FIELD OF THE INVENTION

The present invention relates to an X-ray tube for radiographic imaging, an X- ray imaging system for examining a region of interest of an object, a method for radiographic imaging of an object, as well as a computer program element and a computer readable medium.

BACKGROUND OF THE INVENTION

In radiographic imaging, usually a large field of view is illuminated during an examination to provide detailed information relating to a narrow region of interest. However, the peripheral part of the field of view is usually provided for reference only. In case of X-ray imaging of a patient, a reduction of the photon flux to the periphery can save patient dose. US 6,542,576 B2 describes a beam shaping filter attenuating the X-ray beam appropriately to reduce the X-ray dose to the patient. SUMMARY OF THE INVENTION

However, it has been shown that moving parts in the beam for filtering the beam means the integration of complex mechanical means.

There may be a need to provide radiographic imaging with reduced object dose which can easily be integrated in the work flow.

The objective 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 X-ray tube, the X-ray imaging system, the method, the computer program element and the computer readable medium.

According to an exemplary embodiment of the invention, an X-ray tube for radiographic imaging is provided, comprising a cathode, an anode, means for deflection of an electron beam, an X-ray aperture with an X-ray transparent opening, and an envelope housing the cathode and the anode. The means for deflection are adapted such that the electron beam from the cathode can be deflected such that the electron beam hits the anode in at least two focal spot positions, wherein the at least two focal spot positions are arranged with different distances to the opening in the X-ray aperture. The aperture is fixedly arranged inside the envelope. An X-ray beam is generated at each focal spot, wherein the X-ray beam is emanating in a viewing direction. At least for one focal spot the X-ray beam is emanating through the opening in the X-ray aperture.

According to an exemplary embodiment, the aperture is provided in addition to an X-ray window provided in the envelope of the X-ray tube. The aperture is arranged inside the envelope, whereas the X-ray window is provided within the envelope allowing the radiation to pass through the envelope, which envelope is X-ray opaque except for the X-ray window.

According to an exemplary embodiment of the invention, the means for deflection are adapted such that the electron beam can be deflected in the viewing direction and/or in a direction which is perpendicular to the viewing direction and perpendicular to a slit direction, wherein the opening in the X-ray aperture is provided as a slit.

According to an exemplary embodiment of the invention, an X-ray imaging system for examining a region of interest of an object is provided, comprising an X-ray tube according to one of the above described exemplary embodiments, a detector and a processing unit. The processing unit is adapted to control the emission of an electron beam from a cathode, and to control the means for deflection. The detector is adapted to provide image data about a region of interest of an object to the processing unit.

According to an exemplary embodiment of the invention, a method for radiographic imaging of an object is provided, comprising the following steps:

a) radiating an object with a first X-ray beam delivered by a first focal spot; b) radiating an object with a second X-ray beam delivered by a second focal spot; and

c) detecting X-ray radiation with a detector arranged behind the object;

During step a) means for deflection are deflecting the electron beam from a cathode such that the electron beam hits an anode in the first focal spot position. During step b) the means for deflection are deflecting the electron beam from the cathode such that the electron beam hits the anode in the second focal spot position. The at least two focal spot positions are arranged with a different distance to an opening in an X-ray aperture, wherein the aperture is fixedly arranged inside an envelope housing the cathode and the anode.

During step a) and/or step b) an X-ray beam is generated emanating in a viewing direction through the opening in the X-ray aperture towards the detector. According to an exemplary embodiment of the invention, the X-rays in step a) are provided as a first sub-pulse, and the X-rays in step b) are provided as a second sub-pulse, wherein the first and the second sub-pulse are provided with different X-ray energies.

According to an exemplary embodiment of the invention, the first sub-pulse is provided with the first focal spot being arranged with a smaller distance to the opening than the second focal spot, and wherein the first sub-pulse is provided with a lower energy than the second sub-pulse.

