CABLING ARRANGMENT, COIL APPARATUS AND APPARATUS FOR INFLUENCING AND/OR DETECTING MAGNETIC PARTICLES

Abstract

The present invention relates to a cabling arrangement and a coil apparatus for use in an MPI apparatus to increase the sensitivity of the apparatus. The cabling arrangement comprises a first AC terminal (320), a second AC terminal (330), wherein said first and second AC terminals are configured for coupling an AC voltage between them, a first internal terminal (301), a second internal terminal (302), wherein said first and second internal terminals are configured for coupling a DC voltage between them, a first subset of one or more first conductors (401) coupled between the first internal terminal and the second AC terminal, and a second subset of one or more second conductors (402) coupled between the second internal terminal and the second AC terminal, wherein said first and second conductors are arranged to form a coil (125, 126, 127; 601, 602, 603, 604) to generate a magnetic field in a zone of interest.

Claims

1. A cabling arrangement for generating a magnetic field, comprising: a first AC terminal; a second AC terminal, wherein the first and second AC terminals are arranged to couple an AC voltage between the first AC terminal and the second AC terminal; a first internal terminal; a second internal terminal, wherein the first and second internal terminals are arranged to couple a DC voltage between the first internal terminal and the second internal terminal; a first subset of one or more first conductors, wherein the first subset is coupled between the first internal terminal and the second AC terminal; and a second subset of one or more second conductors, wherein the second subset is coupled between the second internal terminal and the second AC terminal, wherein the cabling arrangement enables the superposition of the AC and DC voltages in the first and second conductors, wherein the said first and second conductors are arranged to form a coil wherein the coil is arranged to generate a magnetic field in a zone of interest.

2. The cabling arrangement as claimed in claim 1, further comprising coupling elements coupled between the first AC terminal and the first internal terminal and between the first AC terminal and the second internal terminal, wherein the said coupling elements comprise: at least a first capacitor coupled between the first AC terminal and the first internal terminal, and at least a second capacitor coupled between the first AC terminal and the second internal terminal.

3. The cabling arrangement as claimed in claim 1, wherein the first AC terminal is coupled to the first and the second internal terminal.

4. The cabling arrangement as claimed in claim 1, further comprising a first capacitive arrangement coupled between the first AC terminal and the first and second internal terminals; and a second capacitive arrangement coupled between the second AC terminal and the first and second sets of conductors.

5. The cabling arrangement as claimed in claim 1, further comprising: a first inductor coupled to the first internal terminal such that the first inductor and the first subset of conductors are located on either side of the first internal terminal; and a second inductor coupled to the second internal terminal such that the second inductor and the second subset of conductors are located on either side of the second internal terminal, wherein the cabling arrangement is further configured for coupling the DC voltage to the first inductor and to the second inductor.

6. The cabling arrangement as claimed in claim 1, wherein the first subset comprises two or more first conductors and the second subset comprises two or more second conductors, wherein the first and second conductors are mechanically arranged substantially in parallel forming a cable, wherein the cable is wound as a coil.

7. The cabling arrangement as claimed in claim 6, wherein all first conductors are mechanically arranged adjacent to each other and all second conductors are mechanically arranged adjacent to each other.

8. The cabling arrangement as claimed in claim 6, wherein the first conductors and the second conductors are mechanically arranged alternately.

9. The cabling arrangement as claimed in claim 6, further comprising: at least a third internal terminal; and at least three coupling terminals coupled between the first and second subsets and the second AC terminal, wherein each of first ends of the first conductors of the first subset are coupled to different internal terminals and each of second ends of the first conductors of the first subset are coupled to different coupling terminals, wherein each of first ends of the second conductors of the second subset are coupled to different internal terminals and each of second ends of the second conductors of the second subset are coupled to different coupling terminals.

10. The cabling arrangement as claimed in claim 9, further comprising: a series coupling of one or more capacitors coupled between each internal terminal and the first AC terminal; and/or a series coupling of one or more capacitors coupled between each coupling terminal and the second AC terminal.

11. The cabling arrangement as claimed in claim 1, wherein the first subset comprises a single first conductor wound as a first coil and the second subset comprises a single second conductor wound as a second coil, wherein the first coil and the second coil are arranged coaxially.

12. A coil apparatus, comprising: a cabling arrangement as claimed in claim 1; and a DC voltage or a DC current source coupled between the first and second internal terminals via switches .

13. An apparatus for influencing and/or detecting magnetic particles in a field of view, which apparatus comprises: a selection apparatus comprising: a selection field signal generator circuit; and selection field elements; wherein the selection field generator circuit and the selection field elements are arranged to generate a magnetic selection field, wherein the magnetic selection field has a pattern in space of its magnetic field strength such that a first sub-zone has a low magnetic field strength where the magnetization of the magnetic particles is not saturated and a second sub-zone has a higher magnetic field strength where the magnetization of the magnetic particles is saturated are formed in the field of view, a drive apparatus comprising: a drive field signal generator circuit; and drive field, wherein the drive field coils are arranged to change the position in space of the two sub-zones in the field of view using a magnetic drive field, wherein the magnetic field is arranged so that the magnetization of the magnetic material changes locally, and one or more coil apparatuses as claimed in claim 12 forming the drive field coils, wherein the drive field signal generator circuit is coupled to the AC terminals of the one or more cabling arrangements of the one or more coil apparatuses.

