X-ray imaging system

Abstract

An X-ray imaging system is disclosed, comprising: a target; an electron beam source configured to provide an electron beam for interacting with the target to generate X-ray radiation; electron optics configured to alternately direct the electron beam to at least a first location and a second location 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 region for receiving a sample to be exposed to the generated X-ray radiation, the sample location region being located 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 being configured to create an image of the sample located in the sample location region based on the X-ray radiation originating from the first and second locations. The electron beam is moved between the 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. A corresponding method is also disclosed.

Description

Technical Field

[0001] The present invention relates to an X-ray imaging system in which an electron beam interacts with a target to generate X-ray radiation. Background Art

[0002] US Pat. No. 5,835,561 discloses a prior art X-ray system that utilizes multiple spots, from which X-ray radiation is generated through the interaction between an electron beam and a target. In this system, each spot is associated with one of multiple collimating elements. The collimating elements are placed in front of the emission face of the X-ray tube, with the aperture axis of each collimating element pointing toward the center of the detector array and providing a divergence that precisely covers the detector array. This generates multiple pencil beams, one from each collimating aperture, each penetrating a different portion of the object being imaged. As the electron beam scans across the target, only a single X-ray pencil beam passes through the object and reaches the detector array at any given moment. The number of collimating apertures (corresponding to the number of spots used to generate X-ray radiation) generally corresponds to the number of pixels in the reconstructed image. This prior art system aims to provide real-time X-ray imaging of cardiac patients at a lower radiation exposure than earlier systems. While such a system is useful in situations where it is desirable to maintain low X-ray exposure, such as when imaging living subjects, it is less advantageous when high X-ray power is desired.

[0003] An advanced scanning beam digital X-ray system for cardiac angiography is disclosed in "Scanning Beam Digital X-ray (SBDX) System for Cardiac Angiography" (Proc. SPIE 3659, Medical Imaging 1999; Physics of Medical Imaging, May 28, 1999, Solomon et al.). The electron beam is scanned across a transmission target located behind a focused source collimator. The collimator is a network of apertures whose axes are aligned with the center of the detector array, and the divergence of the X-ray beam passing through the collimator apertures is matched to the detector size.

[0004] X-ray phase contrast imaging is disclosed in "Stationary electromagnetic phase stepping versus mechanical phase stepping in X-ray phase contrast imaging using a compact source" (Phys.Med.Biol.60(2015), pp. 3031-3043, author Harmon et al.), and it is mentioned that the X-ray focus can be offset by a magnetic field so that the focus moves in a direction perpendicular or parallel to the grating lines depending on the direction of the magnetic field. Summary of the Invention

[0005] The present invention is based on the recognition that an improved X-ray imaging system can be obtained if the electron beam is directed alternately to different locations on the target and the detector array used to detect the generated X-ray radiation is configured to perform time-resolved photon counting so that photons detected during a specific time range can be attributed to a corresponding specific location on the target. Thus, embodiments of the present 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 to each respective location for a duration short enough that thermal equilibrium does not establish, it is possible to apply a higher power than can be continuously deposited at any particular location on the target. In other words, the duration of the continuous exposure period for any one location on the target should be shorter than the time limit.

[0007] By further ensuring that the electron beam returns, or at least approximately returns, to a particular location on the target within a sufficiently short time that the location has not cooled significantly since the electron beam was last directed at that location, excessive thermal cycling of the target is avoided, and problems associated with thermally induced mechanical stresses are avoided. To determine the time period within which the electron beam should return to the same location (i.e., what is considered a sufficiently short time), a characteristic time scale can be determined for each situation. In other words, the time elapsed between successive periods in which the electron beam is directed to each of the first and second locations on the target, respectively, should be shorter than a time limit.

[0008] Alternatively, instead of alternating the same electron beam between different positions, two separate electron beams may be used which are alternately switched on and off.

