JP3661329B2 - Biomagnetic measurement device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a biomagnetic measurement apparatus that measures a minute magnetic field generated along with a biological activity current source in a subject and obtains the biological activity current source in the subject based on the measurement data.
[0002]
[Prior art]
With the development of superconducting device technology in recent years, a biomagnetic measuring device using a high-sensitivity magnetometer called SQUID (Superconducting Quantum Interference Device) is being put into practical use as one of medical diagnostic devices, It is expected to be useful for elucidation and diagnosis of cardiovascular diseases.
[0003]
In this biomagnetism measurement device, based on the measured magnetic field data, the position, orientation, size, etc. of the bioactive current source in the coordinate system with reference to the magnetometer can be estimated by, for example, the least square method or the least norm method. (Jukka Sarvas "Basic mathemtical and electromagnetic concepts of the biomagnetic inverse problem", Phys. Med. Biol., 1987, vol.32, No.1, 11-22, Printed by the UK).
[0004]
On the other hand, the obtained bioactive current source is written side by side on a medical image such as an MRI image obtained by a magnetic resonance tomography apparatus (MRI apparatus) or an X-ray tomographic image obtained by an X-ray CT apparatus. Since it is possible to specify the physical position of the affected part in the living body, it is possible to accurately grasp the positional relationship between the position information of the bioactive current source in the coordinate system based on the magnetometer and the medical image is important.
[0005]
For this reason, a magnetic field source called PROBE POSITION INDICATOR is placed at a clear position on the head surface, such as the nasal root or under both ears. Way
(1) S. Ahlfors, et al, "MAGNETOMETER POSITION INDICATOR FOR MULTI
CHANNEL MEG ", Advances in Biomagnetism, Edited by SJWilliamson et al, Plenum Press, New York 693-696, 1989
(2) Neuromag-122 Preliminary Technical Data, August 1991
(3) “Apparatus and method for performing biomagnetic measurement” (Japanese Patent Laid-Open No. 1-503603)
(4) “Position detection device for biomagnetic field measurement device” (Japanese Patent Publication No. 5-55126)
Has been proposed.
[0006]
In these methods, a DC current is first applied to the first oscillation coil among the three or more oscillation coils attached to the body surface of the subject, and the magnetic fields generated from the oscillation coils are mutually transmitted. The first one for the magnetometer group is detected by a plurality of magnetometers whose positional relationships are known, and the strength of the current applied to the oscillation coil, the strength of the magnetic field detected by each magnetometer, and the positional relationship between the magnetometers. The position of the oscillation coil is obtained. Then, this operation is sequentially applied to the second and subsequent oscillation coils, the positions of all the oscillation coils are obtained, and the position of the subject with respect to the magnetometer group is determined.
[0007]
Here, in the case of multiple types of examinations, there are many cases where there are a plurality of regions of interest in the brain to be measured.In such a case, the magnetometer is moved to an optimal position for each region of interest, or magnetic field measurement is performed, or Therefore, it is necessary to dispose the magnetometer so that the plurality of regions of interest fall within the sensitivity range.
[0008]
[Problems to be solved by the invention]
However, if the position of the magnetometer is adjusted for each region of interest, it is necessary to calculate the relative position with respect to the oscillation coil for each position of the magnetometer, and much labor and inspection time are required.
[0009]
If the magnetometer is positioned so that multiple regions of interest fall within the sensitivity range, the relative position between the oscillating coil and the magnetometer can be determined once, but in some magnetometers far away from the region of interest Thus, accurate magnetic field data cannot be obtained, and therefore, the relative position of the oscillation coil with respect to the magnetometer, that is, the subject cannot be obtained with high accuracy.
[0010]
The present invention was devised to solve the above problem, and even when there are a plurality of regions of interest, the relative positional relationship of the subject can be accurately identified without moving the magnetometer. An object of the present invention is to provide a biomagnetism measuring apparatus that can be used.
