Patent Publication Number: US-2016223626-A1

Title: Coil arrangement of mpi system or apparatus

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus for influencing and/or detecting magnetic particles in a field of view, in particular a magnetic particle imaging apparatus. Further, the present invention relates to a coil arrangement, in particular for use in such a magnetic particle imaging apparatus. 
     BACKGROUND OF THE INVENTION 
     Magnetic Particle Imaging (MPI) is an emerging medical imaging modality. The first versions of MPI were two-dimensional in that they produced two-dimensional images. Newer versions are three-dimensional (3D). A four-dimensional image of a non-static object can be created by combining a temporal sequence of 3D images to a movie, provided the object does not significantly change during the data acquisition for a single 3D image. 
     MPI is a reconstructive imaging method, like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). Accordingly, an MP image of an object&#39;s volume of interest is generated in two steps. The first step, referred to as data acquisition, is performed using an MPI scanner. The MPI scanner has means to generate a static magnetic gradient field, called the “selection field”, which has a (single or more) field-free point(s) (FFP(s)) or a field-free line (FFL) at the isocenter of the scanner. Moreover, this FFP (or the FFL; mentioning “FFP” in the following shall generally be understood as meaning FFP or FFL) is surrounded by a first sub-zone with a low magnetic field strength, which is in turn surrounded by a second sub-zone with a higher magnetic field strength. In addition, the scanner has means to generate a time-dependent, typically spatially nearly homogeneous magnetic field. Actually, this field is obtained by superposing a rapidly changing field with a small amplitude, called the “drive field”, and optionally a slowly varying field with a large amplitude, called the “focus field”. By adding the time-dependent drive field and optional focus field to the static selection field, the FFP may be moved along a predetermined FFP trajectory throughout a “volume of scanning” surrounding the isocenter. The scanner also has an arrangement of one or more, e.g. three, receive coils and can record any voltages induced in these coils. For the data acquisition, the object to be imaged is placed in the scanner such that the object&#39;s volume of interest is enclosed by the scanner&#39;s field of view, which is a subset of the volume of scanning. 
     The object contains magnetic nanoparticles or other magnetic non-linear materials; if the object is an animal or a patient, a tracer containing such particles may be administered to the animal or patient prior to the scan. During the data acquisition, the MPI scanner moves the FFP along a deliberately chosen trajectory that traces out/covers the volume of scanning, or at least the field of view. The magnetic nanoparticles within the object experience a changing magnetic field and respond by changing their magnetization. The changing magnetization of the nanoparticles induces a time-dependent voltage in each of the receive coils. This voltage is sampled in a receiver associated with the receive coil. The samples output by the receivers are recorded and constitute the acquired data. The parameters that control the details of the data acquisition make up the “scan protocol”. 
     In the second step of the image generation, referred to as image reconstruction, the image is computed, or reconstructed, from the data acquired in the first step. The image is typically a discrete 3D array of data that represents a sampled approximation to the position-dependent concentration of the magnetic nanoparticles in the field of view. The reconstruction is generally performed by a computer, which executes a suitable computer program. Computer and computer program realize a reconstruction algorithm. The reconstruction algorithm is based on a mathematical model of the data acquisition. As with all reconstructive imaging methods, this model can be formulated as an integral operator that acts on the acquired data; the reconstruction algorithm tries to undo, to the extent possible, the action of the model. 
     Such an MPI apparatus and method have the advantage that they can be used to examine arbitrary examination objects—e. g. human bodies—in a non-destructive manner and with a high spatial resolution, both close to the surface and remote from the surface of the examination object. Such an apparatus and method are generally known and have been first described in DE 101 51 778 A1 and in Gleich, B. and Weizenecker, J. (2005), “Tomographic imaging using the nonlinear response of magnetic particles” in Nature, vol. 435, pp. 1214-1217, in which also the reconstruction principle is generally described. The apparatus and method for magnetic particle imaging (MPI) described in that publication take advantage of the non-linear magnetization curve of small magnetic particles. 
