Abstract:
With a view toward implementing a thermal controlling method for making reversible a temperature characteristic of a magnetic field generator using permanent magnets small in Hcj, a magnetic field generator whose temperature characteristic is reversible, using permanent magnets small in Hcj, and an MRI apparatus provided with such a magnetic field generator, there is provided a method for controlling the temperature of a magnetic field generator having a pair of disc-shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween, and a yoke that forms return passes for magnetic fluxes of the permanent magnets, comprising the steps of raising the temperature from room temperature to a temperature higher than the room temperature, maintaining the temperature higher than the room temperature, and lowering the temperature from the temperature higher than the room temperature to the room temperature, whereby the temperature characteristics of the permanent magnets are made reversible.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Chinese Application No. 200610121228.1 filed Jul. 14, 2006. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a thermal controlling method, a magnetic field generator and an MRI (Magnetic Resonance Imaging) apparatus, and more specifically to a method for controlling the temperature of a permanent magnet-type magnetic field generator, a permanent magnet-type magnetic field generator, and an MRI apparatus provided with such a magnetic field generator. 
         [0003]    An MRI apparatus acquires a magnetic resonance signal under a magnetic field generated by a magnetic field generator and reconstructs an image, based on the magnetic resonance signal. As one apparatus of the magnetic field generator, there is known one using permanent magnets. A pair of disc-shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween is used in such a magnetic field generator. As the permanent magnet, a magnet composed of an Nd—Fe—B alloy, i.e., a Neodymium magnet is used. 
         [0004]    A magnetic field strength of the magnetic field generator varies depending upon ambient temperatures on the basis of the temperature characteristics of the permanent magnets. Therefore, the magnetic field generator is used in a state in which its temperature is being raised to a temperature higher than room temperature, thereby to avoid the influence of a change in room temperature (refer to, for example, a Patent Document 1). 
         [0005]    [Patent Document 1] Japanese Unexamined Patent Publication No. 2000-287950 
         [0006]    BH curves of a permanent magnet vary with temperature. As shown in  FIG. 8(   a ), for example, a BH curve given by a linear curve L 1  at a temperature T 1  results in a linear curve L 2  parallel-moved downward at a temperature T 2  (&gt;T 1 ). This change is reversible and the BH curve is restored to the linear curve L 1  if the temperature is returned to T 1 . 
         [0007]    However, the BH curve at the temperature T 2  becomes partly non-linear as indicated by a dashed line L 3 . When an operating point P of the permanent magnet is placed on such a non-linear region, the BH curve is not returned to the linear curve L 1  even though the temperature is restored to T 1 . This is because the operating BH curve at the temperature T 2  results in a linear curve L 4  parallel-moved further down as indicated by a broken line L 4 . 
         [0008]    On the other hand, when the operating point P of the permanent magnet is placed on a linear region L 21  even where the BH curve at the temperature T 2  has a non-linear region L 22  as shown in  FIG. 8(   b ), the BH curve is restored to a linear curve L 1  if the temperature is returned to T 1 . That is, the temperature characteristic of the permanent magnet is made reversible when the operating point is placed on the linear region of the BH curve, whereas when the operating point is placed on the non-linear region of the BH curve, the temperature characteristic thereof is rendered irreversible. 
         [0009]    The operating point of the permanent magnet is determined depending upon demagnetization. The smaller the demagnetization, the higher the operating point (the higher the magnetic flux density). The larger the demagnetization, the lower the operating point (the lower the magnetic flux density). As the operating point becomes high (demagnetization becomes small), it is easy to fall into the linear region. As the operating point becomes low (demagnetization becomes large), it is easy to fall into the non-linear region. 
         [0010]    A non-linear region is small at a magnet large in Hcb/Br and Hcj (thus a linear region is large). A non-linear region is large at a magnet small in Hcb/Br and Hcj (thus a linear region is small). They will be shown in  FIGS. 9(   a ) and  9 ( b ) respectively. In both figures, a graph having a slope corresponds to a BH curve, and a slope-free curve corresponds to a JH curve. Incidentally, Hcb indicates retentivity as to a magnetic flux density B, Br indicates residual magnetism, and Hcj indicates retentivity as to magnetization J. Since Hcj is also large when Hcb/Br is large, Hcb/Br is typified by Hcj below. 
         [0011]    The magnet large in Hcj may preferably be used to make the temperature characteristic reversible. However, such a magnet becomes extremely expensive because it contains Dysprosium corresponding to a rare element. On the other hand, the magnet small in Hcj is relatively low in cost because it does not contain Dysprosium. 
