Shim coil design for human brain magnetic resonance imaging using the stream function method

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Journal Title

Journal ISSN

Volume Title

Sähkötekniikan korkeakoulu | Master's thesis

Authors

Date

2018-10-11

Department

Major/Subject

Biosensing and Bioelectronics

Mcode

ELEC3045

Degree programme

LST - Master's Programme in Life Science Technologies (TS2013)

Language

en

Pages

44

Series

Abstract

Objective Magnetic Resonance Imaging (MRI) is a very common imaging method for studying the human brain. A fundamental requirement of MRI is the magnetic field homogeneity in the volume that is to be studied. Shim coils are used to increase the magnetic field homogeneity. In this study, we propose two shim coil designs that increase the magnetic field homogeneity in human brain. Approach We use off-resonance maps obtained from human subjects and use the target field and stream function methods to design shim current patterns. We propose a novel smoothness constraint in our method to ensure a smooth distribution of shim currents. This method was applied to design two head shim coils with either a helmet or a cylindrical geometry. Main Results Using the target field method with a stream function and a smoothness constraint, we designed the shim current distribution on a helmet shim coil. Its performance was compared with a cylindrical shim coil of similar size. While the shimming performance from a cylindrical shim coil was 5% better than that from a helmet shim coil, it needs 30% higher shim current magnitude. Significance The smoothness constraint we propose to be used in stream function method is a novel constraint that significantly simplifies the shim current patterns. We applied the target field and stream function methods for shim coil design on a helmet shaped coil which has never been done to our best knowledge. We showed the importance of current distribution symmetry and placing the shim currents as close to the magnetic field inhomogeneity as possible.

Description

Supervisor

Ilmoniemi, Risto

Thesis advisor

Lin, Fa-Hsuan

Keywords

magnetic resonance imaging, shimming, stream function method, shim coil design

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Citation

Permanent link to this item

https://urn.fi/URN:NBN:fi:aalto-201810175434

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[dipl] Sähkötekniikan korkeakoulu / ELEC