JP2007105171A - Magnetic resonance apparatus and sensor head - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance apparatus excellent in a spatial resolution, as the magnetic resonance apparatus using a permanent magnet as a means for generating a static magnetic field. <P>SOLUTION: The magnetic resonance apparatus has the sensor head provided with a static magnetic field generation part 20 having a yoke and a pair of permanent magnets 21a and 21b fixed to it and forming a U-shape as a whole. A reception coil is provided on the center part. The gap of the pair of permanent magnets 21a and 21b is formed so that the interval of the magnets 21a and 21b along an X axis direction becomes maximum on the inner side of the static magnetic field generation part 20. At the maximum part, a magnetic flux density can be locally formed to be small. When the reception coil is placed near the part and observation is performed, since an equal magnetic flux density line curves to a Y axis side, the degree of separating from the reception coil is large, thus the reception sensitivity of a peripheral part is reduced and nuclear magnetic resonance signals are received only from the vicinity of the part provided with the reception coil. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a magnetic resonance apparatus that uses a permanent magnet as a static magnetic field generating means and is characterized by the shape of the permanent magnet.

  2. Description of the Related Art Conventionally, a magnetic resonance apparatus that uses a nuclear magnetic resonance phenomenon to image the distribution of water or fat in a measurement object, such as a human body, has been put into practical use and applied to medical fields. Such a magnetic resonance apparatus measures a density of a specific atom by receiving a nuclear magnetic resonance signal generated by irradiating a measurement object with a static magnetic field and a radio wave (high-frequency magnetic field) from a receiving coil. The frequency condition of the high-frequency magnetic field for causing the resonance is expressed as f = (γ / 2π) B where B is the magnetic flux density of the static magnetic field. The value of the constant γ is determined by each atom, and is 42.56 MHz / T (Tesla) for a hydrogen atom, for example. When the atoms in the measurement object are placed in an environment that satisfies the conditions of the high-frequency magnetic field, such a magnetic resonance phenomenon occurs and a nuclear magnetic resonance signal can be received. As can be seen from this relational expression, a nuclear magnetic resonance phenomenon does not occur in a portion having a specific static magnetic flux density unless a specific frequency is applied. If the above-described distribution of magnetic flux density is known in advance, a portion having a constant magnetic flux density can be identified on a one-to-one basis. Then, if a high-frequency magnetic field with an appropriate frequency is applied to the specific part where the magnetic flux density is known, the nuclear magnetic resonance signal can be received from the receiving coil, and the atomic density distribution of the part can be obtained. it can.

  Therefore, when the magnetic flux density is inclined with respect to the coordinates of the measurement target, the portion where the nuclear magnetic resonance signal can be generated can be specified by the difference in magnetic flux density. Then, by changing the frequency of the high-frequency magnetic field, it is possible to obtain the atomic density distribution corresponding to the portion corresponding to the value of the magnetic flux density, and to obtain a spatial atomic distribution as a whole.

As a means for generating a static magnetic field of such a magnetic resonance apparatus, one using a permanent magnet has been put into practical use (for example, see Patent Document 1). A large number of auxiliary magnets such as electromagnets and auxiliary coils are arranged between permanent magnets in order to change the magnetic flux density (see, for example, Patent Document 2).
JP 2005-27763 A JP 2005-095479 A

  However, such a magnetic resonance apparatus in which an auxiliary coil is arranged has a problem that it is difficult to reduce the size. That is, there is a problem that the mounting and the occupied space for arranging the auxiliary coil are increased in the sensor head that irradiates the measurement object with a static magnetic field and a radio wave (high frequency magnetic field). On the other hand, if this auxiliary coil is not installed, a portion with a uniform magnetic flux density is likely to be generated, and the nuclear magnetic resonance signal will be generated for the atoms of the entire portion with the uniform magnetic flux density, There was a problem that the part could not be specified in detail and the spatial resolution was lowered.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic resonance apparatus that uses a permanent magnet as a means for generating a static magnetic field and has excellent spatial resolution.

  The present invention is configured as follows.

(1) The present invention
A space is provided in a part of a magnetic circuit having a permanent magnet or a permanent magnet and a yoke, or a support member made of a non-magnetic material is inserted into the space, and a leakage static magnetic field formed in that portion is measured. A static magnetic field generator applied to
The direction perpendicular to the closed loop of the lines of magnetic force formed in the magnetic circuit is the X-axis direction,
The direction of the line of magnetic force in the gap or the support member is the Z axis,
A receiving coil provided at a position where the X-axis coordinate is located inside the static magnetic field generation unit and the Z-axis coordinate is located at a substantially central portion of the gap or the support member;
A sensor head having
The static magnetic field generator is configured such that, at the position of the X coordinate where the receiving coil is provided, the gap or the support member has an interval of X so that the interval in the Z-axis direction is maximized along the X-axis direction. This is a magnetic resonance apparatus that is non-uniformly formed along the axial direction.

  If comprised in this way, a pair of permanent magnet will be the said space | gap part or the part by which the nonmagnetic material is inserted in this space | gap along the X-axis direction (in addition, the nonmagnetic material is inserted in this space | gap) However, it is considered that the static magnetic field is almost unchanged. The same applies hereinafter.) In the part where the distance between the two magnets is narrow, the magnetic flux density is high in the vicinity of the part. In a portion where the gap between both magnets is wide, the magnetic flux density can be lowered in the vicinity of that portion. Therefore, the magnetic flux density can be changed without providing the auxiliary coil.

  However, even if the magnetic flux density is inclined in this way by the configuration of the present invention, in fact, the magnetic flux density cannot be varied at all points in the space, and a portion where the magnetic flux density is uniform is generated. The portions having a uniform magnetic flux density are arranged on a certain surface in a three-dimensional space (hereinafter, this surface is referred to as “equal magnetic flux density surface”). Then, all the points on the equal magnetic flux density surface satisfy the nuclear magnetic resonance condition, and the nuclear magnetic resonance reaction can occur from the portion. According to the present invention, the point is away from the receiving coil. By utilizing the fact that the receiving sensitivity is remarkably lowered, it is possible to measure only the measurement point close to the receiving coil.

