JP2008022877A - Magnetic resonance imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an economical magnetic resonance imaging device which can correct irregular magnetic fields such as an eddy current magnetic field and/or a residual magnetic field by a simple device constitution, simplify the correcting control of the same, and reduce image deterioration by the irregular magnetic field. <P>SOLUTION: An irregular magnetic field characteristic data of every photographing space component for correcting the irregular magnetic field induced by the impression of a gradient magnetic field is measured (S201). A primary gradient component and a polarized component of the irregular magnetic field in the photographing space are calculated out from the measured characteristic data and the average value of the primary gradient component and the polarized component are calculated out from the calculated value. The current for correcting the irregular magnetic field from the calculated average value is calculated out (S202), and the irregular magnetic field is corrected by superposing the same on a gradient magnetic field current (S203). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus, and in particular, various image quality degradation caused by distortion of a gradient magnetic field waveform or magnetic field fluctuation caused by an eddy current magnetic field generated when a gradient magnetic field is applied or an irregular magnetic field due to a residual magnetic field. The present invention relates to a technique for correcting a spatially and temporally changing magnetic field induced by the irregular magnetic field for preventing the irregularity.

A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) is an apparatus that applies a high-frequency magnetic field to a subject placed in a static magnetic field, thereby collecting and imaging nuclear magnetic resonance signals generated from the subject. In order to add position information to the NMR signal, a gradient magnetic field is applied superimposed on the static magnetic field.
As the gradient magnetic field, a gradient magnetic field in the triaxial direction is usually used, and the timing of applying the gradient magnetic field, the waveform of the gradient magnetic field, and the application amount are determined by the imaging method.

  In an MRI apparatus that obtains a tomographic image by controlling such a gradient magnetic field pulse, the waveform and application timing of the gradient magnetic field pulse are controlled accurately and flexibly, and the desired application amount (the area surrounded by the gradient magnetic field pulse waveform and the time axis). ) Must be applied.

  However, an eddy current magnetic field is induced by the application of the gradient magnetic field pulse, and is generated along with the application of the gradient magnetic field pulse causing the change, and then temporally varies (time-dependent) and has a spatial distribution. (Spatial dependence).

In addition, in an MRI apparatus using a permanent magnet, a ferromagnetic material having a magnetic hysteresis (hysteresis) characteristic is used for the apparatus structure (magnetic circuit). ) Residual magnetic field is also generated.
Even if the application of the gradient magnetic field pulse is stopped, the residual magnetic field remains in accordance with the waveform of the applied gradient magnetic field pulse, the applied direction, and the application history, and as a result, the static magnetic field in the imaging space is complicated. To distort.

  The eddy current magnetic field and residual magnetic field irregular magnetic field generated in this way (unless otherwise misunderstood, the eddy current magnetic field and residual magnetic field are collectively referred to as the irregular magnetic field) and the applied amount of the gradient magnetic field pulse is set to a desired value. As a result, the echo signal intensity is lowered, and artifacts such as distortion and ghost are generated on the reconstructed image, thereby degrading the image quality.

Patent Document 1 discloses this irregular magnetic field correction technique for reducing deterioration of a reconstructed image caused by the irregular magnetic field.
The correction technique of Patent Document 1 is irregular by measuring the eddy current magnetic field induced by the gradient magnetic field pulse and the irregular magnetic field of the residual magnetic field temporally and spatially by changing the direction and waveform of applying the gradient magnetic field pulse. Calibration data relating to various magnetic field dependencies is acquired, and correction current is immediately applied to the shim coil, localized coil, or gradient magnetic field pulse generation system based on the calibration data for correction.
JP 2004-261591 A

  However, the correction of the eddy current magnetic field disclosed in Patent Document 1 can ideally correct the measured eddy current magnetic field, but to implement an eddy current correction system including a shim coil. However, since the device configuration is very complicated, the cost of the device is increased and there is a problem in economic efficiency.

