X-ray source and X-ray imaging apparatus

Abstract

An X-ray source for emitting an X-ray beam is proposed. The X-ray source comprises an anode and an emitter arrangement comprising a cathode for emitting an electron beam towards the anode and an electron optics for focusing the electron beam at a focal spot on the anode. The X-ray source further comprises a controller configured to determine a switching action of the emitter arrangement and to actuate the emitter arrangement to perform the switching action, the switching action being associated with a change of at least one of a position of the focal spot on the anode, a size of the focal spot, and a shape of the focal spot. The controller is further configured to predict before the switching action is performed, based on the determined switching action, the size and the shape of the focal spot expected after the switching action. Further, the controller is configured to actuate the electron optics to compensate for a change of the size and the shape of the focal spot induced by the switching action.

Claims

1. An X-ray source for emitting an X-ray beam, the X-ray source comprising: an anode; an emitter arrangement comprising a cathode for emitting an electron beam towards the anode and an electron optics for focusing the electron beam at a focal spot on the anode; and a controller configured to determine a switching action of the emitter arrangement and to actuate the emitter arrangement to perform the switching action, the switching action being associated with a change of at least one of a position of the focal spot on the anode, a size of the focal spot on the anode, and a shape of the focal spot on the anode; wherein the controller is configured to predict before the switching action is performed, based on the determined switching action, the size and the shape of the focal spot expected after the switching action.

2. The X-ray source according to claim 1, wherein the controller is configured to predict a change of the size and the shape of the focal spot induced by the switching action.

3. The X-ray source according to claim 1, wherein the switching action comprises at least one of changing a voltage supplied to the cathode, changing a current supplied to the cathode, changing a position of the focal spot on the anode by deflecting the electron beam with the electron optics, and switching the X-ray beam on.

4. The X-ray source according to claim 1, wherein the controller is configured to predict the size and the shape of the focal spot based on predicting a width and a height of the focal spot.

5. The X-ray source according to claim 1, wherein the controller is configured to predict the size and the shape of the focal spot based on a model modelling a width and a height of the focal spot as a function of a current supplied to the cathode, a voltage supplied to the cathode, and a heat load of the anode.

6. The X-ray source according to claim 1, wherein the controller is configured to predict the size and the shape of the focal spot based on predicting a heat load of the anode; and/or wherein the controller is configured to determine, based on a pre-determined cooling rate of the anode, a heat load of the anode expected after the switching action.

7. The X-ray source according to claim 1, wherein the emitter arrangement further comprises a grid interposed between the cathode and the anode, wherein the grid is configured to blank out the electron beam in an on-state of the grid and to transmit the electron beam in an off-state of the grid; and wherein the controller is configured to switch the grid between the on-state and the off-state by providing a grid switch signal to the grid.

8. The X-ray source according to claim 7, wherein the controller is configured to determine the switching action based on a pre-determined grid switch profile, the grid switch profile defining a modulation of an intensity of the X-ray beam based on a sequence of at least one off-state of the grid and at least one on-state of the grid.

9. The X-ray source according to claim 7, further comprising: a feedback control with a focal spot sensor for acquiring an image of the focal spot based on detecting X-ray radiation emitted from the anode, the acquired image being indicative of the shape, the size and the position of the focal spot on the anode; and wherein the controller is further configured to analyze the image acquired with the focal spot sensor to determine a change of at least one of the size, the shape and the position of the focal spot; and wherein the controller is configured to adjust, by actuating the electron optics, at least one of the size, the shape and the position of the focal spot if a change of at least one of the size, the shape and the position of the focal spot is determined after the switching action is performed.

10. The X-ray source according to claim 9, wherein the controller is configured to analyze the image acquired by the focal spot sensor when the grid is in the off-state and the electron beam impinges onto the anode; and/or wherein the controller is configured to discard the image acquired by the focal spot sensor when the grid is in the on-state and the electron beam is blanked out by the grid.

11. The X-ray source according to claim 9, wherein the controller is configured to determine, based on the grid switch signal, if the image of the focal spot is acquired by the focal spot sensor during the off-state of the grid.

