GLEICH, Bernhard (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
WEIZENECKER, Juergen (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
DAHNKE, Hannes (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
PHILIPS INTELLECTUAL PROPERTY & STANDARDS GmbH (20099 Hamburg, 20099, DE)
BORGERT, Joern (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
GLEICH, Bernhard (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
WEIZENECKER, Juergen (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
DAHNKE, Hannes (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
| CLAIMS: 1. System for generating a system function for use in the reconstruction of images in an imaging device for imaging the distribution of a tracer material in an object, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said system comprising: an acquisition means (220) for acquiring a first portion (SFn) of the system function, said first portion describing dynamics of the tracer material, an imaging device (210) for acquiring a second portion (SFSc) of the system function, said second portion (SFSc) describing characteristics of the imaging device, combination means (230) for combining said first and second portions to generate said system function. 2. System as claimed in claim 1, wherein said acquisition means (220) is provided at the manufacturer or distributor of the tracer material. 3. System as claimed in claim 1, wherein said acquisition means (220) comprises a system calibration unit. 4. System as claimed in claim 1, wherein said acquisition means (220) is adapted for acquiring said first portion (SFTr) of the system function for each new batch of tracer material. 5. System as claimed in claim 1, further comprising a first storage means (250) for storing said first portion (SFn) of the system function, in particular in electronic form, for instance on a record carrier or on the internet, or in printed form, for instance on a label or a document accompanying the tracer material. 6. System as claimed in claim 1, wherein said imaging device (210) comprises a second storage means (216) for storing its second portion (SFSc) of the system function. 7. System as claimed in claim 1, wherein said imaging device (210) and/or the manufacturer of the tracer material comprise a delivery means (226, 212) for delivering the first portion (SFn) of the system function and the second portion (SFSc) of the system function, respectively, to a central storage means (260) for storing said first and/or second portions of the system function. 8. System as claimed in claim 7, wherein said combination means (230) is provided at said central storage means. 9. System as claimed in claim 1, further comprising a first identification means (224) for assigning a first identifier (250) to the first portion (SFTr) of the system function, wherein said first identifier is provided for delivery to the user of the imaging device and is required by the user to obtain the first portion (SFTr) of the system function. 10. System as claimed in claim 9, further comprising a second identification means (214) for assigning a second identifier (270) to the second portion (SFSc) of the system function, wherein said second identifier is provided for delivery to the user of the imaging device and wherein said first identifier and said second identifier are required by the user to obtain the first and the second portion of the system function or to obtain said system function. 11. Method for generating a system function for use in the reconstruction of images in an imaging device for imaging the distribution of a tracer material in an object, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said method comprising the steps of: acquiring a first portion (SFTr) of the system function, said first portion (SFTr) describing dynamics of the tracer material, acquiring a second portion (SFSc) of the system function, said second portion describing characteristics of the imaging device, combining said first and second portions to generate said system function. 12. Device (220) for providing a first portion of a system function for generating a system function for use in the reconstruction of images in an imaging device for imaging the distribution of a tracer material in an object, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said device comprising: an acquisition means (221) for acquiring said first portion (SFTr) of the system function, said first portion describing dynamics of the tracer material, a first identification means (224) for assigning a first identifier (250) to the first portion (SFTr) of the system function, a first delivery means (222) for delivering said first identifier to the user of the imaging device, and a second delivery means (226) for delivering said first portion (SFTr) of the system function either to the user on request by the user, said request including said first identifier, or to a central storage means. 13. Method for providing a first portion (SFn) of a system function for generating a system function for use in the reconstruction of images in an imaging device for imaging the distribution of a tracer material in an object, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said method comprising the steps of: acquiring said first portion (SFTr) of the system function, said first portion describing dynamics of the tracer material, assigning a first identifier to the first portion (SFTr) of the system function, delivering said first identifier to the user of the imaging device, and delivering said first portion of the system function either to the user on request of the user, said request including said first identifier, or to a central storage means. 14. Imaging device (300) for imaging an object in a field of view and for reconstructing an image representing the distribution of a tracer material in an object by use of a system function, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said imaging device comprising: an acquisition means (310) for acquiring a second portion (SFSc) of the system function, said second portion describing characteristics of the imaging device, - a request means (320) for requesting a first portion (SFTr) of the system function from the manufacturer of the tracer material or from a central storage means, in particular by use of a first identifier assigned to the first portion (SFTr) of the system function, said first portion describing dynamics of the tracer material, a combination means (330) for combining said first and second portions to generate said system function. 15. Imaging device (350) for imaging an object in a field of view and for reconstructing an image representing the distribution of a tracer material in an object by use of a system function, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said imaging device comprising: an acquisition means (310) for acquiring a second portion of the system function, said second portion describing characteristics of the imaging device, a delivery means (360) for delivering said second portion (SFSc) of the system function to a central storage means, where said first and second portions of said system function are combined into said system function, a request means (370) for requesting said system function from said central storage means, in particular by use of a first identifier assigned to the first portion (SFn) of the system function, said first portion (SFn) describing dynamics of the tracer material, and/or a second identifier assigned to the second portion of the system function. |
FIELD OF THE INVENTION
The present invention relates to a system and a method for generating a system function for use in the reconstruction of images in an imaging device for imaging the distribution of a tracer material in an object, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device.
