Patent Description:
Magnetic resonance imaging apparatuses have recently been provided with a superconducting magnet, which is an electromagnet using a superconductor, as a static magnetic field magnet that generates a static magnetic field in an imaging area where a subject is placed. The superconducting magnet is typically fabricated by placing a coil of a superconductor (hereinafter referred to as a "superconducting coil") in a refrigerant container filled with liquid helium serving as a refrigerant.

When cooled with the liquid helium and transitioned to a superconducting state, the superconducting coil has an electrical resistance of <NUM>, and as a result, a large current can flow therethrough. Therefore, the superconducting magnet can generate a stronger magnetic field than an ordinary electromagnet.

<CIT> discloses a permanent current switch apparatus with permanent current switches connected in parallel.

A permanent current switch apparatus configured to be electrically connected to a superconducting coil via a superconducting wire, the permanent current switch apparatus including.

The thermal permanent current switches may each include: a heater; and a switch part configured to switch the thermal permanent current switch between a superconducting state and a normal conducting state by heat emitted by the heater. The permanent current switch apparatus may further include a heater connecting wire configured to connect the heaters provided to the respective thermal permanent current switches in parallel to an external power source configured to supply electric power to the heaters.

The heaters may be divided into a plurality of groups such that an electric current flowing through each of the heaters is equal and be connected in parallel to the external power source.

The number of the heaters included in each of the groups may be equal.

A magnetic resonance imaging apparatus including a superconducting magnet, wherein.

The thermal permanent current switches of the magnetic resonance imaging apparatus may each include a heater and a switch part configured to switch the thermal permanent current switch between a superconducting state and a normal conducting state by heat emitted by the heater. The permanent current switch apparatus may further include a heater connecting wire configured to connect the heaters provided to the respective thermal permanent current switches in parallel to an external power source configured to supply electric power to the heaters.

The heaters of the magnetic resonance imaging apparatus may be divided into a plurality of groups such that an electric current flowing through each of the heaters is equal and be connected in parallel to the external power source.

The number of the heaters included in each of the groups of the magnetic resonance imaging apparatus may be equal.

A permanent current switch apparatus according to an embodiment is electrically connected to a superconducting coil via a superconducting wire, the permanent current switch apparatus including a plurality of parallel structures with thermal permanent current switches connected in parallel, the thermal permanent current switches being capable of switching between conducting and interrupting an electric current flowing through the superconducting wire. The parallel structures are connected in series.

Embodiments of a permanent current switch and a magnetic resonance imaging apparatus are described below in greater detail with reference to the accompanying drawings. While the embodiments below describe a case where the permanent current switch is used for a static magnetic field magnet of the magnetic resonance imaging apparatus, the embodiments are not intended to limit the present invention. In the following description, a magnetic resonance imaging apparatus is also referred to as an "MRI apparatus".

The configuration of an MRI apparatus <NUM> according to the present embodiment is described first. <FIG> is a configuration diagram of the configuration of the MRI apparatus <NUM> according to the present embodiment. As illustrated in <FIG>, the MRI apparatus <NUM> includes a static magnetic field magnet <NUM>, a gradient coil <NUM>, an RF coil <NUM>, a couchtop <NUM>, a gradient power source <NUM>, a transmitter <NUM>, a receiver <NUM>, a sequence control apparatus <NUM>, and a computer system <NUM>.

The static magnetic field magnet <NUM> generates a static magnetic field in an imaging area where a subject is placed. The static magnetic field magnet <NUM> is an example of a superconducting magnet. The static magnetic field magnet <NUM> includes a vacuum chamber <NUM>, a refrigerant container <NUM>, and a superconducting coil <NUM>.

The vacuum chamber <NUM> is formed in a substantially cylindrical shape, and the inside of the cylindrical wall is maintained in a vacuum. The space formed on the inner side of the cylinder of the vacuum chamber <NUM> serves as the imaging area where the subject is placed. The refrigerant container <NUM> is formed in a substantially cylindrical shape and is housed in the vacuum chamber <NUM>. The refrigerant container <NUM> contains a refrigerant, such as liquid helium, inside the wall of the cylinder. The superconducting coil <NUM> is disposed in the refrigerant container <NUM> and is immersed in the liquid helium. The superconducting coil <NUM> generates a static magnetic field in the imaging area on the inner side of the cylinder of the vacuum chamber <NUM>.

