JP6929667B2 - Magnetic resonance imaging device and static magnetic field correction method - Google Patents

Description

Embodiments of the present invention relate to a magnetic resonance imaging apparatus and a static magnetic field correction method.

A magnetic resonance imaging device, that is, an MRI (Magnetic Resonance Imaging) device is a device that images a subject on a top plate and acquires internal information of the subject as image data.

The MRI apparatus excites the nuclear spin of an imaging site placed in a static magnetic field with a high frequency signal of Larmor frequency, that is, an RF (Radio Frequency) signal. Then, the MRI apparatus receives the magnetic resonance signal generated from the imaging site in association with the excitation, that is, the MR (Magnetic Resonance) signal with the coil, and reconstructs the image data to generate the image data.

In order to obtain an image with good image quality with an MRI apparatus, spatial uniformity of a static magnetic field is required. Therefore, for example, when the MRI apparatus is installed, the metal shim is arranged at a predetermined position in the gradient magnetic field coil unit to correct the uniformity of the static magnetic field.

On the other hand, during the operation of the MRI apparatus, the temperature of the metal shim also rises as the temperature of the gradient magnetic field coil rises, and the magnetic permeability of the metal shim changes. As the magnetic permeability changes, the static magnetic field also changes, resulting in a shift in the center frequencies of, for example, water and fat. Such a shift of the center frequency is also called an F0 shift.

Japanese Unexamined Patent Publication No. 2012-249765

An object to be solved by the present invention is to provide an MRI apparatus and a static magnetic field correction method capable of suppressing a center frequency shift while maintaining the uniformity of the static magnetic field.

The MRI apparatus according to the present embodiment includes a static magnetic field magnet that generates a static magnetic field, and a metal shim that corrects the uniformity of the static magnetic field by arranging a predetermined number at each position in the static magnetic field. The metal shim comprises a coil that generates a magnetic field by supplying power and corrects the uniformity of the static magnetic field by active shiming, regardless of whether or not the uniformity of the static magnetic field meets a predetermined criterion. The predetermined number at each of the above positions is determined so that the central frequency shift of the magnetic resonance signal due to the temperature change of the metal shim falls within a predetermined range.

The schematic diagram which shows the whole structure of the MRI apparatus which concerns on this embodiment. The figure which shows the cross section of the gantry device provided in the MRI apparatus which concerns on this embodiment, that is, the I-I cross section of the gantry device shown in FIG. The perspective view for demonstrating the schematic structure of the gradient magnetic field coil unit provided in the MRI apparatus which concerns on this embodiment. FIG. 3 is a perspective view for explaining a detailed configuration example of a plurality of shim trays provided in the MRI apparatus according to the present embodiment. The figure which shows the number of metal shims provided in the MRI apparatus which concerns on this embodiment. The figure which shows the degree of influence on the F0 shift for each pocket in the MRI apparatus which concerns on this embodiment. The figure which shows the procedure of the static magnetic field correction method in the MRI apparatus which concerns on this embodiment as a flowchart. The figure which shows the concept of the measurement target area in the MRI apparatus which concerns on this embodiment. (A) to (D) are diagrams showing an example of a change in the distribution of the magnetic field in the Z-axis direction in the MRI apparatus according to the present embodiment. The figure which shows the procedure of the static magnetic field correction method in the 1st modification of the MRI apparatus which concerns on this Embodiment as a flowchart. (A) to (C) are diagrams showing an example of a change in the distribution of the magnetic field in the Z-axis direction in the first modification of the MRI apparatus according to the present embodiment. (A) and (B) are diagrams showing an example of a change in the distribution of the magnetic field in the Z-axis direction in the prior art. The figure which shows the procedure of the static magnetic field correction method in the 2nd modification of the MRI apparatus which concerns on this Embodiment as a flowchart. (A) to (C) are diagrams showing an example of a change in the distribution of the magnetic field in the Z-axis direction in the second modification of the MRI apparatus according to the present embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a schematic view showing the overall configuration of the MRI apparatus according to the present embodiment.

FIG. 1 shows an MRI apparatus 1 according to the present embodiment. The MRI apparatus 1 includes a magnet mount 100, a control cabinet 300, a sleeper apparatus 400, and a console 500. The magnet mount 100, the control cabinet 300, and the sleeper device 400 are generally provided in a soundproofed inspection room. The examination room is also called the photography room. The console 500 is provided in the control room. The control room is also called the operation room.

The magnet mount 100 includes a static magnetic field magnet 10, a shim coil unit 11, a gradient magnetic field coil unit 12, and a WB (Whole Body) coil 13. These members are housed in a cylindrical housing.

The control cabinet 300 includes a gradient magnetic field power supply 31 (X-axis 31x, Y-axis 31y, Z-axis 31z), an RF transmitter 32, an RF receiver 33, and a sequence controller 34.

The static magnetic field magnet 10 of the magnet mount 100 is roughly classified into a tunnel type in which the magnet has a cylindrical magnet structure and an open type in which a pair of magnets are arranged one above the other with an imaging space in between. Here, a case where the static magnetic field magnet 10 is a tunnel type will be described, but the case is not limited to that case.

The static magnetic field magnet 10 has a substantially cylindrical shape, and generates a static magnetic field in a bore in which a subject, for example, a patient is carried. The bore is the space inside the cylinder of the magnet mount 100. The static magnetic field magnet 10 is composed of a housing for holding liquid helium, a refrigerator for cooling the liquid helium to an extremely low temperature, and a superconducting coil inside the housing. The static magnetic field magnet 10 may be composed of a normal conduction magnet or a permanent magnet. Hereinafter, a case where the static magnetic field magnet 10 has a superconducting coil will be described.

The static magnetic field magnet 10 has a built-in superconducting coil, and the superconducting coil is cooled to an extremely low temperature by liquid helium. The static magnetic field magnet 10 generates a static magnetic field by applying a current supplied from a static magnetic field power source to the superconducting coil in the excitation mode. After that, when the mode shifts to the permanent current mode, the static magnetic field power supply is disconnected. Once transitioned to the permanent current mode, the static magnetic field magnet 10 continues to generate a large static magnetic field for a long period of time, for example, one year or more.