     Drive coils are needed in MPI to generate the rapidly changing magnetic field (f˜25 kHz . . . 200 kHz or even higher), which has a typical amplitude of 20 mT peak or less. The energy stored in the bore is proportional to the volume, hence rises with the third dimension of the radius. For a human size application, with a bore diameter of approximately 40 cm (for a first experimental demonstrator and more for future products), the energy is around 10 J (peak). The reactive power is the product of this times the angular frequency ω=2*pi*f, so P react ˜2 MW. This reactive power can be oscillated between magnetic field in the coil and electric field in the series capacitors by any product of current and voltage. As a typical example, U pk ˜15 kV, I pk ˜250 A, both of which are challenging to operate. 
     Therefore the power needed in such systems has typically a very high value, and an optimization of its use can thus significantly reduce the power consumption costs and increase the security of the patients. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an apparatus for influencing and/or detecting magnetic particles in a field of view, i.e. an MPI apparatus, that enables the examination of such larger subjects (human beings, animals), in particular for adult human beings. Further, it is an object of the present invention to provide a coil arrangement which is more suitable for the examination of larger subjects (human beings, animals), in particular for adult human beings, by use of an MPI apparatus. 
     In a first aspect of the present invention an apparatus for influencing and/or detecting magnetic particles in a field of view is presented comprising:
         selection elements comprising a selection field signal generator unit and selection field elements 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 where the magnetization of the magnetic particles is not saturated and a second sub-zone having a higher magnetic field strength where the magnetization of the magnetic particles is saturated are formed in the field of view,   drive elements comprising a drive field signal generator unit and at least one drive field coil 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, said at least one drive field coil being arranged generally around a central longitudinal axis, passing through the field of view,       

     wherein at least one drive field coil is formed by a major cable arranged around the central longitudinal axis, wherein the major cable comprises mainly a plurality of minor cables or wires which are positioned angularly differently around the central longitudinal axis such that in a first angular sub-range the ratio of height to width of the major cable&#39;s cross-section is different than in a second angular sub-range. 
     In another aspect of the present invention a coil arrangement for use in such an apparatus is presented comprising a major cable arranged around a central longitudinal axis, passing through a field of view in an angular range, wherein the major cable comprises a plurality of minor cables or wires forming said major cable which are positioned angularly differently around the central longitudinal axis such that in a first angular sub-range the ratio of height to width of the major cable&#39;s cross-section is different than in a second angular sub-range. 
     Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed apparatus and the claimed coil arrangement have similar and/or identical preferred embodiments as defined in the dependent claims. 
     For sake of simplicity, and without any limitation whatsoever, in the following section of this specification, “cable” will refer to said “major cable” and “wires” will refer to said “minor cables or wires”. 
     The patient&#39;s chest/trunk is generally placed inside the drive field coil arrangement which typically comprises one or several drive field coils (generally one coil or coil pair per one of the three spatial directions). To this end, the patient might actually slide into the generator by means of a patient support. The drive field coils occupy space between the patient and the selection field elements, which generally comprises selection field coils and/or permanent magnets which are arranged above and below the patient forming an open structure in a similar way as known from an open MRI apparatus. There are various trade-offs for the space between the upper and lower half of the selection field elements. 
     According to the present invention the drive field coil arrangement comprising one or various drive field coils has a maximum internal bore size (extending around said central longitudinal axis) allowing the patient to comfortably slide in. Further, the outer diameter is, at least in the direction facing the selection field elements, as small as possible allowing other components of the apparatus, in particular the selection field elements and preferably provided focus field coils to be arranged as close as possible to the patient. This is achieved according to the present invention by providing that at least one drive field coil, preferably all drive field coils, are slim at positions adjacent the selection field elements compared to positions not facing the selection field elements. In other words, the ratio of height to width is made low to make the cable and thus the drive coil slim at a certain position and the ratio of height to width is made high to make the cable and thus the drive coil thicker at a certain position. 
