A homogeneous RF-shielded magnet for low-field magnetic resonance studies

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Smith, Kyla Marie
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Today’s clinical magnetic resonance imaging (MRI) systems have B0-fields of 1.5 − 7.0T, and research usage time on these high-field systems is limited and expensive. The development of novel MRI techniques, such as transmit array spatial encoding (TRASE), can be accelerated by access to in-house, small-scale, low-field systems. This thesis therefore develops a method for designing a low-field magnet with excellent field homogeneity. The value of this method is its versatility: while it is used here to design a uniform field within a cylindrical volume, the method can also be applied to other geometries and to produce any desired field profile. An overview of the design method, and the research performed in this thesis, is as follows. The magnetic scalar potential Φ is chosen inside what will be the magnet volume to give the desired field profile; outside, however, an outer boundary surface is set-up and Φ is solved for numerically from the Laplace equation using a finite element method within the region between the two surfaces. The discontinuity in the magnetic scalar potential ∆Φ at the surface of the inner volume gives the required surface current distribution of the magnet, which is then discretized into wires. Biot-Savart field calculations are used to simulate the field and quantify its homogeneity. The cylindrical magnet of 25-cm radius and 1-m length designed here is found to have a theoretical homogeneity of < 1 ppm over a 20-cm diameter spherical volume.
Magnetic Resonance, Transmit Array Spatial Encoding (TRASE) MRI, Magnet design, Low field