JP5465583B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a technique for improving image quality of a magnetic resonance imaging apparatus using an open magnet.

  An MRI imaging method that uses a nuclear magnetic resonance (NMR) phenomenon to obtain a tomographic image of a human body and obtain internal information thereof is widely used in the medical field and the research field. This MRI imaging method requires a magnet that generates a uniform magnetic field strength in the space in which the imaging region is arranged because it needs to reflect the internal structure of the imaging region of the subject. Many of the magnets used in an MRI apparatus having a high magnetic field strength have a structure using a cylindrical superconducting solenoid coil. MRI apparatuses using superconducting magnets are widely used because they have high image quality and high functionality due to high magnetic field strength and high magnetic field uniformity. However, the MRI apparatus with this structure cannot be imaged unless the subject enters a narrow and long tunnel-like gantry, and is not suitable for examination of patients and children who complain of claustrophobia.

  On the other hand, in an MRI system consisting of a pair of magnets that sandwich the space where the subject is placed, called an open MRI device, from the top and bottom, make the most of the features of the magnet and widen the gantry opening to make the test gentle The environment is realized. In addition, since the direction of the magnetic flux is vertical, using a special receiver coil can provide a higher S / N ratio than the magnetic field strength. On the other hand, an open MRI system that has both high image quality and high functionality is highly expected.

  Since this open MRI apparatus has a structure composed of two divided magnets, it has a paired structure in which a coil as a gradient magnetic field generating means and an RF coil that generates a high-frequency magnetic field are also divided. As a connection means for each of the divided coils, there is a conventional technique described in Patent Document 1 as a known technique. More specifically, the current is supplied to the upper gradient coil only by connecting a soft cable wiring from the lower side.

JP 2004-344182 A

  However, the above prior art has the following problems. In other words, the open-structured magnet with the above-described subject placement space widely open and with a strong magnetic field strength is switched at high speed to implement the above-described high image quality and high function, and It is necessary to incorporate a unit that generates a high gradient magnetic field. Further, since the gradient magnetic field coil for generating the gradient magnetic field is also an open type, it is configured on a flat plate instead of the conventional cylindrical shape.

  In order to improve the strength, increase the speed, and open the structure, it is necessary to electrically connect the two gradient magnetic field generating coils that are divided into flat plates. A cable was used as described in Document 1.

  These cables vibrate due to electromagnetic force because high voltage and high current flow in the vicinity of the imaging space where the magnetic field is generated. For this reason, a sheet metal and a stopper serving as a guide for firmly fixing the cable are used.

  However, when using a gradient magnetic field generation unit that can generate high voltage and high current using a magnet with stronger magnetic field strength for higher image quality and higher functionality, this cable transmits the high voltage and high current to the gradient coil. Even if the sheet metal, the strength of the fixture, and the fixing area are increased, the cable slightly vibrates, and image noise caused by this vibration is superimposed on the echo signal acquired in MR imaging and reconstructed. There was a phenomenon that artifacts appeared in MRI images.

  The present invention has been made to solve the above problems, and the object of the present invention is to suppress vibration with respect to the wiring used to connect two pairs of gradient magnetic field coils and to use an open magnet. It is to improve the image quality of the magnetic resonance imaging apparatus.

In order to solve the above-described problems, the MRI apparatus of the present invention has a pair of static magnetic fields arranged opposite to each other with the space interposed therebetween in order to generate a static magnetic field in the space in which the subject is arranged. Generating means, a gradient magnetic field generating means that is disposed on the space side of the pair of static magnetic field generating means and generates a gradient magnetic field in the space, a first portion, and a second that is more rigid than the first portion. A cable for supplying a current to the gradient magnetic field generating means, and the second part is disposed closer to the space than the first part. Features.

The second portion is disposed at a portion connecting the static magnetic field generating means disposed opposite to each other.
Also, the second portion is located a plurality of, there is covered with resin.
The second portion covered with the resin is installed on the static magnetic field generating means via a metal rigid body.
In addition, a space is provided between the metal rigid body and the static magnetic field generating means.

