Method for investigating specimens using a tomographic imaging device

Abstract

To provide a method for examining specimens using tomographic imaging.SOLUTION: Provided is a method that includes a step for providing a specimen and a source, a step for directing a beam of radiation from the source toward the specimen and a step for detecting a radiation beam having passed through the specimen. This method further includes a step for moving at least one of the specimen and the source in order to provide a relative motion of the source to the specimen, and a step for enclosing the specimen and imaging the specimen along a series of different visual axis that intersects a virtual reference plane existing at essentially the center of it, with the combined movement and imaging steps generating a sampling shape Gs on the virtual reference plane. The movement and the imaging steps are adjusted so that the sampling shape Gs includes a plurality of segments Ls that are spaced from each other.SELECTED DRAWING: Figure 4b

Description

[Technical Field]

[0001] overview The present invention relates to a method of investigating a specimen using a tomographic imaging device, in which a specimen and a source are provided and the beam of radiation from the source is directed towards the specimen.

[0002] The invention also relates to a tomographic imaging device that can be used in carrying out such a method, and to a charged particle microscope equipped with such a tomographic imaging device.

[0003] In tomography (also called computed tomography (CT)), a source and a (diametrically opposed) detector are used to view a specimen along different lines of sight (visual axes) to obtain transparent views of the specimen from various perspectives. These are then used as input to a mathematical procedure that produces a reconstructed "volume image" of (part of) (an interior of) the specimen. To achieve the set of different lines of sight suggested here, one could choose to do, for example: (a) The source and detector are kept stationary and the specimen is moved relative to them. (b) The specimen is kept stationary and the source is moved relative to it. In this case, you can choose to either: - moving the detector synchronously with the source, or The detector is realized as a (static) array of sub-detectors whose positions are matched to correspond to the different positions that the source may assume.

[0004] Regardless of whether the source or specimen moves, their relative motion can be described using (for example) a specimen-centered coordinate system or reference frame. The beam of radiation traversing the specimen can be considered, for example, as being substantially cone-shaped for a 2D detector (thus producing so-called cone-beam tomography) or substantially resembling a segment of a disk for a 1D detector (thus producing so-called fan-beam tomography). The associated viewing axis suggested here is considered to correspond to the optical axis along which the beam propagates (from the source through the specimen to the detector), which essentially corresponds to the position of the central ray of that beam. To achieve sufficient sample penetration, the radiation used generally includes X-rays.

[0005] The tomographic imaging referred to herein can be performed using a standalone device, which is traditionally the case, for example, in medical imaging applications where the specimen (e.g., human or animal) is macroscopic. Standalone CT tools can also be used to perform so-called "microCT," which uses a microfocus source to image microscopic specimens in geology / petrology, biological (tissue or pharmacology) studies, etc. Aiming for even higher resolution, so-called "nanoCT" instruments have also been developed. These can be implemented as standalone tools, but can also be implemented, for example, as add-on modules to a charged particle microscope (CPM). In this case, the charged particle beam of the CPM can be used to irradiate a metal target, causing the production of bremsstrahlung X-rays, which are used to perform the desired tomography. The use of charged particles to perform the desired tomography is also contemplated. The present disclosure is not limited to X-ray tomography but also includes other types of tomography, such as electron tomography.

[0006] It should be noted that as referred to herein in the context of CPM, the phrase "charged particle" should be interpreted broadly to encompass: - electrons, as in the case of transmission electron microscopes (TEM), scanning electron microscopes (SEM), and scanning transmission electron microscopes (STEM). Ions, which may be positive (e.g., Ga or He ions) or negative. Such ion beams can be used for imaging purposes, but are also often used for surface modification, such as in focused ion beam (FIB) milling, ion beam induced deposition (IBID), and ion beam induced etching (IBIE). -Other charged particles such as protons and positrons.

[0007] It should also be noted that in addition to imaging and / or surface modification, the charged particle beam in a CPM may also have other functions, such as performing spectroscopy and / or examining diffractograms.