[0009] The target can be a solid target or a liquid target. The target can be implemented as a transmission target or a reflection target. In an embodiment in which 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 the target, the different positions between which the electron beam moves alternately are on the liquid jet. The single liquid jet can be implemented as a flat jet for use in transmission mode, wherein the electron beam moves between different positions in a direction substantially perpendicular to the direction of travel of the liquid jet. If two or more liquid jets are used as targets, the different positions between which the electron beam moves alternately can be on different liquid jets. The solid reflection target can be implemented as a rotating target, i.e., a rotating anode, in which case the electron beam can move between different positions in a direction substantially perpendicular to the direction of rotation of the target.

[0010] When X-ray radiation is detected in a time-resolved manner, each image captured by the detector can be associated with a corresponding location on the target. The images associated with the different locations can then be combined into a single image of the region where X-ray radiation from the different locations overlaps, while maintaining the resolution and clarity obtained from a single X-ray spot. Thus, at least first and second images of the same portion of the object are captured, where the first image is captured using X-ray radiation from a first location on the target, and the second image is captured using X-ray radiation from a second location on the target. These images are then combined into a single image of the portion of the object in post-processing.

[0011] In embodiments of the present invention using a solid target, the electron beam preferably alternates between different locations on the target at a sufficiently high rate (with a short period or time between successive exposures) to simulate a thermal steady state in the target. Thus, mechanical stresses due to thermal cycling are reduced, which in turn may increase the useful life of the target. In other words, the electron beam alternates between different locations at such a high rate that any temperature changes during the duty cycle are negligible. Thus, embodiments of the present invention allow for increasing the power of the electron beam, and thereby the power of the generated X-ray radiation, while mitigating potential problems associated with thermal stresses due to temperature cycling of the target material.

[0012] In embodiments where the electron beam alternates between two different positions, it is convenient to set the duty cycle for these two positions to 50%, meaning the electron beam resides at the corresponding position half of the time (neglecting the time required for the electron beam to move between the two different positions). By keeping the dwell time short, the amount of thermal cycling can be limited. However, in embodiments 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 is exposed only rarely. However, in practice, the number of available positions will be limited. This limitation may be related to detector capacity (e.g., how many timestamp values ​​can be stored per pixel), 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 preferred that no more than 10 μs, for example, less than 5 μs, or preferably less than 1 μs, elapse between successive exposures at the corresponding positions. Allowing longer times between exposures either risks damaging the target due to excessive thermal cycling or necessitates limiting the applied power to mitigate this risk.

[0013] Techniques for moving an electron beam between different locations are well known in the art, such as using electrostatic deflection. Existing techniques can be used to move an electron beam between different locations at very high rates. Therefore, the upper limit on the speed at which an electron beam can alternate between different locations on a target is not determined by the available technology for moving the electron beam, but rather by the temporal resolution of the detector, since the photons generated from each corresponding location must be combined to allow the captured images to be combined into a composite image during post-processing reconstruction.

[0014] In the detailed description below, an expression describing the characteristic time scale for heat generation and dissipation in the target will be given. Preferably, the electron beam moves between positions on the target with a time period less than 10 times this characteristic time scale, more preferably less than 5 times this characteristic time scale.

[0015] In an alternative embodiment, the target can comprise different materials at different locations, thus providing the opportunity to image the sample with different X-ray spectra during a single run. The output would be at least two images acquired simultaneously at high power using different spectra from only slightly different angles. This time-resolved detection can be used to associate each detection event with a corresponding X-ray spectrum, eliminating the need for energy discrimination in the detector. This can be advantageous over 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 present invention will now be described in more detail with reference to the accompanying drawings, in which:

[0017] Figure 1 A typical setup according to an embodiment of the present invention is shown;

[0018] Figure 2 schematically illustrates an X-ray imaging system according to an embodiment; and

[0019] Figure 3 The method according to the invention is schematically illustrated. DETAILED DESCRIPTION

[0020] In projection radiography, images are created by capturing X-ray radiation that has passed through an object and been absorbed to varying degrees by the object. Figure 1 A setup illustrating the principles of the invention is shown schematically. Figure 1The electron beam (not shown) is incident on the target 110 to generate X-ray radiation, and after passing through the sample 120 located in the sample position area, the X-ray radiation is detected using the X-ray detector array 130. The area where the electron beam interacts with the target to generate X-ray radiation can be called an X-ray spot, and the size of the spot can be called the spot size of the X-ray source. The image captured by the X-ray detector array is geometrically magnified M times.