14. The apparatus as claimed in claim 13, further comprising a drive apparatus and a receiving apparatus, the apparatus comprising: the drive field signal generator unit; a signal receiving unit; and a drive-receiving coil, wherein the drive-receiving coil are configured for changing the position in space of the two sub-zones in the field of view by means of a magnetic drive field so that the magnetization of the magnetic material changes locally and for acquiring detection signals, wherein the detection signals depend on the magnetization in the field of view, wherein the magnetization is influenced by the change in the position in space of the first and second sub-zone, wherein the one or more coil apparatuses form the drive-receiving coil.

15. A method for influencing and/or detecting magnetic particles in a field of view, which method comprises: generating a magnetic selection field, the magnetic selection field having a pattern in space of its magnetic field strength such that a first sub-zone has a low magnetic field strength where the magnetization of the magnetic particles is not saturated and a second sub-zone has a higher magnetic field strength where the magnetization of the magnetic particles is saturated are formed in the field of view, changing the position in space of the two sub-zones in the field of view using a magnetic drive field so that the magnetization of the magnetic material changes locally, and forming the drive field coils using one or more coil apparatuses as claimed in claim 12, applying DC currents to drive field coils formed by one or more cabling arrangements as claimed in claim 1 and used to generate the magnetic drive field, the DC currents being configured to bring magnetic contaminations of the first and second conductors of the one or more cabling arrangements into saturation and to generate no additional static magnetic field or only in the first sub-zone.

16. The cabling arrangement as claimed in claim 1, further comprising: a first inductor coupled to the first internal terminal such that the first inductor and the first subset of conductors are located on either side of the first internal terminal; wherein the cabling arrangement is further configured for coupling the DC voltage to the first inductor and to the second inductor.

17. The cabling arrangement as claimed in claim 1, further comprising: a second inductor coupled to the second internal terminal such that the second inductor and the second subset of conductors are located on either side of the second internal terminal, wherein the cabling arrangement is further configured for coupling the DC voltage to the first inductor and to the second inductor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings

[0043] FIG. 1 shows a first embodiment of an MPI apparatus,

[0044] FIG. 2 shows an example of the selection field pattern produced by an apparatus as shown in FIG. 1,

[0045] FIG. 3 shows a second embodiment of an MPI apparatus,

[0046] FIG. 4 shows a third and a fourth embodiment of an MPI apparatus,

[0047] FIG. 5 shows a block diagram of an MPI apparatus according to the present invention,

[0048] FIG. 6 shows a circuit diagram of a first embodiment of a coil apparatus according to the present invention,

[0049] FIG. 7 shows a circuit diagram of a second embodiment of a coil apparatus according to the present invention,

[0050] FIG. 8 shows a circuit diagram of a third embodiment of a coil apparatus according to the present invention,

[0051] FIG. 9 shows a circuit diagram of an embodiment of sets of capacitors for used in a coil apparatus according to the present invention,

[0052] FIG. 10 shows various implementations of the arrangement of the conductors in a cable according to the present invention,

[0053] FIG. 11 shows a circuit diagram of a fourth embodiment of a coil apparatus according to the present invention,

[0054] FIG. 12 shows an arrangement of saddle coils using a cabling arrangement according to the present invention, and

[0055] FIG. 13 shows an arrangement of split solenoid coils using a cabling arrangement according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0056] Before the details of the present invention shall be explained, basics of magnetic particle imaging shall be explained in detail with reference to FIGS. 1 to 4. In particular, four embodiments of an MPI scanner for medical diagnostics will be described. An informal description of the data acquisition will also be given. The similarities and differences between the different embodiments will be pointed out. Generally, the present invention can be used in all these different embodiments of an MPI apparatus.

[0057] The first embodiment 10 of an MPI scanner shown in FIG. 1 has three pairs 12, 14, 16 of coaxial parallel circular coils, these coil pairs being arranged as illustrated in FIG. 1. These coil pairs 12, 14, 16 serve to generate the selection field as well as the drive and focus fields. The axes 18, 20, 22 of the three coil pairs 12, 14, 16 are mutually orthogonal and meet in a single point, designated the isocenter 24 of the MPI scanner 10. In addition, these axes 18, 20, 22 serve as the axes of a 3D Cartesian x-y-z coordinate system attached to the isocenter 24. The vertical axis 20 is nominated the y-axis, so that the x- and z-axes are horizontal. The coil pairs 12, 14, 16 are named after their axes. For example, the y-coil pair 14 is formed by the coils at the top and the bottom of the scanner. Moreover, the coil with the positive (negative) y-coordinate is called the y.sup.+-coil (y.sup.−-coil), and similarly for the remaining coils. When more convenient, the coordinate axes and the coils shall be labelled with x.sub.1, x.sub.2, and x.sub.3, rather than with x, y, and z.

[0058] The scanner 10 can be set to direct a predetermined, time-dependent electric current through each of these coils 12, 14, 16, and in either direction. If the current flows clockwise around a coil when seen along this coil's axis, it will be taken as positive, otherwise as negative. To generate the static selection field, a constant positive current I.sup.s is made to flow through the z.sup.+-coil, and the current −I.sup.s is made to flow through the z.sup.−-coil. The z-coil pair 16 then acts as an anti-parallel circular coil pair.

