Electromagnetic sensor layout proposal method and layout proposal device, and current source position estimation method

Abstract

This method for suggesting the positioning of electromagnetic sensors involves suggesting the positioning of a plurality of electromagnetic sensors positioned on the surface of a living body around a target region inside a subject, using a forward model for estimating electromagnetic fields produced at the positions of the plurality of electromagnetic sensors by biological activity inside the subject, the method including: a step for setting calculation conditions including the position of the target region and the positions at which the plurality of electromagnetic sensors can be positioned; a step for calculating, on the basis of the calculation conditions that have been set, suggested positioning for all or some of the plurality of electromagnetic sensors, which are for estimating the position of a current source inside the subject; and a step for outputting the suggested positioning.

Description

[Technical Field]

[0001] The present disclosure relates to a method and device for proposing placement of electromagnetic sensors, and a method for estimating the position of a current source. [Background technology]

[0002] There is a known technology for detecting biological activity within a living body using multiple electromagnetic sensors placed on the surface of the subject's body. For example, in magnetoencephalography (MEG), multiple sensors placed on the subject's head can detect brain magnetic fields that arise in response to neural activity. To clarify brain function, it is desirable to accurately estimate the location of the brain's active areas, which act as current sources (hereinafter referred to as "current source estimation"). One way to improve the accuracy of current source estimation is to measure brain magnetic fields with high sensitivity. To measure brain magnetic fields with high sensitivity, the positional relationship between the sensor and the head is important. Specifically, it is important to place the sensor close to the brain's active areas and to place the sensor in an appropriate position depending on brain activity.

[0003] Conventional magnetoencephalography uses multiple superconducting quantum interference device (SQUID) sensors, each of which requires liquid helium cooling. Each SQUID sensor is placed in a liquid helium-filled dewar, and the placement of each SQUID is fixed.

[0004] Meanwhile, active development has been underway on compact magnetic sensors that operate at room temperature, such as optically pumped magnetometer (OPM) sensors, tunnel magnetoresistance (TMR) sensors, and nitrogen-vacancy center in diamond (NVC) sensors. These sensors do not require cooling with liquid helium and can be easily repositioned, making it easy to reposition the sensor according to the location of brain activity, which was previously difficult. For example, Japanese Patent Application Laid-Open Publication No. 2020-151023 discloses a method for identifying the location of the Brodmann's region of a subject's head from magnetic resonance imaging (MRI) images of the head, and then placing one OPM sensor on the surface of the head near the identified Brodmann's region. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-151023 Summary of the Invention [Problem to be solved by the invention]

[0006] Even when current source estimation is performed using a small electromagnetic sensor that operates at room temperature, such as an OPM sensor, it is desirable to place multiple sensors on the surface of the living body, rather than just one, as with conventional SQUIDs. If the number of sensors is limited for some reason, it is desirable to place the limited number of sensors in appropriate positions. However, JP 2020-151023 A does not mention the appropriate placement of multiple sensors.

[0007] Furthermore, even in cases where the placement of each sensor is fixed, such as in the case of SQUIDs, there has not been sufficient consideration given to which of the outputs of a plurality of sensors should be used.

[0008] The present disclosure has been made to solve the above problems, and one object of the present disclosure is to propose an arrangement of multiple sensors suitable for current source estimation. Another object of the present disclosure is to provide a method for optimally utilizing the outputs of multiple sensors. [Means for solving the problem]

[0009] The method for proposing the placement of electromagnetic sensors according to the present disclosure is a method for proposing the placement of multiple electromagnetic sensors to be placed on the biological surface surrounding a target area inside a subject, using a forward model for estimating the electromagnetic field generated at the positions of the multiple electromagnetic sensors due to biological activity inside the subject, and includes the steps of setting calculation conditions including the position of the target area and possible positions for placing the multiple electromagnetic sensors, calculating a proposed placement of all or part of the multiple electromagnetic sensors for estimating the position of a current source inside the subject based on the set calculation conditions, and outputting the proposed placement.

[0010] The current source position estimation method according to the present disclosure is a method for estimating the position of a current source inside a subject using the detection results of multiple electromagnetic sensors that are placed on the biological surface of the subject and each detects an electromagnetic field generated by biological activity taking place in a target area inside the subject, and includes the steps of setting calculation conditions including the position of the target area and the placement of the multiple electromagnetic sensors, calculating priorities of all or some of the multiple electromagnetic sensors based on the calculation conditions, and estimating the position of the current source using the detection results of the electromagnetic sensors selected based on the priorities.

[0011] The electromagnetic sensor placement proposing device according to the present disclosure is a device that proposes the placement of multiple electromagnetic sensors to be placed on the biological surface surrounding a target area inside a subject using a forward model for estimating the electromagnetic field generated at the positions of the multiple electromagnetic sensors due to biological activity inside the subject, and is equipped with an input unit to which calculation conditions including the position of the target area and possible positions for placing the multiple electromagnetic sensors are input, an arithmetic unit that calculates a proposed placement of all or part of the multiple electromagnetic sensors for estimating the position of a current source inside the subject based on the calculation conditions, and an output unit that outputs the proposed placement. [Effects of the Invention]

[0012] According to the placement proposal method and placement proposal device of the present disclosure, it is possible to propose a placement of a plurality of sensors suitable for current source estimation.

