Magnet arrangement for generating a selection magnetic field, apparatus with a magnet arrangement and method for generating a selection magnetic field

Abstract

A magnet arrangement for generating a selection magnetic field with a gradient and a field-free region in a sample volume includes: a Maxwell magnet system with two ring magnets, which are arranged in coaxial fashion and at a distance from one another on a common Z-axis extending through the sample volume, a focus field coil arrangement with at least one focus field coil pair for displacing the field-free region within the sample volume, and a drive coil for generating an MPI drive field. The magnet arrangement comprises a quadrupole magnet system with at least one quadrupole ring for generating a quadrupole magnetic field, the quadrupole magnet system being arranged coaxially with respect to the Maxwell magnet system. Using the magnet arrangement according to the invention, it is possible to switch between a selection magnet field with a field-free point and a field-free line without undesirably increasing the field gradient.

Claims

1. A magnet arrangement for generating a selection magnetic field with a gradient and a field-free region in a sample volume, the magnet arrangement comprising: a Maxwell magnet system with two ring magnets, which are arranged in coaxial fashion and at a distance from one another on a common Z-axis which extends through the sample volume (PV), a focus field coil arrangement with at least one focus field coil pair for displacing the field-free region within the sample volume, a drive coil for generating an MPI drive field, and a quadrupole magnet system with at least one quadrupole ring for generating a quadrupole magnetic field, the quadrupole magnet system being arranged coaxially with respect to the Maxwell magnet system, and being rotatably mounted such that it is rotatable about the Z-axis.

2. The magnet arrangement according to claim 1, wherein the quadrupole magnet system comprises at least two concentric quadrupole rings which are rotatable relative to one another about the Z-axis and which are able to be coupled to one another.

3. The magnet arrangement according to claim 1, wherein the distance between the ring magnets of the Maxwell magnet system is variable.

4. The magnet arrangement according to claim 1, wherein the focus field coil arrangement comprises two stationary focus field coil pairs.

5. An apparatus for carrying out MPI imaging and/or magnetic fluid hyperthermia using magnetic particles with a magnet arrangement according to claim 1.

6. A method for generating a selection magnetic field with a gradient and a field-free region in a sample volume comprising providing a magnet arrangement according to claim 1 and superposing a magnetic field of the Maxwell magnet system and the quadrupole field of the quadrupole magnet system.

7. The method according to claim 6, wherein a shape of the field-free region is altered by adjusting a ratio of a quadrupole gradient strength of the quadrupole magnet system to a gradient strength of the Maxwell magnet system.

8. The method according to claim 6, further comprising adjusting a gradient strength of the Maxwell magnet system by varying a spacing of the ring magnets of the Maxwell magnet system or by shorting the ring magnets in soft magnetic fashion.

9. The method according to claim 6, further comprising rotating the field-free region by rotating the quadrupole magnet system within an angular range about an axis Z′.

10. The method according to claim 9, wherein a relative position of the Z′-axis is displaced into a target zone by a coil arrangement.

11. The method of claim 6 further comprising performing MPI imaging using the superposed magnetic fields in the sample volume.

12. The method of claim 6 further comprising performing magnetic fluid hyperthermia using the superposed magnetic fields in the sample volume.

13. A magnet arrangement for generating a selection magnetic field with a gradient and a field-free region in a sample volume, the magnet arrangement comprising: a Maxwell magnet system with two ring magnets, which are arranged in coaxial fashion and at a distance from one another on a common Z-axis which extends through the sample volume (PV), wherein the ring magnets of the Maxwell magnet system are partly shorted in soft magnetic fashion, a focus field coil arrangement with at least one focus field coil pair for displacing the field-free region within the sample volume, a drive coil for generating an MPI drive field, and a quadrupole magnet system with at least one quadrupole ring for generating a quadrupole magnetic field, the quadrupole magnet system being arranged coaxially with respect to the Maxwell magnet system.

14. A magnet arrangement for generating a selection magnetic field with a gradient and a field-free region in a sample volume, the magnet arrangement comprising: a Maxwell magnet system with two ring magnets, which are arranged in coaxial fashion and at a distance from one another on a common Z-axis which extends through the sample volume (PV), a focus field coil arrangement with at least one focus field coil pair for displacing the field-free region within the sample volume, wherein the focus field coil pair is rotatably mounted such that it is rotatable relative to the Maxwell magnet system about the Z-axis, a drive coil for generating an MPI drive field, and a quadrupole magnet system with at least one quadrupole ring for generating a quadrupole magnetic field, the quadrupole magnet system being arranged coaxially with respect to the Maxwell magnet system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a perspective illustration of a magnet arrangement with a Maxwell magnet pair with axially magnetized ring magnets for generating a static field-free point and a continuously wound drive-field drive coil.

