WO2005076222A1 - Correction of artifacts caused by high attenuating objects - Google Patents

Correction of artifacts caused by high attenuating objects Download PDF

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Publication number
WO2005076222A1
WO2005076222A1 PCT/IB2005/050457 IB2005050457W WO2005076222A1 WO 2005076222 A1 WO2005076222 A1 WO 2005076222A1 IB 2005050457 W IB2005050457 W IB 2005050457W WO 2005076222 A1 WO2005076222 A1 WO 2005076222A1
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data
material
correction
up table
beam
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PCT/IB2005/050457
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French (fr)
Inventor
Udo Van Stevendaal
Roland Proksa
Thomas Köhler
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Philips Intellectual Property & Standards Gmbh
Koninklijke Philips Electronics N. V.
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Application filed by Philips Intellectual Property & Standards Gmbh, Koninklijke Philips Electronics N. V. filed Critical Philips Intellectual Property & Standards Gmbh
Publication of WO2005076222A1 publication Critical patent/WO2005076222A1/en

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    • GPHYSICS
    • G06COMPUTING; CALCULATING; COUNTING
    • G06TIMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T11/002D [Two Dimensional] image generation
    • G06T11/003Reconstruction from projections, e.g. tomography
    • G06T11/005Specific pre-processing for tomographic reconstruction, e.g. calibration, source positioning, rebinning, scatter correction, retrospective gating

Abstract

Neglecting the effect of beam-hardening in a CT scanner system in the reconstruction process leads to beam-hardening artifacts, such as cupping, streaks and flares. By correcting beam-hardening artifacts caused by high attenuating objects on the basis of a physical correction method, which uses the data in the shadow of the high attenuating objects without disregarding any physical information, an improved artifact correction may be provided, leading to a better image quality and less streak artifacts in the reconstructed images. Furthermore, by using projections, which have been filtered adaptively, the noise of line integrals through high attenuating objects may be reduced.

Description

Correction of artifacts caused by high attenuating objects

The present invention relates to the field of image processing, for example, in medical applications. In particular, the present invention relates to a method of artifact correction in a data set, to data processing devices and to respective computer programs. In a CT scanner with a polychromatic source of radiation, such as a polychromatic x-ray source, a polychromatic x-ray beam passes through matter and low-energy photons are absorbed as the linear attenuation coefficient generally decreases with energy. As a result, the beam gradually becomes harder, i.e. its mean energy increases. The harder the beam, the less it is further attenuated. Therefore, the total attenuation is no longer a linear function of the thickness of the absorber. Neglecting this effect in the reconstruction process leads to well known beam- hardening artifacts, such as cupping, streaks and flairs. While cupping is corrected by a first order non-linear correction, streaks caused by bones are corrected by a second-pass beam hardening correction. Artifacts caused by the presence of metal objects, such like implants for example, are corrected, if at all, by mathematical correction methods, which assume metal objects as being opaque. The data, which corresponds to projection lines through these objects are defined as missing data. This missing data is replaced by synthetic data. The projection completion method leads to problems in the regions where projection lines through bone and other high attenuating objects, such like metal, overlap.

