JP4820063B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus and a gradient magnetic field forming apparatus.

  2. Description of the Related Art A magnetic resonance imaging (MRI) apparatus is known as an apparatus that can take a tomographic image of a subject by using a nuclear magnetic resonance (NMR) phenomenon. Magnetic resonance imaging apparatuses are used in various fields such as medical applications and industrial applications.

  When taking a tomographic image of a subject using a magnetic resonance imaging apparatus, first, the subject is placed in an imaging space where a static magnetic field is formed, and the direction of spin of protons in the subject is determined statically. Align in the direction of the magnetic field to obtain the magnetization vector. Thereafter, by irradiating an electromagnetic wave having a resonance frequency and a gradient magnetic field, a nuclear magnetic resonance phenomenon is generated to change the magnetization vector of the proton, and a magnetic resonance signal from the proton that returns to the original magnetization vector is received. Then, the magnetic resonance imaging apparatus generates a tomographic image of the subject based on the received magnetic resonance signal.

When the static magnetic field is formed by a permanent magnet, a pair of permanent magnets are arranged to face each other so as to sandwich a space in which the subject is accommodated. And in order to adjust the static magnetic field which a permanent magnet produces | generates, a pair of magnetic shunt plate is arrange | positioned on each surface which a pair of permanent magnet has opposed. And the gradient coil is arrange | positioned on each surface where a pair of magnetic shunt plates are facing (for example, refer patent document 1).
JP 2000-126152 A

  As described above, when a static magnetic field is formed by a permanent magnet, since the gradient coil is close to the magnetic shunt plate, an eddy current is generated in the magnetic shunt plate by the gradient magnetic field generated by the gradient coil. Since the temporal and spatial behavior of the magnetic field is altered, the tomographic image may be distorted and the image quality may be reduced.

  For this reason, the gradient pulse is corrected so that the influence of the eddy current is reduced. Conventionally, the gradient pulse is corrected on the assumption that the eddy current is proportional to the gradient pulse amplitude.

  However, when the amplitude of the gradient pulse increases, the generation rate of the eddy current is not proportional to the amplitude of the gradient pulse, and the generation rate changes. For this reason, conventionally, the influence of eddy currents cannot be sufficiently reduced, and it has been difficult to improve image quality due to distortion of tomographic images.

  Therefore, an object of the present invention is to provide a magnetic resonance imaging apparatus and a gradient magnetic field forming apparatus capable of suppressing the influence of eddy currents to prevent tomographic image distortion and improving the image quality.

  To achieve the above object, the magnetic resonance imaging apparatus of the present invention forms a gradient magnetic field and a high-frequency magnetic field in a space in which a subject is accommodated and a static magnetic field is formed, and based on a magnetic resonance signal from the subject. In the magnetic resonance imaging apparatus for generating a tomographic image of the subject, the means for forming the gradient magnetic field includes a gradient coil that generates the gradient magnetic field, a gradient pulse applied to the gradient coil, and the gradient coil Gradient driving means for driving, and correction means for correcting the gradient pulse so that the influence of the eddy current is reduced in the gradient magnetic field, and the correction means sets the amplitude and time constant of the eddy current to the gradient. Storage means for storing corresponding to the amplitude of the pulse, and the storage means stores the gradient pulse based on the amplitude and time constant of the eddy current stored corresponding to the amplitude of the gradient pulse. It is corrected.

  According to the magnetic resonance imaging apparatus of the present invention described above, the correction means includes the storage means for storing the amplitude and time constant of the eddy current corresponding to the amplitude of the gradient pulse. Then, the correcting unit corrects the gradient pulse so that the influence of the eddy current in the gradient magnetic field is reduced based on the amplitude and time constant of the eddy current stored by the storage unit corresponding to the amplitude of the gradient pulse.