12. A method for operating an X-ray source, comprising: determining a switching action of an emitter arrangement, the switching action being associated with a change of at least one of a position of a focal spot on an anode, a size of the focal spot, and a shape of the focal spot; predicting, based on the determined switching action, the size and the shape of the focal spot expected after the switching action is performed; and actuating the emitter arrangement to perform the switching action.

13. 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 the method according to claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The subject-matter of the invention will be explained in more detail in the following with reference to exemplary embodiments which are illustrated in the attached drawings, wherein:

(2) FIG. 1 shows schematically an X-ray imaging apparatus according to an exemplary embodiment of the invention:

(3) FIG. 2 shows schematically an X-ray source according to an exemplary embodiment of the invention;

(4) FIG. 3 shows schematically an X-ray source according to an exemplary embodiment of the invention;

(5) FIG. 4 shows a flow chart illustrating steps of a method for operating an X-ray source according to an exemplary embodiment of the invention.

(6) In principle, identical or like parts are provided with identical or like reference symbols in the figs.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) FIG. 1 shows schematically an X-ray imaging apparatus 100 according to an embodiment of the invention. The X-ray imaging apparatus may refer to any type of imaging system, such a digital X-ray imaging system, a dual or multi energy X-ray imaging system, Computed Tomography (CT), spectral CT, interventional X-R (IXR), digital-X-R (DXR), a C-arm system, and/or multimodality systems like PET/CT.

(8) The X-ray imaging apparatus 100 comprises an X-ray source 10 for generating and/or emitting an X-ray beam 101 towards an object 103 to be examined and an X-ray detector 102 for detecting at least a part of the X-ray beam 101 that has passed through the object 103. The X-ray source 10 can refer to any X-ray source, such as e.g. an X-ray tube, a stereo X-ray tube or any other type. The X-ray source 10 will be described in more detail in subsequent figs.

(9) Likewise, the X-ray detector 102 can refer to any suitable X-ray detector 102. Particularly, the X-ray detector 102 can comprise a scintillator for converting X-ray photons into visible light. Further, the X-ray detector 102 can comprise one or more detecting elements for detecting light emitted from scintillator. Further, 102 can comprise a direct conversion detector for converting X-ray photons into electrical charges, and can comprise detecting elements for charge integration or single photon counting.

(10) Further, the X-ray imaging apparatus 100 comprises a controller 22 operationally coupled to the X-ray detector 102 and the X-ray source 10. However, the controller 22 may be part of the X-ray source 10, as described with reference to subsequent figs., and/or part of the X-ray detector 102. Also, the controller 22 may refer to a controller arrangement 22 having one or more sub-controllers, modules and/or units.

(11) FIG. 2 shows schematically an X-ray source 10 according to an exemplary embodiment of the invention.

(12) The X-ray source 10 comprises a housing 11 and an anode 12 arranged in the housing 11. The anode 12 may be any type of anode. For instance, the anode 12 may be a rotatable and/or movable anode 12.

(13) The X-ray source 10 further comprises an emitter arrangement 14 with a cathode 16 for emitting an electron beam 15 towards the anode 12. The emitter arrangement 14 further comprises an electron optics 18 for focusing the electron beam 15 on the anode 12 and/or on an outer surface thereof. The electron optics 18 may be configured to generate an electric field and/or a magnetic field to deflect the electron beam 15 and to focus the electron beam 15 on the anode 12.

(14) The electron beam 15 is focused at a focal spot 20 on the anode 12. Therein, the focal spot 20 may refer to an area and/or region of the anode 12, in which the electron beam 15 impinges onto the anode 12. When impinging onto the anode 12, the electron beam 15 generates X-ray radiation and/or X-ray photons emitted from the anode 12 and/or from the focal spot 20. At least a part of the emitted X-ray photons can pass through an X-ray window 17 of the X-ray source 10 to form the X-ray beam 101.

(15) The X-ray source 10 further comprises a controller 22 operatively coupled to the emitter arrangement 14, the electron optics 18 and/or the cathode 16.

