JPH0779804B2 - Equivalent dipole measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is equivalent to measuring an electric field or a magnetic field on the surface of a living body and calculating the position or vector component of an assumed equivalent dipole inside the living body from the measured value. The present invention relates to a dipole measuring device.

[Conventional technology]

BACKGROUND ART Conventionally, an electroencephalograph, an electromyogram, an evoked potential adding device, etc. have been used as a device for measuring an electric field appearing on the surface of a living body by the nerve activity of the living body. Recently, the electric field or magnetic field generated on the surface of the living body with the nerve activity of the living body is measured,
An equivalent dipole method for estimating the active site in the living body has been proposed. The principle of this direction is as follows. That is, for example, when cells at each active site of the brain are stimulated, electromotive force is generated to generate an electric field or magnetic field on the scalp. Correlate each part of the brain with an electric dipole from such an electric field or magnetic field, and calculate the position of this dipole and the vector component from the above electric field or magnetic field to estimate the position of active brain cells By doing so, the activity state of the brain is tracked. In the equivalent dipole method for estimating such a dipole, initially, for example, the head is assumed to be a perfect sphere and the skull is equal because of the relation that the electric field or magnetic field generated by the dipole is repeatedly calculated. The calculation was performed on the assumption that it was in such an infinite conductor. That is, a method of calculating an electric field or a magnetic field by assuming a homogeneous conductive sphere in which a homogeneous brain is present in the head and assuming a concentric or an eccentric spherical shell has been proposed. However, the method of approximating a non-spherical skull with a spherical model not only involves errors in the calculation of electric and magnetic fields, but it was not clear where the estimated position of the equivalent dipole corresponds to in the brain.

Therefore, a method of removing the influence of the cavity such as the eye hole and the ear hole that disturbs the homogeneity in the skull (Japanese Patent Application No. 62-285728 and Japanese Patent Application No. 63-285728).
86464, etc.) and a method of removing the influence of the head shape (Japanese Patent Application No. 6)
2-285728, Japanese Patent Application No. 63-182163, etc.) have been proposed.

[Problems to be Solved by the Invention]

In the conventional equivalent dipole method, for example, in the head, the influence of the cavity that disturbs the homogeneity in the skull can be eliminated. However, the influence of the skull cannot be ignored in calculating the position and vector component of the equivalent dipole. That is,
The skull has a different conductivity or magnetic permeability from the other tissue-filled portions in the skull, and an error will occur if the skull is calculated as the same conductivity or magnetic permeability as the other tissue-filled portions. Further, for example, in the chest, an error occurs due to the influence of ribs.

The present invention has been made in order to solve such a drawback of the conventional equivalent dipole method, and in addition to the conventional cavity, calculation using all substances that cause inhomogeneity of the skull, ribs, etc. It is an object of the present invention to provide an equivalent dipole measuring device capable of correcting an error and accurately estimating the position and vector component of an equivalent dipole.

[Means for Solving the Problems]

The equivalent dipole measuring device of the present invention actually calculates the position or vector component of the equivalent dipole obtained by calculating from the measured value of the electric field or magnetic field generated on the surface of the living body by the dipole power source installed inside the living body. Storage means for storing the correction amount obtained by comparing with the position or vector component of the dipole power source, detection means for detecting an electric field or magnetic field on the surface of the living body, and the detection means for detecting the electric field on the surface of the living body. Computation means for computing the position or vector component of the equivalent dipole assumed inside the living body from the magnitude of the electric field or magnetic field, and the correction corresponding to the position or vector component of the equivalent dipole obtained by the computation means. And a correction means for correcting the position or vector component of the equivalent dipole by reading the quantity from the storage means.

[Action]

In the equivalent dipole measuring device of the present invention, the electric field or magnetic field generated on the surface of the living body is measured by the dipole power source installed inside the living body, and the position or vector component of the equivalent dipole is calculated from the measured value. And the deviation between the estimated value by this calculation and the actual position or vector component of the dipole power source is stored in advance in the storage means as the correction amount of the estimated value of the equivalent dipole position or vector component by the calculation. .

Next, from the magnitude of the electric field or magnetic field on the surface of the living body detected by the detecting means, the position or vector component of the assumed equivalent dipole inside the living body is calculated by the calculating means and estimated.

Then, the correction amount corresponding to the position or vector component of the equivalent dipole estimated by calculating the detection value of the detecting unit by the calculating unit is read from the storage unit, and the position or vector component of the equivalent dipole is estimated. The value is corrected by the correction means to obtain the correct equivalent dipole position or vector component.

