(Radiology. 1999;211:886-888.)
© RSNA, 1999
Platinum Subdural Grid: MR Imaging Compatibility Testing1
Emanuel Kanal, MD,
Carolyn Cidis Meltzer, MD,
P. David Adelson, MD and
Mark P. Scheuer, MD
1 From the Departments of Radiology (E.K., C.C.M.), Psychiatry (C.C.M.), and Neurology (M.P.S.), University of Pittsburgh Medical Center, 200 Lothrop St, Rm D132, Pittsburgh, PA 15213-2582; and the Department of Neurosurgery, Children's Hospital of Pittsburgh, Pa (P.D.A.). Received April 23, 1998; revision requested July 2; revision received August 21; accepted December 15. Address reprint requests to E.K.
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Abstract
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A platinum subdural grid electrode (used for seizure monitoring) was tested for potential heating during magnetic resonance (MR) imaging at 1.5 and 3.0 T. Fast spin-echo imaging with high radio-frequency transmitted power was performed while temperature measurements for the grid phantom were recorded by means of fiberoptic thermometry. Temperature variations remained within 1.0°C. MR imaging examination in patients in whom such grid electrodes are implanted is expected to result in clinically insignificant temperature changes in the vicinity of the grid electrode or its leads.
Index terms: Head, MR, 10.1214 • Magnetic resonance (MR), physics • Magnetic resonance (MR), safety • Radiology and radiologists, iatrogenic injury
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Introduction
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Since the introduction of magnetic resonance (MR) imaging as a clinical imaging tool in the early 1980s, there has been concern about possible adverse interactions between the MR imaging environment and implanted devices, especially those that contain metallic components. Various techniques have been used to demonstrate the advisability of permitting patients with different implanted devices to undergo diagnostic MR imaging studies (1–7). MR safety considerations include possible ferromagnetic interactions between the device and the relatively strong static magnetic fields of the MR imager, potential induced voltages from the rapidly switched gradient magnetic fields used in the MR imaging process, and potential heating of the device or patient tissue (power deposition) as a result of the radio-frequency (RF) magnetic fields transmitted during an MR examination.
Patients with seizure disorders are commonly referred for MR imaging instead of computed tomography owing to the superior depiction of soft-tissue contrast and increased detectability of epileptogenic lesions (8). In patients with medically intractable seizures, a platinum subdural grid electrode is surgically implanted in the subdural space to localize the epileptogenic focus, often in preparation for surgery to control epilepsy. The metallic grid electrode could act as an RF antenna during MR imaging, with resultant heating of the grid electrode or patient tissue. Therefore, we designed a study to test for temperature alterations that might occur in the immediate vicinity of the grid electrode during MR imaging studies with high power deposition.
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Materials and Methods
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A platinum subdural grid electrode (Wyler; Ad-Tech Medical Instrument, Racine, Wis) was dangled from one of its eight leads and slowly introduced into the bore of an unshielded 3.0-T whole-body MR imaging unit (Signa; GE Medical Systems, Milwaukee, Wis). The grid electrode was observed for deflection. The same procedure was then repeated by holding the grid electrode with the leads dangling to observe possible deflection of any of the eight leads.
A phantom was then constructed from a 5 x 8.75-inch (12.70 x 22.23-cm) plastic container filled with gelatin (Knox, unflavored; Nabisco Foods, East Hanover, NJ). The phantom was filled to a depth of 1 inch (2.54 cm) with 635 mL of the warm gelatin solution and then left to cool to room temperature. The grid electrode was implanted within this gelatin phantom while the solution was still liquid to help avoid the introduction of air bubbles into the phantom. To increase the coupling of the RF oscillating magnetic fields transmitted by the MR imager and the metallic leads of the grid electrode, the eight 15-inch (38.10-cm) leads were coiled into a roughly circular configuration with a 5-inch (12.70-cm) diameter and also embedded in the gelatin phantom (Figure). The radius of the arc formed by the grid electrode and the distal end of the eight grouped leads was roughly 8–12 inches (20–30 cm). The probes of the four-channel fiberoptic thermometry system (model 3000-4; Luxtron, Mountain View, Calif) were positioned as illustrated in the Figure. This system has been found to maintain accuracy of better than 0.1°C in the strong static and time-varying magnetic fields used in the MR imaging environment (9,10).
