WO2021028277A1

WO2021028277A1 - Superconducting switch state detection for superconducting magnet control

Info

Publication number: WO2021028277A1
Application number: PCT/EP2020/071944
Authority: WO
Inventors: John Robert Rogers
Original assignee: Koninklijke Philips N.V.
Priority date: 2019-08-13
Filing date: 2020-08-05
Publication date: 2021-02-18

Abstract

A controller (250/350/360) for detecting closure of a persistent current switch (207) connected to a sealed superconducting magnet (206) includes a detection circuit (351/361). The detection circuit (351/361) of the controller (250/350/360) measures (S430) a variable voltage across the persistent current switch (207) when a power supply is supplying current to the sealed superconducting magnet (206). The detection circuit (351/361) of the controller (250/350/360) also determines (S440) when the variable voltage decreases to a predetermined threshold. The detection circuit (351/361) of the controller (250/350/360) moreover detects (S450) closure of the persistent current switch (207) based on the variable voltage decreasing to the predetermined threshold.

Description

SUPERCONDUCTING SWITCH STATE DETECTION FOR SUPERCONDUCTING

MAGNET CONTROL

BACKGROUND

[001] Magnetic resonance imaging (MRI) systems include a superconducting magnet to align and realign hydrogen nuclei (protons) in water molecules in a subject being imaged. A strong first magnetic field is applied to align the proton "spins" of the hydrogen nuclei, which can then be realigned systematically by applying a second magnetic field. Magnetic resonance imaging systems can include radio frequency (RF) coils to then selectively apply a Bi magnetic field in a transmit stage. In a receive stage, the hydrogen nuclei return to an original position (i.e., the position before the selective application of the Bi magnetic field) and emanate a weak radio frequency signal which can be picked up and used to produce images. Magnetic resonance imaging systems also typically include a persistent current switch (PCS). A PCS is provided in a circuit arrangement in parallel with the superconducting magnet, allowing maintenance of a stable magnetic field for a long time.

[002] Conceptually, a known magnetic resonance imaging system can be envisioned as including a magnet housing as an exterior layer, a body coil housing immediately inside the magnet housing, a field gradient coil housing immediately inside the body coil housing, and an RF coil housing immediately inside the field gradient coil housing. A body coil in the body coil housing provides a main uniform, static, magnetic field as the first magnetic field that excites and aligns the hydrogen nuclei. The field gradient coils in the field gradient coil housing are used to vary (e.g., distort) the static magnetic field by applying the second magnetic field using a field gradient. The radio frequency coil in the RF coil housing is used to apply the Bi magnetic field. The result of the application of the Bi magnetic field is to manipulate orientations of the proton "spins" of the hydrogen nuclei within an imaging zone.

[003] In a strict sense, superconductivity is characterized as a state in which a material has zero electrical resistance at temperatures below a critical temperature threshold and in which impacts occur such as ejection of a weak magnetic field of/from the material. The superconducting magnet is energized by direct current power (DC power) from a power supply as the PCS presents resistance. A typical power supply may provide 500 amperes of direct current. Afterwards, the PCS is switched to short-circuit (i.e., presents low electrical impedance) to the direct current (DC) power source by returning to the superconducting state which allows the maintenance of the stable magnetic field.

[004] In a previous generation of magnetic resonance imaging systems, superconducting windings (coils) of the superconducting magnet were typically constructed as superconducting fibers embedded in a copper matrix and immersed in liquid helium to maintain the superconducting windings (coils) of the superconducting magnet below the critical temperature. In this previous generation, the PCS was heated above the critical temperature of the superconductor material used in the PCS to make the PCS present resistance as the superconducting windings (coils) of the superconducting magnet are energized by the direct current power. Typically the PCS has a critical temperature in the range of 6 to 9 Kelvin. In the previous generation of magnetic resonance imaging systems, the PCS was also immersed in liquid helium which allowed reversion to the superconducting state quickly when the heater was turned off. A newer type of "sealed" magnetic resonance imaging system has been developed in which the PCS is not immersed in liquid helium. Instead, in the newer magnetic resonance imaging system, a sealed cooling system is charged with gas helium at an elevated temperature. In this newer magnetic resonance imaging system, the PCS is cooled by an external refrigeration system that takes significantly longer to cool the PCS before the magnetic resonance imaging can be performed. Additionally, the amount of time required to cool the PCS may vary based on manufacturing variability in the superconducting magnet and the external cooling system. It is possible to determine which of the superconducting state and the resistive state the PCS is in by measuring the temperature, however sensors to measure cryogenic (very low) temperatures accurately are expensive and there may be a gradient in temperature across the PCS.

[005] FIG. 1 illustrates a known circuit arrangement of a known magnetic resonance imaging system. In FIG. 1, the circuit arrangement 110 has a magnet system 105 that includes the coils 106 of the superconducting magnet and the PCS 107. The coils 106 of the superconducting magnet in the circuit arrangement of the magnetic resonance imaging system in FIG. 1 may be representative of the coils in the known magnetic resonance imaging system described above. The power supply 108 provides a current in a circuit to the PCS 107. Significant voltage is present when the PCS 107 presents resistance, but no significant voltage is present when the PCS 107 is in the superconducting state. A typical example of the power supply 108 will have a small current variation caused by imperfect current regulation in the power supply 108. The small current variation may be alternating current (AC) at or above 50 Hertz. When the coils 106 of the superconducting magnet are being energized essentially all of the alternating current component of the current will flow through the PCS 107 due to the inductive impedance of the coils 106 of the superconducting magnet. The presence of small current variation is well known and typical examples of the magnet system 105 are designed with “ripple” current at sufficiently low levels that it does not prevent the PCS 107 from closing.