  That is, the time and space dependence of the measured eddy current magnetic field is expressed by, for example, the coefficient of each order term of the spherical harmonic function and its time change, and calibration for correcting the eddy current magnetic field in each of x, y, and z directions. Data is prepared, a correction current for each spatial component is obtained for each direction using this data, and the correction current is supplied to the irregular magnetic field correction coil corresponding to each spatial component to correct the irregular magnetic field.

  In this case, as the irregular magnetic field correction coil, a shim coil according to each order term of the spherical harmonic function is used, and further, a localized coil customized for each MRI apparatus is used. In order to correspond to each order term, the coil becomes complicated and the calibration data becomes enormous.

  As described above, since it is a very complicated device configuration to be implemented using the technique according to Patent Document 1, even if a defective magnetic field component of the second order term or more of the spherical harmonic function actually exists, the imaging space In most cases, only the primary gradient magnetic field component measured at the center of B and the B0 component (constant term), which is a component having no spatial dependence, are compensated.

However, image quality degradation due to the spatial dependence of the eddy current magnetic field exists significantly, and it is often insufficient to compensate only for the primary gradient magnetic field component and B0 component of the eddy current magnetic field measured near the center of the imaging space. . For example, a typical phenomenon is that a useful image quality is obtained near the center of the shooting space, but the image quality deteriorates as the distance from the center increases.
The problem of correcting the eddy current magnetic field is the same in the correction of the residual magnetic field disclosed in Patent Document 1.

  The present invention has been made in view of the above problems, and it is possible to correct an eddy current magnetic field and / or a residual magnetic field, which is an irregular magnetic field, with a simple device configuration and a simple correction control. An object is to provide an economical magnetic resonance imaging apparatus with reduced deterioration.

  In order to achieve the above object, a magnetic resonance imaging apparatus according to the present invention includes a static magnetic field generating unit that applies a static magnetic field to a measurement space, and a gradient magnetic field that applies a gradient magnetic field in each of a slice direction, a phase encoding direction, and a frequency encoding direction. A generating coil, a correction coil for correcting the irregular magnetic field induced by the application of the gradient magnetic field, and characteristic data of the irregular magnetic field for each imaging space component, and a current supplied to the correction coil is controlled based on the characteristic data In the magnetic resonance imaging apparatus provided with the correction magnetic field control means for correcting and controlling the irregular magnetic field, the correction magnetic field control means calculates a first gradient component and a polarization component of the irregular magnetic field from the characteristic data. Means, an average value calculating means for calculating an average value of the primary gradient component and the polarization component calculated by the component calculating means, And correcting the irregular magnetic field and a compensation current calculation means for calculating a current supplied to the correction coils from the average value of the polarization components and calculated the primary gradient components in the average value calculating means.

The component calculation means of the magnetic resonance imaging apparatus is a means for calculating a primary gradient component and a polarization component of the irregular magnetic field in a direction parallel to the axis to be corrected for the irregular magnetic field.
Further, the component calculation means includes an extraction means for extracting the irregular magnetic field data of the excitation cross-sectional area from the characteristic data, and the primary gradient of the excitation cross-section to be corrected for the irregular magnetic field from the irregular magnetic field data extracted by the extraction means It is a means for calculating a component and a polarization component.

  As described above, the irregular magnetic field is corrected using only the primary gradient component and the polarization component of the irregular magnetic field (eddy current magnetic field and / or residual magnetic field) in the imaging space induced by the application of the gradient magnetic field. Further, finer correction than in the prior art is possible, the apparatus configuration and the irregular magnetic field correction control are simplified, and image quality deterioration due to the irregular magnetic field can be reduced.

  In addition, even if the characteristic data is incorrect magnetic field data at a sparse position with respect to the imaging space, the extraction means surrounds at least two sections that are close to the excitation cross section from the incorrect magnetic field data at the sparse position. A configuration may be provided that includes selection means for selecting incorrect magnetic field data of one cross section and means for calculating irregular data of the excitation cross section by interpolation using the incorrect magnetic field data selected by the selection means.

