INHALATION METRIC FOR CHEST X-RAY IMAGES

Abstract

In order to enhance enhanced X-ray image inhalation quality monitoring, a metric is proposed hat reproducibly provides an index of ribs visible to be used in the assessment of the inhalation state. In an example, a detected diaphragm in a chest X-ray image may be projected into an atlas that contains labels for all intercostal spaces, namely spaces between rib centerlines. A spatial representation of both the clavicle and the ribs is provided in the atlas, a cumulative histogram is built for all points, i.e. pixels, of the diaphragm, for every point a rib label counter of the rib in the rib label map at that point is incremented as well as all ribs above it, the rib label counter is normalized by a division by the number of points, the median (or a different quantile) may be taken of this distribution serving as an inhalation index. An objective metric of inhalation state is thus achieved.

Claims

1. A computer implemented method for X-ray image inhalation quality monitoring, comprising: receiving a chest X-ray image of a patient; detecting rib bones in the chest X-ray image; identifying a plurality of intercostal spaces each representing a space between adjacent detected rib bones; detecting the diaphragm in the chest X-ray image and generating a diaphragm line comprising a plurality of points representing a path of a diaphragm in the chest X-ray image; applying a score assigning rule to assign a point score to each point of the diaphragm line based on a respective intercostal space where the point is located in the image; and determining an inhalation index based on the point scores of the points of the diaphragm line.

2. The method according to claim 1, further comprising: comparing the determined inhalation index with a pre-determined X-ray image inhalation quality monitoring rule specifying an image inhalation state in terms of the inhalation index, and providing an X-ray image inhalation quality measure based on the comparison.

3. Method according to claim 1, further comprising building a statistical atlas by co-registration of training images that comprise a plurality of chest X-ray images of one or more patients; and registering the statistical atlas with the chest X-ray image of the patient to build a patient specific rib-bone model of the patient.

4. The method according to claim 3, wherein the plurality of intercostal spaces is identified in the patient specific rib-bone model.

5. The method according to claim 3, further comprising mapping the detected diaphragm into the patient specific rib-bone model to generate the diaphragm line based on the registration.

6. The method according to claim 1, wherein the inhalation index is determined based on the point scores of the points of a right lung diaphragm line representing a path of the diaphragm in a right lung field portion and/or a left lung diaphragm line representing a path of the diaphragm in a left lung field portion.

7. The method according to claim 1, further comprising: labelling the plurality of intercostal spaces with intercostal labels; and/or labelling the detected rib bones with rib labels; wherein the score assigning rule defines: a correspondence between point scores and intercostal labels; and/or a correspondence between point scores and rib labels.

8. The method according to claim 1, wherein the score assigning rule further defines a correspondence between point scores and a spatial relation of a point in an intercostal space with respect to two detected rib bones that define the intercostal space; or wherein the score assigning rule provides interpolated rib labels and/or interpolated intercostal labels at each position in the statistical atlas such that a point score can be obtained for a given image pixel of the chest X-ray image by registering the statistical atlas with the chest X-ray image.

9. The method according to claim 1, wherein the inhalation index comprises at least one of measures of central tendency of the point scores of the diaphragm line.

10. An apparatus for X-ray image inhalation quality monitoring, comprising: an input configured to receive a chest X-ray image of a patient; and a processor configured for: detecting rib bones in the chest X-ray image; identifying a plurality of intercostal spaces each representing a space between adjacent detected rib bones; detecting the diaphragm in the chest X-ray image and generating a diaphragm line comprising a plurality of points representing a path of a diaphragm in the chest X-ray image; applying a score assigning rule to assign a point score to each point of the diaphragm line based on a respective intercostal space where the point is located in the image; and determining an inhalation index based on the point scores of the points of the diaphragm line.

11. The apparatus according to claim 10, wherein the processor is further configured to compare the determined inhalation index with a pre-determined X-ray image inhalation quality monitoring rule specifying an image inhalation state in terms of the inhalation index, and providing an X-ray image inhalation quality measure based on the comparison.

12. The apparatus according to claim 10, further comprising: an output configured to output at least one of the determined inhalation index and the X-ray image inhalation quality measure.

