Figure 1: Left Stimulus induced afferent electrical current (green) conducted along neuronal pathway (blue) reaches cortical surface. Right Current flow at neuronal level (Pyramid cells) induces surrounding magnetic field (red).
Figure 2: Orientation of the current flow and induced magnetic field in relation to cortical surface and one MEG sensor element.
Figure3: Measured magnetic field response at the sensor position after electrical stimulation of the median nerve.
Figure 4: Left Magnetic isofield map and time course of the somatosensory response at 20 ms, Right topological display of MEG channels.
Figure 5: Result of MSI procedure shows origin of specific brain activity overlaid with individuals MRI and rendered brain surface. Those coordinates of origin will be transferred to the frameless stereotactic device in the neurosurgical suite.
Figure illustrations credit: Michael Weisend, Albuquerque
|What is Magnetic Source Imaging (MSI)?
Magnetic Source Imaging is a non-invasive method to examine the function and structure of the brain. The technique takes advantage of the inherent magnetic properties of the brain, without the need to expose patients to any harmful agents. MSI technology allows to evaluate the normal function of the brain, and it provides a means for detecting abnormalities caused by disease. MSI is complimentary to other brain imaging methods such as positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) and it provides unique information on the spatio-temporal dynamics of brain activity. MSI is the modality of choice for:
- preoperative mapping of brain areas supporting sensory, motor and language function,
- characterization and localization of epileptiform activity,
- characterization and localization of abnormal slow wave activity, and
- definition of normal and disordered information processing.
The MSI examination is performed in two steps - Magnetoencephalography (MEG) and Magnetic Resonance Imaging (MRI). MEG is a method for detecting the very weak magnetic signals that are generated by the natural electrical activity of the brain. MEG is ideal for examining brain function in relationship to behavior and for identifying functional abnormalities such as epilepsy. MRI is the method of choice for providing images of the structure of the brain. MRI is ideal for detecting brain damage caused by conditions such as tumors and strokes. When MEG and MRI data are combined in the MSI examination, it provides a detailed picture of the relationship between behavior, brain structure, and brain function. This allows physicians to provide patients with better care, tailored to their individually needs.
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How does MEG work?
Magnetoencephalography (MEG) takes advantage of the fact that all electrical currents, be they in power lines, or brain cells, generate a surrounding magnetic field. Using special sensors it is possible to measure the tiny magnetic signals produced by the brain, even though these are more than one billion times smaller than the magnetic field generated by a light bulb. Different parts of the brain produce different patterns of magnetic waves. When the brain has been affected by disease, abnormal magnetic signals may be produced, and MEG can be used to determine which brain regions are malfunctioning. MEG can also be used to identify specific functional regions of the brain such as auditory and visual cortex. When a patient is presented with a stimulus (for example, a sound or a picture), specific portions of the brain are normally activated in characteristic sequence. By examining how the neuromagnetic activity changes during stimulation, it is possible to pinpoint the location of functional regions and to determine if the sequence of activation has been perturbed by disease.
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How Does MRI Work?
Solid objects are actually made from atoms that are composed of even smaller particles of matter such as protons and electrons. The proton of the hydrogen atom has the important property that it behaves like a very tiny bar magnet that spins. When an object is placed within a large magnetic field, the protons of the object align along the direction of the field, or exactly opposite to it. The ratio of protons in these two states -- the net magnetization -- can be modulated by application of radiofrequency magnetic signals. Using carefully constructed sequences of radio-frequency pulses and specialized detection coils, the net magnetization at different points in space can be determined. Through examination of how the magnetization of the tissues of the head change as a function of combined static and radio-frequency magnetic fields, a computer can construct an image of the brain.
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MSI -- Putting It All Together
Using simple computer methods, information from MEG and MRI are integrated to form magnetic source localization images that provide a detailed structural-functional blueprint of each patient's brain.
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The basic principles of MSI and MEG
When enough brain cells are active together, the generated magnetic field can be measured using special super-cooled sensors, even though the strongest magnetic signal from the brain is more than one billion times smaller than that generated by an ordinary light bulb. The device used to measure the brain's magnetic signature is known as a biomagnetometer. CAMT houses a whole-head biomagnetometer unit that contains 306 sensors arrayed about the head.
The output of each sensor is a waveform showing how the local magnetic field changes with time. By examining these waveforms, abnormal brain signals, such as epileptic spikes can be identified. The presented data set (Fig. 4) shows approximately one second of data after electrical stimulation of the right median nerve. It is difficult to view all 306 signals at the same time so data are viewed in a condensed format. The panel on the right shows the waveforms from 122 sensors.
By examining the pattern of magnetic activity associated with that stimulation, it is possible to infer from where in the brain that activity originated. Red shows magnetic flux exiting the head, and blue shows flux entering the head. The yellow arrow shows the time point of the first cortical response after stimulation onset.
The functional MEG data are integrated with structural MRI data to generate magnetic source localization images. The green circle shows the location of the neurons that generated the response following stimulation of the right median nerve. MSI can also be used to map auditory, visual, and motor areas of the brain in relationship to brain tumors and epileptic foci. This is especially useful for patients being evaluated for neurosurgery where it is important that the neurosurgical intervention avoid damaging critical brain regions.
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Who Can Benefit From MSI?
MSI evaluation can be of benefit in several neurological and psychiatric disorders, especially when physicians are considering neurosurgical intervention or other therapies that should be initiated only in the presence of objective evidence of brain dysfunction. MSI is clinically useful in the evaluation of patients with the following disorders:
- Brain Tumors,
- Vascular Malformations,
- Epilepsy and epileptic syndromes,
- Head Trauma,
- Schizophrenia & Depression, and
- Learning Disabilities.
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