  By correcting the irregular magnetic field in this way, the illegal magnetic field measurement time is reduced and the amount of illegal magnetic field data is reduced, so that the total processing time is reduced, thereby contributing to the improvement of the imaging throughput.

Further, the correction coil is the gradient magnetic field coil, and the correction current is superimposed on a gradient magnetic field current flowing through the gradient magnetic field coil.
Thus, it is not necessary to use a special coil as the correction coil, and the apparatus configuration can be simplified.

  Furthermore, the present invention can also be applied to a magnetic resonance imaging apparatus provided with shim coils and / or localized coils that make the static magnetic field generated by the static magnetic field generating means uniform. In this case, the correction coil is the gradient magnetic field coil and the shim coil and / or the localized coil, and the correction current calculating means uses the correction current calculated by the means as the shim coil and / or the localized coil. What is necessary is just to share and supply to the said gradient magnetic field coil.

  According to the present invention, the irregular magnetic field is corrected using only the primary gradient component and the polarization component of the irregular magnetic field (eddy current magnetic field and / or residual magnetic field) in the imaging space induced by the application of the gradient magnetic field. Therefore, the correction control of the irregular magnetic field is simple and the correction control is also simple, and it is possible to provide a practical magnetic resonance imaging apparatus with reduced image quality deterioration due to the irregular magnetic field.

  And supplying a correction current for the irregular magnetic field superimposed on a gradient magnetic field current flowing through the gradient magnetic field coil, or a shim coil and / or a localized coil for making the gradient magnetic field coil and the static magnetic field generating means uniform. Since the special correction coil is not required, the apparatus configuration can be further simplified and the apparatus cost can be reduced.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

<Overall configuration of MRI system>
First, an outline of an MRI apparatus to which the present invention is applied will be described with reference to FIG.
FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus to which the present invention is applied.
This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject, and as shown in the figure, a static magnetic field generation system 1, a gradient magnetic field generation system 2, a transmission system 3, and a reception system 4 The signal processing system 5, the sequencer 6, the central processing unit (CPU) 7, and the operation unit 8 are configured.

  The static magnetic field generation system 1 generates a uniform static magnetic field in a space around the subject 9 in the body axis direction (horizontal magnetic field method) or in a direction perpendicular to the body axis (vertical magnetic field method). A permanent magnet type, normal conducting type or superconducting type static magnetic field generating means is arranged around the.

  The gradient magnetic field generation system 2 includes a gradient magnetic field coil 10 wound in three axes of X, Y, and Z, and a gradient magnetic field power source 11 that drives each gradient magnetic field coil. Accordingly, when the gradient magnetic field power supply 11 of the coils in the three-axis directions is driven, gradient magnetic field pulses in the respective directions are applied to the subject 9.

  More specifically, a slice direction gradient magnetic field pulse is applied in one of X, Y, and Z to set a slice plane for the subject 9, and the phase encode direction gradient magnetic field pulse and frequency are set in the remaining two directions. Encoding direction gradient magnetic field pulses are applied to encode position information in the respective directions into the echo signal.

The transmission system 3 irradiates a high-frequency magnetic field pulse (a high-frequency pulsed electromagnetic wave, hereinafter referred to as an RF pulse) in order to cause an NMR phenomenon to occur in the nuclear spins of the atoms constituting the biological tissue of the subject 9. It comprises an oscillator 12, a modulator 13, a high frequency amplifier 14, and a high frequency coil 15 on the transmission side.
The high-frequency pulse output from the high-frequency oscillator 12 is amplitude-modulated by the modulator 13 at a timing according to a command from the sequencer 6, amplified by the high-frequency amplifier 14, and then transmitted to the high-frequency coil 15 on the transmission side arranged close to the subject 9 By supplying, the subject 9 is irradiated with the RF pulse.

