Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1999-08-24
2003-03-25
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S225000
Reexamination Certificate
active
06538436
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to multichannel signal measuring. More particularly, the present invention relates to a novel and improved method for collecting multichannel signals comprising of the signal of interest and of superposed background interference contributions which may be much larger than the signal of interest.
DESCRIPTION OF THE RELATED ART
Performing many simultaneous measurements on a subject, i.e. multichannel detection, is sometimes essential in order to obtain sufficient information on the issue under examination. We consider, in particular, the detection of biomagnetic fields associated with the function of human brain or heart. Modern magnetometers for this purpose comprise about 100 channels to enable accurate localization of neuro- or cardiographic sources. Biomagnetic fields are very weak in comparison to the background magnetic fields in the surroundings, so that the problem of resolving the real signal from environmental interference is technically very challenging (M. Hämäläinen et. al., “Magnetoencephalography—theory, instrumentation, and applications to noninvasive studies of the working human brain”, Rev. Mod. Phys. vol. 65, no 2 April 1993.).
Prior art of protecting very sensitive instruments against external interference include basically five methods: 1) use of passive shielding elements surrounding the instrument (magnetically shielded room in the biomagnetic application), 2) use of active elements canceling the interfering environmental signal (large scale compensation coils in magnetic measurements), 3) reducing the relative sensitivity of the sensors to typical background signals (use of gradiometers instead of magnetometers), 4) use of additional sensors to estimate the background interference in order to separate it out from the signals, and 5) numerical processing of the multichannel data to separate true signal from external interference.
In method 1), when applied to biomagnetic measurements, the instrument is placed inside a shielding room having walls made of high permeability metal alloy (mu-metal). In the low frequency range, relevant to biomagnetic signals, the shielding factor of such a room is limited to about 100-1000 by reasonable amount and finite permeability of mu-metal. At high frequencies the shielding may be improved by adding layers of highly conducting material, such as aluminium (V. O. Kelh{overscore (a)} et. al., “Design, Construction, and Performance of a Large-Volume Magnetic Shield”, IEEE Trans. on Magnetics, vol. MAG-18, no 1, January 1982.).
In studies of human subjects, possibly patients in a hospital, the magnetically shielded room has to be relatively large, leading to a heavy and expensive construction. Sufficient shielding requires multilayer structure with total wall thickness of about 0.6 m. Thus, the outer dimensions of the room must be on the order of 4 m×5 mm×3.5 m to provide enough space for the instrument and comfortable conditions for the patient on a bed, and possibly for medical personnel taking care of the patient. Especially, the need of 3.5 m in height (two floors) is inconvenient in a typical hospital environment.
Method 2), when large compensation coils are used (EP 0 514 027, M. Kazutake et al. “Magnetic noise reducing device for a squid magnetometer”) resembles the passive shielding with high permeability material. The shielding current, which in mu-metal is generated as a natural response to an exposure to magnetic field, is now generated artificially in a control system and driven into coils with dimensions comparable to those of a typical shielded room. As a realization of such a system, three orthogonal Helmholtz pairs may be used. The external field to be eliminated is measured outside the coil system by field sensing elements, such as fluxgates, whose output is converted by a proper control system into electrical currents fed into the compensation coils. This kind of active shielding is far lighter and less expensive than a typical passive shield. It also performs best at low frequencies, where passive shielding of magnetic fields is most difficult.
The major disadvantage of method 2) is the very restricted geometry of the shielding currents. In practice, a compensation coil system can reject the field of distant sources only, which produce nearly uniform field at the site of the instrument. It may also be difficult to find the optimal positions for the field sensing elements, and if the environmental conditions change, the system may have to be readjusted.
Method 3), regarding the biomagnetic application, is based on the fact that the gradients of a magnetic field decrease more rapidly as a function of the distance from the source than the field itself. Therefore, the signal to background ratio is increased by measuring the difference of magnetic flux between two adjacent locations instead of the flux itself: the signal arising from the nearby object of study (e.g. a brain) is enhanced in comparison to the disturbance signal from an interfering source further away.
In principle, method 3) provides total immunity against uniform interference fields. In practice, however, the balance of best gradiometers is limited to at best 1/1000 because of technical difficulties in controlling the geometry of the sensors. In addition, the interfering fields are never strictly uniform. If the disturbing source is located a distance l away (typically 1-10 m) and the baselength of the gradiometer is h (typically 0.01-0.1 m), the background signal of the sensor is damped roughly by a factor of h/l compared to a magnetometer with the same loop size.
The most severe drawback of method 3) is that it rejects part of the signal arising from the object of study as well. This is especially unfavorable when the biomagnetic field is nearly uniform on the length scale of the sensor. This is to some extent the case in cardiac studies, and when a neuromagnetic source is located deep below the scull. For this reason, magnetometers would be preferred instead of gradiometers in many biomagnetic measurements (M. Hämäläinen et. al., “Magnetoencephalography—theory, instrumentation, and applications to noninvasive studies of the working human brain”, Rev. Mod. Phys. vol. 65, no 2 April 1993).
In method 4) (U.S. Pat. No. 5,187,436 A, J. A. Mallick “Noise cancellation method in a biomagnetic measurement system using an extrapolated reference measurement”, and U.S. Pat. No. 5,020,538, N. H. Morgan et al., “Low Noise Magnetoencephalogram system and method”, and DE 4131947, G. M. Daalmans, “Mehrkanalige SQUID—Detektionseinrichtung mit St{overscore (o)}rfeldunterdr{overscore (u)}ckung”, and DE 4304516, K. Abraham-Fuchs, “Verfahren zum Bestimmen einer Characteristischen Feldverteilung einer ortsfesten St{overscore (o)}rquelle”, and WO 93/17616, K. Abraham-Fuchs, “Disturbances suppression process during position and/or direction finding of an electrophysiological activity”, and EP 0481 211, R. H. Koch, “Gradiometer having a magnetometer which cancels background magnetic field from other magnetometers”, and U.S. Pat. No. 5,657,756, J. Vrba et al., “Method and systems for obtaining higher order gradiometer measurements with lower order gradiometers”) the apparatus is equipped with additional background sensors, which are so arranged that they do not receive any substantial input from the object of study. They are usually placed further away from the actual sensor array. From the signals of these sensors an estimate of the interfering background field is calculated—for example up to the desired order in the Taylor expansion of the field—and then properly extrapolated and subtracted from the signals of the actual measuring channels.
The relatively large distance between the background sensors and the actual sensors and the inaccuracy in the calibration and relative location and orientation of the sensors are the main drawback of this method, because these factors limit the degree of achievable compensation. Especially, correct interpretation and use of the background sensor outputs is practically
Ahonen Antti
Kajola Matti
Parkkonen Lauri
Simola Juha
Tuoriniemi Juha
Altera Law. Group LLC
Neuromag Oy
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