Patent No. 5752514 Biomagnetism measuring method and apparatus

 

Patent No. 5752514

Biomagnetism measuring method and apparatus (Okamura, et al., May 19, 1998)

Abstract

A biomagnetism measuring method and apparatus for determining a positional relationship of an examinee with fluxmeters in a short time. A current supply unit simultaneously supplies alternating currents of different frequencies to a plurality of oscillator coils attached to the examinee, respectively. The fluxmeters detect magnetic fields simultaneously formed by the oscillator coils supplied with the currents. Field data thereby obtained are applied through a data collecting unit to a field analyzer for frequency analysis to recognize field strengths due to the respective oscillator coils for the respective fluxmeters. The field analyzer computes positions of the oscillator coils relative to the fluxmeters from the field strengths recognized for the respective oscillator coils and known values of the currents supplied to the respective oscillator coils. Based on this positional information, information indicative of positions of bioelectric current sources measured in a separate process is displayed as superposed on an MRI image of a site of interest of the examinee.

Notes:

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a biomagnetism measuring method and apparatus for measuring minute magnetic fields formed by bioelectric current sources in the body of an examinee or patient, and determining the bioelectric current sources based on data of the measurement.

(2) Description of the Related Art

With the recent developments in the superconducting device technique, biomagnetism measuring apparatus utilizing high-sensitivity fluxmeters called SQUID (Superconducting Quantum Interface Device) are being implemented as one type of apparatus for use in medical diagnosis. These biomagnetism measuring apparatus are expected to be useful in elucidating the functions of the brain and in diagnosing diseases of the circulatory organs.

The biomagnetism measuring apparatus is used to deduce, from data of magnetic fields measured, and by a least square or least norm method, positions, orientations and sizes of bioelectric current sources in a coordinate system where fluxmeters serve as a reference (Jukka Sarvas "Basic mathematical and electromagnetic concepts of the biomagnetic inverse problem", Phys. Med. Biol., 1987, vol. 32, No. 1, 11-22, printed in the UK).

A magnetoencephalogram thus obtained may be combined with medical images such as MRI images obtained from a magnetic resonance imaging apparatus (MRI apparatus) or X-ray images obtained from a radiographic CT apparatus, to determine a physical position of a disease or the like in the living body. It is important to grasp positional information of bioelectric current sources in the coordinate system based on fluxmeters, and their positional relationship with the medical images.

For this purpose, magnetic field generators called probe position indicators are arranged in proper positions on the surface of the head, such as near the root of the nose or under the ears. The following methods have been proposed to determine a positional relationship between the bioelectric current sources and the examinee:

(1) S. Ahlfors et al., "MAGNETOMETER POSITION INDICATOR FOR MULTI CHANNEL MEG", Advances in Biomagnetism, edited by S. J. Williamson et al., Plenum Press, New York 693-696, 1989;

(2) Neuromag-122 Preliminary Technical Data, Aug. 1991;

(3) "Method and Apparatus for Measuring Biomagnetism" (Japanese Patent Publication (Unexamined) No. H1-503603); and

(4) "Position Detector for a Biomagnetic Field Measuring Apparatus" (Japanese Patent Publication (Examined) No. H555126).

In these methods, three or more oscillator coils are pasted to the body surface of an examinee. First, a direct current is applied to the first oscillator coil. A magnetic field formed by the first oscillator coil is detected by a plurality of fluxmeters whose mutual positional relationship is known. The position of the first oscillator coil relative to the group of fluxmeters is determined from the strength of the current applied to the oscillator coil, field strengths detected by the respective fluxmeters, and the positional relationship among the fluxmeters. The operation is successively applied to the second and subsequent oscillator coils to determine positions of all of the oscillator coils, thereby to determine the position of the examinee relative to the group of fluxmeters.

However, three stages of operation are required for determining the position of each oscillator coil, which are (1) determining a proper amount of current to realize a field strength detectable by the fluxmeters, (2) supplying the determined proper current to the oscillator coil, and (3) detecting the magnetic field formed by the oscillator coil. In the conventional methods, these operations must be repeated for the number of oscillator coils pasted to the body of the examinee, and no less than several tens of seconds are consumed in determining the position of the examinee. This results in a drawback that a long examination time is required for measuring bioelectric current sources.

In particular, a proposal has been made for facilitating superposition on MRI images by using an increased number of oscillator coils (Japanese Patent Publication (Unexamined) No. H8-98821). However, such a method requires a still longer time for determining the position of the examinee.

SUMMARY OF THE INVENTION

This invention has been made having regard to the state of the art noted above, and its object is to provide a biomagnetism measuring method and apparatus for determining a positional relationship of an examinee with fluxmeters in a short time.

