Purpose: To improve transcranial magnetic stimulation of deep brain structures. Conventional TMS systems are unable to directly stimulate such structures, instead relying on intrinsic neuronal connections to activate deep brain nuclei. 

Methods: An MRI was built using modular electropermanent magnets (EPM) with rise times of less than 10 microseconds. Each EPM is individually controlled with respect to timing and magnitude. Electromagnetic simulations were performed to examine pulse sequences for stimulating the deep brain, in which various groups of the 101 EPMs making up a helmet-shaped system would be actuated in sequence. 

Results: Sets of EPMs could be actuated so that the electric field would be 2 volts/cm within a 1-cm region of interest in the center of the brain with a rise time of about 50 microseconds. Based on prior literature, this value should be enough to stimulate neurons (Z. DeDeng, Clin. Neurophysiology 125:6, 2014). The same EPM sequences applied 6V/cm electric fields to the cortex with rise and fall times of less than 5 microseconds, which according to prior human studies (IN Weinberg, Med. Physics, 39:5, 2012) should not stimulate neurons. The EPM sets could be combined tomographically within neuronal integration times to selectively excite bands, spots, or arcs within the deep brain. 

Conclusions: A combined MRI/TMS system with individually programmed electropermanent magnets has been designed that can selectively stimulate arbitrary locations in the brain, including deep structures that cannot be directly stimulated with conventional surface TMS coils. The system could also stimulate entire pathways. The ability to follow TMS with MRI pulse sequences should be helpful in confirming localization and extent of the magnetic fields and to measure physiological neuronal activation.