SCATTERING OF THE MAGNETIC RESONANCE IMAGING RADIOFREQUENCY FIELD BY IMPLANTED MEDICAL DEVICES

Abstract

The radiofrequency field used in Magnetic Resonance Imaging is scattered by implanted medical devices. The scattered field is concentrated in the tissue surrounding the implant and conduction currents will flow in the tissue resulting in potentially hazardous heating. Patients with medical implants can undergo diagnostic or interventional MRI procedures and thus the scattering of the MRI RF field by medical implants merits a detailed investigation. In this thesis, scattering by various types of implants has been investigated. The scattered field of a deep brain stimulation lead can be very intense near the electrodes stimulating the brain. The lead is just like an antenna excited by an incident electromagnetic field in a dissipative medium. The greatest concern regarding MRI induced heating is when the lead length approaches the resonant length. The factors that determine the resonant length of a lead are examined. The finite element method is used to find the near field for the lead immersed in nonhomogeneous tissue and connected to an implantable pulse generator as well as for varying distances of the connecting portion of the lead from the air-tissue interface. Electric field, SAR, dissipated powers and induced temperature rise distributions have been obtained in the brain tissue surrounding the electrodes. It is shown that the presence of the IPG can significantly change the induced temperature rise and that the near proximity of the air-tissue interface results in a reduction in the induced temperature rise. The computed values are in good agreement with in-vitro measurements made in the laboratory. Similar analyses and computations have been carried out for an implanted vagus nerve stimulation lead device. Current distributions in the twin-strand lead have been computed. SAR and temperature rise distributions have been obtained around the twin electrodes which are placed on the left vagus nerve. A model implant embedded in nonhomogeneous tissue has been investigated. The nature of the embedding tissue is varied and the current distribution in the implant, the scattered field, and the temperature rise distributions in the tissue surrounding the electrodes has been computed. It has been found that the induced temperature rise is significantly lower for tissue with a lower conductivity and permittivity such as fat than for tissue with a higher conductivity and permittivity such as muscle. The interaction of the MRI RF field with orthopedic implants is investigated. As specific case studies, the scattered fields due to a bone support frame implant and a hip joint implant are computed. It is found that the greatest MRI- induced heating occurs at the tips of long metal parts where the length and thickness of a metal part and its tips determine the amount of induced heating. For the bone support frame, the induced surface current density distributions on the steel pins and the spatial electric field distributions in the surrounding tissue have been obtained. For the hip joint, the maximum temperature rise is at the elliptical tip of the long cylindrical limb of the joint where it joins the femur. The spatial electric field and temperature rise distributions around intravascular stents of various lengths have also been obtained. The maximum temperature rise occurs in the tissue surrounding the tips of a stent. The induced heating effect increases with increasing length.