Temperature-control system for MR apparatuses with a permanent magnet arrangement

Abstract

A temperature-control system for an NMR magnet system. A permanent magnet arrangement (1) with a central air gap (2) generates a homogeneous static magnetic field inside the air gap. A probehead (3) transmits RF pulses and receives RF signals from a test sample (0). An H0 coil changes the amplitude of the static magnetic field. A shim system (4) in the air gap further homogenizes the magnetic field. A first insulation chamber (5) surrounds and thermally shields the permanent magnet arrangement and includes an arrangement (6) controlling a temperature T1 of the first insulation chamber. The shim system, the H0 coil and the NMR probehead are arranged outside the first insulation chamber in the air gap. A heat-conducting body (7) is arranged between the shim system and the H0 coil on one side and the permanent magnet arrangement on the other, thereby enhancing field stability and suppressing drift.

Claims

1. A temperature-control system for nuclear magnetic resonance (NMR) magnet system, comprising: a permanent magnet arrangement with a central air gap and configured to generate a homogeneous static magnetic field having an amplitude in a measuring volume inside the central air gap, an NMR probehead configured to transmit radio frequency (RF) pulses and to receive RF signals from a test sample, an H0 coil configured to change the amplitude of the static magnetic field, a shim system arranged in the central air gap and configured to further homogenize the magnetic field in the measuring volume, a first insulation chamber surrounding and thermally shielding the permanent magnet arrangement, wherein the first insulation chamber comprises at least one arrangement controlling a temperature T1 of the first insulation chamber, and wherein the shim system, the H0 coil and the NMR probehead are arranged outside the first insulation chamber in the central air gap, and at least one heat-conducting body defining two sides and arranged between the shim system and the H0 coil on one of the sides and the permanent magnet arrangement on the other of the sides.

2. The temperature-control system as claimed in claim 1, further comprising at least one arrangement controlling a temperature T2 of the heat-conducting body arranged outside the first insulation chamber.

3. The temperature-control system as claimed in claim 2, wherein the first insulation chamber that envelopes the permanent magnet arrangement comprises a wall having at least two respective, thermally separated faces, each of which comprises at least one respective sensor that determines a surface temperature T1i of the wall, and wherein the respective faces are thermally controlled independently of one another, one of the at least two thermally separated faces encompassing the central air gap that encompasses the measuring volume and actively thermally separates the shim system, the H0 coil and the NMR probehead from the permanent magnet arrangement.

4. The temperature-control system as claimed in claim 2, wherein the first insulation chamber that envelopes the permanent magnet arrangement comprises a wall, and wherein at least one of the arrangements controlling the temperature T1 of the first insulation chamber or the temperature T2 of the heat-conducting body is a thermoelectric cooler and comprises a heat exchanger, which ensures a flow of heat between the wall of the first insulation chamber during operation.

5. The temperature-control system as claimed in claim 1, wherein the first insulation chamber that envelopes the permanent magnet arrangement comprises a wall, and wherein the wall of the first insulation chamber is configured as a shielding arrangement for magnetically shielding the magnetic field with respect to an external space.

6. The temperature-control system as claimed in claim 5, wherein the shielding arrangement comprises a passive thermal insulation on an outer side of the first insulation chamber.

7. The temperature-control system as claimed in claim 1, wherein the heat-conducting body consists essentially of a homogenization body and a heat-conducting device.

8. The temperature-control system as claimed in claim 7, wherein the heat-conducting device comprises at least one heat pipe.

9. The temperature-control system as claimed in claim 7, wherein the homogenization body is thermally connected to the shim system and the H0 coil.

10. The temperature-control system as claimed in claim 7, wherein at least part of the homogenization body is arranged between the shim system and the permanent magnet arrangement.

11. The temperature-control system as claimed in claim 2, wherein the arrangement controlling the temperature T2 in the central air gap comprises a heater and/or a thermometer that measures the temperature T2, and is thermally connected to the shim system and/or to a heat-conducting system.

12. The temperature-control system as claimed in claim 1, wherein the first insulation chamber is encompassed by a second insulation chamber, which is at a temperature T3 and with which the temperature T3 inside the second insulation chamber is insulated with respect to an ambient temperature TR outside the second insulation chamber.

13. The temperature-control system as claimed in claim 12, further comprising a temperature control device on an outer wall of the second insulation chamber, which controls the temperature T3 inside the second insulation chamber with respect to the ambient temperature TR outside the second insulation chamber.