The gradient coil <NUM> is formed in a substantially cylindrical shape and is fixed to the inner side of the static magnetic field magnet <NUM>. The gradient coil <NUM> generates a gradient magnetic field in X-, Y-, and Z-axis directions set in the imaging area by an electric current supplied from the gradient power source <NUM>.

The RF coil <NUM> is fixed to the inner side of the gradient coil <NUM> such that it faces across a subject P. The RF coil <NUM> irradiates the subject P with RF pulses transmitted from the transmitter <NUM> and receives magnetic resonance signals emitted from the subject P by excitation of hydrogen nuclei.

The couchtop <NUM> is horizontally movably provided on a couch, which is not illustrated, and is moved into the imaging area with the subject P placed thereon when imaging is performed. The gradient power source <NUM> supplies an electric current to the gradient coil <NUM> based on instructions from the sequence control apparatus <NUM>.

The transmitter <NUM> transmits RF pulses to the RF coil <NUM> based on instructions from the sequence control apparatus <NUM>. The receiver <NUM> detects magnetic resonance signals received by the RF coil <NUM> and transmits raw data obtained by digitizing the detected magnetic resonance signals to the sequence control apparatus <NUM>.

The sequence control apparatus <NUM> scans the subject P by driving the gradient power source <NUM>, the transmitter <NUM>, and the receiver <NUM> under the control of the computer system <NUM>. When receiving raw data from the receiver <NUM> as a result of scanning, the sequence control apparatus <NUM> transmits the raw data to the computer system <NUM>.

The computer system <NUM> collectively controls the MRI apparatus <NUM>. Specifically, the computer system <NUM> includes an input unit, a sequence controller, an image reconstructor, a storage unit, a display unit, and a main controller, for example. The input unit receives various inputs from an operator. The sequence controller causes the sequence control apparatus <NUM> to perform scanning based on imaging conditions input by the operator. The image reconstructor reconstructs an image based on the raw data transmitted from the sequence control apparatus <NUM>. The storage unit stores therein the reconstructed image and other data. The display unit displays various kinds of information, such as the reconstructed image. The main controller controls operations of the functional units based on instructions from the operator.

The configuration of the static magnetic field magnet <NUM> is described with reference to <FIG> is a block diagram of an example of the configuration of the static magnetic field magnet <NUM> according to the present embodiment. As illustrated in <FIG>, the static magnetic field magnet <NUM> includes the vacuum chamber <NUM>, the refrigerant container <NUM>, the superconducting coil <NUM>, a protection circuit <NUM>, current leads <NUM>, a PCS <NUM>, and a magnet excitation and demagnetization power source <NUM>.

The protection circuit <NUM> protects the superconducting coil <NUM> by consuming the electric current flowing through the superconducting coil <NUM> when a quench occurs in the superconducting coil <NUM>. The protection circuit <NUM> is a protective resistance element or a diode bank, for example. The protection circuit <NUM> is disposed in a normal temperature environment outside the refrigerant container <NUM>.

When the superconducting coil <NUM> is excited, in the present embodiment, the magnet excitation and demagnetization power source <NUM> is disconnected from the current leads <NUM>. To demagnetize the superconducting coil <NUM>, the magnet excitation and demagnetization power source <NUM> is reconnected to the current leads <NUM>.

The current leads <NUM> supply an electric current from the magnet excitation and demagnetization power source <NUM> at room temperature (hereinafter, also referred to as normal temperature environment) to the superconducting coil <NUM> cooled by a refrigerant, such as liquid helium (hereinafter, also referred to as low temperature environment). The current leads <NUM> connect the superconducting coil <NUM> to the protection circuit <NUM>. The current leads <NUM> are made of a high-temperature superconductor. With this structure, the current leads <NUM> have lower conductivity in the normal temperature state and have higher conductivity in the low temperature state.

Therefore, the current leads <NUM> are less likely to conduct heat during normal operation, whereby the amount of heat entering into the refrigerant container <NUM> from the outside can be reduced. In other words, the current leads <NUM> can suppress evaporation of the refrigerant caused by heat entering from the outside of the refrigerant container <NUM>.

By contrast, when a quench occurs in the superconducting coil <NUM>, the current leads <NUM> are cooled to be in a superconducting state by vaporization of the refrigerant in the refrigerant container <NUM>. Therefore, an electric current automatically flows from the superconducting coil <NUM> to the protection circuit <NUM> via the current leads <NUM> when a quench occurs, whereby the protection circuit <NUM> can be reliably protected. In other words, the current leads <NUM> can serve as switches that supply an electric current from the superconducting coil <NUM> to the protection circuit <NUM> when a quench occurs.

