WO2008135897A2

WO2008135897A2 - Detection device for detecting radiation and imaging system for imaging a region of interest

Info

Publication number: WO2008135897A2
Application number: PCT/IB2008/051628
Authority: WO
Inventors: Roland Proksa
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2007-05-04
Filing date: 2008-04-28
Publication date: 2008-11-13
Also published as: WO2008135897A3

Abstract

The invention relates to a detection device for detecting radiation, wherein the detection device (6) comprises at least one first region (14) and at least one second region (15). In the at least one first region detector elements (16) are present, which are different from detector elements (17) present in the at least one second region (15). Preferentially, the at least one second region (15) is an inner region and the at least first region (14) is an outer region of the detection device (6) and in the at least one second region (15) photon-counting detector elements are present. In the at least one first region (14) preferentially non-energy- resolving detector elements are present. The invention relates further to an imaging system for imaging a region of interest comprising this detection device.

Description

Detection device for detecting radiation and imaging system for imaging a region of interest

FIELD OF THE INVENTION

The invention relates to a detection device for detecting radiation and an imaging system comprising this detection device. The invention relates further to a corresponding imaging method and computer program for imaging a region of interest.

BACKGROUND OF THE INVENTION

Detection devices for detecting radiation are used in computed tomography systems, wherein a radiation source and the detection device rotate around a region of interest for illuminating the region of interest from different directions and for acquiring detection data, which are used by a reconstruction unit for reconstructing an image of the region of interest from the acquired detection data. The detection device is one of the most expensive components of a computed tomography system because of the high technical requirements of such a detection device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a detection device for detecting radiation having reduced technical requirements. It is a further object of the present invention to provide an imaging system and an imaging method using a detection device and a computer program for imaging a region of interest, in which the technical requirements for a detection device are reduced.

In an aspect of the present invention a detection device for detecting radiation is presented, wherein the detection device comprises at least one first region and at least one second region, wherein in the at least one first region detector elements are present, which are different from detector elements present in the at least one second region. The invention is based on the idea that an object, which is present in a field of view, which is traversed by radiation detected by the detection device, is generally not homogenously distributed over the whole field of view. Thus, the properties of the radiation incident on the detection device differ in different regions on the detection device, because radiation detected by different regions on the detection device has traversed different regions within the field of view. For example, a part of the radiation could have traversed an object within the field of view and another part of the radiation could have traversed a region of the field of view, in which the object is not present. The technical requirements of the detection device depends on the radiation, which has to be detected. In the prior art, the detection device comprises one kind of detector elements, which are adapted such that they can cope with the properties of the radiation, which is supposed to reach the detection device. In particular, all detector elements are adapted such that they can cope with the properties of the radiation of all possible properties of the radiation, which might be incident on the detection device, having the highest technical requirements, in order to detect the radiation satisfactorily. But, the prior art does not consider that on different regions of the detection device the properties of the radiation, which are supposed to be incident on the detection device, are different. The detection device in accordance with the invention comprises different detector elements in different regions allowing to use in each region detector elements, which are needed for detecting the radiation, which is supposed to be incident on the detection device in the respective region. Detector elements having higher technical requirements, which might be needed in some regions on the detection device, are not needed in other regions. Thus, in each region detector elements can be located, which meet the technical requirements for the properties of the radiation, which is supposed to be incident on the respective region, and detector elements fulfilling high technical requirements have to be placed only in regions on the detection device, where they are really needed. This reduces the overall technical requirements of the detection device.

It is preferred that the detector elements in the at least one second region are energy-resolving detector elements. Preferentially the detector elements in the at least one second region are photon-counting detector elements. This allows determining detection data in the at least one second region energy-resolved. If the detection device is used in a computed tomography system, these energy-resolved detection data can be used for spectral computed tomography methods.

