Anode with unevenly distributed slots

Abstract

There is provided an anode disc (200) for use as rotating anode in an X-ray tube. The anode disc comprises a center region (220) including a center (210), wherein the center (210) intersects an axis of rotation when the anode disc is in use. The anode disc (200) further comprises an outer circumference (230) surrounding the center region (220), and at least three slots (201, 202, 203) for reducing stress when the anode disc is in use. Each slot extends from the outer circumference towards the center region. According to embodiments of the invention, the slots (201, 202, 203) are unevenly distributed along the outer circumference, such that each circumferential distance (c12, c23, c31) between two neighboring slots is different. By providing an anode with slots that are unevenly distributed along the circumference, the risk of patterns in X-ray intensity distribution is reduced. Furthermore, when images from multiple integration periods are combined to form a reconstructed image, possible disturbance from the unevenly or randomly distributed slots will be noise that can be filtered out or will even out, as compared to repeating patterns that may grow with each integration. Thus, imaging quality may be improved.

Description

FIELD OF THE INVENTION

[0001] The invention relates to an anode disc for use as a rotating anode in an X-ray tube, an X-ray tube comprising the anode disc, an X-ray imaging system, and a method of manufacturing the anode disc.BACKGROUND OF THE INVENTION

[0002] A rotating anode plate is implemented in an X-ray tube used in diagnostic medical equipment, such as computed tomography (CT) systems. Under normal operating conditions, the anode plate is subjected to large mechanical compression and tensile stresses resulting from the anode's high rotational speed, as well as extreme thermal loading resulting from heat generated from an incident electron beam impinging the anode's surface, in order to generate X-rays. These mechanical and thermal stresses degrade the anode surface, leading to, for example, cracking or warping of the anode plate over time. The usable lifetime of the anode, and accordingly, the X-ray tube, is reduced by these effects. To reduce or overcome degrading of the anode, slots that penetrate the thickness of the anode are commonly used.

[0003] US2009 / 086916A1 describes an anode plate for an X-ray tube. The anode plate includes an outer edge, a center region, and a plurality of slots disposed along the outer edge and extending toward the center region with each of the plurality of slots including a slot end. The anode plate further includes slot termination material disposed around a least a portion of the periphery of one or more of the slot ends, the slot termination material operable to reduce the tension stress or compression stress at the slot end.

[0004] Although the slots are helpful to reduce the risk of (thermo)mechanical failure of the anode, the slots may negatively impact X-ray imaging quality.SUMMARY OF THE INVENTION

[0005] It is, inter alia, an object of the invention to provide an anode disc for improved imaging quality when used in an X-ray tube in an imaging system. The invention is defined by the independent claims. Advantageous embodiments are defined in the dependent claims.

[0006] According to a first aspect of the invention, there is provided an anode disc for use as rotating anode in an X-ray tube. The anode disc comprises a center region including a center, wherein the center intersects an axis of rotation when the anode disc is in use. The anode disc further comprises an outer circumference surrounding the center region, and at least three slots for reducing stress when the anode disc is in use. Each slot extends from the outer circumference towards the center region. According to embodiments of the invention, the slots are unevenly distributed along the outer circumference, such that each circumferential distance between two neighboring slots is different.

[0007] The slots in the rotating anode provide improved thermomechanical stability, e.g. by preventing cracks in the anode due to thermal expansion. In conventional anodes used for X-ray imaging, such as the anode schematically illustrated in Fig. 1, slots may negatively influence image quality. Due to a correlation between the rotation frequency of the anode, hence the frequency of the slots passing the electron beam, and the sampling rate of the X-ray detector, the slots of such an anode may cause patterns in X-ray intensity distribution, which can lead to image artifacts. By providing an anode with slots that are unevenly distributed along the circumference, the risk of patterns in X-ray intensity distribution is reduced. Furthermore, when images from multiple integration periods are combined to form a reconstructed image, possible disturbance from the unevenly or randomly distributed slots will be noise that can be filtered out or will even out, as compared to repeating patterns that may grow with each integration. Thus, imaging quality may be improved.

