Patent Publication Number: US-9903837-B2

Title: Removal of background in MPI

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/IB2012/057350, filed on Dec. 14, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/570,854, filed on Dec. 15, 2011. These applications are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an apparatus and a method for detecting magnetic particles in a field of view which enable the removal of background signals. The present invention relates particularly to the field of magnetic particle imaging. 
     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) field-free point (FFP) 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, 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 a slowly varying field with a large amplitude, called the “focus field”. By adding the time-dependent drive and focus fields 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 must contain magnetic nanoparticles or other magnetic non-linear materials; if the object is an animal or a patient, a contrast agent containing such particles is 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 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. 
     Background signal variations and spurious signals, which occur both in MPI calibration scans and object scans, can strongly compromise image quality. Different sources of background signal exhibit different spectral behavior, so that background signal contributions are not homogeneously spread over the measured bandwidth, but vary in intensity and temporal pattern between different frequency components. 
     Further, the design of the MPI apparatus and methods described so far are not yet optimal for human beings. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an apparatus and method for detecting magnetic particles in a field of view that enable the examination of such larger subjects (human beings, animals), in particular for adult human beings, and that allow the removal of background signals. 
     In a first 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 means 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 means comprising a drive field signal generator unit and drive field coils 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, 
     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, and 
     reconstruction means for reconstructing an image of the field of view from detection signals, said detection signals including a plurality of frequency components, wherein one or more frequency components are selected and/or weighted by use of a frequency component specific signal quality factor obtained from background signal measurements and wherein only selected and/or weighted frequency components are used for reconstruction of the image. 
     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: 
     i) selection-and-focus means for generating a magnetic selection-and-focus 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 and for changing the position in space of the field of view within an examination area, said selection-and-focus means comprising at least one set of selection-and-focus field coils, and a selection-and-focus field generator unit for generating selection-and-focus field currents to be provided to said at least one set of selection-and-focus field coils for controlling the generation of said magnetic selection-and-focus field,
 
wherein said at least one set of selection-and-focus field coils comprises
 
     at least one inner selection-and-focus field coil being formed as a closed loop about an inner coil axis, and 
     a group of at least two outer selection-and-focus field coils arranged at a larger distance from said inner coil axis than said at least one inner selection-and-focus field coil and at different angular positions, each being formed as a closed loop about an associated outer coil axis, and 
     ii) drive means comprising a drive field signal generator unit and drive field coils 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,
 
ii) 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, and
 
iv) reconstruction means for reconstructing an image of the field of view from detection signals, said detection signals including a plurality of frequency components, wherein one or more frequency components are selected and/or weighted by use of a frequency component specific signal quality factor obtained from background signal measurements and wherein only selected and/or weighted frequency components are used for reconstruction of the image.
 
