Abstract:
A magnetic resonance apparatus in which magnetic metal pieces are accommodated in an accommodation section so as to correct uniformity in a main magnetic field, includes an acquisition unit which acquires temperature information related to at least one of a temperature of the magnetic metal pieces accommodated in the accommodation section, a temperature of the accommodation section, and a temperature of a position in the vicinity of the accommodation section, and a temperature adjustment unit which adjusts the temperature of the magnetic metal pieces to a target temperature by preheating the magnetic metal pieces on the basis of the temperature information acquired by the acquisition unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-280515, filed Oct. 13, 2006, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a magnetic resonance apparatus in which a magnetic metal part is accommodated in an accommodation section for correction of uniformity in a main magnetic field. 
   2. Description of the Related Art 
   A magnetic resonance apparatus is provided with a magnet (a permanent magnet or an electromagnet) for generating a main magnetic field and a gradient magnetic field system (a gradient coil) for generating a gradient magnetic field. The main magnetic field is a static magnetic field and should desirably have high uniformity. In order to maintain the uniformity of the main magnetic field, shimming is performed. Shimming is roughly classified into passive shimming and active shimming. In the passive shimming, a magnetic metal piece (iron piece or the like) called a shim is arranged in the vicinity of the magnet, thereby adjusting magnetic field distribution of the main magnetic field. More specifically, arrangement of a plurality of magnetic metal pieces is contrived, whereby uniformity of the main magnetic field is maintained. In the active shimming, by adjusting a current to be caused to flow through a coil (shim coil), a correction magnetic field for uniformizing the main magnetic field is generated. 
   Then, in the passive shimming, it is known that non-uniformity of the main magnetic field is caused by variation of the temperature of the magnetic metal piece. It is also known that an offset is caused in the main magnetic field by the variation of the temperature of the magnetic metal piece. As a cause of the temperature variation, heat generation from a gradient coil and heat generation caused in a magnetic metal by an eddy current induced by generation a gradient magnetic field are mainly considered. When the temperature of the magnetic metal is changed by such heat generation, the magnetic susceptibility of the magnetic metal is changed and, as a result of this, the intensity of the main magnetic field is changed locally or entirely. 
   A technique is known in which a correction magnetic field for correcting non-uniformity or the like of the main magnetic field is included in a gradient magnetic field (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2-206436). In this technique, a temperature of a magnet is detected, and a correction amount is determined in accordance with the temperature. Further, an offset of a current value corresponding to the correction amount is added to a current for generating a gradient magnetic field which is originally required, and the resultant current is supplied to a gradient coil. 
   In the above-mentioned prior art technique, the main magnetic field is corrected by the gradient magnetic field, and hence the correction amount and the correction resolution have their limits, and there has been no guarantee that the uniformity and intensity of the main magnetic field could have been kept constant. 
   BRIEF SUMMARY OF THE INVENTION 
   Under these circumstances, it has been required to make the passive shimming effectively function and maintain the main magnetic field stable. 
   According to a first aspect of the present invention, there is provided a magnetic resonance apparatus in which magnetic metal pieces are accommodated in an accommodation section so as to correct uniformity in a main magnetic field, comprising: an acquisition unit which acquires temperature information related to at least one of a temperature of the magnetic metal pieces accommodated in the accommodation section, a temperature of the accommodation section, and a temperature of a position in the vicinity of the accommodation section; and a temperature adjustment unit which adjusts the temperature of the magnetic metal pieces to a target temperature by preheating the magnetic metal pieces on the basis of the temperature information acquired by the acquisition unit. 
   According to a first aspect of the present invention, there is provided a magnetic resonance apparatus in which magnetic metal pieces are accommodated in an accommodation section so as to correct uniformity in a main magnetic field, comprising: acquisition unit which acquires temperature information related to at least one of a temperature of the magnetic metal pieces accommodated in the accommodation section, a temperature of the accommodation section, and a temperature of a position in the vicinity of the accommodation section; and temperature adjustment unit which adjusts the temperature of the magnetic metal pieces to a target temperature higher than the normal temperature on the basis of the temperature information acquired by the acquisition unit. 
   Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
       FIG. 1  is a schematic view of a part of an MRI apparatus according to an embodiment of the present invention. 
       FIG. 2  is a perspective view showing the configuration of a gradient coil unit shown in  FIG. 1 . 
       FIG. 3  is a cross-sectional view of the gradient coil unit shown in  FIG. 1  in the XY-plane. 