     In an embodiment in which the selection field elements are arranged above and below the patient, the at least one drive field coil is thus made slim in the vertical direction at the positions above and below the patient, while it is less slim at the positions on the left and right side of the patient. For this purpose the cable forming the at least one drive field coil does not, as conventionally, have a fixed cross-sections having a fixed shape but at least the shape of the cross-section changes along the longitudinal direction of the cable, while preferably the cross-section (i.e. the area of the cross-section) is kept constant. 
     In this context it shall be noted that there are various embodiments of drive field coils, in particular solenoid coils, which complete surrounds the field of view in an angular range of 360°, and saddle coils, which only surround the field of view in a smaller angular range of less than 180°, e.g. in the range of 90° to 160°. The angular sub-ranges are to be understood as portions of the respective (total) angular range and can be as small as only a few degrees (i.e. only a certain position). Generally, a sub-range is to be understood as an angular range between 5° and 90°, preferably between 15° and 75°. 
     In an embodiment the first angular sub-range is shifted by an angle in the range of 75° to 105°, in particular by an angle of substantially 90°, with respect to the second angular sub-range. Thus, at the sides of the patient (in particular under the axles when the apparatus is used for heart imaging) the cable is made thicker but with smaller width compared to the area above the chest and below the back of the patient where the cable is made thinner but with larger width. 
     In another embodiment the plurality of wires are arranged such that the ratio of height to width of the cable&#39;s cross-section has a first substantially identical value in oppositely arranged first and third angular sub-ranges (e.g. above and below the patient), which is different from a second substantially identical value in oppositely arranged second and fourth angular sub-ranges (e.g. at the sides of the patient). Thus, in the desired directions space can be saved. 
     This is preferably further achieved in an embodiment according to which the first angular sub-range is arranged facing a selection field element and the value of the ratio of height to width of the cable&#39;s cross-section is smaller in the first angular sub-range than in the second angular sub-range. 
     Preferably, multiple windings of the cable are arranged adjacent to each other in a z-direction substantially perpendicular to the longitudinal direction of the cable, wherein said windings are arranged closer together in the second angular sub-range than in the first angular sub-range. This is particular important if space in the z-direction, which corresponds to the longitudinal axis of the patient, is short, e.g. under the axles of the patient. 
     In the first angular sub-range the positions of the windings are displaced with respect to the positions of the windings in the second angular sub-range according to another preferred embodiment. In this way it is possible to design the peak of the coils sensitivity to be nearer to or ideally at a particular region of interest, e.g. the heart of the patient. 
     As explained above, the drive field coils are used to create high frequency (25 kHz up to 100 kHz or higher) magnetic drive fields for activating magnetic particles in the body in view of their detection for imaging purpose. Conventionally, drive field coils are realised with many windings, leading to a high inductance. However, this conventional design cannot be used anymore for the human-size MPI apparatus, as the voltage (e.g. 40 kVpk) is far too high and will accordingly hardly comply with the medical instrumentation standard (IEC 60601-1). In a preferred embodiment said plurality of wires are twisted one of the other along the cable (in other words around the longitudinal axis of the cable), in particular as a Rutherford cable. This solution provides for an inductance with fewer windings made of a thicker, so-called “Rutherford”-like cable, which has a flat appearance, and in which each wire sees each position equally often. Such a Rutherford cable mimics a perfect RF-Litz wire. Further, said wires are preferably Litz wires comprising a plurality of strands to have a low-loss cable type. 
     As already mentioned the at least one drive field coil is a solenoid coil or a saddle coil. Preferably, said drive field coils forming a drive field coil arrangement, comprise two pairs of saddle coils arranged around a central symmetry axis perpendicular to said central longitudinal axis and a solenoid coil arranged around said central symmetry axis. Some or all of the drive field coils are designed as explained above for the at least one drive field coil. 