  In order to achieve the above object, a gradient magnetic field forming apparatus of the present invention is a gradient magnetic field forming apparatus for forming a gradient magnetic field in a space in which a subject is accommodated in a magnetic resonance imaging apparatus, wherein the gradient coil generates the gradient magnetic field. And gradient driving means for driving the gradient coil by applying a gradient pulse to the gradient coil, and correction means for correcting the gradient pulse so that the influence of eddy current is reduced in the gradient magnetic field, The correcting means includes storage means for storing the amplitude and time constant of the eddy current corresponding to the amplitude of the gradient pulse, and the amplitude of the eddy current stored by the storage means corresponding to the amplitude of the gradient pulse. And the gradient pulse is corrected based on the time constant.

  According to the gradient magnetic field forming apparatus of the present invention described above, the correction means includes storage means for storing the amplitude of the eddy current and the time constant corresponding to the amplitude of the gradient pulse. Then, the correcting unit corrects the gradient pulse so that the influence of the eddy current in the gradient magnetic field is reduced based on the amplitude and time constant of the eddy current stored by the storage unit corresponding to the amplitude of the gradient pulse.

  According to the present invention, it is possible to provide a magnetic resonance imaging apparatus and a gradient magnetic field forming apparatus capable of suppressing the influence of eddy currents to prevent tomographic image distortion and improving the image quality.

  Hereinafter, an example of an embodiment according to the present invention will be described with reference to the drawings.

  FIG. 1 is a configuration diagram showing the configuration of the magnetic resonance imaging apparatus of the present embodiment.

  As shown in FIG. 1, the magnetic resonance imaging apparatus includes a static magnetic field magnet unit 12, a gradient coil unit 13, an RF coil unit 14, a search coil unit 15, an RF drive unit 22, a gradient drive unit 23, The data collection unit 24, the control unit 25, the cradle 26, the eddy current measurement unit 27, the gradient pulse correction unit 28, the data processing unit 31, the operation unit 32, and the display unit 33 are included. In the magnetic resonance imaging apparatus of the present embodiment, the search coil unit 15 and the eddy current measurement unit 27 are removed after measuring the eddy current.

  Hereinafter, each component will be sequentially described.

  The static magnetic field magnet unit 12 is provided to form a static magnetic field in the imaging space 11 in which the subject is accommodated. The static magnetic field magnet unit 12 is configured such that the direction of the static magnetic field is along a direction Z perpendicular to the body axis direction of the subject 40.

  FIG. 2 is a configuration diagram showing the configuration of the static magnetic field magnet unit 12.

  As shown in FIG. 2, the static magnetic field magnet unit 12 includes a yoke 101, a permanent magnet 102, and a magnetic shunt plate 103.

  The yoke 101 includes an upper yoke 101a, a lower yoke 101b, and a side yoke 101c. The upper yoke 101a and the lower yoke 101b are arranged so as to sandwich the imaging space 11 in which the subject is accommodated, and the end portions are supported by the side yoke 101c. Further, the upper yoke 101a, the lower yoke 101b, and the side yoke 101c are magnetically connected.

  The permanent magnets 102 are arranged in pairs on the upper yoke 101a and the lower yoke 101b so as to sandwich the imaging space 11 in which the subject is accommodated. The permanent magnet 102 is magnetically connected to the upper yoke 101a and the lower yoke 101b, and a static magnetic field is generated in the imaging space 11 in which the subject is accommodated by the permanent magnet 102 and the yoke 101.

  The magnetic shunt plates 103 are respectively arranged on the surfaces where the pair of permanent magnets 102 are facing each other. The magnetic shunt plate 103 is provided to adjust the static magnetic field generated by the permanent magnet 102 and the yoke 101 in the imaging space 11 in which the subject is accommodated. The magnetic shunt plate 103 is made of a ferromagnetic material such as iron, and includes a bottom plate portion 111 and a protrusion 112. The bottom plate portion 111 is a substantially concentric disk, and an annular protrusion 112 is provided so as to surround the bottom plate portion 111. In the magnetic shunt plate 103, the protruding portion 112 is thicker than the bottom plate portion 111, and the protruding portion 112 has a lower magnetic resistance than the bottom plate portion 111. For this reason, the magnetic shunt plate 103 compensates for the strength of the static magnetic field by the protrusion 112 and makes the static magnetic field uniform.