The PCS <NUM> is an example of a permanent current switch apparatus. The PCS <NUM> according to the present embodiment is a permanent current switch apparatus including a superconducting wire <NUM> including switch parts <NUM>, which will be described later, and thermal permanent current switches <NUM> (refer to <FIG>). The switch part <NUM> is a part of the superconducting wire <NUM> serving as a switch that switches the superconducting wire <NUM> between a superconducting state and a normal conducting state by heat emitted by a heater. The PCS <NUM> is turned on when the switch part <NUM> is in the superconducting state and is turned off when the switch part <NUM> is in the normal conducting state. The PCS <NUM> is connected in parallel to the superconducting coil <NUM>.

When the PCS <NUM> is turned on, the superconducting coil <NUM> and the PCS <NUM> form a closed loop if the magnet excitation and demagnetization power source <NUM> is disconnected, for example. When the PCS <NUM> is turned off, the magnet excitation and demagnetization power source <NUM> can supply an electric current to the superconducting coil <NUM>, for example.

When the PCS <NUM> is turned on, in the present embodiment, the state of the magnet excitation and demagnetization power source <NUM> transitions to a disconnected state. The state of the magnet excitation and demagnetization power source <NUM>, however, is not limited thereto. The magnet excitation and demagnetization power source <NUM> may have an output of close to <NUM>, for example. In other words, the magnet excitation and demagnetization power source <NUM> simply needs to be in such a state that the superconducting coil <NUM> and the PCS <NUM> can form a closed loop.

The PCS <NUM> also includes heaters <NUM> (refer to <FIG>), which will be described later. The heater <NUM> is connected to an external power source outside the static magnetic field magnet <NUM>. To excite and demagnetize the superconducting coil <NUM>, the heater <NUM> controls turning on/off the PCS <NUM> by raising and lowering the temperature of the switch part <NUM>. The configuration of the PCS <NUM> will be described later.

The magnet excitation and demagnetization power source <NUM> is a power source used to excite or demagnetize the superconducting coil <NUM>. The magnet excitation and demagnetization power source <NUM> is disposed in a normal temperature environment outside the refrigerant container <NUM>. When the superconducting coil <NUM> is excited or demagnetized, the magnet excitation and demagnetization power source <NUM> is connected to the superconducting coil <NUM> via the current leads <NUM>.

When the superconducting coil <NUM> is excited or demagnetized, the refrigerant in the refrigerant container <NUM> evaporates because the heaters <NUM> emit heat to control the PCS <NUM>. The number of switch parts <NUM> of the PCS <NUM> is determined by considering the amount of evaporation of the refrigerant because refrigerants are typically expensive.

At this time, the evaporated refrigerant cools the current leads <NUM> and brings them into a superconducting state. Therefore, when the superconducting coil <NUM> is excited or demagnetized, the magnet excitation and demagnetization power source <NUM> can stably supply an electric current to the superconducting coil <NUM> via the current leads <NUM>.

The configuration of the PCS <NUM> is specifically described with reference to <FIG>. A configuration of a PCS <NUM> useful for understanding the PCS <NUM> according to the present embodiment is described first with reference to <FIG> by way of comparison with the configuration of the PCS <NUM> illustrated in <FIG>. <FIG> is a schematic of the configuration of the PCS <NUM> different from the PCS <NUM> useful for understanding the embodiment of <FIG>.

As illustrated in <FIG>, the PCS <NUM> includes thermal permanent current switches PC1 to PC8, a superconducting wire SU, and a heater connecting wire HL, for example. In the following description, the thermal permanent current switches PC1 to PC8 may be referred to simply as thermal permanent current switches PC when they are not particularly distinguished.

The thermal permanent current switch PC1 includes a switch part SW1 and a heater HT1. In the following description, the switch parts SW1 to SW8 may be referred to simply as switch parts SW when they are not particularly distinguished. The heaters HT1 to HT8 may be referred to simply as heaters HT when they are not particularly distinguished. The switch part SW is a part serving as a switch that switches the switch part SW between the superconducting state and the normal conducting state by heat emitted by the heater HT.

The thermal permanent current switch PC switches between conducting and interrupting an electric current flowing through the switch part SW. The thermal permanent current switches PC1 to PC4 are connected in series to constitute a switch series structure IL1. Similarly, the thermal permanent current switches PCS to PC8 constitute a switch series structure IL2. The switch series structure IL1 and the switch series structure IL2 are connected in parallel by the superconducting wire SU.