[006] By way of explanation, the previous generation of superconducting magnet systems in the known magnetic resonance imaging system of FIG. 1 does not measure or use the component of the voltage based on the alternating current component of the current and may deliberately filter out the alternating current component of the voltage in order to measure only the direct current component of the voltage. In the previous generation of superconducting magnets, the coils 106 of the superconducting magnet and the PCS 107 are immersed in a helium bath, and the PCS 107 could have an open resistance on the order of 10 to 100 Ohms. When ramping (or energizing) the superconducting magnets in the previous generation, a voltage on the order of 6 volts was applied and the current in the superconducting magnets increased by the relationship V=L*di/dt, where V is voltage, L is inductance, and di/dt is the change in inductance over the change in time. While energizing the superconducting magnets, some direct current flowed through the PCS 107. When the PCS 107 reached the desired current the voltage across the PCS 107 started to drop as the current redistributes to the coils 106 of the superconducting magnet with zero resistance in the superconducting state. When voltage of the PCS 107 was on the order of 100 mV or less in this prior generation, the heater that heated the PCS 107 was turned off. If the resistance of the PCS 107 is 10 Ohms, then the current of the PCS 107 is less than 10 mA. In other words, all but 10 mA of a typical supply current of 500 amperes flowed though the coils 106 of the superconducting magnet. The voltage across the PCS 107 will continue to decrease at an exponential rate determined by the resistance and the magnet inductance (L/R time constant on the order of 50 Henry / 10 Ohms or 5 seconds) of the PCS 107 until the PCS 107 becomes superconducting and the voltage quickly drops to near zero. The transition was relatively easy to detect. Accordingly, in the previous generation of superconducting magnets, any high frequency component of the voltage of the PCS 107 voltage was likely to be noise and was not used or even measured. Since determining the exact time of closure of the PCS 107 was not required, the voltage of the PCS 107 could be averaged or filtered to eliminate any high frequency signals above 50 Hertz.

[007] As noted above, a newer type of magnetic resonance imaging system has been developed in which the PCS 107 is not immersed in liquid helium, and cooling of a PCS 107 by external refrigeration system takes significantly longer to cool the PCS 107 before the magnetic resonance imaging can be performed. Additionally, the amount of time required to cool the PCS 107 may vary based on manufacturing variability in the superconducting magnet and the external cooling system. Accordingly, new ways for determining closure of the PCS 107 with a reasonable level of precision are required.

SUMMARY

[008] According to an aspect of the present disclosure, a controller for detecting closure of a persistent current switch connected to a sealed superconducting magnet includes a detection circuit. The detection circuit of the controller measures a variable voltage across the persistent current switch when a power supply is supplying current to the sealed superconducting magnet. The detection circuit of the controller also determines when the variable voltage decreases to a predetermined threshold. The detection circuit of the controller moreover detects closure of the persistent current switch based on the variable voltage decreasing to the predetermined threshold. [009] According to another aspect of the present disclosure, a system for detecting closure of a persistent current switch connected to a sealed superconducting magnet includes a superconducting filament, a current source, and a detection circuit. The superconducting filament is integrated with the persistent current switch. The current source is dedicated to powering only the superconducting filament. The detection circuit measures a voltage across the superconducting filament due to the current source while a power supply is supplying current to the sealed superconducting magnet. The detection circuit also determines when the voltage across the superconducting filament decreases to a predetermined threshold. The detection circuit further detects closure of the persistent current switch based on the voltage decreasing to the predetermined threshold.

[010] According to yet another aspect of the present disclosure, a method of detecting closure of a persistent current switch connected to a sealed superconducting magnet includes measuring, by a detection circuit, variable voltage across the persistent current switch when a power supply is supplying current to the sealed superconducting magnet. The method also includes determining when the variable voltage decreases to a predetermined threshold. The method further includes detecting closure of the persistent current switch based on the variable voltage decreasing to the predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS [Oil] The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.

[012]

[013] FIG. 1 illustrates a known circuit arrangement of a known magnetic resonance imaging system.

[014] FIG. 2 illustrates a circuit arrangement of a magnetic resonance imaging system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment.

[015] FIG. 3A illustrates a controller for a system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment.

[016] FIG. 3B illustrates another controller for a system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment. [017] FIG. 3C illustrates a circuit arrangement of a system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment. [018] FIG. 4 illustrates a process for superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment.

[019] FIG. 5 illustrates a general computer system, on which a method of superconducting switch state detection for superconducting magnet control can be implemented, in accordance with a representative embodiment. [020] FIG. 6 illustrates another circuit arrangement of a magnetic resonance imaging system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment.

DETAILED DESCRIPTION

[021] In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. Descriptions of known systems, devices, materials, methods of operation and methods of manufacture may be omitted so as to avoid obscuring the description of the representative embodiments. Nonetheless, systems, devices, materials and methods that are within the purview of one of ordinary skill in the art are within the scope of the present teachings and may be used in accordance with the representative embodiments. It is to be understood that the terminology used herein is for purposes of describing particular embodiments only and is not intended to be limiting. The defined terms are in addition to the technical and scientific meanings of the defined terms as commonly understood and accepted in the technical field of the present teachings.

