DE102010034917B4 - Method and device for determining the influence of shadowing on x-ray imaging, as well as corresponding data carrier and computer tomograph - Google Patents

Abstract

Method for determining the influence of shadowing on X-ray imaging of an examination object (P) as a function of a data record representing the examination subject (P), the data set comprising data about a section (22) of the examination subject (P) having a threshold absorption coefficient above it in that, for a projection virtual series (30, 30 ', 31-33, 51, 54) having a plurality of projections (30, 30', 31-33, 51, 54), the different positions of a beam path (41-46; 52 , 55) relative to the examination object (P), depending on the data set, information about shadowing by the section (22) is determined, wherein one of the virtual projection series (30, 30 ', 31-33; 51, 54) is determined in order to quantify the influence of shading by the section (22) on a reconstruction.

Description

The invention relates to the field of X-ray imaging. In particular, the invention relates to methods and apparatuses which can be used for X-ray imaging on an examination subject which has an X-ray-strongly absorbing section, in particular a metal section.

Three-dimensional image or object detection is widely used in medical technology. 3D volume data are often used to prepare for therapeutic and / or surgical procedures. An example of an application is the trauma construction, for example for the treatment of bone fractures, in which further treatment steps can be planned on the basis of pre-recorded 3D volume data of the examination subject.

Methods and apparatus for 3D X-ray imaging are known with which 3D volume data of the examination object can be reconstructed. Exemplary methods include, but are not limited to computed tomography or cone-beam computed tomography. In 3D X-ray imaging, metal objects or other objects with high X-ray absorption can adversely affect the reconstructed data. For example, so-called metal artifacts can occur. The degree of deterioration of reconstruction depends on various factors, for example the dose of the individual projection images or the number of sample imaging shots. In X-ray imaging using a C-arm device, conventional positioning for a scan is often chosen. For example, the object is positioned in the isocenter. The C-arm is positioned perpendicular to the table with an angulation of 0 °. The scan can be done for example by an orbital movement of the C-arm of 190 °.

However, depending on the geometry and location of the metal object in the patient, pre-set scan parameters, such as table orientation and angulation, may cause metal artifacts to be more pronounced.

From the DE 10 2008 050 570 A1 For example, a method is known for generating a 3D image data set of a body containing an X-ray opaque object.

The US 2007/0211853 A1 describes a method and a device for X-ray imaging, in which or in which X-ray recordings are carried out with different energy levels of the X-ray radiation and, depending on the image data recorded in the process, it is concluded that the presence of a metallic object.

It is often difficult for a user to estimate how much a metal object affects reconstructed volume data. It is also often difficult to estimate if and by what change in positioning a pickup device metal artifacts can be reduced.

The invention has for its object to provide a method and an apparatus, as well as a corresponding data carrier and computer tomograph, which allow a user to assess quantitatively the influence of metal objects or other strongly absorbing objects in X-ray imaging.

According to the invention, a method, a data carrier with a computer program stored thereon, a device and a computer tomograph are provided, as specified in claims 1, 22, 23 and 25. The subclaims define embodiments.

In one aspect, a method is provided for determining the effect of shading on x-ray imaging of an examination subject. The method uses a data record representing the examination object as input data. The data set includes data about a portion of the examination subject having a threshold absorption coefficient. For a virtual projection series with a plurality of projections, which correspond to different positions of a ray path relative to the examination subject, information about shading by the section is computationally determined depending on the data set. Depending on the information obtained, a parameter assigned to the virtual projection series is determined in order to quantify the influence of the shading by the section on a reconstruction.

The method considers that the influence, for example, of a metal object on the quality of a reconstruction depends on the positional relationship between the metal object and a scanning geometry in the data acquisition. The method quantifies the influence of metal shading. This makes it possible to quantitatively determine, before or after X-ray imaging, how strong the influence of shading is on a reconstruction.

The data about the portion of the examination subject having a threshold absorption coefficient may be data about a portion of metal.

For example, the parameter determined for the virtual projection series can be exactly one numeric value. The characteristic can be determined such that it lies, for example, in the interval from 0 to 1 or another known interval. The reconstruction for which the influence of the shading is determined does not necessarily have to be calculated. In particular, without appropriate data acquisition using the data set, it can be determined in advance whether strong metal artifacts are to be expected for a particular scan geometry, ie for a specific projection series.

The virtual projection series can be selected depending on an X-ray imaging device that was used for data acquisition or is still to be used. For example, if a C-arm device is used, the different layers may correspond to different angular positions of a fan beam or beam cone.

The determination of the information about the shading by the section can be done pixel by pixel or voxelweise. In this case, for example, a location-dependent information about the influence of shading is first determined for all voxels of a volume. It can be based on the pixel or voxel resolution, which should also be used for the actual reconstruction. The information about the shading can be determined depending on projection matrices that describe the image of an object volume positioned on a patient table in pixels of a detector for the X-ray recording device used.

A virtual projection series or virtual projection is understood to mean a projection series or projection which is determined by calculation. An actual data acquisition, in which the position of the beam path was selected relative to the examination object according to the projection series, can take place before or after the computational determination of the parameter. However, this is not mandatory.

