Secondary emission compensation in X-ray sources

Abstract

Disclosed is an x-ray imaging system comprising an x-ray source, a sample position, and a detector arranged to detect x-ray radiation downstream of the sample position, the x-ray source comprising an electron source arranged to provide an electron beam, a target arranged to produce x-ray radiation upon impact by the electron beam, the target comprising a substrate and a target layer at least partially covering the substrate, the target layer arranged to produce x-ray radiation upon impact by the electron beam, means for directing the electron beam to a first position on the target layer and a second position selected from a position on the target where the electron beam directly impinges on the substrate and a position on an electron beam dump arranged such that x-ray radiation produced by interaction between the electron beam and the electron beam dump does not substantially reach a detector, and a controller arranged to use the detector to direct the electron beam to the first position to record a first image and to direct the electron beam to the second position to record a second image, and to generate an image that differs between the first image and the second image. A method for x-ray imaging is also disclosed.

Description

[Technical field]

[0001] The present disclosure relates to methods for x-ray imaging and related x-ray imaging systems. [Background technology]

[0002] It is well known in the art that secondary radiation can cause problems in X-ray sources. See, for example, US Patent Application Publication No. 2011 / 0176663. Secondary radiation is generally understood as any radiation other than the desired radiation, and can originate, for example, from X-ray scattering or from unintended electron beam interactions in the system. Typically, efforts are focused on reducing the amount of secondary radiation generated, which can be accomplished, for example, by mechanical design to avoid a line of sight from elements that can generate secondary radiation. Such reduction of secondary radiation can also be achieved by careful selection of materials, for example, by providing coatings of low X-ray producing materials (e.g., carbon) on elements that would otherwise generate secondary radiation. Furthermore, commonly used calibration procedures for X-ray detectors can reduce the effects of secondary radiation at least to some extent.

[0003] Furthermore, a procedure known as flat-field correction usually consists of exposing the detector to uniform radiation. Nominally, this results in an equal readout from each pixel on the detector. However, various imperfections result in variations between the pixels. To compensate for this, the values ​​recorded during the uniform illumination can be used to adjust the image recorded thereafter. However, if the assumed uniform illumination comprises a contribution from a secondary radiation source, i.e. it is not actually uniform, this procedure can reduce the effect of the secondary radiation during imaging. The problem with this approach is that when a sample is present between the X-ray source and the detector, the secondary radiation is at least partially absorbed by the sample. Therefore, the assumption of uniform illumination during flat-field calibration is not valid, and the compensation may not be able to eliminate the effect of the secondary radiation when applied to an image comprising a sample.

[0004] A somewhat similar problem is related to scattered X-ray radiation reaching the detector. This can be particularly problematic when performing CT scans. A method to compensate for this, proposed in EP 3939510, is to calculate a simulated image produced by the scattered radiation and subtract this simulated image from the acquired image to obtain an image with reduced scattering. In this type of application, it is not clear how to actually obtain an image produced only from the scattered radiation, i.e. how the simulation was used.

[0005] US 2005 / 0123097 discloses a method in which two images of the same object are acquired with different X-ray energy spectra. The two images are subtracted to enhance a particular feature, for example to subtract an undesirable image of fatty tissue in a mammogram from an image of a potential cancerous lesion, or to remove a bone image from an X-ray image of a chest. The two images are produced by providing a target of a composite material and directing an electron beam at different parts of the target to obtain different X-ray energy spectra. Also disclosed are embodiments in which different target materials are provided as layers overlapping each other, sometimes on a substrate that is transparent to X-rays. Summary of the Invention

[0006] The present disclosure proposes a method for compensating for undesired secondary X-ray radiation that may be created in an X-ray source, for example, by an aperture (a beam limiting element for the electron beam) and / or an electron beam interacting with a target substrate. Such secondary emissions of X-ray radiation adversely affect the image quality that may be obtained when using an X-ray source for illumination. The proposed solution involves recording two images, one in which the electron beam is directed at the target and one in which the beam is directed such that only secondary radiation is emitted. A final image is then formed by generating a difference between the two images.

