The distance from center at which the magnetic field drops to 10% of its peak value. This defines how far from center the field is still meaningful. It's half of the Effective Field Diameter. Tissue beyond this radius receives less than 10% of peak intensity. Primarily determined by coil diameter and geometry, not drive voltage or pulse parameters.
Also known as: 10% Falloff Distance
Seen from above, a coil lays its field down as a footprint that is strongest over the center and fades with distance. The dashed ring is the usable edge, the distance where the field has fallen to a tenth of its peak. The effective radius reaches from the center to that edge, the effective diameter is twice it, and the shaded disc inside is the effective field area, the surface the coil usefully covers.