       FIG. 4  is a perspective view showing the configuration of a gradient coil unit provided with a cooling mechanism that can be utilized as cooling unit for a magnetic metal piece. 
       FIG. 5  is a cross-sectional view of the gradient coil unit provided with the cooling mechanism that can be utilized as cooling unit for a magnetic metal piece in the XY-plane. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of the present invention will be described below with reference to the accompanying drawings. 
     FIG. 1  is a schematic view of a part of a magnetic resonance imaging (MRI) apparatus  100  according to the embodiment of the present invention. 
   The MRI apparatus  100  of this embodiment comprises a gantry  10 , a magnet  11 , a gradient coil unit  12 , a gradient power supply  22 , a radio frequency coil (RF coil)  23 , a transmitter  24 , a receiver  25 , a sequencer  26 , a system controller  27 , an input unit  28 , an computation unit  29 , a storage unit  30 , a display unit  31 , a heater controller  32 , and a flow rate controller  33 . In addition, the MRI apparatus  100  includes a bed (not shown) which is disposed adjacent to the gantry  10 . The gantry  10  is typically formed in such a manner that a substantially cylindrical imaging space  10   a  is formed in the center thereof so as to allow the space  10   a  to penetrate the gantry  10 . The axial direction of the imaging space  10   a  is defined as the Z direction, and the remaining two directions which are perpendicular to the Z direction and are perpendicular to each other are defined as the X direction (lateral direction) and the Y direction (vertical direction). In  FIG. 1 , only half the gantry  10  cut away by the YZ-plane is shown. 
   The magnet  11  and the gradient coil unit  12  are accommodated in the gantry  10 . The magnet  11  generates the main magnetic field (static magnetic field) Bo in the imaging space  10   a . A superconducting magnet is typically used as the magnet  11 . The entire shape of the magnet  11  is in a substantially cylindrical shape. A magnet bore (hereinafter referred to as a bore)  11   a  is formed inside the magnet  11 . The central axis of the bore  11   a  coincides with the central axis of the imaging space  10   a . The gradient coil unit  12  is disposed in the bore  11   a . The gradient magnetic coil  12  includes three sets of coils for receiving a supply of driving currents corresponding to the X-, Y-, and Z-axes, respectively, from the gradient power supply  22  and generating the gradient magnetic fields respectively corresponding to the X-, Y-, and Z-axes. 
   At the time of imaging, the RF coil  23  is placed inside the imaging space  10   a . The transmitter  24  and the receiver  25  are connected to the RF coil  23 . The transmitter  24  supplies a pulse current oscillating at the Larmor frequency to the RF coil  23  under the control of the sequencer  26 . The receiver  25  receives a magnetic resonance (MR) signal through the RF coil  23  and subjects the signal to various signal processing operations, thereby generating corresponding digital signals. A top plate  34  of the bed is arranged so that it can be moved toward and away from the imaging space  10   a  in the gantry  10 , and an inspection object  200  is placed on the top surface thereof. 
   The sequencer  26  operates under the control of the system controller  27  that controls the entire MRI apparatus  100 . The input unit  28  is connected to the system controller  27 . The operator can select a desirable pulse sequence from a plurality of pulse sequences using a method such as a spin echo (SE) method or an echo planar imaging (EPI) method through the input unit  28 . The system controller  27  sets a selected pulse sequence to the sequencer  26 . The sequencer  26  controls the application timing and intensity of the gradient magnetic field in each of the X-, Y-, and Z-axes directions and the application timing, amplitude, and duration time of a high-frequency magnetic field, in accordance with the set pulse sequence. 
   The computation unit  29  receives an MR signal (digital data) generated by the receiver  25  and performs Fourier transform for arranging actually measured data in the two-dimensional Fourier space formed by a memory incorporated therein, and reconstructing the image, thereby generating image data or spectrum data. The storage unit  30  stores the computed image data. The display unit  31  displays the image. 
   The heater controller  32  controls an exothermic amount of heat generated by heaters (to be described later) incorporated in the gradient coil unit  12 . The flow rate controller  33  controls the flow rate of a cooling liquid flowing through cooling pipes (to be described later) incorporated in the gradient coil unit  12 . Incidentally, the cooling liquid is cooled by a cooling unit (not shown). The system controller  21  is provided with a function of controlling the heater controller  32  and the flow rate controller  33  so as to maintain the temperature of the magnetic metal incorporated in the gradient coil unit  12  at a target temperature on the basis of a temperature value measured by a sensor (to be described later) incorporated in the gradient coil unit  12 . 