(2) FIG. 2 shows a perspective illustration of a magnet arrangement with a Maxwell magnet pair with radially magnetized ring magnets for generating a static field-free point and a drive-field drive coil consisting of two spaced apart partial coils.

(3) FIG. 3 shows a perspective illustration of a magnet arrangement according to the invention with an axially magnetized Maxwell magnet system and a quadrupole magnet system, and a field-free line generated by the magnet arrangement.

(4) FIG. 4 shows a perspective illustration of a magnet arrangement according to the invention with a radially magnetized Maxwell magnet system and a quadrupole magnet system, and a field-free line generated by the magnet arrangement.

(5) FIG. 5 shows the magnet arrangement of FIG. 3 and a field-free line generated by the magnet arrangement and rotated in the XY-plane.

(6) FIG. 6 shows a cross section through a magnet arrangement according to the invention which comprises a quadrupole magnet system with two quadrupole rings (with Q=M/2), wherein the magnetizations of the quadrupole rings are aligned in the same way and a field-free line is generated (FFL mode).

(7) FIG. 7 shows a cross section through a magnet arrangement according to the invention which comprises a quadrupole magnet system with two quadrupole rings (with Q=0), wherein the magnetizations of the quadrupole rings are aligned counter to one another and a field-free point is generated (FFP mode).

(8) FIG. 8 shows a cross section through a magnet arrangement according to the invention which comprises a quadrupole magnet system with two quadrupole rings (with O<Q<M/2), wherein the magnetizations of the quadrupole rings are rotated with respect to one another through an angle α and an ellipsoidal field-free region is generated.

(9) FIG. 9 shows a cross section through a magnet arrangement according to the invention which comprises a quadrupole magnet system with two quadrupole rings (with Q=M/2), wherein the magnetizations of the quadrupole rings are rotated with respect one another through an angle α and a field-free line is generated.

(10) FIG. 10 shows a longitudinal section through a magnet arrangement according to the invention, in which the magnetic rings of the Maxwell magnet system are arranged a distance of 2*D1 with respect to one another.

(11) FIG. 11 shows a longitudinal section through a magnet arrangement according to the invention, in which the magnetic rings of the Maxwell magnet system are arranged a distance of 2*D2 with respect to one another.

(12) FIG. 12 shows a longitudinal section through a magnet arrangement according to the invention which has soft magnetic cylinder structures in the Maxwell magnet system.

(13) FIG. 13 shows a cross section through a magnet arrangement according to the invention which has a quadrupole magnet system and a rotatable focus field coil pair, which generates a field-free line that is displaced in the X′-direction.

(14) FIG. 14 shows a cross section through a magnet arrangement according to the invention which has a quadrupole magnet system and two stationary focus field coil pairs, which generates a field-free line that is displaced in the X′-direction.

(15) FIG. 15 shows a field-free point generated in the FFP mode in a target zone by the magnet arrangement according to the invention (not shown), and a cross section through a hyperthermia coil.

(16) FIG. 16 shows field-free lines which intersect a target zone and were generated in the FFL mode by the magnet arrangement according to the invention (not shown), and a cross section through a hyperthermia coil.

(17) FIG. 17 shows a perspective illustration of a magnet arrangement according to the invention which has a Maxwell electromagnet system and a quadrupole electromagnet system with two quadrupole electromagnets which are rotated with respect to one another through 45°, and a field-free line that was generated by the magnet arrangement and rotated in the XY-plane.

DETAILED DESCRIPTION

(18) FIG. 1 shows a magnet arrangement with a Maxwell magnet system MM with two axially magnetized ring magnets R1, R2. The ring magnets R1, R2 are arranged coaxially in relation axis Z and at a distance from one another along this axis Z. To excite magnetizable particles, a one-part drive coil DF is located around a sample volume PV. This drive coil DF can also serve to detect a magnetization response generated within the scope of an MPI measurement. The Maxwell magnet system MM generates a rotationally symmetric Maxwell gradient field of gradient strength M with a field-free point FFP in the sample volume PV.

