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Shielded Room

The usual amplitude of magnetic fields created by the brain are extremely small, they do not exceed a few hundred femto tesla (10−15 T). Compared with this the Earth's magnetic field is between 10−4 and 10−5 T and an MRI is usually 1.5-3 T.

Ross Devlin, Senior Service Specialist, MEGIN, says ... The background earth’s magnetic field is about 0.5 Gauss or 50micro T = 50x10-3 T. This is 11 orders of magnitude higher than expected brain signals (10-14) If the sensors were able to function in that level of field without being completely saturated, which is the most likely scenario, then the background noise would be so high as to make the data unusable.

To attenuate the external magnetic noise the MEG is housed inside a magnetically shielded room (MSR). An MSR is an enclosure with a shell comprising layers of high permeability metals that are also good electrical conductors. This attenuates (absorbs) the spurious magnetic and electrical fields emanating from outside sources. All objects within the room are non-metallic/non-magnetic. It is imperative that neither the MEG operator, nor the participants bring any metal or magnetic objects into the MSR. This is important for two reasons:

Firstly, the sensors (SQUIDs) are ultrasensitive in the sense that they can pick up changes in magnetic field in the range of femtoteslas. Bringing magnetic objects close to the MSR can cause flux traps in the SQUIDs. This may lead to a long and expensive service break and delay the measurements.

Secondly, bringing metal/magnetic objects into the MSR will distort your measurements.

Therefore, before entering the MSR, please remove any potentially magnetic objects that you may be carrying. These include belts, keys, watches, coins, hairpins, eyeglasses and pieces of clothing with metallic parts. Also bringing cameras, flashlights, mobile phones or any other electrical equipment inside the MSR is strictly forbidden. If you want to use any untested equipment or objects (e.g. chairs, cushions, etc.) this has to be tested first, in the presence of an experienced MEG operator only. In the test, somebody waves the study object inside the closed MSR and somebody else watches the raw data display. Start from the door and then proceed towards the MEG little by little. If your colleague sees artifacts on the raw data display, then the object is magnetic and you should avoid bringing it inside the MSR.

Please check whether your participant has removed all of the following items before entering the MSR:

  • Phones
  • Keys
  • Bra’s containing metal (e.g. underwired bra’s) @
  • Eye make-up containing metals (e.g. eye shadow/ mascara)
  • Hair clips/pins
  • Earrings and piercings
  • Rings
  • Bracelets/ necklaces containing metal
  • Clothing with metal zips & metal buttons
  • @ Hospital scrubs available if dresses/jeans need to be changed out of.
  • Any other metal objects in pockets, on clothing, or in hair

Ask your participant if they have a dental retainer/brace, amalgam(mercury/metal alloy) fillings, or are wearing coloured contact lenses, or "Quick Attach" eyelashes.