     In another embodiment the drive elements comprise a carrier structure carrying said drive field coils on its outer surface and/or its inner surface, preferably comprising grooves for receiving cables forming said drive field coils. Thus, the drive field coils have a fixed structure and are pre-formed. In an alternative embodiment the drive field coils are flexible and can be placed around the patient as needed. 
     Advantageously, said at least one drive field coil is a saddle coil, wherein the plurality of wires forming the cable of said at least one drive field coil are twisted one to the other along the cable (in other words around the longitudinal axis of the cable), in particular as a Rutherford cable, while the cable is arranged on the outer surface or inner surface of the carrier structure to form said at least one drive field coil. Thus, the cable of the at least one drive field coil is not pre-formed on a workbench and then brought into the right form, which might be difficult in case of a saddle coil since the cable can only be bent in one direction but may be hard to bend in the other direction. Thus, the cable is formed (i.e. the wires are twisted to form the cable) on the fly while the cable is brought into the right form for forming the at least one drive field coil which makes it easier to bend the cable in the right form. 
     In still another embodiment the apparatus further comprises a connection cable for connecting the at least one drive field coil with the drive field signal generator unit, said connection cable having an unvaried cross-section and a transition unit for connecting the cable forming said at least one drive field coil with the connection cable. 
     In another aspect of the present invention an apparatus for influencing and/or detecting magnetic particles in a field of view is presented, which apparatus comprises:
         selection elements comprising a selection field signal generator unit and selection field elements 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 where the magnetization of the magnetic particles is not saturated and a second sub-zone having a higher magnetic field strength where the magnetization of the magnetic particles is saturated are formed in the field of view,   drive elements comprising a drive field signal generator unit and at least one drive field coil 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, said at least one drive field coil being arranged generally around a central longitudinal axis passing through the field of view,       

     wherein at least one drive field coil is formed by a major cable arranged around the central longitudinal axis, wherein the major cable comprises a plurality of Litz wires comprising a plurality of strands, said Litz wires being twisted one to the other along the major cable, in particular as Rutherford cable. 
     The apparatus according to this aspect primarily provides the advantages explained above in the context of Rutherford cables. 
     For receiving detection signals for determining the distribution of magnetic particles within the examination area and, thus, for generating images of the examination area, e.g. of the heart region of a patient, the apparatus further comprises a receiving means comprising at least one signal receiving unit and at least one receiving coil for acquiring detection signals, which detection signals depend on the magnetization in the field of view, which magnetization is influenced by the change in the position in space of the first and second sub-zone. 
     It is preferably proposed that the MPI apparatus employs combined selection-and-focus field coils, which is based on the idea to combine focus field coils and the selection field coils that are generally provided as separate coils in the known MPI apparatus into a combined set of selection-and-focus field coils. Hence, a single current is provided to each of said coils rather than separate currents as conventionally provided to each focus field coil and each selection field coil. The single currents can thus be regarded as two superposed currents for focus field generation and selection field generation. The desired location and movement of the field of view within the examination area can be easily changed by controlling the currents to the various coils. Not all selection-and-focus field coils must, however, always be provided with control currents, as some coils are only needed for certain movements of the field of view. 
     The proposed apparatus further provides more freedom of how and where to arrange the coils with respect to the examination area in which the subject is place. It is particularly possible with this arrangement to build an open scanner that is easily accessible both by the patient and by doctors or medical personnel, e.g. a surgeon during an intervention. 
     With such an apparatus the magnetic gradient field (i.e. the magnetic selection field) is generated with a spatial distribution of the magnetic field strength such that the field of view comprises a first sub-area with lower magnetic field strength (e.g. the FFP), the lower magnetic field strength being adapted such that the magnetization of the magnetic particles located in the first sub-area is not saturated, and a second sub-area with a higher magnetic field strength, the higher magnetic field strength being adapted such that the magnetization of the magnetic particles located in the second sub-area is saturated. Due to the non-linearity of the magnetization characteristic curve of the magnetic particles the magnetization and thereby the magnetic field generated by the magnetic particles shows higher harmonics, which, for example, can be detected by a detection coil. The evaluated signals (the higher harmonics of the signals) contain information about the spatial distribution of the magnetic particles, which again can be used e.g. for medical imaging, for the visualization of the spatial distribution of the magnetic particles and/or for other applications. 