[0013] According to the position estimation method of the present disclosure, the position of a current source can be estimated with high accuracy using the detection result of a sensor with a high priority selected from among a plurality of electromagnetic sensors. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a diagram schematically illustrating the overall configuration of a data processing system used in a method for proposing an arrangement of electromagnetic sensors. [Figure 2] FIG. 1 is a diagram conceptually illustrating a method for proposing placement of OPM sensors. [Figure 3] 10 is a flowchart illustrating an example of a procedure for a placement proposal process. [Figure 4] FIG. 10 is a diagram showing an example of sensor arrangement and measurement results when magnetoencephalography is performed twice using 11 OPM sensors. [Figure 5] FIG. 5 is a diagram comparing the measurement accuracy of the first measurement (uniform arrangement) shown in FIG. 4 with the measurement accuracy of the second measurement (optimal arrangement). DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated.

[0016] [Embodiment 1] <System configuration> 1 is a diagram schematically illustrating the overall configuration of a data processing system 1 used in a method for proposing an electromagnetic sensor placement according to this embodiment. In this embodiment, a case will be described in which an OPM sensor 2, which is a small magnetic sensor that operates at room temperature, is used as an example of an electromagnetic sensor.

[0017] The data processing system 1 includes a plurality of OPM sensors 2, an input device 3, an output device 4, and a processing device 10. The data processing system 1 is configured to estimate the location of biological activity (current source) in the brain of a subject using the detection results from the plurality of OPM sensors 2.

[0018] Each of the multiple OPM sensors 2 is placed on the surface of the subject's head by a user (such as a medical professional) and detects magnetic fields generated at each placement position due to biological activity occurring in the subject's brain. The multiple OPM sensors 2 may be in direct contact with the surface of the subject's head, or may not be in direct contact with the surface of the subject's head. For example, a cover member that holds the sensors 2 may be attached to the subject's head. Note that FIG. 1 shows an example in which four OPM sensors 2 are placed on the surface of the subject's head.

[0019] Since each OPM sensor 2 operates at room temperature, it does not need to be placed in a Dewar like the SQUID described above. Therefore, the placement of multiple OPM sensors 2 can be easily changed according to brain activity.

[0020] The input device 3 is, for example, a pointing device such as a keyboard or a mouse, and accepts input information (such as calculation conditions described below) from the user. The information input to the input device 3 is sent to the processing device 10. The output device 4 is, for example, a liquid crystal display (LCD) panel, and is a display that displays information to the user.

[0021] The processing device 10 includes, as main hardware elements, a sensor interface 11, an input interface 12, an output interface 13, a storage device 14, and an arithmetic unit 15. The processing device 10 may be realized, for example, by a general-purpose computer, or may be realized by a computer (such as a server) dedicated to the data processing system 1.

[0022] Sensor interface 11 is an interface for connecting processing device 10 and multiple OPM sensors 2, and realizes input and output of signals between processing device 10 and multiple OPM sensors 2. Input interface 12 is an interface for connecting processing device 10 and input device 3, and realizes input and output of signals between processing device 10 and input device 3. Output interface 13 is an interface for connecting processing device 10 and output device 4, and realizes input and output of data between processing device 10 and output device 4.

[0023] The storage device 14 stores information (programs, etc.) used for processing by the arithmetic device 15. Note that input information (such as calculation conditions described below) input by the user to the input device 3 is stored in the storage device 14.

[0024] The calculation device 15 has a CPU (Central Processing Unit) and performs "current source estimation" to estimate the position of the current source (place of biological activity) in the subject's brain using information stored in the memory device 14 and the measurement results of multiple OPM sensors 2.

[0025] The arithmetic device 15 displays the position of the current source obtained by current source estimation as the place of biological activity on the output device 4. By looking at the content displayed on the output device 4, the user can grasp the position of the place of biological activity in the brain of the subject.

[0026] <Proposal of sensor placement suitable for current source estimation> In order to accurately estimate the position of current sources in the brain, it is desirable to measure the brain magneto- electrical signals with high sensitivity using multiple OPM sensors 2. In order to measure the brain magneto-electrical signals with high sensitivity, the positional relationship between the multiple OPM sensors 2 and the current sources in the brain is important. Specifically, it is important to arrange the multiple OPM sensors 2 in appropriate positions according to the positions of the current sources in the brain.

[0027] Arithmetic device 15 according to the present embodiment is configured to perform processing for proposing an arrangement of OPM sensors 2 suitable for current source estimation (hereinafter also referred to as "arrangement proposal processing"). Hereinafter, the arrangement of OPM sensors 2 proposed by the arrangement proposal processing will also be simply referred to as "proposed arrangement."

[0028] FIG. 2 is a diagram conceptually showing a method for proposing the placement of the OPM sensor 2 by the placement proposal process. In the placement proposal process, the steps of setting calculation conditions, calculating the proposed placement, and outputting the proposed placement are carried out in this order.

[0029] [Calculation condition settings] In the step of setting the calculation conditions, the calculation conditions used to calculate the proposed placement are set based on the input information input to the input device 3 and the information stored in the storage device 14. The calculation conditions include "possible sensor placement positions," "number of sensors s," "sensor SNR (Signal-to-Noise Ratio)," "brain model," "target region," "objective function f," etc.

[0030] "Possible sensor placement positions" is data indicating possible positions on the surface of the subject's head where OPM sensors 2 can be placed. Specifically, "possible sensor placement positions" includes the maximum number of OPM sensors 2 that can be placed (hereinafter also referred to as "maximum placement number m") and the coordinates of the possible placement positions. The coordinates of the possible sensor placement positions can be determined, for example, based on image data of the subject's head obtained by an MRI or a three-dimensional scanner.

[0031] 2 shows an example in which the coordinates of the sensor placement positions are discrete values, but it is also possible to treat the coordinates of the sensor placement positions as continuous values ​​by defining the head surface coordinates in a form similar to an equation for a sphere or a plane. Also, the coordinates of the sensor placement positions may be virtual positions where the OPM sensor 2 cannot actually be placed.

