Development of a Computational Method for Assessing Static Field Induced Torque on Medical Implants
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.