JP4503405B2 - Superconducting magnet apparatus and magnetic resonance imaging apparatus using the same - Google Patents

Description

  The present invention relates to a superconducting magnet apparatus for a magnetic resonance imaging (hereinafter abbreviated as “MRI”) apparatus, and more particularly to a technique for stably operating without quenching while reducing the size and weight.

Many attempts have been made to improve the efficiency of the superconducting magnet device for the MRI apparatus, that is, to increase the central magnetic field strength with respect to (current × number of turns) of the superconducting coil.
One way to achieve that goal is to place the superconducting coil (current distribution) as close to the center of the magnetic field as possible. That is, for the same amount of current, the closer to the magnetic field center, the higher the magnetic field can be generated. Conversely, if the central magnetic field strength is the same, the amount of current can be reduced as the superconducting coil is arranged closer to the magnetic field center. As a result, it is possible to reduce the electromagnetic force and the empirical magnetic field, which is very effective for reducing the magnet manufacturing cost.

Regarding the above problems, there are the following known techniques.
(Patent Document 1) describes an open access magnetic resonance imaging device that maintains a superconducting wire at a temperature lower than the temperature at which the superconducting state is maintained by cooling the superconducting coil by heat transfer (see FIGS. 1 and 8). Is disclosed. The components (such as superconducting coils) of the device 10 are cooled by complete conduction by making heat conductive contact with a plurality of solid thermal conductors such as cables and bars connected to the cryocooler 100. As a result, the components are lowered to the required temperature by the cryocooler 100 without consuming liquid coolant such as helium.

  (Patent Document 2) discloses a double-doughnut-type open superconducting magnet (see FIGS. 1 and 5) in which the superconducting coil is placed close to the imaging space by placing it outside the helium tank and the magnetic field generation efficiency is increased. . As an example, a superconducting main coil 342 is provided that is disposed outside the Dewar 352 and is in thermal contact with the Dewar 352 by solid conduction. Similarly, superconducting backing coil 380 is disposed outside Dewar 352 and is in thermal contact with Dewar 352 in solid conduction.

The superconducting main coil 342 and the superconducting backing coil 380 are arranged outside the dewar 352 and cooled by the cryogenic fluid 340 in the dewar 352 by solid conduction from the wall of the dewar 352, so that the coil is structured. In particular, the cost of the magnet 310 can be reduced by reducing the amount of superconducting main coil that can be placed vertically in the magnet open space 374 and required for the same strength magnetic field.
JP 07-327961 A JP 09-182731 A

However, the above patent document does not disclose a solution to the following problem.
In the superconducting magnet disclosed in (Patent Document 1), since a refrigerant such as helium is not used, when the superconducting coil is cooled to a predetermined temperature by the refrigerator, when the refrigerator is stopped due to a power failure or failure, Since the heat capacity of the coil itself and the heat conductor in contact with the coil itself is small, the temperature may rise in a short time of several tens of minutes, which may cause quenching.

The superconducting magnet disclosed in (Patent Document 2) can be configured when the number of coils is as small as about four as shown in FIG. The structure becomes complicated. In addition, it is necessary to cool parts that are less stable than general superconducting wires, such as permanent current switches (PCS) and superconducting connections, as much as possible, but this is taken into consideration. Absent.
Accordingly, the present invention provides a superconducting magnet device for an MRI apparatus that has a high magnetic field generation efficiency, is small and lightweight, and operates stably without quenching even when the number of superconducting coils is relatively large, such as five or more. With the goal.

In order to solve the above problems, the superconducting magnet device of the present invention is configured as follows. That is,
Has a static magnetic field generating means including a plurality of superconducting coils, and container for cooling to a predetermined temperature by accommodating the plurality of superconducting coils, a for generating a uniform static magnetic field in a predetermined region,
The receiving container includes a coolant tank for containing a coolant for cooling the plurality of superconducting coils, in a superconducting magnet device including a vacuum vessel, a for housing the refrigerant container, the plurality of superconducting coils at least one of the inner is an external superconducting coil disposed between the front Symbol refrigerant container and the vacuum container and the cooling container and the external superconducting coil is disposed a heat transfer member thermally connects It is characterized by being.

