JP2500715B2 - Living activity current source estimation device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for estimating the position, orientation and size of a bioactivity current source from measurement data of a magnetic field generated by a bioactivity current.

[0002]

2. Description of the Related Art When an external stimulus such as light or sound is given to a living body, a signal (active current) is generated in a sensory nerve.
The magnetic field formed by this biological activity current is SQUI
D (Superconducting Quantum
Interference Device (superconducting quantum interferometer) is used for measurement, and the position, size, and direction of the biological activity current source are estimated from the measurement data. The estimated biological activity current source (hereinafter simply referred to as current source) is X
It is displayed on a tomographic image in the body taken by a line CT apparatus or an MRI apparatus, and is used for specifying the physical position of the affected area or the like.

For example, the following is known as a conventional current source estimating device. (A) Assuming that the conductivity of the head, that is, the conductivity of the inside of the brain and the conductivity of the skull are all constant by modeling the head as a sphere, the orientation of the head in any direction ，
Assume a current source of arbitrary size. Magnetic field data obtained by numerical calculation with an assumed current source, and the SQUI
The squared error from the measurement data obtained by the D sensor is obtained, and the position, orientation, and size of the current source are changed so that the error is minimized. The position, orientation, and size of the current source thus obtained are estimated to be the current source of the magnetic field measured by the SQUID sensor.

(B) First, the shape of the head is represented by a combination of a plurality of minute elements (for example, triangles) and is precisely modeled, and the area represented by each element (for example, skull, brain, head) The conductivity is set for each of the skins). Assuming a current source of any direction and size at any point inside the brain, calculate the magnetic field by the assumed current source in consideration of conductivity (a calculation method such as the boundary element method is used) . The square error between the calculated magnetic field data and the measurement data obtained by the SQUID sensor is obtained, and the position, orientation, and size of the current source are changed so that the error is minimized. The position, orientation, and size of the current source thus obtained are estimated to be the current source of the magnetic field measured by the SQUID sensor.

[0005]

Generally, the bioactive current generated in the brain of a living body is distributed inside the brain having conductivity, and the distribution state depends on the conductivity of the head and the shape of the head. To do. The magnetic field data measured by the SQUID sensor is data in which the magnetic field from such a distributed current and the magnetic field from the current source are mixed. According to the magnetic field data obtained by numerical calculation, the magnetic field from the assumed current source,
The magnetic field from the distributed current obtained from the conductivity at the assumed position and the shape is calculated, and the sum of these magnetic field data is used as the magnetic field data used for estimation.

However, in the apparatus described in the conventional example (a), the head is modeled as a sphere, and the magnetic field data is calculated on the assumption that the conductivity of the head is constant. In reality, the conductivity inside the brain is not constant, and there is a marked difference in the conductivity between the brain tissue and the skull. The head shape is also not a sphere. From the difference between these conductivity and the head shape, that is, the difference in the magnetic field due to the distributed current,
The numerically calculated magnetic field data includes many errors, and the estimation error of the position, orientation, and size of the current source was also significant.

Therefore, when the apparatus described in the conventional example (a) is used, only the magnetic field in the direction perpendicular to the head surface is handled as the numerical calculation data. By doing so, it is known that the influence of the distributed current can be reduced, and the error amount of the numerical calculation data can be reduced. However, as described above, the actual measurement data also includes the magnetic field data from the distributed current, and when the numerical calculation data is only the magnetic field in the direction perpendicular to the head surface, The accuracy of the estimation was low because the magnetic field was not taken into consideration, and only the rough position, orientation, and size of the current source could be grasped.

On the other hand, in the apparatus described in the conventional example (b), the shape of the head is accurately modeled, and the electric conductivity is set in accordance with each part. As described above, the magnetic field from the current source and the magnetic field from the distributed current are taken into consideration, the error from the measurement data is small, and the estimation accuracy is high. However, there is a problem that the numerical calculation amount of the magnetic field data considering the shape and the conductivity of the whole region of the precisely modeled brain is huge, and the estimation time is remarkably extended.

The present invention has been made in view of the above circumstances, and it is possible to eliminate the error due to the above-mentioned numerical calculation and to estimate the living body current source capable of performing highly accurate estimation with a small amount of calculation. The purpose is to provide a device.

