Neurostimulators: Potential for excessive heating of deep brain stimulation electrodes during magnetic resonance imaging

We are grateful to the editors of the Journal of Magnetic Resonance Imaging for the opportunity to alert the medical community about an emergent matter pertaining to magnetic resonance imaging (MRI) and patients with neurostimulators. In general, MRI is regarded as an extremely safe, noninvasive diagnostic technique (1,2). Maintaining a safe MR environment is a daily challenge and an important responsibility for MR healthcare workers. The types of biomedical devices that are encountered in patients continue to grow. Experimental data must be obtained for these devices using in vitro testing techniques to establish safe MR conditions and parameters. This is particularly important for devices that are electronically activated because of the obvious inherent risks (1–3). Currently, there is a heightened interest in the use of chronic deep brain stimulation (DBS) of the thalamus, globus pallidus, and the subthalamic nucleus for treatment of medically refractory movement disorders, chronic pain, and other types of neurological conditions (4). Thus, the number of patients receiving implantable pulse generators (IPGs) and DBS electrodes is rapidly growing. Obviously, it is desirable to be able to use MRI in patients with neurostimulators, as well as to use MR guidance techniques to optimally place DBS electrodes (4–6). However, to date only a limited number of studies have addressed MR safety issues for implantable IPGs and DBS electrodes (7–10). The MR safety issues that exist for neurostimulators include magnetic field interactions, heating, induced electrical currents, and functional disruption of the operational aspects of these devices (1–4,7–11). One of the most crucial MR safety concerns is related to excessive heating of electronic implants associated with MRI (1,2,11,12). Radiofrequency (RF) and pulsed gradient magnetic fields used for MRI induce currents in the body (11). It is well known that electrically conducting implants can locally increase these currents and, under certain MR operational conditions, generate excessive heating of biomedical devices (11). For example, Achenbach et al (12) reported a temperature increase at the tip of the pacing electrode of up to 63.1° C during 90 seconds of scanning. Thus, there is supportive experimental evidence that MRI can rapidly produce an exorbinant temperature rise in conducting materials. The MR safety implications of this are obvious. The present safety guideline for management of a patient with an implantable neurostimulator is that the individual should not undergo an MRI procedure unless prior testing has defined criteria for the safe use of this imaging modality (1,2). Although the product insert information for one of the most commonly used neurostimulation systems (Medtronic Activa Tremor Control System, Medtronic, Minneapolis, MN, 1997) explicitly states that patients with this system should not be exposed to the electromagnetic fields produced by MRI, it is known that MRI is often used in patients with these devices (unpublished observations, A.R. Rezai, 2001). Numerous centers are performing MRI examinations on patients with DBS implants with no apparent adverse effects; however, the MR parameters used for these procedures have not been reported. Fortunately, it appears that there have been no serious adverse events involving patient injury associated with the use of MRI in individuals with this neuromodulation device. We recently initiated a series of experiments designed to define MRI conditions and parameters that would permit the safe use of this imaging modality in patients with neurostimulators and DBS electrodes. In vitro experiments were performed at 1.5 and 3.0 T using a fluoroptic thermometry system to record temperatures at multiple positions for various configurations of IPGs and DBS electrodes placed in a specially constructed phantom (i.e., used to simulate the human torso) filled with a tissue-equivalent conducting gel. Studies were conducted using both transmit/receive head coils and body coils. Preliminary findings revealed that certain MR conditions and parameters produced relatively minor temperature alterations, while others generated excessive temperature increases at the tip of the DBS electrodes that could result in serious patient injury. In fact, the threshold level known to be associated with the production of thermal lesions was exceeded in some cases with the use of the transmit/receive body coil, while the use of the transmit/receive head coil at relatively low wholebody specific absorption rate (SAR) levels tended to generate temperature alterations that would be safe for patients undergoing MR procedures. If appropriate neurostimulator settings (i.e., IPG amplitude programmed to zero and IPG turned off) and MR system parameters are used, quantitative evidence from our work and other previously published studies (4,6–11), along with pertinent clinical experience, has *Address reprint requests to: F.G.S., 7511 McConnell Ave., Los Angeles, CA 90045. E-mail: www.MRIsafety.com Received July 11, 2001; Accepted July 13, 2001. JOURNAL OF MAGNETIC RESONANCE IMAGING 14:488–489 (2001)