(16) The X-ray source 10 shown in FIG. 2 may advantageously be utilized in imaging applications, in which a characteristic of the X-ray beam 101 and/or the X-ray beam 101 may be adjusted and/or modified during an imaging task and/or during acquisition of an X-ray image. Particularly, the X-ray source 10 depicted in FIG. 1 may be configured for kV-peak or kVp switching, where an energy and/or energy distribution of the X-ray beam 101 is modified and/or varied during the imaging task, as e.g. used for dual energy X-ray imaging. In kV-peak switching applications, an electrical power supplied to the cathode 16 may be switched in a corresponding switching action between at least two power levels. For instance, a voltage supplied to the cathode 16 may be changed from e.g. 80 kV to 140 kV during a single imaging task and/or during a switching action. Alternatively or additionally a current supplied to the cathode 16 may be changed during a switching action. Therein, the actual switching action usually takes place in a rather short time scale, e.g. in the range of microseconds to milliseconds.

(17) However, any change of the voltage and/or current supplied to the cathode 16 may affect at least one of a shape, a size and a position of the focal spot 20 on the anode 12. This in turn may affect the X-ray beam 101 and/or a characteristic of the X-ray beam 101, e.g. due to a changing heat load and/or temperature of the anode 12 and/or due to different properties of the electrons of the electron beam 15 impinging onto the anode 12 at different energies. To ensure a high image quality of an X-ray image, it may be favorable to precisely control the shape and the size of the focal spot 20. This can be accomplished by the X-ray source 10 according to the present invention, as described in the following.

(18) The controller 22 is configured to determine the switching action of the emitter arrangement 14. Determining the switching action may comprise determining one or more values of parameters for operating the X-ray source 12, such as e.g. the voltage and/or the current supplied to cathode 16. The controller 22 may also determine other parameters of the switching action, such as e.g. a control signal provided to the electron optics 18, a time period, in which the switching action is performed, and/or a time instant, at which the switching action is performed. Such parameters of the switching action may be input to the X-ray source 10 by a user and/or may be retrieved by the controller from a data storage 24 and/or data stored therein. Further, the controller 22 is configured to actuate the emitter arrangement 14 to perform the switching action.

(19) Before actuating the emitter arrangement 14 to perform the switching action, the controller 22 determines, calculates and/or predicts the shape and the size of the focal spot 20 expected after the switching action. Based on the determined switching action, the controller 22 can predict and/or estimate the shape and size of the focal spot 20 that will expectedly be present after the switching action is performed. Alternatively or additionally, the controller 22 is configured to determine a change of the size and the shape of the focal spot 20 induced by the switching action. Accordingly, the controller 22 may be configured to determine a relative change of the size and shape of the focal spot, e.g. relative to a size and shape of the focal spot 20 before and/or prior to the switching action and/or during a current operation of the X-ray source 12.

(20) Further, the controller 22 is configured to determine, based on the predicted size and shape of the focal spot 20 and/or based on the predicted change in the size and shape of the focal spot 20, a predictive control signal. The controller 22 may then provide the predictive control signal to the electron optics 18 directly before, during, at termination and/or when the switching action is completed in order to adjust the electron beam 15, thereby taking into account the switching action and the changes in size and shape of the focal spot induced therewith. This allows to proactively control the shape and the size of the focal spot 20 before any change in the focal spot 20 due to the performed switching action occurs or is noticeable. Accordingly, the change in size and shape of the focal spot 20 induced by the switching action can be compensated for, before this change actually occurs. Hence, a more precisely controlled X-ray beam 101 and/or a better image quality can be provided.

(21) Generally, the controller 22 can be configured to determine the switching action and to predict the size and shape of the focal in response to determining the switching action. Further, the controller 22 can be configured to determine the predictive control signal in response to predicting the size and shape of the focal spot 20. Moreover, the controller 22 can be configured to actuate, based on the predictive control signal, the electron optics 18 in accordance with the predicted size and shape of the focal spot 20. In response thereto, the controller 22 can then initiate the switching action and/or actuate the emitter arrangement 14 to perform the switching action.

