MRI safety: Radiofrequency field induced heating of implanted medical devices
The RF field in Magnetic Resonance Imaging (MRI) induces currents in the human body, and the principal bioeffect is tissue heating. The presence of an electrically conducting medical implant will concentrate the RF-induced currents that may cause additional and possibly dangerous tissue heating. For this reason, metallic implants are generally considered to be contraindicated for MR imaging. However, patients with metallic implants often need further examinations using MR procedures. ^ In this work, MRI-induced heating of medical implants was evaluated by in vitro measurements and numerical computations. As a specific example of a medical device, a Deep Brain Stimulation (DBS) system was investigated for MRI-induced heating. Novel concepts of transfer functions and current density imaging were developed to analyze the dependence of heating on the incident field distribution and to quantify the induced current distributions on metallic wires that simulated medical leads. ^ The impact of concentration of the gelling agent in a saline-based phantom on the MRI-induced temperature rise was measured using a DBS system. It was determined that an appropriate gelling agent is required to avoid convection in order to accurately simulate the thermal properties of body tissues for measurements of RF-induced heating with medical implants. ^ Numerical methods based on integral equations to analyze the interaction between the RF-induced electric field with bare and insulated wires were developed. The reaction integral equation (RIE) was used for straight wires and the electric field integral equation (EFIE) for curved wires. Currents induced on the wire were calculated using Galerkin's method of moments. The electric field in the tissue surrounding the wire was calculated from the induced currents to determine the power deposition. From the power deposition, the temperature rise was calculated using the bioheat equation. The method was applied to a practical DBS lead and there was good agreement between measurements and calculations. ^ A transfer function between the incident electric field and the scattered field at the electrode was developed to investigate in detail how the incident field contributes to the heating of implanted medical leads.^
John Nyenhuis, Purdue University.
Engineering, Electronics and Electrical
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