In a further preferred embodiment, the detector elements in the at least one first region are non-energy-resolving detector elements. The technical requirements for energy-resolving detector elements, in particular, for photon-counting detector elements, are high. If the detection device comprises in the at least one first region non-energy-resolving detector elements, which comprise lower technical requirements, the technical requirements of the overall detection device compared to a detection device comprising energy-resolving detector elements distributed over the whole detection device are further reduced. In a preferred embodiment, the detector elements in the at least one first region have another dynamic range than the detector elements in the at least one second region. It is further preferred that the detector elements in the at least one first region are capable of detecting radiation having an intensity higher than any intensity of the radiation which is detectable by the detector elements in the at least one second region. As already mentioned above, an object in a field of view, which might be traversed by radiation, which has to be detected by the detection device, is generally not homogenously distributed in the field of view. Thus, radiation being incident on the detection device in different regions has generally different intensities. In particular, an intensity of the radiation traversing an object differs generally in orders of magnitude from an intensity of radiation, which does not traverse an object. It is therefore possible to have different kinds of detector elements in different regions on the detection device, wherein the different kinds of detector elements can cope with different dynamic ranges, without reducing the quality of detection data acquired by the detection device, and it is not needed to have the same kind of detector elements, which can cope with the whole possible dynamic range of the radiation, which is supposed to be incident the detection device, distributed over the whole detection device.

It is further preferred that the at least one first region is an outer region and the at least one second region is an inner region of the detection device. This allows using the detection device having reduced technical requirements, if radiation having traversed a first object should be detected, which is located within a second object, like a human heart within a patient. If the second object is located within a field of view defined by the radiation detectable by the detection device, the radiation, which has traversed the first object, will be incident on the detection device in an inner region and the radiation, which has traversed only the second object and not the first object, will be incident on the detection device on an outer region. Thus, in the at least one second region being an inner region, detector elements can be provided, which have higher technical requirements than detector elements provided in the at least one first region being an outer region, because for imaging the first object high quality detection data are desired. If in the at least one second region being an inner region of the detection device energy-resolving detector elements, in particular, photon-counting detector elements, are provided, the first object can be reconstructed using spectral computed tomography methods, while, if in the at least one first region being an outer region non- energy-resolving detector elements are provided, the complete second object can be imaged by conventional imaging methods. If an object is located centrally within the field of view, the at least one second region being an inner region can be formed such that the radiation, which has traversed this object, is incident on the at least one second region, and the at least one first region being an outer region can be formed such that radiation, which has not traversed the object, is incident on the at least one first region. Since the object has to be imaged, the detector elements in the at least one second region have preferentially higher technical requirements than the detector elements in the at least one first region. For example, as already mentioned above, also in this case, the at least one second region can comprise energy-resolving detector elements, in particular photon-counting detector elements, and the at least one first region can comprise non-energy-resolving detector elements. This allows, for example, imaging the whole object using spectral computed tomography methods, wherein energy-resolving detector elements are only needed in the at least one second region and not in the at least one first region.

In a further aspect of the invention, an imaging system for imaging a region of interest is presented, wherein the imaging system comprises: - a radiation source for emitting radiation for traversing the region of interest, - a detection device for detecting the radiation after having traversed the region of interest, wherein the detection device comprises at least one first region and at least one second region, wherein in the at least one first region detector elements are present, which are different from detector elements present in the at least one second region.

It is preferred that the at least one second region is adapted for detecting detection data, which are sufficient for reconstructing an image of the region of interest, wherein the imaging system further comprises a reconstruction unit for reconstructing an image of the region of interest from the detection data of the at least one second region. Since the at least one second region is adapted for detecting detection data, which are sufficient for reconstructing an image of a region of interest, detector elements having high technical requirements like energy-resolving detector elements, are only required in the at least one second region and not in the at least one first region. Therefore, an image of the region of interest can be reconstructed using a detection device, of which the overall technical requirements are reduced, without a degradation of image quality.

It is further preferred that the reconstruction unit is further adapted for reconstructing an image of the field of view from the detection data of the at least one first region and of the at least one second region. This allows reconstructing an image of the whole field of view from the detection data, wherein the quality of the image of the whole field of view can differ from the quality of the image of the region of interest, because the at least one first region and the at least one second region comprise different detector elements. In a further aspect of the invention an imaging method for imaging a region of interest is presented, wherein the imaging system comprises:

- emitting radiation for traversing the region of interest by a radiation source,

- detecting the radiation after having traversed the region of interest by a detection device comprising at least one first region and at least one second region, wherein in the at least one first region detector elements are present, which are different from detector elements present in the at least one second region.

In a further aspect of the invention a computer program for imaging a region of interest is presented, wherein the computer program comprises program code means for causing an imaging system as defined in claim 8 to carry out the steps of the method as claimed in claim 11, when the computer program is run on a computer controlling the imaging system.

