JPH1119061A - Disturbed magnetic field compensation method and magnetic resonance camera apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce restriction of the position of a magnetic sensor by correcting the effect of a compensation magnetic field contained in a disturbed magnetic field detection signal detected outside a space where a magnetic field is formed by a signal as a function of the compensation magnetic field to compensate for changes in the magnetic field in the space. SOLUTION: A magnetic field at an installing location of a magnetic sensor is measured by a magnetic field measuring section 40 and a leakage magnetic field calculating part 44 of a current adjusting part 42 calculates a leakage magnetic field based on an output current I of a current output part 26. A calculated value of the leakage magnetic field is subtracted from a measured value of the magnetic field measuring part 40 by a subtraction part 46 to erase the leakage magnetic field from the measured value of the magnetic field measuring section 40 and a compensation magnetic field calculating part 48 calculates a disturbed magnetic field at a magnet center of a magnetostatic field space. An output current calculating part 52 inputs an output current value I calculated into a current output part 26 to feed an output current I to compensation coils 8 and 8'. As a result, compensation coils 8 and 8' generate compensation magnetic fields Bci to cancel disturbed magnetic fields Bdi at the magnet center.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a disturbance magnetic field compensation method and a magnetic resonance imaging apparatus, and more particularly, to a disturbance magnetic field compensation method for compensating a disturbance magnetic field in a space where a predetermined magnetic field is formed, and a disturbance magnetic field compensation means. The present invention relates to a magnetic resonance imaging apparatus having the same.

[0002]

2. Description of the Related Art Magnetic resonance imaging
In a Ming (MRI) apparatus, a gradient magnetic field and a high-frequency magnetic field are applied to an imaging target (subject) accommodated in a static magnetic field space, and a magnetic resonance signal in which spins of atoms of the subject are generated is measured. An image of the subject is generated (reconstructed) based on the measurement signal.

In order to obtain a proper image, the static magnetic field is required to be highly stable. The static magnetic field is affected by the magnetic field environment where the magnetic resonance imaging apparatus is placed. For example, when a magnetized moving object such as a vehicle passes near the installation location of the magnetic resonance imaging apparatus, the magnetic field environment fluctuates, and a disturbance is given to the static magnetic field.

[0004] If the static magnetic field is disturbed, image distortion or the like is generated. To prevent this, disturbance magnetic field compensation is performed. In particular, in a static magnetic field generator using a permanent magnet, a normal conductive magnet, or the like, compensation is indispensable because the influence of a disturbance magnetic field is large. Even in a static magnetic field generator using superconductive magnets,
Disturbance magnetic field compensation is performed as needed.

Conventionally, a Helmholtz disposed around a static magnetic field space by detecting a disturbance magnetic field by arranging a magnetic sensor near or in a static magnetic field space and detecting the disturbance magnetic field based on the detection signal. A current is passed through the coil (Helmholtz coil) to generate a magnetic field (compensation magnetic field) that cancels the disturbance magnetic field.

[0006]

In the above-described disturbance magnetic field compensating apparatus, the magnetic sensor is arranged in the vicinity of the static magnetic field generating apparatus or in the static magnetic field space. A gradient magnetic field also acts. Rather, these fields are generally stronger than the disturbing fields.

Therefore, it is necessary to carefully determine the position of the magnetic sensor in consideration of the influence of the static magnetic field and the gradient magnetic field. That is, there are many restrictions on the position of the magnetic sensor, and sufficient compensation cannot always be performed.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for compensating a disturbance magnetic field with less restriction on the position of a magnetic sensor and a disturbance magnetic field compensation means with less restriction on the position of a magnetic sensor. To realize a magnetic resonance imaging apparatus having the above.

[0009]

[Means for Solving the Problems]

(1) A first invention for solving the above-mentioned problem detects a disturbance magnetic field outside a space where a predetermined magnetic field is formed,
A compensation magnetic field that compensates for the influence of the compensation magnetic field included in the disturbance magnetic field detection signal with a signal that is a function of the compensation magnetic field and generates a compensation magnetic field that compensates for a magnetic field change in the space due to the disturbance magnetic field based on the corrected disturbance magnetic field detection signal. To be characterized.

(2) According to a second aspect of the present invention, a disturbance magnetic field is detected outside a space where a predetermined magnetic field is formed, and a disturbance magnetic field detection signal and a signal given by a function of a compensation magnetic field are used. Generating a compensation magnetic field for compensating for a magnetic field change in the space due to the disturbance magnetic field based on the difference between

(3) According to a third aspect of the present invention, a disturbance magnetic field is detected at a plurality of locations outside a space where a predetermined magnetic field is formed, and the disturbance magnetic field is detected based on the disturbance magnetic field detection signal. Determine the position and magnetic moment of the source, determine the disturbing magnetic field strength in the space based on the position and magnetic moment of the disturbing magnetic field source, and determine the disturbing magnetic field strength in the space based on the disturbing magnetic field strength in the space. Generating a compensating magnetic field that compensates for the magnetic field change of
It is characterized by the following.

