JP2752884B2 - Life activity current source estimation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for estimating the position, orientation, and size of a biological activity current source, and more particularly to a method for estimating a biological activity current source using a minimum norm method.

[0002]

2. Description of the Related Art When a living body is stimulated, the polarization formed across a cell membrane is broken and a living activity current flows. This biological activity current appears in the brain and heart, and is recorded as an electroencephalogram and an electrocardiogram. The magnetic field generated by the biological activity current is recorded as a magnetoencephalogram and a magnetocardiogram.

Recently, as a device for measuring a minute magnetic field in a living body, SQUID (Superconducting Quantum Inter
A sensor using an ference device (superconducting quantum interferometer) has been developed. This sensor is placed outside the head, and the sensor can non-invasively measure a small magnetic field generated by a current dipole (hereinafter simply referred to as a current source), which is a biological activity current source, generated in the brain. Estimate the position, direction, and size of the current source related to the lesion from the measured magnetic field data,
The estimated current source is displayed on a tomographic image obtained by an X-ray CT apparatus or an MRI apparatus and used for specifying a physical position of an affected part or the like.

Conventionally, as one of current source estimation methods, there is a method using a minimum norm method (for example, WHKullman
n, KDJandt, K. Rehm, HASchlitt, WJDallas and
WESmith, Advances in Biomagnetism, pp.571-574, P
lenum Pless, New York, 1989).

Hereinafter, a conventional current source estimating method using the minimum norm method will be described with reference to FIG. As shown in FIG. 3, a multi-channel SQUID sensor 1 is provided near the subject M. Multichannel SQUID sensor 1 is accommodated by immersing a number of magnetic sensors (pickup coil) S ₁ to S _m in the refrigerant such as liquid nitrogen in a container called a dewar.

On the other hand, a large number of grid points (1) to (n) are set in, for example, the brain, which is a diagnosis target area of the subject M, and an unknown current source (current dipole) is assumed at each grid point. each current source represented by 3-dimensional Betokuru _VP j (j = _1~n). Then, the magnetic field intensities B _{i to} B _m detected by the magnetic sensors S _{1 to} S _m of the SQUID sensor 1 are represented by the following equation (1). Equation (1) is based on Biot-Savart's law and S
Each magnetic sensor _S 1 to S _m of QUID sensor 1 are each configured by a coil, therefore, the magnetic sensor _S 1 ~
S _m is derived from detecting the magnetic field component in the axial direction of each coil.

[0007]

(Equation 1)

In equation (1), VP _j = (P _jx , P _jy , P _jz ) α _ij = (α _ijx, α _ijy, α _ijz ). Note that α _ij is the magnetic sensors S ₁ to S ₁ when a current source having a unit size in the X, Y, and Z directions is placed on a grid point.
Is a known coefficient which represents the strength of the magnetic field detected by each position of the S _m.

[B] = (B ₁ , B ₂ ,..., B _m ) [P] = (P _1x , P _1y , P _1z , P _2x , P _2y , P _{2z ,.}
(P _nx , P _ny , P _nz ), the expression (1) can be rewritten into a linear relational expression like the expression (2). [B] = A [P] (2)

In the equation (2), A is a matrix having 3n × m elements represented by the following equation (3).

[0011]

(Equation 2)

Here, when the inverse matrix of A is represented by A ⁻ ,
[P] is represented by the following equation (4). (P) = A ^- (B) ......... (4)

The simultaneous equation represented by the equation (4) is obtained by comparing the number m of equations (the number of magnetic sensors S _{1 to} S _m , for example, several 10 to several hundred) with the number n of unknowns (for each grid point). Since there are many assumed current sources (for example, several hundred to several thousand), no solution can be obtained. Then, the vector [P]
Of minimizing the norm | [P] | Then, the above equation (4) is expressed as the following equation (5). [P] = A ⁺ [B] (5) where A ⁺ is a generalized inverse matrix expressed by the following equation (6). ^{A + = A t (AA t} ) -1 ......... (6) However, A ^t is the transpose matrix of A.

