Dipole-Fed Rectangular Dielectric Resonator Antennas for Magnetic Resonance Imaging at 7 T: The Impact of Quasi-Transverse Electric Modes on Transmit Field Distribution

Shortened dipole antennas based on rectangular dielectric blocks play an important role in ultrahigh field magnetic resonance imaging (UHF-MRI) radio frequency (RF) coil design. However, the generally assumed direct contact with the subject is difficult to maintain in typical in vivo settings. We have previously observed that certain dielectrically shortened dipole antennas can produce a substantially altered transmit field distribution with a very low transmit efficiency when the block and the sample are physically separated. Therefore, the aim of this study was to determine a) why certain designs of dielectrically shortened dipole antennas can produce an inefficient transmit field when the block and the sample are physically separated and b) how this depends on key parameters such as rectangular block geometry, dielectric constant, loading geometry, and RF feeding. In this work, two main types of quasi-transverse dielectric modes were found in different rectangular block geometries and interpreted as TE11δz (MR efficient) and TE1δδy (MR inefficient), and their impact on in vivo MRI experiments involving the human head, calf, and wrist was explored. This study shows, for the first time, why certain antennas preserve their transmit field efficiency despite physical separation from the sample. We conclude that the proposed approach has the potential to provide new insights into dipole antenna design for UHF-MRI.

Anchorage Zone Stresses Model of Interior Diaphragm for Segmental Box Girder Bridge

Post-tensioning tendons in segmental box girder bridges are usually anchored in very massive diaphragms. This paper presents an analytical investigation of the anchorage zone stresses for an interior pier diaphragm and a rectangular block model (model of diaphragm anchorage zone). The failure load and load-displacement relation for the two cases were compared, a typical diaphragm for a segmental box girder bridge with external prestressing tendons and rectangular block model were analyzed by finite element program (ABAQUS). The anchorage devices were subjected to a prestressing design load which was based on the ultimate strength of the tendons. The magnitude and distribution of stresses were plotted through different paths for both the diaphragm and the block model; also the contours of stress distribution for the diaphragm and block model were compared. The results showed that the stresses and failure load for the diaphragm and rectangular block model were very close.

RESOLUÇÃO DE PROBLEMAS NAS AULAS DE MATEMÁTICA: PROMOVENDO A APRENDIZAGEM SIGNIFICATIVA DO CONCEITO DE VOLUME DE BLOCOS RETANGULARES / PROBLEM SOLVING IN MATH CLASSES: PROMOTING THE MEANINGFUL LEARNING OF THE RECTANGULAR BLOCK VOLUME CONCEPT

Neste artigo objetivou-se compreender como a Metodologia de Resolução de Problemas pode contribuir para a aprendizagem significativa do conceito de volume de blocos retangulares. Para isto, por meio da pesquisa participante, considerou as etapas da metodologia de Resolução de Problemas e aportes teóricos de autores que discutiram a Aprendizagem Significativa. Participaram desta pesquisa vinte alunos de uma escola estadual localizada município de Ilhéus, na Bahia. Na etapa de análise foram utilizados os materiais produzidos por esses alunos, bem como os registros de suas interações realizadas no diário de campo. Com os resultados foi possível constatar que a metodologia da Resolução de Problemas mostrou-se de grande relevância, proporcionando a discussão de pré-requisitos importantes para promover a aprendizagem significativa do conceito de volume de blocos retangulares. Também possibilitou uma melhor interação entre entre os alunos, resultando em suas participações ativas durante o processo de aprendizagem.

A downward buoyant force experiment

In hydrostatics, the Archimedes principle predicts an upward force whenever a body is submerged in a liquid. In contrast to common sense, this physical law is not free of exceptions, as for example when the body touches the container. This is more evident when a rectangular block less dense than the liquid rests on the bottom, with no liquid underneath it, a case in which a downward force is expected, according to a recent work by the first author. In the present work, we describe a simple, low-cost experiment which allows the detection of such force. This counterintuitive result shows the inadequacy of Archimedes' principle for treating "contact" cases.