In a preferred embodiment of the superconducting magnet device according to the present invention, the plurality of superconducting coils include one or more main coils, one or more uniformity control coils, and one or more shield coils . It is at least one of the one or more main coils.

In a preferred embodiment of the superconducting magnet device according to the present invention, the external superconducting coil has at least one of the one or more uniformity control coils, and the uniformity control coil is the heat conduction coil. It is characterized by being thermally connected to the member .

The external superconducting coil includes at least one of the one or more shield coils.
The magnetic resonance imaging apparatus includes the superconducting magnet.

  As described above, according to the present invention, since a helium tank is provided and liquid helium is contained therein, even when the refrigerator is stopped, quenching does not occur in a short time, and stable. Operation is possible. In addition, since the superconducting coil other than the main coil, the PCS, and the superconducting connection portion can be immersed in the body helium, it is possible to ensure superconducting stability.

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

  First, an outline of the MRI apparatus according to the present invention will be described. FIG. 8 shows a perspective view of the overall basic configuration of an embodiment of the horizontal magnetic field type MRI apparatus according to the present invention. However, the present invention is not limited to the horizontal magnetic field method, and can be applied to other vertical magnetic field methods. This MRI apparatus uses a nuclear magnetic resonance (NMR) phenomenon to obtain a tomographic image of an object. As shown in FIG. 8, various apparatuses for inducing an NMR phenomenon in an object and receiving NMR signals. A gantry 51 that houses a subject, a table 52 on which a subject is placed, a power source that controls various devices in the gantry and a housing 53 that houses various control devices, and a received NMR signal to process a step-up image of the subject. It consists of processing devices 54 to be reconfigured, and each is connected by a power source / signal line 55. The gantry and the table are arranged in a shield room that shields a high-frequency electromagnetic wave and a static magnetic field (not shown), and the casing 53 and the processing device 54 are arranged outside the shield room.

  FIG. 9 is a block diagram showing the overall configuration of an embodiment of the MRI apparatus according to the present invention. This MRI apparatus uses a nuclear magnetic resonance (NMR) phenomenon to obtain a tomographic image of a subject, and as shown in FIG. 9, the MRI apparatus includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, A transmission system 5, a reception system 6, a signal processing system 7, a sequencer 4, and a central processing unit (CPU) 8 are provided.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. Thus, a superconducting static magnetic field generation source is arranged around the subject 1. The static magnetic field generation system 2 is accommodated in the gantry 51.

  The gradient magnetic field generation system 3 includes a gradient magnetic field coil 9 wound in the three axes of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil. The gradient magnetic fields Gx, Gy, Gz are applied in the three axis directions of X, Y, and Z by driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the sequencer 4 described later. At the time of imaging, a slice direction 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 1, and the remaining two orthogonal to the slice plane and orthogonal to each other A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded into an echo signal. The gradient magnetic field coil 9 is accommodated in the gantry 51, and the gradient magnetic field power source 10 is accommodated in the casing 53.

  The sequencer 4 is a control means that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence, and operates under the control of the CPU 8 to collect tomographic image data of the subject 1. Various commands necessary for the transmission are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6. The sequencer 4 is accommodated in the housing 53.

  The transmission system 5 irradiates an RF pulse to cause nuclear magnetic resonance to the nuclear spins of atoms constituting the biological tissue of the subject 1, and includes a high frequency oscillator 11, a modulator 12, a high frequency amplifier 13, and a transmission side And a high frequency coil (transmitting coil) 14a. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13 and then placed close to the subject 1. By supplying to the high frequency coil 14a, the subject 1 is irradiated with the RF pulse. In general, the high-frequency coil 14 a is accommodated in the gantry 51, and the others are accommodated in the housing 53.

The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and receives a high-frequency coil (receiving coil) 14b and a signal amplifier 15 on the receiving side. And a quadrature detector 16 and an A / D converter 17. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15, The signal is divided into two orthogonal signals by the quadrature phase detector 16 at the timing according to the command from the sequencer 4, and each signal is converted into a digital quantity by the A / D converter 17 and sent to the signal processing system 7.
Generally, the device group constituting the receiving system 6 is accommodated in a gantry 51.