[0059] It should be noted here that the arrangement of the axes and the nomenclature given to the axes in this embodiment is just an example and might also be different in other embodiments. For instance, in practical embodiments the vertical axis is often considered as the z-axis rather than the y-axis as in the present embodiment. This, however, does not generally change the function and operation of the device and the effect of the present invention.

[0060] The magnetic selection field, which is generally a magnetic gradient field, is represented in FIG. 2 by the field lines 50. It has a substantially constant gradient in the direction of the (e.g. horizontal) z-axis 22 of the z-coil pair 16 generating the selection field and reaches the value zero in the isocenter 24 on this axis 22. Starting from this field-free point (not individually shown in FIG. 2), the field strength of the magnetic selection field 50 increases in all three spatial directions as the distance increases from the field-free point. In a first sub-zone or region 52 which is denoted by a dashed line around the isocenter 24 the field strength is so small that the magnetization of particles present in that first sub-zone 52 is not saturated, whereas the magnetization of particles present in a second sub-zone 54 (outside the region 52) is in a state of saturation. In the second sub-zone 54 (i.e. in the residual part of the scanner's field of view 28 outside of the first sub-zone 52) the magnetic field strength of the selection field is sufficiently strong to keep the magnetic particles in a state of saturation.

[0061] By changing the position of the two sub-zones 52, 54 (including the field-free point) within the field of view 28 the (overall) magnetization in the field of view 28 changes. By determining the magnetization in the field of view 28 or physical parameters influenced by the magnetization, information about the spatial distribution of the magnetic particles in the field of view 28 can be obtained. In order to change the relative spatial position of the two sub-zones 52, 54 (including the field-free point) in the field of view 28, further magnetic fields, i.e. the magnetic drive field, and, if applicable, the magnetic focus field, are superposed to the selection field 50.

[0062] To generate the drive field, a time dependent current I.sup.D.sub.1 is made to flow through both x-coils 12, a time dependent current I.sup.D.sub.2 through both y-coils 14, and a time dependent current I.sup.D.sub.3 through both z-coils 16. Thus, each of the three coil pairs acts as a parallel circular coil pair. Similarly, to generate the focus field, a time dependent current I.sup.F.sub.1 is made to flow through both x-coils 12, a current IF.sub.2 through both y-coils 14, and a current I.sup.F.sub.3 through both z-coils 16.

[0063] It should be noted that the z-coil pair 16 is special: It generates not only its share of the drive and focus fields, but also the selection field (of course, in other embodiments, separate coils may be provided). The current flowing through the z.sup.±-coil is I.sup.D.sub.3+I.sup.F.sub.3±I.sup.S. The current flowing through the remaining two coil pairs 12, 14 is I.sup.D.sub.k+I.sup.F.sub.k, k=1, 2. Because of their geometry and symmetry, the three coil pairs 12, 14, 16 are well decoupled. This is wanted.

[0064] Being generated by an anti-parallel circular coil pair, the selection field is rotationally symmetric about the z-axis, and its z-component is nearly linear in z and independent of x and y in a sizeable volume around the isocenter 24. In particular, the selection field has a single field-free point (FFP) at the isocenter. In contrast, the contributions to the drive and focus fields, which are generated by parallel circular coil pairs, are spatially nearly homogeneous in a sizeable volume around the isocenter 24 and parallel to the axis of the respective coil pair. The drive and focus fields jointly generated by all three parallel circular coil pairs are spatially nearly homogeneous and can be given any direction and strength, up to some maximum strength. The drive and focus fields are also time-dependent. The difference between the focus field and the drive field is that the focus field varies slowly in time and may have a large amplitude, while the drive field varies rapidly and has a small amplitude. There are physical and biomedical reasons to treat these fields differently. A rapidly varying field with a large amplitude would be difficult to generate and potentially hazardous to a patient.

[0065] In a practical embodiment the FFP can be considered as a mathematical point, at which the magnetic field is assumed to be zero. The magnetic field strength increases with increasing distance from the FFP, wherein the increase rate might be different for different directions (depending e.g. on the particular layout of the device). As long as the magnetic field strength is below the field strength required for bringing magnetic particles into the state of saturation, the particle actively contributes to the signal generation of the signal measured by the device; otherwise, the particles are saturated and do not generate any signal.

[0066] The embodiment 10 of the MPI scanner has at least one further pair, preferably three further pairs, of parallel circular coils, again oriented along the x-, y-, and z-axes. These coil pairs, which are not shown in FIG. 1, serve as receive coils. As with the coil pairs 12, 14, 16 for the drive and focus fields, the magnetic field generated by a constant current flowing through one of these receive coil pairs is spatially nearly homogeneous within the field of view and parallel to the axis of the respective coil pair. The receive coils are supposed to be well decoupled. The time-dependent voltage induced in a receive coil is amplified and sampled by a receiver attached to this coil. More precisely, to cope with the enormous dynamic range of this signal, the receiver samples the difference between the received signal and a reference signal. The transfer function of the receiver is non-zero from zero Hertz (“DC”) up to the frequency where the expected signal level drops below the noise level. Alternatively, the MPI scanner has no dedicated receive coils. Instead the drive field transmit coils may be used as receive coils as is the case according one embodiment according to the present invention using combined drive-receiving coils.