  Hereinafter, the operation of measuring only the measurement point close to the receiving coil according to the present invention will be described in more detail. In the present invention, the gap between the static magnetic field generating portions) is formed to be a maximum in the periphery of the receiving coil, and in this portion, the magnetic flux density is the lowest in the periphery, The magnetic flux density of the static magnetic field increases along the X-axis direction along the X-axis direction (see FIG. 3 described later), which is a direction perpendicular to the U-shaped surface. (See FIG. 5A, which will be described later). On the other hand, the direction perpendicular to the bottom surface of the sensor head and away from the receiving coil (this direction is defined as the Y-axis direction. The Y-axis direction is perpendicular to the X-axis and the Z-axis. See FIG. 3 below.) The magnetic flux density decreases along the line (see FIG. 6B, which will be described later). Here, if the equal magnetic flux density surface is considered on the XY plane, it becomes one equal magnetic flux density line. However, considering the tendency of these magnetic flux densities, the line extends from the receiving coil along the X-axis direction. As the distance increases, the direction is the Y-axis direction, that is, the direction away from the receiving coil (see FIG. 7C, which will be described later). As described above, the farther away from the receiving coil, the lower the receiving sensitivity becomes. Therefore, when considered in the XY plane, it is possible to receive nuclear magnetic resonance signals only at points near the receiving coil along the X-axis direction. Therefore, the resolution in the X-axis direction can be improved.

  Further, the X-axis coordinate is fixed in the direction closest to the receiving coil and considered in the Y-axis-Z-axis plane. If the X-axis coordinate and the Y-axis coordinate are fixed, the magnetic flux density of the static magnetic field in the Z-axis direction is minimized at the substantially central portion of the gap between the magnets, and increases as it approaches the edge of each magnet (described later, FIG. 6 (C), the maximum point in this figure is the magnetic flux density at the end of each magnet.) Here, the magnetic flux density of the static magnetic field in the Z-axis direction becomes one uniform magnetic flux density line when the uniform magnetic flux density surface is considered on the Y-axis-Z-axis plane. As the line moves away from the receiving coil along the Z-axis direction, the line becomes the Y-axis direction, that is, the direction away from the receiving coil (see FIG. 7D, which will be described later). In the present invention, since the Z-axis coordinate of the receiving coil is provided at a position that becomes a gap between the magnets or a substantially central portion of the support member, if considered in the Y-axis-Z-axis plane, the Z-axis direction is The nuclear magnetic resonance signal can be received only at points near the receiving coil. Therefore, in the central portion where the receiving coil is provided, the resolution is also provided in the Z-axis direction.

As described above, since a nuclear magnetic resonance signal can be received only at a point near the receiving coil, high resolution can be obtained without providing an auxiliary coil. Furthermore, by varying the frequency of the high-frequency magnetic field, it is possible to obtain a nuclear magnetic resonance signal distribution in the Y-axis direction. According to the present invention, it is possible to obtain a three-dimensional space distribution by scanning in the X-axis-Z-axis plane direction.
Therefore, according to the present invention, it is possible to configure a magnetic resonance apparatus that is small and excellent in spatial resolution. Moreover, since it is not necessary to provide an auxiliary coil, it is not necessary to add a power circuit for that purpose.

  The leakage static magnetic field is a static magnetic field leaking from a gap or a support member inserted in the gap (the same applies hereinafter).

  The magnetic circuit of the present invention includes, for example, a U-shaped permanent magnet, a magnetic circuit formed in a U-shape by two permanent magnets and a yoke, and a magnetic circuit having the configuration (2) described later. .

  Further, as in the configuration of the present invention, it is only necessary that “the gap in the Z-axis direction of the gap is maximal around the reception coil”, and the Z-axis direction interval is a place other than the circumference of the reception coil. There may be a local maximum point.

(2) The present invention
The magnetic circuit of the static magnetic field generating unit is provided with a cylindrical permanent magnet having magnetic poles formed along a circumferential surface, and provided in close proximity to both sides of the permanent magnet, A pair of yokes having a recess for housing a part thereof,
The air gap or the support member in which the leakage static magnetic field is formed is provided between the ends of the pair of yokes.

  The present invention is one of the magnetic circuits having the configuration (1), wherein the distance between the pair of yokes is formed non-uniformly, so that the magnetic flux density is increased along the cylindrical axis direction of the permanent magnet. In any case, an inclination can be provided in the magnetic flux density without providing an auxiliary coil to make a change. When a magnetic circuit is configured as in the present invention, the state of the gradient of the static magnetic field is considered to be as shown in (1) above.

  Therefore, according to the present invention, the nuclear magnetic resonance signal can be received only at points near the receiving coil, as in the configuration (1), so that high resolution can be obtained without providing an auxiliary coil. . According to the present invention, it is possible to configure a magnetic resonance apparatus that is small in size and excellent in spatial resolution. Moreover, since it is not necessary to provide an auxiliary coil, it is not necessary to add a power circuit for that purpose.

  The “magnetic pole formed along the circumferential surface” of the “cylindrical magnet” may be arranged, for example, in the diametrical direction, or N poles at both ends in the diametrical direction, and inclined by 90 degrees. A configuration in which S poles are arranged at both ends in the diameter direction is conceivable.

(3) The present invention
A space is provided in a part of a magnetic circuit having a permanent magnet or a permanent magnet and a yoke, or a support member made of a non-magnetic material is inserted into the space, and a leakage static magnetic field formed in that portion is measured. A static magnetic field generator applied to
The direction perpendicular to the closed loop of the lines of magnetic force formed in the magnetic circuit is the X-axis direction,
The direction of the line of magnetic force in the gap or the support member is the Z axis,
A receiving coil provided at a position where the X-axis coordinate is the inside of the static magnetic field generation unit and the Z-axis coordinate is the gap or a substantially central portion of the support member;
With a sensor head with
The static magnetic field generator is configured such that, at the position of the X coordinate where the receiving coil is provided, the gap or the support member has an interval of X so that the interval in the Z-axis direction is maximized along the X-axis direction. This is a sensor head that is non-uniformly formed along the axial direction.

  If the magnetic resonance apparatus is configured by using the sensor head of the present invention configured as described above, the nuclear magnetic resonance signals can be received only at the points near the receiving coil as in the above-described configuration (1). Even without providing an auxiliary coil, high resolution can be obtained. According to the present invention, a magnetic resonance apparatus excellent in spatial resolution can be configured. Moreover, since it is not necessary to provide an auxiliary coil, it is not necessary to add a power circuit for that purpose.

  In addition, you may comprise the structure of (3) as a probe inserted in a body, or its part. In this case, as described above, the effect of the present invention that is small and excellent in spatial resolution can be exhibited in a body that is spatially limited.

