Abstract:
The present invention relates to a magnetic resonance examination system ( 10 ) and to a method of operating such a magnetic resonance examination system ( 10 ). In particular the present invention relates to a magnetic resonance examination system ( 10 ) comprising a superconducting main magnet ( 20 ) surrounding an examination region ( 18 ) and generating a main magnetic field in the examination region ( 18 ), and further comprising a magnetic field gradient system ( 30 ) selectively causing alternating gradient magnetic fields in the examination region ( 18 ), said magnetic field gradient system ( 30 ) being coupled to the main magnet ( 20 ). In order to provide a technique to reliably detect a quench of the superconducting main magnet ( 20 ) of such a magnetic resonance examination system ( 10 ) a detecting device ( 91 ) is suggested for detecting an emerging quench of the main magnet ( 20 ), said detecting device ( 91 ) being adapted to operate in different modes depending on the mode of operation of the magnetic resonance examination system (e.g. ramp-up, ramp-down or continuous operation).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a magnetic resonance examination system and to a method of operating such a magnetic resonance examination system. In particular the present invention relates to a magnetic resonance examination system comprising a superconducting main magnet surrounding an examination region and generating a main magnetic field in the examination region, and further comprising a magnetic field gradient system selectively causing alternating gradient magnetic fields in the examination region, said magnetic field gradient system being coupled to the main magnet. 
     BACKGROUND OF THE INVENTION 
     Recently, new magnetic resonance examination system designs have been proposed, in which the main magnet system comprises magnet coils of superconductive material, and in which the magnetic field gradient system is located at the outside of the superconducting coil system and a weak-iron flux conduction system is provided to guide the magnetic gradient flux into the patient bore. A detailed description of such a magnetic resonance examination system is given in International Patent Application published as WO2005/124381 A2. A main advantage of such new magnetic resonance examination systems is a reduced scanner acoustic noise (“silent imaging”). Furthermore, the superconductive coils of the main magnet are closer to the examination region without compromising the effective bore size. Accordingly, less superconductive material is needed, which reduces the overall costs of the magnetic resonance examination system. 
     From conventional magnetic resonance examination systems, it is known to use a balanced bridge circuit to detect emerging quenches, i.e. loss of superconducting state in part of the coil. However, the arrangement of the magnetic field gradient system outside of the main magnet of the magnetic resonance examination system may induce voltages in the main magnet, which could misleadingly be interpreted as a quench. Prior art quench detecting methods are therefore not suitable to ensure a reliable detection of a quench of the superconducting main magnet of such a magnetic resonance examination system. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a technique to reliably detect a quench of the superconducting main magnet of a magnetic resonance examination system with a magnetic field gradient system being coupled to the main magnet in a way that the gradient magnetic field can induce voltages in the main magnet. 
     The object of the present invention is achieved by a magnetic resonance examination system, comprising a superconducting main magnet surrounding an examination region and generating a main magnetic field in the examination region, and further comprising a magnetic field gradient system selectively causing alternating gradient magnetic fields in the examination region, said magnetic field gradient system being coupled to the main magnet, wherein the magnetic resonance examination system comprises a detecting device for detecting an emerging quench of the main magnet, said detecting device being adapted to operate in different modes depending on the mode of operation of the magnetic resonance examination system. 
     This object is also achieved according to the invention by a method of operating a magnetic resonance examination system, said system comprising a superconducting main magnet surrounding an examination region and generating a main magnetic field in the examination region, and further comprising a magnetic field gradient system selectively causing alternating gradient magnetic fields in the examination region, said magnetic field gradient system being coupled to the main magnet, wherein the method comprises the step of detecting an emerging quench of the main magnet, said detecting being carried out in different modes depending on the mode of operation of the magnetic resonance examination system. 
     The object of the present invention is also achieved by a computer program to be executed in a computer, said program comprising computer instructions to detect an emerging quench of the main magnet of a magnetic resonance examination system with a magnetic field gradient system being coupled to the main magnet, said detection being carried out in different modes depending on the mode of operation of the magnetic resonance examination system, when the computer program is executed in the computer. 
