In order to fully understand the function of neuronal networks, it is not sufficient to only monitor activation during task performance using neuroimaging methods. To decide whether a network node is critical for the function under investigation, one needs to disturb it and observe the consequences, both behaviorally and neuronally.
To monitor human brain function noninvasively, functional MRI has become widely popular over the last two decades. To stimulate a human brain area noninvasively, transcranial magnetic stimulation (TMS) is the method of choice. Ideally, stimulation of a human brain is performed during fMRI scanning as this allows monitoring of the neuronal effect of TMS. However, TMS inside an MR bore is not easy to implement as it is an environment hostile to electronic devices. Only a handful of laboratories managed to combine TMS and fMRI in a satisfactory manner. TMS during fMRI scanning can increase our understanding of the role of functional neuronal networks in controlling behavior, and is also a novel method to investigate deviating network properties in neuropsychiatric diseases that so far remain elusive.
We have recently developed a solution to apply TMS inside a 3T Philips Achieva scanner. To this end, we had to overcome three main issues: electrical interference between MR image formation and TMS operation, electromechanical issues resulting from generating a secondary magnetic field inside an MRI bore, and image distortions due to the mere presence of the TMS coil. The first tests on groups of healthy volunteers have been performed succesfully. The above picture shows fMRI activation for supra-motor-threshold TMS vs sub-motor-threshold TMS, and in blue the tube with contrast fluid around the TMS coil can be seen, for one of the healthy participants. The yellow rods indicate estimated TMS magnatic field maximum, and the blue rods indicate the direction of induced currents. A clinical study on patient groups is currently in preparation.
A.D. de weijer (2012). A multimodal approach to investigate the hallucinating brain. PhD thesis