[0007] It is usually preferable to eliminate all secondary radiation, i.e., in the ideal case, all emitted X-ray radiation comes from the interaction between the focused electron beam and the target at the desired location. In practice, however, some secondary radiation is always generated somewhere within the X-ray source. Secondary radiation can be thought of as having two components, which affect the generated X-ray image somewhat differently, as discussed below.

[0008] One type of secondary radiation is generated some distance upstream of the target (as seen along the electron beam). This type of secondary radiation usually appears as a well-defined spot in the final image. If a set of images is acquired with the object at different rotation angles and these images are used to create a three-dimensional reconstruction of the object, this type of secondary radiation can cause ring-like features in the reconstruction. In a sense, the image is created from two well-defined x-ray sources, one where a focused electron beam strikes the target and one where the electron beam interacts with some other structure upstream of the target. One way to isolate this type of secondary radiation is to block the electron beam, i.e., to deflect it away from the target, where substantially no x-ray radiation is emitted from the x-ray source. If the structure in which the secondary radiation is produced is located upstream of the beam deflector, the contribution from the secondary radiation to the final image will be the same when the electron beam is directed towards the target as when the beam is blocked, and its effect on the final image can be eliminated by generating a difference between the two images (blocked and unblocked electron beam).

[0009] Another type of secondary radiation is generated near the target, for example, by interaction between the electron beam and the target substrate, or between electrons scattered from the target and the backscatter collector. This type of radiation is usually manifested as an overall blurring of the resulting X-ray image. Because the secondary radiation is generated near the exit window, it is emitted over a wide range of angles, which in a sense can make the effective X-ray spot larger and less clearly defined. To be able to separate the contribution from this type of secondary radiation, the electron beam must impinge on the target and / or the target substrate under conditions such that the X-ray radiation created by the interaction between the electron beam and the target does not emanate from the source.

[0010] One example of a secondary radiation source is an aperture or other beam limiting element in the electron beam path. The purpose of having an aperture that geometrically limits the electron beam is to have a well-defined electron beam size in some respects and / or to limit the angle at which the electrons propagate further in the system. The dimensions of the aperture are determined, among other things, by the requirements for the electron beam spot size at the target and the total electron beam throughput. However, the electrons that do not pass through the aperture will interact with the material that defines the aperture (i.e., the beam limiting element) and generate X-ray radiation, at least to some extent. To avoid this, the material that defines the aperture may be made of or at least partially coated with a material that has a low X-ray generation efficiency. An alternative may be to provide the aperture as a hole in a relatively thick, dense material, so that the X-ray radiation generated by the electrons striking this material is absorbed (e.g., reabsorbed in the material) and does not reach the exit window. Although it may be difficult to completely eliminate the secondary radiation using these approaches, at least some reduction may be achieved. Another approach is to not have a line of sight between the aperture and the exit window through which the X-ray radiation emerges from the source. The latter option may require some added complexity in some cases, for example in conductive structures where the X-rays, by definition, emerge in the same direction as the electron beam. Furthermore, downstream of the aperture there must not be any other object other than the target with which the electron beam can interact to generate X-ray radiation that can exit the source.

[0011] A further potential secondary radiation source is the interaction between electrons backscattered from the target and surrounding parts of the X-ray source. It is known in the art to provide a backscatter collector (or trap) to capture the scattered electrons and the energy they carry. The interaction between the backscatter collector or some other part of the X-ray source and the electrons may further generate secondary radiation that may be emitted through the exit window. To eliminate or at least reduce this contribution to the emitted radiation, surface treatments (see WO2020 / 052773) and / or geometric constraints (see US Patent Application Publication No. 2012 / 0170715) may be applied.

[0012] In the case where the X-ray source comprises electron optics for focusing and deflecting the electron beam, and where the target comprises a portion where a low X-ray producing substrate is exposed, the inventive principles disclosed herein can be implemented by directing the electron beam to the intended working position, i.e., onto the target layer, e.g., comprising tungsten (W), to take a first image, and directing the electron beam to the location where the substrate, e.g., comprising diamond, is exposed (i.e., there is no target layer on the substrate), to take a second image. By moving the sample so that it is imaged at the same location on the detector for each image, the image generated by the X-rays emerging from the target layer can be extracted by generating a difference image between the images (e.g., by subtracting corresponding pixel values ​​of the two images). If the shift in the image position on the detector is small compared to the pixel resolution, the sample may not need to be moved. Secondary radiation generated from any other structure in the X-ray source, e.g., the material defining the aperture, is present in both images and is therefore excluded in generating the difference images.