       SUMMARY OF THE INVENTION 
       [0012]    Therefore, an object of the invention is to provide a thermal controlling method for making reversible a temperature characteristic of a magnetic field generator using permanent magnets small in Hcj, a magnetic field generator whose temperature characteristic is reversible, using permanent magnets small in Hcj, and an MRI apparatus equipped with such a magnetic field generator. 
         [0013]    The invention according to one aspect, for solving the above problem is a method for controlling a temperature of a magnetic field generator having a pair of disc-shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween, and a yoke which forms return passes for magnetic fluxes of the permanent magnets, comprising the steps of raising the temperature from a room temperature to a temperature higher than the room temperature, maintaining the temperature higher than the room temperature, lowering the temperature from the temperature higher than the room temperature to the room temperature, and making temperature characteristics of the permanent magnets reversible. 
         [0014]    Each of the permanent magnets may preferably operate in a non-linear region of a BH curve in that it may be a magnet small in Hcj. 
         [0015]    Preferably, the temperature raising step has a first temperature rise step for raising the temperature from the room temperature to 35° C., and a second temperature rise step for raising the temperature from 35° C. to 45° C., the temperature lowering step has a first temperature fall step for lowering the temperature from 45° C. to 35° C., and a second temperature fall step for lowering the temperature from 35° C. to the room temperature, and the temperature maintaining step has a first maintenance step for maintaining a temperature of 35° C. over two hours after the first temperature rise step, a second maintenance step for maintaining a temperature of 45° C. over two hours after the second temperature rise step, and a third maintenance step for maintaining the temperature of 35° C. over one hour after the first temperature fall step, in that reversibility of the temperature characteristic of the magnetic field generator is improved. 
         [0016]    The room temperature may preferably range from 10° C. to 25° C. in that validity is obtained. 
         [0017]    The temperature controlling may preferably be performed prior to shimming of a magnetic field in that workability is good. 
         [0018]    The invention according to another aspect, for solving the above problem is a magnetic field generator comprising a pair of disc-shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween, and a yoke which forms return passes for magnetic fluxes of the permanent magnets, wherein each of the permanent magnets includes a first portion relatively large in demagnetization, which is constituted of a magnetic material relatively large in Hcj, and a second portion relatively small in demagnetization, which is constituted of a magnetic material relatively small in Hcj. 
         [0019]    The invention according to a further aspect, for solving the above problem is an MRI apparatus comprising a magnetic field generator including a pair of disc-shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween, a yoke which forms return passes for magnetic fluxes of the permanent magnets, gradient magnetic field coils, and RF coils, wherein each of the permanent magnets includes a first portion relatively large in demagnetization, which is constituted of a magnetic material relatively large in Hcj, and a second portion relatively small in demagnetization, which is constituted of a magnetic material relatively small in Hcj. 
         [0020]    Preferably, the first portion is a peripheral edge portion of the permanent magnet, and the second portion is a portion located inside of the peripheral edge portion of the permanent magnet in that adaptability to a demagnetization strength is good. 
         [0021]    Each of the permanent magnets is preferably an Nd—Fe—B magnet in that its performance is good. 
         [0022]    Each of the permanent magnets preferably has a pole piece smaller than a magnetic face thereof in area in that magnetic field uniformity is improved. 
       EFFECTS OF THE INVENTION  
       [0023]    According to the invention related to one aspect, the thermal controlling method is a method for controlling a temperature of a magnetic field generator having a pair of disc-shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween, and a yoke which forms return passes for magnetic fluxes of the permanent magnets, comprising the steps of raising the temperature from a room temperature to a temperature higher than the room temperature, maintaining the temperature higher than the room temperature, lowering the temperature from the temperature higher than the room temperature to the room temperature, and making temperature characteristics of the permanent magnets reversible. It is therefore possible to implement a thermal controlling method which makes reversible the temperature characteristic of a magnetic field generator using permanent magnets small in Hcj. 
         [0024]    According to the invention related to another aspect, the magnetic field generator is a magnetic field generator comprising a pair of disc-shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween, and a yoke which forms return passes for magnetic fluxes of the permanent magnets, wherein each of the permanent magnets includes a first portion relatively large in demagnetization, which is constituted of a magnetic material relatively large in Hcj, and a second portion relatively small in demagnetization, which is constituted of a magnetic material relatively small in Hcj. It is therefore possible to realize a magnetic field generator whose temperature characteristic is reversible, using permanent magnets small in Hcj. 