[0032] The "number of sensors s" is the number of OPM sensors 2 actually placed on the surface of the subject's head. The number of sensors s is a value equal to or less than the maximum number of sensors m. In the example shown in Figure 2, the number of sensors s is "4."

[0033] The "sensor SNR" is the SNR of the OPM sensor 2. The "sensor SNR" is used to determine the "constant λ" used in the placement calculation described later.

[0034] A "brain model" is a model of the subject's brain structure using triangular meshes, etc. The brain model may be generated based on the results of measuring the subject's brain by MRI, or may be generated by assuming the subject's brain to be a standard brain and substituting an existing standard brain model.

[0035] The "target region" is a location within the brain model where biological activity is expected to occur. In the calculation of the proposed placement described below, the vertices within the target region within the brain model are treated as the positions of the current source (current dipole). The number of vertices included in the target region may be one or more.

[0036] The "objective function f" is a function used in the calculation of the proposed placement, which will be described later. As will be described later, the "objective function f" is determined depending on the "target area."

[0037] In this embodiment, the "objective function f" is set to a function that maximizes the diagonal sum (sum of diagonal components) of the vertices in the target region. However, the objective function is not limited to this, and any of the functions listed in Table 1 of Olaf Hauk et al., "Towards an objective evaluation of EEGMEG source estimation methods - The linear approach (Olaf Hauk)" may also be used as the objective function.

[0038] [Calculate proposed placement] In the step of calculating the proposed layout, a sensor layout suitable for the set calculation conditions is derived as the proposed layout by theoretical calculation. In this embodiment, each proposed layout is assigned a "priority p" indicating the order of importance. The calculation method for the proposed layout will be described in detail later.

[0039] [Output of proposed placement] In the step of outputting the proposed layout, data of the proposed layout obtained from the theoretical calculation is output. For example, when the data of the proposed layout is output to the output device 4, the proposed layout is displayed on the output device 4.

[0040] In the example shown in FIG. 2, the proposed placements are displayed by priority. Specifically, the most important placement is displayed as "priority p=1", and the placement with priority 1 is displayed above it. The second most important placement is displayed as "priority p=2", and the placements with priorities 1 and 2 are displayed above it. The third most important placement is displayed as "priority p=3", and the placements with priorities 1 to 3 are displayed above it. The fourth most important placement is displayed as "priority p=4", and the placements with priorities 1 to 4 are displayed above it.

[0041] [Placement proposal processing procedure] FIG. 3 is a flowchart showing an example of the procedure of the placement proposal process described above.

[0042] First, the arithmetic unit 15 sets calculation conditions (step S10) based on the input information input to the input device 3 and the information stored in the storage device 14. As described above, the calculation conditions include the possible sensor placement positions (maximum placement number m, coordinates of each placement), the number of sensors s, the sensor SNR, the brain model, the target region, the objective function f, etc.

[0043] Next, the arithmetic unit 15 sets the priority p to an initial value of "1" (step S11). After that, the arithmetic unit 15 moves the process to step S30.

[0044] Next, the arithmetic device 15 arbitrarily selects one sensor position whose priority has not yet been determined from among the multiple sensor placement positions (step S30).

[0045] Next, arithmetic device 15 calculates forward model matrix G (step S31). Forward model matrix G is a matrix indicating a forward model solution that expresses the correspondence between the current dipole moment of a current source when the vertices of the brain model are given a current dipole moment to form a current source, and the magnetic field generated at the position of OPM sensor 2 (magnetic field measured by OPM sensor 2). In other words, forward model matrix G satisfies the following equation (1).

[0046]

number

[0047] In equation (1), "B" is the magnetic field vector generated at the position of the OPM sensor 2, "J" is the density vector of the true current of the current source in the brain, and "G" is the forward model matrix (a matrix that indicates the forward model solution). The forward model matrix G is a matrix that converts the current of the current source into a magnetic field generated at the position of the OPM sensor 2, and is also called a "lead field matrix" in the field of magnetoencephalography. This forward model matrix G corresponds to an example of a "forward model" in this disclosure.

[0048] The calculation device 15 calculates the forward model matrix G p is calculated using the following formula (2).

[0049]

number

[0050] Specifically, the calculation device 15 calculates the forward model matrix G p-1 The matrix obtained by adding the forward model matrix g corresponding to one sensor whose priority has not been determined in step S30 to the matrix p is called the forward model matrix G p It is calculated as follows.

[0051] Forward model matrix G p The matrix size of is (priority p) × (number of vertices v). Therefore, when the priority p reaches the number of sensors s, the forward model matrix G s is (number of sensors s) × (number of vertices v). The number of vertices v is the number of vertices in the brain model (the number of points where current dipoles simulating neural activity are placed). When determining the position of priority p=1, there are no sensor positions with a determined priority, and the forward model matrix G0 is an empty matrix. Therefore, the forward model matrix G1 of priority 1 is G1=g.

[0052] Next, the calculation device 15 calculates an estimated model matrix R using the forward model matrix G calculated in step S31 (step S32). The estimated model matrix R is a matrix corresponding to an estimated model for estimating the position of a current source in the brain from the detection results of the OPM sensor 2 placed on the head surface. The estimated model matrix R satisfies the following equation (3).

[0053]

number

[0054] In equation (3), "J" is the density vector of the true current of the current source in the brain, "J'" is the density vector of the estimated current of the current source in the brain, and "R" is the estimated model matrix (a matrix indicating the estimated model solution). The estimated model matrix R is a matrix that converts the true current (true value) of the current source into a magnetic field generated at the sensor position, and further converts the converted magnetic field back into the estimated current of the current source (estimated solution). The estimated model matrix R is also called a "resolution matrix" in the field of magnetoencephalography. This estimated model matrix R corresponds to an example of the "estimation model" of the present disclosure.

