For NBS mapping, a TMS coil is placed over the head. The coil generates a brief magnetic field when activated. This magnetic field passes unhindered through your skull generating a weak electric field in the underlying cortex. If the electric field stimulates a part of your brain responsible for movement, it will activate a muscle. Just like you would do voluntarily.
In NBS mapping, the stimulation intensity is adjusted to stimulate just above your motor threshold. This way, only a small number of neurons are activated with each stimulating pulse - giving NBS the ability to accurately locate individual motor functions in the cortex.
Mapping the motor cortex with NBS
After the initial set-up process which aligns your head to the 3D model of your MRI head scan, NBS is used to find your motor cortex and determine your resting motor threshold (RMT) from your hand muscle.
Electrodes are placed on the skin over the muscles your physician wishes to study. These muscles may include the foot, leg and facial muscles, as well as those for the hand, arm and shoulder. The muscles will be chosen based on the location of the tumor in your brain. The electrodes are normal surface electrodes which use gel or paste.
Moving and activating the coil, the clinician will first find the area in your brain vital for controlling the thumb muscle. Next, the clinician determines how weak the stimulation intensity can be while still causing your thumb to twitch slightly. Knowing this intensity, called the motor threshold, allows the clinician to personalize the mapping to be both reliable and comfortable for you.
The clinician will continue to activate the coil, moving it gradually over the head. Normally, only the areas around a tumor are mapped, so the tumor location determines which additional muscles will be monitored (usually the hand, arm and leg muscles on one side).
A session typically takes 15-30 minutes. It is important that your muscles are relaxed during the whole session, and yot relieve any muscle tension. Thnaks to neuronavigation of the coil and the head you may move around in the chair—the experience is unlike having to be immobile in an MRI scanner.
After the mapping session
Normally, your physician will review the results of the NBS mapping with you. Responses to stimuli will be shown as colored markers in a 3D model of the brain. Taking into account other diagnostic information, the physician will discuss your treatment options. NBS results can be of significant help when discussing possible trade-offs between treatment risks and expected benefits
Navigation of the electric field
An MRI scan is processed to show the anatomy of your brain in 3D. Navigation of the electric field inside your brain is achieved by "linking" landmarks of your head to those in your MRI-scan. The clinician touches your landmarks with a "tracker" pointer and enters the data to the system. You only need wear a small "tracker" on the forehead. Once the setup procedure is completed, the NBS clinician can see the exact location of the electric field to be generated inside your brain. During mapping you can move your head and relax. Whenever the clinician moves the coil over your head, the stimulating electric field can be seen moving, overlaid in the 3D model of your brain.