By solving the above equation (5), the current sources VP _j on each grid point
Are estimated, and the one having the largest value among them is assumed to be close to the true current source. This is the principle of the current source estimation method using the minimum norm method.

Further, there has been proposed a method of repeatedly obtaining the minimum norm solution while subdividing the grid points in order to improve the position resolution of the minimum norm method (for example, Y.Oka).
da, J.Huang and C.Xu, 8th International Conference
on Biomagnetism, Munster, August 1991). Hereinafter, FIG.
This method will be described briefly with reference to FIG.

FIG. 4 is an enlarged view of a part of the lattice point group N shown in FIG. 3, and the symbol J in the figure represents a true current source estimated using the above-described minimum norm method. Is a grid point where a current source close to Around this grid point J, a subdivided grid point group M (indicated by small black dots in FIG. 4) is additionally set. Then, a current source closer to the true current source is estimated by using the same method as described above, with the newly set grid point group M included in the initially set grid point group N.

[0017]

However, the prior art having such a structure has the following problems. According to the conventional method shown in FIG. 4, the subdivided grid point group M is newly set in addition to the initially set grid point group N, so that the number of grid points increases. for that reason,
There is a disadvantage that the number of elements of the vector [P] in the equation (5) increases, and the calculation accuracy of the minimum norm solution decreases.

The present invention has been made in view of such circumstances, and a biological activity current source capable of accurately estimating a current source while maintaining the calculation accuracy even when the minimum norm method is repeatedly used. It aims to provide an estimation method.

[0019]

The present invention has the following configuration to achieve the above object. That is, the present invention detects a magnetic field generated by a biological activity current with a plurality of magnetic sensors to estimate a physical quantity including any of the position, size, and direction of the biological activity current source. Set many grid points in
The relational expression between the unknown current source on each grid point and the magnetic field data measured by each magnetic sensor is added with a condition that the norm of a vector having the current source at each grid point as an element is minimized. In the biological activity current source estimating method using the method of obtaining the physical quantity of each current source (minimum norm method), a number of grid points are set in the subject,
A first step of obtaining a current source on each grid point by the minimum norm method, and, among the current sources of each grid point obtained in the first step, in the vicinity of a grid point where a current source having a large value exists. A second step of moving another group of grid points; and a third step of obtaining a current source on each of the moved grid points using the minimum norm method.

[0020]

The operation of the present invention is as follows. The current source estimated in the first step is not a true current source, but a current source close thereto. Therefore, in the second step, the group of grid points set in the first step is brought close to the grid point where the estimated current source exists, and in the third step, the current source on each grid point moved is set to the minimum norm. Estimate by the method. In other words, since the true current source is estimated by moving the positions of the grid points without changing the number of grid points, the true current source can be accurately calculated while maintaining the calculation accuracy of the minimum norm solution. Presumed.

[0021]

An embodiment of the present invention will be described below with reference to the drawings. Also in the present embodiment, similarly to the conventional method shown in FIG. 3, the multi-channel SQUID sensor 1 is arranged close to, for example, the brain of the subject M, and a small magnetic field generated in the brain by a biological activity current source is generated in the SQUID sensor 1. The measurement is performed non-invasively and magnetic field data is collected. Hereinafter, the true current source is estimated based on the collected magnetic field data by the procedure shown in the flowchart of FIG.

First, similarly to the conventional example shown in FIG. 3, a three-dimensional grid point group N is uniformly set in, for example, a brain which is a diagnosis target area (step S1).

Then, using the minimum norm method described above,
A current source (minimum norm solution) at each grid point is obtained (step S2).

Next, among the current sources at the respective grid points obtained in step S2, another group of grid points is moved to the vicinity of a grid point where a current source having a large value exists (step S3).
FIG. 2 shows this state. In the figure, the symbol N indicates the step S
This is the first grid point group set at 1. The grid points indicated by crosses in the figure are the grid points where the current source having a large value exists among the current sources obtained in step S2.
By moving the other lattice point group toward this lattice point, a lattice point group N ₁ having the same number of lattice points as the lattice point group N and a close interval is obtained.