     In an embodiment a computer program is presented comprising program code means for causing a computer to control an apparatus according to the present invention to carry out the steps of the method according to the present invention when said computer program is carried out on the computer. 
     Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method and computer program have similar and/or identical preferred embodiments as the claimed coil arrangement and as defined in the dependent claims. 
     The present invention is based on the idea to treat selected or each frequency component of detection signals individually for background determination and removal. The detection signals generally depend on the magnetization in the field of view, but also on the background signal of the system. Slowly varying background signal can be determined by dedicated background measurements, e.g. interspersed in a scan at moments when no magnetic tracer material is in the sensitive imaging volume, as proposed according to a preferred embodiment. Background signal variations that are more rapid than the time period between background measurements cannot be removed by this method. However, frequencies that strongly suffer from rapidly varying background signals can be suppressed or removed in image reconstruction. This is possible due to the large number of frequency components used for reconstruction and the fact that similar information is encoded at different mix frequencies if more than on drive field is applied at different frequencies in MPI and unlike other medical imaging modalities such as MRI, where valuable image information would get lost if some frequency components were not used in the reconstruction. 
     According to a preferred embodiment said reconstruction means is adapted for selecting frequency components by use of a signal quality threshold, wherein only those frequency components are selected having a signal quality factor above said signal quality threshold. This provides an efficient solution for selecting frequency components so that also rapidly varying background signals (that reduce signal quality, such as the SNR, i.e. the signal-to-noise ratio) are suppressed and not used in the reconstruction. 
     Generally, according to the invention signal quality factors and signal quality thresholds are used. In an embodiment an SNR factor and an SNR threshold are used, in other embodiments other signal quality factors and signal quality thresholds are used. In another embodiment a signal quality factor and signal quality threshold may be used that are e.g. based on background measurements, which are used to identify spurious signals which may then be suppressed by selecting an appropriate signal quality factor and signal quality threshold. In still another embodiment a noise factor and noise threshold may be used directed to the suppression of noise. In still another embodiment a signal quality factor and signal quality threshold are used that are directed to the selection or weighing of spatial frequency components that are most important or best for the reconstruction of an image, while other spatial frequencies are suppressed or less weighted. 
     According to another embodiment said reconstruction means is adapted for weighting all or selected frequency components by its frequency component specific signal quality factor. Also in this way the result of the reconstruction can be improved since frequency components of rapidly varying background signals are less weighted than frequency components not related to rapidly varying background signals. 
     There are various embodiments for obtaining background signals representing slowly and/or rapidly moving background. In an embodiment said apparatus is adapted for performing background signal measurements interspersed with system function measurements and for deriving said signal quality factors from said background signal measurements. This provides a quite fast method for obtaining background signal. 
     While generally the time intervals during which background signals are measured are shorter than the time intervals during which system function data are measured, according to a preferred embodiment said apparatus is adapted for performing background signal measurements during longer time intervals than system function measurements. This increases the accuracy of the background signal measurement and ensures that rapidly varying background can reliably be removed. 
     In another embodiment said reconstruction means is adapted for removing slowly varying background signals from system function measurements by determining frequency specific background signals from said background signal measurements and subtracting these frequency specific background signals from the corresponding frequency components of said system function measurements. 
     To further improve the removal of background signals, according to a preferred embodiment said reconstruction means is adapted for interpolating said background signal measurements and for using the interpolated background signal measurements for determining said frequency specific background signals. 
     A still further improvement of the removal of background signals is achieved by an embodiment according to which said apparatus is adapted for repeating said background signal measurements interspersed with system function measurements and for averaging the obtained background signal measurements, and wherein said reconstruction means ( 152 ) is adapted for determining said frequency specific background signals from said averaged background signal measurements. 
     A still further improvement of the removal of background signals is achieved by an embodiment according to which said apparatus is adapted for repeating said background signal measurements at different time scales. 
     Preferably, said apparatus is adapted for performing said background signal measurement before and/or after receiving detection signals for reconstructing an image of the field of view. Thus, a high accuracy by the removal of currently appearing background signals can be obtained. 
     Another option for removal of background includes performing calibration measurements, wherein a first calibration measurement is performed while a magnetic sample is moved through the field of view and a second calibration measurement is performed without any a magnetic material in the field of view. 
     The preferably proposed MPI apparatus employing combined selection-and-focus field coils is based on the idea to combine the 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, but 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. 
     According to a preferred embodiment said closed loops of the outer selection-and-focus field coils have a contour in the form of a ring segment. In other words, the windings of each of said outer selection-and-focus field coils are wound as a closed loop, which is arranged along an angular area around said at least one inner selection-and-focus field coil, which angular area covers a ring segment of a ring enclosing said at least one inner selection-and-focus field coil. 
     Preferably, said at least one set of selection-and-focus field coils comprises a group of at least four outer selection-and-focus field coils. Generally, even more selection-and-focus field coils may be provided which are preferably arranged at the same distance from the inner coil axis but at different angular positions around said inner coil axis. 
     For instance, in an embodiment it is provided that said at least one set of selection-and-focus field coils comprises a group of four outer selection-and-focus field coils being arranged at the same distance from the inner coil axis but angularly displaced by 90° with respect to each other. In further embodiments even more groups of outer selection-and-focus field coils, the coils of the various groups being arranged at different distances from the inner coil axis. 
     In another embodiment said at least one set of selection-and-focus field coils comprises a first inner selection-and-focus field coil and a second inner selection-and-focus field coil being formed as a closed loop about said inner coil axis and having a larger diameter than said first inner selection-and-focus field coil. Even more inner selection-and-focus field coil formed as closed loops about the inner coil axis at different distances may be provided. These inner selection-and-focus field coil are generally more effective for generation of the magnetic selection and focus fields and are, hence, generally provided with control currents all the time during operation of the apparatus. 
     Preferably, said at least one inner selection-and-focus field coil and/or said outer selection-and-focus field coils are split into at least two, in particular at least four, coil segments, wherein coil segments of a coil are arranged adjacent to each other in the direction of the associated coil axis and wherein adjacent coil segments are electrically connected. In this way the desired current density can be controlled to be higher at certain areas, in particular closer to the examination area, i.e. said coils segments are preferably arranged such that in the direction of the associated coils axis the obtained current density increases with decreasing distance from the examination area. This further increases the efficiency of the generated magnetic fields. 
     For the purpose of controlling the desired current density different measures with respect to the coil segments can be taken. In particular, one or more coil segments of a coil arranged closer to the examination area are, compared to one or more coil segments of the same coil arranged farther away from the examination area, made of a different material, have thicker windings, are more compact and/or have a higher thickness in the direction of the associated coil axis. 
     In a preferred embodiment said selection-and-focus means further comprises at least one pole shoe having a number of pole shoe segments carrying the various selection-and-focus field coils and a pole shoe yoke connecting said pole shoe segments. Such a pole shoe does not only serve as a mechanical carrier for the various coils, but also for increasing the efficiency of the magnetic fields by conducting the magnetic flux. 
