Limits of Near-Field Directivity for a Wire Array

Transcranial magnetic stimulation (TMS) systems require greater spatial resolution to enable more advanced therapeutic techniques and diagnostics procedures. Highly directive near-field arrays have been proposed to increase the ratio of magnetic field penetration depth to focality of TMS systems. We theoretically evaluate an infinite wire array and the current density required to produce highly directive field patterns. The results are shown to be applicable for practically sized arrays. We give examples of the current density distribution and resulting fields. Finally, we elaborate on how the density of the elements in the near-field array and the maximum allowable current magnitude limit the achievable directivity of the array.