X-ray Imaging Systems

Abstract

An X-ray imaging system is disclosed, comprising: a target; an electron beam source configured to provide an electron beam that interacts with the target to generate X-ray radiation; electron optics configured to alternately direct the electron beam to at least first and second locations on the target; an X-ray detector array configured to receive the X-ray radiation generated at the first and second locations on the target; a sample location area for receiving a sample exposed to the generated X-ray radiation; the sample location area positioned in a region where the X-ray radiation generated at the first location overlaps with the X-ray radiation generated at the second location; and a processing unit coupled to the X-ray detector array, the processing unit configured to generate an image of the sample positioned in the sample location area based on the X-ray radiation originating from the first and second locations. The electron beam is moved between locations such that the time between successive exposures of the first and second locations is less than 10 μs, thereby reducing thermally induced mechanical stress. Corresponding methods are also disclosed.

Description

[Technical field]

[0001] The present invention relates to x-ray imaging systems in which an electron beam interacts with a target to produce x-ray radiation. [Background technology]

[0002] A prior art X-ray system utilizing multiple spots where X-ray radiation is generated by the interaction between the electron beam and the target is disclosed in U.S. Pat. No. 5,835,561. In this system, each spot is associated with one of multiple collimator elements. The collimator elements are placed in front of the emitting face of the X-ray tube, each collimator element having its aperture axis directed toward the center of the detector array and providing a divergence such that the detector array is just covered. Thus, multiple pencil beams can be generated, one from each collimator aperture, each pencil beam passing through a different part of the object being imaged. As the electron beam is scanned across the target, at any instant, there is only a single X-ray pencil beam that passes through the object to the detector array. The number of collimator apertures (corresponding to the number of spots where X-ray radiation is generated) typically corresponds to the number of pixels in the reconstructed image. The objective of this prior art system is to provide real-time X-ray imaging of cardiac patients with even lower radiation exposure than previous systems. Such systems can be useful in situations where it is desirable to keep x-ray exposure low, such as when imaging living organisms, but are less advantageous in situations where high x-ray power is required.

[0003] In "Scanning-beam digital X-ray (SBDX) system for cardiac angiography" (Proc. SPIE 3659, Medical Imaging 1999; Physics of Medical Imaging, 28 May 1999), authors Solomon et al. disclose an advanced scanning-beam digital X-ray system for cardiac angiography. An electron beam is scanned across a transmission target positioned behind a focused source collimator. The collimator is a grid of apertures whose axes are aligned with the centers of the detector array, and the divergence of the X-ray beam through the collimator apertures matches the size of the detector.

[0004] In "Motionless electromagnetic phase stepping versus mechanical phase stepping in x-ray phase-contrast imaging with a compact source" (Phys. Med. Biol. 60(2015), p.3031-3043), authors Harmon et al. disclose x-ray phase-contrast imaging and note that the x-ray focal spot can be shifted by a magnetic field to move the spot in a direction perpendicular or parallel to the grid lines, depending on the direction of the magnetic field. Summary of the Invention

[0005] The invention is based on the realization that an improved X-ray imaging system can be obtained if an electron beam is alternately directed to different positions on a target and a detector array for detecting the generated X-ray radiation is configured to perform time-resolved photon counting, such that photons detected during a specific time frame can be attributed to a respective specific position on the target. Thereby, embodiments of the invention can provide a higher photon flux without increasing the local heat load of the target or increasing the spot size.

[0006] By ensuring that the electron beam is directed at each respective location for a duration short enough that thermal equilibrium is not established, a higher power may be applied to any particular location on the target than can be deposited continuously. In other words, the period of continuous exposure of any one location on the target should have a duration shorter than the time limit.

[0007] By further ensuring that the electron beam returns to a particular location on the target, or at least approximately, within a time short enough to prevent the location from cooling significantly since the electron beam was last directed to that location, excessive thermal cycling of the target is avoided and problems associated with thermally induced mechanical stresses are avoided. To determine within what period of time the electron beam should return to the same location (i.e., what is considered to be a sufficiently short time), a characteristic time scale for each situation can be determined. In other words, the time that elapses between successive periods during which the electron beam is directed to each of the first and second locations on the target, respectively, should be shorter than the time limit.

[0008] Alternatively, rather than alternating the same electron beam between different positions, two separate electron beams can be used that are alternately switched on and off.