[0013] If the steering and shaping of the electron beam is performed downstream (with respect to the electron beam propagation direction) from the structure generating the secondary radiation, the contribution from this secondary radiation will be the same in the two images.

[0014] In other embodiments, the electron beam may be directed in the same way when recording the first and second images, and instead the target is moved between image acquisitions, whereby a first image is exposed with the electron beam directed at the target layer, and a second image is recorded with the electron beam directed such that only secondary radiation contributes to the image. In this way, there is no need to move the sample between images, since the relative orientation between the electron beam, sample, and detector is the same for the two images. The X-ray source in this case comprises an actuator arranged to move the target with sufficient precision, typically in the order of μm, in a vacuum-tight arrangement.

[0015] X-ray radiation produced by electrons penetrating the target layer, i.e., X-ray radiation produced from the interaction of the electron beam with the substrate supporting the target layer rather than the target layer itself, produces an image that is substantially the same as that produced by electrons impinging directly on the substrate, i.e., on bare portions of the substrate, if scattering from the target layer can be neglected.

[0016] In the case where negligible secondary radiation is generated by the interaction between the electron beam and structures located upstream of the beam deflection element, a first image may be acquired with the electron beam directed towards the target layer and a second image may be acquired with the electron beam directed towards the exposed surface of the target substrate. To compensate for the shift in image position on the detector, the sample may be moved accordingly or the acquired images may be shifted (aligned) so that they overlap before producing different images. Since secondary radiation that is unrelated to the electron beam striking the target is negligible, no noticeable artifacts are created by aligning the images.

[0017] In cases where a significant portion of the incoming electrons are absorbed and / or scattered in the target layer, more complicated procedures may be required. By deflecting the electron beam into a location where the generated X-ray radiation cannot leave the X-ray source or at least cannot reach the detector, such as a shielded electron dump, an image can be obtained that is created only from secondary radiation generated from other structures in the X-ray source (e.g., a beam limiting element for the electron beam). The electron dump may be located as any part of the source that can withstand the thermal load imposed by the electron beam, where any X-ray radiation generated by the impinging electron beam does not reach the detector. By generating a different image (e.g., by subtracting pixel values ​​of this image from corresponding pixel values ​​of an image acquired when the electron beam is directed towards the target layer), the effect of secondary radiation generated in the beam limiting element can be eliminated, or at least substantially reduced. To also eliminate the effect of X-ray radiation generated in the substrate, an image can be acquired by directing the electron beam towards the substrate but away from the target layer and moving the sample so that it is imaged at the same position on the detector as when the electron beam is directed towards the target layer. Contributions from secondary radiation generated in the beam limiting element can also be eliminated from this image by subtracting the image acquired with the electron beam directed towards the electron dump. The resulting image can then be scaled to compensate for absorption and / or scattering in the target layer. After scaling, pixel values ​​of the image can be subtracted from corresponding pixel values ​​of a different image generated between the image recorded with the electron beam directed towards the target layer and the image generated with the electron beam directed towards the electron dump. The scale factor may be known from the target design or may be determined during a calibration procedure. It will thus be appreciated that a suitable target comprises a substrate that is only partially covered with a target layer (e.g. made from tungsten), with some parts of the substrate having no such target layer.

