Patent Publication Number: US-2009238328-A1

Title: X-ray tube with oscillating anode

Description:
The present invention relates to an X-ray generating tube, which is adapted to generate X-ray originating from at least two spatially different focal spots. In particular, the present invention relates to an X-ray tube being used for computed tomography. 
     The present invention further relates to a computed tomography system being equipped with such an X-ray generating tube. 
     Further, the present invention relates to a method for operating an X-ray generating tube. 
     In some circumstances, it is desirable to provide a computed tomography (CT) apparatus with an X-ray source, which is capable of rapidly shifting a focal spot emitting X-rays from one place to another with respect to the patient being examined. It has been proposed to effect such shifting by electromagnetic or electrostatic deflection of the electron beam of the X-ray tube. 
     U.S. Pat. No. 4,002,917 and U.S. Pat. No. 4,010,371 disclose various CT arrangements in which such electron beam deflection is used to shift radiation paths laterally across the examined slice of a patient&#39;s body, longitudinally of said slice, or to hold the radiation in a certain disposition relative to the patient despite a physical rotation of the X-ray tube around the patient. 
     U.S. Pat. No. 4,162,420 discloses an X-ray tube including an envelope enclosing a flat-edged anode disc, which is rotatable and axially relocatable. The X-ray tube further encloses an electron beam source for projecting electrons along a beam axis toward the edge of the anode disc. The beam source is disposed to direct its beam at an acute angle of incidence to the edge of the anode disc and to produce X-rays, which are transmitted through a window in the envelope. The anode is elastically supported by means of two springs, wherein a first spring is attached at an upper end of an anode shaft and a second spring is attached at a lower end of the anode shaft. Thereby, the anode may be linearly shifted in an oscillating manner with respect to the envelope. 
     U.S. Pat. No. 4,107,563 discloses an X-ray generating tube, which is especially suitable for to be used in a CT apparatus. The X-ray generating tube comprises a rotating anode, which can be linearly shifted along a rotational axis of the anode in an oscillatory manner. The anode oscillation is realized by means of a so-called figure-of-eight groove, which is formed at a shaft of the rotating anode and which mechanically interacts with pegs being provided at a bearing of the rotating shaft. When the anode is shifted with respect to an envelope of the X-ray tube, a focal spot representing the origin of the generated X-ray is also moved with respect to the envelope. The described X-ray generating tube has the disadvantage that the oscillatory movement is directly connected with the rotational movement of the anode such that only a continuous displacement of the focal spot is possible. However, there are applications in particular in the field of CT, which require a fast switching of an X-ray focal spot between a first focal spot position and a second focal spot position. 
     There may be a need for an improved X-ray tube, which allows for a fast switching of an X-ray focal spot between a first focal spot position and a second focal spot position. 
     This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims. 
     According to a first aspect of the invention there is provided an X-ray tube, in particular for generating X-rays being used for computed tomography. The provided X-ray tube comprises (a) an electron source, adapted for generating an electron beam projecting along a beam axis, (b) an electron deflection device for deflecting the generated electron beam, (c) a control unit being coupled to the electron deflection device for spatially controlling the beam axis and (d) an anode, which is arranged within the beam axis such that the electron beam impinges onto a focal spot of a surface of the anode. Thereby, the anode is movable along a z-axis in an oscillating manner, the surface of the anode is oriented oblique with respect to the z-axis, and the control unit is adapted to spatially control the focal spot in such a manner that the focal spot moves essentially in a discrete manner between a first focal spot position having a first z-coordinate and a second focal spot position having a second z-coordinate being different from the first z-coordinate. 