  According to the present invention, it is possible to provide a gradient in the magnetic flux density without providing an auxiliary coil in order to provide a gradient in the magnetic flux density. However, even if the magnetic flux density is inclined, an equal magnetic flux density surface having a uniform magnetic flux density is generated, and a nuclear magnetic resonance signal can be generated from any part of the surface. However, according to the present invention, as the X-axis and Z-axis coordinates move away from the center portion of the receiving coil, the equal magnetic flux density surface is locally reflected in the Y-axis direction, that is, the direction away from the receiving coil. Can be formed. Then, since sensitivity can be weakened in a portion far from the equal magnetic flux density surface receiving coil, the resonance phenomenon is less likely to occur with respect to an extra portion of atoms, and the portion to be measured can be specified in detail. Therefore, a magnetic resonance apparatus excellent in spatial resolution can be configured.

The mechanical configuration of the magnetic resonance apparatus according to the first embodiment will be described with reference to FIG. FIG. 1 is an external configuration diagram of a magnetic resonance apparatus according to an embodiment of the present invention. As shown in FIG. 1, the magnetic resonance apparatus 1 includes a sensor head 2 that can move on a guide rail 25 and a measurement target support 3 on which the measurement target 100 is placed. The sensor head 2 includes a yoke 22 and a pair of permanent magnets 21a and 21b, and a static magnetic field generator 20 formed in a U shape as a whole, and an RF coil 23 (Radio Frequency) that generates a high-frequency magnetic field. It has. The sensor head 2 includes a sensor head support 24 that supports the sensor head 2 by a known means. The sensor head support 24 suspends and supports the static magnetic field generator 20 and the RF coil 23. ing. Further, as described above, the sensor head itself rides on the guide rail 25 and can move in the longitudinal direction (Z-axis direction) of the measurement target 100 installed on the measurement target support 3. A ball screw (not shown) and a ball screw rotation motor (not shown) are provided, and positioning in the longitudinal direction (Z-axis direction) is performed by driving the ball screw rotation motor.
The magnets 21 a and 21 b in FIG. 1 have both magnetic poles in the direction (Y-axis direction) orthogonal to the bottom surface 101 of the sensor head facing the measurement target 100, and generate a static magnetic field for the measurement target 100. The RF coil 23 generates a high-frequency magnetic field when a driving current having a specific frequency flows by a high-frequency circuit system 51 shown in FIG.

  Here, the coordinate system and positional relationship of the magnetic resonance apparatus of the first embodiment will be briefly described with reference to FIG. As shown in FIG. 1, the coordinate system is defined by an orthogonal coordinate system including an X axis, a Y axis, and a Z axis. The Z axis is the longitudinal direction of the measurement object 100 and is the direction in which the sensor head 2 moves along the guide rail 25 as described above. The Y axis is orthogonal to the Z axis and is the perspective direction with respect to the measurement object 100. The X-axis is perpendicular to the Y-axis and Z-axis and is a direction from the surface 103 on the back side of the apparatus near the back surface portion supporting the sensor head 2 toward the surface 102 on the near side of the apparatus. Hereinafter, the surfaces 102 and 103 both indicate the surface of the sensor head. A bottom surface 101 represents the bottom surface of the sensor head.

  Next, the function of the magnetic resonance apparatus of the first embodiment will be described with reference to FIG. As described above, the magnetic resonance apparatus of the first embodiment uses the nuclear magnetic resonance phenomenon to measure the distribution of molecules 100 such as water and fat in the human body (not limited to human bodies and organisms). It is a device that acquires data. Further, the nuclear magnetic resonance phenomenon is obtained by receiving a nuclear magnetic resonance signal generated by irradiating the measurement object 100 with a static magnetic field and a radio wave (high-frequency magnetic field) by a receiving coil and acquiring the density of a specific atom. It is. The frequency condition of the high-frequency magnetic field for causing this resonance is expressed as f = (γ / 2π) B where B is the magnetic flux density of the static magnetic field, and the value of the constant γ is determined by each atom. Yes. Therefore, the nuclear magnetic resonance phenomenon occurs only at a specific frequency for a specific magnetic flux density. Therefore, by providing a gradient of the magnetic flux density of the static magnetic field at the position of the measuring object 100 and measuring the relationship between the magnetic flux density and the part of the measuring object 100 in advance, the magnetic flux density and the part of the measuring object 100 can be related. it can. If it does so, when the frequency of a high frequency magnetic field is given to the RF coil 23, the site | part which a nuclear magnetic resonance phenomenon produces can be specified. Furthermore, by changing the frequency of the high-frequency magnetic field, the relationship between the part to be measured and the density of atoms at the part can be obtained via the magnetic flux density and the frequency of the high-frequency magnetic field.

  Next, the internal configuration of the magnetic resonance apparatus of the first embodiment will be described with reference to FIG. FIG. 2 is an internal configuration diagram of the magnetic resonance apparatus of the first embodiment.

  The high frequency circuit system 51 of FIG. 2 generates a drive current having a specific frequency and sends the current to the RF coil 23.

The high frequency circuit system 51 includes an oscillation unit 511, a transmission unit 512, and a high frequency amplifier 513.
The oscillating unit 511 can be composed of, for example, a highly stable crystal oscillator, and sends a signal generated thereby to the transmitting unit 512.
The transmission unit 512 includes a frequency synthesizer, a frequency converter, and a pulse generator. The frequency synthesizer frequency-converts the reference signal generated by the oscillation unit 511 to generate a signal having a predetermined frequency. The frequency converter frequency-converts the signal having the predetermined frequency to generate a high-frequency signal having a predetermined frequency (hereinafter referred to as “a”). On the other hand, the pulse generator generates a pulse signal (pulse width of 1 to 20 μs, one cycle from 0.5 seconds to 1 second) separately from the high-frequency signal a (hereinafter referred to as “b”). These signals a and b are combined by being input to the modulator, and a high frequency signal (RF pulse) in which high frequencies are intermittent is generated.
The high frequency amplifier 513 amplifies the RF pulse converted by the transmission unit 512 and sends a drive current to the magnetic circuit system 52.
The magnetic circuit system 52 includes a static magnetic field generation unit 20 that generates a static magnetic field and an RF coil 23 that generates a high-frequency magnetic field. When the above-described predetermined relationship is satisfied, a nuclear magnetic resonance signal can be received from a portion of the measurement object 100 placed in an environment that satisfies the relationship.
In the apparatus of the first embodiment, the RF coil 23 also serves as a receiving coil, and the RF coil 23 receives a nuclear magnetic resonance signal.