     Preferably, said at least one pole shoe comprises a at least one inner pole shoe segment carrying said at least one inner selection-and-focus field coil and at least two outer pole shoe segments arranged at a larger distance from said inner coil axis and each carrying one of said at least two outer selection-and-focus field coils. Thus, the design of the pole shoe is adapted to the design of the selection-and-focus field coils to optimally support the efficiency of the magnetic field generation. 
     Preferably, said at least one pole shoe comprises at least four outer pole shoe segments each carrying an outer selection-and-focus field coil. Thus, for each outer selection-and-focus field coil an outer pole shoe segment is provided for guiding the magnetic field of the associated selection-and-focus coil. Thus, in an embodiment for the corresponding design of the outer selection-and-focus coils said at least one pole shoe comprises four outer pole shoe segments each carrying an outer selection-and-focus field coil, said outer pole shoe segments being arranged at the same distance from the inner coil axis but angularly displaced by 90° with respect to each other. Still further, each outer pole shoe segment preferably has a cross section in the form of a ring segment. 
     In still another embodiment, in which said selection-and-focus coil comprises a second inner selection-and-focus coil, said at least one pole shoe comprises a second inner pole shoe segment in the form of a closed ring around said first inner pole shoe segment, said second inner pole shoe segment carrying said second inner selection-and-focus field coil. 
     In a preferred embodiment at least the at least one inner pole shoe segment and head portions of the outer pole shoe segments facing the examination area are made from a soft-magnetic material having a high saturation induction, in particular FeCo, FeSi, Fe, FeNi, Dy, Gd or an alloy thereof such as Fe 49 V 1.9 Co 49 . Preferably, the complete pole shoe should be made of the best soft-magnetic material that best guides the magnetic flux. However, for cost reasons only part of the pole is made from this material to have the best saturation magnetization there. The tail portions of the outer pole shoe segments facing away from the examination area and the pole shoe yoke are made from a soft-magnetic material having a lower saturation induction than the material of the inner pole shoe segments, in particular FeSi, FeNi, Permalloy or an alloy thereof such as Fe 73.5 Cu 1 Nb 3 Si 15.5 B 7 . 
     Further, in an embodiment the pole shoes are made from magnetically conductive sheets, wherein sheets forming the pole shoe segments and an adjacent head portion of the pole shoe yoke are arranged in a direction parallel to the inner coil axis. The sheets are used to suppress eddy currents and are arranged to be conduct the magnetic flux. 
     Preferably, sheets forming the tail portion of the pole shoe yoke are arranged in a direction perpendicular to the inner coil axis. This allows guidance of the magnetic flux while eddy currents are suppressed. 
     In an embodiment said selection-and-focus means further comprises a pole shoe bearing connecting said pole shoes mechanically, said pole shoe bearing being made of a magnetically conductive material. Said pole shoe bearing is preferably also made from magnetically conductive sheets that are arranged adjacent to each other in the same direction as sheets forming the portion of the pole shoe to which the pole shoe bearing is connected. The pole shoe bearing should both provide a mechanical stability and a good magnetic flux. 
     In an advantageous embodiment said at least one inner pole shoe segment and said at least one inner selection-and-focus field coil are arranged at a larger distance from the examination area than said outer pole shoe segments and said outer selection-and-focus field coils. This provides the advantage that there is more space for arranging the drive field coils, particularly in case of an apparatus comprising two opposingly arranged sets of selection-and-focus field coil and two opposingly arranged pole shoes, since the drive field coils are preferably not arranged adjacent the outer pole shoe segments. 
     A cross section perpendicular to said inner coil axis through a head portion of said second inner pole shoe segment facing said examination area preferably covers a smaller area than a parallel cross section through a tail portion of said second inner pole shoe segment facing away from said examination area. This increases the gradient field strength obtainable for a given electrical current strength. 
     In another embodiment the outer diameter of said head portion of the second inner pole shoe segment decreases in the direction of the inner coil axis with decreasing distance from the examination area. This provides a higher magnetic flux density on the surface facing the examination area and thus allows providing higher gradients of the magnetic field within the examination area. 
     Further, in an embodiment a cross section perpendicular to said inner coil axis through a head portion of said outer pole shoe segments facing said examination area covers a larger area than a parallel cross section through a tail portion of said outer pole shoe segments facing away from said examination area. This measure also contributed to achieving a higher magnetic flux density on the surface facing the examination area. 
     Another measure contributing to achieving a higher magnetic flux density on the surface facing the examination area is that the distance of the inner diameter of said head portion of the outer pole shoe segments from the inner coil axis decreases in the direction of the inner coil axis with decreasing distance from the examination area. 
     Preferably, the arrangement of the coils of a set of selection-and-focus field coils is rather flat, wherein said outer coil axes are parallel to each other and to the inner coil axis. This arrangement of the coils is space-saving, relatively easy to manufacture and allows to more easily calculate and/or simulate the achievable magnetic fields. 
     In an embodiment said selection-and-focus means comprises 
     i1) a first set of selection-and-focus field coils, 
     i2) at least one second set of selection-and-focus field coils, and 
     i3) a selection-and-focus field generator unit for generating selection-and-focus field currents to be provided to said first and said sets of selection-and-focus field coils for controlling the generation of said magnetic selection-and-focus field. Preferably, one second set of selection-and-focus field coils arranged on the opposite side of the examination area than said first set of selection-and-focus field coils is used resulting in an apparatus where the examination area is accessible from at least one side. This allows easy positioning of a patient within the examination area, e.g. by just lifting the patient from a transport bed to a patient table arranged in the examination area. This also avoids the need of having many coils arrange coaxially around the examination area so that the examination area has the form of tunnel in between into which the patient has to be moved like in conventional MRI scanners. Patients will thus feel less uncomfortable than in those conventional MRI scanners. 
     In other embodiments more than two sets of selection-and-focus field coils are provided which are arranged at different angular positions around the examination area. For instance, in case of three sets, they are preferably displaced by an angle of 120° with respect to each other. 
     Preferably, the selection-and-focus field coils of the first set are identical to the selection-and-focus field coils of the at least one second set. Further, in case of two sets, the various coils of one set are preferably arranged exactly opposite to each respective coils of the other set which also support a more easy calculation of the achievable magnetic fields. 
     In an embodiment said selection-and-focus field generator unit is configured to generate selection-and-focus field currents individually for each selection-and-focus field coil of said at least one sets of selection-and-focus field coils. This provides the highest flexibility for generating the desired magnetic fields, but also requires the highest number of generator units/channels. 
     To reduce the number of generator units/channels it is proposed in a preferred embodiment that said selection-and-focus field generator unit is configured to generate selection-and-focus field currents individually for each pair of selection-and-focus field coils of said first and second sets of selection-and-focus field coils, wherein a pair comprises the opposingly arranged selection-and-focus field coils of the two sets. 
     Another proposal for reducing the number of generator units/channels provides that said selection-and-focus field generator unit is configured to generate selection-and-focus field currents individually for each pair of outer selection-and-focus field coils of said at least one set of selection-and-focus field coils, wherein a pair comprises two opposingly arranged outer selection-and-focus field coils of the same set of selection-and-focus field coils. 
     Preferably, as mentioned above briefly, the apparatus comprises at least two pole shoes arranged on different sides of said examination area, each pole shoe having a number of pole shoe segments carrying the various selection-and-focus field coils and a pole shoe yoke connecting said pole shoe segments. 
     To shield the at least one set of selection-and-focus field coils from magnetic fields generated by the drive field coils an inner surface of said at least one set of selection-and-focus field coils facing said examination area is covered by a shielding. This shielding particularly prevents a disturbance of the measurement signal, which would occur if the drive field interacts with the soft-magnetic material. 
     As mentioned above said drive field coils are arranged in the area between said first inner selection-and-focus field coils of the twos 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. 
     Preferably, said drive field coils are smaller in a direction perpendicular to the inner coil axis than the distance in said direction between to opposing outer selection-and-focus field coils. Further, preferably, said drive field coils comprises two pairs of saddle coils arranged around a central symmetry axis perpendicular to said inner coil axis and a solenoid coil arranged around said central symmetry axis. 
     For receiving detection signals need 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. 
    