[0018] In the above discussion, it was assumed that the image associated with the secondary radiation generated at the aperture (beam limiting element) can be considered the same regardless of sample movement. In most practical cases, this is an acceptable assumption. If this is not the case, i.e. if the image of the sample created by the secondary radiation changes significantly upon sample movement, more complex methods can be used. A total of four images can be recorded. The first image is then taken with the sample in the first position and the electron beam pointed towards the target. The second image is taken with the sample still in the first position and the electron beam pointed so that only the secondary radiation from the aperture reaches the detector. The third image is taken with the electron beam deflected towards the part of the target where the substrate is exposed, and the sample is moved to a second position so that the image of the sample is in the same place as in the first image. The fourth image is taken with the sample in the second position and the electron beam pointed as in the second image. By subtracting the second image from the first image to generate a fifth image and the fourth image from the third image to generate a sixth image, respectively, the effect from secondary radiation generated at the aperture is eliminated (or at least reduced). Then, by subtracting the sixth image, appropriately scaled to compensate for scattering and / or absorption in the target layer, from the fifth image, the effect from secondary radiation generated by electrons interacting with the substrate is also eliminated. As discussed above, subtracting an image from another image means that a difference between two images is generated, for example, by subtracting pixel values ​​of one image from corresponding pixel values ​​of the other image. For example, a difference between a first image and a second image can be generated by subtracting pixel values ​​of the second image from corresponding pixel values ​​of the first image. The pixel values ​​can be scaled before subtraction in some embodiments.

[0019] For X-ray sources where secondary radiation produced by electrons backscattered from the target contributes significantly to the image formed on the detector, it may be advantageous to provide a region on the target where X-ray radiation is not emitted towards the sample and detector when the electron beam is directed at the target. This may be accomplished, for example, by providing local X-ray shielding for the conductive target outside the target, or by providing a region of the target that has no line of sight to the exit window. By directing the electron beam at such a location, only X-ray radiation produced by backscattered electrons reaches the detector, thus forming an image that can be subtracted from an image acquired by directing the electron beam to the intended target location.

[0020] A corresponding procedure can be performed to eliminate the contribution from backscattered electrons to an image formed by directing the electron beam towards the exposed substrate of a conductive target. Once the backscattering contribution has been eliminated, the image acquired by directing the electron beam towards the substrate can be subtracted, possibly after appropriate scaling, from the image acquired by directing the electron beam towards the target layer.

[0021] In an embodiment in which the image substantially only comprises X-ray radiation generated in some structures upstream of the electron beam deflector (e.g. in an aperture), the image may be thresholded before subtraction from another image. The thresholding may be performed such that only pixels that have actually received secondary radiation are used in the subtraction, other pixel values ​​are set to zero (i.e., effectively the corresponding pixels in the original image remain unchanged after the subtraction). A suitable threshold may be found, for example, as a multiple of the standard deviation of the pixel values ​​or as a fraction of the maximum pixel value in the image. This may be advantageous as the amount of noise in the final image may be less than in the case where all pixel values ​​were used in generating the different images (i.e. in the subtraction procedure). Other means for reducing noise may also be used, such as local median filtering of pixel values ​​or other well-known image processing algorithms.

[0022] The methods outlined above may be applied when performing CT scans. In particular, as discussed above, images in which the electron beam is blocked may be recorded for at least some of the sample orientations used during the scan in order to compensate for secondary radiation generated at the aperture. It may be advantageous to acquire compensation images for a subset of the sample orientations used for the entire scan, for example, good reconstruction results were obtained by acquiring a blocked image for every tenth sample orientation. Instead of blocking the electron beam, it may be directed at the part of the target where the substrate is exposed, and in this way a somewhat sharper image may be obtained after subtraction of the compensation image. However, by blocking the electron beam, the total X-ray dose applied to the sample may be less.

[0023] The different approaches described above can be combined in different ways, depending on what contributions to the secondary emission need to be taken into account.

[0024] In the following detailed description, reference is made to the accompanying drawings. [Brief description of the drawings]

[0025] [Figure 1] 1 is a schematic diagram of an X-ray system according to the present invention; [Diagram 2] FIG. 13 illustrates how the electron beam can be directed to different locations on the target and how the sample can be moved accordingly. [Diagram 3] FIG. 1 shows a schematic diagram of a method for X-ray imaging. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Although the inventive concepts presented herein have been discussed and summarized to provide a thorough understanding thereof, specific implementations are described to further illustrate how the present invention may be practiced.