Further, the present invention relates to a device and a method for providing a first portion of such a system function.
Still further, the present invention relates to imaging devices and imaging methods for imaging an object in a field of view and for reconstructing an image representing the distribution of a tracer material in an object by use of a system function, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device.
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 into 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 or Magnetic Resonance Imaging. Accordingly, an MP image of an object'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) at the isocenter of the scanner. Moreover, this FFP 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's volume of interest is enclosed by the scanner's field of view, which is a subset of the volume of scanning.
The object must contain magnetic nanoparticles; if the object is an animal or a patient, a contrast agent (also called "tracer material" in the following) 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 Al 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.
Generally, it is possible for anybody to produce a tracer material that is suitable for MPI. Characteristics of the tracer material are described, for instance, in the applications EP1738773 Al and EP1738774 Al . To result in optimal image quality, each portion of the tracer material (i.e. each batch) has to be characterized in combination with the scanner (generally, the imaging device) used, i.e. the system function (SF) of the imaging device in combination with the tracer response to a given trajectory of the FFP has to be acquired. Hence, it is required to generate and provide the system function and to safeguard the provision of suitable tracer materials.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system and a method for generating a system function for use in the reconstruction of images in an imaging device for imaging the distribution of a tracer material in an object, by which the system function can be easily acquired and by which the provision of tracer material can be safeguarded.
It is a further object of the present invention to provide a device and a corresponding method for providing a first portion of such a system function and to provide a corresponding imaging device and method.
In a first aspect of the present invention a system for generating a system function for use in the reconstruction of images in an imaging device for imaging the distribution of a tracer material in an object is presented, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said system comprising:
an acquisition means for acquiring a first portion of the system function, said first portion describing dynamics of the tracer material,
an imaging device for acquiring a second portion of the system function, said second portion describing characteristics of the imaging device,
combination means for combining said first and second portions to generate said system function. The present invention is based on the recognition that the information generally combined within the system function comprises information about the tracer material and information about the imaging device, which second portion generally comprises information about the reception characteristics, filters used in the imaging device and filters for the excitation fields. Further, it has been recognized that the information about filters and reception characteristics can be separated from the remaining information contained in the system function by use of knowledge about the filter characteristics and the coil sensitivities of the reception coils as well as general standard knowledge.
One possible solution for obtaining the system function would be to determine the excitation fields for all positions in all existing imaging devices (scanners) for each possible trajectory of the FFP (also called sequence). In a system calibration unit the resulting system functions could be recorded using the tracer material to be calibrated, which resulting system functions could then be delivered to the user along with the tracer material. Such a solution would, however, be impracticable and highly time consuming.
A much simpler and more practical solution is hence proposed according to the present invention according to which a sufficient amount of information about the imaging device as well as about the tracer material is acquired and is then combined later to generate the desired system function. For this purpose, in the system according to the present invention appropriate acquisition means, an imaging device and a combination means are provided. Thus, based on, for instance, measurements of data allowing to determine the different portions of the system function and to derive there from the complete system function. Generally, the behaviour and the interaction of the tracer material and the excitation fields depend on each other. This dependency is generally non-trivial since the tracer material itself generates electromagnetic fields in response to the excitation fields, which again have an influence on the tracer material.