2 receives the nuclear magnetic resonance signal from the RF coil 23, amplifies it, and digitizes it.
The arithmetic processing system 54 includes an image reconstruction unit 541 and a data processing unit 542.
The image reconstruction unit 541 converts the reception data generated by the nuclear magnetic resonance signal reception unit 53 into digital image data.
The data processing unit 542 D / A converts the digital image data and sends a video output to the display. The display 55 displays the video output.
The operation unit 57 receives an operation operation of the magnetic resonance apparatus 1. The system control unit 56 receives an instruction from the operation unit 57 and controls the above-described units (reference numerals 51, 53, and 54).

Next, the static magnetic field generator 20 shown in FIG. 2 will be described in detail.
First, the coordinate system and positional relationship of the static magnetic field generation unit 20 described below will be described with reference to FIG. FIG. 3 shows a diagram representing the coordinate system of the permanent magnet of the magnetic resonance apparatus. The shape of the permanent magnet shown in FIG. 3 pertains to the magnets 210a and 210b of the conventional magnetic resonance apparatus. However, for the sake of easy explanation, the description will be given with this shape (the first described below). The coordinate axes of the apparatus of the embodiment are also the same). The XYZ coordinate system shown in FIG. 3 coincides with FIG. 1, and the Y-axis direction is a direction toward the lower side of the apparatus 1 of FIG. 1 (that is, a direction approaching the measurement object 100). The upper surfaces of the permanent magnets 210a and 210b correspond to the bottom surface 101 of the sensor head. The X axis is a direction from the back surface 103 of the apparatus 1 toward the front surface 102 of the apparatus (see FIG. 1). As shown in FIG. 3, the surface 102 on the front side of the apparatus and the surface 103 on the back side of the apparatus parallel to this form a U-shape as a whole of the static magnetic field generator 20, and follow this U-shape as will be described later A closed magnetic circuit is formed, and the X-axis is in a direction perpendicular to the U-shape. The Z-axis direction is a direction toward the right hand side of the apparatus as shown in FIG. 3, the Z axis is a direction in which the magnets 210 a and 210 b are arranged, and is a direction in which a static magnetic field is generated in the gap 18 of the static magnetic field generation unit 20. The origin of the coordinate axes is at the center of the bottom surface 101 of the sensor head. The end points of the permanent magnets 210a and 210b on the bottom surface 101 are (x _a , 0, −z _a ), (−x _a , 0, −z _a ), (−x _a ) in the XYZ coordinate system, as shown in FIG. _a , 0, z _a ), (x _a , 0, z _a ). The coordinates advanced _{y a} in the Y-axis direction from the origin of the coordinate axes is _{(0, y} a, 0). From _{(0, y a,} 0) position, ± _{x a,} ± _{y a} advanced _{coordinates, (- x a, y a} , 0), (x a, y a, 0), (0, y a , −z _a ), (0, y _a , z _a ).

  The magnets 210a and 210b have both magnetic poles formed in the Y-axis direction, and the magnetic poles of the surface 101 on the bottom side of the sensor head are opposite to each other in the magnet 210a and the magnet 210b. Is the S pole and the magnet 210b is the N pole. Thereby, the static magnetic field generation unit 20 forms a one-round magnetic circuit.

  Next, the shape of the permanent magnet of the static magnetic field generation unit 20 of the magnetic resonance apparatus of the first embodiment will be described with reference to FIG. FIG. 4 shows an example of a permanent magnet of the magnetic resonance apparatus of the first embodiment. The permanent magnets 21a and 21b shown in FIG. 4 correspond to the permanent magnets 210a and 210b shown in FIG. 3 and have the same positional relationship and coordinate system, and will be described below with reference to the coordinate system of FIG. . As shown in FIG. 4, the upper surfaces of the permanent magnets 21a and 21b correspond to the sensor head 101, and the right surface corresponds to the front surface 102 (see FIG. 1). The permanent magnets 21a and 21b have both magnetic poles formed in the Y-axis direction, which is a direction perpendicular to the bottom surface 101 of the sensor head, and the surfaces of the bottom surface 101 of the sensor head are respectively S and N poles. The static magnetic field generator 20 includes a yoke 22 and a pair of permanent magnets 21a and 21b fixed to the yoke 22, and has a U-shape as a whole, forming a magnetic circuit of one circumference. ing. Therefore, a static magnetic field is generated on the bottom surface 101 of each of the two magnets which are the U-shaped tips. Further, the static magnetic field leaks in the Z-axis direction in the gap 108 of the static magnetic field generation unit 20.

  Then, as shown in FIG. 4, the direction perpendicular to the front surface 102, which is the U-shaped surface, that is, the X-axis direction (similar to FIG. 3, from the rear surface 103 to the front side of the device. The magnets 21a and 21b are formed so as to have inclined portions in which the interval between the magnets is linearly changed (in the direction toward the surface 102). This inclined portion has a large gap between the magnets at the center in the X-axis direction. Along the X-axis direction, the distance between the magnets is not uniform but non-uniform as in the magnets 210a and 210b of the conventional magnetic resonance apparatus shown in FIG. Then, in a portion where the distance between both magnets is wide, the magnetic flux density is low in the vicinity of that portion, and in a portion where the distance between both magnets is narrow, the magnetic flux density can be increased in the vicinity of that portion. Therefore, the magnetic flux density can be changed without providing an auxiliary coil. In the apparatus of the first embodiment, it is necessary to provide an RF coil 23 (not shown) as a receiving coil at a portion where X = 0 and Z = 0 in the central portion where the distance between both magnets is wide. This will be described later with reference to FIG.

Note that the air gap 108 between the magnets 21a and 21b in FIG. 4 corresponds to the “air gap” of the present invention.
A closed magnetic circuit is formed as the entire static magnetic field generator 20. The X axis in FIGS. 3 and 4 is a direction perpendicular to the loop of the magnetic circuit in which the magnetic lines of force are closed, and this X axis corresponds to the “X axis” of the present invention.