(4) According to a fourth aspect of the present invention, there is provided a magnetic field forming means for forming a magnetic field in a predetermined space, a detecting means for detecting a disturbance magnetic field outside the predetermined space, and the detecting means. Correction means for correcting the influence of the compensation magnetic field included in the disturbance magnetic field detection signal detected by a signal that is a function of the compensation magnetic field, and the predetermined magnetic field generated by the disturbance magnetic field based on the disturbance magnetic field detection signal corrected by the correction means. A compensating means for generating a compensating magnetic field for compensating a magnetic field change in the space, a gradient magnetic field forming means for forming a gradient magnetic field in the predetermined space, a high frequency magnetic field forming means for forming a high frequency magnetic field in the predetermined space, It is characterized by comprising a measuring means for measuring a magnetic resonance signal from a predetermined space, and an image generating means for generating an image based on the magnetic resonance signal measured by the measuring means. To.

(5) According to a fifth aspect of the present invention, there is provided a magnetic field forming means for forming a magnetic field in a predetermined space, a detecting means for detecting a disturbance magnetic field outside the predetermined space, and the detecting means. Compensation means for generating a compensation magnetic field for compensating a magnetic field change in the predetermined space due to the disturbance magnetic field based on a difference between a disturbance magnetic field detection signal detected and a signal that is a function of the compensation magnetic field, A gradient magnetic field forming means for forming a gradient magnetic field, a high frequency magnetic field forming means for forming a high frequency magnetic field in the predetermined space, a measuring means for measuring a magnetic resonance signal from the predetermined space, and the magnetic field measured by the measuring means. Image generating means for generating an image based on the resonance signal.

(6) According to a sixth aspect of the present invention, there is provided a magnetic field forming means for forming a magnetic field in a predetermined space, and a detecting means for detecting a disturbance magnetic field at a plurality of locations outside the predetermined space. First calculating means for obtaining a position and a magnetic moment of a disturbance magnetic field source based on a plurality of disturbance magnetic field detection signals detected by the detection means, and a position of the disturbance magnetic field source obtained by the first calculation means And a second calculating means for obtaining a disturbance magnetic field strength in the predetermined space based on the magnetic moment, and a second calculating means for obtaining the disturbance magnetic field strength in the space based on the disturbance magnetic field strength in the space obtained by the second calculating means. Compensating means for generating a compensating magnetic field for compensating for a change in magnetic field, gradient magnetic field forming means for forming a gradient magnetic field in the predetermined space, and high-frequency magnetic field forming for forming a high-frequency magnetic field in the predetermined space A stage, a measuring means for measuring magnetic resonance signals from said predetermined space, characterized by comprising an image generating means for generating an image, the based on the magnetic resonance signals the measuring means has measured.

[0015] In any one of the fourth to sixth inventions, the static magnetic field forming means may include a pair of magnets facing each other across a space and a pair of magnets respectively attached to opposing magnetic poles of the pair of magnets. A pole piece, the gradient magnetic field forming means having a pair of gradient coils respectively attached to end faces of the pair of pole pieces, and the compensating means surrounding the pair of gradient coils. It is preferable to have a pair of compensating coils attached to the camera in that the disturbance magnetic field is efficiently compensated for at the position closest to the imaging space, and the static magnetic field generating unit and the surrounding scenery are not spoiled.

Further, in any one of the fourth to sixth aspects of the present invention, the static magnetic field forming means may include a pair of magnets facing each other across a space and a pair of magnets respectively attached to the opposing magnetic poles of the pair of magnets. A pair of pole pieces, wherein the gradient magnetic field forming means comprises a pair of gradient coils respectively mounted on end faces of the pair of pole pieces, and wherein the compensation means comprises
It is preferred to have a pair of compensating coils mounted around the pair of pole pieces, as this reduces coupling between the gradient and compensating coils.

Further, in any one of the fourth to sixth aspects of the present invention, the static magnetic field forming means may include a pair of magnets facing each other across a space and a pair of magnets attached to the pair of magnets facing each other. A pair of pole pieces, wherein the gradient magnetic field forming means comprises a pair of gradient coils respectively mounted on end faces of the pair of pole pieces, and wherein the compensation means comprises
It is preferred to have a pair of compensating coils mounted around a pair of magnets in that coupling between the gradient coil and the compensating coil is further reduced.

Further, in any one of the fourth to sixth inventions, the static magnetic field forming means may comprise a pair of magnets facing each other with a space therebetween and a frame constituting a magnetic circuit for the pair of magnets. And a compensating means having a pair of compensating coils attached to the outer surface of the frame so as to face each other with the pair of magnets interposed therebetween, depending on the internal structure of the static magnetic field generating unit. This is preferable in that a compensation coil can be provided without any restrictions.

Further, in any one of the fourth to sixth inventions, the static magnetic field forming means may comprise a pair of magnets facing each other across a space and a frame constituting a magnetic circuit for the pair of magnets. It is preferable that the compensating means include a compensating coil wound around the frame body in terms of easy design of the compensating coil.

