Alternative mapping to track the brain’s behaviour: nTMS

In previous weeks, you have already been introduced to intraoperative mapping. Now we will look at preoperative mapping with navigated transcranial magnetic stimulation, short nTMS. The message of transcranial magnetic stimulation may be known from other fields and is applicable in a variety of participant groups– in research, on the functionality of the brain in healthy participants, in post-stroke aphasia rehabilitation, in other clinical groups apart from low-grade glioma patients such as epilepsy patients, in treating neuropathic pain and tinnitus, and in a range of psychiatric disorders like schizophrenia. But what is crucial for us is how we can use navigated TMS for language mapping purposes.

Frank Zanow, the CEO of ANT Neuro BV in Berlin, our research and development partners, is here to talk you through that. This is the visor neuronavigation system for TMS. Transcranial magnetic stimulation is a method to stimulate superficial brain structures noninvasively. A TMS coil is placed at the skull and delivers a strong electromagnetic pulse through the skull to the underlying patch of brain tissue. Parameters such as frequency, intensity, duration of the stimulus, and orientation determine the overall effect of TMS stimulation. Single pulse and low frequency TMS stimulation decreases cortical excitability. Whereas high frequency stimulation with 10 hertz or more increases brain excitability. Visor2 is a stereotactic navigation solution that allows performing MRI guided navigated TMS.

It tracks the position of a TMS coil and the patient relative to each other with an infrared camera system. The patient is coregistered based on anatomical landmarks in his individual MRI, which is loaded into the software. The user observes the MRI, the brain structures, and the TMS coil position in a three dimensional interactive display. The focal point of the TMS coil and the estimated induced electric field allow him to precisely navigate the TMS coil to the targeted area.

Once the TMS coil is correctly placed over the desired target, the TMS stimulator is activated and electromagnetic pulses are delivered with selected intensity and repetition rate.

When we place the TMS coil over the motor cortex of the right hemisphere, we can activate neurons in the left hand area. You can see the thumb of the left hand twitching in response to the stimulus. This so-called motor mapping allows us to precisely localise motor function. It is increasingly being used in noninvasive presurgical mapping of brain function and helps greatly to improve surgical outcome. When we aim at mapping speech function, we cause the participant to become aphasic for a short moment. How this precisely works will be shown in the next video.