JP2005270327A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus for eliminating a disorder generated by the displacement of a resonance frequency following an unbalance of heterogeneity of a static magnetic field. <P>SOLUTION: The magnetic resonance imaging apparatus is equipped with a frequency displacement detecting section 16 which applies the static magnetic field and high-frequency waves on a sample having a smaller time constant than water and a different resonant frequency of water to detect a FID signal generated in the sample, and a frequency transition calculating section 205 which measures a temporal transition of the resonant frequency of the sample from the standard time on the basis of the detected FID signal. Also, a reconstruction section 204, a gradient magnetic field coil driving apparatus 17, a transmission section 18 and a controlling section 202 to control a shim coil controlling section or the like are equipped to correct the influence by the magnetic field heterogeneity in a prescribed space on the basis of the temporal transition of the resonant frequency of the sample from the standard time. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic resonance imaging apparatus for eliminating a problem caused by a change in resonance frequency accompanying disturbance of static magnetic field inhomogeneity.

  A magnetic resonance imaging apparatus utilizes a phenomenon in which energy of a high-frequency magnetic field rotating at a specific frequency is resonantly absorbed when a group of nuclei having a specific magnetic moment is placed in a uniform static magnetic field. It is a device that visualizes chemical and physical microscopic information of a substance or observes a chemical shift spectrum.

  As an image diagnostic technique using this magnetic resonance imaging apparatus, for example, there is a technique called fMRI (functional MRI). This fMRI is a technique for imaging brain functions based on changes in local hemodynamics associated with neural activity, and executes high-input gradient magnetic fields and long-term continuous scanning. In such a scan, the temperature of the iron shim in the gradient magnetic field coil rises, thereby changing the magnetic permeability, and disturbing the static magnetic field inhomogeneity. This disturbance of the static magnetic field inhomogeneity affects imaging as, for example, distortion or position shift of an EPI (Echo Planer Imaging) image.

  As a technique for solving this problem, there has been proposed a method of correcting a position shift by estimating a zero-order component (corresponding to a position shift of the entire image) of a magnetic field distribution from collected echo data. In addition, a method of correcting the resonance frequency shift by hardware has been proposed separately.

  However, the above technique only corrects the 0th-order component of the magnetic field distribution, and does not correct higher-order components higher than the first order. Further, it does not correspond to a change in resonance frequency corresponding to the influence (about 1 Hz / min) on the scan after fMRI imaging. In particular, the latter may cause poor fat suppression (Chess / SPIR).

  The present invention has been made in view of the above circumstances, and eliminates problems caused by displacement of the resonance frequency due to disturbance of inhomogeneity of the static magnetic field, including the influence of the first and higher order components of the magnetic field distribution. An object of the present invention is to provide a magnetic resonance imaging apparatus capable of

  In order to achieve the above object, the present invention takes the following measures.

An aspect of the present invention is an imaging in which a static magnetic field is applied to a subject arranged in a predetermined space, and a high frequency and a gradient magnetic field are applied according to a predetermined pulse sequence to detect a magnetic resonance signal generated in the subject. In a magnetic resonance imaging apparatus that repeatedly executes an operation, the static magnetic field and the high frequency are applied to a substance having a time constant (T ₁ , T ₂ ) smaller than that of the subject tissue and a resonance frequency different from that of the subject tissue. Thus, based on the time transition of the resonance frequency of the substance from the reference time and the time transition of the resonance frequency of the substance from the reference time, the magnetic field in the predetermined space is measured. A magnetic resonance imaging apparatus comprising: a correction unit that corrects the influence of uniformity.

  As described above, according to the present invention, it is possible to realize a magnetic resonance imaging apparatus for solving the problems caused by the displacement of the resonance frequency due to the disturbance of the inhomogeneity of the static magnetic field.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, components having substantially the same function and configuration are denoted by the same reference numerals, and redundant description will be given only when necessary.

  FIG. 1 is a diagram showing a configuration of a magnetic resonance imaging apparatus 10 according to the present embodiment. As shown in FIG. 1, the magnetic resonance imaging apparatus 10 includes a static magnetic field magnet 11, a cooling system control unit 12, a gradient magnetic field coil 13, a whole body radio frequency (RF) coil 14, a high frequency receiving coil 15, and a frequency displacement detection unit 16. , A gradient magnetic field coil drive device 17, a transmission unit 18, a reception unit 19, a calculation device 20, and a display unit 24.