[0067] The embodiment 10 of the MPI scanner shown in FIG. 1 has a cylindrical bore 26 along the z-axis 22, i.e. along the axis of the selection field. All coils are placed outside this bore 26. For the data acquisition, the patient (or object) to be imaged is placed in the bore 26 such that the patient's volume of interest—that volume of the patient (or object) that shall be imaged—is enclosed by the scanner's field of view 28—that volume of the scanner whose contents the scanner can image. The patient (or object) is, for instance, placed on a patient table. The field of view 28 is a geometrically simple, isocentric volume in the interior of the bore 26, such as a cube, a ball, a cylinder or an arbitrary shape. A cubical field of view 28 is illustrated in FIG. 1.

[0068] The size of the first sub-zone 52 is dependent on the strength of the gradient of the magnetic selection field and on the field strength of the magnetic field required for saturation, which in turn depends on the magnetic particles. For a sufficient saturation of typical magnetic particles 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 amounting to 50×10.sup.3 A/m.sup.2, the first sub-zone 52 in which the magnetization of the particles is not saturated has dimensions of about 1 mm (in the given space direction).

[0069] The patient's volume of interest is supposed to contain magnetic nanoparticles. Prior to the diagnostic imaging of, for example, a tumor, the magnetic particles are brought to the volume of interest, e.g. by means of a liquid comprising the magnetic particles which is injected into the body of the patient (object) or otherwise administered, e.g. orally, to the patient.

[0070] Generally, various ways for bringing the magnetic particles into the field of view exist. In particular, in case of a patient into whose body the magnetic particles are to be introduced, the magnetic particles can be administered by use of surgical and non-surgical methods, and there are both methods which require an expert (like a medical practitioner) and methods which do not require an expert, e.g. can be carried out by laypersons or persons of ordinary skill or the patient himself/herself. Among the surgical methods there are potentially non-risky and/or safe routine interventions, e.g. involving an invasive step like an injection of a tracer into a blood vessel (if such an injection is at all to be considered as a surgical method), i.e. interventions which do not require considerable professional medical expertise to be carried out and which do not involve serious health risks. Further, non-surgical methods like swallowing or inhalation can be applied.

[0071] Generally, the magnetic particles are pre-delivered or pre-administered before the actual steps of data acquisition are carried out. In embodiments, it is, however, also possible that further magnetic particles are delivered/administered into the field of view.

[0072] An embodiment of magnetic particles comprises, for example, a spherical substrate, for example, of glass which is provided with a soft-magnetic layer 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 which protects the particle against chemically and/or physically aggressive environments, e.g. acids. The magnetic field strength of the magnetic selection field 50 required for the saturation of the magnetization of such particles is dependent on various parameters, e.g. the diameter of the particles, the used magnetic material for the magnetic layer and other parameters.

[0073] In the case of e.g. a diameter of 10 μm with such magnetic particles, 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 of a material having a lower saturation magnetization is chosen or when the thickness of the layer is reduced.

[0074] In practice, magnetic particles commercially available under the trade name Resovist (or similar magnetic particles) are often used, which have a core of magnetic material or are formed as a massive sphere and which have a diameter in the range of nanometers, e.g. 40 or 60 nm.

[0075] For further details of the generally usable magnetic particles and particle compositions, the corresponding parts of EP 1224542, WO 2004/091386, WO 2004/091390, WO 2004/091394, WO 2004/091395, WO 2004/091396, WO 2004/091397, WO 2004/091398, WO 2004/091408 are herewith referred to, which are herein incorporated by reference. In these documents more details of the MPI method in general can be found as well.

[0076] During the data acquisition, the x-, y-, and z-coil pairs 12, 14, 16 generate a position- and time-dependent magnetic field, the applied field. This is achieved by directing suitable currents through the field generating coils. In effect, the drive and focus fields push the selection field around such that the FFP moves along a preselected FFP trajectory that traces out the volume of scanning—a superset of the field of view. The applied field orientates the magnetic nanoparticles in the patient. As the applied field changes, the resulting magnetization changes too, though it responds nonlinearly to the applied field. The sum of the changing applied field and the changing magnetization induces a time-dependent voltage V.sub.k across the terminals of the receive coil pair along the x.sub.k-axis. The associated receiver converts this voltage to a signal S.sub.k, which it processes further.

[0077] Like the first embodiment 10 shown in FIG. 1, the second embodiment 30 of the MPI scanner shown in FIG. 3 has three circular and mutually orthogonal coil pairs 32, 34, 36, but these coil pairs 32, 34, 36 generate the selection field and the focus field only. The z-coils 36, which again generate the selection field, are filled with ferromagnetic material 37. The z-axis 42 of this embodiment 30 is oriented vertically, while the x- and y-axes 38, 40 are oriented horizontally. The bore 46 of the scanner is parallel to the x-axis 38 and, thus, perpendicular to the axis 42 of the selection field. The drive field is generated by a solenoid (not shown) along the x-axis 38 and by pairs of saddle coils (not shown) along the two remaining axes 40, 42. These coils are wound around a tube which forms the bore. The drive field coils also serve as receive coils.

