JPH04135536A - Position detecting device of living body magnetic field measuring instrument - Google Patents

Abstract

PURPOSE:To detect the position relation of a multi-channel flux meter and a living body with high accuracy by a simple constitution by installing one coil being equal approximately to an external form of the living body to the living body, executing an electric conduction to this coil and measuring a generated magnetic field by a multi-channel SQUID fluxmeter. CONSTITUTION:A feeble magnetic field generated from a living body 1 is detected by a pickup coil 32 cooled by liquid helium 12, converted to a voltage corresponding to input magnetic field strength by a SQUID element 31, and converted to a signal being proportional to the input magnetic flux quantity by a flux locked loop circuit 13. Subsequently, an output of each channel is recorded in a computer 15. On the other hand, to a position measuring coil 11, a current is allowed to flow from a position measuring coil control circuit 16, and when a magnetic field is generated, it is measured by a SQUID fluxmeter 3, and a position of the SQUID fluxmeter 3 by a body motion of the living body 1. In this case, a shape of the position measuring coil 11 is almost the same as an external form of the living body 1.

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 technique #i]

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-"T or less, and can measure magnetic flux generated from living tissues such as muscles, heart, lungs, and brain. It is well known that biological functions can be diagnosed through measurements.Furthermore, a multi-channel magnetometer has been developed to measure the magnetic flux generated due to nerve activity currents, and to estimate the distribution of nerve activity currents. This is true, for example, in the r1989 International Superconductivity Electronics Conference J (1989 International Superconductivity Electronics Conference J).
nal SuperconductjvityE 1
electronics conference), p.
p4 Q-45. June 2-13. 1989. Tokyo
Japan, ). However, multichannel magnetometers cannot obtain morphological information of living tissues, and MRJ and
By superimposing the neural activity current distribution on a morphological image such as an ICT image, active sites in the living body are identified. Therefore, alignment with the morphological image is important. FIG. 2(a) is an example of the prior art shown in Japanese Patent Application Publication No. 1-503603. A plurality of transmitters 4 for generating electromagnetic signals are provided on the cryostat 2 for maintaining the 5QUID magnetometer 3 at an extremely low temperature, and a receiver 5 for receiving the electromagnetic signals generated from the transmitters 4 is further provided. It is provided on a plurality of individual bodies 1. The above transmitter 4 is shown in Fig. 2(b)
As shown in the figure, it consists of three mutually orthogonal coils 6 made of conductive wire that act as an antenna. Furthermore, the receiver 5 similarly consists of three mutually orthogonal coils 7 made of conductive wire, as shown in FIG. 2c. Using the transmitter 4 and the receiver if&5. The six-axis position of each receiver 5 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 5QUID 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 3 (a) shows IE, Transaction on Magnetics, MnG-23.
Volume, No. 2, (1987), pp. 1319-13
Page 22 (IEEE. Trans, Magnetics, Vol, MA
G-23, No. 2, (1987) pp 1319-1322). It consists of a plurality of 5QUID magnetometers 3, a cryostat 2 that cools them to an extremely low temperature, and a position measurement coil 10 for deriving the positional relationship between the 5QUID magnetometers 3 and the human body 1. Position measurement coil 1
A plurality of coils 10 are installed on the human body 1, and the magnetic field generated from the coil 10 is measured by a plurality of 5QUID magnetometers 3, and the coil 1 is
Estimate the 0 position. From the plurality of coils 10, the relationship between the biological coordinate system and the coordinate system set in the 5QUID magnetometer can be obtained. FIG. 3(b) shows the structure of the position measuring coil 10. Consists of a solenoid coil made of conductive wire.
To simplify the coil position estimation algorithm, 5QU
When this coil 10 is viewed from the ID flux meter 3, it is designed so that 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 5QUID magnetometer 10, so one-position measurement and biomagnetic field measurement cannot be performed simultaneously, but a dedicated receiver for receiving position signals is not required. It becomes possible to reduce the scale of the measurement system.

[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 determine the relationship between the coordinate system provided in the 5QUID magnetometer and the coordinate system provided in the living body, we also determined the relationship between the 5QUID magnetometer and the transmitter provided in the cryostat, and the positions of the transmitter and receiver. This requires two operations to determine the relationship, which may reduce positional accuracy. 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. That is, the position of the position measurement coil in the coordinate system provided in the 5QUID magnetometer: (X oH'/ oHZ 6 )
is 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. 3(C), one plane (for example, the xy plane) is determined by three points, and one axis (for example, two axes) directions perpendicular to this plane are determined. Furthermore, with the midpoint of the two magnetic dipoles on the plane as the origin,
One axis is determined in the direction connecting the remaining magnetic dipole 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 any one of them is attached out of position, an incorrect coordinate position will be set. Furthermore, when considering superimposition with MHI 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, since it is difficult to accurately estimate where the three position measurement coils 10 are attached in an MRI or XCT image, it is not possible to accurately determine the relationship between the two coordinate systems. For this reason, the neural activity current distribution obtained with the 5QUID magnetometer is
It is not possible to superimpose the image on the I or XCT image with high accuracy. Although accuracy can be improved by arranging a large number of position measuring coils 10 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. flll! [Means for Solving the Problem] The above object is to attach a single coil that is approximately the same as the external shape of the living body, that is, it is not circular, and to measure the magnetic field generated by energizing the coil using a multi-channel 5QUID magnetometer. This is achieved by measuring.

