Claims:1. A system (100, 200) for reducing voltage stress, the system comprising:
a power conversion unit comprising:
one or more converters (126, 212) disposed between a first rail (216) and a second rail (224), wherein the one or more converters (212) comprise a plurality of switching units (214);
a cold plate (226) configured to dissipate heat from the plurality of switching units (214); and
a compensation unit (206) operatively coupled to the cold plate (226) of the power conversion unit, wherein the compensation unit (206) is configured to modify a voltage of the cold plate (226) based on a reference voltage to reduce voltage stress in the plurality of switching units (214).

2. The system (100, 200) of claim 1, wherein the compensation unit (206) is configured to:
measure the voltage of the cold plate (226); and
determine the reference voltage for the cold plate (226) based on a voltage corresponding to the first rail and a voltage corresponding to the second rail.

3. The system (100, 200) of claim 2, wherein the compensation unit (206) is configured to determine the reference voltage based on an average value of the voltage corresponding to the first rail and the voltage corresponding to the second rail.

4. The system (100, 200) of claim 1, wherein the cold plate (226) comprises at least one of an air-cooled cold plate and a liquid-cooled cold plate.

5. The system (100, 200) of claim 1, wherein the compensation unit (206) comprises at least one of an electrical subunit (242) and a controlling subunit (244).

6. The system (100, 200) of claim 5, wherein the electrical subunit (242) comprises at least one of a passive circuit element and an active circuit element.

7. The system (100, 200) of claim 5, wherein the electrical subunit (242) is operatively coupled to a safety ground (252).

8. The system (100, 200) of claim 1, wherein the compensation unit (206) is configured to modify the voltage of the cold plate by setting the voltage of the cold plate to the reference voltage.

9. The system (100, 200) of claim 8, wherein the compensation unit (206) is further configured to alter a voltage across the plurality of switching units based on the modified voltage of the cold plate to reduce the voltage stress in the plurality of switching units.

10. A compensation unit (124, 206) operatively couplable to a power conversion unit having one or more converters (212) disposed between a first rail (216) and a second rail (224), wherein the one or more converters (212) comprise a plurality of switching units (214) disposed on a cold plate (226), the compensation unit (206) comprising:
a controlling subunit (244) configured to measure a voltage of the cold plate (226); and
an electrical subunit (242) coupled to the cold plate (226), wherein the electrical subunit (242) is configured to modify the measured voltage of the cold plate based on a reference voltage to reduce voltage stress in the plurality of switching units.

11. The compensation unit (124, 206) of claim 10, wherein the electrical subunit (242) comprises a plurality of impedances (246, 250).

12. The compensation unit (124, 206) of claim 11, wherein a first end of a first impedance (246) of the plurality of impedances (246, 250) is coupled to the first rail (216) and a first end of a second impedance (250) of the plurality of impedances (246, 250) is coupled to the second rail (224).

13. The compensation unit (124, 206) of claim 12, wherein a second end of the first impedance (246) of the plurality of impedances (246, 250) and a second end of the second impedance (250) of the plurality of impedances (246, 250) are coupled to a common point (254).

14. The compensation unit (124, 206) of claim 10, wherein the electrical subunit (242) is configured to determine the reference voltage for the cold plate based on a voltage corresponding to the first rail and a voltage corresponding to the second rail.

15. A method (600) for reducing voltage stress in a plurality of switching units of one or more converters, wherein the one or more converters are disposed between a first rail and a second rail, the method (600) comprising:
measuring (602), using a compensation unit, a voltage of a cold plate, wherein the cold plate is configured to dissipate heat from the plurality of switching units disposed thereon;
determining (604), using the compensation unit, a reference voltage for the cold plate based on a voltage corresponding to the first rail and a voltage corresponding to the second rail; and
modifying (606), using the compensation unit, the measured voltage of the cold plate based on the determined reference voltage to reduce the voltage stress in the plurality of switching units.

16. The method (600) of claim 15, wherein determining (604) the reference voltage for the cold plate comprises computing an average value of the voltage corresponding to the first rail and the voltage corresponding to the second rail.

17. The method (600) of claim 15, wherein modifying (606) the measured voltage of the cold plate comprises setting the measured voltage of the cold plate to the determined reference voltage to reduce a voltage across the plurality of switching units.

18. A system (100, 200) for reducing voltage stress, comprising:
an imaging unit (106) comprising:
an acquisition subsystem configured to acquire image data;
one or more gradient power supplies (116), wherein at least one of the one or more gradient power supplies (116) comprises:
one or more converters (126, 212) disposed between a first rail (216) and a second rail (224), wherein the one or more converters (126, 212) comprise a plurality of switching units (214);
a cold plate (226) configured to dissipate heat from the plurality of switching units (214);
a plurality of gradient amplifiers (118), wherein each of the plurality of gradient amplifiers (118) is operatively coupled to a corresponding gradient power supply of the one or more gradient power supplies (116);
a processing subsystem in operative association with the acquisition subsystem and configured to process the acquired image data; and
a compensation unit (124, 206) operatively coupled to the one or more gradient power supplies (116), wherein the compensation unit (124, 206) is configured to modify voltage of the cold plate based on a reference voltage to reduce voltage stress in the plurality of switching units.

19. The system (100, 200) of claim 18, wherein the compensation unit (124, 206) is further configured to:
measure a voltage of the cold plate; and
determine the reference voltage for the cold plate based on a voltage corresponding to the first rail and a voltage corresponding to the second rail.

20. The system (100, 200) of claim 18, wherein the compensation unit (124, 206) is configured to modify the voltage of the cold plate by setting the voltage of the cold plate to the reference voltage.