In FIG. 4, the Z axis is the direction in which the magnets 21 a and 21 b are arranged, and a static magnetic field is formed in this direction in the gap portion of the static magnetic field generation unit 20. The direction of the Z axis corresponds to the “Z axis” of the present invention.
Further, the magnetic poles of the magnets 21a and 21b need to be opposite to each other on the bottom surface 101, but the magnet 21a may be an N pole and the magnet 21b may be an S pole at the position of the bottom surface 101.
In the apparatus according to the first embodiment, the pair of magnets 21a and 21b and the yoke 22 are used. However, the static magnetic field generating unit 20 may be formed using a U-shaped magnet or the like. It suffices if there is a gap between the circuit and a part thereof. Furthermore, the structure which inserts the nonmagnetic support member in the space | gap 108 may be sufficient. Even if a non-magnetic support member is inserted, the static magnetic field leaking from this portion is considered to be the same as in the apparatus of this embodiment.
In the device of the present embodiment, the above-described “inclined portion in which the interval between the magnets is linearly changed” is used, but it is not necessarily required to change linearly.

  Next, the state of the gradient of the magnetic flux density will be described in more detail with reference to FIG. FIG. 5 shows the magnetic flux density and magnetic pole arrangement in the X direction for the embodiment of the permanent magnet shown in FIG. FIG. 5A shows the relationship between the position [mm] in the X-axis direction and the magnetic flux density B [T]. A solid line B1 indicates the apparatus of this embodiment, and a broken line B0 indicates the conventional apparatus (details will be described later). FIG. 5B is a diagram illustrating the shape of the magnet according to the first embodiment with a triangular projection and a diagram illustrating the arrangement of magnetic poles. As shown in FIG. 5B, the magnet 21a has an S pole on the bottom surface 101 of the sensor head, and the magnet 21b has an N pole on the bottom surface 101 of the sensor head.

Here, the X-axis coordinate, the Y-axis coordinate, and the Z-axis coordinate of the measurement point of the magnetic flux density in FIG. In the X-axis coordinates, the positions of ± 50 mm, which are the positions of the straight lines shown by the dotted lines in FIG. 5A, correspond to both ends of the permanent magnets 21a and 21b, as shown in FIG. 5B. FIG. 5A shows the magnetic flux density B ₁ (B ₁ is the data of the magnetic resonance apparatus 1 of the first embodiment, the solid line along the X-axis direction when the positions of the Y-axis coordinates and the Z-axis coordinates are fixed. ), B ₀ (B ₀ is the data of the conventional magnetic resonance apparatus shown in FIG. 3, and is shown by a dotted line). Similar to that shown in FIG. 3, Y-axis coordinate of the measurement point, Y = y _a, i.e., from the bottom surface 101 of the sensor head downward (i.e., toward the measurement target 100) located at a position y _a mobile The Z-axis coordinate is Z = 0, that is, the central portion of the static magnetic field generation unit 20.

  As indicated by the solid line in the graph, in the static magnetic field generation unit 20 according to the magnetic resonance apparatus of the first embodiment, the magnetic flux density is inclined or changed along the X-axis direction inside the both end faces of the magnet. The magnetic flux density varies depending on the position of the X coordinate. As shown in FIG. 5B, this is because the distance between the permanent magnets 21a and 21b is inclined along the X-axis direction. That is, as described above, when the gap between the magnets 21a and 21b is widened, the magnetic flux density is weakened, and when narrowed, the magnetic flux density is strong.

As shown in FIG. 5A, since the magnetic flux density is different along the X axis, the frequency of the high frequency magnetic field for causing the nuclear magnetic resonance phenomenon at the position of X = 0 in FIG. Corresponds to a magnetic flux density of 0.3 [T], which is 12.8 MHz as indicated by the reference numeral 514. That is, when a pulse having a frequency of 12.8 MHz is applied to the RF coil 23 in the vicinity of X = 0 on the line Y = Y _a and Z = 0, the nuclear magnetic resonance signal is transmitted only from the point where X = 0. Occurs. If such a relationship is calculated and grasped in advance, any location in the X-axis direction when a receiving unit 53 (shown in FIG. 2) receives a nuclear magnetic resonance signal when a specific frequency is given. It can be specified whether the signal is a nuclear magnetic resonance signal generated from the. The X-axis coordinate of the position where the receiving coil is provided is that the gap between the magnets 21a and 21b is maximum with respect to the X-axis-Z plane, that is, the magnetic flux density is fixed along the X-axis (Y and Z coordinates are fixed). .) The position of X = 0 which is the minimum is set. The Z-axis coordinate of the position where the receiving coil is provided is the center of the air gap 108, that is, the position of Z = 0 where the magnetic flux density becomes minimum along the Z-axis (X and Y coordinates are fixed) ( The reason will be described later.)

  Next, the magnetic flux density distribution of the static magnetic field generation unit 20 of the apparatus according to the first embodiment will be described with reference to FIG. FIG. 6 is a diagram illustrating the magnetic flux density distribution of the static magnetic field generation unit 20 of the apparatus according to the first embodiment. FIG. 6A is the same as FIG. 5A, and the solid line indicates the device of the first embodiment, and the broken line indicates the device of the conventional form. 6B and 6C show the magnetic flux density in the Y-axis direction and the magnetic flux density in the Z-axis direction, and the graphs of the same shape are obtained for both the conventional apparatus and the first embodiment. Conceivable.

  6A and 6B are the same as the positions shown in FIGS. 3 and 5. The coordinate values other than the horizontal coordinate axis at the magnetic flux density measurement points in FIGS. 6A and 6B are the values shown in the upper right of each graph.

  Here, as shown in FIGS. 6A, 6 </ b> B, and 6 </ b> C, the magnetic flux density is different in the Y-axis and Z-axis directions. Although it is thought that the frequency-to-position relationship can be grasped on a one-to-one basis, in reality, a surface or curved surface that is a set of parts having the same magnetic flux density is generated in a three-dimensional space, and the nuclear magnetic resonance reaction occurs in all the parts Occurs and the resolution is lowered. In the apparatus according to the first embodiment, the RF coil 23 as a receiving coil has a higher sensitivity as the distance is shorter from the measurement target, and uses the fact that the sensitivity rapidly decreases as the distance increases. Hereinafter, this will be described with reference to FIG.