[0078] To give a few typical parameters of such an embodiment: The z-gradient of the selection field, G, has a strength of G/μ.sub.0=2.5 T/m, where μ.sub.0 is the vacuum permeability. The temporal frequency spectrum of the drive field is concentrated in a narrow band around 25 kHz (up to approximately 250 kHz). The useful frequency spectrum of the received signals lies between 50 kHz and 1 MHz (eventually up to approximately 15 MHz). The bore has a diameter of 120 mm. The biggest cube 28 that fits into the bore 46 has an edge length of 120 mm/√2≈84 mm.

[0079] Since the construction of field generating coils is generally known in the art, e.g. from the field of magnetic resonance imaging, this subject need not be further elaborated herein.

[0080] In an alternative embodiment for the generation of the selection field, permanent magnets (not shown) can be used. In the space between two poles of such (opposing) permanent magnets (not shown) there is formed a magnetic field which is similar to that shown in FIG. 2, that is, when the opposing poles have the same polarity. In another alternative embodiment, the selection field can be generated by a mixture of at least one permanent magnet and at least one coil.

[0081] FIG. 4 shows two embodiments of the general outer layout of an MPI apparatus 200, 300. FIG. 4A shows an embodiment of the proposed MPI apparatus 200 comprising two selection-and-focus field coil units 210, 220 which are basically identical and arranged on opposite sides of the examination area 230 formed between them. Further, a drive field coil unit 240 is arranged between the selection-and-focus field coil units 210, 220, which are placed around the area of interest of the patient (not shown). The selection-and-focus field coil units 210, 220 comprise several selection-and-focus field coils for generating a combined magnetic field representing the above-explained magnetic selection field and magnetic focus field. In particular, each selection-and-focus field coil unit 210, 220 comprises a, preferably identical, set of selection-and-focus field coils. Details of said selection-and-focus field coils will be explained below.

[0082] The drive field coil unit 240 comprises a number of drive field coils for generating a magnetic drive field. These drive field coils may comprise several pairs of drive field coils, in particular one pair of drive field coils for generating a magnetic field in each of the three directions in space. In an embodiment the drive field coil unit 240 comprises two pairs of saddle coils for two different directions in space and one solenoid coil for generating a magnetic field in the longitudinal axis of the patient.

[0083] The selection-and-focus field coil units 210, 220 are generally mounted to a holding unit (not shown) or the wall of room. Preferably, in case the selection-and-focus field coil units 210, 220 comprise pole shoes for carrying the respective coils, the holding unit does not only mechanically hold the selection-and-focus field coil unit 210, 220 but also provides a path for the magnetic flux that connects the pole shoes of the two selection-and-focus field coil units 210, 220.

[0084] As shown in FIG. 4a, the two selection-and-focus field coil units 210, 220 each include a shielding layer 211, 221 for shielding the selection-and-focus field coils from magnetic fields generated by the drive field coils of the drive field coil unit 240.

[0085] In the embodiment of the MPI apparatus 201 shown in FIG. 4B only a single selection-and-focus field coil unit 220 is provided as well as the drive field coil unit 240. Generally, a single selection-and-focus field coil unit is sufficient for generating the required combined magnetic selection and focus field. Said single selection-and-focus field coil unit 220 may thus be integrated into a (not shown) patient table on which a patient is placed for the examination. Preferably, the drive field coils of the drive field coil unit 240 may be arranged around the patient's body already in advance, e.g. as flexible coil elements. In another implementation, the drive field coil unit 240 can be opened, e.g. separable into two subunits 241, 242 as indicated by the separation lines 243, 244 shown in FIG. 4b in axial direction, so that the patient can be placed in between and the drive field coil subunits 241, 242 can then be coupled together.

[0086] In still further embodiments of the MPI apparatus, even more selection-and-focus field coil units may be provided which are preferably arranged according to a uniform distribution around the examination area 230. However, the more selection-and-focus field coil units are used, the more will the accessibility of the examination area for placing a patient therein and for accessing the patient itself during an examination by medical assistance or doctors be limited.

[0087] FIG. 5 shows a general block diagram of an MPI apparatus 100 according to the present invention. The general principles of magnetic particle imaging explained above are valid and applicable to this embodiment as well, unless otherwise specified.

[0088] The embodiment of the apparatus 100 shown in FIG. 5 comprises various coils for generating the desired magnetic fields. First, the coils and their functions in MPI shall be explained.

[0089] For generating the combined magnetic selection-and-focus field, selection-and-focus means 110 are provided. The magnetic selection-and-focus field has a pattern in space of its magnetic field strength such that the first sub-zone (52 in FIG. 2) having a low magnetic field strength where the magnetization of the magnetic particles is not saturated and a second sub-zone (54 in FIG. 4) having a higher magnetic field strength where the magnetization of the magnetic particles is saturated are formed in the field of view 28, which is a small part of the examination area 230, which is conventionally achieved by use of the magnetic selection field. Further, by use of the magnetic selection-and-focus field the position in space of the field of view 28 within the examination area 230 can be changed, as conventionally done by use of the magnetic focus field.