21. The system (100, 200) of claim 20, wherein the compensation unit (124, 206) is further configured to reduce a voltage across the plurality of switching units based on the modified voltage of the cold plate.
, Description:BACKGROUND
[0001] Embodiments of the present specification generally relate to a converter/rectifier having one or more switching units, and more particularly to systems and methods for reducing voltage stress in the switching units employed in a converter/rectifier of medical imaging devices, such as a magnetic resonance imaging (MRI) system.
[0002] Typically, MRI systems are used in medical applications to generate images of soft tissues in the human body. The MRI system includes various components such as a gradient unit, a radio frequency (RF) transmit chain, a RF receive chain, a system control unit, a patient handling unit, and the like. The gradient unit includes three gradient coils corresponding to three axes (for example, X-axis, Y-axis, and Z-axis), three gradient amplifiers corresponding to the three gradient coils, and three power supplies for powering the three gradient amplifiers. Each power supply provides multiple direct current (DC) voltages (for example, 650 volts, 650 volts, and 350 volts).
[0003] In addition, each gradient amplifier includes a plurality of multilevel converters. Each multilevel converter includes multiple converter bridges. The multiple DC voltages supplied by the power supply are fed to each of the multiple converter bridges. The converter bridges are configured to control a magnitude and shape of a current flowing through a corresponding gradient coil. Typically, the current flowing through the gradient coil has a magnitude of several hundreds of amperes (for example, 900 A) and a slew rate of about 2 A/µs.
[0004] Lately, MRI systems are being used to acquire neuro scans that entail a higher magnitude of current (> 1000 A) through the gradient coil and a higher value of slew rate (for example 3 A/µs) for achieving a good image quality. To achieve such high slew rates, desired values of the DC voltage to be provided by the power supply also increase significantly. This increase in the values of DC voltage results in a substantive increase in voltage stress on an insulation of the switching units and insulation of high frequency transformers, typically used in the gradient power supply.
[0005] Typically, high frequency rectifiers are used in the power supply to rectify the high frequency voltage produced by the high frequency transformers. The switching units of the rectifiers are mounted on a cold plate. If the cold plate on which the switching units are mounted is connected to a safety ground, then the switching units that are located at the extremities of a leg of the rectifier are exposed to a voltage that is higher than an isolation voltage rating of the switching units. This results in failure of the corresponding switching units and in turn failure of the power supply.
[0006] Currently available techniques entail the use of a thermal pad coupled between the switching units and the cold plate to avoid failure of the switching units due to high voltage stress. However, use of the thermal pad results in degradation of the thermal performance of the switching units and in turn the thermal performance of the power supply. Certain other presently available techniques avoid failure of the switching units due to high voltage stress via use of an isolated cold plate. However, in this scenario, since the cold plate voltage is not defined with respect to a safety ground, the reduction in voltage stress is not assured under all conditions. In addition, it is not desirable to use an isolated cold plate. Other techniques call for the use of higher voltage switching units (for example, 1700 V) with higher isolation voltage rating to withstand higher voltage stress. However, the higher voltage switching units are slower and have higher conduction and reverse recovery losses.
BRIEF DESCRIPTION
[0007] In accordance with aspects of the present specification, a system for reducing voltage stress, is presented. The system includes a power conversion unit. The power conversion unit includes one or more converters disposed between a first rail and a second rail, where the one or more converters include a plurality of switching units. Further, the power conversion unit includes a cold plate configured to dissipate heat from the plurality of switching units. Furthermore, the system includes a compensation unit operatively coupled to the cold plate of the power conversion unit, where the compensation unit is configured to modify a voltage of the cold plate based on a reference voltage to reduce voltage stress in the plurality of switching units.
[0008] In accordance with another aspect of the present specification, a compensation unit operatively couplable to a power conversion unit having one or more converters disposed between a first rail and a second rail, where the one or more converters include a plurality of switching units disposed on a cold plate. The compensation unit includes a controlling subunit configured to measure a voltage of the cold plate. Further, the compensation unit includes an electrical subunit coupled to the cold plate, where the electrical subunit is configured to modify the measured voltage of the cold plate based on a reference voltage to reduce voltage stress in the plurality of switching units.
[0009] In accordance with yet another aspect of the present specification, a method for reducing voltage stress in a plurality of switching units of one or more converters is presented. The one or more converters are disposed between a first rail and a second rail. The method includes measuring, using a compensation unit, a voltage of a cold plate, where the cold plate is configured to dissipate heat from the plurality of switching units disposed thereon. Also, the method includes determining, using the compensation unit, a reference voltage for the cold plate based on a voltage corresponding to the first rail and a voltage corresponding to the second rail. Further, the method includes modifying, using the compensation unit, the measured voltage of the cold plate based on the determined reference voltage to reduce the voltage stress in the plurality of switching units.
[0010] In accordance with yet another aspect of the present specification, a system for reducing voltage stress is presented. The system includes an imaging unit. The imaging unit includes an acquisition subsystem configured to acquire image data. Further, the imaging unit includes one or more gradient power supplies. The at least one of the one or more gradient power supplies include one or more converters disposed between a first rail and a second rail, where the one or more converters include a plurality of switching units. Further, the one or more gradient power supplies include a cold plate configured to dissipate heat from the plurality of switching units. The imaging unit also includes a plurality of gradient amplifiers, where each of the plurality of gradient amplifiers is operatively coupled to a corresponding gradient power supply of the one or more gradient power supplies and a processing subsystem in operative association with the acquisition subsystem and configured to process the acquired image data. The system further includes a compensation unit operatively coupled to the one or more gradient power supplies, where the compensation unit is configured to modify voltage of the cold plate based on a reference voltage to reduce voltage stress in the plurality of switching units.
DRAWINGS
[0011] These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
[0012] FIG. 1 is a block diagram illustration of an exemplary imaging system configured to use systems and methods of FIGs. 2-6;
[0013] FIG. 2 is a diagrammatical representation of an exemplary system for reducing voltage stress in a switching unit in the imaging system of FIG. 1, according to aspects of the present specification;
[0014] FIG. 3 is a diagrammatical representation of one embodiment of a compensation unit for use in the system of FIG. 1, according to aspects of the present specification;
[0015] FIGs. 4 and 5 are graphical representations of an electrical parameter corresponding to the imaging system with and without use of the compensation unit of FIG. 3, according to aspects of the present specification;
[0016] FIG. 6 is a flow chart representing an exemplary method for reducing voltage stress in a switching unit of the imaging system of FIG. 1, according to aspects of the present specification; and
[0017] FIG. 7 is a block diagram illustration of an exemplary imaging system in the form of a magnetic resonance imaging (MRI) system, according to aspects of the present specification.
DETAILED DESCRIPTION
