Abstract:
A ceiling board free from trouble due to bending is to be provided. In the ceiling board to be supported horizontally by its peripheral parts as fixed parts, the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of Japanese Application No. 2001-173896 filed Jun. 8, 2001. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a ceiling board, a designing method for the ceiling board, a manufacturing method for the ceiling board, a magnet cover, a magnet system and a magnetic resonance imaging apparatus, and more particularly to a horizontally supported ceiling board, a designing method for such a ceiling board, a manufacturing method for such a ceiling board, a magnet cover having such a ceiling board, and a magnet system having such a cover, and a magnetic resonance imaging apparatus provided with such a magnet system. 
   In a magnetic resonance imaging (MRI) apparatus, the subject of imaging is brought into the internal space of its magnet system, i.e. an imaging space in which a magnetostatic field is formed, a gradient magnetic field and a high frequency magnetic field are applied to the subject, magnetic resonance signals are generated from spins therein, and an image is reconstructed on the basis of the signals so received. 
   A magnet system of a vertical magnetic field type has a magnet body having an upper pole section and a lower pole section opposite to each other in the vertical direction with the imaging space between them. The magnet body is housed in a magnet cover. The magnet cover has a ceiling board covering the end face of the upper pole section and a floor board covering the end face of the lower pole section, and the space between these ceiling board and floor board serves as the imaging space. In a full human body size magnetic resonance imaging apparatus, the diameter of the upper pole section and the lower pole section is almost 1 m, and the ceiling board and the floor board have correspondingly large diameters. 
   In order to efficiently form a required magnetic field space, as a magnet system has its upper pole section and lower pole section to be opposite to each other at the minimum required distance between them, the distance between the ceiling board and the floor board tends to be short. Therefore, when the ceiling board is bent by its own weight, its distance from the floor board is correspondingly shortened, making it more difficult for a patient with a head coil on, for instance, to be brought into the imaging space. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to realize a ceiling board free from trouble due to bending, a designing method for such a ceiling board, a manufacturing method for such a ceiling board, a magnet cover having such a ceiling board, a magnet system having such a magnet cover, and a magnetic resonance imaging apparatus provided with such a magnet system. 
   (1) According to one aspect of the invention to solve the problem noted above, there is provided a ceiling board to be supported horizontally by its peripheral parts as fixed parts, the ceiling board characterized in that the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   (2) According to another aspect of the invention to solve the problem noted above, there is provided a ceiling board to be supported horizontally by its peripheral parts and central part as fixed parts, the ceiling board characterized in that the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   (3) According to another aspect of the invention to solve the problem noted above, there is provided a designing method for a ceiling board to be supported horizontally by its peripheral parts as fixed parts, the ceiling board designing method characterized in that the quantity of dome shaping in a direction reverse to the direction of bending of the part between the fixed parts by gravity is equalized to the quantity of bending of the part between the fixed parts by the part&#39;s own weight. 
   (4) According to another aspect of the invention to solve the problem noted above, there is provided a designing method for a ceiling board to be supported horizontally by its peripheral parts and central part as fixed parts, the ceiling board designing method characterized in that the quantity of dome shaping in a direction reverse to the direction of bending of the part between the fixed parts by gravity is equalized to the quantity of bending of the part between the fixed parts by the part&#39;s own weight. 