     The MPI apparatus according to the present invention are based on a new physical principle (i.e. the principle referred to as MPI) that is different from other known conventional medical imaging techniques, as for example nuclear magnetic resonance (NMR). In particular, this new MPI-principle, does, in contrast to NMR, not exploit the influence of the material on the magnetic resonance characteristics of protons, but rather directly detects the magnetization of the magnetic material by exploiting the non-linearity of the magnetization characteristic curve. In particular, the MPI-technique exploits the higher harmonics of the generated magnetic signals which result from the non-linearity of the magnetization characteristic curve in the area where the magnetization changes from the non-saturated to the saturated state. 
     The drive field coils are preferably arranged in the area between said first inner selection-and-focus field coils of the two sets of selection-and-focus field coils. The drive field coils may be designed such that they are (fixedly or movable) arranged between the two sets of selection-and-focus field coils. In other embodiments, the drive field coils are somewhat flexible and can be arranged on the desired portion of the patient&#39;s body before the patient is placed inside the examination area. 
    
    
     
       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. In the following drawings 
         FIG. 1  shows a first embodiment of an MPI apparatus, 
         FIG. 2  shows an example of the selection field pattern produced by an apparatus as shown in  FIG. 1 , 
         FIG. 3  shows a second embodiment of an MPI apparatus, 
         FIG. 4  shows a third and a fourth embodiment of an MPI apparatus, 
         FIG. 5  shows a block diagram of an MPI apparatus according to the present invention, 
         FIG. 6  shows two views of a first embodiment of a drive field coil according to the present invention, 
         FIG. 7  shows two views of a second embodiment of a drive field coil according to the present invention, 
         FIG. 8  shows a perspective view and a cross-section through an embodiment of a cable for use in a drive field coil according to the present invention, 
         FIG. 9  shows how the cable shall be flat around the bore, 
         FIG. 10  shows an embodiment of a saddle coil pair for use as drive field coil according to another embodiment of the present invention, and 
         FIG. 11  shows a connection cable for externally connecting a drive field coil. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     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′-coil (y-coil), and similarly for the remaining coils. When more convenient, the coordinate axes and the coils shall be labelled with x 1 , x 2 , and x 3 , rather than with x, y, and z. 
     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&#39;s axis, it will be taken as positive, otherwise as negative. To generate the static selection field, a constant positive current I S  is made to flow through the z + -coil, and the current −I S  is made to flow through the z − -coil. The z-coil pair  16  then acts as an anti-parallel circular coil pair. 
     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. 
     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&#39;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. 
     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 . 
     To generate the drive field, a time dependent current I D   1  is made to flow through both x-coils  12 , a time dependent current I D   2  through both y-coils  14 , and a time dependent current I D   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 F   1  is made to flow through both x-coils  12 , a current I F   2  through both y-coils  14 , and a current I F   3  through both z-coils  16 . 
     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 ± -coil is I D   3 +I F   3 ±I S . The current flowing through the remaining two coil pairs  12 ,  14  is I D   k +I F   k , k=1, 2. Because of their geometry and symmetry, the three coil pairs  12 ,  14 ,  16  are well decoupled. This is wanted. 
     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. 
     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. 
     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 are used as receive coils as is the case according to the present invention using combined drive-receiving coils. 
     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&#39;s volume of interest—that volume of the patient (or object) that shall be imaged—is enclosed by the scanner&#39;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 . 
     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 3  A/m 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). 
     The patient&#39;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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 k  across the terminals of the receive coil pair along the x k -axis. The associated receiver converts this voltage to a signal S k , which it processes further. 
     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. 
     To give a few typical parameters of such an embodiment: The z-gradient of the selection field, G, has a strength of G/μ 0 =2.5 T/m, where μ 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 150 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. 