The receiving system 4 detects an echo signal (NMR signal) emitted by the NMR phenomenon of the nuclear spin constituting the biological tissue of the subject 9, and receives a high-frequency coil 16 on the receiving side, an amplifier 17, and quadrature detection And an A / D converter 19, and an echo signal (NMR signal) that is a response of the subject 9 due to the RF pulse irradiated from the high-frequency coil 15 on the transmitting side is arranged close to the subject 9. It is detected by the high frequency coil 16 on the receiving side.
This echo signal is amplified by the amplifier 17 and then separated into two orthogonal signals by the quadrature detector 18 at the timing according to the command from the sequencer 6, and each is converted into a digital quantity by the A / D converter 19. And sent to the signal processing system 5 as echo data.

  The signal processing system 5 performs image reconstruction calculation using the echo data detected by the reception system 4, and displays and records the obtained image, and includes image reconstruction including Fourier transformation of the echo data. CPU 7 that controls the process and sequencer 6; ROM (read-only memory) 20 that stores time-lapse image analysis processing and measurement programs and invariant parameters used in the execution; and measurement obtained in the previous measurement The RAM (Timely Write Read Memory) 21 that temporarily stores parameters and echo data detected by the receiving system 4 and the image used to set the region of interest and stores the parameters for setting the region of interest, and the CPU 7 The magneto-optical disk 22 and the magnetic disk 24 for recording the configured image data, and the image data read from the magneto-optical disk 22 or the magnetic disk 24 as video And a display 23 for displaying as a tomographic image.

  The sequencer 6 is a control means that repeatedly applies an RF pulse and a gradient magnetic field pulse in a predetermined pulse sequence. The sequencer 6 operates under the control of the CPU 7 and transmits various commands necessary for collecting echo data of a tomographic image of the subject 9. 3. Send to gradient magnetic field generation system 2 and reception system 4.

The operation unit 8 inputs control information for processing performed by the signal processing system 5, and includes a trackball or mouse 25 and a keyboard.
The operation unit 8 is disposed in the vicinity of the display 23, and an operator interactively controls processing of the MRI apparatus through the operation unit 8 while looking at the display 23.

The radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as widely used in clinical practice.
By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  Hereinafter, in the MRI apparatus having the above configuration, eddy current and residual magnetic field characteristics are measured using a known technique (for example, JP-A-10-272120, International Publication WO2004 / 004563), and based on the measured data. The present invention for correcting the irregular magnetic field will be specifically described.

<< First Embodiment >>
FIG. 2 is a flowchart of a process for correcting an eddy current magnetic field and a residual magnetic field according to the first embodiment of the present invention.

(1) Measurement of characteristic data of eddy current magnetic field and residual magnetic field (S201)
First, in step S201, the eddy current magnetic field and the residual magnetic field are measured using the above-described known technique, and a data list in a format suitable for each MRI apparatus is created for the eddy current and the residual magnetic field characteristics using the measurement results. This is stored as characteristic data in the magnetic disk 24 of FIG. The characteristic data is obtained by quantifying variables (time constant, amplitude) that characterize the irregular magnetic field, and this characteristic data is obtained by measuring the spatial dependence of the eddy current and the residual magnetic field including the position information over the imaging space. Is.

The format of the characteristic data may be spatially continuous using three-dimensional measurement as shown by 301 in FIG. 3, or the imaging space is covered by performing two-dimensional measurement at appropriate intervals as shown by 302. May be.
Further, the MRI phantom used for measuring the eddy current and the residual magnetic field may be an arbitrary one that occupies a necessary area of the imaging space.

(2) Calculation of eddy current magnetic field and residual magnetic field correction values (S202)
This step S202 is the main processing in the first embodiment of the present invention, and the correction values of the eddy current magnetic field and the residual magnetic field are calculated by the following processing using the characteristic data measured in step S201.

