MOIRÉ MARKER FOR X-RAY IMAGING

Abstract

The present invention relates to a computer-implemented method of determining a rotational position of an object in a coordinate system of an x-ray imaging device. An x-ray image is generated of an object to which a Moiré marker for x-ray imaging is attached. Subsequently, the Moiré pattern generated by the Moiré marker is analysed and the rotational position of the marker and hence of the object is determined in a calculative manner. The Moiré marker for x-ray imaging includes a pattern which results in a significantly different appearance when being observed from slightly different perspectives. One embodiment example of the Moiré marker for x-ray imaging consists of two layers with patterns produced by a material that shields x-ray as good as possible like for example lead, surrounded and spaced apart by material that is highly transparent in x-ray like for example air or light plastics. The size of the openings in the pattern shall preferably be small compared to the distance of the two layers such that a small change in orientation of the marker results in a fairly significant change in the structure of the second layer seen through the aperture of the first layer. Multiple structures with different hole sizes and layer distances can be used to have a larger working range while maintaining accuracy.

Claims

1. A computer-implemented method of determining a rotational position of an object in a coordinate system of an x-ray imaging device, the method comprising the steps: providing one x-ray image of the object, to which a Moiré marker for x-ray imaging is attached, the x-ray image being imaged by the x-ray imaging device, wherein the Moiré marker for x-ray imaging generates a Moiré pattern of x-ray signal intensities on the image and the Moiré pattern is indicative for an angle between the Moiré marker and an x-ray propagation direction of the x-ray imaging device; and determining, based on the Moiré pattern of signal intensities, the rotational position of the object in the coordinate system of the x-ray imaging device.

2. The method according to claim 1, wherein the step of determining the rotational position of the object comprises: determining at least one point in the Moiré pattern of x-ray signal intensities; and using the determined at least one point as an input of a pre-defined relation describing a dependency of the Moiré pattern from the angle between the Moiré marker and the x-ray propagation direction of the x-ray imaging device.

3. The method according to claim 2, wherein the determined at least one point represents an x-ray signal intensity minimum of the Moiré pattern or an x-ray signal intensity maximum of the Moiré pattern.

4. The method according to claim 2 wherein the relation is a stored x-ray intensity distribution detected by an x-ray sensor of the x-ray imaging device as a function of the angle between the Moiré marker and the x-ray propagation direction of the x-ray imaging device.

5. The method according to claim 1, further comprising generating a control signal for positioning the imaged object relative to the x-ray imaging device based on a result of the determination of the rotational position of the object.

6. The method according to claim 5, further comprising repeating the method until a pre-defined position condition describing a desired position of the object in the coordinate system of the x-ray imaging device is reached.

7. The method according to claim 5, further comprising using the generated control signal to cause a movement of the object and wherein the object is a medical robot, a medical instrument, medical device, a patient support device.

8. The method according to any claim 1, wherein the x-ray image is an x-ray projection image, and the determination of the rotational position of the object takes into account, in a calculative manner, a spatial divergence of an x-ray beam emitted by the x-ray imaging device.

9. The method according to claim 1, wherein in the provided x-ray image the Moiré marker and a further marker are attached to the object as marker array, and the method further comprising automatically identifying the further marker in the provided x-ray image.

10. The method according to claim 9, the method further comprising using the automatically identified further marker in the provided x-ray image for calculating a translational position of the Moiré marker within the coordinate system of the x-ray imaging device.

11. The method according to claim 9, wherein the further marker is of an x-ray opaque material and has a ball shape, a cuboid shape, a pyramidal shape, a disc shape, or any combination thereof.

12. The method according to claim 1, wherein the step of determining the rotational position further comprises comparing at least the Moiré pattern of the x-ray signal intensities generated by the Moiré marker in the x-ray image with a target pattern of x-ray intensities to be generated by the Moiré marker.

13. The method according to claim 12, further comprising repeating the method until a pre-defined match between the generated Moiré pattern in the provided x-ray image and the target pattern is achieved.

14. The method according to claim 1, further comprising automatically detecting the Moiré pattern of x-ray signal intensities in the x-ray image with an image processing algorithm.