[0027] By way of introduction, FIG. 1 shows a schematic of an exemplary imaging system comprising an X-ray source 100. The illustrated X-ray source includes an electron gun 102 configured to form an electron beam 114. The electron beam emitted from the electron gun 102 is aligned and shaped using an alignment coil 104 and a stigmator coil 106, respectively, before reaching a beam limiting element in the form of an aperture 108. As discussed in the Summary of the Invention above, such an aperture has the purpose of limiting the geometric spread of the electron beam before it enters the electron beam optics downstream (relative to the electron beam propagation direction). The aperture 108 typically has a diameter, i.e., opening size, of the order of 1 mm. After passing through the aperture 108, the electron beam reaches the electron optics, which typically comprise one or more focusing lenses 110 and one or more deflectors 112. The electron beam optics are used to direct and focus the electron beam to an intended location on a target 116. The target 116 typically comprises a high-Z (Z>20) material that generates X-rays 118 upon impact of electrons. A typical target material for this type of X-ray source is tungsten (W), which is present as a film on a substrate material made of, for example, diamond. These components of the X-ray source are enclosed in a low-pressure (vacuum) enclosure 120. The generated X-ray radiation 118 can be used to image a sample or object 130 located at a sample location using a detector 140. The X-ray source also includes a controller 150, which is configured, according to the principles disclosed herein, to direct the electron beam to a first location and record a first image using the detector 140, and to direct the electron beam to a second location and record a second image, and generate a difference between the first and second images, for example, by subtracting pixel values ​​of the second image from corresponding pixel values ​​of the first image. The controller 150 can also be responsible for controlling the lens 110 and the deflector 112 to direct the electron beam to said location. As shown in FIG. 1, a controller 150 can be operatively coupled to the electron-optical systems 110 , 112 and the detector 140 .

[0028] Thus, an X-ray imaging system according to an embodiment comprises the X-ray source 100 discussed above, a sample position (indicated in FIG. 1 by sample 130), and a detector 140. The X-ray source 100 comprises an electron source 102 arranged to provide an electron beam, a target 116 arranged to produce X-ray radiation upon impact by the electron beam, means such as electron optics 110, 112 for directing the electron beam to a first position on the target and a second position (which may or may not be on the target), and a controller 150 arranged to use the detector to direct the electron beam to the first position and record a first image, and direct the electron beam to the second position and record a second image, producing different images between the first and second images. The imaging system may also comprise an actuator (not shown) arranged to move the target relative to the electron beam, such that directing the electron beam to the first and second positions may involve steering / deflecting the electron beam using the electron optics and / or moving the target. The imaging system may also include a manipulator for moving the X-ray source and the sample position relative to one another. Such a manipulator may be, for example, a translation stage 135 to which the sample may be mounted as shown in FIG. 1. Other examples include placing the X-ray source on a translation stage or providing the X-ray source with feet that allow translation. Preferably, the manipulator is also operably connected to the controller 150, as shown diagrammatically by the dotted line of the translation stage 135 in FIG. 1.

[0029] FIG. 2 shows the targets 210, 220, the sample / object 230, and the detector 240 in more detail. According to the principles disclosed herein, the effect of unwanted X-ray radiation on the captured image is reduced by subtracting one or more reference images from the primary image. The unwanted X-ray radiation is generally referred to herein as secondary emission radiation. To capture the primary image and one or more reference images, the electron beam is moved (deflected) between different locations to capture the various images of interest. In FIG. 2, three different electron beam locations are shown, a first location is indicated as 214a, where the electron beam strikes the target layer 220 to intentionally produce X-ray radiation, a second location is indicated as 214b, where the electron beam does not strike the target layer 220, but instead strikes the underlying substrate 210 directly, and a third location is indicated as 214c, where the electron beam strikes the electron dump 250. As will be appreciated, arrows 214a, 214b, and 214c in FIG. 2 represent the same electron beam directed to different locations. When the electron beam impinges directly on the substrate 210 as shown in 214b, some X-ray radiation may be generated by the interaction between the electron beam and the substrate material. It will be appreciated that X-ray radiation may also be generated by the interaction between the electron beam and the substrate when the electron beam is directed to the target material 220 as shown in 214a, since some electrons may pass through the target material 220. On the other hand, when the electron beam impinges on the electron dump 250, the X-ray radiation generated from the interaction between the electron beam and the electron dump does not exit the X-ray source, or at least, the X-ray radiation generated from the interaction between the electron beam and the electron dump does not reach the detector. Thus, when the electron beam impinges on the electron dump 250, the X-ray radiation reaching the object 230 and the detector 240 mainly consists of the X-ray radiation generated at the aperture 108 located upstream of the target (see FIG. 1).