The shim coil unit 11 has a substantially cylindrical shape like the static magnetic field magnet 10, and is installed inside the static magnetic field magnet 10. The shim coil unit 11 targets the non-uniform component of the static magnetic field as a correction target. The shim coil unit 11 includes a plurality of shim coils (for example, 5 channels shown in the lower part of FIG. 2) having different non-uniform components of the static magnetic field to be corrected.

The gradient magnetic field coil unit 12 has a substantially cylindrical shape like the static magnetic field magnet 10, and is installed inside, for example, the shim coil unit 11. The gradient magnetic field coil unit 12 applies a gradient magnetic field to the patient by the electric power supplied from the gradient magnetic field power supply 31. The gradient magnetic field coil unit 12 may be configured to correct, for example, the first-order non-uniform component of the static magnetic field.

Here, since the eddy current generated by the generation of the gradient magnetic field hinders the imaging, ASGC (Actively Shielded Gradient Coil) for the purpose of reducing the eddy current is used as the gradient magnetic field coil unit 12. The gradient magnetic field coil unit 12 includes a main coil 12a (shown in the lower part of FIG. 2) for generating each gradient magnetic field in the X-axis, Y-axis, and Z-axis directions, and a shim tray capable of accommodating a plurality of metal shims. It is a gradient magnetic field coil provided with a unit 12b (shown in the lower part of FIG. 2) and a shield coil 12c (shown in the lower part of FIG. 2) for suppressing a leakage magnetic field.

Here, a configuration example of the static magnetic field magnet 10, the shim coil unit 11, and the gradient magnetic field coil unit 12 will be described with reference to FIGS. 2 to 5.

FIG. 2 is a diagram showing a cross section of the gantry device provided in the MRI apparatus according to the present embodiment, that is, an I-I cross section of the gantry device shown in FIG. The upper part of FIG. 2 shows the I-I cross section of the gantry device shown in FIG. 1, and the lower part of FIG. 2 is an enlarged view of the broken line portion of the upper part.

As shown in FIG. 2, the shim coil unit 11 has a substantially cylindrical shape like the static magnetic field magnet 10, and is installed inside the static magnetic field magnet 10. Shim coil unit 11, as shown in the lower part of FIG. 2, "ZX", "ZY", forms the structure of the secondary shim "XY", ^"X 2 -Y ^2", and ^{"Z 2".} The shim coil unit 11 includes a resin layer formed in a cylindrical shape, a ZX channel shim coil 11a wound on the outer peripheral surface thereof, a ZY channel shim coil 11b wound on the outer peripheral surface thereof, and an outer peripheral surface thereof. and shim coils 11c of XY channels that are wound on, the shim coil 11d of wound on the ^X 2 -Y ² channels on its outer peripheral surface, a shim coil 11e of the ^{Z 2} channels that are wound on the outer circumferential surface, It has a resin layer formed on its outer peripheral surface.

With such a configuration, the shim coil unit 11 can generate a correction magnetic field of 5 channels. The order of winding the 5-channel shim coils 11a to 11e is not limited to this case. Further, the shim coil unit 11 may include shim coils having more than 5 channels, for example, shim coils capable of generating a correction magnetic field of 13 channels or 18 channels.

Each of the 5-channel shim coils 11a to 11e forms a flexible substrate so as to form a required coil pattern on, for example, an insulating base. Then, the 5-channel shim coils 11a to 11e are arranged on the outer peripheral surface of the inner resin layer in a state of being laminated in order.

The ZX channel shim coil 11a has a coil pattern in which a magnetic field having a magnetic field direction substantially the same as the ZX component of the static magnetic field generated by the static magnetic field magnet 10 is generated as a correction magnetic field. The ZY channel shim coil 11b has a coil pattern in which a magnetic field having a magnetic field direction substantially the same as the ZY component of the static magnetic field generated by the static magnetic field magnet 10 is used as a correction magnetic field. The shim coil 11c of the XY channel has a coil pattern in which a magnetic field having a magnetic field direction substantially the same as the XY component of the static magnetic field generated by the static magnetic field magnet 10 is generated as a correction magnetic field. X ² -Y ² channel shim coil 11d has a coil pattern to generate a magnetic field having substantially the same magnetic field direction as X ² -Y ² component of the generated static magnetic field by the static magnetic field magnet 10 as a correction magnetic field. The Z ² channel shim coil 11e has a coil pattern in which a magnetic field having a magnetic field direction substantially the same as ^{the Z 2} component of the static magnetic field generated by the static magnetic field magnet 10 is generated as a correction magnetic field.

Correcting the non-uniformity of the static magnetic field by supplying electric power to the shim coils 11a to 11e of the shim coil unit 11 to generate a correction magnetic field is called active shimming. Since the patient is transported into the bore during imaging, the uniformity of the static magnetic field is disturbed by the influence of the patient. Therefore, by active shimming using the shim coil unit 11, it is possible to correct the non-uniformity of the static magnetic field while the patient to be imaged is conveyed in the bore.

As described above, each shim coil 11a to 11e actively shims the secondary component of the non-uniformity of the static magnetic field manually or by a computer by the electric power supplied from the shim power source (not shown). The shim coils 11a to 11e correct the static magnetic field formed by the static magnetic field magnet 10 so as to satisfy the desired uniformity in a predetermined three-dimensional region.

Further, the gradient magnetic field coil unit 12 has a main coil 12a for generating each gradient magnetic field in the X-axis, Y-axis, and Z-axis directions, and a shim tray arranged outside the main coil 12a and capable of accommodating a plurality of metal shims. It includes a unit 12b and a shield coil 12c arranged outside the unit 12b for suppressing a leakage magnetic field.

FIG. 3 is a perspective view for explaining a schematic configuration of a gradient magnetic field coil unit 12 provided in the MRI apparatus according to the present embodiment. FIG. 3 shows a configuration example of the gradient magnetic field coil unit 12 shown in FIG.