    
     
       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 perpendicular cross sections through an embodiment of a selection-and-focus field coil arrangement for the third and fourth embodiments of the MPI apparatus, 
         FIG. 7  shows two perpendicular cross sections through an embodiment of a pole shoe arrangement for the third and fourth embodiments of the MPI apparatus, 
         FIG. 8  shows a perspective view of the embodiment of a pole shoe arrangement shown in  FIG. 7 , 
         FIG. 9  shows two perpendicular cross sections through an embodiment of a selection-and-focus field coil arrangement for the third and fourth embodiments of the MPI apparatus, 
         FIG. 10  shows an enlarged of one of the cross sections through an embodiment of one set of selection-and-focus field coils of the selection-and-focus field coil arrangement shown in  FIG. 9 , 
         FIG. 11  shows a perspective view of another embodiment of a pole shoe arrangement for the third and fourth embodiments of the MPI apparatus, 
         FIG. 12  shows a perspective view of another embodiment of a selection-and-focus field coil arrangement for the third and fourth embodiments of the MPI apparatus, 
         FIG. 13  shows a perspective view of still another embodiment of a selection-and-focus field coil arrangement for the third and fourth embodiments of the MPI apparatus, 
         FIG. 14  shows a diagram showing the gradient field strength as a function of electrical power for the third and fourth embodiments of the MPI apparatus, 
         FIG. 15  illustrates background signal measurements according to a first preferred embodiment, 
         FIG. 16  illustrates background interpolation, smoothing and correction, 
         FIG. 17  shows different slices through a volume acquired in a calibration scan, and 
         FIG. 18  shows the signal content of different frequency components. 
     
    
    
     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. 
     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 contrast agent 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 1304542, 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 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 ,  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. 
     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 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 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 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. 
     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. 
     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. 
     While generally selection field coils and focus field coils are implemented as separate elements according to the present invention, according to a preferred embodiment of the present invention said selection-and-focus field coils  114  comprise at least one inner selection-and-focus field coil  115  being formed as a closed loop about an inner coil axis  115   a , and a group of at least two outer selection-and-focus field coils  116 ,  117  arranged at a larger distance from said inner coil axis  115   a  than said at least one inner selection-and-focus field coil  115  and at different angular positions, each being formed as a closed loop about an associated outer coil axis  116   a ,  117   a  as shown in  FIGS. 6A and 6B  showing perpendicular cross sections. Preferably, two additional outer selection-and-focus field coils  118 ,  119 , each being formed as a closed loop about an associated outer coil axis  118   a ,  119   a  are provided as indicated by the dashed lines in  FIG. 6B . 
     It is generally possible according to the present invention that the selection-and-focus field means only comprises various coils as shown in  FIG. 6 . However, it is preferred according to the present invention that the selection-and-focus field means are a combination of magnetic material in the form of one or more pole shoes, particularly soft-magnetic material, and electromagnetic coils. The at least one pole shoe serves for conducting the magnetic flux and, thus, for increasing the generation of the required magnetic fields. 
     An embodiment of a pole shoe arrangement is shown in  FIGS. 7 and 8 , wherein  FIGS. 7A and 7B  show two perpendicular cross sections through the pole shoe arrangement  300  and  FIG. 8  shows a perspective view of the pole shoe arrangement  300 . In this embodiment of the pole shoe arrangement  300  two pole shoes  310 ,  320  are provided which are connected via a pole shoe bearing  330  mechanically carrying and magnetically coupling the two pole shoes  310 ,  320 . While the pole shoes  310 ,  320  shown in these figures will, in this embodiment, have the geometrical properties shown here, the particular shape of the pole shoe bearing  330  is only shown here as a simple example, while the particular shape for a practical application will be determined by construction parameters like the required stability. 
     As shown in  FIGS. 7 and 8  each pole shoe  310 ,  320  comprises at least one, here in this embodiment two, inner pole shoe segments  311 ,  312  and  321 ,  322 , respectively, and at least two, here in this embodiment four, outer pole shoe segments  313 - 316  and  323 - 326 , respectively. Further, each pole shoe  310 ,  320  comprises a pole shoe yoke  317  and  327 , respectively, that connects the various pole shoe segments of the same pole shoe. 
     All pole shoe segments of a common pole shoe are coaxially arranged about the common inner coil axis  115   a  wherein the second inner pole shoe segments  312 ,  322  are arranged as rings around the respective inner pole shoe segment  311 ,  321 . The outer pole shoe segments  313 - 316  and  323 - 326 , respectively, are each designed in form of a ring segment arranged at the same distance around the inner coil axis  115   a  but have different angular positions as shown in  FIG. 7B . 
     Such an arrangement of pole shoes, on which the various coils of the selection-and-focus field coils are arranged as will be shown and explained below, is advantageous for achieving the desired movement of the selection-and-focus field coil (the first sub-zone  52 ). The segmentation of the outer pole shoe segments, here in two to four segments (generally at least two segments, but also more segments are possible), is particularly advantageous for movement of the FFP along the x- and y-direction. 
     In a practical implementation, the distance d i  between the inner pole shoe segments  311 ,  321  (in z-direction) is at least so large that a patient as well as drive field coils can be arranged there between. This means that the distance d i  should be at least 40 cm, preferably at least 45 cm. The distance d o  between the outer pole shoe segments b can be slightly smaller since there between no drive field coils are generally arranged. Hence, the distance d o  should be at least 25 cm, preferably at least 40 cm. 
