'Electromagnetic Simulation Used to Evaluate' by als

Electromagnetic Simulation Used to Evaluate

Safety of Combining EEG and MRI

Electromagnetic simulation software is being used to investigate the safety

Angelone and Bonmassar used electromagnetic simulation to measure the

of an important research technique that integrates electroencephalography

RF energy absorbed by the human head in an integrated EEG-MRI

(EEG) with magnetic resonance imaging (MRI). The integration of these two

experiment. They selected the XFDTD Version 6.1 software from Remcom

analysis methods has the potential to improve investigations of brain activity

Inc., State College, Pennsylvania, which incorporates a full-wave, three-

because EEG offers high temporal resolution while MRI offers high spatial

dimensional solver based on the finite difference time domain (FDTD)

resolution. But concerns have arisen about temperature increases in sensitive

method. According to the Federal Communications Commission of the

brain tissues that could be caused by the current induced

United States as stated in OET Bulletin 65,

in the EEG electrodes by the radio frequency (RF)

"Evaluating Compliance with FCC

power generated by MRI. Leonardo Angelone and Dr.

Guidelines for Human Exposure to

Giorgio Bonmassar, researchers at the Massachusetts

Radiofrequency Electromagnetic Fields",

General Hospital/Massachusetts Institute of

Supplement C: "Currently, the finite-

Technology/Harvard Medical School Athinoula A.

difference time-domain (FDTD) algorithm

Martinos Center for Biomedical Imaging, Charlestown,

is the most widely accepted computational

Massachusetts, simulated the RF power dissipated in the

method for SAR modeling." Furthermore, in

human head in an integrated MRI-EEG software

FCC Part 95 Section 603(f) it is stated:"

simulation. Their results (Angelone et al. 2004) showed

Applications for equipment authorization of

that in particular cases, such as high magnetic MRI

devices operating under this section must

fields and use of metallic EEG leads, the specific

contain a finite difference time domain

absorption rate (SAR), which measures tissue exposure

(FDTD) computational modeling report

to RF, is four to seven times higher than in MRI alone,

showing compliance with these provisions

indicating that power levels need to be reduced in

for fundamental emissions." Remcom

integrated experiments.

provides FDTD heterogeneous head and

body models, together with software that

One of the most important challenges in brain imaging is to model the

allows repositions of the limbs. In addition

sources of brain activity during different visual, auditory or motor tasks.

they provide the Specific Anthropomorphic

Brain mapping with MRI has the highest spatial resolution of current non-

Mannequin (SAM) head as a CAD file that

invasive imaging techniques. The spatial resolution of MRI is typically

may be oriented and meshed at any desired

millimeters in the case of human subjects. However, because MRI measures

resolution within XFDTD to aid customers in

primarily a hemodynamic response with a time constant on the order of

complying with FCC limits on SAR.

seconds, the precise mechanics of information exchange within the brain,

which occur on a millisecond scale, remain hidden. EEG, on the other hand,

Modeling an MRI-EEG experiment

can provide temporal accuracy in the required millisecond range but the

spatial accuracy is only on the order of centimeters. Researchers have been

working to integrate MRI and EEG in order to combine the spatial resolution

Due to the particular spatial resolution needed for their research, Angelone

available with MRI and the temporal resolution offered by EEG.

and Bonmassar developed their own high-resolution head models by

meshing the anatomical MRI data of two adult male subjects. The brain was

Safety concerns arise

segmented into cerebrospinal fluid, gray matter, and white matter, using a

hybrid method combining watershed algorithms and deformable surface

However, this emerging research technology raises rare but real safety

techniques.

issues. The use of electrodes in an MRI environment is in many respects

similar to the presence of metallic implants in an MRI environment, which

Using the XFDTD Geometric Modeler they constructed a FDTD model of a

has already been addressed in several studies. These studies have shown that

bird cage coil composed of sixteen 300 mm perfect electrical conductor rods,

when heating of tissues are present, these depend in the area of the implant is

closed by two 260 mm diameter, 1 mm thick loops at each end, and placed

a function of the dimensions, orientation, shape, and location of the implant

symmetrically around the head. Using the standard features available in

in the patient. Furthermore, in the case of metallic wire, which is particularly

XFDTD, a circular excitation was simulated, driving the current generators

relevant to the integrated EEG-MRI case, the location of heating in the tissue

placed on the centers of the rods with 1 A peak-to-peak amplitude and a

is usually concentrated in a small volumetric area near the tip of the wire.

22.58 degree phase-shift between any two adjacent generators. The surface

These results highlight the importance of specific studies involving EEG

coil used was a circular perfect electrical conductor, oriented in the XZ plane

electrodes in the presence of an RF field.

with a diameter of 140 mm and thickness of 1 mm. The current source, a