JP5148146B2 - Magnetic resonance imaging system - Google Patents

Description

The present invention relates to a magnetic resonance imaging equipment having a function of correcting the non-uniformity component of the static magnetic field.

  In magnetic resonance imaging (MRI), extremely high static magnetic field uniformity is required in various imaging methods, and so-called shimming for correcting the static magnetic field inhomogeneity is important. Shimming is roughly divided into passive shimming and active shimming. In passive shimming, an iron piece (shim) or the like is arranged in a static magnetic field generated by a static magnetic field magnet, thereby making the static magnetic field in the imaging region uniform. Active shimming makes the static magnetic field in the imaging region uniform by superimposing the correction magnetic field generated by the shim coil on the static magnetic field generated by the static magnetic field magnet. Since active shimming is important in recent magnetic resonance imaging, this active shimming will be described below and simply referred to as shimming.

Static magnetic field inhomogeneities include zero order components X ⁰ , Y ⁰ , Z ⁰ , primary components X ¹ , Y ¹ , Z ¹ and secondary components X ² , Y ² , Z ² , XY, ZY, ZX, etc. It can be expressed separately for each component. There are also higher order components above the tertiary component.

  The shimming is generally performed for each of the non-uniform components as described above. However, since a shim coil corresponding to one-to-one is required for the component to be corrected, correction is generally performed by narrowing down the component, such as up to the primary component or up to the secondary component.

In the shimming calculation, signal collection for measuring the magnetic field distribution in the region of interest is actually performed, and the magnetic field distribution is developed for each magnetic field component to be shimmed to obtain a correction magnetic field that cancels out the inhomogeneous component of the static magnetic field. Then, a current value to be supplied to each shim coil, that is, a correction value is calculated in order to generate the correction magnetic field. For example, assuming that the reference value of the uniform magnetic field strength is B ₀ , the magnetic field distribution is obtained up to the primary component and shimming is performed. For further simplicity, the X direction and the Y direction are not considered in the expansion formula of the magnetic field distribution for the Z direction. If only the above is considered, the correction value of the first-order component is −C _{1 and} the correction value of _the zero-order component is −− with respect to the expression C ₁ Z + C ₀ + B ₀ as a result of expanding the magnetic field distribution to the first order. It is given as C _0.
Japanese Patent Publication No. 3-51172

  However, since the current that can be set in the shim coil has an upper limit, if the shimming correction value exceeds the allowable upper limit, appropriate correction cannot be performed. Of course, even in these cases, since the magnetic field distribution is calculated by expanding the components, the shimming correction value can be obtained, but because it is the result of calculation under the influence of restrictions such as the allowable current value of the shim coil. There is a fear that the obtained correction magnetic field does not match the actual magnetic field distribution.

  The present invention has been made in consideration of such circumstances, and an object of the present invention is to obtain an appropriate correction magnetic field and improve the static magnetic field uniformity.

The magnetic resonance diagnostic imaging apparatus according to the first aspect of the present invention includes a unit for generating a static magnetic field, a measuring unit for measuring a spatial magnetic field distribution related to the static magnetic field , and a plurality of components to be corrected. A first calculating means for calculating a shimming value based on the intensity distribution characteristic, and a shimming value exceeding a predetermined threshold value for each of the plurality of components among the plurality of shimming values calculated by the first calculating means; And a confirmation means for confirming whether or not a shimming value exceeding the threshold value is included in the confirmation means, the shimming value exceeding the threshold value is calculated by the first calculation means. shimming values for components under the conditions set out in 0, second calculating means for calculating on the basis of the shimming values for components other than the component that defines the shimming value to 0 in the intensity distribution characteristic , When the confirmation means confirms that contains no shimming value that exceeds the threshold value, each of the plurality of shimming values by the first calculation means has calculated the shimming values for use in this shimming When the confirmation means confirms that a shimming value exceeding the threshold is included, the shimming value exceeding the threshold is set to 0 for the current shimming for the component calculated by the first calculation means. And a determination means for determining the shimming value calculated by the second calculation means as a shimming value used for the current shimming .

  According to the present invention, an appropriate correction magnetic field can be obtained, and the static magnetic field uniformity can be improved.