[0018] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this specification belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The term “or” is meant to be inclusive and mean one, some, or all of the listed items. The use of “including,” “comprising” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. Furthermore, terms “circuit” and “circuitry” and “controlling unit” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together to provide the described function. Also, the term “operatively coupled,” as used herein, includes wired coupling, wireless coupling, electrical coupling, magnetic coupling, radio communication, software based communication, or combinations thereof.
[0019] As will be described in detail hereinafter, various embodiments of exemplary systems and methods for reducing voltage stress in a switching unit are presented. Although, systems and methods for reducing voltage stress in a switching unit of a medical imaging device are presented hereinafter, use of the present systems and methods for reducing voltage stress in switching units in other applications such as, but not limited to, a power converter employed in motor drives, high voltage direct current applications, and the like, is also envisioned. The medical imaging device may be a magnetic resonance imaging (MRI) device, a computed tomography (CT) device, and the like. This reduction in the voltage stress in the switching units advantageously aids in minimizing failures of the switching units and any systems that employ the switching units. Use of the systems and methods presented hereinafter provides a cost effective and simpler solution for reducing the voltage stress in the switching units without compromising on the thermal or electrical performance of the switching units.
[0020] Turning now to the drawings, and referring to FIG. 1 a block diagram illustration of an exemplary imaging system configured to use systems and methods of FIGs. 2-6, is presented. The system 100 includes a power source 102, a power distribution unit (PDU) 104, and a medical imaging device 106. The medical imaging device 106 may be a magnetic resonance imaging (MRI) unit, in one embodiment.
[0021] In the present specification, the terms ‘MRI unit,’ ‘imaging unit,’ ‘imaging device,’ and ‘medical imaging device’ may be used interchangeably. Also, the terms ‘potential of cold plate,’ ‘voltage of the cold plate,’ and ‘cold plate voltage’ may be used interchangeably. Similarly, the terms ‘voltage of the switching units,’ ‘switching unit voltages,’ ‘voltage across the switching units,’ ‘voltage at terminals of the switching units,’ and ‘voltage at terminals of the switching units with respect to corresponding base plates’ may be used interchangeably. Further, terms ‘voltage of the first rail’ and ‘first rail voltage’ may be used interchangeably. Also, terms ‘voltage of the second rail’ and ‘second rail voltage’ may be used interchangeably.
[0022] The power source 102 is coupled to the medical imaging device 106 via the PDU 104. The power source 102 is configured to provide an alternating current (AC) input. In one embodiment, the PDU 104 is a low frequency power distribution unit (LFPDU). However, in other embodiment, the PDU 104 may be a high frequency power distribution unit (HFPDU).
[0023] Moreover, in the example of FIG. 1, the medical imaging device 106 includes components such as a gradient unit 108, a radio frequency (RF) transmit chain 110, a RF receive chain 112, and a patient handling unit 114. In one embodiment, the gradient unit 108 includes a gradient power supply 116, a gradient amplifier 118, a ripple filter 120, and a gradient coil 122.
[0024] In the example of FIG. 1, the gradient unit 108 is depicted as including a single gradient power supply, a corresponding gradient amplifier, and a corresponding gradient coil. However, the gradient unit 108 of the medical imaging device 106 may include multiple gradient power supplies, corresponding gradient amplifiers, and corresponding gradient coils. The gradient power supply 116 may be an X-axis power supply, a Y-axis power supply, or a Z-axis power supply. Also, the gradient amplifier 118 may be an X-axis amplifier, a Y-axis amplifier, or a Z-axis amplifier. The gradient coil 122 may be X-axis coil, a Y-axis coil, or a Z-axis coil.
[0025] The gradient power supply 116 is configured to generate isolated direct current (DC) voltages. The isolated DC voltages generated by the gradient power supply 116 is provided to gradient amplifier 118. Further, the gradient amplifier 118 is coupled to gradient coil 122 via ripple filter 120.
[0026] Furthermore, the gradient amplifier 118 includes a multilevel converter. The multilevel converter in turn includes a plurality of converter bridges 130. The converter bridges 130 of the gradient amplifier 118 are configured to control a magnitude and shape of the current flowing to the gradient coil 122. Furthermore, the ripple filter 120 is configured to filter any switching ripple and corresponding harmonics in the current being supplied to the gradient coil 122. Also, rails such as a first rail (not shown) and a second rail (not shown) are used to couple the gradient power supply 116 to the gradient amplifier 118. It may be noted that these rails experience the maximum voltage stress. Moreover, switching of switches in the converter bridges 130 of the gradient amplifier 118 results in variations in first rail voltage and second rail voltage with respect to a safety ground (not shown).
[0027] The gradient power supply 116 includes one or more converters 126, where the converters 126 are operatively coupled to one another in series. Also, these converters 126 are disposed between the first rail and the second rail. In one embodiment, the converters 126 are rectifiers. The rectifier may be a high-speed rectifier.
[0028] Furthermore, each converter 126 of the gradient power supply 116 includes one or more switching units (not shown). Each switching unit of the gradient power supply 116 in turn includes a switching device, such as a semiconductor switch. Further, each switching unit is disposed on a corresponding base plate (not shown). In one embodiment, the semiconductor switch is a non-controllable semiconductor switch, such as, a diode. In another embodiment, the semiconductor switch is a controllable semiconductor switch. Also, the controllable semiconductor switch may include a partially controlled semiconductor switch or a fully controlled semiconductor switch. The term “base plate” is used to refer to a substrate on which a switching unit is disposed. Typically, the base plate is a metallic plate, such as a copper plate.
[0029] It may be noted that the switching unit is typically insulated from the base plate, thereby resulting in a parasitic capacitance between the switching unit and the base plate. Further, each switching unit along with its base plate is disposed on a cold plate 125. Additionally, a capacitive path is established between the switching units and the cold plate 125. Accordingly, any change in voltage of the cold plate 125 impacts the voltage at terminals of the switching units with respect to corresponding base plates. In a presently contemplated configuration the gradient power supply is shown to include a single cold plate 125, however, use of multiple cold plates is envisaged.
[0030] The cold plate 125 is configured to dissipate heat generated by switching units of the gradient power supply 116. A combination of the cold plate 125 and the converters 126 having switching units may be referred to as a power conversion unit.
[0031] As noted hereinabove, the first rail voltage and the second rail voltage vary with the switching of the switches of the converter bridges 130 of the gradient amplifier 118. These variations in the first and second rail voltages with respect to safety ground in turn result in variations in the voltage at the terminals of the switching units with respect to corresponding base plates. In some instances, the variations in the first and second rail voltages may result in an increase in the voltage at the terminals of the switching units with respect to corresponding base plates. In this example, in certain instance, the voltage at the terminals of the switching units of the gradient power supply 116 may be equal to or exceed an isolation voltage rating of the switching units of the gradient power supply 116. The term “isolation voltage rating,” is used to refer to a maximum value of voltage at the terminals of the switching units with respect to corresponding base plates, that the switching units are configured to withstand without breaking down. This increase in the voltage at the terminals of the switching units disadvantageously results in an increase in voltage stress in the switching units of the gradient power supply 116. Hence, it is desirable to reduce the voltage at the terminals of the switching units of the gradient power supply 116 to reduce the voltage stress in the switching units of the gradient power supply 116.