[0030] Regardless of whether the electron beam is directed at location 214a, 214b, or 214c, a background of X-ray radiation generated at aperture 108 is present at object 230 to be imaged. Such background may be reduced in the captured image by obtaining a difference between the image captured by the electron beam at location 214a and the image captured by the electron beam at location 214c. Obtaining the difference between the images may involve, for example, obtaining the difference between corresponding pixel values ​​of each of the images.

[0031] When the electron beam is directed towards the substrate 210, as shown at 214b in FIG. 2, the X-ray radiation that reaches the object 230 may include both contributions from radiation generated at the aperture 108 and contributions generated within the substrate 210.

[0032] Since X-ray radiation is generated at two different locations when an image captured by the electron beam at location 214b, i.e. impinging directly on the substrate 210, is accompanied, it can be assumed a priori that the object 210 must be correspondingly moved to allow comparison with (or subtraction from) an image captured by directing the electron beam towards the target material 220 as shown in FIG. 214a. FIG. 2 shows the deflection of the electron beam 214a,b between points separated by a distance BD on the target. The electron beam is shown as a solid arrow 214a impinging on a first point and as a dotted arrow 214b impinging on a second point on the target. As discussed, the first point is located in a region where the electron beam 214a impinges on the target layer 220, and the second point is located in a region where the electron beam 214b does not impinge on the target layer 220, but instead impinges directly on the substrate 210. In order to obtain an image on the detector 240 at the same position with respect to the two electron beam impact points, the sample or object 230 must be moved a distance OD, which can be calculated as:

[0033]

number

[0034] where ODD is the distance from the sample to the detector (object-detector distance) and SDD is the distance from the target to the detector (source-detector distance). Thus, the distance the sample must be moved between image acquisitions can be calculated by considering a congruent triangle, so that the displacement of the electron beam on the target is scaled by the ratio between the distance between the sample and the detector and the distance between the source and the detector. In many practical applications, the distance between the source and the sample, which corresponds to SDD-ODD as shown in FIG. 2, is small compared to the distance SDD from the source to the detector, which means SDD≈ODD, and therefore the sample can be moved by a distance OD that is substantially equal to the displacement BD of the electron beam without having a noticeable difference in the final result. As an example, the distance ODD between the sample and the detector may be 100 times the distance between the source and the sample (SDD-ODD). In such a case, the sample must be moved by essentially 99% of the electron beam displacement. If the beam displacement is on the order of 10 μm, the sample must be moved by about 9.9 μm. A difference of 0.1 μm may be neglected, especially since the usual pixel resolution on the detector is in the range of 100 μm. Thus, considering a magnification of about 100 in this example, an error of 0.1 μm corresponds only to one tenth of a pixel. A similar argument can be applied when considering thicker samples. The two sides of a thick sample are at different distances from the detector and therefore must basically be scaled differently for the two sides, although in practice an average scaling can be used without significantly impairing the image.

[0035] 3 shows a method for X-ray imaging using an X-ray source comprising an electron source configured to provide an electron beam and a target configured to produce X-ray radiation upon impact by the electron beam. The method comprises directing the electron beam to a first location on the target 401, recording a first X-ray image using a detector while directing the electron beam to the first location 402, directing the electron beam to a second location 403, recording a second X-ray image using the detector while directing the electron beam to the second location 404, and generating a different image between the first image and the second image 405. As discussed above, for both the first and second positions of the electron beam, the X-ray radiation reaching the detector comprises X-ray radiation generated by interaction between the electron beam and a beam limiting element (aperture), and any features or noise in the captured images may be reduced by generating such a different image between the two captured images.

[0036] To further reduce effects caused by secondary radiation, a scale factor may be determined from the respective exposure times used in capturing the first and second images, and then the pixel values ​​of the first image and / or the second image may be scaled prior to generating the different images such that the difference in exposure times between the first and second images may be compensated for.

[0037] To further improve the image quality of the final differing image, pixel values ​​of the second image that are below a predetermined threshold may be set to zero before the differing image is generated.

[0038] Preferably the method involves moving the object to be imaged such that the image position of the object on the detector is substantially the same for the first and second images.