[022] It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements or components, these elements or components should not be limited by these terms. These terms are only used to distinguish one element or component from another element or component. Thus, a first element or component discussed below could be termed a second element or component without departing from the teachings of the inventive concept. [023] The terminology used herein is for purposes of describing particular embodiments only and is not intended to be limiting. As used in the specification and appended claims, the singular forms of terms ‘a’, ‘an’ and ‘the’ are intended to include both singular and plural forms, unless the context clearly dictates otherwise. Additionally, the terms "comprises", and/or "comprising," and/or similar terms when used in this specification, specify the presence of stated features, elements, and/or components, but do not preclude the presence or addition of one or more other features, elements, components, and/or groups thereof. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[024] Unless otherwise noted, when an element or component is said to be “connected to”, “coupled to”, or “adjacent to” another element or component, it will be understood that the element or component can be directly connected or coupled to the other element or component, or intervening elements or components may be present. That is, these and similar terms encompass cases where one or more intermediate elements or components may be employed to connect two elements or components. However, when an element or component is said to be “directly connected” to another element or component, this encompasses only cases where the two elements or components are connected to each other without any intermediate or intervening elements or components.

[025] In view of the foregoing, the present disclosure, through one or more of its various aspects, embodiments and/or specific features or sub-components, is thus intended to bring out one or more of the advantages as specifically noted below. For purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. However, other embodiments consistent with the present disclosure that depart from specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparatuses are within the scope of the present disclosure.

[026] As described herein, superconducting switch state detection for superconducting magnet control provides an ability to accurately determine the specific time a PCS is or will be closed, as well as to eliminate an expensive temperature sensor such as a Ruthenium Oxide sensor. Superconducting switch state detection for superconducting magnet control also provides for certainty based on measured characteristics rather than predictions based on averages of similar superconducting magnets without regard for construction variables or performance reduction due to wear or operating conditions.

[027] FIG. 2 illustrates a circuit arrangement of a magnetic resonance imaging system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment.

[028] As shown in FIG. 2, the circuit arrangement 210 includes a magnet system 205 and a power supply 208. The magnet system 205 includes coils 206 of a superconducting magnet, a PCS 207 connected in parallel with the coils 206 of the superconducting magnet, and a controller 250.

[029] Notably, the controller 250 in FIG. 2 is shown within the magnet system 205, but a controller 250 for a PCS is typically mounted in an enclosure attached to the outside of the cryostat vessel. The controller 250 may be located based on the characteristics of the controller 250. For example, when a controller 250 includes an analog circuit that directly measures variable voltage across the PCS the controller 250 may include one or more components directly in contact with the PCS and one or more other components located in an enclosure attached to the outside of the cryostat vessel. In embodiments described herein, electronics of a controller 250 are not actually inside the cryostat vessel, and in some embodiments the controller 250 may be located separate from the magnet system 205.

[030] The power supply 208 energizes the superconducting magnet with the coils 206 primary with a direct current. However, the power supply 208 may be designed to provide a small (low amplitude) alternating current as a “ripple” current at sufficiently low levels so as to not prevent the PCS 207 from closing. The power supply 208 is representative of a typical power supply for a superconducting magnet, and the "ripple" current may be a small current variation which is caused by the imperfect current regulation in the power supply 208.

[031] The superconducting magnet with the coils 206 has an inductance typically in the range of 10 to 100 Henry. After the superconducting magnet is energized and in a superconducting state, it can be used for magnetic resonance imaging. In FIG. 2 and most embodiments based on FIG. 2, the magnetic resonance imaging system that includes the magnet system 205 may employ so- called “cryofree” superconducting magnets as the superconducting magnet that includes the coils

206 magnet system 205. Such "cryofree" superconducting magnet systems may be referred to herein as sealed superconducting magnet systems with a sealed superconducting magnet. The term "sealed" means that the superconducting magnet system is closed and may not include any mechanism for a user to add new cryogenic material to the system. Such sealed superconducting magnet systems with sealed superconducting magnets typically have a smaller volume of cryogenic material (e.g., 1 liter of liquid helium) when compared with previous non-sealed systems which employed relatively large volumes of cryogenic material (e.g., 1000 liters of liquid helium). However with the more recent generation of superconducting magnets that are sealed, the PCS

207 takes much longer to close, i.e., on the order of an hour. This means that the current will redistribute to the coils and the voltage across the PCS 207 will reach near zero before the PCS 207 closes. [032] The PCS 207 is connected across the superconducting magnet which includes the coils 206 and which has an inductance typically in the range of 10 to 100 Henry. When the superconducting magnet is being energized essentially the entirety of the alternating current component of the current will flow through the PCS 207 due to the inductive impedance of the superconducting magnet which includes the coils 206. While the PCS 207 is in the “normal” or resistive state this should be measurable as a small voltage that will become zero when the PCS 207 transitions to the superconducting state.

[033] Accordingly, superconducting switch state detection for superconducting magnet control as described herein for several embodiments (see FIG. 3 A and FIG. 3B) is based on the use of the parasitic high frequency current (> 50 Hertz) that is caused by the switching noise present on the power supply 208. Because this alternating current is high frequency, it will only flow through the PCS 207 since the PCS 207 has low inductance. Accordingly, in at least the embodiments of FIG. 3A and FIG. 3B, the voltage of the PCS 207 is specifically not discarded, and instead the noise voltage across the PCS 207 is used in the manner described herein. In the previous generation of superconducting magnets (i.e., previous to sealed superconducting magnets) there was not a particular benefit to gain using the teachings described herein, and there could be a benefit to filtering the PCS voltage when using the previous generation of superconducting magnets.