[0008] With respect to the relative motion between the specimen and the source used to achieve different lines of sight, the following is conventionally used: -Circular scanning, where the source follows a planar trajectory around the specimen and images are captured along this trajectory at a relatively high sampling rate (i.e., quasi-continuously). This type of scanning is applicable in situations where only a relatively thin "slice" of the specimen needs to be imaged, for example, when performing a cone-beam CT scan of the human dentition. -helical scanning, in which the source follows a coiled (spiral) path around the (longitudinal) axis of the specimen, and images are recaptured along this path at a relatively high sampling rate (i.e. quasi-continuously). This type of scanning is applicable in situations where a relatively elongated portion of the specimen needs to be imaged, for example when performing a CT scan of (part of) the human spine. This is usually achieved by combining a circular motion (of e.g. the source) with a simultaneous translational motion (of the specimen). As an alternative to conventional curvilinear scanning trajectories (such as the circular / spiral scanning path just mentioned), it is also possible to use grid-like data acquisition trajectories, as described for example in European Patent No. EP 3 133 554 B1.

[0009] While prior art techniques such as these have produced acceptable results in the past, the present inventors have worked extensively to provide innovative alternatives to the conventional approaches, and the result of this effort is the subject of the present invention.

[0010] It is an object of the present invention to provide an innovative tomographic imaging technique, and more particularly to provide faster acquisition times while maintaining the same level of acquired data.

[0011] These and other objects are achieved by the method defined in claim 1. According to this method, a specimen and a source are provided, and a beam of radiation is directed from the source toward the specimen. A flux of radiation transmitted through the specimen is detected. At least one of the specimen and the source is moved to provide relative motion of the source with respect to the specimen. The specimen may thus be imaged with a series of different visual axes that intersect with a virtual reference plane that surrounds the specimen and is substantially centered thereon. The intersection point of each of the visual axes may be considered as this reference plane, and a set of such intersection points corresponding to the series of visual axes may thus be generated. The combined moving and imaging steps generate a sampling shape on the virtual reference plane. The actual sampling shape includes a set of intersection points corresponding to the series of visual axes.

[0012] As defined herein, the moving and imaging steps are adjusted so that the sampling shape includes multiple spaced-apart line segments. In this regard, a single line segment, as defined herein, is formed by multiple intersections positioned adjacent to one another, such that a (virtual) line segment is created by a set of associated intersections. The intersections within a single line segment are positioned relatively close to one another to allow for continuous relative motion, e.g., continuous detector motion. For example, adjacent intersections may have a center-to-center distance between one and three times the diameter of the single intersection, effectively forming a virtual line segment. Alternatively, the intersections within a single line segment may partially overlap, i.e., the intersections have a center-to-center distance less than one time the diameter of the single intersection, effectively forming a real line segment. The virtual and real line segments allow for continuous relative motion.

[0013] The distance between adjacent line segments is relatively large, i.e., adjacent line segments have a distance at least substantially greater than the distance between intersections within a single line segment. The distance between adjacent line segments may be on the order of half the length of a single line segment or more, for example, greater than the length of two line segments, for example, greater than five line segment lengths. Larger distances between line segments are also contemplated.

[0014] The line segment includes multiple intersections, and the number of intersections in a single line segment may be between 3 and 30, more preferably between 5 and 20, such as 10 or 15.

[0015] The use of multiple, separate segments provides a novel and inventive type of scanning trajectory that can be used, for example, in cone-beam tomography. Within each line segment, the spacing between views (i.e., the spacing between intersections) is small enough to allow continuous relative motion, e.g., continuous detector motion. In embodiments, the impact of gaps between segments on image quality can be minimized by uniformly distributing the segments across the 2D space of the line of sight to maximize data sufficiency. The trajectories defined herein enable scanning at speeds approaching those possible with continuous trajectories, while providing much more complete coverage of the space of possible views, similar to grid-sampled (point-by-point) trajectories.

[0016] Conventional space-filling trajectories (SFTs), as described in, for example, EP 3 133 554 B1, waste 90% of the scan time during motion. The multi-segment trajectories disclosed herein reduce this to 50% by taking fewer, larger steps. The time to acquire a full scan with the trajectories disclosed herein is reduced by 15 to 3 minutes compared to a spiral trajectory, without reducing the acquired data. If each segment contains N images (i.e., N intersections), then compared to SFT, the motion occurs N times less frequently, and the segment spacing is N times shorter, so that acceleration-limited motion over space s is

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[0017] Advantageous embodiments are the subject of the dependent claims and are discussed below.

[0018] In one embodiment, the method includes sequentially imaging at least one of the plurality of spaced apart line segments.

[0019] In one embodiment, the method includes ceasing imaging between the plurality of spaced apart line segments.

[0020] In one embodiment, the sampling geometry comprises an array of said spaced apart line segments, which may be selected and arranged such that the array consists of a regular grid of spaced apart line segments.

[0021] In one embodiment, the moving step includes combined longitudinal (z) and tangential (θ) movement.