As shown in FIG. 3, the shim tray unit 12b of the gradient magnetic field coil unit 12 has a substantially cylindrical shape, and is sandwiched between the substantially cylindrical main coil 12a and the shield coil 12c. A plurality of slots 71 are formed at substantially equal intervals in the circumferential direction of the shim tray unit 12b. The number of slots 71 formed in the shim tray unit 12b is not particularly limited, but FIG. 3 shows a case where the number of slots 71 is 24.

The slot 71 is a through hole formed by forming openings on both end surfaces of the shim tray unit 12b and forming the entire length in the longitudinal direction (major axis direction) of the shim tray unit 12b. A shim tray 72 can be inserted into the slot 71. The shim tray 72 is fixed to the substantially central portion of the shim tray unit 12b. The substantially central portion of the shim tray unit 12b in the Z-axis direction is also the central portion of the gradient magnetic field coil unit 12 in the Z-axis direction. The shim tray 72 is formed of, for example, a resin which is a non-magnetic and non-conducting material, and has a substantially rod shape.

FIG. 4 is a perspective view for explaining a detailed configuration example of a plurality of shim trays 72 provided in the MRI apparatus according to the present embodiment.

As shown in FIG. 4, a plurality of pockets 72a are formed at predetermined intervals in the longitudinal direction of the shim tray 72. The number of pockets 72a is not particularly limited.

A required number of metal shims 72b are housed in the pockets 72a for the purpose of equalizing the static magnetic field of the imaging region in the bore. The material of the metal shim 72b is, for example, a silicon steel plate or a permendur (alloy of iron and cobalt). Adjusting the number of metal shims 72b stored in each pocket 72a when the MRI apparatus 1 is installed to make the static magnetic field in the imaging region in the bore uniform is called passive shiming.

The static magnetic field magnet 10 (shown in FIG. 2) is designed and manufactured so that the static magnetic field in the bore is as uniform as possible, but in reality, it is affected by magnet manufacturing errors and structures around the bore. Therefore, it is difficult to make the static magnetic field completely uniform without some adjustment. In addition, the degree of non-uniformity of the static magnetic field differs depending on the individual device and also depends on the surrounding environment of the device installation location. For this reason, passive shim using the metal shim 72b is usually performed for each installation of the device.

FIG. 5 is a diagram showing the number of metal shims 72b provided in the MRI apparatus according to the present embodiment in association with the pocket number and the slot number.

In FIG. 5, a different slot number i is assigned to the slot 71 (shown in FIG. 3) for each position in the circumferential direction. For example, when the number of slots 71 is 24, a number from 1 to 24 is assigned to the slot number i. Further, with respect to the pocket 72a (shown in FIG. 4), a different pocket number j is assigned to each position in the Z-axis direction. For example, when the number of pockets 72a is 15, a number from 1 to 15 is assigned to the pocket number j. The number of metal shims 72b at the position of slot number i and the position of pocket number j is written as _{Si, j.} Hereinafter, a case where the number of slots 71 is 24 and the number of pockets 72a is 15 will be described as an example.

Returning to the description of FIG. 1, the WB coil 13, also called a whole-body RF (Radio Frequency) coil, is installed inside the gradient magnetic field coil unit 12 in a substantially cylindrical shape so as to surround the patient. The WB coil 13 transmits the RF pulse transmitted from the RF transmitter 32 toward the patient. On the other hand, the WB coil 13 receives, for example, a magnetic resonance signal emitted from a patient by excitation of hydrogen nuclei, that is, an MR (Magnetic Resonance) signal.

The MRI apparatus 1 may include a local coil 20 as shown in FIG. 1 in addition to the WB coil 13. The local coil 20 is also called a local RF coil. The local coil 20 is placed close to the patient's body surface. The local coil 20 may include a plurality of coil elements. Since these plurality of coil elements are arranged in an array inside the local coil 20, they are sometimes called a PAC (Phased Array Coil).

There are several types of local coil 20. For example, the local coil 20 includes a body coil (Body Coil) installed on the chest, abdomen, or leg of the patient as shown in FIG. 1 and a spine coil (Spine Coil) installed on the back side of the patient. There is a type. In addition, the local coil 20 also has a type such as a head coil for imaging the patient's head and a foot coil for imaging the foot. The local coil 20 also includes a wrist coil (Wrist Coil) for imaging the wrist, a knee coil (Knee Coil) for imaging the knee, and a shoulder coil (Shoulder Coil) for imaging the shoulder. Many types of the local coil 20 are reception-only coils, but some local coils 20 are transmission / reception coils that perform both transmission and reception. For example, the transmission / reception coil also exists in the head coil and the knee coil as the local coil 20.

The gradient magnetic field power supply 31 includes gradient magnetic field power supplies 31x, 31y, and 31z for each channel that drive the coils that generate the X-axis, Y-axis, and Z-axis gradient magnetic fields. The gradient magnetic field power supplies 31x, 31y, and 31z output necessary current waveforms independently for each channel according to the command of the sequence controller 34. Thereby, the main coil 12a of the gradient magnetic field coil unit 12 can apply the gradient magnetic field in the X-axis, Y-axis, and Z-axis directions to the patient.

The RF transmitter 32 generates an RF pulse based on an instruction from the sequence controller 34. The RF pulse is transmitted to the WB coil 13 and applied to the patient. An MR signal is generated from the patient by applying an RF pulse. This MR signal is received by the local coil 20 or the WB coil 13.

The MR signal received by the local coil 20, more specifically, the MR signal received by each coil element in the local coil 20 is transmitted to the RF receiver 33. When the local coil 20 is configured to be able to transmit an MR signal to the RF receiver 33 via a cable, the MR signal received by each coil element is sent to the RF receiver 33 via a cable provided inside the sleeper body 40. Be transmitted. The output path of each coil element and the output path of the WB coil 13 are called channels. Therefore, each MR signal output from each coil element or the WB coil 13 may be referred to as a channel signal. The channel signal received by the WB coil 13 is also transmitted to the RF receiver 33. The MR signal received by each coil element in the local coil 20 may be wirelessly transmitted to the RF receiver 33.