   (5) According to another aspect of the invention to solve the problem noted above, there is provided a manufacturing method for a ceiling board to be supported horizontally by its peripheral parts as fixed parts, the ceiling board manufacturing method characterized in that the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   (6) According to another aspect of the invention to solve the problem noted above, there is provided a manufacturing method for a ceiling board to be supported horizontally by its peripheral parts and central part as fixed parts, the ceiling board manufacturing method characterized in that the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   (7) According to another aspect of the invention to solve the problem noted above, there is provided a magnet cover, for a magnet body having an upper pole section and a lower pole section opposite to each other with a space between them, having a ceiling board covering the end face of the upper pole section and a floor board covering the end face of the lower pole section, the magnet cover characterized in that the ceiling board is horizontally supported by its peripheral parts as fixed parts, and the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   (8) According to another aspect of the invention to solve the problem noted above, there is provided a magnet cover, for a magnet body having an upper pole section and a lower pole section opposite to each other with a space between them, having a ceiling board covering the end face of the upper pole section and a floor board covering the end face of the lower pole section, the magnet cover characterized in that the ceiling board is horizontally supported by its peripheral parts and central part as fixed parts, and the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   (9) According to another aspect of the invention to solve the problem noted above, there is provided a magnet system having a magnet body having an upper pole section and a lower pole section opposite to each other with a space between them and a magnet cover having a ceiling board covering the end face of the upper pole section and a floor board covering the end face of the lower pole section, the magnet system characterized in that the ceiling board is horizontally supported by its peripheral parts as fixed parts, and the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   (10) According to another aspect of the invention to solve the problem noted above, there is provided a magnet system having a magnet body having an upper pole section and a lower pole section opposite to each other with a space between them and a magnet cover having a ceiling board covering the end face of the upper pole section and a floor board covering the end face of the lower pole section, the magnet system characterized in that the ceiling board is horizontally supported by its peripheral parts and central part as fixed parts, and the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   (11) According to another aspect of the invention to solve the problem noted above, there is provided a magnetic resonance imaging apparatus provided with a magnet system, a patient supporting apparatus for supporting a patient so as to enable him or her to be brought into and out of an imaging space of the magnet system, a signal acquisition means for acquiring magnetic resonance signals from the patient by using the magnet system, and an image generating means for generating an image on the basis of the magnetic resonance signals, the magnetic resonance imaging apparatus characterized in that the magnet system has a magnet body having an upper pole section and a lower pole section opposite to each other with the imaging space between them and a magnet cover having a ceiling board covering the end face of the upper pole section and a floor board covering the end face of the lower pole section, the ceiling board is horizontally supported by its peripheral parts as fixed parts, and the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   (12) According to another aspect of the invention to solve the problem noted above, there is provided a magnetic resonance imaging apparatus provided with a magnet system, a patient supporting apparatus for supporting a patient so as to enable him or her to be brought into and out of an imaging space of the magnet system, a signal acquisition means for acquiring magnetic resonance signals from the patient by using the magnet system, and an image generating means for generating an image on the basis of the magnetic resonance signals, the magnetic resonance imaging apparatus characterized in that the magnet system has a magnet body having an upper pole section and a lower pole section opposite to each other with the imaging space between them and a magnet cover having a ceiling board covering the end face of the upper pole section and a floor board covering the end face of the lower pole section, the ceiling board is horizontally supported by its peripheral parts and central part as fixed parts, and the part between the fixed parts, in a state wherein it is bent by its own weight, is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal. 
   According to any of the aspects of the invention described in (1) through (12), as the part between the fixed parts of the ceiling board is formed in a dome shape in a direction reverse to the direction of bending by the part&#39;s own weight so that the board surface be horizontal, the board will never be bent farther downward beyond a horizontal state. Therefore, no trouble due to bending will arise. Incidentally, it is desirable for the ceiling board to have a circular main face with a view to improving its isotropy. 
   According to the present invention, it is possible to realize a ceiling board free from trouble due to bending, a designing method for such a ceiling board, a manufacturing method for such a ceiling board, a magnet cover having such a ceiling board, a magnet system having such a magnet cover, and a magnetic resonance imaging apparatus provided with such a magnet system. 
   Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an apparatus, which is an example of mode of implementing the present invention. 
       FIG. 2  is a schematic diagram showing external looks of a magnet cover. 
       FIG. 3  is another schematic diagram showing external looks of the magnet cover. 
       FIG. 4  is a schematic diagram of the A—A section in FIG.  2 . 