It shall be understood that the detection device of claim 1, the imaging system of claim 7, the imaging method of claim 10 and the computer program of claim 11 have similar and/or identical preferred embodiments as defined in the dependent claims.

It shall be understood that preferred embodiments of the invention can also be any combination of the dependent claims with the respective independent claim.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings:

Fig. 1 shows schematically and exemplarily an representation of an imaging system for imaging a region of interest,

Fig. 2a shows schematically an arrangement of a detection device and a radiation source of the imaging system in a plane perpendicular to an axis of rotation,

Fig. 2b shows schematically a top view on the detection device seen from the radiation source,

Fig. 3a shows schematically an arrangement of the detection device and a radiation source of another embodiment of an imaging system for imaging a region of interest in a plane perpendicular to an axis of rotation,

Fig. 3b shows schematically a top view on the detection device seen from the radiation source,

Fig. 4 shows a flow chart illustrating an embodiment of an imaging method for imaging a region of interest, Fig. 5 shows schematically and exemplarily an emission spectrum of the radiation source,

Fig. 6 shows schematically and exemplarily a spectra of different attenuation effects, and Fig. 7 shows a flow chart illustrating another embodiment of an imaging method for imaging a region of interest.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 shows schematically and exemplarily an imaging system for imaging a region of interest being, in this embodiment, a computed tomography system. The computed tomography system includes a gantry 1 which is capable of rotating around an axis of rotation R, which extends parallel to the z direction. A radiation source 2, for example a X- ray tube, is mounted on the gantry 1. In this embodiment, the radiation source 2 emits polychromatic radiation. The radiation source 2 is provided with a collimator device 3 which forms a conical radiation beam 4 from the radiation emitted by the radiation source 2. In other embodiments, the collimator device 3 can be adapted for forming a radiation beam having another shape, for example, having a fan shape.

The radiation traverses an object (not shown in Fig. 1), such as a patient or a technical object, in a region of interest in a cylindrical examination zone 5. After having traversed the region of interest, the radiation beam 4 is incident on a detection device 6 having in this embodiment a two-dimensional detection surface, wherein the detection device 6 is mounted on the gantry 1. In another embodiment, the detection device 6 can comprise a one-dimensional detection surface.

The detection device 6 comprises detector elements 16, 17; 116, 117, which are schematically and exemplarily shown in Figures 2a to 3b.

Fig. 2a shows an arrangement of the radiation source 2 and the detection device 6 together with an object 18 in the field of view in a plane perpendicular to the axis of rotation. The detection device 6 comprises detector elements 17, which are located in a second region 15, and detector elements 16, which are located in two first regions 14. The first regions 14 are outer regions and the second region 15 is an inner region. The second region 15 is formed such that radiation, which has traversed the object 18, is incident on the second region 15 on the detection device 6. If the imaging system is used for a certain type of objects, for example, for human patients, the second region 15 is preferentially formed such that for a human patient of arbitrary human size the radiation, which has traversed the human patient, is incident on the second region 15. For determining the second region 15 such that the above mentioned condition is fulfilled, average human patient sizes together with standard deviations can be determined and the second region 15 can be formed such that radiation traversing an object having sizes within the determined standard deviations is incident on the second region 15. Fig. 2b shows schematically a surface of the detection device 6 as seen from the radiation source 2.

The second region 15 is preferentially formed such that, during an acquisition of computed tomography detection data, the detection data acquired by the detector elements 17 located in the second region 15 are sufficient to reconstruct an image of the object 18, preferentially for all possible acquisition geometries of the imaging system, in particular for a circular or helical relative movement between the radiation source and the object. If the imaging system is used for human patient, the second region 15 is preferentially formed such that for human patients 18 having an arbitrary human size the above mentioned sufficiency condition is fulfilled. Alternatively or in addition, the second region 15 is formed such that the above mentioned sufficiency condition is fulfilled for a human patient 18 having sizes within standard deviations from averagely sized human patients.

The above mentioned sufficiency condition is, for example, that each voxel, which represents a part of the human patient 18, has been seen from the radiation source 2 during the acquisition of detection data of the detector elements 17 by radiation incident on the second region 15 over an angular range of at least 180°.