With reference to FIG. 7, a description will be given of the equal magnetic flux density lines of the magnetic flux density of the static magnetic field generation unit 20 of the apparatus of the first embodiment. Here, a line representing a portion having the same magnetic flux density is called an equal magnetic flux density line. FIG. 7 is a conceptual diagram for explaining the state of the equal magnetic flux density line of this magnetic flux density. FIG. 7A shows changes in magnetic flux density when the X-axis coordinates are changed when the Y-coordinates are fixed to values a to h, respectively. FIG. 7B shows changes in the magnetic flux density when the Y-axis coordinates are changed with X = 0, X ₁ , X ₂ , X ₃ , and X ₄ fixed. 7A and 7B correspond to FIGS. 6A and 6B. _X -4 to X ₄ in FIG. 7 (A), all X-coordinate magnet 21a as shown in FIG. 5 (A), the located in the portion of the inside of 21b (shown in dotted lines.) FIG. 7 (A ) Is an enlarged view of that part. When comparing FIGS. 6A and 6B and comparing them as shown in FIGS. 7A and 7B, the position and magnetic flux density of (X = 0, Y = d) are as follows. An equal magnetic flux density line that is equal to B _d is obtained on the XY plane as a curve of B _d in FIG. Similarly, when FIGS. 6B and 6C are compared, as shown in FIG. 7D, an equal magnetic flux density line having the same magnetic flux density, which is the same magnetic flux density B _d , is obtained. Here, the dotted lines shown in FIG. 7D are the positions of the end sides of the magnets 21a and 21b. Actually, since the equal magnetic flux density lines continue in the X-axis and Z-axis directions, a surface having the same magnetic flux density is generated in a three-dimensional space (hereinafter referred to simply as “equal magnetic flux density surface”). Called). From these considerations, it is considered that the Y-coordinate is locally small in the equal magnetic flux density surface connecting the aforementioned equal magnetic flux density lines when X = 0 and Z = 0.

  Here, the position where the RF coil 23 as the receiving coil is placed will be described with reference to FIG. In the apparatus of the first embodiment, the RF coil 23 as a receiving coil is placed at X = 0 and Z = 0 (the coordinate system is shown in FIGS. 3 to 5). As described above, the position where the RF coil 23 is provided is a position where the X-axis coordinate is inside the static magnetic field generation unit (any of X = −50 to X = 50 in FIG. 5A). And the Z-axis coordinate is at the center of the position of the gap portion 108 (shown in FIG. 4). Further, as shown in FIG. 5, the position where the receiving coil is provided is in the vicinity of a portion where the distance between the pair of permanent magnets 21a and 21b is the largest in the portion close to the receiving coil. To do.

  Returning to FIG. 7 again, the relationship between the equiflux degree surface, the positional relationship of the RF coil 23 as the receiving coil, and the sensitivity of the nuclear magnetic resonance signal will be described. When a high frequency magnetic field frequency of 12.8 MHz (shown in FIG. 5) is applied to the RF coil 23 as a transmission coil, all points on the curve shown in FIG. For all points on the curve shown in FIG. 7 (D), the condition for causing the nuclear magnetic resonance phenomenon is satisfied. However, as described below, there is a difference in sensitivity depending on the distance between the equal magnetic flux density surface and the receiving coil, and a nuclear magnetic resonance signal can be received only in the vicinity of X = 0 and Z = 0. This will be described below.

  As described above, in the RF coil 23 as a receiving coil, the X-axis direction is set to the position of X = 0 where the gap in the Z-axis direction is enlarged, and the Z-axis direction is the center of the gap 108 (see FIG. 4). Some Z = 0. As shown in FIGS. 7C and 7D, the Y-coordinates of the aforementioned equal magnetic flux density lines are locally small when X = 0 in the XY plane and Z = 0 in the YZ plane. Therefore, even if all the equal magnetic flux density surfaces become resonance points, the sensitivity between X = 0 and Z = 0, where the distance between the equal magnetic flux density surface and the RF coil 23 as the receiving coil is close, is relatively high. become. On the other hand, in the peripheral part, the equal magnetic flux density surface of this magnetic flux density is radially warped in the Y-axis direction as the distance from the central part increases. Even if a nuclear magnetic resonance signal can be received under certain conditions, a signal acquired from a portion other than the central portion is far away from the receiving coil at a distance 104 (shown in FIG. 7C). The sensitivity to receive is dull. Therefore, it is possible to receive a nuclear magnetic resonance signal only in the vicinity of X = 0 and Z = 0.

  As described above, the resolution in the X-axis and Z-axis directions can be improved as described with reference to FIG. Further, in the Y-axis direction, as shown in FIG. 6B, the magnetic flux density varies depending on the position of the Y-axis. Therefore, by changing the frequency of the RF coil 23 as a transmission coil, resonance occurs in accordance with this frequency. Since the regions are different, a nuclear magnetic resonance signal can be received by specifying a portion in the Y-axis direction. Furthermore, by scanning the sensor head 2 in the XZ plane direction with respect to the upper surface of the measurement object 100, it is possible to acquire three-dimensional atomic distribution data as a whole.

  Instead of scanning the sensor head, the measurement object 100 side may be moved, or it may be moved manually.

  Further, as shown in FIG. 4, in the apparatus of the first embodiment, the portion where the gap between the two magnets is wide is placed in the central portion where X = 0 and Z = 0, but the X-axis coordinates are static. It suffices if it is inside the magnetic field generation unit 20, and if it is used in FIG. However, as described above, it is necessary to provide a receiving coil at an X coordinate position where the distance between the two magnets is wide. Further, the Z-axis coordinate of the position where the receiving coil is provided is set near the center of the gap 108 as described above. By doing so, as described above, the Y coordinate of the equal magnetic flux density line can locally approach the sensor head 2 on the YZ plane, and the sensitivity of the central portion Z = 0 portion can be increased. .