13-15. (canceled)

16. A non-transitory computer-readable medium having executable instructions stored thereon which, when executed by at least one processor, cause the at least one processor to perform a method for X-ray image inhalation quality monitoring, the method comprising: receiving a chest X-ray image of a patient; detecting rib bones in the chest X-ray image; identifying a plurality of intercostal spaces each representing a space between adjacent detected rib bones; detecting the diaphragm in the chest X-ray image and generating a diaphragm line comprising a plurality of points representing a path of a diaphragm in the chest X-ray image; applying a score assigning rule to assign a point score to each point of the diaphragm line based on a respective intercostal space where the point is located in the image; and determining an inhalation index based on the point scores of the points of the diaphragm line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] These and other aspects of the invention will be apparent from and elucidated further with reference to the embodiments described by way of examples in the following description and with reference to the accompanying drawings, in which

[0071] FIG. 1A shows an example of a chest X-ray image.

[0072] FIG. 1B shows another example of a chest X-ray image.

[0073] FIG. 2 shows a flowchart representative of a method of X-ray image inhalation quality monitoring according to some embodiments of the present disclosure.

[0074] FIG. 3A shows an example of a patient specific rib-bone model.

[0075] FIG. 3B shows an example of a patient specific rib-bone model with identified intercostal spaces.

[0076] FIG. 3C shows an example of a patient specific rib-bone model with an identified diaphragm.

[0077] FIG. 4 shows a schematic block diagram of an exemplary implementation of an apparatus for X-ray image inhalation quality monitoring according to some embodiments of the present disclosure.

[0078] FIG. 5 depicts an exemplary implementation of an X-ray imaging system according to some embodiments of the present disclosure.

[0079] It should be noted that the figures are purely diagrammatic and not drawn to scale. In the figures, elements, which correspond to elements already described, may have the same reference numerals. Examples, embodiments or optional features, whether indicated as non-limiting or not, are not to be understood as limiting the invention as claimed.

DETAILED DESCRIPTION OF EMBODIMENTS

[0080] A chest X-ray acquisition is typically performed with a patient having inhaled completely and holding breath. In images not compliant to this rule, the lung parenchyma may look different, tissue may cluster to form opacities that can easily be taken as sign of an abnormality. The European guideline on X-ray acquisition states that a posterior-anterior (PA) chest should be performed at full inspiration (as assessed by the position of the ribs above the diaphragmeither 6 anteriorly or 10 posteriorly) and with suspended respiration.

[0081] When assessing image quality with respect to the patient inhalation state, a radiologist often decides intuitively and only counts in cases of doubt. In order to automate this task, algorithms can be used to do the assessment of image quality criteria like the inhalation state. An automatic quality check may be more reproducible, more robust, and more accurate. When implementing the guideline in an algorithm it turns out that counting ribs above the diaphragm is not easily possible. Typically, the ribs close to the diaphragm intersect with it instead of being located either clearly above or below it.

[0082] For example, FIGS. 1A and 1B shows two examples of chest X-ray images. In particular, FIG. 1A shows a case where the decision is still quite easy. In this case the rib centerlines 10 do not intersect with the diaphragm 12. Only rib centerlines 14 might be considered intersecting. FIG. 2B shows a case where depending on the criterion when to consider a rib above the diaphragm may results in a difference of two rib counts on both sides. In this case two rib centerlines 14 on both sides intersect with the diaphragm 12 and the only ones fully above the diaphragm are the rib centerlines 10.

[0083] FIG. 2 shows a flowchart representative of a method 100 for enhanced X-ray image inhalation quality monitoring according to some embodiments of the present disclosure.

[0084] In step 110, i.e. step a), a chest X-ray image of a patient is received. In an example, the chest X-ray image may be received from a digital recording plate, namely a digital flat-panel X-ray detector. In another example, the X-ray image may be received from a digital scanning apparatus for scanning a film-exposure of the region of interest.

[0085] In step 120, i.e. step b), rib bones are detected in the chest X-ray image. There is a number of methods to segment ribs in chest radiographs. In an example, an edge-based method may be used to detect rib bones. This method first extracts the rib pixels using an edge detection algorithm, and then groups the candidate pixels/lines into a complete rib boundary by applying e.g., a curve fitting technique or a geometric model such as parabolas. In another example, a classification scheme is used to classify rib/non-rib pixels. For example, the posterior ribs in chest radiographs may be segmented using iterated contextual pixel classification. Starting from an initial rib segmentation obtained from pixel classification, this method updates it by reclassifying every pixel, based on the original features and, additionally, class label information of pixels in the neighborhood of the pixel to be re-classified. In a further example, a prior-information based segmentation method may be used to detect rib bones. One way to incorporate prior knowledge is to use a prototype atlas, which comprises a set of model chest X-ray images and their corresponding rib-bone boundaries.