         [0025]    According to the invention related to yet another aspect, the MRI apparatus is an MRI apparatus comprising a magnetic field generator including a pair of disc-shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween, a yoke which forms return passes for magnetic fluxes of the permanent magnets, gradient magnetic field coils, and RF coils, wherein each of the permanent magnets includes a first portion relatively large in demagnetization, which is constituted of a magnetic material relatively large in Hcj, and a second portion relatively small in demagnetization, which is constituted of a magnetic material relatively small in Hcj. It is therefore possible to implement an MRI apparatus equipped with a magnetic field generator whose temperature characteristic is reversible, using permanent magnets small in Hcj. 
         [0026]    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 
         [0027]      FIG. 1  is a block diagram of an MRI apparatus showing one example of the best mode for carrying out the invention. 
           [0028]      FIG. 2  is a diagram illustrating the construction of a magnetic field generator. 
           [0029]      FIG. 3  is a diagram depicting BH curves of a permanent magnet. 
           [0030]      FIG. 4  is a diagram showing a process of thermal controlling. 
           [0031]      FIG. 5  is a diagram illustrating one example of a further detailed process of thermal controlling. 
           [0032]      FIGS. 6(   a ) and  6 ( b ) are diagrams showing a magnetic flux density distribution and a magnetic field strength distribution at a magnetic pole face of the permanent magnet. 
           [0033]      FIG. 7  is a diagram depicting the construction of the permanent magnet. 
           [0034]      FIGS. 8(   a ) and  8 ( b ) are diagrams illustrating BH curves of a permanent magnet. 
           [0035]      FIGS. 9(   a ) and  9 ( b ) are diagrams showing BH curves of a permanent magnet. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    A best mode for carrying out the invention will hereinafter be explained in detail with reference to the figures. Incidentally, the present invention is not limited to the best mode for carrying out the invention. A block diagram of an MRI apparatus is shown in  FIG. 1 . The present apparatus is one example of the best mode for carrying out the invention. One example of the best mode for carrying out the invention related to the MRI apparatus is shown according to the construction of the present apparatus. 
         [0037]    As shown in  FIG. 1 , the present apparatus has a magnetic field generator  100 . The magnetic field generator  100  has main magnetic field magnet units  102 , gradient coil units  106  and RF (radio frequency) coil units  108 . 
         [0038]    Any of the main magnetic field magnet units  102 , gradient coil units  106  and RF coil units  108  comprises paired ones opposed to one another with a space interposed therebetween. Further, any of them has a substantially disc shape and is placed with its central axis held in common. Each of the gradient coil units  106  is one example of a gradient magnetic field coil employed in the invention. Each of the RF coil unit  108  is one example of an RF coil employed in the invention. 
         [0039]    A target  1  is placed on a table  500  in an internal bore of the magnetic field generator  100  and carried in and out. The table  500  is driven by a table driver  120 . 
         [0040]    Each of the main magnetic field magnet units  102  forms a static magnetic field in the internal bore of the magnetic field generator  100 . The direction of the static magnetic field is approximately orthogonal to the direction of a body axis of the target  1 . That is, each of the main magnetic field magnet units  102  forms a so-called vertical magnetic field. Each of the main magnetic field magnet units  102  is configured using a permanent magnet. 
         [0041]    The gradient coil units  106  produce three gradient magnetic fields for respectively causing the intensities of static magnetic fields to have gradients or slopes in the directions of three axes vertical to one another, i.e., a slice axis, a phase axis and a frequency axis. Each of the gradient coil units  106  has unillustrated 3-systematic gradient coils in association with the three gradient magnetic fields. 
         [0042]    Each of the RF coil units  108  transmits an RF pulse (radio frequency pulse) for exciting a spin in a body of the target  1  to a static magnetic field space. Further, the RF coil unit  108  receives therein a magnetic resonance signal which produces the excited spin. The RF coil units  108  may perform transmission and reception by either the same coil or discrete coils. 
         [0043]    A gradient driver  130  is connected to the gradient coil units  106 . The gradient driver  130  supplies a drive signal to each of the gradient coil units  106  to generate a gradient magnetic field. The gradient driver  130  has unillustrated 3-systematic drive circuits in association with the 3-systematic gradient coils in the gradient coil unit  106 . 
         [0044]    An RF driver  140  is connected to the RF coil units  108 . The RF driver  140  supplies a drive signal to each of the RF coil units  108  to transmit an RF pulse, thereby exciting the spin in the body of the target  1 . 
         [0045]    A data acquisition unit  150  is connected to each of the RF coil units  108 . The data acquisition unit  150  takes in or captures signals received by the RF coil units  108  by sampling and collects or acquires the same as digital data. 