The heater HT heats the switch part SW. The thermal permanent current switch PC switches the switch part SW between the superconducting state and the normal conducting state by adjusting the heating of the heater HT and raising and lowering the temperature of the switch part SW. The heater HT is connected in series to an external power source PW by the heater connecting wire HL.

The following describes the state of the electric current in a case where a quench occurs in the thermal permanent current switch PC when the PCS <NUM> is in the superconducting state. First, a case where no quench occurs in any of the thermal permanent current switches PC1 to PC8 is described.

In this case, when the electric current flowing through the PCS <NUM> is Isc, the electric current Isc is divided between the switch series structure IL1 and the switch series structure IL2 connected in parallel as illustrated in <FIG>. Therefore, the electric current flowing through the switch series structure IL1 is approximately Isc/<NUM>, and the electric current flowing through the switch series structure IL2 is approximately Isc/<NUM>. Next, a case where a quench occurs in any one of the thermal permanent current switches PC1 to PC8 is described.

<FIG> illustrates the configuration of the PCS <NUM> of <FIG> as a comparison with the configuration of the PCS <NUM> illustrated in <FIG>. <FIG> is a schematic of the state of the electric current when a quench occurs in one of the thermal permanent current switches PC included in the PCS <NUM> different from the PCS <NUM> according to the embodiment. <FIG> illustrates a case where a quench occurs in the thermal permanent current switch PC2 constituting the switch series structure IL1 due to failure FP1. In this case, the thermal permanent current switch PC2 serves as a resistance R.

When the thermal permanent current switch PC2 serves as the resistance R, all the electric current flowing through the PCS <NUM> is commutated to the switch series structure IL2 composed of the thermal permanent current switches PCS to PC8 where no quench occurs as illustrated in <FIG>. Therefore, the electric current flowing through the switch series structure IL1 is <NUM>, and the electric current flowing through the switch series structure IL2 is Isc.

As a result, the PCS <NUM> can maintain the superconducting state of one of the paths, thereby maintaining the operation of the static magnetic field magnet <NUM> in a permanent current mode. In other words, the PCS <NUM> in this case is redundant because it can maintain the operation of the static magnetic field magnet <NUM> in the permanent current mode if a quench occurs in any one of the thermal permanent current switches PC1 to PC8.

Next, a case where a quench occurs in each of the switch series structure IL1 and the switch series structure IL2 is described.

<FIG> illustrates the configuration of the PCS <NUM> of <FIG> as a comparison with the configuration of the PCS <NUM> illustrated in <FIG>. <FIG> is a schematic of the state of the electric current when a quench occurs in each of the switch series structure IL1 and the switch series structure IL2 included in the PCS <NUM> different from the PCS <NUM> according to the embodiment. <FIG> illustrates a state where a quench occurs in the thermal permanent current switch PC2 constituting the switch series structure IL1 due to failure FP1, and a quench occurs in the thermal permanent current switch PC8 constituting the switch series structure IL2 due to failure FP2.

In this case, the thermal permanent current switch PC2 and the thermal permanent current switch PC8 serve as resistances R. When the thermal permanent current switch PC2 and the thermal permanent current switch PC8 serve as the resistances R, the redundancy of the PCS <NUM> is lost, and the electric current Isc flowing through the PCS <NUM> is attenuated and decreases to <NUM> as illustrated in <FIG>.

In other words, if a quench occurs in the thermal permanent current switch PC of one of the switch series structures IL1 and IL2 while a quench is occurring in the thermal permanent current switch PC of the other, the static magnetic field magnet <NUM> fails to maintain the operation in the permanent current mode.

In addition, the configuration of the PCS <NUM> different from the PCS <NUM> has the problem of failing to excite or demagnetize the superconducting coil <NUM> if the heater connecting wire HL is not conductive due to disconnection or other causes. The following describes the effects caused when disconnection occurs in the heater connecting wire HL included in the PCS <NUM> different from the PCS <NUM>.

<FIG> illustrates the configuration of the PCS <NUM> of <FIG> as a comparison with the configuration of the PCS <NUM> illustrated in <FIG>. <FIG> is a schematic of a state where disconnection occurs in the heater connecting wire HL included in the PCS <NUM> different from the PCS <NUM> according to the embodiment. <FIG> illustrates a case where disconnection occurs at part of the heater connecting wire HL due to failure FH1.