The RF receiver 33 AD (Analog to Digital) converts the channel signal from the local coil 20 and the WB coil 13, that is, the MR signal, and outputs the channel signal to the sequence controller 34. The digitally converted MR signal is sometimes called raw data.

The sequence controller 34 takes an image of the patient by driving the gradient magnetic field power supply 31, the RF transmitter 32, and the RF receiver 33, respectively, under the control of the console 500 described later. When the sequence controller 34 receives the raw data from the RF receiver 33 by imaging, the sequence controller 34 transmits the raw data to the console 500.

The sequence controller 34 includes a processing circuit (not shown). This processing circuit is composed of, for example, a processor that executes a predetermined program and hardware such as an FPGA (Field Programmable Gate Array) and an ASIC (Application Specific Integrated Circuit).

The sleeper device 400 has a sleeper body 40 and a top plate 41. The sleeper body 40 can move the top plate 41 in the vertical direction and the horizontal direction. Before imaging, the patient placed on the top plate 41 is moved to a predetermined height. After that, at the time of photographing, the top plate 41 is moved in the horizontal direction to move the patient into the opening of the magnet mount 100.

The console 500 includes a processing circuit 50, a storage circuit 51, a display 52, and an input circuit 53. The console 500 functions as a host computer.

The processing circuit 50 means a processing circuit such as a dedicated or general-purpose CPU (Central Processing Unit) or MPU (Micro Processor Unit), an integrated circuit for a specific application (ASIC), and a programmable logic device. Examples of the programmable logic device include circuits such as a simple programmable logic device (SPLD: Simple Programmable Logic Device), a complex programmable logic device (CPLD: Complex Programmable Logic Device), and a field programmable gate array (FPGA). The processing circuit 50 realizes a function described later by reading and executing a program stored in the storage circuit 51 or directly incorporated in the processing circuit 50.

Further, the processing circuit 50 may be composed of a single processing circuit or a combination of a plurality of independent processing circuits. In the latter case, the storage circuit 51 may have a plurality of storage circuits corresponding to the plurality of processing circuits, and the storage circuit 51 may have one storage circuit corresponding to the plurality of processing circuits. May be good.

The storage circuit 51 includes semiconductor memory elements such as a RAM (Random Access Memory) and a flash memory (Flash Memory), a hard disk, an optical disk, and the like. The storage circuit 51 may include a portable medium such as a USB (Universal Serial bus) memory and a DVD (Digital Video Disk). The storage circuit 51 stores various processing programs (including an OS (Operating System) and the like in addition to the application program) used in the processing circuit 50, data necessary for executing the program, and medical images. Further, the OS may include a GUI (Graphical User Interface) that makes extensive use of graphics for displaying information on the display 52 to the operator and allows basic operations to be performed by the input circuit 53.

The display 52 is a display device such as a liquid crystal display panel, a plasma display panel, and an organic EL (Electro Luminescence) panel.

The input circuit 53 is a circuit for inputting a signal from an input device such as a pointing device (mouse or the like) or a keyboard that can be operated by an operator, and here, the input device itself is also included in the input circuit 53. .. When the input device is operated by the operator, the input circuit 53 generates an input signal corresponding to the operation and outputs the input signal to the processing circuit 50. The MRI apparatus 1 may include a touch panel in which the input device is integrally configured with the display 52.

Subsequently, the static magnetic field correction method in the MRI apparatus 1 according to the present embodiment will be described.

First, the relationship between the metal shim 72b and the F0 shift will be described. FIG. 6 shows the degree of influence of each pocket 72a on the F0 shift.

The horizontal axis of the graph of FIG. 6 is the pocket number j, and the vertical axis is the F0 shift influence coefficient Kj. The F0 shift influence coefficient Kj is an amount obtained by normalizing the F0 shift amount per unit number of metal shims 72b arranged in each pocket 72a with the F0 shift amount of pocket number 8. The F0 shift amount is a value obtained by obtaining the value at which F0 changes for each pocket 72a when the same temperature change is applied to the same number of metal shims 72b.

Referring to the graph of FIG. 6, the absolute value of the F0 shift influence coefficient Kj is large near the pocket number 8, that is, near the center of the magnetic field, and the absolute value of the F0 shift influence coefficient Kj is small near the end of the magnet away from the center of the magnetic field. .. Further, the F0 shift influence coefficient Kj of the metal shim 72b arranged near the magnet end portion away from the magnetic field center is opposite to the F0 shift influence coefficient Kj of the metal shim 72b near the magnetic field center. For example, when the metal shim 72b is warmed, the metal shim 72b near the center of the magnetic field raises F0, and the metal shim 72b near the end of the magnet away from the center of the magnetic field lowers F0. As described above, the metal shim 72b has a different direction and size for changing the F0 shift depending on the position in the Z-axis direction from the center of the magnetic field.

In passive shim, the arrangement of the metal shim 72b is determined so that the non-uniformity of the static magnetic field falls within a predetermined allowable range at the installation stage. In general, since many metal shims 72b are often arranged near the center of the magnetic field, the F0 shift amount tends to increase when the temperature of the metal shims 72b rises as a whole. If the amount of the metal shim 72b arranged near the end of the magnet away from the center of the magnetic field can be increased, the F0 shift in the upward direction due to the metal shim 72b near the center of the magnetic field can be suppressed.

At the time of completing the passive shimming, if the non-uniformity of the static magnetic field is to be finally satisfied, the amount of metal shims 72b that can be placed near the end of the magnet away from the center of the magnetic field is limited. be. In the method for adjusting the static magnetic field according to the present embodiment, the amount of metal shims 72b that can be arranged near the magnet end portion away from the center of the magnetic field is increased on the premise that the correction is finally performed by active shimming. At the time of completing the passive shimming, the non-uniformity of the static magnetic field is not within the tolerance that should be finally settled, but the active shimming that follows the passive shimming finally corrects the static magnetic field.

FIG. 7 is a diagram showing a procedure of the static magnetic field correction method in the MRI apparatus 1 according to the present embodiment as a flowchart.