[0009] The target may be a solid target or a liquid target. The target may be implemented as a transmission target or a reflection target. In embodiments where the target is implemented as a liquid target, the target is preferably one or more liquid jets. If a single liquid jet is used as a target, the different positions to which the electron beam is moved alternately are on that liquid jet. The single liquid jet may be implemented as a flat jet used in a transmission mode, where the electron beam is moved between different positions along a direction substantially perpendicular to the direction of movement of the liquid jet. If two or more liquid jets are used as targets, the different positions to which the electron beam is moved alternately can be on different liquid jets. The solid reflection target may be implemented as a rotating target, i.e. a rotating anode, and in such a case the electron beam may be moved between different positions along a direction substantially perpendicular to the direction of rotation of the target.

[0010] When the X-ray radiation is detected in a time-resolved manner, each image captured by the detector can be associated with a respective one of the different locations on the target. The images relating to the different locations can then be combined into a single image of the area where the X-ray radiation from the different locations overlap, while maintaining the resolution and sharpness that would be obtained from a single X-ray spot. Thus, at least first and second images of the same portion of the object are captured, the first image being captured using X-ray radiation from a first location on the target, and the second image being captured using X-ray radiation from a second location on the target. The images are then combined into a single image of the portion of the object in post-processing.

[0011] In embodiments of the invention using solid targets, the electron beam is preferably alternated between different locations on the target at a sufficiently high rate (short duration or time between successive exposures) to emulate a thermal steady state in the target. This reduces mechanical stresses due to thermal cycling, which may lead to an increased useful life of the target. In other words, the electron beam is alternated between different locations at such a high rate that the temperature change during the duty cycle is negligible. Thus, embodiments of the invention allow for an increase in the power of the electron beam, and therefore of the generated X-ray radiation, while at the same time reducing potential problems related to thermal stresses due to temperature cycling of the target material.

[0012] In an embodiment where the electron beam alternates between two distinct positions, the duty cycle can be conveniently set to 50% for both positions, i.e. the electron beam stays at each position for half the time (ignoring the time required to move the electron beam between the two positions). By shortening the dwell time, the amount of thermal cycling can be limited. However, in an embodiment where the electron beam alternates between more than two positions, the time between two successive exposures will be longer for a given dwell time. In principle, the number of positions can be arbitrarily large, in which case each position will be exposed very little. However, in practice, the number of available positions is limited. The limitation may be related to the detector capacity (e.g. how many time-stamped values ​​each pixel can store), power supply limitations, or target size. Regardless of the reason for limiting the number of positions, it is advantageous to limit the thermal stresses induced in the target due to temperature cycling. It is preferable that at most 10 μs, e.g. less than 5 μs, or preferably less than 1 μs, elapse between successive exposures at each position. Increasing the time between exposures either runs the risk of damaging the target through excessive thermal cycling or forces limitations on the applied power to mitigate this risk.

[0013] Techniques for moving an electron beam between different positions are commonly known in the art, for example the use of electrostatic deflection. An electron beam can be moved between different positions very quickly using existing techniques. Thus, the upper limit on how fast an electron beam can be alternated between different positions on a target is not determined by the available techniques for moving the electron beam, but rather by the time resolution of the detector, since the photons generated from each respective position should be combined together to allow the captured images to be combined into a composite image during post-processing reconstruction.

[0014] In the detailed description below, equations for characteristic time scales that describe the generation and dissipation of heat in the target are derived. Preferably, the electron beam moves between locations on the target in a period that is less than 10 times such characteristic time scale, more preferably less than 5 times such characteristic time scale.

[0015] In an alternative embodiment, the target may contain different materials at different locations, thus offering the opportunity to image the sample with different X-ray spectra during one run. The power is at least two images taken with different spectra from only slightly different angles acquired simultaneously at high power. Time resolved detection can be used to relate each detection event to its respective X-ray spectrum, therefore no energy discrimination in the detector is required. This may be advantageous compared to having multiple separate X-ray sources and detectors to achieve this. A typical application of this embodiment is bone mineral density measurement. [Brief description of the drawings]

[0016] The invention will now be described in more detail with reference to the accompanying drawings. [Figure 1] FIG. 1 illustrates a typical setup according to one embodiment of the present invention. [Diagram 2]FIG. 2 illustrates a schematic diagram of an X-ray imaging system according to an embodiment. [Diagram 3] FIG. 3 illustrates diagrammatically a method according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In projection radiography, an image is created by capturing X-ray radiation that passes through an object and is absorbed to various degrees by the object. Figure 1 shows a schematic of a setup illustrating the principles of the invention. An electron beam (not shown in Figure 1) is incident on a target 110 generating X-ray radiation, which is detected using an X-ray detector array 130 after passing through a sample 120 positioned in a sample location area. The area where the electron beam interacts with the target to generate X-ray radiation can be referred to as the X-ray spot, and the size of this spot can be referred to as the spot size of the X-ray source. The image captured by the X-ray detector array is geometrically magnified by a factor M

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[0018] To achieve short exposure times with sufficient contrast, the total X-ray flux must be high enough. The X-ray flux can be increased by increasing the power of the electron beam impinging on the target. However, increasing the electron beam power while maintaining the size of the electron beam spot increases the power density delivered to the target. If the power density becomes too high, it can cause permanent damage to the target. Therefore, in order to apply more electron beam power to the target and thereby increase the total X-ray flux, it has been necessary in the prior art to increase the source size. On the other hand, this leads to a decrease in image sharpness, as mentioned above.

