WO2010067272A1

WO2010067272A1 - Arrangement for influencing and/or detecting magnetic particles in a region of action

Info

Publication number: WO2010067272A1
Application number: PCT/IB2009/055477
Authority: WO
Inventors: Holger Timinger
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2008-12-11
Filing date: 2009-12-03
Publication date: 2010-06-17

Abstract

The present invention relates to an arrangement for influencing and/or detecting magnetic particles (100) in a region of action (300), which comprises selection means (210) for generating a magnetic selection field (211) having a pattern in space of its magnetic field strength such that a first sub-zone (301) having a low magnetic field strength and a second sub-zone (302) having a higher magnetic field strength are formed in the region of action (300). The arrangement further comprises drive means (220) for changing the position in space of the two sub-zones (301, 302) in the region of action (300) by means of a magnetic drive field (221) so that the magnetization of the magnetic material changes locally. The arrangement further comprises receiving means (230) for acquiring detection signals, which detection signals depend on the magnetization in the region of action (300), which magnetization is influenced by the change in the position in space of the first and second sub- zone (301, 302). The selection means (210) and/or the drive means (220) are embedded into a cooling assembly (400) comprising a plurality of interconnected hollow vessels (250). Said hollow vessels (250) encapsulate and extend along said selection means legs (212) and/or said drive means legs, and the hollow vessels (250) are at least partly filled with a cooling liquid (270).

Description

ARRANGEMENT FOR INFLUENCING AND/OR DETECTING MAGNETIC PARTICLES IN A REGION OF ACTION

FIELD OF THE INVENTION

The present invention relates to an arrangement for influencing and/or detecting magnetic particles in a region of action.

BACKGROUND OF THE INVENTION

An arrangement of this kind is known from German patent application DE 101 51 778 Al. In the arrangement described in that publication, first of all a magnetic field having a spatial distribution of the magnetic field strength is generated such that a first sub-zone having a relatively low magnetic field strength and a second sub-zone having a relatively high magnetic field strength are formed in the examination zone. The position in space of the sub-zones in the examination zone is then shifted, so that the magnetization of the particles in the examination zone changes locally. Signals are recorded which are dependent on the magnetization in the examination zone, which magnetization has been influenced by the shift in the position in space of the sub-zones, and information concerning the spatial distribution of the magnetic particles in the examination zone is extracted from these signals, so that an image of the examination zone can be formed. Such an arrangement has the advantage that it can be used to examine arbitrary examination objects - e. g. human bodies - in a non-destructive manner and without causing any damage and with a high spatial resolution, both close to the surface and remote from the surface of the examination object.

A similar arrangement and method is known from Gleich, B. and Weizenecker, J. (2005), "Tomographic imaging using the nonlinear response of magnetic particles" in nature, vol. 435, pp. 1214-1217. The arrangement and method for magnetic particle imaging (MPI) described in that publication takes advantage of the non- linear magnetization curve of small magnetic particles.

Known arrangements of this type have shown the disadvantage that during operation, loss in the system is introduced, in particular due to resistive current transport and eddy currents. Especially for superconducting coils, which are used in such arrangements for generating the various magnetic fields, the temperature of the coils is crucial for a stable operation and an optimal efficiency of the system.

Cooling systems for coils are already known from magnetic resonance (MR) systems, where, as disclosed in US 7,015,692 B2, a cryostatic cooling system is provided surrounding the whole RF coil arrangement.

An adaptation of such cooling systems to MPI systems mentioned above is although not advantageous since the structure and the arrangement of the coils used in MPI systems significantly differs from MR systems. Furthermore, the cooling system known from US 7,015,692 B2 has the disadvantage of preventing access from directions other than the patient bore. Additionally, the cooling of the RF coil arrangement is realized by a separate cooling circuit which is not optimally efficient due to indirect cooling.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an arrangement of the kind mentioned initially, wherein the system loss during operation is minimized and a stable operation at a nearly constant temperature level is ensured.