[034] The controller 250 is shown in close proximity to the coils 206 and the PCS 207 in FIG. 2. However, the controller 250 may be provided apart from the circuit arrangement 210, such as when connected by a communications wire that carries digital signals generated by an analog-to-digital converter that is proximate to the PCS 207. Additionally, a controller 250 may include a set of elements that work together, such as a memory that stores instructions and a processor such as a microprocessor that executes the instructions to implement processes described herein. A controller 250 may also be or include a processor such as an application-specific integrated circuit (ASIC) that automatically performs logical functions without necessarily requiring software, such as based on analog detections of voltage levels reaching a threshold so as to trigger a reaction. [035] As described herein a controller such as the controller 250 may include a detection circuit, and in the embodiments of FIGs. 3 A and 3B the detection circuit measures a variable voltage across the persistent current switch when a power supply is supplying current to the sealed superconducting magnet so as to power up the sealed superconducting magnet. The variable voltage may refer to a voltage that results from the relatively high-frequency (> 50 Hz) alternating current. As an example, an analog-to-digital converter may convert voltage readings to digital values which are analyzed to identify the amplitude of the variable voltage that varies based on the high-frequency alternating current. Analog-to-digital converters may be used to convert analog signals to digital signals for digital analysis in most or all embodiments herein.

[036] As explained previously, the variable voltage measured by a controller 250 may be due to a small (low amplitude) alternating current provided from the power supply 208 as a “ripple” current, and the variable voltage is present when the PCS 207 presents resistance when the material of the PCS 207 is above the critical temperature. The alternating current from the power supply 208 may be present independent of the temperature of the PCS 207 or the superconducting magnet, and independent of whether the PCS 207 and/or the superconducting magnet are in a superconducting state. On the other hand, since the variable voltage measured by the controller 250 is present only when the PCS 207 presents resistance, the variable voltage will become zero as/when the PCS 207 transitions to the superconducting state.

[037] FIG. 3A illustrates a controller for a system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment.

[038] As shown in FIG. 3 A, the controller 350 includes a detection circuit 351. The detection circuit 351 includes an ADC 353 (analog-to-digital controller), a processor 352 and a memory 357. The ADC 353 detects analog voltage readings from a PCS such as the PCS 207 in FIG. 2, and converts the detected analog voltage reading from an analog reading to a digital value. The ADC 353 outputs the digital signal with the digital value to the processor 352. The analog voltage readings are based on variable voltages across the PCS, which in turn vary based on alternating current which may alternate at a high frequency and based on whether the PCS is in the resistive state or the superconducting state. The processor 352 retrieves or otherwise receives software instructions from the memory 357 and executes the instructions to determine when the PCS is closed based on the amplitude of the variable voltage readings decreasing below a predetermined threshold.

[039] The controller 350 in FIG. 3A may be used as the basis for a first set of embodiments different from a second set of embodiments in FIG. 3B and a third set of embodiments in FIG. 3C. In the first set of embodiments based on FIG. 3 A, the transition to the superconducting state may be detected by measuring the voltage across the PCS with sufficient resolution to detect power supply ripple with the ADC 353. The variable voltage that is measured by the detection circuit 351 results from the variable current from a power supply such as the power supply 208 in FIG. 2. Digital signal processing by the processor 352 is used to detect the PCS transition to superconducting state. The controller 350 may then then wait a predetermined period for the PCS to cool sufficiently to conduct the full operating current of the superconducting magnet in the superconducting state.

[040] FIG. 3B illustrates another controller for a system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment. The controller 360 includes a detection circuit 361. The detection circuit 361 includes an analog circuit 364, a processor 362 and a memory 367. The analog circuit 364 includes a low-pass filter 365, a high-pass filter 366, and an ADC 363. The ADC 363 detects analog voltage readings from a PCS such as the PCS 207 in FIG. 2, and converts the detected analog voltage reading from an analog reading to a digital value. The ADC 363 outputs the digital signal with the digital value to the processor 362. The detected analog voltage readings may be isolated already by the high-pass filter 366 and by the low-pass filter 365, though the readings from the high-pass filter 366 are the readings of interest for most purposes explained herein. The filtered variable voltage readings from the high-pass filter 366 are based on variable voltages across the PCS, which in turn vary based on alternating current which may alternate at a high frequency and based on whether the PCS is in the resistive state or the superconducting state. The processor 362 retrieves or otherwise receives software instructions from the memory 367 and executes the instructions to determine when the PCS is closed based on the amplitude of the variable voltage readings decreasing below a predetermined threshold.

[041] The controller 360 in FIG. 3B may be used as the basis for a second set of embodiments different from the first set of embodiments in FIG. 3A and the third set of embodiments in FIG. 3C. In the second set of embodiments based on FIG. 3B, the transition to the superconducting state may be detected using separate analog circuits designed to isolate and measure the alternating current and direct current components of the voltage across the PCS, so that the alternating current component serves as the basis for determining the PCS closure. [042] the superconducting filament 379 is a separate superconducting filament built into the corresponding PCS. The transition to the superconducting state may be determined by making measurements using a small current source such as a dedicated current source that is not part of the main magnet circuit. The dedicated current source may provide, for example, direct current only to the superconducting filament 379, and the detection circuit 371 may detect voltage from the superconducting filament 379.

[043] In each of the controller 350 in FIG. 3A, the controller 360 in FIG. 3B and the controller

370 in FIG. 3C, logical operations are performed on digital data by processors executing instructions from memories. However, logical operations may also be performed for simple tasks by an application-specific integrated circuit, which may include circuitry for detecting when a threshold is reached as described herein.

[044] FIG. 3C illustrates a circuit arrangement of a system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment. [045] The circuit arrangement 310 includes a superconducting filament 379, a controller 370 and a current source 378. The controller 370 includes a detection circuit 371. The detection circuit

371 includes an ADC 373, a memory 376 and a processor 372.