[0022] In one embodiment, successively spaced apart line segments indicate continuity in said longitudinal direction.

[0023] In one embodiment, the successively spaced line segments exhibit a discontinuity in the tangential direction.

[0024] In one embodiment, the discontinuity corresponds to a tangential rotation of between about 30 and 90 degrees, and more preferably between 45 and 75 degrees.

[0025] In one embodiment, for a single line segment, the longitudinal displacement is in the range of 1-5% of the total sample height.

[0026] In one embodiment, for a single line segment (Ls), the displacement in the tangential direction is in the range of 5 to 25 degrees.

[0027] According to one aspect, there is provided a tomographic imaging device comprising: a specimen holder for holding a specimen; a source for generating a radiation beam that can be directed at the specimen; a detector for detecting the radiation flux transmitted from the source through the specimen (S); a stage arrangement for generating relative movement of the source with respect to the specimen, thereby enabling the source-detector to image the specimen along a series of different viewing axes that surround the specimen and intersect a virtual reference plane that is substantially at its center, thereby generating a sampling shape; - a controller for controlling operation of the tomographic imaging apparatus.

[0028] A tomographic imaging device as defined herein is adapted to carry out the method as defined herein.

[0029] According to one aspect, there is provided a charged particle microscope comprising a tomographic imaging device as defined herein. [Brief explanation of the drawings]

[0030] The invention will now be elucidated in detail on the basis of exemplary embodiments and the accompanying schematic drawings.

[0031] [Figure 1] A perspective view of a specimen undergoing tomographic imaging is rendered to help explain certain geometric aspects of the present invention. [Figure 2] 1A and 1B are schematic renderings of prior art tomographic trajectories corresponding to conventional helical scanning. [Figure 3] 1A and 1B are schematic renderings of prior art tomographic trajectories corresponding to conventional matrix scans; [Figure 4a] 1 shows a schematic diagram of a scanning trajectory according to the method defined herein; [Figure 4b] 1 shows a schematic diagram of a scanning trajectory according to the method defined herein; [Figure 5] A longitudinal section elevation of a particular type of CPM can be rendered to implement embodiments of the present invention using a CT module. [Figure 6] A CT module suitable for use in a CPM such as that shown in Figure 2 is shown.

[0032] FIG. 1 renders a perspective view of a specimen S undergoing tomographic imaging and serves to explain certain geometric aspects of the present invention. In this illustration, an elongated specimen S (which may be, for example, macroscopic, micron-scale, or nanometer-scale) has an associated longitudinal axis L. A radiation source Sx produces a beam B of radiation (usually x-rays) propagating along an axis Vi, which may be considered the line of sight or line of sight. As shown here, Vi is substantially perpendicular to the longitudinal axis L. Upon passing through a portion of the specimen S, the beam B impinges on a (diametrically opposed) detector D. The detector D may be, for example, a silicon drift detector (SDD), a silicon lithium (Si(Li)) detector, a pixelated detector, or other suitable detector. The beam B may be considered to be (for example) cone-shaped or fan-shaped, depending on the effective shape that the detector D "presents" to the source Sx. The detector D forms an electronic image of the portion of the specimen S, which may be stored in electronic memory. This procedure is then repeated for a series of different viewing axes Vi, allowing the specimen S to be viewed along different lines of sight, and the various images acquired in this manner are then used as input to a mathematical reconstruction procedure to create a tomographic image. The various viewing axes Vi are achieved by using a stage device that creates relative motion between the source Sx and the specimen S, for example, by creating translational / rotational motion of the source Sx / detector D and / or specimen S in a predetermined manner. Such a stage device may comprise, for example, one or more linear motors, piezoelectric actuators, stepper motors, voice coil motors, pneumatic / hydraulic actuators, etc., and can be easily adjusted by one skilled in the art to suit the needs of a given setup. In the particular embodiment shown here, the stage device A is capable of translating / rotating the specimen S relative to the source Sx / detector D.

[0033] Also shown in this figure is a virtual reference plane Sr. In this case, the cylinder axis coincides with the longitudinal axis L. The radius of this reference plane Sr, Rsr, is selected to be less than or equal to the distance Rsx of the source Sx from the axis L. A visual axis Vi intersects this reference plane Sr at an intersection point Pi. The visual axis Vi may project linearly along L, and the projected visual axis Vi' intersects a virtual disk-shaped termination surface St at the lower end of the surface Sr. Associated with the reference plane Sr is a cylindrical coordinate system (R, θ, Z). The set of intersection points Pi {Pi} corresponding to the series of visual axes Vi can be considered to represent a "data acquisition trajectory," such as the circular or spiral scanning path described above, or, for example, a grid-like trajectory as described in the aforementioned patent application EP 3 133 554 B1.