In a further aspect of the present invention a device for providing a first portion of a system function for generating a system function for use in the reconstruction of images in an imaging device for imaging the distribution of a tracer material in an object is presented, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said device comprising:
an acquisition means for acquiring said first portion of the system function, said first portion describing dynamics of the tracer material,
a first identification means for assigning a first identifier to the first portion of the system function, a first delivery means for delivering said first identifier to the user of the imaging device, and
a second delivery means for delivering said first portion of the system function either to the user on request by the user, said request including said first identifier, or to a central storage means.
Said device could be set up at the manufacturer or distributor of tracer material. The device and the corresponding method make sure that the provision of suitable tracer material is safeguarded and that the receiver of the tracer material also receives the correct portion of the system function relevant for this (batch of) tracer material. Preferably, a unique identifier is used as first identifier. Further, said device and method provide a simple handling of the data relevant for generating the system function.
In a further aspect of the present invention an imaging device for imaging an object in a field of view and for reconstructing an image representing the distribution of a tracer material in an object by use of a system function is presented, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said imaging device comprising:
an acquisition means for acquiring a second portion of the system function, said second portion describing characteristics of the imaging device,
a request means for requesting a first portion of the system function from the manufacturer of the tracer material or from a central storage means, in particular by use of a first identifier assigned to the first portion of the system function, said first portion describing dynamics of the tracer material,
a combination means for combining said first and second portions to generate said system function.
In still a further aspect of the present invention an imaging device for imaging an object in a field of view and for reconstructing an image representing the distribution of a tracer material in an object by use of a system function is presented, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by the imaging device, said imaging device comprising:
- an acquisition means for acquiring a second portion of the system function, said second portion describing characteristics of the imaging device,
a delivery means for delivering said second portion of the system function to a central storage means, where said first and second portions of said system function are combined into said system function, a request means for requesting said system function from said central storage means, in particular by use of a first identifier assigned to the first portion of the system function, said first portion describing dynamics of the tracer material, and/or a second identifier assigned to the second portion of the system function.
According to these aspects of the present invention different embodiments of imaging devices, which may be part of the above-described system and which may cooperate with the above-described device for providing the first portion of the system function, e.g. set up at the manufacturer or distributor of the tracer material, are presented. Those imaging devices may be set up at a hospital. By the proposed devices it is ensured again that the suitable tracer material is used along with the correct system function.
In further aspects of the present invention methods, which correspond to the system and the devices as explained above, are presented. Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed devices and methods have similar and/or identical preferred embodiments as the claimed system and as defined in the dependent claims.
According to a preferred embodiment the acquisition means of the claimed system is provided at the manufacturer or distributor of the tracer material. Hence, the acquisition of the first portion of the system function describing the dynamics of the tracer material are separated, in time and in space, from the acquisition of the second portion of the system function describing characteristics of the imaging device. Preferably, for said acquisition a system calibration unit is used. Such a system calibration unit may be provided for normal or hybrid system calibration, i.e. used both calibration measurements and simulated or measured magnetic field data. Such calibration measurements may be made using such a dedicated system calibration unit, which may be able to emulated an arbitrary MPI system in each point of space by use of three orthogonal offset fields, without the need for mechanically moving a probe. This significantly reduces the total amount of time required for such a calibration. Further, the quality of the hybrid system calibration is potentially better than using a system calibration based on actual measurements (by mechanically moving a probe to all points in space), since the sensing coil(s) can be arranged much closer to the probing particle and since, consequently, the signal-to-noise ratio is significantly larger.
According to a further embodiment the acquisition means is adapted for acquiring said first portion of the system function for each new batch of tracer material. This ensures that for each new batch the correct system function can be generated and used and avoids any inaccuracies due to an incorrect system function not matching with the actual batch of tracer material.
In still a further embodiment a first storage means is provided for storing said first portion of the system function, in particular in electronic form, for instance on a record carrier or on the internet, or in printed form, for instance on a label or a document accompanying the tracer material. Thus, different ways are provided how the first portion of the system function can be transmitted to the user or another instance where the first portion is combined with the second portion to generate the system function.
Further, a second storage means is provided in the imaging device for storing its second portion of the system function. In particular, if the second portion is combined with a first portion to generate the system function this second storage means is required. It also enables that this second portion of the system function can already be acquired at the manufacturer of the imaging device, but also later when the imaging device is set up at the user's place.
Preferably, the combination means is provided at the manufacturer of the tracer material, at the manufacturer of the imaging device or at the user of the imaging device. Such combination means may, for instance, be implemented by hard- and/or software elements adapted for combining said first and second portions into said system function. Preferably, a model is used for said combination.
