Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Medical Biophysics

Supervisor

Chronik, Blaine A.

Abstract

The objective of this thesis is the development of a computational method for finding the torque induced on an object when placed in the static magnetic field of an MR scanner. As a preliminary step, the classic EM problems of a sphere and infinitely long cylinder of linear material was modeled in commercially available simulation software. Upon verification of the parameters implemented, the second step is the simulation of simple objects with realistic material properties, stainless-steel cylinders. Physical cylinders were machined to match those in the simulations and underwent the ASTM standard method for measuring induced torque. An adjacent study that was also performed was finding the measurement uncertainty in a prototype ASTM abiding apparatus, separate from the one used for experimental verification.

It was found that the sphere and infinitely long cylinder models differed less than 5% from the analytical solutions. Implementing the correct material properties, magnetic susceptibility in particular, to the grades of stainless-steel used in this study was particularly challenging. However, when the experimentally measured results were used to find the necessary susceptibility values for the computational methods, it was found to be in agreement with literature values. The following computationally-found torque values agreed within 10% difference from the experimentally measured values. The induced torque increased linearly with the length of the cylinders and the square of magnetic susceptibility.

In the uncertainty analysis of the torque measurement apparatus described in ASTM F2213-17, it was found that the apparatus described in the ‘Pulley Method’ offered a lower instrument uncertainty than the apparatus described in the ‘Torsional Spring Method’. This study emphasized on the contribution of static friction and is important to consider should the apparatus be used in the future to verify computational results.

Summary for Lay Audience

Magnetic Resonance Imaging (MRI) is a method of visualizing the inside of the human body by using a variety of magnets to create a complex electromagnetic environment, or MR environment, contained within the scanning room. Since the 1990s, MRI has seen widespread adoption around the world and has since received a reputation being a safe imaging method due, in part, to the intense scrutiny that MRI technicians place on what is allowed into the scanning room.

A common signage at any MRI site is the warning that ‘The Magnet is Always On’. When foreign material, anything not already contributing to the MR environment, enter the MRI site, it may interact with the magnetic fields being generated. Material of any kind have magnetic properties. Pure iron for example, can fly across the scanning room, like a projectile, due to the displacement force exerted on it by the scanner’s magnetic fields. Human tissue, on the other hand, is so weakly magnetic that they appear to be inert until extremely high magnetic field strengths, far above what is currently clinically approved.

A significant population of patients who may benefit from MRI exams are those living with medical implants. However, the magnetic properties of the implant are not always known. Testing for implant safety is often laborious since implants often have many components allowing for innumerable configurations making it impractical to rely on experimental testing alone. Therefore, the objective of this thesis is the development of the capability to assess medical implants quickly and accurately by computational means. The particular interaction explored in this thesis is the induced rotational force on an implant from the magnetic field generated by the main magnet.

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