[0055] The calculation device 15 calculates the estimated model matrix R of the priority p. p is calculated using the following formula (4).

[0056]

number

[0057] In equation (4), "G p ' is the forward model matrix G p is the transposed matrix of , where "λ" is a constant determined according to the sensor SNR, and "I" is the identity matrix. p The matrix size is (number of vertices v) × (number of vertices v).

[0058] Next, the calculation unit 15 calculates the difference matrix ΔR p is calculated using the following equation (5) (step S33).

[0059]

number

[0060] In equation (5), "R p " is the estimated model matrix R of the priority p, and "R p-1 ” is the estimated model matrix R of priority p−1.

[0061] Next, the calculation device 15 calculates the difference matrix ΔR for all sensor positions whose priorities have not yet been determined. pIt is determined whether or not the following has been calculated (step S34).

[0062] For all sensor positions whose priorities have not yet been determined, the difference matrix ΔR p has not been calculated (NO in step S34), the calculation device 15 returns the process to step S30, and calculates the difference matrix ΔR p The processes of steps S30 to S34 are repeated until is calculated.

[0063] For all sensor positions whose priorities have not yet been determined, the difference matrix ΔR p is calculated (YES in step S34), the calculation device 15 calculates the difference matrix ΔR p The objective function f(ΔR p ) is identified, and the identified sensor position is set as the position of priority p (step S40). The objective function f is determined according to the position of the target region, as will be described later.

[0064] Next, the calculation device 15 determines whether the priority order p has reached the number of sensors s (step S41). If the priority order p has not reached the number of sensors s (NO in step S41), the calculation device 15 proceeds to step S20 to increment the priority order p by 1, and then repeats the processes of steps S30 to S34. The processes of steps S30 to S34 are repeated until the priority order p reaches the number of sensors s.

[0065] By repeating this process, the sensor placements with the priorities p=1 to s are determined sequentially in descending order of priority.

[0066] For example, assume that the maximum number of placements m is "10" and the number of sensors s is "3." In this case, first, the position of priority p=1 is determined, then the position of priority p=2 is determined, and finally the position of priority p=3 is determined.

[0067] At the stage of determining the position with priority level p=1, the priorities of all 10 possible placement positions have not yet been determined, so the forward model matrix G1, estimated model matrix R1, and difference matrix ΔR1 for determining the position with priority level p=1 are calculated separately for each of the 10 possible placement positions. Then, the position at which the objective function f based on the difference matrix ΔR1 is optimal is set as the position with priority level p=1.

[0068] Once the position with priority level p=1 is set, a process for determining the position with the next priority level p=2 is performed. At this stage, the forward model matrix G2, the estimated model matrix R2, and the difference matrix ΔR2 are calculated separately for each of the remaining nine possible placement positions excluding the position with priority level p=1. At this time, G2, R2, and ΔR2 are calculated using G1, R1, and ΔR1 used to determine the position with the previous priority level p=1. Therefore, G2, R2, and ΔR2 are values ​​that reflect the influence of placing a sensor at the position with priority level p=1. Then, the position at which the objective function f based on ΔR2 is optimized is set as the position with priority level p=2. Therefore, the position with priority level p=2 is set taking into account the position with the previous priority level p=1.

[0069] Once the position with priority p=2 is set, a process for determining the final position with priority p=3 is performed. At this stage, the forward model matrix G3, the estimated model matrix R3, and the difference matrix ΔR3 are calculated separately for each of the remaining eight possible placement positions excluding the positions with priority p=1,2. At this time, G3, R3, and ΔR3 are calculated using G2, R2, and ΔR2 for the immediately preceding priority p=2. G2, R2, and ΔR2 are then calculated using G1, R1, and ΔR1 for the even immediately preceding priority p=1. Therefore, G3, R3, and ΔR3 are values ​​that reflect the influence of the positions with priority p=1,2. Then, the position at which the objective function f based on ΔR3 is optimized is set as the position with priority p=3. Therefore, the position with priority p=3 is set taking into account the positions with priorities p=1,2 up to the immediately preceding priority p=1.

[0070] When the priority p reaches the number of sensors s (YES in step S41), the calculation device 15 outputs the sensor arrangements of the priorities p=1 to p=s as proposed arrangements (step S50). For example, the calculation device 15 causes the output device 4 to display each of the proposed arrangements of the priorities p=1 to p=s.

[0071] Through the above-described placement suggestion process, the processing device 10 can provide the user with a proposed placement of the s OPM sensors 2 (a sensor placement suitable for estimating the position of a current source in the subject's brain). The user can accurately estimate the position of the current source by placing the s OPM sensors 2 according to the proposed placement displayed on the output device 4. If the proposed placement includes a virtual position where an OPM sensor 2 cannot actually be placed, the OPM sensor 2 can be placed in a position that is closest to the virtual position and where the OPM sensor 2 can actually be placed.

[0072] [Calculation of forward model matrix G] The calculation method for the forward model matrix G will be explained in detail below. As described above, the forward model matrix G is a matrix that satisfies "B = GJ" shown in equation (1). Specifically, the forward model matrix G is a matrix that converts the current density vector "J" of a current source in the brain into the magnetic field vector "B" generated at the position of the OPM sensor 2.

[0073] The relationship between the current generated in the current source in the brain and the magnetic field generated at the position of the OPM sensor 2 can be expressed as the following equation (6) using Biot-Savart's law.