     This aspect of the invention is based on the idea that an essentially discrete switching of the focal spot between two different z-positions can be achieved even if there is a continuous and non-discrete oscillating movement of the anode. Thereby, the focal spot is moved over the surface of the anode in such a manner that the two focal spots have different radial distances with respect to the z-axis. Since the surface of the anode is oriented oblique with respect to the z-axis the radial focal spot movement caused by the electron deflection device also contributes to the variation of the focal spot along the z-direction. Thereby, by adequately operating the anode movement and the control unit in a synchronized manner, the contribution of the moving anode to the focal spot movement along the z-direction and the contribution of the electron beam deflection to the focal spot movement along the z-direction can be superimposed in such a manner that an essentially discrete switching of the focal spot along the z-direction may be achieved. This even holds if the anode movement and/or the electron beam deflection are not discrete. In other words, the electron beam deflection may compensate for a non-discrete movement of the anode. 
     By contrast to a focal spot displacement by means of the electron deflection device only, the described combined focal spot displacement being based on both the movement of the anode and the radial deflection of the electron beam provides the advantage that the difference of the radial distance of the two focal spots with respect to the z-axis is much smaller. Therefore, when operating the X-ray tube in a discrete focal spot switching mode the radial distance between the corresponding focal spot and an object being placed outside of the z-axis varies only slightly. This has the advantage that in many applications the radial focal spot movement may be neglected in a good approximation. 
     In particular when the described X-ray tube is used for increasing the sampling rate of digital X-ray attenuation data acquired e.g. by means of a computed tomography apparatus, an increased spatial resolution may be achieved within a wide region of interest. In this respect, an increase of the sampling rate may be achieved if for each projection angle of the X-ray source with respect to the object under examination two datasets are acquired. Thereby, a first dataset is acquired when the X-rays originate from the first focal spot and a second dataset is acquired when the X-rays originate from the second focal spot. 
     A further advantage of the focal spot displacement by means of both the electron deflection device and the mechanical motion of the anode is the fact that the requirements regarding the electron beam deflection unit are relaxed. This is based on the matter of fact that a major part of the z-movement of the focal spot is facilitated by the mechanical anode motion as compared to a focal spot z-movement caused solely by deflecting the electron beam. 
     According to an embodiment of the invention the anode is rotatable around the z-axis. This may provide the advantage that the concentration of the heat load of the anode may be reduced significantly because even when the electron beam generates only two discrete focal spots the heat load generated by a high-energy electron beam is distributed over a wide region on the anode surface. 
     According to a further embodiment of the invention the X-ray tube further comprises a spring element, which is arranged in between the anode and an envelope of the X-ray tube. This may provide the advantage that in particular a harmonic oscillation of the anode can be provided easily by means of simple elastic elements. 
     Preferably, the X-ray tube comprises at least two spring elements whereby a first spring element is attached to an upper portion of the anode and a second spring element is attached to an lower portion of the anode. This may provide the advantage that apart from providing an oscillatory movement the two springs may also contribute to a stable guidance of the anode parallel to the z-axis. Therefore, an unaccepted tilting of the anode may be prevented in a simple and effective manner. 
     It has to be pointed out that the spring element may be realized by mechanical and/or by electric respectively magnetic devices. For instance magnetic spring elements have the advantage that an abrasion or deterioration is negligible. 
     According to a further embodiment of the invention the X-ray tube further comprising a drive means, which is coupled to the anode in order to generate and/or to maintain an oscillatory movement of the anode. The drive means may be coupled mechanically and/or magnetically to the anode. A pure magnetic coupling has the advantage that the drive means may be realized without any movable mechanical parts. 
     According to a further embodiment of the invention the drive means is adapted to oscillate the anode with a frequency being essentially equal to a resonance frequency of the oscillating anode. This has the advantage that only little forces are needed to keep the anode oscillating at the desired frequency. Therefore, an essentially discrete switching of the focal spot positions may be realized without using complex mechanical apparatuses. 
     In this respect it is clear than apart from the mass respectively the weight of the anode also the spring constant of the spring element has a strong influence on the resonance frequency. Therefore, by taking into account the moving mass, the spring element or the spring elements have to be designed such that the resonant frequency of the system matches a predetermined focal spot frequency. In this context the focal spot frequency designates the frequency with which the focal spot is discretely switched between the first focal spot position and the second focal spot position and vice versa. 