It is an object of the present invention to provide for an improved correction of artifacts caused by high attenuating objects. In accordance with an exemplary embodiment of the present invention as set forth in claim 1 , the above object may be solved by a method of artifact correction in a data set, wherein the data set is acquired by means of a polychromatic source of electro-magnetic radiation generating a polychromatic beam and by means of a radiation detector detecting the polychromatic beam. According to this exemplary embodiment of the present invention, a first material in the data set is identified, wherein the first material is a material which causes a defect or artifact in the data set. After that, correction image data is generated, wherein the correction image data is data which results from the first material. In a following step, the correction image data is subtracted from an original image, resulting in a corrected image. Advantageously, this may allow for a correction of artifacts caused by the first material, such as prosthesis or dental fillings, on the basis of physical laws and by using the data in the shadow of the first material completely. No information is disregarded. In other words, a physical correction method for metal or other artifacts is provided, which may be performed by means of a second-pass beam-hardening correction. According to another exemplary embodiment of the present invention as set forth in claim 2, the identification of the first material in the data set is performed by reconstructing the data set, resulting in the original image, setting a threshold for the first material and performing a segmentation of the first material in the original image, resulting in a first-material-only image and the original image. Advantageously, the threshold may be set by a user. By setting the threshold, the user may indirectly select which material in the data set is considered as a first material causing artifacts which have to be corrected. Another exemplary embodiment of the present invention is set forth in claim 3, wherein, after identifying the first material in the data set and before generating correction image data, the original image and the first-material-only image are forward- projected. Therefore, forward-projected images of the original data and of the first- material-only data are obtained, which may be further processed. According to another exemplary embodiment of the present invention as set forth in claim 4, the generation of the correction image data comprises the steps of generating correction projection data on the basis of a multi-dimensional look-up table and generating correction image data from the correction projection data by filtering and back-projecting the correction projection data. Advantageously, according to an aspect of this exemplary embodiment of the present invention, artifacts caused by the first material are corrected by using the data in the shadow of the first material completely. Furthermore, by filtering the correction projection data and back-projecting the correction projection data, noise of the line integrals (which may lead to a large error in the estimation of the path length through the high attenuating object) may be reduced, resulting in an improved image quality. According to another exemplary embodiment of the present invention as set forth in claim 5, the multi-dimensional look-up table is a two-dimensional look-up table, wherein the look-up table comprises first material attenuation values. The lookup table is generated on the basis of an energy spectrum of the beam and on the basis of corresponding water absorption coefficients and corresponding first material absorption coefficients. Advantageously, such a multi-dimensional look-up table may allow for an improved artifact correction and therefore an improved image quality without artifacts, such as cupping, streaks and flairs caused by beam-hardening or the heel- effect. According to another exemplary embodiment of the present invention as set forth in claim 6, the multi-dimensional look-up table is a three-dimensional look-up table, which comprises first material attenuation values and second material attenuation values. The look-up table is generated on the basis of an energy spectrum of the beam and on the basis of corresponding water absorption coefficients, corresponding first material absorption coefficients and corresponding second material absorption coefficients. Advantageously, this provides a correction of artifacts which are not only caused by a first material but which are caused by a first material and a second material. According to another exemplary embodiment of the present invention as set forth in claim 7, the first material is one of a metal, a bone, a dental filling which is not a metal, a prosthesis which is not a metal and an implant which is not a metal. Thus, artifacts resulting from high attenuating objects like metal objects, bone, dental fillings and the like may be corrected according to this exemplary embodiment. According to another exemplary embodiment of the present invention as set forth in claim 8, the polychromatic source of electromagnetic radiation is an x-ray source and the CT scanner system has one of a fan beam geometry and a cone beam geometry. The application of polychromatic x-rays is advantageous, since polychromatic x-rays are easy to generate and provide a good image resolution. Furthermore, since the geometry of the CT scanner system may be of different designs, such as, for example, cone beam or fan beam geometry, a method of an exemplary embodiment of the present invention may be applied to a plurality of different scanner systems and may not be limited to CT scanner systems. According to another exemplary embodiment of the present invention as set forth in claim 9, a data processing device is provided, which comprises a memory for storing a data set and a data processor for performing artifact correction in the data set, wherein the data processor is adapted for performing the following operation: loading the data set acquired by means of a polychromatic source of electro-magnetic radiation generating a polychromatic beam and by means of a radiation detector detecting the polychromatic beam; identifying a first material in the data set, wherein the first material is a material which causes artifacts in the data set; generating correction image data, wherein the correction image data is data which results from the first material; and subtracting the correction image data from an original image resulting in a corrected image. Advantageously, this may allow for improved image quality of images which comprise artifacts caused by a first material, such as a metal. According to another exemplary embodiment of the present invention as set forth in claim 10, a data processing device is provided, which is part of a CT scanner system and wherein the CT scanner system has one of a fan beam geometry and a cone beam geometry. Furthermore, the data processing device is adapted for generating a multi-dimensional look-up table and for generating the correction image data on the basis of the multi-dimensional look-up table. Advantageously, using a multi-dimensional look-up table for correction of the artifacts in the data set may allow for an improved artifact correction and therefore for an improved image quality without artifacts caused by beam-hardening or heel-effect. The present invention relates also to a computer program which may, for example, be executed on a processor, such as an image processor. Such computer programs may be part of, for example, a CT scanner system. The computer programs according to exemplary embodiments of the present invention are set forth in claims 11 and 12. These computer programs may preferably be loaded into working memories of data processors. The data processors are thus equipped to carry out exemplary embodiments of the methods of the present invention. The computer programs may be stored on a computer readable medium such as a CD-ROM. The computer programs may also be presented over a network such as the Worldwide Web, and may be downloaded into the working memory of a data processor from such networks. It may be seen as the gist of an exemplary embodiment of the present invention that beam-hardening artifacts caused by high attenuating objects are corrected on the basis of a physical correction method, which uses the data in the shadow of the high attenuating objects without disregarding any physical information. This may provide for a better image quality in the reconstructed images. These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter. Exemplary embodiments of the present invention will be described in the following, with reference to the following drawings:

Fig. 1 shows a simplified schematic representation of an embodiment of a computed tomography (CT) scanner according to the present invention. Fig. 2a shows a reconstructed image slice of a water cylinder with titanium inserts, including image artifacts, which may be caused by the beam-hardening effect (left), the reconstructed image slice after a second-pass beam-hardening correction (center) and a simulation with monochromatic x-ray rediation (right). Fig. 2b shows a reconstructed image slice of a water cylinder with titanium and bone inserts, including image artifacts, which may be caused by the beam-hardening effect (left), a second-pass beam- hardening correction considering only metal according to an exemplary embodiment of the present invention (center) and a polychromatic correction considering metal and bone (right). Fig. 2c shows a reconstructed image slice of a water cylinder with titanium and bone inserts, including image artifacts, which may be caused by the beam-hardening effect (left), a second-pass beam- hardening correction considering metal and bone (right) according to an exemplary embodiment of the present invention and a monochromatic simulation and reconstruction considering metal and bone (right). Fig. 3 shows a flow-chart of an exemplary embodiment of a method of generating a two-dimensional look-up table according to the present invention. Fig. 4 shows a flow-chart of an exemplary embodiment of a method of generating a three-dimensional look-up table according to the present invention. Fig. 5 shows a flow-chart of an exemplary embodiment of a method of operating the CT scanner system or the data processing system according to the present invention. Fig. 6 shows a flow-chart of an other exemplary embodiment of a method of operating the CT scanner system or the data processing system according to the present invention. Fig. 7 shows an exemplary embodiment of an image processing device according to the present invention, for executing an exemplary embodiment of a method in accordance with the present invention.

