JPH0555126B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a biomagnetic field measuring device that detects magnetic signals generated from a living body, and particularly to a device that can accurately detect the positional relationship between a multichannel magnetometer and a living body.

[Conventional technology]

A magnetometer consisting of a superconducting quantum interference device (SQUID) and a superconducting coil is capable of measuring extremely weak magnetic fields with a magnetic flux density of 10 ^-12 T or less, and is capable of measuring extremely weak magnetic fields such as muscles, heart, lungs, etc.
It is well known that biological functions can be diagnosed by measuring magnetic flux generated from biological tissues such as the brain. Furthermore, a multi-channel magnetometer has been used to measure the magnetic flux generated due to nerve activity currents, thereby providing a device for estimating the nerve activity current distribution. This is true, for example, at the 1989 International Superconductivity Electronics Conference.
Superconductivity Electronics Conference),
pp40−45, June12−13, 1989, Tokyo Japan.)
It is described in However, multichannel magnetometers cannot obtain morphological information about living tissues, and by superimposing neural activity current distributions on morphological images such as MRI or X-ray CT images, it is possible to identify active sites in living bodies. is being carried out. Therefore, alignment with the morphological image is important. FIG. 2a shows an example of the prior art disclosed in Japanese Patent Publication No. 1-503603. A plurality of transmitters 4 that generate electromagnetic signals are provided on the cryostat 2 for maintaining the SQUID magnetometer 3 at an extremely low temperature, and a receiver 5 for receiving the electromagnetic signals generated from the transmitters 4. are provided on the plurality of individuals 1.
As shown in FIG. 2b, the transmitter 4 described above is composed of three mutually orthogonal coils 6 made of conductive wire and functioning as an antenna. Furthermore, receiver 5
Similarly, as shown in FIG. 2c, it consists of three mutually orthogonal coils 7 made of conductive wire. When transmitter 4 and receiver 5 are used, each receiver 5
The six-axis position relative to the transmitter 4 can be measured. Position information of each transmitter 4 is obtained from angle information from a plurality of receivers 5. This method can determine the position of the cryostat 2 with respect to the human body 1 in real time by using a frequency different from the measurement signal band of the SQUID magnetometer. Furthermore, since the positional relationship between the cryostat 4 and the magnetometer 3 is uniquely determined, the position of the magnetometer 3 with respect to the human body 1 can be obtained in real time. Figure 3a is from I.E.I., Transactions on Magnetics, M.A.G. Vol. 23, No. 2, (1987), p. 1319.
Page 1322 (IEEE, Trans.Magnetics, Vol.MAG
23, No. 2, (1987) pp 1319-1322). plural
The SQUID magnetometer 3, the cryostat 2 that cools it to an extremely low temperature, and the position measurement coil 10 for deriving the positional relationship between the SQUID magnetometer 3 and the human body 1.
Consisting of A plurality of position measurement coils 10 are installed on the human body 1, and the magnetic field generated from the coils 10 is measured by a plurality of SQUID magnetometers 3, and the coils 10 are
Estimate location. From the plurality of coils 10, the relationship between the biological coordinate system and the coordinate system set in the SQUID magnetometer can be obtained. FIG. 3b shows the structure of the position measuring coil 10. It consists of a solenoid coil made of conductive wire, and in order to simplify the coil position estimation algorithm, it is designed so that when viewed from the SQUID magnetometer 3, it can be regarded as a magnetic dipole. This can be achieved by reducing the diameter of the solenoid coil. In this method, position measurement is performed using the SQUID magnetometer 3, so position measurement and biomagnetic field measurement cannot be performed simultaneously, but since a dedicated receiver for receiving position signals is not required, the measurement system It becomes possible to reduce the scale of

[Problem to be solved by the invention]