(19) FIG. 2 likewise shows a magnet arrangement with a Maxwell magnet system MM and a drive coil DF for generating a field-free point FFP. In contrast to the magnet arrangement shown in FIG. 1, the magnetic rings R1, R2 of the Maxwell magnet system MM are radially magnetized in FIG. 2. The drive coil DF is in two parts and situated axially outside of the sample volume PV. This facilitates the central arrangement of further components in or around the sample volume PV.

(20) FIG. 3 and FIG. 4 show magnet arrangements according to the invention. According to the invention, a quadrupole magnet system QM is provided in addition to the Maxwell magnet system MM. The gradient B1 of the selection magnetic field according to the invention is therefore composed of a quadrupole gradient field G.sub.Quadrupole and a Maxwell gradient field G.sub.Maxwell. The superposition according to the invention of the Maxwell gradient field G.sub.Maxwell and the quadrupole gradient field G.sub.Quadrupole allows the generation of a field-free region, the shape of which deviates from the punctiform shape of the field-free point FFP, which is generated by the Maxwell magnet system MM on its own. In exemplary fashion, FIG. 3 and FIG. 4 show a field-free region in the form of a field-free line FFL, the elongate extent of which extends in the Y-direction.

(21) The Maxwell magnet system (Maxwell permanent magnet system or Maxwell coil magnet system) comprises two identical, spaced apart, axially or radially magnetized magnetic rings R1, R2, has a maximum gradient strength M and generates a Maxwell gradient field with a static field-free point FFP, as shown in FIG. 1 and FIG. 2. The Maxwell gradient field generated can be represented by a gradient matrix G:

(22) $\mathrm{GMM}=\left[\begin{array}{ccc}-\frac{M}{2}& 0& 0\\ 0& -\frac{M}{2}& 0\\ 0& 0& M\end{array}\right].$

(23) In the embodiments shown in FIG. 3 and FIG. 4, the quadrupole magnet system QM comprises the ring magnet R3, which is arranged coaxially between the magnetic rings R1, R2 of the Maxwell magnet system MM and which is magnetized such that it generates a quadrupole gradient field in the sample volume PV. The Z-axis forms the axis of symmetry, both of the Maxwell magnet system MM and of the quadrupole magnet system QM. The quadrupole magnet system QM is arranged coaxially symmetrically between the magnetized magnetic rings R1, R2 of the Maxwell magnet system MM (which are axially magnetized in FIG. 3 and radially magnetized in FIG. 4).

(24) With the addition (in the axis of symmetry of the Maxwell magnet system MM) of the quadrupole ring R3 which generates a quadrupole gradient field G.sub.Quadrupole with the quadrupole gradient strength Q=−M/2 and the gradient matrix

(25) $G_{\mathrm{Quadrupole}}=\left[\begin{array}{ccc}-Q& 0& 0\\ 0& Q& 0\\ 0& 0& 0\end{array}\right],$
the field-free point FFP from FIG. 1 and FIG. 2 generated by the Maxwell magnet system MM degenerates to an axially true field-free line FFL (see FIG. 3 and FIG. 4) with the gradient matrix

(26) $G=\left[\begin{array}{ccc}-M& 0& 0\\ 0& 0& 0\\ 0& 0& M\end{array}\right]$
(along FFL extent in the Y-direction).

(27) The quadrupole ring R3 preferably is a quadrupole permanent magnet system in Halbach dipole configuration (wherein, preferably, the parameter required to calculate the magnetization is k=3) or a quadrupole coil magnet system.

(28) To be able to record FFL projections over the entire angular range β=0 . . . 180° in the XY-plane, the quadrupole magnet system QM is rotatably mounted such that it can rotate about its axis of symmetry Z. A rotation of the quadrupole magnet system QM about the axis Z through an angle β causes rotation of the field-free line FFL through the angle β in the XY-plane, as illustrated in FIG. 5. The field-free line FFL is now aligned in the Y′-direction (rotating coordinate system X′Y′Z′).

(29) The quadrupole magnet system QM can have a plurality of quadrupole rings R3a, R3b, which can be rotated relative to one another. This allows the quadrupole strength Q of the quadrupole gradient field to be varied. FIGS. 6-9 show those magnet arrangements with a quadrupole magnet system QM which each have two quadrupole rings R3a, R3b, with each quadrupole R3a, R3b generating a partial quadrupole field. In the examples shown, the quadrupole rings R3a, R3b are arranged concentrically in the mirror plane XY and around the isocentre of the Maxwell magnet system MM with its two ring magnets R1, R2.