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

  FIG. 1 is a diagram showing a configuration of a magnetic resonance imaging apparatus (MRI apparatus) according to the present embodiment. The MRI apparatus shown in FIG. 1 includes a static magnetic field magnet 1, a gradient magnetic field coil 2, a gradient magnetic field power supply 3, a bed 4, a bed control unit 5, a transmission RF coil unit 6, a transmission unit 7, a reception RF coil unit 8, and a reception unit. 9. A computer system 10, a shim coil unit 11, a shim coil power supply, a shim controller 13 and a sequencer 14 are provided.

  The static magnetic field magnet 1 has a hollow cylindrical shape and generates a uniform static magnetic field in an internal space. As the static magnetic field magnet 1, for example, a permanent magnet, a superconducting magnet or the like is used.

  The gradient magnetic field coil 2 has a hollow cylindrical shape and is disposed inside the static magnetic field magnet 1. The gradient magnetic field coil 2 is a combination of three types of coils corresponding to the X, Y, and Z axes orthogonal to each other. The gradient magnetic field coil 2 generates a gradient magnetic field in which the above three coils are individually supplied with electric current from the gradient magnetic field power supply 3 and the magnetic field intensity changes along the X, Y, and Z axes. The Z-axis direction is, for example, the same direction as the static magnetic field. The gradient magnetic fields of the X, Y, and Z axes respectively correspond to, for example, the slice selection gradient magnetic field Gs, the phase encoding gradient magnetic field Ge, and the readout gradient magnetic field Gr. The slice selection gradient magnetic field Gs is used to arbitrarily determine an imaging section. The phase encoding gradient magnetic field Ge is used to change the phase of the magnetic resonance signal in accordance with the spatial position. The readout gradient magnetic field Gr is used for changing the frequency of the magnetic resonance signal in accordance with the spatial position.

  The static magnetic field magnet 1 and the gradient magnetic field coil 2 are accommodated in a gantry. In the gantry, a cavity is formed inside the gradient magnetic field coil 2, and the inside of this cavity becomes an imaging region.

  The subject P is inserted into the imaging region while being placed on the top plate 41 of the bed 4. The top 41 of the bed 4 is driven by the bed control unit 5 and moves in the longitudinal direction and the vertical direction. Usually, the bed 4 is installed such that the longitudinal direction thereof is parallel to the central axis of the static magnetic field magnet 1.

  The transmission RF coil unit 6 includes at least one coil and is disposed inside the gradient coil 2. The transmission RF coil unit 6 receives a high frequency pulse from the transmission unit 7 and generates a high frequency magnetic field (rotating magnetic field).

  The transmission unit 7 transmits a high-frequency pulse corresponding to the Larmor frequency to the transmission RF coil unit 6.

  The reception RF coil unit 8 includes at least one coil and is disposed inside the gradient coil 2. The reception RF coil unit 8 receives a high-frequency magnetic resonance signal emitted in the process of relaxing the spin of the target nucleus of the subject excited by the influence of the high-frequency magnetic field. An output signal from the reception RF coil unit 8 is input to the reception unit 9. The reception RF coil unit 8 may also be used as the transmission RF coil unit 6.

  The receiving unit 9 amplifies the magnetic resonance signal output from the reception RF coil unit 8, performs quadrature detection, and further performs analog / digital conversion to generate magnetic resonance signal data.

  The computer system 10 includes an interface unit 101, a data collection unit 102, a reconstruction unit 103, a storage unit 104, a display unit 105, an input unit 106, and a main control unit 107.

  The interface unit 101 is connected to the gradient magnetic field power source 3, the bed control unit 5, the transmission unit 7, the reception RF coil unit 8, the reception unit 9, and the like. The interface unit 101 inputs and outputs signals exchanged between these connected units and the computer system 10.

  The data collection unit 102 collects digital signals output from the reception unit 9 via the interface unit 101. The data collection unit 102 stores the collected digital signal, that is, magnetic resonance signal data in the storage unit 104.

  The reconstruction unit 103 performs post-processing, that is, reconstruction such as two-dimensional Fourier transform (2DFT), on the magnetic resonance signal data stored in the storage unit 104, and the spectrum of the desired nuclear spin in the subject P. Find data or image data.