[0090] The selection-and-focus means 110 comprises at least one set of selection-and-focus field coils 114 and a selection-and-focus field generator unit 112 for generating selection-and-focus field currents to be provided to said at least one set of selection-and-focus field coils 114 (representing one of the selection-and-focus field coil units 210, 220 shown in FIGS. 4A, 4B) for controlling the generation of said magnetic selection-and-focus field. Preferably, a separate generator subunit is provided for each coil element (or each pair of coil elements) of the at least one set of selection-and-focus field coils 114. Said selection-and-focus field generator unit 112 comprises a controllable current source (generally including an amplifier) and a filter unit which provide the respective coil element with the field current to individually set the gradient strength and field strength of the contribution of each coil to the magnetic selection-and-focus field. It shall be noted that the filter unit 114 can also be omitted. Further, separate focus and selection means are provided in other embodiments.

[0091] For generating the magnetic drive field the apparatus 100 further comprises drive means 120 comprising a drive field signal generator unit 122 and a set of drive field coils 124 (representing the drive coil unit 240 shown in FIGS. 4A, 4B) for changing the position in space and/or size of the two sub-zones in the field of view by means of a magnetic drive field so that the magnetization of the magnetic material changes locally. As mentioned above said drive field coils 124 preferably comprise two pairs 125, 126 of oppositely arranged saddle coils and one solenoid coil 127. Other implementations, e.g. three pairs of coil elements, are also possible.

[0092] The drive field signal generator unit 122 preferably comprises a separate drive field signal generation subunit for each coil element (or at least each pair of coil elements) of said set of drive field coils 124. Said drive field signal generator unit 122 preferably comprises a drive field current source (preferably including a power amplifier) and a filter unit for providing a time-dependent drive field current to the respective drive field coil.

[0093] The selection-and-focus field signal generator unit 112 and the drive field signal generator unit 122 are preferably controlled by a control unit 150, which preferably controls the selection-and-focus field signal generator unit 112 such that the sum of the field strengths and the sum of the gradient strengths of all spatial points of the selection field is set at a predefined level. For this purpose the control unit 150 can also be provided with control instructions by a user according to the desired application of the MPI apparatus, which, however, is preferably omitted according to the present invention.

[0094] For using the MPI apparatus 100 for determining the spatial distribution of the magnetic particles in the examination area (or a region of interest in the examination area), particularly to obtain images of said region of interest, signal detection receiving means, in particular a receiving coil, and a signal receiving unit 140, which receives signals detected by said receiving means, are provided. One to three separate receiving coils 124 are provided in an MPI apparatus as receiving means.

[0095] According to other embodiments of the present invention, however, one to three of said drive field coils 124 (or drive field coil pairs) act (simultaneously or alternately) as receiving coils for receiving detection signals, wherein these drive field coils are then called “drive-receiving coils” herein. The generation of magnetic drive fields and the detection of detection signals may then be performed simultaneously or alternately. Preferably, all three drive-receiving coils (or coil pairs) may then act as receiving coils.

[0096] One to three receiving units 140—one per receiving coil (or coil pair)—are provided in practice, but more than three receiving coils and receiving units can be also used, in which case the acquired detection signals are not 3-dimensional but K-dimensional, with K being the number of receiving coils.

[0097] Said signal receiving unit 140 comprises a filter unit 142 (also called Rx filter) for filtering the received detection signals. The aim of this filtering is to separate measured values, which are caused by the magnetization in the examination area which is influenced by the change in position of the two part-regions (52, 54), from other, interfering signals (in particular crosstalk of the fundamental frequency). To this end, the filter unit 142 may be designed for example such that signals which have temporal frequencies that are smaller than the temporal frequencies with which the drive coil(s) is (are) operated, or smaller than twice these temporal frequencies, do not pass the filter unit 142. The signals are then transmitted via an amplifier unit 144 (also called LNA, Low-Noise-Amplifier) to an analog/digital converter 146 (ADC).

[0098] The digitized signals produced by the analog/digital converter 146 are fed to an image processing unit (also called reconstruction means) 152, which reconstructs the spatial distribution of the magnetic particles from these signals and the respective position which the first part-region 52 of the first magnetic field in the examination area assumed during receipt of the respective signal and which the image processing unit 152 obtains from the control unit 150. The reconstructed spatial distribution of the magnetic particles is finally transmitted via the control means 150 to a computer 154, which displays it on a monitor 156. Thus, an image can be displayed showing the distribution of magnetic particles in the field of view of the examination area.

[0099] In other applications of the MPI apparatus 100, e.g. for influencing the magnetic particles (for instance for a hyperthermia treatment) or for moving the magnetic particles (e.g. attached to a catheter for moving the catheter or attached to a medicament for moving the medicament to a certain location) the receiving means may also be omitted or simply not used.

[0100] Further, an input unit 158 may optionally be provided, for example a keyboard. A user may therefore be able to set the desired direction of the highest resolution and in turn receives the respective image of the region of action on the monitor 156. If the critical direction, in which the highest resolution is needed, deviates from the direction set first by the user, the user can still vary the direction manually in order to produce a further image with an improved imaging resolution. This resolution improvement process can also be operated automatically by the control unit 150 and the computer 154. The control unit 150 in this embodiment sets the gradient field in a first direction which is automatically estimated or set as start value by the user. The direction of the gradient field is then varied stepwise until the resolution of the thereby received images, which are compared by the computer 154, is maximal, respectively not improved anymore. The most critical direction can therefore be found respectively adapted automatically in order to receive the highest possible resolution.