The object is achieved according to the present invention by an arrangement for influencing and/or detecting magnetic particles in a region of action, comprising: - selection means comprising permanent magnets and/or coils having a number of selection means legs for generating a magnetic selection field having a pattern in space of its magnetic field strength such that a first sub-zone having a low magnetic field strength and a second sub-zone having a higher magnetic field strength are formed in the region of action, - drive means comprising coils having a number of drive means legs for changing the position in space of the two sub-zones in the region of action by means of a magnetic drive field so that the magnetization of the magnetic material changes locally, and

- receiving means for acquiring detection signals, which detection signals depend on the magnetization in the region of action, which magnetization is influenced by the change in the position in space of the first and second sub-zone, wherein:

- the selection means and/or the drive means are embedded into a cooling assembly comprising a plurality of interconnected hollow vessels, - said hollow vessels encapsulate and extend along said selection means legs and/or said drive means legs, and

- the hollow vessels are at least partly filled with a cooling liquid.

According to the present invention, it is to be understood that the selection means and/or the drive means and/or the receiving means can at least partially be provided in the form of one single coil or solenoid. However, it is preferred according to the present invention that separate coils are provided to form the selection means, the drive means and the receiving means. Furthermore according to the present invention, the selection means and/or the drive means and/or the receiving means can each be composed of separate individual parts, especially separate individual coils or solenoids, provided and/or arranged such that the separate parts form together the selection means and/or the drive means and/or the receiving means. Especially for the drive means and/or the selection means, a plurality of parts, especially pairs for coils (e.g. in a Helmholtz or Anti-Helmholtz configuration) are preferred in order to provide the possibility to generate and/or to detect components of magnetic fields directed in different spatial directions.

With the cooling assembly in which the selection means and/or the drive means are embedded it is possible to establish an efficient cooling of the selection means and/or the drive means, and thereby to reduce the loss in the system which is caused by resistive current transport, eddy-currents, and so-called AC losses, which are losses caused by superconducting coils due to changing the magnetic field and the current. Since the cooling assembly comprises a plurality of interconnected hollow vessels which encapsulate and extend along each leg of the selection means and/or each leg of the drive means, said legs can be directly cooled in order to ensure a stable operation. In contrast to a simple cooling system where the magnetic field generation means as a whole are immersed in a cooling liquid bath, the structure of the interconnected hollow vessels, as proposed by the present invention, allows a very efficient cooling, especially if superconducting coils are used as magnetic field generation means. The vessel structure is furthermore advantageous since significantly less cooling medium is required than in a full-body arrangement. The accurate amount of cooling liquid used in the vessel system depends on the actual coil configuration and desired operating conditions. Another advantage is reflected in the very space-saving design of the hollow vessels wherein the form of the hollow vessels is directly adapted to the legs of the magnetic selection means. This allows a very good exploitation of the space within the arrangement. In particular, the design enables patient access not only from two sides along the main patient bore, but also from four additional orientations. Alternatively, this space can be used to house additional system components.

According to an embodiment of the present invention, it is preferred that the cooling liquid has an evaporating temperature in the range of the working temperature of the selection means or the drive means. The loss of the magnetic field generation means causes a temperature increase, which in turn will cause the cooling liquid to evaporate. The thereby created vapor then transfers within the interconnected hollow vessels to the cold end of the vessel structure, where it condenses again and returns back to the warm parts of the vessel structure. This type of heat transport exhibits extremely large thermal conductivity and small temperature differences between cold and warm parts within the vessel structure. Due to the pressure differences of the cold and warm parts a high velocity transfer of the evaporated cooling liquid is generated. This generates an optimal heat convection between the magnetic generation means and the evaporated cooling liquid.

Another significant advantage is that the transport of the cooling liquid within the vessel structure is absolutely self-regulated, so that no special pump is necessary. Such a cryostatic system is especially efficient for cooling superconducting coils and permanent magnets. The temperature of the cold end thereby depends on the desired operating temperature. The operating temperature can furthermore be adapted by choosing the type of the cooling liquid, which could be in this case water or alcohol, if operated at room temperature, or argon, helium, neon or nitrogen if operated at cryogenic temperatures. Depending on the actually used fluid, the cooling liquid can be in direct contact to the magnetic field generation means, which is preferable. Otherwise the cooling liquid must be fed through additional vessels which are separated from the magnetic generation means.