[0074]

number

[0075] In equation (6), "B" is the magnetic field vector generated at the position of the OPM sensor 2, "r" is the position (coordinate) of the OPM sensor 2, "J" is the current density vector of the current source in the brain, and "r'" is the position (coordinate) of the current source in the brain.

[0076] The elements of the forward model matrix G can be found by calculating equation (6) above. "J" is a current density vector, and if its direction is not specified, a huge number of equations would have to be calculated. However, in the field of current source estimation, it is considered reasonable to assume that current flows perpendicular to the brain model, so "J" can be uniquely determined. The position "r" of the OPM sensor 2 can be determined by the possible sensor placement positions set in the calculation conditions. The position "r'" of the current source can be determined by the vertex position of the brain model set in the calculation conditions. Therefore, by calculating the Biot-Savart equation for one current source (vertex) the maximum number of sensor placements m, the elements of the forward model matrix G for that one current source can be calculated.

[0077] Specifically, Biot-Savart's law can be written for one current source up to the maximum number m of sensors arranged, as shown in the following equation (7).

[0078]

number

[0079] Here, by setting the magnitude of the current of the current source to a unit current, it is possible to find the magnetic field B generated at the position of the OPM sensor 2 per unit current.

[0080] Furthermore, for ease of understanding, consider a case where the OPM sensor 2 has sensitivity in only one axial direction. In this case, "B = GJ" shown in the above equation (1) becomes the following equation (8) when the components of each matrix are visualized.

[0081]

number

[0082] The calculation device 15 can calculate the components of the forward model matrix G using the above-mentioned method.

[0083] [Relationship between target domain and objective function f] The "target region" set in the calculation conditions is used to set the "objective function f." The relationship between the target region and the objective function f will be explained in detail below. Note that, as described above, the objective function f is a function based on the difference matrix ΔR. However, since the difference matrix ΔR is obtained by taking the difference of the estimated model matrix R, for ease of understanding, the following explanation will be given assuming that the objective function f is based on the estimated model matrix R.

[0084] As described above, the estimation model matrix R is a matrix that satisfies "J' = RJ" shown in equation (3). Specifically, the estimation model matrix R is a matrix that converts the true current J into the estimated current J'.

[0085] If we visualize the components of each matrix in equation (3), we get equation (9) below.

[0086]

number

[0087] In equation (9), "j1~j n " is the true current at each vertex of the brain model, and "j1'~j n ' ' is the estimated current at each vertex of the brain model.

[0088] If the target region is a region including the four vertices of the brain model (vertices 1, 2, 3, and 4), the first to fourth rows and first to fourth columns of the estimated model matrix R are matrix elements related to the target region, as shown in equation (10) below.

[0089]

number

[0090] A closer look at the estimation model matrix R reveals the influence of the true current at vertex 1 on the estimated current at vertices 1 to v, and the influence of the true currents at vertices 1 to v on the estimated current at vertex 1. Considering these, one ideal form of the estimation model matrix R is a matrix that has values ​​only in the diagonal elements where true values ​​exist. In other words, the objective function f can be set to a function that maximizes the diagonal sum (the sum of the diagonal elements) of the vertices in the target region. In view of this, the calculation device 15 determines the objective function f according to the position of the target region (more specifically, the vertices in the target region).

[0091] When the target region to be optimized is taken broadly and vertices 1 to 4 are included within the target region, the function that maximizes the diagonal sum of elements r11 to r44 of the estimated model matrix R can be set as the objective function f, as shown in the following equation (11). In situations where the region where brain activity occurs can be predicted but the detailed location has not been narrowed down, it is conceivable to take a broad target region like this.

[0092]

number

[0093] On the other hand, if the target region to be optimized is narrowed to include only vertex 1, the objective function f can be set to a function that maximizes the diagonal element r11 of the estimated model matrix R. In situations where the region where brain activity occurs can be predicted with pinpoint accuracy, it is expected that the target region will be narrowed in this way.

[0094] Moreover, the objective function f may be set to a function that maximizes the ratio between the diagonal sum (sum of diagonal components) and the off-diagonal sum (sum of off-diagonal components) of the target region shown in the following equation (12): Alternatively, the objective function f may be set to a function that maximizes the ratio between the diagonal sum of the target region and other elements.

[0095]

number

[0096] As described above, arithmetic device 15 according to this embodiment calculates the forward model matrix G and the estimated model matrix R based on calculation conditions including the target region, possible sensor placement positions, etc. Then, arithmetic device 15 sets and outputs a proposed placement (priority order p) of OPM sensors 2 using, as an index, an optimum objective function f set based on estimated model matrix R, etc. By placing multiple OPM sensors 2 according to the proposed placement, the user can efficiently perform highly sensitive magnetoencephalography.

[0097] According to the layout proposal method of this embodiment, it is possible to propose a layout of multiple OPM sensors 2 suitable for current source estimation. This makes it possible to efficiently optimize sensor layout, which was difficult with conventional SQUIDs. Furthermore, it becomes possible to perform highly sensitive (highly accurate) magnetoencephalography. It is also possible to reduce the number of sensors required for current source estimation, thereby reducing the hardware cost of the magnetoencephalography system.

[0098] [Embodiment 2] In the second embodiment, in order to efficiently perform even more highly accurate magnetoencephalography using a limited number of OPM sensors 2, the magnetoencephalography is performed twice as follows.

[0099] FIG. 4 is a diagram showing an example of sensor arrangement and measurement results when eleven OPM sensors 2 are used to perform two magnetoencephalographic measurements.

