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Archived Comments for: Temperature measurement on neurological pulse generators during MR scans

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  1. Letter to the Editor

    Ashwini Sharan, Thomas Jefferson University

    21 January 2003

    To the Editor:

    The rapid acceptance and application of neurostimulators for the treatment of movement disorders, pain, epilepsy, and other emerging areas has raised the important issue of the safety of these devices related to magnetic resonance (MR) procedures, including routine imaging and fMRI. The most significant concern is the possibility of generating excessive heating of the components of the neurostimulation systems (1,2). As the number of investigations addressing these concerns increase, it will be paramount to detail the experimental design to allow for scientific assessment and standardization of the methodology, to allow reproducibility and more importantly, to ensure patient safety.

    The first study to assess the safety of deep brain stimulation (DBS) systems was performed by Rezai et al.(1) In this investigation, two scenarios for a neurostimulation deep brain stimulation (DBS) system were evaluated consisting of bilateral model 3387 lead, model 7495-51 extension, and model 7426-Soletra neural pulse generator (Medtronic, Minneapolis, MN) with and without 2.5cm loops positioned at the skull with either 2 or 4 loops depending on the scenario under the implantable pulse generator. Additionally, both whole body and local specific absorption rates (SAR) of radiofrequency power deposition were reported for the 1.5-T MR system (Siemens Medical Systems, Iselin, NJ).

    Using the transmit/receive head RF coil, the highest temperature ranged from 2.3-7.1˚C and using the transmit/receive body RF coil, the highest temperature ranged from 2.5-25.3˚C with the highest temperature recorded at the distal electrode contact in each case. Notably, all temperature measurements were recorded on the electrodes of the neurostimulation system in the study by Rezai et al. (1), not on the IPGs as was indicated by Kaniz et al. (3). Based on these findings, detailed MR safety recommendations regarding the tested neurostimulation systems were made.

    Kaniz et al. (3) recently reported on a slightly different scenario. In their description, a unilateral model 3387 lead, model 7495-51 extension, and Itrel-III neural pulse generator (Medtronic, Minneapolis, MN) with 3.5 cm loop at the skull was tested on a 1.5-T MR system (Siemens Medical Systems, Iselin, NJ) and a 3.0-T MR system (Bruker). For a local SAR of 2.0 W/kg (this is not consistently stated in the Kainz et al. report) a temperature rise of 2.1˚C was similar to that reported by Rezai et al. in experiment 6 with a local SAR of 2.1 W/kg resulting in a temperature rise of 2.3˚C. Both studies reported on maximum temperature elevations that occurred at the distal electrode.

    A few concerns must be addressed. It appears that both studies have illustrated a similar local SAR in defining what should be a tolerable temperature rise by a patient. This is not accurately reflected in the Kaniz et al. report. Furthermore, Rezai et al. reported on maximum temperature rise of 25˚C at the distal electrode while using a transmit/receive body RF coil with whole body SAR at 3.9 W/kg while the MR imaging procedure was performed over the IPG. In no case, did Rezai et al. (1) measure the temperature of the IPG, as this was not deemed to be problematic based on a prior analysis of the MR safety for neurostimulation systems (Unpublished observations, 2001).

    There are a number of issues of concern that need to be more clearly addressed by Kainz. et al. They must clarify in their manuscript if it is the whole body or local SAR at 2W/kg which has been tested since the RF energy delivered to the phantom will significantly differ and impact the resulting temperature increase for the electrode. Detailed measurements of the phantom should be provided especially for the purposes of reproducibility as well as for the comparison of the 1.5-T and 3.0-T experimental designs.

    It is known that the specific length and geometry of the lead exposed to the RF field can result in significant changes in temperatures (1). This was demonstrated with the use of a transmit/receive body RF coil. The conclusion regarding the loops must be interpreted carefully as neither reports were testing the specific hypothesis on the interaction of the loop with the MR environment. The assumption that the lead and/or extension should be arranged in the form a “meander” should not have been drawn. Finally, comparison of unilateral and bilateral systems may introduce even another level of complexity.

    Other MR safety issues for neurostimulation systems regarding magnetic field interactions, induced voltages, programming changes, device functionality, and image artifacts need to be systematically addressed. Finally, it must be emphasized that a comprehensive checklist of precautions will be absolutely necessary to insure safety for patients undergoing MR procedures.

    Ashwini D. Sharan, MD (Philadelphia, PA)

    Ali R. Rezai MD (Cleveland, OH)

    John Nyenhuis, Ph.D. (West Lafayette, IN)

    Jean Tkach, PhD (Cleveland, OH)

    Paul Ruggieri, M.D. (Cleveland, OH)

    Greg Hrdlicka, BSEE (Minneapolis, MN)

    Paul Stypulkoski, PhD (Minneapolis, MN)

    Daniel Finelli, M.D. (Cleveland, OH)

    Ken Baker, Ph.D. (Cleveland, OH)

    Frank G. Shellock, Ph.D. (Los Angeles, CA)

    1 Rezai AR, Finelli D, Nyenhuis JA, Hrdlicka G, Tkach J, Sharan A, Rugieri P, Stypulkowoski PH, Shellock FG. Neurostimulation Systems for Deep Brian Stimulation: In Vitro Evaluation of Magnetic Resonance Imaging-Related Heating at 1.5 Tesla. J Magn Reson Imaging 2002; 15;241-250.

    2 Finelli D. Rezai AR. Rugieri P. Tkach J. Nyenhuis JA. Hrdlicka G. Sharan A. Stypulkowski PH. Shellock FG. MR-related Heating of Deep Brain Stimulation Electrodes: An In Vitro Study Using a Head Transmit / Receive Coil Establishing the Relationship Between SAR and Temperature Elevation, With Analysis of Clinical Imaging Sequences. AJNR). 2002 Nov/Dec: 23; 1-7

    3 Kainz W, Neubauer G, Uberbacher R, Alesch F, Chan DD. Temperature Measurement on Neurological Pulse Generators during MR Scans. BioMed

    Competing interests

    None declared

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