The data set may be, for example, a 3D data set determined by a metal segmentation, in which voxels representing one metal have a first value (eg 1) and all other voxels have a second value (eg 0). Metal segmentation is understood to mean a procedure by means of which, in a 2D sectional image data set or 3D volume data set, for example a set of Hounsfield values, the pixels or voxels corresponding to a material with a high absorption coefficient, in particular a metal, can be determined. The data record can also be constructed differently as long as it can be determined which section of the represented volume or the represented layer corresponds to a metal object. For example, the record may contain the Hounsfield values from which segmentation can be used to determine the section corresponding to the metal object.

The computational determination of the information may include determining, for pixels or voxels of the reconstruction, whether the pixel or voxel in the projections of the virtual projection series is respectively arranged on a projection beam passing through the section. The shadowing that occurs when the pixel or voxel is placed on a projection beam passing through the section results in loss of information and can cause artifacts. As mentioned, the reconstruction itself does not have to be calculated. According to the method, the influence of shading by the section which would occur when a scan is actually carried out can be calculated.

The computational determination of the information may include determining, for pixels of a slice image or voxels of the volume, a number of projections of the virtual projection series in which the corresponding pixel or voxel is arranged on a projection beam passing through the portion. This number provides pixel- or voxel-wise a measure of how many of the projections would result in information loss through metal shading.

The characteristic quantity determined for the virtual projection series can be determined as a function of a sum of the number of projections determined for the pixels or voxels of the reconstruction in which the pixel or voxel is arranged on a projection beam passing through the section of the examination subject. Thus, the pixel or voxelweise determined influence of Metallabschattung is easily converted into a global characteristic for the virtual projection series or reconstruction, which can be determined in actual implementation of the projection series. A position-dependent weighting of the pixel- or voxel-determined number of projections in which the pixel or voxel is arranged on a projection beam passing through the section of the examination object can be undertaken.

For determining the information about the influence of the shading, for each pixel or voxel of the reconstruction, reliability information proportional to a number of projections of the virtual projection series may be determined where the corresponding pixel or voxel is not on a plane passing through the portion of the examination subject projection beam is arranged. A normalization by division by the total number of projections of the virtual projection series can be made. A value of this local measure for the influence of shadowing close to 1 then indicates that information loss by shading occurs for the specific virtual projection series only in a small proportion of projections.

The characteristic assigned to the virtual projection series can then be determined by averaging the reliability information about the pixels or voxels of the reconstruction. As a result, a global parameter for the virtual projection series or the reconstruction, which can be determined when the projection series is actually executed, can be generated in a simple manner.

In embodiments, the influence of shading can first be quantified in pixel or voxel manner. For this purpose, several projection images can be calculated based on the data record. In particular, the data set may be a data set resulting from a 3D metal segmentation in which all voxels corresponding to the metal object have a first value and all other voxels have a second value. The projection images can be generated as black and white projections. By unfiltered back projection of the projections, a 3D data field can be generated in which the value of each voxel depends on the number of projections of the virtual projection series with the corresponding voxel lying on a projection beam passing through the metal object. Then averaging can be done via voxels to determine the parameter for the virtual projection series.

The determination of the parameter can be carried out after performing a data acquisition with an X-ray recording device to quantify the influence of Metallabschattung. For this purpose, 3D volume data can be reconstructed from a plurality of recordings which were recorded with the X-ray recording device. A 3D segmentation of the 3D volume data can be made to generate the data set. The virtual projection series can be chosen such that it corresponds to the actual position between the beam path and the examination subject during data acquisition. In this way, the influence of metal shading on the reconstructed 3D volume data can be quantified.

The parameter can be output, for example, via a display of a computer tomograph. For example, the characteristic may be normalized to be in the interval from 0 to 1. From the output absolute value, a user can estimate whether the influence of the metal shading on the already acquired data is tolerable or a new data acquisition must be made.

The parameter quantifying the shading can be determined not only for one but also for several virtual projection series. This allows a comparison of how strongly metal shading influences the associated reconstruction in different virtual projection series.

It is possible to determine a maximum or minimum of the parameters determined for different virtual projection series. An optimization procedure, for example a so-called "simulated cooling algorithm" (also referred to as "simulated annealing"), can be carried out in order to determine the extremum of the parameter. The extremum of the characteristic can be output together with the parameter which was determined for the virtual projection series corresponding to a data acquisition performed. As a result, information can be communicated to the user as to what extent the metal shading has an effect on the already existing reconstruction in comparison to a reconstruction in which the influence of the metal shading is smaller, in particular minimal.

It is also possible to provide information about the virtual projection series, which leads to a reconstruction with the smallest possible influence of the metal shading. The information may be provided in the form of parameters describing settings for an x-ray cradle. For example, a starting or end position of a movable component of the X-ray recording device relative to a patient table can be determined. In one embodiment, positioning of a C-arm relative to a patient table may be determined. For example, the position of the C-arm relative to the table as well as the angulation can be determined, which provides an extremely characteristic reconstruction.

To determine the characteristic variable for different virtual projection series, the examination object represented by the data record can be rotated virtually. As a result, for example, different relative positions between the examination object and a scan plane of a C-arm device can be tested out.