A further embodiment of a detection device is shown in Figures 3 a and 3b. Fig. 3a shows schematically an arrangement of the radiation source 2, the radiation beam 4, the object 18 and a detection device 106 in a plane perpendicular to the axis of rotation. The object 18 contains a further object 19, which is located in a region of interest and which has to be reconstructed. Preferentially, the object 18 is a human patient, which contains the object 19, which is preferentially a human heart. The detection device 106 comprises an inner region 115 with detector elements 117 being the second region and two outer regions 114 with detector elements 116 being first regions of the detection device 106. The second region 115 is formed such that, during the acquisition of detection data, the detection data acquired by the detector elements 117 of the second region are sufficient for reconstructing the object 19, in particular the human heart 19. If the imaging system is used for reconstructing an image of human heart, the second region 115 is formed such that for a predetermined position of the human patient 18 within the field of view, during an acquisition of detection data, detection data are acquired by the detector elements 117 in the second region 115, which are sufficient for reconstructing the human heart 19. The size of the second region 115 in Fig. 3 a and Fig. 3b is shown only schematically. Preferentially, the second region 115 is large enough for fulfilling the above mentioned sufficiency condition for a reconstruction of a human heart having an arbitrary human size. Alternatively, the second region 115 is formed such that the above mentioned sufficiency condition for reconstructing the human heart 19 is fulfilled for each human heart having sizes within standard deviations from an averagely sized human heart. Fig. 3b shows schematically a surface of the detection device 106 seen from the radiation source 2.

In Fig 2a, the object 18 is preferentially horizontally centred with respect to the orientation given in Fig. 2a, i.e. on the surface of the detection device 6 the second region is preferentially centred with respect to a direction perpendicular to the axis of rotation. Or, in other words, the surface of the detection device 6 has an substantially rectangular shape (possible small curvatures are not considered for illustration reasons) having two short and two long sides and the second region is centred with respect to a direction pointing from one of the short sides to the other of the short sides. In addition, the object 18 is preferentially also vertically centred with respect to the orientation shown in Fig. 2a, in particular, with respect to the field of view.

In Fig 3a, the object 19 is preferentially horizontally centred with respect to the orientation given in Fig. 3a, i.e. on the surface of the detection device 106 the second region is preferentially centred with respect to a direction perpendicular to the axis of rotation. Or, in other words, the surface of the detection device 106 has an substantially rectangular shape (possible small curvatures are not considered for illustration reasons) having two short and two long sides and the second region is centred with respect to a direction pointing from one of the short sides to the other of the short sides. In addition, the object 19 is preferentially also vertically centred with respect to the orientation shown in Fig. 3 a, in particular, with respect to the field of view.

In the second regions 17, 117 energy-resolving detector elements are located, wherein in the first regions 14, 114 non-energy-resolving detector elements are located. The energy-resolving detector elements are preferentially photon-counting detector elements which work, for example, on the principal of counting the incident photons and output a signal that shows the number of photons in different energy windows. Such an energy- resolving detection device is, for instance, described in Llopart, X., et al. "First test measurements of a 64k pixel readout chip working in a single photon counting mode", Nucl. Inst, and Meth. A, 509 (1-3): 157-163, 2003 and in Llopart, X., et al., "Medipix2: A 64-k pixel readout chip with 55 μm square elements working in a single photon counting mode",

IEEE Trans. Nucl. Sci. 49(5): 2279-2283, 2002. Preferably, the energy-resolving detector element are adapted such that each detector element provides at least two energy-resolved detection signals for at least two different energy windows. However, it is favourable to have an even higher energy resolution in order to enhance the sensitivity and noise robustness of the imaging system.

Referring again to Fig. 1, the gantry 1 is driven at a preferably constant but adjustable angular speed by a motor 7. A further motor 8 is provided for displacing the object, for example, a patient who is arranged on a patient table in the examination zone 5, parallel to the direction of the axis of rotation or the z axis. These motors 7, 8 are controlled by a control unit 9, for instance, such that the radiation source 2 and the examination zone 5, in particular, the region of interest, move relative to each other along a helical trajectory. It is also possible, that the object or the examination zone 5, in particular the region of interest, is not moved and that the radiation source 2 is rotated, i.e. that the radiation source 2 travels along a circular trajectory relative to the region of interest. The data acquired by the detector elements 17, 117 in the second region 15, 115 of the detection device 6, 106 are, in this embodiment, energy-dependent detection data. The data acquired by the detector elements