         [0046]    A controller  160  is connected to the table driver  120 , the gradient driver  130 , the RF driver  140  and the data acquisition unit  150 . The controller  160  controls the table driver  120  to data acquisition unit  150  respectively to execute shooting or imaging. 
         [0047]    The controller  160  is configured using a computer or the like, for example. The controller  160  has a memory. The memory stores a program and various data for the controller  160  therein. The function of the controller  160  is implemented by allowing the computer to execute the program stored in the memory. 
         [0048]    The output side of the data acquisition unit  150  is connected to a data processor  170 . The data acquired by the data acquisition unit  150  is inputted to the data processor  170 . The data processor  170  is configured using a computer or the like, for example. The data processor  170  has a memory. The memory stores a program and various data for the data processor  170  therein. 
         [0049]    The data processor  170  is connected to the controller  160 . The data processor  170  ranks ahead of the controller  160  and generally controls it. The function of the present apparatus is implemented by allowing the data processor  170  to execute the program stored in the memory. 
         [0050]    The data processor  170  causes the memory to store the data captured by the data acquisition unit  150 . A data space is defined in the memory. The data space forms a Fourier space. The Fourier space is also called “k-space”. The data processor  170  transforms data in the k-space into inverse Fourier form to thereby reconstruct an image for the target  1 . 
         [0051]    A display unit  180  and an operation or control unit  190  are connected to the data processor  170 . The display unit  180  comprises a graphic display or the like. The operation unit  190  comprises a keyboard or the like provided with a pointing device. 
         [0052]    The display unit  180  displays a reconstructed image and various information outputted from the data processor  170 . The operation unit  190  is operated by an operator and inputs various commands and information or the like to the data processor  170 . The operator is able to control the present apparatus on an interactive basis through the display unit  180  and the operation unit  190 . 
         [0053]    An outer appearance of one example of the magnetic field generator  100  is shown in  FIG. 2  by a perspective view. The magnetic field generator  100  is one example of the best mode for carrying out the invention. One example of the best mode for carrying out the invention related to the magnetic field generator is shown according to the construction of the magnetic field generator  100 . The magnetic field generator  100  is also one example of a magnetic field generator according to the invention. 
         [0054]    As shown in  FIG. 2 , the magnetic field generator  100  includes a pair of main magnetic field magnet units  102  supported by a yoke  200 . Each of the main magnetic field magnet units  102  is one example of a permanent magnet employed in the invention. The yoke  200  is one example of a yoke employed in the invention. 
         [0055]    The main magnetic field magnet units  102  respectively have substantially disc-shaped or short cylindrical outer shapes. The yoke  200  forms return passes used for the pair of main magnetic field magnet units  102  and is constituted of a ferromagnetic material such as soft iron in a substantially C form. Incidentally, the shape of the yoke  200  is not limited to the C form. 
         [0056]    The pair of main magnetic field magnet units  102  is supported in parallel and coaxially in such a manner that magnetic poles opposite in polarity to each other are opposed to each other. Thus, a vertical magnetic field is formed between both magnetic poles. The direction of the magnetic field is assumed to be a z direction below. While the pair of gradient coil units  106  and the pair of RF coil units  108  are respectively provided over magnetic pole faces of the pair of main magnetic field magnet units  102 , their illustrations are omitted. 
         [0057]    Each of the main magnetic field magnet units  102  comprises a permanent magnet  122  and a pole piece  124 . The permanent magnet  122  is a magnet composed of an Nd—Fe—B alloy, i.e., a Neodymium magnet. The Neodymium magnet does not contain a rare element like dysprosium and is a magnet relatively small in Hcj. 
         [0058]    The pole piece  124  is made up of soft iron. The diameter of the pole piece  124  is smaller than the diameter of the magnetic pole face of the main magnetic field magnet unit  102 . That is, the permanent magnet  122  has a pole piece smaller in area than its magnetic pole face. 
         [0059]    BH curves of the permanent magnet  122  are shown in  FIG. 3 . As shown in  FIG. 3 , the BH curve is given by a linear curve L 1  at a temperature T 1 , whereas the BH curve results in a combination of a linear curve L 2  parallel-moved downwardly and a non-linear curve L 3  at a temperature T 2  (&gt;T 1 ). 
         [0060]    An operating point P of the permanent magnet  122  is placed on the non-linear curve L 3 . Thus, an operating BH curve at the temperature T 2  results in a linear curve L 4  parallel-moved further down as indicated by a broken line L 4 . 
         [0061]    Returning the temperature to Ti from this state yields the BH curve as a broken line L 1 ′. The broken line L 1 ′ is parallel to the linear curve L 1  but becomes a linear curve displaced in position. When, however, the temperature is raised to T 2  again and is returned to T 1 , the BH curve is restored to the broken line L 1 ′. This is the same no matter how many times it is repeated. 