[0010]

The present invention has the following constitution in order to achieve the above object. That is, the biological activity current source estimation apparatus according to the present invention includes: (a) a multi-axis pickup coil that includes a large number of three-axis pickup coils that measure orthogonal three-direction components of a magnetic field generated by a biological activity current that appears in a region of interest in the living body. comprising a channel SQUID (superconducting quantum interference device) sensors, store the (b) the measured magnetic field data of said measured three directions for each channel (each triaxial pickup coil) of the multi-channel SQUID sensor, (c) Based on the relationship between the directions of the three coils forming the triaxial pickup coil and the direction perpendicular to the surface of the region of interest of the living body, the stored three-direction measurement magnetic field data is used to determine the direction perpendicular to the surface of the region of interest. seeking direction of the magnetic field data (vertical measuring magnetic field data) for each channel, such, measuring the vertical for each channel obtained above (d) Storing magnetic field data, (e) the assumed at any position within the region of interest, any resulting from virtual current source any direction size, a virtual magnetic field data in a direction perpendicular to the surface of the region of interest ( Vertical virtual magnetic field data) is obtained for each channel, and (f) a biological activity current source that minimizes the square error between the calculated vertical virtual magnetic field data and the stored vertical measured magnetic field data is set to the region of interest. Estimating in a simplified model, (g) Assuming the estimated biological activity current source in a model that accurately represents the region of interest, the three directions of the magnetic field generated by the biological activity current source. obtains a virtual magnetic field data corresponding to the component (3 directions virtual field data) for each channel, (h) said the three-way virtual field data and the the stored three directions measured magnetic field data for each channel JP multiplication error estimate the smallest bioelectric current sources in a precise model of the region of interest, that
To collect.

[0011]

The function of the present invention is as follows. 3 measured with multi-channel SQUID sensor
The measured magnetic field data in the direction is stored for each channel .
The calculated from the stored measured magnetic field data, magnetic field data in the direction perpendicular to the surface of the region of interest (vertical measurement field data)
Then, the vertically measured magnetic field data is stored . Hand, it assumed at any position related heart region, any resulting from virtual current source any direction size, a virtual magnetic field data in a direction perpendicular to the surface of the region of interest (vertical virtual field data) Ru required for each channel. Estimated in front Symbol the obtained vertical virtual field data and previous SL stored vertically measurement model squared error between the magnetic field data representing the smallest bioelectric current sources in a simplified manner the region of interest. In this way, the biological activity current source estimated by using only the magnetic field data in the direction perpendicular to the surface of the living body reduces the magnetic field from the biological activity current distributed in the living body and reduces the error in the numerical calculation. The estimation accuracy is not very high, and it is a provisionally roughly estimated biological activity current source. Assuming in the estimated model of bioelectric current sources have precisely represents the region of interest, a virtual magnetic field data (three-way virtual field data) corresponding to the three directional components of the magnetic field generated by the bioelectric current sources Calculate for each channel. And it estimates the previous SL bioelectric current sources squared error between three-way virtual field data and previous SL the stored three directions measured magnetic field data for each channel is minimized at the precise model of the region of interest. Estimation of this is done only in the vicinity of the position of bioelectric current sources that are <br/> roughly estimated earlier, between the vertebral scheduled do not intended to be performed on the entire area of the living body to be measured Is short.

[0012]

EXAMPLES Hereinafter, it described based on an embodiment of the invention with reference to the accompanying drawings. [1-1] Multi-channel SQU including a large number of triaxial pickup coils 1 as shown in FIG.
Using the ID sensor 2 (see (b) in the figure), data of the magnetic field formed by the bioactive current appearing in the brain of the subject m is measured. The triaxial pickup coil 1 includes coils 1a and 1a for respectively detecting the magnitudes of magnetic fields in three directions.
b and 1c are wound around the top and bottom of the cylindrical bobbin 3,
The structure shown in FIGS. 2 (a) and 2 (b) is not limited to that shown in FIG. 1 (a).

When the number of channels of the multi-channel SQUID sensor 2 is n, the magnetic field data in each direction measured for each channel are B1a, ..., Bna, B1b,
· · ·, Bnb, and B1c, · · ·, a Bnc, stores these measurement data Bra, Brb, the file as shown in FIG. 3 as Brc.