       FIG. 5  is a schematic diagram showing external looks of a magnet body. 
       FIG. 6  is another schematic diagram showing external looks of the magnet body. 
       FIG. 7  is a schematic diagram of the B—B section in FIG.  5 . 
       FIG. 8  is a plan of a ceiling board. 
       FIG. 9  is a schematic diagram showing a sectional shape of the ceiling board. 
       FIG. 10  is another plan of a ceiling board. 
       FIG. 11  is another schematic diagram showing a sectional shape of the ceiling board. 
       FIG. 12  is a chart of a flow to compute the quantity of shaping. 
       FIG. 13  is a drawing illustrating the sequence of steps for manufacturing the ceiling board. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A mode of carrying out the present invention will be described in detail below with reference to drawings.  FIG. 1  is a block diagram of a magnetic resonance imaging apparatus. This apparatus is an example of the mode of implementing the invention. The configuration of this apparatus represents an apparatus, which is an example of the mode of implementing the invention. 
   As shown in the diagram, the apparatus has a magnet system  100 . The magnet system  100  has main magnetic field sections  102 , gradient coil sections  106  and RF (radio frequency) coil sections  108 . The main magnetic field sections  102 , the gradient coil sections  106  and the RF coil sections  108  are all paired ones, one opposite the other with a space between them. Each is substantially disk-shaped, all of them being arranged around a common central axis. The magnet system  100  is an example of the mode of implementing the invention, a magnet system embodying the invention. The magnet system  100  also is an example of implementing a magnet system according to the invention. 
   The main magnetic field section  102 , the gradient coil section  106  and the RF coil section  108  in the magnet system  100  are housed in a magnet cover to be described afterwards. 
   A subject  1 , mounted on a table  500 , is brought into and out of the internal space (bore) of the magnet system  100 . The table  500  is an example of realizing a patient supporting apparatus according to the invention. The table  500  is driven by a table drive section  120 . 
   The main magnetic field section  102  forms a magnetostatic field in the internal space of the magnet system  100 . The direction of the magnetostatic field is substantially orthogonal to that of the bodily axis of the subject  1 . Thus it forms a so-called vertical magnetic field. The main magnetic field section  102  is configured of, for instance, a permanent magnet or the like. Incidentally, it may obviously be configured as a superconducting electromagnet, an ordinarily conducting electromagnet or the like, instead of a permanent magnet. 
   The gradient coil section  106  creates three gradient magnetic fields for giving gradients to respective magnetostatic field intensities in the directions of three mutually normal axes, i.e. a slice axis, a phase axis and a frequency axis. The gradient coil section  106  has three lines of gradient coils (not shown), each matching one or another of the three gradient magnetic fields. 
   The RF coil section  108  transmits to the magnetostatic field space an RF pulse (radio frequency pulse) for exciting spins within the body of the subject  1 . The RF coil section  108  also receives magnetic resonance signals generated by the excited spins. The RF coil section  108  may either be a single coil for both transmission and reception or include separate coils, one for transmission and the other for reception. 
   To the gradient coil section  106  is connected with a gradient drive section  130 . The gradient drive section  130  provides a drive signal to the gradient coil section  106  to cause a gradient magnetic field to be generated. The gradient drive section  130  has three lines of drive circuits (not shown), respectively matching the three lines of gradient coils in the gradient coil section  106 . 
   To the RF coil section  108  is connected with an RF drive section  140 . The RF drive section  140  provides a drive signal to the RF coil section  108  to transmit an RF pulse, and excites spins within the body of the subject  1 . 
   To the RF coil section  108  is connected with a data collecting section  150 . The data collecting section  150  takes in receive signals received by the RF coil section  108  by sampling, and collects them as digital data. 
   To the table drive section  120 , the gradient drive section  130 , the RF drive section  140  and the data collecting section  150  is connected with a control section  160 . The control section  160  controls the units from the table drive section  120  through the data collecting section  150  to carry out imaging. The part including the gradient drive section  130 , the RF drive section  140  and the data collecting section  150  constitutes an example of realizing a signal acquisition means according to the present invention. 
