Conditional simulation of random fields by successive residuals

This paper presents a new approach to the LU decomposition method for the simulation of stationary and ergodic random fields. The approach overcomes the size limitations of LU and is suitable for any size simulation. The proposed approach can facilitate fast updating of generated realizations with new data, when appropriate, without repeating the full simulation process. Based on a novel column partitioning of the L matrix, expressed in terms of successive conditional covariance matrices, the approach presented here demonstrates that LU simulation is equivalent to the successive solution of kriging residual estimates plus random terms. Consequently, it can be used for the LU decomposition of matrices of any size. The simulation approach is termed conditional simulation by successive residuals as at each step, a small set (group) of random variables is simulated with a LU decomposition of a matrix of updated conditional covariance of residuals. The simulated group is then used to estimate residuals without the need to solve large systems of equations.

Winding patterns for actively shielded shim coils with asymmetric target-fields

A method is presented for calculating the currents and winding patterns required to design independent zonal and tesseral shim coils for magnetic resonance imaging. Both actively shielded and unshielded configurations are considered, and the region of interest can be located asymmetrically with respect to the coil's length. Streamline, target-field and Fourier-series methods are utilized. The desired target-field is specified at two cylindrical radii, on and inside a circular conducting cylinder of length 2L and radius a. The specification is over some asymmetric portion pL < z < qL of the coil's length (-1 < p < q < 1). Arbitrary functions are used in the outer sections, -L < z < pL and qL < z < L, to ensure continuity of the magnetic field across the entire length of the coil. The entire field is then periodically extended as a half-range cosine Fourier series about either end of the coil. The resultant Fourier coefficients are then substituted into the Fourier-series expressions for the internal and external magnetic fields, and current densities and stream functions on both the primary coil and shield. A contour plot of the stream function directly gives the required coil winding patterns. Spherical harmonic analysis and shielding analysis on field calculations from a ZX shim coil indicate that example designs and theory are well matched.

Calculation of electric fields induced by body and head motion in high-field MRI

In modern magnetic resonance imaging (MRI), patients are exposed to strong, nonuniform static magnetic fields outside the central imaging region, in which the movement of the body may be able to induce electric currents in tissues which could be possibly harmful. This paper presents theoretical investigations into the spatial distribution of induced electric fields and currents in the patient when moving into the MRI scanner and also for head motion at various positions in the magnet. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic, full-body, male model. 3D field profiles from an actively shielded 4T magnet system are used and the body model projected through the field profile with a range of velocities. The simulation shows that it possible to induce electric fields/currents near the level of physiological significance under some circumstances and provides insight into the spatial characteristics of the induced fields. The results are extrapolated to very high field strengths and tabulated data shows the expected induced currents and fields with both movement velocity and field strength. (C) 2003 Elsevier Science (USA). All rights reserved.

Influence of magnetically-induced E-fields on cardiac electric activity during MRI: A modeling study

In modern magnetic resonance imaging (MRI), patients are exposed to strong, time-varying gradient magnetic fields that may be able to induce electric fields (E-fields)/currents in tissues approaching the level of physiological significance. In this work we present theoretical investigations into induced E-fields in the thorax, and evaluate their potential influence on cardiac electric activity under the assumption that the sites of maximum E-field correspond to the myocardial stimulation threshold (an abnormal circumstance). Whole-body cylindrical and planar gradient coils were included in the model. The calculations of the induced fields are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic, whole-body model. The potential for cardiac stimulation was evaluated using an electrical model of the heart. Twelve-lead electrocardiogram (ECG) signals were simulated and inspected for arrhythmias caused by the applied fields for both healthy and diseased hearts. The simulations show that the shape of the thorax and the conductive paths significantly influence induced E-fields. In healthy patients, these fields are not sufficient to elicit serious arrhythmias with the use of contemporary gradient sets. However, raising the strength and number of repeated switching episodes of gradients, as is certainly possible in local chest gradient sets, could expose patients to increased risk. For patients with cardiac disease, the risk factors are elevated. By the use of this model, the sensitivity of cardiac pathologies, such as abnormal conductive pathways, to the induced fields generated by an MRI sequence can be investigated. (C) 2003 Wiley-Liss, Inc.