(22) To determine the shape and the size of the focal spot 20 expected after the switching action, the controller 22 can determine a width and a height of the focal spot 20 expected after the switching action. For instance, the controller 22 can determine, based on the determined switching action, the voltage and/or the current supplied to the cathode 16 after the switching action, and the controller can estimate and/or calculate the width and the height of the focal spot 20. Also, a time period (or a length thereof) that the X-ray beam was on and/or off, which may e.g. be determined based on a switching profile describing one or more switching actions performed during image acquisition, can be taken into account to calculate the width and the height of the focal spot 20. Accordingly, the width and the height of the focal spot 20 may be a function of the voltage and/or current supplied to the cathode 16. Such functional relationship may be pre-determined, e.g. based on measurements, and the functional relationship may be stored in the data storage 24. Also, the width and the height of the focal spot 20 may be determined by the controller 22 based on a model modelling the height and the width of the focal spot 20 as a function of voltage and/or current supplied to the cathode. Alternatively or additionally a look-up table may be stored in the data storage 24 and the controller may determine the width and the height of the focal spot 20 based on the look-up table.

(23) Alternatively or additionally, the controller 22 can be configured to determine a heat load and/or a temperature of at least a part of the anode 12 expected after the switching action. Based on the determined heat load and/or the temperature, the controller 22 may then predict the shape and the size of the focal spot 20 after the switching action.

(24) The controller 22 may determine the heat load and/or the temperature e.g. based on a pre-determined cooling rate and/or a cooling curve of the anode, based on which a temperature of the anode may be calculated as a function of current and/or voltage supplied to the cathode 16. Therein, also a previous operation of the X-ray source, such as e.g. a time period preceding switching action, in which time period the X-ray beam was switched off, may be taken into account. The cooling rate and/or the cooling curve may be stored in the data storage 24. Accordingly, the controller 22 may be configured to predict the size and the shape of the focal spot 20 based on the voltage supplied to the cathode 16 after the switching action, based on the current supplied to the cathode 16 after the switching action and/or based on the heat load expected after the switching action. For this purpose, the controller 22 may be configured to predict the shape and the size of the focal spot 20 based on a model modelling the width and the height of the focal spot 20 as a function of the voltage supplied, the current and/or the heat load of the anode. Alternatively or additionally, a look-up table may be stored in the data storage 24, and the controller 22 may determine the width and the height of the focal spot 20 expected after the switching action based on the look-up table. It is to be noted that in addition to changing the voltage and/or current supplied to the cathode 16 in the frame of the switching action, also a filter and/or filter grating may be moved into the X-ray beam 101 and/or out of the X-ray beam 101 in the course of the switching action.

(25) Further, alternatively or additionally to the kV-peak switching described above, also dynamic focal spot positioning may be applied, wherein a position of the focal spot 20 on the anode may be changed, by deflecting the electron beam 15 with the electron optics 18, in a separate switching action or simultaneously with changing the voltage and/or current supplied to the cathode 16. Analogue to the description above, the controller 22 is configured to predict the size and shape of the focal spot 20 inferred by changing a position of the focal spot 20 on the anode 12 and to actuate the electron optics 18 to compensate for such a change in position.

(26) Alternatively or additionally, grid switching may be applied, wherein the X-ray beam 101 is switched on and/or off during an imaging task, as described in more detail in FIG. 3.

(27) It is to be noted that the exemplary embodiment of the X-ray source 10 illustrated FIG. 2 can also comprise further components, e.g. as described with reference to FIG. 3. Particularly, also the X-ray source 10 of FIG. 2 can comprise a focal spot sensor 28 and/or a feedback control 30, as described with reference to FIG. 3 in the following.

(28) FIG. 3 shows schematically an X-ray source 10 according to an exemplary embodiment of the invention. If not stated otherwise, the X-ray source 10 of FIG. 3 comprises the same features, functions and/or elements as the X-ray source 10 described with reference to FIGS. 1 and 2.

(29) In the embodiment depicted in FIG. 3, the emitter arrangement 14 comprises a grid 26 interposed between the cathode 16 and the anode 12. In an on-state of the grid 26, the grid 26 blanks out the electron beam 15, and hence switches the X-ray beam 101 off. In an off-state of the grid 26, the electron beam 15 can pass through the grid 26 and impinge onto the anode 12, such that the X-ray beam 101 is on. The controller 22 is operatively coupled to the grid 26 and is configured to switch the grid 26 between the on state and the off-state based on providing a grid switch signal to the grid 26. The grid switch signal may refer to a pulse width modulation signal for actuating the grid 26. Such grid switching may particularly be advantageous for generating pulsed X-ray beams and/or for dose modulation techniques, in which an intensity of the X-ray beam 101 is varied based on switching the grid 26 between the on-state and the off-state. A sequence of on-states and off-states of the grid 26 may e.g. be defined in a grid switch profile that may be stored in the data storage 24 and/or provided to the controller 22. The controller 22 may then determine the grid switch signal based on the grid switch profile and actuate the grid 16 accordingly.