  The static magnetic field magnet 11 is a magnet that generates a static magnetic field, and generates a uniform static magnetic field. For example, a permanent magnet or a superconducting magnet is used as the static magnetic field magnet 11 and is cooled by a cooling system (not shown).

  The cooling system control unit 12 controls a cooling system (not shown) for cooling the static magnetic field magnet 11.

  The gradient magnetic field coil 13 is a magnetic field coil having a shorter axis than the static magnetic field magnet 11 and is provided inside the static magnetic field magnet 11. The gradient coil 13 forms a gradient magnetic field having a linear gradient magnetic field distribution in three directions X, Y, and Z orthogonal to each other based on the pulse current supplied from the gradient coil drive device 17. The signal generation site (position) is specified by the gradient magnetic field generated by the gradient magnetic field coil 13.

  In this embodiment, the Z-axis direction is the same as the direction of the static magnetic field (the body axis direction of the subject). In the present embodiment, the gradient magnetic field coil 13 and the static magnetic field magnet 11 are assumed to be cylindrical. Further, the gradient coil 13 may be disposed in a vacuum by a predetermined support mechanism. This is because the vibration of the gradient magnetic field coil 13 generated by applying the pulse current is not propagated to the outside as a sound wave from the viewpoint of noise reduction.

  The whole-body RF coil 14 is a coil that applies a high-frequency pulse for generating a magnetic resonance signal to the imaging region of the subject. For example, when photographing the abdomen or the like, it is also used as a receiving coil.

  The high-frequency receiving coil (RF receiving coil) 15 is a movable surface coil having a dedicated shape for each part. The high-frequency receiving coil 15 is disposed in the vicinity of the imaging region by a predetermined instrument or a photographer's hand, and receives a magnetic resonance signal generated in the subject.

  A plurality of frequency displacement detectors 16 are provided in the high frequency receiving coil 15 to detect the frequency displacement from the reference time. The frequency displacement detection unit 16 includes a core unit 160 as a sample for detecting frequency displacement, and a minute coil unit 161 that functions as a reception coil.

  FIG. 2 illustrates the configuration of the frequency displacement detector 16, and FIG. 3 illustrates an installation example of the frequency displacement detector 16 on the high frequency receiving coil 15.

As shown in FIG. 2, the frequency displacement detection unit 16 is an antenna for receiving a NMR signal generated in the core unit 160 surrounding the core unit 160 having a small cylindrical shape having a diameter of about 5 mm, for example. It has a certain small coil part 161. The core portion 160 is made of a material having a short T ₁ and T ₂ , generating a proton signal due to magnetic resonance, and a chemical shift amount larger than that of water. Preferable examples include Me4Si (chemical shift amount: 4.37 ppm) and ABS resin (T ₂ = 1.6 ms). In addition, the size of the core portion 160 needs to be sufficiently small to prevent the high-frequency receiving coil 15 from detecting the proton signal generated by the core portion 160.

  As shown in FIG. 3, the frequency displacement detector 16 is installed, for example, at the opening peripheral edge of the high-frequency receiving coil 15. Since the purpose is to detect the distribution of magnetic fields (disturbances in static magnetic field inhomogeneity), the frequency displacement detector 16 is preferably installed symmetrically. Moreover, in order to correct the primary and higher components of the magnetic field inhomogeneity in the coordinate axis directions, it is preferable that at least eight frequency displacement detectors 16 are installed.

  The gradient coil device power source 17 generates a pulse current for forming a gradient magnetic field and supplies the pulse current to the gradient magnetic field coil 13. Further, the gradient coil device power source 17 controls the polarity of the gradient magnetic field by switching the direction of the pulse current supplied to the gradient coil 13 according to the control of the control unit 202.

  The transmitting unit 18 includes an oscillating unit, a phase selecting unit, a frequency converting unit, an amplitude modulating unit, and a high frequency power amplifying unit (each not shown), and applies a high frequency pulse corresponding to the Larmor frequency to the whole body RF coil 14. Send. Due to the high frequency generated from the whole-body RF coil 14 by the transmission, the magnetization of the predetermined nucleus of the subject is in an excited state.

  The reception unit 19 includes an amplification unit, an intermediate frequency conversion unit, a phase detection unit, a filter, and an A / D converter (not shown), and each magnetic resonance received from the high frequency reception coil 15 or the frequency displacement detection unit 16. The signal (high frequency signal) is individually subjected to predetermined signal processing. That is, the reception unit 19 performs amplification processing, intermediate frequency conversion processing using a transmission frequency, phase detection processing, filter processing, A, on the magnetic resonance signal emitted when the nuclear magnetization relaxes from the excited state to the ground state. / D conversion processing is performed.

