Calibration of radiotherapy devices

Abstract

This document discloses a computer-implemented method for calibrating an imaging apparatus mounted on a rotatable frame, wherein the imaging apparatus includes a detector and an imaging radiation source configured to emit imaging radiation in a field of view, the emitted imaging radiation having an imaging radiation axis. The method includes performing a calibration process comprising acquiring a first projected image of a calibration phantom positioned at a first location within the field of view, determining the projected position of the calibration phantom in the first projected image, and determining a relative rotation angle α between the detector and the imaging radiation source based on the determined projected position of the calibration phantom and the distance between one or more pixels in the first projected image representing the intersection point between the detector and the imaging radiation axis when the imaging apparatus is in a preferred configuration.

Description

Technical Field

[0001] This disclosure relates to a calibration method and related system, and more particularly to a method for calibrating an imaging device mounted on a rotatable gantry. Background Technology

[0002] Radiation therapy can be described as using ionizing radiation (such as X-rays) to treat the human or animal body. Radiation therapy is commonly used to treat cancer, such as treating tumors in a patient or subject. In this type of treatment, ionizing radiation is used to irradiate and thus destroy or damage the cells that make up part of the tumor.

[0003] Modern radiotherapy equipment typically includes a gantry that supports both the therapeutic radiation source and the imaging system. The imaging system usually consists of an imaging radiation source and a detector. The gantry is rotatable, allowing both the therapeutic radiation source and the imaging system to rotate around the patient.

[0004] In a mathematically "ideal" system, the relative distances between each component of the imaging system and each other component mounted to the gantry are fixed and stable, and the relative orientations of these components do not change. However, in a "real" or "practical" system, the relative distances and orientations of the components of the imaging apparatus are not fixed and stable. As the imaging system rotates within the gantry of the radiotherapy machine, the imaging system, the gantry, and the mechanical devices through which the imaging system is coupled to the gantry may experience small mechanical offsets or "deflections," which are functions of the gantry angle. These deflections of the imaging apparatus components are exacerbated by other heavy components mounted to the gantry (such as the treatment radiation source), resulting in additional deflections of the gantry as a function of the gantry rotation angle.

[0005] This deflection presents problems. If the images obtained via the imaging system are to be used to inform radiotherapy, the accurate relationship between the images and the alignment of the radiotherapy equipment must be obtained. A particularly important factor is the position of the isocenter of the treatment radiation source in each projected image. Furthermore, since 2D projected images taken at each gantry rotation angle can be assembled to form a 3D image as part of the reconstruction process, the accuracy and resolution of the resulting 3D image can be negatively affected by the deflection and offset of various components as a function of the gantry angle.

[0006] Imaging systems used in radiotherapy are known to be calibrated by generating so-called "deflection maps." These maps describe the degree of "sagging" or deflection as a function of gantry angles. Deflection maps are generated by positioning a ball-bearing phantom at the isocenter of the treatment radiation source and acquiring projected images at each of several gantry angles. The distance between the center of each projected image and the projected position of the ball-bearing phantom at each gantry angle is recorded. This map can then be used in 3D reconstruction to generate more accurate 3D images.

[0007] Existing flexure mapping can generate accurate 3D reconstructions that meet clinical and regulatory guidelines and provide a foundation for safe radiotherapy. While existing systems and methods can provide accurate 3D reconstructions, there remains a persistent need for improvements in image quality and for consistently high-quality results.

[0008] Although the current method is effective, it is not yet possible to determine, for example, whether a relative rotational offset occurs between the components of the imaging device when the gantry rotates.

[0009] The present invention aims to address these and other shortcomings encountered in the prior art. Summary of the Invention

[0010] The invention is set forth in the independent claims. Optional features are described in the dependent claims. Attached Figure Description

[0011] Specific embodiments will now be described by way of example only, with reference to the accompanying drawings, in which:

[0012] Figure 1 A radiotherapy apparatus or device including an imaging device is described according to this disclosure;

[0013] Figure 2 A calibration pattern according to this disclosure is depicted;

[0014] Figures 3a-3e Different configurations of the imaging apparatus are described, including varying degrees of translational and rotational offsets;

[0015] Figure 4 It is a diagram depicting the configuration of the imaging apparatus according to this disclosure;

[0016] Figure 5 It is a flowchart depicting the method according to this disclosure;

[0017] Figure 6 It is a flowchart depicting the method according to this disclosure;

[0018] Figure 7It depicts the separation of the traditional SLG / MLG gain map into an X-ray field map and a pure gain map;

[0019] Figure 8 A radiotherapy system according to this disclosure is described;

[0020] Figure 9 Exemplary computer-readable media suitable for storing instructions according to this disclosure are depicted. Detailed Implementation

