Frequency dependence of the functional MRI response after electrical median nerve stimulation.

Localizing sensorimotor areas with high resolution functional MRI is of considerable interest for a wide range of medical applications from the preoperative planning of neurosurgical interventions to determining the course of neuroplastic reorganisation after brain lesions. We examined the effect of the stimulation frequency on the blood oxygen level dependent (BOLD) fMRI response and on perfusion weighted fMRI using electrical median nerve stimulation at 5, 15, 40, and 100 Hz. BOLD fMRI was performed using a single shot gradient echo EPI sequence to acquire 15 contiguous slices. For the qualitative flow sensitive studies, a single slice inversion recovery prepared spin echo echoplanar sequence (IR-SE EPI) was used. In the primary sensorimotor cortex, a linear increase of the fMRI-BOLD response, affecting both the number of activated pixels and the amplitude of the signal changes, was seen with increasing stimulation frequencies. The qualitative in-flow sensitive studies, using the IR-SE EPI sequence, indicate that the tissue perfusion also increases over the same range of frequencies. This implicates that larger fMRI responses can be obtained if electrical median nerve stimulation is performed at higher frequencies. The results are compared with electrophysiological data, which show a decrease of the early somatosensory evoked potentials at higher frequencies.