  The computing device 20 includes a storage unit 201, a control unit 202, a data collection unit 203, a reconstruction unit 204, a frequency transition computation unit 205, and an input unit 207.

  The storage unit 201 stores, for each patient, magnetic resonance image data and the like after reconstruction of magnetic resonance signal data before reconstruction obtained via the reception unit 19.

  The control unit 202 has a CPU, a memory, and the like (not shown), and statically or dynamically controls the magnetic resonance imaging apparatus as a control center of the entire system. In particular, when performing parallel imaging, the control unit 202 performs control for receiving and processing magnetic resonance signals in parallel by a plurality of RF receiving coils having different sensitivity distributions.

  Further, the control unit 202, based on the resonance frequency shift amount in the imaging section estimated by the frequency transition calculation unit 205, the image reconstruction unit 204, the gradient coil device power source 17, the transmission unit 18, and not shown. Each shim coil control unit is controlled.

  The data collection unit 203 collects the digital signal sampled by the reception unit 19.

  The reconstruction unit 204 performs post-processing, that is, reconstruction such as Fourier transform, and obtains spectrum data or image data of a desired nuclear spin in the subject. When parallel imaging is performed, the reconstruction unit 204 reconstructs images from the magnetic resonance signals from the coils, and then uses the sensitivity distribution of the coils to obtain a plurality of images. As post-processing, a development process is performed to generate one image. Here, parallel imaging uses a plurality of RF receiving coils with different sensitivity distributions, executes a sequence with phase encoding thinned out, and performs an unfolding process to remove aliasing artifacts by matrix calculation, thereby shortening the imaging time. Technology.

  The post-processing executed by the reconstruction unit 204 is executed in consideration of the frequency displacement obtained by the frequency transition calculation unit 205 as necessary. That is, the reconfiguration unit 204 executes Fourier transform or the like reflecting the frequency displacement obtained by the frequency transition calculation unit 205.

  The frequency transition calculation unit 205 records a center frequency (hereinafter referred to as CF) detected by each frequency displacement detection unit 16 at a reference time (for example, at shimming time), and by each frequency displacement detection unit 16 at the time of diagnostic image shooting. A difference value from the detected CF is calculated. The frequency transition calculation unit 205 calculates the transition of the resonance frequency from the reference time of the core unit 160 based on the difference value.

  In addition, the frequency transition calculation unit 205 estimates the shift amount of the resonance frequency in the imaging section by the above-described method based on the obtained transition of the resonance frequency from the reference time of the core unit 160.

  The input unit 207 has an input device (mouse, trackball, mode switch, keyboard, etc.) for capturing various instructions, instructions, and information from the operator.

  The display unit 24 is an output device that displays spectrum data, image data, or the like input from the arithmetic unit 22 via the control unit 202.

(Correction process to correct the influence of disturbance of static magnetic field inhomogeneity)
Next, correction processing for correcting the influence of disturbance of static magnetic field inhomogeneity executed by the magnetic resonance imaging apparatus will be described. This correction process is realized by each process of measurement of the resonance frequency shift amount using the frequency displacement detector 16, estimation of the resonance frequency shift amount in the imaging section, and correction of the resonance frequency shift amount. Hereinafter, each process will be described.

(Measurement of resonance frequency shift of sample)
The measurement of the resonance frequency shift amount of the sample (core unit 160) using the frequency displacement detector 16 is executed based on the phase displacement of the received signal (FID signal) from the reference time. That is, for example, if the reference time is set to shimming in the present embodiment, when a spatial non-selection pulse that excites only the core unit 160 without applying the tissue of the subject is applied during the shimming, the core unit 160 is applied. Generates a short T ₁ , T ₂ FID signal (proton signal), and the microcoil unit 161 detects this. The detection timing of the signal by the micro coil unit 161 is controlled by a Pin diode (not shown).

  The reason why the reference time is set to shimming as described above is that the static magnetic field uniformity is the highest during shimming.

  Next, a spatial non-selection pulse, more specifically, an RF pre-pulse according to a timing chart as shown in FIG. 4 is applied before the main imaging, and the FID signal generated from each core unit 160 is converted into each micro coil unit. 161. Thereafter, a spoiler pulse is applied to prevent signals other than the FID signal from each core unit 160 from remaining.

