Modeling RF heating of active implantable medical devices during MRI using safety index
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Magnetic Resonance Imaging (MRI) is known as a safe imaging modality that can be hazardous for patients with active implantable medical devices, such as a pacemakers or deep brain stimulators. The primary reason for that is the radio frequency (RF) heating at the tips of the implant leads. In the past, this problem has been analyzed with phantom, animal and human experiments. The amount of temperature rise at the lead tip of these implants, however, has not been theoretically analyzed. In this thesis, a simple approximate formula for the safety index of implants, which is the temperature increase at the implant lead tip per unit deposited power in the tissue without the implant in place, was derived. For that purpose, an analytical quadrature birdcage coil model was developed and the longitudinal incident electric field distribution inside the body was formularized as follows: ER H z () R = −ω µ − in which ω is the angular frequency, µ is the magnetic permeability of the tissue, H- is the left hand rotating component of the RF magnetic field and R is the radial distance from the center of the body. This formula was examined by simulations and phantom experiments. The analytical, simulation and experimental results of that model are in good agreement.Then, depending on the quadrature birdcage coil model safety index (SI) formula for active implants with short leads was derived as shown below: 2 max 2 peak 1 ( ) 2 j t b T SI Rl Ae f Dv SAR c R θ α ∆ == + where ∆Tmax is the maximum temperature increase in the tissue, SARpeak is the maximum deposited power in the body when there is no implant in the body, α is the diffusivity of the tissue, ct isthe heat capacity of the tissue, Rb is radius of the body, R is the radial distance from the center of the body, l is the length of the implant lead, A is the area of the curvature of the lead, θ is the angle that curvature of the implant makes with the radial axis, and f(Dv) is the perfusion correction factor, which is function of the diameter of the electrode and perfusion. The safety index formula was tested by simulations. Simulation results showed that the theoretical safety index formula approximates and identifies the RF heating problem of active implants with short leads accurately. The safety index formula derived in this thesis is valid for only short wires. However, the formulation for long wires is currently under investigation. Despite the fact that the results obtained for short leads can not be generalized for the safety of patients with active implants, it is believed that this study is the first step towards safety of these patients. Using safety index as a measure of safety is very beneficial to ensure the safety of patients with active implants. Because, it uses the MR scanner-estimated deposited power that does not take the existence of the implant in the patient body into account. This formulation is the first study illustrating the advantage of the safety index metric for RF heating studies of active implants.