[0032] As noted hereinabove, any change in the cold plate voltage impacts the voltage at the terminals of the switching units of the gradient power supply 116. In accordance with aspects of the present specification, the voltage stress in the switching units of the gradient power supply 116 is reduced by modifying the cold plate voltage. In one embodiment, the cold plate voltage is set to an average value of the first and second rail voltages. Modifying the cold plate voltage based on the first and second rail voltages aids in reducing the voltage at terminals of the switching units of the gradient power supply 116 to a value lower than the isolation voltage rating of the switching units of the gradient power supply 116 at any instant in time.
[0033] In accordance with aspects of the present specification, the system 100 includes an exemplary compensation unit 124 coupled to the cold plate 125 and configured to modify the cold plate voltage. In particular, the compensation unit 124 is configured to set the cold plate voltage to the average value of the first and second rail voltages. Additionally, the compensation unit 124 is configured to measure the cold plate voltage and determine a reference voltage of the cold plate 125 based on the first and second rail voltages. The operation of the compensation unit 124 will be explained in greater detail with respect to FIG. 2 and FIG. 6.
[0034] In one embodiment, the compensation unit 124 includes an electrical subunit (not shown) and a controlling subunit (not shown). The electrical subunit may include two impedances coupled to each other. Each of these impedances include at least one of one or more passive circuits element and one or more active circuit elements. The electrical subunit of the compensation unit 124 will be described in greater detail with respect to FIG. 3. In certain embodiments, the controlling subunit of the compensation unit 124 includes one or more processing units.
[0035] In the example of FIG. 1, the compensation unit 124 is depicted as being integral to the gradient power supply 116. However, in another embodiment, the compensation unit 124 may be disposed at a determined distance from the gradient power supply 116
[0036] Referring now to FIG. 2, a diagrammatical representation 200 of one embodiment of a portion of the system 100, according to aspects of the present specification, is presented. More particularly, one embodiment of the compensation unit 124 of FIG. 1 along with a portion of the medical imaging device 106 of FIG. 1 is depicted in FIG. 2. Also, in the example of FIG. 2, for ease of illustration, only a subset of components of the medical imaging device 106, such as a gradient unit 201 is depicted. Reference numeral 206 is generally representative of one embodiment of the compensation unit 124 of FIG. 1.
[0037] The compensation unit 206 is configured to reduce voltage stress in switching units used in the gradient unit 201. In a presently contemplated configuration, the compensation unit 206 includes an electrical subunit 242 and a controlling subunit 244.
[0038] The gradient unit 201 includes a gradient power supply 202, a gradient amplifier 204, and a gradient coil 222. The gradient power supply 202 includes an input power converting circuit 210, high frequency transformers 256, 258, and one or more converters 212. In one embodiment, the input power converting circuit 210 is a DC to AC converter. Reference numeral 208 is generally representative of a voltage Vin that is provided to the gradient power supply 202. The voltage Vin 208 is provided from an AC power source via a main disconnect panel (not shown), a power distribution unit (not shown), and a rectification unit (not shown).
[0039] Further, the input power converting circuit 210 is configured to provide power to the converters 212 via the high frequency transformers 256, 258. The converters 212 are operatively coupled to each other in series to generate a determined value of voltage to be provided to the gradient amplifier 204. The converters 212 are disposed between a first rail 216 and a second rail 224. The terms “first rail” and “second rail” are used to refer to rails that are employed for coupling the gradient power supply 202 to the gradient amplifier 204 via a ripple filter 240. It may be noted that these rails 216, 224 experience the maximum voltage stress.
[0040] Also, in one example, these converters 212 may be categorized into two sets - a first set of converters and a second set of converters. The first set of converters 212 is coupled between the first rail 216 and a first intermediate rail 218, and the second set of converters 212 is coupled between a second intermediate rail 220 and the second rail 224. The terms “first intermediate rail” and “second intermediate rail” are used to refer to intermediate rails that are employed for coupling the gradient power supply 202 to the gradient amplifier 204. It may be noted that the voltage stress experienced by the first and second intermediate rails 220, 224 is lower than the voltage stress experienced by the first and second rails 216, 224.
[0041] In one embodiment, each of the converters 212 is a high-speed rectifier. The converters 212 include a plurality of switching units 214. In certain embodiments, each of the switching units 214 includes a switching device, such as a semiconductor switch. In one embodiment, the semiconductor switch is a non-controllable switch, such as a diode. In another embodiment, the semiconductor switch is a controllable switch. Each switching unit 214 is disposed on a corresponding base plate.
[0042] Further, each switching unit 214 along with its base plate is disposed on a cold plate 226. The cold plate 226 may be alternatively referred to as a heat sink. The cold plate 226 is configured to dissipate any heat generated by the switching units 214. Furthermore, the cold plate 226 is an uni-potential surface. The cold plate 226 may include an air-cooled cold plate or a liquid-cooled cold plate. In one embodiment, the cold plate 226 includes a tubular, pipe-like structure configured to allow flow of a coolant. Additionally, in other embodiments, the cold plate 226 is an aluminum pipe, a copper pipe, and the like. The coolant may be a gaseous coolant or a liquid coolant. The liquid coolant includes water, oil, and the like and the gaseous coolant includes hydrogen.
[0043] The gradient power supply 202 provides multiple isolated DC voltages to the gradient amplifier 204. The use of high frequency transformers 256, 258 in the gradient power supply 202 facilitates generation of isolated DC voltages at output terminals 228, 230 of the gradient power supply 202. The isolated DC voltage obtained at the output terminal 228 is represented as V1 and the isolated DC voltage obtained at the output terminal 230 is represented as V2. In one embodiment, a value of voltage V1 is equal to a value of voltage V2.
[0044] In addition, the gradient amplifier 204 includes a plurality of converter bridges 232, 234. The converter bridges 232, 234 include a plurality of switches 236. Each of the plurality of switches 236 includes a controllable semiconductor switch. The gradient amplifier 204 is configured to control a current provided to a corresponding gradient coil 222. This current aids in creating a magnetic field with a desired gradient in a medical imaging device, such as the medical imaging device 106 of FIG. 1. This magnetic field is used in conjunction with excitation of RF coils of the medical imaging device to generate images of a patient’s body.
[0045] Additionally, the switches 236 are transitioned between different states based on a determined switching pattern. The switching pattern may be determined by a controller. The switching/transitioning of the switches 236 in the converter bridges 232, 234 aids in controlling a magnitude and shape of a current flowing through the gradient coil 222, which in turn influences a quality of an image generated by the medical imaging device 106. In particular, a slew rate of the current flowing through the gradient coil 222 affects the quality of the resulting image. The term “slew rate” refers to a rate of change of current per unit of time.
[0046] Furthermore, in one embodiment, a ripple filter 240 is used to operatively couple the gradient amplifier 204 and the gradient coil 222. The ripple filter 240 is configured to filter any switching ripple and corresponding harmonics in the current supplied to the gradient coil 222.
[0047] Moreover, the switching of the switches 236 of the gradient amplifier 204 results in variations in the first and second rail voltages with respect to a safety ground. In one embodiment, the potential of the safety ground may be zero volt. In certain instances, the switching of the switches 236 may result in the first rail voltage attaining a maximum positive value, while the second rail voltage attains a maximum negative value. In one embodiment, the maximum positive value may be higher than +3V1/2, where V1 is an isolated DC voltage generated by the gradient power supply 202 and V1 > 1.2 kV. Also, in one embodiment, the maximum negative value may be lower than -3V1/2.