In this way, in the equivalent dipole measuring apparatus of the present invention, since the estimated value by the calculation is corrected by the correction amount obtained by installing the actual dipole power source in the living body, the homogeneity in the living body is disturbed. Accurate equivalent dipole position and vector components can be estimated by removing all the influences of the cavity and bone.

〔Example〕

 Examples of the present invention will be described below.

FIG. 1 is a block diagram showing the configuration of an embodiment of the equivalent dipole measuring device of the present invention. This embodiment will be described with reference to FIG.

First of all, about 15 CT tomographic images (16) are taken using X-ray CT in order to accurately grasp the shape and dimensions of the living body (1) body surface, for example, the head, and then this CT tomographic image Reading the two-dimensional dimensions of (16) one by one into the computer (9) via the input port (14) using the pickup (17) of the digitizer (18), and obtaining the three-dimensional head shape from the signal. To Also, the electrode position signal input device (1
Input as 3D coordinates of x, y, z from 9). If a shape sensor developed by the inventor of the present invention (see, for example, Japanese Patent Application No. 63-182162) is used instead of X-ray-CT, the head shape can be obtained more easily and quickly.

Next, for example, about 21 electrode groups (5) are attached to the head of the living body (1), and the potential based on the nerve activity in the brain is detected by the potential detecting means (10). The measurement potential from the electrode (5) is supplied to the analog-digital converter (A / D) (8) via the amplifier (6) and the multiplexer (7), and the digitized measurement potential is input port (14). Is supplied to the computer (9) via. The computer (9) has a control unit (9a) and an arithmetic unit (9b), and has an address bus (11
The a) and the data bus (11b) are connected to the ROM (12), RAM (13), input port (14), and output port (15).
The ROM (12) and RAM (13) are storage means for storing the program necessary for signal processing and for storing the data from the digitizer (18), the electrode position signal input device (19) and the potential detection means (10). is there. The computing unit (9b) of the computer (9) has a computing unit and an equivalent dipole setting unit.
The input port (14) is connected to an external storage device (20) that stores a program for obtaining an equivalent dipole, and the output port (15) displays the calculation result of the computer (9).
A display means (22) such as a CRT and a printer (21) for storing data and waveforms displayed on the display means (22) are connected.

The operation of this embodiment having the above-mentioned structure will be described with reference to the flowchart of FIG.

In FIG. 2, although not shown, the power supply is turned on to set the equivalent dipole measuring device (23) of this embodiment to the initial state as shown in the first step ST ₁ . In the next second step ST ₂ , various calculation programs and signal processing programs, which will be described later, are read from the external storage device (20) and stored in the RAM (13) in the computer (9). Such a program is a non-volatile memory in the computer (9).
If it is stored in advance in the ROM (12), the second step ST ₂ becomes unnecessary.

In the next third step ST ₃ , for example, the head shape dimension of the living body (1) is input. X-ray CT as an example of skull shape dimension measurement
Approximately 15 CT tomographic images (16) with a slice interval of 15 mm are made for one person using. This CT tomographic image (16) measures several parameters such as the circumference, width, and anterior-posterior length of the skull for each individual, and if it adopts the method of applying to several standard models prepared, each person This simplifies the measurement because it eliminates the need to take a CT tomographic image to measure the skull.
Of course, each individual skull may be measured. The two-dimensional image of the 15 CT tomographic images (16) sliced in this way is taken out by the pickup (17) for each tomographic image (16), and the digitizer (18) is used to input from the computer (9) from the input port (14). ) And store it in RAM (13). In this case, when the slices are stacked three-dimensionally, the "deviation"
It is advisable to specify the intersection points of three straight lines perpendicular to the slice section for each slice so that the above will not occur.

In the fourth step ST ₄ , the computer (9) performs catch calculation and converts it into three-dimensional data of the skull based on the thus inputted head shape dimension.

In the next fifth step ST ₅ , 2 placed on the head of the living body (1)
From the electrode position signal input device (19) such as the keyboard shown in FIG. 1, x, y in order to make the position of one electrode (5) about one electrode correspond to the three-dimensional head shape obtained in the fourth step ST ₄ . , z-axis three-dimensional coordinates, and enter the RAM (1
Store in 3).