It can be seen as the gist of the invention to provide an adaptable focal spot in combination with a fixed aperture close to the focal spot. By adapting the focal spot position, as well as the focal spot shape, it is possible to shape the X-ray beam. An X-ray beam generated from one of the at least two focal spot positions is provided with a wider angle to radiate a field of view. A second X-ray beam from another focal spot provides image information of a region of interest only. The change between the at least two beam shapes is achieved with deflection means as electron beam manipulation means. Thus, image information in the field of view, can be provided applying a lower X-ray dose, wherein the region of interest, being a portion of the field of view, can be radiated with a higher dose to receive the desired image information.

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 another exemplary embodiment of an X-ray imaging system according to the invention.

Fig. 3 schematically shows a perspective view of an X-ray tube for radiographic imaging according to the invention.

Figs. 4 and 5 schematically show aspects for generating X-ray beams at different focal spot positions in a cross-section according to an exemplary embodiment of the invention. Fig. 6 schematically shows further aspects for generating different X-ray beams in a cross-section according to an exemplary embodiment of the invention.

Fig. 7 schematically shows further aspects for generating different X-ray beams in a view perpendicular to the cross-sections of Figs. 4 to 6.

Figs. 8 and 9 show further aspects for generating different X-ray beams in a view according to Fig. 7 according to an exemplary embodiment of the invention.

Figs. 10 and 11 show further aspects of an embodiment according to the invention in relation with X-ray flux and patient dose.

Fig. 12 schematically illustrates aspects according to an exemplary embodiment of the invention relating to projection shift.

Fig. 13 shows further aspects of an exemplary embodiment of the invention.

Figs. 14 and 15 show further exemplary embodiments according to the invention.

Figs. 16 to 19 schematically illustrate method steps of exemplary embodiments of methods according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 illustrates an X-ray imaging system 10 for examining a region of interest of an object according to the invention. An X-ray tube 12 and a detector 14, as well as a processing unit 16 are provided.

The processing unit 16 is adapted to control the emission of an electron beam from a cathode and to control means for deflection, which will be described in relation with

Figs. 3 et seq. The detector 14 is adapted to provide image data about a region of interest of an object to the processing unit 16.

For example, the X-ray tube 12 and the detector 14 are arranged on opposing ends of a C-arm structure 18 which is movably mounted by a support 20 which allows sliding movements of the C-arm 18 and rotating movements of the C-arm 18. Further, a table 22 is provided to receive an object of interest 24, for example a patient.

Further, a display unit 26 is arranged in the vicinity of the table 22 such that image information 28 as well as the input of control commands by a staff member is provided. The display unit 26 can be integrally formed with the processing unit. For example, the processing unit 16 and the display unit 26 can be suspended from a ceiling of an examination room with a hanging support 30. Further, it should be noted that the support 20 for the C-arm structure 18 can also be supported by a not further shown hanging support, suspending the C-arm arrangement from a ceiling. Of course, it can also be provided as a standing support, or mounted to a wall.

The object of interest, for example the patient 24, can be arranged between the X-ray tube 12 as an X-ray source and the detector 14 such that X-ray radiation emanating from the X-ray tube towards the detector passes the object of interest, whereby the X-ray radiation is attenuated accordingly. Thus, image information of the object of interest can be provided.

It must be noted that the C-arm is only shown as an example. Of course, other X-ray image acquisition systems are also possible, either with static X-ray sources or with movable sources and detectors such as a CT with a circular gantry.

For example, Fig. 2 shows a gantry 32 as a further example for an X-ray imaging system 10. The gantry is provided with the X-ray tube 12 and the detector (not further shown), mounted rotatably such that a circular movement around an object of interest, for example the patient 24, is possible in order to receive image information for different viewing directions, and in particular to receive a plurality of 2D image data to reconstruct three-dimensional volume data.

Fig. 2 schematically shows an example of the above-mentioned X-ray tube in a perspective view. The X-ray tube 12 comprises a cathode 50 and an anode 52. The anode 52 can be provided as a rotating anode, as shown in Fig. 2. Further, means for deflection 54 for deflecting an electron beam 56 are provided, but not further shown in Fig. 3, but in Figs. 4 and 5.