Fig. 1 shows an exemplary embodiment of a CT (computed tomography) scanner system according to the present invention. With reference to this exemplary embodiment, the present invention will be described for the application in medical imaging. However, it should be noted that the present invention is not limited to the application in the field of medical imaging, but may be used in applications such as baggage inspection to detect hazardous materials, such as explosives, in items of baggage or other industrial applications such as material testing. The scanner depicted in Fig. 1 is a cone beam CT scanner. The CT scanner depicted in Fig. 1 comprises a gantry 1, which is rotatable around a rotational axis 2. The gantry is driven by means of a motor 3. Reference numeral 4 designates a source of radiation such as an x-ray source, which, according to an aspect of the present invention, emits a polychromatic radiation. Reference numeral 5 designates an aperture system which forms the radiation beam emitted from the radiation source to a cone shaped radiation beam 6. The cone beam 6 is directed such that it penetrates and object of interest 7 arranged in the center of the gantry 1, i.e. in an examination region of the CT scanner and impinges onto the detector 8. As may be taken from Fig. 1, the detector 8 is arranged on the gantry 1 opposite to the source of radiation 4, such that the surface of the detector 8 is covered by the cone beam 6. The detector 8 depicted in Fig. 1 comprises a plurality of detector elements. During a scan of the object of interest 7, the source of radiation 4, the aperture system 5 and the detector 8 are rotated along the gantry 1 in the direction indicated by arrow 16. For rotation of the gantry 1 with the source of radiation 4, the aperture system 5 and the detector 8, the motor 3 is connected to a motor control unit 17, which is connected to a calculation unit 18. In Fig. 1, the object of interest is disposed on a conveyor belt 19. During the scan of the object of interest 7, while the gantry 1 rotates around the item of baggage 7, the conveyor belt 19 displaces the object of interest 7 along a direction parallel to the rotational axis 2 of the gantry 1. By this, the object of interest 7 is scanned along a helical scan path. The conveyor belt 19 may also be stopped during the scans to thereby measure single slices. Instead of providing a conveyor belt 19, for example, in medial applications where the object of interest 7 is a patient, a movable table is used. However, it should be noted that in all of the described cases it is also possible to perform a circular scan, where there is no displacement in a direction parallel to the rotational axis 2, but only the rotation of the gantry around the rotational axis 2. The detector 8 is connected to the calculation unit 18. The calculation unit 18 receives the detection result, i.e. the read-outs from the detector elements of the detector 8 and determines a scanning result on the basis of these read-outs. The detector elements of the detector 8 may be adapted to measure the attenuation caused to the cone beam 6 by the object of interest 7. Furthermore, the calculation unit 18 communicates with the motor control unit 17 in order to coordinate the movement of the gantry 1 with motor 3 and 20 or with the conveyor belt 19. The calculation unit 18 may be adapted for reconstructing an image from read-outs of the detector 8. The image generated by the calculation unit 18 may be output to a display (not shown in Fig. 1) via an interface 22. The calculation unit which may be realized by a data processor may also be adapted to perform an artifact correction in the image based on the read-outs from the detector elements of the detector 8. According to an aspect of the present invention, this correction may be performed by generating a multi-dimensional look-up table and by generating a correction image data on the basis of the multi-dimensional look-up table, which is subtracted from an original image, resulting in a corrected image. Furthermore, the calculation unit may be adapted for performing artifact correction in the data set by performing the following operation: loading the data set acquired by means of the polychromatic radiation source 4 and by means of the radiation detector 8. After that, a first material in the data set, for example, a metal which causes artifacts due to its high attenuation is identified. Then, correction image data, which is data resulting from the first material, is generated and subtracted from the original image data, resulting in a corrected image. Furthermore, as may be taken from Fig. 1 , for example, the calculation unit 18 may be connected to a loudspeaker 21, for example, to automatically output an alarm. In the following, the theoretical background of the beam-hardening effect will be described in further detail. When a polychromatic x-ray beam, such as the cone beam 6, passes through matter, low-energy photons are absorbed, as the linear attenuation coefficient decreases with energy. As a result, the beam gradually becomes harder, i.e. its mean energy increases. The harder the beam, the lower the attenuation. Therefore, the total attenuation is no longer a linear function of the thickness of the absorber. Neglecting this effect in the reconstruction process leads to well-known beam-hardening artifacts, such as cupping, streaks and flairs. According to an exemplary embodiment of the method according to the present invention, a second-pass beam-hardening correction is performed in order to correct beam-hardening artifacts caused by metal objects. The advantage of this physical correction method in contrast to mathematical methods is that it is based on physical laws and uses the data in the shadow of the metal completely. No information is disregarded. The method may lead to a reduction of beam-hardening artifacts such as streak artifacts and to a better image quality in the reconstructed images. Line integrals through matter are typically very noisy, which may result in a large error in the estimation of the path length through metal. This may be improved by using