[0021]

[0022] Where SDD is the source-to-detector distance, and SOD is the source-to-object distance. To achieve maximum clarity in the captured image, the spot size of the X-ray source should be as small as possible. Any finite-size X-ray source will produce an image unsharpness given by

[0023] Unsharpness

[0024] where d is the source size, e.g. the diameter of the X-ray spot.

[0025] To achieve short exposure times and sufficient contrast, the total X-ray flux must be sufficiently high. This can be achieved by increasing the power of the electron beam striking the target. However, increasing the electron beam power while maintaining the electron beam spot size increases the power density delivered to the target. If the power density is too high, permanent damage to the target may occur. Therefore, in order to apply more electron beam power to the target and thereby increase the total X-ray flux, the prior art requires increasing the source size. This, on the other hand, results in reduced image clarity, as described above.

[0026] Therefore, any attempt to increase the total photon flux by increasing the electron beam power and making the X-ray source spot size larger (to limit the peak power delivered to the target) will inevitably lead to a reduction in image clarity.

[0027] To resolve these seemingly conflicting requirements, embodiments of the present invention utilize a movable electron beam that is alternately directed to at least a first location 110a and a second location 110b on target 110. From a thermal perspective, this results in a larger electron spot, as the applied power is distributed over a larger area of ​​the target. However, image clarity is not necessarily adversely affected, as the spot size at each location does not need to be increased.

[0028] To maintain the Figure 1To obtain a high resolution (e.g., a spot at 110a or 110b as shown), the detected photons should be associated with the specific location from which they originated, i.e., the detector 130 and the electron beam should be configured so that it can be determined whether the X-ray radiation received by the X-ray detector 130 at any moment originated from the first location 110a or the second location 110b. For an object 120 located in the region where the radiation from the separate locations 110a, 110b overlaps, a high-resolution composite image can then be reconstructed in post-processing.

[0029] Any target has a certain limit on how much power can be applied continuously to a certain area without damaging the target. However, higher powers can be applied intermittently, provided that the time period during which such higher powers are applied is short on the time scale of heating and cooling the target. The average power applied to a specific location on the target will be the electron beam power (which can be assumed to be constant) multiplied by the duty cycle, that is, the fraction of time 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 specific location. This limit can be increased, for example, by providing better cooling of the target.

[0030] For example, consider a target that can handle a continuous heat load 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 pointed at the corresponding position 50% of the time) and neglecting the time required for the electron beam to move between the two positions, the electron beam power can be increased by a factor of 2. In this way, the average power applied to the corresponding position will still be P0, but the total power delivered to the target will be P0 divided by the duty cycle, which is equal to 2P0. This can be generalized to more than two positions. For the case where the electron beam alternates between n positions, the duty cycle will be 1 / n, and the maximum total power that can be delivered to the target becomes nP0 (again assuming that the time required for the electron beam to move between positions can be neglected). In order to be able to achieve the maximum total power without damaging the target, the positions need to be sufficiently spaced so that the power applied to one position does not cause heating of adjacent positions. Assuming again that the maximum allowed power for a spot size D is P0, it can be considered that the higher power nP0 applied during a short period of time is distributed over a spot of size D. eff By stipulating that the average power density should be the same, it can be seen that D eff Must be scaled to Therefore, the distance between adjacent positions can be chosen to be greater than In the case where the electron beam moves continuously along a closed path over the target, the duty cycle can be approximated as the electron beam spot diameter D divided by the path length (L). Therefore, the total power delivered to the target in this case will be POL / D.

[0031] In order to be able to apply a higher electron beam power than the target can continuously absorb at a particular location, the higher power should be applied for a time shorter than a certain time limit. This time limit will depend on the type and characteristics of the target, as described below.