  According to the present invention, it is possible to suppress the vibration with respect to the wiring used for connecting two pairs of gradient magnetic field coils and improve the image quality of the magnetic resonance imaging apparatus using the open magnet.

1 is an overall configuration diagram of an MRI apparatus to which the present invention is applied. The figure which connected two gradient magnetic field coils with the metal rod. The figure which shows the more detailed cross-section of 102 which is a static magnetic field generating magnet. FIG. 3 is an enlarged view of a cross section A-A ′ in FIG. 2. FIG. 3 is an enlarged view of a portion indicated by a frame B in FIG. FIG. 3 is a diagram showing a structure of a C-C ′ cross section in FIG. 2. The figure which shows the various modified examples of arrangement | positioning of a metal bar.

Hereinafter, embodiments of the present invention will be described in detail.
FIG. 1 is an overall configuration diagram of an MRI apparatus to which the present invention is applied. This MRI apparatus has a static magnetic field generating magnet 102 arranged so as to sandwich a space in which the subject 101 is placed, a gradient magnetic field coil 103 arranged inside the static magnetic field generating magnet 102, and further arranged inside thereof. A high-frequency coil 104 and a detection coil 105 that detects an NMR signal generated from the subject 101. The gradient magnetic field coil 103 and the high frequency coil 104 have a pair of upper and lower plate-like structures so as not to disturb the open shape. Furthermore, a sequencer 106 for controlling the operation timing of each coil, a computer 107 for controlling the apparatus and processing NMR signals to perform imaging, and a transfer table 108 for arranging the subject 101 in the central space of the static magnetic field generating magnet 102 are provided. ing.

  The static magnetic field generating magnet 102, the gradient magnetic field coil 103, the high-frequency coil 104, the detection coil 105, and the transfer table 108 are installed in a shield room 109 that is shielded from electromagnetic waves. Electromagnetic shielding is the resonance frequency (corresponding to the magnetic field strength of the static magnetic field generating magnet 102) of the nucleus used for the inspection (usually hydrogen nuclei are used) for the purpose of preventing the induction coil 105 from inducing external electromagnetic waves. It has an attenuation factor of about 70 dB in the band. Various coils and static magnetic field generating magnets 102, the connection between the transfer table 108 and the power supply and control equipment outside the shield room 109 are connected via a filter circuit 110 grounded to the shield room so as not to draw external noise into the shield room 109, The outside is connected by a coaxial cable covered with a shield layer (the distinction between coaxial cables is not shown in the figure).

  The gradient magnetic field coil 103 is composed of three sets of coils wound so as to change the magnetic flux density in the three axial directions x, y, and z that are orthogonal to each other, each connected to the gradient magnetic field power supply 111, and the gradient magnetic field generating means Configure. The gradient magnetic field power supply 111 is driven in accordance with a control signal from a sequencer 106, which will be described later, and the current value flowing through the gradient magnetic field coil 103 is changed to change the gradient magnetic fields Gx, Gy, Gz consisting of three axes into the arrangement space of the subject 101. It is superimposed on the static magnetic field. This gradient magnetic field is used to identify the spatial distribution of NMR signals obtained from the imaging region of the subject 101.

  The high-frequency coil 104 is connected to a high-frequency power amplifier 112 for flowing a high-frequency current, and generates a high-frequency magnetic field for resonantly exciting hydrogen nuclei in the imaging region of the subject 101. The high frequency power amplifier 112 is also controlled by a control signal from the sequencer 106.

  The detection coil 105 is connected to the receiver 113 and constitutes a means for detecting the NMR signal. The receiver 113 amplifies and detects the NMR signal detected by the detection coil 105 and converts it into a digital signal that can be processed by the computer 107. The operation timing of the receiver 113 is also controlled by the sequencer 106.