  As shown in FIG. 2, the gradient coil section 13 is disposed in a pair on the side where the pair of magnetic shunt plates 103 are facing each other. The gradient coil unit 13 forms a gradient magnetic field in the imaging space 11 in which a static magnetic field is formed. The gradient coil unit 13 forms a gradient magnetic field in the imaging space 11 in which a static magnetic field is formed, and adds position information to the magnetic resonance signal received by the RF coil unit 14. In FIG. 2, only one gradient coil unit 13 is shown, but the gradient coil unit 13 forms three types of gradient magnetic fields: a slice selection gradient magnetic field, a read gradient magnetic field, and a phase encoding gradient magnetic field. Have 3 systems.

  As shown in FIG. 1, the RF coil unit 14 is disposed so as to surround the entire head, which is an imaging region of the subject 40, and is configured to be used for both transmission and reception. The RF coil unit 14 transmits an RF signal, which is an electromagnetic wave, in order to excite the spin of protons in the imaging region of the subject 40 in the imaging space 11 in which the static magnetic field is formed by the static magnetic field magnet unit 12 to generate a high frequency. Create a magnetic field. The RF coil unit 14 receives electromagnetic waves generated from the excited protons in the subject 40 as magnetic resonance signals. In addition, although the RF coil unit 14 is used for both transmission and reception in the present embodiment, the transmission coil and the reception coil may be provided independently.

  As shown in FIGS. 1 and 2, the search coil unit 15 is configured as a pair. Each of the search coil portions 15 is disposed at a substantially center between a pair of gradient coils 13 facing each other and corresponding to an end portion of the gradient coil 13. The search coil unit 15 detects the induced electromotive force of the magnetic field generated by the eddy current in the imaging space 11 and outputs it to the eddy current measurement unit 27. The search coil unit 15 is configured to measure eddy currents in three types of gradient magnetic fields: a slice selection gradient magnetic field, a read gradient magnetic field, and a phase encoding gradient magnetic field. Then, the search coil unit 15 is removed after measuring the eddy current.

  The RF drive unit 22 drives the RF coil unit 14 to form a high-frequency magnetic field in the imaging space 11, and includes a gate modulator (not shown), an RF power amplifier (not shown), and an RF oscillator (not shown). Have Based on the control signal from the control unit 25, the RF drive unit 22 modulates the RF signal from the RF oscillator into a signal having a predetermined timing and a predetermined envelope using a gate modulator. The RF signal modulated by the gate modulator is amplified by the RF power amplifier and then output to the RF coil unit 14.

  The gradient driving unit 23 applies a gradient pulse having a predetermined amplitude to the gradient coil unit 13 based on a control signal from the control unit 25 to drive the gradient coil unit 13 so that the gradient magnetic field is generated in the imaging space 11 where the static magnetic field is formed. generate. The gradient drive unit 23 includes three systems of drive circuits (not shown) corresponding to the three systems of gradient coils of the gradient coil unit 13.

  The data collection unit 24 collects a magnetic resonance signal received by the RF coil unit 14 based on a control signal from the control unit 25, and a phase detector (not shown) and an analog / digital converter (not shown). And have. The phase detector phase-detects the magnetic resonance signal received by the RF coil unit 14 using the output of the RF oscillator of the RF drive unit 22 as a reference signal, and outputs it to the analog / digital converter. The analog / digital converter converts the analog magnetic resonance signal output from the phase detector into a digital signal and outputs the digital signal to the data processing unit 31.

  The control unit 25 is configured by a computer, and based on an operation signal input from the operation unit 32 via the data processing unit 31, the RF drive unit 22, the gradient drive unit 23, the data collection unit 24, and the eddy current measurement. A control signal is output to each unit 27 for control. The control unit 25 receives an operation signal based on a predetermined pulse sequence from the operation unit 32 via the data processing unit 31, and sends a control signal to the RF drive unit 22, the gradient drive unit 23, and the data collection unit 24 based on the operation signal. Output the magnetic resonance signal. Further, the control unit 25 receives an operation signal for measuring eddy current from the operation unit 32 via the data processing unit 31 and outputs a control signal to the gradient driving unit 23 and the eddy current measurement unit 27 based on the operation signal. Then, the eddy current is measured.