     The pole shoes are generally made of soft-magnetic material. Preferably, the two inner pole shoe segments  311 ,  312  and  321 ,  322 , respectively, and head portions  313   h - 314   h  and  323   h - 324   h  (see  FIG. 7A ; the head portions of the other outer pole shoe segments are not explicitly shown in this figure) are made from a soft-magnetic material and have a high saturation induction, in particular FeCo, Fe, Dy, Gd or an alloy thereof such as Fe 49 V 1.9 CO 49  (such as the material known under the trade name Vacoflux48). Alternatively, FeNi may be used, but this material has a lower saturation induction. Preferably, the tail portions  313   t ,  314   t  and  323   t ,  324   t  of the outer pole shoe segments (the tail portions of the outer pole shoe segments  315  or  316 ,  325 ,  326  are not explicitly shown) facing away from the examination area and the pole shoe yoke are made from the same material. However, for cost reasons it is possible to make them from a soft-magnetic material having a lower saturation induction than the material of the inner head pole shoe segments, in particular FeSi, FeNi, Permalloy or an alloy thereof such as Fe 73.5 Cu 1 Nb 3 Si 15.5 B 7  (commonly known as Nanoperm). 
       FIG. 9  shows two perpendicular cross sections through an embodiment of a selection-and-focus field coil arrangement  400  in which the various selection-and-focus field coils are mounted on a pole shoe arrangement  300  as shown in  FIGS. 7 and 8 . 
       FIG. 10  shows an enlarged view of a single selection-and-focus field coil sub-unit  410  which will be used to explain further details thereof. The first inner pole shoe segment  311  carries the first inner selection-and-focus field coil  115  which is formed as a ring around said first inner pole shoe segment  311 . A second inner selection-and-focus field coil  113  is formed as another ring coil which is carried by the second inner pole shoe segment  312 , which itself is formed as a ring around said first inner selection-and-focus field coil  115 . Four outer selection-and-focus field coils  116 ,  117  (only two outer selection-and-focus field coils are shown in  FIGS. 9 and 10 ; the other two outer selection-and-focus field coils are not shown in  FIGS. 9 and 10 ) are carried by respective outer pole shoe segments  313 ,  314 ,  315 ,  316 . Each of said outer selection-and-focus field coils  116 ,  117  is wound around its associated outer pole shoe segment  313 ,  314 ,  315 ,  316  so that the current flows around the respective outer pole show segment. Each outer pole shoe segment  313 ,  314 ,  315 ,  316  has the form of a ring segment arranged at different angular positions around the inner coil axis  115   a.    
     Thus, the selection-and-focus field coil arrangement  400  shown in  FIG. 9A  comprises in total twelve selection-and-focus field coils, six coils (the coils  113 ,  115 - 119 ) in the upper selection-and-focus field coil sub-unit  410  and six coils (the coils  133 ,  135 ,  136 ; the remaining two coils are not visible in  FIG. 9A ) in the lower selection-and-focus field coil sub-unit  420 . This number shall, however, only be understood as an exemplary number. Other numbers are possible as well. Generally, at least six, preferably at least eight, selection-and-focus field coil units are desired. 
     Preferably, for each selection-and-focus field coil a single selection-and-focus field generator sub-unit is provided so that each selection-and-focus field coil can be individually controlled by providing an individual current to the selection-and-focus field coil. However, it is also possible to couple selection-and-focus field coils together and provide them with a common current so that the number of selection-and-focus field generator sub-units can be reduced. For instance, in an embodiment the two outer selection-and-focus field coils  116  and  117  are provided by a common current. Similarly, the other two outer selection-and-focus field coils are coupled together. This means that for such an selection-and-focus field coil arrangement in total eight selection-and-focus field generator sub-units are required. 
     In another embodiment, the two oppositely arranged selection-and-focus field coils of two different selection-and-focus field coil sub-units  410 ,  420  are coupled together and provided with a common current. For instance, the two (in  FIG. 9 ) outer selection-and-focus field coils on the right-hand side may be coupled together and be provided with the identical current. The same holds for the other outer selection-and-focus field coils. 
     Preferably, according to an embodiment one or more of the selection-and-focus field coils are split into at least two, in particular at least four, coil segments, wherein coil segments of a coil are arranged adjacent to each other in the direction of the associated coil axis (which means, in the direction of the inner coil axis  115   a  if all coil axes are parallel as in the depicted embodiment) and wherein adjacent coil segments are electrically connected. Preferably, as shown in  FIGS. 9 and 10 , all selection-and-focus field coils are split into several coil segments as indicated by multiple coil sample division lines in  FIGS. 9A and 10 . 
     For instance, the first inner selection-and-focus field coil  115  is split into four coil segments indicated by letters A, B, C, D in  FIG. 10 . Similarly, the second inner selection-and-focus field coil  113  and the various outer selection-and-focus field coils  116 ,  117  are split into a plurality of coil segments indicated by letters A, B, C, etc. 
     This splitting of the selection-and-focus field coils into several segments enables the realization of different current densities along the respective selection-and-focus field coil. The following table summarizes, as an exemplary embodiment, the maximum current densities for each coil segment. These exemplary values for the current densities are obtained from simulation runs taking into account that different locations of the selection-and-focus field coil require large currents in different coils. Over all, the total of electrical power was at −100 kW. The maximum power in the first inner selection-and-focus field coil was 49 kW, while no more than 38 kW was used for the currents in the second inner selection-and-focus field coil. In each of the outer selection-and-focus field coils no more than 20 kW dissipated. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 curd [A/m 2 ] 
                 curd [A/mm 2 ] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 113A 
                 3.9104E+07 
                 39,1042 
               