[046] The superconducting filament 379 is integrated with a corresponding PCS, so that when the corresponding PCS is cooled by an external cooling system to reach the superconducting state the superconducting filament 379 will also reach the superconducting state. The superconducting filament 379 and the corresponding PCS may be made of the same material, bonded together by a bonding mechanism or even molded together at the time the corresponding PCS is manufactured. The ADC 373 detects analog voltage readings from the superconducting filament 379 which is integrated with a PCS such as the PCS 207 in FIG. 2. The ADC 373 converts the detected analog voltage reading from an analog reading to a digital value. The ADC 373 outputs the digital signal with the digital value to the processor 372. The detected analog voltage readings may be isolated already by a high-pass filter (now shown) similar to the high-pass filter 366 in FIG. 3B. The analog voltage readings are based on variable voltages across the PCS, which in turn vary based on alternating current which may alternate at a high frequency and based on whether the PCS is in the resistive state or the superconducting state. The processor 372 retrieves or otherwise receives software instructions from the memory 377 and executes the instructions to determine when the PCS is closed based on the amplitude of the variable voltage readings decreasing below a predetermined threshold.

[047] The current source 378 may be a dedicated current source that provides a direct current or a variable (alternating) current only to the superconducting filament 379. The dedicated current may be a small current that causes a small voltage reading when the superconducting filament 379 is in a resistive state and that approaches a zero reading when the superconducting filament 379 is in a superconducting state. When the voltage across the superconducting filament 379 approaches zero, this will reflect that the corresponding PCS which is integrated with the superconducting filament 379 is in a superconducting state, and this in turn will reflect that the corresponding PCS is closed.

[048] The controller 370 in FIG. 3C may be used as the basis for a third set of embodiments different from the first set of embodiments in FIG. 3 A and the second set of embodiments in FIG. 3B. In the third set of embodiments based on FIG. 3C, the superconducting filament 379 is a separate superconducting filament built into the corresponding PCS. The transition to the superconducting state may be determined by making measurements using a small current source such as a dedicated current source that is not part of the main magnet circuit. The dedicated current source may provide, for example, direct current only to the superconducting filament 379, and the detection circuit 371 may detect voltage from the superconducting filament 379.

[049] In each of the controller 350 in FIG. 3A, the controller 360 in FIG. 3B and the controller 370 in FIG. 3C, logical operations are performed on digital data by processors executing instructions from memories. However, logical operations may also be performed for simple tasks by an application-specific integrated circuit, which may include circuitry for detecting when a threshold is reached as described herein.

[050] FIG. 4 illustrates a process for superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment.

[051] The process of FIG. 4 starts at S410 in a default state, which in the embodiment of FIG. 4 is a resistive state when the persistent current switch is open. The default state S410 may be the state when a PCS is not cooled and is therefore above the critical temperature and open.

[052] At S420, the process of FIG. 4 includes initiating a cooling system for the persistent current switch. The cooling system is not shown in the circuit arrangement 210 of FIG. 2 but may be controlled manually or by a machine to perform S420.

[053] At S430, the process of FIG. 4 includes measuring, by a detection circuit, variable voltage across the persistent current switch when a power supply is supplying power to a superconducting magnet. The measuring at S430 is performed by the controller 350 in FIG. 3 A and by the controller S360 in FIG. 3B. In the embodiments based on FIG. 3C however, the measuring by a detection circuit of the controller S370 is of voltage that is based on direct current rather than on alternating current.

[054] At S440, the process of FIG. 4 includes determining when the variable voltage decreases to a predetermined threshold.

[055] In many embodiments based on the process of FIG. 4, the predetermined threshold is substantially zero. The term "substantially" may reflect that reaching a voltage of exactly zero may be very difficult even when the resistivity of the persistent current switch is almost entirely reduced in the superconductive state. Accordingly, the term "substantially zero" may be between absolute zero and 100 millivolts (mV) for direct current, though the upper limit for consideration of the threshold may be lower than 100 millivolts. As a result of the predetermined threshold being substantially zero, the detection circuit determines when the variable voltage decreases substantially to zero.

[056] At S450, the process of FIG. 4 includes detecting closure of the persistent current switch based on the variable voltage decreasing to the predetermined threshold. The variable voltage decreasing to the predetermined threshold corresponds to the superconducting state in which the persistent current switch is closed.

[057] At S460, the process of FIG. 4 includes outputting a notification of closure of the persistent current switch. The notification of closure may be by an audible signal, a visual signal such as a light illuminating, and/or by a data signal provided to a communications device or a computer monitor in order to alert an operator of closure of a PCS. The closure may be detected in advance at S450, when there is a known delay between the voltage reaching the predetermined threshold and the PCS actually closing. Accordingly, a notification at S460 may be delayed by a predetermined amount such as 30 seconds after the detection at S450.

[058] At S470, the process of FIG. 4 concludes with performing MRI procedures. The MRI procedures are performable based on confirmation of closure of the PCS at S460. [059] FIG. 5 illustrates a general computer system, on which a method of superconducting switch state detection for superconducting magnet control can be implemented, in accordance with a representative embodiment.

[060] The general computer system of FIG. 5 shows a complete set of components for a communications device or a computer device. However, a "controller" as described herein may be implemented with less than the set of components of FIG. 5, such as by a memory and processor combination.

[061] The computer system 500 can include a set of instructions that can be executed to cause the computer system 500 to perform any one or more of the methods or computer-based functions disclosed herein. The computer system 500 may operate as a standalone device or may be connected, for example, using a network 501, to other computer systems or peripheral devices. In embodiments, a computer system 500 may be used to perform logical processing based on digital signals received via an analog-to-digital converter as described herein for embodiments.