According to an embodiment of the present invention, it is preferred that the hollow vessels are made of an electrical insulator, in particular glass-fϊber-reinforced plastic, and/or are surrounded by a thermal insulator, in particular an additional vacuum vessel or polystyrene- like material. With the electrical insulator and/or the surrounding with a thermal insulator it is possible to prevent eddy currents. Resistive current transport, which causes system loss, is thereby also prevented. This allows an optimal generation of the magnetic fields with minimal system loss.

According to an embodiment of the present invention, it is furthermore preferred that the interconnected hollow vessels comprise a wick structure. This wick structure has the advantage that it supports the transport of the re-condensed cooling liquid to regions within the vessel structure, which otherwise cannot be reached by the cooling liquid. Such a cooling structure can be realized, for example, by a wire mesh. In this case, the transport of the cooling liquid does no longer depend on gravity, but is driven by capillary forces, and therefore the transport from the cold to the warm parts of the vessel structure is substantially improved. This leads to an enhanced but still controlled ascendancy of the cooling liquid along the wick structure within the interconnected hollow vessels.

In a preferred embodiment of the present invention, focus means are provided for changing the position in space of the region of action, comprising coils having a number of focus means legs. Since the operating range of the drive means generating the magnetic field which changes the position in space of the two sub-zones is limited to the region of action, the introduction of focus means is advantageous since the position in space of the region of action can be changed. The operating range, in which magnetic particles can be influenced and/or detected, is therefore substantially enlarged so that for example longer blood vessels can easily be examined with the arrangement according to the present invention. It has to be noted that the magnetic field generated by the focus means has preferably a lower frequency and a higher amplitude than the magnetic field generated by the drive means. This has mainly the purpose of having a low interference between both magnetic fields. It is also possible to combine focus means and selection means within one field generation means. Alternatively, the focus mans can also be combined with the drive means within one field generation means. In this case high currents are needed if the combined field generation means are realized by coils.

In a further preferred embodiment of the present invention, the focus means are embedded into the cooling assembly, said hollow vessels encapsulating and extending along said focus means legs. Encapsulating the focus means legs by the hollow vessels has the advantage that the focus means are also stabilized at a mainly constant temperature and that a stable operation with minimal system losses can be realized due to an efficient cooling. It is to be understood that the focus means, respectively the focus means legs, can thereby also be encapsulated by the same hollow vessels which encapsulate the selection means legs and/or the drive means legs. Furthermore, it is preferred according to the present invention that the selection means, the drive means, the receiving means and the focus means comprise permanent magnets, resistive coils or superconducting coils. The magnetic generation means mentioned above are thereby preferably realized by opposing pairs of permanent magnets, resistive coils or superconducting coils, wherein each pair generates a magnetic field for one of the three main spatial directions. The selection means and the focus means can, as already mentioned above, be realized by the same pairs of permanent magnets, resistive coils or superconducting coils. It is to be understood that the magnetic fields can also be provided by a combination of coils and permanent magnets. According to an embodiment of the present invention, it is furthermore preferred that the interconnected hollow vessels together form the edges of a hollow cube having openings at each of the six lateral surfaces. This is advantageous because it allows the access of the object to be examined from all six sides. Especially, when used for the examination of human patients, the openings at each surface serve as patient bore and provide an easy access to the center of the arrangement. This also represents, in contrast to known MR and CT systems, a very open design, which is comfortable and does not constrict the patient by a closed tube.

It is furthermore preferred according to the present invention that the interconnected hollow vessels comprise a cooling unit or heat exchanger. The cooling unit has the advantage that it supports the condensation of the evaporated cooling liquid at the cold end and therefore intensifies the convective flow within the vessel structure. The heat exchanger can also be introduced in order to amplify the heat transport.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. Therein:

Fig. 1 shows a schematic view of the principle layout of a magnetic particle imaging (MPI) arrangement, Fig. 2 shows an example of the field line pattern produced by an arrangement according to the present invention, Fig. 3 shows an enlarged view of a magnetic particle present in the region of action,

Figs. 4a and 4b show the magnetization characteristics of such particles, Figs. 5a and 5b show schematic views of an embodiment of the selection means (Fig. 5a) and the focus means (Fig. 5b), embedded into a cooling assembly according to the present invention, and Fig. 6 shows an enlarged detail of the embodiment shown in Fig. 5b. DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows an arbitrary object to be examined by means of a MPI arrangement 10. The reference numeral 350 in Fig. 1 denotes an object, in this case a human or animal patient, who is arranged on a patient table, only part of the top of which is shown. Prior to the application of the method according to the present invention, magnetic particles 100 (not shown in Fig. 1) are arranged in a region of action 300 of the inventive arrangement 10. Especially prior to a therapeutical and/or diagnostical treatment of, for example, a tumor, the magnetic particles 100 are positioned in the region of action 300, e.g. by means of a liquid (not shown) comprising the magnetic particles 100 which is injected into the body of the patient 350.

