Original-experimentalDeterminants of gradient field-induced current in a pacemaker lead system in a magnetic resonance imaging environment
Introduction
Magnetic resonance imaging (MRI) has emerged as an important diagnostic tool with an increasing number of clinical applications. The majority of patients can safely undergo MRI and reap the benefits; however, MRI is contraindicated in patients with implantable devices such as pacemakers and defibrillators.1, 2, 3 Studies have addressed concerns about device displacement and lead heating, and a number of patients with implanted devices have successfully undergone MRI without serious consequences.4, 5, 6, 7 Fontaine et al8 reported a case of rapid cardiac pacing occurring in a patient during MRI and raised the possibility of MRI-related induction of currents in the pacemaker leads. MRI systems use three types of magnetic fields to generate images: (1) a strong static magnetic field, (2) a radiofrequency (RF) time-varying magnetic field, and (3) a time-varying gradient magnetic field. According to the laws of physics (i.e., Maxwell’s equations), the time-varying magnetic fields can generate time-varying electric fields. For a 1.5 T-MRI system, the RF resonance frequency for hydrogen is 64 MHz. This high-frequency (RF) time-varying electromagnetic field potentially can transfer energy into implanted electronic devices and cause thermal injury to tissue near the tissue–electrode interface.9, 10, 11, 12 MRI gradient fields, on the other hand, have a much lower frequency (1–10 kHz) and are used to provide spatial information. The interaction between the time-varying MRI gradient field and the conductive loop formed by the pacemaker-lead system can be considered an instance of electromagnetic induction, per Faraday’s law. Whether the gradient fields can induce sufficient electromotive force and current in the implanted system to result in undesired cardiac stimulation is unknown. The purpose of this study was twofold: (1) to determine the magnitude of MRI low-frequency (gradient field)–induced current in an implanted pacemaker-lead system and (2) to investigate the in vivo determinants of low-frequency–induced current in an animal model.
Section snippets
In vitro experiments
Gradient fields present small transient increments or decrements in the main magnetic field. The magnitude of these transients varies as a function of location in the scanner, but the gradient fields have a vector orientation along the z-axis (main axis of the magnet cylinder). To demonstrate the z-directed component of the x gradient, we mapped the x gradient using a custom-made current recorder. The current recorder was a battery-operated MRI-compatible data acquisition unit, which digitized
In vitro experiments
Figure 4 shows the mapped x gradient field as a function of its position in the x and z planes. The maximum low-frequency–induced current recorded was <5 mA with the loop of wire in the x–z plane (Figure 3A) as the loop’s cross-sectional area is minimally exposed to a z-directed dB/dt. However, low-frequency–induced current for a loop of wire in the x–y plane varied with position along the x-axis (Figure 3B). In this situation, the loop’s cross-sectional area was maximally oriented to a z
Discussion
This study investigated the determinants of low-frequency–induced current in a pacemaker lead system in an MRI environment, with several interesting findings. Based on results of the in vivo experiments, it can be concluded that the return pathway for low-frequency–induced current generated in a pacemaker lead system due to MRI gradient fields under normal implant conditions is PG case–lead–tissue–PG case. The impedance path from the PG case internally to the lead is critical in controlling the
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S.R. Wedan is a consultant for Boston Scientific. T. Lloyd currently is employed by Imricor Medical Systems.