(Operation) In the first or fourth aspect of the invention, the disturbance magnetic field detection signal is corrected with a signal that is a function of the compensation magnetic field, and the disturbance magnetic field detection signal excluding the influence of the compensation magnetic field is obtained. Generate a magnetic field.

In the second or fifth invention, the compensation magnetic field is generated such that the feedback signal, which is a function of the compensation magnetic field, is balanced with the disturbance magnetic field detection signal. In the third or sixth aspect, the position and magnetic moment of the disturbance magnetic field source are determined based on the disturbance magnetic field detection signals detected at a plurality of locations, the influence of the position and the magnetic moment on the magnetic field in a predetermined space is determined, and the influence is compensated. Generate a compensating magnetic field.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiment.

FIG. 1 shows a block (bloc) of the magnetic resonance imaging apparatus.
k) Show the figure. This device is an example of an embodiment of the present invention. The configuration of the present apparatus shows an example of an embodiment relating to the apparatus of the present invention. Further, an example of an embodiment relating to the method of the present invention is shown by the operation of the present apparatus.

(Construction) In the present apparatus, as shown in FIG. 1, the static magnetic field generating section 2 forms a uniform static magnetic field in its internal space. The static magnetic field generation unit 2 is an example of an embodiment of a magnetic field forming unit in the present invention. The static magnetic field generating unit 2 includes a pair of magnetic generators such as permanent magnets (not shown), which are opposed to each other in the vertical direction with a space therebetween, and form a static magnetic field (vertical magnetic field) in the opposed space. I have. The magnetic generator is not limited to a permanent magnet, and may be a normal conductive magnet, a superconductive magnet, or the like.

The static magnetic field generating section 2 includes a transmitting coil section 4,
4 ', gradient coil sections 6, 6' and compensation coil section 8,
8 'are provided, and are similarly opposed in the up-down direction with an interval.

In the static magnetic field space between the transmitting coil sections 4 and 4 ', a body coil section 10 having a substantially cylindrical shape is provided.
Is arranged. The central axis of the body coil section 10 is orthogonal to the direction of the static magnetic field. The subject 1 is placed in a generally cylindrical space formed inside the body coil unit 10.
2 is carried in by carrying means (not shown). The body axis of the subject 12 is orthogonal to the direction of the static magnetic field.

Above the static magnetic field generator 2, a magnetic sensor 14 is arranged at a predetermined distance. Magnetic sensor 1
4 is suspended from, for example, a ceiling of an examination room in which the present apparatus is installed, by supporting means (not shown). The distance from the upper end of the static magnetic field generator 2 is, for example, 1.5 m. By setting the distance to be equal to or longer than this, it is possible to make it hardly affected by the gradient magnetic field generated by the gradient coil units 6, 6 '.

As the magnetic sensor 14, for example, a hall
A device using a (Hall) element is used. Hall elements are preferred because of their high magnetic detection sensitivity. Of course, not only the Hall element but also an appropriate magnetic detecting means such as a coil having a magnetic core or an air-core coil can be used. The number of the magnetic sensors 14 is not limited to one, and a plurality of magnetic sensors may be provided at appropriate locations, and these may be used selectively or simultaneously.

A transmitting section 16 is connected to the transmitting coil sections 4 and 4 '. The transmitting coil units 4, 4 'and the transmitting unit 16
It is an example of embodiment of the high frequency magnetic field formation means in this invention. The transmission unit 16 supplies a drive signal to the transmission coil units 4 and 4 ′ to generate an RF (radio frequency) magnetic field, thereby exciting a spin of a specific atom in the body of the subject 12.

A gradient driving unit 18 is provided in the gradient coil units 6, 6 '.
Is connected. The gradient coil units 6, 6 'and the gradient driving unit 18 are an example of the embodiment of the gradient magnetic field forming means in the present invention. The gradient driving unit 18 includes gradient coil units 6 and 6 ′.
To generate a gradient magnetic field. The generated gradient magnetic fields are of three types: a slice gradient magnetic field, a readout gradient magnetic field, and a phase encode gradient magnetic field.

The body coil section 10 detects a magnetic resonance signal generated by excited spins in the subject 12. The receiving unit 20 is connected to the body coil unit 10. The body coil unit 10 and the receiving unit 20 are an example of an embodiment of a measuring unit according to the present invention. The receiving section 20 receives the signal detected by the body coil section 10.

The receiving section 20 has an analog / digital (ana
log-to-digital) converter 22 is connected. The analog-to-digital converter 22 converts an output signal of the receiver 20 into a digital signal. analog·
The digital converter 22 is connected to a computer 22.

The computer 24 receives a digital signal from the analog-to-digital converter 22 and stores it in a memory (not shown). A data space is formed in the memory. This data space constitutes a two-dimensional Fourier space. The computer 24 performs a two-dimensional inverse Fourier transform on the data in the two-dimensional Fourier space to perform the subject 12
Is generated (reconstructed). The computer 24 is an example of an embodiment of an image generating unit according to the present invention.