14. The temperature-control system as claimed in claim 1, wherein the probehead comprises an arrangement setting a temperature TS of the test sample in the measuring volume.

15. The temperature-control system as claimed in claim 14, wherein the temperature TS of the test sample in the measuring volume is set independently of a temperature of a wall of the first insulation chamber enveloping the permanent magnet arrangement.

16. The temperature-control system as claimed in claim 14, further comprising an insulation system arranged between the shim system and at least one RF coil of the NMR probehead.

17. The temperature-control system as claimed in claim 16, wherein the insulation system is configured with a flush gas flow, a temperature of which is controlled at a temperature TF, arranged to flow through the NMR probehead, the shim system or the H0 coil.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is shown in the drawings and will be explained in more detail on the basis of embodiments, in which:

(2) FIG. 1 is a schematic vertical sectional view of a first embodiment of the temperature-control system according to the invention comprising an insulation chamber, a temperature controller and heat-conducting body;

(3) FIG. 2 shows a permanent magnet arrangement, the temperature of which is controlled as per the invention, comprising additional T2 control, a piece of thermal insulation and a closed air gap;

(4) FIG. 3 shows an embodiment of the invention comprising a second insulation chamber, a closed air gap as well as additional T3 control;

(5) FIG. 4 shows an embodiment having two insulation chambers, an open air gap, T1, T2 and T3 control and in which the temperature of the heat-conducting body is controlled in relation to the ambient temperature;

(6) FIGS. 5A and 5B are schematic vertical sections through two respective embodiments of the NMR probehead according to the invention, comprising the thermal insulation system and temperature-controlled flush gas flow and temperature-controlled VT gas flow for controlling the temperature of the sample directly (FIG. 5A) or indirectly (FIG. 5B);

(7) FIGS. 6A, 6B and 6C show cross sections through three respective designs of probeheads comprising a shim system, heat-conducting bodies and homogenization bodies; and

(8) FIG. 7 is a schematic vertical sectional view of a temperature-control system according to the prior art.

DETAILED DESCRIPTION

(9) FIGS. 1 to 6C of the drawings are each a schematic view of different details of preferred embodiments of the temperature-control system according to the invention, while FIG. 7 shows a generic arrangement according to the prior art.

(10) Such a temperature-control system for an NMR magnet system comprises a permanent magnet arrangement 1 having a central air gap 2 for generating a homogeneous static magnetic field in a measuring volume inside the central air gap 2, an NMR probehead 3 for transmitting RF pulses and receiving RF signals from a test sample 0 via RF coils 13, an NMR frequency lock comprising frequency detection in the NMR probehead 3 and an H0 coil for changing the amplitude of the static magnetic field, and a shim system 4 in the central air gap 3 for further homogenizing the magnetic field in the measuring volume.

(11) Due to a lack of space and for the sake of greater clarity, in all the present patent drawings the H0 coil is not shown specifically and therefore has not been given its own reference numeral either. In the schematic vertical sectional views, the H0 coil spatially (for the viewer) coincides with the schematically depicted shim system 4 in each case.

(12) In such arrangements according to the prior art, the temperature of the permanent magnet arrangement can be directly or indirectly controlled, the waste heat of the shim system and the H0 coil is, however, always directly or indirectly transferred to the magnet since no thermal separation is provided.

(13) The present invention improves these arrangements known per se and expands them by the following elements associated with the invention:

(14) The temperature-control system according to the invention is characterized in that a first insulation chamber 5 surrounds the permanent magnet arrangement 1 so as to thermally shield it, in that the first insulation chamber 5 comprises one or more arrangements 6 controlling a temperature T1 of the first insulation chamber 5, the shim system 4, the H0 coil and the NMR probehead 3 being arranged outside the first insulation chamber 5 in the central air gap 2, and in that at least one heat-conducting body 7 is arranged between the shim system 4 and the H0 coil on one side and the permanent magnet arrangement 1 on the other side.

(15) A first particularly simple embodiment of the invention is shown schematically in FIG. 1.

(16) Since the degree of heat conduction of the magnet and the pole piece material is usually relatively poor, in the case of selective temperature control, a temperature gradient can form in the magnet material that has a negative impact during NMR measurements. For the same reason, convection is intended to be avoided. Therefore, according to the invention, heaters and thermoelectric coolers, as are described in the prior art, on the magnet are dispensed with. Instead, homogeneous heat radiation and heat conduction through the medium encompassing the magnet (generally fluid/gas/insulation body) is desired. Therefore, in a preferred embodiment, the temperature of the magnet is not controlled, and instead the temperature of the environment is fully and homogeneously controlled. In particular, the temperature of the wall in the air gap is also controlled.