First, when the MRI apparatus 1 is installed, the magnetic field characteristics of the static magnetic field magnet 10 are measured. For example, the magnetic fields at multiple points in the center of the bore are measured in a state where all of the plurality of shim pockets 72a are empty, that is, in a state where no metal shims 72b are arranged (step ST1). Since the time of step ST1 is before passive shimming, the uniformity of the measured static magnetic field is not guaranteed. The measurement is performed using an arbitrary magnetic field measuring device. Further, in the measurement of the magnetic field, the shim pockets 72a do not necessarily have to be all empty, and some metal shims 72b may be provided in advance.

FIG. 8 is a diagram showing a concept of a measurement target region in the MRI apparatus 1 according to the present embodiment.

As shown in FIG. 8, it is assumed that there is a sphere having a predetermined diameter (for example, a sphere having a diameter of 50 cm) near the center of imaging. _{The magnetic field B Z_bare} (r, θ, φ) of the measurement target region including this sphere is measured in the spherical coordinate system shown in FIG.

Returning to the description of FIG. 7, the processing circuit 50 (or a computer outside the MRI apparatus 1) executes an optimization calculation based on the measured value of the magnetic field in step ST1 when the MRI apparatus 1 is installed, and each pocket 72a _{Calculate the numbers Si and j} of the metal shims 72b to be stored in (steps ST2 to ST6). In the optimization calculation, the objective function f (x) and the constraint condition g (x) are set as a function of the parameter x (x is usually a multivariate), and the objective function f (x) is satisfied while satisfying the constraint condition g (x). Is a method of determining the parameter x such that is the minimum (or maximum). The constraint condition g (x) represents a predetermined allowable range Bd of static magnetic field non-uniformity in the MRI apparatus 1 (shown in FIG. 9 to be described later). As the constraint condition, not only the constraint condition g (x) but also a plurality of constraints may be set.

The parameter x is a value obtained by optimizing _{the numbers S i and j} of the metal shim 72b, respectively. Then, as the constraint condition g (x), for example, an inequality indicating that the index representing the non-uniformity of the magnetic field is within a predetermined range, that is, the spatial variation component is within a predetermined range is set. _{Further, the numbers S i and j} of the metal shim 72b are set as the objective function f (x). By performing the optimization calculation with these settings, the number of metal shims 72b is minimized so that the index indicating the non-uniformity of the magnetic field is suppressed within a predetermined range. S _{i and j} are optimized respectively.

First, in step ST2, the processing circuit 50 sets the objective function f ( _{Si, j} ) as shown in the following equation (1). The objective function f (S _{i, j} ) is expressed as a function (a function representing the sum) of _{the numbers S i, j} of the metal shim 72b.

Next, the processing circuit 50 sets the constraint condition g (x) for homogenizing the magnetic field in step ST3. VRMS (Volume Root Mean Square) can be mentioned as an example of an index of a constraint condition for uniformizing a magnetic field.

In general, the theoretical formula of the magnetic field (spherical coordinates) in the bore is given by the following formula using the Legendre function expansion.

Here, P _n ^m (cos (θ)) is a Legendre function of order (n, m) (0 ≦ m ≦ n). Further, _Ang ^m and B _n ^m are values called harmonics of order (n, m), and are quantities similar to the Fourier coefficient in the Fourier series expansion. The order (n, m) = (0,0) corresponds to a perfectly uniform magnetic field. Harmonics A _n ^m of degree (n ≠ 0, m ≠ 0 ), the magnitude of B _n ^m, magnetic field inhomogeneities (spatial variation component) is represented.

From the magnetic field B _{Z_bare} (r, θ, φ) measured in step ST1 and the theoretical formula of the magnetic field represented by the above equation (2), there is no metal shim 72b based on the measured values (state before adjustment). ) Harmonics _An ^m _{_bare} and B _n ^m _{_bare} (hereinafter referred to as unadjusted harmonics) can be obtained. _{For example, the pre-adjustment harmonics An} ^m _{_bare} and B _n ^m _{_bare} can be obtained by fitting the measured values to a theoretical formula. Unadjusted harmonics A _n ^m _{_bare,} as the value of _{B n} ^m _bare is large, it means that non-uniformity of the magnetic field at that time (before adjustment) is large.

Passive shimming is the number S _i of the metal shim _72b, by the proper _j, to correct the spatial distribution of the magnetic field, as small as possible before adjustment harmonics A _n ^m _{_bare,} the value of _{B n} ^m _bare This is a method to improve the uniformity of the magnetic field.

Next, in step ST4, the processing circuit 50 sets a constraint condition other than the constraint condition g (x) as needed. Other constraints are, for example, that the number of metal shims 72b is non-negative and the number of metal shims 72b that can be accommodated in each pocket 72a. Then, in step ST5, the processing circuit 50 executes an optimization calculation that minimizes or maximizes the objective function under the set constraints. After execution of the optimization calculation, distribution _S 1,1 _{~S 24,15} of the metal shim 72b is calculated in step ST6.

9 (A) to 9 (D) are diagrams showing a flow in which the _{static magnetic field B Z in the Z-axis direction is made uniform in the MRI apparatus 1 according to the present embodiment.}

FIG. 9A shows an example of the distribution _{of the magnetic field B Z} in the Z-axis direction before the arrangement of the metal shim 72 b. Magnetic field _{B Z} is represented harmonics _A ² _{0 _bare} the _(A ² _{0 _bare> 0)} is a proportional constant, as a function proportional to the square of the Z position. _{A 2} ⁰ _bare is an element of harmonic _A ⁿ _{m _bare.} Since FIG. 9A shows before passive shimming, the non-uniformity of the magnetic field does not fall within the allowable range Bd of the non-uniformity of the magnetic field that should be finally settled.

On the other hand, FIG. 9B shows the Z-axis direction when the metal shims 72b are tentatively arranged according to _{the numbers Si and j of the metal shims 72b determined by the above-mentioned optimization calculation with respect to FIG. 9A.} An example of the distribution of the magnetic field B _{Z in the above is shown.} FIG. 9B corresponds to a state in which the non-uniformity of the magnetic field is within a predetermined allowable range Bd due to the arrangement of the metal shims 72b by passive shiming.