The X-ray tube 12 also comprises an X-ray aperture 58 with an X-ray transparent opening 60. Further, an envelope 62 housing the cathode and the anode is provided.

The means for deflection 54 are adapted such that the electron beam from the cathode towards the anode can be deflected such that the electron beam hits the anode in at least two focal spot positions 62, 64, wherein the at least two focal spot positions are arranged with different distances to the opening 60 in the X-ray aperture 58, which is indicated with reference numeral 66.

The aperture 58 is fixedly arranged inside the envelope 52.

An X-ray beam is generated at each focal spot, wherein the X-ray beam is emanating in a viewing direction 68. For example, a first X-ray beam 70 is generated from a first focal spot position 62, which X-ray 70 is indicated with its shape with two dotted lines 72. Further, a second X-ray beam 74 can be generated at a second focal spot position 64, which second X-ray beam shape is indicated with further dotted lines 76.

The X-ray beam for at least one focal spot is emanating through the opening in the X-ray aperture.

According to the example shown in Fig. 3, both X-ray beams 70, 74 are emanating through the opening 60 in the X-ray aperture 58.

With reference to Fig. 4, the aspect of deflecting an electron beam will be explained further.

Fig. 4 schematically shows the anode 52 in form of a rotating anode disk, indicated with an axis of rotation 78 as well as a circular arrow 80. The anode 52 comprises a disk body 82 with an inclined surface portion 84 arranged in a circular manner in the vicinity of the outer edge. For example, the inclined surface can have an angle 85 to a line perpendicular to the rotating axis 78, i.e. an angle 85 in relation with the radial direction.

According to the invention, an electron beam 56 from a cathode (not further shown) in the direction of the anode 52 can be deflected by the means for deflection 54, which are only schematically shown.

With the means of deflection 54, for example, it is possible to hit the anode 52 at a first focal spot position FSPl, and at a second focal spot position FSP2, being spaced apart from each other with a distance 86. The deflected electron beam for the first focal spot position FSPl is indicated with an arrow 88, the electron beam hitting the second focal spot position FSP2 is indicated with a further arrow 90.

Since the anode surface is inclined, as mentioned above, the two focal spot positions FSPl, FSP2 not only have a distance 68 in radial direction to each other, but also a different height 92.

As indicated in the lower left part of Fig. 4, in case of a rotating anode, the radial direction is also referred to as r-axis 94, the direction of the rotating axis is also referred to as z-axis 96. The direction perpendicular to both the z-axis 96 and the radial direction, or r-axis 94, is referred to as x-axis 98.

Fig. 5 shows a further exemplary embodiment for an anode disk 52 having a disk body 82. Instead of the inclined surface 84, the anode disk shown in Fig. 5 has a stepped edge portion 100 with a first step 102 and a second step 104. For example, the first focal spot position FSPl can be arranged on the first step 102, and the second focal spot position FSP2 can be provided on the second step 104. For example steps and inclined conical parts may be combined. As mentioned above in relation with Fig. 4, the means for deflection 54 are provided to deflect the electron beam 56 such that the electron beam hits the respective focal spot positions FSPl, FSP2.

Of course, it is also possible to provide an anode disk with a stepped edge portion with a plurality of steps. According to a further aspect, not shown, the steps can also be provided with inclined surface portions.

In the following, the invention shall be described in more detail with reference to Fig. 6. As can be seen, the X-ray aperture 58 with the X-ray transparent opening 60 is provided in close vicinity to the anode disk 52. The X-ray transparent opening 60 is provided as a slit having a slit opening shaped in the direction of the z-axis 96.

In case of the electron beam 56 hitting the first focal spot position FSPl, a first X-ray beam 106 is generated emanating through the opening 60 in the X-ray aperture 58.