   The control section  160  is configured by using, for instance, a computer or the like. The control section  160  has a memory (not shown). The memory stores programs for the control section  160  and various data. The functions of the control section  160  are realized by the execution of programs stored in the memory by the computer. 
   The output side of the data collecting section  150  is connected to a data processing section  170 . Data collected by the data collecting section  150  are inputted to the data processing section  170 . The data processing section  170  is configured by using, for instance, a computer or the like. The data processing section  170  has a memory (not shown). The memory stores programs for the data processing section  170  and various data. 
   The data processing section  170  is connected to the control section  160 . The data processing section  170  is positioned superior to, and exercises general control over, the control section  160 . The functions of this apparatus are realized by the execution of programs, stored in the memory, by the data processing section  170 . 
   The data processing section  170  stores data, collected by the data collecting section  150 , into the memory. A data space is formed within the memory. This data space constitutes a two-dimensional Fourier space. The Fourier space will also be referred to as the k-space hereinafter. The data processing section  170  reconstructs an image of the subject  1  by subjecting data in the k-space to a two-dimensional inverse Fourier transform. The data processing section  170  constitutes an example of realizing an image generation means according to the present invention. 
   To the data processing section  170  is connected with a display section  180  and an operating section  190 . The display section  180  is configured of a graphic display or the like. The operating section  190  is constituted of a keyboard or the like, provided with a pointing device. 
   The display section  180  displays reconstructed images and various information outputted from the data processing section  170 . The operating section  190  is operated by the user to input various commands, information and so forth to the data processing section  170 . The user interactively operates this apparatus through the display section  180  and the operating section  190 . 
   The magnet cover will be described now.  FIG. 2 , FIG.  3  and  FIG. 4  schematically illustrate the external looks of the magnet cover.  FIG. 2  shows a front view;  FIG. 3 , a profile; and  FIG. 4 , an A—A section with reference to FIG.  2 . The magnet cover illustrated in these drawings is an example of magnet cover in the mode of implementing the invention. As shown in these drawings, the magnet cover has an upper cover  112 , a lower cover  114  and a pair of support covers  116 . 
   The bottom face of the upper cover  112  constitutes a ceiling board  112   a . The top face of the lower cover  114  constitutes a floor board  114   a . The ceiling board  112   a  is an example of ceiling board in the mode of implementing the invention. The floor board  114   a  is an example of floor board in the mode of implementing the invention. 
   A patient  1  is brought into the space between the ceiling board  112   a  and the floor board  114   a  using the table  500 . The ceiling board  112   a  and the floor board  114   a  are configured of a non-magnetic and electrically insulating material, such as plastics for instance. 
   A magnet body is housed in such a magnet cover.  FIG. 5 , FIG.  6  and  FIG. 7  schematically illustrate the external looks of the magnet body.  FIG. 5  shows a front view;  FIG. 6 , a profile; and  FIG. 7 , a B—B section with reference to FIG.  5 . As shown in these drawings, the magnet body has an upper structure  111 , a lower structure  113  and a pair of supports  115 . The magnet body shown in these drawings is an example of magnet body in the mode of implementing the invention. 
   The bottom part of the upper structure  111  constitutes a columnar upper pole section  111   a . The top part of the lower structure  113  constitutes a columnar lower pole section  113   a . Each of the upper pole section  111   a  and the lower pole section  113   a  includes a main magnetic field section  102 , a gradient coil section  106  and an RF coil section  108 . The upper pole section  111   a  and the lower pole section  113   a  are examples of the upper pole section and the lower pole section, respectively, in the mode of implementing the invention. 