[0008] To further improve thermomechanical stability, the anode disc preferably comprises more than three unevenly distributed slots, such as at least seven slots, or even more preferably at least nine slots.

[0009] The anode disc may comprise Tungsten or Molybdenum. The anode may comprise an alloy, such as Titanium Zirconium Molybdenum (TZM), Tungsten Rhenium etc. The anode disc may comprise a supportive backpack, such as a supportive backpack made of graphite, carbon-fiber-carbon composites, silicon carbide, or other ceramic materials. Each slot forms a thin opening in the anode disc. A slot may have a width in the range of 0.1 mm - 2 mm. A slot penetrates the anode disc from the 'front' side of the anode facing the electrode beam when in use in an X-ray tube, through the entire thickness of the anode or through only part of the thickness of the anode.

[0010] According to an embodiment, each slot extends radially from the outer circumference towards the center.

[0011] Radial slots may be particularly advantageous for improving thermomechanical stability. By having unevenly distributed radial slots, disturbance to the X-ray intensity distribution is reduced.

[0012] According to an embodiment, no two angles between radial directions of neighboring slots have a common divisor. In an embodiment, each angle between the radial directions of two neighboring slots may be a prime number.

[0013] By avoiding a common divisor between any two of the neighboring angles, the risk of intensity patterns due to the slots may be even further reduced. As an example, the following angles that are all prime numbers may be used between neighboring pairs of 8 slots (in any order), adding up to 360 degrees: 37 degrees, 41 degrees, 43 degrees, 47 degrees, 53 degrees, 59 degrees, 61 degrees, and 19 degrees.

[0014] According to a second aspect of the invention, there is provided an X-ray tube for use in an X-ray imaging system, wherein the X-ray tube comprises the anode disc according to the first aspect.

[0015] According to a third aspect of the invention, there is provided an X-ray imaging system. The X-ray imaging system comprises the X-ray tube according to the second aspect, and an X-ray detector for detecting X-rays.

[0016] In the X-ray imaging system, it is advantageous to use an X-ray tube with the anode disc as described above. It allows freedom in selection of anode rotation speeds and / or detector integration periods without causing disturbing patterns in the images.

[0017] According to a fourth aspect of the invention, there is provided a method for manufacturing the anode disc according to the first aspect, wherein the method comprises: receiving an intermediate anode disc without multiple slots; and creating the at least three slots in the intermediate anode disc to form the anode disc.

[0018] According to an embodiment, the at least three slots are created using electrical discharge machining.

[0019] Electrical discharge machining (EDM), such as wire EDM, allows to create the set of unevenly distributed slots with suitable precision in the anode disc.

[0020] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Fig. 1 schematically illustrates a known anode disc. Fig. 2 schematically illustrates an anode disc according to embodiments of the invention. Fig. 3 schematically illustrates another anode disc according to embodiments of the invention. Fig. 4 schematically illustrates another anode disc according to embodiments of the invention. Fig. 5 schematically illustrates an X-ray imaging system according to embodiments of the invention. Fig. 6 schematically illustrates a manufacturing method according to embodiments of the invention. DESCRIPTION OF EMBODIMENTS

[0022] Slots are commonly used in anode disks for x-ray tubes to reduce the risk of mechanical failure of the anode during high energy applications. Fig. 1 schematically illustrates an example of a known anode disc 100. The anode has a center 110 around which it rotates when in use in an X-ray tube. In other words, the axis of rotation for the anode disc when in use goes through the center 110. The anode disc has a central region 120 and an outer circumference 130. The slots 101 extend from the outer circumference radially towards the center 110, as illustrated with the dotted lines. The slots sometimes have a circular end opening at the 'inside' of the slot. Although the slots are effective in reducing the risks of e.g. thermal cracks in the anode disc during use, and hence to improve the thermomechanical stability, the inventors have found that the slots may cause X-ray intensity patterns that negatively influence image quality. This negative effect is due to the relationship between the rotational frequency of the anode and the sampling rate (integration period) of the detector. For combinations of rotational frequency and sampling rate, repeating patterns in the X-ray distribution are formed. Such patterns may multiply when reconstructing images from multiple detector integration periods. A work-around during imaging may be to limit imaging to only combinations of anode rotation speed and detector integration periods which do not introduce or at least reduce the patterns. However, such a work-around may limit the versatility and performance of the X-ray imaging.