[0032] Depending on the target material used and the power levels involved in the various embodiments of the invention, the scheme of the invention may introduce problems related to the target lifetime due to repeated cooling and heating of the target material. Such thermal cycling may in some cases generate thermally induced mechanical stresses which in turn may lead to permanent damage to the target. In order to avoid this, the electron beam may be alternated between different positions on the target at a sufficiently high rate (with a short period or time between successive exposures) in order to simulate 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 and the electron beam spot size. It can be assumed that the time scale for target heating depends on the following characteristics

[0033]

[0034]

[0035] The relevant time scales can be derived by estimating the energy deposited in the target by the electron beam and the heat power conducted away by the target. It can be noted that although the power of the electron beam affects the temperature level reached in the target, the characteristic time scale depends only on the properties of the target and the geometry of the electron beam. For a transmission target (a target where the electron beam impinges on one side and collects X-ray radiation from the opposite side, i.e., "transmissive") of thickness t, the volume in which the heat is deposited can be approximated as a cylinder (not necessarily circular) with a 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 to be the geometric mean of the spot sizes. Then, neglecting numerical factors, the amount of energy deposited in the target per degree of temperature increase can be written as

[0036] Cρδ 2 t (1)

[0038] The temperature increase within the volume struck by the electron beam will drive a heat flow from that volume through the area A of the cylinder's envelope (for transmission targets, heat conduction in the direction of the electron beam can be neglected). Heat needs to travel some average distance l to escape from the cylinder; this distance is approximately determined by the size of the electron beam spot. Then, neglecting numerical factors, the power conducted away from the volume struck by the electron beam per degree of temperature increase can be written as

[0039]

[0040] By dividing the deposited energy per degree temperature rise according to expression (1) by the dissipated power per degree temperature rise according to expression (2), the characteristic time scale τ can be written as

[0041]

[0042] In practice, transmission targets typically consist of a thin film of an X-ray generating material (e.g., tungsten) placed on a substrate (e.g., diamond) selected for its thermal and mechanical properties. In this case, there will also be heat transfer across the end face of the cylinder struck by the electron beam. This contribution to heat transfer will depend on the heat transfer from the target material to the substrate material and the thermal conductivity of the substrate material. Since the substrate is arranged to increase thermal conductivity, more power will be consumed per degree of temperature increase, and thus the characteristic time scale will be shorter than for a similar target without a substrate. Therefore, the estimate provided above can be regarded as an upper limit for the characteristic time scale of the transmission target.

[0043] For a reflective target (a target where the electron beam strikes one side of the target and collects the X-ray radiation from the same side, i.e., "reflective") with a finite electron penetration depth h, the volume in which the heat from the electron beam is deposited can be approximated as a cylinder whose base area is determined by the electron beam spot size and whose height is given by the penetration depth. Similar to expression (1) above, therefore, neglecting any numerical factors, the energy per degree of temperature increase can be written as

[0044] Cρδ 2 h (4)

[0046] The power dissipated from this cylinder will travel through an area A defined by the cylinder's envelope and its end faces. The average distance l that the heat must travel is determined by the electron beam spot size and the electron penetration depth. These two lengths can be considered as parallel paths, so the reciprocal of the average distance is the sum of the reciprocals of these two lengths. Therefore, neglecting any numerical factors, the power dissipated per degree of temperature rise can be written as

[0047]

[0048] For the case of a transmissive target, the characteristic time scale can be obtained by dividing the deposited energy per degree temperature rise according to expression (4) by the dissipated power per degree temperature rise according to expression (5).

[0049]

[0050] where S is the geometric factor of the reciprocal length of the dimension, given by

[0051]

[0052] For large electron spots, it can be assumed that δD>>Dh, S is reduced to h / δ 2 And the time scale is mainly determined by the penetration depth

[0053]

[0054] For a tightly focused electron spot, it can be assumed that h>>Dδ, S decreases to 1 / h, and the time scale is mainly determined by the electron spot size

[0055]

[0056] In the latter case, heat conduction through the end faces of the heating cylinder can be neglected and the situation becomes the same as for a transmission target, so the characteristic time scale in this limit is the same as for a transmission target, see expression (3) above.