A current output section 26 is connected to the compensation coils 8, 8 '. Compensation coils 8, 8 'and current output unit 26
Is an example of some embodiments of the compensation means in the present invention. The compensating coils 8, 8 'are composed of, for example, Helmholtz coils or the like. The current output section 26 supplies a current to the compensation coils 8, 8 '. The current output unit 26 is also connected to the computer 24, through which a signal indicating an output current value is input to the computer 24. The compensation coils 8, 8 'generate a magnetic field corresponding to the supplied current. The direction of the magnetic field is the same as the direction of the static magnetic field. The magnetic field generated by the compensation coils 8, 8 'is a compensation magnetic field. The compensating magnetic field will be described later.

A magnetic field measuring section 28 is connected to the magnetic sensor 14. The magnetic sensor 14 and the magnetic field measuring unit 28 are an example of an embodiment of a detecting unit according to the present invention. The detection signal of the magnetic sensor 14 is input to the magnetic field measurement unit 28.
The magnetic field measuring unit 28 calculates the magnetic field strength based on the input signal.

When a plurality of magnetic sensors 14 are used, the magnetic field strength is determined for each magnetic sensor. The magnetic field measurement unit 28 is connected to the computer 24. The magnetic field intensity signal obtained by the magnetic field measuring unit 28 is input to the computer 24.

The computer 24 also has a control unit 30
Is connected. The control unit 30 includes a transmitting unit 16, a gradient driving unit 18, a receiving unit 20, an analog / digital converting unit 22.
And a current output unit 28.

The control unit 30 receives a command from the computer 24, and based on the command, transmits the transmission unit 16 and the gradient drive unit 1.
8, a control signal is supplied to the receiving unit 20, the analog / digital converting unit 22, and the current output unit 28, respectively.

The computer 24 also has a display unit 32
And the operation unit 34 are connected. The display unit 32 displays various information including a reconstructed image output from the computer 24. The operation unit 34 is operated by an operator, and inputs various commands and information to the computer 24.

FIG. 2 shows an example of a specific arrangement of the compensation coils 8 and 8 ′ in the static magnetic field forming section 2. As shown in the figure, the static magnetic field generation unit 2 has a magnetic circuit 200. The magnetic circuit 200 is configured to form a frame with a magnetic material such as soft iron, for example. The magnetic circuit 200 has an upper plate 202, a lower plate 204, a right pillar 206, and a left pillar 208.

Permanent magnets 210 and 210 'are attached to the lower surface of the upper plate 202 and the upper surface of the lower plate 204, respectively, facing each other. The permanent magnets 210, 210 'are magnetized in the same direction, forming a vertical magnetic field in the space facing them.

Pole pieces 212 and 212 'are respectively attached to the mutually facing magnetic pole faces of the permanent magnets 210 and 210'. The pole pieces 212 and 212 'are made of a magnetic material such as soft iron, and have a thickness profile (pro).
file) is configured to make the distribution of the magnetic field strength uniform. Gradient coil sections 6, 6 'and transmission coil sections 4, 4' are attached to the end faces of the pole pieces 212, 212 ', respectively.

The compensating coils 8, 8 'are provided with pole pieces 212, 2
At the end face of 12 ', it is attached around the gradient coils 6, 6', respectively. The compensation coils 8, 8 'are Helmholtz coils. This arrangement is preferable in that the disturbance magnetic field is efficiently compensated for at the position closest to the imaging space. The static magnetic field generator 2 and the surrounding scenery are composed of the compensation coil 8,
It is preferable because it is not damaged by 8 '.

FIGS. 3 to 6 show other examples of specific arrangements of the compensation coils 8, 8 '. In each figure, FIG.
The same reference numerals are given to the same parts as those described above, and the description is omitted. FIG. 3 shows an example in which the compensation coils 8, 8 'are mounted around the pole pieces 212, 212', respectively. In this arrangement, the magnetic flux of the gradient coils 6, 6 'and the like is absorbed by the pole pieces 212, 212' and the linkage magnetic flux with the compensating coils 8, 8 'is reduced, as compared with the case of FIG. Is preferred in that the coupling is reduced.

FIG. 4 shows that the compensation coils 8, 8 '
This is an example in which they are mounted around 10, 210 ', respectively. This arrangement is preferable in that coupling with the gradient coils 6, 6 'and the like is further reduced as compared with the case of FIG.

FIG. 5 shows that the compensation coils 8 and 8 ′ are connected to the magnetic circuit 2.
This is an example in which the upper plate portion is attached to the upper surface of the upper plate portion and the lower surface portion of the lower plate portion. Since the magnetic flux of the compensation coils 8, 8 'is passed through the magnetic circuit 200, a compensation magnetic field can be efficiently generated even in this arrangement. This arrangement is preferable in that the compensation coils 8 and 8 'can be provided without being restricted by the internal structure of the static magnetic field generation unit 2, as compared with the above arrangements. In addition, the landscape of the static magnetic field generation unit 2 is not significantly impaired.

FIG. 6 shows that the compensation coils 8 and 8 ′ are connected to the magnetic circuit 2.
This is an example of mounting around the right column portion 206 and the left column portion 208 of FIG. Even with this arrangement, the magnetic flux of the compensation coils 8, 8 'is passed through the magnetic circuit 200, so that a compensation magnetic field can be efficiently generated.