In an embodiment the imaging device and/or the manufacturer (or distributor) of the tracer material comprise a delivery means for delivering the first portion of the system function and the second portion of the system function, respectively, to a central storage means for storing said first and/or second portions of the system function. This reduces the amount of information that needs to be transmitted to the user from the manufacturer or distributor of the tracer material and/or the manufacturer of the imaging device. All the data can be collected at such a separate instance, i.e. the central storage means which may, for instance, be a separate service provider or security device. Said central storage means may be accessible only for registered users and/or may only transmit data on request, in particular including a first and/or second identifier as provided according to further embodiments. Further, also the combination means may be provided at the central storage means so that the complete system function is generated there, which then only needs to be transmitted to the final user.
Hence in an embodiment a first identification means is provided for assigning a first identifier to the first portion of the system function, wherein said first identifier is provided for delivery to the user of the imaging device and is required by the user to obtain the first portion of the system function. Still further, in a more elaborate embodiment, a second identification means is provided for assigning a second identifier to the second portion of the system function, wherein said second identifier is provided for delivery to the user of the imaging device and wherein said first identifier and said second identifier are required by the user to obtain the first and the second portion of the system function or to obtain said system function.
The present invention relates finally also to imaging methods, corresponding to the imaging devices as defined above, for imaging an object in a field of view and for reconstructing an image representing the distribution of a tracer material in an object by use of a system function, wherein the system function describes the mapping between tracer distribution and imaging signals acquired by an imaging device.
In an embodiment said imaging method comprises the steps of: acquiring a second portion of the system function, said second portion describing characteristics of the imaging device,
requesting a first portion of the system function from the manufacturer of the tracer material or from a central storage means, in particular by use of a first identifier assigned to the first portion of the system function, said first portion describing dynamics of the tracer material,
- combining said first and second portions to generate said system function.
In another embodiment said imaging method comprises the steps of:
acquiring a second portion of the system function, said second portion describing characteristics of the imaging device,
delivering said second portion of the system function to a central storage means, where said first and second portions of said system function are combined into said system function,
requesting said system function from said central storage means, in particular by use of a first identifier assigned to the first portion of the system function, said first portion describing dynamics of the tracer material, and/or a second identifier assigned to the second portion of the system function. 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 block diagram of an MPI apparatus according to the present invention,
Fig. 5 shows a schematic block diagram generally illustrating the generation of the system function,
Fig. 6 shows a schematic block diagram illustrating a first embodiment of a system and method for generating the system function according to the present invention,
Fig. 7 shows a schematic block diagram illustrating a second embodiment of a system and method for generating the system function according to the present invention,
Fig. 8 shows a schematic block diagram illustrating a third embodiment of a system and method for generating the system function according to the present invention,
Fig. 9 shows a schematic block diagram illustrating a fourth embodiment of a system and method for generating the system function according to the present invention, and
Fig. 10 schematically shows two embodiments of an imaging device according to the present invention.
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, two 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 two embodiments will be pointed out.
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 l s 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'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'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°i 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 i 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° 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, oils, again oriented along the x-, y-, and z-axes. These coil pai
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 a 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 is saturated and does not generate any signal.
The embodiment 10 of the MPI scanner has at least one further pair, preferably three further pairs, of parallel circular c rs, 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.
The embodiment 10 of the MPI scanner shown in Fig. 1 has a cylindrical bore 26 along the z-axis 22, i.e. along the axis of the selection field. All coils are placed outside this bore 26. For the data acquisition, the patient (or object) to be imaged is placed in the bore 26 such that the patient's volume of interest - that volume of the patient (or object) that shall be imaged - is enclosed by the scanner's field of view 28 - that volume of the scanner whose contents the scanner can image. The patient (or object) is, for instance, placed on a patient table. The field of view 28 is a geometrically simple, isocentric volume in the interior of the bore 26, such as a cube, a ball, a cylinder or an arbitrary shape. A cubical field of view 28 is illustrated in Fig. 1.
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 50xl0 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'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.
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 μιη 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 μιη 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 (i.e. all fields add together) 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 Xk-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/μο = 2.5 T/m, where μο 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 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. 4 comprises various sets of coils for generating the desired magnetic fields. First, the coils and their functions in MPI shall be explained.