[0057] As a numerical example, it is assumed that the target is made of tungsten with C = 130 J / (kg K), ρ = 19300 kg / m 3 , κ = 173 W / (m K), and assuming both an electron spot size and a penetration depth of 5 μm, the above expression (6) gives a characteristic time scale of about 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 about 230 ns.

[0058] For liquid jet targets, the situation is somewhat different. The main contribution to target cooling at the electron beam spot is the flow of target material. The characteristic time scale of this process can be written as

[0059]

[0060] where δ is the spot size of the electron beam at the liquid jet target along the direction of travel of the liquid jet, and v jet is the velocity of the liquid jet.

[0061] In this case, a rotating anode with realistic parameters can be treated as a stationary, reflective solid target. To achieve target velocities of the same order of magnitude as those typically used for liquid jets (of the order of hundreds of m / s), an impractically large and / or rapidly rotating anode 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 scope of the present invention, a circular spot moving in a direction perpendicular to the direction of rotation can alternatively be used. This can also achieve more favorable emission angles than are typically used with rotating anodes.

[0062] In order to be able to intermittently apply a higher power to a certain position on the target than the target can withstand under long-term exposure, the continuous exposure period of any position on the target (before the electron beam moves away from that position) is preferably less than 10 times the characteristic time scale derived above, and more preferably less than 5 times the characteristic time scale.

[0063] In order to protect the target from damage caused by thermal cycling, it is also preferred that the time period for switching the electron beam between different locations on the target is preferably less than 10 times the characteristic time scale derived above, and more preferably less than 5 times the characteristic time scale. In other words, the time that elapses between successive periods in which the electron beam is directed to each location on the target (in other words, the time that elapses between successive exposures) is preferably less than 10 times the characteristic time scale, and more preferably less than 5 times the characteristic time scale.

[0064] As previously mentioned, there are limits on the electron power that can be applied to each corresponding electron beam spot location. Assuming that the maximum allowable continuous power is P0 in an embodiment with n locations, the applied power can be set to nP0 when the duty cycle is 1 / n. Based on the above calculations, it can be assumed that the electron beam spot should reside at each corresponding location for a characteristic time scale τ, limited by the target's heat capacity and thermal conductivity. This means that the electron beam should return to each corresponding location after nτ seconds. During each exposure, a certain amount of energy τnP0 will be applied to each corresponding location, accompanied by a corresponding temperature increase. Conversely, if the dwell time is shorter than τ, a smaller amount of energy will be applied during each exposure, resulting in a lower temperature increase. If the duty cycle and applied power are controlled solely by the number of locations, 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 location will also be reduced. Preferably, the time between successive exposures at the same location is less than 10 μs, for example, less than 5 μs, or even less than 1 μs.

[0065] In one embodiment, the characteristic time scale is 90 ns, as calculated above. Assuming the electron beam alternates between two positions and the dwell time is set equal to the characteristic time scale, the time between successive exposures will be 180 ns if the time required for the electron beam to move between these positions is ignored. Conversely, if the dwell time is set to ten times the characteristic time scale, the time between successive exposures will be 1.8 μs.

[0066] In an embodiment with a slightly larger spot size, the characteristic time scale is 230 ns. If the spot alternates between 10 different positions, the time between successive exposures will be 2.3 μs. If the spot alternates between 100 different positions and the dwell time is set to 100 ns, the time between successive exposures will be 10 μs.