[0034] In Figures 2-4B, the reference plane Sr is unfolded (unwound around L) to form a flat surface Sr' with an associated planar Cartesian coordinate system (Y, Z), which allows Y = θR.

[0035] Turning first to Figure 2, this shows the prior art situation corresponding to conventional helical scanning, in which a source Sx traces a helical path relative to axis L (i.e., simultaneously orbits around L and displaces it parallel to L), and images are captured quasi-continuously (i.e., at a high sampling rate) along a series of closely spaced visual axes Vi. Widening the resulting helical path of reference plane Sr yields the result shown in Figure 2, where a series of closely spaced intersection points Pi lie along a (curved) linear path C (the exploded view at the top right of the figure shows the closely spaced spacing between successive points Pi). The angle of attack of the helical path defines the distance M″ measured in the z direction between imaged points with the same Y coordinate. Note the extreme lack of uniformity / isotropy in this situation: the relatively large distance M″ results in a concentration of points along path C (highly directional), with no points at all in the intervening region C′.

[0036] Figure 3 illustrates a situation described in EP 3 133 554 B1, in which the relative motion of the source Sx and specimen S, as well as the accompanying sampling (image capture) frequency / spacing, are selected to form a two-dimensional lattice (matrix, net) M of points Pi areally distributed over (at least a portion of) the surface Sr' with a substantially uniform distribution. Associated with this lattice M are unit cells M', which can be considered the basic "building blocks" of the repeated lattice M. Adjacent points Pi have a distance Δo in the Y direction and a distance Δz in the Z direction. This sampling trajectory introduces a highly optimized "space-filling" scanning trajectory consisting of visual angles evenly distributed around the z-theta space of possible source positions. This produces excellent data, and post-correction for unplanned motion allows for ultra-sharp images with submicron resolution and high-quality images from a reduced data volume. However, because a space-filling trajectory requires large motions (typically 5 degrees) between each visual angle, fast scanning is limited by the stage acceleration. This can significantly slow down scan times. As an example, for a total imaging time of 15 minutes, a total of about 12 minutes may be required to move the stage to all subsequent imaging positions.

[0037] 4A and 4B, an embodiment of a scan trajectory consistent with the present disclosure is shown.

[0038] FIG. 4A shows that the virtual reference plane Sr' includes a sampling shape Gs that includes multiple spaced-apart line segments Ls. Here, a total of five spaced-apart line segments Ls1-Ls5 are provided. Each of the line segments Ls1-Ls5 includes several intersection points Pi. Thus, each line segment Ls1-Ls5 includes multiple intersection points Pi. A line segment can include a total of 10-20 intersection points Pi, or even more, if desired. As shown in the inset "A" of line segment Ls5, adjacent intersection points Pi may exhibit some overlap such that the intersection points Pi are connected and a true line segment Ls5 is formed by the intersection points Pi. On the other hand, as shown in the inset "B" of line segment Ls5, the intersection points Pi may also be spaced apart from each other. In this manner, a virtual line segment is formed. In option "B", the distance between the intersection points Pi is on the order of 1 to 3 times the diameter of a single intersection point, effectively forming imaginary line segments, and may be spaced apart from each other by relatively short distances, the distance being on the order of the diameter of the intersection points Pi.

[0039] As shown in Figure 4a, the second line segment Ls2 is spaced from the first line segment Ls1 by a distance Δo in the Y direction (tangential direction) and a distance Δz in the Z direction (longitudinal direction). In this manner, an array of line segments Ls can be formed. The imaging trajectory shown in Figure 4A is relatively fast while maintaining the amount of acquired data.

[0040] Turning now to Figure 4B, a comparison is shown between the spiral H scan trajectory, the point matrix P scan trajectory, and the line segment L scan trajectory, as defined herein. It can be seen that the line segment L scan trajectory can cover a larger area of ​​the reference plane Sr'. The use of line segments allows for movement and imaging to occur simultaneously, improving speed compared to the matrix P scan trajectory.

[0041] The sampling geometry G shown in Figure 4B effectively comprises an array AL of spaced line segments L. The array shown comprises a more or less regular grid of spaced line segments L, although irregular grids are also contemplated. The line segments extend in both the longitudinal (z) and tangential (θ) directions. Line segments may also extend only tangentially or only longitudinally. However, extending in both the tangential and longitudinal directions increases sample coverage. The scanning trajectory disclosed herein improves data collection efficiency (requiring minimal motion) by collecting multiple closely spaced views within each segment while distributing the segments relatively evenly throughout z-theta space, preserving the primary benefit of SFT.

[0042] From Figure 4B, it can be seen that the z-coordinate of the upper end of the first line segment Ls1 coincides with the z-coordinate of the lower end of the second line segment Ls2. Therefore, the consecutively spaced line segments (Ls) exhibit continuity in the longitudinal z-direction. A jump in the tangential coordinate occurs between the first line segment Ls1 and the second line segment Ls2. Therefore, the consecutively spaced line segments (Ls1-Ls2) exhibit discontinuity in the tangential direction. Effectively, the scan trajectory shown in Figure 4B includes longitudinal and tangential motion during imaging of the line segments, and only tangential motion during the movement from the first line segment Ls1 to the second line segment Ls2. This reduces the amount of movement required, increasing the speed at which the sample can be imaged.

[0043] The tangential discontinuity may correspond to a tangential rotation of approximately 30-90°, and more preferably between 45-75°. For a single line segment (Ls), the longitudinal displacement may range from 1-5% of the total height of the sample. For a single line segment (Ls), the tangential displacement may range from 5-25°. The total number of line segments used to image the specimen may range from 5-100. In the embodiment shown in FIG. 4B, the number of line segments is approximately 35. The higher the number, the greater the sample coverage.

[0044] FIG. 5 is a highly schematic diagram of an embodiment of a CPM 1 that can be used in connection with the present invention. More specifically, it shows one embodiment of a STEM, but could equally usefully be, for example, an ion-based microscope, or, for example, a TEM, in the context of the present invention. The microscope 1 comprises a particle-optical column / illuminator 3 that produces a beam of charged particles 5 (in this case, an electron beam) propagating along a particle-optical axis 5′. The particle-optical column 3 is mounted in a vacuum chamber 7 that includes a specimen holder 9 and associated stage / actuator 11 for holding / positioning a specimen 13. The vacuum chamber 7 is evacuated using a vacuum pump (not shown). Using a voltage source 15, the specimen holder 9, or at least the specimen 13, can be biased (floated) to a predetermined potential relative to ground, if desired.

[0045] The particle-optical column 3 comprises an electron source 17 (e.g., a Schottky emitter), (electrostatic / magnetic) lenses 19, 21 (typically more complex than shown in the schematic diagram) for focusing the electron beam 5 onto the specimen 13, and a deflection unit 23 for performing beam deflection / scanning of the beam 5. When the beam 5 strikes / scans the specimen 13, it triggers the emission of various types of "stimulated" radiation, such as backscattered electrons, secondary electrons, X-rays, and cathodoluminescence (infrared, visible, and / or ultraviolet photons). One or more of these radiation types can then be detected / recorded using one or more detectors, which can form an image, spectrum, or diffractogram, typically by organizing a "map" (or "matrix") of the detector output as a function of the scan position on the specimen. The diagram shows two such detectors 25, 27, which can be realized, for example, as follows: The detector 25 may for example be an electronic detector (such as a solid state photomultiplier), an X-ray detector (such as an SDD or Si(Li) sensor) or a photodetector (such as a photodiode). The detector 27 is a segmented electron detector, comprising a number of independent detector segments (e.g. quadrants) arranged around a central aperture 29 (allowing the passage of the beam 5). Such a detector can be used, for example, to investigate the (angular dependence of) the output (secondary or backscattered) electron flux emerging from the specimen 13.

[0046] These are merely examples and one skilled in the art will appreciate that other detector types, numbers, and shapes / configurations are possible.

[0047] Microscope 1 further comprises a controller / computer processing unit 31 for controlling, among other things, lenses 19 and 21, deflection unit 23, and detectors 25, 27, and for displaying information collected from detectors 25, 27 on display unit 33 (such as a flat panel display), such control being via control lines (bus) 31'. Controller 31 (or another controller) can further be used to perform various mathematical operations such as combination, integration, subtraction, false coloring, edge enhancement, and other operations known to those skilled in the art. Furthermore, automatic recognition processes (e.g., as used in particle analysis) may be included in such operations.

[0048] Also noted is a vacuum port 7', which can be opened to introduce / remove items (components, specimens) into / from the interior of the vacuum chamber 7, or in addition, for example, auxiliary devices / modules can be attached thereto (not shown). The microscope 1 may be provided with multiple such ports 7', if desired.

[0049] In the context of the present invention, the microscope 1 can also be equipped with an in situ CT module 7″, as shown in FIG. 6. In this figure, the specimen holder 9 of the CPM is equipped with a metal target 13′ that is positioned (using the actuator 11) so that the electron beam 5 impinges on it, thus producing bremsstrahlung X-rays in various directions. The figure shows a beam B of such X-rays propagating unilaterally from the target 13′ (effective source Sx) to the module 7″, where they pass through the specimen S and impinge on a detector D. Compare FIG. 1. The specimen S is mounted on a stage arrangement A that allows the specimen S to be positioned / moved (usually translated and rotated) relative to the source Sx.

[0050] Such a CT module 7″ may be permanently present in the vacuum enclosure 7 (ab initio) or may be an add-on module that can be fitted (after manufacture of the CPM1) onto / in a spare vacuum port 7, for example.

Claims

1. 1. A method of investigating a specimen (S) using tomographic imaging, comprising: - providing a sample (S) and a source (Sx); - directing a beam of radiation (B) from said source (Sx) towards said specimen (S); - detecting the radiation flux transmitted through said specimen (S); - moving at least one of said specimen (S) and said source (Sx) to provide a relative movement of said source (Sx) with respect to said specimen (S); - imaging the specimen (S) along a virtual reference plane (Sr) surrounding the specimen and a series of different visual axes (Vi) intersecting a central axis of the virtual reference plane (Sr), the virtual reference plane (Sr) being a cylindrical surface associated with a cylindrical coordinate system (R, θ, Z) and having a radius (Rsr) from a longitudinal axis (L) which is the cylinder axis, the combination of the moving and imaging steps generating a sampling shape (Gs) on the virtual reference plane (Sr), the moving and imaging steps are adjusted so that the sampling shape (Gs) includes a plurality of spaced apart line segments (Ls); the moving step includes combined longitudinal (Z) and tangential (θ) movement; A method in which successive spaced apart line segments (Ls) exhibit continuity in said longitudinal direction (Z).

2. The method of claim 1, wherein the plurality of spaced apart line segments (Ls) comprises a first line segment (Ls1) and a second line segment (Ls2) spaced apart from the first line segment (Ls1).

3. The method of claim 1 or 2, comprising the step of sequentially imaging at least one of said plurality of spaced apart line segments (Ls).

4. The method according to any one of claims 1 to 3, comprising the step of ceasing imaging between said plurality of spaced apart line segments (Ls).

5. A method according to any one of claims 1 to 4, wherein said sampling geometry (Gs) comprises an array (AL) of said spaced apart line segments (Ls).

6. 6. A method according to claim 5, wherein said array (AL) consists of a regular grid of spaced line segments (Ls).

7. 7. The method according to any one of claims 1 to 6, wherein successive spaced line segments (Ls) exhibit a discontinuity in the tangential direction (θ).

8. The method of claim 7, wherein the discontinuity corresponds to a rotation of the tangential direction (θ) between 30 and 90 degrees or between 45 and 75 degrees.

9. 9. The method according to any one of claims 1 to 8, wherein for a single line segment (Ls), the displacement in the longitudinal direction (Z) is in the range of 1 to 5% of the total sample height.

10. 10. The method according to any one of claims 1 to 9, wherein for a single line segment (Ls), the displacement in the tangential direction (θ) is in the range of 5 to 25°.

11. 1. A tomographic imaging device comprising: a specimen holder for holding a specimen (S); a source (Sx) for generating a beam (B) of radiation that can be directed towards said specimen (S); a detector (D) for detecting the radiation flux transmitted from said source (Sx) through said specimen (S); a stage arrangement (A) for generating a relative movement of a source (Sx) with respect to said specimen (S), thereby enabling said source and detector (D) to image said specimen (S) along a virtual reference plane (Sr) surrounding said specimen (S) and a series of different visual axes (Vi) intersecting the central axis of said virtual reference plane (Sr), thereby generating a sampling shape (Gs), said virtual reference plane (Sr) being a cylindrical surface associated with a cylindrical coordinate system (R, θ, Z) and having a radius (Rsr) from a longitudinal axis (L) which is the cylindrical axis; a controller for controlling the operation of said tomographic imaging device, Tomographic imaging device, characterized in that the device is arranged to carry out the method according to any one of claims 1 to 10.

12. A charged particle microscope comprising a tomographic imaging device according to claim 11.

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