16, 116 in the first regions 14, 114 are, in this embodiment, non-energy-dependent detection data. The detection data are provided to a reconstruction unit 10 for reconstructing an image of the region of interest from the provided energy-dependent detection data and preferentially for reconstructing an image of the whole field of view from the energy-dependent detection data and the non-energy-dependent detection data. Also the reconstruction unit 10 is preferentially controlled by the control unit 9. The reconstructed image is provided to a display unit 11 for displaying the image. The reconstruction unit 10 comprises a calculation unit 12 for determining at least one attenuation component from the detection data acquired by the detector elements

17, 117 in the second regions 15, 115. An attenuation component of the detection data is caused by only one or by several physical effects, for example, one attenuation component can be a component caused by the Compton effect and another attenuation component can be a component caused by the photo-electric effect. A further attenuation component can be a component caused by a k-edge present in the region of interest. Alternatively or in addition, an attenuation component can be a component caused by, for example, an absorption of a certain material within the region of interest. For example, an attenuation component can be a component caused by the absorption of a certain material and another attenuation component can be a component caused by the absorption of another material.

The reconstruction unit 10 further comprises a backprojection unit 13 for backprojecting the calculated attenuation components of the detection data and preferentially also for backprojecting detection data of the first regions 14, 114 and the second regions 15, 115 for reconstructing an image of the region of interest and preferentially also of the field of view.

In the following an embodiment of an imaging method for imaging a region of interest in accordance with the invention will be described in more detail with respect to a flow chart shown in Fig. 4.

In this embodiment, the imaging system with the detection device 6 as shown in Figures 2a and 2b is used. In step 201, the radiation source 2 rotates around the axis of rotation R or the z direction and the object is not moved, i.e. the radiation source 2 travels along a circular trajectory around the object. In another embodiment, the radiation source 2 can move along another trajectory, for example, a helical trajectory relative to the object. The radiation source 2 emits radiation, in this embodiment X-ray radiation, traversing the object, which is present in a field of view. The radiation, which has traversed the object, is detected by the detection device 6, which generates detection data. Thus, in step 201 detection data are acquired. In step 202, the detection data are transmitted to the calculation unit 12 of the reconstruction unit 10. The calculation unit 12 determines at least one attenuation component of the detection data from the energy-dependent detection data acquired in the second region 15. In particular, in this embodiment, the calculation unit 12 determines a Compton component, a photo-electric effect component and a k-edge component of the object present in the field of view. The k-edge component can also be a k-edge component of a substance, like a contrast agent, which, for example, is based on iodine or gadolinium, present within the object. The determination of the attenuation components will now be explained in more detail.

The input to the calculation unit 12 are energy-resolved detection data d_t of a plurality, in this embodiment at minimum three, energy bins. These detection data d_t show a spectral sensitivity D₁ (E) of the i-th energy bin b_t Furthermore, the emission spectrum T (E) of the polychromatic radiation source 2 is generally known or can be measured prior to step 201. An example of such an emission spectrum T (E) of a polychromatic radiation source is schematically and exemplarily shown in Fig. 5. In the calculation unit 12 the generation of the detection data d_t is modeled as a linear combination of the photo-electric effect with spectrum P (E) , the Compton effect with spectrum C (E) and a k-edge with spectrum K (E) . Spectra P (E) , C (E) and K (E) are exemplarily and schematically shown in

Fig. 6.

The generation of the detection data can be modeled by following system of equations:

d, = IdE T(E) D₁ (E)_Qχ_V[-(p_≠otoP(E)+p_ComptmC(E)+p_k__edgeK(E))], (1)

wherein p_photo, Pc_ompt<m ^and P_ledge ^{are me} density length products of the photo-electric component, the Compton component and the k-edge component, respectively.

Since at least three detection signals d_vd₂,d₃ are available for the at least three energy bins b_vb₂,b₃ , a system of at least three equations is formed having three unknowns, which are the three density length products, which can thus be solved with known numerical methods in the calculation unit 12. If more than three energy bins are available, it is preferred to use a maximum likelihood approach that takes the noise statistics of the measurements into account. Generally, three energy bins are sufficient. In order to increase the sensitivity and noise robustness, however, it is preferred to have more detection signals for more energy bins.