First, for the eddy current magnetic field correction, a technique disclosed in International WO2004 / 004563, which is a typical known technique, is used.
This is a method of calculating correction values using two points on the reference three planes of the XY plane, YZ plane, and ZX plane in FIG. 3 as the characteristic data format of step S201, for example, an eddy current in the X direction. When obtaining the correction value, the characteristic data of the XZ plane at Y = 0 is used.

That is, assuming that the magnetic field change at position x and time ti is B (x, ti) = g (ti) x + B0 (ti), the magnetic field changes B (x1, ti) and B (x2 at arbitrary two points x1 and x2 , ti), the primary gradient component g (ti) and the polarization component B0 (ti) are calculated using (Expression 1) and (Expression 2).
g (ti) = [B (x2, ti) −B (x1, ti)] / (x2−x1) (1)
B0 (ti) = [B (x2, ti) + B (x1, ti)] / 2− [g (ti) (x2 + x1)] (2)
In the above (Equation 2), if two points satisfying x1 = −x2 are selected, B0 (ti) = [B (x2, ti) + B (x1, ti)] / 2 and B0 (ti) can be calculated more easily. Become.

The difference between the processing in this step S202 and the prior art is that, for example, when obtaining an eddy current correction value in the X direction, the first-order gradient component g (ti) and the polarization component using the above (Formula 1) and (Formula 2) B0 (ti) is calculated for the XZ plane in all Y directions, and an average value thereof is obtained and used as a correction value for the eddy current magnetic field.
That is, assuming that the primary gradient component obtained at each position in the Y direction is g (yj, ti) and the polarization component is B0 (yj, ti), the correction values obtained by this method are (Equation 3), (Equation 3) 4)

  The same processing is applied to the residual magnetic field correction and is performed for all physical axes (X, Y, Z).

(3) Transfer of correction values and correction of eddy current magnetic field and residual magnetic field (S203)
The correction values obtained in step S202 are stored as a data list in an appropriate storage device of the MRI apparatus, for example, the magnetic disk 24 in FIG. 1, and transferred to the irregular magnetic field correction apparatus 30 shown in FIG. 4 before imaging.

  The irregular magnetic field correction device 30 corrects each gradient magnetic field coil 33 (same as 10 in FIG. 1) of the gradient magnetic field generation system 2 in accordance with the time / space dependence of the irregular magnetic field obtained as described above. Immediately apply current to correct irregular magnetic field.

  That is, when the gradient magnetic field pulse waveform calculated by the sequencer 6 is input to the irregular magnetic field adjustment device 30a of the irregular magnetic field adjustment device 30, the irregular magnetic field adjustment device 30a performs steps corresponding to the input gradient magnetic field pulse waveform. The correction current is calculated with reference to the correction data set in the correction data table 30b, which is the correction value data table obtained in S202, and the correction magnetic field is supplied to each gradient magnetic field coil 33 to correct the irregular magnetic field.

  In an actual pulse sequence, gradient magnetic field pulses are applied in the x, y, and z directions, so that the irregular magnetic field induced by the application of gradient magnetic field pulses in each direction is superimposed for each spatial component and changes over time. To go. Therefore, in order to correct such superimposed irregular magnetic fields, the correction data is prepared for each spatial component in each of the x, y, and z directions, and a correction current for each spatial component is obtained for each direction. The correction current obtained for each spatial component is applied to the gradient magnetic field pulse for correction.

<< Second Embodiment >>
FIG. 5 is a flowchart of a process for correcting an eddy current magnetic field and a residual magnetic field according to the second embodiment of the present invention.
The second embodiment of the present invention does not average the correction values and hold them as apparatus-specific correction values as in the first embodiment, but directly uses the MRI apparatus as the correction value according to the position of the imaging space. Is stored in the storage device (for example, the magnetic disk 24 in FIG. 1), and the correction value is extracted from the storage device in accordance with the excitation cross section (imaging position).