[062] In a networked deployment, the computer system 500 may operate in the capacity of a server or as a client user computer in a server-client user network environment, or as a peer computer system in a peer-to-peer (or distributed) network environment. The computer system 500 can also be implemented as or incorporated into various devices, such as a stationary computer, a mobile computer, a personal computer (PC), a laptop computer, a tablet computer, or any other machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. The computer system 500 can be incorporated as or in a device that in turn is in an integrated system that includes additional devices. In an embodiment, the computer system 500 can be implemented using electronic devices that provide voice, video or data communication. Further, while the computer system 500 is illustrated in the singular, the term "system" shall also be taken to include any collection of systems or sub-systems that individually or jointly execute a set, or multiple sets, of instructions to perform one or more computer functions. [063] As illustrated in Fig. 5, the computer system 500 includes a processor 510. A processor for a computer system 500 is tangible and non-transitory. As used herein, the term “non-transitory” is to be interpreted not as an eternal characteristic of a state, but as a characteristic of a state that will last for a period. The term “non-transitory” specifically disavows fleeting characteristics such as characteristics of a carrier wave or signal or other forms that exist only transitorily in any place at any time. A processor is an article of manufacture and/or a machine component. A processor for a computer system 500 is configured to execute software instructions to perform functions as described in the various embodiments herein. A processor for a computer system 500 may be a general-purpose processor or may be part of an application specific integrated circuit (ASIC). A processor for a computer system 500 may also be a microprocessor, a microcomputer, a processor chip, a controller, a microcontroller, a digital signal processor (DSP), a state machine, or a programmable logic device. A processor for a computer system 500 may also be a logical circuit, including a programmable gate array (PGA) such as a field programmable gate array (FPGA), or another type of circuit that includes discrete gate and/or transistor logic. A processor for a computer system 500 may be a central processing unit (CPU), a graphics processing unit (GPU), or both. Additionally, any processor described herein may include multiple processors, parallel processors, or both. Multiple processors may be included in, or coupled to, a single device or multiple devices. [064] A “processor” as used herein encompasses an electronic component which is able to execute a program or machine executable instruction. References to the computing device comprising “a processor” should be interpreted as possibly containing more than one processor or processing core. The processor may for instance be a multi-core processor. A processor may also refer to a collection of processors within a single computer system or distributed amongst multiple computer systems. The term computing device should also be interpreted to possibly refer to a collection or network of computing devices each including a processor or processors. Many programs have instructions performed by multiple processors that may be within the same computing device or which may even be distributed across multiple computing devices.

[065] Moreover, the computer system 500 may include a main memory 520 and a static memory 530, where memories in the computer system 500 may communicate with each other via a bus 508. Memories described herein are tangible storage mediums that can store data and executable instructions and are non-transitory during the time instructions are stored therein. As used herein, the term “non-transitory” is to be interpreted not as an eternal characteristic of a state, but as a characteristic of a state that will last for a period. The term “non-transitory” specifically disavows fleeting characteristics such as characteristics of a carrier wave or signal or other forms that exist only transitorily in any place at any time. A memory described herein is an article of manufacture and/or machine component. Memories described herein are computer-readable mediums from which data and executable instructions can be read by a computer. Memories as described herein may be random access memory (RAM), read only memory (ROM), flash memory, electrically programmable read only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), registers, a hard disk, a removable disk, tape, compact disk read only memory (CD-ROM), digital versatile disk (DVD), floppy disk, blu-ray disk, or any other form of storage medium known in the art. Memories may be volatile or non-volatile, secure and/or encrypted, unsecure and/or unencrypted.

[066] “Memory” is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. Examples of computer memory include, but are not limited to RAM memory, registers, and register files. References to “computer memory” or “memory” should be interpreted as possibly being multiple memories. The memory may for instance be multiple memories within the same computer system. The memory may also be multiple memories distributed amongst multiple computer systems or computing devices.

[067] As shown, the computer system 500 may further include a video display unit 550, such as a liquid crystal display (LCD), an organic light emitting diode (OLED), a flat panel display, a solid-state display, or a cathode ray tube (CRT). Additionally, the computer system 500 may include an input device 560, such as a keyboard/virtual keyboard or touch-sensitive input screen or speech input with speech recognition, and a cursor control device 570, such as a mouse or touch- sensitive input screen or pad. The computer system 500 can also include a disk drive unit 580, a signal generation device 590, such as a speaker or remote control, and a network interface device 540.

[068] In an embodiment, as depicted in Fig. 5, the disk drive unit 580 may include a computer- readable medium 582 in which one or more sets of instructions 584, e.g. software, can be embedded. Sets of instructions 584 can be read from the computer-readable medium 582. Further, the instructions 584, when executed by a processor, can be used to perform one or more of the methods and processes as described herein. In an embodiment, the instructions 584 may reside completely, or at least partially, within the main memory 520, the static memory 530, and/or within the processor 510 during execution by the computer system 500.

[069] In an alternative embodiment, dedicated hardware implementations, such as application- specific integrated circuits (ASICs), programmable logic arrays and other hardware components, can be constructed to implement one or more of the methods described herein. One or more embodiments described herein may implement functions using two or more specific interconnected hardware modules or devices with related control and data signals that can be communicated between and through the modules. Accordingly, the present disclosure encompasses software, firmware, and hardware implementations. Nothing in the present application should be interpreted as being implemented or implementable solely with software and not hardware such as a tangible non-transitory processor and/or memory.