The first conventional technique requires a transmitter and a receiver, and requires a position measurement system in addition to the biomagnetic field measurement system. As the system scale expands, equipment becomes more expensive. Furthermore, in order to show the relationship between the coordinate system provided in the SQUID magnetometer and the coordinate system provided in the living body, we also investigated the relationship between the SQUID magnetometer and the transmitter provided in the cryostat, and the relationship between the transmitter and receiver. This requires two operations to determine the positional relationship, and there is a possibility that the positional accuracy will decrease. On the other hand, in the second conventional technique, since the position measuring coil 10 is a magnetic dipole, only point information can be obtained. In other words, the position of the position measurement coil in the coordinate system provided in the SQUID magnetometer: (x ₀ ,
y ₀ , z ₀ ) are obtained. Therefore, it is impossible to express the coordinate system of the living body with one coil.
To express one coordinate system using magnetic dipoles, at least three magnetic dipoles are required. That is, as shown in FIG. 3c, one plane (for example, the xy plane) is determined by three points, and one axis (for example, the z-axis) direction perpendicular to this plane is determined. Furthermore, the midpoint of the two magnetic dipoles on the plane is set as the origin, and one axis is determined in the direction connecting the remaining magnetic dipoles and this origin. As a result, a coordinate system is set for the living body 1. In this method, the three position measuring coils 10 are each attached to the living body separately, so if the attachment position of any one of them is shifted, an incorrect coordinate position will be set. Furthermore, when considering superimposition with MRI and XCT images, it is necessary to clarify the relationship between the coordinates of each image and the coordinate system obtained in the above discussion. However, the three position measurement coils 10
However, it is difficult to accurately estimate where it is attached in MRI or XCT images, so
It is not possible to accurately determine the relationship between two coordinate systems. For this reason, it is not possible to accurately superimpose the neural activity current distribution obtained with a SQUID magnetometer on an MRI or XCT image. A large number of position measurement coils 10
Although the precision can be improved by placing the image on the living body 1 and determining the external shape of the living body, there is a problem that the system scale becomes large and the device becomes expensive. An object of the present invention is to solve the problems of the prior art described above, and to obtain a position detection device for a biomagnetic field measurement device that can accurately detect the positional relationship between a multichannel magnetometer and a living body with a simple configuration. It is in. Another object of the present invention is to provide a position setting method that can accurately derive the relative relationship between a coordinate system provided in a medical image and a coordinate system obtained by the position detecting device.

[Means to solve the problem]

The above purpose is to attach a single coil to a living body that is approximately the same as the external shape of the living body, that is, it is not circular.
This is achieved by measuring the magnetic field generated by energizing the coil with a multichannel SQUID magnetometer.

[Effect]

Nerve activity current distributions obtained using multichannel magnetometers are superimposed on MRI and X-ray CT images to identify active areas within the body. This identification requires the following two operations. First of all, the position of the SQUID magnetometer must be calculated based on the reference coordinate system set in the living body.
Then, biomagnetic field measurements are performed using this position as a reference, and the neural activity current distribution is calculated from the magnetic field strength by solving an inverse problem. Second, determine the relationship between the reference coordinate system and the coordinate system of the medical image to be displayed in an overlapping manner, and align the images. In X-ray CT, a plane consisting of the outer corner of the eye and the upper end of the ear is usually used as a reference plane, so the reference coordinate system may be set on this plane. A method for setting the reference coordinates will be explained using FIG. 4a. A coil having approximately the same shape as the outer circumference of the living body is attached to the living body. The plane determined by this coil becomes the reference plane. Next, since the living body is symmetrical about the midline, the straight line that is line-symmetrical to the coil is defined as the x-axis, and the midpoint of the line segment formed by the intersection of this x-axis and the coil is defined as the origin. The z-axis and y-axis are uniquely determined. Next, the position measurement method of the multi-channel SQUID magnetometer will be described below. Since the mutual positions of the superconducting coils of the magnetometer are fixed, we do not give the positions of each superconducting coil, but rather the coordinate system set for the multichannel magnetometer system and the coordinate system set for the living body. Let us find the positional relationship with. This constraint reduces the number of variables, making it possible to significantly reduce the amount of calculation. Let the coordinate system provided in the living body be (x, y, z), and the coordinate system provided in the superconducting coil be (X, Y, Z). The relationship between these coordinate systems is as shown in Figure 4b: r→=R→+r→ ₀ ...(1) x=x ₀ +R→・a→ =x ₀ +XA→・a→+YB→・a →+ZC→・a→ y=y ₀ +R→・b→ =y ₀ +XA→・b→+YB→・b→+ZC→・b→ z=z ₀ +R→・z→ =z ₀ +XA→・c→ +YB→・c→+ZC→・c→ However, a→, b→, c→, A→, B→, C→ are each x
unit vector in the axis, y-axis, z-axis, x-axis, y-axis, z-axis direction. is given by The number of variables is the inner product of x ₀ , y ₀ , z ₀ and each unit vector. When a current is passed through the coil 11, a magnetic field is generated depending on the coil shape and the amount of current. Its strength is uniquely given by Biot-Sa-Bar's law. H→(r→)= _s I/4π ds×r/r ^Cubic equation: FOM= _o 〓 ⁱ⁼¹ (H→(r→ _i )・N→ _i + H _i ^m ) ² H→(r _i ): Estimated magnetic field N at the i-th coil position → _i : Normal vector of the i-th coil H _i ^m : Variable that minimizes the measured data of the i-th coil (x ₀ , y ₀ , z ₀ , The inner product of each unit vector) is determined. (However, this FOM assumes that the superconducting coil is a magnetometer. In the case of a differential coil, the estimated magnetic field uses differential data in the n _i direction.) The variable that minimizes the FOM and (1 ) The position of the SQUID magnetometer can be obtained from the equation. The alignment of the coordinate system provided in the living body and images from other image diagnostic apparatuses is performed by the following procedure. Coil 1 is shown on the image taken on the same plane as coil 11.
1 are displayed in an overlapping manner, and the coordinate positions are adjusted so that the outline of the image and the image of the coil 11 are approximately overlapped. This variation indicates the positional relationship between the coordinate system set on the living body and the coordinate system set on the image. This method uses two-dimensional images to obtain positional relationships, so positional accuracy is improved. Furthermore, since only one coil 11 is used, the system size can be small. By using the non-magnetic coil 11, the position can be measured during biomagnetic signal measurement, so it is also possible to eliminate the influence of the subject's body movement. Furthermore, by setting the frequency of the current flowing through the position measurement coil to a frequency band different from that of the biomagnetic field, it is possible to capture the SQUID magnetometer position information while measuring the biomagnetic field, monitoring the body movements of the living body in real time. can do.