(30) In FIG. 6-7, the assumption is made that the partial quadrupole fields of the quadrupole rings R3a, R3b are of equal strength at the centre.

(31) In FIG. 6, the quadrupole rings R3a, R3b are aligned such that their partial quadrupole fields superpose constructively and consequently generate a maximum quadrupole gradient field G.sub.Quadrupole with a gradient strength Q. If the condition Q=M/2 is satisfied, a selection magnetic field with a field-free line in the XY-plane arises (FFL mode).

(32) If the quadrupole rings R3, R3 are rotated through α=90° in relation to the arrangement shown in FIG. 6, as shown in FIG. 7, the two partial quadrupole fields of the two quadrupole rings R3a, R3b are aligned counter to one another and cancel one another (Q=0). In this case, a selection magnetic field with a field-free point FFP arises, corresponding to the Maxwell gradient field G.sub.Maxwell with a maximum gradient strength M generated by the Maxwell magnet system MM (FFP mode).

(33) If the two quadrupole rings R3a, R3b are rotated against one another through an angle 0<α<90, the partial quadrupole fields are only partly cancelled (0<Q<M/2) and a selection magnetic field with a field-free region FFR in the form of an ellipsoid arises, the elongate extent of which is rotated through the angle γ<α in relation to the Y-direction, as shown in FIG. 8. The quadrupole strength Q can be set continuously by way of the relative rotation of the two quadrupole rings R3a, R3b relative to one another.

(34) FIG. 9 shows a field-free line FFL that is tilted through the angle γ in relation to the Y-axis. This is achieved by virtue of, firstly, the quadrupole rings R3a, R3b being rotated against one another through the angle α and, at the same time, the Maxwell gradient strength M of the Maxwell magnet system MM being reduced accordingly such that the condition Q=M/2 is satisfied.

(35) What is decisive for the shape of the field-free region is the ratio of the gradient strengths of the gradient fields generated by the Maxwell magnet system and the quadrupole magnet system.

(36) To change the gradient strength in the FFP mode, it is possible to vary the spacing of the two ring magnets R1, R2 in the case of a Maxwell magnet system with permanent magnets or to vary the current density in the case of a Maxwell magnet system with magnetic coils. FIG. 10 and FIG. 11 show such a variation.

(37) FIG. 10 shows a magnet arrangement with a Maxwell magnet system MM, in which the ring magnets R1, R2 are arranged axially within the sample volume PV at a distance D1 from the isocentre of the magnet arrangement while in FIG. 11 the ring magnets R1, R2 are arranged at a maximum distance from one another (at the edge of the sample volume PV) at a distance D2 from the isocentre of the magnet arrangement. The change in the distance of the ring magnets R1, R2 from the centre of the magnet arrangement changes the resultant Maxwell gradient strength M of the Maxwell magnet system MM.

(38) A further option for changing the Maxwell gradient strength M is shown in FIG. 12. In this case, soft magnetic cylinder structures K1, K2 are arranged around the ring magnets R1, R2. Depending on the axial positioning (at a distance of KD1 from the isocentre of the magnet arrangement in FIG. 12) of the soft magnetic cylinder structures K1, K2, the Maxwell gradient strength M of the Maxwell magnet system MM can be influenced to a greater or lesser extent.

(39) To change the Maxwell gradient strength in the FFL mode, the spacing of the two ring magnets R1, R2 can be varied in the case of a Maxwell magnet system MM with permanent magnets as described above or the current density can be varied in the case of a Maxwell magnet system MM with magnetic coils, wherein the requirement Q=−M/2 must be observed. In order to observe the requirement Q=−M/2, the quadrupole gradient strength Q can be realized by the above-described rotation of the two concentrically radially nested quadrupole rings R3a, R3b in the case of a quadrupole permanent magnet system or by an appropriate adaptation of the current density in the case of a quadrupole coil magnet system.

(40) In the case of 0<Q<M/2, the arising field-free region FFR is a flattened ellipsoid with a long extent in the Y′-direction, medium extent in the X′-direction and minimal extent in the Z-direction.

(41) A field-free line FFL in the z-direction arises in the case M=0, Q≠0.

(42) To be able to record projections of the field-free region over the entire field of view at each angle β=0 . . . 180°, the field-free region (to the extent it has an elongate extent) must be displaced orthogonally to its long extent in the XY-plane. This can be realized by focus field coil arrangement FF.