If disconnection occurs in part of the heater connecting wire HL, a heater current Ih fails to be applied to the heaters HT1 to HT8 because the heaters HT1 to HT8 are connected in series to the external power source PW. As described above, the PCS <NUM> switches the switch parts SW between the superconducting state and the normal conducting state by adjusting the heating of the heaters HT1 to HT8 and raising and lowering the temperature of the superconducting wire SU.

In other words, if disconnection occurs in part of the heater connecting wire HL, the PCS <NUM> fails to adjust the heating of the heater HT1 and raise and lower the temperature of the superconducting wire SU. As a result, the static magnetic field magnet <NUM> fails to excite or demagnetize the superconducting coil <NUM>.

To address this, the PCS <NUM> according to the present embodiment has a configuration to solve the problem of the PCS <NUM> of <FIG> described above. Specifically, in the PCS <NUM> according to the present embodiment, a plurality of switch parallel structures each composed of a plurality of thermal permanent current switches connected in parallel are connected in series. The switch parallel structure is an example of a parallel structure. The following describes the configuration of the PCS <NUM> according to the present embodiment with reference to <FIG>.

<FIG> is a schematic of an example of the configuration of the PCS <NUM> according to the embodiment. As illustrated in <FIG>, the PCS <NUM> includes thermal permanent current switches 160a to <NUM>, a superconducting wire <NUM>, and a heater connecting wire <NUM>. In the following description, the thermal permanent current switches 160a to <NUM> may be referred to simply as thermal permanent current switches <NUM> when they are not particularly distinguished.

Similarly to the PCS <NUM> different from the PCS <NUM>, the thermal permanent current switch <NUM> switches between conducting and interrupting an electric current flowing through the superconducting wire <NUM>. The thermal permanent current switches 160a and 160b are connected in parallel by the superconducting wire <NUM> to constitute a switch parallel structure JP1.

Similarly, the thermal permanent current switches 160c and 160d constitute a switch parallel structure JP2, the thermal permanent current switches 160e and 160f constitute a switch parallel structure JP3, and the thermal permanent current switches <NUM> and <NUM> constitute a switch parallel structure JP4. The switch parallel structures JP1, JP2, JP3, and JP4 are connected in series by the superconducting wire <NUM>.

While the superconducting wire <NUM> according to the present embodiment is made of Cu/NbTi, the material of the superconducting wire <NUM> is not limited thereto. The superconducting wire <NUM> may be made of CuNi/NbTi, for example.

The thermal permanent current switch 160a includes a switch part 161a and a heater 162a. In the following description, the switch parts 161a to <NUM> may be referred to simply as switch parts <NUM> when they are not particularly distinguished. The heaters 162a to <NUM> may be referred to simply as heaters <NUM> when they are not particularly distinguished.

Similarly to the switch part SW of the PCS <NUM> different from the PCS <NUM>, the switch part <NUM> is a part serving as a switch that switches the switch part <NUM> between the superconducting state and the normal conducting state by heat emitted by the heater <NUM>.

In the present embodiment, while the switch part <NUM> is fabricated by removing Cu from Cu/NbTi constituting the superconducting wire <NUM>, the configuration of the switch part 161a is not limited thereto. The switch part 161a may be made of CuNi/NbTi, for example. Similarly to the heater HT of the PCS <NUM> different from the PCS <NUM>, the heater <NUM> heats the superconducting wire <NUM>.

The heaters <NUM> are divided into a plurality of groups and are connected in parallel to the external power source PW. The heaters <NUM> are connected such that the electric current flowing through the heaters <NUM> is equal. In the PCS <NUM>, for example, the electric current flowing through the heaters <NUM> is made equal by making the number of heaters <NUM> connected in parallel to the external power source PW by the heater connecting wire <NUM> equal.

With this configuration, the heaters can heat the superconducting wire <NUM> at the corresponding positions without generating any temperature difference. Therefore, the switch parts <NUM> can switch the switch parts <NUM> at the corresponding positions between the superconducting state and the normal conducting state at substantially the same timing.

In the present embodiment, the heaters 162a, 162b, 162c, and 162d constitute a heater group HG1. The heaters 162e, 162f, <NUM>, and <NUM> constitute a heater group HG2. The heater group HG1 and the heater group HG2 are connected in parallel to the external power source PW by the heater connecting wire <NUM> independent of the superconducting wire <NUM>.

The parallel connection configuration of the heaters <NUM> is not limited to the configuration described above. For example, the number of heaters <NUM> of the heater group HG1 may be two, and the number of heater groups of the heater group HG2 may be six in the example described above. The number of heater groups is not limited to two. The number of heater groups may be three or more, for example.