     According to a further embodiment of the invention the drive means is adapted to oscillate the anode with a frequency being slightly bigger than a resonance frequency of the oscillating anode. This has the advantage that undesired anode vibrations may be reduced and that the anode may be oscillated in a piston like manner predominately along the z-axis. 
     Preferably, the oscillation frequency being slightly bigger than the resonance frequency is defined with respect to a curve exhibiting the resonance behavior of the oscillating system as a function of the driving frequency. Typically, the resonance behavior can be well approximated by a Lorenz curve having a maximum ω M  and a width Δω. Thereby, the width Δω strongly depends on the damping of the oscillating system. 
     Oscillating the anode with a frequency being slightly bigger than the resonance frequency means that the anode is oscillated with a frequency within a predetermined frequency range being defined by a lower frequency ω 1  and an upper frequency ω 2 . Thereby, ω 1 , may be equal to ω M  and ω 2  may be equal to ω M +Δω. Preferably, ω 1  is equal to ω M +Δω/20 and ω 2  is equal to ω M +Δω/2. More preferably, ω 1  is equal to ω M +Δω/10 and ω 2  is equal to ω M +Δω/4. 
     According to a further aspect of the invention there is provided a computed tomography system comprising (a) a rotatable holder being rotatable around a rotation axis and (b) an X-ray tube according to any one of the embodiments described above, wherein the X-ray source is mounted at the rotatable holder in such a manner that the z-axis is oriented essentially parallel to the rotation axis. The computed tomography system further comprises (c) an X-ray detection device comprising a plurality of detector elements, the X-ray detection device being mounted at the rotatable holder opposite to the X-ray source with respect to the rotation axis. 
     This aspect of the invention is based on the idea that the above-described X-ray tube may be used advantageously for computed tomography wherein digital image reconstruction is based on the acquisition of at least two attenuation datasets wherein each dataset has been obtained with a different projection angle with respect to the object under examination. The spatial resolution of a reconstructed image strongly depends on the spatial resolution of the X-ray detection device, i.e. the spatial separation of the detector elements. When an essentially discrete switching of the focal spot position is carried out, for each projection angle, i.e. for each angular position of the rotational holder, two X-ray attenuation datasets may be acquired. Thereby, each voxel of the object under examination is penetrated with two different angles such that switching the focal spot position yields more detailed information regarding the attenuation respectively the absorption of the object under examination as compared to a data acquisition with one focal spot only. 
     According to an embodiment of the invention the first focal spot position is spatially separated from the second focal spot position in such a manner that a first fan of X-rays originating from the first focal spot, crossing the rotation axis and impinging on a row of various detector elements is interleaved with a second fan of X-rays originating from the second focal spot, crossing the rotation axis and impinging on the row of various detector elements. 
     Preferably, the computed tomography system allows for a predominantly symmetric interleaving such that the sampling rate of X-ray attenuation data may be doubled. Thereby, neighboring X-ray rays crossing the center of rotation are separated from each other by a distance being half of the distance between neighboring X-ray in the case when only one focal spot is used. 
     It has to be pointed out that in particular when the two focal spots have predominantly the same or at least a similar radial distance with respect to the z-axis, the so-called half-row sampling, which corresponds to a symmetric interleaving, might be realized not only within a region corresponding to a small section of the rotational axis. The symmetric interleaving might rather be realized within a wide region along the rotation axis. 
     It has to be mentioned that it is not necessary that the computed tomography system employs an X-ray tube which generates a fan beam. The computed tomography system might also take benefit from a cone beam geometry wherein a two dimensional detector array is used in order not only to detect X-rays crossing the rotation axis but also to detect X-rays passing the rotation axis. Thereby, the interleaving being symmetric for X-rays crossing the rotation axis might degenerate with an increasing distance between the rotation axis and the X-ray passing the rotation axis. However, as compared to a single focal spot X-ray tube the sampling rate of X-ray attenuation data with the described dual focal spot X-ray tube will anyway be increased significantly such that images with a higher spatial resolution may be reconstructed. A further advantage compared to a single focal spot X-ray tube is the fact that so-called splay or windmill artifacts may be reduced. 
