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Received Feb 21, 2018; Accepted May 9, 2018
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1. Introduction
Transcranial magnetic stimulation (TMS) is a noninvasive and painless method for activating neurons in the brain and can be used as a probe of higher brain functions and an intervention for neurological and psychiatric disorders [1]. Several coils were designed to stimulate different brain regions for different treatments (depression and Parkinson’s disease) but, due to the electric field rapid attenuation deep in the brain, TMS has been restricted to superficial cortical targets, around 2-3 cm in depth [2]. However, recent studies show that treatment for depressions can also consider nonsuperficial brain areas of 3-5 cm depth [3], as well as deeper regions of 6-8 cm depth [4, 5].
Using traditional TMS, with circular or figure of eight (Fo8) coils, regions of deep brain cannot be reached, as the electric field decreased rapidly as a function of tissue depth for this type of coils [6]. Thus, much higher stimulation amplitudes were needed to stimulate deeper neuronal regions. However, such high intensities at the sources may raise many safety concerns and can cause local discomfort due to the direct activation of nerves and muscles in the scalp [7]. Coil designs suitable for deep TMS, such as double-cone coil [8], Halo coil [9], and H-coil [10] were developed to circumvent these limitations. The double-cone coil provides deeper field penetration and has been used to target the anterior cingulate cortex with the transsynaptic activation [11]. The Halo coil has been designed to increase the magnetic field at depth in the brain when used together with the existing Fo8 and circular coils typically used for TMS [9, 12]. The coil design will be a combination of two TMS coils mostly used to increase the deep penetration of the electric field: the double-cone coil and the Halo coil. Locations of activation in the brain are related to the area where the induced electric field is maximal. These locations, in turn, depend on the coils’ placement and geometry as well as the head model for simulation studies [13]. Despite its importance and...