     FIG. 2  is a perspective view showing the outline configuration of the gradient coil unit  12 . 
   As shown in  FIG. 2 , the gradient coil unit  12  is provided with a plurality of pockets  13 , and includes a plurality of cooling pipes  14 , a plurality of heaters  15 , and a sensor  16 . 
   Each of the pockets  13  has a through-hole shape formed along the axis of the gradient coil unit  12 , and the magnetic metal pieces are arranged therein as needed. Incidentally, in  FIG. 2 , four pockets  13  are shown. The number of pockets  13  may be arbitrary. Although the number of pockets  13  is physically limited, the larger number of the pockets is desirable. This is because the degree of freedom of arrangement of the magnetic metal pieces is enhanced, and accuracy of correction of magnetic field uniformity can be improved. The desirable number of the pockets  13  is, for example, 12 or 24. Each of the cooling pipes  14  is arranged in each of the pockets  13  at a position adjacent to the inner circumferential side of the gradient coil unit  12  in parallel with each pocket  13 . Each of the cooling pipes  14  serves as a flow path of the cooling liquid for cooling the magnetic metal pieces arranged in the pockets  13 . A flow rate of the cooling liquid flowing through the cooling pipe  14  is controlled by the flow rate controller  33 . Each of the heaters  15  is arranged in each of the pockets  13  at a position adjacent to the outer circumferential side of the gradient coil unit  12  in parallel with each pocket  13 . Each of the heaters  15  heats the magnetic metal pieces arranged in each of the pocket  13 . The heating temperature of the heater  15  is controlled by the heater controller  32 . The sensor  16  is arranged in the vicinity of any pocket  13 . The sensor  16  measures the temperature of the magnetic metal piece arranged in the pocket  13 . The sensor  16  sends a signal indicative of the measured temperature value to the system controller  27 . As the sensor  16 , a semiconductor sensor or a thermocouple can be used. 
     FIG. 3  is a cross-sectional view of the gradient coil unit  12  in the XY-plane. 
   As shown in  FIG. 3 , the gradient coil unit  12  is segmented into, from the inner circumferential side, a main coil layer  12   a , shim layer  12   b , and a shield coil layer  12   c . Further, the pockets  13 , cooling pipes  14 , and heaters  15  are all provided in the shim layer  12   b . The shim layer  12   b  is formed by molding a resin into a cylindrical shape together with the pockets  13 , cooling pipes  14 , and heaters  15 . The main coil layer  12   a  is formed by molding a resin into a cylindrical shape together with three types of main coils (X-main coil, Y-main coil, and Z-main coil) for generating gradient magnetic fields each of which changes in the magnetic field intensity along corresponding one of the X-, Y-, and Z-axes by being supplied with currents from the gradient power supply  22 . The shield coil layer  12   c  is formed by molding a resin into a cylindrical shape together with three types of shield coils (X-shield coil, Y-shield coil, and Z-shield coil) for generating magnetic fields for shielding a leakage magnetic field from the main coil layer  12   a . That is, the gradient coil unit  12  is so-called an actively shielded gradient coil (ASGC). 
   Incidentally, in  FIG. 3 , only cross-sectional contours of the main coil layer  12   a  and the shield coil layer  12   c  are shown, and their detailed internal structures are omitted from the drawing. 
   Operations of the MRI apparatus  100  having the configuration described above will be described below. 
   At the time of imaging, the magnet  11  generates the main magnetic field in the imaging space  10   a . The main magnetic field is normally required to exhibit field intensity of about several kilogauss to several tens of kilogauss (several tesla). The main magnetic field is required to have spatial uniformity in addition to the intensity. The spatial region which is required to have a uniform magnetic field is generally a spherical region having a diameter of about 50 cm. Where the intensity of the main magnetic field is 1.5 tesla, the spatial uniformity is required to be equal to or less than several tens of ppm at any position of the spatial region. 
   The magnet  11  is manufactured so that it can generate a magnetic field fulfilling the above conditions. However, the magnetic field generated by the magnet  11  is affected by magnetic materials existing in the peripheral regions so as to be distorted. Thus, for example, magnetic metal pieces are appropriately arranged in the pockets  13  as a part of installation work or maintenance work of the MRI apparatus  100  so that the non-uniformity of the main magnetic field can be corrected. That is, by arranging magnetic metal pieces in the pockets  13 , the magnetic field distribution of the main magnetic field is changed by the influence of the magnetic metal pieces. Accordingly, by appropriately arranging magnetic metal pieces in such a manner that the change in the magnetic field distribution acts to correct the non-uniformity of the main magnetic field, the non-uniformity of the main magnetic field can be corrected. 