Instrumentation used to measure residential magnetic fields and currents

The equipment used to measure magnetic fields and, electric currents in residences is described. The instrumentation consisted of current transformers, magnetic field probes and locally designed and, built signal conditioning modules. The data acquisition system was capable of unattended recording for extended time periods. The complete system was calibrated to verify its response to known physical inputs. (C) 2003 ISA-The Instrumentation Automation Society.

Adapted Gaussian basis sets for atoms from Li through Xe generated with the generator coordinate Hartree-Fock method

Utiliza-se o m��todo coordenada geradora Hartree-Fock para gerar bases Gaussianas adaptadas para os ��tomos de Li (Z=3) at�� Xe (Z=54). Neste m��todo, integram-se as equa����es de Griffin-Hill-Wheeler-Hartree-Fock atrav��s da t��cnica de discretiza����o integral. Comparam-se as fun����es de ondas geradas neste trabalho com as fun����es de ondas Roothaan-Hartree-Fock de Clementi e Roetti (1974) e com outros conjuntos de bases relatados na literatura. Para os ��tomos estudados aqui, os erros em nossas energias totais relativos aos limites num��ricos Hartree-Fock s��o sempre menores que 7,426 milihartree.

Modelo de escoamento multif��sico para estudo da intera����o onda/sedimento

Modelos de escoamento multif��sico s��o amplamente usados em diversas ��reas de pesquisa ambiental, como leitos fluidizados, dispers��o de g��s em l��quidos e v��rios outros processos que englobam mais de uma propriedade f��sico-qu��mica do meio. Dessa forma, um modelo multif��sico foi desenvolvido e adaptado para o estudo do transporte de sedimentos de fundo devido �� a����o de ondas de gravidade. Neste trabalho, foi elaborado o acoplamento multif��sico de um modelo euleriano n��o-linear de ondas do tipo Boussinesq, baseado na formula����o num��rica encontrada em Wei et al. (1995), com um modelo lagrangiano de part��culas, fundamentado pelo princ��pio Newtoniano do movimento com o esquema de colis��es do tipo esferas r��gidas. O modelo de ondas foi testado quanto �� sua fonte geradora, representada por uma fun����o gaussiana, p��-pist��o e p��-batedor, e quanto �� sua intera����o com a profundidade, atrav��s da n��o-linearidade e de propriedades dispersivas. Nos testes realizados da fonte geradora, foi observado que a fonte gaussiana, conforme Wei et al. (1999), apresentou melhor consist��ncia e estabilidade na gera����o das ondas, quando comparada �� teoria linear para um kh ��� ��� . A n��o-linearidade do modelo de ondas de 2�� ordem para a dispers��o apresentou resultados satisfat��rios quando confrontados com o experimento de ondas sobre um obst��culo trapezoidal, onde a deforma����o da onda sobre a estrutura submersa est�� em concord��ncia com os dados experimentais encontrados na literatura. A partir da��, o modelo granular tamb��m foi testado em dois experimentos. O primeiro simula uma quebra de barragem em um tanque contendo ��gua e o segundo, a quebra de barragem �� simulada com um obst��culo r��gido adicionado ao centro do tanque. Nesses experimentos, o algoritmo de colis��o foi eficaz no tratamento da intera����o entre part��cula-part��cula e part��cula-parede, permitindo a evid��ncia de processos f��sicos que s��o complicados de serem simulados por modelos de malhas regulares. Para o acoplamento do modelo de ondas e de sedimentos, o algoritmo foi testado com base de dados da literatura quanto �� morfologia do leito. Os resultados foram confrontados com dados anal��ticos e de modelos num��ricos, e se mostraram satisfat��rios com rela����o aos pontos de eros��o, de sedimenta����o e na altera����o da forma da barra arenosa

