JP6151583B2 - Static magnetic field generating magnet and magnetic resonance imaging apparatus - Google Patents

Description

  The present invention relates to a magnetic resonance imaging (hereinafter abbreviated as MRI) apparatus for displaying an image of an arbitrary cross section of a subject by utilizing a nuclear magnetic resonance phenomenon (hereinafter abbreviated as NMR), and in particular, the uniformity of a static magnetic field space. It relates to technology to improve.

  An MRI system that generates a static magnetic field using a static magnetic field generating magnet can obtain an image using a nuclear magnetic resonance phenomenon that occurs when a subject placed in a uniform static magnetic field space is irradiated with a high-frequency magnetic field pulse. I can do it.

  In the MRI apparatus, one of the factors for improving the image quality is to improve the spatial uniformity of the static magnetic field in the imaging space. For this reason, in a static magnetic field generating magnet used in an MRI apparatus, magnetic field adjustment is performed at the time of assembly and apparatus installation in order to make the static magnetic field generated in the imaging space spatially uniform.

  Among these, the magnetic field adjustment of the static magnetic field generating magnet at the installation stage is the amount of magnetic field uniformity adjusting material made of magnetic material arranged on the static magnetic field generating magnet so that the static magnetic field inhomogeneity component caused by various installation space conditions is reduced. Adjust.

  Specifically, the magnetic field adjustment of the static magnetic field generating magnet is performed by measuring the magnetic field distribution in the imaging space with a magnetic sensor, calculating the arrangement amount of the magnetic field uniformity adjusting material by calculating from the measurement result, This series of operations of placing the magnetic field uniformity adjusting material in the static magnetic field generating magnet is repeated until the desired uniformity is achieved.

  In general, the magnetic field uniformity adjusting material is mounted on the static magnetic field adjusting unit in a space between the magnetic field generating source of the static magnetic field generating magnet and the gradient magnetic field generating unit arranged inside thereof. (For example, Patent Document 1).

JP 2008-289703 A

  In recent years, in order to obtain higher image quality, it is necessary to adjust the static magnetic field uniformity with higher definition. Since the amount of magnetic field adjustment is determined by the mass of the magnetic field uniformity adjusting material, if high-precision magnetic field adjustment is required, the magnetic field uniformity outside the rating that is finer than the rated magnetic field uniformity adjusting material defined by the static magnetic field adjustment unit It is necessary to use a conditioner. The same situation is considered to occur even with the shim tray disclosed in Patent Document 1.

  This non-rated magnetic field uniformity adjusting material can be adhered and disposed in the static magnetic field adjusting unit, for example, but since it is out of the rating, there is a possibility of moving during imaging due to the influence of the magnetic field in the static magnetic field adjusting unit. If it moves, it may cause non-uniform static magnetic field and electrical noise, which may adversely affect the image. Further, as a result of the magnetic field uniformity adjustment again, when replacing the magnetic field uniformity adjusting material once arranged, it may be difficult to perform the replacement work by removing the adhesive.

  Accordingly, the present invention has been made to solve the above-described problems, and an object thereof is to enable high-precision and easy magnetic field adjustment performed by arranging a magnetic field uniformity adjusting material in a static magnetic field adjustment unit. And

  In order to solve the above problems, the present invention includes a static magnetic field generation unit that generates a static magnetic field, a plurality of static magnetic field adjustment units that are equipped with a magnetic field uniformity adjusting material and adjust the spatial uniformity of the static magnetic field, Each of the plurality of static magnetic field adjustment units has a plurality of pockets of different sizes, and the magnetic field uniformity adjusting material has a plurality of types of different sizes suitable for the sizes of the pockets. The magnetic field uniformity adjusting material is fixed to the pocket by the cap portion and arranged.

  According to the static magnetic field generating magnet and the MRI apparatus of the present invention, the magnetic field adjustment performed by arranging the magnetic field uniformity adjusting material in the static magnetic field adjustment unit can be performed with high accuracy and ease.

1 is an overall configuration diagram of an MRI apparatus according to the present invention. 1 is a block diagram of an MRI apparatus according to the present invention. The figure explaining the arrangement position in a static magnetic field adjustment part and its static magnetic field generating magnet. (a) is a figure which shows the detail of a static magnetic field adjustment part. (b) is sectional drawing which cut (a) by the B-B 'line. The flowchart which shows the processing flow of static magnetic field adjustment.