[0019] Therefore, any attempt to increase the total photon flux by increasing the electron beam power (to limit the peak power delivered to the target) and making the X-ray source spot size larger will inevitably result in a decrease in image sharpness.

[0020] To resolve these seemingly conflicting requirements, an embodiment of the present invention uses a movable electron beam that is alternately directed to at least a first location 110a and a second location 110b on the target 110. From a thermal standpoint, this corresponds to using a larger electron spot, since the applied power is distributed over a larger area of ​​the target. However, image sharpness is not necessarily adversely affected, since there is no need to increase the spot size at each location.

[0021] In order to maintain the high resolution available from a single spot (spot 110a or 110b as shown in FIG. 1), detected photons should be associated with the specific location from which they originated, i.e., the detector 130 and electron beam should be configured to be able to determine whether the X-ray radiation received by the X-ray detector 130 at any instant in time originated from the first location 110a or the second location 110b. For objects 120 located in an area where radiation from the separate locations 110a, 110b overlap, a high definition composite image can then be reconstructed in post-processing.

[0022] Any target has some limit on how much power can be applied continuously to a particular area without damaging the target. However, higher powers can be applied intermittently, provided that the periods during which such higher powers are applied are short on the time scale for heating and cooling of the target. The average power applied to a particular location on the target is the electron beam power (which can be assumed to be constant) multiplied by the duty cycle, i.e., the fraction of time that each location on the target is hit by the electron beam. The total power that can be delivered to the target without causing damage may also limit the power that can be applied to each particular location. This limit can be increased, for example, by providing better cooling of the target.

[0023] As an example, consider a target capable of handling a continuous heat load of P0 distributed over a circular spot of diameter D. Assuming that the electron beam is deflected alternately between two different positions with a duty cycle of 50% (i.e., the electron spot is directed at each position for 50% of the time) and ignoring the time required to move the electron beam between the two positions, the electron beam power can be increased by a factor of two. In this way, the average power applied to each location is still P0, but the total power delivered to the target is P0 divided by the duty cycle, which is equal to 2P0. This may be generalized to more than two positions. If the electron beam alternates between n positions, the duty cycle is 1 / n, and the maximum total power that can be delivered to the target is nP0 (again, assuming that the time required to move the electron beam between positions can be ignored). To be able to achieve the maximum total power without damaging the target, the positions need to be spaced far enough apart that the power applied to one position does not contribute to the heating of an adjacent position. Furthermore, if we assume that the maximum allowable power for a spot size D is P0, then a higher power nP0 applied for a short period of time will result in a spot size D eff By stipulating that the average power density should be the same, D eff teeth,

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[0024] To be able to apply a higher electron beam power than can be continuously absorbed by the target at a particular location, the time during which the higher power is applied should be less than some time limit, which depends on the type and characteristics of the target, as explained below.

[0025] Depending on the target material used and the power levels included in various embodiments of the invention, the scheme of the invention may result in problems with target life due to repeated cooling and heating of the target material. Such thermal cycling may in some circumstances result in thermally induced mechanical stresses, which in turn may result in permanent damage to the target. To avoid this, the electron beam can be alternated between different locations on the target at a sufficiently high rate (short duration or time between successive exposures) to emulate a thermal steady state in the target. The characteristic time scale for target heating is given by the thermal conductivity and volumetric heat capacity of the target as well as the electron beam spot size. It can be assumed that the time scale for target heating depends on the following properties: C Target heat capacity per unit mass [J / (kg K)] ρ Target density [kg / m 3 ] κ Target thermal conductivity [W / m K] δ Electron beam spot size [m] h Electron penetration depth of the target [m] t Thickness of the transmission target [m]