               
                   
                 113B 
                 3.0290E+07 
                 30,2900 
               
               
                   
                 113C 
                 1.4279E+07 
                 14,2788 
               
               
                   
                 113D 
                 1.2366E+07 
                 12,3658 
               
               
                   
                 115A 
                 1.4485E+07 
                 14,4853 
               
               
                   
                 115B 
                 1.3682E+07 
                 13,6820 
               
               
                   
                 115C 
                 1.2966E+07 
                 12,9664 
               
               
                   
                 115D 
                 1.2250E+07 
                 12,2499 
               
               
                   
                 115E 
                 1.1529E+07 
                 11,5291 
               
               
                   
                 115F 
                 1.0699E+07 
                 10,6994 
               
               
                   
                 115G 
                 9.9520E+06 
                 9,9520 
               
               
                   
                 115H 
                 8.9570E+06 
                 8,9570 
               
               
                   
                 115I 
                 1.0142E+07 
                 10,1418 
               
               
                   
                 115J 
                 7.8558E+06 
                 7,8558 
               
               
                   
                 115K 
                 4.5355E+06 
                 4,5355 
               
               
                   
                 115L 
                 4.7809E+06 
                 4,7809 
               
               
                   
                 117A 
                 7.0403E+06 
                 7,0403 
               
               
                   
                 117B 
                 7.0148E+06 
                 7,0148 
               
               
                   
                 117C 
                 6.9895E+06 
                 6,9895 
               
               
                   
                 117D 
                 6.9645E+06 
                 6,9645 
               
               
                   
                 117E 
                 6.9398E+06 
                 6,9398 
               
               
                   
                 117F 
                 6.9153E+06 
                 6,9153 
               
               
                   
                 117G 
                 6.8911E+06 
                 6,8911 
               
               
                   
                 117H 
                 6.8671E+06 
                 6,8671 
               
               
                   
                 117I 
                 6.8434E+06 
                 6,8434 
               
               
                   
                 117J 
                 6.8199E+06 
                 6,8199 
               
               
                   