The above object is fulfilled, according to this invention, by a biomagnetism measuring method for measuring, with a plurality of fluxmeters, fine magnetic fields formed by bioelectric current sources in the body of an examinee, and determining at least positions of the bioelectric current sources based on field data obtained, the method comprising the steps of:

determining positions relative to the fluxmeters of a plurality of oscillator coils attached to the examinee; and

measuring, with the fluxmeters, fine magnetic fields formed by bioelectric current sources in the body of an examinee, and determining positions relative to the fluxmeters of the bioelectric current sources based on field data obtained;

wherein the step of determining positions of the plurality of oscillator coils includes:

simultaneously supplying alternating currents of different frequencies to the oscillator coils attached to the examinee, respectively;

measuring, with the fluxmeters, fine magnetic fields formed by the oscillator coils supplied with the alternating currents;

analyzing frequencies of field data measured with the fluxmeters, and computing field strengths due to the oscillator coils for individual ones of the fluxmeters; and

computing positions of the oscillator coils relative to the fluxmeters from the field strengths computed.

In the method according to this invention noted above, positions of the oscillator coils relative to the plurality of fluxmeters are computed by simultaneously oscillating the oscillator coils with different frequencies. Compared with the method in which the oscillator coils are oscillated successively, the method according to this invention can quickly measure the positions of the oscillator coils, i.e. positions of the examinee relative to the plurality of fluxmeters. In particular, a processing time is hardly variable even where numerous oscillator coils are used, to realize a quick and accurate measurement of bioelectric current sources. After measuring positions of the oscillator coils relative to the fluxmeters and positions of bioelectric current sources relative to the fluxmeters, the relationship between these positions enables the positions of bioelectric current sources to be identified relative to the oscillator coils or relative to the examinee.

The positions of the oscillator coils may be determined, for example, from field data obtained by simultaneously oscillating the oscillator coils only once. In order to increase the precision of measurement, the positions of the oscillator coils may be determined from averages of field data obtained by simultaneously oscillating the oscillator coils a plurality of times.

The step of analyzing frequencies of field data measured with the fluxmeters, and computing field strengths due to the oscillator coils for individual ones of the fluxmeters, may be executed to determine field strengths of frequencies corresponding to frequencies assigned to the oscillator coils as the field strengths due to the oscillator coils. It is also preferable to determine field strengths having maximum values in frequency ranges including respective frequencies assigned to the oscillator coils as the field strengths due to the oscillator coils. Then, the field strengths due to the oscillator coils may be determined with high precision even where slight variations occur with oscillating frequencies of the oscillator coils.

The step of computing positions of the oscillator coils relative to the fluxmeters from the field strengths computed, may be based on a least square method for computing positions of the oscillator coils.

The method according to this invention may further comprise the step of identifying positions of the bioelectric current sources on a medical image of a site of interest of the examinee based on the positions of the oscillator coils relative to the fluxmeters and the positions of the bioelectric current sources relative to the fluxmeters. This is useful for diagnostic and other purposes in that the positions of the bioelectric current sources are recognized on a medical image of a site of interest of the examinee. In this case, the oscillator coils are attached to positions corresponding to positions of markers attached to the site of interest before the medical image is picked up of the site of interest, the positions of the bioelectric current sources being identified on the medical image by relating the positions of the oscillator coils determined to the positions of the markers on the medical image, respectively.

In another aspect of this invention, a biomagnetism measuring apparatus is provided for measuring, with a plurality of fluxmeters, fine magnetic fields formed by bioelectric current sources in the body of an examinee, and determining at least positions of the bioelectric current sources based on field data obtained, the apparatus comprising:

a plurality of oscillator coils attachable to the examinee;

a current supply unit for outputting alternating currents of different frequencies to the oscillator coils, respectively;

a plurality of fluxmeters for measuring magnetic fields formed by the oscillator coils supplied with the alternating currents;

a field recognizer for analyzing frequencies of field data provided by the fluxmeters, and recognizing field strengths due to the oscillator coils for the fluxmeters, respectively; and

a field analyzer for computing positions of the oscillator coils relative to the fluxmeters from the field strengths recognized for the oscillator coils.

In the apparatus according to this invention noted above, the current supply unit supplies alternating currents of different frequencies to the oscillator coils, respectively. The fluxmeters detect magnetic fields formed by the oscillator coils. The field recognizer carries out a frequency analysis of field data received from the fluxmeters, to recognize field strengths due to the respective oscillator coils for the respective fluxmeters. The field analyzer computes positions of the oscillator coils relative to the fluxmeters from the field strengths recognized for the respective oscillator coils.

The oscillator coils are not limited to any particular type.

For example, each oscillator coil may be formed by printing metal on an insulating base or by winding a metal wire around a bobbin.

The current supply unit may, for example, include a plurality of AC sources, a plurality of amplifiers connected to the AC sources, respectively, and a controller for controlling oscillating frequencies of the AC sources and amplification degrees of the amplifiers.

Each of the fluxmeters used in this invention, preferably, includes a pickup coil and a superconducting quantum interface device connected thereto.

The apparatus according to this invention may further comprise an image storage for storing medical images of a site of interest of the examinee, the field analyzer being operable to identify the positions of the bioelectric current sources on the medical images by relating the positions of the oscillator coils computed to positions of markers on the medical images.

Preferably, a display is provided for displaying, in superposition, the medical images read from the image storage, and the positions of the bioelectric current sources identified by the field analyzer.

The apparatus according to this invention may further comprise a stimulator for applying at least one of light, sound and electric stimuli to the examinee.

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The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.