MEG versus Other Brain Imaging Technologies

MEG is a completely noninvasive, non-hazardous technology allowing for functional imaging of the brain's electrophysiology at a millisecond temporal resolution. Localization of electrical activity has an accuracy of approximately 2 mm. MEG measures the intercellular currents of the neurons in the brain giving a direct information about the brains activity, spontaneously or to a given stimulus. Measurement preparation and collection times are relatively short and can be performed by a technician with a minimum of training.

Other brain imaging technologies include EEG, x-ray computed tomography (CT), MRI, PET, and SPECT, which generally measure anatomy or blood flow, neither of which gives direct information about brain function. EEG, which shares MEG's fine temporal resolution, cannot accurately localize sources, unless electrodes are placed on the cortex itself. For comparison, the following is a list of imaging techniques contrasted to MEG:

Magnetic Resonance Imaging (MRI)

Gives anatomical information, no information about function.
Possible hazard, especially to children or pregnant women, due to high RF and magnetic fields.
Cost of equipment comparable to MEG.
Cost of procedure comparable to MEG.

Computed Tomography (CT)

CT has a limited role in the investigation of partial epilepsy because MRI is superior to CT in demonstrating brain tumors, vascular malformations, and focal brain atrophy. The diagnostic value of MRI in visualizing mesial temporal sclerosis and atrophy is under study. MRI is useful postoperatively to assess the extent of surgical resection.

Gives anatomical information, no information about function.
Possible hazard, especially to children or pregnant women, due to x-rays.
Cost of equipment comparable to MEG.
Cost of procedure comparable to MEG.

Electroencephalography (EEG)

EEG displays the electrical activity of the brain. Nerve cells in the brain are constantly creating very small electrical signals, whether a patient is waking or sleeping. EEG techniques such as depth or subdual electrode recordings, which are used at present to locate the seizure-provoking area.These invasive methods carry some risk, their use requires considerable expertise and, above all, they cause discomfort and inconvenience to the patient.

Measures electrophysiology of extracellular currents. The patten of extracellular current flow is affected singnificantly by he differing electrical conductivity properties of the brain, cerebrospinal fluid, skull, and scalp.
Localization accuracy is impaired by distortions created by the conductivity of the scalp and inhomogeneous tissue conductivity.
Time consuming to place and localize large numbers of electrodes. Compared with EEG, MEG is a much faster technique: instead of first pasting a set of electrodes on the scalp, requiring at least half an hour, the subject's head is just put inside the helmet.
Sensitive to both radial and tangential current sources. MEG signals reflect current flow in the apical dendrites of pyramidal cells oriented tangential to the skull surface, EEG reflects both tangential and radial activity.
Cost of equipment much less than MEG.
Cost of procedure more than MEG (long preparation time).

Subdural Electrocorticography (ECoG)

Measures electrophysiology of extracellular currents.
Highly invasive technique suitable for clinical cases only.
Possible hazard due to need for general anesthetic and surgical procedure.
Sensitive to both radial and tangential current sources.
Cost of equipment less than MEG.
Cost of procedure much more than MEG (craniotomy required).

Stereotactic Electroencephalograph Potentials (SEEP)

Measures electrophysiology of extracellular currents.
Highly invasive technique suitable for clinical cases only.
Possible hazard due to need for general anesthetic and surgical procedure.
Sensitive to both radial and tangential current sources.
Cost of equipment less than MEG.
Cost of procedure much more than MEG (craniotomy required).

Functional Magnetic Resonance Imaging (fMRI)

fMRI is a new application of MRI technology that allows the study of functional activity in the brain. Localized intrinsic signal changes that correlate with increases in neuronal activity are non-invasively detected allowing the acquisition of high resolution images that are dependent on brain activity rather than anatomy.

Measures blood flow or blood volume, rather than electrophysiology.
Does not allow for spontaneous measurements of Alpha rhythms etc.
Does not allow for millisecond time resolution.
Possible hazard, especially to children or pregnant women, due to very high RF and high magnetic fields.
Cost of equipment comparable to MEG.
Cost of procedure comparable to MEG.

Positron Emission Tomography (PET)

PET measures regional cerebral metabolism and blood flow. PET imaging has been quite successful in identifying the focus as an area of hypometabolism between attacks. This observation may be used in selecting patients with partial and secondarily generalized seizures for resective surgery. Because of the high costs and complexities of PET, this technology has been confined to a limited number of centers.

Measures metabolism of oxygen or sugar, rather than electrophysiology.
Does not allow for spontaneous measurements of Alpha rhythms etc.
Does not allow for millisecond time resolution.
Possible hazard, especially to children or pregnant women, due to ionizing radiation from ingested radionuclides.
Measurements cannot be repeated, after annual maximum dose is reached (generally one examination)
Cost of equipment much greater than MEG (to perform PET a cyclotron or other accelerator is required).
Cost of procedure much greater to MEG (five or more Ph.D. level staff are required to operate cyclotron, make radiopharmaceuticals and measure patients).

Single Photon Emission Computed Tomography (SPECT)

SPECT can also be used for functional imaging of the brain because it demonstrates regional cerebral blood flow, which is linked to cerebral metabolism and can therefore be used to identify the epileptic focus. SPECT uses conventional and readily available equipment and radiopharmaceuticals. These compounds can be used to study both ictal and interictal states. In the past decade, this relatively affordable technology has become widely available. More work is needed to determine whether SPECT is as sensitive as PET in localizing the epileptic regions.

Measures blood flow, rather than electrophysiology.
Does not allow for spontaneous measurements of Alpha rhythms etc.
Does not allow for millisecond time resolution.
Possible hazard, especially to children or pregnant women, due to ionizing radiation from ingested radionuclides.
Cost of equipment less than MEG.
Cost of procedure comparable to MEG.

Regional Cerebral Blood Flow (rCBF)

Measures blood flow, rather than electrophysiology.
Does not allow for spontaneous measurements of Alpha rhythms etc.
Does not allow for millisecond time resolution.
Possible hazard, especially to children or pregnant women, due to ionizing radiation from ingested radionuclides.
Cost of equipment less than MEG.
Cost of procedure comparable to MEG.