  The computer 107 performs operations such as image reconstruction and spectrum calculation using the NMR signals converted into digital quantities, and controls the operation of each unit of the MRI apparatus via the sequencer 106 at a predetermined timing. The computer 107, the display device 114 for displaying processed data, and the console 115 for performing operation input constitute an arithmetic processing system.

  In the case of an MRI apparatus having an open structure as shown in FIG. 1, the gradient magnetic field coil 103 divided into two vertically and the gradient magnetic field power supply 111 are connected to each axis x, y, and z. In order to connect the two gradient magnetic field coils 103, by using a metal rod 116 having higher rigidity than a normal cable as shown in FIG. 2, it is less expensive than the case of Patent Document 1 in which a cable is used for connection. In addition, a connection that does not generate vibrations can be performed. 2 is the lower part of the metal bar 116, but may be anywhere on the metal bar by the wiring path. In FIG. 2, the two metal rods are described corresponding to the positive and negative directions of the current direction. Actually, for the X-axis direction gradient coil, the Y-axis gradient coil There are a total of six types for the Z-axis gradient magnetic field coil, and the remaining four are omitted.

  Further, in FIG. 2, the static magnetic field generating magnet 102 has a cross-sectional structure as shown in FIG. 3 in more detail. In FIG. 3, 102a is a superconducting coil that constitutes a static magnetic field generating magnet, and 102b is a connecting tube that mechanically or electrically connects static magnetic field generating magnets arranged so as to sandwich a space in which a subject is placed above and below. It is. FIG. 2 is a view of FIG. 3 as viewed from the right side or the left side in the horizontal direction, and a metal rod 116 is disposed along the connecting pipe 102b. Thereby, it becomes possible to wire without impairing openness. However, regarding the horizontal position of the metal rod 116 arranged along the connecting pipe, when the metal rod 116 is thick, it is arranged at an equal interval. A member (for example, a cooling pipe or the like in the case of cooling a gradient magnetic field coil by a water cooling method) may be disposed at a vacant position. In addition, the six metal rods 116 have a positive direction and a negative direction from the viewpoint of canceling the magnetic field induced by the metal rod 116 for each of the X axis direction, the Y axis direction, and the Z axis direction. Adjacent to each other.

  That is, according to the present embodiment, in order to generate a static magnetic field in a space in which the subject is disposed, the static magnetic field generating means disposed vertically opposite to each other with the space interposed therebetween, and the static magnetic field generation In the magnetic resonance imaging apparatus provided with the gradient magnetic field generating means that is disposed on the space side of the means and generates a gradient magnetic field in the space, a cable for supplying a current to the gradient magnetic field generating means includes a first portion And a second portion (metal rod 116) having higher rigidity than the first portion, and the second portion is disposed closer to the space than the first portion. As described above, the reason why the position close to the space is constituted by a highly rigid metal rod is as follows. That is, since the static magnetic field strength is high at a position close to the space due to the structure of the static magnetic field generating magnet, it is considered that the force that causes vibration due to the Lorentz force acting on the current supplied to the gradient coil is large. This is because it is necessary to increase the rigidity for prevention. The second part is arranged at a part connecting the static magnetic field generating means arranged opposite to each other. In addition, the second portion is disposed in the vicinity of the portion (connecting tube 102b) that mechanically connects the static magnetic field generating means disposed opposite to each other. As a result, it is possible to minimize the reduction in the opening of the magnet by providing the metal rod.

  On the other hand, an enlarged view of the A-A ′ cross section in FIG. 2 is as shown in FIG. In FIG. 4, 116a is a metal rigid body (metal bar) constituting a metal rod, and 117a and 117b are resins configured to cover the metal rod 116a. Each metal rigid body 116a has a positive direction and a negative direction for each of the X axis, the Y axis, and the Z axis, and a total of six are arranged. In order to prevent the occurrence of static electricity, the outside is covered with the resins 117a and 117b. Specifically, the resin 117a has a concave surface on one side, and a metal rigid body 116a is sandwiched between them, and the resin 117b is covered with a cover and fastened with a screw. This ensures insulation and secures service work.