In this case, the compensation coils 8, 8 'are not Helmholtz coils, but are, for example, solenoid coils. This arrangement is preferable in that the compensating coils 8, 8 'do not need to satisfy the conditions of the Helmholtz coil and the design is easy. In addition, the landscape of the static magnetic field generation unit 2 is not significantly impaired.

In this apparatus, the magnetic sensor 14, the magnetic field measuring unit 28, the compensation coils 8, 8 ', the current output unit 26, the computer 24, and the control unit 30 constitute a compensation magnetic field generation system. Among them, the compensation coils 8, 8 'and the current output unit 2
6. The computer 24 and the control unit 30 are an example of an embodiment of a compensating unit in the present invention. FIG. 7 is a block diagram focusing on signal processing of the compensation magnetic field generation system.

In FIG. 7, the magnetic field measuring unit 40 measures the magnetic field at the place where the magnetic sensor 14 is installed. The magnetic field measurement unit 40 is configured by the magnetic sensor 14 and the magnetic field measurement unit 28.

The compensating coils 8, 8 'generate a compensating magnetic field Bci at the center (magnet center) of the static magnetic field space according to the current I supplied from the current output unit 26. The leakage magnetic field Bcs of the compensation magnetic field Bci is generated at the place where the magnetic sensor 14 is installed. Therefore, the magnetic field measured by the magnetic field measuring unit 40 when the disturbance magnetic field arrives is the algebraic sum of the disturbance magnetic field Bds and the leakage compensation magnetic field Bcs.

The output current I of the current output unit 26 is adjusted by the current adjusting unit 42 according to the disturbance magnetic field. Current adjuster 4
Reference numeral 2 is composed of a computer 24 and a control unit 30. In the current adjusting unit 42, the leakage magnetic field Bcs at the installation location of the magnetic sensor 14 is calculated by the leakage magnetic field calculation unit 44 based on the output current I of the current output unit 26.

To calculate the leakage magnetic field Bcs, for example, first, a compensation magnetic field Bci generated in the magnet center by the compensation coils 8 and 8 ′ in accordance with the output current I is obtained, and then the position of the compensation magnetic field Bci of the magnetic sensor 14 is calculated. Magnetic field Bc
It is performed in the procedure of obtaining s.

To calculate the compensation magnetic field Bci, the magnetic field generation coefficient Gh of the compensation coils 8, 8 'is used. Leakage magnetic field B
For calculating cs, the magnetic field leakage coefficient Gc at the position of the magnetic sensor 14 is used. The magnetic field generation coefficient Gh is a known value determined by the design of the compensation coils 8, 8 '. The magnetic field leakage coefficient Gc is obtained by actual measurement or the like when the apparatus is installed in an examination room, for example.

The subtraction section 46 subtracts the leakage magnetic field calculation value of the leakage magnetic field calculation section 44 from the measurement value of the magnetic field measurement section 40 and inputs the result to the compensation magnetic field calculation section 48. The leakage magnetic field calculation unit 44 and the subtraction unit 46 are an example of an embodiment of a correction unit in the present invention. By subtraction, the magnetic field measurement unit 40
The leakage magnetic field Bcs is erased from the measured value Bds + Bcs of the above, and the measured value of only the disturbance magnetic field Bds is used as the compensation magnetic field calculation unit 48.
Is input to

The compensating magnetic field calculator 48 calculates the disturbance magnetic field Bdi at the magnet center in the static magnetic field space based on the input signal. The magnetic sensor 14 is used to calculate the disturbance magnetic field Bdi.
Is used as the influence coefficient Gd of the disturbance magnetic field Bds at the position of. The influence coefficient Gd is obtained by actual measurement or the like when the apparatus is installed in an examination room.

The calculated value of the disturbance magnetic field Bdi is calculated by the sign inversion unit 5.
The signal is input to the output current calculation unit 52 via 0. The output current calculator 52 calculates an output current value I based on the input signal. To calculate the output current value I, the reciprocal of the magnetic field generation coefficient Gh of the compensation coils 8, 8 'is used. As a result, an output current value I that generates a magnetic field having the same strength and the opposite direction to the disturbance magnetic field Bdi in the compensation coils 8, 8 'is obtained.

The calculated output current value I is supplied to the current output unit 26
Is input to The current output unit 26 outputs an output current I corresponding to the input signal to the compensation coils 8, 8 '. Due to this current I, the compensation coils 8, 8 'generate a compensation magnetic field Bci in the magnet center in the static magnetic field space, which has the same strength as the disturbance magnetic field Bdi and has the opposite direction. Thereby, the disturbance magnetic field Bdi in the magnet center is canceled. That is,
The disturbance magnetic field compensation in the static magnetic field space is performed.

FIG. 8 is a block diagram showing another configuration example of the compensation magnetic field generation system. 8, the same reference numerals are given to the same portions as in FIG. 7, and the description is omitted. The current adjustment unit 54 uses the feedback magnetic field calculation unit 56 to output the current I
Is used to calculate the feedback magnetic field Bfb.