[070] In accordance with various embodiments of the present disclosure, the methods described herein may be implemented using a hardware computer system that executes software programs. Further, in an exemplary, non-limited embodiment, implementations can include distributed processing, component/object distributed processing, and parallel processing. Virtual computer system processing can be constructed to implement one or more of the methods or functionalities as described herein, and a processor described herein may be used to support a virtual processing environment.

[071] The present disclosure contemplates a computer-readable medium 582 that includes instructions 584 or receives and executes instructions 584 responsive to a propagated signal; so that a device connected to a network 501 can communicate voice, video or data over the network 501. Further, the instructions 584 may be transmitted or received over the network 501 via the network interface device 540.

[072] FIG. 6 illustrates another circuit arrangement of a magnetic resonance imaging system with superconducting switch state detection for superconducting magnet control, in accordance with a representative embodiment.

[073] As shown in FIG. 6, the circuit arrangement 610 includes a magnet system 605, and a power supply 608. The circuit arrangement 610 is similar in some ways to the circuit arrangement 210 in FIG. 2, but also includes a superconducting filament 679 and a secondary current source 678. The magnet system 605 includes coils 606 of a superconducting magnet, a PCS 607 connected in parallel with the coils 606 of the superconducting magnet, and the controller 670. The controller 670 is similar to the controller 370 in the embodiment of FIG. 3C.

[074] In the embodiment of FIG. 6, the secondary current source 678 is dedicated to providing a current across the PCS 607, and the current from the secondary current source 678 may be direct current or alternating current. When the dedicated current is a variable current it may vary at a predetermined frequency or within a predetermined bandwidth that can be readily filtered by an analog filter and/or readily detected by a processor of a detection circuit of the controller 670. That is, the predetermined frequency of the dedicated current from the secondary current source 678 may result in a variable voltage that is measured by the detection circuit of the controller 670. In any event, the dedicated current from the secondary current source 678 may be directly measured as a voltage across the superconducting filament 679 by an ADC of the controller 670. When the PCS 607 changes to a superconducting state, the superconducting filament 679 will also change to the superconducting state and the voltage across the superconducting filament 679 will approach zero volts. Accordingly, closure of the PCS 607 can be detected using the superconducting filament 679 and the secondary current source 678 even when the secondary current source 678 is providing direct current.

[075] In some embodiments based on FIG. 6, the circuit arrangement 610 is part of a system for detecting closure of the PCS 607. The coils 606 are part of a sealed superconducting magnet, and the PCS 607 is connected to the sealed superconducting magnet. The system which includes the circuit arrangement 610 includes the superconducting filament 679, the secondary current source 678, and the detection circuit of the controller 670. The superconducting filament 679 is integrated with the PCS 607, such as by being attached to or even molded as part of the PCS 607. The secondary current source 678 is dedicated to powering only the superconducting filament 679. The detection circuit of the controller 670 measures a voltage across the superconducting filament 679 due to the secondary current source 678 while the power supply 608 is supplying current to the sealed superconducting magnet that includes the coils 606. The detection circuit of the controller 670 determines when the voltage across the superconducting filament 679 decreases to a predetermined threshold (at or close to zero volts). The detection circuit of the controller 670 also detects closure of the PCS 607 based on the voltage decreasing to the predetermined threshold. [076] In FIG. 6, the secondary current source 678 may be a source of direct current or variable (alternating) current to the superconducting filament 679. The secondary current source 678 may be provided specifically so that the detection circuit of the controller 670 can determine when the voltage across the superconducting filament 679 approaches zero as the PCS 607 reaches the superconducting state due to the cooling by an external cooling system. [077] Accordingly, superconducting switch state detection for superconducting magnet control described herein enables accurate identification of the timing when a PCS closes, which in turn enables efficient use of a magnetic resonance imaging system for magnet resonance imaging. [078] Although superconducting switch state detection for superconducting magnet control has been described with reference to several exemplary embodiments, it is understood that the words that have been used are words of description and illustration, rather than words of limitation. Changes may be made within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of superconducting switch state detection for superconducting magnet control in its aspects. Although superconducting switch state detection for superconducting magnet control has been described with reference to particular means, materials and embodiments, superconducting switch state detection for superconducting magnet control is not intended to be limited to the particulars disclosed; rather superconducting switch state detection for superconducting magnet control extends to all functionally equivalent structures, methods, and uses such as are within the scope of the appended claims.

[079] For example, in the embodiments of FIGs. 3A and 3B, the noisy alternating current component across a PCS is measured, whereas in the embodiments based on FIG. 3C, the voltage across a filament powered directly by direct current from a secondary power source is measured. Thus, superconducting switch state detection for superconducting magnet control can be with alternating current or direct current. Similarly, analog-to-digital converters are described for several embodiments but may be used for any embodiment described herein. Additionally, digital processors that executes software instructions are described for several embodiments but may be used for any embodiment described herein. The same is true also of application-specific integrated circuits that may be used for some or all functions otherwise performed by the digital processors for the superconducting switch state detection for superconducting magnet control.

[080] The illustrations of the embodiments described herein are intended to provide a general understanding of the structure of the various embodiments. The illustrations are not intended to serve as a complete description of all of the elements and features of the disclosure described herein. Many other embodiments may be apparent to those of skill in the art upon reviewing the disclosure. Other embodiments may be utilized and derived from the disclosure, such that structural and logical substitutions and changes may be made without departing from the scope of the disclosure. Additionally, the illustrations are merely representational and may not be drawn to scale. Certain proportions within the illustrations may be exaggerated, while other proportions may be minimized. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive.

[081] One or more embodiments of the disclosure may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any particular invention or inventive concept. Moreover, although specific embodiments have been illustrated and described herein, it should be appreciated that any subsequent arrangement designed to achieve the same or similar purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all subsequent adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the description.

