded relatively easily. However, this transgression can be rectified by a non-bolus injection of the contrast agent media.     

TLD measurements and Monte Carlo simulations for glandular dose and scatter fraction assessment in mammography: A comparative study

The main purpose of this study was to validate and compare Mean Glandular Dose (MGD) values obtained using Monte Carlo simulations with experimental values obtained from Entrance Surface Dose (ESD) and depth dose measurements performed in a Hospital mammography unit. ESD and depth dose were measured using ThermoLuminescent Dosimeters (TLDs), and a tissue equivalent mammography phantom recommended by the American College of Radiology (ACR). Measurements and Monte Carlo simulations were also compared with the MGD calculated using the Automatic Exposure Control (AEC) system of the mammographic unit. In the simulations the Doppler energy broadening effect was also taken into account. The simulated ESD are about 5%–10% higher than the measured ESD values. The deviation between the measured and simulated MGD values in the phantom is of about 15%. The MGD evaluated using the AEC system is smaller both with respect to the Monte Carlo simulation and experimental result by a factor of about 15% and 25% respectively. Moreover the BackScatter Factor (BSF) estimated by Monte Carlo simulations was used for the MGD calculation according to the Wu’s method. Finally the inclusion of the energy broadening effect on MGD calculation produces negligible variations on the simulated results.     

因子分析法校下在ICP—AES中光谱干扰及与其他方法的比较

本文将目标变换因子分析用于电感耦合等离子的子发射光谱分析中光谱干扰的校下在，对因子分析法校正谱线重叠和结构背影干扰作屯较详细的讨论。     

Correlation effects in the diffusion and electrical conductivity of an interacting lattice gas

A simple and effective self-consistent scheme is proposed for the determination of the average potentials which allows thermodynamic functions and other characteristics of lattice systems in equilibrium to be calculated with high accuracy and short computing time. This scheme has been used for analysis of the expressions for the coefficients of diffusion and electrical conductivity obtained on the basis of the modern statistical theory of nonequilibrium processes. Results of the simulations are correlated with the data of Monte Carlo simulations obtained using parallel vector algorithms on a Cray TZE computer of the Max Plank Society (Germany).     

A methodology for multivariate simulation with massively parallel computing systems for NPP safety assessment: VARIA code

The paper is focused on the practical application of parallel computing techniques in uncertainty assessment in simulation of heat transfer, mechanical and some other problems related to deterministic analysis of NPP safety. A methodology is developed and implemented in VARIA computer code that performs simultaneous run of multiple simulations on a parallel computing system with further statistical analysis of the array of their results. The current version of the code allows automated preparation and execution of multivariate simulations of thermal and mechanical behavior of pressurized water reactor structures by best-estimate (BE) codes in the scope of NPP safety assessment under severe accident conditions. The number of simultaneously launched tasks is limited only by the computer cluster capacity. The VARIA code is verified on multivariate simulation with HEFEST code of thermal behavior of a core melt in the VVER-440 reactor vessel during a severe accident. The influence of the variation of input parameters (decay heat value and coefficients of the applied convective heat transfer model) on the simulation results is studied. It is concluded that the potential field of applying the program is beyond the scope of analyzing severe accidents at NPP and includes also software product quality assurance and analysis of uncertainties of obtained simulation results.     

On the calculation of proton-proton coupling by π-electrons

A new method for molecular dynamics computer simulations, called the multiple time-step (MTS) method, is described, in which two or more time steps of different lengths are used to integrate the equations of motion in systems governed by continuous potential functions. With this method computing speeds have been increased by factors of three to eight over conventional molecular dynamics methods in simulations of monatomic and polyatomic fluids, with only marginal increases in computer storage.     

Computation of the hindrance factor for the diffusion for nanoconfined ions: molecular dynamics simulations versus continuum-based models

We report the self-diffusion coefficients and hindrance factor for the diffusion of ions into cylindrical hydrophilic silica nanopores (hydrated silica) determined from molecular dynamics (MD) simulations. We make a comparison with the hindered diffusion coefficients used in continuum-based models of nanofiltration (NF). Hindrance factors for diffusion estimated from the macroscopic hydrodynamic theory were found to be in fair quantitative agreement with MD simulations for a protonated pore, but they strongly overestimate diffusion inside a deprotonated pore.     