[0086] An exemplary implementation of the atlas is described in conjunction with FIG. 3A, which illustrates a patient specific rib-bone model 20 of the patient. To build the patient-specific rib-bone model, a statistical atlas is built by co-registration of training images that comprise a plurality of chest X-ray images of one or more patients. The training images may have manually delineated rib-bone boundaries. Each pixel in the statistical atlas may be assigned with a rib index by construction. Then, the statistical atlas is registered with the chest X-ray image of the patient to build a patient specific rib-bone model of the patient. In other words, the statistical atlas is registered to the chest X-ray image of the patient, yielding a transformation for each pixel, which allows the corresponding atlas rib mask to be transformed and treated as segmentation for the rib-bone of the patient. In this way, a rib index for each pixel is transformed from the statistical atlas to the chest X-ray image, thus forming a patient-specific rib-bone model in form of a density of rib centerlines 30 in the atlas space.

[0087] Optionally, as illustrated in FIG. 3A, the rib bones may be numbered using a rib-labelling algorithm. For example, the rib centerlines 30 may be labelled according to their rib position. Conventionally, the rib nearest a patient head is denoted 1. The true ribs depend downwards as ribs 1 to 7. The false ribs 8 to 12 depend onwards. Ribs 11 and 12 are sometimes known as floating ribs.

[0088] In step 130, i.e. step c), a plurality of intercostal spaces is identified. Each intercostal space represents a space between adjacent detected rib bones.

In an example, if an edge-based method or a classification scheme is used for rib-bone detection in step b), the intercostal space may be identified in the chest X-ray image.

[0089] In another example, if an atlas is used for rib-bone detection, the intercostal space may be identified in the patient specific rib-bone model of the patient.

[0090] An exemplary implementation of the atlas for identifying intercostal spaces is described in conjunction with FIG. 3B. In this example, the plurality of intercostal spaces 40 is identified in the patient specific rib-bone model 20 in FIG. 3A. Each intercostal space 40 corresponds to a respective space between adjacent rib centerlines 30.

[0091] Optionally, as illustrated in FIG. 3B, the intercostal spaces may be numbered using an intercostal-space-labelling algorithm. For example, the intercostal space below the rib centerline X is denoted 10. The intercostal space between the rib centerline IX and X is denoted 9.

[0092] In step 140, i.e. step d), a diaphragm line representing a path of a diaphragm in the chest X-ray image is generated. Typically, in the chest X-ray image, the region of interest is a human thorax. The diaphragm follows a curved path through the bottom of the thorax area. The diaphragm divides different textures of tissue, and has a distinct shape, having a right-hand-side dome which is slightly higher than a dome of the left side of the diaphragm. Therefore, a line following the path of the diaphragm in the chest X-ray image is amenable to detection by image processing techniques. Although a human diaphragm is a single, connected boundary, it is possible to detect distinct sessions of the diaphragm, such as a left section and a right section.

[0093] The diaphragm may be detected by gradient detection, texture detection of regions inside the thorax, a water-filling algorithm, or by deploying knowledge learned from training images concerning the expected shapes to be found in the image. The shape of the diaphragm follows a curved line, and is thus amenable to modelling using a spline approach.

[0094] An exemplary example of a diaphragm is described in conjunction with FIG. 3C. Similar to FIGS. 3A and 3B, the diaphragm 50 is provided in the atlas space. This may be done firstly by detecting the diaphragm in the chest x-ray image, e.g., by gradient detection, or a water-filling algorithm, and then by mapping the detected diaphragm into the atlas space with the same mapping based on the registration in FIG. 3A. In the illustrated example in FIG. 3C, both the left lung diaphragm line 50a and the right lung diaphragm line 50b are located in the tenth intercostal space. In some examples, such as a thoracic X-ray showing bad-quality inspiration as illustrated in FIG. 3D, the left lung diaphragm line 50a may be located in two or more intercostal spaces.