In each of the virtual projection series, the conditions of the X-ray recording device can be taken into account. In particular, only those virtual projection series that can be carried out in reality with a given X-ray recording device can be tested. at For example, in a C-arm device, the C-arm, the table and the patient constrain possible positions between X-ray source, examination subject and detector. These restrictions can be taken into account as boundary conditions, for example in an optimization procedure.

According to a further aspect, a data carrier is specified on which a computer program is stored, which comprises a command sequence which, when executed by an electronic computing device, causes the computing device to carry out the method according to one aspect or exemplary embodiment of the invention. The computer program can be loaded, for example, into the memory of a control and evaluation computer of a device for x-ray imaging, for example a computer tomograph. The computer program may be present as source code or as a compiled instruction sequence. The computer program allows the device to be set up in a program to carry out the method. The data carrier can be, for example, a CD-ROM, a DVD, a magnetic tape, a flash memory or a USB stick or any other non-transient data carrier on which the computer program is stored as electronically readable control information.

According to a further aspect, an apparatus for determining the influence of shadowing on an X-ray imaging of an examination subject is specified. The device comprises a memory for storing a data record representing the examination subject. The data set includes data about a portion of the examination subject having a threshold absorption coefficient. The device comprises a computing device coupled to the memory, which is set up to determine information about shading by the section for a virtual projection series with a plurality of projections, depending on the data record. The different projections correspond to different layers of a beam path relative to the object to be examined. The computing device is furthermore set up in order to determine, depending on the determined information, a characteristic variable associated with the virtual projection series in order to quantify the influence of the shadowing by the section on a reconstruction.

The device makes it possible to mathematically quantify the impact that a metal object has on the quality of a reconstruction, depending on the positional relationship between the metal object and a scanning geometry used in data acquisition. This makes it possible to quantitatively determine, before or after X-ray imaging, how strong the influence of shading is on a reconstruction.

The apparatus may be arranged to perform the method according to one of the aspects or according to one of the embodiments.

According to a further aspect, there is provided a computed tomography apparatus comprising a line or area detector for capturing images of the examination subject and a device coupled to the line or area detector according to one of the aspects or any of the embodiments.

Embodiments of the invention are suitable for determining the influence of metal shading on one or more reconstructions. Application areas are in particular in medical technology.

Hereinafter, embodiments of the invention will be explained in detail with reference to the drawings.

1 is a schematic representation of a computed tomography device with an apparatus according to an embodiment.

2 is a schematic representation of a projection of a virtual projection series to explain the method according to an embodiment.

3 is a schematic representation of another projection of the virtual projection series of 2 ,

4 is a schematic representation of a plurality of projection images for explaining the method according to an embodiment.

5 is a schematic representation of projections of another virtual projection series in an embodiment for explaining the method according to an embodiment.

6 FIG. 10 is a flowchart illustration of a method according to an embodiment. FIG.

7 FIG. 10 is a flowchart illustration of a method according to an embodiment. FIG.

8th FIG. 10 is a flowchart illustration of a method according to an embodiment. FIG.

9 - 11 illustrate sections through 3D reconstructions that are for a conventional scanning geometry and one with methods and devices Scanning of the invention were generated.

The features of the embodiments described below may be combined with each other unless expressly stated otherwise.

In the following, embodiments of the invention will be described in the context of an X-ray imaging in which an X-ray recording device for capturing 2D images is set up, from which 3D reconstructions are calculated. However, the methods and apparatuses according to embodiments of the invention may be applied to other fields as well. For example, the methods and apparatus can be applied when reconstructing 3D volume data from multiple 1D shots. The methods and apparatus can also be used when reconstructing 2D image data from multiple 1D images.

1 is a schematic representation of a computer tomograph 1 with a device for determining the influence of Metallabschattungen according to an embodiment. The computer tomograph 1 includes an X-ray recording device 2 and a device 11 , which quantitatively evaluates the influence of metal shading. The X-ray recording device 2 includes an X-ray source 4 and a detector 5 for detecting X-rays after passing through a subject P under examination from a table 9 is supported. The X-ray recording device 2 can have various configurations. For example, the computer tomograph can be configured as a cone-beam computer tomograph or as a fan-beam computed tomography. Other embodiments of the X-ray recording device 2 are possible. Illustrated in FIG 1 an embodiment in which the x-ray source 4 and the detector 5 on a C-arm 3 are attached. The detector 5 For example, it can be designed as a line detector or as an area detector. A drive device 7 is provided to the X-ray source 4 and the detector 5 relative to the table 9 to reposition with the examination object P. A movement about an axis perpendicular to the plane of 1 is schematic at 8th shown. Another position of the C-arm with X-ray source and detector is at 3 ' shown with broken lines. An additional drive means may be provided to the table 9 relative to the C-arm 3 to translate. A control device 10 controls the X-ray source 4 , the detector 5 and the drive device 7 , The X-ray source 4 and the detector 5 are positioned in a plurality of different positions relative to the examination subject P. In each of the positions, a data acquisition takes place in which an image is captured.