Sixth step ST in ₆ living body as shown in FIG. 1 (1) 21 before and after the electrode group on the head is the (5) is placed, is performed potential measurement based on brain neuronal activity. The nerve activity potential measured in this manner may be an evoked potential for various stimuli such as electrical stimulation, light stimulation, and sound stimulation, or a nerve activity potential in the state where no stimulation is applied, and the measured value is an amplifier. (6) →
It is supplied as digital data from the input port (14) to the computer (9) via the multiplexer (7) → A / D (8) and is stored in the RAM (13).

In the seventh step ST ₇ , the potential of one sample lock is extracted from the potentials of nerve activity and designated in the computer (9).

In the next eighth step ST ₈ , the transfer matrix at the designated electrode (5) position, assuming that the current dipole is placed at a predetermined position in the skull, is calculated by the calculation unit (9b) of the computer (9) and the current is calculated. Calculate the electric potential at each electrode position generated by the dipole. Generally, assuming that the source of nerve action potential is a current dipole, the potential Vc generated on the scalp by the current dipole
Is expressed by equation (1).

Vc = A (r) · p (1) where p is the vector component of the current dipole, r is the position of the current dipole, and A (r) is the transmission of M rows and 3 columns, where M is the number of electrodes. Matrix (function of dipole position r).

Here, the potential generated from the current dipole that is assumed when the brain in the skull is considered to be an infinitely uniform medium is φ∞. From this potential, as shown in FIG. Consider conversion of the electric potential of a heterogeneous medium into the cavity (2) such as the eye and ear canals and the brain (24).

In FIG. 3, Ψ ₀ : potential in tissues other than brain and cavity Ψ ₁ : potential in brain Ψ ₂ : potential in cavity Ψ _out : potential outside the skull Ω ₀ : region of tissue other than brain and cavity Ω ₁ : Brain region Ω ₂ : Cavity region Ω _out : Extracranial region σ ₀ : Conductivity of tissues other than brain and cavity σ ₁ : Brain conductivity σ ₂ : Cavity conductivity σ _out : Extracranial Conductivity of S ₀ , S ₁ , S ₂ : Assuming the boundary with each region, the current dipole is placed in the region Ω ₁ and this region is assumed to be an infinite homogeneous medium. If the generated potential is φ∞, φ∞ is given by the equation (2).

Here, σ ₁ is the conductivity of the brain, which is an infinite uniform medium, and γ m is the electrode attachment position area, and if there is a current outlet in the area, the electric potential can be described in that area by Poisson's equation. That is, within the region Ω Here, σ is the conductivity, I is the strength of the current flow, φ is the potential, and the Poisson's equation is difficult to solve by the boundary element method, so the following equation is defined.

If this equation (4) is used, Poisson's equation becomes the following Laplace equation, which can be solved by the boundary element method.

As the boundary condition of Expression (5), the boundaries S ₀ , S ₁ , and
Since the potential and the current density are the same on S ₂ , the following equation holds.

Here, n represents an outward normal line.

By solving the above equations (5) and (6) using the boundary element method, the electric potential in the heterogeneous medium can be obtained.

Since determine the direct current dipole from the measured potential of the next neural activity measured in the ninth step ST sixth step ST ₆ in ₉ (a V _m) is difficult obtain a current dipole in the following widen method.

Letting S be the squared error between the above-mentioned measured potential V _m and the potential V _c in the inhomogeneous medium obtained by equation (1), S is expressed by equation (7).

_{S = (V m -V c)} t · (V m -V c) ... (7) where t is the transpose matrix.

Position r of the current dipole that minimizes this squared error S
And the vector component p is obtained. The vector p that minimizes the equation (7) when the position r of the current dipole is arbitrarily fixed is obtained from the equation (1) as follows.

p = (A ^t A) ⁻¹ · A ^t · V _m (8) When the vector component p is selected in this way, the squared error S is a function of only the position r of the current dipole S ₀ = V _m ^t -(E _M -A (A ^-t A) ^-1 · A ^t ) V _m (9) Here, E _M is obtained as a unit matrix of order M.

Obtain the position r of the current dipole tenth step ST ₁₀ the square error S ₀ of the next to a minimum, the determination of whether the square error is less than the reference value is made by the computer (9).

If this squared error is greater than the reference, move the position of the current dipole by the simplex method as shown in the 11th step ST ₁₁ and return to the 8th step ST ₈ to perform this operation until the value of the squared error converges. repeat. The simplex method described above is one of the nonlinear optimization methods, and iterative calculation is performed to obtain an approximate solution. When this iterative calculation is performed, for example, a regular tetrahedron is set in the skull, an equivalent dipole is assumed at the four vertex positions of the regular tetrahedron, and the unit at the electrode position on the scalp where the equivalent dipole is generated is , The squared error from the measured potential is calculated for each equivalent dipole, and the vertex with the largest squared error among them is moved in the direction in which the squared error becomes smaller. At this time, the algorithm for deciding where to move is performed according to equation (10).