  It should be noted that the measurement of the FID signal from each core unit 160 accompanying the pre-pulse application is preferably executed a plurality of times at a predetermined time interval, preferably just before each main photographing, from the viewpoint of improving accuracy. In addition, it is assumed that the data collection time of the FID signal from each core unit 160 (TE: time from the 90-degree pulse application time to the completion of data collection) is 1 ms to 2 ms.

After data collection, a complex phase difference Δφ [degree] is calculated for 1 to 2 ms (that is, per 1 ms) of each obtained FID signal after detection, and the core unit 160 after a certain time has elapsed from the reference time. The resonance frequency shift amount Δf _core [Hz] is calculated by the following equation (1), assuming that the complex phase difference at the reference time is Δφ0 and the complex phase difference after a predetermined time has elapsed is Δφ1.

However, the unit of t = 0.001 s, Δφ0, Δφ1 is degree. Therefore, for example, when t = 0.001 s and Δφ1−Δφ0 = 2.78 [Hz / degree], the range that can be measured without aliasing is −500 Hz to +500 Hz.

With such a configuration, it is possible to grasp the shift amount (transition) Δf _core with time of the resonance frequency of the core unit 160 from the reference time. In particular, by measuring the FID signal from the core unit 160 in accordance with the spatial non-selection pulse application timing that is performed immediately before each main photographing, the shift amount of the resonance frequency with time in the main photographing performed immediately after the main photographing can be reduced. It can be grasped easily and quickly.

Further, the core portion 160 generates a proton signal by magnetic resonance, and has a chemical shift amount different from that of water (resonance frequency is different from that of water) as shown in FIG. 5, and a material whose T ₁ and T ₂ values are shorter than that of water. (Me4Si, ABS resin, etc.) is used. Therefore, even when a pre-pulse is applied, it is possible to measure the frequency transition of the core unit 160 without affecting the subject tissue to be imaged.

  Further, in the measurement of the FID signal using the frequency displacement detection unit 16, since the core unit 160 is closest, the microcoil 161 surrounding each core unit 160 sufficiently detects the proton signal generated by the core unit 160. be able to. On the other hand, the proton signal generated by the core unit 160 is very small for the high-frequency receiving coil 15 to detect because the size of the core unit 160 is sufficiently small. Therefore, it is not a problem for diagnostic image imaging.

Further, in the diagnostic image imaging pulse, TE is longer than T _{2 of the} core portion 160. For this reason, the frequency displacement detection unit 16 does not detect a signal generated by diagnostic image imaging, and does not cause a problem with the proton signal detection from the core unit 160.

  In this measurement process, for simplification of hardware, it is not necessary to collect data of all sensors at all timings at the same time, connecting multiple sensors to the collection system of one sensor and switching the sensors. May be used.

(Estimation of resonance frequency shift amount in the imaging section)
Regarding the estimation of the resonance frequency shift amount in the imaging section, there are a method by measurement weight addition of the resonance frequency shift amount of the core section 160 and a method by calculation of the expansion coefficient of the spherical elliptic function indicating the static magnetic field distribution.

  The estimation method based on the measurement weighted addition of the resonance frequency shift amount of the core unit 160 is to weight and add the resonance frequency shift values of the sensors installed at n locations (in this embodiment, n = 8). The weighting coefficient corresponding to the weight of the detection value of each sensor is defined according to the position of the sensor and the distance to the imaging section.

That is, if the frequency shift in n sensors is Δfi [Hz] (i = 1, 2,..., N), the frequency shift Δf _slice [Hz] in the imaging section is expressed by the following equation (2). be able to.

The frequency shift Δf _slice in the imaging cross section estimated in this way is used in a resonance frequency shift amount correction process to be described later.

On the other hand, the method of calculating the expansion coefficient of the spherical elliptic function indicating the static magnetic field distribution uses the frequency shift in n sensors using Δfi [Hz] (i = 1,2, ..., n) Is developed into n-th order (n = 1, 2,...) Magnetic field distribution components by the spherical harmonic function at. Here, zero-order (constant), primary (X, Y, Z), 2-order ^{(Z 2, X 2, Y} 2, ZX, ZY, XY) and request the expansion coefficients of the magnetic field distribution components to However, the order of the component for which the expansion coefficient is actually obtained is determined by the number of frequency displacement detectors 16, the installation location, the presence or absence of shim coils, and the like. The zeroth-order component corresponds to the offset of the excitation frequency (center frequency), the first-order component corresponds to the offset of the gradient magnetic field output, and the second-order (and subsequent) higher-order components correspond to current changes in the shim coil.