  The storage unit 104 stores magnetic resonance signal data and spectrum data or image data for each patient.

  The display unit 105 displays various information such as spectrum data or image data under the control of the main control unit 107. As the display unit 105, a display device such as a liquid crystal display can be used.

  The input unit 106 receives various commands and information input from the operator. As the input unit 106, a pointing device such as a mouse or a trackball, a selection device such as a mode change switch, or an input device such as a keyboard can be used as appropriate.

  The main control unit 107 includes a CPU, a memory, and the like (not shown), and comprehensively controls the MRI apparatus of the present embodiment.

  The shim coil unit 11 uses high-order inhomogeneous magnetic field components that are not directly or indirectly corrected by the FUC (field uniformity correction) method as correction targets. The shim coil unit 11 includes a plurality of shim coils each having different inhomogeneous magnetic field components of the static magnetic field to be corrected.

  FIG. 2 is a layout view of the laminated state of the shim coils of the shim coil unit 11 as viewed from the axial direction.

  In FIG. 1, the gradient coil 2 and the shim coil unit 11 are shown apart from each other in order to facilitate the distinction. ing.

As shown in FIG. 2, the shim coil unit 11 has a secondary shim structure of “ZX”, “ZY”, “XY”, “X ² -Y ² ”, and “Z ² ”. More specifically, the shim coil unit 11 includes an inner resin layer 20, shim coils 21, 22, 23, 24, 25 and a resin tape layer 26.

The inner resin layer 20 is formed in a cylindrical shape on the outer peripheral surface of the gradient magnetic field coil 2. Each of the shim coils 21, 22, 23, 24, and 25 is formed, for example, by forming a flexible substrate on an insulating base so as to form a required coil pattern. The shim coils 21, 22, 23, 24, 25 are arranged on the outer peripheral surface of the inner resin layer 20 in a state of being sequentially laminated. The shim coil 21 has a coil pattern that generates a magnetic field having substantially the same magnetic field direction as the ZX component of the static magnetic field generated by the static magnetic field magnet 1. The shim coil 22 has a coil pattern that generates a magnetic field having substantially the same magnetic field direction as the ZY component of the static magnetic field generated by the static magnetic field magnet 1. The shim coil 23 has a coil pattern that generates a magnetic field having substantially the same magnetic field direction as the XY component of the static magnetic field generated by the static magnetic field magnet 1. The shim coil 24 has a coil pattern that generates a magnetic field having substantially the same magnetic field direction as the X ² -Y ² component of the static magnetic field generated by the static magnetic field magnet 1. The shim coil 25 has a coil pattern that generates a magnetic field having substantially the same magnetic field direction as the Z ² component of the static magnetic field generated by the static magnetic field magnet 1. The resin tape layer 26 is configured by winding a resin tape on the outer peripheral surface of the shim coil 25. The resin tape layer 26 protects and insulates the shim coils 21, 22, 23, 24, and 25.

  The shim coil unit 11 having the structure shown in FIG. 2 is described in detail in Japanese Patent Laid-Open No. 8-196518.

  With such a configuration, the shim coil unit 11 can generate a five-channel correction magnetic field. There are also realized shim coil units equipped with more shim coils and capable of generating 13-channel and 18-channel correction magnetic fields.

  The shim coil power supply 12 supplies a current (shim current) independently to each of the plurality of shim coils of the shim coil unit 11. The shim coil power supply 12 sets the shim current to a magnitude corresponding to the shimming value output from the shim controller 13.

  The shim controller 13 takes in the magnetic resonance signal data from the receiving unit 9, obtains a spatial magnetic field distribution based on the data, and obtains a shimming value (correction amount) for each component based on the magnetic field distribution. The shim controller 13 supplies the shimming value of the non-uniform component to be corrected by the shim coil unit 11 to the shim coil power supply 12 while changing it according to the movement of the partial area where data is collected. The shim controller 13 supplies the shimming value of the first-order non-uniform component as an offset value to the sequencer 14 while changing it according to the movement of the partial area where data is collected. Further, the shim controller 13 shims the 0th order component by adjusting the reference frequency of the quadrature phase detection in the receiving unit 9 according to the 0th order component, that is, the shimming value with respect to the resonance frequency shift.