Where X is the vertex position of the tetrahedron X _h is the vertex position where the squared error is maximum X _m is the center of gravity at all vertices except X _h α, β, γ are constants X _r , X _e , X _c Value after calculation by formula While calculating these three formulas, each vertex of the tetrahedron is moved to the direction in which the square error becomes smaller, and the vertex is stopped when the stop condition is satisfied. The position at the time of this stop is the position obtained by calculation.

In this way, when the squared error value converges to a “YES” state and falls below the reference value, the current dipole at that position is used as an equivalent dipole by calculation, and a dipole power source is actually inserted into the living body. reads out the correction amounts corresponding to the position of the equivalent dipole obtained Te from the memory (12 step ST _12), corrects the equivalent dipole by the calculation by the correction amount (13 step ST
₁₃ ).

This correction is performed as follows.

First, the correction amount is obtained in advance. To do that, do the following:

An electrode is installed at a predetermined position inside the living body (for example, inside the skull), for example, at a test point TP ₁ between the skull (25) and the brain (24) to form a current dipole power source as shown in FIG. Then, the potential detecting means (10) described above detects the potential distribution generated by the current dipole power source and estimates the position of the equivalent dipole by the same procedure as described above based on the detected potential distribution. Then, the position difference between the position where the current dipole power source actually exists (test point TP ₁ ) and the equivalent dipole calculated and estimated from the potential distribution on the scalp can be obtained. In this way, a plurality of test points TP ₂ and TP ₃ in the skull are obtained.
…… If the actual current dipole power supply is inserted into TP ₈ and the deviation from the position of the equivalent dipole is calculated by the calculation at the test point, the error of the equivalent dipole position over almost the entire skull can be calculated.

The correction amount corresponding to the position of the equivalent dipole obtained in advance is stored in the external storage device (20) or the RAM (13), and if the equivalent dipole position is obtained by measuring the potential distribution on the scalp of the living body. , The displacement amount corresponding to this position, that is, the correction amount is read from the external storage device (20) or RAM (139 (twelfth step ST ₁₂ ), and the position is corrected (
13 steps ST ₁₃ ), the calculation error due to the skull is corrected,
The exact position of the equivalent dipole is required.

When the position of the equivalent dipole corrected in this way is obtained, the position is stored in a memory such as the RAM (13) (first
4 steps ST ₁₄ ).

Next, the vector component p shown in the eighth equation of the equivalent dipole at the position determined in the 14th step ST ₁₄ is calculated by the calculation section (9b) of the computer (9) as shown in the 15th step ST ₁₅ .

Although the vector component is not corrected by the actual measurement value in the flow chart of the present embodiment, it goes without saying that the vector component can be corrected similarly to the position correction.

Calculating a dipole degree representing the degree of or potential determined from electric dipole with respect to the position of the next 16 step ST ₁₆ the actually measured skull surface was degree approximation. The dipole degree d is calculated by the equation (11).

Here, M is the number of electrodes.

Then determined in advance the value of the dipole of d, it is determined whether or limit value in the 17 step ST _17. For example, the limit value of the dipole degree d is set to 95% or more, and the value of 95% or more is valid. If it is smaller than 95%, the process returns to the seventh step ST ₇ and the sampling value at the next time point is designated. If the dipole degree d is 95% or more, as shown in the 18th step ST ₁₈ , CR of the display means is displayed.
The position of the electrode dipole and the vector component are displayed on the T (22) in a three-dimensional figure of the head.

In the present embodiment, the control operations as described above are carried out.To summarize these control operations, a current dipole is assumed at a certain position in the skull, and the potential generated at each electrode position from the current dipole is assumed. Calculation is performed using the equation (1). Then, the squared error S ₀ between the potential V _m actually measured at each electrode and the potential V _c calculated from the current dipole is calculated. Next, the position of the current dipole is slightly shifted and the squared error is obtained in the same manner as above. In this way, while gradually changing the position of the current dipole, the position where the squared error is minimized is found, and the position is determined as the position of the current dipole. The correction amount corresponding to the position of the current dipole thus estimated by calculation is read out from the list of correction amounts actually obtained by actually inserting the current dipole power source into the skull in advance, and the position of the equivalent dipole is calculated. Correct and find the correct equivalent dipole position. In addition, it is possible to trace the nerve activity state by finding the dipole degree that shows the degree of approximation of the potential obtained from the current dipole with respect to the measured potential and displaying the current dipole on the CRT. is there.