The structure of liquid bromine

The atom-atom correlation function for liquid bromine, derived from the X-ray diffraction studies reported in the preceding paper, is compared with the results of Monte Carlo computer simulations and reference site interaction model (RISM) calculations for various bromine-like model molecules. The models used included the two-centre Lennard-Jones interaction model, both by itself and with idealized point quadrupole and quadrupole-induced dipole interaction terms in the computer simulations, and two and three-centre hard-sphere models in RISM. None of these models lead to atom-atom correlation functions which are in good agreement with that obtained from experiment. This suggests that additional information about the interactions between bromine molecules will be needed before the structure of liquid bromine can be completely understood. Finally, by comparing the results of our computer simulations with RISM results for analogous models, we obtain some insight into the accuracy of the reference site interaction model.     

Transitioning model potentials to real systems

The parameters of two pair potentials that describe argon over its entire liquid phase at a fixed pressure were optimized through a novel application of constant temperature and pressure molecular dynamics (NPT-MD) and Monte Carlo (NPT-MC) computer simulations. The forms of these potentials were those of a modified Lennard-Jones potential and a Lennard-Jones potential. The optimized potential determined using NPT-MD simulations reproduces experimental densities, internal energies and enthalpies with an error less than 1% over most of the liquid range and yields self-diffusion coefficients that are in excellent agreement with experiment. The results using the potential determined by NPT-MC simulations are in almost as good agreement with deviations from experiment of no more than 5.89% for temperatures up to vaporization. Additionally, molar volumes predicted using this potential at pressures in the range 100–600 atm and over temperatures in the range 100–140K were within 0.83% of experimental values. These results show that, when properly parametrized, Lennard-Jones-like potentials can describe a system well over a large temperature range. Further, the method introduced is easy to implement and is independent of the form of the interaction potential used.     

Relaxation and transport in FCHC lattice gases

FCHC lattice gases are the basic models for studying flow problems in three-dimensional systems. This paper presents a self-contained theoretical analysis and some computer simulations of such lattice gases, extended to include an arbitrary number of rest particles, with special emphasis on non-semi-detailed balance (NSDB) models. The special FCHC lattice symmetry guarantees isotropy of the Navier-Stokes equations, and enumerates the 12 spurious conservation laws (staggered momenta). The kinetic theory is based on the mean field approximation or the nonlinear Boltzmann equation. It is shown how calculation of the eigenvalues of the linearized Boltzmann equation offers a simple alternative to the Chapman-Enskog method or the multi-time-scale methods for calculating transport coefficients and relaxation rates. The simulated values for the speed of sound in NSDB models slightly disagree with the Boltzmann prediction.     

Thermodynamics of the Stockmayer fluid in an applied field

The thermodynamic properties of the Stockmayer fluid in an applied field are studied using theory and computer simulation. Theoretical expressions for the second and third virial coefficients are obtained in terms of the dipolar coupling constant (λ, measuring the strength of dipolar interactions as compared to thermal energy) and dipole–field interaction energy (α, being proportional to the applied field strength). These expressions are tested against numerical results obtained by Mayer sampling calculations. The expression for the second virial coefficient contains terms up to λ⁴, and is found to be accurate over realistic ranges of dipole moment and temperature, and over the entire range of the applied field strength (from zero to infinity). The corresponding expression for the third virial coefficient is truncated at λ³, and is not very accurate: higher order terms are very difficult to calculate. The virial coefficients are incorporated in to a thermodynamic theory based on a logarithmic representation of the Helmholtz free energy. This theory is designed to retain the input virial coefficients, and account for some higher order terms in the sense of a resummation. The compressibility factor is obtained from the theory and compared to results from molecular dynamics simulations with a typical value λ = 1. Despite the mathematical approximations of the virial coefficients, the theory captures the effects of the applied field very well. Finally, the vapour–liquid critical parameters are determined from the theory, and compared to published simulation results; the agreement between the theory and simulations is good.     