  The sequencer 14 controls each operation timing of the transmission unit 7, the reception unit 9, and the gradient magnetic field power supply 3 to execute a pulse sequence for determining shimming values and a pulse sequence for imaging. When executing the pulse sequence for imaging, the sequencer 14 adds the offset value to the reference value and supplies the added value to the gradient magnetic field power supply 3. The gradient magnetic field power supply 3 shims the primary component by supplying a gradient magnetic field current corresponding to the added value to the gradient magnetic field coils of the X, Y, and Z axes.

  Next, the operation of the MRI apparatus configured as described above will be described.

  In this embodiment, both shimming using a shim coil and shimming called a so-called FUC method by giving an offset to a gradient magnetic field are targets. The FUC method directly corrects the first-order inhomogeneous component of the static magnetic field by superimposing an offset on the gradient magnetic fields of the X, Y, and Z axes. In the present embodiment, this FUC method is used. It is possible to indirectly correct higher-order, that is, second-order non-uniform components by using. In the present embodiment, similarly to the FUC method, it is possible to indirectly correct components other than the non-uniform component that each shim coil directly corrects.

  Next, a method for determining shimming values will be described.

There are the following methods for determining the shimming value (shim current value) .

  (1) Obtain a magnetic resonance signal from a target region without superimposing a gradient magnetic field, and obtain a shim current value having the longest signal decay time constant.

  (2) Obtaining a magnetic resonance signal from the target region without superimposing a gradient magnetic field, Fourier transforming this magnetic resonance signal, and obtaining a shim current value that minimizes the frequency band of the converted data.

  (3) Obtain the magnetic field distribution spatially as a phase map, expand (decompose) this magnetic field distribution for each magnetic field component to be shimmed, and obtain a magnetic field strength that makes the magnetic field distribution stable for each magnetic field component. The shim current value required for this is obtained.

  Since the best method among these methods is the method (3), the present embodiment is based on the method (3). In this method, the object of interest is, for example, a very thin 3 mm thick slice region. Assuming that the slice direction is the most common Z direction, if the shim current value for the component representing the non-uniformity in the Z direction is obtained from the magnetic field distribution of only one thin slice region, the accuracy decreases. Is concerned. Therefore, in the present embodiment, this concern is solved by obtaining the shim current value from the magnetic field distribution of a wide area, that is, the entire slice area.

  FIG. 3 is a flowchart showing a procedure for the shim controller 13 to determine the shimming value.

  In step Sa1, the shim controller 13 sets all the components to be corrected as targets for calculating shimming values. In step Sa2, the shim controller 13 calculates a shimming value of each component to be calculated. A known process can be applied to the calculation of the shimming value. In step Sa3, the shim controller 13 confirms whether or not the shimming value obtained by the above calculation includes a value exceeding a predetermined threshold value for each component. The threshold value is set to a value corresponding to the maximum amount of current that the shim coil power supply 12 can supply to each shim coil of the shim coil unit 11 or a value slightly smaller than that.

  If there is no shimming value exceeding the threshold value, the shim controller 13 proceeds from step Sa3 to step Sa4. In step Sa4, the shim controller 13 determines the value calculated as described above as a shimming value to be used for the current shimming.

  On the other hand, if there is a shimming value exceeding the threshold, the shim controller 13 proceeds from step Sa3 to step Sa5. In step Sa5, the shim controller 13 sets the shimming values of some of the components to be corrected to a predetermined value. In step Sa6, the shim controller 13 sets components other than the partial components as calculation targets. In step Sa7, the shim controller 13 calculates the shimming value of each component to be calculated under the condition that the shimming values of some components are set to predetermined values. In step Sa8, the shim controller 13 determines the predetermined value determined for the partial component in step Sa5 and the value calculated in step Sa7 as shimming values for use in the current shimming.

  For example, the partial component may be a component whose value calculated in step Sa2 exceeds a threshold or a higher-order component. The predetermined value may be 0, for example.

  Thus, according to the present embodiment, when shimming is not stable by the conventional method of directly correcting all components, it is possible to perform correction indirectly by limiting the components, thereby further improving the stability of shimming. Can do.