[0021] Figure 1 A radiotherapy system 100 is shown, and particularly suitable for providing image-guided radiotherapy (IGRT). The radiotherapy system 100 includes a rotatable gantry 102 to which treatment equipment and imaging devices are mounted. The treatment equipment and imaging devices are attached to the gantry 102 such that they can rotate with the gantry 102, i.e., they rotate as the gantry rotates. In the example shown, the treatment equipment and imaging devices are mounted on the gantry such that the treatment beam propagates in a direction substantially perpendicular to the imaging beam direction. Figure 1 The diagram also schematically depicts a tatami mat 110, which can also be referred to as a patient positioning device, on which the patient 112 can lie during radiotherapy.

[0022] The treatment device includes a treatment beam source 114 and a treatment beam target 116. The treatment beam source 114 is configured to emit or guide therapeutic radiation, such as MV energy radiation. As those skilled in the art will understand, the treatment beam source 114 may include other components, such as a collimator configured to shape the therapeutic beam. Once the therapeutic radiation is emitted from the source 114 and optionally passes through the patient 112, the radiation continues toward the treatment beam target 116, where it is blocked / absorbed. The treatment beam target 116 may include an imaging panel (not shown). Thus, the treatment beam target can form part of an electronic field imaging device (EPID).

[0023] The imaging apparatus includes an imaging beam source 118 and an imaging panel 120. The imaging beam source 118 is configured to emit or guide imaging radiation, such as X-rays and / or kV energy radiation. The imaging beam source 118 may include an X-ray tube or other suitable X-ray source. The imaging radiation source 118 may include an aperture (not shown) defining a field of view for the imaging radiation. Once the imaging radiation is emitted from the imaging beam source 118 and optionally passes through the patient 112, the radiation continues toward the imaging panel 120. The imaging panel 120 may be referred to as a radiation detector or a radiation intensity detector. The imaging panel 120 is configured to generate signals indicating the intensity of radiation incident on the imaging panel 120. In use, these signals indicate the intensity of radiation that has passed through the patient 112. These signals can be processed to form an image of the patient 112. A 2D image taken in this way at a particular gantry rotation angle is called a projected image. By taking images around the patient at multiple angles, a 3D image of the patient can be generated, for example, using tomographic reconstruction techniques.

[0024] This article mentions isocenters for treatment equipment or equivalent treatment of radiation source 114. Isocenters are located in... Figure 1 The isocenter is marked with an "X". It can be described as a point or region in space through which the treatment beam passes at all gantry rotation angles. Importantly, the location of the treatment beam isocenter in the 2D and 3D images generated by the imaging system is accurately known, and the images are accurate and high-resolution, for example, to enable clinicians to accurately configure the equipment and position the patient for treatment.

[0025] Figure 2A calibration pattern 200 according to an embodiment of the present disclosure is depicted. The calibration pattern 200 includes a first plurality of marks 210a-210d and a second plurality of marks 220a-220d. Marks 210, 220 may take the form of a spot (e.g., a circular shape). The calibration pattern 200 may be formed on a sheet, film, or other 2D structure or substrate. The calibration pattern 200 may be formed on a high-absorption substrate (e.g., a copper sheet). Marks 210, 220 may be formed from low-absorption spots or holes. Alternatively, the calibration pattern 200 may be formed on a low-absorption substrate (e.g., PVC), wherein marks 210, 220 are high-absorption spots, such as metal spots or spheres. More generally, the majority of the calibration pattern 200 is formed of a material having a first radiometric impermeability or radiometric density, while marks 210, 220 have a second different radiometric impermeability (or radiometric density). The second radiometric density may be achieved, for example, by using materials of different thicknesses or a second different material. Although the figure depicts a specific number of marks within each of the first plurality of marks 210 and the second plurality of marks 220, this is not necessarily the case, and those skilled in the art will understand that the method of this disclosure may utilize a calibration pattern 200 with fewer or more marks contained within the first plurality of marks 210 and the second plurality of marks 220.

[0026] In use, the calibration pattern 200 is positioned within the field of view of the imaging device to be calibrated. In one embodiment, the calibration pattern 200 is positioned at the imaging radiation source, and in particular, the calibration pattern 200 may be positioned above, i.e., adjacent to, the aperture defining the field of view of the imaging radiation source. This positioning means that a majority of the imaging radiation passing through the aperture and entering the field of view will pass through the calibration pattern 200. In use, the imaging axis of the imaging radiation will pass through the calibration pattern 200, thereby defining an intersection point on the calibration pattern 200. Optionally, the imaging radiation axis may be the central axis of the radiation, and may be referred to herein as such. If the calibration pattern 200 is centered above the aperture, the intersection point will be located at or very close to the center of the calibration pattern 200. The calibration pattern 200 is positioned such that each mark 210, 220 is visible in the projected image acquired using the imaging device. The projection positions of the marks 210, 220 on the imaging panel can be determined by appropriate processing of the projected image, and these determined positions can be used as part of the calibration process, which will be described in more detail herein.

