JP2804961B2 - Equivalent current dipole tracking device in the head - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an equivalent head current dipole tracking apparatus suitable for use in brain wave measurement and the like.

[Summary of the Invention]

The present invention relates to a head equivalent current dipole tracking device suitable for use in brain wave measurement and the like, avoiding the head cavity, a potential measuring means for measuring a plurality of potentials, and a medium in the head where the medium is uneven. Assuming a current dipole at an arbitrary position, calculating means for calculating a potential corresponding to each of a plurality of electrodes formed by the current dipole, an actually measured value of the potential measuring means, and a value calculated by the calculating means. A square error calculating means for calculating a square error, and an equivalent current dipole setting means for obtaining a position and a vector component of the current dipole that minimizes the square error value obtained from the square error calculating means and for setting the equivalent current dipole. The intra-cranial equivalent current dipole tracking device is characterized in that the potential based on neural activity can be measured without being affected by the intracranial cavity.

[Conventional technology]

2. Description of the Related Art Conventionally, an electroencephalograph, an electromyograph, an evoked potential adding device, and the like have been used as devices for measuring a potential appearing on a body surface due to a nerve activity of a living body. 2. Description of the Related Art Recently, an equivalent dipole method has been proposed in which a potential generated on a body surface due to a nerve activity of a living body is measured, and an active site in the living body is estimated. In this method, for example, when cells in each active site of the brain are stimulated, an electromotive force is generated to generate a potential distribution on the scalp. From such a potential distribution, each part is made to correspond with an electric dipole, and the position of the dipole and the vector component are calculated from the above-mentioned potential distribution to estimate the position of the active brain cell, thereby obtaining a brain. The activity status is tracked. In the equivalent dipole method for estimating such a dipole, since the potential distribution generated by the dipole is repeatedly calculated, conventionally, for example, the head is assumed to be a perfect sphere in order to calculate the potential distribution. The calculations were performed assuming that the skull was in a uniform infinite conductor. Further, there has been proposed a method of calculating a potential distribution assuming a homogeneous conductive sphere or a concentric or eccentric spherical shell having a homogeneous brain in the head.

In order to perform such an electric potential measurement, for example, an international electrode arrangement method (10-20) as shown in FIG.
Law) is known. FIGS. 6A to 6E show the electrode arrangement procedure viewed from the upper surface of the head. First, (a) the root of the nose (nasion), ie, the median portion on the suture line of the forehead and the occipital pole (inion), ie, the back Divide the median line along the skull surface connecting the intersection of the line of the demarcation term of the skull and the median sagittal plane into 10%, 20%, 20%, 20%, 20% and 10% as shown in Fig. 6A. And FPz, Fz, Cz, Pz, and Oz from the root of the nose.

(B) A crossing line along the surface of the head passing through the Cz point and connecting the left and right pre-auricular points (a recess in the root of the cheekbone in front of the tragus) is taken as the sixth line.
Divide into 10%, 20%, 20%, 20%, 20%, 10% as shown in Fig.
Let T ₃ , C ₃ , Cz, C ₄ , T ₄ from the left ear.

(C) a sixth the total length of the coronal line connecting the street FPz and Oz to T ₃ Figure C
10% as the 20%, 20%, 20%, 20%, divided into 10%, the procerus side from _{FPz, FP 1, F 7,} T 3 at the left hemisphere, T _5, O ₁ And

(D) through C _3, from the front 4 equal portions at the right hemisphere as the full length of the inner coronal line connecting the FP ₁ and O ₁ 6 Figure D
Let F ₃ , C ₃ , P ₃ .

(E) The same procedure as (c) and (d) is performed in the left hemisphere to obtain FP ₂ , F ₈ , T ₄ , T ₆ , O ₂ and F ₄ , C ₄ , P ₄ (No. 6) Fig.C, D
(See Reference) Electrodes were measured at such points by placing electrodes at each point.

FIG. 7 schematically shows the relationship between these electrodes and the position of the brain.

In FIG. 7, there is a brain (4) in the head (1) and a root of the nose (5)
The relation between each electrode and the position of the brain (4) is shown for the hemisphere, as well as the relation between the head and the occipital pole (6).