[0095] In step 150, i.e., step e), a score-assigning rule is applied to assign a point score to each point of the diaphragm line based on a respective intercostal space where the point is located. The score-assigning rule defines a correspondence between point scores and intercostal spaces.

[0096] For example, the score-assigning rule may define a correspondence between point scores and intercostal labels. In an example, the point score of one point of the diaphragm line is equal to the label of the intercostal space where that point is located. For example, if one point of the diaphragm line is located with the tenth intercostal space, a point score of 10 is then assigned to this point. In the exemplary example of FIG. 3C, as the left lung diaphragm line 50a is located in the tenth intercostal space, the points of the left lung diaphragm line 50a will be assigned with a point score of 10. This also applies to the right lung diaphragm line 50b. In another example, the score-assigning rule may define that a correspondence between point scores and rib labels, e.g., that the point score of one point of the diaphragm line is equal to an average of the rib labels of the adjacent ribs that defines the intercostal space where the point is located.

[0097] Optionally, the score-assigning rule may further define a correspondence between point scores and a spatial relation of a point in an intercostal space with respect to two detected rib bones that define the intercostal space. In other words, the point score of a point may be further adjusted based on its relative position in the intercostal space. An exemplary implementation of the correspondence between the point scores and the position within the respective intercostal space is illustrated in FIG. 3D, which illustrates a simple version of a right half of a thorax cavity with a level of inspiration which would result in a poor quality image. A point 60 of the diaphragm line 50b is located between the eighth rib centerline and the ninth rib centerline. The point 60 has a distance a from the eighth rib centerline and a distance b from the ninth rib centerline. In an example, the point score of this point 60 may be calculated using the following formula:

[00001]$\mathrm{point}\mathrm{score}=x+\frac{a-b}{a+b}.Math.0.5$

where x is the intercostal label of the intercostal space where the point is located.

[0098] Alternatively, the score assigning rule provides interpolated rib labels and/or interpolated intercostal labels at each position in the statistical atlas such that a point score can be obtained for a given image pixel of the chest X-ray image by registering the statistical atlas with the chest X-ray image. For example, the atlas provides interpolated rib indices at each position that may be directly read out for a given image pixel using the transformation.

[0099] In step 160, i.e., step f), an inhalation index is determined based on the point scores of the points of the diaphragm line. Optionally, the inhalation index may be determined based on the point scores of the points of a right lung diaphragm line 50b representing a path of the diaphragm in a right lung field portion and/or a left lung diaphragm line 50a representing a path of the diaphragm in a left lung field portion.

[0100] Examples of the inhalation index may include, but not limited to, the mean, median, and mode of the point scores, i.e., the pixels, of the diaphragm line. In an example, a cumulative histogram may be built for all points of the left and/or right diaphragm lines. For every point the rib label counter of the rib in the rib label map at that point is incremented as well as all ribs above it. Having all points processed, all rib label counters may be normalized by a division by the number of the points, thereby having a value ranging between 0 and 1. Finally, the median (or mode) may be taken of this distribution serving as the inhalation index.

[0101] Optionally, the inhalation index may be output. In an example, the inhalation index may be output on a display, such as a computer display screen. This may allow an operator to gauge the quality of the image themselves. In another example, the inhalation index may be output for further analysis, e.g., for yielding an X-ray image inhalation quality measure as explained below.

[0102] Optionally, step 170, i.e. step g) may be provided for comparing the determined inhalation index with a pre-determined X-ray image inhalation quality monitoring rule specifying an image inhalation state in terms of the inhalation index, to yield an X-ray image inhalation quality measure.

[0103] The X-ray inhalation quality monitoring rule may be provided as data records linked to specific imaging standards, such as the European Guidelines on Quality Criteria for Diagnostic Images, to enable regional or national imaging standards to be followed. Alternatively or additionally, the X-ray image inhalation quality monitoring rule may be pre-programmed by a medial imaging professional, either to a pre-defined standard, or to a specific protocol required by a healthcare facility.