【Example】

An embodiment of the present invention will be described below with reference to FIG. A weak magnetic field generated from the living body 1 or the position measurement coil 11 is detected by a pickup coil 32 cooled with fluid helium 12, and converted by the SQUID element 31 into a voltage according to the input magnetic field strength.
This output is converted by a flux locked loop (FLL) circuit 13 into a signal proportional to the amount of input magnetic flux. The output of each channel is A/D converted by an A/D conversion circuit 14 and recorded in a computer 15 as digital data. The material of the cryostat 2 that keeps the SQUID magnetometer 3 at an extremely low temperature is glass, stainless steel, FRP, etc.; is low,
It is lightweight. With this in mind, we used FRP.
A current is applied to the position measuring coil 11 from the position measuring coil control circuit 16 according to instructions from the computer 15, and a magnetic field is generated. Since the generated magnetic field is measured by the same SQUID magnetometer 3 as the biomagnetic field, the computer 15 controls so that only one of them is measured. In this example, after measuring the biomagnetic field for a certain period of time, the position measuring coil 11 was energized to monitor the position of the SQUID magnetometer 3 due to body movements of the living body 1. Furthermore, position measurement coil 1
The frequency of the current flowing through the magnetic field 1 must be within the frequency band of the biomagnetic field, and in this example, it was set to 20 Hz. In order not to disturb the biomagnetic field, the material was made of non-magnetic copper. In this example, the shape is an ellipse because it is approximately the same as the outer shape of the living body 1 and the strength of the generated magnetic field is easy to calculate. The position of the position measurement coil 11 is preferably on the plane 17 that connects the outer corner of the eye and the upper end of the root of the ear. It was placed on a flat surface that could not be entered. In the above, the present invention has described a case where the frequency of the current flowing through the position measurement coil 11 is set to the frequency band of the biomagnetic field, but it is not limited to this, and it is also possible to set it to a frequency band different from the biomagnetic field. In this case, the FLL circuit 13 requires a filter that passes only the biomagnetic field signal and a filter that passes only the position measurement signal. Furthermore, the number of input channels of the A/D converter is required for biomagnetic field signals and position measurement signals. This makes it possible to measure the position at the same time as measuring the biomagnetic field, making it possible to monitor the movement of the human body 1 in real time.

【Effect of the invention】

According to the present invention, the positional relationship between the multichannel magnetometer and the living body can be accurately detected by attaching a coil different from a circular one to the living body. Furthermore, when superimposing the neural activity current distribution obtained by a multichannel SQUID magnetometer on a medical image, the position measurement coil used is approximately equal to the outer circumference of the living body, so alignment is extremely easy.

[Brief explanation of the drawing]

FIG. 1 shows one embodiment of the present invention, FIG. 2 a shows the configuration of a conventional technology, FIGS. 2 b and c show the configuration of a conventional transmitter and receiver, respectively, and FIG. 3 a shows another conventional technology. 3b shows another prior art position measuring coil, FIG. 3c shows another prior art reference coordinate system setting method, and FIG. 4a shows the reference coordinate system setting method of the present invention. Figure 4b shows the reference coordinate system and multi-channel SQUID.
It is a figure which shows the relationship with the coordinate system on a magnetometer, respectively. 1... Biological body, 3... SQUID magnetometer, 11...
Position measurement coil, 13...FLL circuit, 15...
Computer, 16...Position measurement coil control circuit.

Claims (1)

[Claims] 1. A biomagnetic field that includes a plurality of magnetometers each consisting of a magnetic sensing coil and a superconducting quantum interference device that detect a biomagnetic field generated by a living body, and that calculates a current distribution caused by the neural activity of the living body. In the measuring device, a position measuring coil is attached to a specific position of the living body and has a shape that is approximately the same as the external shape of the living body at that position, and the relative position of a magnetic sensing coil with respect to the living body is detected prior to detecting the biomagnetic field. A position detecting device for a biomagnetic field measuring device, comprising means for supplying current to the position measuring coil in order to perform the position measuring coil. 2. The measurement target of the biomagnetic field is the biological head,
2. The biomagnetic field measuring device according to claim 1, wherein the position detection coil is mounted parallel to a reference plane that connects the outer corner of the eye of the living body and the base of the upper end of the ear shell.