The device 11 includes a computing device 12 , a display device 13 and a memory 14 , The memory 14 Contains a data record containing data that specifies the location of a metal object in a study object. For example, the data set may be a set of volume data in which voxels corresponding to the metal object are 1 and all other voxels are 0. At the computer tomograph 1 determines the computing device 12 not only the influence of Metallabschattungen, but also works as an evaluation computer, which reconstructed from the plurality of captured images 3D volume data of the examination subject. The device 11 However, it does not have to be coupled to an X-ray recording device, but instead can be designed as a separate arithmetic unit. According to embodiments of the invention, the computing device is 12 set up so that they arithmetically dependent on that in the memory 14 stored record a characteristic that is assigned to a virtual projection series determined. The parameter is determined so that it gives a quantitative measure of the influence of metal shading in a reconstruction that can be determined from the projection series. The determined parameter can be displayed via the display device 13 be issued.

In one embodiment, the computing device is 12 is set up in such a way that, in order to determine the parameter, it first determines information for all voxels within a volume which represents a local measure of the influence of metal shading. In one embodiment, the information for a voxel may be proportional to a number of projections of the virtual projection series in which the corresponding voxel is not disposed on a projection beam passing through the metal object. In one embodiment, the information for each voxel may be equal to the number of projections of the virtual projection series in which the corresponding voxel is not located on a projection beam passing through the metal object divided by the total number of projections in the virtual projection series. The computing device 12 may suitably combine the thus determined spatially resolved information into a single characteristic associated with the virtual projection series. For this purpose, the computing device 12 For example, calculate an average of the spatially resolved information about the voxels of the volume. The parameter determined in this way represents a quantitative measure of the influence of the metal shading for a projection series or the reconstruction that can be determined therefrom. A value of the characteristic thus determined close to 1 indicates that the metal shading has a minor influence on the reconstruction having. A value of the thus determined characteristic, which is much smaller than 1, indicates that the metal shading has a significant influence on the reconstruction.

In one embodiment, the computing device 12 be set up so that it determines the characteristic in the manner described not only for one, but for several virtual projection series. For example, to debug various virtual projection series, the volume represented by the record may be rotated virtually. The virtual projection series may depend on the characteristics of the X-ray cradle 2 to get voted. For example, all of the virtual projection series can be chosen to have design constraints on the possible relative positions of the patient table 9 , X-ray source 4 and detector 5 consider. These design constraints, for example, lead to constraints that are subject to the entries of projection matrices that can be used in computing the projections of a virtual projection series.

In one embodiment, the computing device 12 be set up so that it automatically performs an optimization with respect to the determined characteristic over different virtual projection series. For example, if the global characteristic is defined as described above such that a value of the characteristic close to 1 indicates a small influence of metal shading, maximizing the characteristic under boundary conditions by which the geometry of the X-ray recording device 2 is taken into account.

The determination of the parameter for a virtual projection series according to embodiments will be made with reference to FIG 2 - 5 described in more detail. The steps described may be performed by the computing device 12 be performed.

2 and 3 show a schematic representation of a volume 21 , where the volume in 3 90 ° in one position 21 ' is turned. The volume represents a section of an object under investigation with a metal object 22 dar. A projection 30 a virtual projection series is in 2 shown. Another projection 30 ' The virtual projection series is in 3 shown. The presented projections 30 . 30 ' need not be actually captured, but can be computationally computed based on the dataset that specifies which voxels are from the metal object 22 are occupied.

It becomes a voxel distribution of the volume 21 performed. For all voxels of volume 21 For example, it is determined whether the voxels in the various projections of the virtual projection series each lie on projection beams passing through the metal object 22 run. If a voxel on one through the metal object 22 running beam, this leads to information loss due to Metallabschattung when performing an actual data acquisition. For each of the voxels, at least the number of projections of the projection series in which the voxel is not arranged on a projection beam passing through the metal object can be determined.

At the in 2 shown projection 30 becomes the metal object 22 in an area 34 a virtual projection image 31 displayed. The projection beams are made to rotate about 90 ° about a C-arm 48 relative to the in 2 Corresponding beam path correspond. The one by a voxel 23 running projection beam 41 passes through the metal object 22 , The one by a voxel 24 running projection beam 43 does not pass through the metal object 22 , The one by a voxel 25 running projection beam 45 does not pass through the metal object 22 , The Metallabschattung would in the projection 30 to a loss of information regarding the voxel 23 to lead.

At the in 3 shown projection 30 ' becomes the metal object 22 in an area 35 a virtual projection image 32 displayed. The one through the voxel 23 running projection beam 42 does not pass through the metal object 22 , The one through the voxel 24 running projection beam 44 also does not run through the metal object 22 , The one through the voxel 25 running projection beam 46 passes through the metal object 22 , The Metallabschattung would in the projection 30 ' to a loss of information regarding the voxel 25 to lead.

Based on the number of projections of the virtual projection series in which the voxels 23 - 25 of the volume 21 each on through the metal object 22 are arranged extending projection beams, the influence of Metallabschattung can be determined in a spatially resolved on a reconstruction. Even though exemplarily only three voxels 23 - 25 can be described, the steps described for an overlap of the volume 21 with voxels, ie for all voxels. For example, by averaging the spatially resolved information, a global characteristic assigned to the virtual projection series can be determined.