[0104] The X-ray image inhalation quality measure may be a discretized category, for example selected from the group of bad, medium, good. Each category of the X-ray image inhalation quality measure may correspond to a particular range of the inhalation index. For example, the category good corresponds to the inhalation index ranging from 9.2 and upper, the category medium corresponds to the inhalation index ranging from 8.1 to 9.2, and the category bad corresponding to the remaining inhalation index. The X-ray image inhalation quality measure may be a continuous value, for example a percentage. This may allow an experienced operator to gauge the quality of the image themselves.

[0105] Optionally, the X-ray image inhalation quality measure may be output. For example, the X-ray image inhalation quality measure may be output on a display, such as a computer display screen, or a display of a mobile device. In another example, the X-ray image inhalation quality measure may be output as an illuminated warning light, or a warning sound. In a further example, the X-ray image inhalation quality measure may be output as a bitmap watermark inserted in a portion of an X-ray image provided by an X-ray apparatus.

[0106] FIG. 4 shows a schematic block diagram of an exemplary implementation of an apparatus 200 for X-ray image inhalation quality monitoring according to some embodiments of the present disclosure.

[0107] The apparatus 200 comprises an input unit 210 and a processing unit 220. The apparatus 200 may be part of, or include an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor and/or memory that execute one or more software or firmware programs, a combinational logical circuit, and/or other suitable components that provide the described functionality.

[0108] The input unit 210 is configured to receive a chest X-ray image of a patient, e.g., from an analog or digital X-ray detector.

[0109] The processing unit 220 is configured for detecting rib bones in the chest X-ray image, e.g., using an edge detection algorithm, a classification scheme, or a prior-information based segmentation method, such as an atlas based segmentation method. For example, the processing unit 220 may be configured for building a statistical atlas by co-registration of training images that comprise a plurality of chest X-ray images of one or more patients, and registering the statistical atlas with the chest X-ray image of the patient to build a patient specific rib-bone model of the patient.

[0110] The processing unit 220 is further configured for identifying a plurality of intercostal spaces. Each intercostal space represents a space between adjacent detected rib bones. In an example, the processing unit 220 may be configured for identifying the plurality of intercostal spaces in the patient specific rib-bone model.

[0111] The processing unit 220 is further configured for generating a diaphragm line representing a path of a diaphragm in the chest X-ray image. The diaphragm may be detected by gradient detection, texture detection of regions inside the thorax, a water-filling algorithm, or by deploying knowledge learned from training images concerning the expected shapes to be found in the image. Optionally, the processing unit 220 may be configured for detecting the diaphragm in the chest X-ray image, e.g., using gradient detection, or a water-filling algorithm, and mapping the detected diaphragm into the patient specific rib-bone model to generate the diaphragm line based on the registration.

[0112] The processing unit 220 is further configured for applying a score assigning rule to assign a point score to each point of the diaphragm line based on a respective intercostal space where the point is located. The score assigning rule defines a correspondence between point scores and intercostal spaces. For example, the processing unit 220 may be configured for labelling the plurality of intercostal spaces with intercostal labels and/or labelling the detected rib bones with rib labels. The score assigning rule may define (i) a correspondence between point scores and intercostal labels and/or (ii) a correspondence between point scores and rib labels. Optionally, the score assigning rule may further define a correspondence between point scores and a spatial relation of a point in an intercostal space with respect to two detected rib bones that define the intercostal space.

[0113] The processing unit 220 is further configured for determining an inhalation index based on the point scores of the points of the diaphragm line. The inhalation index comprises at least one of measures of central tendency, such as the mode, median, or the mean, of the point scores of the diaphragm line.

[0114] Optionally, the processing unit 220 may be further configured to compare the determined inhalation index with a pre-determined X-ray image inhalation quality monitoring rule specifying an image inhalation state in terms of the inhalation index, to yield an X-ray image inhalation quality measure, which may be a discretized category, for example selected from the group of bad, medium, good.

[0115] Optionally, the apparatus 200 may further comprise an output unit 220. The output unit 220 may be configured to output at least one of the determined inhalation index and the X-ray image inhalation quality measure. For example, at least one of the determined inhalation index and/or the X-ray image inhalation quality measure may be output on a display.

[0116] FIG. 5 illustrates an exemplary implementation of an X-ray imaging system 300 according to some embodiments of the present disclosure.