Further, in case of the electron beam 56 hitting the second focal spot position FSP2, a second X-ray beam 108 is generated emanating through the opening 60 in the X-ray aperture 58.

As already mentioned above, the aperture 58 is provided in addition to an X- ray window 110 provided in the envelope 62 of the X-ray tube.

The aperture 58 is arranged inside the envelope.

Further, the aperture 58 may be provided with a cooling 113 to dissipate heat from the aperture 58. It is noted that this is an option for the other described embodiments, too. However, the aperture 58 basically functions also without the aperture-cooling, which is why the cooling 113 is connected to the aperture 58 with a dotted line.

Due to the different locations of the focal spot positions FSPl, FSP2, the first and second X-ray beams 106, 108 hit different areas on a detector 112.

Since Fig. 6 shows a cross-section in the z-direction, the effect of the different detector areas is shown in relation with the z-direction only.

For example, Fig. 7 schematically illustrates the aspect of different detector areas with a top view on the anode 52. Of course, the term "top view" only relates to the view on the anode and not the actual orientation of the anode in space when in operation. As mentioned above, the first and second focal spot positions FSPl, FSP2 are provided with the distance 86 to each other in radial direction. Thus, different distances 66 to the opening 60 in the aperture 58 are provided, for example FSPl with first radial distance 114, FSP2 with a second radial distance 116. Because of the different radial distances to the opening 60, it is possible to generate the first X-ray beam 106 having a smaller width Wl, indicated with an arrow 118, and the second X-ray beam 108 with a second width W2, indicated with an arrow 120. Thus, different area sizes on the detector 112 are hit by the respective X-ray beams.

With reference to Fig. 6, it should be noted that due to the different heights on the inclined anode surface, the two X-ray beams 106, 108 are provided with different orientations in the z-direction. The different heights are indicated with an arrow 121, also labelled with Δζ, wherein the radial distance 86 is indicated with ΔΓ.

For example, the anode disk can be provided with a focal track. As an example, Fig. 7 shows a first focal track 122 for the first focal spot position FSP1 and a second focal track 124 for the second focal spot position FSP2.

For example, in case the focal spot size is 0.3 (IEC) at an inclination of the anode surface of 7 degrees, this would mean a real length of 5 mm. When the distance between the focal spot and the aperture is 5 mm, the anode diameter being 20 cm, and a Δ¾ of 2 cm, i.e. a distance between the first and the second focal spot position of 2 cm, this would lead to a width Wl of the region of interest of 13 cm on the detector, and a width W2 of the field of view of 40 cm in a given distance between the detector and the aperture.

According to a further exemplary embodiment, although not shown, a single focal track is provided for both focal spot positions.

According to a further example (not shown), a number of focal spot positions is provided, for example two or more focal spot positions.

According to an example, a respective number of focal tracks on the anode disk are provided.

As mentioned above, the X-ray aperture is provided in a peripheral position, close to the focal track.

According to a further example, not shown, the X-ray aperture is provided close to and on top of the focal spot track.

For example, the X-ray aperture is provided between the first and the second focal spot position, wherein a second X-ray aperture with a second transparent opening is provided such that the X-ray beam of the first and the second focal spot is emanating to the second opening in the X-ray aperture, wherein the X-ray beam of the first focal spot is emanating to the X-ray aperture, as well. According to a further example, not shown, at least two focal spot positions are each arranged with a different radial distance with a different height along a height direction parallel to the rotating axis, as shown in Fig. 6, for example.

According to a further example, the at least two focal spot positions are each arranged with a direction radial distance, but having the same height along a height direction parallel to the rotating axis. For example, the anode disk is provided with segments with focal track portions having the same height for different radial distances (not shown).

For example, the anode rotation is synchronized with an X-ray pulsing (see also below).

As indicated above, the opening in the X-ray aperture is provided as a slit 126, as indicated in Fig. 8, which slit is arranged in a z-direction perpendicular to the viewing direction. In case of a rotating anode, the slit is arranged parallel to the rotating axis.