(30) The X-ray source 10 depicted in FIG. 3 further comprises a focal spot sensor 28 for acquiring an image of the focal spot 20 based on detecting X-ray radiation emitted from the focal spot 20 and e.g. transmitted through a window 29 in the housing 11. During operation of the X-ray source 10, the controller 22 analyzes the images acquired with the focal spot sensor 28 to monitor the shape, the size and the position of the focal spot 20. Further, the controller 22 determines a change in size, shape and/or position of the focal spot 20 based on analyzing images acquired with the focal spot sensor 28. Moreover, the controller 22 actuates the electron optics 18 to compensate for a change in size, shape and/or position of the focal spot 20. Accordingly, the focal spot sensor 28 and the controller 22 or a dedicated module, part, section, sub-controller or unit of the controller 22 form a feedback control 30 of the X-ray source 10 for stabilizing the focal spot 20 in terms of its shape and size based on actuating the electron optics 18. The feedback control 30 may be particularly advantageous for compensating thermal expansion of the anode 12 during operation

(31) However, the actuation of the electron optics 18 based on images of the focal spot sensor 28 is not to be confused with the prediction of the shape and size of the focal spot before the switching action is performed and the corresponding actuation of the electron optics 18 based on the predicted shape and size. For the predictive control, the controller 22 may determine the switching action based on the grid switch signal and/or based on the grid switch profile and predict the shape and the size of the focal spot 20 for the determined switching action. Determining the switching action may comprise determining a time period of an on-state and/or an off-state of the grid 26, to a voltage supplied to the cathode, to a current supplied to the cathode, and/or to a control signal supplied to the electron optics 18. Based on the predicted shape and size of the focal spot 20, the controller 22 may actuate the electron optics 18 to compensate for the switching action, as described with reference to FIG. 2.

(32) Before switching the X-ray beam 101 on and/or off by means of the grid 26, i.e. during a grid-switch operation of the X-ray source 10, the controller 22 can determine based on the switching signal and/or based on the grid switch profile a duty-cycle, the on-states and/or the off-states of the grid 26 in advance. This information can then be used to predict the shape and size of the focal spot 20, to proactively determine the predictive control signal and/or a correction for the changes in size and shape of the focal spot 20, as described in detail with reference to FIG. 2. In contrast to this proactive and/or predictive control, the adjustment of the electron beam 15 by means of the feedback control 30 and/or the focal spot sensor 28 may serve to fine-tune the electron beam 15 and/or the electron optics 18, e.g. to compensate for thermal expansion during off-states of the grid 26, as will be described in more detail in the following.

(33) The feedback control 30 depicted in FIG. 3 may be considered as being based on the following insights and findings. When using a grid 26, e.g. in a grid switch X-ray source 10 or grid switch tube, in combination with focal spot sensing by means of the focal spot sensor 28 and the corresponding feedback control 30, it may be desirable to take certain precautions in order to ensure that periods of time, in which the electron beam 15 is blanked-out, are not interpreted as a loss of intensity by the controller 22 when analyzing an image of the focal spot sensor 28. Accordingly, a sampling rate of the feedback control 30 may be irregular when using a grid 26 for grid switching. Apart from that, a period of time between consecutive X-ray images may vary, and hence, also a temperature of the anode 12 may vary between acquisitions of two consecutive X-ray images. Such temperature differences may in turn affect the size and the shape of the focal spot 20, which would cause the feedback control 30 to work on correcting large deviations or changes, e.g. when compared to an X-ray source 10 operating continuously. This might be particularly the case if the grid 26 is used for dose modulation, in which for example a certain gantry rotation angle may cause a significantly larger heat load on the anode 12 than in other rotation angles. Furthermore, when using dynamic focal spot positioning, e.g. for filter modulation to tune the spectral filtration via a filter, may also switch between two positions of the focal spot 20 with two different intensities. Moreover, e.g. in C-arc systems a dose control may be directly influenced by the measured beam intensity. E.g. in case of a frame to frame modulation the feedback control 30 would normally regulate the voltage and/or current according to given dose changes and time constants. All these aspects should preferably be taken into account in the feedback control 30 to control the shape and size of the focal spot 20 in order to ensure a high image quality.