  In addition, the predetermined value may be a threshold value, and correction for some components may be performed within a possible range. However, by setting the predetermined value to 0 as in the present embodiment, a part of the predetermined value may be corrected. If the correction amount for the component is set to 0 and correction is performed with other components, the static magnetic field can be corrected appropriately.

  This embodiment can be variously modified as follows.

  The processing of the above steps Sa1 to Sa4 may be omitted, and the shimming values of some components may be fixed to a predetermined value from the beginning. Thereby, for example, by fixing the shimming value of the component of ZX to a certain value and fixing the shimming value of the second or higher order component other than ZX to 0 and obtaining the shimming values of the remaining components, ZX and It becomes possible to indirectly correct the ZX component and the higher-order component by using only the components less than the second order. In addition, for example, when the shimming value of a component of a specific order is 0 and the shimming values of other components are solved, the component of the specific order can be indirectly corrected with a component other than the specific order as a result. The method of the present invention can increase the degree of freedom of combination of components in shimming correction.

  Further, in order to find a converged solution with higher magnetic field uniformity than the correction value obtained by one shimming calculation, for example, when performing shimming up to the second order, first, the shimming value of the second order component It is also possible to repeat several times that the shimming value of the remaining component is obtained by setting to 0, and then the secondary component that is obtained by fixing the obtained component and is set to 0 first is obtained. Of course, in each shimming calculation, which component is fixed to which value is arbitrary. Thereby, when an optimal solution cannot be obtained by one calculation, a more converged solution can be obtained by repeating the calculation, and the stability of shimming can be further improved.

  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.

1 is a diagram showing a configuration of a magnetic resonance imaging apparatus (MRI apparatus) according to an embodiment of the present invention. The layout which looked at the lamination | stacking state of each shim coil of the shim coil unit 11 in FIG. 1 from the axial direction. The flowchart which shows the procedure in which the shim controller 13 in FIG. 1 determines a shimming value.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet, 2 ... Gradient magnetic field coil, 3 ... Gradient magnetic field power supply, 4 ... Bed, 5 ... Bed control part, 6 ... Transmission coil unit, 7 ... Transmission part, 8 ... Reception coil unit, 9 ... Reception part, DESCRIPTION OF SYMBOLS 10 ... Computer system, 11 ... Shim coil unit, 12 ... Shim coil power supply, 13 ... Shim controller, 14 ... Sequencer.

Claims (3)

Means for generating a static magnetic field;
Measuring means for measuring a spatial magnetic field distribution related to the static magnetic field;
First calculating means for calculating a shimming value for each of a plurality of components to be corrected based on the intensity distribution characteristics;
Confirmation means for confirming whether or not a plurality of shimming values calculated by the first calculation means include a shimming value exceeding a predetermined threshold for each of the plurality of components;
When the confirmation means confirms that a shimming value exceeding the threshold value is included, the shimming value exceeding the threshold value is under the condition that the shimming value for the component calculated by the first calculation means is set to zero. A second calculating means for calculating a shimming value for a component other than the component having the shimming value set to 0 based on the intensity distribution characteristics;
When the confirmation unit confirms that no shimming value exceeding the threshold is included, each of the plurality of shimming values calculated by the first calculation unit is determined as a shimming value to be used for the current shimming. When the confirmation means confirms that a shimming value exceeding the threshold value is included, for the component in which the shimming value exceeding the threshold value is calculated by the first calculation means, 0 is set for the current shimming. A magnetic resonance imaging apparatus comprising: determination means for determining a shimming value to be used and determining a shimming value calculated by the second calculation means as a shimming value to be used for the current shimming . The measuring means spatially determines the magnetic field distribution as a phase map,
The first calculation means expands the magnetic field distribution obtained as the phase map for each magnetic field component to be shimmed, and shimming values required to obtain a magnetic field strength that makes the magnetic field distribution stable for each magnetic field component. The magnetic resonance imaging apparatus according to claim 1, wherein: The magnetic resonance imaging apparatus according to claim 1, wherein the measuring unit measures a magnetic field distribution of the entire plurality of slice regions.

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