(43) FIG. 13 shows an embodiment in which the focus field coil arrangement FF comprises a focus field coil pair FF-X′a/FF-X′b that is rotatable about the Z-axis. The focus field coil pair FF-X′a/FF-X′b generates a focus field in the X′-direction. This brings about a displacement, for example of a field-free line FFL in the X′-direction. The focus field coil pair FF-X′a/FF-X′b can preferably be mechanically coupled to the quadrupole magnet system such that these can be rotated together. This ensures that the displacement by the focus field coil arrangement FF is implemented perpendicular to the elongate extent of the field-free region (the field-free line FFL in this case).

(44) As an alternative to a rotatable focus field coil pair FF-X′a/FF-X′b, the focus field coil arrangement FF may also have a plurality of stationary focus field coil pairs FF-Xa/FF-Xb, FF-Ya/FF-Yb, as shown in FIG. 14. By the superposition of the field components in the X- and Y-direction generated by the two focus field coil pairs FF-Xa/FF-Xb, FF-Ya/FF-Yb, it is possible to generate a resultant magnetic field in the X′-direction. By way of an appropriate control of the coil currents of the focus field coil pairs FF-Xa/FF-Xb, FF-Ya/FF-Yb, it is consequently possible to displace the field-free line from its rest position in the centre in the X′-direction.

(45) Thus, the shape and the alignment of the field-free region can be influenced by the magnet arrangement according to the invention. In particular, it is possible to alternate between an FFL mode, in which a field-free line FFL is generated, and an FFP mode, in which a field-free point is generated.

(46) By means of the magnet arrangement according to the invention it is possible to easily alternate between the FFP mode and the FFL mode. To obtain the FFP mode, the quadrupole magnet system QM must be deactivatable. This can be realized by deactivation of the current in the case of an electromagnet or by an appropriate rotation of the two concentrically radially nested quadrupole rings R3a, R3b in the case of a permanent magnet quadrupole, as shown in FIG. 7.

(47) Alternating between FFP mode and FFL mode facilitates in particular the use of the magnet arrangement according to the invention in different fields of use. Thus, the magnet arrangement according to the invention can be used, for example, both for MPI measurements and for hyperthermia applications.

(48) To excite the magnetizable particles for imaging (e.g., MPI) in the FFL mode, a sufficient amplitude (|B|>5 mT) and sufficient frequency (10 kHz<f.sub.DF<200 kHz) or appropriately high dB/dt must be applied to at least one field component (Bx′, By′, Bz). This is implemented by the drive coil DF in the case of imaging and by the hyperthermia coil in the case of hyperthermia. In the process, the field-free line FFL is moved from its rest position. To generate particle signal only on a field-free line FFL, a sufficient amplitude (|B|>5 mT) and sufficient frequency (10 kHz<fly<200 kHz) or an appropriately high dB/dt must be applied to a field component By′ [rotating coordinate system]. In this case, the field-free line FFL remains static in its position (“needle excitation”), i.e., without location smearing by the drive field; only the isoline (MBI) of the field-free line FFL varies its diameter and hence the field-free line FFL pulses.

(49) To measure the particle response, at least one reception coil system must be designed with the sensitivity of at least one field component (Bx′, By′, Bz) (ideally the same field component as the excitation field component). By way of example, the drive coil DF (transreceive process) or dedicated receiver coil (receive-only process) can be used as receiver coil system.

(50) In the case of a magnetic fluid hyperthermia (MFH) application, an energy influx (energy deposition) in the form of a development of heat should be implemented by losses in the re-magnetization of the hyperthermia particles in a target zone Z1. Using a selection magnetic field allows the hyperthermia volume to be restricted by virtue of the hyperthermia particles outside of the field-free region FFR remaining in saturation, with the hyperthermia particles in the region swept by the field-free region FFR and hyperthermia excitation experiencing a significant change in magnetization. Spatial encoding of the MFH by way of a static selection magnetic field can be used to keep the energy influx by MFH local. However, the sample volume PV may simultaneously also contain regions where there should be no energy deposition where possible (protection zone Z2) but which may contain hyperthermia particles. To excite the magnetizable particles for the hyperthermia, a sufficient amplitude (|B|>3 mT) and sufficient frequency (100 kHz<f.sub.hyper<10 MHz) or appropriately high dB/dt must be applied to at least one field component (Bx, By, Bz).

(51) FIG. 15 and FIG. 16 show two different options for implementing a hyperthermia application.

