Overview of MRFs
Magnetic fields preceding voluntary movements are generated by neurons oriented tangential to the scalp in motor cortex contralateral to the limb moved. MRFs can be recorded and their intracranial sources localized with much greater fidelity and reliability than movement-related potentials. The location of MRF sources in the anterior wall of the central sulcus has been con- firmed in several studies through direct comparisons with the results of direct electrocortical-stimulation mapping.
MRFs are used clinically to localize the central sulcus and the somatotopic organization of the primary motor cortex or to evaluate motor function in patients with either organic or functional brain diseases before surgical interventions such as craniotomy, stereotactic, or radiosurgical procedures, and/or with suspected abnormalities in the descending corticospinal pathways.
Extension of the index finger is most commonly used for eliciting MRFs. Extension of the entire hand may also be used. Movement can be self-paced or externally cued. During a self-paced recording session, the patient lies supine, hand at his/her side resting on a nonmagnetic pad, and is asked to briskly flex the index finger at a rate of approximately one movement every 3 to 5 seconds (see Fig. 21.1). In the context of externally cued procedures, the patient is asked to react by pushing a button or performing hand extension as quickly as possible to a visual or tactile cue that is presented every 2 to 3 seconds. At most centers, somatosensory and motormapping are performed in separate sessions.