Magnetic resonance apparatus and operating method therefor for actively regulating heating in the apparatus

ABSTRACT

In a method for the operation of a magnetic resonance apparatus having a gradient coil system as well as a shim system with a number of shim plates with respectively allocated heaters, at least one information signal indicating a change in shape and/or position of the gradient coil system is determined or employed, the regulation of the heaters, and thus the temperatures of the respective shim plates ensuing dependent thereon.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the relevant field-generating components of a magnetic resonance apparatus.

FIG. 2 is a schematic illustration of the temperature regulation of the shim plates in accordance with the invention.

FIG. 3 is a sectional view of a shim plate in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an inventive magnetic resonance apparatus , with only the relevant field-generating elements being shown. A basic field magnet (for example, an axial superconducting air coil magnet with active stray field shielding) is shown that generates a uniform magnetic basic field in an interior volume. The interior of the superconducting basic field magnet is composed of superconducting coils that are situated in liquid helium. The basic field magnet is surrounded by a two-shell (double-walled) jacket that is usually composed of stainless steel. The inner shell, which contains the liquid helium and partly serves as winding body for the magnetic coils, is suspended at the outer shell, which is at room temperature, via weakly thermally conductive GFK rods. A vacuum prevails between the inner and outer shell. The cylindrical gradient coil system is concentrically introduced into the inside of the basic field magnet into the inside of a carrying tube, by means of carrying elements . The carrying tube is limited toward the outside and toward the inside by means of two inner shells .

The gradient coil system has three sub-windings that respectively generate gradient fields that are spatially perpendicular to one another, each field being proportional to the current in the coil that generates it. As shown in FIG. 1, the gradient coil system has an x-coil , a y-coil and a z-coil that are respectively wound around a coil carrier (for example, a GFK tube) and thus gradient fields are respectively generated in the direction of the Cartesian axes x, y, z. Each of these coils is equipped with its own power supply in order to generate current pulses with exact amplitude and at the exact time required by the sequence programmed in the pulse sequence controller. The required currents lie between about 250-450A. Since the gradient switching times should be as short as possible, current rise rates on the order of magnitude of 250 kA/s are required.

Since the gradient coil system usually is surrounded by conductive structures (for example, magnet vessel of stainless steel), eddy currents are generated therein due to the pulsed (charging) fields, the eddy currents in turn entering into interaction with the basic magnetic field and modifying it. The homogeneity of the basic field in the measurement volume is of fundamental significance in magnetic resonance tomography. A shim system (a portion thereof is shown in FIG. 2) is provided in order to homogenize this field. This shim system includes a number of shim devices (one thereof is shown in FIG. 2) arranged concentrically around the gradient coil. Each shim device has a thermally and electrically non-conductive carrier rail , primarily an injection molded part of, for example, GFK material, in which a number of chamber-like receptacles are formed, wherein respective shim elements are arranged. Each shim plate (one thereof is shown magnified in FIG. 3) is composed of the actual shim plate . In the example according to FIG. 3, a regulating device and a heater , regulated via the former, are provided at each side of the shim plate . A non-magnetic, thermally conductive plate (which is not shown in detail in FIG. 3) can be provided between the shim plate and the heater . The heater can be, for example, a foil heater, a plate heater or a substrate heater with heat conductors applied (for example, in serpentine form) on the foil, the plate or the substrate. A temperature sensor (not shown in detail) that directly or indirectly (via the intervening plate) acquires the temperature of the shim plate is situated at the same side as the coil. The temperature sensor is connected to the regulating device that regulates the operation of the heater in order to thus set the temperature of the shim plate to a prescribed reference value temperature, as discussed below.

Since each shim plate has a separate heater , it is thus possible to separately manage the temperature of each shim plate . In this way, a constant temperature of the shim plates can be maintained, this being required for the homogenization of the basic field. Moreover, the individual control possibility also allows a reaction to changes in shape and/or position of the gradient coils or of the gradient coil system including the coils , and as well as the coil carrier . During operation, the gradient coils , and are subjected to high currents that lead to significant heating and can definitely lead to a change in shape and position. This has disadvantageous effects on the homogeneity of the basic magnetic field.

In order to counteract this, as shown in FIG. 2, information signals I, I′, I″ and I′″ are determined in various possible ways, these supplying information about whether a change in shape and/or position of the gradient coil system occurs. These information signals—only one thereof may already be sufficient—are acquired, for example, via sensors that are preferably arranged uniformly distributed at the gradient coil system , on the coil carrier . For example, temperature sensors, magnetic field sensors (that detect the magnetic fields generated by the gradient coils , and ) or expansion (strain) sensors can be employed as these sensors. An information signal in the form of an operating parameter of the gradient can likewise be employed. For example, the operating currents of the coils , and can be used; i.e., the current or pulse sequence of the coils , and is taken into consideration. Alternatively or additionally, it is possible to analyze changes in shape and/or position (which are expressed in image modifications) in the framework of an image analysis of the images registered with the magnetic resonance apparatus . Suitable analysis algorithms are employed therefor.