[0048] For ease of explanation, the topmost switching unit 214 of the gradient power supply 202 may be represented as D1 and the bottommost switching unit 214 may be represented as D2. The variations in the first and second rail voltages with respect to safety ground results in variations in the voltage at terminals of the switching units 214 with respect to the corresponding base plates. By way of example, any increase in the first and second rail voltages with respect to safety ground results in an increase in the voltage at the terminals of the switching units 214 with respect to the corresponding base plates. This increase in voltage at terminals of the switching units 214 disadvantageously increases voltage stress in the switching units 214. Hence, it is desirable to reduce the voltage at the terminals of the switching units 214 to reduce the voltage stress in the switching units 214.
[0049] As noted hereinabove, each switching unit 214 is disposed on a corresponding base plate (not shown). Also, since the switching unit 214 is insulated from the base plate, a parasitic capacitance is established between the switching unit 214 and the base plate. Additionally, a capacitive path is also established between the switching units 214 and the cold plate 226. By way of example, the capacitive path is established between the switching unit D1 and the cold plate 226. Moreover, any change in the cold plate voltage impacts the voltage at the terminals of the switching units 214 with respect to the corresponding base plates.
[0050] In one example, if cold plate 226 is connected to the safety ground and the first rail voltage is higher than +3V1/2 and the second rail voltage is lower than -3V1/2, then the voltage at the terminals of topmost switching unit D1 with respect to the base plate is higher than +3V1/2. In this scenario, the voltage at the terminals of topmost switching unit D1 may exceed the isolation voltage rating of switching unit D1 and in turn damage the switching unit D1.
[0051] In such a scenario, it is desirable to reduce the voltage at the terminals of all the switching units 214 disposed on the cold plate 226. In accordance with aspects of the present specification, the potential of the cold plate 226 is set to an average value of first rail voltage and the second rail voltage. The average value of the first and second rail voltages may be referred to as a reference voltage. As noted hereinabove, any change in the cold plate voltage impacts the voltage at the terminals of the switching units 214. Therefore, setting the cold plate voltage to the reference voltage results in a drop/reduction in the voltage at the terminals of topmost switching unit D1 bottommost switching unit D2 with respect to corresponding base plates, thereby averting any damage to the switching units D1 and D2. In a similar manner, the voltage at terminals of other switching units 214 is also reduced and thus, any damage to the other switching units 214 is also averted.
[0052] In one embodiment, the compensation unit 206 is employed to set the voltage of the cold plate 226 to the reference voltage. In particular, the electrical subunit 242 of the compensation unit 206 is employed to set the voltage of the cold plate 226 to the reference voltage. The electrical subunit 242 includes first and second impedances 246, 250. First ends of the first and second impedances 246, 250 are respectively coupled to the first and second rails 216, 224. Further, a second end of the first impedance 246 is coupled to a second end of the second impedance 250 at a point 254. The point 254 may be referred to as a common point. Also, the common point 254 is coupled to the cold plate 226. The first and second impedances 246, 250 may have substantially similar values of impedance.
[0053] Reference numeral 252 is representative of a safety ground. In the medical imaging device, such as the medical imaging device 106 of FIG. 1, the ripple filter 240 is typically coupled to the safety ground 252. In one embodiment, if a value of an impedance between the cold plate 226 and the safety ground 252 is a known value, the first and second impedances 246, 250 are selected such that values of the first and second impedances 246, 250 are at least one order lower than the impedance between the cold plate 226 and the safety ground 252. In one example, the selected values of the first and second impedances 246, 250 are at least ten times lower than the value of the impedance between the point 254 and the safety ground 252. Accordingly, if the value of the impedance between the cold plate 226 and the safety ground 252 is Z1, then the selected values of the first impedance 246 and the second impedance 250 may be Z1/10 or lower.
[0054] However, in certain embodiments where the impedance between the cold plate 226 and the safety ground 252 is not a known value, the electrical subunit 242 may include a third impedance 248 that is coupled between the common point 254 and the safety ground 252. It may be noted that the value of the third impedance 248 is selected such that the value of the third impedance 248 is at least ten times greater than the values of the first and second impedances 246, 250.
[0055] The first, second, and third impedances 246, 250, 248 include passive circuit elements such as resistors, capacitors, and inductors. Moreover, in certain embodiments, the impedances 246, 248, and 250 may include a series-parallel combination of resistors, capacitors, and inductors. In yet another embodiment, the impedances 246, 248, and 250 may include a series-parallel combination of resistors, capacitors, inductors, and an active circuit element, such as transistors or any other semiconductor based switches.
[0056] As noted hereinabove, the first ends of the first and second impedances 246, 250 are respectively coupled to the first and second rails 216, 224. Further, second ends of the first and second impedances 246, 250 are coupled to one another at the common point 254. Therefore, at any instant in time, the voltage at the first ends of the first and second impedances 246, 250 may be equal to the first and second rail voltages, respectively. By way of example, the voltage at the first end of the first impedance 246 may be represented as Vm and the voltage at the first end of the second impedance 250 may be represented as Vn. Further, in this example, if the value of impedance between the cold plate 226 and the safety ground 252 is Z1, the value of the first and second impedances 246, 250 is Z1/100, and the potential at the safety ground 252 is zero volts, then a voltage measured at the common point 254 may have a value of approximately (Vm+Vn)/2. In particular, the value of voltage measured at the common point 254 has a value corresponding to an average of the first and second rail voltages. Additionally, since the cold plate 226 is coupled to the common point 254, the voltage of the cold plate 226 is set to the voltage at the point 254. Accordingly, the compensation unit 206 and the electrical subunit 242 in particular is configured to set the value of the cold plate voltage to the average value of the first and second rail voltages.
[0057] Although in the example of FIG. 2, the controlling subunit 244 is shown as being integral to the compensation unit 206, in certain embodiments, the controlling subunit 244 may be a standalone unit which may be disposed remotely from the compensation unit 206. In certain scenarios, the operation of the controlling subunit 244 may be adversely impacted by the existence of a high magnitude magnetic field in the medical imaging device 106. In such situations, it is desirable to position the controlling subunit 244 at a determined distance from the medical imaging device 106. In this example, the controlling subunit 244 may be communicatively coupled to the compensation unit 206 and in particular, to the electrical subunit 242.
[0058] Additionally, the compensation unit 206 and in particular, the controlling subunit 244 is configured to measure the cold plate voltage. Further, the controlling subunit 244 is also configured to measure the first and second rail voltages.
[0059] Additionally, the controlling subunit 244 may also be configured to continually monitor the cold plate voltage for any variations based on the variations in the first and second rail voltages. In one embodiment, the controlling subunit 244 may continually monitor the cold plate voltage to determine if the value of the cold plate voltage is equal to the reference voltage. If the cold plate voltage is equal to the reference voltage, it may be indicative of an optimal operation of the compensation unit 206. The term “optimal operation” of the compensation unit 206 refers to an operation of the compensation unit 206 which enables reduction in the voltage stress in the switching units 214 by varying the cold plate voltage based on any variations in the first and second rail voltages.
[0060] However, if it is determined that the cold plate voltage fails to vary with variations in the first and second rail voltages within a determined time period and the voltage at the terminals of the switching units 214 with respect to ground is substantially equal to or greater than the isolation voltage rating of the switching units 214, a corrective action may be triggered by the compensation unit 206. Additionally, the compensation unit 206 is configured to also initiate corrective action if a difference between the cold plate voltage and the reference voltage at a given instant in time is greater than a determined threshold value.