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Figure 1. Photograph shows the platinum subdural grid electrode and the approximate positions of the thermometry probes (1–3). The ends of the wires (4) were coiled during the phantom experiment.
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The phantom was imaged at 3.0 T with a fast spin-echo pulse sequence (repetition time of 3,000 msec, echo time of 17 msec [effective], field of view of 24 x 24 cm, 5-mm section thickness, 1-mm gap, 192 phase- x 256 frequency-encoding steps, and echo train length of 32). The pulse sequence was repeated continually for a total imaging time of 20 minutes 31 seconds.
The phantom was then imaged at 1.5 T (Signa; GE Medical Systems) with the same pulse sequence.
With the phantom at room temperature, the temperature at each of the four probes was recorded immediately before the initiation of and immediately after the cessation of each pulse sequence, as well as at 4–5-minute intervals during the MR imaging time. By using the fiberoptic thermometry probes, the temperatures could be recorded without interrupting the imaging procedure. Finally, the temperatures were recorded at 4–5-minute intervals after the procedure, for a total of 18 minutes with the 3.0-T imager and 1 hour 46 minutes with the 1.5-T imager.
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Results
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With the 3.0-T whole-body MR imager, deflection of the grid electrode or its leads was not observed at any location.
The temperature readings are shown in the Table. With both the 3.0- and 1.5-T imagers, the intraprobe and interprobe temperatures at all times varied by less than 1.0°C.
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Discussion
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Since MR imaging has been shown to be effective for detecting lesions that may be associated with epileptogenic foci, interest has increased in performing MR imaging in patients with implanted brain grid electrodes. The safety of MR imaging in this setting is dependent on several factors, including the potential ferromagnetic interactions of the grid electrode with the static magnetic fields and gradients of the MR imager, induced voltages within the electrically conductive leads of the grid electrode that result from the gradient magnetic fields of the MR imaging process that vary rapidly over time, and heating of the grid electrode or patient tissue that may be associated with ohmic losses from the RF power being transmitted during the MR imaging procedure.
The lack of deflection of the grid electrode or its leads at 3.0 T demonstrates that electrode translation or torque would not be reasonably expected from exposure of such devices to MR imaging devices approved by the U.S. Food and Drug Administration (FDA). Despite use of a clinical pulse sequence with relatively high RF transmitted power that was prolonged for clinically unrealistic scan times, the lack of clinically significant heating of the electrodes or the surrounding gel suggests that tissue heating from clinical imaging sequences at levels approved by the FDA is highly unlikely. No test was performed to assess induced voltages within the leads or grid, although such voltage induction is theoretically possible. Nevertheless, sufficient voltage induction might be expected, at worst, to trigger epileptogenic activity.
Limitations of this study include the fact that the position(s) of the grid electrode and its leads in a patient can be quite variable, and thus every potential configuration was not precisely simulated in this study. However, we attempted to simulate a worst-case scenario by coiling the leads into a loop with a large radius perpendicular to the transmitted RF power. Further, we studied the grid electrode at 1.5 and 3.0 T, which has concomitantly higher RF resonant frequencies and, therefore, substantially higher RF power depositions. Thus, we attempted to simulate worst-case scenarios for transmitted RF power that could be realistically anticipated in clinical MR imaging devices to which patients with such implanted grid electrodes could be exposed.
MR imaging examination of patients with an implanted platinum subdural grid electrode is expected to result in clinically insignificant temperature changes in the vicinity of the grid electrode or its leads. No adverse ferromagnetic properties were observed with this grid electrode or its leads. It seems reasonable to conclude that patients with such grid electrodes might be safely examined with standard MR imaging equipment and sequences, although the potential for epileptogenic sequelae as a result of the MR imaging process itself has not been fully addressed for such patients.
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Footnotes
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Abbreviations: FDA = Food and Drug Administration
RF = radio frequency
Author contributions: Guarantor of integrity of entire study, E.K.; study concepts, all authors; study design, E.K., C.C.M.; definition of intellectual content, E.K., C.C.M.; literature research, E.K., C.C.M.; experimental studies, E.K., C.C.M.; data acquisition and analysis, E.K., C.C.M.; manuscript preparation, E.K.; manuscript editing, E.K., C.C.M.; manuscript review, all authors.
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Abstract
Introduction
