Miniaturized coils for noninvasive magnetic stimulation: A numerical comparison in terms of focality and penetration depth

Miniaturized multi-layered coils (mCoils) were designed to achieve focal noninvasive magnetic stimulation of neural tissue. Thus, overcoming the well-known spatial resolution limitations of standard transcranial magnetic stimulation (TMS). The first mCoils were manufactured using novel flex circuit technologies, and their efficacy and safety have been assessed experimentally in previous studies using peripheral nerve stimulation of healthy subjects. This paper examines the second upgraded generation of the mCoils, the mTMS coils, specifically designed to target the brain cortex by doubling the number of layers and increasing the coil diameter. Additionally, an iron core was introduced to increase the induced E-field intensity inside the brain. The mTMS coils were studied to estimate their focality and penetration depth inside a virtual head model. The results show that despite a larger coil diameter, the mTMS coils maintain the original mCoil focality.