[Problems to be solved by the invention]

According to the above-described conventional equivalent dipole method,
For example, it is assumed that the head is a pseudo spherical body or a spherical shell, and that an infinitely uniform medium, that is, a conductor having the same conductivity as the brain exists uniformly outside the head, or Assuming that the portion is a spherical body or a spherical shell, and a uniform medium, that is, a brain, is present in the spherical body, a problem arises in calculating the potential distribution. The problem is that since the inside of the head is a uniform medium, the position of the designated equivalent dipole and the accuracy of the vector component are not sufficient. This cause will be described with reference to FIG. FIG. 8 considers a head (1) as a living body (1), and considers a cavity (2) such as an eye socket or an external auditory canal in the head (1). If the vector component direction of the true value (3a) of the equivalent dipole points to the cavity (2) as shown in FIG. 8 as the designated equivalent dipole, the calculated value of this equivalent dipole ( 3b) is affected by the cavity (2) and moves away from the cavity (2) from its true position, and its vector component becomes smaller than its true value. On the other hand, when the vector component direction of the true value (3a ') of the equivalent dipole is parallel to the cavity (2), this equivalent dipole (3b') is affected by the cavity (2) and becomes true. As the position approaches the cavity (2) from the position, its vector component becomes larger than the true value (3a '). However, since the conventional equivalent dipole method does not consider these points, there is a problem that the position of the equivalent dipole and the accuracy of the vector component are deteriorated.

Such a problem naturally occurs in the electroencephalogram measurement other than specifying an active site in the brain by approximating it with an electric dipole.

The present invention has been made to solve the above-mentioned problems, and its purpose is to track the flow of electrical information in a living body percutaneously or to perform electroencephalogram measurement. Another object of the present invention is to provide an in-head equivalent current dipole tracking device for highly accurately estimating the position and vector component of an equivalent dipole to be estimated or an electroencephalogram.

[Means for solving the problem]

The present invention provides a potential measuring means for measuring a plurality of potentials, avoiding the head cavity as shown in FIG.
Assuming a current dipole at an arbitrary position in the head where the medium is non-uniform, a calculation means for calculating potentials respectively corresponding to a plurality of electrodes formed by the current dipole, and an actual measurement value of the potential measurement means , A square error calculating means for calculating a square error between the calculated value of the calculating means, and a position and a vector component of a current dipole which minimizes a square error value obtained from the square error calculating means, and an equivalent current dipole. And a head-equivalent current dipole tracking device characterized by having an equivalent current dipole setting means.

[Action]

The in-head equivalent current dipole tracking device according to the present invention has a nose root (5).
The plane connecting the occipital pole (6) and the occipital pole (6) is rotated so as not to cross the cavity (2). For example, a plurality of electrodes (7) are arranged based on the International Electrode Placement Method. EEG measurements or dipole approximation of active sites can be performed.

〔Example〕

Hereinafter, a case where the equivalent current dipole in the living body is estimated by the in-head equivalent current dipole tracking device of the present invention will be described.

FIG. 2 shows a system diagram in the case of tracking the activity state of brain cells in the head (1). Hereinafter, FIG. 2 will be described in detail.

First, in order to accurately grasp the shape and size of the head (1), X
A line-CT is used to take about 15 CT tomographic images (16), and then the two-dimensional dimensions of the CT tomographic images (16) are input one by one using a pickup (17) of a digitizer (18). 34)
And read it into a computer (29) via the computer to obtain a three-dimensional head shape from the signal. Each electrode position corresponding to the three-dimensional head shape is input as x, y, and z three-dimensional coordinates from an electrode position signal input device (19) such as a keyboard.

Next, electrodes are arranged on the head (1) by a method different from the international electrode arrangement method and described in FIG. In FIG. 1, the mid-sagittal of the horizontal plane (12) that cuts off the head (1) as shown by the broken line connecting the nose (5) and the occipital pole (inion) (6) In FIG. 1, a horizontal plane (12) orthogonal to the plane (11) (the vertical plane passing from the root of the nose (5) to the occipital pole (6)) is clockwise centered on the bisector (14) of this horizontal plane. Rotate
Angle of the nose (5) upward and occipital pole (6) downward θ = 10-
After setting the rotating horizontal plane (13) so as not to cross the cavity (2) at about 20 °, each of the electrode groups (7) is mounted based on the rotating horizontal plane (13) by, for example, a division method using an international electrode arrangement method. The potential of nerve activity in the brain is measured in the same way
Measure in 3).

