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

Localizing sensorimotor areas with high resolution functional MRI is of con siderable interest for a wide range of medical applications from the preope rative 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 stimu lation at 5, 15, 40, and 100 Hz. BOLD fMRI was performed using a single sho t gradient echo EPI sequence to acquire 15 contiguous slices. For the quali tative flow sensitive studies, a single slice inversion recovery prepared s pin echo echoplanar sequence (LR-SE EPI) was used. Ln the primary sensorimo tor cortex, a linear increase of the fMRI-BOLD response, affecting both the number of activated pixels and the amplitude of the signal changes, was se en with increasing stimulation frequencies. The qualitative in-flow sensiti ve studies, using the IR-SE EPI sequence, indicate that the tissue perfusio n also increases over the same range of frequencies. This implicates that l arger fMRI responses can be obtained if electrical median nerve stimulation is performed at higher frequencies. The results are compared with electrop hysiological data, which show a decrease of the early somatosensory evoked potentials at higher frequencies. Hum. Brain Mapping 9:106-114, 2000. (C) 2 000 Wiley-Liss, Inc.