projections which have been filtered adaptively. The artifact correction code requires a two-dimensional look-up table which maps measured attenuation values and the proportion of total attenuation due to metal into attenuation error. This table has to be computed prior to running the metal correction code or artifact correction code. In order to compute the table, it is necessary to know the x-ray energy spectrum of the scanner at hand, standard water absorption tables and metal absorption tables for each x-ray energy level in the spectrum. The generation of the look-up table is described in the following with reference to Fig. 3. The look-up table generation may be performed in the calculation unit 18 in the CT scanner system of Fig. 1. Then, with reference to Fig. 4, an artifact correction scheme is described using the 2-dimensional look-up table generated in accordance with the method described with reference to Fig. 3. It should be noted that such a two-dimensional look-up table is generated for each voltage setting of the tube, i.e. the source of radiation. An easy approach for a polychromatic correction method for metal artifacts is the derivation from second-pass beam-hardening correction for bone. In this case, the metal correction code requires a two-dimensional look-up table that maps measured attenuation values and the proportion of total attenuation due to metal into attenuation error. The result is shown as image slice 55 in Fig. 2a. Image slice 55 is an image slice of a 400 mm water cylinder with four titanium inserts 51, 52, 53, 54, including image artifacts, which may be caused by the beam-hardening effect resulting from the titanium inserts 51, 52, 53, 54. The beam-hardening artifacts in the reconstructed image 55 occur as streaks. As may be seen from Fig. 2a, without any correction of the hardening, i.e. without any artifact correction according to an exemplary embodiment of the present invention, a strong streak pattern can be observed. After performing a second-pass beam-hardening correction according to a method of an exemplary embodiment of the present invention, the streaks disappear (see image slice 55). The remaining artifacts arise for other reasons, for example, aliasing. This can be seen from image slice 56 which is a result from a monochromatic simulation and reconstruction. The correction code requires a multi-dimensional look-up table, e.g. a two-dimensional or a three-dimensional look-up table, that maps measured attenuation values and the proportion of total attenuation due to metal and/or bone, respectively. This table may be computed prior to running the correction code. In order to compute the table, it is necessary to know the x-ray energy spectrum of the scanner system at hand, standard water absorption tables and metal and/or bone absorption tables for each x-ray energy level in the spectrum. Fig. 2b shows a reconstructed image slice of a water cylinder with titanium 61, 63 and bone inserts 62, 64, including image artifacts, which may be caused by the beam-hardening effect (see image slice 58). In the presence of bone and metal, a consecutive polychromatic correction does not yield sufficient results, as may be observed from image slice 59 in Fig. 2b. Since no correlation between metal and bone is regarded in the look-up table, streak artifacts remain between these components in the corrected image. Therefore, a polychromatic correction has to be done for metal and bone simultaneously. Image slice 60 in Fig. 2b shows the result if metal and bone are taken into account when generating a three-dimensional look-up table. The streaks between the metal and bone contents have disappeared. The remaining artifacts have other reasons, e.g. aliasing. A further example is depicted in Fig. 2c. A reconstructed image slice 65 of a water cylinder with titanium and bone inserts, including image artifacts, which may be caused by the beam-hardening effect is shown. Image slice 66 shows the result of a polychromatic beam-hardening correction considering metal and bone according to an exemplary embodiment of the present invention. Furthermore, image slice 67 shows a monochromatic simulation and reconstruction considering metal and bone. A prerequisite for performing a second-pass beam-hardening correction for two non-water components may be the generation of a three-dimensional look-up table, which may be a raw binary data file comprising, according to an exemplary embodiment of the present invention, 32 x 32 x 32 floating point numbers. These numbers may be organized as 32 metal attenuation values for each of the 32 bone attenuation values for each of the total attenuation values. One table is generated for each voltage setting of the tube. But it should be understood that the raw binary data file may comprise any other quantity of floating point numbers, e.g. 512 x 512 x 512 floating point numbers. If the artifacts which have to be filtered out of the image arise from three different materials (e.g. bone, dental fillings and metal implants), a 4-dimensional lookup table may be implemented in a scanner system, a method for artifact correction, a data processing device and a computer program according to an exemplary embodiment of the present invention. The all-dominant generation of a look-up table is described in the following. Fig. 3 shows a flow-chart of an exemplary embodiment of a method of generating a two-dimensional look-up table according to the present invention. Before the start in step SI, the spectrum of the x-ray source is determined for a given tube voltage. After that, in step S2, the x-ray spectrum table is read by the data processing device and written into arrays e(i) and s(i), where e(i) is the energy and s(i) is the amount of that energy in the spectrum. After that, in step S3 the normalized x-ray spectrum sn(e) is computed: β m _ _______ Then the water absorption coefficient table and the metal absorption coefficient table are read and written into array w(i) and m(i), respectively (step S4). In step S5 the water only log-attenuation Aw(i) for a range of water thicknesses is computed: A* (t) = - log , 0 < i < KV

