Original contribution
Bilateral neurostimulation systems used for deep brain stimulation: in vitro study of MRI-related heating at 1.5 T and implications for clinical imaging of the brain

https://doi.org/10.1016/j.mri.2005.02.007Get rights and content

Abstract

Deep brain stimulation (DBS) is used increasingly in the field of movement disorders. The implanted electrodes create not only a prior risk to patient safety during MRI, but also a unique opportunity in the collection of functional MRI data conditioned by direct neural stimulation. We evaluated MRI-related heating for bilateral neurostimulation systems used for DBS with an emphasis on assessing clinically relevant imaging parameters. Magnetic resonance imaging was performed using transmit body radiofrequency (RF) coil and receive-only head RF coil at various specific absorption rates (SARs) of RF power. In vitro testing was performed using a gel-filled phantom with temperatures recorded at the electrode tips. Each DBS electrode was positioned with a single extension loop around each pulse generator and a single loop at the “head” end of the phantom. Various pulse sequences were used for MRI including fast spin-echo, echo-planar imaging, magnetization transfer contrast and gradient-echo techniques. The MRI sequences had calculated whole-body averaged SARs and local head SARs ranging from 0.1 to 1.6 W/kg and 0.1 to 3.2 W/kg, respectively. Temperature elevations of less than 1.0°C were found with the fast spin-echo, magnetization transfer contrast, gradient-echo and echo-planar clinical imaging sequences. Using the highest SAR levels, whole-body averaged, 1.6 W/kg, local exposed-body, 3.2 W/kg, and local head, 2.9 W/kg, the temperature increase was 2.1°C. These results showed that temperature elevations associated with clinical sequences were within an acceptable physiologically safe range for the MR conditions used in this evaluation, especially for the use of relatively low SAR levels. Notably, these findings are highly specific to the neurostimulation systems, device positioning technique, MR system and imaging conditions used in this investigation.

Introduction

The use of implantable neurostimulation systems used for deep brain stimulation (DBS) has become increasingly common in the treatment of refractory movement disorders including Parkinson's disease, essential tremor and dystonia [1], [2], [3]. Although treatment efficacy has been clearly established, there are potential risks for patients with neurostimulation systems undergoing MRI procedures related to the possibility of excessive MRI-related heating, device movement, dislodging leads and electrodes, induced currents and program interference [1], [4], [5], [6], [7], [8]. Importantly, recent studies by Rezai et al. [4] and Finelli et al. [5] reported that MRI-related heating of the tips of DBS electrodes can result in substantial increases in temperatures under certain conditions. Accordingly, this aspect of MRI safety for neurostimulation systems used for DBS is considered to be of the utmost importance.

Magnetic resonance imaging has an increasing role in the evaluation of patients for DBS surgery and particularly in the ongoing management of patients with neurostimulation systems. Furthermore, the implantation of DBS electrodes may be facilitated by using MRI-guided stereotactic localization in order to achieve the most effective location in treating PD [9], [10]. Magnetic resonance imaging has also been used in many clinical scenarios, both related and unrelated to the implantation of neurostimulation systems, for example, verification of lead position, assessment of patients with poor surgical outcomes or in the management of unrelated problems in patients with implanted neurostimulators [9], [11]. Therefore, to perform MRI procedures without posing a risk to the patient, further studies are clearly needed to determine safety guidelines as well as to expand the current recommendations to include different device configurations and MR system settings.

Rezai et al. [4] and Finelli et al. [5] conducted in vitro MRI-related heating studies using a 1.5-T MR system with bilateral DBS systems positioned in a gel-filled phantom. They reported that temperature increases measured at the electrode tip were dependent on the type of radiofrequency (RF) coil used, the level of RF power and how the electrodes and leads were positioned. Although temperature elevations were found to be clinically insignificant in association with clinical sequences used for brain imaging [5], studies were limited to the use of a transmit–receive RF head coil. Importantly, MRI-related heating associated with the use of a transmit body RF coil and receive-only RF head coil (i.e., the RF coil configuration commonly used by recently installed MR systems) has not been reported for implanted neurostimulation systems used for DBS. Because the use of the body RF coil tends to involve higher levels of RF power and over a larger anatomic region, the risks are likely to be greater for this MRI configuration.

Therefore, the purpose of the present study was to characterize MRI-related heating for bilateral neurostimulation systems used for DBS using a 1.5-T MR system and imaging with a transmit body RF coil and receive-only RF head coil, with an emphasis on assessing clinically relevant imaging parameters. The issue for MRI-related heating relates to the amount of the neurostimulation system that is contained within the transmitting coil (body vs. head).

Section snippets

Materials and methods

In vitro testing was performed using a 1.5-T/64-MHz MR system (Sonata MRI, NUMARIS/4 software, Version Syngo MR2002B; Siemens Medical Solutions, Erlangen, Germany) with a transmit body RF coil and receive-only RF head coil. A plastic phantom designed to approximate the size and shape of the human head and torso was filled with a semisolid gel (5.85 g polyacrylic acid and 0.8 NaCl per liter of distilled water) prepared to simulate the electrical conductivity and thermal convection properties of

Results

Table 1 summarizes the results of this investigation. For the use of the transmit body RF coil and receive-only RF head coil, the highest temperatures recorded at the distal electrodes in these experiments ranged from 19.7 to 21.8°C. The highest temperature changes recorded at the distal electrodes ranged from +0.2 to +2.1°C. The highest temperature change measured by the reference probes ranged from +0.1 to +0.3°C.

In general, the temperatures increased to the highest levels within the first 14

Discussion

Deep brain stimulation of the globus pallidus interna and STN has grown increasingly common in recent years, in part due to advances in knowledge concerning the subcortical pathophysiology of Parkinson's disease [16]. In consideration of the efficacy of using neurostimulation systems for DBS in ameliorating the cardinal motor symptoms of PD, it is likely that the number of patients receiving these devices will continue to increase [17]. Therefore, it will become more common for radiologists to

Conclusion

Magnetic resonance imaging-related heating was assessed for bilateral neurostimulation systems used for DBS to assess a relatively high level of RF exposure and clinically relevant imaging scenarios. The findings indicated that temperature elevations associated with MRI procedures performed using clinical relevant pulse sequences were within a physiologically acceptable range, especially if the level of RF power deposition is restricted during the MRI procedure.

Notably, the findings presented

Acknowledgments

Roongroj Bhidayasiri, M.D., MRCP, is supported by Lilian Schorr Postdoctoral Fellowship of Parkinson's Disease Foundation (PDF) and Parkinson's Disease Research, Education and Clinical Center (PADRECC) of West Los Angeles Veterans Affairs Medical Center. We thank Dr. Sopon Iamsirithaworn for his help with the statistical analysis. Mark S. Cohen, Ph.D., is supported in this research under NIDA grant award DA15549. Frank G. Shellock, Ph.D., is supported by National Institutes of Health, grant

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