With reference to 4 a method is described by means of which, according to an embodiment for voxels of a volume, the number of projections of a virtual projection series can be calculated, in which the voxel is positioned on a projection beam which is projected through a Metal object goes. For this purpose, based on a data set that contains data about the position of the metal object, computationally multiple virtual projection images 31 - 33 determined. The virtual projection images 31 - 33 are 2D binary data fields, which can also be called black-and-white projections. The virtual projection images 31 - 33 exhibit in areas 34 - 36 in which projecting rays passing through the metal object terminate, the pixel value 1 and otherwise the pixel value 0. By an unfiltered back projection of the thus generated virtual projection images 31 - 33 a set of volume data is generated in which the value h (x, y, z) of a voxel with coordinates (x, y, z) indicates the number of projections of the virtual projection series in which the voxel is on a projection beam passing through the metal object lies. Of course, a quantity g (x, y, z) = n-h (x, y, z) can also be determined accordingly, where n is the total number of projections of the virtual projection series. The value g (x, y, z) is equal to the number of projections of the virtual projection series in which the voxel is not on a projection beam passing through the metal object. By normalization, a spatially resolved reliability information g (x, y, z) / n can be determined which, for the various voxels, indicates the proportion of the projections of the virtual projection series in which the voxel does not lie on a projection beam passing through the metal object. By averaging over the voxels, a global characteristic can be determined. A local weighting may be made, for example, if certain volume ranges are particularly important for a therapeutic or surgical procedure. The averaging can also be done only on those voxels for which a suppression of metal artifacts is particularly desired.

The determination of the global characteristic can be repeated for further virtual projection series. 5 illustrates the determination of the further characteristic for another virtual projection series. This was the volume 21 relative to the in 2 and 3 shown position by 90 ° about an axis 49 shot to test another virtual projection series. Of course, equivalently, rotation of the scan plane relative to the volume could also be equivalent 21 be made. The next virtual projection series includes several projections. In a projection image 51 , in which the central ray of the cone of rays is horizontal, becomes the metal object 22 in an image area 53 displayed. In a projection image 54 , in which the central ray of the cone of rays is vertical, becomes the metal object 22 in an image area 56 displayed. The number of voxels in the volume 21 ' taking in the projection beams 52 . 55 can be summed to obtain a parameter quantifying the influence of metal shading.

By determining the parameters for various virtual projection series, for example as part of an optimization procedure, an optimal parameter can be determined which, in the context of the X-ray recording device 2 given boundary conditions for the examination object can be achieved. Furthermore, it can be determined for which scanning geometry, for example for which initial position of a C-arm relative to a patient table, the influence of metal shading is low, in particular minimal.

6 is a flowchart representation of a method 60 for determining a parameter that quantifies the influence of metal shading. The method may be performed by the computing device 12 be performed.

at 61 a virtual projection series is selected. The projection series comprises several projections, which correspond to different positions between a beam path and an examination subject. The different positions can be indicated, for example, by the angles between a central ray of a ray fan or ray cone with coordinate axes of a patient coordinate system. The virtual projection series can be selected depending on the specific application of the method. If, for example, a data acquisition has already taken place on the examination object with an X-ray recording device, the virtual projection series can be selected such that the different projections correspond to the angles under which the different recordings were recorded during the data acquisition. The virtual projection series at 61 can also be chosen as part of an optimization procedure. In this case, the virtual projection series can be selected depending on at least one parameter determined for an earlier virtual projection series.

at 62 For all voxels of a volume, reliability information is determined which indicates the influence of metal shading on the different voxels of the volume in a spatially resolved manner. The reliability information is provided for in 61 selected virtual projection series and depending on a data set indicating the location of a metal object in the examination object. The reliability information may be as described with reference to 2 - 5 for example, be determined so that the reliability information for each voxel indicates the proportion of the projections of the virtual projection series in which the corresponding voxel is not arranged on a projection beam passing through the metal object.

at 63 becomes a global one depending on the reliability information determined for the voxels Parameter for the selected virtual projection series. For this purpose, for example, the reliability information about voxels can be averaged.

7 is a flowchart representation of a method 70 for determining a parameter that quantifies the influence of metal shading. The method may be performed by the computing device 12 be performed. The procedure 70 can be used to implement the steps 62 and 63 of the procedure 60 be used. In the method, a spatially resolved information g (x, y, z) is determined for voxels of a volume and, depending on this, a parameter for a virtual projection series is determined.

at 71 an iteration is initialized via projections of a virtual projection series. The index p designates one of the n projections of the virtual projection series.

at 72 an iteration is initialized via voxels of one volume. The voxel resolution can be chosen to be identical to a voxel resolution, which is used for the reconstruction of 3D volume data from acquired X-ray images.

at 73 It checks whether the voxel with coordinates (x, y, z) in the p.-th projection lies on a projection beam that also passes through the metal object. If the voxel in the pth projection does not lie on a projection beam that also passes through the metal object, then 74 the value g (x, y, z) of a data field at the location of the voxel (x, y, z) is incremented by one. Then the procedure will be added 75 continued. If at 73 if the voxel in the pth projection is detected to lie on a projection beam that also passes through the metal object, the method continues without incrementing g (x, y, z) 75 continued.

at 75 it is checked whether all voxels of the volume have already been run through. If not all voxels have been run through, join 76 by changing the coordinate triplet the next voxel is selected. The process returns 73 back.