   However, the magnetic susceptibility of a magnetic metal piece is changed according to the temperature. When the susceptibility of the magnetic metal pieces arranged in the pockets  13  is changed, the influential condition of the magnetic metal pieces is changed with respect to the main magnetic field, and the non-uniformity of the main magnetic field is lowered. Thus, in order to suppress the lowering of the non-uniformity of the main magnetic field, the system controller  27  performs temperature control described below. 
   (Basic Operation) 
   When heat generated from the gradient coil  12  is transferred to the magnetic metal pieces in the pockets  13  and the temperature of the magnetic metal pieces becomes higher than a predetermined temperature or when the temperature of the magnetic metal pieces becomes lower than the predetermined temperature in the standby state, a value of the temperature of the magnetic metal pieces measured by the sensor  16  is transmitted to the system controller  27 . When it is necessary to raise the temperature of the magnetic metal pieces, the system controller  27  issues an instruction to operate the heaters  15  and raise the temperature of the magnetic metal pieces to the predetermined temperature to the heater controller  32 . On the contrary, when it is necessary lower the temperature of the magnetic metal pieces, the system controller  27  issues an instruction to increase the flow rate of the cooling liquid in the cooling pipes  14  to lower the temperature of the magnetic metal pieces to the predetermined temperature to the flow rate controller  33 . The heater controller  32  or the flow rate controller  33  performs control so as to maintain the temperature of the magnetic metal pieces at the fixed temperature on the basis of the instruction from the system controller  27 . As a result of this, the temperature of the magnetic metal pieces can be maintained substantially constant at all times, and hence variation in the magnetic susceptibility of the magnetic metal pieces caused by the variation in the temperature is reduced. Accordingly, the uniformity of the main magnetic field can be maintained. 
   The temperature control will be described below more specifically. 
   (At the Time of Arrangement Work of the Magnetic Metal Pieces) 
   When the work for arranging the magnetic metal pieces in the pockets  13  is performed, the system controller  27  recognizes the fact on the basis of, for example, an instruction or the like issued by an operator. Further, in this case, the system controller  27  controls the heater controller  32  and the flow rate controller  33  in such a manner that the temperature of the magnetic metal pieces measured by the sensor  16  becomes the predetermined target temperature. Here, the target temperature is set higher than the normal temperature. Furthermore, the target temperature is set lower than a temperature at which the resin or the like located around the magnetic metal pieces is degenerated. The range of the target temperature that satisfies such conditions is normally about 40 to 80° C. 
   At the time of the arrangement work of the magnetic metal pieces, normally, large heat generation is not caused in the gantry  10 . For this reason, the temperature of the magnetic metal pieces arranged in the pockets  13  is about the normal temperature in many cases. That is, the temperature of the magnetic metal pieces is lower than the target temperature in many cases. Thus, in such a state, the system controller  27  controls the exothermic amount of the heaters  15  through the heater controller  32  in such a manner that the temperature of the magnetic metal pieces arranged in the pockets  13  is raised to the target temperature. If the temperature of the magnetic metal pieces is higher than the target temperature for some reason or if the temperature of the magnetic metal pieces becomes higher than the target temperature as a result of heating by the heaters  15 , the system controller  27  controls the flow rate of the cooling liquid in the cooling pipes  14  through the flow rate controller  33  such that the temperature of the magnetic metal pieces arranged in the pockets  13  is lowered to the target temperature. 
   In this manner, the work for arranging the magnetic metal pieces in the pockets  13  is performed in the state where the temperature of the magnetic metal pieces becomes the target temperature. That is, the magnetic metal pieces are arranged in such a manner that the uniformity of the main magnetic field becomes high when the temperature of the magnetic metal pieces is the target temperature. 
   (At the Time of Imaging) 
   At the time of imaging, the current supply is switched to the main coils and the shield coils of the gradient coil unit  12  at a high speed. As a result of this, the main coil and the shield coil generate heat, and the magnetic metal pieces are heated by the generated heat. The magnetic metal pieces are also heated by the eddy current produced therein by the magnetic field. Hence, the system controller  27  increases the cooling power of the cooling liquid in accordance with the temperature rise of the magnetic metal pieces on the basis of the measurement result of the sensor  16 , and controls the flow rate of the cooling liquid in the cooling pipes  14  so that the temperature of the magnetic metal pieces can be maintained at the target temperature through the flow rate controller  33 . 