                   
               
            
           
         
       
     
     Preferably, the coil segments are arranged such that in the direction of the associated coil axis in the obtained current density increases with decreasing distance from the examination area. Various embodiments accessed are to obtain this. Preferred embodiments include that one or more coil segments of the coil arranged closer to the examination area are, compared to one or more coil segments of the same coil arranged further away from the examination area, made of a different material, have thicker windings, are more compact and/or have a higher thickness in the direction of the associated coil axis. For instance, the ratios of the current densities of the different coil segments are used to determine how the wire cross sections should be varied within each coil. In practice, however, deviations from the theoretical values are certainly required since manufacturers of wires generally provide only a limited number of cross section values. 
     It can further be observed from  FIGS. 9 and 10  that in this preferred embodiment a cross section perpendicular to the inner coil axis  115   a  through a head portion  312   h  of the second inner pole shoe segment  312  facing the examination area, i.e. a cross section along line X shown in  FIG. 10 , covers a smaller area than the parallel cross section through a tail portion  312   t  of said second inner pole shoe segment  312  facing away from said examination area, i.e. along line Y shown in  FIG. 10 . 
     Preferably, the outer diameter of said head portion  312   h  of the second inner pole shoe segment  312  decreases in the direction of the inner coil axis  315   a  with decreasing distance from the examination area  230 . In other words, the outer edges of the head portion  312   h  are inclined in the direction of the inner coil axis  315   a.    
     Still further, a cross section perpendicular to the inner coil axis  315   a  through a head portion  313   h ,  314   h  of the outer pole shoe segments  313 ,  314  (the same holds for the other outer pole shoe segments not explicitly shown in  FIG. 10 ) facing said examination area, i.e. along line X, covers a larger area than a parallel cross section through the tail portion  313   t ,  314   t  of said outer pole shoe segments  313 ,  314  facing away from the examination area, i.e. a cross section along line Y. 
     Still further, the distance of the inner diameter of said head portions  313   h ,  314   h  of the outer pole shoe segments  313 ,  314  (the same holds for the other, not shown outer pole shoe segments) from the inner coil axis  315   a  decreases in the direction of the inner coil axis  115   a  with decreasing distance from the examination area  330 . In other words, the inner edges of the head portions  313   h ,  314   h  are inclined in the direction of the inner coil axis  115   a.    
     As shown, the second inner selection-and-focus field coil  113  and the outer selection-and-focus field coils  116 ,  117  (the same holds for the other not shown outer selection-and-focus field coils) are moved around the respective pole shoe segment assembling the same outer shape than the corresponding pole shoe segment, which is, however, not necessarily required. 
     These measures provide for the highest flux density on the surface of the inner pole shoe segments  311 ,  312  and the inner selection-and-focus field coils  113 ,  115  facing the examination area, particularly to obtain a high gradient of the magnetic field. It shall be noted, that also the outer edges of the outer pole shoe segments can be inclined into the direction of the inner coil axis  115   a  to further increase this effect. 
     For moving the field of view  28  through the examination area, which is conventionally achieved by use of the magnetic focus field, it is generally not required to provide all selection-and-focus field coils with currents. In particular, for moving the field of view  28  in the upper or lower direction, i.e. along the inner direction of the inner coil axis  115   a , mainly the two inner selection-and-focus field coils  115 ,  113  are used. For instance, if a movement of the field of view  28  is desired from the upper selection-and-focus field coil sub-unit  410  in the direction of the lower selection-and-focus field coil sub-unit  420  a current provided to the first inner selection-and-focus field coil of the lower selection-and-focus field coil sub-unit  420  and to the current provided to the second inner selection-and-focus field coil of the upper selection-and-focus field coil sub-unit  410  are increased. Alternatively or in addition the current provided to the first inner selection-and-focus field coil of the upper selection-and-focus field coil sub-unit  410  and the current provided to the second inner selection-and-focus field coil of the lower selection-and-focus field coil sub-unit  420  are decreased. The outer selection-and-focus field coils need not necessarily be used for such a movement. 
     If a movement of the field of view  28  is desired in a direction perpendicular to the inner coil axis  115   a , the outer selection-and-focus field coils are additionally provided with currents. In particular, by said outer selection-and-focus field coils an additional magnetic field is generated in a direction along the desired direction of movement and perpendicular to the inner coil axis  115   a . For instance, if a movement from left to right is desired in  FIG. 9A  a magnetic field is additionally generated having a north pole on the left side and a south pole on the right side (or vice versa). By the amplitude of the current provided to the outer selection-and-focus field coils it can than be controlled how far the field of view  28  shall be moved in this direction. 
     The above explanation only provides a brief general idea how movement of the field of view can generally be achieved. In practice, of course, the currents need to be controlled precisely which is, however, only a matter of implementation which strongly depends on the exact layout of the overall arrangement. 
     With respect to the pole shoes it shall be noted that they are preferably made from magnetically conductive sheets, wherein sheets forming the inner pole shoe segments  311 ,  312  and an adjacent head portion  317   h  of the pole shoe yoke  317  of the pole shoe  310  (the same holds for the inner pole shoe segments and the pole shoe yoke of the other pole shoe  320 ) are arranged along a direction parallel to the inner coil axis  315   a . Sheets forming the tail portion  317   t  of the pole shoe yoke  317  (the same holds for the other pole shoe yoke  327 ) are preferably arranged in a direction substantially perpendicular to the inner coil axis  315   a . This provides for an optimum connectivity of the magnetic flux. 
     In case of using two or more pole shoes that are connected by a pole shoe bearing  330 , as shown in  FIG. 8 , it is preferred that also the pole shoe bearing  330  is made from magnetically conductive sheets that are arranged adjacent to each other in the same direction as sheets forming the portion of the pole shoe to which the pole shoe bearing is connected. For instance, if the pole shoe bearing connects to the head portions of the pole shoe yokes, the sheets of the pole shoe bearing are preferably arranged in a direction perpendicular to the inner coil axis. The sheets forming the pole shoe bearing are also arranged in a direction perpendicular to the inner coil axis  315   a  at least at the connection to the pole shoe yokes. Generally, the sheets should be arranged such that the best magnetic flux connectivity is achieved. 
       FIG. 11  shows a perspective view of an embodiment of a pole shoe arrangement  500 . Compared to the pole shoe arrangement  300  shown in  FIG. 8 , the outer pole shoe segments are not formed in this embodiment as ring-shaped segments, but the outer pole shoe segments  512 - 517  (of the first pole shoe  510 ) and  522 - 527  (of the second pole shoe  520 ) are formed as bar-shaped cylinders, preferably in the same shape as the inner pole shoe segments  511 ,  521 . The advantages of such an arrangement are primarily cost savings since only one or two kinds of pole shoes need to be manufactured. The main advantage is realized, if at least a second ring of pole shoes (not shown in  FIG. 11 ) is arranged around the central pole shoe. In other embodiments, also further forms of the pole shoe segments, in particular of the outer pole shoe segments, are useable. 
       FIG. 12  shows a perspective view of another embodiment of a selection-and-focus field coil arrangement  600 . In this embodiment the pole shoe arrangement  500  depicted in  FIG. 11  is used, wherein each pole shoe segment is provided with an individual selection-and-focus field coil wound around a ring-shaped coil  611 - 617  (for the upper selection-and-focus field coil sub-unit  610 ; the same holds for the lower selection-and-focus field coil sub-unit  620 ). 
     There are further embodiments of selection-and-focus field coil arrangements. For instance, in still another embodiment of a selection-and-focus field coil arrangement  600 ′ shown in  FIG. 13  a large cylindrical magnetic field coil  631 ,  632  is placed outside of and around each selection-and-focus field coil sub-unit  610 ,  620 . Further, it is possible to arrange one or more additional magnetic field coil(s)  640  around the magnetic bearing  630  to further strengthen the magnetic field. 
     It shall be noted that in addition to the various selection-and-focus field coils additionally a permanent material in each selection-and-focus field coil sub-unit may be provided to further strengthen the generation of the magnetic selection field for generating the selection-and-focus field coil. This permanent magnet would preferably be located close to the examination zone substituting parts of the soft-magnetic material. 
     Further, it shall be noted that the cooling means are preferably provided for cooling some or all of the coils. The cooling means may use a cooling fluid like water or oil. The coils may be made from copper or aluminum, but it is also possible to make them from superconductive material, which would then be cooled by use of an appropriate cooling material such as helium. In case of high temperature superconductive conductors the cooling can be achieved by use of gaseous helium. In case of low temperature superconductive conductors the cooling can be achieved by use of liquid helium. 
     Using the geometry described above different simulation runs were performed. The results obtained this way are summarized in the following. 
     For the FFP located at the center of the geometry a gradient field strength of 2.5 T/m was obtained with an electrical power of 30 kW. Using 90 kW of electrical power the gradient field strength increased to 3.3 T/m.  FIG. 14  shows how the gradient field strength increases with the electrical power. For these simulations only the inner selection-and-focus field coils. No current flew in the outer selection-and-focus field coils. In particular, the electrical power in the second inner selection-and-focus field coils was four times larger than in the first inner selection-and-focus field coils. 
     With respect to movement in z-direction, using the inner selection-and-focus field coils the FFP could be placed on the z-axis at a distance of 10 cm from the origin. With a total power consumption of 92 kW the gradient field strength obtained was 2.5 T/m. The electrical power was distributed among the coils as follows. For the pole shoe in the direction in which the FFP was moved the first inner selection-and-focus field coil dissipated 49 kW, while no current flew in the second inner selection-and-focus field coil. For the pole shoe in the other direction the first inner selection-and-focus field coil dissipated 5 kW, while 38 kW were necessary in the second inner selection-and-focus field coil. 
     With respect to movement in x- and/or y-direction, using the outer selection-and-focus field coils the FFP can be moved along x and/or y. For instance, in one of the simulations, the FFP was placed on the x-axis at a distance of 10.1 cm from the origin. Here a total electrical power of 100 kW was used. A power of 40 kW dissipated in four of the outer selection-and-focus field coil, while the remaining 60 kW were used in the inner selection-and-focus field coils. The gradient field strength obtained was 2.2 T/m. Nevertheless, the gradient was rather inhomogeneous. Using common computation methods the values obtained are G x =−0.69 T/m and G y =−1.51 T/m. 
     For certain applications (MR) it is desirable to generate a magnetic field which does not have an FFP but is rather homogeneous. Simulations were therefore performed in which the current direction in one of the inner pole-shoes was reversed. Using all coils and different distributions of the available power (100 kW) the maximum observed field strength at the origin was 0.45 T. The field strength increases along z and decreases along x/y. To compute the energy stored in the magnetic field the integral
 