The loop at 73 - 76 can for example be done by a back projection of computationally generated binary projection images whose pixels have the value 1, if in the pixel a projection beam ends by the metal object, and whose pixels otherwise have the value 0.

at 77 checks if all the projections of the virtual projection series have been evaluated. If all projections have not yet been evaluated, then 78 the index p increments and the process returns 72 back. If all projections have been evaluated, then the method will be included 79 continued.

at 79 The virtual projection series parameter, which is a numerical value that quantifies the impact of metal shading by the metal object on the virtual projection series or a reconstruction that can be obtained from it, is determined. The characteristic at 79 can be determined by averaging g (x, y, z) / n over the voxels of the volume. Optionally, a spatial weighting can be made.

The at 79 determined parameter can be used in various ways. In one embodiment, the parameter can be output to a user. As a result, after a data acquisition, a user can estimate how much metal shading influences a reconstruction obtained from the recordings of the data acquisition. In further embodiments, the determined parameter can be used for an optimization procedure.

8th is a flowchart representation of a method 80 , in which parameters for several different virtual projection series are calculated. The method may be performed by the computing device 12 be performed. The method can be used to determine the characteristic that belongs to the virtual projection series, in which the influence of the metal shading is minimal. The method can also be used to determine parameters for performing data acquisition so that the influence of metal shading remains low.

at 81 be received images from an X-ray recording device, such as the X-ray recording device 2 , were recorded. A 3D reconstruction is calculated.

at 82 are determined depending on a segmentation of the 3D reconstruction voxels representing a section with a high absorption coefficient, in particular a metal object.

at 83 For example, a virtual projection series is selected such that the different layers of the fan beam or beam cone relative to the examination object in the virtual projection series correspond to the positions that were present during the data acquisition by the x-ray recording device.

at 84 the parameter for the selected virtual projection series is determined. The determination of the parameter can be carried out according to the method according to one of the embodiments of the invention, for example with the method 70 from 7 ,

at 85 it is checked whether an abort criterion is met. The termination criterion can For example, be a termination criterion of an optimization procedure. The abort criterion may include a comparison of the characteristic with a threshold. If the abort criterion is not met, then 86 selected another virtual projection series. The selection of the further virtual projection series can be carried out in such a way that possible relative positions between the examination object and the X-ray recording device are tested out. The selection of the further virtual projection series can take place as part of an optimization procedure, for example a "simulated cooling" algorithm.

If at 85 If the determination is made that the termination criterion has been met, the method is added 87 continued. at 87 becomes an extreme value for the various virtual projection series 84 determined characteristics determined. Alternatively or additionally, the associated virtual projection series can also be determined. If the characteristic is as with reference to 7 described is such that the characteristic in the interval from 0 to 1 and a value close to 1 indicates that the shading has a small influence, the maximum value of the characteristic is determined.

In other words, in each case the extreme value of the parameter which indicates the lowest possible influence of metal shading and / or the associated virtual projection series is determined.

at 88 the extremal value of the determined parameter and / or parameter is output for the associated virtual projection series. The parameters for the virtual projection series, which allows reconstruction with minimal impact of metal shading, can be output as parameters for use in a new data acquisition. For example, the parameters may be specified to indicate the positioning of the detector or C-arm relative to the patient table and the angulation at the beginning of a scan if a C-arm device is used for data acquisition.

at 89 optionally a new data acquisition can be made. The re-data collection can be done using the 88 determined parameters, which lead to a small influence of Metallabschattungen be performed. Even without re-recording, a user can at 88 Use determined parameters advantageous if a data acquisition to be made to another object to be examined, in which the metal object is positioned similarly. For example, for a patient having a screw implant with a particular orientation, parameters for a scan may be determined that result in little influence of metal shading. If X-ray imaging is to be performed on another patient with a similar screw implant, this can be done immediately using the parameters determined in order to minimize the influence of metal shading.

The methods of the various embodiments can be used for various applications. For example, after every 3D scan with a computer tomograph, feedback may be given on how good the current positional relationship between an X-ray recording device, for example the C-arm of a C-arm device, and the examination object is. After passing through an optimization method, it is optionally also possible to output information as to which parameter is possible for a scan performed with the x-ray recording device for a position between the c-arm and the object to be examined which minimizes the influence of the metal shading.

In a further comparable imaging, the determined position, which minimizes the influence of the metal shading, can be used as the initial configuration in order to achieve a better image quality.

If the difference between the parameter obtained for the data acquisition and the optimal parameter exceeds a threshold value, the scan can be repeated with new angle parameters. The new angle parameters can be selected depending on the determined with an optimization procedure virtual projection series. If the difference between the parameter obtained for the data acquisition and the optimal parameter is small, a new data acquisition can be dispensed with.

If the position of metal objects used, for example, during a surgical procedure is known in advance, a positioning can already be computed before the data acquisition is carried out, for which the influence of the metal shading is low.