   Incidentally, when a standby state continues for a long period, the temperature of the magnetic metal pieces is about the normal temperature in some cases. In such a case, the system controller  27  controls the exothermic amount of the heaters  15  through the heater controller  32  so as to raise the temperature of the magnetic metal pieces to the target temperature. When the magnetic metal pieces are heated as described above, imaging may be started before the temperature of the magnetic metal pieces reaches the target temperature or may be started after waiting for the temperature of the magnetic metal pieces to reach the target temperature. In the former case, the imaging time can be shortened. In the latter case, imaging of a high image quality which is less affected by the variation in the main magnetic field can be performed. 
   When the imaging start time is determined in advance, the system controller  27  may control the heaters  15  through the heater controller  32  so as to raise the temperature of the magnetic metal pieces up to the target temperature before the imaging start time. By performing such a procedure, imaging of a high image quality which is less affected by the variation in the main magnetic field can performed, and the imaging time can be shortened. Needless to say, even in the standby state, the flow rate of the cooling liquid and the exothermic amount of the heaters  15  may be controlled by the system controller  27  so as to maintain the temperature of the magnetic metal pieces at the target temperature. 
   As described above, according to the MRI apparatus  100 , the temperature of the magnetic metal pieces is stably maintained at substantially the target temperature at the imaging time, and hence variation in the uniformity of the main magnetic field caused by variation in the temperature of the magnetic metal pieces hardly occurs during imaging. As a result of this, it becomes possible to perform imaging in the stable main magnetic field, and obtain an image of a high image quality. 
   Further, in the MRI apparatus  100 , even when the work for arranging the magnetic metal pieces is performed, the temperature of the magnetic metal pieces arranged in the pockets  13  is stably maintained at the target temperature. Therefore, when the arrangement of the magnetic metal pieces is appropriately performed at the time of the work, the state where the uniformity in the main magnetic field is high can be maintained at the time of imaging. As a result of this, it becomes possible to obtain a high quality image in which spatial unevenness in the image quality is small. 
   By the way, cooling of the gradient coil unit or the like has also been performed in the prior art. In such cooling in the prior art, it has been regarded as being desirable to lower the temperature of the object to be cooled as low as possible, and hence it has been regarded as being necessary to prepare a large cooling power. However, in the MRI apparatus  100 , the target temperature is set as a temperature higher than the normal temperature, and the temperature of the magnetic metal is maintained at a somewhat high temperature. For this reason, the MRI apparatus  100  has only to be provided with a cooling power smaller than that required on the basis of conventional common sense. However, in the MRI apparatus  100 , the target temperature is set lower than a temperature at which the resin or the like located around the magnetic metal pieces is degenerated, and hence members located around the magnetic metal pieces are never degenerated by maintaining the temperature of the magnetic metal pieces at a high temperature. 
   This embodiment can be variously modified and implemented as described below. 
   (1) The sensor  16  may be arranged at any position as long as it can measure a temperature related to the temperature of the magnetic metal piece. Needless to say, the sensor  16  may be arranged at any position in the vicinity of the magnetic metal pieces. However, the sensor  16  may be arranged at any position, as long as it is a position, for example, in a position at which a correlation between the temperature at the position and the variation in the temperature of the magnetic metal pieces can be observed (for example, a position at which 1.5 times the temperature variation appears when the temperature variation of the magnetic metal pieces becomes twice). In this case, by giving a correlation coefficient between the sensor  16  and the temperature of the magnetic metal to the system controller  27  in advance, the system controller can determine the temperature of the magnetic metal on the basis of the correlation coefficient and the value of the temperature measured by the sensor  16 . 
   (2) In the embodiment described above, the MRI apparatus  100  includes the cooling pipes  14  serving as cooling units and the heater  15  serving as heating units. However, a cooling unit and a heating unit may be provided in the MRI apparatus  100 . In this case, a cooling pipe serving as a cooling unit and a heater serving as a heating unit may be arranged in a spiral form along the central axis of the cylindrical shape of the gradient coil unit  12 , thereby cooling or heating all the magnetic metal pieces. In this case, the cooling pipe and the heater may be arranged along one of the inner circumferential surface and the outer circumferential surface of the gradient coil unit  12 , or may be arranged between the inner circumferential surface of the magnet  11  and the outer circumferential surface of the gradient coil unit  12 . That is, any type of configuration may be employed as long as the magnetic metal pieces can be cooled or heated. Incidentally, the cooling medium may be a gas, such as the air. 
