Experimental and Theoretical Analysis of the Induced Voltage along Implant Leads due to Gradient Fields

Introduction: Magnetic resonance imaging (MRI) is an important imaging technique to diagnose various diseases. However, patients with an implanted medical device, e.g. pacemakers, cannot benefit from this diagnostic modality. To analyze the adverse effects of MRI on patients with implants, various experimental studies have been performed [1]. It is observed that due to radiofrequency pulses, heating at the tip of the implant lead can occur and due to gradient fields, undesired stimulation can be observed. The harmful effects of gradient and static magnetic fields on the pacemaker also include asynchronous pacing, inhibition of pacing output, damage at the pacemaker circuitry causing changes at the program, or movement of the device [1]. In this study, we present a mathematical formulation that shows the risk of undesired stimulation due to the induced electric field by the gradient fields. During MRI, it is known that gradient fields induce an electric field inside the conductive medium. In this study, to analyze the safety aspect of the induced electric field on the implant lead, we use the simplified analytical electric field expressions obtained for a homogenous body model [2]. With the help of these expressions for x, y and z gradient coils, approximate voltage values to occur on the lead are derived analytically and these values are compared with the values obtained from realistic experiments. Experimental results show that, if we know the path of the implant lead we can determine the voltage induced on it using the simplified expressions. Theory and Method: The simplified electric field expression for a homogenous cylindrical body model was found with the assumption of the uniform gradient field distribution [2]: ( ) { } ( ) { } { } 2 2 2 2 2 2 0 0 ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ( , , , ) ( ) 0.5 0.25 ( ) 0.25 0.5 ( ) 0.5 0.5 x x y z y x y z z x y E x y z t G t xya y x a yza G t y x a xya xza G t yza xza ρ ρ ′ ′ ′ = + − + − − + + − − + + − r .