The information signal or signals I, . . . , I′″ are forwarded to a control device that has a processor that processes the information signal or signals. Dependent on the quality of the information signals, the processor selects a reference temperature value S1, S2 . . . Sn from a look-up table that serves for maintaining a constant temperature and simultaneously serves for the compensation of a field drift resulting from a change in shape and/or position of the gradient coil system . This selected reference temperature value is forwarded to each regulating device one of which being shown in FIG. 2 as an example. In the regulating device , the given reference temperature value is then compared to the actual temperature value measured via the temperature sensor at the shim plate and a corresponding regulation of the operation of the heater (only one thereof per shim plate is likewise shown in FIG. 2) is made. The heaters are connected to a suitable current source via which the power is delivered for heating purposes.

The regulation of the heaters and thus the setting of the temperature of the shim plates ensues dependent on at least one information signal that indicates a change in shape and/or position of the gradient coil system in order to be able to compensate inhomogeneities of the basic magnetic field that result therefrom. At the same time, a direct heating of the shim plates adequately maintains a constant temperature, insofar as this is required.

Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.

CLAIMS

1. A method for operating a magnetic resonance apparatus comprising the steps of: operating a basic field magnet of a magnetic apparatus for generating a basic magnetic field; operating a gradient coil system of said magnetic resonance apparatus for generating at least one gradient field, said gradient coil system being subject to a change in at least one of shape and position during operation thereof; shimming said basic magnetic field with a plurality of ferromagnetic shim plates, disposed at said coil system, with a shimming effect for homogenizing said basic magnetic field, said change causing a change in said shimming effect; providing a plurality of heaters respectively in thermally conductive relation with said plurality of shim plates; obtaining an information signal during operation of said gradient coil system indicative of said change in at least one of shape and position of the gradient coil system; and regulating said heaters during operation of said gradient coil system dependent on said information signal for compensating said change in said shimming effect for substantially restoring said shimming effect for homogenizing said basic magnetic field.

2. A method as claimed in claim 1 comprising measuring said change with at least one measurement element, and obtaining said information signal from said measurement element.

3. A method as claimed in claim 2 comprising measuring at least one magnetic field, with said measurement element, generated by said gradient coil system during operation of said gradient coil system.

4. A method as claimed in claim 2 comprising measuring a temperature of said gradient coil system with said measurement element during operation of said gradient coil system.

5. A method as claimed in claim 2 comprising measuring an expansion of said gradient coil system with said measurement element during operation of said gradient coil system.

6. A method as claimed in claim 2 comprising employing a plurality of measurement sensors distributed over said gradient coil system for respectively obtaining a plurality of locally resolved information signals.

7. A method as claimed in claim 6 comprising uniformly distributing said measurement elements over said gradient coil system.

8. A method as claimed in claim 6 comprising distributing said plurality of measurement elements over an interior and an exterior of said gradient coil system.

9. A method as claimed in claim 1 comprising operating said gradient coil system with at least one operating parameter, and using said operating parameter as said information signal.

10. A method as claimed in claim 8 comprising operating said gradient coil system with an operating current, as said operating parameter, and using a value characterizing said operating current as said information signal.

11. A method as claimed in claim 10 comprising obtaining a further information signal representing a criterion for cooling said gradient coil system during operation of said gradient coil system, and regulating said heaters during operation of said gradient coil system dependent on said information signal characterizing said operating current and said further information signal.

12. A method as claimed in claim 1 comprising, as a result of operating said gradient coil system obtaining an excitation response of a subject exposed to said gradient field, and obtaining said information signal by analyzing said excitation response.

13. A method as claimed in claim 1 wherein regulation of said heaters has an energy consumption associated therewith, and comprising regulating said heaters dependent on said information signal and said energy consumption.

14. A method as claimed in claim 1 wherein the step of regulating said heaters dependent on said information signal comprises setting a reference temperature respectively for said shim plate dependent on said information signal, and operating said heaters to cause an actual temperature of the respective shim plates to match said reference temperature.