Next, the shape of the equal magnetic flux density lines of the magnetic flux density will be described with reference to FIG. FIG. 8 is a conceptual diagram for obtaining the shape of the equal magnetic flux density line in the X-axis-Y-axis plane shape. FIGS. 8A and 8B correspond to FIGS. 7A and 7B. FIG. 8A shows the case where the X-axis coordinate is changed with Y = d fixed. (In FIG. 8A, unlike FIG. 7, only a graph fixed at Y = d is shown for simplicity). FIG. 8B shows changes in magnetic flux density when the Y-axis coordinates are changed while X = 0, X ₁ , X ₂ , X ₃ , and X ₄ are fixed. Similar to FIGS. 7A and 7B, FIG. 8A and FIG. 8B are added together, and the equal magnetic flux density line where the magnetic flux density becomes equal to the position of (X = 0, Y = d). (Indicated by the dotted line in FIGS. 5A and 6A, the magnetic flux density is a constant value _Bd ). In the static magnetic field generation unit 20 according to the conventional apparatus, unlike the apparatus of the first embodiment, the equal magnetic flux density line has a constant Y-axis coordinate along the X-axis direction and is a straight line. This is because the gaps between the magnets 210a and 210b are uniform as shown in FIG. Here, when a specific frequency of the high-frequency magnetic field is applied to the RF coil 23, the conditions for causing the nuclear magnetic resonance phenomenon are satisfied at all points on the straight line shown in FIG. . As described above, as in the apparatus of the first embodiment, the equal magnetic flux density surface (defined in the description of FIG. 7) is radially warped in the Y-axis direction as it is away from X = 0 and Z = 0. Unlike the static magnetic field generator 20 according to the conventional apparatus, as shown in FIG. 8, the Y-axis coordinates of the isomagnetic flux density line of the magnetic flux density are constant along the X-axis direction. Therefore, when the RF coil 23 as a receiving coil is provided at X = 0 and Z = 0, the portion where the reaction of nuclear magnetic resonance occurs in the peripheral portion of X = 0 is as shown in FIG. Although it is certain that the distance 104 from the receiving coil increases as the distance from X = 0 increases, the distance is far greater than that of the apparatus of the first embodiment (shown in FIG. 7C). There is little to be. Therefore, the sensitivity for receiving the nuclear magnetic resonance signal is not so different from X = 0 at the center. Therefore, a nuclear magnetic resonance reaction occurs up to the extra part, and a nuclear magnetic resonance signal is received from the peripheral part, so that the resolution is not sufficient in the X-axis direction.

  In addition to the embodiment shown in FIG. 4, a pair of permanent magnets 21a and 21b is considered to be formed with non-uniform intervals along a direction perpendicular to the U-shaped surface. be able to. For example, in FIG. 4, it is formed so as to be widest at the center of the X-axis, but even if X = 0, the distance between these magnets is maximized at any position inside the magnets 21a and 21b. It is also conceivable to form as described above. However, in that case, it is necessary to provide the RF coil 23 as a receiving coil in the vicinity of the position of the X coordinate where the magnets 21a and 21b are widest as in the example of the apparatus of the present embodiment shown in FIG. This is because, as described above, it is necessary to provide the receiving coil at a position where the magnetic flux density is minimal with respect to the direction of the XZ plane (see FIG. 7).

  Next, the mechanical configuration of the magnetic resonance apparatus of the second embodiment will be described with reference to FIG. FIG. 9 is an external configuration diagram of the magnetic resonance apparatus of the second embodiment. The configuration and internal configuration of the magnetic resonance apparatus 1 of the second embodiment are substantially the same as those of the apparatus of the second embodiment shown in FIGS. 1 and 2, but the shape and function of the sensor head 2 are different. The sensor head 2 of the apparatus of FIG. 9 is fixed to a rotary shaft 28 that is rotatably supported, and has a cylindrical permanent magnet 21c having a pair of magnetic poles in the diameter direction, a motor 26 that rotates the permanent magnet 21c, A peripheral surface of the permanent magnet 20 is provided with a pair of yokes 22a and 22b disposed to face each other with upper and lower nonmagnetic materials 27a and 27b interposed therebetween. The pair of yokes 22a and 22b has a recess that accommodates a part of the circumference of the permanent magnet, and the gap between the permanent magnet 21c and the nonmagnetic bodies 27a and 27b and the pair of yokes 22a and 22b is very small. In this way, the amount of leakage magnetic flux in this portion is prevented from increasing. For example, a stepping motor is used as the magnet rotation motor 26 so that the rotation angle of the permanent magnet 12 can be accurately controlled. Also, use a high torque one so that it can rotate rapidly. The magnet rotation motor 26 is controlled using a motor drive unit (not shown), and the angle of the permanent magnet 21c is precisely controlled.

  The static magnetic field generation unit 20 (corresponding to the static magnetic field generation unit 20 of the apparatus of the first embodiment shown in FIG. 4 and mainly includes a pair of yokes 22a and 22b and a magnet 21c) is configured as described above. Thus, in this apparatus, a magnetic circuit is formed by the yoke 22a２２permanent magnet 21c 磁 気 yoke 22b, and the yoke is provided in the peripheral space of the nonmagnetic material 27b provided below the permanent magnet 21c and fixing between the yokes 22a and 22b. Magnetic flux leaks from the bottom surfaces of 22a and 22b, thereby forming a static magnetic field. An RF coil 23 for generating a predetermined high-frequency magnetic field (rotating magnetic field) and receiving a resonance signal is disposed in the static magnetic field generated by the static magnetic field generator 20.

  Next, the magnet 21c and the yokes 22a and 22b of the magnetic resonance apparatus 1 according to the second embodiment will be described in more detail with reference to FIGS. FIG. 10 shows an example of the magnet 21c of the magnetic resonance apparatus 1 of the second embodiment, and represents a cross-sectional view cut along the AA cross section (position including the central axis of the magnet 21c) in FIG. Yes. In the BB cross section shown in the embodiment of FIG. 10, the positional relationship is such that the motor 26 is on the front side of the apparatus and the RF coil 23 is on the lower side of the apparatus. In the embodiment shown in FIG. 10, by processing the shape of the magnet 21c, the distance between the magnet 21c and the yokes 22a and 22b is formed larger than the other portions in the central portion in the X-axis direction. In this case, the magnetic flux density at the center in the X-axis direction can be made the lowest, and the magnetic flux density can be inclined. When a magnet is formed in this way, the static magnetic field of the magnetic resonance apparatus 1 of the second embodiment is considered to be the same as that of the apparatus of the first embodiment. Therefore, similarly to the device of the first embodiment, if the RF coil 23 as a receiving coil is provided in the central portion of the X-axis direction where the magnetic flux density in the X-axis direction is the lowest, the same as the device of the first embodiment. It is thought that there is an effect of. That is, in the apparatus of the second embodiment, as in the apparatus of the first embodiment, the above-described equal magnetic flux density surface having the magnetic flux density exists, and this equal magnetic flux density surface is represented by X = 0, Z in FIG. = 0 It is considered that the warpage is locally warped in the Y-axis direction as it goes away from 0. Therefore, the sensitivity is high when only X = 0 and Z = 0, and the sensitivity is low for the other. Therefore, in the XZ plane direction, it is considered that the nuclear magnetic resonance signal can be received only in the vicinity of the point where X = 0 and Z = 0. It is done.

