Mapping of motor cortex gyral sites non-invasively by transcranial magnetic stimulation in normal subjects and patients.
Transcranial stimulation offers the opportunity to replace a standard clinical method of invasive brain cortical mapping. A figure '8' magnetic coil with a 1-3-cm area of stimulation is combined with a new method of creating magnetic resonance imaging (MRI) scans which represent the cortical surface (mapping cuts) and thus establishing common landmarks between the MRI and the scalp for cortical mapping. This assigns gyral sights to scalp sites of transcranial stimulation. We report the development and evaluation of these techniques in 6 normals and 10 patients. Cortical maps were shown to enlarge during voluntary contraction. They were variable between individuals, as reported for direct cortical mapping. Maps showed a degree of short and longer term consistency within an individual. The maps were influenced by lesions. They also provided clinical information of the relation of functioning cortex to brain tumors that could influence clinical management including surgical decisions. A consideration of safety of the method in comparison to direct brain stimulation indicates that it falls within existing parameters for electrical stimulation of the brain, and could reduce the risks of the present standard by lowering charge density and the need for invasive methods. Broader application could lead to development of functional imaging methods, which would be a useful preoperative planning tool and investigative method.