[0101] In MPI, the non-linearity of materials does not (or at least less) become effective if the material is driven into (or nearly into) magnetic saturation. The present invention applies this observation to the cabling, used particularly for the drive field coils 124, and applies dedicated DC currents to bring the magnetic contaminations of the cabling into saturation. In order not to generate an additional static magnetic field, which would shift the field-free point, the DC currents are applied such that effectively no AC (far-) field is generated. The static magnetic field is therefore mainly confined to within the cabling. The DC current shall be in the order of magnitude of the AC current (i.e. drive field) in order to substantially reduce the harmonics generated by the magnetic nonlinearities within the cabling.

[0102] FIG. 6 shows a circuit diagram of a first embodiment of a coil apparatus 1 according to the present invention. The coil apparatus 1 comprises a first embodiment of a cabling arrangement 1000 and a DC voltage or current source 600 directly coupled between a first internal terminal 301 and a second internal terminal 302. The cabling arrangement 1000 comprises a first AC terminal 300, a second AC terminal 310, the first internal terminal 301 and the second internal terminal 302. A cabling 1400 (in particular conductors and/or inductors) is connected between the first internal terminal 301, the second internal terminal 302 and the second AC terminal 310. The cabling 1400 comprises a first subset of one or more first conductors 401 coupled between the first internal terminal 301 and the second AC terminal 310 and a second subset of one or more second conductors 402 coupled between the second internal terminal 302 and the second AC terminal 310. Further, the cabling arrangement 1000 is configured for coupling a DC voltage between the first internal terminal 301 and the second internal terminal 302, in this embodiment via the DC voltage (or current) source 600.

[0103] In one exemplary embodiment the cabling 1400 represents the conductors of the drive field coils (125, 126, and 127 in FIG. 5), which are preferably—due to the high current—realized by a plurality of parallel conductors, each e.g. consisting of 23000×20 μm Litz wires making up a Rutherford cable. These parallel conductors typically have considerable magnetic coupling. In such an embodiment the first and second conductors 401, 402 are mechanically arranged substantially in parallel and forming a cable, wherein said cable is wound as a coil.

[0104] Due to the high voltage, a first set 500 of capacitors 501 is coupled to the first AC terminal 300, in particular in series, and a second set 510 of capacitors 511 is coupled to the second AC terminal 310, in particular in series. To the other ends 320, 330 of the first and second sets 500, 510 of conductors the drive field signal generator unit (122 in FIG. 5) is coupled in case of using the present invention for implementing the drive field coils (124).

[0105] In the embodiment of the cabling 1400 presented in FIG. 6 the first subset of conductors 401 and the second subset of conductors 402 are mechanically essentially in parallel. The AC current (provided by the drive field generator) indicated by arrows 410 flows in parallel trough the conductors 401, 402. A DC current, indicated by arrows 420 additionally flows through them, but with a current direction within the conductors 401 being antiparallel to the current direction in the conductors 402. This way, the magnetic field essentially cancels out when observed from further away, wherein it is considered that the FOV is essentially in the far field.

[0106] The DC current flow is a result of a DC voltage applied by a voltage source 600, which is transparent to AC currents (or is made transparent by additional capacitors connected in parallel). This voltage source 600 can e.g. be a DC charged capacitor or electric battery. The positive side of the battery is connected to the first internal terminal 301 and the negative side of the battery is connected to the second internal terminal 302.

[0107] FIG. 7 shows a circuit diagram of a second embodiment of a coil apparatus 2 according to the present invention. The coil apparatus 2 comprises a second embodiment of a cabling arrangement 2000 including a cabling 2400. The DC current (or voltage) source 600 delivers its current to the internal terminals 301 and 302 via optional inductors 700, 701 and switches 800, 801. As in the first embodiment 1, the DC current source 600 might be a battery, an accumulator, a (rechargeable) (super)-capacitor, or a dedicated current source, or parallel connections of these, and is typically connected to further AC circuitry to receive its energy, optionally solely at intervals when no imaging takes places.

[0108] In order to separate internal terminal 301 from internal terminal 302, which are at different DC potential, but at identical AC potential, the last capacitor of the capacitors of the first set 520 is split into a first capacitor 502 and a second capacitor 503. Further, the conductors 401, 402 are alternately arranged, i.e. pairs of one first conductor 401 and one second conductor 402 are arranged adjacent to each other.

[0109] FIG. 8 shows a circuit diagram of a third embodiment of a coil apparatus 3 according to the present invention. The coil apparatus 3 comprises a third embodiment of a cabling arrangement 3000 including a cabling 3400. This embodiment provides a higher resistance to the voltage source 600, for an example with 6 parallel conductors 401 to 406.

[0110] In this embodiment a third internal terminal 303 and a fourth internal terminal 304 are provided. Further, three coupling terminals 311, 312, 313 are provided between the subsets of conductors 401-406 and the second AC terminal 310. A first end of the first conductors 401, 403, 405 of the first subset are coupled to different internal terminals 301, 302, 303 and a second end of the first conductors 401, 403, 405 of the first subset are coupled to different coupling terminals 311, 312, 313, and a first end of the second conductors 402, 404, 406 of the second subset are coupled to different internal terminals 302, 303, 304 and a second end of the second conductors 402, 404, 406 of the second subset are coupled to different coupling terminals 311, 312, 313.