(1) Measurement of eddy current magnetic field and residual magnetic field characteristic data (S501)
As in the first embodiment, the eddy current magnetic field and the residual magnetic field are measured using a known technique, and the eddy current and the residual magnetic field characteristics are used for the data list in a format suitable for each MRI apparatus using the measurement result. Is stored in the magnetic disk 24 of FIG. 1 as characteristic data.

(2) Extraction of irregular magnetic field data (S502)
The irregular magnetic field data in the region matching the excitation position / width (excitation section) in the clinical measurement set by inputting from the operation unit 26 in FIG. 1 is extracted from the data list in step S501 before imaging.
For example, in the case of two-dimensional measurement as shown in FIG. 6, data corresponding to the position of the excitation section 601 is extracted from the correction data 602 measured in advance over the imaging space of the MRI apparatus.

(3) Calculation of irregular magnetic field correction value of excitation site (S503)
The average value of the irregular magnetic field data extracted in step S502 is calculated and used as the irregular magnetic field correction value of the excitation site.
This irregular magnetic field correction value is a value averaged over the excitation cross section using the above-described (formula 3) and (formula 4).

(4) Transfer of correction values and correction of eddy current magnetic field and residual magnetic field (S504)
The correction values obtained in step S503 are stored as a data list in an appropriate storage device of the MRI apparatus, for example, the magnetic disk 24 in FIG. 1, and transferred to the irregular magnetic field correction apparatus 30 shown in FIG. 4 before imaging.
The irregular magnetic field correction device 30 corrects each gradient magnetic field coil 33 (same as 10 in FIG. 1) of the gradient magnetic field generation system 2 in accordance with the time / space dependence of the irregular magnetic field obtained as described above. Immediately apply current to correct irregular magnetic field.

In step S502, the correction data 602 shown in FIG. 6 is incorrect magnetic field data measured over the entire imaging space, similar to 301 in FIG. In order to reduce the amount of illegal magnetic field data, illegal magnetic field data at a position sparse with respect to the imaging space may be used as indicated by 702 in FIG.
In this case, incorrect magnetic field data close to the excitation cross section is selected from 702, and in the next step S503, the incorrect magnetic field is obtained by interpolation to determine a correction value.

The processing in steps S502, S503, and S504 is performed for each slice before the start of imaging, and the position-dependent correction value calculated is not permanently stored in the MRI apparatus, but is updated for each imaging.
In the case of 3D imaging, the same processing as in 2D imaging is performed for each slab, and a correction value corresponding to the excitation slab position is calculated to correct the irregular magnetic field.

As described above in the first embodiment and the second embodiment, by correcting the correction magnetic field by appropriately calculating the correction data of the eddy current magnetic field and the residual magnetic field, which are irregular magnetic fields suitable for the imaging conditions, , Finer correction is possible.
Accordingly, the correction control is simplified by a simple apparatus configuration, and it is possible to obtain a practical magnetic resonance imaging apparatus by reducing image quality deterioration due to an irregular magnetic field.

  In the first and second embodiments, the example of correcting the irregular magnetic field of both the eddy current magnetic field and the residual magnetic field has been described. However, when it is not necessary to correct both the eddy current magnetic field and the residual magnetic field, either one of them is corrected. It is sufficient to correct the magnetic field.

Furthermore, the present invention can also be applied to an MRI apparatus provided with shim coils and / or localized coils that correct static magnetic field inhomogeneities due to a static magnetic field generation system.
In this case, the irregular magnetic field correction device 30 ′ composed of the irregular magnetic field adjustment device 30a ′ and the correction data table 30b ′ is configured as shown in FIG. 8, and the correction calculated in the first embodiment or the second embodiment is performed. A correction current based on the data may be distributed to the gradient magnetic field coil 31 (10 in FIG. 1) and the shim coil and / or the local coil 32 so as to correct the irregular magnetic field.