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

  FIG. 1 shows an overall perspective view of an embodiment of a horizontal magnetic field type MRI apparatus according to the present invention.

  This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject. As shown in FIG. 1, a gantry 31 that houses various devices for inducing a NMR phenomenon in a subject and receiving NMR signals. A bed 32 having a top plate 36 on which the subject is placed, a housing 33 that houses a power source for driving various devices in the gantry and various control devices for control, and a subject by processing received NMR signals And a processing device 34 for reconstructing a tomographic image of the same, which are respectively connected by a power source / signal line 35. The operation panel 37 for operating the movement of the top plate 36 is disposed on the front left and right of the opening of the gantry 31 and includes a switch for operating the movement of the top plate 36.

  The gantry 31 and the table 32 are disposed in a shield room that shields high-frequency electromagnetic waves and static magnetic fields, and the housing 33 and the processing device 34 are disposed outside the shield room.

  FIG. 2 shows a block configuration diagram in which the configuration of the MRI apparatus of FIG. 1 is disassembled for each more detailed function. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject 101. As shown in FIG. 2, a static magnetic field generating magnet 102, a gradient magnetic field coil 103, a gradient magnetic field power supply 109, and an RF transmission coil 104 and RF transmitter 110, RF receiver coil 105 and signal processor 107, measurement control unit 111, overall control unit 112, display / operation unit 118, and top plate on which the subject 101 is mounted generates a static magnetic field. And a bed 106 to be taken in and out of the magnet 102.

  The static magnetic field generating magnet 102 generates a uniform static magnetic field in the direction perpendicular to the body axis of the subject 101 in the vertical magnetic field method and in the body axis direction in the horizontal magnetic field method. A permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the.

  The gradient magnetic field coil 103 is a coil wound in the three-axis directions of X, Y, and Z that are the real space coordinate system (stationary coordinate system) of the MRI apparatus, and each gradient magnetic field coil is a gradient magnetic field that drives it. A current is supplied to the power source 109. Specifically, the gradient magnetic field power supply 109 of each gradient coil is driven according to a command from the measurement control unit 111 described later, and supplies a current to each gradient coil. Thereby, gradient magnetic fields Gx, Gy, and Gz are generated in the three-axis directions of X, Y, and Z. The gradient magnetic field coil 103 and the gradient magnetic field power supply 109 are included in the gradient magnetic field generator.

  When imaging a two-dimensional slice plane, a slice gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 101, orthogonal to the slice plane and orthogonal to each other. Phase encoding gradient magnetic field pulse (Gp) and frequency encoding (reading) gradient magnetic field pulse (Gf) are applied in the remaining two directions, and position information in each direction is encoded in the nuclear magnetic resonance signal (echo signal). The

  The RF transmission coil 104 is a coil that irradiates the subject 101 with an RF pulse, and is connected to the RF transmission unit 110 and supplied with a high-frequency pulse current. As a result, an NMR phenomenon is induced in the spins of atoms constituting the living tissue of the subject 101. Specifically, the RF transmission unit 110 is driven in accordance with a command from the measurement control unit 111 (to be described later), amplitude-modulates and amplifies the high-frequency pulse, and then the RF transmission unit 104 is placed near the subject 101 after being amplified. By supplying, the subject 101 is irradiated with the RF pulse. The RF transmitter coil 104 and the RF transmitter 110 are included in the RF pulse generator.

  The RF receiving coil 105 is a coil that receives an echo signal emitted by the NMR phenomenon of spin that constitutes the living tissue of the subject 101, and is connected to the signal processing unit 107 so that the received echo signal is sent to the signal processing unit 107. Sent.

  The signal processing unit 107 performs detection processing of the echo signal received by the RF receiving coil 105. Specifically, in accordance with a command from the measurement control unit 111 described later, the signal processing unit 107 amplifies the received echo signal and divides it into two orthogonal signals by quadrature detection, For example, 128, 256, 512, etc.) are sampled, and each sampling signal is A / D converted into a digital quantity. Therefore, the echo signal is obtained as time-series digital data (hereinafter referred to as echo data) composed of a predetermined number of sampling data. Then, the signal processing unit 107 performs various processes on the echo data, and sends the processed echo data to the measurement control unit 111.

  The measurement control unit 111 mainly transmits various commands for collecting echo data necessary for reconstruction of the tomographic image of the subject 101 to the gradient magnetic field power source 109, the RF transmission unit 110, and the signal processing unit 107. And a control unit for controlling them.