[0100] In the first measurement, since it is unclear where in the brain the current source (true abnormal region) is located, the entire brain is treated as the region to be examined and 11 OPM sensors 2 are evenly arranged on the surface of the head. Then, the position of the current source is tentatively determined from the measurement results of the 11 evenly arranged OPM sensors 2. In the example shown in Figure 4, the first measurement results show more measurement results with stronger intensity in the left brain than in the right brain, so the left brain is tentatively determined as the tentative position of the current source. Note that the part of the left brain where intensity above a predetermined value is concentrated may also be determined as the tentative position of the current source.

[0101] In the second measurement, the provisional position (left brain) of the current source tentatively measured in the first measurement is set as the "target area," and the placement proposal process described above is performed, with the 11 OPM sensors 2 repositioned according to the proposed placement obtained in the placement proposal process. The current source position is then officially measured from the measurement results of the repositioned 11 OPM sensors 2. In the second measurement, by repositioning the OPM sensors 2 to the optimal position based on the first measurement result, the measurement result of the maximum intensity is observed in a position closer to the true value (true abnormal area). In other words, the accuracy of the current source estimation has improved in the second measurement.

[0102] Figure 5 compares the measurement accuracy of the first measurement (uniform placement) with that of the second measurement (optimal placement) shown in Figure 4. The estimation accuracy of the current source can be said to be higher when the position error between the true value and the estimated value (i.e., the shortest distance between the coordinate j1 of the maximum intensity in the true value and the coordinate j2 of the maximum intensity in the estimated current) is smaller, and when the spatial spread of the estimated current (the maximum length of the area where the estimated current is measured) is smaller.

[0103] As shown in Figure 5, in the first measurement (uniform placement), the position error between the true and estimated values ​​was 33.16 mm, and the spatial spread of the estimated current was 48.28 mm, whereas in the second measurement (optimal placement), the position error between the true and estimated values ​​was 5.18 mm, and the spatial spread of the estimated current was 34.38 mm, which is an improvement. In particular, the position error between the true and estimated values ​​was significantly improved (approximately 28 mm), and with optimal placement, the maximum intensity of the estimated current was only about 5 mm away from the true value (the anomalous region that is the true source of activity).

[0104] As described above, in the second embodiment, in the first measurement, multiple OPM sensors 2 are evenly arranged on the head surface to tentatively measure the positions of the current sources, the tentatively measured positions of the current sources are set in the target region to perform an arrangement proposal process, and the multiple OPM sensors 2 are rearranged according to the proposed arrangement obtained in the arrangement proposal process to perform the second current source position determination. This makes it possible to efficiently perform high-precision MEG measurement using a limited number of OPM sensors 2.

[0105] In the first provisional measurement, it is sufficient to measure the provisional position of the current source, and it is not necessarily limited to using the OPM sensor 2. For example, in the first provisional measurement, the provisional position of the current source may be measured using fMRI (functional Magnetic Resonance Imaging) or the like. Furthermore, a doctor's opinion may be involved in determining the provisional position of the current source.

[0106] [Variation 1] For example, in step S50 of Figure 3 described above, in addition to displaying the proposed layout of each priority on the output device 4, the position error (position error between the true current and the estimated current) between the target area and the result of current source estimation for each proposed layout may be displayed on the output device 4.

[0107] In this way, the user can determine the number s of sensors to be used for current source estimation after checking whether the positional error in the proposed placement of each priority level is within an acceptable range by looking at the output device 4. Therefore, it is possible to reduce the number s of sensors to be used for current source estimation as needed, thereby reducing the hardware cost of the MEG measurement system.

[0108] [Variation 2] The sensors to which the placement proposal method according to the present disclosure can be applied are not limited to the OPM sensor 2. The placement proposal method according to the present disclosure can also be applied to, for example, a fluxgate sensor, an MR (Magneto Resistance) sensor, an MI (Magneto Impedance) sensor, a coil-type sensor, or an NVC (Nitrogen-Vacancy Center in Diamond) sensor instead of or in addition to the OPM sensor 2.

[0109] [Variation 3] The field to which the placement proposal method according to the present disclosure can be applied is not limited to the field of magnetoencephalography, but can also be applied to fields measuring biomagnetic fields other than those of the brain (such as magnetic fields generated by biological activity in the heart, spinal cord, peripheral nerves, or muscles).

[0110] [Variation 4] The sensors to which the layout suggestion method according to the present disclosure can be applied are not limited to magnetic sensors. That is, the layout suggestion method according to the present disclosure can also be applied to potential sensors that detect potentials generated by electrical activity of a living body, such as electroencephalogram (EEG) sensors or electromyography (EMG) sensors.

[0111] [Variation 5] The placement proposal method according to the present disclosure can also be applied to SQUIDs whose positions are difficult to change. As described above, SQUIDs are placed in a dewar filled with liquid helium, making it difficult to easily change their positions. However, it is possible to pre-position a large number (e.g., tens or hundreds) of SQUIDs on the subject's head. The above-described placement proposal method can be applied to the process of proposing the positions of SQUIDs useful for current source estimation from among the large number of SQUIDs pre-positioned on the subject's head.

[0112] Specifically, multiple SQUIDs may be placed on the subject's head in advance, calculation conditions may be set including the position of the target area and the placement of the multiple SQUIDs, the priorities of all or some of the multiple SQUIDs may be calculated based on the calculation conditions, and the detection results of the electromagnetic sensors selected based on the priorities may be used to estimate the position of the current source in the subject's brain.

[0113] In this way, a SQUID with a high priority can be selected from among the many SQUIDs pre-positioned on the subject's head, and the current source can be estimated using the measurement results of the selected SQUID. Therefore, even when using a SQUID whose position is difficult to change, accurate current source estimation can be performed by pre-positioning a large number of electromagnetic sensors on the subject's head.