     Technical effects necessary according to the invention can thus be realized on the basis of the instructions of the computer program in accordance with the invention. Such a computer program can be stored on a hard disk, on a carrier such as a CD-ROM or it can be available over the Internet or another computer network. Prior to executing the computer program is loaded into the computer by reading the computer program from the carrier, for example by means of a CD-ROM player, or from the internet, and storing it in the memory of the computer. The computer includes inter alia a central processor unit (CPU), a bus system, memory means, e.g. RAM or ROM etc., storage means, e.g. floppy disk or hard disk units etc. and input/output units. Alternatively, the inventive computer program could be implemented in hardware, e.g. using one or more integrated circuits. 
     A core idea of the invention is to provide a magnetic resonance examination system with a quench detection functionality, which operates in different modes depending on the mode of operation of the magnetic resonance examination system. In other words, the quench detection can be adapted to different behaviors of the magnetic resonance examination system or of parts of it during different modes of operation. As a result, a very flexible quench detection functionality is provided, which can be used, for example, if the main magnet of the magnetic resonance examination system shows a different behaviour during ramp-up or ramp-down on the one hand and during continuous operation on the other hand. Thus, with the present invention a reliable quench detection is achieved in case of the magnetic field gradient system being coupled to the main magnet. Because of the quench detection, the magnet will not be damaged as a result of a quench. Furthermore, because the quench detection technique can be realized using simple and robust components only, a very reliable quench detection can be ensured. As a result, a very safe and reliable magnetic resonance examination procedure can be provided. In general, the present application can be applied to all magnetic resonance examination systems, in which the gradient coil system can cause interfering voltages in the main magnet. 
     These and other aspects of the invention will be further elaborated on the basis of the following embodiments which are defined in the dependent claims. 
     The number of different modes, the detecting device is able to operate in, is not limited according to the present invention. For example, if there are two, three, or four modes of operation of the magnetic resonance examination system, the detecting device can be adapted to operate in two, three, or four different operating modes respectively. 
     According to a preferred embodiment of the invention the detecting device is adapted to operate in a first operating mode during ramp-up and ramp-down of the main magnet, in which mode the detecting device compares a first measurement signal (U M,B ) with a first threshold, and the detecting device is further adapted to operate in a second operating mode during continuous operation of the main magnet, in which mode the detecting device compares a second measurement signal (U M,A ) with a second threshold. The first threshold and the second threshold are defined preferably beforehand during setup of the magnetic resonance examination system. However, the thresholds can also be determined dynamically by the detecting device based on the known setup of the magnetic resonance examination system. 
     According to a preferred embodiment of the invention said first measurement signal is a bridge voltage over the resistance of an electrical bridge circuit coupled to the coil(s) of the main magnet. In this case the electrical bridge preferably divides the coil(s) of the main magnet into two symmetric coil sections. In other words, there is a balanced bridge voltage in case of no quenches, i.e. the bridge voltage is zero, if the bridge is in a balanced condition. In case of a quench in the main magnet, the bridge voltage becomes non-zero. 
     According to a preferred embodiment of the invention said second measurement signal is the total voltage over the main magnet. In order to ensure, that the total voltage can be used as second measurement signal during continuous operation of the main magnet, the main magnet is preferably adapted in a way that the magnetic field gradient system do not cause substantial induced voltages in the main magnet during continuous operation of the main magnet. In other words, it has to be assured, that the total voltage over the main magnet is not corrupted by changing gradient magnetic fields, which are inherent to the magnetic examination system with gradient coils coupled to the main magnet. However, the gradient coils have a symmetry relative to the midplane of the magnet, hence no net-coupling occurs. 
     According to a preferred embodiment of the invention the detecting device comprises a switching means for automatically switching the detecting device between the first operating mode and the second operating mode, said switching means being adapted to receive signals indicating the mode of operation of the magnetic resonance examination system. Preferably a transistor or a relay is used as a switching means. Thereby the switching means receives signals from the power supply control, which specifies the mode of operation (ramp-up, ramp-down, or continuous operation). According to the mode of operation of the magnetic resonance examination system, the switching means selects the mode of operation of the detecting device. Alternatively, the switching means can be implemented as a digital switch in a computer system. According to such an implementation, the control signal of the power supply, which specifies the mode of operation of the magnetic resonance examination system, is passed to the computer system and a software program is used to set the corresponding mode of operation of the detecting device. 