According to a further aspect of the invention, the means for deflection, which are not further shown in Figs. 7, 8, and 9, are adapted such that the electron beam can be deflected in the viewing direction, as mentioned above, and/or in an x-direction, which X- direction is perpendicular to the viewing direction. The x-direction is also perpendicular to the z-direction, i.e. perpendicular to the rotating axis. This is indicated with a respective axis diagram in Fig. 9, but is also applicable for Fig. 8.

For example, the focal spot can be provided to be minimal in a direction orthogonal to a line between the focal spot and the centre of the region of interest.

As can be seen, the two focal spot positions are arranged with different distances to the opening in the X-ray aperture in viewing direction and with different lateral distances to the viewing direction.

With reference to Fig. 9, the first focal spot position FSPl and the second focal spot position FSP2 are provided with different radial distances in relation to the aperture 58, wherein the difference, i.e. the distance between two focal spot positions, is indicated with an arrow 128, also indicated with Δ¾. Further, the first focal spot position FSPl is also deflected with an offset 130 to the rotating axis 78, which is also indicated with Axf _s .

Thus, it is possible to shift a region radiated by the first X-ray beam 106 with respect to the central viewing axis, for example the bisecting line of the field of view.

According to a further exemplary embodiment of the invention, the means for deflection are adapted such that the focal spot can be provided with a focal spot shape 131 in which a projected length 132 is larger than a projected width 134. For example, the projected length and the projected width have a ratio of larger than 3/1.

According to a further exemplary embodiment, also shown in Figs. 8 and 9, the means for deflection are adapted such that the focal spot can be skewed, which is shown for the first focal spot position FSP1.

It is noted that the skewing of the focal spot is not necessary for the other aspects shown in Figs. 8 and 9. Of course, it is also possible to skew the second focal spot in the second focal spot position. Thus, it must be noted that the offset in lateral direction to the viewing direction can be applied in combination with the skewing However, according to the invention, the skewing can also be applied without lateral offset, and the lateral offset can be applied without the skewing.

It is further noted that more than two focal spots can be provided.

According to the invention, the angulation, the size, and/or length of the first focal spot are different from the angulation, size, and/or length of the second focal spot.

According to a further exemplary embodiment, not shown, the shape of the X- ray transparent opening itself and/or the edge of the X-ray transparent opening in the X-ray aperture is adapted such that the beam is further shaped.

For example, the X-ray aperture is provided with a plurality of different opening shapes and/or edge formations, such that an opening shape and/or edge formation can be selected for a particular application.

According to a further example, the beam is shaped by the geometry of the X- ray aperture, i.e. the form of the X-ray aperture.

According to a further example, the beam is shaped by the edge profile surrounding the opening of the X-ray aperture.

According to a further example, the beam is shaped by the material thickness of the X-ray aperture, for example the thickness of the X-ray opaque material as well as the thickness of the X-ray transparent material forming the opening.

For example, a chamfered or slanted edge of the opening can provide a shape of the beam at least at the edge zones of the beam.

According to a further example, the material for the opening in the aperture shield can be adapted to provide a spectral filtering, e.g. of the inner and/or outer areas or zones of the beam. For example, the opening comprises a stepwise or continuously adapted material with different filtering characteristic. As further indicated in Figs. 8 and 9, the second X-ray beam 108 defines the field of view, and image of which, indicated with reference numeral 136 is shown below in Fig. 8. The first X-ray beam 106 defines a region of interest, which is indicated with a line frame 138. For example, a stent 140 is located in the region of interest and thus detailed information about the region of interest is required, whereas the field of view surrounding the region of interest needs to be shown only with fewer details, i.e. with less information.

Thus, the second X-ray beam 108 can be provided with lower energy in order to save dose, wherein the first X-ray beam 106 is provided with more energy than the second X-ray beam in order to provide sufficient information.