The calculation of the feedback magnetic field Bfb is based on the measured value of the magnetic field measuring unit 40 based on the current value I that generates the compensating magnetic field Bci which cancels the disturbance magnetic field Bdi at the magnet center in the static magnetic field space. Balanced return magnetic field value Bf
This is performed so that b is obtained. The calculation coefficient Gf for obtaining such a feedback magnetic field value Bfb from the current value I is obtained, for example, by actual measurement when the apparatus is installed in an examination room.

The difference between the feedback magnetic field value Bfb and the magnetic field measurement value Bds + Bcs of the magnetic field measurement unit 40 is obtained by the subtraction unit 58 and input to the subtraction unit 60. Subtraction unit 60
Is given a reference value of 0. The subtraction unit 60 inputs the difference between the reference value 0 and the input signal to the current control unit 62. The current control unit 62 outputs, for example, a PID based on the input signal.
It outputs a feedback control signal such as a (proportional, integral, or differential) control signal to control the current output unit 26.

Since the reference value is set to 0,
The current control unit 62 determines that the feedback magnetic field value Bfb is the magnetic field measurement value Bd
The output current I is controlled so as to be balanced with s + Bcs. As a result, a compensation magnetic field Bci that cancels the disturbance magnetic field Bdi is generated in the magnet center in the static magnetic field space. That is, disturbance magnetic field compensation is performed.

FIG. 9 is a block diagram showing a configuration example of a compensation magnetic field generation system when two magnetic sensors are used. In FIG. 9, the same parts as those in FIG. As shown in FIG. 9, two magnetic field measuring units 40 and 4
The measurement signal of 0 ′ is input to the disturbance magnetic field source identification unit 66 of the current adjustment unit 64.

The disturbance magnetic field source identification unit 66 is configured to determine the position of the disturbance magnetic field generation source and the magnetic moment (magneti) at the position based on the input signals from the two magnetic field measurement units 40 and 40 '.
c moment). The disturbance magnetic field source identification unit 66 is an example of an embodiment of the first calculation unit in the present invention.

The calculation is performed by, for example, the magnetic field measurement units 40 and 4
0 ′ magnetic field components (Bx1, By1, Bz1), (Bx1, By1, Bz1) detected by the magnetic sensors 14 and 14 ′, respectively.
, By2, Bz2) and the three-dimensional coordinates (x1, y1, z1), (x2, y2,
Assuming that z2) is a known value, the three-dimensional coordinates (x, y, z) of the position of the disturbance magnetic field generation source and the three-axis direction components (mx, my, mz) of the magnetic moment at that position are unknown.
This is performed by solving a system of simultaneous equations.

The calculated position and magnetic moment of the disturbance magnetic field generation source are input to the compensation magnetic field calculation unit 68. The compensation magnetic field calculation unit 68 calculates a disturbance magnetic field Bdi in the magnet center in the static magnetic field space based on the position and the magnetic moment of the disturbance magnetic field generation source. The compensating magnetic field calculator 68 is an example of an embodiment of the second calculator in the present invention. If the position and magnetic moment of the disturbance magnetic field source are known, the magnetic field generated in the magnet center can be obtained by calculation.

Alternatively, instead of calculating each time, the magnetic field intensity at the magnet center based on the unit magnetic moment at each point of the lattice space assumed around the static magnetic field generating unit 2 is calculated in advance, and it is tabulated. May be stored in a memory and used for calculating a disturbance magnetic field. This is preferable in that the calculation time is reduced.

The calculated value of the disturbance magnetic field Bdi is input to the output current calculation unit 52 via the sign inversion unit 50 as in the case of FIG. Hereinafter, similarly to the case of FIG.
2. The current output unit 26 and the compensation coils 8, 8 '
A compensation magnetic field Bci having the same strength as the disturbance magnetic field Bdi but in the opposite direction is generated at the magnet center in the static magnetic field space. That is, disturbance magnetic field compensation is performed.

The number of magnetic field measurement points is not limited to two. If the number of magnetic field measurement points is three or more, more accurate identification of a disturbance magnetic field source or identification of a plurality of disturbance magnetic field sources can be performed. It is desirable that the magnetic field measurement point is sufficiently separated from the static magnetic field forming unit 2 so as not to be affected by the leakage of the compensation magnetic field. Of course, this is not the case when the influence of the leakage compensation magnetic field is removed in the same manner as in the case of FIG. 7 and the measurement signal after the removal is used for identifying the disturbance magnetic field source.

As described above, the disturbance magnetic field can be effectively compensated for while using the magnetic sensor 14 arranged at a position distant from the static magnetic field generation unit 2. Magnetic sensor 1
4 can be arranged at an appropriate place if it is at least a certain distance from the static magnetic field generation unit 2, and there is little restriction on the position.

[Operation] The operation of the present apparatus will be described. As one specific example of magnetic resonance imaging, spin echo (spin ech)
o) A case where imaging is performed by the method will be described. In the spin echo method, for example, a pulse sequence as shown in FIG. 10 is used.

FIG. 10 is a schematic diagram of a pulse sequence when collecting magnetic resonance signals (spin echo signals) for one view. Such a pulse sequence is repeated, for example, 256 times to acquire a spin echo signal of 256 views.

The execution of the pulse sequence and the collection of the spin echo signal are controlled by the control unit 30. In addition,
This apparatus can perform magnetic resonance imaging by not only the spin echo method but also various other techniques.

As shown in FIG. 10 (6), the pulse sequence is divided into four periods (a) to (d) along the time axis. First, in the period (a), RF excitation is performed by the 90 ° pulse P90 as shown in (1). The RF excitation is performed by the transmission coil units 4 and 4 ′ driven by the transmission unit 16.

At this time, a slice gradient magnetic field Gs is applied as shown in (2). The application of the slice gradient magnetic field Gs is performed by the gradient coil units 6 and 6 ′ driven by the gradient driving unit 18. Thereby, the spin of a predetermined slice in the body of the subject 12 is excited (selective excitation).

Next, in the period (b), a phase encoding gradient magnetic field Gp is applied as shown in (3). The application of the phase encoding gradient magnetic field Gp is also performed by the gradient coil units 6 and 6 ′ driven by the gradient driving unit 18. As a result, spin phase encoding is performed.

During the phase encoding period, the spin gradient is rephased by the slice gradient magnetic field Gs as shown in (2).
(rephase) is performed. Further, as shown in (4), a read gradient magnetic field Gr is applied, and the spin dephase (d
ephase) is performed. The application of the read-out gradient magnetic field Gr is also performed by the gradient coil units 6 and 6 ′ driven by the gradient driving unit 18.
It is performed by

Next, in the period (c), a 180 ° pulse P180 is applied as shown in (1), whereby the spin is inverted. The spin reversal is performed by the transmitter 1
This is performed by the transmission coil units 4 and 4 ′ that are RF-driven in 6.

Next, in a period (d), a read gradient magnetic field Gr is applied as shown in (4). As a result, as shown in (5), the spin echo signal is
From 2

This spin echo signal is transmitted to the body coil 1
0, and the detection signal is input to the receiving unit 20. The output signal of the receiving unit 20 is converted into an analog / digital converter 22.
Is converted into a digital signal and input to the computer 24. The computer 24 stores the input data in a memory.

Such an operation is sequentially performed on, for example, 256 views. The intensity of the phase encoding gradient magnetic field Gp is changed for each view, and a different phase encoding is performed for each view. As a result, spin echo data satisfying the two-dimensional Fourier space is collected in the memory. The above data collection operation is called a scan.

Even if a disturbance magnetic field is generated during scanning, the magnetic sensor 14, the magnetic field measuring unit 28, the compensation coil units 8, 8 ',
The above-described disturbance magnetic field compensation is performed by the compensation magnetic field generation system including the current output unit 26, the control unit 30, and the computer 24. Therefore, spin echo data not affected by the disturbance magnetic field is collected.

Based on the spin echo data satisfying the two-dimensional Fourier space collected in the memory, the computer 2
4 performs image reconstruction. The reconstructed image is displayed on the display unit 32 as a visible image. Since the spin echo data is not affected by the disturbance magnetic field, no inconvenient phenomenon such as distortion of the reconstructed image occurs.

[0084]

As described above in detail, according to the present invention, a disturbance magnetic field is detected outside a predetermined magnetic field space, and disturbance compensation is performed in a predetermined magnetic field space based on a signal obtained by processing the detection signal. Since the magnetic field is generated, it is possible to realize a disturbance magnetic field compensation method with less restriction on the position of the magnetic sensor and a magnetic resonance imaging apparatus including a disturbance magnetic field compensation unit with less restriction on the position of the magnetic sensor.

[Brief description of the drawings]

FIG. 1 is a block diagram of a device according to an example of an embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration example of a static magnetic field generation unit in an apparatus according to an embodiment of the present invention;

FIG. 3 is a diagram illustrating a configuration example of a static magnetic field generation unit in the device according to an example of the embodiment of the present invention;

FIG. 4 is a diagram illustrating a configuration example of a static magnetic field generation unit in an apparatus according to an example of an embodiment of the present invention.

FIG. 5 is a diagram illustrating a configuration example of a static magnetic field generation unit in the device according to an example of the embodiment of the present invention;

FIG. 6 is a diagram illustrating a configuration example of a static magnetic field generation unit in the device according to an example of the embodiment of the present invention;

FIG. 7 is a block diagram of a compensation magnetic field generation system in the device according to an example of the embodiment of the present invention;

FIG. 8 is a block diagram of a compensation magnetic field generation system in the apparatus according to the embodiment of the present invention;

FIG. 9 is a block diagram of a compensation magnetic field generation system in the apparatus according to an example of the embodiment of the present invention.

FIG. 10 is a schematic diagram showing an example of a pulse sequence executed by the device according to the embodiment of the present invention;

[Explanation of symbols]

 Reference Signs List 2 static magnetic field generating section 4, 4 'transmitting coil section 6, 6' gradient coil section 8, 8 'compensating coil section 10 body coil section 12 subject 14 magnetic sensor 16 gradient driving section 18 transmitting section 20 receiving section 22 analog / digital conversion Unit 30 control unit 24 computer 26 current output unit 28 magnetic field measurement unit 30 control unit 32 display unit 34 operation unit 200 magnetic circuit 202 upper plate unit 204 lower plate unit 206 right column unit 208 left column unit 210, 210 'permanent magnet 212, 212 'Pole piece 40 Magnetic field measuring unit 42, 54, 64 Current adjusting unit 44 Leakage magnetic field calculating unit 46, 58, 60 Subtracting unit 48, 68 Compensating magnetic field calculating unit 50 Sign inverting unit 52 Output current calculating unit 56 Feedback magnetic field calculating unit 62 Current control unit 66 Disturbance magnetic field source identification unit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yusuke Ito 127-7-4 Asahigaoka, Hino-shi, Tokyo Inside GE Yokogawa Medical Systems Co., Ltd. (72) Inventor Yuji Inoue 4-7-1, Asahigaoka, Hino-shi, Tokyo 127 GE Yokogawa Medical System Co., Ltd.

Claims (6)

[Claims] 1. A method for detecting a disturbance magnetic field outside a space in which a predetermined magnetic field is formed, and correcting an influence of a compensation magnetic field included in a disturbance magnetic field detection signal by a signal that is a function of the compensation magnetic field. A disturbance magnetic field compensation method, comprising: generating a compensation magnetic field for compensating a magnetic field change in the space due to the disturbance magnetic field based on a magnetic field detection signal. 2. A disturbance magnetic field is detected outside a space in which a predetermined magnetic field is formed, and a disturbance magnetic field is detected in the space by the disturbance magnetic field based on a difference between a disturbance magnetic field detection signal and a signal given by a function of a compensation magnetic field. A disturbance magnetic field compensation method, comprising: generating a compensation magnetic field for compensating a magnetic field change. 3. A disturbance magnetic field is detected at each of a plurality of positions outside a space where a predetermined magnetic field is formed, and a position and a magnetic moment of a disturbance magnetic field generation source are obtained based on the disturbance magnetic field detection signals. Finding a disturbance magnetic field strength in the space based on the position and magnetic moment of the source, and generating a compensation magnetic field that compensates for a magnetic field change in the space due to the disturbance magnetic field based on the disturbance magnetic field strength in the space. A disturbance magnetic field compensation method. A magnetic field forming means for forming a magnetic field in a predetermined space; a detecting means for detecting a disturbance magnetic field outside the predetermined space; and a compensation magnetic field included in a disturbance magnetic field detection signal detected by the detection means. Correcting means for correcting the influence by a signal which is a function of the compensating magnetic field; and generating a compensating magnetic field for compensating a magnetic field change in the predetermined space due to the disturbing magnetic field based on the disturbance magnetic field detection signal corrected by the correcting means. Compensating means, gradient magnetic field forming means for forming a gradient magnetic field in the predetermined space, high-frequency magnetic field forming means for forming a high-frequency magnetic field in the predetermined space, and measuring means for measuring a magnetic resonance signal from the predetermined space A magnetic resonance imaging apparatus comprising: an image generation unit configured to generate an image based on the magnetic resonance signal measured by the measurement unit. 5. A magnetic field forming means for forming a magnetic field in a predetermined space, a detecting means for detecting a disturbance magnetic field outside the predetermined space, and a function of a disturbance magnetic field detection signal detected by the detecting means and a compensation magnetic field. A compensating means for generating a compensating magnetic field for compensating a magnetic field change in the predetermined space due to the disturbance magnetic field based on a difference from a certain signal; a gradient magnetic field forming means for forming a gradient magnetic field in the predetermined space; A high-frequency magnetic field forming unit that forms a high-frequency magnetic field in the space; a measuring unit that measures a magnetic resonance signal from the predetermined space; and an image generating unit that generates an image based on the magnetic resonance signal measured by the measuring unit. , A magnetic resonance imaging apparatus. 6. A magnetic field forming means for forming a magnetic field in a predetermined space; a detecting means for detecting a disturbance magnetic field at a plurality of locations outside the predetermined space; and a plurality of disturbance magnetic field detection signals detected by the detecting means. Calculating a position and a magnetic moment of a disturbance magnetic field generation source based on the following: a disturbance in the predetermined space based on the position and the magnetic moment of the disturbance magnetic field generation source obtained by the first calculation means Second calculating means for obtaining a magnetic field strength; and compensating means for generating a compensation magnetic field for compensating a magnetic field change in the space due to the disturbance magnetic field based on the disturbance magnetic field strength in the space obtained by the second calculation means. A gradient magnetic field forming means for forming a gradient magnetic field in the predetermined space; a high frequency magnetic field forming means for forming a high frequency magnetic field in the predetermined space; and magnetic resonance from the predetermined space. Magnetic resonance imaging apparatus comprising: the measuring means for measuring, by comprising, an image generating device which generates an image based on the magnetic resonance signals the measuring means has measured Nos.