Figure imgf000014_0001
where Kw is the number of rows in the look-up table, pw the density of water (e.g. 1,0 g/cm ) and Δw = Amax/(μwKw). The maximum attenuation may be chosen to be Amax = 15,0. The linear attenuation coefficient of water is μw. The number of rows Kw in the look-up table may be, e.g., 32 or 512 or any other number. In steps S6, the linearized water only log-attenuation Cw(i) is computed: Ow( = ri w O ≤ i ≤ Kw After that, in step S7 a mixture of water and metal which gives the same log-attenuation Aw(i) is computed, wherein the thickness of water in that mixture is

^ = a-(' - i^i) where Km is the number of columns in the look-up table. Furthermore, the thickness of the metal is Me(ij), such that

Aw(i) ± e = - log ( sa(n) exp(-tf (n)pwVT(2, j) - m(n)ρmMe(i,j)) \ where pm is the density of the metal. In a further step (S8), the linearized log-attenuation of the mixture is computed for a range of water and metal thicknesses: M(.,j) = μwW(i,j) + μι_Me(itj) wherein μm is the linearized log-attenuation coefficient of a metal. After that, in step S9, the difference in linearized mixture log-attenuation and linearized water only log-attenuation is computed. This results in the so-called error look-up table or two-dimensional look-up table. C(i,j) = M(i,j) + Cw(i,j) Once a look-up table has been generated, it may be implemented in a second-pass beam-hardening correction algorithm as described with respect to Fig. 4. The look-up table generation ends with step S10. Fig. 4 shows a flow-chart of an exemplary embodiment of a method of generating a three-dimensional look-up table according to the present invention. For a description of steps SI - S6 please refer to Fig.3. In steps S7 to S10, a mixture of water, bone and metal is computed that gives the same log-attenuation Aw(i) via the following steps: (a) Let the thickness of water in that mixture be

W(i,j,k) = iΔ [l τ τr .for is <1 , Kh-l Km-lJ ' κm-ι- Kb-1' and 0 <i < K„ ,0 < j < Kh ,0 < k < Km t where Kb = 32 is the number of columns and Km = 32 is the third dimension of the look-up table. This means that an X-ray beam passes through bone with a fraction of j / (Kb - 1) and through metal with a fraction of k / (Km - 1) of the entire path through the object (step S7). (b) Determine the remaining water-only log-attenuation, if the fraction k / (Km - 1) of the entire path is caused by metal (step S8):

e(i, k) = - log ,

Figure imgf000015_0001
0<i<KvandO<k<Km. (c) Find a bone thickness B (i, j, k) such that