As an example of an embodiment of the present invention, an arrangement 10 is shown in Fig. 2 comprising a plurality of coils forming a selection means 210 whose range defines the region of action 300 which is also called the region of treatment 300. For example, the selection means 210 is arranged above and below the patient 350 or above and below the table top. For example, the selection means 210 comprise a first pair of coils 210', 210", each comprising two identically constructed windings 210' and 210" which are arranged coaxially above and below the patient 350 and which are traversed by equal currents, especially in opposed directions. The first coil pair 210', 210" together are called selection means 210 in the following. Preferably, direct currents are used in this case. The selection means 210 generate a magnetic selection field 211 which is in general a gradient magnetic field which is represented in Fig. 2 by the field lines. It has a substantially constant gradient in the direction of the (e.g. vertical) axis of the coil pair of the selection means 210 and reaches the value zero in a point on this axis. Starting from this field- free point (not individually shown in Fig. 2), the field strength of the magnetic selection field 211 increases in all three spatial directions as the distance increases from the field-free point. In a first sub- zone 301 or region 301 which is denoted by a dashed line around the field- free point the field strength is so small that the magnetization of particles 100 present in that first sub-zone 301 is not saturated, whereas the magnetization of particles 100 present in a second sub-zone 302 (outside the region 301) is in a state of saturation. The field-free point or first sub-zone 301 of the region of action 300 is preferably a spatially coherent area; it may also be a punctiform area or else a line or a flat area. In the second sub-zone 302 (i.e. in the residual part of the region of action 300 outside of the first sub-zone 301) the magnetic field strength is sufficiently strong to keep the particles 100 in a state of saturation. By changing the position of the two sub-zones 301, 302 within the region of action 300, the (overall) magnetization in the region of action 300 changes. By measuring the magnetization in the region of action 300 or a physical parameters influenced by the magnetization, information about the spatial distribution of the magnetic particles in the region of action can be obtained. In order to change the relative spatial position of the two sub-zones 301, 302 in the region of action 300, a further magnetic field, the so-called magnetic drive field 221, is superposed to the selection field 211 in the region of action 300 or at least in a part of the region of action 300.

Fig. 3 shows an example of a magnetic particle 100 of the kind used together with an arrangement 10 of the present invention. It comprises for example a spherical substrate 101, for example, of glass which is provided with a soft-magnetic layer 102 which has a thickness of, for example, 5 nm and consists, for example, of an iron-nickel alloy (for example, Permalloy). This layer may be covered, for example, by means of a coating layer 103 which protects the particle 100 against chemically and/or physically aggressive environments, e.g. acids. The magnetic field strength of the magnetic selection field 211 required for the saturation of the magnetization of such particles 100 is dependent on various parameters, e.g. the diameter of the particles 100, the used magnetic material for the magnetic layer 102 and other parameters.

In the case of e.g. a diameter of 10 μm, a magnetic field of approximately 800 A/m (corresponding approximately to a flux density of 1 mT) is then required, whereas in the case of a diameter of 100 μm a magnetic field of 80 A/m suffices. Even smaller values are obtained when a coating 102 of a material having a lower saturation magnetization is chosen or when the thickness of the layer 102 is reduced.

For further details of the preferred magnetic particles 100, the corresponding parts of DE 10151778 are hereby incorporated by reference, especially paragraphs 16 to 20 and paragraphs 57 to 61 of EP 1304542 A2 claiming the priority of DE 10151778.

The size of the first sub-zone 301 is dependent on the one hand on the strength of the gradient of the magnetic selection field 211 and on the other hand on the field strength of the magnetic field required for saturation. For a sufficient saturation of the magnetic particles 100 at a magnetic field strength of 80 A/m and a gradient (in a given space direction) of the field strength of the magnetic selection field 211 amounting to 160 10³

A/m2, the first sub-zone 301 in which the magnetization of the particles 100 is not saturated has dimensions of about 1 mm (in the given space direction).

