A novel passive functional MRI paradigm for preoperative identification of the somatosensory cortex

The purpose of this study was to assess the feasibility of a novel passive functional magnetic resonance imaging (fMRI) paradigm for activation analysis of the somatosensory cortex utilizing a specifically designed conductor for electrical stimulation of the median and tibial nerves. Thirteen healthy volunteers underwent electrical stimulation of these nerves with defined frequencies and intensities in a block-designed fashion. Electrical stimuli were applied by two custom-designed magnetoelectrically protected coaxial leads, taking into account the technical difficulties of the application of electrical current in the fMRI environment. Activation effects were analysed in real-time mode and validated by statistical parametric mapping. The shielded conductors suppressed electromagnetically derived artefacts nearly completely. The measurements revealed maximum cortical activation when applying a stimulation frequency of 3 Hz and an intensity of 3 mA above motor threshold. Simultaneous stimulation of both the median and tibial nerves enhanced identification of the central region significantly. A standardized setup for the clinical environment was evolved. With this passive paradigm, the identification of the somatosensory cortex was possible in all evaluated cases. The presented technical setup and paradigm is a reliable and fast method for preoperative identification of the somatosensory cortex and may represent a feasible paradigm for generation of pre- and intraoperative fMRI in functionally disabled patients.