'Electromagnetic Simulation Used to Evaluate' by als

sinusoidal generator of 1 A peak-to-peak amplitude with internal resistance

For more information, contact Remcom: 315 South Allen Street, Suite

of 50 ohms, was placed on the lowest point of the ring.

222, State College, PA 16801; Telephone: 1-814-861-1299, 1-888-773-

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SAR is the variable typically used to quantify the effects on tissue exposure

861-1308, 1-888-973-6266 (1-888-9REMCOM) Toll-Free in U.S. and

to RF signals. SAR is defined as the time derivative of the incremental

Canada; e-mail: info@remcom.com; Web site: www.remcom.com

energy absorbed by an incremental mass contained in a volume of given

density (NCRP 1981). Again using standard postprocessing features in the

REFERENCES

Bio-pro module, XFDTD was used to compute the electric and magnetic

fields and the SAR. Simulations were performed with surface and birdcage

MRI coils; 16, 31, 62, and 124 electrodes; and at 128 and 300 MHz. An

Angelone, L.M., Potthast., A., Iwaki, S., Segonne, F., Belliveau, J. W.,

Athlon PC was used for the calculations.

Bonmassar, G. (2004). Metallic electrodes and leads in simultaneous EEG-

MRI:

Specific

Absorption

Rate

(SAR)

simulation

studies.

Simulation shows higher SAR values

Bioelectromagnetics, Vol. 25 (4): 285-295.

FDA. 2003. Criteria for Significant Risk Investigations of Magnetic

The results confirmed an average of up to seven times the original value in

Resonance Diagnostic Devices. Center for Devices and Radiological Health.

averaged SAR on the skin as well as an increase in the tissues adjacent to the

July 14. http://www.fda.gov/cdrh/ode/guidance/793.pdf.

electrodes. The difference in SAR between the electrodes and no-electrodes

conditions was greater with the bird cage coil than with the surface coil. The

Mirsattari SM, Lee DH, Jones D, Bihari F, Ives JR. 2004. MRI compatible

peak 1 g averaged SAR values were highest at 124 electrodes, increasing to

EEG electrode system for routine use in the epilepsy monitoring unit and

as much as two orders of magnitude at 300 MHz compared to the original

intensive care unit. Clin Neurophysiol 115(9):2175-80.

value. At 300 MHz, there was a fourfold increase of SAR averaged over the

bone marrow, and a sevenfold increase in the skin. The study shows that the

NCRP. 1981. Radiofrequency electromagnetic fields: properties, quantities

presence of nonmagnetic high conductive metallic EEG electrodes can

and units, biophysical interaction, and measurement. Bethesda, MD:

increase the peak SAR on the subject by as much as 172 times.

National Council Radiation Protection and Measurements. Report nr 67.

The simulation also showed that with the electrodes and leads, because of

the RF induced currents along the leads, the electric field increases near the

electrodes. Thus, the electrodes increase the electric field on the skin and on

the surrounding tissues. This generates an increase of peak SAR values for

both surface and bird cage coils, relative to that produced by the RF of the

coil only.

A growing number of laboratories are performing EEG recordings during

MRI. The research described here indicates that the pulsed RF fields that are

used to elicit MRI signals from tissue may pose a safety hazard by inducing

currents in the EEG electrodes/leads. Safety issues, a relatively minor

problem at 1.5 Tesla (Mirsattari et al., 2004), may be present at higher fields

when using purely metallic EEG electrodes/leads. The electromagnetic

simulations performed by Angelone and Bonmassar quantified the resulting

SAR for different RF coil types, numbers of electrodes and frequencies.

Their results, computed by using the FDTD technique available in the

Remcom software, confirmed an increase of up to seven times the averaged

SAR on the skin and as much as 172 times for peak 1 g averaged SAR

compared to MRI performed without metallic EEG electrodes. The FDA

guidelines (FDA 2003) for use in MRI environments recommend SAR levels

lower than 3 W/kg averaged over the head for 10 minutes and 8 W/kg in any

gram of tissue in the head for 5 min. The conclusions to draw from this study

are that in order to comply with the FDA recommendation, the maximum

input power used for an MRI sequence needs to be reduced when using

metallic EEG electrodes/leads respect to the case without EEG

electrodes/leads.