Congratulations on participating in one of the most cutting edge brain imaging technologies available today. Participating in a MEG session is a completely painless, non-invasive procedure. The total length of your session with the MEG varies depending on the application of the study. A presurgical somatosensory, motor, auditory and/or language mapping study takes about an hour and a half to complete, an epilepsy exam about two to three hours, and a cognitive research study may take anywhere from one to three hours, depending on the study. An additional structural MRI may be scheduled as well, but should take less than 30 minutes.
When the patient arrives for his/her MEG session, they will be greeted and will sign any consent forms that may be required. After explaining a short overview of the procedure, the patient will be briefly put into the closed MEG room while the technologist confirms that the patient does not have any noise issues that would make data collection less effective. Noise issues could stem from having metallic objects in the body (braces, shrapnel, aneurism clip, etc.). Noise could also stem from removable sources (underwire bra, jewelry, etc.), which can then be removed for the actual MEG data collection session.
After being approved for the session, patients are brought into an adjacent room where four head position indicator (HPI) coils are affixed to the head, typically with collodion glue but sometimes with tape, to ensure that the coils will not move. These HPI coils communicate with the MEG machine to determine how close the head is to the sensors that are collecting the brain data. Up to five pairs of electrodes are also applied: horizontal eye electrodes (HEOG), vertical eye electrodes (VEOG), electrocardiography electrodes (ECG), electromyography electrodes (EMG), and reference (REF) and ground (GND) electrodes. Horizontal EOG electrodes are placed in the temple areas on each side of the face, one placed more superior and the other more inferior but in otherwise mirrored locations. Vertical EOG electrodes are placed above and below the patient’s left eye, as vertically aligned as possible. If used, ECG electrodes are placed on the chest, and EMG electrodes are placed along the wrist. Reference and ground electrodes are placed upon the mastoid bone behind each ear (reference on left). These electrodes are affixed using electrode cream and tape.
Following HPI coil and electrode application, the patient is led back into the MEG Acquisition Suite where the placement of the HPI coils is recorded via a process known as digitization. During digitization, patients are asked to sit in a chair and wear a pair of goggles that have a localizing reference sensor attached to the side. The technologist then uses a digital pen attached to the goggle system in combination with the MEG Acquisition software to digitally mark the location of all HPI coils, measured with respect to the localizing reference sensor attached to the goggles. In addition to the HPI coil locations, the basic anatomical landmarks of nasion and left and right pre-auricular points are also digitally recorded. These landmarks aid in the coregistration of the MEG data to an anatomical, structural MRI, which is used during later analysis phases. Beyond the three landmarks, up to 200 additional points are taken defining the shape of the head all over, and including nose and chin, to enhance the coregistration process. The patient is asked to remain still during this process to ensure that the digital representation of their head shape and the HPI coils is most accurate.
Following digitization, all patients are instructed on the various tasks they will be participating in once inside the MEG machine. Patients are told what to expect from each task, as well as how to best perform the task to enhance our localization procedures. Of utmost importance is the need to remain as still as possible and with the head as close to the sensors as possible, with limited movement that causes muscular artifact such as blinking and jaw tension. Experimental protocols typically build in time for relaxation and to blink during a non-relevant time in the task, so as to facilitate comfort while completing the MEG session.
Once in the MEG room, the patient is seated in the chair and the HPI coils and electrodes are plugged into the machine. They are given a blanket if cold. Each task has different equipment, which the technologist enters the room and sets up before each task. Some examples are button pads to respond to a cognitive task, electrodes placed along the median nerve to stimulate somatosensory regions, or ear phones for an auditory task. Patients also have a table in front of them on which to rest their arms at all times. Once patients are comfortable, the chair is pumped up such that the head is raised into the dewar of the MEG machine. To acquire optimal data, the patient’s head must be as close as possible to the sensors contained in the dewar. Achieving this requires the patient’s head to be firmly touching the top of the dewar, so care is taken to ensure that the placement of the head is optimal for data collection while not being too uncomfortable for the patient.
Once the patient is situated comfortably, data collection can begin. First the technician checks all channels to ensure that there is minimal noise affecting the signals. If there is noise, a method of cleaning the channel is quickly employed before beginning the session. Once this is completed, the patient’s head position with respect to the sensors is measured. During the first scan, we purposefully ask the patient to blink their eyes, look left to right with their eyeballs without moving their head, swallow, clench their jaw, and close their eyes for a small period of time. The purpose of the preparatory scan is to collect a snapshot of each of these types of artifacts, useful in later analysis. Following this preparatory scan, each task is run, with head position being re-measured before each new task. At the conclusion of all tasks, the patient is removed from the MEG room and the technician removes the HPI coils with acetone (if affixed by collodion) as well as the electrodes and cleans any remaining glue off of the patient. This concludes the patient’s MEG experience.
For optimal mapping capabilities, MEG analysis employs the use of structural MRI. The structural MRI aids in localization by constraining the estimated MEG source solutions to the individual’s anatomy. The procedures for the MRI are set by the clinical center associated with the scan; however, the patient can expect to speak with a technician briefly about their medical history, following which they may be asked to wear scrubs. The patient will lie on the MRI table and have the helmet put in place around their head. Typically, the head is helped into a stable position by several pillows situated inside the helmet. All patients wear ear plugs due to the loud noise of an MRI, and a blanket is placed over their bodies for comfort. An emergency squeeze ball is always provided if there is a case of panic. Ideally for the MEG data processing, the MRI would be collected the same day as and after the MEG session, but it is not necessary. A previously collected structural MRI may be sufficient for MEG processing