Shielding

MSR

The Room itself

The magnetically shielded room (MSR) is a product of Vacuumschmelze (Hanau, Germany). The shielding effect is provided by two layers of mumetal, the inner layer being 3mm and the outer layer being 2mm. Predicted shielding performance was of -60dB at 1Hz; actual performance exceeds this prediction. The exterior dimensions of the room are 2.9 x 3.5 x 2.9m, and the inner dimensions are 2.4 x 3.0 x 2.4m.

Active shielding

In addition to the offline CALM noise reduction system described above, we also have an active shielding system (“active compensation coils”) designed by Vacuumschmelze and incorporated into the MSR. The system senses the incoming (extramural) signal and produces signals of compensating voltages to annul as much of that signal as possible. The system consists of:

• 6 compensating coils, one on each surface of the MSR
• A three-axis fluxgate sensor that detects the disrupting extramural noise
• A controller that tunes the fluxgate sensor, digitizes the reference (input) signal, calculates the canceling (compensating) voltages, converts these voltages to analog signals and feeds these voltages to the compensating coils

Because the dewar/MEG sensors are not centered within the room, the precise locations of the compensation coils were adjusted by Dr. Jochen Bork of Vacuumschmelze so as to maximize the cancellation effect.

OTHER SHIELDING

RF/EM shielding

To shield our cables proceeding from the MSR, we used Kitagawa Industries 300-series cable shielding.

To shield miscellaneous small parts, we used 3M copper foil tape (1mil/.00254mm), products 1181 and 1182 (1″ and 2″ wide, with single and double-sided conductive adhesive, respectively). Our local distributor for 3M was Bristol Tape Corp. (Fall River, MA).

Digitization

We record the location of the marker coils and electrodes on the subjects, as well as three fiducial locations (the nasion, and the left and right preauricular points), before MEG recording takes place. Additionally, we record the shape of the subject’s head using a 3D laser scanning system, which generally allows 50,000-200,000 points to be collected. The hardware used for the digitization process is a Polhemus (Colchester, VT) FastSCAN COBRA 3D laser system. One receiver is placed on the subject during digitization and a transmitter is placed behind the subject. The use of the receiver on the subject permits accurate correction for subject movement.

We use MATLAB software to convert the file generated by the FastSCAN software to the format required by our analysis methods for source localization.