[0067] Figure 2 An X-ray imaging system 200 according to the invention is schematically shown. It comprises a detector 230 for recording an image of, for example, an object or sample 220, the image being generated by X-ray radiation from the corresponding electron beam spot position (see Figure 1 110a and 110b in FIG. 110b). System 200 includes an X-ray target 210 and an electron source 201 for generating an electron beam 1. Electron source 201 generally includes a cathode 202 powered by a voltage source 700 and includes an electron source 203, such as a thermionic, hot field, or cold field charged particle source. Electron beam 1 from electron source 201 can be accelerated toward an acceleration aperture 204, at which point beam 1 enters a region having electron optics, which can include an arrangement of alignment plates 205, a lens 206, and a deflection plate 207. Variable characteristics of alignment plates 205, deflection plates 207, and lens 206 can be controlled by signals provided by controller 500. As shown, deflection and alignment devices 207, 205 are operable to accelerate electron beam 1 in at least two lateral directions.

[0068] The various components mentioned above may be located inside the housing 600 , with the possible exception of the voltage source 700 and the controller 500 , which may be located outside the housing 600 as shown in the figure.

[0069] Downstream of the electron optics, the electron beam 1 can strike the X-ray target 210 to generate X-ray radiation. The X-ray radiation generated by the interaction between the electron beam 1 and the target 210 can exit the housing 600 via, for example, the X-ray window 208 in a direction generally toward the sample 220 and the detector array 230.

[0070] The system may also optionally include a sensor device 240 for measuring / detecting electrons downstream of the target 210. Such a sensor may be, for example, a conductive plate connected to ground via an ammeter 242, which provides an approximate measure of the total current carried by the electron beam 1 downstream of the target 210. It will be appreciated that the controller 500 has access to the actual signal from the ammeter 242.

[0071] The size of the detector 230 can determine the available size of the sample position area. Since the present invention relies on imaging the sample using X-ray radiation originating from different positions of the target 210, the volume in which the X-ray radiation and the detector's field of view overlap defines the available sample position area. To avoid the detector limiting the image quality, the pixel size should be small compared to the unsharpness that is necessarily introduced by the finite size of the X-ray source (i.e., the spot). However, the required pixel resolution can be determined based on the characteristic length scale of the object to be inspected and the magnification of the X-ray imaging system. In order to classify the incoming X-ray radiation according to its point of origin, the detector must be able to record a timestamp for each detection event, or to merge together detection events that occur within a certain time window or a set of time windows. The temporal resolution of the detector should preferably be short compared to the time that the electron beam is at each corresponding spot position. An example of a potentially advantageous detector technology is the Timepix3 chip developed by the Medipix collaboration hosted by CERN. In general, detectors that provide processing capabilities associated with each pixel can be advantageously used in the present invention.

[0072] Assuming that the detector creates a timestamp for each detection event, the detection events can be associated with the electron beam positions, provided that these are known at these corresponding times. For example, the electron beam motion can be predefined and known to the post-processing system. The first detection event can then be considered the start time of the electron beam motion pattern, and the detection events can be mapped onto this pattern. Another embodiment can use a trigger signal provided by the X-ray source or the detector to mark the start of the sequence. In yet another embodiment, the X-ray source generates a protocol of the electron beam position as a function of time, which is sent to the post-processing software together with the records from the detector. The post-processing software can then classify the detection events into 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.

[0073] Creating a high-resolution, high-contrast image from two images obtained at two locations for the X-ray spot can be accomplished by translating one image relative to the other by a certain amount.

[0074]

[0075] Where b is the distance between the electron beam spot positions on the target. An expression for Δ can be derived from basic geometry. After the translation, the two images can be added together. Those skilled in the art will recognize that adding the two images together can be performed in a variety of ways and can also include filtering to improve the signal-to-noise ratio. One example is to add each individual pixel and divide the result by two. In this way, the images are given equal weight, and the pixel values ​​remain within the dynamic range of the original image.

[0076] Embodiments in which the electron beam moves continuously over the target, rather than necessarily stopping at a specific location, are also encompassed by the present invention. Examples of such embodiments include an electron beam that moves 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 can introduce motion blur, similar to the blur caused by a finite-sized X-ray spot discussed above. Therefore, the speed of the electron beam should be set so that the motion blur introduced during the time between detector readouts can be considered negligible. In embodiments in which the electron beam moves between different locations at a constant frequency, images collected at the same frequency will be generated from X-ray radiation emitted from the same location. Therefore, by shifting the positions of these images according to a prescribed motion during post-processing, images with high resolution and contrast can be obtained. If the exact timing is unknown—that is, if the post-processing system cannot access the electron beam position at a specific moment—different hypotheses can be tried until no further improvement in image contrast is achieved. This can be thought of as finding the phase of the electron beam motion when the frequency and amplitude are known.