[0011]
[Means for Solving the Problems]
In order to achieve the above-described object, the present invention measures a micro magnetic field generated along with a bioactive current source in a subject, and obtains a bioactive current source in the subject based on the measurement data. A measuring device comprising: a plurality of oscillation coils attached to the subject; current supply means for supplying a predetermined current to the plurality of oscillation coils; a magnetometer for measuring a magnetic field generated from each oscillation coil; Based on the magnetic field data obtained for each oscillating coil, magnetic field analysis means for calculating the relative position of each oscillating coil with respect to the magnetometer, the position calculating error is obtained from the obtained relative position of each oscillating coil, and the error Coil selection means for selecting a coil that is within an allowable range when calculating the position of the coil, so as to perform alignment with the subject image such as MRI from the position information of the selected oscillation coil It is characterized in that form.
[0012]
The coil selection means calculates an area of a triangle obtained by an arbitrary combination from among the oscillation coils having an allowable position calculation error, and selects three oscillation coils having the maximum area. To do.
[0013]
Further, the coil selection means calculates a combination in which the volume of the triangular pyramid obtained by any combination is maximized among the oscillation coils having a position calculation error within an allowable range, and the four oscillation coils have the maximum volume. It is characterized by selecting.
[0014]
The coil selection means obtains the position calculation error as a goodness-of-fit value.
[0015]
The coil selection means obtains the position calculation error as a correlation coefficient γ between the “measurement magnetic field” and the “theoretical magnetic field from the analysis position”.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a schematic configuration diagram of a biomagnetic measurement apparatus according to an embodiment of the present invention. In the figure, a sensor unit 1 includes a plurality of high-sensitivity magnetometers S1 to Sm, each of which includes a pickup coil and a SQUID, housed together with a refrigerant in a dewar. Deployed close to M's head.
[0017]
The oscillating coils C1 to Cn are attached to portions that are characteristic in specifying the subject M, such as the nasal root and the lower ears, and are each made of an insulator such as a ceramic plate as shown in FIG. A coil C formed by printing a metal on the formed substrate 31 to form a coil portion 32, or a coil C ′ formed by winding a metal wire 34 around a bobbin 33 as shown in FIG.
The current supply unit 7 supplies a predetermined known current to each of the oscillation coils C1 to Cn. For example, the current supply unit 7 may be configured to simultaneously output alternating currents having different frequencies with individually designated intensities. A predetermined current may be separately supplied to each of the oscillation coils C1 to Cn at a time interval of. In the former case, the position measurement time of the oscillation coils C1 to Cn can be greatly reduced, but it is necessary to separately perform frequency analysis and calculate the magnetic field strength given to the magnetometers S1 to Sm by the individual oscillation coils Ci. In the embodiment described below, a predetermined current is separately supplied to each of the oscillation coils C1 to Cn at predetermined time intervals.
[0018]
The data collection unit 2 performs A / D conversion on the magnetic field data generated from the oscillation coils C1 to Cn measured by the magnetometers S1 to Sm and outputs the converted data to the oscillation coil position calculation unit 3 of the computer 6.
[0019]
The computer 6 analyzes the measured magnetic field data and controls the operation of the current supply unit, and mainly includes an oscillation coil position calculation unit 3, an oscillation coil selection unit 4, and a magnetic field analysis unit 5.
[0020]
The oscillating coil position calculating unit 3 uses a known least square method or the like to measure the magnetic flux meter S1 from the magnetic field data obtained from the magnetic flux meters S1 to Sm caused by the respective oscillating coils C1 to Cn. The relative positions of the oscillation coils C1 to Cn with respect to ~ Sm are calculated.
[0021]
The oscillating coil selector 4 selects an oscillating coil to be used for matching with an MRI image or the like from an error or the like regarding the relative position of each oscillating coil C1 to Cn with respect to the magnetometers S1 to Sm calculated by the oscillating coil position calculator 3. .