[Effect]

The nerve activity current distribution obtained by a multichannel magnetometer is superimposed on an MRI or XIXCT image to identify the active site in the living body. This identification requires the following two operations. First of all, based on the reference coordinate system set for the living body,
5 Calculate the position of the QUID magnetometer. Then, biomagnetic 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, the relationship between the reference coordinate system and the coordinate system of the medical image to be displayed in an overlapping manner is determined and alignment is performed. In X1ICT, the plane consisting of the outer corner of the eye and the root of the upper end of the ear is usually used as the reference plane, so the reference coordinate system may be set on this plane. Figure 4 (a) shows how to set the reference coordinates.
). 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 set as the X-axis, and the midpoint of the line segment formed by the intersection of this X-axis and the coil is set as the origin. The two axes and the y-axis are uniquely determined. Next, a method for measuring the position of the multi-channel 5QUID magnetometer will be described below. The mutual positions of the superconducting coils of the magnetometer are fixed, so rather than giving the positions of each superconducting coil, we can calculate the coordinate system set for the multichannel magnetometer system and the coordinate system set for the living body. Let us find the positional relationship between. 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 (xt yt 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 4 (b), → → x=x, +R−a → → → → → →
==xo+XA-a+YB-a+ZC-a→ → y=3'o+R-b → → → → → →
=:y, +xA-b1YB -b+ZC-b→ → zzzo1RaZ → → → → → →
=zo+XA-c tenYB-c+ZC-cHowever, B,
b, Ct A + B + C are the X axis,
Unit vectors in the y-axis, Z-axis, X-axis, Y-axis, and Z-axis directions. is given by The number of variables is the inner product of Xo, yow Zo, 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-S-Barr's law. Next, the following formula: H(rt): Estimated magnetic field N+ at the xth coil position
: Normal vector H♂ of the i-th coil = Determine the variable (XO? yar Zo, inner product of each unit vector) that minimizes the measurement data of the j-th coil. (However, this FOM is based on the assumption that the superconducting filter is a magnetometer. In the case of a differential coil, the estimated magnetic field is n, and the differential data in the direction is used.) -) The position of the 5QUID 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. The coil 11 is displayed superimposed on an image photographed on the same plane as the coil 11, and the coordinate position is adjusted so that the outline of the image and the image of the coil 11 are approximately superimposed. 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, one 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 5QUID magnetometer position information without measuring the biomagnetic field, allowing the body movements of the living body to be measured in real time. can be monitored.

【Example】

An embodiment of the present invention will be described below with reference to FIG. The weak magnetic field generated from the living body 1 or the position measurement coil 11 is
Pickup coil 32 cooled with liquid helium 12
is detected by the 5QUID element 31 and converted 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 the computer 5 as digital data. 5 The material of the cryostat 2 that keeps the QUID magnetometer 3 at an extremely low temperature is glass, stainless steel, FRP, etc., but in this example, it is 1) non-magnetic, 2) has high transfer efficiency when liquid He is injected, and 2) liquid He. Low evaporation rate ■Light weight. With this in mind, FRP was used. A current is applied to the position measurement coil 11 from the 1-position measurement coil control circuit 16 according to instructions from the computer 15, and a magnetic field is generated. The generated magnetic field is 5QUID, which is the same as the biomagnetic field.
Since it is measured by the magnetometer 3, 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 5-position measurement coil 11 was energized to monitor the position of the 5QUID magnetometer 3 due to the body movement 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 embodiment, 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 ear. However, since the position measurement coil 11 is within the field of view, the position measurement coil 11 is attached on a plane parallel to the plane 17. ] It is assumed to be on a plane where -1 does not enter. In the above, the present invention has described the case where the frequency of the current flowing through one position measurement coil is set to the frequency band of the biomagnetic field, but it is not limited to 29, and it is also possible to set it to a frequency band different from the biomagnetic field. It is. In this case, F L
L circuit] 3 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 for position measurement signals. This makes it possible to measure the position at the same time as measuring the biomagnetic field.
body movements can be monitored in real time. [Effects 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 the multi-channel 5QU4D 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 an embodiment of the present invention, FIG. 2(a) shows the configuration of the prior art, FIGS. 2(b) and (C) show the configuration of the transmitter and receiver of the prior art, respectively, and FIG. 3 (a) Method of setting the reference coordinate system, FIG. 4(a) shows the method of setting the reference coordinate system of the present invention, and FIG. 4(b) shows the relationship between the reference coordinate system and the coordinate system on the multi-channel 5QUID magnetometer. FIG. 1... Biological body, 3...5 QUID magnetometer, 11...
Position measurement coil, 13...F L L circuit, 15...
- Computer, 16... Position measurement coil control circuit.

Claims (1)

[Claims] 1. A living body 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 the living body, and calculates the current distribution caused by the neural activity of the living body. The magnetic field measuring device includes a position measuring coil that 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 a magnetic sensing coil that measures the relative position of the magnetic sensing coil to the living body prior to detecting the biological magnetic field. A position detection device for a biomagnetic field measurement device, comprising means for supplying current to the position measurement coil for detection. 2. The object to be measured of the biomagnetic field is the head of the living body, and the position detection coil is mounted parallel to the reference plane with a plane connecting the outer corner of the eye and the base of the upper end of the ear flap of the living body as a reference plane. The biomagnetic field measuring device according to claim 1.