15. A Moiré marker for x-ray imaging comprising a pattern structure of at least a first and a second material, wherein the first material has a higher x-ray opacity than the second material, and wherein the pattern structure of the first and the second material is configured for generating a Moiré pattern of x-ray signal intensities in an x-ray image when being imaged by an x-ray imaging device.

16. The Moiré marker for x-ray imaging according to claim 15, wherein the pattern structure of the first and the second material is configured to allow a determination of a rotational position of the Moiré marker from the x-ray image of the Moiré marker.

17. The Moiré marker for x-ray imaging according to claim 15, wherein the pattern structure further comprises: a first layer with a first pattern of the first material and second material; and a second layer with a second pattern of the first material and second material.

18. The Moiré marker for x-ray imaging according to claim 17, wherein the first layer and the second layer are separated from each other by a first distance, the second material of the first pattern and the second material of the second pattern has a first width between two adjacent pattern elements of the first material, and the first distance is larger than the first width.

19. The Moiré marker for x-ray imaging according to claim 18, wherein the pattern structure comprises: a third layer with a third pattern of the first and second material, a fourth layer with a fourth pattern of the first and second material, wherein the third layer and the fourth layer are separated from each other by a second distance, the second material of the third pattern and the second material of the fourth pattern has a second width between two adjacent pattern elements of the first material, the second distance is larger than the width, and a ratio of the first width to the first distance is different from a ratio of the second width to the second distance.

20. The Moiré marker for x-ray imaging according to claim 15, wherein the first material comprises at least one of lead, tin, bismuth, tungsten, iodine, gold, tantalum, yttrium, niobium, molybdenum, ruthenium, rhodium, barium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium, rhenium, osmium, iridium, or bismuth, and the second material comprises at least one of air, plastic material, carbon, a composite of a thermoplastic resin with carbon-fiber reinforcement, a thermoplastic polymer, like e.g. PEEK.

21. A marker array for x-ray imaging, the array comprising: the Moiré marker for x-ray imaging according to claim 15; and an x-ray marker of an x-ray opaque material having a ball shape, a cuboid shape, a pyramidal shape, a disc shape, or any combination thereof.

22. A system for determining a rotational position of an object in a coordinate system of an x-ray imaging device, the system comprising: a calculation unit configured to: provide one x-ray image of the object, to which a Moiré marker for x-ray imaging is attached, the x-ray image being imaged by the x-ray imaging device, wherein the Moiré marker for x-ray imaging generates a Moiré pattern of x-ray signal intensities on the image and the Moiré pattern is indicative for an angle between the Moiré marker and an x-ray propagation direction of the x-ray imaging device; and determine the rotational position of the object in the coordinate system of the x-ray imaging device based on the Moiré pattern of signal intensities.

23. The system according to claim 22, the system further comprising the x-ray imaging device, wherein the calculation unit is further configured to generate a control signal for positioning the imaged object relative to the x-ray imaging device based on a result of the determination of the rotational position of the object.

24. The system according to claim 22, further comprising a Moiré marker for x-ray imaging, the Moiré marker comprising a pattern structure of at least a first and a second material, the first material having a higher x-ray opacity than the second material, wherein the pattern structure of the first and the second material is configured for generating a Moiré pattern of x-ray signal intensities in an x-ray image when being imaged by an x-ray imaging device.

25. (canceled)

26. A non-transitory computer-readable medium that, when executed by a computer or when loaded onto a computer, causes the computer to perform the computer-implemented method of determining a rotational position of an object in a coordinate system of an x-ray imaging device according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0090] In the following, the invention is described with reference to the appended Figures which give background explanations and represents specific embodiments of the invention. The scope of the invention is however not limited to the specific features disclosed in the context of the figures.

[0091] FIG. 1 schematically shows a system for determining a rotational position of an object in a coordinate system of an x-ray imaging device using a Moiré marker for x-ray imaging according to an exemplary embodiment of the present invention.

[0092] FIG. 2 schematically shows two embodiments of a Moiré marker for x-ray imaging according to two different embodiments of the present invention.

[0093] FIG. 3 schematically shows five different Moiré patterns of x-ray signal intensities generated in an x-ray image with a Moiré marker for x-ray imaging according to an exemplary embodiment of the present invention for five different rotational positions of the marker in the coordinate system of the x-ray imaging device.

