FIELD OF THE INVENTION 
     The present invention relates to an X-ray detector, an X-ray detector array, an X-ray imaging system and a corresponding method as well as a computer program element and a computer readable medium for a multi-frame X-ray detection with distributed X-ray sources, and in particular to respective devices and a method which can integrate multiple frames in parallel. 
     BACKGROUND OF THE INVENTION 
     X-ray imaging systems having a plurality of distributed X-ray sources are used to allow a fast switching between several focal spot positions without the need for moving the respective X-ray tube. 
     From U.S. Pat. No. 7,139,367 B1, a time-share digital integration method and apparatus for processing X-ray images is known, according to which a method and apparatus for processing signals output from an array of X-ray radiation detectors receive X-ray radiation emanating from an object irradiated by a source beam of X-ray radiation, to thereby produce a visual image of internal object features, utilizing repetitive sampling a couple of times and digital accumulation of signals output from each detector to increase signal-to-noise ratio of the images by a certain factor. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to avoid a fast temperature increase of a focal spot of distributed X-ray sources. 
     The object of the present invention is solved by the subject-matter of the independent claims, wherein further embodiments are incorporated in the dependent claims. 
     It should be noted that the following described aspects of the invention apply also for the detector, the detector array, the imaging system, the method, the program element and the computer readable medium. 
     According to an aspect of the invention, an X-ray detector comprises an X-ray sensitive element having a charge output, a charge determination device being adapted to accumulate a plurality of temporally distributed partial charges according to a respective one of a plurality of spatially distributed X-ray sources, a readout device being adapted for reading out the charge determination device, wherein the charge determination device is connected to the charge output of the X-ray sensitive element via an accumulation activation device, wherein the charge accumulation device is connected to a readout line via a readout activation device. 
     Such a detector allows to decrease the focal spot temperature of the spatially distributed X-ray sources by accumulating temporally distributed partial charges, so that the entire charge quantity being necessary for a signal-to-noise ratio of a certain quality can be distributed over a plurality of sub-frames, so that each of the plurality of spatially distributed X-ray sources, in particular the focal spots thereof, are only impacted for a duration of a certain time. The X-ray detector of the invention allows accumulating the distributed partial charges in order to obtain the total charges for each of the plurality of spatially distributed X-ray sources, in particular the focal spots thereof, and to provide a respective output signal. 
     According to another aspect of the invention, there is provided a method for multi-frame X-ray detection with distributed X-ray sources, comprising synchronously activating a first charge accumulation section for a duration of a first predetermined sub-frame and a first X-ray source out of a plurality of distributed X-ray sources, synchronously switching activating from the first charge accumulation section to a second charge accumulation section for a duration of a second predetermined sub-frame and from a first X-ray source to a second X-ray source out of a plurality of distributed X-ray sources, repeating synchronously activating and synchronously switching activating until lapse of a predetermined time frame for each of the first and second charge accumulation section, the time frame consisting of a plurality of sub-frames, for the accumulation of a plurality of temporally distributed partial charges according to an origin of a respective one of the plurality of spatially distributed X-ray sources, and sequentially reading out the first charge accumulation section and the second charge accumulation section after lapse of the predetermined time frame for each of the first and second charge accumulation section. 
     This allows a synchronous switching of a respective X-ray source and a respective detector section for only a limited duration of a sub-frame, respectively, in order to avoid the increase of a focal spot temperature. 
     It should be noted that the present invention is not limited to a first and second charge accumulation section, a first and second predetermined sub-frame, a first and second X-ray source, etc., but also may include a plurality of charge accumulation sections, a plurality of predetermined sub-frames, a plurality of X-ray sources etc., the activation may by successively synchronized. 
     It can be seen as the gist of the invention to establish a specific relation between the focal spot position and a rule for accumulating the respective partial measurements, e.g. temporally distributed partial charges, belonging to the same focal spot positions, and to keep the focal spot temperature low by only activating the focal spot for a limited time according to a sub-frame. Reading out the accumulated charges only after accumulating a plurality of distributed partial charges for each focal spot reduces the required data rate and improves signal to noise ratio. 
     These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will be described in the following with reference to the following drawings. 
         FIG. 1  illustrates an X-ray imaging system according to an exemplary embodiment of the invention. 
         FIG. 2  illustrates a schematic view of a multi-frame X-ray detector and respective distributed X-ray sources according to an exemplary embodiment of the invention. 
         FIG. 3  illustrates a time schedule for integration periods and readout periods according to an exemplary embodiment of the invention. 