[0061] The corrective action may include adding or removing at least one of one or more passive circuit elements and one or more active circuit elements from the first and second impedances 246, 250 of the electrical subunit 242. In one embodiment, a switching mechanism may be used to add or remove at least one of one or more passive circuit elements and one or more active circuit elements from the first and second impedances 246, 250 of the electrical subunit 242. The addition or removal of the passive circuit elements and/the active circuit elements from the first and second impedances 246, 250 results in an increase or decrease in the value of the first and second impedances 246, 250.
[0062] In addition, the controlling subunit 244 in the compensation unit 206 includes one or more processing devices configured to perform the functions of the controlling subunit 244. As used herein, the term “controlling subunit” refers not only to integrated circuits being included in a computer, but also refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), application-specific processors, digital signal processors (DSPs), Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), and/or any other programmable circuits.
[0063] In the example of FIG. 2, a single gradient power supply, gradient amplifier and gradient coil are presented. However, the medical imaging device may include multiple gradient power supplies and corresponding gradient amplifiers and gradient coils. Additionally, although the system 200 of FIG. 2 is employed to reduce the voltage stress in the switching units of the medical imaging device, such as the medical imaging device 106 of FIG. 1, similar systems and methods may be employed in any power converter operating at high voltages (kilovolt range) and high frequencies (hundreds of Hz to tens of kHz).
[0064] Turning now to FIG. 3, a diagrammatical representation 300 of one embodiment of a portion of a compensation unit for use in the system 100 of FIG. 1, according to aspects of the present specification, is presented. In particular, FIG. 3 depicts one example of a circuit configuration of an electrical subunit, such as the electrical subunit 242 of the compensation unit 206 of FIG. 2.
[0065] The electrical subunit 300 includes a first impedance 302 and a second impedance 304. In one embodiment, values of the first and second impedances 302, 304 may be substantially equal. The first and second impedances 302, 304 are coupled to one another at a point 306. The point 306 may be operatively coupled to a cold plate, such as the cold plate 226 of FIG. 2. The point 306 may be referred to as a common point. Each of the first and second impedances 302, 304 includes a respective first leg 308 and a second leg 310, where the first leg 308 is disposed in parallel with the second leg 310. The first legs 308 include a resistive element R2cmp and the second legs 310 include a series combination of a capacitive element Ccmp and a resistive element Rcmp. Further, in certain embodiments, a third impedance (such as impedance 248 of FIG. 2) may be coupled between the common point 306 and a safety ground (such as the safety ground 252 of FIG. 2). Moreover, the first and second impedances 302, 304 are selected such that the values of the first and second impedances 302, 304 are ten times lower than the value of an impedance between the common point 306 and the safety ground.
[0066] Although in the example of FIG. 3, an electrical subunit 300 is shown as including passive circuit elements such as resistances and capacitances, use of active circuit elements along with passive circuit elements in the electrical subunit 300 is envisaged.
[0067] FIGs. 4 and 5 are representative of performance of electrical parameters corresponding to the medical imaging device 106 of FIG. 1 with and without use of the compensation unit 206 of FIG. 2. FIGs. 4 and 5 are described with respect to components of FIG. 2.
[0068] FIG. 4 is a graphical illustration 400 of a simulation result of a voltage at the terminals of the switching unit D1 with respect to the base plate of the switching unit D1 when the compensation unit 206 is not employed. Reference numeral 402 represents a y-axis indicative of a voltage (V) in volts and reference numeral 404 represents an x-axis indicative of time (T) in seconds. Further, reference numeral 406 represents a voltage curve of the switching unit D1 with respect to time. In one example, the cold plate 226 is at a ground potential and the voltage of the first rail 216 is about 2.5 kV. As clearly depicted in FIG. 4, in this scenario, without the use of the compensation unit 206, the voltage at the terminals of switching unit D1 with respect to the corresponding base plate is about 2.5 kV. This voltage of 2.5 kV is substantially equal to the isolation voltage rating of the switching unit D1. Consequently, the high voltage may result in a failure of the switching unit D1 and other switching units 214 in the converters 212 of the gradient power supply 202. Hence, it is desirable to reduce the voltage at the terminals of switching units 214 with respect to corresponding base plates.
[0069] FIG. 5 represents a graphical illustration 500 of a simulation result of a voltage at the terminals of the switching unit D1 with respect to the corresponding base plate when the compensation unit 206 of FIG. 2 is used. Reference numeral 502 represents a y-axis indicative of a voltage (V) in volts and reference numeral 504 represents an x-axis indicative of time (T) in seconds. Also, reference numeral 506 represents a voltage curve of the switching unit D1 with respect to time.
[0070] As noted hereinabove, any increase in the first rail voltage results in a corresponding increase in the reference voltage, where the reference voltage is an average value of the first and second rail voltages. Also, the cold plate voltage is set to the reference voltage. Consequently, any increase in the first rail voltage results in a corresponding increase in the cold plate voltage. This increase in the cold plate voltage in turn results in a decrease in the voltage across the terminals of the switching unit D1 with respect to the base plate. In one example, the increase in the cold plate voltage may result in a voltage at the terminals of the switching unit D1 to reduce to a value lower than the isolation voltage rating of the switching unit D1, thereby reducing the voltage stress in the switching unit D1. Implementing the compensation unit 206 as described hereinabove aids in reducing the voltage stress in the switching units 214 of the converters 212 and consequently, averting any failure of the switching units 214 of the converters 212 in the gradient power supply 202.
[0071] FIG. 6 is a flow chart representing a method 600 for reducing voltage stress in a switching unit of the medical imaging device 106 of FIG. 1, according to aspects of the present specification. The method of FIG. 6 is described with respect to components of FIGs. 1-2.
[0072] The method begins at block 602, where a cold plate voltage is measured. In certain embodiments, the cold plate voltage is measured using the compensation unit 206. In particular, the cold plate voltage is measured continually using the controlling subunit 244 of the compensation unit 206 during the operation of the medical imaging device 106.
[0073] The compensation unit 206 and the controlling subunit 244, in particular, may further be configured to continually monitor the cold plate voltage for any variations in response to variations in the first and second rail voltages. In addition, the controlling subunit 244 is configured to monitor the voltage at the terminals of the switching units 214 to verify if the voltage exceeds the isolation voltage rating of the switching units 214. If it is verified that there are no variations in the cold plate voltage in response to variations in the first and second rail voltages and/or the voltage at the terminals of the switching units 214 is substantially equal to the isolation voltage rating of the switching units 214 over a determined period of time, a corrective action may be initiated after the determined period of time. Consequently, any false alarms may be avoided. The corrective action may include adding or removing at least one of one or more passive circuit elements and one or more active circuit elements from the impedances, such as the first and second impedances 246, 250 of the electrical subunit 242. The addition or removal of the passive circuit elements and/or the active circuit elements from the impedances 246, 250 may cause a change in value of the impedances 246, 250. The change in value of impedances may aid in accurately determining the reference voltage of the cold plate.