Each magnetic field distribution component can be obtained by calculating each component of the coefficient matrix A _ij of the matrix calculation represented by the following equation (3).

B _i = A _ij x _j (3)
Here, B _i is a frequency shift amount (i = 1, 2,..., Number of measurement points) at a measurement point, and A _ij is a coefficient matrix (j = 1, 2,..., The number of magnetic field distribution components). ) And x _j is a magnetic field distribution component.

  Using the coefficients of the magnetic field distribution components thus obtained (in this case, the 0th order, 1st order, and 2nd order components), the offset of the excitation frequency at the time of main imaging as a correction process of the resonance frequency shift amount described later, Gradient field output offset and shim coil current are controlled respectively.

(Resonance frequency shift correction process)
A correction process using the resonance frequency shift amount in the imaging section estimated by the frequency transition calculation unit 205 will be described. This correction processing can be classified into two types: a post-processing performed on the magnetic resonance signal obtained by the main photographing and a processing performed before the main photographing.

  The correction processing performed after the main photographing is a post-processing that reflects the frequency shift Δf in the photographing section calculated according to the above-described equation (2), that is, image reconstruction by Fourier transform. In addition, the correction processing performed before the main imaging means that the offset of the excitation frequency (center frequency) is calculated using the zero-order magnetic field distribution component calculated according to the equation (3), and the gradient magnetic field output is calculated using the primary magnetic field distribution component. The offset is used to control the applied current to the shim coil using the primary magnetic field distribution component to correct the distortion of the static magnetic field.

In principle, the two correction processes are not compatible. However, for example, the zero-order magnetic field distribution component is corrected using the frequency shift Δf _slice in the imaging section calculated according to the above-described equation (2), and the remaining first-order and second-order magnetic field distribution components are represented by equation (3). It is also possible to employ a configuration in which a part is combined and executed within a consistent range, such as controlling the offset of the gradient magnetic field output and the applied current to the shim coil using this.

(Shooting operation)
Next, a series of imaging operations including a correction process for correcting the disturbance of the static magnetic field inhomogeneity will be described.

  FIG. 6 is a flowchart showing the flow of each process executed in a series of imaging operations by the magnetic resonance imaging apparatus. As shown in the figure, first, the subject is mounted on the top, the subject is placed at an appropriate position in the static magnetic field magnet 11, and various imaging conditions are input from the input unit 207 (step S1). .

  Next, shimming for adjusting the static magnetic field distribution is performed (step S2), and the spatial non-selection pulse shown in FIG. 4 is applied in conjunction with the shimming to start from the core unit 160 at the reference time (during shimming). FID signals (reference FID signals) are collected (step S3).

  Next, for example, a pre-scan by applying a spatial non-selection pulse for the purpose of measuring the resonance frequency shift amount is performed (step S4), and the FID signal and the reference FID signal detected from the core unit 160 detected immediately thereafter are performed. Based on this, the calculation of the resonance frequency shift amount is executed (step S5).

  Next, based on the calculated resonance frequency shift amount of the core section 160, the correction amount related to the imaging section, that is, the frequency shift amount in the imaging section and each expansion coefficient up to the second order are calculated (step S6). In addition, based on the obtained expansion coefficients, the excitation frequency offset, gradient magnetic field output offset, and shim coil current are set so that the non-uniformity of the magnetic field distribution shifted from the reference time is returned to the reference time state. To perform actual photographing (step S7). At this time, the frequency shift amount in the imaging section is set as the expansion coefficient of the zeroth-order component.

  Next, post-processing such as image reconstruction is performed on the magnetic resonance signal obtained by the main imaging (step S8), and the obtained image is displayed on the display unit 24 (step S9).

  Hereinafter, when the main imaging is repeatedly executed, each process from S4 to S9 is repeatedly executed, and a correction process for correcting the influence due to the disturbance of the static magnetic field inhomogeneity is executed in each main imaging.

  According to the configuration described above, the following effects can be obtained.

  According to this magnetic resonance imaging apparatus, a sample (core part) is used to estimate the resonance frequency shift amount in the imaging section from the reference time immediately before the main imaging, and based on this, post-processing is performed in consideration of the shift amount And at least one of the excitation frequency offset of the imaging section, the offset of the gradient magnetic field output, and the control of the current applied to the shim coil. Therefore, even when the resonance frequency of the imaging cross section changes due to non-uniformity of the static magnetic field during imaging, an MRI image corresponding to an image taken immediately after shimming is always obtained without being affected by this. Can be provided. In particular, even in sequences other than EPI, stable fat suppression can be realized by executing the correction process.