9 - 11 show different cuts through 3D reconstructions. It is at 91 . 92 . 93 a section through a 3D reconstruction, which was created from a conventional Scangeometrie in which the C-arm is at an angle of 90 ° to the table and the angulation is 0 °. at 101 . 102 . 103 FIG. 3 shows a section through a 3D reconstruction, which was generated from a scanning geometry, in which the parameters for the scan were determined by a method according to an exemplary embodiment. In the illustrated example, an initial positioning was determined in which the C-arm in is at an angle of 60 ° to the table and the angulation is 0 °. The section plane in the section views 91 and 101 in 9 is equal. The section plane in the section views 92 and 102 in 10 is equal. The section plane in the section views 93 and 103 in 11 is equal. A metal screw is positioned so that in the conventional scan, its reconstruction in the sectional images 91 - 93 is shown lying in the scan plane.

In 9 disappears in the cut 91 the reconstruction determined with conventional scanning parameters, the screw in LEFT-RIGHT (left-right) direction of the cut 91 and leaves an artifact in screw diameter size in the LEFT-RIGHT (left-right) direction throughout the reconstructed volume. On the other hand is in the cut 101 the artifact is reduced. The circular screw cross section is recognizable.

In 10 is in the cut 92 also in FOOT-HEAD (foot-head) direction the actual screw body is erased due to metal shading. Only one external thread of the screw is visible. In the cut 102 the screw is clearly visible.

In 11 points the screw in the cut 93 in the ANTERIOR POSTERIOR direction has a weakening coefficient similar to that of the fabric. In the cut 103 the screw has a coefficient of attenuation that is significantly different from that of the surrounding tissue.

As by comparing the cuts 101 - 103 with the cuts 91 - 93 As can be seen, artefacts can be reduced relative to a patient table by positioning a C-arm relative to a patient table, as determined by a method according to one embodiment, and thus the 3D image quality can be increased. The scan protocol for determining the two different 3D reconstructions is identical.

While exemplary embodiments have been described in detail with reference to the figures, in further embodiments, modifications of these embodiments can be realized. While embodiments have been described in the context of C-arm devices, devices and methods in accordance with embodiments of the invention may be used with other devices in which 3D volume data is reconstructed from 1D or 2D images. While exemplary embodiments have been described in detail in which initially a spatially resolved reliability information is determined for different voxels of a volume and a characteristic for a virtual projection series has been determined by averaging over it, the characteristic can also be determined otherwise in further embodiments. For example, for several projections of a virtual projection series, the number of voxels arranged on a projection beam passing through the metal object can be determined, the voxel numbers determined for the different projections can be added, and then divided by the total number of voxels and the number of projections be normalized. While embodiments have been described in the context of reconstructing 3D volume data from 2D images, the methods and apparatus can also be used when capturing 1D images and reconstructing 3D volume data therefrom. While embodiments have been described in the context of reconstructing 3D volume data from 2D images, the methods and apparatus can also be used when capturing 1D images and reconstructing 2D slice images therefrom. While embodiments have been described in which the parameter for the virtual projection series is determined by averaging a location-dependent reliability information, further factors can be taken into account in determining the characteristic. For example, a weighting can take place. Voxels that are more relevant to a planned surgical or therapeutic intervention may be included in the determination of the parameter with a higher weighting than less relevant voxels. The identification of the more relevant voxels can be done via user input.

The described methods and apparatus can be combined with methods for the reduction of metal artefacts, for example with that in the DE 10 2008 050 570 A1 described method. Thus, even after applying such a correction method, the user can be provided with information about the influence of metal shading on a data acquisition performed or to be performed.

Embodiments of the invention allow the quantification of the influence of metal shading. Applications are in particular in medical technology.

LIST OF REFERENCE NUMBERS

P

object of investigation

1

CT Scanner

2

X Cradle

3, 3 '

C-arm

4

X-ray source

5

X-ray detector

6

cone beam

7

drive

8th

Movement of the C-arm

9

table

10

control device

11

Device for determining the shadowing effect

12

computing device

13

display

14

Storage

21, 21 '

volume

22

metal object

23-25

voxel

30, 30 '

projection

31-33

virtual projection image

34-36

image area

41-46

projection beam

48, 49

coordinate axes

51

virtual projection image

52

projection lines

53

image area

54

virtual projection image

55

projection lines

56

image area

60

method

61-63

steps

70

method

71-79

steps

80

method

81-89

steps

91-93

Cut through 3D reconstruction

101-103

Cut through 3D reconstruction

Claims (25)