     According to a further aspect of the invention there is provided a method for operating an X-ray tube, in particular for operating an X-ray tube being used for computed tomography. The provided method comprises (a) moving an anode along a z-axis in an oscillating manner, wherein the anode comprises a surface being oriented oblique with respect to the z-axis, (b) directing an electron beam being emitted from an electron source along a beam axis such that the electron beam impinges onto a focal spot of the surface and (c) spatially controlling the beam axis by means of an electron deflection device in such a manner that the focal spot moves essentially in a discrete manner between a first focal spot position having a first z-coordinate and a second focal spot position having a second z-coordinate being different from the first z-coordinate. 
     This aspect of the invention is based on the idea that by combining two movements namely the oscillating movement of the anode along the z-axis and a radial variation of the focal spot on the surface being oriented oblique with respect to the z-axis an essential discrete z-switching of the focal spot may by achieved even if the at least one of the movements is carried out in a non discrete manner. Thereby, a discrete focal spot switching may be realized without any mechanical step-wise motion. This has the advantage that the essential discrete X-ray focal point switching might be realized with a very simple mechanical system, which need not to be designed such stable that the system is capable of withstanding abrupt momentum transfers or jerky leaps caused by a stepwise motion of the anode. 
     According to an embodiment of the invention the first focal spot position is spatially separated from the second focal spot position in such a manner that a first fan of X-rays originating from the first focal spot, crossing the rotation axis and impinging on a row of various detector elements is interleaved with a second fan of X-rays originating from the second focal spot, crossing the rotation axis and impinging on the row of various detector elements. 
     Preferably, the focal spot variation allows for a symmetric interleaving such that the sampling rate of X-ray attenuation data may be doubled. As has already been mentioned above, in the case of symmetric interleaving neighboring X-ray rays crossing the center of rotation are separated from each other by a distance being half of the distance between neighboring X-ray originating from a single focal spot only. 
     It has to be pointed out that in particular when the two focal spots have predominantly the same or at least a similar radial distance with respect to the z-axis, the symmetric interleaving might be realized not only within a small section of the rotation axis. The symmetric interleaving might rather be realized within a wide region along the rotation axis. 
     According to a further embodiment of the invention the anode is moved in a sinusoidal manner. This may provide the advantage that the anode carries out a smooth harmonic motion, which causes only a comparatively small momentum transfer to a suspension for the anode. This in turn may provide the advantage that the essential discrete X-ray focal point switching might be realized with a very simple mechanical system, which need not to be designed very stable. 
     It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to apparatus type claims whereas other embodiments have been 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. 
    
    
     
       The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited. 
         FIG. 1   a  shows a CT system according to a preferred embodiment of the invention in a simplified cross sectional view oriented perpendicular to a rotational axis. 
         FIG. 1   b  shows the X-ray beams originating from two different focal spots of the X-ray source of the CT system shown in  FIG. 1   a  in a simplified cross sectional view oriented parallel to the rotational axis. 
         FIG. 2  shows an X-ray generating tube comprising an oscillating anode and an electron beam deflection unit. 
         FIG. 3  shows a diagram depicting the discrepancy between an ideal step wise variation of the focal spot along the z-axis and an harmonic mechanic motion of the anode. 
         FIGS. 4   a  and  4   b  illustrate the influence of a radially varying focal spot position on the interleaving between a first fan of X-rays originating from a first focal spot and a second fan of X-rays originating from a second focal spot. 
     
    
    
     The illustration in the drawing is schematically. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit. 
       FIG. 1   a  shows a CT scanner  100  comprising a rotatable holder  101  in which an X-ray source  105  and an X-ray detection device  115  are incorporated. The holder  101  is rotated around a rotational axis  102  by means of a drive motor  104  and a drive mechanism. The drive mechanism is symbolized by means of three drive rollers  103 . The rotation of the holder  101  may be accomplished in a continuous or in a stepwise manner. 