[0094] FIG. 4 schematically shows a flow diagram of a computer-implemented method of determining a rotational position of an object in a coordinate system of an x-ray imaging device according to different embodiments of the present invention.

[0095] FIGS. 5A to 5E schematically show five embodiments of the present invention, in which one or more Moiré marker for x-ray imaging are depicted.

DESCRIPTION OF EMBODIMENTS

[0096] FIG. 1 schematically shows a system 100 for determining a rotational position of an object in the coordinate system of an x-ray imaging device 106. The x-ray imaging device 106 comprises a calculation unit 111, which is configured for carrying out the computer-implemented method of determining the rotational position of the object in the coordinate system as has been disclosed hereinbefore in detail. In particular, this calculation unit 111 can be provided with an x-ray image of the object to which the Moiré marker 101 is attached. The detector 109 detects the x-ray intensities that result from the propagation of the x-ray beams 108 that are propagating from the x-ray source 107 through the object, not shown here, to which the Moiré marker for x-ray imaging 101 is attached. The Moiré marker 101 generates a Moiré pattern of x-ray signal intensities on the image. The Moiré pattern is indicative for the angle between the Moiré marker 101 and the x-ray propagation direction of the x-ray beams 108. The calculation unit 111 is configured for determining, based on the Moiré pattern of signal intensities, the rotational position of the marker 101 and hence of the object to which the marker is attached in the coordinate system of the x-ray imaging device 106.

[0097] As can be gathered from FIG. 1, the x-ray image is an x-ray projection image. Consequently, the determination of the rotational position of the object takes into account, in a calculative manner, the spatial divergence 110 of the x-ray beams 108 emitted by the x-ray imaging device 106. If desired, a further marker, which is a non-Moiré marker, and which is made of an x-ray opaque material and preferably has a ball shape, a cuboid shape, a pyramidal shape, a disc shape or any combination thereof, is also attached to the object and preferably also fixed to the Moiré marker 101. The captured x-ray image can then be expected for the location of the spherical marker. From this position, the position of the Moiré pattern can be deduced and the rotational position of the Moiré marker can be determined from the distribution of the signal intensities, as has been described hereinbefore in detail and will be described in more detail with respect to for example the embodiment of FIG. 3.

[0098] As can be seen from FIG. 1, the Moiré marker 101 comprises a pattern structure of at least a first and a second material. The first material has a higher x-ray opacity than the second material. Due to this pattern structure, a Moiré pattern is generated in the x-ray image which allows for the determination of the rotational position of the Moiré marker 101 from the x-ray image that is generated on the detector 109. The pattern structure of the Moiré marker 101 comprises a first layer 102 with a first pattern of the first material 104a and a second material 105a. A second layer 103 is comprised with the second pattern of the first material 104b and second material 105b. The first layer 102 and the second layer 103 are separated from each other by a first distance d.sub.1. In the first pattern and preferably also in the second pattern, the second material has a first width w.sub.1 between two adjacent pattern elements of the first material. It should be noted, that the first distance d.sub.1 is larger than the first width w.sub.1 in order to provide for a proper angle resolutions for small angle changes of the angle between the marker and the propagation direction of the x-ray beams 108. It should also be noted that between different layers of the Moiré marker a material in the area of diameter d.sub.1 can be used that is different from the material with “high x-ray opacity” and the “material with low x-ray opacity”, as will be explained in detail hereinafter. Thus, a third, other material or also a material combination can be used in this section of the marker.

[0099] The embodiment of FIG. 1 uses a compact Moiré marker 101 (not shown at scale here) for x-ray imaging and allows estimating the position of the marker in the x-ray machine coordinate system very accurately from only one x-ray image. Such a Moiré marker for x-ray imaging comprises a pattern, which results in a significantly different appearance when being observed from slightly different perspectives. In this embodiment, said Moiré marker 101 for x-ray imaging consists of two layers 102, 103 with patterns produced by a material that shields x-ray as good as possible like for example lead, surrounded and spaced apart by material that is highly transparent in x-ray like air or like plastics. The size of the openings in the pattern is small compared to the distance of the two levels of the patterns such that a small change in orientation of the Moiré marker results in fairly significant change in the structure of the second layer seen through the aperture of the first layer. An example is to use 0.5 mm openings with a layer distance of 25 mm. Moreover, it should be noted that the double layer Moiré marker for x-ray imaging shown in FIG. 1, is schematically identical to the double layer Moiré marker for x-ray imaging shown on the left hand side of FIG. 2, which will be described now in detail.