When a further magnetic field - in the following called a magnetic drive field 221 is superposed on the magnetic selection field 210 (or gradient magnetic field 210) in the region of action 300, the first sub-zone 301 is shifted relative to the second sub-zone 302 in the direction of this magnetic drive field 221; the extent of this shift increases as the strength of the magnetic drive field 221 increases. When the superposed magnetic drive field 221 is variable in time, the position of the first sub-zone 301 varies accordingly in time and in space. It is advantageous to receive or to detect signals from the magnetic particles 100 located in the first sub-zone 301 in another frequency band (shifted to higher frequencies) than the frequency band of the magnetic drive field 221 variations. This is possible because frequency components of higher harmonics of the magnetic drive field 221 frequency occur due to a change in magnetization of the magnetic particles 100 in the region of action 300 as a result of the non-linearity of the magnetization characteristics.

In order to generate these magnetic drive fields 221 for any given direction in space, there are provided three further coil pairs, namely a second coil pair 220', a third coil pair 220" and a fourth coil pair 220'" which together are called drive means 220 in the following. For example, the second coil pair 220' generates a component of the magnetic drive field 221 which extends in the direction of the coil axis of the first coil pair 210', 210" or the selection means 210, i.e. for example vertically. To this end the windings of the second coil pair 220' are traversed by equal currents in the same direction. The effect that can be achieved by means of the second coil pair 220' can in principle also be achieved by the superposition of currents in the same direction on the opposed, equal currents in the first coil pair 210', 210", so that the current decreases in one coil and increases in the other coil. However, and especially for the purpose of a signal interpretation with a higher signal to noise ratio, it may be advantageous when the temporally constant (or quasi constant) selection field 211 (also called gradient magnetic field) and the temporally variable vertical magnetic drive field are generated by separate coil pairs of the selection means 210 and of the drive means 220.

The two further coil pairs 220", 220'" are provided in order to generate components of the magnetic drive field 221 which extend in a different direction in space, e.g. horizontally in the longitudinal direction of the region of action 300 (or the patient 350) and in a direction perpendicular thereto. If third and fourth coil pairs 220", 220'" of the Helmholtz type (like the coil pairs for the selection means 210 and the drive means 220) were used for this purpose, these coil pairs would have to be arranged to the left and the right of the region of treatment or in front of and behind this region, respectively. This would affect the accessibility of the region of action 300 or the region of treatment 300. Therefore, the third and/or fourth magnetic coil pairs or coils 220", 220'" are also arranged above and below the region of action 300 and, therefore, their winding configuration must be different from that of the second coil pair 220'. Coils of this kind, however, are known from the field of magnetic resonance apparatus with open magnets (open MRI) in which a radio frequency (RF) coil pair is situated above and below the region of treatment, said RF coil pair being capable of generating a horizontal, temporally variable magnetic field. Therefore, the construction of such coils need not be further elaborated herein.

The arrangement 10 according to the present invention further comprise receiving means 230 that are only schematically shown in Fig. 1. The receiving means 230 usually comprise coils that are able to detect the signals induced by magnetization pattern of the magnetic particles 100 in the region of action 300. Coils of this kind, however, are known from the field of magnetic resonance apparatus in which e.g. a radio frequency (RF) coil pair is situated around the region of action 300 in order to have a signal to noise ratio as high as possible. Therefore, the construction of such coils need not be further elaborated herein. In an alternative embodiment for the selection means 210 shown in Fig. 1, permanent magnets (not shown) can be used to generate the gradient magnetic selection field 211. In the space between two poles of such (opposing) permanent magnets (not shown) there is formed a magnetic field which is similar to that of Fig. 2, that is, when the opposing poles have the same polarity. In another alternative embodiment of the arrangement according to the present invention, the selection means 210 comprise both at least one permanent magnet and at least one coil 210', 210" as depicted in Fig. 2.