     According to a preferred embodiment of the invention the detecting device is adapted to control a protecting means to protect the main magnet in case of a quench. In particular the detecting means is adapted to disconnect the power supply of the main magnet if an emerging quench is detected. Preferably the detecting device is adapted to operate a switch to disconnect the power supply of the main magnet in such a case. The magnetic resonance examination system is preferably arranged in a way, that the main magnet is ramped-down safely by dissipating the stored electromagnetic energy outside the main magnet&#39;s windings. This means limits an uncontrolled dissipation of the electromagnetic energy in the main magnet&#39;s windings, which can damage or destroy the main magnet. Instead, most of the magnetic energy is safely dissipated in special resistors. 
     Preferably, the magnetic resonance examination system comprises a number of large electrical resistors connected to the main magnet in a way that the electric current through the superconducting coil of the main magnet is directed through these resistors in case the power supply of the main magnet is disconnected. In this case said resistors act as a kind of “energy absorber” by receiving the current from the superconducting coil and transforming said current into heat. In other words, the electrical resistors dissipate significant amounts of energy outside the main magnet&#39;s windings and, thus, local heating in the region where the quench started is minimized. Thus, in case of a false trigger of the detecting device, i.e. an unnecessary disconnecting of the power supply of the main magnet, the main magnet remains in the superconducting state, and can be put into operation again without any unnecessary delay. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will be described in detail hereinafter, by way of examples, with reference to the following embodiments and the accompanying drawings; in which: 
         FIG. 1  shows a schematic block diagram of a magnetic resonance imaging system, 
         FIG. 2  shows a perspective view of the magnetic field generating parts of a magnetic resonance imaging system, 
         FIG. 3  shows a perspective view of a magnetic resonance imaging system of  FIG. 2 , in which a portion of the vacuum jacket and the support tube of the main field magnet have been removed to reveal the main magnet coils and the magnetic field gradient system, 
         FIG. 4  shows a schematic electrical circuit illustrating components of the magnetic resonance imaging system, 
         FIG. 5  shows a flow chart of the method according to the invention, 
         FIG. 6  shows a diagram of measurement signals during ramp-up of the main magnet, 
         FIG. 7  shows a diagram of measurement signals during ramp-up, with a starting quench, 
         FIG. 8  shows a diagram of measurement signals during continuous operation of the main magnet, with a starting quench, and 
         FIG. 9  shows a schematic electrical circuit illustrating another embodiment of the present invention, 
         FIG. 10  shows a schematic electrical circuit illustrating yet another embodiment of the present invention, and 
         FIG. 11  shows a schematic electrical circuit illustrating yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     As illustrated in  FIG. 1 , the magnetic resonance imaging (MRI) scanner  10  comprises a superconducting main magnet  20 , which surrounds an examination region  18  (see  FIG. 2 ) and generates a main magnetic field in the examination region  18 . Furthermore the MRI scanner  10  comprises a magnetic field gradient system  30  that enables spatial localization of the MRI signals. The magnetic field gradient system  30  selectively causes alternating gradient magnetic fields in the examination region  18 , and is disposed outside of the main magnet  20 , as described below in more detail. Furthermore the MRI scanner  10  comprises a radio frequency (RF) system  60  that transmits energy and receives signal information, and a computer system  70  to control the scanner&#39;s components and subsystems. 
     With reference to  FIGS. 2 and 3 , the MRI scanner  10  includes a housing made up of an outer flux return shield  12  and an inner bore tube  14 . The outer flux return shield  12  and the inner bore tube  14  are sealed together to define a vacuum jacket  16 . An examination region  18  lies inside of the bore tube  14 . The patient or other imaging subject is positioned in the examination region  18 . A main magnet  20  is disposed inside of the vacuum jacket  16 . The magnet  20  is made using superconducting material, e.g. a high Tc superconductor operating at 30 to 100 K. The main magnet  20  includes a plurality of spaced apart generally annular magnet windings segments  22 , six segments in the embodiment of  FIG. 3 . Each windings segment  22  includes a number of turns of an electrical conductor, preferably a superconductor. Typically, the main magnet  20  is closer to the bore tube  14  than to the flux return shield  12 . 
     The windings segments  22  of the main magnet  20  are designed to produce a substantially spatially uniform magnetic field in the examination region  18  in which the main magnetic field vector is directed along an axial or z-direction parallel to the axis of the bore tube  14 . The outer flux return shield  12  is made of a ferromagnetic material and provides a flux return path for completing the magnetic flux loop. That is, magnetic flux generated by the main magnet  20  follows a closed loop that passes through the inside of the bore tube  14  including the examination region  18  and closes back on itself by passing through the flux return shield  12 . As a result, there exists a low magnetic field region within the vacuum jacket  16  between the magnet  20  and the flux return shield  12 . 