[0100] FIG. 2 schematically shows a first Moiré marker for x-ray imaging 200 and a second Moiré marker for x-ray imaging 206. The first Moiré marker 200 comprises a first layer 201 with a first pattern of the first material and the second material and comprises a second layer 202 with a second pattern of the first material and the second material. In particular, the first layer 201 comprises three concentrically arranged rings 203, 204 and 205. They can for example be made of the material lead, tin, bismuth, tungsten, iodide, gold, tantalum, yttrium, niobium, molybdenum, ruthenium, rhodium, barium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium, rhenium, osmium, iridium, bismuth. The gaps between these concentrically arranged rings may be filled with the second material that could be selected from air, plastic material, carbon, a composite of a thermoplastic resin with carbon-fibre reinforcement, a thermoplastic polymer, like e.g. PEEK, or any combination thereof, to mention only some exemplary embodiments.

[0101] The same material combinations can be used also for the single layer Moiré marker for x-ray imaging 206. This Moiré marker comprises only a single layer of a pattern structure, which is made of three concentrically arranged rings 209, 208 and 207.

[0102] FIG. 3 schematically shows a diagram where on the x-axis the detector position on the x-ray detector is presented with reference sign 301. The y-axis 302 depicts the detected x-ray intensity. Hence, in FIG. 3, the intensity distribution including the Moiré pattern 300 over the x-ray detector is shown for five different rotational positions of a Moiré marker when being imaged by an x-ray imaging device. In other words, FIG. 3 shows five relations 303 describing the dependency of the Moiré pattern 300 from the angle between the Moiré marker and the x-ray propagation direction of the x-ray imaging device. As can be seen in FIG. 3, the relation 303 respectively comprise x-ray signal intensity minima 305 and x-ray signal intensity maxima 304. The diagram shown in FIG. 3 depicts the x-ray absorption in percent for different marker angles at 500 mm distance to the x-ray source using 0.5 lead with 1 mm hole distance and 25 mm layer distance. In other words, a Moiré marker as shown in FIG. 1 has been used for generating the diagram in FIG. 3.

[0103] FIG. 4 schematically shows a flow diagram of a computer-implemented method of determining a rotational position of an object in a coordinate system of an x-ray imaging device. The method described in the context of FIG. 4 particularly comprises three different embodiments that can be used separately, but which can also be combined. In particular, the method steps S3, S4 and S5 follow the method steps S1 and S2a and S2b. This will be described now in more detail.

[0104] The method comprises the step of providing at least one x-ray image of the object in step S1. A Moiré marker for x-ray imaging is attached to the object. The Moiré marker for x-ray imaging, as has been described before, generates a Moiré pattern of x-ray signal intensities on the image that is provided. The Moiré pattern is indicative for an angle between the Moiré marker and an x-ray propagation direction of the x-ray imaging device. In step S2, the rotational position of the object in the coordinate system of the x-ray imaging device is determined based on the Moiré pattern of signal intensities in the x-ray image provided. The step S2 comprises the further two sub-steps S2a and S2b. In particular, at least one point in the Moiré pattern of x-ray signal intensities is determined in the x-ray image during step S2a. Moreover, the determined at least one point is used as an input in step S2b when putting this determined point into a pre-defined relation describing the dependency of the Moiré pattern from the angle between the Moiré marker and the x-ray propagation direction of the x-ray device. Therefore, the result of step S2 is the determined rotational position of the object. This result can now be used either only for step S3, or only for step S4 or only for step S5, but this can also be combined. In step S3, a control signal for positioning the imaged object relative to the x-ray imaging device is generated based on the result of the determination of the rotational position, i.e. the outcome of method step S2. This control signal can be used to cause a movement of the object, as has been described hereinbefore. For example, a medical robot, a medical instrument, a medical device, a patient support device like a patient couch and/or the patient may be moved based on the use of this control signal. It may be checked after step S3 whether the desired position of the object is already achieved. If this is denied, then the method comprising steps S1, S2 and S3 can be repeated until a pre-defined position condition describing the desired position of the object in the coordinate system of the x-ray imaging device is reached. Alternatively or also in addition, the result of the method step S2 can also be used to verify the alignment of a medical instrument in step S4. However, in step S5, one could also use the outcome of the step S2, i.e. the determined rotational position of the marker and of the object for tracking an object during for example image guided surgery. This is depicted in FIG. 4 by reference sign S5.

