Safe electrical stimulation of the cochlear nerve at the promontory during functional magnetic resonance imaging

The purpose of this study was to evaluate possibilities and technical risks for combining intended electrical stimulation of the cochlear nerve and functional magnetic resonance imaging (fMRI). Theoretical considerations and experiments indicate that fMRI can be performed safely during electrical stimulation. A nerve stimulator was developed with minimized length of electrical conductors, current limiting resistance, high inner impedance of a current source, radio frequency (RF)-shielding, and avoidance of ferromagnetic materials. This nerve stimulator transfers the optically encoded stimulating current signal via a fiber optic cable located near the area of stimulation. There, the optical signal drives an MRI-compatible current source. This set-up was tested with transtympanic electrical stimulation of the cochlear nerve at the promontory during an fMRI examination. No hazardous effects could be detected. The stimulation resulted in activation of the Heschl's gyrus. Compared to the conventional promontory testing this method may allow a more objective examination of cochlear implant candidates. Magn Reson Med 42:371-378, 1999.