[0074] At block 604, a reference voltage corresponding to the cold plate 226 may be determined. In certain embodiments, the compensation unit 206 is configured to determine the reference voltage. More particularly, the electrical subunit 242 is configured to determine the reference voltage based on the first rail voltage and the second rail voltage.
[0075] As previously noted with respect to FIG. 2, the electrical subunit 242 includes first and second impedances 246, 250 coupled to one another at the common point 254. In certain embodiments, a third impedance 248 may be coupled between the common point 254 and the safety ground 252. The value of first and second impedances 246, 250 may be ten times lower than the value of the third impedance 248.
[0076] Moreover, while the first ends of the first and second impedances 246, 250 are coupled to the first and second rails 216, 224, respectively, the second end of both the first and second impedances 246, 250 are coupled to each other via the common point 254. Accordingly, the voltage at the first ends of the first and second impedances 246, 250 is equal to the first and second rail voltages. Further, in one example, if the value of impedance between the common point 254 and the safety ground 252 is Z1 and if the value of the first and second impedances 246, 250 is Z1/100 and the potential at the safety ground 252 is zero volts, a value of a voltage at the common point 254 may be approximately (Vm+Vn)/2. The voltage Vm is representative of the voltage at the first end of the first impedance 246 and Vn is representative of the voltage at the first end of the second impedance 250. In particular, the electrical subunit 242 is configured to generate an average value of the first and second rail voltages at the common point 254. The average value of the first and second rail voltages is representative of a reference voltage for the cold plate 226.
[0077] Moreover, at block 606, the cold plate voltage is modified based on the determined reference voltage. In particular, the cold plate voltage may be set to the determined reference voltage by the compensation unit 206. As noted hereinabove with respect to FIG. 2, the first and second rail voltages vary due to the switching of the switches 236 of the gradient amplifier 204. By way of example, if the first rail voltage and the second rail voltage vary from a first instance of time t = tx to a subsequent second instance of time t = ty, voltages Vx and Vy corresponding to the time instances tx, and ty measured at the common point 254 also vary from time tx to ty. Further, since the cold plate 226 the is coupled to the common point 254, the voltage of the cold plate 226 is set to the voltage at the common point 254. Thus, from time tx to ty, the voltage of the cold plate 226 may be changed from a value Vx to a value Vy.
[0078] This change in the cold plate voltage in turn results in change in the voltage appearing at the terminals of the switching units 214 with respect to corresponding base plates. In one embodiment, the voltage at terminals of the switching units 214 with respect to corresponding base plates is maintained below the corresponding isolation voltage rating of the switching units 214. This reduces the voltage stress in the switching units 214 and thereby prevents any failure of the switching units 214.
[0079] Referring to FIG. 7, a block diagram 700 of an imaging system including a medical imaging device, such as an MRI unit 701 and an exemplary compensation unit 748 is depicted. In the example of FIG. 7, one embodiment of the medical imaging device 106 of FIG. 1 is depicted. The MRI unit 701 includes an acquisition subsystem configured to acquire image. In one embodiment, the acquisition subsystem may include a scanner 704. Furthermore, a processing subsystem in operative association with the acquisition subsystem and configured to process the acquired image data. In one embodiment, the processing subsystem includes a scanner control circuitry 706. The MRI unit 701 is illustrated diagrammatically as including the scanner 704, the scanner control circuitry 706, and the system control circuitry 708. While the MRI unit 701 may include any suitable MRI scanner or detector, in the illustrated embodiment the system 700 includes a full body scanner including a patient bore 710 into which a cradle 712 may be positioned to place a patient 702 in a desired position for scanning. The scanner 704 may be of any suitable field strength, including scanners varying from 0.5 Tesla to 3 Tesla and beyond. As used herein, the term “patient” is used to refer to a human person or an animal that is the subject of the imaging application.
[0080] Additionally, the scanner 704 may include a series of associated coils for producing controlled magnetic fields, for generating radio-frequency (RF) excitation pulses, and for detecting emissions from gyromagnetic material within the patient 702 in response to such pulses. In the diagrammatical view of FIG. 7, a primary magnet coil 714 may be provided for generating a primary magnetic field generally aligned with patient bore 710. A series of gradient coils 716, 718, and 720 may be grouped in a coil assembly for generating controlled magnetic field gradients during examination sequences as will be described in greater detail hereinafter. A RF coil 722 may be provided for generating radio frequency pulses for exciting the gyromagnetic material. In the embodiment illustrated in FIG. 7, the RF coil 722 also serves as a receiving coil. Thus, the RF coil 722 may be coupled with driving and receiving circuitry in passive and active modes for receiving emissions from the gyromagnetic material and for applying RF excitation pulses, respectively. Alternatively, various configurations of receiving coils may be provided separate from the RF coil 722. Such coils may include structures specifically adapted for target anatomies, such as head coil assemblies, and so forth. In one example, an anterior array of coils may be provided. Moreover, this anterior array of coils may be disposed on or about the patient 702. In addition, receiving coils may be provided in any suitable physical configuration, including phased array coils, and so forth.
[0081] In a presently contemplated configuration, the gradient coils 716, 718 and 720 may have different physical configurations adapted to their function in the imaging system 700. As will be appreciated by those skilled in the art, the coils include conductive wires, bars or plates that are wound or cut to form a coil structure that generates a gradient field upon application of control pulses as described below. The placement of the coils within the gradient coil assembly may be done in several different orders. In one embodiment, a Z-axis coil may be positioned at an innermost location, and may be formed generally as a solenoid-like structure that has relatively little impact on the RF magnetic field. Thus, in the illustrated embodiment, the gradient coil 720 is the Z-axis solenoid coil, while gradient coils 716 and 718 are Y-axis and X-axis coils, respectively.
[0082] The coils of the scanner 704 may be controlled by external circuitry to generate desired fields and pulses, and to read signals from the gyromagnetic material in a controlled manner. As will be appreciated by those skilled in the art, when the material, typically bound in tissues of the patient 702, is subjected to the primary field, individual magnetic moments of the paramagnetic nuclei in the tissue partially align with the field. While a net magnetic moment is produced in the direction of the polarizing field, the randomly oriented components of the moment in a perpendicular plane generally cancel one another. During an examination sequence, an RF frequency pulse is generated at or near the Larmor frequency of the material of interest, resulting in rotation of the net aligned moment to produce a net transverse magnetic moment. Further RF signals are emitted and are detected by the scanner 704 and processed for reconstruction of the desired image.
[0083] The gradient coils 716, 718 and 720 may be configured to serve to generate precisely controlled magnetic fields, the strength of which vary over a defined field of view, typically with positive and negative polarity. When each coil is energized with known electric current, the resulting magnetic field gradient is superimposed over the primary field and produces a desirably linear variation in the Z-axis component of the magnetic field strength across the field of view. The field varies linearly in one direction, but is homogenous in the other two. The three coils have mutually orthogonal axes for the direction of their variation, enabling a linear field gradient to be imposed in an arbitrary direction with an appropriate combination of the three gradient coils.
[0084] The pulsed gradient fields perform various functions integral to the imaging process. Some of these functions are slice selection, frequency encoding and phase encoding. These functions may be applied along the X-axis, Y-axis, and Z-axis of the original coordinate system or along other axes determined by combinations of pulsed currents applied to the individual field coils.