On the other hand, as described above, when the temperature of the metal shim 72b rises, an F0 shift occurs. The magnitude of the F0 shift and the direction of the F0 shift (whether the F0 shifts in the positive direction or the negative direction) differ depending on the position of the metal shim 72b.

Therefore, in order to suppress the F0 shift that occurs when the metal shims 72b are tentatively arranged according to _{the number Si, j} of the metal shims 72b determined so that the static magnetic field uniformity falls within the predetermined allowable range Bd, a step is taken. In ST7, pockets near the end of the magnet, specifically, at positions where the F0 shift influence coefficient (shown in FIG. 6) is negative (for example, when the number of pockets is 15, the numbers 1 to 4, 12 to 15). One or more metal shims 72b are intentionally added to at least one of the pockets). When the metal shim 72b is added, an example of the distribution of the magnetic field B _Z in the Z-axis direction is shown in FIG. 9 (C).

At this time, the number of metal shims 72b is added within a range that can be corrected by the active shim performed later. Relative distribution _S 1,1 _{~S 24,15,} by adding place the metal shim 72b at step ST7, as shown in FIG. 9 (C), and finally the magnetic field inhomogeneity tolerance Bd to be met It may come off from, but the installation is completed in this state. The allowable range Bd of the static magnetic field non-uniformity corresponds to the constraint condition g (x) in the above-mentioned optimization process.

Next, after the patient is placed in the imaging region, the processing circuit 50 performs active shimming in step ST8 to correct the magnetic field so that the static magnetic field non-uniformity falls within a predetermined allowable range Bd. For example, with respect to harmonics of claim _A ² _{0 _shim} deviating from added by constraints g of metal shim 72b in step ST7 (x), satisfying the constraints g (x) in the active shimming by ^{Z 2-channel} shim coil 11e The magnetic field is corrected as follows. _{An example of the distribution of the magnetic field B Z} in the Z-axis direction when active shimming by the shim coil 11e is executed is shown in FIG. 9 (D).

As a result, the distribution _S 1,1 with respect _{to S 24,15,} while the metal shim 72b is added to the outer position pocket, homogeneity of the magnetic field shim coil 11e constraints are complementary corrected active shimming under the condition g (X) is satisfied. That is, it can be expected that the F0 shift in the MR scan is suppressed while maintaining the uniformity of the static magnetic field.

According to the MRI apparatus 1, with respect to the characteristics of the static magnetic field magnet 10 that term harmonics are found to be _{A 2} ⁰ _bare> ^0, while maintaining the uniformity of the static magnetic field can be suppressed F0 shift.

(First modification)
_{In the first embodiment described above, the numbers Si and j} of the metal shims 72b are optimized so that the uniformity of the static magnetic field falls within the range Bd to be finally satisfied (shown in FIG. 9B). Further, a metal shim 72b was added near the end of the magnet. On the other hand, in the first modification, the permissible range of static magnetic field non-uniformity is represented on the premise of complementary correction by active shiming later at the time of the optimization calculation of _{the numbers Si and j of the metal shim 72b.} Set constraints.

If terms of harmonics of the static magnetic field magnet 10 having a characteristic such that _{A 2} ⁰ _bare> ^0, the uniformity of the static magnetic field to be satisfied finally when trying to achieve a passive shimming, the metal shim 72b in the outer position The number is limited. Therefore, for example, a constraint condition that deteriorates the uniformity of the static magnetic field is intentionally applied in the optimization calculation so that more metal shims 72b are arranged near the end of the magnet. Since the uniformity of the static magnetic field to be finally satisfied is not guaranteed at the stage where the metal shim 72b is arranged, active shiming is performed later.

FIG. 10 is a diagram showing a procedure of the static magnetic field correction method in the first modification of the MRI apparatus 1 according to the present embodiment as a flowchart. In FIG. 10, the same steps as those shown in FIG. 7 are designated by the same reference numerals, and the description thereof will be omitted.

In step ST13, the processing circuit 50 sets the constraint condition h (x), which is different from the constraint condition g (x) set by step ST3 shown in FIG. 7. The constraint condition h (x) is a constraint condition that allows deterioration of the magnetic field uniformity within a range in which the magnetic field can be corrected by active shimming, and a large number of metal shims 72b are arranged near the magnet end. Next, the processing circuit 50 sets other constraint conditions as necessary in step ST14, as in step ST4 shown in FIG. 7. Next, the processing circuit 50 executes an optimization calculation that minimizes the objective function under the set constraint condition h (x) in step ST15, similarly to step ST5 shown in FIG. Then, processing circuit 50, as in step ST6 shown in FIG. 7, in step ST16, the number _{S i} of the metal shim _{72b, j} were determined respectively, and calculates a distribution _S 1,1 _{~S 24,15.} Then, in step ST17, the metal shim 72b is disposed in accordance with the distribution _S 1,1 _{~S 24,15} calculated in step ST16.

11 (A) to 11 (C) are diagrams showing an example of a change in the distribution _{of the magnetic field B Z} in the Z-axis direction in the first modification of the MRI apparatus 1 according to the present embodiment.

FIG. 11A shows an example of the distribution _{of the magnetic field B Z} in the Z-axis direction before the arrangement of the metal shim 72 b. FIG. 11 (A) is the same as FIG. 9 (A). FIG. 11 (B) to FIG. 11 (A), the case where the metal shim 72b is disposed according to the distribution _S 1,1 _{~S 24,15,} shows an example of the distribution of the magnetic field _{B Z} in the Z axis direction.

Here, in the case of having a characteristic that the static magnetic field magnet 10 is Harmonics _A ² _{0 _bare} positive, the MR scan is performed with the metal shim 72b is disposed according to the distribution _S 1,1 _{~S 24,15,} The F0 shift is suppressed. In this case, the arrangement of the metal shim 72b in accordance with the distribution S _1,1 ~S _24,15, there is a case where out of the final field inhomogeneity tolerance Bd to be satisfied, complete the installation remains in this state Let me.