         FIG. 4  illustrates a structure of an X-ray detector according to an exemplary embodiment of the invention. 
         FIG. 5  illustrates a further structure of an X-ray detector according to an alternative exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates an X-ray imaging system comprising of an X-ray source  90  and an X-ray detector  1  and an X-ray detector array  2 , respectively. The X-ray detector  1 ,  2  and the X-ray source  90  are geometrically arranged to each other in a predetermined position. Thus, it is possible to obtain an efficient imaging without the need for repeated adjustment. The X-ray detector and the X-ray source are connected to a control unit  80  (not shown in  FIG. 1 ) wherein the X-ray imaging system may comprise a display unit  4  allowing to immediately display the results of the examination. 
       FIG. 2  illustrates a schematic view of an arrangement of an X-ray source  90  having a plurality of focal spots  91 ,  92 . The detector  1  receives the radiation irradiated from either of the focal spots  91 ,  92 . The focal spots  91 ,  92  are activated sequentially, and the detector  1  is multiplexed respectively, in order to distinguish between the radiation irradiated from the first focal spot  91  and a subsequent focal spot  92 . The activation of the respective one out of a plurality of focal spots  91 ,  92  and a respective multiplexing and synchronizing of the detector  1  can be carried out by a control unit  80 , so that the detector  1  can be used to take images from a plurality of focal spots  91 ,  92 , or respective X-ray sources. The control device  80  may also be adapted to control a repeated synchronous multiplexed activation of the plurality of charge accumulating units for a respective duration of a sub-frame, wherein multiple temporally distributed sub-frames belonging to a respective one of each of the plurality of charge accumulating units form a time frame. The detector can be read out via a readout line  50  to provide the recorded images to a display device  4 . It should be noted that the control unit may also be adapted to synchronize the readout of the detector  1 . Reading out only after a plurality of accumulated sub-frames for each of the focal spots and accumulating units reduces the required data rate and improves the signal to noise ratio. 
     As the focal spots  91 ,  92  possibly increase very fast to temperatures of about 1000° C. within a very short time of a couple of microseconds, each focal spot can be activated only for a very limited time. However, this very limited time may be not sufficient for providing an imaging signal having a sufficient signal-to-noise ratio, so that it is desired to elongate the total activation time of each of the focal spots. The idea of using distributed X-ray sources rather than rotating anode tubes for e.g. tomographic X-ray imaging systems allows to use a stationary anode having a plurality of X-ray foci, which are distributed for example along a line or an arch, as can be schematically seen from  FIG. 2 . The foci can be switched very fast, which is of relevance for tomographic systems, as no mechanical movement is needed to take projection images from different angles. Thus, a projection of images from different foci and angles, respectively, can in principle be acquired in rapid succession without motion blur. Such a tomosynthesis system, e.g. for mammography, may require only a limited angular coverage and with this respect, is useful for use of distributed X-ray sources, as no significant enlargement of a detector area is required. However, also in case of a computer tomography system, where projection images with an angular coverage of 180° plus a possible fan angle need to be acquired, for example a combination of a distributed source together with a rotating detector, might be a cost-effective solution, as then an increase of the detector area is not required. Furthermore, such a configuration would also allow the use of a focused anti-dash scatter grid, which is useful for large coverage CT. 
     X-ray detectors with a rotating anode currently used in X-ray imaging systems usually work in an integrating mode. In this mode, the X-ray photon energy entering a detector pixel is integrated for a fixed time period called a frame. After that, the image (i.e. the pixel values) is read out and integration of a new frame starts. However, the speed of such a detector is given by the frame rate, which is the number of images it can generate per time. In a tomographic imaging system having a distributed X-ray source and an integrating detector, the overall time required to generate a sequence of projection images from each focus is determined entirely by the detector speed, since mechanical movement is not required. 
     In case, the anode of an X-ray imaging system is stationary, the focal spot temperature will increase very fast to about 1000° C. within a couple of microseconds when using a tube power currently used in the CT or CV systems. As a consequence, the available tube power is very limited in such a system and the advantage of a fast exposition is not effective, as the images become too noisy. The whole sequence has to be repeated several times, such that images from the same location can be averaged in order to arrive at an acceptable signal-to-noise level. In order to utilize the fast exposition potential of the distributed source, a faster switching between the foci is needed in order to keep the available tube power high. This however requires a much faster X-ray detector with an integration period of a couple of microseconds, for example 10 to 15 microseconds, which would cause an incredibly high data rate. When using a shorter time frame integration time without increasing the data rate, the signal-to-noise ratio can be kept better, so that the detector becomes more ideal for X-ray imaging systems with distributed X-ray sources. 
     The present invention provides for a device and a method having a special integrating X-ray detector, which can integrate multiple frames in parallel. Such an operation mode is illustrated in  FIG. 3 .  FIG. 3  illustrates the activation of several foci, here illustrated exemplarily as focus  1 , focus  2  and focus  3 . Each focus is activated only for a limited integration period, which corresponds to a sub-frame  100 . Such a sub-frame is of only limited duration of a couple of microseconds, so that for each focal spot it can be avoided a fast increase of the temperature beyond the safe operating point. By repeating the integration periods after a certain time, each focus and at the same time each of the corresponding allocated detector sections receive the full extent of charge as the sum of partial charges in a sub frame, which is needed for a sufficient signal-to-noise ratio. The sum of each of the sub-frame  100  for each focal spot resulting in the entire time frame which is needed for a sufficient signal-to-noise ratio, so that after reaching the integrated duration of the entire time frame, a readout procedure is carried out for the readout period. Thus, the entire integration time can be kept high, but the temporal impact on the focal spot can be kept low without the need of a read out for each sub-frame, as the readout period starts after a plurality of sub-frames for each focal spot. According to the following example, the total required integration time may be for example 100 microseconds per view for each focal position. This total required integration time than is split up into several periods of for example 10 to 15 microseconds. Each integration period is short enough to prevent overheating of the anode. After an integration period, the active focal spot changes and the integrated signal is kept in a storage device. The integration is continued when the focus returns to this position. After a sufficient number of integration periods, the detector frames are read out. Thus, data transfer occurs at the same data rate as state of the art detectors, for example 10.000 frames per second, whereas the integration periods, i.e. the sub-frames are much smaller, e.g. 10 to 15 microseconds, rather than 100 microseconds. 
       FIG. 4  illustrates an exemplary embodiment of the invention. This embodiment may be used for a flat panel X-ray detector. For this purpose, an X-ray sensitive element  10  is provided, for example in form a photo diode. Such a photodiode can be provided with a scintillator for converting X-ray radiation into radiation for which the photodiode is sensitive. Such scintillators may be made for example of a material comprising CsI having a Tl doping. It should be noted that the X-ray sensitive element also may be an element having a Se or PbO or other photoconductor layer. The X-ray sensitive element is provided with a charge output  11 . Further, the detector comprises a charge determination device  30 , being adapted to accumulate a plurality of temporally distributed partial charges according to a respective one of a plurality of spatially distributed X-ray sources. The charge determination device  30  is connected to the charge output  11  of the X-ray sensitive element  10  via an accumulation activation device  20 . The charge accumulation device is further connected to a readout line via a readout activation device  40 . This readout device  40  is adapted for reading out the charge determination device. 
     The accumulation activation device may be realized for example by a plurality of transfer transistors  21 ,  22 , which can be activated according to the time pattern shown in  FIG. 3 . The charge accumulation device  30  may be structured as a plurality of parallel charge accumulation units  31 ,  32  each of which again may comprise a charge accumulation capacity. Thus, the charges delivered from the X-ray sensitive element, e.g. a photo diode  10  can be sequentially provided to a respective storing capacity  31   a ,  32   a , allocated to a respective charge accumulation unit  31 ,  32 . The voltage level of the respective capacitance  31   a ,  32   a  can control the respective amplifier  31   b ,  32   b  so as to operate as a source follower. The respective voltage corresponding to the signal of the pixel with respect to a particular focal spot can be read out with a readout transistor or switch  41 ,  42  connecting sequentially the respective accumulation unit  31 ,  32  to the readout line  50 . A reset device (not shown) can be provided in parallel to the respective capacitance  31   a ,  32   a  in order to reset the respective capacitance to a default state. The default state may be a fully charged capacity which will be discharged during accumulation or an empty capacity which will be charged during accumulation. The reset devices may be for example provided in form of a switch so that the respective capacitor can be discharged and the gate of the respective amplifier  31   b ,  32   b  operating as a source follower can be brought to the Level of VDD. 
     In practice, before data is acquired, the photo diode  10  of each pixel, which photo diode converts the incoming light into charge, is usually reset, i.e. set to a predefined voltage. The acquisitions of several sub-frames belonging to the same X-ray focus, which together form the signal of the signal frame, can be staggered in time, as described with respect to  FIG. 4 . With the help of at least two charge transfer devices  21 ,  22 , the charge belonging to a certain X-ray focal spot is transferred to the associated storage capacitor  31   a ,  32   a . The charge transfer devices maybe realized as charge transfer transistors. The photo diode should maintain the voltage irrespective of the charge condition of the capacitors  31   a ,  32   a . When realizing the charge transfer devices as transistors, the gates thereof can be kept on a lower but fixed voltage above the threshold voltage of the transistor. In case the photo diode does not provide a current, the source terminal of the transistor is a threshold voltage below the gate voltage. In case the photo diode provides charges, i.e. current, the source voltage decreases by a minute amount, the transistor starts to be conductive and transfers the charges to the charge storage capacitor. When the current of the photodiode stops, the voltage of the source increases to the initial voltage and the current reduces to zero. Thus the charge transfer device keeps the voltage of the photo diode constant whereas the drain can take almost any arbitrary voltage levels. Thus, a feedback effect to the photodiode can be avoided. When the first focal spot is activated, a first charge transfer transistor  21  is activated by pulling its gate TG 1  to an intermediate voltage. The charge transfer transistor  21  then transfers the charge to a first storage capacitor  31   a . When the second focal spot is activated, a second charge transfer transistor  22  is activated, while the other charge transfer transistor are deactivated, by pulling its gate TG 2  to an intermediate voltage so that the charge from the photo diode can then be transferred to the second storage capacitor  32   a . For each focal spot position there is one branch with a charge transfer transistor and a storage capacitor. The charge is stored on the storage capacitor  31   a ,  32   a  and can be read out as a charge, i.e. like in a passive pixel with a readout switch, which is not shown in  FIG. 4 , or the charges can be converted into a voltage on the storage capacitor  31   a ,  32   a  and the corresponding voltage is read out, i.e. like in an active pixel with a source follower operating as an amplifier  31   b ,  32   b  and a respective readout switch  41 ,  42 , as shown in  FIG. 4 . 
     An alternative embodiment of the invention is illustrated with respect to  FIG. 5 .  FIG. 5  also illustrates an X-ray sensitive element  10  having a charge output  11 , as described above with respect to  FIG. 4 . The charge determination device  30  being adapted to accumulate a plurality of temporally distributed partial charges according to a respective one of a plurality of spatially distributed X-ray sources by using for example a common charge unit integrator  33 . This charge unit integrator  33  can be used for each of the focal spots. However, it should be understood, that also a full charge unit integrator (i.e. each with its own event counter and time counter) can be provided for each of the focal spots, which, however, will increase the number of charge unit integrators. Such a charge unit integrator  33  can be realized by an operational amplifier with a charge capacity being capable of storing a certain charge unit, wherein the charge capacity Q is reset when full, wherein the reset event is counted by a counter. The recognition of the full charge capacity, the triggering of the reset and the counting of the resets can be carried out by a discriminator with counter  36 . The reset may be triggered via a respective line  37 , wherein the counting result may be transferred to a latch for event counter  34  via a respective connection  38 . The latch for event counter  34  then distributes the counts to the respective focal accumulator  34   a ,  34   b  . . .  34   n . Thus a multiplexing of the counting may be realized, which may be functionally considered as a multiplex counter. This multiplex counter is adapted to count resets of the charge unit integrator according to a respective one of the plurality of spatially distributed X-ray sources  91 ,  92  as a measure of the total charge quantity of a respective partial charge. A control device (not shown in  FIG. 5 , but described with respect to  FIG. 2 , may control a repeated synchronous multiplexed activation of the plurality of charge accumulating units for a respective duration of a sub-frame, either in form of discrete units  31 ,  32  as described with respect to  FIG. 4 , or in form of a multiplex counter  34 ,  34   a ,  34   b ,  36  as described with respect to  FIG. 5 , wherein multiple temporally distributed sub-frames form a time frame. 
     Thus, the charge capacitor maintains the charge Q and is repeatedly reset according to a pattern also shown on top of  FIG. 5 . Thus, the charge quantity can be determined by counting the respective number of resets during a sub-frame as a measure of the totally accumulated charges. When switching for the next focal spot, the counter counts the resets for the next focal spot. This procedure will be repeated for each of the focal spots. Thus the charge quantity can be determined by counting the resets, i.e. the charge packets of the capacitor. In case this procedure is not sufficiently exact enough owing to the quantization of the charges, the charge estimation can be improved based on the respective duration of the sub-frame. For this purpose a time counter can be provided, as will be described in the following and known from R. Luhta et al, A new 2D-tiled detector for multi-slice CT, SPIE 2006 (6142), where however the linkage between different focal spot positions is not disclosed. 