1 is a block diagram showing the overall configuration of an MRI apparatus to which the present invention is applied. 6 is a flowchart of processing for correcting an eddy current magnetic field and a residual magnetic field according to the first embodiment of the present invention. Characteristic data format of eddy current magnetic field and residual magnetic field. Schematic of an irregular magnetic field correction system. 9 is a flowchart of processing for correcting an eddy current magnetic field and a residual magnetic field according to the second embodiment of the present invention. Explanatory drawing which extracts the correction data of an excitation cross section from two-dimensional measurement data. Explanatory drawing which calculates the correction data by selecting the improper magnetic field data close to the excitation cross section from the sparse two-dimensional measurement data. 1 is a schematic diagram of an irregular magnetic field correction system when the present invention is applied to an MRI apparatus equipped with shim coils or localized coils.

Explanation of symbols

  1 Static magnetic field generation system, 2 Gradient magnetic field generation system, 3 Transmission system, 4 Reception system, 5 Signal processing system, 6 Sequencer, 7 Central processing unit (CPU), 8 Operation unit, 9 Subject, 10 Gradient magnetic field coil, 11 Gradient magnetic field power supply, 15 high frequency irradiation coil, 16 high frequency receiving coil, 20 ROM, 21 RAM, 23 display, 24 magnetic disk, 25 trackball or mouse, 26 keyboard, 30 irregular magnetic field correction device, 30a irregular magnetic field correction adjustment device, 30b Correction data table, 31 gradient coils, 32 shim coils and / or localized coils

Claims (6)

  A static magnetic field generating means for applying a static magnetic field to the measurement space, a gradient magnetic field generating means for applying a gradient magnetic field in each of the slice direction, the phase encoding direction, and the frequency encoding direction, and the irregular magnetic field induced by the application of the gradient magnetic field are corrected. Magnetic field comprising characteristic data of the irregular magnetic field for each correction coil and imaging space component, and correction magnetic field control means for correcting and controlling the irregular magnetic field by controlling a current supplied to the correction coil based on the characteristic data In the resonance imaging apparatus, the correction magnetic field control means includes: a component calculation means for calculating a primary gradient component and a polarization component of the irregular magnetic field from the characteristic data; a primary gradient component and a polarization component calculated by the component calculation means; An average value calculating means for calculating an average value, and an average value of the primary gradient component and the polarization component calculated by the average value calculating means Magnetic resonance imaging apparatus comprising a correction current calculating means for calculating a current supplied to the correction coils.   2. The magnetic resonance imaging according to claim 1, wherein the component calculating means is a means for calculating a first-order gradient component and a polarization component of an irregular magnetic field in a direction parallel to an axis to be corrected for the irregular magnetic field. apparatus.   The component calculation means includes extraction means for extracting irregular magnetic field data of the excitation cross section region from the characteristic data, and a primary gradient component of the excitation cross section to be corrected for the irregular magnetic field from the irregular magnetic field data extracted by the extraction means; The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a means for calculating a polarization component.   The characteristic data is fraudulent magnetic field data at a sparse position with respect to the imaging space, and the extraction means includes at least two cross sections surrounding the cross section close to the excitation cross section from the fraudulent magnetic field data at the sparse position. 4. The magnetic resonance imaging apparatus according to claim 3, further comprising: selection means for selecting incorrect magnetic field data; and means for calculating irregular data of the excitation cross section by interpolation using the incorrect magnetic field data selected by the selection means.   5. The magnetic resonance imaging apparatus according to claim 1, wherein the correction coil is the gradient magnetic field coil, and the correction current is supplied by being superimposed on a gradient magnetic field current flowing through the gradient magnetic field coil. 6. .   Further, a shim coil and / or a localized coil that makes the static magnetic field generated by the static magnetic field generating means uniform is provided, and the correction coil is the gradient magnetic field coil, the shim coil, and / or the localized coil, and calculates the correction current. 6. The magnetic resonance imaging apparatus according to claim 5, wherein the means comprises correction current sharing means for supplying the correction current calculated by the means to the shim coil and / or the localized coil and the gradient coil.

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