  Specifically, the measurement control unit 111 operates under the control of the overall control unit 112 described later, and the gradient magnetic field power source 109, the RF transmission unit 110, and the signal processing unit 107 are controlled based on control data of a predetermined pulse sequence. Control to repeatedly perform the irradiation of the RF pulse and the application of the gradient magnetic field pulse to the subject 101 and the detection of the echo signal from the subject 101 to reconstruct an image of the imaging region of the subject 101 Control the collection of required echo data. In the repetition, the application amount of the phase encoding gradient magnetic field is changed in the case of two-dimensional imaging, and the application amount of the slice encoding gradient magnetic field is further changed in the case of three-dimensional imaging. Values such as 128, 256, and 512 are normally selected as the number of phase encodings, and values such as 16, 32, and 64 are normally selected as the number of slice encodings. With these controls, echo data from the signal processing unit 107 is output to the overall control unit 112.

  The overall control unit 112 controls the measurement control unit 111 and controls various data processing and processing result display and storage, and includes an arithmetic processing unit (CPU) 114, a memory 113, and a magnetic disk. And the like, and a network IF 116 that interfaces with an external network. Further, an external storage unit 117 such as an optical disk may be connected to the overall control unit 112.

  Specifically, when the measurement control unit 111 collects echo data by executing an imaging sequence and the echo data is input from the measurement control unit 111, the arithmetic processing unit 114 converts the encoded information applied to the echo data. Based on this, it is stored in an area corresponding to the k space in the memory 113. Hereinafter, the statement that the echo data is arranged in the k space means that the echo data is stored in an area corresponding to the k space in the memory 113. A group of echo data stored in an area corresponding to the k space in the memory 113 is also referred to as k space data.

  Then, the arithmetic processing unit 114 performs processing such as signal processing and image reconstruction by Fourier transform on the k-space data, and displays the resulting image of the subject 101 on the display / operation unit 118 described later. The data is recorded in the internal storage unit 115 or the external storage unit 117, or transferred to an external device via the network IF 116.

  The display / operation unit 118 includes a display unit for displaying the reconstructed image of the subject 101, a trackball or a mouse and a keyboard for inputting various control information of the MRI apparatus and control information for processing performed by the overall control unit 112. Etc., and an operation unit. The operation unit is disposed in the vicinity of the display unit, and an operator controls various processes of the MRI apparatus interactively through the operation unit while looking at the display unit.

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

<Configuration of static magnetic field adjustment unit>
Next, an example of the static magnetic field adjusting unit of the present invention in the static magnetic field generating magnet and the MRI apparatus using the magnet will be described with reference to FIGS.

  FIG. 3 shows a horizontal magnetic field type MRI apparatus in which a cylindrical static magnetic field generating magnet 102 generates a static magnetic field in its inner space, and a gradient magnetic field coil 103 is connected to the inner space of the static magnetic field generating magnet 102. Placed inside. In addition, a static magnetic field adjustment unit 150 is disposed in each of a plurality of A portions (insertion holes of the static magnetic field adjustment unit) provided in the circumferential direction in the space between the static magnetic field generating magnet 102 and the gradient magnetic field coil 103. . FIG. 3 is an example in which the static magnetic field adjustment units 150 are arranged at 24 locations, but the present invention is not limited to 24 locations, and may be more or less than this. Further, a static magnetic field space 130 (imaging space) is formed inside the gradient magnetic field coil 103, and a subject is inserted into the static magnetic field space 130 so that imaging for diagnosis can be performed.

  The static magnetic field adjustment unit 150 includes a tray unit having a plurality of concave pockets of different sizes, and a plurality of types of magnetic field uniformity adjustment materials (hereinafter referred to as shim materials) that are different in size from each other in accordance with the size of each pocket. A dummy magnetic field uniformity adjusting material (hereinafter abbreviated as a dummy material) of the same size, and a cap portion for fixing the shim material and the dummy material in the pocket. The shim material is made of a magnetic material such as iron, and the dummy material is made of a non-magnetic material such as resin.

  FIG. 4 is a detailed view of one of the plurality of static magnetic field adjustment units 150 arranged in each part A in FIG. (a) is a perspective view of the static magnetic field adjustment unit 150, and (b) is a cross-sectional view of the static magnetic field adjustment unit 150 of FIG.