[0027] The first plurality of marks 210a-210d includes at least one first mark 210a. The first mark 210a is positioned at or near the center of the calibration pattern. Each of the first plurality of marks 210a-210d may be positioned at or near the center of the calibration pattern 200. At least one pair of marks 210a, 210b are separated along the x-axis of the calibration pattern 200, and at least another pair of marks 210b, 210d are separated along...

[0028] The y-axis of pattern 200 is separated. Each of the first plurality of marks can be equidistantly positioned at intersections (e.g., center points) in the calibration pattern 200.

[0029] As will be described below, the first plurality of markers 210a-210d are positioned to facilitate the identification of specific points in the projected image acquired by the imaging device. This point is the projected location of a point in the calibration pattern, for example, a point where a specific axis of the imaging radiation intersects (i.e., crosses) the calibration pattern 200. Therefore, this point can be described as an intersection point. Figure 2 In the depicted calibration pattern, the first plurality of marks 210a-210d are positioned to facilitate identification of the projection position of a point located between the first plurality of marks 210a-210d. This point is located at the center of the projection pattern 200. For a calibration pattern 200 that is properly aligned and centered at the imaging source aperture, the central axis of the imaging radiation will pass through this point at the center of the calibration pattern 200.

[0030] The positional relationship between at least one first mark 210a and / or each of the first plurality of marks 210a-210d and their intersections in the calibration pattern is known. For example, in Figure 2 In the depicted calibration pattern 200, the positional relationship is simply that a first plurality of marks 210a-210d are positioned around and equidistant from the center of the calibration pattern. Therefore, by using the projection positions of the first plurality of marks 210a-210d in the projected image, the projection position of the center of the calibration pattern 200 in the image can be determined. Because the first plurality of marks 210a-210d include marks separated on both the x-axis and y-axis, this determination is independent of the direction or degree of offset or rotation of the panel or kV imaging source.

[0031] A second set of marks 220a-220d are positioned at the corresponding corners of the calibration pattern 200. These marks 220a-220d are positioned away from the center, one at each corner of the rectangular or square pattern 200.

[0032] The markings 210, 220, and their projected positions enable users or software programs to determine whether there is a relative offset between the source and the panel, and / or whether a relative rotation has occurred between the source and the panel, and to distinguish between translational and rotational offsets. This contrasts with previous methods and systems, which could not distinguish these offsets.

[0033] Figures 3a-3e An imaging apparatus is depicted, comprising an imaging radiation source 318 (e.g., a kV imaging radiation source, such as a kV X-ray tube) and a detector 320 (e.g., a flat panel detector). Under the effect of gravity, as the gantry rotates, the components of the imaging apparatus undergo relative translational and rotational offsets relative to each other to varying degrees. Calibration pattern 315, such as... Figure 2 The calibration pattern depicted in the figure is positioned at the kV source, such that the imaging radiation passes through the calibration pattern 315.

[0034] In the method of this disclosure, a calibration phantom is also positioned within the field of view of the imaging device. The calibration phantom can be a spherical phantom, such as a ball bearing phantom, although other shapes of phantoms can be used. The calibration phantom can be positioned along the axis of rotation of the gantry, in a position such that when the imaging device is in... Figure 3a In the mathematically ideal configuration described herein, its projection position on panel 320 is substantially at the center of panel 320, and therefore at the center of any resulting projected image. This position can be the isocenter of the imaging radiation beam. When the imaging device forms part of a radiotherapy machine, the phantom is positioned at the isocenter of the therapeutic radiation beam. In an ideal system, the isocenters of the imaging and therapeutic radiation beams should coincide. An optical lamp can be positioned on the therapeutic radiation source to enable the clinician to locate the isocenter of the therapeutic beam. As will be understood upon reading the calibration method disclosed herein, positioning the calibration phantom at the isocenter of the therapeutic beam allows the projection position of the isocenter to be determined in the projected image. This assists the clinician in radiotherapy planning.

