Developing a more focal magnetic stimulator. Part I: Some basic principles.

Some general properties of currents induced by magnetic stimulators in volume conductors of any shape are first reviewed. Then, the property of focality (concentration at some internal point) of the current induced in a spherical model of the head is discussed, due to coils of various orientations and configurations. It is shown that one important property for focality is the complete absence of the radial component of current, regardless of the coil used. The theoretically computed current distributions in the sphere produced by three different coils are illustrated. These are (1) a circular coil parallel to the head (very nonfocal). (2) a circular coil erect on the head (more focal), and (3) a figure-eight coil parallel to the head (most focal). A computational search for yet more focal coil configurations shows that the figure-eight focality can only be altered somewhat, but basically not improved. A plot of focality of the figure-eight coil versus the diameter of its coils shows an improvement in focality with decreasing diameter down to one cm. At this diameter, the induced current at a level 2 cm below the coil is concentrated in a band 1.5 cm wide (half-maximum points). Finally, it is noted that as the diameter of the figure-eight coil is decreased, the current in the coil necessary for stimulation increases rapidly, with increasing engineering problems.