     The CT scanner  100  further comprises a table  112 , which is arranged such that an object under examination  110  may be positioned in the center of the holder  101 . The table  112  may be movable with respect to the gantry  101  in a direction parallel to the rotational axis  102  such that different portions of the object under examination  110  can be examined. 
     The X-ray detection device  115  contains at least one row of interconnected detector elements, wherein the row extends parallel to the rotational axis. The detector elements can all be read out separately via a preamplifier  118  and a data processing device  125 . The data processing device  125  is capable of converting the measured detector signals. By measuring attenuation signals under a variety of different projection or viewing angles of the X-ray source  105  with respect to the object  110 , the data processing device  125  is capable of reconstructing a three dimensional representation of the object  110 . The reconstructed images may be outputted by means of a monitor  126  and/or by means of a printer  127 . 
     The data processing device  125  is further coupled with a motor control unit  120 , which is used for controlling the movement of the rotatable holder  101  in a rotation direction indicated by an arrow  117 . 
     The X-ray source  105  is an X-ray tube comprising an anode  106 . The anode  106  is elongated in a direction parallel to the rotational axis  102 . An electron beam emitted by a cathode, which is not indicated here, can be directed discretely onto one of two X-ray focal spots, onto a first X-ray focal spot  106   a  and onto a second X-ray focal spot  106   b . Preferably, these two focal spots  106   a  and  106   b  are oriented as close as possible in a row parallel to the rotational axis  102  such that in  FIG. 1   a  the two focal spots  106   a  and  106   a  cannot be visually discriminated from each other. As a consequence, also a first radiation beam  107  originating from the first X-ray focal spot  106   a  and a second radiation beam  108  originating from the first X-ray focal spot  106   b  can also not be discriminated from each other. 
     The data processing device  125  is further coupled with an electronic control unit (not depicted) in order to provide for a synchronization between the data acquisition and spatially switching the electron beam between the two focal points  106   a  and  106   b.    
       FIG. 1   b  shows an enlarged representation of the X-ray tube  105 , the object under examination  110  and the X-ray detection device  115  in a cross sectional view parallel to the rotational axis  102 . The two focal spots  106   a  and  106   b  are oriented in a row essentially parallel to the rotational axis  102 . A discrete switching of the X-ray focal spot between the two focal spots  106   a  and  106   b  has the effect that the object  110  is sequentially illuminated with the X-ray beams  107  and  108  under slightly different projection angles. Therefore, each detector element  116  of the X-ray detection device  115  can detect two different X-ray attenuation line integrals, a first line integral extending between the first focal spot  106   a  and the detector element  116  and a second line integral extending between the second focal spot  106   b  and the detector element  116 . As a consequence, for each projection angle of the system X-ray source  105  and X-ray detection device  115  with respect to the object  110  i.e. for each angular position of the gantry  101  two different datasets may be acquired which can be combined in an appropriate manner such that the spatial resolution of the CT scanner  100  can be enhanced. 
       FIG. 2  shows an X-ray tube  205 , which is adapted to generate X-rays originating from different X-ray focal spots. The X-ray tube  205  comprises an anode  206  having a shaft  230 . The shaft  230  is guided in such a manner that the shaft  230  may be both shifted linearly along a z-axis and rotated around the z-axis. A rotational drive  231  is provided in order to allow for a rotational movement of the anode  206 . In order to allow for a linear movement of the anode  206  an oscillatory drive  241  is provided. Both drives  231  and  241  may interact with the shaft  230  by means of a mechanical and/or a magnetic interaction. 
     The X-ray tube  205  further comprises an electron source  250 , which is arranged laterally with respect to the z-axis. According to the embodiment described here, the electron source is a hot cathode  250 , which during operating generates an electron beam  255 . The electron beam impinges onto a top surface of the anode  206 . Thereby, a focal spot is defined. The top surface is oriented oblique with respect to the z-axis such that from the focal spot an X-ray beam  258  projects radially outwards from the z-axis. 