[0039] Depending on the circumstances, it may also be preferable to use image processing (known per se) to align the first and second images with one another before generating the different images.

[0040] The second position may correspond to a position on the target where the electron beam directly impinges on the target substrate. In such a case, the method further comprises directing the electron beam to a third position and capturing a third image, where the third position corresponds to an electron beam dump positioned such that at least no X-ray radiation reaches the detector, such that X-ray radiation produced by the interaction between the electron beam and the electron beam dump is not substantially emitted from the X-ray source. A fourth image may then be created by subtracting pixel values ​​of the third image from corresponding pixel values ​​of the second image. The resulting pixel values ​​of the fourth image may then be scaled by a scale factor, and the scaled fourth image and the third image may be subtracted from the first (primary) image. In this way, secondary emissions generated both in the aperture and in the target substrate are compensated for.

[0041] The arrangement, source and method according to the invention may be used for different types of X-ray microscopy, radiography, fluoroscopy or CT scanning.

Claims

1. An X-ray source; The sample position, a detector positioned to detect X-ray radiation downstream of the sample location, The X-ray source comprises: an electron source arranged to provide an electron beam; a target arranged to produce X-ray radiation upon impact by the electron beam, wherein the target comprises a substrate and a target layer at least partially covering the substrate, the target layer arranged to produce X-ray radiation upon impact by the electron beam; means for directing the electron beam to a first location on the target layer and a second location selected from a location on the target where the electron beam directly impinges on the substrate and a location on the electron beam dump positioned such that x-ray radiation produced by interaction between the electron beam and an electron beam dump does not substantially reach the detector; a controller arranged to use the detector to direct the electron beam to the first location and record a first image, and to direct the electron beam to the second location and record a second image, generating a difference between the first image and the second image; An X-ray imaging system comprising:

2. The system of claim 1 , wherein the means for directing comprises a deflector positioned to deflect the electron beam.

3. 3. The system of claim 1 or 2, wherein the means for directing comprises an actuator arranged to move the target relative to the electron beam.

4. The system of claim 1 , further comprising a manipulator for moving the X-ray source and the sample position relative to each other.

5. The system of claim 1 , further comprising a beam limiting element disposed between the electron source and the target.

6. 6. The system of claim 5, wherein x-ray radiation generated from an interaction between the electron beam and the beam-limiting element reaches the detector when the electron beam is directed to the first location and the second location.

7. 1. A method for x-ray imaging using an x-ray source comprising an electron source configured to provide an electron beam and a target comprising a substrate and a target layer at least partially covering the substrate, the target layer being positioned to produce x-ray radiation upon impact by the electron beam, the method comprising: directing the electron beam to a first location on the target layer; recording a first x-ray image using a detector while directing the electron beam at the first location; directing the electron beam to a second location; recording a second x-ray image using the detector while directing the electron beam at the second location; generating a first different image between the first image and the second image; the second position is selected from a position on the target where the electron beam impinges directly on the substrate and a position on the electron beam dump positioned such that x-ray radiation produced by an interaction between the electron beam and an electron beam dump does not substantially reach the detector.

8. determining a scale factor from an exposure time of each of the first and second images; The method of claim 7 , further comprising: scaling at least one pixel value of the first image and the second image before generating the first different image.

9. 9. The method of claim 7 or 8, further comprising performing the step of setting pixel values ​​of the second image that are below a predefined threshold to zero, and thereafter generating the first different image.

10. 10. The method of claim 7, further comprising moving the object to be imaged such that a position of an image of the object on the detector is substantially the same for the first image and the second image.

11. The method of claim 7 , further comprising aligning the first image and the second image prior to generating the first different image.

12. directing the electron beam to a third location and recording a third image, wherein the first location corresponds to a location on the target configured to produce x-rays; the second position corresponds to the electron beam dump being positioned such that x-ray radiation produced by an interaction between the electron beam and the electron beam dump does not substantially reach the detector; the third position corresponds to a position on the target that is not configured to produce x-rays; generating a second different image between the third image and the second image; and Scaling pixel values ​​of the second different image by a predetermined scale factor; The method of claim 7 , further comprising: generating a third different image between the first different image and the scaled second different image.

Images