[082] The Abstract of the Disclosure is provided to comply with 37 C.F.R. § 1.72(b) and is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, various features may be grouped together or described in a single embodiment for the purpose of streamlining the disclosure. This disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter may be directed to less than all of the features of any of the disclosed embodiments. Thus, the following claims are incorporated into the Detailed Description, with each claim standing on its own as defining separately claimed subject matter.

[083] The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to practice the concepts described in the present disclosure. As such, the above disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments which fall within the true spirit and scope of the present disclosure. Thus, to the maximum extent allowed by law, the scope of the present disclosure is to be determined by the broadest permissible interpretation of the following claims and their equivalents and shall not be restricted or limited by the foregoing detailed description.

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Claims

CLAIMS:

1. A controller (250/350/360/670) for detecting closure of a persistent current switch (207) connected to a sealed superconducting magnet (206), comprising: a detection circuit (351/361) that measures (S430) a variable voltage across the persistent current switch (207/607) when a power supply (208) is supplying current to the sealed superconducting magnet (206), that determines (S440) when the variable voltage decreases to a predetermined threshold, and that detects (S450) closure of the persistent current switch (207/607) based on the variable voltage decreasing to the predetermined threshold.

2. The controller (250/350/360) of claim 1, wherein the detection circuit (351/361) comprises: a processor (352/362) that determines (S440) when the variable voltage decreases to the predetermined threshold and that detects (S450) closure of the persistent current switch (207) based on the variable voltage decreasing to the predetermined threshold.

3. The controller (250/350/360) of claim 2, wherein the detection circuit (351/361) comprises: an analog-to-digital converter (353/363) that converts the variable voltage to a digital signal and outputs the digital signal to the processor (352/362).

4. The controller (250/350/360) of claim 2, wherein the detection circuit (351/361) further comprises: a memory (357/367) that stores instructions executed by the processor (352/362) to determine (S440) when the variable voltage decreases to the predetermined threshold and to detect (S450) closure of the persistent current switch (207) based on the variable voltage decreasing to the predetermined threshold.

5. The controller (250/360) of claim 1, wherein the detection circuit (361) comprises: an analog circuit (364) including a high-pass filter (366) that isolates the variable voltage across the persistent current switch (207) to obtain a filtered variable voltage, and a processor (362) that determines (S440) when the variable voltage decreases to the predetermined threshold based on the filtered variable voltage, and that detects (S450) closure of the persistent current switch (207) based on the variable voltage decreasing to the predetermined threshold.

6. The controller (250/350/360) of claim 1, wherein a state of the persistent current switch (207) without external cooling is open, and the persistent current switch (207) is closed by an external cooling system that cools the persistent current switch (207).

7. The controller (250/350/360) of claim 1, wherein the predetermined threshold is substantially zero so that the detection circuit (351/361) determines (S440) when the variable voltage decreases substantially to zero.

8. The controller (670) of claim 1, further comprising: a secondary current source (678) that is dedicated to providing a variable current across the persistent current switch (207/607) at a predetermined frequency, wherein the variable voltage that is measured by the detection circuit (351/361) results from the variable current at the predetermined frequency.

9. A system for detecting closure of a persistent current switch connected to a sealed superconducting magnet, comprising: a superconducting filament integrated with the persistent current switch; a current source dedicated to powering only the superconducting filament; a detection circuit that measures a voltage across the superconducting filament due to the current source while a power supply is supplying current to the sealed superconducting magnet, that determines when the voltage across the superconducting filament decreases to a predetermined threshold, and that detects closure of the persistent current switch based on the voltage decreasing to the predetermined threshold.

10. A method (FIG. 4) of detecting closure of a persistent current switch (207) connected to a sealed superconducting magnet (206/606), comprising: measuring (S430), by a detection circuit (351/361), variable voltage across the persistent current switch (207) when a power supply (208) is supplying current to the sealed superconducting magnet (206); determining (S440) when the variable voltage decreases to a predetermined threshold, and detecting (S450) closure of the persistent current switch (207) based on the variable voltage decreasing to the predetermined threshold.

11. The method of claim 10, wherein the determining (S440) and the detecting (S450) are performed using a processor (352/362).

12. The method of claim 11, wherein the measuring is performed using an analog-to-digital converter (353/363) that converts the variable voltage to a digital signal and outputs the digital signal to the processor (352/362).

13. The method of claim 11, wherein the processor (352/362) executes instructions retrieved from a memory (357/367) to determine (S440) when the variable voltage decreases to the predetermined threshold and to detect (S450) closure of the persistent current switch (207) based on the variable voltage decreasing to the predetermined threshold.

14. The method of claim 10, further comprising: filtering, using a high-pass filter (366), the variable voltage across the persistent current switch (207) to obtain a filtered variable voltage; determining (S440), using a processor (362), when the variable voltage decreases to the predetermined threshold based on the filtered variable voltage, and detecting (S450), using the processor (362), closure of the persistent current switch (207) based on the variable voltage decreasing to the predetermined threshold.

15. The method of claim 10, wherein a state of the persistent current switch (207) without external cooling is open, and the persistent current switch (207) is closed by an external cooling system that cools the persistent current switch (207).

16. The method of claim 10, wherein the predetermined threshold is substantially zero so that the detection circuit (351/361) determines (S440) when the variable voltage decreases substantially to zero.

17. The method of claim 10, further comprising: providing, from a secondary current source (678), a variable current across the persistent current switch (207) at a predetermined frequency, wherein the variable voltage that is measured by the detection circuit (351/361) results from the variable current at the predetermined frequency.