For generating the magnetic selection field explained above, selection means are provided comprising a set of selection field coils 116, preferably comprising at least one pair of coil elements. The selection means further comprises a selection field signal generator unit 110. Preferably, a separate generator subunit is provided for each coil element (or each pair of coil elements) of the set 116 of selection field coils. Said selection field signal generator unit 110 comprises a controllable selection field current source 112 (generally including an amplifier) and a filter unit 114 which provide the respective section field coil element with the selection field current to individually set the gradient strength of the selection field. However, since the selection field is generally static, the filter unit 114 can also be omitted. Preferably, a constant current is provided. If the selection field coil elements are arranged as opposite coils, e.g. on opposite sides of the field of view, the selection field currents of the opposite coils are preferably oppositely oriented.
The selection field signal generator unit 110 can be controlled by a control unit 150, which preferably controls the selection field current generation 110 such that the sum of the field strength and the sum of the gradient strength of all spatial fractions of the selection field is maintained 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 the generation of a magnetic focus field the apparatus 100 further comprises focus means comprising a set of focus field coils, preferably comprising three pairs 126a, 126b, 126c of oppositely arranged focus field coil elements. Said magnetic focus field is generally used for changing the position in space of the first and second sub-zones. The focus field coils are controlled by a focus field signal generator unit 120, preferably comprising a separate focus field signal generation subunit for each coil element (or at least each pair of coil elements) of said set of focus field coils. Said focus field signal generator unit 120 comprises a focus field current source 122 (preferably comprising a current amplifier) and a filter unit 124 for providing a focus field current to the respective coil of said subset of coils 126a, 126b, 126c which shall be used for generating the magnetic focus field. The focus field current unit 120 is also controlled by the control unit 150. With the present invention, the filter unit 124 may also be omitted. For generating the magnetic drive field the apparatus 100 further comprises drive means comprising a subset of drive field coils, preferably comprising three pairs 136a, 136b, 136c of oppositely arranged drive field coil elements. The drive field coils are controlled by a drive field signal generator unit 130, preferably comprising 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. Said drive field signal generator unit 130 comprises a drive field current source 132 (preferably including a current amplifier) and a filter unit 134 (which may also be omitted with the present invention) for providing a drive field current to the respective drive field coil. The drive field current source 132 is adapted for generating a time- dependent current and is also controlled by the control unit 150.
It should be noted that in the embodiment of the apparatus 10 shown in Fig. 1 identical coils are preferably used for generating the magnetic drive field and the magnetic focus field.
For 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.
Further, an input unit 158 is provided, for example a keyboard. A user is therefore 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.
As becomes clear from the above, the image quality is ultimately dependent not only on the instrumentation (e.g. scanner, coils, electronics), but also on the performance of the tracer material (e.g. magnetization, ability to follow a changing external magnetic field). To result in optimal image quality, the tracer material, in particular each portion of the tracer material (batch), should be characterized in combination with the scanner used, i.e. the system function (SF) should be acquired. .
Fig. 5 shows a schematic block diagram generally illustrating the generation of the system function. For each batch of tracer material 200 a system function SF has to be acquired with the scanner (imaging device) 210 that is later used for the imaging. The system function SF then contains information about the scanner characteristics (coil sensitivity and geometry, electronics,...) as well as about the dynamics of the tracer material (also depending on the sequence including the drive field strength used). The system function SF may thus be factorized in a part SFs c describing the scanner characteristics and a part SFn describing the tracer dynamics. These two parts can therefore be acquired separately.
If the acquisition of the tracer part SFn of the system function is performed using a dedicated device 220 (generally an acquisition means), in particular a System
Calibration Unit (SCU), then the effort can be further reduced, as such a dedicated device 220 can perform the acquisition of the data needed in a fraction of the time that is needed when performed in a scanner 210. This is illustrated in the schematic block diagram of Fig. 6 depicting a first embodiment of a system and method for generating the system function according to the present invention.
The acquisition of the tracer characteristics SFn is done for every new batch of tracer material 200 with the SCU 220, preferably at the manufacturer or distributor of the tracer material 200. The acquisition of the scanner characteristics SFs c has to be done only once (or at least after each major modification of the scanner) in terms of a so-called
'calibration' and is done at the actual scanner 210 by use of a test probe 202 of tracer material, preferably at the user's place, but can also be done at the manufacturer of the scanner 210. Again, the scanner part SFs c and the tracer part SFn are finally combined by a combination unit 230, preferably at the user's place. Said combination unit 230 may also be part of the scanner 210 (imaging device) or may be provided at the manufacturer or distributor of the tracer material.