Studies on mosquitoes (Diptera: Culicidae) and anthropicenvironment: 5- Breeding of Anopheles albitarsis in flooded rice fields in South-Eastern Brazil

Studies on breeding Anopheles albitarsis and association with rice growth in irrigated paddy fields were carried out during the rice cultivation cycle from December 1993 to March 1994. This period corresponded to the length of time of permanent paddy flooding. Breeding occurred in the early stage up until five weeks after transplantation when rice plant height was small. That inverse correlation may give potential direction to control measures.

Studies on mosquitoes (Diptera: Culicidae) and anthropic environment: 6 - Breeding in empty conditions of rice fields in South-Eastern Brazil

Studies on culicid breeding in empty rice fields were carried out during the cultivation cycle from May to November 1993. This period corresponded to stages 1 and 2, when empty conditions prevailed. Breeding occurred in stage 1 and the first part of stage 2, corresponding respectively to fallow uncultivated and ploughing situations. No breeding was found to take place during the second part of stage 2 when transient floods and harrowing occurred. The predominant species were Aedes scapularis, Culex nigripalpus and Cx. mollis. The Pilosus Group of Culex (Melanoconion) was found at lower densities. Some epidemiological considerations are presented.

Maryam Mirzakhani : a primeira mulher vencedora da Medalha Fields

Este artigo apresenta uma breve biografia de Maryam Mirzakhani, a primeira mulher vencedora da Medalha Fields, pr��mio institu��do desde 1936 e que �� concedido de quatro em quatro anos a matem��ticos com menos de 40 anos.

A Medalha Fields

A Medalha Fields �� um pr��mio desconhecido (ou quase) para a maioria das pessoas. Algumas ter��o ouvido dizer que �� semelhante ao Pr��mio Nobel da Matem��tica. Outras dir��o que �� um pr��mio de excel��ncia s�� para os matem��ticos. A Medalha Fields, oficialmente conhecida como Medalha Internacional de Descobrimentos Proeminentes em Matem��tica, �� um conceituado pr��mio concedido a dois, tr��s ou quatro matem��ticos com menos de 40 anos e entregue durante o Congresso Internacional de Matem��tica que se realiza de quatro em quatro anos. �� equiparada, em termos de prest��gio, ao Pr��mio Nobel.

Risk Assessment of Chronic Exposure to Magnetic Fields near Electrical Apparatus

Confer��ncia: 9th International Symposium on Occupational Safety and Hygiene (SHO) Guimaraes, Portugal - FEB 14-15, 2013

Remote lab experiments: opening possibilities for distance learning in engineering fields

Remote experimentation laboratories are systems based on real equipment, allowing students to perform practical work through a computer connected to the internet. In engineering fields lab activities play a fundamental role. Distance learning has not demonstrated good results in engineering fields because traditional lab activities cannot be covered by this paradigm. These activities can be set for one or for a group of students who work from different locations. All these configurations lead to considering a flexible model that covers all possibilities (for an individual or a group). An inter-continental network of remote laboratories supported by both European and Latin American institutions of higher education has been formed. In this network context, a learning collaborative model for students working from different locations has been defined. The first considerations are presented.

Pre-harvest cane burning and health: the association between school absences and burning sugarcane fields

The objective of this study was to investigate an association between pre-harvest sugarcane burning and respiratory diseases in children under five years of age. The following data were collected in five schools in the city of Araraquara, SP, Southeastern Brazil, between March and June 2009: daily records of absences and the reasons stated for these absences, total concentration of suspended particulate matter (��g/m3), and air humidity. The relationship between the percentage of school absences due to respiratory problems and the concentration of particulate matter in March and from April to June presented a distinct behavior: absences increased alongside the increase in particulate matter concentration. The use of school absences as indicators of this relationship is an innovative approach.