In step 203, at least one of the density length products determined in step 202 is backprojected by the backprojection unit 13 for reconstructing an image of the object, in particular, of a region of interest within the object. For each attenuation component determined in step 202, i.e. for each density length product, an image of the object can be reconstructed by backprojecting the respective density length product. Alternatively or in addition, images of different density length products or, before backprojecting, different density length products can be combined for generating a combination image being an image of the object generated by the combined attenuation components. For the reconstruction of the object, in this embodiment, only detection data are used, which correspond to detector elements 17 located in the second region 15. In addition, the backprojection unit 13 can be adapted for reconstructing an image of the whole field of view from the detection data of the detector elements 17 located in the first region 15 and from the detector elements 16 located in the first regions 14, wherein before backprojecting the detection data of the energy-resolving detector element 17 are transformed, for example, by integrating over energy, to non-energy-resolving detection data, which correspond to detection data, which would have been detected by the detector elements 16, if they would have been located in the second region 15.

In step 204, the reconstructed images are transmitted to the display unit 11 for displaying the reconstructed images.

In the following a further embodiment of an imaging method for imaging a region of interest in accordance with the invention will be described in more detail with reference to a flow chart shown in Fig. 7.

In this embodiment, the detection device 106 as schematically shown in Figures 3a and 3b is used.

In step 301, detection data are detected by detector elements 116 located in the first regions 114 and by the detector elements 117 located in the second region 115 of the detection device 106, while the radiation source 2 travels along circular or helical trajectory relative to the object 18. This acquisition of detection data is similar to the acquisition described above with reference to step 201.

In step 302, the detection data are transmitted to the calculation unit 12 of the reconstruction unit 10. The calculation unit 12 determines attenuation components of the detection data, in particular, a Compton component, a photo-electric effect component and a k-edge component, as described above with reference to step 202.

In step 303, the determined attenuation components, i.e., in this embodiment, the density length products of the Compton effect, the photo-electric effect and a k-edge, are transmitted to the backprojection unit 13. The backprojection unit 13 reconstructs an image of the human heart 19 from the detection data, which have been detected by the detector elements 117 in the second region 115 of the detection device 106.

Preferentially, only one of the density length products is used by the backprojection unit 13 for reconstructing an image of the human heart 19. If a contrast agent is present within the human heart 19, for example, for reconstructing an image of the vessel structure of the human heart, the determined k-edge component of the detection data is preferentially used for reconstruction an image of the human heart 19. It is further preferred that known cardiac gating techniques are used for reconstructing an image of the human heart 19 and that these cardiac gating techniques preferentially use an electrocardiogram detected during the acquisition of the detection data in step 301.

It is also preferred that a further image is reconstructed, which shows not only the human heart 19, but also a surrounding part of the human patient 18, for example, the entire thorax of the human patient 18. Before performing this reconstruction, preferentially the detection data detected by the detector elements 117 located in the second region 115 of the detection device 106 are transformed from energy-dependent detection data into non- energy-dependent detection data, which would have been detected in the second region 115, if also in this second region 115 non-energy-resolving detector elements 116 are used as in the first regions 114. An image of the entire thorax of the human patient 18 is reconstructed by backprojecting the detection data detected by the detector elements 116 located in the first regions 114 and the transformed detection data of the second region 115. Thus, at the same time an image of the human heart 19 can be reconstructed using the energy-resolved detection data and an image of the entire thorax of the human patient 18 can be reconstructed using the detection data from the first regions 114 and the second region 115.

In step 304, the reconstructed images are transmitted to the display unit 11 for displaying the reconstructed images.

Some medical imaging protocols require a high intensity of radiation, but some photon-counting detector elements can not detect the high intensity of direct radiation in a computed tomography system, i.e. of radiation, which is not attenuated by an object located in the field of view. This intensity of the direct radiation is out of the intensity range, which is detectable by the photon-counting detector elements by photon-counting. But, if the radiation is attenuated by an object located in the field of view, the corresponding intensity is lowered and can be detected by the photon-counting detector elements by photon-counting. Thus, photon-counting detector elements are preferentially located in the second region and conventional non-energy-resolving detector elements are preferentially located in the first regions. It is therefore not needed to locate photon-counting detector elements, which are more expensive and have higher technical requirements than conventional non-energy- resolving detector elements, over a whole surface of a detection device, which reduces the overall costs and technical requirements of the detection device.

