Coil design considerations for deep transcranial magnetic stimulation

Abstract

Objectives To explore the field characteristics and design tradeoffs of coils for deep transcranial magnetic stimulation (dTMS). Methods We simulated parametrically two dTMS coil designs on a spherical head model using the finite element method, and compare them with five commercial TMS coils, including two that are FDA approved for the treatment of depression (ferromagnetic-core figure-8 and H1 coil). Results Smaller coils have a focality advantage over larger coils; however, this advantage diminishes with increasing target depth. Smaller coils have the disadvantage of producing stronger field in the superficial cortex and requiring more energy. When the coil dimensions are large relative to the head size, the electric field decay in depth becomes linear, indicating that, at best, the electric field attenuation is directly proportional to the depth of the target. Ferromagnetic cores improve electrical efficiency for targeting superficial brain areas; however magnetic saturation reduces the effectiveness of the core for deeper targets, especially for highly focal coils. Distancing winding segments from the head, as in the H1 coil, increases the required stimulation energy. Conclusions Among standard commercial coils, the double cone coil offers high energy efficiency and balance between stimulated volume and superficial field strength. Direct TMS of targets at depths of ~ 4 cm or more results in superficial stimulation strength that exceeds the upper limit in current rTMS safety guidelines. Approaching depths of ~ 6 cm is almost certainly unsafe considering the excessive superficial stimulation strength and activated brain volume. Significance Coil design limitations and tradeoffs are important for rational and safe exploration of dTMS.

Theta burst magnetic stimulation over the pre-supplementary motor area improves motor inhibition

“…Using a large double cone coil we localized the left leg motor area starting at a point 2 cm posterior to Cz. The rationale for using a cone shaped TMS coil was to ensure that stimulation reached the pre-SMA which is a deeper structure [30,31]. Other smaller coils give larger focality over larger coils but this advantage diminishes with increasing target depth as in our case.…”

The Efficacy of High-Frequency Repetitive Transcranial Magnetic Stimulation for Improving Apathy in Chronic Stroke Patients

“…The first was a technical reason. Although a double-cone coil can stimulate deeper regions than an 8-figure coil, which is generally used for rTMS [12], it is difficult to stimulate very deep regions such as the orbital-medial PFC. Even with this coil, it is not impossible to stimulate very deep regions when the stimulation intensity is markedly high.…”

Recent Developments in Noninvasive Neuromodulation for Mood and Anxiety Disorders

“…In addition, baseline sgACC hyperconnectivity to DMN on fMRI (83)(84)(85), and baseline rostral ACC (rACC) glucose uptake on PET (14), predicated TMS antidepressant response. Current TMS devices are not able to reach sgACC directly, even with deep TMS (86)(87)(88). However, it is plausible to effect changes in sites remote from the coil via transsynaptic action.…”

“…Focusing the TMS field in depth is not possible due to the physics inherent in electromagnetic induction (86,87). It is important to note that the safety guidelines covering the selection of parameters of stimulation to reduce the risk of seizure with TMS were based on the figure 8 coil and do not directly apply to other coil geometries.…”

“…Several approaches to extend the reach of TMS to access deeper structures have been explored through novel coil designs. The H-coil was designed to reach deeper brain regions (dTMS), such as the medial prefrontal cortex (mPFC) (87,89). This device was approved by the FDA with a dose of HF-TMS over the left DLPFC (12,90).…”