The following describes the state of the electric current when a quench occurs in the thermal permanent current switch <NUM> when the PCS <NUM> having the configuration described above is in the superconducting state. First, a case where no quench occurs in any of the thermal permanent current switches 160a to <NUM> is described.

In this case, when the electric current flowing through the PCS <NUM> is Isc, the electric current Isc is divided between the thermal permanent current switches <NUM> connected in parallel as illustrated in <FIG>. Therefore, the electric current flowing through the thermal permanent current switch 160a of the switch parallel structure JP1 is approximately Isc/<NUM>, and the electric current flowing through the thermal permanent current switch 160b is approximately Isc/<NUM>.

Similarly, the electric current flowing through the thermal permanent current switch 160c of the switch parallel structure JP2 is approximately Isc/<NUM>, and the electric current flowing through the thermal permanent current switch 160d is approximately Isc/<NUM>. Similarly, the electric current flowing through the thermal permanent current switch 160e of the switch parallel structure JP3 is approximately Isc/<NUM>, and the electric current flowing through the thermal permanent current switch 160f is approximately Isc/<NUM>. Similarly, the electric current flowing through the thermal permanent current switch <NUM> of the switch parallel structure JP4 is approximately Isc/<NUM>, and the electric current flowing through the thermal permanent current switch <NUM> is approximately Isc/<NUM>.

Next, a case where a quench occurs in any one of the thermal permanent current switches <NUM> is described with reference to <FIG>.

<FIG> is a schematic of an example of a current state when a quench occurs in one of the thermal permanent current switches <NUM> included in the PCS <NUM> according to the embodiment. <FIG> illustrates a case where a quench occurs in the thermal permanent current switch 160c constituting the switch parallel structure JP2 due to failure FP1. In this case, the thermal permanent current switch 160c serves as a resistance R.

When the thermal permanent current switch 160c serves as the resistance R, all the electric current flowing through the switch parallel structure JP2 is commutated to the thermal permanent current switch 160d where no quench occurs as illustrated in <FIG>. Therefore, the electric current flowing through the thermal permanent current switch 160c is <NUM>, and the electric current flowing through the thermal permanent current switch 160d is Isc. The electric current flowed through the thermal permanent current switch 160d is transmitted to the switch parallel structure JP3 in the subsequent stage.

As a result, the PCS <NUM> can maintain the superconducting state of the superconducting wire <NUM>, thereby maintaining the operation of the static magnetic field magnet <NUM> in the permanent current mode. In other words, the PCS <NUM> is redundant because it can maintain the operation of the static magnetic field magnet <NUM> in the permanent current mode if a quench occurs in any one of the thermal permanent current switches 160a to <NUM>.

Next, a case where quenches occur in any two of the thermal permanent current switches <NUM> is described with reference to <FIG>.

<FIG> is a schematic of an example of a current state when quenches occur in two of the thermal permanent current switches included in the PCS <NUM> according to the embodiment. <FIG> illustrates a state where a quench occurs in the thermal permanent current switch <NUM> constituting the switch parallel structure JP4 due to failure FP2 while a quench is occurring in the thermal permanent current switch 160c constituting the switch parallel structure JP2 due to failure FP1. In this case, the thermal permanent current switch <NUM> serves as a resistance R.

When the thermal permanent current switch <NUM> serves as the resistance R, all the electric current flowing through the switch parallel structure JP3 is commutated to the thermal permanent current switch <NUM> where no quench occurs as illustrated in <FIG>. Therefore, the electric current flowing through the thermal permanent current switch <NUM> is Isc, and the electric current flowing through the thermal permanent current switch <NUM> is <NUM>.

As a result, the PCS <NUM> effectively functions and maintains the superconducting state of the superconducting wire <NUM>, whereby the operation of the static magnetic field magnet <NUM> in the permanent current mode can be maintained. In other words, the PCS <NUM> is redundant because it can maintain the operation of the static magnetic field magnet <NUM> in the permanent current mode if quenches occur in two of the thermal permanent current switches 160c and <NUM>.

If a quench occurs in the thermal permanent current switch 160d while a quench is occurring in the thermal permanent current switch 160c constituting the switch parallel structure JP2, the thermal permanent current switch 160d serves as a resistance R, which is not illustrated. When the thermal permanent current switch 160c and the thermal permanent current switch 160d serve as the resistances R, the redundancy of the PCS <NUM> is lost, and the electric current Isc is attenuated by the resistances R and decreases to <NUM>.

