Modeling of radio frequency induced currents on lead wires during MR imaging using a modified transmission line method (MoTLiM)

Abstract

Magnetic resonance imaging (MRI) is widely used diagnosis technique. During MRI radio frequency (RF) fields are utilized to excite the spins. If these RF fields incidence on metallic implants, currents will be induce on the metallic parts of implants. Inside the body these induced currents on metallic implants cause heating of tissue and sometimes cause severe burning of tissue. This phenomena makes MRI hazardous for patients with metallic implants. Much work has been done to understand this phenomena. However, most of these work based on purely experimental or numerical methods. So to understand and to obtain a good intuition on this problem a lot of cases must be solved computationally or tested experimentally. In this study lumped element model of the transmission line is modified in order to model the conductive wires of implants inside the body. This model is based on the similarity between the damped oscillatory behavior of transmission line currents and induced currents on wires inside the body. A voltage source is added to model the effect of the incident electric field. Voltages and currents on a infinitesimally small portion of wire are solved. Solving currents and voltages simultaneously on the modified lumped element model lead to a non-homogeneous differential equation for the current. The solution of this differential equation gives the analytical solution for the induced current on the implant lead. To test the validity of this solution, wire under the uniform incident electric field is solved with the Modified Transmission Line Method (MoTLiM) and compared to Methods of Moment (MoM) solution. The results are also verified using phantom experiments. For experimental verification, the distorted flip angle distribution due to induced currents are measured using flip angle imaging techniques. In addition to this, the flip angle distribution around the wire is calculated using results obtained from MoTLiM. Finally these results are compared and an error analysis is carried out.