How is motor cortex mapping performed?
Navigated Brain Stimulation (NBS) is a transcranial magnetic stimulation technique for accurately locating the areas of the brain vital for voluntary movement or speech. Completely noninvasive, NBS is safe, painless and requires no participation from you the patient. Using the Nexstim NBS System, an operator gently stimulates parts of the cortex in your brain using a wand-like TMS coil, which is rested against your head.
When the TMS coil is activated, a brief magnetic field passes harmlessly through the head, inducing a weak electric field in the cortex under the coil. The strength of the field is adjusted so that the coil activates only a small number of the neurons in the cortex, allowing the operator to finely differentiate between areas of the brain. Responses to the NBS stimuli are recorded from electrodes placed on the skin over selected muscles.
NBS mapping is noninvasive
NBS functional brain mapping is noninvasive, meaning there is no surgery involved whatsoever. NBS mapping is performed for planning purposes. Typically, NBS mapping is made before any decision to operate has been taken. You may have heard of mapping by intraoperative direct electrocortical stimulation, this is a special diagnostic procedure performed during surgery in the operating room after craniotomy. NBS mapping gives results which are as accurate as direct electrocortical stimulation, but without surgery.
Where does NBS mapping take place?
Compared to most imaging equipment, the NBS System is compact and in a moveable cart, so mapping the brain may take place anywhere in the hospital. Normally, however, the NBS System will be in hospital's neurosurgery or neurology facilities. For the mapping, you will be asked to sit in a comfortable, reclining chair.
Who performs NBS mapping?
NBS mapping is made by a trained healthcare professional, who may be a doctor or a nurse. Interpretation of the results, however, will typically be made by your neurosurgeon or a neuro-oncologist in consultation with the whole neurosurgical or neuro-oncologist team.
What happens when I arrive?
Before an NBS mapping session, you will have had an MRI scan of your head taken, either at your own clinic or at the hospital's radiology department. When you arrive for mapping, your MRI head scan is uploaded to the NBS System via the hospital computer network (or from a storage medium). Your physician will have assessed your medical condition and medication, to ensure there is no limitation on NBS mapping (a history of epilepsy or treatment for epilepsy requires special precautions and mapping is not possible in patients with brain implants).
What do I need to do?
You will be asked to sit in a reclining seat with a comfortable headrest. Normally, you will be seated so that you can view from the NBS System screen yourself how the mapping session is proceeding.
Any kind of loose hairstyle is appropriate. Spectacles and metal earrings need to be removed. For motor mapping, small electrodes will be placed on the skin over some of your muscles, typically the hand, wrist, leg and face muscles, for measuring responses to stimulation.
You will be asked to wear a tracker on your forehead with reflecting spheres. The tracker allows you to move around during the mapping - any change in your head position will be taken into account by the System's computer. Because the mapping coil makes clicks during the session, ear plugs are offered for hearing protection and comfort. Setting up the system typically takes less than 2 minutes.
From your MRI-scan, a three-dimensional "rendering" of your head and brain will be displayed on the NBS System screen. The operator will then place a pointer against certain positions on your scalp and enter their locations into the system. This process will align your head with the 3D rendering of your head. Because of the little spheres on your forehead tracker and the equipment, all your head movements and movement of the coil can be followed in real-time using an infrared-based position tracking system. Since the infrared is invisible to the human eye, all you can see is a "camera with two eyes". Although it is called a camera, it does not actually take pictures.
What does the operator do?
The operator will place small electrodes on the skin over muscles of interest. 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 lesion's location. The electrodes are normal surface electrodes which use gel or paste. This method of detecting muscle movement is called electromyography (EMG)The operator will pick up the wand-like coil and rest it gently against your head. You will not feel any weight on your head.
When the operator triggers the coil, it will induce a stimulating electric field in the underlying cortex of your brain. You will hear a click and you may feel a little tingling in your scalp. Depending on the stimulus strength, you may notice a small twitch in a muscle. The strength of the magnetic field is similar to that of an MRI scanner, but lasts only momentarily.
By moving the coil and stimulating your brain several times, the operator will first find the area in your cortex from which you move your main thumb muscle. Once the optimal location has been found, the operator will determine how strong the stimuli need to be, in order to just cause the thumb to twitch. This level is your motor threshold. From this result, the operator can set the optimal stimulation power needed for comfortably and reliably mapping your cortex.
The clinician will continue to activate the coil, moving it gradually over your 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).
How long does it take?
The operator will continue to trigger the coil, while moving it gradually over the top of your head. Typically, the operator will only be mapping the area around the tumor, so the tumor location determines in which of your muscles you may feel a twitch. The tumor location determines the total duration of the session. Mapping of the motor area, the part of the brain responsible for voluntary movement, typically takes 15-30 minutes. However, unlike fMRI studies in a scanner, you are not in a confined space and you may move around in the chair.
What happens after the session?
Once the session is over the operator will remove the head tracker, the earplugs and the electrodes and you are free to leave the examination seat.
What happens next?
Later you will be invited to review the results of the mapping with your physician, together with results from your other scans and test. The NBS mapping results will be shown to you in a 3D view of your own brain, taken from the original MRI scan. Your motor responses to the stimuli are shown as colored markers. The results of the NBS mapping will be explained to you by your physician and the physician will discuss the implications for treatment. Experience has shown that the NBS mapping results are a significant help for patients and doctors when they come to discuss treatment and the possible trade-offs between possible risks and expected benefits of the various treatment alternatives.