  Further, in the present magnetic resonance imaging apparatus, the above correction processing is executed immediately before each main photographing. Therefore, it is possible to realize a suitable correction process that is suitable for the actual photographing performed immediately after.

  Furthermore, in this magnetic resonance imaging apparatus, the resonance frequency transition of the imaging section due to the static magnetic field inhomogeneity is grasped in hardware using the frequency displacement detection coil. Therefore, the present correction process can be used regardless of the scan sequence without being affected by the sequence. As a result, a suitable MRI image without distortion can always be provided.

  In FIG. 6, the resonance frequency shift amount is measured only once before the main imaging in the pre-scan. However, in the dynamic scan performed continuously in time, the resonance frequency is measured every time phase or every several time phases. Correction may be performed by measuring the shift amount.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

FIG. 1 is a diagram showing a configuration of a magnetic resonance imaging apparatus 10 according to the present embodiment. FIG. 2 illustrates the configuration of the frequency displacement detector 16. FIG. 3 illustrates how the frequency displacement detector 16 is installed. FIG. 4 is a sequence diagram of prepulses applied in the measurement of the resonance frequency shift amount of the core unit 160. FIG. 5 is a diagram showing the distribution of the resonance frequency of the core unit 160 and the resonance frequency of water. FIG. 6 is a flowchart showing the flow of each process that can be executed in a series of imaging operations by the magnetic resonance imaging apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Magnetic resonance imaging apparatus, 11 ... Static magnetic field magnet, 13 ... Gradient magnetic field coil, 14 ... Whole body radio frequency (RF) coil, 15 ... High frequency receiving coil, 16 ... Frequency displacement detection part, 17 ... Gradient magnetic field coil drive device, DESCRIPTION OF SYMBOLS 18 ... Transmission part, 19 ... Reception part, 20 ... Arithmetic unit, 24 ... Display part, 160 ... Core part, 161 ... Micro coil part

Claims (8)

Magnetism that applies a static magnetic field to a subject arranged in a predetermined space, applies a high frequency and a gradient magnetic field according to a predetermined pulse sequence, and repeatedly performs an imaging operation for detecting a magnetic resonance signal generated in the subject. In a resonance imaging apparatus,
By applying the static magnetic field and high frequency to a substance having a time constant (T ₁ , T ₂ ) smaller than that of the subject tissue and a resonance frequency different from that of the subject tissue, a reference time of the resonance frequency of the substance Frequency transition measuring means for measuring the temporal transition from
Correction means for correcting the influence of magnetic field inhomogeneity in the predetermined space based on the temporal transition from the reference time of the resonance frequency of the substance,
A magnetic resonance imaging apparatus comprising:   The magnetic resonance imaging apparatus according to claim 1, wherein the correction unit performs image reconstruction in which a frequency is shifted according to the temporal transition with respect to the magnetic resonance signal detected from the subject. A current shim coil for controlling magnetic field inhomogeneity in the predetermined space;
The correction means includes
Calculate the expansion coefficient of the spherical harmonics based on the time transition,
Controlling at least one of a current for applying the high frequency, a current for applying the gradient magnetic field, and a current applied to the current shim coil based on the expansion coefficient;
The magnetic resonance imaging apparatus according to claim 1. A current shim coil for controlling magnetic field inhomogeneity in the predetermined space;
The correction means includes
For the magnetic resonance signal detected from the subject, perform image reconstruction in which the frequency is changed according to the time change,
Calculate the expansion coefficient of the spherical harmonics based on the time transition,
Controlling at least one of a current for applying the high frequency, a current for applying the gradient magnetic field, and a current applied to the current shim coil based on the expansion coefficient;
The magnetic resonance imaging apparatus according to claim 1.   The frequency transition measuring means includes the substance and a high-frequency coil arranged in the vicinity of the substance, and sensors arranged in at least eight different places near the receiving coil for receiving a magnetic resonance signal from the subject. 5. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is provided.   6. The magnetic resonance imaging apparatus according to claim 1, wherein the substance is Me4Si or ABS resin.   The magnetic resonance imaging apparatus according to claim 1, wherein the reference time is a shimming time using the current shim coil. The frequency transition measuring means performs the measurement of the temporal transition for each photographing operation,
The correction means performs the correction for each shooting operation;
The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus.

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