[1-2] The relationship between the orientations of the coils 1a, 1b, 1c of the triaxial pickup coil 1 and the direction perpendicular to the head surface of the subject m is determined. First, each coil 1a, 1b, 1 with respect to the three-dimensional coordinate system based on the container (referred to as Dewar) of the multi-channel SQUID sensor 2 is used.
Know the direction of c with reference to the design drawing. Next, a light projector is attached to the dewar to irradiate the subject m with a light beam, or small coils are attached to a plurality of positions of the subject m so that the magnetic field generated from the small coils is generated by the multi-channel SQ.
The direction perpendicular to the head surface of the subject m with respect to the Dewar coordinate system is grasped by a method such as detection by the UID sensor 2.

These pieces of information, that is, the directions of the coils 1a, 1b and 1c with respect to the Dewar three-dimensional coordinate system,
From the relationship with the direction perpendicular to the head surface of the subject m with respect to Dewar's three-dimensional coordinate system, the relationship between the orientation of each coil 1a, 1b, 1c and the direction perpendicular to the head surface of the subject m is shown. Ask. For example, as shown in FIG. 4, each coil 1a, 1b,
It is assumed that the directions a, b, and c of 1c are three-axis directions that are orthogonal to each other, and that the direction perpendicular to the head surface of the subject m has a relationship indicated by a symbol d.

[0016] [1-3] measurement data Bra stored in files, Brb, from Brc, the magnetic field data in the direction perpendicular to the head surface of the subject m d (hereinafter, simply referred to as the vertical direction d so) Ask for. That is, from the relationship shown in FIG.
Field data B1d vertical d, · · ·, calculates Bnd by the following Equation 1, and stores the full <br/> § i le as shown in Figure 5 as the magnetic field data Brd.

[0017]

[Equation 1]

[1-4-1] Current source M ₀ of arbitrary size Ij, arbitrary direction φk, at arbitrary position Pi in the brain of subject m
We assume the ijk. Multichannel SQUI the magnetic field data in the vertical direction d generated by the current source _M 0 ijk this
The number of channels n of the D sensor 2 is calculated numerically.
This calculation method is a known method and is a method also used in the related art. Calculate the calculated magnetic field data to B ₀ 1i
_jk, B 0 _{2ijk, ···,} and B 0 NIJK, calculates the square error delta ₀ ijk between the magnetic field data Brd vertical d by the number 2.

[0019]

[Equation 2]

[0020] Next, the current source _M 1 ijk when varying current source _M 0 ijk position Pi and size Ij of the direction φk
Magnetic field data B ₁ 1ijk in the vertical direction d
B _{1 2ijk,} ···, obtained by the B 1 NIJK numerically calculated, calculates the square error delta ₁ ijk between the magnetic field data Brd vertical d using equation 2 above. This Δ ₁ ijk,
The difference from the previously calculated Δ ₀ ijk is obtained, and if the difference becomes small, the position Pi and the magnitude Ij of the current source M ₀ ijk and the change in the direction φk are determined to be correct. A current source Mijk (obj) having the smallest squared error with respect to the magnetic field data Brd of is obtained. Also, Δ
_If the difference between ₁ ijk and Δ ₀ ijk is large, the position Pi and the magnitude Ij of the current source M ₀ ijk and the direction φk are determined to be incorrect, and they are changed to different directions. Then, finally, the current source Mijk having the smallest squared error from the magnetic field data Brd in the vertical direction d.
Get (obj).

The current source Mijk (obj) is a current source estimated using only magnetic field data in a direction perpendicular to the head surface of the subject m. That is, as described in “Problems to be Solved by the Invention”, the magnetic field due to the distributed current is reduced to reduce the error of numerical calculation, but the accuracy of the estimation is not very high. It is a current source. The estimation of the current source Mijk (obj) is the first stage estimation, and the estimated current source Mijk (obj) is the current source M1 (obj). In addition, the current source M1 (ob
j) has a position of P1 (obj) and a size of 11 (ob
j) and the direction is φ1 (obj).

[1-4-2] Another method for estimating the first stage will be introduced here. This is the above [1-4
-1] is a modified example. First, the magnetic field data in the vertical direction d when the position Pi, the magnitude Ij, and the direction φk of the assumed current source Mijk in the brain are changed are numerically calculated and stored in the file as simulation data Sijk.