  The signal processing system 7 performs various data processing and display and storage of processing results, and has an external storage device such as an optical disk 19 and a magnetic disk 18 and a display 20 composed of a CRT, etc. Is input to the CPU 8, the CPU 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display 20, and the magnetic disk 18 of the external storage device. Record in etc. The signal processing system 7 is accommodated in the processing device 54.

  The operation unit 25 inputs various control information of the MRI apparatus and control information of processing performed in the signal processing system 7, and includes a trackball or mouse 23 and a keyboard 24. The operation unit 25 is disposed close to the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

  In FIG. 9, the high-frequency coil 14a and the gradient magnetic field coil 9 on the transmission side face the subject 1 in the static magnetic field space of the static magnetic field generation system 2 into which the subject 1 is inserted, if the vertical magnetic field method is used. If the horizontal magnetic field method is used, the subject 1 is installed so as to surround it. The high-frequency coil 14b on the receiving side is installed so as to face or surround the subject 1.

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

Hereinafter, the present invention in a superconducting magnet apparatus using a superconducting coil as a static magnetic field generation source of the MRI apparatus will be described.
(First embodiment)
First, a first embodiment of the present invention will be described with reference to FIG. FIG. 1 shows a cross-sectional view of a horizontal superconducting magnet. FIG. 1 shows a quarter region, in which a plurality of concentric superconducting coils are arranged around a central axis in the horizontal direction, and a static magnetic field with high uniformity is generated at the center of the magnet. The static magnetic field has a horizontal orientation. The magnet includes a vacuum vessel 101, a heat shield plate 102, a helium tank 103, and a refrigerator (not shown) as components. These are the same as the configuration of the conventional apparatus.

  In the present invention, the superconducting coil for generating a static magnetic field is arranged separately in the helium tank 103 and outside. The coils are roughly classified into a main coil 104 for generating a static magnetic field in the center of the gantry, uniformity control coils 105, 106, and 107 for increasing the uniformity of the static magnetic field, and a leakage magnetic field outside the gantry. The shield coil 108 is composed of three types. As shown in FIG. 1, in this embodiment, the main coil 104 is arranged outside the helium tank 103 and inside the heat shield plate 102. By doing so, the distance from the main coil 104 to the center of the magnetic field can be reduced, and the generation efficiency of the magnetic field can be increased. Or when it is set as the magnetic field generation efficiency similar to the conventional structure, it becomes possible to set it as the gantry (magnet) which improved openness more.

  In order to explain this, the conventional structure shown in FIG. 2 is compared with the structure of the present invention shown in FIG. In both figures, only the vicinity of the main coil 104 is enlarged for the sake of explanation, and the rest is the same as FIG. 2 and 3, the distance from the magnetic field center to the coil is the same. Therefore, the magnetic field generation efficiency is the same. As can be seen from FIG. 3, in this embodiment, since the helium tank 103 does not enter between the heat shield plate 102 and the main coil 104, the inner diameter of the helium tank 103 is equal to the uniformity control coil portion (105 to 107). ).

That is, in the present invention, only the main coil 104 that greatly contributes to the magnetic field generation efficiency is arranged outside the helium tank 103, and the other coils are arranged inside the helium tank 103, thereby solving the conventional problem.
The uniformity control coil generally requires a smaller magnetomotive force (current amount) than the main coil 104. Therefore, even if the distance from the center of the magnetic field is slightly longer, the influence is smaller than that of the main coil.
As a result, the inner diameters of the heat shield plate 102 and the vacuum vessel 101 can be increased. That is, the distance (d) from the central axis to the vacuum vessel 101 can be increased as compared with the conventional example of FIG. As a result, since a wide opening can be realized, an apparatus with high openness can be provided.

