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Master's Dissertation
DOI
10.11606/D.76.2000.tde-06122013-164129
Document
Author
Full name
Rogério Ferreira Xavier
Institute/School/College
Knowledge Area
Date of Defense
Published
São Carlos, 2000
Supervisor
Committee
Tannus, Alberto (President)
Baffa Filho, Oswaldo
Schneider, José Fábian
Title in Portuguese
Desenvolvimento e construção de bobinas de gradiente com blindagem ativa (Active Shielding) com aplicação em imagens por RMN
Keywords in Portuguese
Bobinas gradientes blindadas
Imagens por RMN
Abstract in Portuguese
As bobinas ativamente blindadas (actively shielded) melhoram a performance das bobinas de gradiente ao cancelar o campo magnético na parte externa da bobina primária, evitando a degradação do campo no seu interior devido às correntes induzidas (eddy currents). O presente trabalho resultou na construção de um conjunto de bobinas de gradientes ativamente blindadas (Gx, Gy e Gz ), de geometria cilíndrica e acesso axial, com diâmetro de volume útil de 10cm para imagens (field of view), com campos de gradiente de aproximadamente 8.0 G/cm para r=0, com uma máxima distorção de campo de 5% neste volume, onde foi observado um efetivo cancelamento do campo externo, o que evita correntes induzidas nas partes metálicas internas do magneto OXFORD de 2.0 Tesla horizontal de 30cm de diâmetro, no qual serão empregadas. Alguns resultados prévios foram obtidos com um protótipo transversal, em apenas uma direção, e também com um conjunto de bobinas de gradiente para utilização em obtenção de imagens microscópicas de ESR pulsado. As bobinas foram projetadas utilizando-se a técnica de densidade de corrente, que se baseia na abordagem de campo alvo (target field) e também foi empregado o método de "simulated annealing" para otimização do desenho. Este sistema de gradientes apresenta também uma baixa indutância, o que permitirá chaveamentos rápidos (~0.1 ms). Na análise do campo de gradiente gerado foi utilizado o método de diferença de fase, confirmando a eficiência da técnica de densidade de corrente utilizada nos cálculos para a construção destas bobinas de gradiente
Title in English
Development and construction of Active Shielding gradient coils with application in Magnetic Resonance Imaging
Keywords in English
Magnetic resonance imaging
Shielded gradient coils
Abstract in English
Actively shielded coils are designed to improve the performance of a gradient coil set by canceling the magnetic field in the region outside the body of the coil, avoiding degradation of the field in its interior due to eddy currents induced in the electric vicinity. The present work resulted on the construction of a shielded gradient coil set (Gx, Gy e Gz) with cylindrical geometry and axial access, with volume diameter of 1 O cm (imaging FOV) and gradient field intensity of 8.0 Gauss/cm in r=0. The maximum gradient distortion aim was 5% inside that volume, along with an effective cancellation of the field close to the inner metallic parts of the OXFORD 2.0 Tesla 31 cm bore horizontal magnet. Preliminary results were obtained with a prototype, which included only one transverse channel, and a miniature set with the same geometry was specially designed to perform micro imaging experiments using pulsed Electron Paramagnetic Resonance. Coils were designed using the current density technique, which is based on the target field approach and simulated annealing for optimization. The gradient systems described here present also a low inductance necessary to perform fast switching (~0.1 ms). For the analysis of the performance of the gradients we used the method of phase reference, confirming the efficiency of the current density technique used on the design of the gradient sets
 
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Publishing Date
2013-12-09
 
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