In step S3, a method for bringing another group of lattice points close to a lattice point where a current source having a large value exists is not particularly limited. For example, the following method is exemplified. The magnitude of the current source at each grid point obtained in step S2 is considered as a mass, and it is assumed that an attractive force acts between each grid point due to gravity. Then, each lattice point approaches a lattice point having a large mass, and a lattice point group having a higher density is obtained as the lattice point is closer to a lattice point having a large mass. The moving distance of each grid point is set appropriately.

In step S4, the minimum grid point interval of the grid point group N ₁ obtained by moving in step S3 is:
It is determined whether the distance is equal to or less than a predetermined interval. This interval is appropriately determined according to the estimated position accuracy of the current source.

If [0027] The minimum distance between lattice points is not less than a predetermined value, the process returns to step S2, the grid point group N ₁ obtained by moving the original lattice points, the current source of each lattice point by the least norm method Ask. As described above, the number of grid points of the grid point group N ₁ is the same as the number of grid points of the first grid point group N. In other words, the linear form (5) used in the minimum norm method described above
(Represented below), [P '] = A ⁺ ' [B] (5) The number of elements of the vector [P '] is constant without increasing. This means that the calculation accuracy of the minimum norm solution is maintained. On the other hand, in the second minimum norm method, the existence of the current source is ignored in the hatched area shown in FIG. However, these regions are originally away from the position where the true current source is assumed to exist, and the probability that the true current source exists at the lattice points in these regions is extremely low. Even if the current source is estimated by excluding, the estimation accuracy does not decrease.

Then, similarly to the previous time, the current source at each grid point of the grid point group N ₁ is obtained by the minimum norm method (step S
2) Among them, it is estimated that a current source having a large value is a current source close to a true current source, and another lattice point group is brought closer to a lattice point where this current source exists, and a new lattice point group N ₂ is set (step S3).

The above processing is repeatedly executed, and step S
In 4, when it is determined that the minimum grid point interval is equal to or smaller than a predetermined value, it is estimated that each current source of the grid point group obtained in the final step S2 is a true current source.

[0030]

As is apparent from the above description, according to the present invention, another group of grid points is moved to the vicinity of a grid point where a current source having a large value estimated by the first minimum norm method exists. Then, in the following minimum norm method, the number of grid points is the same as the previous time, and the current source is estimated with only the grid point interval narrowed, so while maintaining the calculation accuracy of the minimum norm solution,
The current source can be accurately estimated.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating a procedure of a life activity current source estimating method according to an embodiment.

FIG. 2 is a diagram showing a state in which lattice points are moved.

FIG. 3 is a diagram provided for explanation of a conventional life activity current source estimating method.

FIG. 4 is a diagram provided for explanation of a conventional life activity current source estimating method.

[Explanation of symbols]

1 ... multichannel SQUID sensor S ₁ to S _m ... magnetic sensor M ... subject N ... first grid point group N _1, N ₂ ... grid point group after movement

Claims (1)

(57) [Claims] A magnetic field generated by a biological activity current is detected by a plurality of magnetic sensors to estimate a physical quantity including any of the position, size, and direction of the biological activity current source. A large number of grid points are set, and the relational expression between the unknown current source on each grid point and the magnetic field data measured by each of the magnetic sensors is minimized with the norm of the vector having the current source at each grid point as an element. In the biological activity current source estimation method using the method of obtaining the physical quantity of each current source (minimum norm method), a large number of grid points are set in the subject, A first method for obtaining a current source on a point by the minimum norm method
A second step of moving another group of lattice points near a lattice point where a current source having a large value exists among the current sources of the respective lattice points obtained in the first step; and Third, the current sources on each of the grid points are determined using the minimum norm method.
And a biological activity current source estimating method.