[0026] The relevant time scales can be inferred by estimating the amount of energy deposited in the target by the electron beam and the thermal power conducted by the target. Note that while the power of the electron beam affects the temperature level reached in the target, the characteristic time scale depends only on the target properties and the geometry of the electron beam. For a transparent target of thickness t (a type of target where the electron beam impinges on one side and the X-ray radiation is collected from the other side, i.e. "transparent"), the volume in which heat is deposited can be approximated by a cylinder with a (not necessarily circular) cross-sectional area defined by the electron beam spot size and a height defined by the target thickness. For non-circular spots, the electron beam spot size δ can be considered as the geometric mean of the spot sizes. The amount of energy deposited in the target per degree of temperature rise, ignoring numerical factors, can be written as: Cρδ 2 t (1)

[0027] The temperature rise in the volume hit by the electron beam causes a heat flow from this volume through the area A of the envelope of the cylinder (for a transparent target, heat conduction in the direction of the electron beam can be neglected). The heat must travel some average distance l to escape the cylinder, an approximation of which is determined by the electron beam spot size. The power transmitted from a volume hit by an electron beam per degree of temperature rise, neglecting numerical factors, can be written as:

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[0028] By dividing the stored energy per degree temperature rise according to equation (1) by the dissipated power per degree temperature rise according to equation (2), the characteristic time scale τ can be written as

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[0029] In practice, a transmission target often consists of a thin film of an x-ray generating material (e.g. tungsten) provided on a substrate (e.g. diamond) chosen for its thermal and mechanical properties. In this case, there is also heat transfer through the end face of the cylinder that is struck by the electron beam. This contribution to the heat transport depends on the heat transfer from the target material to the substrate material and the thermal conductivity of the substrate material. Since the substrate is configured to improve the thermal conductivity, more power is spread per degree of temperature increase and therefore the characteristic time scale is shorter than for a similar target provided without a substrate. The estimates provided above can therefore be considered as an upper limit for the characteristic time scale of a transmission target.

[0030] For a reflective target (a type of target in which the electron beam strikes one side of the target and the X-ray radiation is collected from the same side of the target, i.e., "reflected") with a finite electron penetration depth h, the volume in which the energy from the electron beam is deposited can be approximated by a cylinder with a base area determined by the electron beam spot size and a height given by the penetration depth. Similar to equation (1) above, the energy per degree of temperature rise can therefore be written, neglecting any numerical factors, as follows: Cρδ 2 h (4)

[0031] The power extracted from this cylinder passes through an area A defined by the envelope of the cylinder and its end faces. The average distance l that the heat must travel is determined by both the electron beam spot size and the electron penetration depth. The two lengths can be viewed as parallel paths, so the reciprocal of the average distance is the sum of the reciprocals of the two lengths. Thus, the power dissipated per degree of temperature rise, ignoring any numerical factors, can be written as:

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[0032] For the case of a transparent target, the characteristic time scale can be obtained by dividing the stored energy per degree temperature rise according to equation (4) by the dissipated power per degree temperature rise according to equation (5).

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[0033] As a numerical example, the target is C = 130 J / (kg K) and ρ = 19300 kg / m 3 Assuming that the electron beam spot is made of tungsten with κ=173 W / (mK) and that the electron spot size and penetration depth are both 5 μm, equation (6) above gives a characteristic time scale of approximately 90 ns. For a larger electron beam spot of 20 μm and the same electron penetration depth of 5 μm, the characteristic time scale becomes approximately 230 ns.

[0034] For liquid jet targets, the situation is a bit different. The dominant contribution to the cooling of the target at the electron beam spot is the flow of the target material. The characteristic time scale of this process can be written as:

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[0035] A rotating anode with realistic parameters can be considered in this context as a stationary reflective solid target. To achieve target velocities of the same order as typically used for liquid jets (on the order of hundreds of m / s), impractically large and / or fast rotating anodes would be required. In such applications, a line focus is typically used with the longer dimension of the focus perpendicular to the direction of rotation. Within the concept of the invention, a circular spot moved along a direction perpendicular to the direction of rotation may be used instead. This may further allow more advantageous take-off angles than are typically used with rotating anodes.

[0036] To allow for intermittent application of higher power to a location on the target than the target can withstand under prolonged exposure, the period of continuous exposure of any one location on the target (before the electron beam is moved from that location) is preferably less than 10 times the characteristic time scale derived above, and more preferably less than 5 times the characteristic time scale.

[0037] To protect the target from damage due to thermal cycling, it is also preferred that the period for switching the electron beam between different locations on the target is preferably less than 10 times the characteristic time scale derived above, more preferably less than 5 times the characteristic time scale. In other words, the time that elapses between successive periods during which the electron beam is directed to each of the locations on the target (in other words, the time that elapses between successive exposures) is preferably less than 10 times the characteristic time scale, more preferably less than 5 times.

[0038] As mentioned above, there is a limit to the electron power that can be applied to each electron beam spot position. If the maximum allowable continuous power is P0 in an embodiment including n positions, then when the duty cycle is 1 / n, the applied power may be set to nP0. From the above calculations, it can be assumed that the electron beam spot should remain at each respective position for a time equal to a characteristic time scale τ limited by the heat capacity and thermal conductivity of the target, which means that the electron beam should return to each respective position after nτ seconds. In each exposure, an amount of energy τnP0 will be applied to each respective location, with a corresponding temperature rise. If instead the dwell time is made shorter than τ, a smaller amount of energy will be applied to each exposure, and therefore the temperature rise will be lower. Assuming that the duty cycle and applied power are governed only by the number of positions, the total flux (X-ray photons per second) does not depend on the dwell time. Reducing the dwell time means that the time between successive exposures at each position is also reduced. Preferably the time between successive exposures at the same location is less than 10 μs, such as less than 5 μs, such as less than 1 μs.

[0039] In one embodiment, the characteristic time scale is 90 ns as calculated above. If we assume that the electron beam is alternated between two positions and the dwell time is set equal to the characteristic time scale, then the time between successive exposures will be 180 ns if we ignore the time required to move the electron beam between positions. If the dwell time is instead set to 10 times the characteristic time scale, then the time between successive exposures will be 1.8 μs.

[0040] In an embodiment with a somewhat larger spot size, the characteristic time scale is 230 ns. If the spot is alternated between 10 different positions, the time between successive exposures is 2.3 μs. If the spot is alternated between 100 different positions and the dwell time is set to 100 ns, the time between successive exposures is 10 μs.

[0041] An X-ray imaging system 200 according to the invention is shown diagrammatically in Fig. 2. It comprises a detector 230 for recording an image of, for example, an object or sample 220, made from X-ray radiation from each electron beam spot position (see 110a and 110b in Fig. 1). The system 200 comprises an X-ray target 210 and an electron source 201 generating an electron beam I. The electron source 201 generally comprises a cathode 202 output by a voltage source 700 and includes an electron source 203, for example a thermionic, thermal field or cold field charged particle source. The electron beam I from the electron source 201 may be accelerated towards an acceleration aperture 204, at which point the beam I enters a region with an electron optical system which may comprise an arrangement of alignment plates 205, lenses 206 and an arrangement of deflection plates 207. Variable properties of the alignment plates 205, the deflection plates 207 and the lenses 206 may be controllable by signals provided by a controller 500. As shown, the deflection and alignment means 207, 205 are operable to accelerate the electron beam I in at least two lateral directions.

[0042] As mentioned above, the various components and parts may be located inside the housing 600, with the exception of the voltage source 700 and the controller 500, which may be located outside the housing 600, as shown in the drawings.

[0043] Downstream of the electron optics, the electron beam I may impinge on an X-ray target 210 to generate X-ray radiation. X-ray radiation generated by the interaction between the electron beam I and the target 210 may be directed out of the housing 600, for example through the X-ray window 208, in a direction generally toward the sample 220 and the detector array 230.

[0044] The system may also optionally include a sensor device 240 for measuring / detecting electrons downstream from the target 210. The sensor may be, for example, a conductive plate connected to ground via an ammeter 242, which provides an approximate measurement of the total current driven by the electron beam I downstream of the target 210. It is understood that the controller 500 has access to the actual signal from the ammeter 242.

[0045] The size of the detector 230 may determine the available size of the sample location area. Since the present invention relies on imaging the sample with X-ray radiation originating from different positions of the target 210, the volume of overlap of the X-ray radiation and the field of view of the detector defines the usable sample location area. To avoid detector image quality limitations, the pixel size should be small compared to the blurring inevitably introduced by the finite size of the X-ray source (i.e., spot). However, the required pixel resolution may be determined from the characteristic length scale of the object intended to be inspected and the magnification of the X-ray imaging system. In order to classify the incident X-ray radiation according to its origin, the detector must be able to record a timestamp of each detection event or combine together detection events that occur within a certain time window or set of time windows. The time resolution of the detector should preferably be short compared to the time that the electron beam is located at each of the respective spot sites. An example of a detector technology that may be advantageous is the Timepix3 chip developed within the Medipix collaboration hosted by CERN. In general, any detector that provides processing power related to each pixel can be advantageously used in the present invention.