  The tray unit 453 has an elongated structure in which a plurality of pocket units formed by combining a plurality of pockets 455 and 456 of different sizes are arranged in the insertion direction (static magnetic field direction) into the A unit. The pocket 455 of each pocket unit is larger in size than the pocket 456, and the pocket 456 has a deeper groove than the pocket 455. Accordingly, the large-sized pocket 455 is a pocket for low-definition (rough) uniformity adjustment, and the small-sized pocket 456 is a pocket for high-definition uniformity adjustment. The number of pockets with different sizes may be three or more. Then, the pocket in which the shim material is arranged and the arrangement amount thereof are determined so as to improve the uniformity of the static magnetic field, and the determined amount of the shim material is arranged and positioned in the determined pocket.

  The shim material conforms to the size of the pocket 456 and is placed in the pocket 456 so as to adjust the static magnetic field with high accuracy, and the size of the pocket 455 or the pocket 458 combined with the pocket 455 and the pocket 456. And a large shim material 450 for adjusting a static magnetic field roughly, which is disposed in a pocket 455 or a pocket 458 (that is, straddling the pocket 455 and the pocket 456). In other words, the shim material 451 having a small size is a shim material for adjusting high-definition uniformity, and the shim material 450 having a large size is a shim material for adjusting low-definition (rough) uniformity.

  Furthermore, since the shim material 451 having a small size for adjusting the high-definition uniformity is disposed in the pocket 456 having a small deep groove size, the static magnetic field space 130 is larger than the shim material 450 having a large size for adjusting the low-definition uniformity. It is arranged at a position close to. This makes it easier to adjust the static magnetic field homogeneity with higher precision at a position close to the static magnetic field space 130 where the subject is placed.

  Further, a dummy material 457 having the same size as the shim material 451 and a dummy material 452 having the same size as the shim material 450 are provided. These dummy members 452 and 457 are used by being packed into a space in which the pockets are free in order to prevent the shim members 450 and 451 from moving in the pocket of the tray portion 453.

  Further, if the shim material 450 having a large size for adjusting the low-definition uniformity is a shape that fits into the pocket 458, that is, a size that covers the pocket 455 and the pocket 456, the shim material 450 covers the shim material 451. It will be covered. As a result, the shim material 451 is pushed into the deep groove pocket 456 and does not move.

  In addition, each shim material and each dummy material are fixedly disposed in the pockets 455 and 456 by the cap portion 454. The cap portion 454 serves as a lid for preventing the shim materials 450 and 451 disposed in the pockets 455 and 456 of the tray portion 453 from jumping out. Thereby, the dummy material having the same size as each shim material is positioned in the matching pocket.

  The shim material and the dummy material of the same size are preferably approximately the same size as the corresponding pocket so as not to move under the influence of a magnetic field, but this is not restrictive.

<Flow of static magnetic field adjustment>
Since the static magnetic field adjustment is determined by the mass of the shim material, a plurality of types of shim materials are used in order to improve accuracy. Also, the static magnetic field adjustment as described above is repeated about 3 to 4 times, depending on the assembly accuracy of the static magnetic field generating magnet and the variation of the shim material components. By doing so, the uniformity of the static magnetic field space is improved.

  Therefore, in the static magnetic field adjustment of the present invention, first, the arrangement amount of the large shim material 450 for each large pocket 455 is adjusted to improve the uniformity of the static magnetic field to near the specification. When making fine adjustments that cannot be handled by the large shim material 450, the placement of the small shim material 551 is further adjusted for each small pocket 456 in the tray section 453, and the static magnetic field is adjusted. Perform fine adjustment of uniformity.

  Specifically, the static magnetic field adjustment for improving the static magnetic field uniformity is performed by the processing procedure shown in the flowchart of FIG. Each step will be described in detail below.

  In step 501, the operator places a magnetic sensor in the static magnetic field space (imaging space) 130, and activates measurement of the magnetic field distribution in the static magnetic field space 130.

  In step 502, the measurement control unit 111 measures the magnetic field distribution in the static magnetic field space 130 and acquires measurement data.

  In step 503, the arithmetic processing unit 114 determines whether the static magnetic field uniformity in the static magnetic field space 130 is equal to or less than a predetermined first reference value based on the measurement data acquired in step 502. If it is less than or equal to the first reference value, the process proceeds to step 506, and if greater than the first reference value, the process proceeds to step 504. This first reference value is the static magnetic field uniformity achievable with the shim material 450 having a large size.