A further aspect is also shown in Figs. 8 and 9 with second dotted lines 142 indicating the effect that in case of a shaped focal spot having a length in the viewing direction, the portion of the focal spot arranged closer to the aperture leads to a wider angle of the second X-ray beam, indicated with the second dotted lines 142, whereas the portion of the focal spot arranged away from the aperture leads to a smaller fan shape of the second X- ray beam 106, which is indicated with third dotted lines 144.

Thus, the dosage applied to the region of interest has a shape with inclined edges, as indicated in the lower part of Fig. 9. A first base part 146 and a second base part 148 indicate the dosage resulting from second X-ray beam 108 covering the field of view 136. A central part 150 with a higher extension is having a first inclined side edge 152 and a second inclined side edge 154, indicating the dose resulting from the radiation with the first X-ray beam 106 covering the region of interest 138. In the lower part of Fig. 9, the dosage value is indicated with a vertical axis 156.

Of course, the dose relation shown in Fig. 9 is shown only for illustrating purposes and is not to scale, which is also the case for the drawings described above and below.

According to the invention, image information about the object is provided with at least two X-ray beams. In Figs. 10 and 11, this is compared with a conventional single X-ray beam radiation.

In Fig. 10, a vertical axis 158 indicates power, or X-ray flux respectively. The horizontal axis, indicated with reference numeral 160, indicates time t. In a conventional X- ray radiation, a single pulse 162 hits, for example, a field of view with 40 cm by 40 cm. For example, the pulse is applied for 4 ms (milliseconds).

According to the invention, for providing image information for the field of view, a first pulse, or fie ld-of- view pulse 164 is applied with low energy and low dose for the field of view having a size of 40 cm by 40 cm. For example, this is applied for the duration of 1 ms. After that, a transition period 166 is provided, for example for a duration of 0.5 ms. Then, a second pulse, or region-of- interest pulse 168 is provided with elevated energy and X- ray flux density, however sent to a limited region of interest only. For example, the region of interest comprises an area size of 13 cm by 40 cm. The region-of- interest pulse 168 is applied, for example, for 3 ms.

This leads to respective patient dose with a rising value along a vertical axis 170, as indicated in Fig. 11. A first square 172 indicates the patient dose of the conventional pulse 162 in Fig. 10. A first rectangle 174 indicates the patient dose of the field-of-view pulse 164, and a second rectangle 176 indicates the patient dose resulting from the region-of- interest pulse 168.

In the right part of Fig. 11, the square 172 representing the dose of a conventional X-ray pulse is compared with the dose resulting from the two different pulses and their rectangles 174 and 176 according to the invention. As can be seen, a delta surface area 178 indicates the difference, i.e. the delta area represents a dose saving of approximately 50 %, while providing the same image quality for the region of interest.

Fig. 12 indicates a further aspect of the invention. A projection shift between a first focal spot position FSP1, indicated with a first dotted line 180, and a second focal spot position FSP2, indicated with a second dotted line 182, which projection shift is also indicated with arrow 184 representing Δ¾, leads to a change of the projection for all objects which are off the centre axis of the field of view. If the region of interest is made small enough, and the focal spot is shifted to the region of interest centre beam, this effect can be made sufficiently small.

In Fig. 12, a further arrow 186 indicates the distance between the first focal spot position and the object, also referred to as SOD. A still further arrow 188 indicates the distance between the first focal spot position and the detector, also referred to as SID. The region of interest has a width 190, also referred to as bRoi. Further, an arrow 192 indicates the projection shift difference Δρ in the detector plane. For determining the shift difference Δρ, the following equation can be applied:

Ap ~ b _RO i * Ar _fs * (M-l) / (2 * SOD)

For example, in case of bRoi = 13 cm, Δ¾ = 2 cm, M = 1.3, SOD = 20 cm, this results in ΔΡ = 0.5 mm.