     FIG. 8  shows a plan of the ceiling board  112   a . As illustrated in the drawing, the ceiling board  112   a  has a circular pan section  200 . The pan section  200 , constituting the main body of the ceiling board  112   a , has an adequate shape and size for covering the lower face of the upper pole section  111   a . The pan section  200  has ear sections  202  and  204  on its periphery. The ceiling board  112   a , using holes  202   a  and  204   a  bored in these ear sections  202  and  204 , are fixed to the magnet body the upper structure  111 . 
     FIG. 9  schematically shows the C—C section of the ceiling board  112   a . As illustrated in the drawing, the pan section  200  is shaped to constitute an easily sloped dome. It is so shaped to a sufficient extent for the board surface of the pan section  200  to become horizontal as indicated by the one-dot chain line when the ceiling board  112   a  is made horizontal as it is bent by its own weight. 
   Since this dome shape only makes the board surface horizontal, even if the ceiling board  112   a  is bent by its own weight, its distance from the floor board  114   a  is kept as prescribed, but not shortened. Therefore, no trouble will arise even if a patient with a head coil, i.e. an RF coil for exclusive use on the head, is brought in. 
   The ceiling board  112   a  may have a hole  200   a  bored in its central part as illustrated in  FIG. 10  to fix the central part, using this hole  200   a , to the upper structure  111  of the magnet body by, for instance, screwing. In that case, the pan section  200  is so shaped, as shown in  FIG. 11 , that both sides of the central part constitute a dome shape throughout its circumference. It is so shaped to a sufficient extent for the board surface of the pan section  200  to become horizontal as indicated by the one-dot chain line when the ceiling board  112   a  is made horizontal as it is bent by its own weight. 
   This dome shape prevents, even if the ceiling board  112   a  is bent by its own weight, its distance from the floor board  114   a  from being shortened. Therefore, no trouble will arise even if a patient with a head coil, i.e. an RF coil for exclusive use on the head, is brought in. 
   The extent of this dome shaping can be computed by computerized simulation.  FIG. 12  shows a flow chart of the procedure of computing the extent of shaping. This flow chart shows an example of implementing the designing method according to the present invention. 
   As charted in the drawing, conditions are set at step  300 . This condition setting results in the setting of a model for use in designing by a finite element method. The model has the same size and shape as the ceiling board excluding the quantity of the dome shape. The set conditions also include the Young&#39;s modulus and mechanical constants regarding the ceiling board material, such as the coefficient of rigidity for instance. 
   Then at step  302 , the initial value of the quantity of shaping is inputted. 
   Next at step  304 , the quantity of bending of the ceiling board, having a dome shape formed exactly to the initial value, due to its own weight is computed. The quantity of bending is computed by a finite element method. Out of available finite element methods, a displacement method supposing the quantity of displacement to be unknown is used. 
   Then at step  306 , it is judged whether or not the board surface of the ceiling board is horizontal in the bent state. 
   If the board surface is not horizontal, the quantity of shaping is corrected at step  308 ; at step  304 , the bending of a dome having a new quantity of shaping due to bending is computed and, at step  306 , it is judged whether or not the board surface is horizontal. These steps are repeated thereafter to correct the quantity of shaping consecutively and, when a quantity of shaping that makes the board surface horizontal, the quantity of shaping is determined at step  310 . 
   The ceiling board  112   a  designed in this manner is manufactured in, for instance, the sequence of steps illustrated in FIG.  13 . The sequence shown in the drawing is an example of manufacturing method in the mode of implementing the invention. As shown therein, a mold for use in shaping is produced at step  402 . In the production of the mold, the designed value of the quantity of shaping is used. Next at step  404 , using that mold, the ceiling board  112   a  is shaped. 
   Whereas the present invention has been described so far with reference to examples of a preferred mode of implementing it, persons having usual knowledge in the technical field to which the invention belongs can apply various modifications or substitutions to these examples of the mode of implementation without deviating from the technical scope of the invention. Therefore, the technical scope of the invention includes not only the above-described examples of the mode of implementation but all the modes of implementation covered by the scope of claims for the patent.