  In step 504, the arithmetic processing unit 114 arranges the large shim material 450 for each large pocket 455 of the tray unit 453 in each static magnetic field adjustment unit 150 based on the measurement data acquired in step 502. Determine the amount of each. Thereby, the arrangement amount and arrangement position of the shim material 450 having a large size are determined.

  In step 505, the operator takes out the static magnetic field adjustment unit 150 from the A part (insertion hole of the static magnetic field adjustment unit), and arranges the large shim material 450 for each large pocket 455 determined in step 504. Based on the amount, the large shim material 450 is placed in each large pocket 455, and the static magnetic field adjustment unit 150 is inserted back into the A part (insertion hole of the static magnetic field adjustment unit). Then, the operator activates measurement of the magnetic field distribution in the static magnetic field space 130 again, and proceeds to step 502.

  In step 506, the arithmetic processing unit 114, based on the measurement data acquired in step 502, the static magnetic field uniformity in the static magnetic field space 130 is a predetermined second reference value (a reference that is stricter (lower) than the first reference value). Value) or less. If it is equal to or smaller than the second reference value, the process ends. If it is larger than the second reference value, the process proceeds to step 507. This second reference value is the static magnetic field uniformity achievable with the shim material 451 having a small size.

  In step 507, the arithmetic processing unit 114 arranges the small shim material 451 for each small pocket 456 of the tray unit 453 in each static magnetic field adjustment unit 150 based on the measurement data acquired in step 502. Determine the amount of each. Thereby, the arrangement amount and the arrangement position of the small shim material 451 are determined.

In step 508, the worker selects the small shim material 451 in each small pocket 456 based on the arrangement amount of the small shim material 451 for each small pocket 456 determined in step 507. The static magnetic field adjustment unit 150 is inserted and returned to the A part. Then, measurement of the magnetic field distribution in the static magnetic field space 130 is activated again, and the process proceeds to step 502.
The above is the outline of the processing flow of static magnetic field adjustment.

  As described above, according to the static magnetic field generating magnet and the MRI apparatus of the present invention, it becomes possible to easily fix and install the shim material 451 having a small size, the shim material does not move during imaging, and the static magnetic field Does not adversely affect uniformity and image quality. Furthermore, it is possible to reduce the burden of shim replacement work performed by the operator, and to shorten the work time on site. That is, the magnetic field adjustment performed by arranging the shim material in the static magnetic field adjustment unit can be performed with high accuracy and ease.

  31 Gantry, 32 Sleeper, 33 Case, 34 Processing unit, 35 Power supply / Signal line, 36 Top panel, 37 Operation panel, 101 Subject, 102 Static magnetic field generating magnet, 103 Gradient magnetic field coil, 104 Transmitting RF coil, 105 RF Receiver coil, 106 bed, 107 Signal processing unit, 108 Overall control unit, 109 Gradient magnetic field power supply, 110 RF transmission unit, 111 Measurement control unit, 113 Memory, 114 Arithmetic processing unit (CPU), 115 Internal storage unit, 116 Network IF , 117 External storage unit, 118 Display / operation unit, 150 Static magnetic field adjustment unit, 450 size shim material, 451 size small shim material, 452 size large dummy material, 453 tray unit, 454 cap unit, 455 size Large pocket, 456 small pocket, 457 small dummy

Claims (2)

A static magnetic field generator for generating a static magnetic field;
A plurality of static magnetic field adjustment units that are equipped with a magnetic field uniformity adjusting material and adjust the spatial uniformity of the static magnetic field,
A static magnetic field generating magnet comprising:
Each of the plurality of static magnetic field adjustment units has a plurality of concave pockets of different sizes,
The magnetic field uniformity adjusting material has a plurality of types having different sizes respectively adapted to the size of the pocket, and is arranged fixed to the pocket by the cap portion via the dummy magnetic field uniformity adjusting material of the same size. A static magnetic field generating magnet. A static magnetic field generator for generating a static magnetic field in an imaging space in which the subject is arranged;
A plurality of static magnetic field adjustment units that are equipped with a magnetic field uniformity adjusting material and adjust the spatial uniformity of the static magnetic field,
A magnetic resonance imaging apparatus comprising:
Each of the plurality of static magnetic field adjustment units has a plurality of concave pockets of different sizes,
The magnetic field uniformity adjusting material has a plurality of types having different sizes respectively adapted to the size of the pocket, and is arranged fixed to the pocket by the cap portion via the dummy magnetic field uniformity adjusting material of the same size. A magnetic resonance imaging apparatus.

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