[0085] The slice select gradient determines a slab of tissue or anatomy to be imaged in the patient 702. The slice select gradient field may be applied simultaneously with a frequency selective RF pulse to excite a known volume of spins within a desired slice that process at the same frequency. The slice thickness is determined by the bandwidth of the RF pulse and the gradient strength across the field of view.
[0086] The frequency encoding gradient is also known as the readout gradient, and is usually applied in a direction perpendicular to the slice select gradient. In general, the frequency encoding gradient is applied before and during the formation of the magnetic resonance (MR) echo signal resulting from the RF excitation. Spins of the gyromagnetic material under the influence of this gradient are frequency encoded according to their spatial position along the gradient field. By Fourier transformation, acquired signals may be analyzed to identify their location in the selected slice by virtue of the frequency encoding.
[0087] The phase encode gradient is generally applied before the readout gradient and after the slice select gradient. Localization of spins in the gyromagnetic material in the phase encode direction may be accomplished by sequentially inducing variations in phase of the processing protons of the material using slightly different gradient amplitudes that are sequentially applied during the data acquisition sequence. The phase encode gradient permits phase differences to be created among the spins of the material in accordance with their position in the phase encode direction.
[0088] As will be appreciated by those skilled in the art, a great number of variations may be devised for pulse sequences employing the exemplary gradient pulse functions described hereinabove as well as other gradient pulse functions not explicitly described here. Moreover, adaptations in the pulse sequences may be made to appropriately orient both the selected slice and the frequency and phase encoding to excite the desired material and to acquire resulting MR signals for processing.
[0089] The coils of the scanner 704 are controlled by the scanner control circuitry 706 to generate the desired magnetic field and RF pulses. In the diagrammatical view of FIG. 1, the scanner control circuitry 706 thus includes a control circuit 726 for commanding the pulse sequences employed during the examinations, and for processing received signals. The control circuit 726 may include any suitable programmable logic device, such as a CPU or digital signal processor of a general purpose or application-specific computer. Also, the control circuit 726 may further include memory circuitry 728, such as volatile and non-volatile memory devices for storing physical and logical axis configuration parameters, examination pulse sequence descriptions, acquired image data, programming routines, and so forth, used during the examination sequences implemented by the scanner.
[0090] Interface between the control circuit 726 and the coils of the scanner 704 is managed by amplification and control circuitry 730 and by transmission and receive (T/R) interface circuitry 732. The amplification and control circuitry 730 includes amplifiers for each gradient field coil to supply drive current to the field coils in response to control signals from the control circuit 726. The amplifiers for each gradient field coil may be referred to as a gradient amplifier. The transmit/receive (T/R) interface circuitry 732 includes additional amplification circuitry for driving the RF coil 722. Moreover, where the RF coil 722 serves both to emit the RF excitation pulses and to receive MR signals, the T/R interface circuitry 732 may typically include a switching device for toggling the RF coil between active or transmitting mode, and passive or receiving mode. Moreover, the scanner control circuitry 706 may include interface components 734 for exchanging configuration and image data with the system control circuitry 708. It should be noted that, while in the present description reference is made to a horizontal cylindrical bore imaging system employing a superconducting primary field magnet assembly, the present technique may be applied to various other configurations, such as scanners employing vertical fields generated by superconducting magnets, permanent magnets, electromagnets or combinations of these means.
[0091] The system control circuitry 708 may include a wide range of devices for facilitating interface between an operator or radiologist and the scanner 704 via the scanner control circuitry 706. In the illustrated embodiment, for example, an operator controller 736 is provided in the form of a computer workstation employing a general purpose or application-specific computer. The workstation also typically includes memory circuitry for storing examination pulse sequence descriptions, examination protocols, user and patient data, image data, both raw and processed, and so forth. Further, the workstation may further include various interface and peripheral drivers for receiving and exchanging data with local and remote devices. In the illustrated embodiment, such devices include a conventional computer keyboard 740 and an alternative input device such as a mouse 742. A printer 744 may be provided for generating hard copy output of documents and images reconstructed from the acquired data. Moreover, a computer monitor 738 may be provided for facilitating operator interface. In addition, the system 700 may include various local and remote image access and examination control devices, represented generally by reference numeral 746 in FIG. 7. Such devices may include picture archiving and communication systems, tele-radiology systems, and the like.
[0092] The gradient power supply 724 is coupled to a gradient amplifier (not shown). Further, the gradient amplifier is coupled to a gradient coil via a ripple filter. A combination of the gradient power supply, the gradient amplifier and the gradient coil may be referred to as a gradient unit.
[0093] In accordance with aspects of the present specification, the system 700 also includes an exemplary compensation unit 748. The compensation unit 748 includes an electrical subunit 750 and a controlling subunit 752. The electrical subunit 750 includes a plurality of impedances. Also, in one embodiment the controlling subunit 752 includes a processing device. In the example of FIG. 7, the compensation unit 748 is depicted as being operatively coupled to the MRI unit 701. However, in certain other embodiments, the compensation unit 748 may be an integral part of the MRI unit 701. In particular, the compensation unit 748 may be an integral part of the gradient power supply 724 of the MRI unit 701.
[0094] As described hereinabove with respect to FIG. 2, the compensation unit 748 is configured to reduce voltage stress in switching units in converters of the gradient power supply 724 of the MRI unit 701.
[0095] Furthermore, the foregoing examples, demonstrations, and process steps such as those that may be performed by the system may be implemented by suitable code on a processor-based system, such as a general-purpose or special-purpose computer. It should also be noted that different implementations of the present technique may perform some or all of the steps described herein in different orders or substantially concurrently, that is, in parallel. Furthermore, the functions may be implemented in a variety of programming languages, including but not limited to C++ or Java. Such code may be stored or adapted for storage on one or more tangible, machine readable media, such as on data repository chips, local or remote hard disks, optical disks (that is, CDs or DVDs), memory or other media, which may be accessed by a processor-based system to execute the stored code. Note that the tangible media may comprise paper or another suitable medium upon which the instructions are printed. For instance, the instructions may be electronically captured via optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in the data repository or memory.
[0096] Various embodiments of methods and systems for reducing voltage stress in components of a medical imaging device are presented. In particular, the systems and methods presented herein enable reduction in voltage stress in the switching units employed in the gradient power supply of the medical imaging device, such as a MRI system. Use of the systems and methods presented hereinabove provides a cost effective and simpler solution for reducing the voltage stress in the switching units. Further, the present systems and methods enable use of the MRI systems in high voltage and high power scanning such as neuro scans. The methods and systems for reducing voltage stress in the switching units may also find application in a high voltage direct current (HVDC) station, medium voltage applications, such as in motor drives, MRI systems, and the like.
[0097] While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof.