FIELD OF THE INVENTION

The invention relates to an X-ray imaging system, to an X-ray imaging method, to a computer program element and to a computer readable medium. BACKGROUND OF THE INVENTION

In volume-image guided minimally invasive interventions, maintain breathing state consistency within a single rotational raw data acquisition as well as between successive rotational acquisitions is an important issue. The breath-hold in a single acquisition needs to be stable in order to avoid breathing artefacts in image volumes reconstructed from the raw data. Also, a consistent breathing state between two successive scans will reduce registration errors when combining the information from two scans.

A method to monitor vital signs (a breathing state is an example for this) has been proposed in WO 20131/56908. SUMMARY OF THE INVENTION

There may be a need for an imaging system of related method to reduce artefacts in reconstructed image data.

The object of the present invention is solved by the subject matter of the independent claims where further embodiments are incorporated in the dependent claims. It should be noted that the following described aspect of the invention equally applies to the X- ray imaging method, to the computer program element and to the computer readable medium.

According to a first aspect of the invention there is provided an X-ray imaging system comprising:

an X-ray imaging component;

a video camera configured to acquire a video signal of at least a part of a subject;

an image-support equipment comprising any one or more of a contrast agent injector and/or a catheter device; a photoplethysmographic processing component configured to process the video signal into a physiological signal representative of an evolution of a physiological state of the subject; and

a control unit configured to control, in dependence on at least a part of said physiological signal, an operation of said image-support equipment..

According to one embodiment, the control unit is further configured to effect, based on said physiological signal, a change of an imaging geometry of said X-ray imaging component.

According to one embodiment, a catheter device may comprise an EP ablation catheter, a flow measurement device, a pressure measurement device, or an intravascular imaging device such as an intravascular ultrasound (IVUS) catheter.

According to one embodiment the video camera is attached to or integrated into the X-ray imaging component.

According to one embodiment, the video camera is attached to or integrated into any one of the following: a gantry of said X-ray imaging component, an X-ray sensitive detector of said X-ray imaging component. In particular the (one or more) video camera is attached or integrated into a housing, or a frame or bezel structure, etc., of the X-ray source and/or the X-ray detector of said X-ray imaging component.

According to one embodiment, the camera is arranged in a manner so that at least a part of the part of the subject remains within a field of view of said video camera during at least a part of the X-ray imaging acquisition operation. In particular, in one embodiment, the relevant part (that is, the part relevant for PPG measurements) of the subject remains within in a field of view of said video camera during the whole of the imaging acquisition operation.

According to one embodiment, the subject is a human or animal patient and the part of the subject is a part of the patient's skin.

According to one embodiment, the skin is facial skin of the patient.

An X-ray imaging system, of any one of the previous claims wherein the physiological state is any one of the following: a cardiac state or a breathing state.

According to one embodiment, the X-ray imaging system comprises a reconstruction component configured to implement a reconstruction algorithm to reconstruct a cross-sectional image or a surface model from projection images acquired of the subject OB during the X-ray imaging acquisition, wherein said reconstructor is configured to include a motion compensation component into said reconstruction algorithm whose operation is based on said physiological signal as supplied by the photoplethysmographic processing component.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described with reference to the following drawings wherein:

Figure 1 shows an x-ray imaging system;

Figure 2 shows an embodiment of an x-ray imaging component of the x-ray imaging system;

Figure 3 shows exemplary representations of vital sign signals;

Figure 4 shows a flow chart for x-ray imaging; and

Figure 5 shows an embodiment of the x-ray imaging component of the x-ray imaging system. DETAILED DESCRIPTION OF EMBODIMENTS

With reference to Figure 1 there is shown an imaging system 100 including an imaging component IMX and a signal processing sub-system SPS.

The imaging component IMX is operable to supply x-ray images of at least a part of a region of interest of a subject OB. Overall control of the imaging component IMX and/or the signal processing sub-system SPS is via an operator console (not shown). One or more images supplied by the imaging system 100 can be viewed on a display device MT, or stored on or otherwise processed by a workstation WS comprised by or associated with the imaging system.

In one embodiment, the imaging component IMX is a projective or 3D radiography apparatus, such as a C- or U-arm imager otherwise such as a CT (computed tomography) scanner etc.

The imaging component IMX includes an x-ray imaging component XIC. The x-ray imaging component comprises an x-ray source XR and an x-ray sensitive radiation detector D. In one embodiment, the x-ray imaging component XIC comprises a (in general movable) gantry G and x-ray source XR and/or the detector D is mounted on said gantry G but other systems with no gantry such as ceiling or floor-mounted systems are also envisaged herein. The subject OB to be X-ray imaged (a human or animal patient for instance) resides on an examination table ET or similar support in an imaging examination region (the space between the x-ray source XR and the detector D) during an image acquisition (operation). The x-ray imaging component or at least the X-ray source XR is movable in particular rotatable relative to (in particular around) the subject OB (or at part thereof) to collect, in general, multiple projection images of the subject OB from different projection directions. The x-ray imaging component is configured to assume a range of different "imaging geometries" to effect these different projection directions. Put differently, a given geometry is defined by the mutual spatial arrangement between i) the subject OB and ii) the x-ray source and/or the detector D. The imaging system includes suitable actuator sub-system (servomotor arrangements, etc.) to change the imaging geometry and this change is responsive to corresponding control commands or requests being issued through suitable driver or interface system form an operation console. According to one embodiment, the change is effected by having the gantry move about one or more axis, exemplary indicted for a C-arm imager, a indicates rotation around an axis parallel to the plane of Fig. 1 and β indicates rotation around a second axis (indicted by an encircled 'x') extending into the plane of Figl . Rotation angels α, β are merely illustrative (and in no way limiting) for one way to implement a functionality for imaging geometry change. For instance, change in imaging geometry may include in addition or instead of rotations α, β, one or more translation options and/or there may be more than two rotation axes, or, in simpler embodiments, there is only a single rotation axis, etc. The requests to change or adjust the imaging geometry are issued manually by the user operating suitable control elements or are issued automatically by a pre- programmed imaging protocol executed by the work station or operator console. For instance, in the embodiment of the C-arm or (1st - third generation) CT imager, the imaging geometry is changed by having the gantry rotate in a scan motion around the subject OB in a full or partial revolution thus causing the x-ray source detector orbit around the subject OB whilst the different projection images (one or more per any given projection direction) is being collected.

In other embodiments (e.g., 4th generation CT scanners) the change in imaging geometry is realized by having only the X-ray source in rotation in the gantry whilst the detector remains stationary. In yet other embodiments, it is the x-ray source that is fixed and it is the motion of the detector that constitutes the change in imaging geometry change. The scan motion may not necessarily be arcuate as straight scan motions (or motions with small curvature) passed the subject OB are also envisaged herein for instance in certain mammography scanner systems.

Before carrying out the imaging acquisition, the X-ray imaging component assumes an initial imaging geometry. The initial geometry is then changed in a continuous or sequential motion into a final imaging geometry. The image acquisition operation includes, for a given imaging geometry between the initial and final geometry, operating the X-ray source XR to produce X-ray radiation which emanates from an egress window of the x-ray source and is then projected towards and across the subject of interest OB. The internal structure of the subject modulates a signal onto radiation as it passes through the subject OB. The radiation that emerges at the far side (when viewed from the source XR) of the subject OB is then detected at the detector D. The actual radiation detection is effected by a plurality of radiation sensitive pixels PX arranged in image plane of the detector. The signal modulated by the subject OB onto the radiation may include for instance

absorption/attenuation information. The radiation incident on the detector pixels causes a plurality of electrical signals that are representative of the modulated information. The electrical signals are then processed by data acquisition circuitry DAS that includes in particular A/D conversion circuitry to convert the electric signals into the particular projection image for the given imaging geometry/projection direction). The (digital) projection image is formed from an array of numbers x(i ), with each position (i ) in the array represent a respective detector pixel position and a respective number x at the respective array position representative of the imaging information "seen" by the respective detector pixel associated with said array position (i,j). In one embodiment, the so collected set of projection images is acquired during a single rotation measurement, that is, the projection data acquired without interruption of the movement of the X-ray source XR or is acquired during a single rotation (which may not necessarily be a full rotation).

The collected set of projection images (also referred to as raw data) may then be reconstructed by a reconstructor RECON module into for instance 3D image data by using a filtered -back-projection algorithm or similar. In other embodiments, rather than producing 3D volumetric image data, the reconstructor module RECON is configured to reconstruct 2D surface models from the projection images.

The x-ray imaging system further includes second imaging component (in addition to the X-ray imaging component XIC), but based on non-ionizing radiation (e.g. visible light, infrared (IR) or near- infrared (NIR), or other). In other word, the system (100) includes at least one video camera sub-system VC and a control unit CU. In one embodiment, the control unit includes in addition control sub-components for one or more of the following: collimator, table TL movement. X-ray tube XR settings (in particular mA, kV).

The video camera VC is coupled with a photoplethysmographic (PPG) processing component PP. The video camera VC together with the PP processing allows for better support of the imaging acquisition. More particularly, the camera VC together with the PPG processing component PP forms a contactless or non-obtrusive sensing means for biometric signals or "vital signs" that represent physiological states of the subject OB. The X-ray image acquisition can be synchronized with certain physiological states or vital signs of the (animated) subject OB (which will be referred to hereinafter as a patient OB). Yet more particularly, the video camera VC acquires a sequence of optical images of patient OB (in particular at least a certain part of the patient not necessarily the whole of the patient) and this video signal sequence is then processed by the PPG processor PP into a vital sign signal of the patient, for example into a breathing cycle signal or a cardiac cycle signal examples of which are shown in Figure 3. An example of a breathing signal is shown in the upper graph and another example of a vital sign signal (cardiac cycle signal) is shown in the lower graph as extracted from the video camera sequence using the PPG-processor PP. The curve(s) is/are times series, that is, the curve captures a time-evolution of the respective physiological states, respiratory and cardiac, respectively. Temporal resolution of the signal is given by the camera frame rate (the data in Fig 3 is presented at a 2 msec sampling on the x-axis).

A basic principle operation of the video camera VC-PP processor sub-system is explained in Applicant's WO 20131/56908. This type of unobtrusive vital sign monitoring using a video camera (also referred to as remote PPG), has been found relevant for patient monitoring. Photo-plethysmographic imaging is, for instance, described in more detail in W Verkruysse et al, "Remote plethysmographic imaging using ambient light", Optics Express, Vol. 16, No. 26, December 2008. Remote PPG is based on the principle that temporal variations in blood volume in the skin lead to variations in light absorptions by the skin. Such variations can be registered by a video camera that takes images of a skin area, e.g. the face, while processing calculates the pixel average over a manually selected region (typically part of the cheek in this system). By looking at periodic variations of this average signal, the heart beat rate and respiratory rate can be extracted. Thus, the pulsation of arterial blood causes changes in light absorption. Those changes observed with a photodetector (or an array of photodetectors) of the video camera VC form a PPG signal (also called, among other, a "pleth" wave or curve). Pulsation of the blood is caused by the beating heart, i.e. peaks in the PPG signal correspond to the individual beats of the heart. Therefore, a PPG signal is a heartbeat signal in itself. The normalized amplitude of this signal is different for different wavelengths, and for some wavelengths it is also a function of blood oxygenation. Regular video data have been shown to yield adequate vital signs not only for the heartbeat, but also for other biometrical signals such as respiration rate, or Sp02 (oxygenation of the blood) rate, etc. For instance, it has been found that the respiratory action modulates into the cardiac cycle as recorded by the spectral changes of the skin. In other words, the PPG analysis allows resolving the video signal not only into a cardiac cycle signal but also into a respiratory cycle signal. In one embodiment, the PPG processor includes an additional registration step for registering spatio-temporal variations of the detected light signals as described in the above referenced WO 2013/156908 and incorporated herein it its entirety. In other words, the analyzing of the processing by the PPG component may include as little as A/D converting and/or splitting up of the video signal into its RGB channel components with or without normalizing. In other words, in one embodiment the video signal is essentially directly used as a surrogate for the vital signs. In other embodiments, more involved processing is involved such as Fourier-transforming, e.g., into power levels and/or averaging and /or band-pass filtering. See also section 2 of the above referenced Verkruysse paper for details. PPG processing has the advantage that the signal can be used directly and/or without any additional, auxiliary devices such as markers/stickers/reflector applied to the patient's body.

The vital sign signal is extracted by the PP processor from the video signal and is then forwarded to the control unit CU. The control unit CU includes a suitable decision logic that allows executing certain control operations conditioned on certain events being present in the vital sign signal. For instance, the control unit may include a differentiator to analyze the vital sign curve supplied by the PP processor for slope to find events which correspond to a breath hold state for instance when the signal is a breathing sign signal. Events which slope zero (that is, peaks or troughs) are designated by "X" in Figure 3. The control unit and/or the PP processor can be arranged as software modules that run on the workstation or the operator console associated with the imaging system 100. In other embodiments the PPG processing functionality, that is, processor PP, is integrated in the camera system VC.

In one embodiment the control unit CU is used to control the imaging supporting equipment ISE. A non-exhaustive list of pieces of equipment ISE that can be controlled by the control unit CU includes (either singly or in any combination) a contrast agent injector pump or active invasive device for diagnosis or therapy, as e.g. catheters or a transesophageal ultrasound probe (TEE). Examples for catheter devices are pressure or flow wires to measure flow or pressure in the vascular system, intravascular imaging catheters like intravascular ultrasound (IVUS), intra-cardiac echo (ICE), or optical coherence tomography (OCT), and ablation catheters for electrophysiology (EP) procedures. For instance, the contrast agent injector pump is used to inject a quantity of contrast agent into the patient to enhance contrast of blood vessels and perfused soft tissue. The triggering of the injection can then be controlled by the control unit based on the vital sign signal provided by the camera VC-PPG processor sub-system.

In addition or alternatively, the control unit may operate to control the initiation i.e. the start, of the X-ray imaging acquisition. Only when certain desirable vital states are detected, is the image acquisition triggered. Referring for illustration purposes to the C-arm imager embodiment of imaging system 100, it is proposed herein in one embodiment to first position the C-arm G in a way that a part of the patient's skin, preferably the face, is in the FOV of the (one or more) video cameras(s) VC. The video camera may be attached to the housing or frame of the detector D. Using the camera-based PPG vital signs sensing, the "correct" or desired respiratory and/or cardiac signal event can be detected automatically from the video frame sequence provided by camera VC. For instance, for respiratory critical imaging, the patient OB is educated to stop breathing either in the full inhale or exhale state for the set of scans being carried out. Once the desired breathing state (full inhale) is reached or breath hold onset can be observed from the PPG signal, the C-arm G is moved, that is, is re-positioned to (e.g., by rotation/angulation (α,β) around one or more axes) to start position (that is, the initial imaging geometry) for the subsequent rotational image acquisition ("run") and the volumetric X-ray image acquisition is performed by energizing the X-ray tube whilst the C-arm is moved to assume the final imaging geometry.

In addition or instead of the controlling the support equipment, the control unit

CU may be used for prospective or retrospective gating of the acquisition. In prospective gating, the image acquisition is alternately started and interrupted in dependence on the monitored PPG signal. In retrospective gating, the monitoring action of the vital signs signal continues throughout the (possibly uninterrupted) image acquisition. That is, the video camera VC continues to acquire videographic material of the subject OB during the X-ray image acquisition. The so observed vital sign signal as provided by the PPG processor are used in retrospective gating to essentially tag the sequence of projection images.

The vital sign signals can then be used to filter out those parts from the raw data that have been acquired whilst the subject OB was in a physiological state of interest. Alternatively, the vital sign information associated with the vital sign signal curve can be harnessed to control application of a motion compensation component in the reconstruction algorithm to obtain the volumetric data. Instead of or in addition to volumetric reconstruction, 3D surface reconstruction is also envisaged herein. See for instance U Jandt et al "Automatic generation of time resolved motion vector fields of coronary arteries and 4D surface extraction using rotational x-ray angiography", Phys. Med. Biol. 54 (2009), pp 47-66.

With reference to Figure 2, there is shown an embodiment for a multi-camera system integrated in the x-ray imaging component XIC. Four separate cameras, labeled VC1- 4, are shown in Figure 2. The cameras VCl-4 are attached to a frame or housing of the detector D. The cameras VCl-4 are arranged regularly spaced around the circumference of the frame of the detector D. In one embodiment the frame of the detector is square or rectangular shaped and the cameras are positioned at the respective mid points of the sides, however this is just one possible embodiment. In other embodiments the cameras are arranged in respective corners of the rectangular or squared detector frame. Embodiments with irregular spacings are also envisaged. Although four cameras VCl-4 are shown in Figure 2, this is not limiting as embodiments are envisaged with a single camera or with two, three or more than four cameras. In another alternative embodiment or addition to the camera(s) being arranged at the detector D, there are embodiments where one or more cameras are arranged at the x-ray source XR housing. In yet other embodiments the cameras are arranged somewhere on the gantry G. The cameras VC may be clipped/bolted etc. onto the tube XR or detector D housing or gantry G with the camera bodies visible for an observer as protrusions. Alternatively, the camera(s) VC may be integrated into the housing, frame or bezel structure of XR tube or detector D or gantry, in one embodiment essentially flush with an outer surface of the respective housing or gantry portion so that the camera VC bodies are not visible for an outside observer as protruding appendages. Merely the camera VC's signal ingress window may be visible on the respective housing or gantry portion.

It should be clear from the above that the respective field of views of the camera on the one hand side and the X-ray detector D on the other hand side may not be the same. I Also, it should be clear from the above that the imaging geometry required in order for the respective patient part, for instance the face, to come within the field of view of the camera VC, is different from the geometry required for the X-ray imaging acquisition. In other words, the imaging geometry will be changed so that the desired subject OB part OB comes within the field of view of the camera and the sequence of video signals are then acquired to monitor for vital signs. If the decision logic in the control unit CU detects the event for image acquisition the imaging geometry is (again) changed so as to bring the field of view of the detector into the desired position before commencing with energizing the x-ray tube. The proposed system may be used for dual or single acquisition mode. For a single rotational acquisition the control unit may be configured to start a scan in a defined breath-hold state or a desired cardiac state. During the scan the breathing state can be observed and in case that breathing is started before the end of the scan, this may be taken into account during the reconstruction by for instance automatically start breathing motion compensated reconstruction or by restricting reconstruction to those projection images that were acquired in a consistent (that is, same or sufficiently similar) cardiac or breathing state. In case of a dual phase acquisition (that is, subsequent scan measurements), the control unit may be configured to control the X-ray imaging component such that both scans can be acquired in a consistent breathing or cardiac state. The proposed system may be used to support a range of different image-guided interventions. For instance, to name but one exemplary embodiment, in X-ray guided needle biopsies, fluoroscopy projections acquired for the needle guidance can be made consistent by using projection imagery acquired during breath-hold for instance.

Referring to Figure 4 there is shown a flow chart of a method of X-ray imaging.

At step S410 a video signal is acquired by an x-ray imaging component of an x-ray imaging system of at least a part of a subject OB to be imaged. In one embodiment the subject OB is a human patient and the part corresponds to a patch of skin, for instance facial skin.