[0035] Figure 3aAn imaging apparatus in a first mathematically "ideal" configuration is depicted. This can be referred to as the "preferred" configuration of the imaging apparatus. In this configuration, no component of the imaging apparatus sags or deflects. Without deflection or sag effects, the central imaging radiation axis 330 of the kV beam passes through the ball bearing. The projection position of the phantom coincides with the projection center of the calibration plate. The imaging radiation axis 330 intersects the detector panel 320 at a crossroads 332. This crossroads 332 may be referred to herein as the first crossroads. Depending on the specific arrangement of the imaging apparatus under discussion, this first crossroads 332 may be located at the center of the detector 320. References herein may include "panel center" and / or "projected image center," or the adjustment or performance of image transformations to align various features with the center of the projected image. However, those skilled in the art will understand that, depending on the specific arrangement of the imaging apparatus to be calibrated, the preferred crossroads 332 of the detector and imaging axis may be located at other positions on the detector.

[0036] The intersection 332 can be represented by one or more specific pixels on the panel and therefore in the projected image. When the imaging device is in a preferred configuration, the one or more pixels representing the intersection 332 can be located at the center of the projected image. When the imaging device is in a “preferred” configuration, the one or more pixels representing the intersection 332 are known and can be identified in any projected image, regardless of the actual configuration of the imaging device when the projected image is acquired.

[0037] Figure 3b An imaging apparatus in an alternative configuration is depicted, where the kV source is offset downwards under gravity. The "ideal" (i.e., preferred) position of the tube is... Figure 3b The image is depicted as shaded. Therefore, a relative translational shift has occurred between source 318 and panel 320. The imaging radiation axis 330 no longer passes through the phantom. The projection position of phantom 335 has shifted upwards on the panel. Therefore, a detectable offset will exist between the projection position of the phantom and the center of the projected image. A detectable offset will also exist between these points and the marks in calibration pattern 315.

[0038] Figure 3c An imaging system in an alternative configuration is depicted, where the panel is offset downwards under the influence of gravity. The "ideal" position of panel 320 is... Figure 3c The image is depicted as a shadow. Therefore, a relative translational shift has occurred between the source 318 and the panel 320. The axis of the imaging radiation still passes through the ball, and thus the projected position of the phantom will be aligned with the projected position of the center of the calibration pattern 315, just as when the imaging device is in... Figure 3aThe preferred configuration depicted is as expected. However, a detectable offset will exist when compared to one or more pixels at the center of the projected image. The projected image captured in this configuration can be "corrected" by shifting the image after acquisition so that the projected position of the phantom is located at the center of the image.

[0039] Figure 3d An imaging system in an alternative configuration is depicted, in which the panel rotates under the influence of gravity. Using radiotherapy axis nomenclature known to those skilled in the art, this rotation is relative to the GT direction. The “ideal” position of panel 320 is... Figure 3d The center is depicted as shaded. Therefore, a relative rotational offset has occurred between the source 318 and the panel 320. In this configuration, there is no relative translational offset, and therefore the projected position of the ball will be located at the center of the panel 320, that is, aligned with one or more pixels representing the intersection on the detector panel when the imaging device is in the preferred configuration. However, it is worth noting that the outer corner markings of the calibration pattern (such as...) Figure 2 The distance between the projection positions of the second plurality of markers (220) shown will differ from the expected distance. Once the relative rotation angles are determined using a process that will be described in detail later, the projected images taken in this configuration can be “corrected” by performing intensity normalization and / or image transformation.

[0040] Figure 3e An imaging system in an alternative configuration is depicted, where the kV source rotates under gravity. The “ideal” (i.e., preferred) position of source 318 is depicted as shadowed. Therefore, a relative rotational offset has occurred between source 318 and panel 320. The projected position of the calibration phantom will be aligned with one or more pixels representing the intersection of the imaging radiation axis and detector 320 when the imaging device is in the preferred configuration. However, the distance between the projected positions marked at the outer corners of the calibration pattern in the image will differ from the expected distance. The projected image acquired in this configuration can be “corrected” after acquisition by using the relative rotation angle between the source and panel, and by performing intensity normalization and / or image transformation based on that angle.

[0041] In summary, the relative translational offset of panel 320 relative to source 318 alters the distance between the projected position of the phantom and the image center in the projected image. The fact that a relative translational offset has occurred can be verified by considering whether the projected position of the center of calibration pattern 315 is aligned with the position of the phantom in the projected image. The projected position of the center of calibration pattern 315 can be determined by considering the positions of the first plurality of marks 210a-210d projected in the projected image. A mere translational offset will not cause the distance between the outer corner marks 220a-220d of calibration pattern 315 in the projected image to differ from the expected distance.

[0042] Relative rotational offset can indeed cause the distance between the outer corner marks 220a-220d in the projected image to differ from the expected distance. Therefore, by measuring the distance between these marks 220a-220d in the projected image and comparing it with the corresponding distance measured when the imaging device is in the preferred configuration, it can be determined whether a relative (i.e., relative to the preferred configuration) rotational offset has occurred.