     A magnetic field gradient system  30  is disposed in the low magnetic field region existing outside the main magnet  20  and inside the flux return shield  12 . The magnetic field gradient system  30  incorporates coils and optionally ferromagnetic parts arranged such as to generate gradient fields Gx, Gy, Gz for imaging. These coils and parts are positioned and shaped in a way that on switching the gradients the net voltage induced in the whole magnet circuit is zero. This particular arrangement is described in the U.S. Provisional Patent Application U.S. 60/752,121, filed on Dec. 20, 2005, which as a whole is hereby incorporated by reference. As it can be seen in  FIGS. 2 and 3  a ferromagnetic yoke  32  is employed, which includes three ferromagnetic rings  40 ,  42 ,  44  disposed between the generally annular magnet windings segments  22 . The magnetic field gradient system  30  further includes a plurality of magnetic field gradient coils  34 . These field gradient coils  34  include wire turns or other electrical conductors wrapped around ferromagnetic crossbars  50  which are arranged generally transverse to the ferromagnetic rings  40 ,  42 ,  44  and which are connected with the ferromagnetic rings  40 ,  42 ,  44 . The magnetic field gradient system  30  has a four-fold rotational symmetry provided by arrangement of four crossbars  50  at ninety degree annular intervals. Each crossbar  50  includes magnetic field gradient coils  34  wrapped on either side of the plane of bilateral symmetry. 
     More detailed information about the arrangement of the magnetic field gradient system  30  if the MRI scanner  10  is given in International Patent Application published as WO2005/124381 A2, which as a whole is hereby incorporated by reference. 
     The computer system  70  of the MRI scanner  10  comprises a processing unit  80 , which is adapted according to the invention for performing all tasks of calculating and computing data as well as determining and assessing results. This is achieved according to the invention by means of a computer program  90  comprising computer instructions adapted for carrying out the steps of the inventive method, when the software is executed in the processing unit  80 . In particular, the processing unit  80  is adapted to execute a computer program  90  for detecting an emerging quench of the main magnet  20 , said detecting being carried out in different modes depending on the mode of operation of the MRI scanner  10 . By execution of the computer program  90  a software detecting module  91  is implemented, the functionality of which is discussed in more detail below. All devices, in particular the processing unit  80 , and the implemented software detecting module  91 , are constructed and programmed in a way that the procedures for data processing run in accordance with the method of the invention. The processing unit  80  itself may comprise functional modules or units, which are implemented in form of hardware, software or in form of a combination of both. In other words, the present invention could also be implemented merely using dedicated hardware, without using a computer program. In this case the software detecting module  91  would be implemented as a hardware device showing identical functionality. The computer system  70  of the MRI scanner  10  is connected to an external touch screen monitor  100 , which serves as interface to the MRI scanner&#39;s operator. Alternatively a conventional monitor screen is used in combination with a computer keyboard and/or computer mouse. 
     With reference to  FIG. 4 , the MRI scanner  10  comprises a first power supply  65  for the superconducting coils of the main magnet  20  and a second power supply  66  for the coils of the magnetic field gradient system  30 . By means of a suitable measurement device  67 , e.g. a high-impedance voltmeter, the total voltage U M,A  of the main magnet  20  is measured near the coils of the main magnet  20 . The connection of the power supply  65  for the superconducting coils of the main magnet  20  can be disconnected by means of a switch  69 . The coils of the main magnet  20  are given as a symmetric circuit with inductances L 1 , L 2  and resistances R 1 , R 2 . An electrical bridge  75  is connected to the superconducting coils of the main magnet  20 , dividing the coils of the main magnet  20  into two symmetric coil sections  24 , forming a balanced bridge circuit. A large measuring resistor R M  is provided to pick-off a bridge voltage U M,B  by means of a suitable measurement device  68 , e.g. a high-impedance voltmeter. Preferably, the value of the resistor R M  is in the order of one to several kilo-Ohms. The measuring devices  67 ,  68  are connected to the analogue digital converter  73  of the computer system  70  by means of signal lines  77 . The bridge circuit  75  further comprises two large electrical resistors R 3 , R 4 . The dimensions of the resistors R 3  and R 4  depends on the inductivity L of the main magnet  20  and are preferably chosen in a way which ensures, that the current in the main magnet  20  decreases fast, e.g. in the order of 1 s. Given this time constant, R 3  and R 4  must meet the condition L/( R 3 +R 4 )≈1 s. Resistors R 3  and R 4  are connected in parallel to the coil sections  24  of the main magnet  20 . During ramp-up and continuous operation the switch  69  is closed. In this case, all current is carried by the coils of the main magnet  20 , because the resistances R 3 , R 4  and R M  are much larger than the resistances R 1 , R 2  of the main magnet  20 . 