[0046] If the detector creates a timestamp for each detection event, the detection event can be related to the electron beam position if the electron beam position is known at the corresponding time. The electron beam motion may, for example, be predefined and known to the post-processing system. The first detection event can then be considered as the start time of the electron beam motion pattern, and the detection events can be mapped onto that pattern. Another embodiment can utilize a trigger signal provided by the X-ray source or detector that marks the start of the sequence. In yet another embodiment, the X-ray source generates a protocol of electron beam position as a function of time that is sent to the post-processing software along with the recordings from the detector. The post-processing software can then classify the detection events to electron beam positions based on the times recorded by the X-ray source and the detector, respectively. This requires that the X-ray source and the detector measure time on a common scale.

[0047] The creation of a high-resolution, high-contrast image from two images taken for two positions of the X-ray spot can be achieved by translating one of the images relative to the other by

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[0048] The concept of the invention also includes embodiments in which the electron beam moves continuously over the target and does not necessarily stop at a specific position. An example of such an embodiment includes an electron beam that is moved continuously back and forth at a predetermined frequency, for example by applying a triangular wave drive signal to the deflection plates. Collecting photons emitted from a moving spot means that motion blur may be introduced, similar to the blur caused by the finite size of the X-ray spot as described above. Therefore, the speed of movement of the electron beam should be set such that the motion blur introduced during the time between readouts from the detector is considered negligible. In an embodiment in which the electron beam moves between different positions at a constant frequency, images collected at the same frequency are generated by X-ray radiation emitted from the same position. Thus, by shifting the position of these images in post-processing according to a predetermined movement, images with high resolution and high contrast can be obtained. If the exact timing is not known, i.e., if the post-processing system does not have access to the electron beam position at a specific time, different assumptions can be tried until no further improvement in the image contrast can be achieved. This can be considered as finding the phase of the electron beam motion when the frequency and amplitude are known.

[0049] As an example, consider a setup for imaging a millimeter-sized object such as an insect or a flower bud. The triangular drive signal applied to the deflection plates can be configured to move the electron beam back and forth between two positions on the target, spaced 1 mm apart. The imaging optics can suitably provide a magnification of about 3 to give adequate resolution for propagation-based phase-contrast imaging on the aforementioned Timepix2 chip with 55 μm pixels. A suitable source-to-detector distance is in this case about 2 m. The electrostatic deflection driven by the triangular drive signal can be set to a frequency of 1 MHz and an amplitude that provides a 1 mm separation between the extreme points of the electron beam on the target, such that the spot moves by 3 μm with a time precision of 1.5 ns on the Timepix3 detector. The detected X-ray radiation is then binned into bins of size 1 ns according to their respective time stamps modulo 1 μs. Each bin gives one image when it contains pixel coordinate data. The images can then be combined in post-processing by shifting (i.e. translating) the images according to the original triangular drive signals with different phases to find which phase provides the highest contrast.

[0050] At least two images taken from different directions can be used to obtain a partial tomographic reconstruction of the sample. The techniques developed for tomosynthesis and computed tomography can be used since they were developed for cases where only a limited number of projections are available. The skilled person will understand that it may be advantageous to combine the translation and addition of images as described above with different tomographic techniques in order to extract as much information as possible from the images obtained.

[0051] FIG. 3 illustrates diagrammatically the method according to the invention. X-ray radiation is generated 301 by directing an electron beam onto the target, the electron beam being directed alternately to at least a first and a second location on the target, thereby generating X-ray radiation alternately at the first and the second location. To prevent overheating of the target, the electron beam should not be maintained at the same location for more than a predefined time limit. Preferably, this time limit is less than 10 times, in particular less than 5 times, the characteristic time scale given by equations (6) and (7) above. To further prevent damage to the target due to thermal cycling, the electron beam should be alternated between positions at a sufficiently high speed, i.e. the duration of the alternating movement should be below the time limit. Preferably, the electron beam is directed alternately to the first and the second location on the target with a duration less than 10 times, in particular less than 5 times, the characteristic time scale given by equations (6) and (7) above. In other words, the time between successive exposures should be less than 10 μs, such as less than 5 μs, for example less than 1 μs. In some embodiments, the electron beam is blanked, i.e., blocked, during switching between the first and second positions, so that no x-ray radiation is generated from areas outside the first and second positions. In other embodiments, the electron beam continuously strikes the target as the target is swept between the first and second positions. While this disclosure focuses on examples where there are two primary positions from which x-ray radiation is generated, it will be apparent that more than two positions can be used to generate x-ray radiation, and in embodiments where the electron beam is swept across the target, x-ray radiation is generated from multiple positions. As an example, the electron beam can be directed onto the target according to a predetermined pattern, which can be used to later create an image from the detected x-ray radiation.