According to the invention, the projection shift between field of view FOV and region of interest ROI images is minimized by pixel shift in the detection system. For example, as already mentioned in relation with Fig. 9, in z-direction, this would be zero at anode shadow, whereas it would be maximum at other edge of the field of view. In x- direction, zero would appear in the region of interest centre, and + /- maximum the edges.

According to a further aspect, the resulting distortion due to the projection shift is smoothened by image processing.

According to the invention, it is also possible to modulate the beam flux at the edges of a radiated field of view. This is explained with reference to Figs. 13 following.

As mentioned above, the aperture 58 is provided close to focal spot positions on anode disk. The first focal spot position FSP1 is arranged away from the opening of the aperture 58, whereas the second focal spot position FSP2 is arranged closer to the opening in the aperture 58. According to a further aspect of the invention, it is possible to apply an X-ray beam to a region of interest while moving a focal spot from a first to a second position, which positions have a different distance in radial distance to the aperture 58. It is noted that the electron beam is hitting the anode disk during the transition of the focal spot. Thus, when pulling back the focal spot from the second focal spot position FSP2 to the first focal spot position FSP1, only the outer part of the focal spot contributes to the illumination of the edges of the radiation field, wherein the term "outer part" relates to the portions of the focal spot arranged closer to the aperture. Thus, the beam flux at the edges can be modulated, for example in both orthogonal directions.

As shown in Fig. 13, the focal spot is positioned closer to the aperture at FSP2 for a wide radiation field, and positioned closer to the rotating axis, i.e. with a larger distance to the aperture 58, for a narrow radiation field. A first pair of lines 194 indicates the radiation of a wide radiation field, and a further pair of lines 196 indicates the narrow field. Thus, a portion at the respective edges, indicated with a double arrow 198, has a modulated beam flux during the pulse, when the focal spot is pulled away from the aperture. Further, an arrow 200 indicates the range of deflection applicable to a feedback loop 202 provided to adapt the focal spot position in order to minimize the dose and achieve a sufficient signal-to-noise ratio on the detector. The dose is indicated in lower part of Fig. 13, wherein a portion 204 is having inclined side edges 206 indicating a reduced dose by reducing the radial distance ¾, indicated with arrow 208. In the diagram, a vertical axis 210 represents the X-ray flux, wherein the horizontal axis 212 represents the distance across the detector.

As mentioned above, the deflection in radial distance can create a shift of the centre position of the focal spot and can be considered during reconstruction of the image data. According to a further exemplary embodiment (not shown), the above- described "pulling back" of a focal spot, wherein the focal spot is translated from one position to another while hitting the focal spot with electrons for edge shape manipulation, is provided in addition to generating a second X-ray beam from a further focal spot position. In other words, for example, a field-of view X-ray beam is generated with a manipulated edge as in Fig. 13, and a region-of- interest X-ray beam is generated. The field-of-view X-ray beam can be provided from a first focal spot having a first or initial-position and a second or end- position; the region-of-interest X-ray beam can be provided from a second focal spot position.

According to a further example, the region-of-interest X-ray beam is provided with a translated or bulled-back focal spot and the field-of-view X-ray beam is having a fixed or non-pulled back focal spot.

According to a further example, the field-of-view X-ray beam as well as region-of-interest X-ray beam are provided with a translated or bulled-back focal spot.

In Fig. 14A, the anode disk 52 is shown with a section of the aperture 58.

Further, a focal spot 214 is indicated on the anode disk 52. In Fig. 14B, the anode disk 52 is seen from above, wherein the focal spot 214, for example, having a size of 1.5 mm by 10 mm leads to an X-ray fan beam 216 which is shown with a first pair of lines 218 representing the radiation resulting from the portions of the focal spot arranged away from the aperture 58, and a second pair of lines 220 representing radiation from the portion of the focal spot 214 arranged closer to the opening.

In Fig. 15 A, a further focal spot position 222 is shown on the anode disk 52. In Fig. 15B, it is shown that the radiation coming from the portion of the focal spot 222 closer to the opening in the aperture 58 is radiated through the opening. The radiation resulting from the portion of the focal spot 222 arranged away from the opening is hindered by the aperture to emanate with the same angle, which is indicated with dotted lines 224.