  FIG. 5 is an enlarged view of a portion indicated by a frame B in FIG. As shown in FIG. 5, the metal rigid body 116a protrudes from the resins 117a and 117b at the ends. There is a hole in it, and the cable can be tied.

  FIG. 6 is a cross-sectional structure taken along the line C-C ′ in FIG. 2, and shows a structure in which the metal rigid body 116a and the resins 117a and 117b constituting the metal rod are installed on the magnet body. As shown in FIG. 6, if the resin whose cross section is shown in FIG. 4 is installed on the metal rigid body 118 with the bolt 119a and the metal rigid body 118 is installed on the magnet body with the bolt 119b, the metal rod can be fixed more stably. it can. However, the metal rigid body 118 and the magnet body are spaced apart so as not to contact each other. The reason for this is that when the metal rigid body comes into contact with the magnet body, it will rattle each time a current is applied to the gradient coil. This is because shot noise occurs. However, the metal rigid body 118a and the resins 117a and 117b and the magnet body may not be sandwiched between the metal rigid body 116a and the magnet body. In this case, the sandwiched resin and the magnet body may be brought into contact with each other. .

  Further, in order to attach the metal rigid body 118 to the magnet body, the magnet body is provided with a projection 102c by welding, and a female screw is cut there, but the reason for providing the projection 102c on the magnet body side is This is because when the magnet is a superconducting magnet, the thickness of the vacuum vessel constituting the magnet body can be reduced. Needless to say, if the magnet body is composed of a permanent magnet or the like, a protrusion on the metal rigid body 118 side may be provided. According to the present embodiment, there are a plurality of the second portions, which are covered with resin.

  The present invention can be variously modified without departing from the scope of the invention. For example, it goes without saying that the present invention is used for a permanent magnet as a static magnetic field generating magnet. In FIG. 2, the metal bar 116 is arranged from the upper surface to the lower surface of the static magnetic field generating magnet, but it may be as shown in FIGS. 7 (a) to 7 (c). That is, as shown in FIG. 7 (a), in the case of only the central part in the vertical direction of the magnet, there is an advantage that only a necessary part is provided with a metal bar. As shown in FIG. 7 (b), when only the upper and lower ends of the magnet are used, there is an advantage that this is an effective method for a magnet having a high magnetic flux density at the upper and lower ends of the magnet. When divided, as shown in Fig. 7 (c), use 4 for large magnets in the vertical direction and 3 for small magnets in the vertical direction. There is an advantage that it is a method that can be performed, and there is an advantage that it is a useful method in the sense of avoiding the protrusion when it is in the middle of the vertical direction.

The present invention can be used for an MRI apparatus using an open magnet.

  102 Static magnetic field generating magnet, 103 Gradient magnetic field coil, 104 High frequency coil, 102b Connecting pipe, 116 Metal rod

Claims (5)

To generate the static magnetic field in a space in which a subject is placed, a pair of static magnetic field generating means disposed to face in between the spaces,
Disposed in the space side of said pair of static magnetic field generating means, a gradient magnetic field generating means for generating a gradient magnetic field in said space,
A cable comprising a first part and a second part having a higher rigidity than the first part, and supplying a current to the gradient magnetic field generating means;
With
The magnetic resonance imaging apparatus, wherein the second portion is disposed at a position closer to the space than the first portion. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the second portion is disposed at a portion connecting the pair of static magnetic field generating means. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the second portion includes a plurality of portions, at least one of which is covered with a resin . The magnetic resonance imaging apparatus according to claim 3 , wherein the second portion covered with the resin is installed on the static magnetic field generation unit via a metal rigid body . The magnetic resonance imaging apparatus according to claim 4, wherein a space is provided between the metal rigid body and the static magnetic field generating means .

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