         [0062]    That is, after a temperature history of T 1 -T 2 -T 1  is passed one cycle, the BH curve at the temperature T 1  reaches the broken line L 1 ′ no matter how many times the same cycle is repeated. Thus, once the temperature history of T 1 -T 2 -T 1  is made, the temperature characteristic of the permanent magnet  122  assumes reversibility. Utilizing this phenomenon enables reversing of the temperature characteristic of the magnetic field generator using the permanent magnets small in Hcj. 
         [0063]    A process for reversing the temperature characteristic is shown in  FIG. 4 . The present process is one example of the best mode for carrying out the invention. One example of the best mode for carrying out the invention related to a thermal controlling method is shown based on the present process. 
         [0064]    As shown in  FIG. 4 , the temperature is raised from room temperature to a temperature higher than it at Step S 401 . At Step S 402 , the temperature higher than the room temperature is maintained. At Step S 403 , the temperature is lowered from the temperature higher than the room temperature to the room temperature. The room temperature is equivalent to the temperature T 1 , and the temperature higher than the room temperature corresponds to the temperature T 2 . Incidentally, the room temperature ranges from 10° C. to 25° C., for example. Step S 401  is one example of a temperature rise step according to the invention. Step S 402  is one example of a maintenance step according to the invention. Step S 403  is one example of a temperature fall step according to the invention. 
         [0065]    One example of a further detailed process of the thermal controlling is shown in  FIG. 5 . As shown in  FIG. 5 , the temperature is raised from the room temperature to 35° C. at Step S 501 , the temperature of 35° C. is maintained over two hours at Step S 502 , the temperature is raised from 35° C. to 45° C. at Step S 503 , the temperature of 45° C. is maintained over two hours at Step S 504 , the temperature is lowered from 45° C. to 35° C. at Step S 505 , the temperature of 35° C. is maintained over 1 hour at Step S 506 , and the temperature is lowered from 35° C. to the room temperature at Step S 507 . 
         [0066]    Step S 501  is one example of a first temperature rise step according to the invention. Step S 502  is one example of a first maintenance step according to the invention. Step S 503  is one example of a second temperature rise step according to the invention. Step S 504  is one example of a second maintenance step according to the invention. Step S 505  is one example of a first temperature fall step according to the invention. Step S 506  is one example of a third maintenance step according to the invention. Step S 507  is one example of a second temperature fall step according to the invention. 
         [0067]    With such thermal controlling, the temperature characteristic of the magnetic field generator is reliably reversed. Incidentally, the thermal operation or controlling of the magnetic field generator may be performed before degree-of-uniformity control on a magnetic filed strength distribution, i.e., its shimming. Such thermal controlling may be carried out before shipment of a product when a maker executes it, whereas when a user performs it, such thermal controlling may be carried out upon its acceptance. 
         [0068]    A magnetic flux density distribution and a magnetic field strength distribution at a magnetic pole face of each permanent magnet  122  are respectively shown in  FIGS. 6(   a ) and  6 ( b ). As shown in  FIGS. 6(   a ) and  6 ( b ), the peripheral edge portion of the permanent magnet  122  becomes lower than a portion located inside of it in magnetic flux density and magnetic filed strength. A portion low in magnetic flux density and magnetic field strength is equivalent to a portion low in operating point, i.e., a portion large in demagnetization. A portion high in magnetic flux density and magnetic field strength is equivalent to a portion high in operating point, i.e., a portion low in demagnetization. 
         [0069]    In view of such a magnetic state, the peripheral edge portion  122   b  is constituted of a magnet large in Hcj and the portion  122   a  located inside of it is constituted of a magnet small in Hcj, as shown in  FIG. 7 . By doing so, any operating point can be set so as to fall within a linear region of a BH curve. 
         [0070]    This is because since the linear region of the BH curve is large although the operating point is low at the peripheral edge portion  122   b  large in Hcj, the operating point thereof exists in the linear region, whereas since the operating point is high at the inner portion  122   a  low in Hcj, the operating point thereof exists in the linear region even though the linear region of the BH curve is small. 
         [0071]    Thus, the magnetic field generator  100  has a reversible temperature characteristic. Since, at this time, the most part of the permanent magnet  122  is made up of the magnet large in Hcj and only the peripheral edge portion is made up of the magnet large in Hcj, the permanent magnet  122  results in one much more inexpensive than when the whole part thereof is constituted of the magnet large in Hcj. 
         [0072]    Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.