基于结构光投影的二维S变换轮廓术

S变换是一种集合了窗口傅里叶变换和小波变换优点的时频分析技术,目前一维S变换已成功应用于结构光投影的条纹相位解调中。由于二维S变换可以对图像在两个方向上进行时频分析,具有更优于一维S变换的分析和处理能力。为了完善S变换的条纹相位解调理论,将二维S变换方法引入到基于结构光投影的三维光学测量中,研究了二维S变换在条纹相位解调中的原理及应用,给出了详尽的理论分析,并同一维S变换结果进行了比较。模拟和实验都表明,在条纹图解相中,二维S变换比一维S变换提取的相位精度更高,即使在存在较严重噪声污染的情况下也表现出良好的可靠性,体现出二维S变换提取相位的优势。     

Prediction of energy decay in room impulse responses simulated with an image-source model

A method is proposed that provides an approximation of the acoustic energy decay (energy-time curve) in room impulse responses generated using the image-source technique. A geometrical analysis of the image-source principle leads to a closed-form expression describing the energy decay curve, with the resulting formula being valid for a uniform as well as nonuniform definition of the enclosure's six absorption coefficients. The accuracy of the proposed approximation is demonstrated on the basis of impulse-response simulations involving various room sizes and reverberation levels, with uniform and nonuniform sound absorption coefficients. An application example for the proposed method is illustrated by considering the task of predicting an enclosure's reflection coefficients in order to achieve a specific reverberation level. The technique presented in this work enables designers to undertake a preliminary analysis of a simulated reverberant environment without the need for time-consuming image-method simulations.     

Interdiffusion and thermotransport in Ni–Al liquid alloys

In this paper, we present extensive self-consistent results of molecular dynamics (MD) simulations of diffusion and thermotransport properties of Ni–Al liquid alloys. We develop a new formalism that allows easy connection between results of the MD simulations and the real experiments. In addition, this formalism can be extended to the case of ternary and higher component liquid alloys. We focus on the temperature and composition dependence of the self-diffusion coefficients, interdiffusion coefficients, thermodynamic factor, Manning factor and the reduced heat of transport. The two latter quantities both represent measures of the off-diagonal Onsager phenomenological coefficients. The Manning factor and the reduced heat of transport can be related to experimentally obtainable quantities provided the thermodynamic factor is available. The simulation results for the reduced heat of transport show that for all compositions, in the presence of a temperature gradient, Ni tends to migrate to the cold end. This is in agreement with an available experimental study for a Ni_21.5Al_78.5 melt (only qualitative result is available so far).     

基于相息图和小波变换的数字水印

提出一种基于相息图和小波变换的数字水印方案。利用迭代相位恢复算法将水印图像编码为相息图, 然后将经权重因子调制后的相息图嵌入到宿主图像的三层小波低频系数中, 完成整个水印嵌入过程。在水印提取阶段, 对宿主图像和含水印图像进行三层小波分解, 将得到的低频系数对应相减提取出水印相息图, 然后对此相息图进行傅里叶变换操作, 取其振幅即可提取原始嵌入的水印图像。针对不同权重因子的水印系统, 详细分析和讨论了所提出水印方案的隐蔽性和稳健性。计算机仿真结果验证了该数字水印方案的可行性。     

目标因子法同时测定息喘灵中三组分

息喘灵中三种组分吸收峰发生重叠，采用传统化学分析法测定三组操作繁琐费时，不宜批量分析。本文利用目标因子紫外分光光度法研究不经分离同时测定三组分，参照均匀设计表配五组不同浓度组合的混合物标准溶液，用紫外分光度计测出混合物标准溶液在210，212，214，216，218和220nm六个波长点的吸光度，再在相同条件下测定出样品的吸光度，然后将所得数据输入计算机，用目标因子法进行处理，即处理求得各组分含量，研究结果表明，选择全适参数，可使息喘灵中茶碱、盐酸麻黄碱，异戊比妥回收率达到98．4％－102．6％，样品分析相对偏差为0．49％－5．00％。     

Theoretical study in applications of doublet mechanics to detect tissue pathological changes in elastic properties using high frequency ultrasound

The mathematical framework of a new elastic theory-doublet mechanics (DM)-was reviewed. The fundamental difference between DM and classical continuum mechanics (CCM) is that the former has taken the discrete nature of tissue on the cellular level into account and the latter assumes tissue is uniform and continuous. Theoretical calculations based on DM were performed for reflection coefficients of a substrate-tissue layer-substrate assembly. Results of computer simulations have shown that ultrasound reflection coefficients in the range of 15-30 MHz are sensitive to changes in cell size and elastic moduli of tissue according to DM but not to CCM. Potential experimental applications of this technique to tissue characterization are discussed.