(17) In order to adjust the magnet temperature initially, additional temperature-control arrangements can be positioned directly on the magnet and/or the pole pieces, which are, however, deactivated after a start phase after the MR spectrometer has been switched on. Such additional temperature-control elements are advantageous for considerably decreasing the time needed for initial control of the temperature of the magnet.

(18) FIG. 2 shows a preferred embodiment of the system according to the invention, in which the first insulation chamber 1 is also encompassed by a piece of passive thermal insulation 9 towards the outside. Towards the probehead region, i.e., inside the inner contour of the magnet, there is arranged at least one shim system 4 and one heat-conducting system having at least one heat-conducting body 7. The heat-conducting body 7 is thermally connected to an arrangement 8 controlling the temperature T2 of the heat-conducting body 7, which can be a heater or a thermoelectric cooler having a heat exchanger towards the environment. Similarly to T1, T2 is set to a temperature of between 15° C. and 35° C. A piece of passive insulation can also be attached on the side facing the magnet system and on one or both sides of the heat-conducting body and is used to act as a low-pass filter for the magnet temperature and to restrict the heat-flow to the environment.

(19) The heat-conducting body 7 can comprise a homogenization body 10 made of a material (for example Cu, Al, Al.sub.2O.sub.3, . . . ) having a good thermal conductivity and/or can be designed having a heat-conducting device 17 in the form of a heatpipe, in which a fluid is present, which fluid is evaporated or condensed depending on the temperature inside the heatpipe and heat can therefore be transported. The heat-conducting body 7 is arranged between the shim system 4 and the magnet such that it forms a thermal shield between the permanent magnet arrangement 1 and the shim system and therefore forms the part of the first insulation chamber that is inside the inner contour of the permanent magnet.

(20) In a preferred embodiment, the central air gap 2 is closed on the outside by the NMR probehead 3. This is not a hermetic seal, but the opening on the outside is designed such that minimum air exchange between the environment and the air gap is ensured. When the NMR sample tube is inserted, the gap to the housing is therefore preferably restricted to 1/10 mm.

(21) A temperature is subsequently set in the air gap 2 that is near to T2, which is near to, preferably identical to, the temperature T1, in particular when the dissipation is conducted away by the heat-conducting body with electrical currents in the shim system and the H0 coil. This is advantageous since the test sample temperature is therefore close to room temperature during operation and the amount of time to reach a constant test sample temperature after a test sample has been introduced is as short as possible.

(22) The shim system 4 consists of a multiplicity of (not shown in more detail in the drawings) shim coils and is preferably thermally coupled to the heat-conducting system. Furthermore, this embodiment having a closed probe head can advantageously be operated via a lock sample, which then also comprises a second transmitter/receiver coil that encompasses the lock sample and/or can generate an RF signal in the lock sample and can receive an RF signal from the lock sample, and at least one additional (likewise not shown) H0 coil, which is thermally coupled to the heat-conducting system and which is adjusted to frequency changes in the lock sample with varying currents. Supplying the H0 coils with power would lead to an input of heat into the magnet region, which is why the thermal shielding of the shim system 4 has proven to be particularly advantageous here. Such an increase in temperature by accurately adjusting currents in the shim coils leads to poor convergence of a shim algorithm, for example. Controlling the current in the H0 coil(s) can lead to “(de)shimming” of the magnet because of thermally generated gradients in the magnet material. In alternative embodiments, the shim system and/or H0 coils are not coupled to the heat-conducting body. The heat resulting from electrical dissipation can be removed from the air gap, e.g., in a non-controlled manner, by other heat-conducting bodies, and given to the room temperature, for example through a heat exchanger.

(23) FIG. 3 shows an extended design of the embodiment in FIG. 2. In this case, a second insulation chamber 11 is additionally provided, which encompasses the first insulation chamber 5. Furthermore, in this example, a thermoelectric cooler having a heat exchanger that exchanges heat with the air inside the second insulation chamber 11 is shown as the arrangement 8 controlling the temperature T2. The second insulation chamber 11 is controlled at a temperature T3 with a temperature control device 12, preferably also with a thermoelectric cooler and a heat exchanger to the room temperature.