The frequency ranges usually used for or in the different components of the selection means 210, drive means 220 and receiving means 230 are roughly as follows: The magnetic field generated by the selection means 210 does either not vary at all over the time or the variation is comparably slow, preferably between approximately 1 Hz and approximately 100 Hz. The magnetic field generated by the drive means 220 varies preferably between approximately 25 kHz and approximately 100 kHz. The magnetic field variations that the receiving means are supposed to be sensitive are preferably in a frequency range of approximately 50 kHz to approximately 10 MHz.

Figs. 4a and 4b show the magnetization characteristic, that is, the variation of the magnetization M of a particle 100 (not shown in Figs. 4a and 4b) as a function of the field strength H at the location of that particle 100, in a dispersion with such particles. It appears that the magnetization M no longer changes beyond a field strength + H_c and below a field strength -H_c, which means that a saturated magnetization is reached. The magnetization M is not saturated between the values +H_C and -H_c. Fig. 4a illustrates the effect of a sinusoidal magnetic field H(t) at the location of the particle 100 where the absolute values of the resulting sinusoidal magnetic field H(t) (i.e. "seen by the particle 100") are lower than the magnetic field strength required to magnetically saturate the particle 100, i.e. in the case where no further magnetic field is active. The magnetization of the particle 100 or particels 100 for this condition reciprocates between its saturation values at the rhythm of the frequency of the magnetic field H(t). The resultant variation in time of the magnetization is denoted by the reference M(t) on the right hand side of Fig. 4a. It appears that the magnetization also changes periodically and that the magnetization of such a particle is periodically reversed. The dashed part of the line at the centre of the curve denotes the approximate mean variation of the magnetization M(t) as a function of the field strength of the sinusoidal magnetic field H(t). As a deviation from this centre line, the magnetization extends slightly to the right when the magnetic field H increases from -H_c to +H_C and slightly to the left when the magnetic field H decreases from +H_C to -H_c. This known effect is called a hysteresis effect which underlies a mechanism for the generation of heat. The hysteresis surface area which is formed between the paths of the curve and whose shape and size are dependent on the material, is a measure for the generation of heat upon variation of the magnetization.

Fig. 4b shows the effect of a sinusoidal magnetic field H(t) on which a static magnetic field Hi is superposed. Because the magnetization is in the saturated state, it is practically not influenced by the sinusoidal magnetic field H(t). The magnetization M(t) remains constant in time at this area. Consequently, the magnetic field H(t) does not cause a change of the state of the magnetization.

Fig. 5a shows a schematic view of an embodiment of the selection means 210 which are embedded into a cooling assembly 400 according to the present invention. The cooling assembly 400 comprises a vessel structure with interconnected hollow vessels 250 that form the edges of a hollow cube having openings 260 at each of the six lateral surfaces. These openings 260 provide an easy access to the centre of the arrangement for the object to be examined and, in case of human patients serve as patient bore.

The hollow vessels 250 encapsulate and extend along the legs 212 of the selection means 210, which are in this embodiment realized as permanent magnets that form the edges of a hollow cube. It has to be noted that, depending on the desired application, the selection means 210 can also be realized as resistive coils or superconducting coils. It is also possible that a combination of coils and permanent magnets is used as selection means in order to generate the selection field 211. Furthermore, it can be seen from Fig. 5a that the cooling assembly 400 is only partly filled with a cooling liquid 270. The selection means 210 are therefore only partly immersed into the cooling liquid 270 at the lower bottom of the cubic vessel structure 250. At this warm part the temperature of the cooling liquid 270 is increased due to the heat dissipation of the selection means 210 and is therefore evaporated. The vapor 280 then ascends within the inner part of the vessel structure 250, and is thereby transferred to the upper part of the cubic vessel structure 250, which represents the cold end.

The thereby transferred vapor 280 is then condensed again at the cold end and, due to the symmetric design of the vessel structure 250, it returns to the lower end again. A cooling unit 290 which is indicated by arrows can additionally support the condensation of the vapor 280 and therefore intensify the convective flow within the vessel structure 250. In order to further amplify the heat transport and dependent on the actually used cooling fluid, it is also possible to provide a heat exchanger, which is not explicitly shown here.

