Thomas W. Lloyd

Determinants of gradient field-induced current in a pacemaker lead system in a magnetic resonance imaging environment

BACKGROUND The determinants of low-frequency-induced current by magnetic resonance imaging (MRI) gradient fields in a pacemaker lead system are largely unknown. OBJECTIVE The purpose of this study was to determine the magnitude of MRI low-frequency-induced current in an implanted pacemaker lead system and to investigate in vivo determinants of low-frequency-induced current in an animal model. METHODS Six mongrel dogs underwent conventional single-chamber pacemaker implantation with a current recorder connected in series. Pulse generator (PG) was programmed to VOO 120 bpm with subthreshold output. MRI was performed in a 1.5-T scanner. Low-frequency-induced current was recorded during unipolar pacing, bipolar pacing, and bipolar pacing with the PG case electrically isolated from the pocket. In each mode, low-frequency-induced current was recorded with and without a large loop of additional lead connected in series. RESULTS With a conventional implant, low-frequency-induced current was < or =0.5 mA in all three pacing modes. With five external loops, the magnitude of low-frequency-induced current increased to >30 mA, with consistent myocardial capture in unipolar and bipolar pacing. However, in bipolar pacing with the PG electrically isolated from the pocket, low-frequency-induced current decreased to <0.5 mA with no myocardial capture even with additional looped leads. CONCLUSION Under conventional implant conditions, the magnitude of low-frequency-induced current is <0.5 mA and is unlikely to cause myocardial capture; however, arrhythmia induction cannot be excluded. With sufficient increase in effective loop area (additional looped leads), direct myocardial capture by the low-frequency-induced current is possible. In this study, breaking the return pathway by electrically isolating the PG case from the circuit abolished low-frequency-induced current.

MRI safety assessment of a generic deep brain stimulator

The radio frequency (RF) electromagnetic field of magnetic resonance (MR) scanners can result in significant tissue heating due to the RF coupling with the conducting parts of medical implants. The objective of this paper is to assess the safety of a generic deep brain stimulator (DBS) during MRI scans based on a combined numerical and experimental procedure described in [1]. The evaluation is performed for 1.5 T MR scanners using a generic model of a deep brain stimulator with a helical lead. The results show that the approach is technically feasible and provides sound and conservative information on the potential heating of implants.