(24) The second insulation chamber 11 ensures that the area around the magnet can be used as a heat sink irrespective of how warm it actually is in the room. As a result, the isotherm around the magnet can always be achieved by heating and does not also need to be cooled. This is undoubtedly always sufficient for controlling the temperature T1, since no heat is introduced in the first insulation box except for by the shim system, H0 coil and a variable test sample temperature that may be present. These heat sources are coupled to the heat-conducting body to the greatest possible extent and therefore only affect the control of the temperature T2.

(25) A thermoelectric cooler comprising heat exchangers on both sides of the wall is used to control the temperature at T3 of the second insulation chamber 11. This also fulfils another important purpose: it is responsible for a first reduction in the fluctuations in the room temperature, which allows the temperature of the isothermal wall and the shim system 4/the H0 coils to be accurately controlled at the temperatures T1 and T2, without placing high demands on the temperature of the area around the spectrometer. In order to be able to reach the temperature T3 with as little power as possible, it is reasonable to provide a piece of passive insulation that has sufficient dimensions as the wall of the second insulation chamber.

(26) Furthermore, in these embodiments, a thermoelectric cooler having a heat exchanger for the second insulation chamber 11 can be provided to generate the temperature T2. This is advantageous, for example, if the temperature difference and size of the heat exchanger are insufficient to have enough thermal lift to cool the shim system 4 and/or the H0 coil and/or the heat input as a result of a variable test sample temperature, for example.

(27) Preferably, the first insulation chamber 5 is made of a material having good thermal conductivity. This is advantageous since the heat radiation on the magnet from outside is as homogeneous as possible, and therefore no temperature gradients occur on the magnet.

(28) The second insulation chamber 11 preferably contains an arrangement circulating the air 19 (for example one or multiple fans) inside the second insulation chamber 11. This allows for temperature distributions that are as homogeneous as possible and can ensure a flow of air through the heat exchanger(s).

(29) In an alternative design—as shown in FIG. 4—the arrangement 8 controlling the temperature T2 can also control said temperature with respect to the external temperature. Furthermore, FIG. 4 shows an open sample chamber and therefore the temperature of the sample can also be controlled in this embodiment. A corresponding probehead having the necessary arrangement controlling the temperature of the test samples is not shown in FIG. 4 for reasons of clarity, but is shown separately in FIGS. 5A and 5B.

(30) If the arrangement controlling the temperature T2 in relation to the external temperature and the NMR probehead 3 to the top are formed as separate chambers, the advantage lies in the fact that, at high thermal load, the probehead does not reach the second insulation chamber 11 and therefore the arrangement of the temperature control device 12 controlling the temperature T3 (thermoelectric cooler) of the second insulation chamber 11 can be smaller. This is particularly applicable if, in addition to the dissipation of the shim system 4/H0 coil, a large amount of heat load is also introduced by controlling the sample temperature. The thermal control of the shim system 4 can be carried out in two stages once again, wherein small additional heaters are used. When the heat is dissipated with respect to the room temperature, the stability with which T2 is controlled can be increased by using a cascaded system of thermoelectric coolers.

(31) FIGS. 5A and 5B are cross-sections showing a schematic longitudinal section through the sample chamber comprising the shim system 4 and heat-conducting body 7 in two embodiments in which the temperature of the sample is controlled with a VT gas flow 16.

(32) For optional control of the temperature of the test sample TS (with TS in the range of from −40 to +150° C.), another thermal insulation system 14 is required, which separates the test sample temperature-control space from the shim system 4. This is made of a material having poor thermal conduction (for example a vacuum, an aerogel, foam, glass, plastics material, . . . ) and optionally contains a second gas flow in the form of a temperature-controlled flush gas flow 15 having a temperature TF, which is preferably between 15° C. and 35° C.

(33) In both embodiments shown, a central pipe is additionally provided for controlling the temperature of the sample, which cylindrically encompasses the sample tube that is generally elongate. Such a central pipe is advantageous in that, in addition to the canalization of the VT gas flow, it can also be used to collect escaping liquid and broken glass in the event that a test sample is broken or of a leak in the flow cells/test samples, so that they can be removed more easily.

(34) The thermal insulation system 14 is arranged between the RF coils 13 and the shim system 4.

(35) FIG. 5A shows an embodiment in which the central pipe is made of material having poor thermal conductivity and a piece of insulation is produced between the flush gas and the VT gas. The flush gas/fluid flow regions are arranged between the central pipe and the piece of thermal insulation in this case, since the temperature gradient can therefore be minimized with the piece of thermal insulation. For the purpose of improving temperature gradients inside the test sample, however, it may make more sense to channel the flush gas flow outside the piece of thermal insulation, for example between this and the shim system, as shown in FIG. 5B. It is also feasible to implement a combination of both configurations and to use a third gas flow having a temperature near to TS, or between TS and TF.