     Since the construction of field generating coils is generally known in the art, e.g. from the static BO field of magnetic resonance imaging, this subject need not be further elaborated herein. 
     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. 
       FIG. 4  shows two embodiments of the general outer layout of an MPI apparatus  200 .  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. 
     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. 
     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 . 
     As shown in  FIG. 4 a   , 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 . 
     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&#39;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. 4 b    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. 
     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. 
       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. 
     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. 
     For generating the combined magnetic selection-and-focus field, selection-and-focus elements  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 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. 
     The selection-and-focus elements  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 can also be omitted. 
     For generating the magnetic drive field the apparatus  100  further comprises drive elements  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. 
     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 current amplifier) and a filter unit (which may also be omitted with the present invention) for providing a time-dependent drive field current to the respective drive field coil. 
     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. 
     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  148 , in particular a receiving coil, and a signal receiving unit  140 , which receives signals detected by said receiving means  148 , are provided. Preferably, three receiving coils  148  and three receiving units  140 —one per receiving coil—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. 
     Said signal receiving unit  140  comprises a filter unit  142  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. 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 receiving coil  148  is 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  to an analog/digital converter  146  (ADC). 
     The digitalized 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. 
     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. 
     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. 
     In the known MPI apparatus the patient chest/trunk is placed inside the drive field coil unit. As explained above, the drive field coil unit typically comprises a solenoid coil made of several cables homogeneously wound around the cylinder-like bore in a straight, non-optimized way. For heart imaging this leads to non-optimal coil usage, hence more power is required to generate the requested drive field strength at the intended position of imaging (e.g. the heart). 
     WO 2013/080145 A1, particularly FIG. 19 discloses an MPI apparatus in which the solenoid coil comprises more cables having an increased cross-section area at the intended position of imaging (e.g. the heart). Nevertheless, connecting cables with different cross-section implies to have many lossy interface terminals leading to high loss for such a high-current, high-voltage and high-frequency MPI apparatus. Moreover, this locally larger cable cross-sections lead to a thicker drive field coil which takes away space from the selection coils or the selection- and focus-field coils, respectively, or from the patient, which should be avoided. 
     An embodiment of a drive field coil  300 , in particular a solenoid coil, as used in an embodiment of an MPI apparatus according to the present invention is shown in  FIG. 6A  in a perspective view and, partially, in  FIG. 6B  in a cross-sectional view. According to this embodiment the drive field coil  300  is formed by a cable  310  (only one winding is shown for better visibility, but there are general several windings around the field of view  28 ), which is arranged at least in an angular range around the field of view  28  (here in the angular range of) 360°. In this embodiment the cable  300  is arranged on the outer surface of a carrier structure  305 , e.g. a tubular structure made e.g. of plastic material, which forms the bore  302  into which the patient is placed for examination. The cable  300  comprises a plurality of wires  301  forming said cable  300 , which are arranged such that in a first angular sub-range  320  the ratio of height h 1  to width w 1  of the cable&#39;s cross-section is different than the ratio of height h 2  to width w 2  of the cable&#39;s cross-section in a second angular sub-range  330 . In particular, h 1 &lt;h 2  and w 1 &gt;w 2  in this embodiment. 
     The sub-ranges  320 ,  330  are to be understood as angular ranges that are smaller than the complete angular range (here 360°) in which the cable  300  is arranged. For instance, the first sub-range  320 , which is arranged here in the area of the top of the drive field coil  300 , and the second sub-range  330 , which is arranged here in the area of the side of the drive field coil  300 , are in the range of only a few degrees (i.e. only a certain position), generally between 5° and 90°, preferably between 15° and 75°. 
     In other words, the cable  300  is wound around the cylinder-like patient bore  302  in a non-straight way, having its cross-section shape varying along its lengths. The relative positioning of the wires  301  of the cable  300  is varying one to the other depending on their angular locations around the cylindrical bore  302 . This can particularly be seen in  FIG. 6B  showing how the (in this example eight) wires  301  in the first angular sub-range  320  are arranged next to each other in z-direction forming a thin but broad cable transform into a thicker but less broad cable in the second angular sub-range  330  where two layers of four wires  301  are stacked upon each other. 