[0043] Advantageously, this method can determine whether the offset is translational, rotational, or a combination of both, and this allows for the inclusion of more information in the flexural calibration file. In turn, once these calibration parameters are determined, appropriate intensity normalization and image transformation can be performed on the projected images taken at each gantry angle. This improves the accuracy of the resulting 3D image when multiple such "corrected" projected images form the basis of the 3D reconstruction process. The resulting 3D image exhibits improved uniformity, spatial resolution, and lower contrast sensitivity.

[0044] Figure 4 A radiotherapy system including an imaging device is described. At a high level, radiotherapy systems are similar in form and function to... Figure 1 As depicted in the text, the radiotherapy system includes a low-energy (e.g., kV) imaging radiation source 418, a detector 420 for imaging radiation, a high-energy (e.g., MV) therapeutic radiation source 414, and a detector 416 for therapeutic radiation. Figure 4 The radiotherapy system and imaging system depicted are in a configuration where the imaging source 418 has been translated downwards. A rotational shift of the detector panel 420 has also occurred. Several dimensions are depicted in the figure; these are exemplary dimensions associated with some Elekta™ machines. Those skilled in the art will understand that these measurements will vary between different imaging systems, and that if these dimensions change, the mathematical calculations described below can be adjusted without difficulty.

[0045] Figure 4 The “original” or “ideal” focus 401 of the kV X-ray beam is depicted. This is when the imaging radiation source 418 is not flexed, i.e., in the position... Figure 3a The focal point in the preferred configuration is depicted. The original focal point 401 is described as point P1 in the following equation. Figure 4 Imaging radiation axis 403 is also depicted. Imaging radiation axis 403 is the reference axis of the X-ray beam. The X-ray field of view is approximately conical. When the system is in the preferred or "ideal" configuration, the field of view 404 of the imaging radiation source is... Figure 4 Shading is used in the depiction. When the imaging panel is in the "ideal" configuration, the reference axis 403 is perpendicular to the imaging panel 420, such that the reference axis 403 is as shown in the image. Figure 4The depicted point intersects the ideal position of the detector panel at a right angle. When the system is in the ideal configuration, reference axis 403 is aligned with the central axis of the X-ray beam and passes through the isocenter. Figure 4 The treatment beam axis 405 of the treatment radiation source 414 is also depicted. The treatment beam axis 405 is depicted using a vertical dashed line.

[0046] The following features Figure 4 Middle Marker:

[0047] Ideal / original focus (passive offset) of P1 – kV X-ray beam

[0048] The deflected focal point of the P2 – kV X-ray beam (after source deflection)

[0049] D ts – The offset distance of the pipe from P1 to P2. This is a parameter to be determined.

[0050] The isocenter of the O-MV bundle, and the ball bearing mold is positioned at or near this isocenter.

[0051] O1 – The intersection between the offset kV X-ray beam reference axis (i.e., the axis perpendicular to the ideal detector position and passing through P2) and the therapeutic radiation beam axis 405.

[0052] O2 – A marker point near the edge of the calibration tool. In some implementations, this could be the projection location of one of the outer corner markers in the calibration tool. This could be the location where one of the outer corner markers is projected onto a plane parallel to the detector in its ideal configuration and including the treatment beam axis 405.

[0053] The intersection of the N-axis (passing through points O and P2) with the original (preferred) kV detector panel location. This can be identified in the projected image.

[0054] N0 – The intersection point between the original kV X-ray beam reference axis 403 (which passes through point P1) and the original kV detector panel position. N0 is the intersection point between the detector and the imaging radiation axis 403 when the imaging apparatus is in the preferred configuration. In a preferred embodiment, this may be the center of the panel. One or more pixels representing this intersection point are known in any projected image and can be used as a reference point in the projected image.

[0055] N1 – The intersection between the new offset kV X-ray beam reference axis (which passes through point P2) and the original kV detector panel position.

[0056] N2 – The intersection of the axis (passing through points O2 and P2) with the original (ideal) kV detector panel position.

[0057] α – The angle between the original kV detector panel position (i.e., the panel position when the imaging device is in the preferred configuration) and the new, rotated-offset kV detector panel. This is the parameter to be calculated.

[0058] β – The angle between the new offset kV X-ray beam reference axis (which passes through points P2 and N1) and the true axis passing through points P2 and O (it will be shown below that β equals α).

[0059] The following distances can be defined using the aforementioned characteristics of the imaging system:

[0060] – The distance from O1 to P2. This is the distance from the focal point to the isocenter. This is a known distance and can be measured for the imaging system. In an example depending on the specific radiotherapy equipment, this distance can be equal to 1000 mm, as depicted in the figure.