Fig. 7 illustrates a second embodiment of a system and method for generating the system function according to the present invention. It is depicted in Fig. 7 that the information SFn about the dynamics of the tracer material can best be acquired directly at the manufacturer of the tracer material 200. The provision of the data can in a first
implementation be performed in the form of some kind of storage means 240 storing said data, e.g. a data carrier such as a barcode, any other form of data matrix, an RF chip or a similar technology or a ROM chip implanted in the packaging of the tracer material 200. This data stored in the storage means 240 is then transferred into the scanner 210 which would reconstruct the complete system function from the tracer and the scanner part (stored, e.g., in a storage unit 216 in the scanner 210). This process reduces the effort that has to be spent by the user to nil, as all acquisition of calibration and system data are performed apart from the actual usage of the tracer.
Fig. 8 shows an extension of the process that further decreases the amount of information that has to be transformed together with the packaging of the tracer material 200 to a unique identifier (IDl) 250 as well as increases the safety of the whole process. As explained above the tracer part SFn is acquired by an acquisition unit 221 of the SCU 220. For generating the unique identifier IDl 250 an identification unit 224 is provided, preferably in the SCU 220. To access the information needed, the user needs to be authorized and only authorized batches of tracer material 200 (that have been characterized before and thus carry an IDl 250) can be used to achieve optimal image quality. All data SFn about the tracer material are, in this embodiment, transferred (e.g. by use of a transfer or delivery unit 222, for instance over the internet or any other transmission channel) to and stored in a central data retention centre 260 and only transferred (e.g. over the internet or any other transmission channel) by request from the user, said request (which may also be transferred over the internet or any other transmission channel) including the ID1 250 for identifying for which batch of tracer material the data SFn are desired. Said identifier ID1 250 is delivered to the user before by use of another delivery unit 226. The system function is, however, still constructed at the user.
This process can be rendered even safer, as illustrated in Fig. 9, when next to the tracer dynamics SFn also the scanner characteristics SFs c are transferred (e.g. by use of a transfer or delivery unit 212, for instance over the internet or any other transmission channel) to and stored in the central data retention centre 260. The system (e.g. any other central unit including the combination unit 230 and, possibly, the central data retention centre 260) then can furthermore reconstruct the complete system function and send only the final information (the SF) to the user, e.g. over the internet. Thereby it is ensured that only authorized users using an authorized scanner and an authorized tracer material (equipped with traceable information) will be able to achieve the ultimate image quality, since in an embodiment the user both needs a first unique identifier ID1 250 identifying the batch 200 of tracer material and a second unique identifier ID2 270 identifying the scanner 210. Said second identifier is preferably generated by another identification unit 214 which is, for instance, part of the scanner 210.
Two embodiments of an imaging device according to the present invention, which may be used in the systems and methods according to the present invention as explained above are schematically shown in Figs. 10A and 10B. These imaging devices are generally provided for imaging an object in a field of view and for reconstructing an image representing the distribution of a tracer material in an object by use of a system function.
The first embodiment 300 of the imaging device comprises an acquisition means 310 (which may correspond to the scanner 210) for acquiring a second portion SF Sc of the system function, said second portion describing characteristics of the imaging device, in particular of the used scanner (210), a request means 320 for requesting the first portion SF Tr of the system function from the manufacturer of the tracer material or from a central storage means (260), in particular by use of a first identifier assigned to the first portion SFn of the system function, and a combination means 330 (which may correspond to the combination unit 230) for combining said first and second portions to generate said system function.
The second embodiment 350 of the imaging device comprises also an acquisition means 310 for acquiring a second portion SF Sc of the system function, said second portion describing characteristics of the imaging device. Further, however, it comprises a delivery means 360 for delivering said second portion SF Sc of the system function to a central storage means (260), where said first and second portions of said system function are combined into said system function, and a request means 370 for requesting said (complete) system function SF from said central storage means (260), in particular by use of a first identifier ID1 assigned to the first portion SFn of the system function.
The invention will, for instance, be used for Magnetic Particle Imaging. But it can furthermore be used for any other imaging modality that uses a tracer and that relies on certain knowledge about the tracer (dynamics, characteristics, chemical properties, distribution, ...) that can be measured and provided.
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.