Concerning the material of the hollow vessels 250, it has to be noted that the hollow vessels 250 are preferably made of an electrical insulator, e.g. glass- fiber-reinforced plastic, and/or are surrounded by a thermal insulator, in particular an additional vacuum vessel or polystyrene-like material. This supports the reduction of undesirable eddy currents, which can occur within the generation of the various magnetic fields. However, other materials could be used as well. Fig. 5b shows the selection means 210 which are in this embodiment combined with the focus means 240 and are embedded into the same cooling assembly 400 as already shown in Fig. 5a. The selection, respectively the focus means 240 are in this embodiment realized by coils, which can be either resistive or superconducting coils. Therefore, each hollow vessel 250 in this embodiment encapsulates and extends along two parallel coil legs 212a/242a and 212b/242b of the combined selection 210 and focus means 240, the two coil legs 212a/242a and 212b/242b belonging to different coils which generate perpendicular fractions of the selection field 211 respectively the focus field. Nevertheless, the transport of the cooling liquid 270 works in the same manner as already explained above in the description of Fig. 5a. The transport of the cooling liquid 270 can be additionally supported by a wick structure 410 (see Fig. 6). Due to the optional wick structure 410, which additionally enhances the transport of the vapor 280, the cooling liquid 270 can also be transported to regions within the vessel structure 250, which otherwise cannot be reached, e.g. acute corners. The two arrows in Fig. 6 indicate the direction of the ascending vapor 280. The wick structure 410 is preferably realized as a wire mesh which extends along the inside of the hollow vessels 250. As it can be seen from Fig. 6, the re-condensed vapor 280 can therefore reflow from the cold to the warm end along the wick structure 410. Thus, the cooling system is due to the introduced special setup of the cooling assembly 400, independent of the used magnetic field generation means (permanent magnets, resistive coils or superconducting coils), absolutely self-regulated and does not need the further support of a pump.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

Furthermore, the terms first, second, third and the like in the description and in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described of illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the present description and claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An arrangement for influencing and/or detecting magnetic particles (100) in a region of action (300), comprising:

- selection means (210) comprising permanent magnets and/or coils having a number of selection means legs (212) for generating a magnetic selection field (211) having a pattern in space of its magnetic field strength such that a first sub-zone (301) having a low magnetic field strength and a second sub-zone (302) having a higher magnetic field strength are formed in the region of action (300),

- drive means (220) comprising coils having a number of drive means legs for changing the position in space of the two sub-zones (301, 302) in the region of action (300) by means of a magnetic drive field (221) so that the magnetization of the magnetic material changes locally, and

- receiving means (230) for acquiring detection signals, which detection signals depend on the magnetization in the region of action (300), which magnetization is influenced by the change in the position in space of the first and second sub-zone (301, 302), wherein:

- the selection means (210) and/or the drive means (220) are embedded into a cooling assembly (400) comprising a plurality of interconnected hollow vessels (250),

- said hollow vessels (250) encapsulate and extend along said selection means legs (212) and/or said drive means legs, and - the hollow vessels (250) are at least partly filled with a cooling liquid (270).

2. An arrangement according to claim 1, characterized in that the cooling liquid (270) has an evaporating temperature in the range of the working temperature of the selection means (210) or the drive means (220).

3. An arrangement according to claim 1, characterized in that the hollow vessels (250) are made of an electrical insulator, in particular glass-fiber-reinforced plastic, and/or are surrounded by a thermal insulator, in particular an additional vacuum vessel or polystyrene- like material.

4. An arrangement according to claim 1, characterized in that the interconnected hollow vessels (250) comprise a wick structure (410).

5. An arrangement according to claim 1, characterized in that focus means (240) are provided for changing the position in space of the region of action (300), comprising coils having a number of focus means legs.

6. An arrangement according to claim 5, characterized in that the focus means (240) are embedded into the cooling assembly (400), said hollow vessels (250) encapsulating and extending along said focus means legs.

7. An arrangement according to claim 1 or 5, characterized in that the selection means (210), the drive means (220), the receiving means (230) and the focus means (240) comprise permanent magnets, resistive coils or superconducting coils.

8. An arrangement according to claim 1, characterized in that the interconnected hollow vessels (250) together form the edges of a hollow cube having openings (260) at each of the six lateral surfaces.

9. An arrangement according to claim 1, characterized in that the interconnected hollow vessels (250) comprise a cooling unit (290) or heat exchanger.