In other words, the X-ray fan beam is limited and the rays indicated with the dotted lines do not contribute to the detective signal, which leads to the above described effect shown in Fig. 13.

Fig. 16 schematically illustrates basic steps of a method 500 for radiographic imaging of an object, comprising the following steps: In a first radiation step 510, an object is radiated with an X-ray beam 512 delivered by a first focal spot. In a second radiation step 514, an object is radiated with an X-ray beam 516 delivered by a second focal spot. Further, in a detection step 518, X-ray radiation 520 is detected with a detector arranged behind the object. During the first radiation step 210, means for deflection are deflecting the electron beam from a cathode such that the electron beam hits an anode in the first focal spot position. During the second radiation step 514, the means for deflection are deflecting the electron beam from the cathode such that the electron beam hits the anode in the second focal spot position. The at least two focal spot positions are each arranged with a different distance to an opening in an X-ray aperture, wherein the aperture is fixedly arranged inside an envelope housing the cathode and the anode. During the radiation step 510 and/or radiation step 514, an X-ray beam is generated emanating in a viewing direction through the opening in the X- ray aperture towards the detector.

For example, a first focal spot is adapted to provide a wide field of view image, and the second focal spot is adapted to provide a narrow region of interest image.

For example, the second focal spot has a projected length which is larger than a projected width. As a further example, the second focal spot is angulated, and the X-ray beam emitting from the second focal spot has a centre line, which is arranged inclined to the viewing direction of the X-ray beam emitting from the first focal spot.

The first radiation step 510 is also referred to as step a), the second radiation step 513 as step b), and the detection step 518 as step c).

According to an exemplary embodiment, the X-rays in step a) are provided as a first sub-pulse 522 and the X-rays in step b) as a second sub-pulse 524. For example, the first and the second sub-pulse 522, 524 are provided with different X-ray energies.

It is noted that the first sub-pulse can be applied to be object before the second sub-pulse and vice- versa.

For example, as shown in Fig. 17, step a) and step b) are each provided within separate integration periods of the detector such that a first image data 526 and a second image data 528 are collected in step c). For example, the first and second image data 526, 528 are then combined in a combination step 530 to provide combined image data 532.

According to a further example, shown in Fig. 18, step a) and step b) are provided within an integration period of the detector such that integrated image data 534 is collected in step c).

According to a further exemplary embodiment, not shown, the first sub-pulse is provided with the first focal spot being arranged with a smaller distance to the opening than the second focal spot, wherein the first sub-pulse is provided with a lower energy than the second sub-pulse. It is noted that the terms "first" and "second" in relation with the sub-pulse do not necessarily relate to the positions of the focal spot as described in Figs. 3 et seq.

For example, the first sub-pulse is a field of view pulse, and the second sub- pulse is a region of interest pulse, wherein the region of interest is a sub-portion of the field of view.

According to a further exemplary embodiment, shown in Fig. 19, the second focal spot is positioned such that the region of interest is automatically centered around a marker, for example a predetermined landmark, catheter tip, bolus, stent marker or the like.

In step a), a first radiation step 510 is performed in which the object is radiated with a first X-ray beam, as described above. This leads to image information 536 detected by the detector, which image information is then supplied to an image content recognition sub- step 538 in which a predetermined marker element 540 is detected and in which information is provided for a region of interest being centered to the detected marker element. This information is then provided to the second radiation step 514, wherein the data providing is indicated with an arrow 542. As mentioned above, the object is radiated with an X-ray beam 516 delivered by a second focal spot. This leads to further image information 544 detected in step c). In the following, the image content 536 provided by the first radiation step 510 and the image content information provided by the second radiation step 514 can then be processed further, which is indicated with frame 546 in order to provide image information data 548, which, for example, can be displayed to a user.

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 fulfil 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.

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 fulfil 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.