[0114] [Aspect] It will be understood by those skilled in the art that the above-described embodiments and their modifications are specific examples of the following aspects.

[0115] (Item 1) A method for proposing the placement of electromagnetic sensors according to one embodiment is a method for proposing the placement of multiple electromagnetic sensors to be placed on a biological surface surrounding a target area inside a subject, using a forward model for estimating electromagnetic fields generated at the positions of the multiple electromagnetic sensors due to biological activity inside the subject, and includes the steps of setting calculation conditions including the position of the target area and possible positions for placing the multiple electromagnetic sensors, calculating a proposed placement of all or part of the multiple electromagnetic sensors for estimating the position of a current source inside the subject based on the set calculation conditions, and outputting the proposed placement.

[0116] According to the placement proposal method described in paragraph 1, a proposed placement of sensors is calculated and output based on calculation conditions. This makes it possible to provide a user with a proposed placement of sensors suitable for current source estimation. By placing multiple electromagnetic sensors according to the proposed placement, the user can accurately estimate the position of the current source.

[0117] (Item 2) In the method for proposing placement of an electromagnetic sensor according to item 1, the step of calculating the proposed placement includes the step of calculating priorities of all or some of the possible placement positions.

[0118] According to the placement suggestion method described in Section 2, it is possible to provide the user with priorities for all or part of the possible placement positions.

[0119] (Item 3) The method for proposing an arrangement of an electromagnetic sensor according to item 1 or 2 further includes a step of tentatively measuring a position of a current source. The step of setting calculation conditions includes a step of setting the tentatively measured position of the current source as a position of the target region.

[0120] According to the placement proposal method described in Section 3, the positions of the current sources are tentatively measured before calculating the proposed placement, and the proposed placement is calculated based on the tentatively measured positions of the current sources. This makes it possible to calculate the proposed placement efficiently with high accuracy.

[0121] (Item 4) In the method for proposing an arrangement of electromagnetic sensors according to any one of Items 1 to 3, the step of setting the calculation conditions includes the step of setting parameters of a forward model as the calculation conditions.

[0122] According to the placement proposal method described in Section 4, a proposed placement can be calculated by setting the parameters of the forward model as the calculation conditions.

[0123] (Item 5) In the method for proposing an arrangement of electromagnetic sensors according to any one of Items 1 to 4, the step of calculating the proposed arrangement includes a step of calculating the proposed arrangement using an estimation model for estimating the position of a current source from the detection results of all or some of the multiple electromagnetic sensors. The step of calculating the proposed arrangement includes a step of calculating the proposed arrangement so as to reduce the difference between the estimation result of the estimation model obtained by setting parameters of the estimation model as calculation conditions and the position of the target area.

[0124] According to the placement proposal method described in Section 5, it is possible to calculate a proposed placement so that the difference between the estimation result of the estimation model (position of estimated current) and the position of the target region (position of true current) becomes small.

[0125] (Item 6) In the method for proposing an arrangement of an electromagnetic sensor according to item 5, the step of outputting the proposed arrangement includes the step of presenting to the user a difference between the estimation result of the estimation model and the position of the target area in the proposed arrangement.

[0126] According to the placement proposal method described in Section 6, the user can confirm the difference between the estimation result of the estimation model and the position of the target region as the accuracy of current source estimation in the proposed placement.

[0127] (Item 7) In the method for proposing placement of an electromagnetic sensor described in any one of Items 1 to 6, the calculation conditions include, in addition to the position of the target area and the possible placement positions, at least one of the number of multiple electromagnetic sensors and the SNR (Signal-Noise Ratio) of the multiple electromagnetic sensors.

[0128] According to the placement proposal method described in Section 7, the proposed placement can be calculated taking into account at least one of the number of electromagnetic sensors and the SNR.

[0129] (Item 8) In the method for proposing an arrangement of an electromagnetic sensor described in any one of Items 1 to 7, the electromagnetic sensor is an OPM (Optically Pumped Magnetometer) sensor, a fluxgate sensor, an MR (Magneto Resistance) sensor, an MI (Magneto Impedance) sensor, a coil-type sensor, or an NVC (Nitrogen-Vacancy Center in Diamond) sensor.

[0130] According to the placement suggestion method described in Section 8, a suggested placement of an OPM sensor, a fluxgate sensor, an MR sensor, an MI sensor, a coil-type sensor, or an NVC sensor can be provided to a user.

[0131] (Item 9) In the method for proposing an electromagnetic sensor placement described in any one of Items 1 to 8, the electromagnetic sensor is a magnetic sensor or an electric potential sensor that detects a magnetic field generated by electrical activity in the brain, heart, spinal cord, peripheral nerves, or muscles.

[0132] According to the placement suggestion method described in Section 9, it is possible to provide a user with suggested placements of magnetic sensors or electric potential sensors that detect magnetic fields generated by electrical activity in the brain, heart, spinal cord, peripheral nerves, or muscles.

[0133] (10) In the method for proposing an arrangement of an electromagnetic sensor according to any one of the first to 9, the step of outputting the proposed arrangement includes the step of presenting the proposed arrangement to a user.

[0134] According to the arrangement suggestion method described in paragraph 10, it is possible to present a proposed arrangement to the user. (Item 11) A method for estimating the position of a current source according to one embodiment is a method for estimating the position of a current source inside a subject using the detection results of multiple electromagnetic sensors that are placed on the biological surface of the subject and each detects an electromagnetic field generated by biological activity taking place in a target area inside the subject, and includes the steps of setting calculation conditions including the position of the target area and the placement of the multiple electromagnetic sensors, calculating priorities of all or some of the multiple electromagnetic sensors based on the calculation conditions, and estimating the position of the current source using the detection results of the electromagnetic sensors selected based on the priorities.