     Preferably, not only at the top but also at the bottom (representing a third angular sub-range  340  arranged opposite to the first angular sub-range) the cable  300  has a broad but thin cross-section, and also on the other side opposite the second angular sub-range  330  in a fourth angular sub-range ( 350 , not explicitly shown) the cable  300  has a less broad but thicker cross-section. 
     The variable cross-section shape thus allows reducing the thickness at the top and bottom location of the drive field coil  300  where the selection field coils (or the selection- and focus-field coils) are located. Preferably, on the top and bottom angular sub-ranges the thickness of the cable is minimized, while at the sides of the drive field coil (and the patient), where space is not that much of importance the cable is allowed to be thicker. 
     Another embodiment of a drive field coil  400 , in particular a solenoid coil, as used in an embodiment of an MPI apparatus according to the present invention is shown in  FIG. 7A  in a top view and in  FIG. 7B  in a side view. In these figures four windings of the drive field coil  400  are shown, which are wound around the chest of the patient  1  who is lying on a patient support  2 . 
     It should be noted that there are generally two possible positions for the arms, namely outside the drive field coil  400  (as shown in  FIG. 7 ) or inside the drive field coil. The present invention is independent of the arm position. 
     The windings  411 ,  412 ,  413 ,  414  of the cable  410  forming the drive field coil  400  are arranged such that, in addition to the variation of the height and width along it length as explained above with reference to  FIG. 6 , the windings  411 ,  412 ,  413 ,  414  are arranged closer together in the second and fourth angular sub-ranges  330 ,  350  (i.e. under the axles) than in the first and third angular sub-range  320 ,  340  (i.e. above the chest and below the back). Thus, the total width of the drive field coil in the second and fourth angular sub-ranges  330 ,  350  is not only smaller because of the smaller width of the cable  410  there, but also because the windings are arranged close together. 
     The non-straight arrangement of the windings  411 ,  412 ,  413 ,  414  of the cable  410 , intended for magnetic field generation in the z-direction allows designing the peak of the coil sensitivity to be nearer to or ideally at the heart of the patient  1 . The windings are densely located beneath the axles (left/right of patient body), whilst they extend more towards the neck and chin (below and above the body). 
     The non-straight arrangement of the windings can be employed independently of the variable cross-section shape, but it is advantageous to combine both ideas as it allows to have smooth current density distribution along the drive field coil, which in turn translates into non-peaking induced currents in the patient and hence to a better tolerance with respect to peripheral nerve stimulation. 
     The same ideas can generally also be applied for the other drive field coils, which are preferably designed as saddle coil pairs. Also for such type of coils the cable can be designed to have a variable thickness to width ratio and/or a variable distance between the windings depending on the angular location. 
       FIG. 8A  shows a cross-section through an embodiment of a cable  510  for use in a drive field coil according to the present invention, for instance in a coil  300  or  400  shown above or in other embodiments of drive field coils.  FIG. 8B  shows a perspective view of this cable  510 . The cable  510  comprises a plurality of Litz wires (in this example eight Litz wires  501 - 508 ) each comprising a plurality of strands  515  (e.g.  40000  strands with a diameter of 20 μm). As shown in  FIG. 8B  said Litz wires  501 - 508  are twisted around the longitudinal axis of the cable  510 , in particular as Rutherford cable. 
     The Litz wires  501 - 508  are, in this embodiment, held together by holding elements  520 , e.g. cable binders such that the cable  510  has a flat appearance. Each Litz wire sees each position equally often so that the whole cable mimics a perfect large-cross section RF-Litz wire. From one holding element  520  to the next (e.g. approx. every 6 cm) the Litz wires shift/rotate by one position. 