[0077] For example, consider a setup for imaging millimeter-sized objects, such as insects or flower buds. The triangular wave drive signal applied to the deflection plates can be configured to move the electron beam back and forth between two locations on the target 1 mm apart. The imaging optics can then appropriately provide a magnification of approximately 3x to provide appropriate resolution for propagation-based phase contrast imaging on the aforementioned Timepix2 chip with 55 μm pixels. In this case, a suitable source-to-detector distance might be approximately 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 spacing between the electron beam extremes on the target, causing the spot to move 3 μm in a Timepix3 detector with 1.5 ns timing accuracy. The detected X-ray radiation is then sorted into 1 ns bins according to their respective 1 μs modulo timestamps. Each bin will then produce an image when pixel coordinate data is included. The images can then be combined in post-processing by shifting (i.e., translating) the images by different phases after the original triangular drive signal to find which phase provides the highest contrast.

[0078] A partial tomographic reconstruction of the sample can be obtained using at least two images taken from different directions. Techniques developed for tomosynthesis and computed tomography can be employed, as these techniques were developed for situations where only a limited number of projections are available. Those skilled in the art will recognize that it may be advantageous to combine the image translation and addition discussed above with different tomographic techniques to extract as much information as possible from the images obtained.

[0079] Figure 3 The method according to the present invention is schematically illustrated. X-ray radiation is generated 301 by directing an electron beam onto a target, wherein the electron beam is alternately directed to at least a first and a second position on the target, thereby generating X-ray radiation alternately at the first and second positions. To prevent overheating of the target, the residence time of the electron beam at the same position should not exceed a specified time limit. Preferably, the time limit is less than 10 times, and in particular less than 5 times, the characteristic time scale τ given by the above expressions (6) and (7). To further prevent damage to the target due to thermal cycling, the electron beam should alternate between the positions at a sufficiently high rate, i.e., the time period of the alternating motion should be less than the time limit. Preferably, the electron beam is alternately directed to the first and second positions on the target with a time period that is less than 10 times, and in particular less than 5 times, the characteristic time scale τ given by the above expressions (6) and (7). In other words, the time between successive exposures should be less than 10 μs, for example less than 5 μs, for example less than 1 μs. In some embodiments, during the switching between the first and second positions, the electron beam is blanked, i.e., blocked, 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 it sweeps between a first location and a second location. While this disclosure uses two primary locations for generating X-ray radiation as an example, it is apparent that more than two locations can be used to generate X-ray radiation, and in embodiments where the electron beam sweeps across the target, X-ray radiation will be generated from multiple locations. As an example, the electron beam can be directed onto the target in a predetermined pattern that can be used to subsequently create an image from the detected X-ray radiation.

[0080] The generated X-ray radiation is directed 302 to a sample position area, which is a location where a sample to be studied can be placed. The sample position 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. An X-ray detector array is then used to detect 303 the X-ray radiation that has passed through the sample position area. The detection of the X-ray radiation that has passed through the sample position 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 location and the second location. In embodiments where the electron beam is directed onto the target according to a predetermined pattern, this 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 location and the second location.

[0081] in conclusion

[0082] An X-ray imaging system is disclosed, comprising: a target; an electron beam source configured to provide an electron beam for interacting with the target to generate X-ray radiation; electron optics configured to alternately direct the electron beam to at least a first location and a second location on the target; an X-ray detector array configured to receive the X-ray radiation generated at the first location and the second location on the target; a sample location region for receiving a sample to be exposed to the generated X-ray radiation, the sample location region being located 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 being configured to create an image of the sample located in the sample location region based on the X-ray radiation originating from the first location and the second location. In an embodiment of the present invention, the electron beam is moved so that the exposure time to the X-ray radiation generated each time the electron beam is directed to a particular location is sufficiently short. In a further embodiment, the electron beam is moved between the locations in sufficiently short time periods to reduce thermal cycling and, thereby, reduce thermally induced mechanical stress. A corresponding method is also disclosed.