[0117] The X-ray imaging system 300 comprises an X-ray image acquisition device 310 with an X-ray source 312 and an X-ray detector 314. The X-ray source 312, e.g., rotating anode generators, may be positioned so that radiation is emitted in the direction of an X-ray detector 314, e.g., pixelated flat-panel X-ray detectors, where it is received. Optionally, the X-ray detector 314 may be located in a free-standing detector 320a, or a bed 320b. Thus, the thorax of a patient may be imaged in the standing, or sitting position.

[0118] The X-ray detector 310 may be connected to the apparatus 200 via a physical cable or wirelessly. In the exemplary example in FIG. 5, the apparatus 200 is a personal computer (PC). In another example, the apparatus 200 may be a server with a more powerful computing power to provide the service to multiple users to perform image quality assessment in many examination rooms. In other words, the image quality assessment algorithm may be made available on a server, e.g., a cloud server. In this case, the chest X-ray image taken by the X-ray imaging device 310 may be sent to the apparatus 200 configured to be a remote server, and the inhalation index, and optionally the X-ray image inhalation quality measure, are identified by the remote server. The corresponding results may be sent to a PC or a mobile device for being displayed to the user. In a further example, the image quality assessment algorithm may be made available to a mobile apparatus, e.g., a mobile computing device.

[0119] Optionally, the apparatus 200 may be configured to output at least one of the determined inhalation index and the X-ray image inhalation quality measure to a user via an output 330. In the exemplary example, the output 330 is a display. In other examples, the output may be a lamp, or a simple audible warning.

[0120] Optionally, the apparatus 200 may be provided with a user interface 340, such as a keyboard 340a and a mouse 340b. Further examples of the user interface may include, but not limited to, a touch screen and a joy stick arrangement. The user interface 340 may allow the user to define the score assigning rule and/or the X-ray image inhalation quality monitoring rule. The user interface 340 may be arranged remotely away from the X-ray imaging device 310, for instance outside the imaging room. Alternatively, the user interface may be arranged inside the room.

[0121] In use, a region of interest of a patient is positioned in front of the X-ray source 312. The X-ray source 312 is activated, e.g., via the user interface 340, and an exposure of the patient is made. The apparatus 200 assesses the quality of the image as described above. The apparatus 200 yields an inhalation index, and optionally an X-ray image inhalation quality measure, which is then output to a user on the display 330. In this way, a more reliable and easier assessable determination of the image quality of a chest X-ray image is achieved. The computation of the inhalation index for each chest X-ray image may be carried out without human supervision or interference, thereby avoiding the dependency upon variations or biases in the skill level of different system operators.

[0122] In another exemplary embodiment of the present invention, a computer program or a computer program element is provided that is characterized by being adapted to execute the method steps of the method according to one of the preceding embodiments, on an appropriate system.

[0123] The computer program element might therefore be stored on a computer unit, which might also be part of an embodiment of the present invention. This computing unit may be adapted to perform or induce a performing of the steps of the method described above. Moreover, it may be adapted to operate the components of the above described apparatus. The computing unit can be adapted to operate automatically and/or to execute the orders of a user. A computer program may be loaded into a working memory of a data processor. The data processor may thus be equipped to carry out the method of the invention.

[0124] This exemplary embodiment of the invention covers both, a computer program that right from the beginning uses the invention and a computer program that by means of an up-date turns an existing program into a program that uses the invention.

[0125] Further on, the computer program element might be able to provide all necessary steps to fulfil the procedure of an exemplary embodiment of the method as described above.

[0126] According to a further exemplary embodiment of the present invention, a computer readable medium, such as a CD-ROM, is presented wherein the computer readable medium has a computer program element stored on it which computer program element is described by the preceding section.

[0127] A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

[0128] However, the computer program may also be presented over a network like the World Wide Web and can be downloaded into the working memory of a data processor from such a network. According to a further exemplary embodiment of the present invention, a medium for making a computer program element available for downloading is provided, which computer program element is arranged to perform a method according to one of the previously described embodiments of the invention.

[0129] It has to be noted that embodiments of the invention are described with reference to different subject matters. In particular, some embodiments are described with reference to method type claims whereas other embodiments are described with reference to the device type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters is considered to be disclosed with this application. However, all features can be combined providing synergetic effects that are more than the simple summation of the features.

[0130] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing a claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

[0131] In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.