(52) An excitation in the FFP mode with a hyperthermia excitation with field component Bz is advantageous since the focal point of the hyperthermia hence is minimized (excitation along the maximum gradient strength). To this end, the quadrupole magnet system QM is adjusted such that the partial quadrupole fields cancel (Q=0) and a field-free point FFP is generated as field-free region, as described above with reference to FIG. 7 in the case of permanent magnets. By using the focus field coil arrangement FF (either rotatably mounted coil pair which generates a homogeneous field in the Bx′ field direction in the rotating coordinate system or two orthogonal securely installed coil pairs, each of which generate a homogeneous field with Bx and By), it is possible to displace the field-free point FFP, and hence the hyperthermia volume, in the XY-plane, as illustrated in FIG. 15.

(53) However, it is also possible to use the magnet arrangement according to the invention in the FFL mode for hyperthermia applications. To this end, a field-free line FFL is generated by means of the Maxwell magnet system MM and the quadrupole magnet system QM. In this case, all particles on the entire field-free line FFL are excited. Consequently, this method is only local to a restricted extent. The field-free line is displaced into the target zone Z1 by means of the focus field coil arrangement FF. The energy dose at the point of rotation of the FFL can be maximized when rotating the field-free line FFL (by rotating the quadrupole magnet system QM) during the hyperthermia excitation and an appropriate adaptation of the field amplitude of the focus field coil arrangement FF (either rotatably mounted coil pair which generates a homogeneous field in the Bx′ field direction in the rotating coordinate system or two orthogonal securely installed coil pairs, each of which generate a homogeneous field with Bx and By) (FIG. 16). It is possible to define an angular range which excludes the protection zone Z2 in order to protect the regions contained in the protection zone Z2 from even a brief energy uptake as a result of hyperthermia particles possibly situated therein. Then, the field-free line FFL might only be moved within the defined angular range or there is no hyperthermia excitation of the magnetizable nanoparticles outside of the set angular range (hyperthermia coil is deactivated during the period of time in which the field-free line FFL intersects the protection zone Z2).

(54) In general, it is possible to design the drive coil for generating the MPI drive field in such a way that it can satisfy the above-described functionality of the hyperthermia coil. However, since the frequency for the drive excitation is found in the two-digit kHz range whereas that of the hyperthermia excitation is found in the three-digit kHz range, this would require the use of a double-resonant coil in order to be able to unify both functions. From a technical point of view, it is therefore easier for the two functions to be realized by a separate coil in each case.

(55) Using the magnet arrangement according to the invention it is possible to easily alternate between an axis-true FFP mode and an axis-true FFL mode without unwanted influences on the field gradient. The adjustment of the resultant gradient strength is also facilitated both for a permanent magnet design and for an electromagnet design. Moreover, differently shaped field-free regions are also realizable. This facilitates the use of the magnet arrangement for different applications.

(56) FIG. 17 shows an embodiment of the magnet arrangement according to the invention in which the Maxwell magnet system MM is embodied as a Maxwell electromagnetic coil system and the quadrupole magnet system QM is embodied as a quadrupole electromagnetic coil system with a quadrupole ring (in particular, electromagnetic coils attached in ring-shaped fashion), wherein the quadrupole ring comprises two quadrupole electromagnets Q1, Q2 that are rotated through 45° with respect to one another. The quadrupole magnetic field can be formed by four electromagnetic coils attached in ring-shaped fashion. The field-free line FFL can be rotated electrically if use is made of two independent quadrupole electromagnets (i.e., eight coils), which are rotated through 45° about the Z-axis.

(57) This embodiment of the magnet arrangement according to the invention allows adapting the Maxwell gradient strength M and the quadrupole gradient strength Q merely by way of an appropriate control of the respective electromagnets and a change in the resultant current density. Likewise, this magnet arrangement facilitates a rotation of the field-free region FFR, in particular the field-free line FFL, by way of an appropriate control of the two quadrupole electromagnets Q1, Q2 which are rotated relative to one another. Particularly in combination with stationary focus field coils that are attached orthogonal to one another (in a manner analogous to FIG. 14), the magnetic field arrangement shown in FIG. 17 facilitates rotation of the field-free line FFL without mechanical rotation of the electromagnets Q1, Q2. Hence, this embodiment potentially facilitates higher frame repetition rates since higher FFL rotational speeds that are higher in comparison with mechanical rotations can be achieved in electromagnetic fashion.