At step S420 the video signal is then PP processed (e.g. converted, analyzed etc.) into a physiological signal that represents a revolution of a physiological state of interest of the subject OB. PPG techniques are used to effect this conversion. Said

physiological signal may represent a cardiac or respiratory cycle of the human or animal patient. The field of view of the camera in step S410 is so chosen that the optically imaged part of the subject is a good "tell-tale" for the physiological state one wishes to monitor. As mentioned, for cardiac and respiratory signals, facial skin is a good sample area.

At step S430 an x-ray imaging acquisition by the x-ray imaging component of the subject OB or at least a part thereof, is initiated. The FOV of the x-ray imaging component is in general different from the FOV of the video camera as per step S410. Step S430 may also include any one of: adapting a setting (niA and/or kV) of the X-ray tube, and/or adapting a collimator aperture, and/or adapting a position of the examination table ET. Further an image support equipment (ISE) comprising any one or more of a contrast agent injector and/or a catheter device is being controlled by the control unit, prior to, during or after the image acquisition.

The above steps of acquiring the video signal S410 and the PPG conversion into the physiological signal at step S420 together form essentially a monitoring of the physiological state. In one embodiment the monitoring operation is restricted to a phase prior to the actual image acquisition. Once a desired vital sign event has been detected the acquisition is started and the monitoring stops. However in other embodiments the monitoring extends throughout the actual X-ray image acquisition which can be useful for retrospective or prospective image gating operations. If the monitoring operation is to extend throughout the X-ray acquisition, the video camera is arranged or integrated in the x-ray imaging component or in the X-ray imaging system in such a manner that the relevant subject OB part (for instance face of the patient) remains within the field of view of at least one of the camera's. For instance, one camera may be affixed to the examination table ET or may be attached to a stationary part of the gantry, e.g., a pivot point PVP as shown in Fig 5, pane A). Instead of in addition, one or more cameras may be mounted on the gantry off-pivot-point, but with a sufficient inclination as shown in Fig 5 pane A) so as to ensure coverage (during the whole of image acquisition operation) of relevant part of the patient for PPG monitoring. When the camera is mounted on the X-ray tube XR and/or detector D, the relevant patient part for PPG purposes may remain in the FOV only through a part of the X-ray tube XR rotation, such as for instance during the time when the acquisition trajectory is situated above the examination table ET as shown in panes B),C) of Fig 5.

It may not be necessary in step S410 to use a single camera as the video signals can be supplied by a different camera (form a plurality of cameras) at different time. The cameras are switched for instance to sequentially supply respective video signals. In this manner it may be ensured that the or a patient OB part (relevant for PPG) remains in the FOV of at least one of the cameras during the X-ray acquisition. The hand-over between the cameras can be synchronized with the gantry motion for instance.

In one embodiment the initiation at step S430 of the x-ray image acquisition may include changing the imaging geometry in respect of the x-ray imaging component. This may particularly be the case when the field of view of the video camera used for acquisition of the video signal is different from the field of view required for the initial imaging geometry as prescribed by the imaging protocol to be used for the X-ray acquisition. In other words, the gantry of the X-ray imaging system may be first used to position the video camera relative to the patient OB and (after the proper physiological stated as per PPG has been detected). The gantry is then used to position the x-ray source and/or detector relative to the region of interest one wishes to have X-ray imaged.

A comparable approach as described herein can be used when optimizing the acquisition of DSA (digital subtraction angiography) sequence for low motion states or gated cardiac acquisitions without using an ECG (electrocardiography).

In case that the patient's face can be monitored with a single camera

throughout the rotational acquisition the breathing state can be analyzed throughout the scan and the acquired signal can be used to reduce breathing artefacts in the reconstruction.

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.

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.

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.

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

According to a further exemplary embodiment of the present invention, a computer readable medium, in particular a non-transitory storage 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.

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. 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.

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.

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.

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 fulfill the functions of several items re-cited in the claims. The mere fact that certain measures are re-cited 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.