On the other hand, in the present embodiment, the total number of coils is 10, which is larger than the four coils in (Patent Document 2). Also, because the shaft length is long, in order to cool all these coils by conduction,
(A) The thermal resistance of the conductive member is greatly reduced.
(B) Use a refrigerator with high capacity.
(C) Increase the number of refrigerators.
It is necessary to take measures such as However, in (a), it is necessary to use a material having high thermal conductivity or to increase the cross-sectional area of the conductive member. These have great demerits such as requiring expensive materials and increasing the weight. In the cases (b) and (c), there is a demerit that the cost of the magnet device increases. Furthermore, in the case of (c), the amount of electric power used by the refrigerator increases, leading to an increase in running cost.

  In the present invention, since the helium tank 103 is provided, even when the refrigerator is stopped, the heat input is consumed by the heat of vaporization of the liquid helium, so the temperature rise of the coil becomes moderate. Therefore, as in (Patent Document 1), even when the refrigerator is stopped, quenching does not occur in a short time, and stable operation is possible.

  Since the main coil 104 is thermally connected to the helium tank 103 by a high thermal conductivity material 109, the main coil 104 is cooled to a temperature that exhibits superconducting performance. This temperature need not necessarily be the boiling point temperature of helium, and may be designed according to the wire material characteristics of the superconducting coil to be used. By using a so-called high-temperature superconducting material 109 for the wire, the temperature limitation can be relaxed.

  As the high thermal conductivity material 109, high-purity aluminum, copper, or the like can be used. Further, it is possible to cool the coil more actively by separately burying a helium pipe in the high thermal conductivity material 109 or bringing it into contact. Alternatively, a material having high flexibility and high thermal conductivity (for example, indium) is sandwiched between the helium tank 103 and the high thermal conductivity material 109, thereby improving the mechanical contact property and thereby increasing the heat transfer coefficient. It is also possible to improve. Further, the high thermal conductivity material 109 can be cooled by a refrigerator.

Further, the coil placed in the helium tank 103 can be uniformly cooled by liquid helium, and it is not necessary to provide a complicated mechanism for conduction cooling.
Furthermore, although not shown in the figure, the PCS and the superconducting connection can be immersed in liquid helium, so that the temperature can be kept constant and the superconducting stability can be ensured.

As described above, according to the present embodiment, since the helium tank is provided and liquid helium is contained therein, even when the refrigerator is stopped, the quench does not occur in a short time and is stable. Operation is possible. In addition, since the superconducting coil other than the main coil, the PCS, and the superconducting connection portion can be immersed in the body helium, it is possible to ensure superconducting stability.
Further, since the main coil can be arranged outside the helium tank, the superconducting magnet can be made small and light.

(Second embodiment)
Next, a second embodiment of the present invention will be described with reference to FIG. This embodiment is an example in which the axial length is shortened in addition to the first embodiment. This depends on the axial position and length of the shield coil 108, but the axial end position of the shield coil 108 can be significantly shorter than that of the main coil 104 due to the specifications of the leakage magnetic field. As shown in FIG. 4, the axial length (L) can be reduced to a length limited by the position of the main coil 104. If the position of the shield coil 108 can be further arranged on the inner side than shown in FIG. 4, the axial length of the vacuum vessel on the outer diameter side can be made shorter than that on the inner diameter side.
As described above, according to this embodiment, in addition to the effects of the first embodiment, the axial length of the superconducting magnet device can be shortened.

(Third embodiment)
Next, a third embodiment of the present invention will be described with reference to FIG. In this embodiment, the end shape of the helium tank 103 is tapered. As a result, in addition to the effects of the first embodiment, it is possible to reduce the wall thickness compared to the saddle type, and the weight can be reduced. In addition, the number of manufacturing steps can be reduced.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described with reference to FIG. In this embodiment, in addition to the main coil 104, a part of the uniformity control coil is also arranged outside the helium tank. In general, in order to obtain high magnetic field uniformity, the current of the uniformity control coil (first control coil) 105 adjacent to the main coil 104 is opposite to the current direction of the main coil 104. Therefore, by integrating the main coil 104 and the first control coil 105, it becomes possible to cancel the electromagnetic forces mutually. As a result, in addition to the effects of the first embodiment, the structure for supporting the coil can be simplified, so that the ease of manufacture and the weight can be reduced.

