Magnetic shielding requirements for PET detectors during transcranial magnetic stimulation

Trans-cranial magnetic stimulation (TMS) can be used to stimulate the human cerebral cortex to demonstrate the interconnections between different brain regions. Pulses of 200 μsec, and high magnetic fields 1.5-2.2 Tesla are used for the stimulation. These are sufficient to seriously perturb the operation of the photo-multipliers (PMT) used in positron emission tomography (PET), a functional imaging technique which can quantify cerebral activation. Pulse trains typical of those used in TMS were applied close to PET detectors. When the detectors were not adequately shielded their ability to identify the crystal in which the γ-rays from positron annihilation are detected was severely compromised, not only during the pulse, but for a period of 100 msec afterwards. The use of three layers of well grounded mu-metal was sufficient to allow the detectors to function normally. The authors conclude that if adequate precautions are taken, the use of TMS for direct stimulation of the human brain during PET studies can be performed without degrading the scanner´s performance