  The cradle 26 is a table on which the subject 40 is placed, and can be taken in and out of the imaging space 11 by a cradle driving unit (not shown).

  The eddy current measurement unit 27 is provided to measure the eddy current. The eddy current measurement unit 27 is connected to the search coil unit 15, and the search coil unit is based on a control signal from the control unit 25. 15 is driven. The eddy current measurement unit 27 measures the amplitude and time constant of the eddy current based on the time change of the induced electromotive force detected by the search coil unit 15. Note that the eddy current measurement unit 27 measures eddy currents in three types of gradient magnetic fields: a slice selection gradient magnetic field, a reading gradient magnetic field, and a phase encoding gradient magnetic field. The eddy current measuring unit 27 is removed after measuring the eddy current.

  The gradient pulse correction unit 28 is configured by a computer, and is connected to the control unit 25 and the eddy current measurement unit 27. Further, the gradient pulse correction unit 28 includes an eddy current storage unit 29. The eddy current storage unit 29 stores the amplitude and time constant of the eddy current measured by the eddy current measurement unit 27 in correspondence with the amplitude of the gradient pulse. The eddy current storage unit 29 stores, for example, the amplitude of the eddy current and the time constant as a lookup table associated with the amplitude of the gradient pulse. Then, the gradient pulse correction unit 28 corrects the gradient pulse based on the amplitude and time constant of the eddy current stored by the eddy current storage unit 29 corresponding to the amplitude of the gradient pulse. The gradient pulse correction unit 28 acquires a control signal for controlling the gradient driving unit 23 from the control unit 25 and obtains information on the amplitude of the gradient pulse applied to the gradient coil 13 by the gradient driving unit 23. Then, the gradient pulse correction unit 28 uses the aforementioned lookup table stored in the eddy current storage unit 29, and the amplitude of the eddy current corresponding to the amplitude of the gradient pulse applied to the gradient coil 13 by the gradient driving unit 23. And the time constant. Then, the gradient pulse correction unit 28 deforms the shape of the gradient pulse applied to the gradient coil 13 by the gradient driving unit 23 based on the obtained amplitude and time constant of the eddy current, and the influence of the eddy current is small. Correct as follows. The gradient pulse correction unit 28 performs the above-described correction for each of the three types of gradient magnetic fields, ie, the slice selection gradient magnetic field, the reading gradient magnetic field, and the phase encoding gradient magnetic field.

  The data processing unit 31 is configured by a computer. The data processing unit 31 is connected to the operation unit 32 and receives an operation signal from the operation unit 32. The data processing unit 31 is connected to the control unit 25, and outputs an operation signal input to the operation unit 32 by the operator to the control unit 25. The data processing unit 31 is connected to the data collecting unit 24, obtains a magnetic resonance signal collected and output by the data collecting unit 24, performs image processing on the acquired magnetic resonance signal, and outputs image data. Is generated. Then, the data processing unit 31 outputs the generated image data to the display unit 33.

  The operation unit 32 is configured by operation devices such as a keyboard and a mouse. The operation unit 32 is operated by an operator and outputs an operation signal corresponding to the operation to the data processing unit 31. In the operation unit 32, for example, setting items for pulse sequence and setting items for eddy current measurement are input by the operator.

  The display unit 33 is configured by a display device such as a graphic display. The display unit 33 displays a tomographic image of the subject generated based on the magnetic resonance signal from the subject 40. Here, the display unit 33 acquires image data from the data processing unit 31 and displays a tomographic image based on the image data.

  Note that the gradient coil section in the present embodiment corresponds to the gradient coil of the present invention. Further, the gradient driving unit in the present embodiment corresponds to the gradient driving means of the present invention. Further, the eddy current measurement unit in the present embodiment corresponds to the measurement means of the present invention. Further, the gradient pulse correction unit in the present embodiment corresponds to the correction means of the present invention. Further, the eddy current storage unit in the present embodiment corresponds to the storage unit of the present invention. The display unit in the present embodiment corresponds to the display unit of the present invention.