[0061] – The distance from N1 to O1. This is the distance from the isocenter to the kV detector panel. This is a known distance and can be measured for the imaging system. This distance can be equal to 536 mm, as depicted in the figure.

[0062] – The distance from N1 to P2. This is a known distance and can be measured for imaging systems. In one example, it equals 1000 + 536 = 1536 mm, as depicted in the figure.

[0063] – The distance from O1 to O2.

[0064]

[0065] – The distance from N1 to N2.

[0066]

[0067] Based on the properties of triangles, it can be shown that:

[0068] α + ∠N1 N P2 = 90°

[0069] β + ∠N1 N P2 = 90°

[0070] so:

[0071] α = β (1)

[0072] Furthermore, line N0P1 is parallel to line N1P2, and line OO1 is parallel to line P1P2.

[0073] so:

[0074]

[0075] Based on formula (1) and triangle Features of OO1P2:

[0076]

[0077] According to formula (2), formula (3) can also be written as:

[0078]

[0079] Furthermore, according to triangle NON0 and Similarity of OP2O1:

[0080]

[0081] Furthermore, line N0P1 is parallel to line N1P2, and line OO1 is parallel to line N0N1.

[0082] so:

[0083]

[0084] Based on formulas (2) and (5), formula (4) can also be written as:

[0085]

[0086] In other words:

[0087]

[0088] In formula 4-2, and Both are known values ​​for a specific imaging system. It can be measured from a kV projected image. This allows D to be calculated. ts .

[0089] In formula 3-1, D ts It can be calculated based on formula 4-2. These are known values ​​for a specific imaging system.

[0090] Therefore, based on both Formula 4-2 and Formula 3-1, α and β can be calculated.

[0091] In fact, according to formula (4-1), formula 3-1 can also be written as:

[0092]

[0093] In formula 3-2, It can be measured from a kV projection image, and These are known values ​​for a specific imaging system, thus allowing the calculation of α and β.

[0094] In summary, formula (3-1) is equal to formula (3-2), and either formula can be used to calculate α.

[0095] for Figure 4 The specific Elekta™ radiotherapy system depicted in the image:

[0096] It equals the predefined value of 1000 mm.

[0097] It equals the predefined value of 536 mm.

[0098] Therefore, it can be understood from the above content, and given that The relative translational offset D between the source 418 and the panel 420 can be determined by measuring the kV projection image. ts And the rotation offset α. In fact, the relative rotation offset can be calculated in one of two ways:

[0099] Calculation Method 1: Using Formula 3-1:

[0100]

[0101] Among them, D ts It can be calculated according to formula 4-2:

[0102]

[0103] Calculation Method 2: Using Formula 3-2:

[0104]

[0105] Figure 5 This is a flowchart depicting a method 500 for calibrating an imaging system including a detector and an imaging radiation source according to this disclosure. Step 501, and in some embodiments, step 502, can be performed by a user such as a hospital / medical physicist, clinician, or field service engineer. The remaining steps can be performed by a suitable processor or group of processors. For example, in addition to block 501, these steps can be embodied as computer-executable instructions that, when executed by one or more processors, cause the one or more processors to perform the method. The one or more processors can form part of a system including an imaging apparatus (e.g., a radiotherapy system).

[0106] At position 501, the calibration phantom is positioned in the first position, and the calibration pattern is positioned in the second position. The calibration pattern can be aligned with... Figure 2 The same or similar markings depicted herein, and in particular, may include a first plurality of markings and a second plurality of markings. These locations are within the field of view (FOV) of the imaging system, such that radiation emitted by the radiation source can reach the first and second locations. This means that the projected locations of the markings can be identified in the projected image.

[0107] As mentioned above Figure 2 The calibration pattern can be formed on a radiation-transmitting sheet, and the markings of the pattern can be radiopaque. A second location for placing the calibration pattern can be at the imaging radiation source, for example, next to an aperture defining the field of view of the imaging radiation source. The first location of the calibration phantom can be the isocenter of the imaging device, and / or, in embodiments where the imaging device forms part of a radiotherapy system, the isocenter of the radiotherapy beam.

[0108] Optionally, at frame 502, field-of-view configuration settings are set. For example, it may be desirable to perform a calibration procedure for each of the small, medium, and / or large FOV settings. Alternatively, the size of the calibration pattern can be set and configured to best fit a particular FOV setting (e.g., a small FOV setting), so that the panel can be moved manually or automatically to the appropriate position for detection in the small FOV setting.

[0109] After the steps of box 501 and optionally box 502 have been performed, the imaging system is in a calibration configuration, where the calibration process can be performed as described in the remaining steps of method 500.