     With additional reference to  FIG. 5 , now the method of operating of the MRI scanner  10  is described. In a feeding step  101  both the bridge voltage U M,B  the and total voltage U M,A  of the main magnet  20  are fed into the computer system  70 , where these measurement signals are converted into digital signals by means of an analogue-digital converter  73 . 
     Since the computer system  70  controls the power supply  66  of the main magnet  20 , it permanently controls the operating mode of the MRI scanner  10 . In other words, a control circuit  71  of the computer system  70  may generate operating signals, said operating signals indicating, if the MRI scanner  10  operates in a continuous operation mode or in a ramp-up or ramp-down mode. The software detecting module  91  receives these signals from the control circuit  71  via a signal line  72 . Within the computer program  90  a switching software switching module  92  is implemented for receiving said signals. The software switching module  92  could as well be implemented as part of the software detecting module  91 . Depending on these signals, the software switching module  92  automatically switches the software detecting module  91  between a first operating mode and a second operating mode in a switching step  102 . Now, if the software detecting module  91  is in it&#39;s first operating mode, the bridge voltage U M,B  is used in order to detect a quench of the main magnet  20 , and if the software detecting module  91  is in it&#39;s second operating mode, the total voltage U M,A  is used in order to detect a quench of the main magnet  20 . 
     For detecting an quench, the bridge voltage U M,B  or the total voltage U M,A  is compared by means of the software detecting module  91  with a first or second threshold voltage U q,B  or U q,A , respectively, in a comparing step  103 , said threshold voltages U q,B , and U q,A  being stored in a database of the computer system  70 . The bridge voltage U M,B , which is used during ramp-up and ramp-down, is compared to the threshold voltage U q,B , and the total voltage U M,A  is compared to the threshold value U q,A , respectively. 
     The software detecting module  91  controls the switch  69  via a control line  76 , which connects the computer system  70  and the switch. If a threshold is exceeded the switch is opened in step  105  by means of the software detecting module  91  and the power supply of the main magnet  20  is disconnected. The main magnet  20  is then ramped-down safely by dissipating all stored electromagnetic energy outside the main magnet&#39;s windings, namely in the electrical resistors R 3 , R 4 . Optionally additional heating elements (not shown), e.g. in form of resistors, are used for purposeful heating the superconducting coil of the main magnet  20 , if a quench has been detected. Such heating elements are preferably controlled by the computer system  70 . Due to a homogeneous heating, the electrical resistance of the main magnet  20  can be increased. As a result, the creation of so called hot-spots on the main magnet  20  can be prevented. Hot-spots correspond to locally confined resistive conducting areas. In these areas, the temperature increases due to dissipation which causes additional increase of resistivity and thus additional temperature rise. In particular if the main magnet is made of high Tc superconductive material, in which heat conduction is very slow, heat generation due to dissipation exceeds heat conduction and thus can lead to severe damages in the main magnet  20 . 
     With reference to  FIGS. 6 to 8 , the measurement signals U M,A  (solid line) and U M,B  (dotted line) will be discussed.  FIG. 6  illustrates the developing of both signals during ramp-up of the main magnet  20 . In the ramp-up phase  200  the total voltage U M,A  is large due to the inductance of the main magnet&#39;s coils. Because U M,A  is so large, it cannot be used during this phase for indicating a quench of the main magnet  20 . However, the bridge voltage U M,B  remains zero and unaffected by the ramp-up of the main magnet  20 . Hence, during the ramp-up phase  200 , the bridge voltage U M,B  is employed by the software detecting module  91  as measuring signal to detect an emerging quench. The software detecting module  91  indicates the quench, if the bridge voltage U M,B  exceed the threshold U q,B . 