[0023] The problem with patterns in the X-ray distribution may be reduced or avoided with an anode disc according to the invention. An example of such an anode disc is illustrated in Fig. 2. Similarly to the anode disc in Fig. 1, the anode disc 200 in Fig. 2 has a center 210 around which it rotates when in use in an X-ray tube. The anode disc 200 has a central region 220 and an outer circumference 230. In the example, three slots 201, 202 and 203 are shown. Each of the slots 201, 202, 203 extends from the outer circumference towards the central region 220. In this example, the slots 201, 202, 203 extend radially towards the center 210. Along a circumference of the anode disc 200, a distance may be determined between each pair of neighboring slots. In Fig. 2, a circumferential distance between the first slot 201 and the second slot 202 is labelled c12, the circumferential distance between the second slot 202 and the third slot 203 is labelled c23, and the circumferential distance between the third slot 203 and the first slot 201 is labelled c31. Each circumferential distance c12, c23, c31 between two neighboring slots is different. c12 is different from c23 and c31. c23 is different from c31. The distribution of the slots may also be expressed with the corresponding angle a12, a23, a31 between the extended radial direction of neighboring slot 201, 202, 203, shown as dotted lines. No two of the angles a12, a23, a31 are the same.

[0024] Thanks to the uneven circumferential distribution of the slots, the risk of patterns in the X-ray intensity distribution during imaging may be reduced. This allows for still having a good thermomechanical stability of the anode disc thanks to the slots, while at the same time providing improved imaging quality and / or flexibility in combinations of anode rotations speeds and imaging sampling times.

[0025] The circumferential distribution of the slots 201, 202, 203 may be random, or may be determined for a particular uneven distribution. As an example, the distribution of the slots 201, 202, 203 may be chosen such that no angle a12, a23, a31 or circumferential distance c12, c23, c31 between (the radial direction of) two neighboring slots has a common divisor with any other respective angle a12, a23, a31 or circumferential distance c12, c23, c31. One way to achieve this may be to design the distribution of the slots such that each angle a12, a23, a31 is a different prime number with the sum of all angles being 360 degrees.

[0026] Fig. 3 schematically illustrates another example of an anode disc according to embodiments of the invention. The anode disc 300 has a center 310 around which it rotates when in use in an X-ray tube, a central region 320 and an outer circumference 330. The anode disc 300 in Fig. 3 has four slots 301, 302, 303, 304. Similarly to in Fig. 2, the slots 301, 302, 303, 304 in Fig. 3 are unevenly distributed around the circumference, such that each circumferential distance c12, c23, c34, c41 between neighboring slots is unique. For readability of the figure, only four slots are illustrated. However, for thermomechanical stability it may be advantageous to have at least five unevenly distributed slots, or even more preferably at least seven unevenly distributed slots. Depending on the use cases of the anode, it may be advantageous to have more slots to improve anode lifetime, or fewer slots to diminish interference with the X-ray intensity.

[0027] Fig. 4 schematically illustrates another example of an anode disc according to embodiments of the invention. The anode disc 400 has a center 410 around which it rotates when in use in an X-ray tube, a central region 420 and an outer circumference 430. Contrary to the examples in Fig. 2 and 3, in this case the unevenly distributed slots 401, 402, 403, 404 are not completely radial directed towards the common center 410. Each of the slots 401, 402, 403, 404 extends towards the central region 420 but in this example not following the corresponding dotted radial line towards the center 410. No two circumferential distances c12, c23, c34, c41 between neighboring slots are the same. The slots have an uneven distribution along the circumference. Although the slots are not radial in this case, the distribution of the slots may be described with the corresponding angles between a radial direction from the circumference to the center of each slot 401, 402, 403, 404. I.e. the angles between the corresponding dotted lines in the figure. No two such angles are the same.