Claims

1. An X-ray imaging system, comprising: target; an electron beam source configured to provide an electron beam to interact with the target to produce X-ray radiation; electron optics configured to alternately direct the electron beam to at least a first location and a second location 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 position area for receiving a sample to be exposed to the generated X-ray radiation, the sample position area being located in an area where the X-ray radiation generated at the first position overlaps with the X-ray radiation generated at the second position; as well as a processing unit coupled to the X-ray detector array, the processing unit configured to create an image of a sample located in the sample position region based on the X-ray radiation originating from the first position and the second position; wherein the X-ray detector array and the electron beam source are configured such that it is possible to determine whether the X-ray radiation received by the X-ray detector at any one moment originates from the first location or the second location on the target; and The time between successive exposures of the first position is less than 10 μs, and the time between successive exposures of the second position is less than 10 μs.

2. The system of claim 1, wherein: The time between successive exposures of the first and second positions is less than 5 μs.

3. The system of claim 1, wherein: The electron optics are configured to direct the electron beam alternately to the at least first and second locations on the target such that a duration of consecutive exposure periods at any one location is shorter than a time limit.

4. The system of claim 3, wherein: The target is a solid reflective target and the time limit is 10 times the characteristic time scale τ, which is given by where S is given by and where p 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 at the target, and h is the penetration depth of electrons into the target.

5. The system of claim 3, wherein: The target is a transmission target and the time limit is 10 times the characteristic time scale τ, which is given by where 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 diameter of the electron beam at the target.

6. The system of claim 3, wherein: The target comprises at least one liquid jet, and the time limit is 10 times a characteristic time scale τ, which is given by where δ is the spot size of the electron beam at the target along the direction of travel of the at least one liquid jet, and v jet is the travel speed of the at least one liquid jet.

7. The system of claim 1, wherein: The electron beam source is configured to blank the electron beam during switching between the first position and the second position on the target so that no X-ray radiation is generated from areas of the target outside the first position and the second position.

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

9. An X-ray imaging method, 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 to a sample position region, wherein the sample position region is located in a region where the X-ray radiation generated at the first position overlaps with the X-ray radiation generated at the second position; as well as detecting X-ray radiation that has passed through the sample location region using an X-ray detector array; as well as correlating the direction of the electron beam with detections of X-ray radiation that has passed through the sample location region to determine whether the X-ray radiation received by the X-ray detector array at any moment originates from the first location or the second location, and creating an image based on the X-ray radiation originating from the first location and the second location, The time elapsed between successive exposures of the first position is less than 10 μs, and the time elapsed between successive exposures of the second position is less than 10 μs.

10. The method of claim 9, wherein: The time between successive exposures of the first and second positions is less than 5 μs.

11. 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 the duration of consecutive exposure periods at any one location is shorter than a time limit.

12. The method of claim 11, wherein: The target is a solid reflective target and the time limit is 10 times the characteristic time scale τ, which is given by where S is given by and where p 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 at the target, and h is the penetration depth of electrons into the target.

13. The method of claim 11, wherein: The target is a transmission target and the time limit is 10 times the characteristic time scale τ, which is given by where 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 method of claim 11, wherein: The target comprises at least one liquid jet, and wherein the time limit is 10 times a characteristic time scale τ, which is given by where δ is the spot size of the electron beam at the target along the direction of travel of the at least one liquid jet, and v jet is the travel speed of the at least one liquid jet.

15. The method of claim 9, further comprising blanking the electron beam during switching between the first position and the second position on the target so that no X-ray radiation is generated from areas of the target outside the first position and the second position.

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

17. The method of claim 9, wherein: The target comprises different materials at the first and second locations, the materials generating X-ray radiation having different spectra when the electron beam is directed to the respective locations.

Images