     The emerging of a quench is illustrated in  FIG. 7 . At a certain point in time t q  during the ramp-up phase  200  a quench of the main magnet  20  occurs. In other words, a part of the main magnet changes state from superconductivity to normal resistivity. The course of the total voltage U M,A  remains almost unchanged, since it is largely dominated by the inductance. However, the bridge voltage U M,B  shows a significant change, because the bridge  75  is unbalanced due to a resistivity change either in the upper or in the lower section  24  of the main magnet&#39;s coils. Furthermore, in case of a ramp-down of the main magnet  20 , the same signal behaviour can be obtained. Thus, the bridge voltage U M,B  is employed by the software detecting module  91  as measuring signal to detect an emerging quench also during the ramp-down phase (not shown). 
       FIG. 8  shows both U M,A  and U M,B  during continuous operation phase  300  of the main magnet  20 . As it can be seen on the left hand side of the diagram, the bridge voltage U M,B  is heavily influenced by alternating gradient magnetic fields caused by switching the magnetic field gradient system  30  at a switching time t S , since the magnetic field gradient system  30  induces voltages in the main magnet  20 . U M,A  on the other hand remains constant and unaffected by such alternating gradient magnetic fields, because of the symmetry of the gradient system  30  and main magnet  20  arrangement. In case of an emerging quench at a certain point in time t q  during continuous operation of the main magnet  20 , both U M,A  and U M,B  changes. While a change of U M,B  may be caused by gradient switching, a change of U M,A  reliably indicates a quench. Hence, during the phase of continuous operation, the total voltage U M,A  is employed by the software detecting module  91  as measuring signal to detect an emerging quench. The software detecting module  91  indicates the quench, if the total voltage U M,A  exceed the threshold U q,A . 
     With the present invention, quenches can reliably be detected during ramp-up, ramp-down, and continuous operation in the main magnet  20 . Furthermore, the quench detection is unaffected by the switching of the magnetic field gradient system  30 . 
       FIGS. 9 ,  10 , and  11  illustrate further embodiments of the present invention. In  FIG. 9  diodes  78  are connected in series with the resistors R 3  and R 4  in order to avoid a residual current through the electrical resistors R 3  and R 4 . A switch  69  is employed in the same way as illustrated in  FIG. 4 . In  FIG. 10  a resistor R S  (1 Ohm), and the further resistors R 3 , R 4 , and R M  (each having 1 kilo-Ohm), are used. Furthermore a switch  79  is employed, as shown. During operation, the switch  79  is closed and no current passes R S . In case of a detected quench, the power supply is turned off and the switch  79  is opened. As a result all current passes R S  and is partly dissipated there. Switch  79  is controlled by the computer  70  in the same way as described above. In  FIG. 11  a number of diodes  78  are used instead of resistor R S  (see  FIG. 10 ). The advantage of a cascade of diodes  78  compared to a resistor is the non-linear characteristic current/voltage curve. Therefore, the voltage drop over the diodes  78  (and with this dissipation in the diodes) remains larger for decreasing currents than in the case of using a resistor. 
     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. It will furthermore be evident that the word “comprising” does not exclude other elements or steps, that the words “a” or “an” do not exclude a plurality, and that a single element, such as a computer system or another unit may fulfill the functions of several means recited in the claims. Any reference signs in the claims shall not be construed as limiting the claim concerned. 
     REFERENCE NUMERALS 
     
         
           10  MRI scanner 
           12  flux return shield 
           14  bore tube 
           16  vacuum jacket 
           18  examination region 
           20  main magnet 
           21  temperature sensor 
           22  winding segment 
           24  coil section 
           30  gradient system 
           32  yoke 
           34  gradient coil 
           40  ring 
           42  ring 
           44  ring 
           50  crossbar 
           60  RF system 
           65  main magnet&#39;s power supply 
           66  gradient system&#39;s power supply 
           67  measurement device 
           68  measurement device 
           69  switch 
           70  computer system 
           71  control circuit 
           72  signal line 
           73  analogue digital converter 
           75  electrical bridge 
           76  control line 
           77  signal line 
           78  diode 
           79  switch 
           80  processing unit 
           90  computer program 
           91  software detecting module 
           92  software switching module 
           100  monitor 
           101 - 105  method steps 
           200  ramp-up phase 
           300  continuous operation phase