This simulation data Sijk will be described in more detail. (A) The position Pi and the size Ij are the same, and k (= 1,
2, ..., W) as a parameter, the direction φk is
1, φ2, ..., φw, all different simulation data, (b) The position Pi and the direction φk are the same, and j (= 1,
2, ..., V) as parameters, the magnitude Ij is I
1, I2, ..., Iv, all different simulation data, (c) The size Ij and the direction φk are the same, and i (= 1, 1,
2, ..., u) as parameters, the position Pi is P1,
It consists of a group of all different simulation data such as P2, ..., Pu. This simulation data Sijk is a number (u × v ×) obtained by multiplying the number of parameters u of i, the number of parameters v of j, and the number of parameters w of k.
w) There are only one.

Next, the squared error ΔSijk between the magnetic field data Brd in the vertical direction d obtained from the measurement data and the simulation data Sijk one by one is calculated by the above-mentioned equation 2. This squared error ΔSijk is also (u × v ×
w) There are only one. Next, the minimum value ΔSijk (obj) of the (u × v × w) squared errors ΔSijk is obtained, and (u × v × w) pieces of simulation data Si are obtained.
The simulation data Sijk (obj) corresponding to the minimum value ΔSijk (obj) of jk is calculated. The current source Mijk (obj) corresponding to this simulation data Sijk (obj) becomes the current source M1 (obj) obtained by this first-stage estimation.

[2-1] The shape of the head of the subject m is expressed by a combination of a plurality of minute elements (for example, triangles) and accurately modeled, and the site of each element from an anatomical point of view. The conductivity is set for each. [2-2] The current source M1 (obj) obtained in the first-stage estimation is assumed for the precise model. That is, at the position P1 (obj) in the brain, the size I1 (ob
j), the current source in the direction φ1 (obj) is assumed.

[0026] [2-3-1] current source M1 (obj) of the magnetic field produced by the coil 1a of a three-axis type pickup coil 1, 1b, 1c orientation, i.e., a in FIG. 4, b, c The magnetic field generated in the direction of
The number of channels n of the UID sensor 2 is calculated numerically. This calculation method is also known and is a method that is also used in the related art. Let the calculated magnetic field data Ba ₁ , Ba ₂ , ..., Ban in the a direction be Bsa _1, and b
Direction magnetic field data Bb ₁ , Bb ₂ , ..., Bbn to B
sb ₁ and magnetic field data in the c direction Bc ₁ , Bc ₂ , ...
-, you the Bcn and Bsc _1. These magnetic field data Bs
The squared errors Δa ₁ , Δb ₁ , Δc ₁ between a ₁ , Bsb ₁ , Bsc ₁ and Bra, Brb, Brc that have already been measured and stored in the file are calculated using Equation 2 above.

[0027] The position of the next, current source M1 (obj) P1
(Obj), size I1 (obj), direction φ1 (ob
j), the magnetic field data Bsa _{2 in the} a direction, the magnetic field data Bsb _{2 in} the b direction, and the magnetic field data Bsc ₂ in the c direction generated by the current source M2 (obj) are calculated, and the measured data Bra, The squared errors Δa ₂ , Δb ₂ and Δc ₂ from Brb and Brc are calculated. This squared error Δ
The sum of a ₂ , Δb ₂ and Δc ₂ is compared with the sum of the squared errors Δa ₁ , Δb ₁ and Δc ₁ calculated above, and if the error is small, the current source M1 (obj) Position P1 (obj), size I1 (obj), direction φ1 (o
Assuming that the direction of change of bj) is correct, it is further changed to obtain a current source M (obj) that has the smallest squared error with the measured data Bra, Brb, Brc.

If the sum of the squared errors Δa ₂ , Δb ₂ and Δc _{2 and} the sum of the squared errors Δa ₁ , Δb ₁ and Δc ₁ show a large error, the current source M1 (o
bj) position P1 (obj), size I1 (obj),
Assuming that the direction of the change in the direction φ1 (obj) is wrong, the direction is changed to a different direction, and finally the current source M (obj) having the smallest square error with the measurement data Bra, Brb, Brc is selected. obtain.

