Transcranial Magnetic Stimulation: An Automated Procedure to Obtain Coil-specific Models for Field Calculations.

BACKGROUND: Field calculations for transcranial magnetic stimulation (TMS) are increasingly implemented online in neuronavigation systems and in more realistic offline approaches based on finite-element methods. They are often based on simplified and/or non-validated models of the magnetic vector potential of the TMS coils. OBJECTIVE: To develop an approach to reconstruct the magnetic vector potential based on automated measurements. METHODS: We implemented a setup that simultaneously measures the three components of the magnetic field with high spatial resolution. This is complemented by a novel approach to determine the magnetic vector potential via volume integration of the measured field. RESULTS: The integration approach reproduces the vector potential with very good accuracy. The vector potential distribution of a standard figure-of-eight shaped coil determined with our setup corresponds well with that calculated using a model reconstructed from x-ray images. CONCLUSION: The setup can supply validated models for existing and newly appearing TMS coils.

New coil positioning method for interleaved transcranial magnetic stimulation (TMS)/functional MRI (fMRI) and its validation in a motor cortex study.

PURPOSE: To develop and test a novel method for coil placement in interleaved transcranial magnetic stimulation (TMS)/functional MRI (fMRI) studies. MATERIALS AND METHODS: Initially, a desired TMS coil position at the subject's head is recorded using a neuronavigation system. Subsequently, a custom-made holding device is used for coil placement inside the MR scanner. The parameters of the device corresponding to the prerecorded position are automatically determined from a fast structural image acquired directly before the experiment. The spatial accuracy of our method was verified on a phantom. Finally, in a study on five subjects, the coil was placed above the cortical representation of a hand muscle in M1 and the blood oxygenation level-dependent (BOLD) responses to short repetitive TMS (rTMS) trains were assessed using echo-planar imaging (EPI) recordings. RESULTS: The spatial accuracy of our method is in the range of 2.9 +/- 1.3 (SD) mm. Motor cortex stimulation resulted in robust BOLD activations in motor- and auditory related brain areas, with the activation in M1 being localized in the hand knob. CONCLUSION: We present a user-friendly method for TMS coil positioning in the MR scanner that exhibits good spatial accuracy and speeds up the setup of the experiment. The motor-cortex study proves the viability of the approach and validates our interleaved TMS/fMRI setup.