On the other hand, the change in the distribution of the magnetic field in the Z-axis direction in the prior art will be described. 12 (A) and 12 (B) are diagrams showing an example of a change in the distribution of the magnetic field in the Z-axis direction in the prior art.

12 (A) shows pre-deployment of the metal shim, shows an example of the distribution of the magnetic field B _Z in the Z axis direction. FIG. 12 (A) is the same as FIG. 9 (A). _{FIG. 12 (B) shows the distribution of the magnetic field B Z} in the Z-axis direction when the metal shims are arranged according to the distribution calculated so as to fall within the allowable range Bd of the magnetic field non-uniformity with respect to FIG. 12 (A). An example is shown.

Figure 12 (B), the according to the arrangement of the metal shim according to the prior art, since the correction amount is also reduced by a metal shim when _{A 2} ⁰ _bare is not so large, the number of metal shims which are arranged outside the pocket Will also decrease. That is, according to the arrangement of the metal shims according to the prior art, the effect of suppressing the F0 shift is reduced.

Returning to the explanation of FIG. 10, the processing circuit 50, after the patient is placed in the imaging area, in step ST18, relative terms _A ² _{0 _shim} harmonics, constraints active shimming by ^{Z 2-channel} shim coil 11e The magnetic field is corrected so as to satisfy g (x). _{An example of the distribution of the magnetic field B Z} in the Z-axis direction when active shimming by the shim coil 11e is executed is shown in FIG. 11 (C).

By the arrangement of the metal shims 72b and the active shimming described above, the F0 shift is suppressed, and the uniformity of the magnetic field is complementarily corrected by the active shimming by the shim coil 11e to satisfy the constraint condition g (x). .. That is, it can be expected that the F0 shift in the MR scan is suppressed while maintaining the uniformity of the static magnetic field.

According to a first modification of the MRI apparatus 1, with respect to the static magnetic field magnet 10 that term harmonics are found to be _{A 2} ⁰ _bare> ^0, while maintaining the uniformity of the static magnetic field, can be suppressed F0 shift ..

(Second modification)
In the first embodiment and the first modified example described above, the term harmonics has been described as an example a case in which the _{A 2} ⁰ _bare> ^0. On the other hand, the second modification, in the case section harmonics is _A ² _{0 _bare <0} and becomes the case of the static magnetic field magnet 10 or _A ² _{0 _bare,>} A _{0 A} ² 0 _bare small static magnetic field magnet 10 Indicates static magnetic field correction.

<For the static magnetic field magnet 10 to be 0, or _A ² _{0 _bare>} term of harmonics _A ² _{0 _bare} if 0 is a by _A ² _{0 _bare} is smaller static magnetic field magnet 10, the metal shim 72b arranged central term tends to collect in the position harmonics stronger than if sufficiently large a _{a 2} ⁰ _bare> ^0. In the second modification, performing the active shimming above, uneven characteristics of the static magnetic _field, A ₂ ⁰ _bare> ⁰ is a by correcting once to be sufficiently large. At this stage, the magnetic field uniformity does not have to satisfy the magnetic field uniformity to be finally satisfied. Then, passive shimming is performed to correct the uniformity of the magnetic field. In this case, since the term of harmonics can be performed similarly passive shimming the static magnetic field magnet 10 which is a _{A 2} ⁰ _bare> ^0, than when performing passive shimming in the state of example _{A 2} ⁰ _bare ^<0, more Many metal shims 72b are arranged near the end of the magnet, specifically, at a position where the F0 shift influence coefficient (shown in FIG. 6) is negative, so that the F0 shift can be suppressed.

In the example shown in FIG. 7, if the feature has been optimized calculated for state of the _{A 2} ⁰ _bare> ^0, the metal shim 72b is a result which is disposed near certain extent magnet end the magnet end He said that the number of metal shims 72b placed near the part is insufficient. Therefore, in the second modification, performing the active shimming, when changing the characteristics of the magnetic field from ^{_{<0 A 2 0 _bare> A 2}} 0 _bare to 0, positive and stronger _{polar, A} ^{2 0} _{_Bare} >> 0. By increasing the positive polarity, more metal shims 72b are placed near the ends of the magnets.

FIG. 13 is a diagram showing a procedure of the static magnetic field correction method in the second modification of the MRI apparatus 1 according to the present embodiment as a flowchart. In FIG. 13, the same steps as those shown in FIG. 7 are designated by the same reference numerals, and the description thereof will be omitted. Further, in FIG. 13, it is also possible to change the order of the steps and execute all the steps at the time of installation of the MRI apparatus 1.

Or on production process control design, the static magnetic field magnet 10 the term of harmonics are found to have the properties that make _{A 2} ⁰ _bare ^<0, the processing circuit 50, upon installation of the MRI apparatus 1, in step ST21 when the magnetic field is measured, in shim coil 11e of ^{Z 2} channels, terms of harmonics to adjust the magnetic field such that the _a ² _{0 _bare> 0.}

14 (A) to 14 (C) are diagrams showing an example of a change in the distribution _{of the magnetic field B Z} in the Z-axis direction in the second modification of the MRI apparatus 1 according to the present embodiment.

FIG. 14A shows an example of the distribution _{of the magnetic field B Z} in the Z-axis direction before the arrangement of the metal shim 72 b. Magnetic field _{B Z} is a proportionality constant is the harmonic term ^{_{A 2 0 _bare (A 2 0}} _bare <0), expressed as a function proportional to the square of the Z position. FIG. 14 (B) to FIG. 14 (A), the case where the magnetic field corrected by shim coil 11e of ^{Z 2} channels, shows an example of the distribution of the magnetic field _{B Z} in the Z axis direction.

Returning to the description of FIG. 13, the processing circuit 50 _{determines the numbers Si and j} of the metal shims 72b so as to satisfy the constraint conditions (steps ST2 to ST6). Then, the metal shim 72b is disposed in accordance with the distribution _S 1,1 _{~S 24,15} of the metal shim 72b calculated in step ST6 (step ST27). _{An example of the distribution of the magnetic field B Z} in the Z-axis direction when passive shiming is performed by the metal shim 72b is shown in FIG. 14 (C).