  Further, with reference to FIG. 11, an example different from FIG. 10 will be described for the static magnetic field generation unit 20 of the magnetic resonance apparatus 1 of the second embodiment. FIG. 11 illustrates another example of the static magnetic field generation unit 20 of the magnetic resonance apparatus 1 according to the second embodiment. This example shows an example of the yokes 22a and 22b of the magnetic resonance apparatus 1 according to the second embodiment. FIG. 11A shows a cross-sectional view taken along the plane AA of FIG. The positional relationship is the same as in FIG. FIG. 11B illustrates a cross-sectional view taken along the plane BB in FIG. In Example 1 of FIG. 11A, by processing the shape of the yokes 22a and 22b, the central portion in the X-axis direction is the gap between the magnet 21c and the yokes 22a and 22b along the X-axis direction. It is formed wider than other parts. In Example 2 of FIG. 11B, by processing the shape of the yokes 22a and 22b, the central portion in the X-axis direction is spaced apart from the yokes 22a and 22b along the X-axis direction from other portions. While forming widely, the shape of the nonmagnetic material 27b is shape | molded according to this. In any of these cases of FIG. 11, the magnetic flux density in the central portion in the X-axis direction can be locally lowered, and if the RF coil 23 as a receiving coil is provided in this portion, the implementation of FIG. The effect is similar to the example.

  Note that Example 2 of the static magnetic field generation unit in FIG. 11B corresponds to a “static magnetic field generation unit” in claim 2.

10 and 11, the gap in the central portion in the X-axis direction of the static magnetic field generation unit 20 is enlarged. However, the position where the gap is maximized is not limited to the central portion and is inside the static magnetic field generation unit 20. Any position is acceptable. However, the equal magnetic flux intensity surface in the central portion in the X-axis direction can be locally brought close to the Y-axis, as described above, between the yokes 22a and 22b and the magnet 21c as shown in FIGS. 10 and 11A. Or since it is a part which formed the space | interval between yokes 22a and 22b large as shown in FIG.11 (B), it is necessary to provide the RF coil 23 as a receiving coil in this part.
Further, the form of FIG. 10 and the form of FIG. 11 (B) can be used in combination because the positions of the features are different. Similarly, the form of FIG. 11A and the form of FIG. 11B can be used in combination.

1 is an external configuration diagram of a magnetic resonance apparatus according to a first embodiment. 1 is an internal configuration diagram of a magnetic resonance apparatus of a first embodiment. The figure showing the coordinate system and positional relationship of the permanent magnet of a magnetic resonance apparatus is shown. The figure of the Example of the permanent magnet of the magnetic resonance apparatus of 1st Embodiment is shown. FIG. 5 is a diagram showing the magnetic flux density in the X direction and the magnetic pole arrangement for the embodiment of the permanent magnet shown in FIG. 4. The relationship between the magnetic flux density of the magnetic force of the permanent magnet of the 1st magnetic resonance apparatus of the Y-axis and Z-axis direction and the position of a coordinate is shown. It is a conceptual diagram for calculating | requiring the equal magnetic flux density line of magnetic flux density about the static magnetic field generation | occurrence | production part of the apparatus of 1st Embodiment. It is a conceptual diagram for calculating | requiring the equal magnetic flux density line of magnetic flux density about the static magnetic field generation | occurrence | production part of the apparatus of the conventional form. The external appearance block diagram of the magnetic resonance apparatus of 2nd Embodiment is shown. The Example of the magnet of the magnetic resonance apparatus of 2nd Embodiment is shown. The Example of the yoke of the magnetic resonance apparatus of 2nd Embodiment is shown.

Explanation of symbols

1-magnetic resonance apparatus 2-sensor head 20-static magnetic field generator 21a, 21b, 21c, 210a, 210b-magnet 22, 22a, 22b-yoke 23-RF coil 24-sensor head support 25-guide rail 26-magnet Rotating motors 27a and 27b-Non-magnetic material 28-Rotating shaft 3-Measurement support base 51-High frequency circuit system 511-Oscillating unit 512-Transmitting unit 513-High frequency amplifier 514a, b, c-Frequency 52-Magnetic circuit system 53- Nuclear magnetic resonance signal receiving unit 54-arithmetic processing system 541-image reconstruction unit 542-data processing unit 55-display 56-system control unit 57-operation unit 100-measurement object 101-bottom surface of sensor head 102-front side of the apparatus Surface 103-surface at the back of the device 104-distance 105-distance 106-distance 107-distance 108-gap

Claims (3)

A space is provided in a part of a magnetic circuit having a permanent magnet or a permanent magnet and a yoke, or a support member made of a non-magnetic material is inserted into the space, and a leakage static magnetic field formed in that portion is measured. A static magnetic field generator applied to
The direction perpendicular to the closed loop of the lines of magnetic force formed in the magnetic circuit is the X-axis direction,
The direction of the line of magnetic force in the gap or the support member is the Z axis,
A receiving coil provided at a position where the X-axis coordinate is located inside the static magnetic field generation unit and the Z-axis coordinate is located at a substantially central portion of the gap or the support member;
A sensor head having
The static magnetic field generator is configured such that, at the position of the X coordinate where the receiving coil is provided, the gap or the support member has an interval of X so that the interval in the Z-axis direction is maximized along the X-axis direction. A magnetic resonance apparatus that is non-uniformly formed along the axial direction. The magnetic circuit of the static magnetic field generating unit is provided with a cylindrical permanent magnet having magnetic poles formed along a circumferential surface, and provided in close proximity to both sides of the permanent magnet, A pair of yokes having a recess for housing a part thereof,
The magnetic resonance apparatus according to claim 1, wherein the gap or the support member in which the leakage static magnetic field is formed is provided between end portions of the pair of yokes. A space is provided in a part of a magnetic circuit having a permanent magnet or a permanent magnet and a yoke, or a support member made of a non-magnetic material is inserted into the space, and a leakage static magnetic field formed in that portion is measured. A static magnetic field generator applied to
The direction perpendicular to the closed loop of the lines of magnetic force formed in the magnetic circuit is the X-axis direction,
The direction of the line of magnetic force in the gap or the support member is the Z axis,
A receiving coil provided at a position where the X-axis coordinate is the inside of the static magnetic field generation unit and the Z-axis coordinate is the gap or a substantially central portion of the support member;
With a sensor head with
The static magnetic field generator is configured such that, at the position of the X coordinate where the receiving coil is provided, the gap or the support member has an interval of X so that the interval in the Z-axis direction is maximized along the X-axis direction. A sensor head that is non-uniformly formed along the axial direction.

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