In the above embodiment, the case where the position and the vector component of the equivalent dipole at a specific time are obtained has been described, but the equivalent dipoles at some time points are obtained and stored in the memory, and these are displayed on the same screen. By displaying them at the same time, it is also possible to track the change with time of the equivalent dipole. Further, in the present embodiment, the potential distribution on the scalp is detected, but the strength of the magnetic field on the scalp may be detected, and the present invention can be applied to a so-called SQUID magnetometer. Further, the present invention can be applied to the case where it is desired to remove the influence of electric field or magnetic field disturbance caused by ribs, cavities, etc. in other parts of the living body, such as the chest.

The electric field or magnetic field on the surface of the living body when actually using a dipole power source, which is used when correcting the position or component vector of the equivalent dipole, is not the subject who generated the electric field or magnetic field to be corrected. Usually, it belongs to another subject, and in a strict sense, it is considered that a correction error occurs due to a difference in skull shape, etc. However, the correction error is practically negligible.

Therefore, once data from an actual dipole power source is obtained from a particular subject, it is possible to make corrections based on this data for all other subjects, and this can be applied in an extremely wide range. It is a thing.

〔The invention's effect〕

In the equivalent dipole measuring apparatus of the present invention, the dipole power source is actually inserted into the living body, the electric field or magnetic field on the surface of the living body measured by the dipole power source is measured, and the equivalent value estimated by the calculation from the measured value is calculated. Since the correction amount for correcting the position or vector component of the dipole is obtained in advance and the estimated value of the equivalent dipole from the measured value of the electric field or magnetic field on the surface of the living body is corrected by the correction amount, It is possible to eliminate all errors in the equivalent dipole operation due to bones and cavities such as skulls and ribs that are major factors that disturb the inhomogeneity of the electric field or magnetic field of the The vector component can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of an equivalent dipole measuring device of the present invention, FIG. 2 is a flow chart showing the operation of the embodiment of FIG. 1, and FIG. 3 is a head explaining a heterogeneous medium. FIG. 4 is a schematic sectional view showing an example of a position where a dipole power source is installed. 1 ... Living body, 2 ... Cavity part, 5 ... Electrode group, 6 ... Amplifier, 7 ...
Multiplexer, 8 ... A / D, 10 ... Potential detection means, 12 ... RO
M, 13 ... RAM, 17 ... Pickup, 18 ... Digitizer, 19
... Electrode position signal input device, 20 ... External storage device, 21 ... Printer, 22 ... CRT, 24 ... Brain, 25 ... Skull.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Saburo Honma 1-9-6 Kasuga, Chiba-shi, Chiba (72) Inheir Nakamura 1-9-9 Motohongo-cho, Hachioji-shi, Tokyo Inside Chuo Denshi Co., Ltd. (72) Keiichi Miyamoto 1-9-9 Motohongo-cho, Hachioji-shi, Tokyo Chuo Denshi Co., Ltd. (72) Nobuo Nakagawa 1-9-9 Motohongo-cho, Hachioji, Tokyo Chuo Denshi Co., Ltd. (72) Masafumi Kikuchi, Inventor Masafumi Kikuchi, 1-9-9 Motohongo-cho, Hachioji-shi, Tokyo, Chuo Denshi Co., Ltd. (56) Reference JP-A-60-227732 (JP, A) JP-A-2-147977 (JP, A) JP-A-2-249530 (JP, A) JP-A-1-256931 (JP, A)

Claims (1)

[Claims] 1. A position or vector component of an equivalent dipole obtained by calculation from a measured value of an electric field or a magnetic field generated on the surface of a living body by a dipole power source installed inside a living body is used as an actual dipole power source. A storage unit that stores a correction amount obtained by comparison with the position or vector component, a detection unit that detects an electric field or a magnetic field on the surface of the living body, and a magnitude of the electric field or the magnetic field on the surface of the living body detected by the detection unit. Therefore, the calculating means for calculating the position or the vector component of the assumed equivalent dipole inside the living body, and the correction amount corresponding to the position or the vector component of the equivalent dipole obtained by the calculating means are stored in the storage means. And a correction means for correcting the position or vector component of the equivalent dipole.