Although in the above described embodiments a certain reconstruction method has been described, in accordance with the invention, any other known reconstruction method can be used. For example, for determining attenuation components of the detection data and for reconstructing an image using the determined attenuation components the methods disclosed in an article from R. E. Alvarez, A. Macovski, Phys. Med. Biol. 21 (5), 733, 1976, "Energy- selective reconstructions in X-ray computerised tomography", and in the article "Base material decomposition using triple-energy X-ray computed tomography", P. Sukovic et. al, IEEE IMTC, 1999 can be used. Although in the above described embodiments the reconstruction unit 10 comprises a backprojection unit 13 for backprojecting attenuation components determined by the calculation unit 12, in particular for backprojecting density length products determined by the calculation unit 12 and preferentially for backprojecting detection data, the invention is not limited to a backprojection of these attenuation components and preferentially detection data. Any reconstruction method can be used for reconstructing an image using the determined attenuation components or the detection data, which generates an image from projection data. For example, also a radon inversion can be used for reconstructing an image from the determined attenuation components of the detection data or directly from the detection data, or other conventional reconstruction methods. Although in the above described embodiments the detection device is a detection device of a computed tomography system, the invention is not limited to a detection device for a computed tomography system. The detection device can be any detection device for detecting radiation. For example, it can be a detection device for detecting nuclear radiation, or it can be a detection device of an X-ray imaging projection system or an X-ray imaging C-arm system.

Although in the above described embodiments radiation has been detected, which has traversed a human patient, in particular a human heart, the invention is not limited to an application on a certain object. For example, the detection device can also be adapted for detecting radiation, which has traversed another organ of a human patient or which has passed a technical object, in particular, if the detection device is used in baggage inspection.

Although in the above described embodiments the second region is an inner region, which is centred with respect to a direction pointing from one of the short sides of the rectangular surface of the detection device to the other one of the short sides, the second region can also be offset from the centre of the surface of the detection device, for example, if the object, which has to be examined, is generally not located in the centre of the imaging system which uses the detection device.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and 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 unit may fulfill the function 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.

Calculations and/or determinations performed by one or several units or devices can be performed by any other number of units or devices. For example, the calculation performed by the calculation unit 12 and the backprojection performed by the backprojection unit 13 can be performed by one unit performing both, the calculation and the backprojection.

A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A detection device for detecting radiation, wherein the detection device (6) comprises at least one first region (14) and at least one second region (15), wherein in the at least one first region detector elements (16) are present, which are different from detector elements (17) present in the at least one second region (15).

2. The detection device as defined in claim 1, wherein the detector elements in the at least one second region are energy-resolving detector elements.

3. The detection device as defined in claim 2, wherein the detector elements in the at least one second region are photon-counting detector elements.

4. The detection device as defined in claim 1, wherein the detector elements in the at least one first region are non-energy-resolving detector elements.

5. The detection device as defined in claim 1, wherein the detector elements in the at least one first region have another dynamic range than the detector elements in the at least one second region.

6. The detection device as defined in claim 1, wherein the at least one first region is an outer region and the at least one second region is an inner region of the detection device.

7. An imaging system for imaging a region of interest, the imaging system comprising:

- a radiation source for emitting radiation for traversing the region of interest, - a detection device for detecting the radiation after having traversed the region of interest, wherein the detection device comprises at least one first region and at least one second region, wherein in the at least one first region detector elements are present, which are different from detector elements present in the at least one second region.

8. The imaging system as defined in claim 7, wherein the at least one second region is adapted for detecting detection data, which are sufficient for reconstructing an image of the region of interest, wherein the imaging system further comprises a reconstruction unit (10) for reconstructing an image of the region of interest from the detection data of the at least one second region.

9. The imaging system as defined in claim 8, wherein region of interest is located within a field of view, wherein the reconstruction unit is further adapted for reconstructing an image of the field of view from the detection data of the at least one first region and of the at least one second region.

10. An imaging method for imaging a region of interest, the imaging system comprising:

- emitting radiation for traversing the region of interest by a radiation source, - detecting the radiation after having traversed the region of interest by a detection device comprising at least one first region and at least one second region, wherein in the at least one first region detector elements are present, which are different from detector elements present in the at least one second region.

11. A computer program for imaging a region of interest, the computer program comprising program code means for causing an imaging system as defined in claim 8 to carry out the steps of the method as claimed in claim 10, when the computer program is run on a computer controlling the imaging system.