Therefore, the PCS <NUM> according to the present embodiment can maintain the operation of the static magnetic field magnet <NUM> in the permanent current mode if quenches occur in any two of the thermal permanent current switches <NUM> as long as the quenches do not occur in either of the thermal permanent current switches <NUM> constituting the same switch parallel structure JP.

The following describes the difference in unreliability between the PCS <NUM> according to the present embodiment and the PCS <NUM> different from the PCS <NUM> using the PCS <NUM> according to the present embodiment described with reference to <FIG> and the PCS <NUM> different from the PCS <NUM> described with reference to <FIG> as examples. In the present embodiment, the unreliability indicates the degree to which the permanent current mode fails to be maintained by quenches in two thermal permanent switches in any desired period of time (e.g., ten years).

When the failure probability per thermal permanent current switch PC or <NUM> in two-parallel connection is F2, and the failure probability per thermal permanent current switch PC or <NUM> in one-parallel connection is F1, for example, unreliability F of the PCS <NUM> is expressed by F = 8C1 × F2 × 4C1 × F1. By contrast, the unreliability F of the PCS <NUM> is expressed by F = 8C1 × F2 × F1. Thus, the unreliability of the PCS <NUM> is one fourth of that of the PCS <NUM>.

In other words, the PCS <NUM> according to the present embodiment has higher reliability than the PCS <NUM> different from the PCS <NUM>.

The following describes the effects caused when disconnection occurs in the heater connecting wire <NUM> included in the PCS <NUM> according to the present embodiment with reference to <FIG> is a schematic of an example of a case where disconnection occurs in the heater connecting wire <NUM> included in the PCS <NUM> according to the embodiment.

<FIG> illustrates a case where disconnection occurs in part of the heater connecting wire <NUM> due to failure FH1. If disconnection occurs at the position of the failure FH1, the external power source PW fails to apply the heater current Ih to the heaters <NUM> constituting the heater group HG1.

If disconnection occurs at the position of the failure FH1, however, the external power source PW can apply the heater current Ih to the heater group HG2 because the heater group HG1 and the heater group HG2 in the PCS <NUM> are connected in parallel to the external power source PW. In other words, the static magnetic field magnet <NUM> according to the present embodiment can excite and demagnetize the superconducting coil <NUM> if disconnection occurs at one position of the heater connecting wire <NUM>.

As described above, the MRI apparatus <NUM> according to the present embodiment includes the static magnetic field magnet <NUM> having the superconducting coil <NUM> and the PCS <NUM>. The PCS <NUM> is connected in parallel to the superconducting coil <NUM> and includes the superconducting wire <NUM>. The PCS <NUM> includes the switch parallel structure JP in which a plurality of thermal permanent current switches <NUM> that interrupt an electric current flowing through the superconducting wire <NUM> are connected in parallel. A plurality of switch parallel structures JP are connected in series.

With this configuration, if a quench occurs in one of the thermal permanent current switches <NUM> constituting the switch parallel structure JP, the electric current can flow through another thermal permanent current switch <NUM> constituting the switch parallel structure JP. Therefore, the PCS <NUM> can maintain the superconducting state of the superconducting coil <NUM>. In other words, the PCS <NUM> according to the present embodiment is redundant and can improve the reliability of the static magnetic field magnet <NUM> compared with the case where the thermal permanent current switches <NUM> are connected in series to the superconducting coil <NUM>.

In the PCS <NUM> according to the present embodiment, a plurality of switch parallel structures JP are connected in series. With this configuration, if one thermal permanent current switch <NUM> is quenched while another thermal permanent current switch is being quenched, the PCS <NUM> can maintain the superconducting state of the superconducting coil unless all the thermal permanent current switches <NUM> constituting the same switch parallel structure JP are quenched. Therefore, the PCS <NUM> according to the present embodiment can increase the possibility of maintaining the superconducting state of the superconducting coil <NUM> if two or more thermal permanent current switches <NUM> are quenched. In other words, the PCS <NUM> according to the present embodiment can further improve the reliability of the static magnetic field magnet <NUM>.

The PCS <NUM> according to the present embodiment includes a plurality of the heaters <NUM> and the heater connecting wire <NUM>. The heaters <NUM> raise or lower the temperature of the respective switch parts <NUM>. The heater connecting wire <NUM> connects the heaters <NUM> in parallel to the external power source PW that supplies electric power to the heaters <NUM>.

