David Cohen

Magnetoencephalography: Detection of the Brain's Electrical Activity with a Superconducting Magnetometer

Measurements of the brains magnetic field, called magnetoencephalograms (MEGs), have been taken with a superconducting magnetometer in a heavily shielded room. This magnetometer has been adjusted to a much higher sensitivity than was previously attainable, and as a result MEGs can, for the first time, be taken directly, without noise averaging. MEGs are shown, simultaneously with the electroencephalogram (EEG), of the alpha rhythm of a normal subject and of the slow waves from an abnormal subject. The normal MEG shows the alpha rhythm, as does the EEG, when the subjects eyes are closed; however, this MEG also shows that higher detector sensitivity, by a factor of 3, would be necessary in order to clearly show the smaller brain events when the eyes are open. The abnormal MEG, including a measurenment of the direct-current component, suggests that the MEG may yield some information which is new and different from that provided by the EEG.

Magnetoencephalography: Evidence of Magnetic Fields Produced by Alpha-Rhythm Currents

Weak alternating magnetic fields outside the human scalp, produced by alpha-rhythm currents, are demonstrated. Subject ard magnetic detector were housed in a multilayer magnetically shielded chamber. Background magnetic noise was reduced by signal-averaging. The fields near the scalp are about 1 x 10-9 gauss (peak to peak). A course distribution shows left-right symmetry for the particular averaging technique used here.

MAGNETOCARDIOGRAMS TAKEN INSIDE A SHIELDED ROOM WITH A SUPERCONDUCTING POINT‐CONTACT MAGNETOMETER

A point‐contact (SQUID) magnetometer was used inside a shielded room to record the magnetic field of the human heart, without noise‐averaging. The resulting magnetocardiograms, with the peak signal at about 3 × 10−7 G had a noise level of about 1 × 10−9 G (rms, per root cycle). They approach good medical electrocardiograms in clarity, and are an order‐of‐magnitude improvement in sensitivity over previous magnetic detectors of the heart. These results suggest new medical uses for this magnetometer.

Comparison of the magnetoencephalogram and electroencephalogram

The spatial response of the magnetoencephalogram (MEG) to sources in the brains cortex is compared with that of the electroencephalogram (EEG). This is done using computer modeling of the head which is approximated by 4 concentric spherical regions that represent the brain and surrounding bone and tissue. Lead fields are calculated at points on the cortex for unipolar, bipolar and quadrupolar MEG and EEG measurements. Since lead fields are patterns of the sensitivity of these measurements to a source at various locations and orientations, they provide a convenient means for comparison. It is found that a unipolar MEG has a very different lead field than a unipolar EEG. Hence, this type of MEG detects sources at different locations and orientations than this EEG. Although bipolar MEG and EEG lead fields are found to have similar patterns, the MEG lead field is narrower than that of the EEG and hence sees a smaller area on the cortex than the EEG. This is because the potentials measured by the EEG are smeared by the low-conductivity skull; the magnetic fields measured by the MEG are not smeared. Quadrupolar MEG and EEG lead fields are found to be about the same. The responses of bipolar MEGs and EEGs to distributed sources, which are composed of aligned and randomly oriented dipoles, are compared. It is found that for both types of sources, the MEG sees an area on the cortex which is approximately 0.3 times that for the EEG. Hence, the MEG appears to be useful for detecting a more restricted group of sources than the EEG.

Demonstration of useful differences between magnetoencephalogram and electroencephalogram

For a dipole source, theory predicts 3 useful differences between the MEG and EEG spatial patterns over the head. These are seen when a comparison is made between theoretical MEG and EEG maps, due to the dipole in a spherical model of the head. If true, these differences would allow the MEG to better localize or differentiate neural sources in some ways than does the EEG. A first experimental test of the differences is made here. A comparison is made between MEG and EEG maps due to a neural source which appears to behave as a dipole (N20 of the somatic evoked response). The same 3 differences are seen, therefore the predicted differences are confirmed experimentally. The first 2 differences, due only to the tangential component of the dipole, are that the MEG pattern is rotated by 90 degrees from the EEG pattern and is one-third tighter. The first allows the MEG to localize a tangential dipole better in a preferred direction, across the dipole (while the EEG does so along the dipole); the second allows the MEG to localize somewhat better in its preferred direction than the EEG does in its preferred direction. The third difference is due only to the radial component of the dipole; while the MEG receives no contribution from this component, the EEG pattern is asymmetrical and is heavily weighted by it. This allows the MEG to reveal tangential sources which are obscured by the radial sources in the EEG. For sources which cannot be approximated by a dipole, the MEG-EEG differences will depend on the particular case; however, the spherical model can now be used with more confidence to predict differences in these cases.

