Safety of localizing epilepsy monitoring intracranial electroencephalograph electrodes using MRI: radiofrequency-induced heating

Abstract

Purpose: To investigate heating during postimplantation localization of intracranial electroencephalograph (EEG) electrodes by MRI.

Materials and methods: A phantom patient with a realistic arrangement of electrodes was used to simulate tissue heating during MRI. Measurements were performed using 1.5 Tesla (T) and 3T MRI scanners, using head- and body-transmit RF-coils. Two electrode-lead configurations were assessed: a "standard" condition with external electrode-leads physically separated and a "fault" condition with all lead terminations electrically shorted.

Results: Using a head-transmit-receive coil and a 2.4 W/kg head-average specific absorption rate (SAR) sequence, at 1.5T the maximum temperature change remained within safe limits (<1 degrees C). Under "standard" conditions, we observed greater heating (<or=2.0 degrees C) at 3T on one system and similar heating (<1 degrees C) on a second, compared with the 1.5T system. In all cases these temperature maxima occurred at the grid electrode. In the "fault" condition, larger temperature increases were observed at both field strengths, particularly for the depth electrodes. Conversely, with a body-transmit coil at 3T significant heating (+6.4 degrees C) was observed (same sequence, 1.2/0.5 W/kg head/body-average) at the grid electrode under "standard" conditions, substantially exceeding safe limits. These temperature increases neglect perfusion, a major source of heat dissipation in vivo.

Conclusion: MRI for intracranial electrode localization can be performed safely at both 1.5T and 3T provided a head-transmit coil is used, electrode leads are separated, and scanner-reported SARs are limited as determined in advance for specific scanner models, RF coils and implant arrangements. Neglecting these restrictions may result in tissue injury.

Figure 1

Experimental arrangement without gel. a: Head part of the phantom patient shown with the various implants inserted. b: Entire phantom with a 30-cm ruler for scale. The phantom patient right hand side (RHS) and left hand side (LHS) are indicated. The black plastic components were used to position and hold the electrodes. c,d: The two electrode-tail arrangements investigated are also demonstrated with the tails separated in an “open circuit” (c) and bundled together in a “short circuit” termination (d). e: The different geometry of the “open circuit” tail arrangement required for the body coil/bottom half of the 12-channel head array coil is demonstrated. f,g: A schematic figure of the phantom showing the sites of temperature measurement (in red) used for most experiments (f), and for testing temperature changes with grid position where different measurement sites were used (again in red; g). h,i: The geometry of temperature probes relative to the electrodes is demonstrated for depth (h) and grid contacts (i).

Figure 2

Visualization of the different grid positions tested. The white bars are adjacent to the grid position as seen in an axial scout image. a–d: Parts correspond to the grid electrode positions a–d as reported in Table 4.

Figure 3

Maximum temperature changes at 1.5 and 3T. Dark solid line: electrode tails shorted together; gray dotted line: electrode tails electrically isolated and separated. Horizontal gridlines are always 0.5°C and the horizontal bar indicates the scan period. a: 1.5T GE system, head-transmit coil, depth electrode. b: 1.5T GE system, head coil, grid electrode. c: 3T GE system, head coil, depth electrode. d: 3T GE system, head coil, grid electrode.

Figure 4

Maximum temperature changes using different RF-transmit coil arrangements at 3T. Dark solid line: electrode tails shorted together; gray dotted line: electrode tails electrically isolated and separated. Horizontal gridlines are always 0.5°C and the horizontal bar indicates the scan period. All data are from a 3T Siemens TIM Trio system. a: Head coil, depth electrode. b: Head coil, grid electrode. c: Body coil, depth electrode. d: Body coil, grid electrode.