Method for determining the influence of shadowing on X-ray imaging of an examination object (P) as a function of a data record representing the examination subject (P), the data record relating to a section (P) ( 22 ) of the examination subject (P) having an absorption coefficient exceeding a threshold, characterized in that for a virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) with several projections ( 30 . 30 ' . 31 - 33 ; 51 . 54 ), the different layers of a beam path ( 41 - 46 ; 52 . 55 ) relative to the examination object (P), depending on the data set, information about shading by the section ( 22 ), wherein, depending on the determined information, one of the virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) is determined in order to determine the influence of shading by the section ( 22 ) to quantify a reconstruction. Method according to claim 1, characterized in that the mathematical determination of the information comprises that for pixels or voxels ( 23 - 25 ) the reconstruction is determined whether the pixel or voxel ( 23 - 25 ) in the several projections ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) on one through the section ( 22 ) extending projection beam ( 41 . 46 ) is arranged. Method according to claim 1 or 2, characterized in that the mathematical determination of the information comprises that for pixels or voxels ( 23 - 25 ) of the reconstruction each have a number of projections ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) of the virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) at which the corresponding pixel or voxel ( 23 - 25 ) on one through the section ( 22 ) extending projection beam ( 41 . 46 ) is arranged. Method according to Claim 3, characterized in that the parameter for the virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) depending on a sum of the pixels or voxels ( 23 - 25 ) number of projections ( 30 . 30 ' . 31 - 33 ; 51 . 54 ), where the pixel or voxel ( 23 - 25 ) on one through the section ( 22 ) of the examination subject (P) extending projection beam ( 41 . 46 ) is determined. Method according to one of the preceding claims, characterized in that for each pixel or voxel ( 23 - 25 ) the reconstruction is determined a reliability information proportional to a number of projections ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) of the virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) is where the corresponding pixel or voxel ( 23 - 25 ) not on one through the section ( 22 ) of the examination subject (P) extending projection beam ( 42 - 45 ) is arranged. Method according to claim 5, characterized in that that of the virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) by averaging the reliability information about the pixels or voxels ( 23 - 25 ) is determined. Method according to one of the preceding claims, characterized in that the determination of the information is a computational determination of virtual projection images ( 31 - 33 ; 51 . 54 ) based on the record. Method according to Claim 7, characterized in that an unfiltered rear projection of the virtual projection images ( 31 - 33 ; 51 . 54 ) is determined. Method according to one of the preceding claims, characterized in that the data item representing the examination object (P) is determined on the basis of a plurality of images of the examination subject (P). Method according to claim 9, characterized in that the virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) is chosen so that the projections ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) of the virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) the layers of the beam path relative to the examination subject (P) in the detection of the plurality of recordings. Method according to one of the preceding claims, characterized in that the mathematical determination of the parameter for at least one further virtual projection series ( 51 . 54 ) is carried out. Method according to claim 11, characterized in that for mathematically determining the parameter for the at least one further virtual projection series ( 51 . 54 ) the examination object (P) represented by the data record is virtually rotated. A method according to claim 11 or 12, characterized in that the computational determination of the characteristic is repeated in an optimization procedure. Method according to one of claims 11-13, characterized in that it is determined for which of the virtual projection series ( 30 . 30 ' . 31 - 33 ) and the at least one further virtual projection series ( 51 . 54 ) the characteristic value has an extreme value. Method according to one of claims 11-14, characterized in that the virtual projection series ( 30 . 30 ' . 31 - 33 ) and the at least one further virtual projection series ( 51 . 54 ) depending on by an X-ray recording device ( 2 ) predetermined geometric boundary conditions are set. Method according to one of claims 11-15, characterized in that, depending on the virtual projection series ( 30 . 30 ' . 31 - 33 ) and the at least one further virtual projection series ( 51 . 54 ) parameters for a data acquisition with an X-ray recording device ( 2 ) be determined. Method according to Claim 16, characterized in that the parameters are an initial or end position of a mobile component ( 3 - 5 ) of the X-ray recording device ( 2 ) relative to a patient table ( 9 ). Method according to claim 16 or 17, characterized in that the parameters include a positioning of a C-arm ( 3 ) relative to a patient table ( 9 ). Method according to one of the preceding claims, characterized in that for determining the data set, a 3D metal segmentation of a 3D reconstruction of the examination subject (P) is performed. Method according to one of the preceding claims, characterized in that the data set representing the examination object (P) is a set of 3D volume data of the examination object (P). Method according to one of the preceding claims, characterized in that the data on the section ( 22 ) of the examination object (P) with a threshold value exceeding an absorption coefficient data on a section ( 22 ) are made of metal. Data carrier with computer program stored thereon, the computer program comprising a command sequence which, when executed by an electronic computer ( 12 ) the computing device ( 12 ) for carrying out the method according to one of the preceding claims. Contraption ( 11 ) for determining the influence of shadowing on X-ray imaging of an examination subject (P), comprising a memory ( 14 ) for storing a data record representing the examination object (P), the data record containing data on a section ( 22 ) of the examination object (P) having an absorption coefficient exceeding a threshold, characterized in that the device has one with the memory ( 14 ) coupled computing device ( 12 ) arranged to - for a virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) with several projections ( 30 . 30 ' . 31 - 33 ; 51 . 54 ), the different layers of a beam path ( 41 - 46 ; 52 . 55 ) relative to the examination object (P), depending on the data set, information about shading by the section ( 22 ) and, depending on the information obtained, one of the virtual projection series ( 30 . 30 ' . 31 - 33 ; 51 . 54 ) to determine the influence of shading by the section ( 22 ) to quantify a reconstruction. Contraption ( 11 ) according to claim 23, characterized in that the device ( 11 ) is arranged to carry out the method according to one of claims 2-21. Computer tomograph ( 1 ) comprising a line or area detector ( 5 ) for capturing images, and one with the line or area detector ( 5 ) coupled device ( 11 ) according to claim 23 or 24.

Images