     Forming a Rutherford cable on the lab bench is generally not difficult, but shaping it into the form of (especially) saddle coil is difficult, especially forming the inner winding, with smallest bending radius. The challenge with flat Rutherford cables is, that it is elongated much more in one direction (left-right) than in the other (top-down). Therefore, bending is nearly impossible in the elongated direction, whilst easy in the other. It is mathematically provable that such a saddle coil structure can only be attached around a cylinder-like shape (i.e. the bore into which the patient is to be placed) if the cable “stands”. This type is called a CPE (constant perimeter end coil). However, in order to have an overall flat drive field coil for an MPI apparatus with few windings, it must “lie”.  FIG. 9  shows a computer sketch of a flat Rutherford cable on top of cylindrical bore, forming the upper saddle coil, to show how the cable shall be flat around the bore. 
     In order to achieve this, it is proposed to use a different manufacturing process. The cable shall not be preassembled on the work bench, but in a special form, in which it is pre-bent while rotating it. Alternatively, it can be assembled directly around or on the bore. In both cases, grooves for placing the cable are preferably provided. Further, holding elements (fixtures like cable binders and clips) are preferably used. 
     Thus, preferably for directions of the magnetic drive field orthogonal to the z-direction the drive field coils employ a Rutherford cable containing Litz wires with μm-thin strands, the cables being laid out on the bore according to a saddle coil pair configuration  600 .  FIG. 10  shows such a saddle coil pair configuration  600 , a saddle coil  610 ,  620  comprising three windings of the cable  510 , said three windings being coupled electrically preferably in parallel and formed on the inner or outer surface of a carrier  605 . The matching and tuning circuit can be realized such that the voltages at terminals are symmetric with respect to ground. E.g., if 10 kVpk is the maximum across the inductor, then the terminals would be at +5 kVpk and −5 kVpk. There would be a virtual middle point at 0V. This feature is very useful, as it helps to reduce the voltages between the coils, as there are altogether three of them for the three spatial directions, at different frequencies. Without this symmetric realization, the maximum inter-coil voltage would be 2*10 kVpk=20 kVpk, with this feature it is only 2*5 kVpk=10 kVpk. This helps to reduce insulating distances and materials within the drive field coils, and hence minimize space (which is then available for the patient). 
     Preferably, as shown in  FIG. 11 , two connection cables  360 ,  365  for connecting the at least one drive field coil with the drive field signal generator unit  122  and a transition unit  370  for connecting the cable  310  forming said at least one drive field coil  300  with the connection cable  360 . One connection cable  360  is provided for the current to enter the drive field coil  300  and the other connection cable  365  is provided for the current to exit the drive field coil  300 . The connection cables  360 ,  365  have a dual function: They carry electric current, but also surround the copper cables with a cooling liquid (preferably oil) to keep the connection cables  360 ,  365  cool. 
     Said connection cables  360 ,  365  are preferably Rutherford cables and have an unvaried (i.e. constant) cross-section. Thus, the transition unit  370  converts the connection cables  360 ,  365  into the cable  310  having the variable cross-section which may be achieved by connecting the various Litz wires of the cables via a connection board (not shown) to which the Litz wires are separately fixed. It is alternatively possible to use uninterrupted continuous Litz wire to form both the cable  310  within the drive field coil  300  and the two connection cables  360 ,  365 , so that no connection board is needed. 
     The cable  310  is preferably wound to the inner surface of the carrier  305 , wherein the winding process is preferably started from the middle of the cable (not the end of the cable), which may make it easier to bring the cable into the right form, in particular in case of forming a saddle coil. 
     Preferably, the saddle coils shall be coupled in parallel and not in series in order to keep the voltage low and to allow each coil to have a virtual middle point at 0V. 
     The various above explained aspects can each be used independently for single or all drive field coils, but are preferably used together in a preferred embodiment of an MPI apparatus according to the present invention. Preferably, all cables of all drive field coils are designed as Rutherford cables. 
     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 measures cannot be used to advantage. 
     Any reference signs in the claims should not be construed as limiting the scope.