[0110] At frame 503, a first projected image is acquired. The first projected image is a projected image of the calibration phantom and the calibration pattern. In a preferred embodiment, no other objects are placed within the field of view (FOV) of the imaging device, so the projection positions of the calibration pattern markings and the calibration phantom are visible and can be easily identified in the projected image.

[0111] At box 504, the projection position of each of the first plurality of markers and the second plurality of markers in the first projection image is determined. At box 505, the projection position of the calibration phantom in the first projection image is determined. These steps can be performed using known object / feature detection algorithms embodied in software, or clinicians or other users can mark these positions in the image.

[0112] At box 506, it is determined whether a relative translational offset has occurred between the detector and the imaging radiation source. This determination is performed based on the projected position of the calibration phantom and the position of one or more known pixels in the projected image representing the first intersection point (i.e., the intersection point between the detector and the imaging radiation axis when the imaging device is in the preferred configuration). Optionally, this determination may be verified based on the projected positions of a first plurality of markers or otherwise further performed.

[0113] The positions of the one or more pixels representing the first intersection point are known and can be identified in the projected image. In a preferred embodiment, the first intersection point is located at the center of the detector panel, meaning that the one or more pixels representing the intersection point in the projected image are simply located at the center of the projected image.

[0114] If the imaging device is in the preferred configuration, i.e., if no translational shift occurs at the source or detector, the projection position of the calibration phantom in the first projected image should be aligned with the projection position of the second intersection point, and this alignment should occur at the one or more pixels in the projected image representing the first intersection point. As described above, this can be at the center of the first projected image.

[0115] At box 506, the method may therefore include determining whether a relative translational shift has occurred by determining whether the projected position of the ball in the projected image is located at the one or more pixels in the projected image representing the first intersection point, for example, whether the position of the ball in the projected image is at the center of the image. If the phantom is a ball or other spherical phantom, this may include determining the center of a circular object representing the ball in the first projected image and determining the distance from the center of that circle to the one or more pixels. If the one or more pixels are located at the center of the circular object, it can be determined that no relative translational shift has occurred.

[0116] Determining whether a relative translational offset has occurred can also be accomplished, for example, by referencing a threshold distance. Box 506 may include determining the distance between the one or more pixels representing the first intersection point and the projected position of the ball (e.g., representing the center of a circular feature of the ball in the projected image). This distance is... Figure 4 The middle is described as If the determined distance is below a threshold distance, for example, if the determined distance is less than one or two pixels of the image, then it can be determined that these points are sufficiently aligned with each other and therefore no relative translational shift has occurred. Alternatively, if the distance is greater than the threshold, then it can be determined that a translational shift has occurred.

[0117] In some implementations, the projection positions of the first plurality of marks in the calibration pattern can be used to verify or otherwise help determine whether a relative translational shift has occurred. (See above regarding...) Figure 2 and Figure 4 The second position where the calibration pattern is placed allows the imaging radiation axis to pass through the calibration pattern. This defines a point of intersection of the imaging radiation axis in the calibration pattern, which may be referred to as the second intersection point. The positional relationship between the first plurality of marks 210a-210d and the second intersection point in the calibration pattern is known. This means that the projection positions of the first marks 210a-210d in the projected image can be used to determine the projection position of the second intersection point in the projected image. As discussed elsewhere herein, in a preferred embodiment, this second intersection point in the calibration pattern is located at the center of the calibration pattern.

[0118] In short, a plurality of markers are positioned within the calibration pattern to enable the projection position of the second intersection point to be found in the projected image. If the projected position of the second intersection point coincides with the position of the ball in the projected image, it can be verified / determined that no relative translational offset has occurred.

[0119] Therefore, at box 506, the method may further include determining the distance between the two points in the first projected image, i.e., the distance between the projected position of the second intersection point and the projected position of the calibration phantom. The goal is to determine whether the two points are aligned, which is expected if the imaging device is in a preferred configuration (where the various mechanical components are not translated, drooping, or deflected). This verification step can use a threshold distance similar to that described above. This is accomplished by measuring the distance as described above, i.e., if the distance is below a threshold distance, then it is determined / verified that no relative translational shift has occurred.

[0120] At box 507, based on the second set of tags (e.g.) Figure 2 The projection positions of the "corner marks" (described in the diagram) determine whether there is a relative rotational offset between the detector and the imaging radiation source. One or more distances between the second or more marks in the calibration sheet, and one or more distances from the first or more marks to the second or more marks, are known, as is the expected distance from the imaging radiation source to the detector. This means that one or more expected distances between the projection positions of these marks are known. (As mentioned above regarding...) Figures 3a-3e If there is a relative rotational offset between the panel (detector) and the imaging source, the relative distance between the second plurality of markers may change, or may otherwise differ from the expected distance.