_4M(-, k)±€ = -log ∑ _n(n) exp(--j(n)?wV (., j, k) - b(n)ρhB(i , k)) ) \«=0 / where pb = 1.92 g/cm3 is the density of bone. For a given set of i, k determining a value for the remaining water-only log-attenuation, a combination of the path lengths through water and bone is calculated such that the resulting attenuation is Are(i, k) in the limits of ε = 3.9 x 10"6 (step S9). (d) Find a metal thickness Me (i, j, k) such that

Figure imgf000016_0001

- log j, k) - m(n)ρmMe(i, j, ,

Figure imgf000016_0002
Figure imgf000016_0003
where pm is the density of metal (step S10). In step SI 1, the linearized log-attenuation of the mixture is computed for a range of water, bone and metal thicknesses: M (i,j, k) = ^W(i, j, k) + μh (i , k) + μmMe{i, j, k) where μb = b x pb = 0.44 cm"1 and μm are the linear attenuation coefficients of bone and metal for 56 keV, respectively. The density and the attenuation coefficient of titanium are p„ = 4.49 g/cm3 and μ (56 keV) = 3.9 cm"1, for example. In step SI 2, the difference in linearized mixture log-attenuation and linearized water only log-attenuation is computed. This is the error look-up table C(i>j, k) = M(i > k) - Cvl{i) Fig. 5 shows a flow-chart of an exemplary embodiment of a method of artifact correction. The method of artifact correction starts with step SI, which may be the generation of the two-dimensional look-up table as described with respect to Fig. 3. After that, in step S2, the data set is reconstructed, resulting in the original image. In step S3, a threshold is set for the first material, which is, in the case of this exemplary embodiment, a metal. Advantageously, this threshold is set by a user, but it may also be set by the data processing device. After that, in step S4, a segmentation of the metal in the original image is performed, resulting in a metal-only image and the original image. In step S5, a forward projection of the original image and of the metal-only image is performed. In step S6, correction projection data on the basis of a two-dimensional look-up table is generated and, in step S7, correction image data from the correction projection data is generated by filtering and back-projecting of the correction projection data. After that, in step S8, a correction image data is subtracted from the original image, resulting in a corrected image. The method ends in step S9. Fig. 6 shows a flow-chart of an other exemplary embodiment of a method of operating the CT scanner system or the data processing system according to the present invention. Once a three-dimensional look-up table has been generated (step SI) it may be implemented in a second-pass beam-hardening correction algorithm as follows: In step S2, the data set is reconstructed, resulting in the original image. In step S3, a threshold is set for the first material, which is, in the case of this exemplary embodiment, a metal. Advantageously, this threshold is set by a user, but it may also be set by the data processing device. After that, in step S4, a segmentation of the metal in the original image is performed, resulting in a metal-only image and the original image. After that, in step S5, a range for bone is set and a segmentation of the bone content in the reconstructed image is performed (step S6). Then, a forward projection of the entire image, the bone-only image and the metal-only image is performed in step S7. In step S8, the correction projection data PCOr(v, d) = C(i, j, k) for each view v and each detector element d is generated with the help of the three-dimensional look-up table. The indices of the look-up table are calculated from the forward projected data according to P P P * = -A — Kw, ; = -=- _-_, k = — A„ where Pt, Pb and Pm are the forward projected data of the entire, the bone-only and of the metal-only image, respectively. In step S9, filtering and back-projection of the correction projection data in order to obtain correction image data is performed. After that, in step S10, the correction image data is subtracted from the original image. Fig. 7 depicts an exemplary embodiment of an image processing device according to the present invention, for executing an exemplary embodiment of a method in accordance with the present invention. The image processing device depicted in Fig. 7 comprises a central processing unit (CPU) or image processor 151 connected to a memory 152 for storing a deformable model and an image depicting an object. The image processor 151 may be connected to a plurality of input/output network or diagnosis devices, such as an MR device or a CT device. The image processor is furthermore connected to a display device 154 (for, e.g. a computer monitor) for displaying information or images computed or adapted in the image processor 151. An operator may interact with the image processor 151 via a keyboard 155 and/or other output devices which are not depicted in Fig. 7.