[0022]
The magnetic field analysis unit 5 performs an estimation calculation of the life activity current source by the known least square method and the like, and the position of the oscillation coil obtained by the oscillation coil position calculation unit 3 and selected by the oscillation coil selection unit 4. The positional relationship in which the information is associated with the feature points of the subject M such as the nasal root and the lower ears on the MRI image read out from the image storage unit 7 and the obtained positional information on the life activity current source is associated here. The image is superimposed on the MRI image and displayed on the monitor 9, and is stored in an external memory 8 such as a MOD (magneto-optical disk) or output to a printer (not shown) or the like as necessary.
[0023]
Next, the operation of this embodiment will be described based on the flowcharts of FIGS. 3 and 4 showing the operation of the computer 6.
[0024]
First, the computer 6 instructs the current supply unit 7 to sequentially output a current having a predetermined intensity at predetermined time intervals to the n oscillation coils C1 to Cn (S1).
[0025]
Next, the oscillating coil selection unit 4 detects magnetic field data generated by each of the oscillating coils C1 to Cn from the data collection unit 2 (S2), and is detected by the magnetometers S1 to Sm. Thus, the relative positions of the oscillation coils C1 to Cn with respect to the magnetometers S1 to Sm are calculated (S3).
[0026]
Then, the oscillation coil position calculation unit 3 sets a goodness-of-fit (hereinafter referred to as “GOF”) value indicating the correct relative position of each of the oscillation coils C1 to Cn obtained for each oscillation coil C1 to Cn. Obtain (S4).
[0027]
Here, GOF is an index indicating how reliable the current source position analyzed by the least square method using the magnetic field strength data of all the magnetic detection elements at a certain time is obtained by the following equation.
[0028]
goodness-of-fit value = (1−Σ (Bmi−Bci) ² / Σ (Bmi) ² ) × 100
Bci: Magnetic field intensity that is considered to be detected by the i-th magnetic detection element (each element of the magnetometer) when it is assumed that there is a predetermined oscillation coil at the analyzed position Bmi: actually measured by the i-th magnetic detection element When the GOF value is obtained for each of the oscillation coils C1 to Cn, the oscillation coils C1 to Cn used for alignment with the MRI image are selected using the obtained GOF value (S5).
[0029]
Here, the selection operation of each of the oscillation coils C1 to Cn will be described based on the flowchart shown in FIG.
[0030]
First, the GOF value is compared with a predetermined reference value for each of the oscillation coils C1 to Cn, and only the oscillation coils whose GOF value exceeds the reference value are selected (S51).
[0031]
Here, the reference value is a value indicating a state in which there is no problem in diagnosis for alignment with the MRI image, but the GOF value depends on the signal intensity and the amount of noise. It depends on the type (audience-induced brain, somatosensory brain, etc.), the amount of noise in the system of the device itself, and the amount of environmental noise at the time of measurement.
[0032]
However, the approximate value of the signal intensity is known in specific measurements from past experience, for example, 85.0% for somatosensory brain stimulation (middle wrist nerve stimulation), somatosensory brain stimulation (post-ankle) It is 80.0% for cervical nerve stimulation), 90.0% for auditory-induced brain stimulation, and 90.0% for visual-induced brain stimulation (semi-visual, pattern reversal stimulation).
[0033]
As a result, the magnetometer data that is located away from the region of interest of the subject M and causes an error in the position calculation is eliminated, so that the alignment with the MRI image can be performed more accurately.
[0034]
When an oscillating coil having a GOF value equal to or greater than a reference value is selected, any three combinations of the selected oscillating coils are obtained, and the area of a triangle connecting the three oscillating coils is obtained (S52). For example, when the number of selected oscillation coils is N, the number of arbitrary three combinations is obtained by _N C ₃ , and when N = 5, ₅ C ₃ = 10.
[0035]
When the area of the triangle is obtained for each arbitrary combination, the respective areas are compared, and three oscillation coils that maximize the area of the triangle are selected (S53).
[0036]
As a result, the three oscillating coils that are considered to be the most distant from each other are selected. Therefore, by using the position information of the oscillating coils, alignment with the MRI image can be performed using the minimum position information. It becomes possible to carry out more accurately. In addition, since the oscillation coil is normally attached to the subject's face so as not to become unstable due to hair or the like, when there are few oscillation coils to select, the number of coils attached to the subject during MRI imaging is reduced, Discomfort given to the subject can be reduced.