[0135] According to the placement proposal method described in Section 11, current source estimation is performed using the detection results of electromagnetic sensors with high priority among a large number of electromagnetic sensors. Therefore, even when using electromagnetic sensors (such as SQUIDs) whose positions are difficult to change, by placing a large number of electromagnetic sensors on the biological surface of the subject in advance, current source estimation can be performed with high accuracy using the detection results of sensors with high priority selected from a large number of electromagnetic sensors.

[0136] (Item 12) An electromagnetic sensor placement suggestion device according to one embodiment is a device that uses a forward model for estimating electromagnetic fields generated at the positions of the multiple electromagnetic sensors due to biological activity inside the subject to suggest the placement of multiple electromagnetic sensors to be placed on the biological surface surrounding a target area inside the subject, and is equipped with an input unit to which calculation conditions including the position of the target area and possible positions of the multiple electromagnetic sensors are input, an arithmetic unit that calculates a proposed placement of all or part of the multiple electromagnetic sensors for estimating the position of a current source inside the subject based on the calculation conditions, and an output unit that outputs the proposed placement.

[0137] According to the placement proposal method described in paragraph 12, it is possible to achieve the same effects as the placement proposal method described in paragraph 1.

[0138] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the description of the above embodiments, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0139] 1 data processing system, 2 OPM sensor, 3 input device, 4 output device, 10 processing unit, 11 sensor interface, 12 input interface, 13 output interface, 14 storage device, 15 arithmetic unit.

Claims

1. 1. A method for proposing an arrangement of a plurality of electromagnetic sensors to be placed on a biological surface around a region of interest within a subject, using a forward model for estimating electromagnetic fields generated at positions of the plurality of electromagnetic sensors due to biological activity within the subject, the method comprising: setting a calculation condition including a position of the target area and possible positions of the plurality of electromagnetic sensors; calculating a proposed arrangement of all or a part of the plurality of electromagnetic sensors for estimating a position of a current source inside the subject based on the set calculation conditions; and outputting the proposed placement.

2. The method for proposing placement of an electromagnetic sensor according to claim 1 , wherein the step of calculating the proposed placement includes the step of calculating priorities of all or some of the possible placement positions.

3. further comprising the step of temporarily measuring the location of the current source; 3. The method for proposing an arrangement of electromagnetic sensors according to claim 1, wherein the step of setting the calculation conditions includes a step of setting the temporarily measured position of the current source as the position of the target region.

4. The method for proposing an arrangement of electromagnetic sensors according to claim 1 , wherein the step of setting the calculation conditions includes a step of setting parameters of the forward model as the calculation conditions.

5. the step of calculating the proposed arrangement includes a step of calculating the proposed arrangement using an estimation model for estimating positions of the current sources from detection results of all or some of the plurality of electromagnetic sensors; 2. The method for proposing an arrangement of an electromagnetic sensor according to claim 1, wherein the step of calculating the proposed arrangement includes a step of calculating the proposed arrangement so that a difference between the estimation result of the estimation model obtained by setting parameters of the estimation model to the calculation conditions and the position of the target area is small.

6. The method for proposing an arrangement of electromagnetic sensors according to claim 5 , wherein the step of outputting the proposed arrangement includes a step of presenting to a user a difference between the estimation result of the estimation model and the position of the target area in the proposed arrangement.

7. The method for proposing an electromagnetic sensor placement according to claim 1 , wherein the calculation conditions include, in addition to the position of the target area and the possible placement positions, at least one of the number of the plurality of electromagnetic sensors and the SNR (Signal-Noise Ratio) of the plurality of electromagnetic sensors.

8. 2. The method for proposing an arrangement of an electromagnetic sensor according to claim 1, wherein the electromagnetic sensor is an OPM (Optically Pumped Magnetometer) sensor, a fluxgate sensor, an MR (Magneto Resistance) sensor, an MI (Magneto Impedance) sensor, a coil-type sensor, or an NVC (Nitrogen-Vacancy Center in Diamond) sensor.

9. The electromagnetic sensor 2. The method for proposing placement of electromagnetic sensors according to claim 1, wherein the sensors are magnetic sensors or electric potential sensors that detect magnetic fields generated by electrical activity in the brain, heart, spinal cord, peripheral nerves, or muscles.

10. The method for proposing an arrangement of an electromagnetic sensor according to claim 1 , wherein the step of outputting the proposed arrangement includes the step of presenting the proposed arrangement to a user.

11. A method for estimating a position of a current source inside a subject using detection results of a plurality of electromagnetic sensors arranged on a biological surface of the subject, each of which detects an electromagnetic field generated by biological activity occurring in a target region inside the subject, comprising: setting calculation conditions including the location of the target area and the arrangement of the plurality of electromagnetic sensors; calculating priorities of all or a portion of the plurality of electromagnetic sensors based on the calculation conditions; and estimating the position of the current source using detection results of the electromagnetic sensors selected based on the priority.

12. 1. An apparatus for proposing an arrangement of a plurality of electromagnetic sensors to be placed on a biological surface around a target region inside a subject, using a forward model for estimating an electromagnetic field generated at positions of the plurality of electromagnetic sensors due to biological activity inside the subject, the apparatus comprising: an input unit into which calculation conditions including the position of the target area and possible positions of the plurality of electromagnetic sensors are input; a calculation device that calculates a proposed arrangement of all or a part of the plurality of electromagnetic sensors for estimating a position of a current source inside the subject based on the calculation conditions; and an output unit that outputs the proposed placement.

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