     In order to control the exact position of the focal spot the X-ray tube  205  further comprises an electron deflection device  256 , which is adapted to deflect the electron beam  255 . The electron deflection device  256  may be realized by known electron optic elements such as e.g. magnetic lenses. The electron deflection device  256  is coupled to a control unit, which provides the necessary electric signals to the electron deflection device  256 . 
     Further, the X-ray tube  205  comprises two spring elements  240   a  and  240   b , which are attached to an upper end of the shaft  230  and to a lower end of the shaft  230 , respectively. The spring elements, which may be realized by mechanical and/or magnetic means, are also attached to a not shown support structure of the X-ray tube  205 . The support structure may be for instance an envelope of the X-ray tube  205 . 
     The system anode  206  and the two spring elements  240   a  and  240   b  represent an harmonic oscillator having a resonance frequency which is given by the mass of the anode and by the spring constants of the spring elements  240   a  and  240   b . Therefore, the anode  206  will preferably exhibit a sinusoidal motion along the z-axis. However, it is clear that also a non-perfect sinusoidal movement of the anode  206  may be enforced by the oscillatory drive  241 . However, the stronger the discrepancy between the real movement and a perfect sinusoidal movement is, the bigger are the mechanical forces which act on the support structure of the X-ray tube. This has the effect that it will be become very difficult to control a movement deviating strongly from a harmonic motion. 
     However, when the described X-ray tube  205  is supposed to be used as a dual focus X-ray tube it is desirable that the z-coordinate of the focal spot does not move in a continuous manner. In order to acquire two different X-ray attenuation datasets under slightly different projection angles and to reduce smearing effects in between these two datasets it is rather desirable that the focal spot moves at least essentially in a discrete manner between two focal spots. 
       FIG. 3  shows a diagram depicting the discrepancy between an ideal step wise variation  360  of the focal spot along the z-axis and an harmonic z-motion  361  of the anode  206 . This discrepancy is illustrated by a double-headed arrow. It can be recognized that the discrepancy periodically varies in a synchronized manner with the harmonic motion. Of course, the overall discrepancy will be minimized when the period of the harmonic motion is selected such that it is equal to the period of the step wise z-motion  360 . 
     According to the embodiment described here the discrepancy between the step wise motion  360  and the harmonic motion  361  is compensated by an appropriate deflection of the electron beam  255  such that also a radial movement of the focal spot contributes to a variation of the z-coordinate of the focal spot. In other words, by adequately operating the anode movement and the radial movement of the focal spot in a synchronized manner, the contribution of the moving anode to the focal spot movement along the z-direction and the contribution of the radial electron beam deflection to the focal spot movement along the z-direction can be superimposed in such a manner that an essentially discrete switching of the focal spot along the z-direction may be achieved. 
     It has to be mentioned that in order to achieve a focal spot variation, which is discrete as much as possible, it might be preferable to generate an oscillatory movement of the anode  206 , which slightly deviates from a perfect sinusoidal movement. Thereby, in order to generate an essentially discrete movement of the focal spot the contribution of the anode movement may be increased and the contribution of the radial electron deflection may be decreased. 
     By contrast to known techniques for a quasi discrete switching of an X-ray focal spot by means of electron beam deflection only, the described X-ray tube  205  has the advantage that the radial focal spot variation is reduced. In the following this advantage will be described with reference to the  FIGS. 4   a  and  4   b.    
       FIGS. 4   a  and  4   b  illustrate the influence of a radially varying focal spot position on the interleaving between a first fan of X-rays  407  originating from a first focal spot  406   a  and a second fan of X-rays  408  originating from a second focal spot  406   b.    