Claims

CLAIMS:
1. A method of artifact correction in a data set, wherein the data set is acquired by means of a polychromatic source of electro-magnetic radiation generating a polychromatic beam and by means of a radiation detector detecting the polychromatic beam, the method comprising the steps of: identifying a first material in the data set, wherein the first material is a material which causes artifacts in the data set; generating correction image data, wherein the correction image data is data which results from the first material; and one of subtracting and adding the correction image data from an original image, resulting in a corrected image.
2. A method according to claim 1 , wherein the identification of the first material in the data set is performed by the following steps: reconstructing the data set, resulting in the original image; setting a threshold for the first material; and performing a segmentation of the first material in the original image, resulting in a first-material- only image and the original image.
3. A method according to claim 1, wherein, after identifying the first material in the data set and before generating correction image data, the following step is performed: forward projecting of the original image and of the first-material-only image.
4. A method according to claim 1, wherein the generation of the correction image data comprises the following steps: generating correction projection data on the basis of a multi-dimensional look-up table; and generating correction image data from the correction projection data by filtering and back-projecting of the correction projection data.
5. A method according to claim 4, wherein the multi-dimensional look-up table is a two-dimensional look-up table; wherein the look-up table comprises first material attenuation values; wherein the look-up table is generated on the basis of an energy spectrum of the beam and corresponding water absorption coefficients and corresponding first material absorption coefficients;
6. A method according to claim 4, wherein the multi-dimensional look-up table is a three-dimensional look-up table; wherein the look-up table comprises first material attenuation values and second material attenuation values; wherein the look-up table is generated on the basis of an energy spectrum of the beam and corresponding water absorption coefficients, corresponding first material absorption coefficients and corresponding second material absorption coefficients.
7. A method according to claim 4, wherein the first material is one of a metal, a bone, a dental filling which is not a metal, a prosthesis which is not a metal, and an implant which is not a metal.
8. A method according to claim 1, wherein the polychromatic source of electro-magnetic radiation is an x-ray source; and wherein the CT scanner system has one of a fan beam geometry and a cone beam geometry.
9. A data processing device comprising: a memory for storing a data set; a data processor for performing artifact correction in the data set, wherein the data processor is adapted for performing the following operation: loading the data set acquired by means of a polychromatic source of electro-magnetic radiation generating a polychromatic beam and by means of a radiation detector detecting the polychromatic beam; identifying a first material in the data set, wherein the first material is a material which causes artifacts in the data set; generating correction image data, wherein the correction image data is data which results from the first material; subtracting the correction image data from an original image, resulting in a corrected image.
10. A data processing device according to claim 9, wherein the data processing device is part of a CT scanner system; wherein the CT scanner system has one of a fan beam geometry and a cone beam geometry; wherein the data processing device is adapted for generating a multi-dimensional look-up table; and wherein the data processing device is adapted for generating the correction image data on the basis of the multi-dimensional look-up table.
11. A computer program for performing artifact correction in a data set, wherein the computer program causes a processor to perform the following operation when the computer program is executed on the processor: loading the data set acquired by means of a polychromatic source of electro-magnetic radiation generating a polychromatic beam and by means of a radiation detector detecting the polychromatic beam; identifying a first material in the data set, wherein the first material is a material which causes artifacts in the data set; generating correction image data, wherein the correction image data is data which results from the first material; subtracting the correction image data from an original image, resulting in a corrected image.
12. A computer program according to claim 11 , wherein the processor is part of a CT scanner system; wherein the CT scanner system has one of a fan beam geometry and a cone beam geometry; wherein the data processing device is adapted for generating a multi-dimensional look-up table; and wherein the data processing device is adapted for generating the correction image data on the basis of the multi-dimensional look-up table.
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