15. A method as claimed in claim 14 comprising selecting said reference temperature from a stored look-up table.

16. A magnetic resonance apparatus comprising: a basic field magnet operable to generate a basic magnetic field; a gradient coil system operable to generate at least one gradient field, said gradient coil system being subject to a change in at least one of shape and position during operation thereof; a plurality of ferromagnetic shim plates disposed at said gradient coil system, operable to produce a shimming effect for homogenizing said basic magnetic field, said chance causing a chance in said shimming effect; a plurality of heaters respectively in thermally conductive relation with said plurality of shim plates; and a plurality of regulators respectively allocated to said plurality of heaters, each of said regulators regulating the heater allocated thereto dependent on an information signal indicative of said change in said at least one of shape and position of said gradient coil system, for compensating said chance in said shimming effect for substantially restoring said shimming effect for homogenizing said basic magnetic field.

17. A magnetic resonance apparatus as claimed in claim 16 comprising a control device connected to said regulators and being supplied with said information signal, said control device generating a reference value signal, representing a criterion for a reference temperature value, dependent on said information signal, and said control unit supplying said reference value signal to each of said regulators and each of said regulators regulating the heater allocated thereto dependent on said reference value signal.

18. A magnetic resonance apparatus as claimed in claim 17 wherein said control device contains a look-up table wherein a plurality of reference temperature values are stored respectively allocated to different values of said information signal, and wherein said control unit selects the reference temperature value from said look-up table allocated to the information signal supplied to said control unit.

19. A magnetic resonance apparatus as claimed in claim 16 wherein each of said heaters comprises first and second heater elements respectively disposed on opposite sides of the shim plate in thermally conductive relation therewith, and wherein each of said regulators regulates the first and second heater elements of the heater allocated thereto in common.

20. A magnetic resonance apparatus as claimed in claim 16 wherein each of said heaters comprises a first heater element and a second heater element respectively disposed on opposite sides of the shim plate in thermally conductive relation therewith, and wherein each of said regulators comprises a first element regulator for regulating the first heater element of the heater allocated thereto and a second element regulator for regulating the second heater element of the heater allocated thereto.

21. A magnetic resonance apparatus as claimed in claim 16 comprising a measurement element for obtaining said information signal.

22. A magnetic resonance apparatus as claimed in claim 21 wherein said measurement element is a magnetometer that measures said at least one gradient magnetic field.

23. A magnetic resonance apparatus as claimed in claim 21 wherein said measurement element is a temperature sensor which measures a temperature of said gradient coil system.

24. A magnetic resonance apparatus as claimed in claim 21 wherein said measurement element is a strain sensor which measures expansion and contraction of said gradient coil system.

25. A magnetic resonance apparatus as claimed in claim 16 comprising a plurality of measurement elements distributed over said gradient coil system for respectively obtaining locally resolved information signals.

26. A magnetic resonance apparatus as claimed in claim 25 wherein said plurality of measurement elements is uniformly distributed over said gradient coil system.

27. A magnetic resonance apparatus as claimed in claim 25 wherein said plurality of measurement elements is distributed at an interior and an exterior of said gradient coil system.

28. A magnetic resonance apparatus as claimed in claim 16 wherein said gradient coil system is operated according to at least one operating parameter, and wherein said regulators regulate said heaters dependent on said at least one operating parameter, as said information signal.

29. A magnetic resonance apparatus as claimed in claim 28 wherein said gradient coil system is supplied with operating current, and wherein said regulators regulate said heaters dependent on said operating current.

30. A magnetic resonance apparatus as claimed in claim 29 wherein a further operating parameter of said gradient coil system is a criterion for cooling said gradient coil system, and wherein said regulators regulate said heaters additionally dependent on said further operating parameter.

31. A magnetic resonance apparatus as claimed in claim 16 comprising a scanner, which includes said gradient coil system, for obtaining an image of a subject exposed to said at least one gradient field, and further comprising an image analysis unit for analyzing said image and for generating said information signal dependent on said analysis of said image.

32. A magnetic resonance apparatus as claimed in claim 16 wherein said regulators respectively consume energy, and regulate said heaters additionally dependent on said energy.

33. A magnetic resonance apparatus as claimed in claim 16 wherein said plurality of heaters is respectively in said thermally conductive relation with said plurality of shim plates by direct attachment to the respective shim plates.

34. A magnetic resonance apparatus as claimed in claim 33 wherein each of said heaters comprises a foil heater having a carrier foil with electrical heat-generating conductors applied thereto.

35. A magnetic resonance apparatus as claimed in claim 33 wherein each of said heaters is a plate heater having a carrier plate with electrical heat-generating conductors applied thereto.

36. A magnetic resonance apparatus as claimed in claim 33 wherein each of said heaters is a substrate heater having a ceramic substrate with electrical heat-generating conductors applied thereto.

37. A magnetic resonance apparatus as claimed in claim 36 wherein said ceramic substrate is an Al2O3 substrate.

38. A magnetic resonance apparatus as claimed in claim 33 wherein each of said heaters has electrical heat-generating conductors proceeding in bifilar fashion for suppressing magnetic fields generated by said conductors.

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