     As can be seen from  FIG. 4   a , a variation of the focal spot position which occurs not only along the z-axis but which occurs also radially with respect to the z-axis has an unwanted side effect. Thereby, when varying the focal spot position with Δz the radial distance between the rotational axis  402  and the focal spot position changes from R 1  to R 2  or vice versa. This unwanted effect causes that an interleaving of X-rays  407  originating from the first focal spot  406   a  with X-rays  408  originating from the second focal spot  406   b  occurs within a small region  470   a  only. This region  470   a  extends along a comparatively short section of the rotational axis  402 . 
     Interleaving, which is a known procedure in order to enhance the spatial resolution, is based on the fact that neighboring X-ray rays, which originate from different focal spots, which cross the rotational axis  402  and which impinge on the same detector element  416  of the X-ray detection device  415 , are separated from each other by a distance being half of the distance between neighboring X-rays, which originate from one focal spot only and which impinge on neighboring detector elements  416 . In the case of a symmetric interleaving the sampling rate of X-ray attenuation data may be doubled. 
     As can be seen from  FIG. 4   b , a variation of the focal spot position occurring predominately only along the z-axis has the advantage that the corresponding interleaving region  470   b  is much bigger than the reduced interleaving region  470   a . Due to the constant radial position R of both focal spots  406   a  and  406   b  with respect to the rotational axis  402  a symmetric interleaving may be realized within the comparatively big region  470   b  extending along the z-axis. 
     It has to be pointed out that when using the above-described X-ray tube  205  one can achieve an essential step wise z-variation of the focal spot, whereby the radial variation of the focal spot position can be minimized. Therefore, the above-described X-ray tube  205  allows for an improved interleaving and as a consequence for acquiring X-ray attenuation data with an improved spatial resolution. 
     It has to be mentioned that although the enhanced interleaving effect has been described with reference to a fan beam wherein all rays cross the rotational axis  402 , it is also possible to take benefit from a cone beam geometry wherein a two dimensional detector array is used in order to not only detect X-rays crossing the rotation axis but also to detect X-rays passing the rotation axis in a predetermined distance. Thereby, the interleaving being symmetric for X-rays crossing the rotation axis might degenerate with an increasing distance between the rotation axis and the X-ray passing the rotation axis. However, as compared to a single focal spot X-ray tube the sampling rate of X-ray attenuation data with the described dual focal spot X-ray tube will anyway be increased significantly such that X-ray images with a higher spatial resolution may be provided. 
     It should be noted that the term “comprising” does not exclude other elements or steps and the “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims. 
     LIST OF REFERENCE SIGNS 
     
         
         
           
               100  computer tomography apparatus/CT scanner 
               101  rotatable holder/gantry 
               102  rotational axis 
               103  drive rollers 
               104  drive motor 
               105  X-ray source 
               106  anode 
               106   a  first X-ray focal spot 
               106   b  second X-ray focal spot 
               107  first radiation beam 
               108  second radiation beam 
               110  object under examination 
               112  table 
               115  X-ray detection device 
               116  detector elements 
               117  rotation direction 
               118  preamplifier 
               120  motor control unit 
               125  data processing device (incl. reconstruction unit) 
               126  monitor 
               127  printer 
               205  X-ray source/X-ray tube 
               206  anode 
               230  shaft 
               231  rotational drive 
               240   a/b  spring elements 
               241  oscillatory drive 
               250  electron source/hot cathode 
               255  electron beam 
               255   a  focal spot 
               256  electron deflection device 
               257  control unit 
               258  X-ray beam 
               360  ideal stepwise z-motion 
               361  harmonic z-motion of anode 
               402  rotational axis 
               406   a  first X-ray focal spot 
               406   b  second X-ray focal spot 
               407  first radiation beam 
               408  second radiation beam 
               415  X-ray detection device/row of detector elements  416   
               416  detector element 
               470   a  interleaving region (small) 
               470   b  interleaving region (big) 
             Δz focal spot variation along the z-axis 
             R 1  radial distance between the rotational axis  402  and the first focal spot  406   a    
             R 2  radial distance between the rotational axis  402  and the second focal spot  406   b    
             R radial distance between the rotational axis  402  and both focal spots  406   a ,  406   b