  Hereinafter, an eddy current measurement method for measuring an eddy current using the magnetic resonance imaging apparatus of the present embodiment will be described with reference to FIGS.

  FIG. 3 is a flowchart showing a procedure for creating a lookup table in which the amplitude of the eddy current and the time constant are associated with the amplitude of the gradient pulse in the present embodiment.

  FIG. 4 is a diagram showing a lookup table in the present embodiment. In FIG. 4, FIG. 4 (a) shows a look-up table for the eddy current amplitude and the gradient pulse amplitude. The vertical axis represents the eddy current amplitude and the horizontal axis represents the gradient pulse amplitude. In FIG. 4, FIG. 4B shows a look-up table for the time constant of the eddy current and the amplitude of the gradient pulse, the vertical axis is the time constant of the eddy current, and the horizontal axis is the amplitude of the gradient pulse. It is.

In the gradient magnetic field forming method of this embodiment, as shown in FIG. 3, the amplitude G _k of the test gradient pulse g _k (t) is manipulated in order to test the measurement of the amplitude and time constant of the eddy current. Part 32. Here, the amplitude G _k of the gradient pulse for testing g _k (t) is set to 10 steps (k = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10) at equal intervals. (ST11).

Next, to measure the eddy current components with respect to the gradient pulse _g k of the amplitude _{G k (t) I ki =} A ki · exp (-t / τ ki), and the amplitude _{A ki} of eddy current components _{I ki,} a time constant tau _ki is calculated (ST21). Here, for example, eddy current components are measured with i = 1, 2, 3. First, the operation signal of the gradient pulse g ₁ (t) having the amplitude G ₁ set by the operation unit 32 is output to the control unit 25 via the data processing unit 31. And the control part 25 outputs a control signal to the gradient drive part 23 and the eddy current measurement part 27 based on the operation signal. Then, based on the control signal from the control unit 25, the gradient driving unit 23 applies the gradient pulse g ₁ (t) having the amplitude G ₁ to the gradient coil unit 13 to drive the imaging space in which a static magnetic field is formed. 11 generates a gradient magnetic field. At this time, the eddy current measurement unit 27 drives the search coil unit 15 based on a control signal from the control unit 25. Then, the search coil unit 15 detects the induced electromotive force generated by the gradient magnetic field formed by the gradient coil unit 13 as measurement data and outputs it to the eddy current measurement unit 27. Then, the time variation of the measurement data detected by the search coil unit 15 is fitted to an expression of I _ki = A _ki · exp (−t / τ _ki ) using a nonlinear optimization method such as the least square method, and the vortex The current measuring unit 27 determines the amplitudes A ₁₁ , A ₁₂ , A _{13 of the} eddy current components and the time constants τ ₁₁ , τ ₁₂ , τ ₁₃ . Then, as shown in FIG. 4, the amplitudes A ₁₁ , A ₁₂ , A _{13 of} the eddy current components obtained by the eddy current measuring unit 27 and the time constants τ ₁₁ , τ ₁₂ , τ ₁₃ are converted into gradient pulses g ₁ (t ) is stored in the eddy current storage unit 29 so as to correspond to the amplitude G ₁ of. Then, in the same manner as described above, the amplitude _A 2i _{to A 10i} of eddy current components so as to correspond to the gradient pulse _{g 2 to 10} of each amplitude _G 2 _{~G 10} (t), and the time constant τ _2i ~τ _10i Is stored in the eddy current storage unit 29 to complete a lookup table as shown in FIG. As described above, the eddy currents in the respective gradient magnetic fields of the slice selection gradient magnetic field, the read gradient magnetic field, and the phase encoding gradient magnetic field are measured.

  Hereinafter, an imaging method for imaging a tomographic image of a subject using the magnetic resonance imaging apparatus of the present embodiment will be described.

  First, the subject 40 is placed on the cradle 26. Thereafter, the RF coil unit 14 is installed on the head that is the imaging region of the subject 40. Thereafter, photographing information based on a predetermined pulse sequence is input to the operation unit 32. Then, the operation unit 32 outputs an operation signal based on the photographing information to the control unit 25 via the data processing unit 31.

  Based on the imaging information input to the operation unit 32, the control unit 25 drives the cradle 26 on which the subject 40 is placed in the imaging space 11 in which the static magnetic field is formed by the cradle driving unit. Then, the head that is the imaging region of the subject 40 is carried into the imaging space 11.

At this time, the gradient pulse correction unit 28 acquires a control signal for controlling the gradient driving unit 23 from the control unit 25, and the eddy current storage unit 29 corresponds to the amplitude G _k of the gradient pulse g _k (t). The gradient pulse g _k (t) is corrected based on the eddy current amplitude A _ki and the time constant τ _ki stored as

FIG. 5 is a flowchart for explaining a procedure in which the gradient pulse correction unit 28 corrects the gradient pulse g _k (t). FIG. 6 is a diagram showing a gradient pulse g _k (t) before correction and a gradient pulse g _k ′ (t) after correction. In FIG. 6, the vertical axis indicates the gradient, and the horizontal axis indicates time.

As shown in FIGS. 5 and 6, the gradient pulse correction unit 28 first obtains a control signal for controlling the gradient driving unit 23 from the control unit 25, and the gradient driving unit 23 applies the gradient driving unit 23 to the gradient coil 13. Information on the amplitude G _k in the gradient pulse g _k (t) is obtained (ST111).

Then, the gradient pulse correction unit 28 uses the aforementioned look-up table stored in the eddy current storage unit 29, and the amplitude G _{k of the} gradient pulse g _k (t) applied to the gradient coil 13 by the gradient drive unit 23. Eddy current amplitude A _ki and time constant τ _{ki corresponding} to are obtained (ST121).

Then, the gradient pulse correcting unit 28 determines the shape of the gradient pulse g _k (t) applied to the gradient coil 13 by the gradient driving unit 23 based on the obtained amplitude A _{ki of the} eddy current and the time constant τ _ki . Correction is performed so that the influence of the eddy current is reduced, and a corrected gradient pulse g _k ′ (t) is obtained (ST131). The gradient pulse correction unit 28 _{convolves the} obtained amplitude A _ki and time constant τ _{ki of} the eddy current and the time change rate of the gradient pulse g _k (t), for example, as shown in Equation (1). Then, the result of the convolution and the original gradient pulse g _k (t) are added to obtain a corrected gradient pulse g _k ′ (t).

Thereafter, the control unit 25 controls the gradient drive unit 23 to apply the gradient pulse g _k ′ (t) corrected by the gradient pulse correction unit 28 to the gradient coil unit 13. Then, the control unit 25 controls the RF drive unit 22 to form a high frequency magnetic field by the RF coil unit 14. Then, the control unit 25 causes the RF coil unit 14 to receive the magnetic resonance signal from the subject 40 and causes the data collection unit 24 to collect the magnetic resonance signal. The data collection unit 24 performs phase detection using a phase detector, converts a magnetic resonance signal into a digital signal using an analog / digital converter, and outputs the digital signal to the data processing unit 31. Thereafter, the data processing unit 31 performs image processing on the magnetic resonance signal output from the data collecting unit 24, and the display unit 33 displays an image.

As described above, in the magnetic resonance imaging apparatus of the present embodiment, the gradient pulse correction unit 28 stores the amplitude A _ki and the time constant τ _{ki of the} eddy current corresponding to the amplitude G _k of the gradient pulse g (t). An eddy current storage unit 29 is included. Here, the gradient pulse correction unit 28 is based on the eddy current amplitude A _ki and the time constant τ _ki that the eddy current storage unit 29 stores in correspondence with the amplitude G _k of the gradient pulse g (t). Is corrected so as to reduce the influence of the eddy current magnetic field, and a corrected gradient pulse g ′ (t) is obtained. In the magnetic resonance imaging apparatus, the gradient drive unit 23 applies the corrected gradient pulse g _k ′ (t) to the gradient coil unit 13, and the RF coil unit 14 outputs the magnetic resonance signal from the subject 40. A tomographic image of the subject is generated based on the received magnetic resonance signal. As described above, in this embodiment, the gradient magnetic field is corrected so that the influence of the eddy current magnetic field is reduced, so that the influence of the eddy current is suppressed to prevent distortion of the tomographic image and improve the image quality. Can do.

  In implementing the present invention, the present invention is not limited to the above-described embodiment, and various modifications can be employed.

  For example, in the above embodiment, the case where a permanent magnet is used as the static magnetism generation source is shown, but the present invention is not limited to this, and the present invention can also be applied to the case where a superconducting coil, a normal conducting coil or the like is used. .

  For example, in the above embodiment, the eddy current is measured using the search coil unit. However, the present invention is not limited to this, and the eddy current is measured and obtained from the phase information of the magnetic resonance signal. May be stored in the storage means.

FIG. 1 is a configuration diagram showing the configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention. FIG. 2 is a configuration diagram showing the configuration of the static magnetic field magnet unit according to the embodiment of the present invention.

FIG. 3 is a flowchart showing a procedure for creating a lookup table in the embodiment of the present invention. FIG. 4 is a diagram showing a lookup table in the embodiment according to the present invention. FIG. 5 is a flowchart for explaining the procedure of correcting the gradient pulse in the embodiment according to the invention. FIG. 6 is a diagram showing a gradient pulse before correction and a gradient pulse after correction in the embodiment according to the present invention.

Explanation of symbols

11: Imaging space 12: Static magnetic field magnet unit 13: Gradient coil unit (gradient coil)
14: RF coil section 15: Search coil section 22: RF drive section 23: Gradient drive section (gradient drive means)
24: Data collection unit 25: Control unit 26: Cradle 27: Eddy current measurement unit (measuring means)
28: Gradient pulse correction unit (correction means)
29: Eddy current storage unit (storage means)
31: Data processing unit 32: Operation unit 33: Display unit (display means)
101: Yoke 101a: Upper yoke 101b: Lower yoke 101c: Side yoke 102: Permanent magnet 103: Magnetic shunt plate 111: Bottom plate portion 112: Projection

Claims (6)

A magnetic resonance imaging apparatus that forms a gradient magnetic field and a high-frequency magnetic field in a space in which a subject is accommodated and a static magnetic field is formed, and generates a tomographic image of the subject based on a magnetic resonance signal from the subject. ,
The means for forming the gradient magnetic field comprises:
A gradient coil for generating the gradient magnetic field;
Gradient driving means for driving the gradient coil by applying a gradient pulse to the gradient coil;
Correction means for correcting the gradient pulse so as to reduce the influence of eddy currents in the gradient magnetic field,
The correction means includes storage means for storing the amplitude and time constant of the eddy current that varies nonlinearly with respect to the amplitude of the gradient pulse in correspondence with the amplitude of the gradient pulse, and the storage means includes the gradient pulse. A magnetic resonance imaging apparatus that corrects the gradient pulse based on the amplitude and time constant of the eddy current stored corresponding to the amplitude of the pulse. Measuring means for measuring the amplitude and time constant of the eddy current corresponding to the amplitude of the gradient pulse;
The magnetic resonance imaging apparatus according to claim 1, wherein the storage unit stores an amplitude and a time constant of the eddy current measured by the measurement unit. The gradient driving means applies the gradient pulse of plural kinds of amplitudes to the gradient coil,
The magnetic resonance imaging apparatus according to claim 2, wherein the measurement unit measures eddy currents in the gradient pulses of the plurality of types of amplitudes. The means for forming the static magnetic field includes a pair of permanent magnets that are arranged to face each other so as to sandwich a space in which the subject is accommodated, and generate the static magnetic field. Magnetic resonance imaging device. The means for forming the static magnetic field has a pair of magnetic shunt plates that are arranged on respective surfaces facing the pair of permanent magnets and adjust the static magnetic field generated by the permanent magnets,
The magnetic resonance imaging apparatus according to claim 4, wherein the gradient coil is disposed on each surface of the pair of magnetic shunt plates facing each other. The magnetic resonance imaging apparatus according to claim 1, further comprising display means for displaying a tomographic image of the subject generated based on a magnetic resonance signal from the subject.

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