Simulator for very-low-field magnetic resonance imaging

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Sähkötekniikan korkeakoulu | Master's thesis
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Date

2022-02-02

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Major/Subject

Translational Engineering

Mcode

ELEC3023

Degree programme

AEE - Master’s Programme in Automation and Electrical Engineering (TS2013)

Language

en

Pages

92

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Abstract

Very-Low-Field Magnetic Resonance Imaging (VLF-MRI), working in the range of 1mT to 100mT, has recently gained interest because of its advantages, such as low manufacturing and operational cost, smaller size and portability, and the ability to scan metal implanted or intubated patients. Although numerous simulation software packages are available for studying high field MR imaging, as far as known, no versatile simulator has been especially developed for VLF-MRI. Therefore, this study tries to fill this gap by developing a VLF-MRI simulator based on an existing versatile high-field MRI simulator to be used in the study of challenges in VLF-MRI scanner design. For this purpose, multiple available MRI simulators are studied, and the selected MRiLab software package is modified and further developed into a VLF-MRI simulator. The square and circular Helmholtz coils models are used to simulate hardware-related inhomogeneity in the main magnetic field, and the inverse Fourier transform method for modelling inhomogeneity due to susceptibility deviation in a 20 mT virtual phantom. In addition, a model of electromagnetic interference of an AC power line and a model of the impulse response of induced eddy current fields resulting from switching gradient coils are implemented into the simulator. This study verifies the developed simulator in selected imaging sequences. The study also evaluates the impact of magnetic field inhomogeneity on generating artifacts such as geometrical distortion and banding artifacts in different sequences. In addition, the inhomogeneity result of susceptibility deviation is studied in gradient echo images. In addition, this thesis investigates how electromagnetic interference of a power line degrades the quality of spin echo and gradient echo images in different receiver bandwidths. The results show that spin echo sequence is less sensitive to the interference than gradient echo sequence and sensitivity to the interference reduces in the lower bandwidth. The study of induced eddy currents on b-SSFP images shows that even a small amount of induced eddy current resulting from actively shielded gradient coils causes banding artifacts in the image. The result of this study is further development of a simulator that can be used for several purposes, such as optimization of magnet design of the MRI system based on application and cost, evaluation of magnetic field correction methods, shielding system, pre-emphasis gradient unit of the MRI system, and developing post-processing methods to enhance image quality.

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Supervisor

Laakso, Ilkka

Thesis advisor

Palva, Lauri

Keywords

simulator, VLF-MRI, field inhomogeneity, eddy currents, EMI

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https://urn.fi/URN:NBN:fi:aalto-202202061758

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[dipl] Sähkötekniikan korkeakoulu / ELEC