This current source M (obj) accurately models the head of the subject m, sets the conductivity in accordance with the inside of the brain, the skull, and the like, and sets the magnetic field in three directions. This is the current source estimated using the data. That is, it is a current source estimated with high accuracy using magnetic field data (numerical calculation data close to measurement data) considering a magnetic field from a distributed current that depends on conductivity and head shape. Further, the use of a large number of magnetic field data from three directions instead of one direction also contributes to the high accuracy of estimation.

This second-stage estimation uses an optimization method with the position, size, and direction of the current source roughly estimated in the first stage as initial values. The time spent for processing (the processing time of the above [2-3-1]) can be short.

[2-3-2] Another method for estimating the second stage will be described below. This is a method corresponding to the above [1-4-2], and will be briefly described. First, a current source M1 (obj) obtained by the first-stage estimation is assumed for the precise model of the head, and the current source M1 (obj) is assumed.
Set any area including. Within this area, assuming many current sources with arbitrary positions, sizes, and directions,
Magnetic field data in three directions (a, b, c) by each current source is calculated.

A large number of calculated magnetic field data in each direction and measured magnetic field data Bra, Brb, Br in three directions.
The squared error with respect to c is obtained, and the current source M (obj) having the smallest error in the three directions is obtained. The time is short because the estimation is not performed for the entire area of the brain but only within the set area.

[0033]

As is apparent from the above description, according to the biological activity current source estimating apparatus of the present invention, first, the biological activity current source is temporarily and roughly determined from the magnetic field data in the direction perpendicular to the surface of the biological body. Estimate (first stage estimation), then
Only in the vicinity of the position of the tentative biological activity current source, the estimation is performed by using the magnetic field data in three directions with respect to the precise model in which the conductivity inside the living body is set (second step).
The estimation accuracy can be improved because an error due to numerical calculation and numerical calculation can be eliminated and a large amount of data used for estimation can be obtained. In addition, the calculation of the magnetic field by the numerical calculation with respect to the precise model is performed only near the position of the biological activity current source estimated in the first step , so the amount of calculation is reduced and the estimation time can be shortened.

[Brief description of drawings]

FIG. 1 shows a multi-channel S current source set in the living body.
It is a figure which shows a mode that it measures with a QUID sensor.

FIG. 2 is a diagram showing another example of a triaxial pickup coil.

FIG. 3 is a diagram showing a file in which measurement data is stored.

FIG. 4 is a diagram showing a relationship between a direction of a triaxial pickup coil and a direction perpendicular to a subject's head.

FIG. 5 is a diagram showing a file in which measured magnetic field data in a direction perpendicular to the subject's head is stored.

[Explanation of symbols]

 1 ... 3-axis pickup coil 2 ... Multi-channel SQUID sensor Bra, Brb, Brc ... Measurement data

Claims (1)

(57) [Claims] 1. A multi-channel SQUID (superconducting quantum interferometer) having a large number of three-axis pickup coils for measuring orthogonal three-direction components of a magnetic field generated by a biological activity current appearing in a region of interest in a living body. A sensor , and (b) stores the measured magnetic field data in the three directions measured for each channel (each triaxial pickup coil) of the multi-channel SQUID sensor,
(C) Based on the relationship between the directions of the three coils forming the triaxial pickup coil and the direction perpendicular to the surface of the region of interest of the living body, the stored three-direction measurement magnetic field data is used to determine the region of interest. calculated for each channel in the vertical direction of the magnetic field data (vertical measuring magnetic field data) on the surface,
(D) storing the obtained vertical measurement magnetic field data for each channel, and (e) generating a virtual current source in an arbitrary size and in an arbitrary direction, which is assumed at an arbitrary position in the region of interest. the virtual field data in the direction perpendicular to the surface of the region of interest (vertical virtual field data), determined for each channel
Because the Symbol and (f) the calculated vertical virtual field data
A biological activity current source that minimizes the squared error from the stored vertical measurement magnetic field data is estimated in a model that simply represents the region of interest, and (g) the estimated biological activity current source is the region of interest. , The virtual magnetic field data (three-direction virtual magnetic field data) corresponding to the three-direction components of the magnetic field generated by the biological activity current source is obtained for each channel. A biological activity characterized by estimating , within the precise model of the region of interest, a biological activity current source that minimizes the square error between the three-direction virtual magnetic field data for each channel and the stored three-direction measured magnetic field data. Current source estimation device.