[0111] The last capacitor of the capacitors of the first set 530 is split into four capacitors 502, 503, 504, 505, each coupled between the first AC terminal 300 and one of the internal terminals 301, 302, 303, 304. The second set 540 comprises a first capacitor 511 between the AC terminal 330 and the second AC terminal 310 and three capacitors 512, 513, 514, each coupled between the second AC terminal 310 and one of the coupling terminals 311, 312, 313.

[0112] FIG. 9 shows a circuit diagram of an alternative embodiment of the first and second sets of capacitors, in particular for use in the third embodiment of the coil apparatus 3 shown in FIG. 8. The first set 550 is similar to the first set 530 shown in FIG. 8, wherein each of the capacitors 502, 503, 504, 505 is split into three (alternatively into two or four or more) capacitors. The second set 560 is similar to the second set 540 shown in FIG. 8, wherein each of the capacitors 512, 513, 514 is split into three (alternatively into two or four or more) capacitors. The embodiment provides a more equalized current flow around the peripheries of the capacitor and the cabling elements and hence reduces eddy current losses.

[0113] FIG. 10 shows various implementations of the cabling, i.e. the arrangement of the conductors in a cable according to the present invention. In particular, the cross sections of the cabling, which consists of parallel conductors that carry AC currents in alike, but DC currents of opposite direction, are shown. The conductor with DC currents flowing into the paper plane are marked by “x” and are termed 402. The remaining unmarked conductors with currents flowing out of the paper plane are termed 403. The layout of the conductors shown in the cross section shall be exemplarily for the way the cabling is shaped inside the drive field coil. Various assignments of current direction to the individual positions are possible.

[0114] FIGS. 10A and 10B show an arrangement in which alternatingly each second conductor has the same current direction. Due to (possible, but not necessary) rotation of the cable (which could e.g. be formed as a Rutherford cable), the position changes along the length of the cabling. So the cross section of the cabling as shown in FIG. 10A will become as shown in FIG. 10B at a different position in the cabling.

[0115] FIGS. 10C to 10F show arrangements in which the conductors of identical current direction are clustered. The benefit of the clustering might be to have, on average, a stronger saturation, which might be translated to reduced DC currents and hence power requirements.

[0116] The present invention reduces the magnetic non-linearity of the cabling, used particularly in the drive field coils, that leads to disturbing harmonic background in the detected signal. In preferred embodiments additional advantage is taken of the DC current in tailoring the generated magnetic field such that the AC fields do not (nearly) cancel out in the entire field-of-view but only cancel out in the center of the field-of-view, which is the field-free point. Hence, the field-free point is kept where it was, but additionally a “selection field” appears around the field-free point. This selection field can boost (or theoretically (in some cases) also replace) the selection field as generated by dedicated selection field coils. This feature can be used to further increase the gradient of the selection field (which translates into better resolution), or to reduce power requirements on the dedicated selection field coils.

[0117] FIG. 11 shows a circuit diagram of a fourth embodiment of a coil apparatus 4 according to the present invention. The coil apparatus 4 comprises a fourth embodiment of a cabling arrangement 4000 including a cabling 4400. The coil apparatus 4 is rather similar to the second embodiment of the coil apparatus 2 shown in FIG. 7, but has just two different conductors 401, 402 in parallel. The AC and DC currents have anti-parallel directions in coil 402 and parallel directions in coil 401.

[0118] FIG. 12 shows an arrangement of saddle coils using a cabling arrangement according to the present invention, especially as the cabling 4400 of FIG. 11. In particular a saddle coil pair of two saddle coils 601, 602, which is used as drive field coils for both the z-(anterior-posterior) and the y-(left-right) direction), is formed, wherein coil 601 is formed by the conductor 401 and coil 602 is formed by the conductor 402. It is shown how the magnetic fields 430 from the AC current add up in the entire field-of-view. The magnetic fields 440 from the DC currents oppose, i.e. at the very center, where the field-free point resides, no static field is superposed. However, when approaching conductor 402 (top coil 602) or conductor 401 (bottom coil 601), the magnetic field increases, which is exactly the property of the selection field.

[0119] FIG. 13 shows an arrangement of split solenoid coils using a cabling arrangement according to the present invention, especially as the cabling 4400 of FIG. 11. In particular a solenoid coil of two solenoid coils 603 (formed by conductor 401), 604 (formed by conductor 402) is shown, which is traditionally realized as one coil and split into two halves here. This coil is used for generating AC fields in the x-direction (head-feet). Since the AC selection field has a fixed orientation, e.g. in the z-(anterior-posterior) direction, as drawn in FIG. 12, the selection field generated by the split solenoid coils is different (i.e. orthogonal) in orientation. However, the x-direction (head-feet) selection field from the split solenoid coils is complemented by the (also) different (orthogonal) selection field from the other saddle coil pairs in the y-(left-right)-direction, which in sum generates a selection field in the desired direction (z-, anterior-posterior).

[0120] 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.

[0121] 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 measures cannot be used to advantage.

[0122] Any reference signs in the claims should not be construed as limiting the scope.