(36) FIG. 5A shows an embodiment in which the test sample temperature can be directly set by temperature-control gas. In FIG. 5B, the temperature is indirectly controlled. This can be useful in particular for generating a hermetically sealed sample chamber, as is used for example for toxic test samples in order to prevent contamination in the event of a test sample being broken.

(37) Ideally, the temperature TF of the flush gas flow is approximately equal to the temperature T2 so as to be able to make it possible to input as little heat as possible into the heat-conducting body.

(38) Lastly, FIG. 6A-6C show different cross sections through embodiments of the NMR probehead 3 comprising the shim system 4, in which the heat-conducting body 7 comprises a homogenization body 10 and a heat-conducting device 17 in each case:

(39) FIG. 6A shows a heat-conducting body 7 comprising two homogenization bodies 10 and two heat-conducting devices 17 for transferring the heat to the outside. The task of the homogenization body 10, which is in plate form, is to conduct the resultant heat in the plane to the heat-conducting devices 17. Since the distance is relatively short, the thickness of this body can be reduced.

(40) In FIG. 6B, the heat-conducting devices 17 each comprise two heat pipes, as a result of which the conduction of heat in the direction perpendicular to the cut plane can be significantly increased.

(41) FIG. 6C schematically shows a geometry for Halbach magnets having a concentric structure, for example. In this case, the heat-conducting body does not separately consist of a homogenization body 10 and a heat-conducting device 17, but heat pipes can also be soldered on or hollow-cylindrical heat pipes can be used in order to improve the transport of heat in the direction perpendicular to the plane of the drawing.

(42) The heat-conducting body 7 is preferably thermally connected to a homogenization body 10. This is in particular made of a material having a high degree of heat conduction in order to set as homogeneous a temperature T2 as possible along the entire outer contour of the homogenization body 10 (oriented towards the magnet).

(43) The heat-conducting device 17 is connected to a heat exchanger and is used to transport heat to, or remove it from, the homogenization body 10. The heat-conducting device 17 is preferably made of a material having a high degree of thermal conductivity or constructed as a heat pipe.

(44) The shim system and/or the H0 coil is/are preferably directly thermally coupled to the heat-conducting body 7, in particular to the homogenization body 10. This can therefore ensure that the dissipation that occurs during operation as a result of electrical currents can be guided out of the central air gap efficiently.

(45) An notable difference with respect to known devices of the prior art lies in controlling the temperature T2, which “is seen” from the inner contour of the magnet. Furthermore, it is novel to introduce a piece of thermal insulation between the sample temperature-control chamber and the shim system. It is clear from the above-cited documents that, in the prior art, the piece of insulation is always placed between the test sample and the inner wall of a sample temperature-control chamber, but especially inside the transmitter/receiver coil.

(46) The features of all the above-described embodiments of the invention can also be combined at least for the most part.

LIST OF REFERENCES

Documents Taken into Consideration when Assessing Patentability

(47) [1] US 2011/0137589 A1 [2] U.S. Pat. No. 8,461,841 B2 [3] U.S. Pat. No. 6,489,873 B1 [4] U.S. Pat. No. 6,566,880 B1 and WO 2000/016117 A1 [5] GB 2512328 A [6] U.S. Pat. No. 8,030,927 B2 [7] U.S. Pat. No. 7,297,907 B2 [8] US 2013/0207657 A1 [9] U.S. Pat. No. 9,285,441 B1 [10] US 2011/0037467 A1 [11] US 2018/0038924 A1 [12] US 2016/0077176 A1

LIST OF REFERENCE SIGNS

(48) 0 test sample 1 permanent magnet arrangement 2 central air gap 3 NMR probehead 4 shim system (with integrated H0 coil) 5 first insulation chamber 6 arrangement controlling the temperature T1 7 heat-conducting body 8 arrangement controlling the temperature T2 9 passive thermal insulation 10 homogenization body 11 second insulation chamber 12 temperature control device 13 RF coil 14 thermal insulation system 15 temperature-controlled flush gas flow 16 temperature-controlled VT gas flow 17 heat-conducting device 18 T-sensor for measuring T1 18′ T-sensor for measuring T2 18″ T-sensor for measuring T3 19 arrangement circulating the air