The measured potential from the electrode (7) is passed through an amplifier (26) and a multiplexer (27) to an analog-to-digital converter (A / D).
D) The measured potential supplied to (28) and digitized is supplied to the computer (29) via the input port (34). The computer (29) has a control unit (29a) and a calculation unit (29b), and the address bus (31a) and the data bus (31b) are ROM (32), RAM (33), input port (34), Connected to output port (15). ROM (32) above and R
AM (33) stores programs necessary for signal processing, and digitizer (18), electrode position signal input device (19),
Storage means for storing data from the potential measuring means (25). The computing section (29a) of the computer (29) has computing means, equivalent current dipole setting means and approximation degree computing means. The input port (34) is connected to an external storage device (20) storing a program for obtaining an equivalent dipole, and the like.
The output port (15) is connected to a display means (22) such as a CRT for displaying the calculation result of the computer (29) and a printer (21) for recording data and waveforms displayed on the display means (22). .

The operation of this example in the above configuration will be described with reference to the flowchart of FIG.

In Figure 3, it is set to an initial state as shown in vivo equivalent electric dipole tracking device of the present in the "on" the but power not shown example (23) in a first step ST _1. It reads the next second step ST ₂ the program for program and signal processing for various operations to be described later or the like from the external storage device (20) stored in the RAM (33) in a computer (29). Such program a computer (29) a second step ST ₂ if you put previously stored in a ROM (32) in a non-volatile memory in is not required.

In the next third step ST ₃ to enter the geometry of the head (1). As an example of cranial shape dimension measurement, about 15 CT tomographic images (16) are created for one person at a slice interval of 15 mm using X-ray CT. This CT tomogram (16) measures several parameters such as the skull circumference, width, length in the anterior-posterior direction for each individual, and applies a method to fit several standard models. This eliminates the need to take a CT tomogram to measure one skull, making the measurement easier. Of course, each skull may be measured. The two-dimensional images of the 15 CT tomographic images (16) sliced in this manner are picked up by the pickup (17) for each tomographic image (16) and digitized by the digitizer (1).
8) Using input port (34) to computer (29)
And store it in the RAM (33). In this case, when the slices are stacked three-dimensionally, the intersection of three straight lines perpendicular to the slice cross-section can be designated and placed in each slice so that “displacement” does not occur.

Based on the input head geometry Thus, the fourth step ST ₄ the computer (29) is converted into a three-dimensional data of the skull by the interpolation calculation.

In FIG. 2 to correspond to the next fifth step ST ₅ in the head (1) 21 around the electrodes placed on the (7) a three-dimensional head shape that the position obtained in the fourth step ST ₄ Input from the electrode position signal input device (19) such as a keyboard shown as three-dimensional coordinates of x, y, z axes, and the RAM (33) in the computer (29)
To be stored.

Sixth step ST At ₆ the head as shown in FIG. 2 (2) 2
About one electrode group (7) is placed on the basis of the above-mentioned standard, and the potential measurement based on the nerve activity in the brain is performed. The potential of the nerve activity measured in this way may be an evoked potential for various stimuli such as electrical stimulation, light stimulation, and sound stimulation, or a potential of the nerve activity in a state where no stimulation is applied. Amplifier (2
6) → Multiplexer (27) → Digital data is supplied from input port (34) to computer (29) via A / D (28) and stored in RAM (33).

To specify the computer (29) retrieves the potential of one sample clock from among the potential of the seventh step ST ₇ the neural activity.

The next current dipoles in the eighth step ST ₈ specified electrode (7) when it is assumed that at a predetermined position of the intracranial computing means the position of the transfer matrix computer (29) (29 b) is calculated, the current Calculate the potential at each electrode position where a dipole occurs. Generally, the potential generated on the scalp by the current dipole when the source of the nerve action potential is assumed to be a current dipole
Vc is expressed by equation (1).