[0028] Fig. 5 schematically shows an X-ray imaging system 500. The X-ray imaging system may be e.g. a computed tomography system, a radiography system, a fluoroscopy system etc. The imaging system 500 includes an X-ray tube 550 and a detector 560. The X-ray tube comprises a cathode 510 and a rotating anode 200 with unevenly distributed slots. During operation of the X-ray tube, the anode 200 is rotated with a rotation frequency. The rotation frequency may be on the order of 100-200 Hz. An electron beam 520 is accelerated from the cathode 510 towards the anode 200. When the electron beam 520 impacts with the anode 200 at a focal spot, X-rays 530 and heat are generated. The X-rays may pass through an imaging subject (not shown) and are detected by the detector 560 during an integration period (sampling rate). An integration period may be in the order of microseconds or milliseconds, depending on the use case. In e.g. computed tomography, multiple detector integrations are reconstructed to form the final image. This means that any recurring patterns in the X-ray intensity detected by the detector may multiply in the final image. Thanks to the anode 200 with unevenly distributed slots, the risk of such patterns may be reduced or avoided over a large range of rotation frequency - integration period combinations. Hence, imaging performance with the imaging system may be improved.

[0029] Fig. 6 schematically illustrates a method to manufacture the anode disc discussed above. The method includes receiving 610 an intermediate anode disc. The intermediate anode disc may already have been subjected to other process steps like surface treatment, assembly with a backpack, etc., but the intermediate anode disc does not yet have the slots. The method further includes creating slots in the intermediate anode disc to form the anode disc with unevenly distributed slots. The slots may be created in a random distribution or with a selected set of distances between slots. With computer design, many possibilities are conceivable for achieving the set of unevenly distributed slots. The slots may be created with Electrical discharge machining (EDM), such as wire EDM, or with another suitable technology for machining anode disc materials.

[0030] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim, and using reference signs relating to the embodiment of Fig. 2 should not be construed as an indication that other embodiments, including but not limited to those of Figs. 3 and 4, are excluded: these other embodiments are included. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. Measures recited in mutually different dependent claims may advantageously be used in combination.

Claims

1. Anode disc (200) for use as rotating anode in an X-ray tube, the anode disc comprising: a center region (220) including a center (210), wherein the center intersects an axis of rotation when the anode disc is in use; an outer circumference (230) surrounding the center region; and at least three slots (201, 202, 203) for reducing stress when the anode disc is in use, wherein each slot extends from the outer circumference towards the center region, wherein the slots are unevenly distributed along the outer circumference, such that each circumferential distance (c12, c23, c31) between two neighboring slots is different.

2. The anode disc according to claim 1, wherein each slot extends radially from the outer circumference towards the center (210).

3. The anode disc according to claim 1 or 2, wherein no two angles between radial directions of two neighboring slots have a common divisor.

4. The anode disc according to claim 3, wherein each angle between radial directions of two neighboring slots is a prime number.

5. An X-ray tube (550) for use in an X-ray imaging system, wherein the X-ray tube comprises the anode disc according to any of the preceding claims.

6. An X-ray imaging system (500) comprising: the X-ray tube (550) according to claim 5; and an X-ray detector (560) for detecting X-rays.

7. A method (600) for manufacturing the anode disc according to any claims 1-4, wherein the method comprises: receiving (610) an intermediate anode disc without multiple slots; and creating (620) the at least three slots (201, 202, 203) in the intermediate anode disc to form the anode disc.

8. The method according to claim 7, wherein the at least three slots (201, 202, 203) are created using electrical discharge machining.

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