Magnetic Fields of a Dipole in Special Volume Conductor Shapes

Expressions are presented for the magnetic fields produced by current dipoles in four basic volume conductor shapes. These shapes are the semi-infinite volume, the sphere, the prolate spheroid (egg-shape), and the oblate spheroid (discus-shape). The latter three shapes approximate the shape of the human head and can serve as a basis for understanding the measurements of the brains magnetic fields. The semi-infinite volume is included in order to investigate the effect of the simplest boundary between a conductor and nonconductor. The expressions for the fields are presented in a form which separates the total field into two parts. One part is due to the dipole alone (the dipole field); the other is due to the current generated in the volume conductor by the dipole (the volume current field). Representative plots of the total field and the volume current field are presented for each shape. The results show that for these shapes the component of the total field normal to the surface of the volume conductor is produced completely or in large part by the dipole alone. Therefore, measurement and use of this component will greatly reduce the complexity of determining the sources of electrical activity inside a body from measurements outside the body by removing the necessity of dealing with the volume current field.

Ferromagnetic Contamination in the Lungs and Other Organs of the Human Body

Contaminating particles which are ferromagnetic have been found in the human body. Their distribution was measured by applying an external magnetic field to the torso for a short time, and then, in a shielded room, mapping the steady magnetic field around the torso due to the magnetized particles. Maps of subjects show various distributions, including particles in the stomach from food cans and in the lungs from are welding. The fields from these two sources are strong enough to be detected with a flux-gate magnetometer, without the need for a shielded room. This simplicity of detection of larger amounts of ferromagnetic contamination suggests that this method may be used in two applications: in detecting the presence of large amounts of asbestos (ferromagnetic and harmful) in the lungs of asbestos workers, and in tests of the condition of the lung where FE3O4 dust (ferromagnetic and harmless) would be used as an inhaled tracer material.

Developing a more focal magnetic stimulator. Part I: Some basic principles.

Some general properties of currents induced by magnetic stimulators in volume conductors of any shape are first reviewed. Then, the property of focality (concentration at some internal point) of the current induced in a spherical model of the head is discussed, due to coils of various orientations and configurations. It is shown that one important property for focality is the complete absence of the radial component of current, regardless of the coil used. The theoretically computed current distributions in the sphere produced by three different coils are illustrated. These are (1) a circular coil parallel to the head (very nonfocal). (2) a circular coil erect on the head (more focal), and (3) a figure-eight coil parallel to the head (most focal). A computational search for yet more focal coil configurations shows that the figure-eight focality can only be altered somewhat, but basically not improved. A plot of focality of the figure-eight coil versus the diameter of its coils shows an improvement in focality with decreasing diameter down to one cm. At this diameter, the induced current at a level 2 cm below the coil is concentrated in a band 1.5 cm wide (half-maximum points). Finally, it is noted that as the diameter of the figure-eight coil is decreased, the current in the coil necessary for stimulation increases rapidly, with increasing engineering problems.

Magnetic determination of the relationship between the S-T segment shift and the injury current produced by coronary artery occlusion.

Both the S-T segment shift and the injury current were measured using the direct-current magnetocardiogram (d-c MCG) in seven dogs undergoing coronary artery occlusion. The purpose of the measurements was to clarify the origin of the S-T shift in acute ischemia and infarction. Previous measurements, consisting of d-c electrograms recorded from the exposed epicardial surface in situ, are partially inconsistent; also, they are not necessarily representative of the surface electrocardiogram (ECG), which sums broadly over the myocardium. The d-c MCG allows steady myocardial currents in the intact torso to be measured externally; because the d-c MCG sums broadly over these currents, conclusions drawn from it are applicable to the ECG. Coronary artery occlusion was produced by inflating a tube which, about 1 week earlier, had been surgically installed around the artery and exteriorized. During occlusions carried out in the MIT magnetically shielded room, a sensitive magnetometer recorded the d-c MCG at various locations around the torso. Within 20 seconds after occlusion, equal and opposite S-T segment and base-line (d-c) shifts appeared on the d-c MCG; these shifts were maintained for at least 15 minutes, after which they slowly decreased. Therefore, during the acute ischemia produced by these occlusions, the S-T shift is a secondary result of a primary injury current that is interrupted during the S-T interval.

Measurements of the magnetic fields produced by the human heart, brain, and lungs

Magnetic fields produced by organs of the human body are being measured in the M.I.T. shielded room, using both a SQUID magnetometer and second-derivative gradiometer. Measurements of the field around the human body can yield new information about the organs which generate current, not available to surface electrodes, and also about organs which contain foreign, ferromagnetic particles. Magnetocardiograms of normal and abnormal heart subjects are being analyzed and visually displayed in order to assess their information content. Magnetoencephalograms recorded from normal and abnormal brain subjects are also under analysis. Measurements have been made of magnetite dust in the lung, with two potential medical applications: the first is the use of pure magnetite dust as a deliberately inhaled tracer (harmless) for pulmonary diagnosis; the second is the assessment of the amount of asbestos accumulated in the lungs of heavily-exposed workers, since most asbestos (harmful) occurs with adhered magnetite.