Second-derivative Gradiometer Research Articles

High-Tc second-order gradiometer for magnetocardiography in an unshielded environment

We have fabricated planar first-derivative gradiometers in which an asymmetric flux transformer patterned in a single-layer YBa2Cu3O7−x film is permanently bonded to a directly coupled magnetometer based on a dc superconducting quantum interference device (SQUID). The gradiometer base line is 48 mm. The common mode rejection of uniform magnetic fields is typically one part in a few hundred. The outputs of two such gradiometers are subtracted digitally to form a second-derivative gradiometer. Additionally, three orthogonal SQUID magnetometers are used to reduce the residual response to uniform magnetic fields to better than 100 ppm. The system is able to detect magnetic signals from a human heart in an unshielded environment.

Magnetic field changes associated with caloric stimulation of the human vestibular organ

Caloric stimulation of the ear produces a shift of the vestibular resting potential. The current produced by the change in this potential produces a corresponding change in the associated magnetic field. A sensitive magnetic detector (second-derivative gradiometer) was used to detect a large shift in the magnetic field close to the ears of two normal subjects who underwent cold thermic ear stimulation. The shift in the magnetic field tracked the reflex responses in both individuals. This technique could offer a noninvasive direct measure of end-organ function.

Transient and continuous responses of neuromagnetic fields evoked by peripheral nerve stimulation

We measured neuromagnetic fields evoked by stimulation of the ulnar nerve using an rf SQUID with a second-derivative gradiometer. Responses to transient stimulus obtained at various positions of the lateral head were used to calculate location and strength of an equivalent current-dipole source by least-squares fit. The dipole location corresponded to the primary somatosensory area. We also measured continuous responses to repeated stimuli, where lag of phase of a component synchronous to the stimuli was detected. The phase lag was linear to the frequency of the stimuli in a limited frequency range. The slope of the linear variation gave latent time of the synchronous component, which agreed with the time of main peaks in the transient response.

Somatically evoked magnetic fields in the vicinity of the neck.

This paper describes measurements of somatically evoked magnetic fields in the vicinity of the neck using a SQUID with a second-derivative gradiometer. The evoked fields were observed not only at the head, but also at the back of the shoulder and at the back of the neck when the peripheral nerve of the wrist was stimulated. The field at the neck appeared within about 150 ms after the stimulus with an amplitude of about 1 pT. The waveform of the responses at various points over the neck showed a reversal of the polarity across the midline of the neck. The isofield pattern indicated a current dipole in the vicinity of the spinous process C6 as the signal source. It was suggested that the evoked fields originated from the postsynaptic neuronal activity in the spinal cord.

Adaptive noise cancellation in neuromagnetic measurement systems

The noise spectrum of a superconducting second-derivative gradiometer has been investigated in an urban environment. Data have been acquired from the gradiometer with three orthogonal accelerometers and a triaxial fluxgate magnetometer attached to the dewar. These data have been analyzed by using signal processing techniques, primarily an adaptive noise canceller, to reduce noise in the gradiometer data. Results shown here indicate that the environmental noise and/or the dewar motion noise can be reduced as much as 40 dB in noise power. It is also shown, in the case of poor signal-to-noise ratio (S/N∼1), that a 10 Hz sine wave can be extracted. This technique not only shows promise for noise reduction, but also aids in the identification of noise signals which might be misconstrued as part of the evoked response. Analysis of data containing evoked response is now underway.

A gradiometer for the study of magnetic fields generated by the human heart in a magnetically unshielded environment: preliminary results.

A second-derivative gradiometer together with an RF SQUID magnetometer has been set up for the detection of magnetic fields generated by the human heart. A preliminary version of the device has successfully worked without a magnetically shielded room in a noisy environment. A first balancing of the gradiometer has been attempted. The effect of the residual ambient 50 Hz magnetic field has been reduced electronically. A typical magnetocardiogram and a simultaneously recorded electrocardiogram are shown.

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.

A superconducting second-derivative gradiometer

A balanced superconducting second-derivative gradiometer has been constructed in order to measure magnetic phenomena in an unshielded environment. The essential design features of the gradiometer, including its insensitivity to rotation and its capability of discriminating effectively against magnetic fields from distant sources, are described. Several details of the measured response of the gradiometer are presented to establish a correspondence between the design and the actual performance. Two magnetocardio-grams and a measurement of remanent magnetization in the area of the chest resulting from a 100 Oe magnetizing field are shown as typical results obtained in an unshielded environment. The observation of a magnetic signal synchronized with the heartbeat but which depends upon orientation of the subject and apparatus in the earth's field is also reported.