   (3) A configuration in which the position of the cooling pipe  14  and the position of the heater  15  are replaced with each other in the above embodiment may be employed. Further, a plurality of sensors  16  may be provided. For example, one sensor  16  is provided in each pocket  13 , and temperature control may be performed separately for each of the magnetic metal pieces. 
   (4) As for the cooling unit, a cooling mechanism provided, in the prior art, in the gradient coil unit  12  for cooling the gradient coil unit  12  itself may be diverted to the cooling unit. 
     FIGS. 4 and 5  are views showing the configuration of a gradient coil unit  40  provided with a cooling mechanism that can be utilized as the cooling unit.  FIG. 4  is a perspective view, and  FIG. 5  is a cross-sectional view in the XY-plane. Incidentally, in  FIG. 4 , only half the gradient coil unit  40  cut away by the YZ-plane is shown. The gradient coil unit  40  includes a main coil layer  40   a , a shim layer  40   b , and a shield coil layer  40   c . The main coil layer  40   a  and the shield coil layer  40   c  are equivalent to the main coil layer  12   a  and the shield coil layer  12   c  in the gradient coil unit  12 , respectively. In the shim layer  40   b , a plurality of pockets  41  each of which is equivalent to the pocket  13  are provided in the shim layer  40   b . Furthermore, a plurality of cooling tubes  42  are arranged on a circle formed around the Z-axis. Each of the cooling tubes  42  is arranged in parallel with the Z-axis. The cooling tubes  42  arranged in such as manner that a pocket  41  is interposed between two pairs of cooling tubes  42 . The cooling tube  42  has substantially a rectangular column-shape or a substantially cylindrical shape, and the inside thereof is a flow path through which cooling water flows. The plural tubes  42  are connected to each other at both ends of the gradient coil unit  40  as shown in  FIG. 4  in such a manner that flow paths formed by the tubes  42  are connected to each other in series. 
   (5) As the heating unit, the main coil or the shield coil of the gradient coil unit  12  may be used. That is, when imaging is not performed, the current supply is switched to the main coil or the shield coil of the gradient coil unit  12  so as to cause the main coil or the shield coil to generate heat, thereby making it possible to heat the magnetic metal pieces. Incidentally, in the known imaging methods of the MRI, there are a method in which non-uniformity in the main magnetic field is a less serious problem, and a method in which non-uniformity in the main magnetic field is a serious problem. Thus, when the temperature of the magnetic metal is low, the former imaging method is used, and as a result of this, the temperature of the magnetic metal is raised up to the target temperature, thereafter the latter imaging method is used, whereby even switching of the current supply to the main coil or the shield coil to be performed when imaging is not performed can be omitted. 
   (6) The temperature control of the magnetic metal as in the above embodiment may be performed only at the time of imaging using the latter imaging method of the two imaging methods. 
   (7) It is desirable that the target temperature be a temperature at which the uniformity of the main magnetic field is the highest. However, even when the target temperature is out of such a temperature, by preventing the uniformity of the main magnetic field from varying during the imaging, the image quality can be made better than when the uniformity of the main magnetic field varies. Hence, the target temperature may be made variable automatically or manually. When the target temperature is automatically varied, it can be considered that the target temperature is set rather low when the temperature of the magnetic metal is low, for example, immediately after the start-up of the MRI apparatus  100 , and that the target temperature is set rather high after the temperature of the magnetic metal is sufficiently raised. As a result of this, immediately after the start-up of the apparatus, it becomes possible to shorten the time needed to raise the temperature of the magnetic metal to the target temperature, and hence shorten the time up to the time at which imaging can be started. Further, thereafter, it becomes possible to set the target temperature at a temperature at which the uniformity of the main magnetic field becomes high, and enable imaging of a higher image quality. 
   (8) Even when the temperature of the magnetic metal varies to a certain degree, if the range of the variation is within a certain allowable range, the influence of the variation in the uniformity of the main magnetic field incidental to the variation in the temperature of the magnetic metal, on the image quality becomes small. For this reason, the target temperature may include a range to a certain degree. 
   (9) The present invention can also be applied to a magnetic resonance apparatus in which no imaging is performed. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.