With this configuration, the thermal permanent current switch <NUM> can switch the switch part <NUM> between the superconducting state and the normal conducting state by adjusting the heating of the heater <NUM> and raising and lowering the temperature of the switch part <NUM>. Therefore, the static magnetic field magnet <NUM> according to the present embodiment can excite and demagnetize the superconducting coil <NUM> by the thermal permanent current switches <NUM>.

In the PCS <NUM> according to the present embodiment, the external power source PW and the heaters <NUM> are connected in parallel. With this configuration, if disconnection occurs at one position of the heater connecting wire <NUM>, the PCS <NUM> can apply an electric current to the heaters <NUM> on the side where no disconnection occurs. In other words, the static magnetic field magnet <NUM> according to the present embodiment can excite and demagnetize the superconducting coil <NUM> if disconnection occurs at one position of the heater connecting wire <NUM>.

In the PCS <NUM> according to the present embodiment, the heaters <NUM> are connected in parallel by the heater connecting wire <NUM> such that the electric current flowing through the heaters <NUM> is equal. With this configuration, the heaters can heat the superconducting wire <NUM> at the corresponding positions without generating any temperature difference. Therefore, the switch parts <NUM> can switch the switch parts <NUM> at the corresponding positions between the superconducting state and the normal conducting state at substantially the same timing.

In the PCS <NUM> according to the present embodiment, the number of heaters <NUM> connected in parallel by the heater connecting wire <NUM> is equal. This configuration can facilitate making the electric current flowing through the heaters <NUM> equal.

The embodiment described above can be appropriately modified by changing some of the components or functions of each apparatus. The following describes some modifications according to the embodiment described above as other embodiments. In the following description, the points different from the embodiment described above are mainly explained, and detailed explanation of the points common to the already explained contents is omitted. The modifications to be described below may be implemented individually or in combination as appropriate.

In the embodiment above, the PCS <NUM> in which four switch parallel structures JP each composed of two thermal permanent current switches <NUM> are connected in series has been described. The configuration of the PCS <NUM>, however, is not limited thereto. In the PCS <NUM>, for example, three switch parallel structures JP each composed of three thermal permanent current switches <NUM> may be connected in series.

The PCS <NUM> can maintain the superconducting state of the superconducting coil <NUM> unless all of the thermal permanent current switches <NUM> constituting the same switch parallel structure JP are quenched. Therefore, by increasing the number of thermal permanent current switches <NUM> constituting the switch parallel structure JP, the PCS <NUM> can increase the possibility that the superconducting state of the superconducting coil <NUM> can be maintained even if quenches occur in a plurality of thermal permanent current switches <NUM>.

In other words, the PCS <NUM> according to the present modification can further improve the reliability of the static magnetic field magnet <NUM>.

In the embodiment above, the PCS <NUM> in which two heater groups HG each composed of four heaters <NUM> are connected in parallel has been described. The parallel connection configuration of the heaters <NUM> of the PCS <NUM>, however, is not limited thereto. In the PCS <NUM>, for example, four heater groups HG each composed of two heaters <NUM> may be connected in parallel.

In the PCS <NUM>, the heaters <NUM> are connected in parallel. With this configuration, if disconnection occurs in the heater connecting wire <NUM>, but there is a heater group HG where no disconnection occurs, the PCS <NUM> can apply the electric current to the heaters <NUM> constituting the heater group HG. Therefore, by increasing the number of groups of the heaters <NUM> connected in parallel, the PCS <NUM> can increase the possibility that the superconducting coil <NUM> can be excited and demagnetized if disconnection occurs in the heater connecting wire <NUM>.

In other words, the PCS <NUM> according to the present modification can increase the possibility that the superconducting coil <NUM> can be excited and demagnetized if disconnection occurs in the heater connecting wire <NUM>.

At least the embodiment, the modifications, and the like described above can further improve the reliability of the static magnetic field magnet <NUM>.

Claim 1:
A permanent current switch apparatus (<NUM>, <NUM>) configured to be electrically connected to a superconducting coil (<NUM>) via a superconducting wire (<NUM>), characterized in that the permanent current switch apparatus (<NUM>) comprises
a plurality of parallel structures (JP1, JP2, JP3, JP4) with thermal permanent current switches (<NUM>, PC1-PC8) connected in parallel, the thermal permanent current switches (<NUM>) being capable of switching between conducting and interrupting an electric current flowing through the superconducting wire (<NUM>, SU), wherein
the parallel structures (JP1, JP2, JP3, JP4) are connected in series.