     In addition, the charge accumulating device  30  further may comprise a time counter  35  for determining a duration of a sub-frame during which each of partial charges transfers. This time can be accumulated by a latch time counter  35  according to each of the respective focal spots. Thus, for each of the focal spots a respective number N of reset events and a respective time can be provided so as to determine the accumulated charge. 
     While in flat X-ray detector usually the integrating capacitance is reset at the end of a frame, in CT, one tries to avoid a reset due to the required higher frame rate. A charge determination device  30  may continuously analogue to digital convert the current from the photo diode without any reset between the frames. In order to facilitate the above-described idea of time staggered sub-frames belonging to the same view or focus, the charge determination device  30  can be designed to cope with shorter integration periods in order not to increase the focal spot temperature. This means that the charge packets have to be made smaller so that with given bias current at least two pump events occur during a sub-frame. The charge determination device  30  is designed for dealing with measuring mean currents and therefore mean charges within a much shorter sub-frame. The digital results of the time-counter  35  and the event counter  34  of the staggered sub-frames, which belong to the same focus, will be accumulated so that they can be read out, when all sub-frames of a frame have been seen. The individual event counter and time counter values resulting for each focus of the accumulation may be read out or already processed digitally before read out in order to provide an estimate of the mean charge according to the formula (NP−1)/NT, where NP is the number of pump events provided by the event counter  34 , and NT is the number of clock ticks counted in the time counter  35 . For readout concepts for CT using a simple integrating capacitor, the same approach can be taken as described for flat X-ray detectors above. 
     The accumulated charge can be calculated as
 
 Q total=((NP−1)/NT)*NF*PC,
 
     With NF as the number of sub-frames and PC as the quantity of the partial charges, given the sub-frame is the same for each focal spot. Otherwise NF will be replaced by a sum formula over each individual sub-frame duration. 
     It should be noted that for the devices and the method the number of units is not limited to the number of two, i.e. not limited to the respective first and second element, but may be also of larger numbers. 
     A compromise solution for keeping the circuit complexity and power consumption low may be implementing the charge integrator  30  with a single operational amplifier, however with N different feedback capacitances and reset switches. At a time, only one feedback capacitance (with reset switch) may be connected to the operational amplifier. When the next focal spot position is visited, the corresponding feedback capacitance is connected to the operational amplifier. Thus it is made sure that the residual charge on the feedback capacitance belonging to the focal spot position k is not lost or falsifies the charge measurement for focal spot position k+1. This under certain circumstances may introduce additional kT/C noise. Recharging processes when switching between focal spot positions (and the corresponding feedback capacitances) may be provided with additional means for reduction of substrate noise. 
     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, 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. 
     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. 
     It has to be noted that exemplary embodiments of the invention are described with reference to different subject matters. In particular, some exemplary embodiments are described with reference to apparatus type claims whereas other exemplary embodiments are described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other 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, in particular between features of the apparatus type claims and features of the method type claims is considered to be disclosed with this application. 
     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. 
     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. 
     LIST OF REFERENCE SIGNS 
     
         
           1  X-ray detector 
           2  X-ray detector array 
           3  X-ray imaging system 
           4  display unit 
           10  X-ray sensitive element 
           11  charge output 
           20  accumulation activation device 
           21 ,  22  charge transfer devices 
           30  charge determination device 
           31 ,  32  charge accumulating unit 
           31   a ,  32   a  charge accumulation capacity 
           31   b ,  32   b  amplifier/source follower 
           33  charge unit integrator 
           34  latch for event counter 
           34   a ,  34   b ,  34   n  focus accumulation event counter 
           35  time counter 
           35   a ,  35   b ,  35   n  focus accumulation time counter 
           36  discriminator and counter 
           37  reset line 
           38  count transfer line 
           40  read out device 
           41 ,  42  read out activation switches 
           50  read out line 
           80  control device 
           90  anode 
           91 ,  92  focal spot 
           100  sub-frames
Device and method for synchronously switching activating a first and second charge accumulation section ( 31, 32) for a duration of a first and second predetermined sub-frame and a first and second X-ray source until lapse of a predetermined time frame for each of the first and second charge accumulation section ( 31, 32) for the accumulation of a plurality of temporally distributed partial charges according to an origin of a respective one of the plurality of spatially distributed X-ray sources so as to establish a specific relation between the focal spot position and a rule for accumulating the respective partial measurements, e.g. Temporally distributed partial charges, belonging to the same focal spot positions, and to keep the focal spot temperature low by only activating the focal spot for a limited time according to a sub-frame.