[0052] The generated X-ray radiation is directed 302 to a sample location area where a sample to be investigated can be placed. The sample location area is positioned in an area where the X-ray radiation generated at the first location overlaps with the X-ray radiation generated at the second location. The X-ray radiation passing through the sample location area is then detected 303 using an X-ray detector array. The detection of the X-ray radiation passing through the sample location area is correlated 304 with the direction of the electron beam, and an image is created based on the X-ray radiation originating from the first and second locations. In embodiments where the electron beam is directed on the target according to a predetermined pattern, such correlation between the direction of the electron beam and the detected X-ray radiation can be based on the predetermined pattern as described above. In some embodiments, a partial tomographic reconstruction is created based on the X-ray radiation originating from the first and second locations.

[0053] [Conclusion] An X-ray imaging system is disclosed, the X-ray imaging system comprising a target, an electron beam source configured to provide an electron beam that interacts with the target to generate X-ray radiation, electron optics configured to alternately direct the electron beam to at least first and second locations on the target, an X-ray detector array configured to receive the X-ray radiation generated at the first and second locations on the target, a sample location area for receiving a sample exposed to the generated X-ray radiation, where the sample location area is positioned in a region where the X-ray radiation generated at the first location overlaps with the X-ray radiation generated at the second location, and a processing unit coupled to the X-ray detector array, the processing unit configured to create an image of the sample positioned at the sample location area based on the X-ray radiation originating from the first and second locations. In an embodiment of the invention, the electron beam is moved such that the exposure time of the generated X-ray radiation is sufficiently short each time the electron beam is directed to a particular location. In a further embodiment, the electron beam is moved between locations for a period of time that is sufficiently short such that temperature cycling, and thereby thermally induced mechanical stresses, are reduced. Corresponding methods are also disclosed. The following is a summary of the claims as originally filed: [1] An X-ray imaging system comprising: Target, an electron beam source configured to provide an electron beam that interacts with the target to generate x-ray radiation; an electron-optical system configured to alternately direct the electron beam to at least first and second locations on the target; an x-ray detector array configured to receive x-ray radiation generated at the first and second locations on the target; a sample location area for receiving a sample to be exposed to the generated X-ray radiation, wherein the sample location area is positioned in a region where the X-ray radiation generated at the first location overlaps with the X-ray radiation generated at the second location; a processing unit coupled to the X-ray detector array, the processing unit configured to generate an image of a sample positioned in the sample location area based on the X-ray radiation originating from the first location and the second location; the x-ray detector array and the electron beam source are configured to determine whether the x-ray radiation received by the x-ray detector at any instant in time originates from the first location or the second location on the target; An x-ray imaging system, wherein the time between successive exposures at the first position and the second position is less than 10 μs. [2] The X-ray imaging system of [1], wherein the time between successive exposures of the first position and the second position is less than 5 μs, for example less than 1 μs. [3] The X-ray imaging system of [1] or [2], wherein the electron optical system is configured to alternately direct the electron beam to at least the first and second positions on the target such that a period of successive exposure of any one position has a duration shorter than a time limit. [4] The target is a solid reflective target, and the time limit is:

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[10] The method of [9], wherein the time between successive exposures of the at least first and second locations is less than 5 μs, for example less than 1 μs.

[11] The method of [9] or

[10] , wherein the electron beam is alternately directed to the at least first and second locations on the target such that a period of continuous exposure of any one location has a duration shorter than a time limit.

[12] The target is a solid reflective target, and the time limit is:

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[13] The target is a transparent target, and the time limit is:

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[14] The target includes at least one liquid jet, and the time limit is:

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[15] The method of any one of [9] to

[14] , further comprising blanking the electron beam during switching between the first and second positions on the target such that x-ray radiation is not generated from areas of the target outside the first and second positions.

[16] The method of any one of [9] to

[15] , wherein the electron beam is directed onto the target according to a predetermined pattern, and the image is created based on the predetermined pattern.

[17] The method of any one of [9] to

[16] , wherein the target comprises different materials at the first and second locations that generate X-ray radiation having different spectra when the electron beam is directed at each location.

Claims

1. 1. An X-ray imaging system comprising: Target and an electron beam source configured to provide an electron beam that interacts with the target to generate x-ray radiation; an electron-optical system configured to alternately direct the electron beam to at least first and second locations on the target; an x-ray detector array configured to receive x-ray radiation generated at the first and second locations on the target; a sample location area for receiving a sample to be exposed to the generated X-ray radiation, wherein the sample location area is positioned in a region where the X-ray radiation generated at the first location overlaps with the X-ray radiation generated at the second location; a processing unit coupled to the X-ray detector array, the processing unit configured to generate an image of a sample located in the sample location area based on the X-ray radiation originating from the first location and the second location; the X-ray detector array and the electron beam source are configured to determine whether the X-ray radiation received by the X-ray detector at any instant in time originates from the first location or the second location on the target; A time between successive exposures of each of the first and second positions is less than 10 μs, respectively.

2. 2. The X-ray imaging system of claim 1, wherein the time between successive exposures of the first position and the second position is less than 5 [mu]s, such as less than 1 [mu]s.

3. 2. The x-ray imaging system of claim 1, wherein the electron optics is configured to alternately direct the electron beam to the at least first and second locations on the target such that a period of successive exposure of any one location has a duration shorter than a time limit.

4. the target is a solid reflective target and the time limit is: [0010] is 10 times, preferably 5 times, the characteristic time scale τ, given by Here, S is [0025] is given by 4. The x-ray imaging system of claim 3, wherein ρ is the density of the target, C is the heat capacity per unit mass of the target, κ is the thermal conductivity of the target, δ is the spot diameter of the electron beam on the target, and h is the penetration depth of electrons into the target.

5. The target is a transmissive target, and the time limit is: [0030] is 10 times, preferably 5 times, the characteristic time scale τ, given by 4. The x-ray imaging system of claim 3, wherein ρ is the density of the target, C is the heat capacity per unit mass of the target, κ is the thermal conductivity of the target, and δ is the spot size of the electron beam at the target.

6. The target includes at least one liquid jet, and the time limit comprises: [0045] is 10 times, preferably 5 times, the characteristic time scale τ, given by where δ is the spot size of the electron beam at the target along the direction of movement of the at least one liquid jet, and v ｊｅｔ The X-ray imaging system of claim 3 , wherein: V v v is the velocity of travel of the at least one liquid jet.

7. 2. The x-ray imaging system of claim 1, wherein the electron beam source is configured to blank the electron beam during switching between the first and second positions on the target such that x-ray radiation is not generated from areas of the target outside the first and second positions.

8. 2. The x-ray imaging system of claim 1, wherein the target includes different materials at the first location and the second location that generate x-ray radiation having different energy spectra when the electron beam is directed at the respective locations.

9. 1. A method for x-ray imaging comprising: generating x-ray radiation by directing an electron beam onto a target, wherein the electron beam is alternately directed to at least a first location and a second location on the target, thereby alternately generating x-ray radiation at the first location and the second location; directing the generated X-ray radiation toward a sample location area, where the sample location area is located in an area where the X-ray radiation generated at the first location overlaps with the X-ray radiation generated at the second location; detecting X-ray radiation passing through said sample location area using an X-ray detector array; and correlating a direction of the electron beam with detection of X-ray radiation passing through the sample location area to determine whether the X-ray radiation received by the X-ray detector array at any instant in time originated from the first location or the second location, and generating an image based on the X-ray radiation originating from the first location and the second location; A method wherein the time elapsed between successive exposures of each of said at least first and second locations is less than 10 μs each.

10. 10. The method of claim 9, wherein the time between successive exposures of the at least first and second locations is less than 5 μs, such as less than 1 μs.

11. 10. The method of claim 9, wherein the electron beam is alternately directed to the at least first and second locations on the target such that a period of continuous exposure of any one location has a duration less than a time limit.

12. the target is a solid reflective target and the time limit is: [0050] is 10 times, preferably 5 times, the characteristic time scale τ, given by Here, S is [006] is given by 12. The method of claim 11 , wherein ρ is the density of the target, C is the heat capacity per unit mass of the target, κ is the thermal conductivity of the target, δ is the spot diameter of the electron beam on the target, and h is the penetration depth of electrons into the target.

13. The target is a transmissive target, and the time limit is: [0070] is 10 times, preferably 5 times, the characteristic time scale τ, given by 12. The method of claim 11, wherein ρ is the density of the target, C is the heat capacity per unit mass of the target, κ is the thermal conductivity of the target, and δ is the spot size of the electron beam at the target.

14. The target includes at least one liquid jet, and the time limit comprises: [0080] is 10 times, preferably 5 times, the characteristic time scale τ, given by where δ is the spot size of the electron beam at the target along the direction of movement of the at least one liquid jet, and v ｊｅｔ The method of claim 11 , wherein V is the velocity of travel of the at least one liquid jet.

15. 10. The method of claim 9, further comprising blanking the electron beam during switching between the first and second positions on the target such that x-ray radiation is not generated from areas of the target outside of the first and second positions.

16. The method of claim 9 , wherein the electron beam is directed onto the target according to a predetermined pattern, and the image is created based on the predetermined pattern.

17. 10. The method of claim 9, wherein the target includes different materials at the first and second locations that produce x-ray radiation having different spectra when the electron beam is directed at the respective locations.

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