BackgroundTranscranial magnetic stimulation (TMS) allows non-invasive stimulation of the cortex. In multi-locus TMS (mTMS), the stimulating electric field (E-field) is controlled electronically without coil movement by adjusting currents in the coils of a transducer.ObjectiveTo develop an mTMS system that allows adjusting the location and orientation of the E-field maximum within a cortical region.MethodsWe designed and manufactured a planar 5-coil mTMS transducer to allow controlling the maximum of the induced E-field within a cortical region approximately 30 mm in diameter. We developed electronics with a design consisting of independently controlled H-bridge circuits to drive up to six TMS coils. To control the hardware, we programmed software that runs on a field-programmable gate array and a computer. To induce the desired E-field in the cortex, we developed an optimization method to calculate the currents needed in the coils. We characterized the mTMS system and conducted a proof-of-concept motor-mapping experiment on a healthy volunteer. In the motor mapping, we kept the transducer placement fixed while electronically shifting the E-field maximum on the precentral gyrus and measuring electromyography from the contralateral hand.ResultsThe transducer consists of an oval coil, two figure-of-eight coils, and two four-leaf-clover coils stacked on top of each other. The technical characterization indicated that the mTMS system performs as designed. The measured motor evoked potential amplitudes varied consistently as a function of the location of the E-field maximum.ConclusionThe developed mTMS system enables electronically targeted brain stimulation within a cortical region.
Abstract The operation of a transcranial magnetic stimulation (TMS) coil produces high-intensity impulse sounds. In TMS, a magnetic field is generated by a short-duration pulse in the range of thousands of amperes in the coil. When placed in a strong magnetic field, such as inside an magnetic resonance imaging (MRI) bore, the interaction of the magnetic field and the current in the TMS coil can cause strong forces on the coil casing. The strengths of these forces depend on the coil orientation in the main magnetic field (B0). Part of the energy in this process is dissipated in the form of acoustic noise. To conduct concurrent TMS and functional MRI (fMRI) safely, the sound pressure levels (SPLs) generated by the TMS coil must be quantitatively characterized. Measuring the SPLs of fast and loud impulse sounds accurately in the presence of static and gradient magnetic fields is challenging. In this study, we present a method for such measurements and report the SPLs of two commercial MRI-compatible TMS systems inside a 3T MRI scanner and of a prototype multi-channel TMS (mTMS) system inside a 9.4T small-animal MRI scanner. The mTMS coil allows for changing the direction of the electric field (E-field) without physically moving the TMS coil. We measured the acoustic noise generated by the TMS coils with different E-field orientations relative to the B0 field at different stimulation intensities and locations. The measurements were compared to the sound level measured outside the MRI room. SPLs and spectrum of the click sounds changed depending on coil and induced E-field orientation compared to the B0 field. SPLs exceeding the safety limit of 140 dB(C) was measured with all the devices. Our study provide is an important step towards the safety operation of concurrent TMS-fMRI respecting the auditory limits of small animals and humans. Keywords: TMS, mTMS, fMRI, acoustic noise