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

Here, the potential generated from the current dipole assumed when the brain in the skull is considered to be an infinitely uniform medium is defined as φ 、, and this potential is applied to the skull of the head (1) as shown in FIG. Orbit,
Consider conversion to a potential of a heterogeneous medium in consideration of a cavity (2) such as a nasal cavity or an ear canal and a brain (4).

In Fig. 4, Ψ ₀ : potential in the tissues other than the brain and cavities Ψ ₁ : potential in the brain Ψ ₂ : potential in the cavities Ψ out: potential outside the skull Ω ₀ : area of the tissues other than the brain and cavities Ω ₁ : Brain region Ω ₂ : Cavity region Ωout: Extracranial region σ ₀ : Conductivity of tissues other than brain and cavity σ ₁ : Brain conductivity σ ₂ : Cavity conductivity σout: Extracranial conductivity s ₀ , s ₁ , s ₂ : Assuming that each region is a boundary, the current dipole is placed in the region Ω ₁ and the potential generated from this current dipole when this region is assumed to be an infinite uniform medium Is φ∞, φ∞ is given by equation (1) Here, σ ₁ is the conductivity of the brain, which is an infinitely uniform medium, and the rm is the electrode mounting position area, and if there is a current outlet in the area, the potential can be described by Poisson's equation in that area. That is, within the region Ω Here, σ is the conductivity 1 is the strength of the current source φ is the potential Since the Poisson's equation is difficult to solve by the boundary element method, the following equation is defined.

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

As the boundary conditions of the equation (5), the boundaries S ₀ , S ₁ ,
Since S potential and current density are equal on the _two following equation holds.

Here, n represents an outward normal.

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

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 Vm) obtains the current dipole in described next process so difficult.

Assuming that a square error between the above-described measured potential Vm and the potential Vc in the heterogeneous medium obtained by the equation (1) is S, S is expressed by the equation (7).

S = (Vm−Vc) ^t · (Vm−Vc) ‥‥ (7) where t is a transposed matrix.

The position r and vector component p of the current dipole that minimizes the square error S are obtained. When the position r of the current dipole is fixed arbitrarily, the vector p that minimizes the expression (7) is obtained from the expression (1) as follows.

p = (A ^t A) ⁻¹ · A ^t · V m ‥‥ (8) When the vector component p is selected in this manner, the square error S is expressed as a function of only the position r of the current dipole, and S ₀ = V m ^t · ^{_{(E M -A (a t a}} ) -1 a t) Vm ‥‥ (9) where E _M is obtained as M following matrix.

Obtain the position r of the current dipole tenth step ST ₁₀ the square error S ₀ of the next to minimize square error determination is made as to whether less than the reference value is performed in the computer (29).

If this square error is beyond the move as indicating the position of the current dipole to a 11 step ST ₁₁ by the simplex method, this operation until the value of the square error is returned to the eighth step ST ₈ converges repeat. The simplex method described above is one of the nonlinear optimization methods, and an approximate solution is obtained by performing an iterative calculation. When performing this iterative calculation, for example, a regular tetrahedron is set in the skull, and an equivalent dipole is assumed at four vertex positions of the regular tetrahedron,
The square error between the potential at the electrode position on the scalp where the equivalent dipole occurs and the measured potential is calculated for each equivalent dipole, and the vertex having the largest square error value is calculated.
Move in the direction in which the square error decreases. At this time, the algorithm of where to move is performed according to equation (10).

Where X is the vertex position of the tetrahedron Xh is the vertex position at which the square error is the maximum Xm is the center of gravity α, β, and γ at all vertices excluding Xh The constants Xr, Xe, and Xc are the values after calculation using the above equation Value While calculating these three equations, each vertex of the tetrahedron is moved to the direction where the square error is reduced, and stops when the stop condition is satisfied. The position at the time of the stop is determined as the position finally obtained.

If this value of the squared error as is converged "YES" state becomes equal to or less than a reference value, a current dipole at that location as the thirteenth step ST ₁₂ as an equivalent dipole located a RAM (33 )
And the like.

