RECORDING OF EEG DURING FMRI EXPERIMENTS - PATIENT SAFETY

The acquisition of electroencephalograms (EEG) during functional magne tic resonance imaging (fMRI) experiments raises important practical is sues of patient safety. The presence of electrical wires connected to the patient in rapidly changing magnetic fields results in currents fl owing through the patient due to induced electromotive forces (EMF), b y three possible mechanisms: fixed loop in rapidly changing gradient f ields; fixed loop in a RF electromagnetic field; moving loop in the st atic magnetic field. RF-induced EMFs were identified as the most impor tant potential hazard. We calculated the minimum value of current-limi ting resistance to be fitted in each EEG electrode lead for a represen tative worst case loop, and measured RF magnetic field intensity and h eating in a specific type of current-limiting resistors. The results s how that electrode resistance should be greater than or equal to 13 k Omega for our setup. The methodology presented is general and can be u seful for other centers.