 E= ½∫ V   B·HdV  
 
is evaluated over volume V. Within our simulations the maximum observed energy stored in the magnetic field was below 40 kJ. The maximum was seen in a simulation trying to obtain a homogeneous (MR) field.
 
     In the following the process of reconstruction as proposed according to the present invention and as performed by the reconstruction unit  152  shall be explained in more detail. As explained above, background signal variations and spurious signals, which occur both in MPI calibration scans and object scans, can strongly compromise image quality. Different sources of background signal exhibit different spectral behavior, so that background signal contributions are not homogeneously spread over the measured bandwidth, but vary in intensity and temporal pattern between different frequency components. 
     In MPI, a calibration scan is used to determine the system function (SF) that relates signal response to spatial position of the signal-generating tracer. The calibration scan can be done using a robot that positions a magnetic sample smaller than or equal to the desired resolution at all voxel positions of the imaging volume. A robot-based SF acquisition is rather slow and can be interspersed with background measurements. For instance, if a 3D box-shaped volume with Nx*Ny*Nz voxels is to be measured, the robot has to position the calibration sample at all these positions. For a background determination, the sample can be moved outside the sensitive volume after a line of voxels is measured, so that a reference scan of the empty scanner can be performed. Part of this timing scheme for a 3D SF scan is shown in  FIG. 15 . 
       FIG. 15  shows that background measurements  700  are interspersed between calibration measurements  710  at different robot positions. The steps indicate the robot positions along the three spatial axes. The initial part of a system function acquisition with 14×14×10 voxels is shown. For the background measurements, the calibration sample is removed from the sensitive imaging volume. Between the background measurements, one line along the z direction is measured. 
     By interpolation of the background data, slow background signal variations, e.g. due to drifts, can be removed from the SF data as shown in  FIG. 16 . Interpolation can be just linear, but more advanced methods like Fourier interpolation also allow simple application of a smoothing filter to the data (as shown in  FIGS. 16D and 16E ).  FIG. 17  shows the effect of the background correction on the spatial patterns acquired in the calibration scan. 
     In particular,  FIG. 16  shows background interpolation, smoothing, and correction (in arbitrary units (a.u.), e.g. in volts, over the samples). One frequency component (x channel, component 2*fx−fy+2*fz=1583) of a system function acquisition with 28×28×20=15680 voxels is shown as an example. At the beginning of the scan and after every 20 voxel positions, a background measurement was performed, amounting to 1+28×28=785 background measurements. Over the total number of 15680+785 robot positions, only the background measurements are shown, while the intermediate positions are interpolated.  FIG. 16A  shows the background measurements  700  and the Fourier interpolation  705 .  FIG. 16D  shows a smoothed (low-pass filtered) Fourier interpolation  715 .  FIGS. 16B and 16E  show a zoomed view of the respective data.  FIGS. 16C and 16F  show the measured calibration data before (signals  720 ) and after ( 725 ,  730 ) background subtraction for the two cases interpolation and interpolation with smoothing. 
       FIG. 17  shows xy-slices through the volume acquired in a calibration scan. Again, the x channel signal of frequency component 2*fx−fy+2*fz=1583 is shown.  FIG. 17A  shows three slices at different z positions before background correction;  FIG. 17B  shows the same slices after background correction. At the upper edge of the calibration images (indicated by the arrows), the effect of the correction is visible. 
     It shall be noted that an alternative approach to the above described approach of performing a calibration scan for obtaining background measurements is the use of a system calibration unit that is able to generate local fields of different spatial positions using coils, so that no spatial repositioning of the sample and thus no robot is required. The idea of a system calibration unit (SCU) is to reproduce the fields seen by the magnetic sample at the positions used for the system function at one fixed much like a spectrometer. In an SCU, homogeneous focus fields are used to emulate positional changes. Whereas in an MPI scanner the sample size is limited by the gradient field used to spatially encode the signal, an SCU will have no such limitation. Additionally, highly sensitive receive coils can be used to measure the signal from the stationary sample. The magnetic particle properties will be explicitly accounted for. Thus, an SCU combines the benefits of both a robot based system function and a simulated one. Alternatively to a dedicated SCU, any MPI scanner with focus fields can be used as an SCU. When using an SCU to estimate the system function for another MPI scanner, it is necessary to account for all aspects of the MPI scanner: all the magnetic fields, receive chain characteristics, etc. 
     With the above described method slowly varying background signals can preferably be removed, while changes more rapid than the temporal spacing of the background measurements will not be removed. However, frequencies which are suffering from these changes can be identified. One method is to measure the full system function twice, once with a tracer sample and once without a sample. After removal of the slowly varying background using the above approach, a spectral plot of the signal content at each frequency in the empty measurement reveals the spectral location of spurious signals (cf.  FIG. 18A ). A measure of signal content could be the standard deviation of the signal at a given frequency over the different spatial positions. This information can be used to weight frequency components in the reconstruction process, with the aim of reducing the influence of components with a high background signal level. Moreover, the ratio between the spectral signal content of the measurements with and without sample gives an indication of the ratio between particle signal and background signal, which is related to the SNR (cf.  FIG. 18E ). The SNR spectrum, i.e. frequency-specific SNR factors, can be used for selecting frequency components for reconstruction by applying a threshold (also called SNR threshold). Furthermore, the SNR can act as a scaling factor for weighting frequency components. Generally, however, other signal quality factors and signal quality thresholds can be used as well as explained above. 
     Since the measurement of two system functions is rather lengthy, an alternative approach is to incorporate measurements of the more rapid variations in a system function scan, e.g. by using a longer sampling window for the interspersed background measurements than for the voxel measurements. Referring to  FIG. 15  this would mean that the time window  700  would be made longer. Then a measure of the signal variation (e.g. the standard deviation) can be determined in these time windows and can be the input for a similar treatment as discussed above, with the aim to find an SNR measure for thresholding and weighting. The background determined using this method is shown in  FIG. 18B  and is very similar to the background from the time-consuming empty measurement as shown in  FIG. 18A . 
     Thus,  FIG. 18  shows the signal content at different frequency components determined from the standard deviation over the signal at different positions. The data is acquired on the x receive coil during a 3D Lissajous excitation. The spacing of frequency components is 46.42 Hz, so that the width of the spectrum amounts to 625 kHz.  FIGS. 18D and 18E  show an SNR measure derived from the ratio of the signal shown in  FIG. 18C  over the signals shown in  FIGS. 18A and 18B , respectively.  FIG. 18A  shows the signal content derived from an empty SF measurement.  FIG. 18B  shows the signal content derived from an empty measurements interspersed in a SF acquisition.  FIG. 18C  shows the signal content measured with a calibration sample.  FIG. 18D  shows the SNR from  FIG. 18C / FIG. 18A .  FIG. 18C  shows the SNR from  FIG. 18C / FIG. 18B . 
     To increase SNR in the calibration scan, the volume acquisition sequence is repeated several (e.g.  50 ) times at each robot positions. The above treatment is preferably applied after averaging of these measurements. To further increase the temporal resolution of the measured background signal variations, the above treatment can be applied to the unaveraged data. In principle, also variations during one repetition time of the sequence can be accessed. 
     Thus background variations can be identified at different time scales and this information can be applied to optimize thresholding and weighting of frequency components for reconstruction. 
     The present invention may further be applied for background removal for object (video) scans. Before and after the object or patient is placed in the scanner, background data can be acquired. Interpolation between the two measurements allows background correction for object data. Furthermore, like for the SF, frequency components with fluctuations during the background measurement intervals can be identified and treated accordingly. 
     Background subtraction is a crucial feature in MPI to ensure decent image quality in view of scanner imperfections and spurious signals picked up by the receive system. The present invention provides a solution for efficient background measurement and subtraction. 
     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.