(34) As described above, the focal spot sensor 28 is configured to dynamically monitor the shape and size of the focal spot 20. To avoid any correction of the shape and size of the focal spot 20 based on an image of the focal spot sensor acquired when the X-ray beam is off and/or when the grid is in the on-state, the controller 22 is configured to analyze the image of the focal spot sensor 28 only when the grid is in the off-state and to disregard an image of the focal spot sensor 28 acquired when the grid is in the on-state. The controller 22 may determine whether a given image of the focal spot sensor 28 is acquired during the off-state of the grid 26 based on the grid switch signal and/or based on the grid switch profile. For instance, the grid switch signal may be used to trigger analyzing the image acquired by the focal spot sensor 28, such that the controller 22 only analyzes images of the focal spot sensor acquired when the grid 26 is in the off-state. By way of example, the grid switch signal may be used by the controller 26 to sample-and-hold the image of the focal spot sensor 28 thereby preventing misguiding the feedback control 30 by analyzing dark images when the X-ray beam 101 is off.

(35) Accordingly, by taking only images of the focal spot sensor 28 that are acquired during the off-state of the grid 26 and/or when the X-ray beam 101 is on, the control and/or regulation of the shape and size of the focal spot 20 by means of the feedback control 30 can be significantly improved.

(36) In addition to this feedback control 30, in the frame of the predictive control, the controller 22 can determine and or predict an expected heat load of the anode 12, as described with reference to FIG. 2. The heat load of the anode 12 may be determined e.g. based on the grid-switch profile for at least a subset of switching actions defined therein. Accordingly, the controller 22 may have access to prior knowledge on the immediate requirements in terms of X-ray on periods and can therefore predict, e.g. based on a model modelling the heat load of the anode 12 as a function of current and/or voltage supplied to the cathode, the predictive control signal to compensate for any changes in size and shape of the focal spot, particularly before any variation of the focal spot is noticeable. This may also be advantageous for reducing control transients which might occur after relatively prolonged X-ray off periods, where the feedback control 30 might otherwise still apply corrections which were adequate at a higher anode temperature. Prior knowledge allows the controller 22 to predict the anode temperature at the time that X-ray beam 101 is switched on again, thereby reducing the reaction time of the feedback control 30.

(37) It is emphasized that any features functions and/or functionalities as described with reference to any of FIGS. 1 to 3 can be combined.

(38) FIG. 4 shows a flow chart illustrating steps of a method for operating an X-ray source 10 according to an exemplary embodiment of the invention. If not stated otherwise, the X-ray source 10 comprises the same features, functions and/or elements as the X-ray source 10 described with reference to any of the preceding FIGS. 1 to 3. Particularly, the X-ray source 10 comprises an anode 12 and an emitter arrangement 14 having a cathode 16 for emitting an electron beam 15 and having an electron optics 18 for focusing the electron beam 15 at a focal spot 20 on the anode 12. The X-ray source 10 further comprises a controller 22.

(39) In a step S1 of the method, a switching action of the emitter arrangement 14 is determined by the controller 22, wherein the switching action is associated with a change of at least one of a position of the focal spot 20 on the anode 12, a size of the focal spot 20, and a shape of the focal spot 20.

(40) In a further step S2, based on the determined switching action, the size and the shape of the focal spot 20 expected after performing the switching action is predicted with the controller 22 based on the determined switching action. Optionally, in step S2 the controller 22 may predict a change of the size and shape of the focal spot 20 induced by the switching action.

(41) In a further step S3, the emitter arrangement 14 is actuated by the controller 22 to perform the switching action. Before, during or after performing the switching action, the controller 22 may further actuate the electron optics 18 based on the predicted size and shape of the focal spot 20, such that any change and/or variation in size and shape of the focal spot 20 induced by the switching action is compensated.

(42) 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 and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

(43) 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. 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.