For terms of harmonics to correct the uniformity of the static magnetic field has a _{A 2} ⁰ _bare> ^0, that is, in order to shift to 14 (C) Fig. 14 (B) is close to the magnet ends A metal shim 72b needs to be placed in the pocket of the metal shim 72b (see FIG. 6). As a result, the number of metal shims 72b is large in the pocket near the end of the magnet, resulting in a "metal shim arrangement capable of suppressing the occurrence of F0 shift".

According to a second modification of the MRI apparatus 1, term of harmonics is _{A 2} ⁰ _bare ^<0, or to the static magnetic field magnet ₁₀ A ₂ ⁰ _bare is not so large a positive, the uniformity of the static magnetic field The F0 shift can be suppressed while maintaining the F0 shift.

According to the MRI apparatus and the static magnetic field correction method of at least one embodiment described above, the F0 shift can be suppressed while maintaining the uniformity of the static magnetic field.

If the uniformity of the static magnetic field can be adjusted only by the active shim by the shim coil unit 11 without performing the passive shim by the metal shim 72b, it is not affected by the temperature rise of the metal shim 72b. However, magnets generally have complex static magnetic field inhomogeneities, including the effects of magnetic shields, and correct higher-order magnetic field inhomogeneities to adjust for higher-order static magnetic field inhomogeneities. You need a shim coil that can be used. Since the shim coil is often arranged inside the gradient magnetic field coil unit 12, the bore becomes narrower when a shim coil capable of magnetic field correction up to a higher order is incorporated. Further, as the number of channels of the shim coil increases, the power supply for supplying power to the shim coil becomes large and the power consumption also increases. Therefore, it is better to use passive shimming and active shimming together instead of correcting the uniformity of the static magnetic field only by active shimming from the viewpoint of ensuring the width of the bore and saving power.

Although some embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other embodiments, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are also included in the scope of the invention described in the claims and the equivalent scope thereof.

1 ... Magnetic resonance imaging (MRI) device 10 ... Static magnetic field magnet 11 ... Sim coil unit 11e ... Z ^2- channel shim coil 12 ... Diagonal magnetic field coil unit (ASGC)
12a ... Main coil 12b ... Shim tray unit 12c ... Shield coil 13 ... WB coil 72b ... Metal shim

Claims (7)

A static magnetic field magnet that generates a static magnetic field and
A predetermined number of metal shims are arranged at each position in the static magnetic field, and
A coil that generates a magnetic field by supplying electric power and corrects the uniformity of the static magnetic field by active shimming is provided.
The arrangement of the metal shims is
Than said uniformity of the static magnetic field is shall be determined by the condition giving priority to suppression of the center frequency shift relative to temperature change,
By correcting the combination of the passive shimming and the active shimming while allowing the magnetic field uniformity to not meet the predetermined criteria in the passive shimming due to the arrangement of the metal shims, the magnetic field uniformity is made the predetermined reference. The metal shim is added so as to satisfy the above and suppress the center frequency shift.
The coil corrects the static magnetic field formed by the arrangement of the metal shims so that the static magnetic field has a predetermined uniformity.
Magnetic resonance imaging device. The metal shims, homogeneity of the magnetic field by the passive shimming is to placement of the metal shim when satisfying the predetermined criterion, is arranged more distant position from the center of the static magnetic field,
The magnetic resonance imaging apparatus according to claim 1. Placement of the metal shim, the uniformity of the magnetic field regardless of whether they meet the predetermined criteria, the center frequency shift is determined so as to fall within a predetermined range,
The magnetic resonance imaging apparatus according to claim 1. In the arrangement of the metal shims, the metal shims are added at a position far from the center position of the static magnetic field with respect to the arrangement of the metal shims when the uniformity of the magnetic field satisfies the predetermined criteria by the passive shiming. Arrangement,
The magnetic resonance imaging apparatus according to claim 3. The degree of influence on the center frequency shift differs depending on the position where the metal shim is arranged.
The arrangement of the metal shims is an arrangement in which the metal shims are added at positions where the degree of influence is negative.
The magnetic resonance imaging apparatus according to claim 4. A static magnetic field magnet that generates a static magnetic field and
A predetermined number of metal shims are arranged at each position in the static magnetic field, and
A coil that generates a magnetic field by supplying electric power and corrects the uniformity of the static magnetic field by active shimming is provided.
The arrangement of the metal shims is determined under the condition that the suppression of the center frequency shift with respect to the temperature change is prioritized over the uniformity of the static magnetic field.
The degree of influence on the center frequency shift differs depending on the position where the metal shim is arranged.
In the active shiming before the passive shiming by the arrangement of the metal shims, the characteristics of the static magnetic field are changed so that many of the metal shims are arranged at positions where the degree of influence is negative in the passive shiming.
The coil corrects the static magnetic field formed by the arrangement of the metal shims so that the static magnetic field has a predetermined uniformity.
Magnetic resonance imaging apparatus. A predetermined number of metal shims are placed at each position in the static magnetic field generated by the static magnetic field magnet, and a coil that corrects the static magnetic field by active shimming receives power to generate a magnetic field, thereby making the magnetic field uniform. It is a static magnetic field correction method that corrects the static magnetic field so that the sex meets a predetermined standard.
The arrangement of the metal shims,
Determined under the condition that the suppression of the center frequency shift with respect to the temperature change is prioritized over the uniformity of the static magnetic field.
By correcting the combination of the passive shimming and the active shimming while allowing the magnetic field uniformity to not meet the predetermined criteria in the passive shimming due to the arrangement of the metal shims, the magnetic field uniformity is made the predetermined reference. And determined that the metal shims were added so as to suppress the center frequency shift.
The coil corrects the static magnetic field formed by the arrangement of the metal shims so that the static magnetic field has a predetermined uniformity.
Static magnetic field correction method.

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