Next, operation in the eighth arithmetic unit of the computer (29) as indicating the vector components p to the 13 step ST ₁₃ shown in equation of the equivalent dipole of the position determined by the twelfth step ST ₁₂ (29).

Calculating a dipole degree d which represents the degree to which the potential obtained from the current dipole for the next 14 step ST ₁₄ the actually measured potential is the degree to which the approximation. This dipole degree d is (1
1) It is calculated by the formula

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 15 step ST _15. For example the limit value of the dipole degree d is 90% or more, of 90% or more is effective, is 90% or less to specify a sampling value of the next time returns to the seventh step ST _7. As dipole degree d is shown in 16 step ST _16, if 90% or more, the display unit CRT (22)
Above, the position of the current dipole and the Bertle component are displayed in a three-dimensional figure of the head (1).

The distance ΔR between the equivalent dipole calculated in this way and the true dipole position (determined experimentally) is plotted on the vertical axis, the distance Z from the cavity (2) is plotted on the horizontal axis, and the horizontal plane (12) The characteristic curve (35) in FIG. 5 shows the measurement results when the electrode group (7) is arranged with the angle between the rotating horizontal plane (13) and the horizontal plane (13) as θ = 20 °. . Similarly, a measurement result by the electrode arrangement method based on the international electrode arrangement method is shown by a characteristic curve (36) in FIG.

As can be seen from this characteristic curve, the true dipole position (position of ΔR = 0) and the calculated equivalent dipole position are compared with the conventional international electrode arrangement method in which the cavity (2) is not considered. It can be seen that the deviation is extremely small, the vector component of the equivalent dipole position is obtained with high accuracy, and that the device is an extremely effective head equivalent current dipole tracking device.

In the above embodiment, the case where the neural activity site is estimated as an equivalent dipole has been described.However, in the normal electroencephalogram measurement, the electrode arrangement is performed in consideration of the heterogeneity of the skull. Measurement errors are reduced, and more accurate brain wave diagnosis can be performed.

In the above example, the international electrode placement method was applied.
The method is also applicable to the Hellstron method, which is a simple method of the international electrode placement method, the University of Tokyo pediatrics method, the Okayama University pediatrics method, and various modifications are possible without departing from the gist of the present invention.

〔The invention's effect〕

Since the present invention is configured as described above, it has a feature that information in the brain can be accurately extracted without being affected by the cavity.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining the electrode arrangement method of the present invention,
FIG. 2 is a system diagram showing an example of an in vivo equivalent current dipole display device of the present invention, FIG. 3 is an example of a flowchart of FIG.
The figure is a schematic diagram of the head explaining the heterogeneous medium, and FIG. 5 is a characteristic curve for comparing the electrode arrangement method of the present invention and the conventional electrode arrangement method with respect to the equivalent dipole position obtained by calculation for the true dipole position. Fig. 6, Fig. 6 is an electrode layout diagram showing the procedure of the international electrode placement method, Fig. 7 is a schematic diagram showing the relationship between the brain and the electrodes in the international electrode placement method, and Fig. 8 is for explaining the effect of heterogeneous media. FIG. (1) head, (2) cavity, (5) root of nose, (6)
Is the occipital pole, (7) is an electrode, (12) is a horizontal plane, (13) is a rotating horizontal plane, (18) is a digitizer, (19) is an electrode position signal input device, (22) is display means, and (23) is An in vivo equivalent current dipole tracking device, (25) is a potential measuring means, and (29) is a computer.

Claims (1)

(57) [Claims] 1. A potential measuring means for measuring a plurality of potentials avoiding a head cavity, and a current dipole is assumed at an arbitrary position in the head where a medium is non-uniform. Calculating means for calculating potentials respectively corresponding to the plurality of electrodes to be formed; square error calculating means for calculating a square error between an actually measured value of the potential measuring means and a measured value of the calculating means; Equivalent current dipole in the head characterized by comprising equivalent current dipole setting means for obtaining the position and vector component of the current dipole that minimizes the squared error value obtained from the error calculation means and for setting the equivalent current dipole. Child tracking device.