[0037]
Note that the area S of the triangle can be obtained from the Heron formula by the following equation.
[0038]
S = √ (s (s−a) (s−b) (s−c))
a, b, c: length of each piece of triangle, s: 2 (a + b + c)
In FIG. 1, when selection of the oscillation coil to be used for alignment with the MRI image is completed, next, the biomagnetic data of the subject M, which is the original purpose, is measured, and the magnetic field analysis unit 5 Based on the obtained data, an estimation calculation of the life activity current source is performed (S6).
[0039]
Then, based on the position information of the finally selected three oscillation coils, the coordinate with the MRI image read out from the image storage unit 9 is aligned by performing orthogonal coordinate conversion or the like, and the living body obtained from the display unit 8 is obtained. The active current source is displayed superimposed on the MRI image (S7).
[0040]
In the embodiment described above, the GOF value is used as the position calculation error of each oscillation coil. For example, the correlation coefficient γ between the “measurement magnetic field” and the “theoretical magnetic field from the analysis position” represented by the following equation is used. May be.
[0041]
γ = Σi (Bthi × Bexi) / √ (Σi (Bexi ² ) × Σi (Bthi ² ))
Bexi: measurement magnetic field at the magnetometer Si Bthi: measurement magnetic field that should be generated at the magnetometer Si when generated from the calculated position In the above-described embodiment, an example in which three oscillation coils are finally selected is shown. However, the present invention is not limited to this. For example, when four oscillation coils are selected, a combination that maximizes the volume of the triangular pyramid constituted by them is selected, and an MRI image or the like is obtained using affine coordinate transformation. Or may be aligned with the MRI image or the like based on all the oscillation coils that satisfy the error.
[0042]
As a result, even when the same error occurs in the four oscillation coil positions, there is a merit that the error in spatial correspondence is smaller in the triangular pyramid formed by the four positions than in the case where the three positions are used.
[0043]
Further, in the above-described embodiment, the GOF value or correlation coefficient γ for each oscillation coil is compared with a fixed reference value. Arbitrary combinations of triangles may be obtained after selecting ten or ten pieces.
[0044]
【The invention's effect】
According to the present invention, since the position calculation error of the relative position of each oscillation coil is selected only within the allowable range when calculating the position of each oscillation coil, the magnetometer can be used even when there are a plurality of regions of interest. The relative positional relationship of the subject can be specified with high accuracy without moving the object.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of a biomagnetic measuring apparatus according to the present invention.
FIG. 2 is a diagram showing an embodiment of an oscillation coil according to the present invention.
FIG. 3 is a flowchart showing the operation of the present invention.
FIG. 4 is a flowchart showing the operation of the present invention.
[Explanation of symbols]
M subject
S1 ~ Sm magnetometer
C1 to Cn Oscillation coil 1 Sensor unit 2 Data collection unit 6 Computer 7 Current supply unit

Claims (2)

In the biomagnetic measurement apparatus for measuring a micro magnetic field generated along with the bioactive current source in the subject and obtaining the bioactive current source in the subject based on the measurement data,
A plurality of oscillation coils attached to the subject;
Current supply means for supplying a predetermined current to the plurality of oscillation coils;
A magnetometer that measures the magnetic field generated by each oscillation coil;
Magnetic field analysis means for calculating the relative position of each oscillation coil with respect to the magnetometer based on the magnetic field data obtained for each oscillation coil;
A coil selection means for obtaining a position calculation error from the obtained relative position of each oscillation coil and selecting an error that falls within an allowable range when calculating the position of the oscillation coil;
A biomagnetism measuring apparatus configured to perform alignment with a subject image such as MRI from position information of a selected oscillation coil. The coil selection means calculates an area of a triangle obtained by an arbitrary combination from among the oscillation coils having an allowable position calculation error, and selects three oscillation coils having the maximum area. The biomagnetic measuring device according to claim 1.

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