[0105] FIGS. 5A to 5E schematically show five embodiments of one or two Moiré markers for x-ray imaging, which can of course be combined which each other. As was described before and as will become apparent from the detailed description of the embodiments of FIGS. 5A to 5E, within one Moiré marker for x-ray imaging all the layers are spatially fixed relative to each other, all the layers are parallel and preferably the centres of mass of the layers are located on a virtual axis that extends perpendicular to the layers of the marker.

[0106] FIG. 5A shows a top view of one Moiré marker 500 for x-ray imaging, which comprises two layers with a non-periodic pattern. However, in this top view only the first layer, i.e. the top layer, can be seen. The skilled reader will appreciate that the second layer of the Moiré marker 500 is located below this first layer, as can be gathered for example from the cross sectional view of FIG. 1, in which both layers of a similar Moiré marker with two layers is depicted. The Moiré marker 500 comprises a pattern structure within said first and second layer using a first and a second material, wherein the first material has a higher x-ray opacity than the second material. The pattern structure in the first layer shown in FIG. 5A is provided by 6 concentrically arranged rings 501-506 and one central element 507, wherein rings 501, 503 and 505 and the central element 507 are made of the first material, whereas rings 502, 504 and 506 are made of the second material. As is clear from FIG. 5A, the pattern per layer is not periodic, since the respective widths of the rings are different.

[0107] FIG. 5B shows a cross sectional view through a Moiré marker 508 for x-ray imaging with four layers according to another embodiment of the present invention. It comprises layer 1 and layer 2, which are separated from each other by distance d.sub.1, as well as layer 3 and layer 4, which are separated from each other by distance d.sub.2, which is different from distance d.sub.1. Each layer of layer 1 to layer 4 comprises a periodic or non-periodic pattern made at least of the first and second material that have been described hereinbefore in detail. Layers 2 and 3 are separated from each other by distance d.sub.3, which is different from distances d.sub.1 and d.sub.2.

[0108] As was described hereinbefore, the present invention of course also covers the use of a combination of two or more Moiré markers for x-ray imaging. Thus, FIG. 5C shows a marker array 509 comprising a first and a second Moiré marker 510, 511 for x-ray imaging as two different and separated objects, which are however provided in a fixed position to each other. They may e.g. be mounted to a fixation element. As can be seen from FIG. 5C, the two markers 510, 511 are not positioned in parallel to each other, but in an angled configuration. The skilled person appreciates that both markers 510, 511 can be chosen to be identical in their geometrical design and in the materials used. But of course also a combination of two Moiré markers 510, 511 using for example different pattern structures, e.g. different distances between the two layers, and/or different materials with different x-ray opacity is part of the present invention.

[0109] FIG. 5D shows in a cross sectional view 512 a first Moiré marker for x-ray imaging 513 (“Marker 1”), which houses a second Moiré marker for x-ray imaging 514 (“Marker 2”). In other words, the second marker 514 is provided within, i.e. integrated in, the first marker 513, which circumvents with a ring shape the second marker 514. As can be seen from the cross section in FIG. 5D, both markers have a double layer structure.

[0110] FIG. 5E shows a marker array 515 comprising two double layer Moiré markers for x-ray imaging 516, 517 according to another embodiment of the present invention. The markers 516, 517 are located at positions along the x-ray propagation direction 518, in which the distance between the markers is 0. The two markers can thus have a distance along the x-ray propagation direction 518, or can alternatively positioned next to each other, i.e. at the same height along the x-ray propagation direction 518. The Moiré markers 516, 517 are two different objects, which are however provided in a predetermined, fixed position to each. This can be realized e.g. by mounting them onto a fixation element.

[0111] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from the study of the drawings, the disclosure, and the appended 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 or steps recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope of the claims.