(Fifth embodiment)
Next, a fifth embodiment of the present invention will be described with reference to FIG. In the present embodiment, a part of the main coil 104 and the shield coil 108 is disposed outside the helium tank 103. Depending on the required leakage magnetic field distribution, it is possible to divide the shield coil 108 into two or more (four or more on one side and on both sides). It can be arranged outside the tank 103. Thereby, in addition to the effect of the first embodiment, the axial length (L) can be shortened as compared with the case where all the shield coils 108 are arranged in the helium tank 103.
In general, since the currents of the main coil 104 and the shield coil 108 are in opposite directions, it is possible to reduce the electromagnetic force by structurally coupling both coils as in the case of the fourth embodiment. Become.

The figure explaining the 1st Example of the superconducting magnet apparatus for MRI apparatuses of this invention. The enlarged view of the main coil vicinity in a prior art example. The enlarged view of the main coil vicinity in the 1st Example of the superconducting magnet apparatus for MRI apparatuses of this invention. The figure explaining the 2nd Example of the superconducting magnet apparatus for MRI apparatuses of this invention. The figure explaining the 3rd Example of the superconducting magnet apparatus for MRI apparatuses of this invention. The figure explaining the 4th Example of the superconducting magnet apparatus for MRI apparatuses of this invention. The figure explaining the 5th Example of the superconducting magnet apparatus for MRI apparatuses of this invention. 1 is a perspective view showing the overall configuration of an embodiment of a horizontal magnetic field MRI apparatus according to the present invention. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Subject, 2 ... Static magnetic field generation system, 3 ... Gradient magnetic field generation system, 4 ... Sequencer, 5 ... Transmission system, 6 ... Reception system, 7 ... Signal processing system, 8 ... Central processing unit (CPU), 9 ... Gradient magnetic field coil, 10 ... Gradient magnetic field power supply, 11 ... High frequency transmitter, 12 ... Modulator, 13 ... High frequency amplifier, 14a ... High frequency coil (transmitting coil), 14b ... High frequency coil (receiving coil), 15 ... Signal amplifier, 16 ... Quadrature phase detector, 17 ... A / D converter, 18 ... Magnetic disk, 19 ... Optical disk, 20 ... Display, 21 ... ROM, 22 ... RAM, 23 ... Trackball or mouse, 24 ... Keyboard, 51 ... Gantry, 52 ... Table, 53 ... Case, 54 ... Processing device

Claims (6)

A plurality of superconducting coils and a cylindrical static magnetic field comprising a cylindrical container for cooling and storing the plurality of superconducting coils to a predetermined temperature for generating a horizontal uniform static magnetic field in a predetermined area Generating means,
The storage container includes a cylindrical refrigerant container containing a refrigerant for cooling the plurality of superconducting coils, and a cylindrical vacuum container for storing the refrigerant container ,
At least one of the plurality of superconducting coils is an external superconducting coil disposed outside the refrigerant container and away from the refrigerant container, and between the cooling container and the external superconducting coil, A thermally conductive member for thermal connection is arranged ,
The plurality of superconducting coils have one or more main coils, one or more uniformity control coils, and one or more shield coils,
The external superconducting coil is a superconducting magnet device that is at least one of the one or more main coils,
The shape of the horizontal end of the refrigerant container on the predetermined region side is a saddle shape, and the external superconducting coil as the main coil is housed in the saddle shape position, part of which is from the inner wall of the coolant container. A superconducting magnet device arranged on the predetermined region side. The external superconducting coil has at least one of the one or more uniformity control coils, and the uniformity control coil is thermally connected to the heat conducting member. Item 2. The superconducting magnet device according to Item 1 . The superconducting magnet device according to claim 1 , wherein the external superconducting coil includes at least one of the one or more shield coils. At least in part, superconducting magnet apparatus according to any one of claims 1 to 3, wherein that it is contained in the thermally conductive member of the external superconducting coil. It said plurality of at least one of the superconducting coil, a superconducting magnet apparatus according to claim 1 to 4 further characterized in that arranged inside the shield plate. Magnetic resonance imaging apparatus having a superconducting magnet apparatus according to any one of claims 1 to 5, wherein.

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