[0121] In one example, if the distance between the projected positions of every two corner markers in the calibration tool differs from the expected distance, this indicates that a relative rotation has occurred, and the method continues to box 508. This process can be described as comparing the distance between two or more markers in a second plurality of markers in the projected image with one or more expected distances if the imaging device is in a preferred configuration. If not, for example, if a deviation exceeding a threshold exists, it is determined that a relative rotation offset has occurred.

[0122] This check can be performed on each pair of adjacent tags for a second or more tags. (See reference) Figure 2 The calibration pattern depicted can measure and check the following four distances: between the two top marks 220a and 220b, between the two bottom marks 220c and 220d, between the upper right and lower right marks 220b and 220d, and between the upper left and lower left marks 220a and 220c.

[0123] At box 508, both relative translational offset and relative rotational offset are determined. These determinations are based on the distance between the projection position of the calibration phantom and the one or more pixels in the first projected image representing the first intersection point; in other words, using... To carry out.

[0124] The relative translational offset between the detector and the imaging radiation source is denoted as D. ts Furthermore, the relative rotational offset between the detector and the imaging radiation source is denoted as α (=β). Refer to the above regarding... Figure 4 The described calculation methods 1 and 2 can be used... Determine D ts ,Should It can be measured in the projected image, and the angle α can be used directly. Or D ts To calculate. Therefore, the box can include determining. Alternatively, use the value determined in an earlier step of the method (e.g., box 506).

[0125] In some implementations, depending on previous steps in the workflow, only certain aspects of block 508 may be performed; for example, if it is determined at block 506 that no relative translational offset has occurred, then D may be omitted at block 508. ts The calculation of α can be omitted at box 508 if it is determined that no relative rotation offset has occurred at box 507.

[0126] Optionally, at box 509, the relative translation offset and relative rotation offset are recorded. For example, D tsAnd α can be recorded in a text file or other data log for later reference. This text file can be used when determining, for example, how to "correct" the projected image before it is formed as part of the reconstruction process. For example, D ts It can be used as the basis for translational image transformations of all future acquired projected images, and α can be used as the basis for rotational image transformations. In one example, D ts It can be broken down into x-dimensional and y-dimensional data, with each dimension recorded in a text file or other data log.

[0127] Steps 502 to 509 define a calibration process that can be performed at each of the multiple rack angles, allowing D to be recorded for each of the multiple rack angles. ts And α. There can be N rack angles. In one example, the process can be performed at 36 rack angles, which are equidistantly distributed around the entire 360° rotation range of the rack. D ts The α values ​​are recorded for each rack angle in a text file or other data log. The result is a significantly improved flexure plot compared to existing ones, which records not only translational flexure but also relative rotational flexure. When future projected images are acquired, they are bucketed based on the rack angle that is "closest" to them in the flexure plot. For example, a future projected image acquired at rack angle 184° is corrected according to an image transformation based on the D values ​​recorded for rack angle 180°. ts And α is determined.

[0128] Figure 6 Process 600 according to this disclosure is described. Process 600 and Figure 5 The processes described in Method 500 are the same or similar, and are intended to provide a different visualization of the workflow of Method 500 to aid understanding. Method 600 assumes that the ball phantom and calibration tool have been placed in place, for example, the ball is at the center of a radiotherapy device, and the calibration tube is positioned at the X-ray tube output window. The detector panel has been moved to the appropriate FOV position, for example, a small FOV position. kV radiation has been delivered to acquire a projection image. The position of the ball in the projection image, as well as the positions of the center and corner markers of the calibration tool, have been identified in the projection image. The following distances have been measured: the distance between one or more center markers of the calibration tool and the ball in the projection image, and the distance between the corner markers of the calibration tool and the ball in the projection image. The process can then begin at box 601.

[0129] At box 601, determine if the ball is centered in the projected image. If not, proceed to box 602a. If yes, proceed to 602b. At boxes 602a and 602b, based on the known physical distances in the calibration pattern, determine if the distance between each pair of diagonal markers is as expected.

[0130] Boxes 603a-603d represent conclusions based on boxes 601 and 602. For example, if the ball is centered in the projected image but at a different distance at 602 than expected, this means that at the rack angle in question, there is no translational offset, but a relative rotational offset exists (see box 603c). Depending on the scene, the rotational angle is determined at box 604 based on one or more measured distances, and the translational offset is determined at box 605 based on one or more measured distances.

[0131] At box 606, the calculated offset and / or rotation values ​​are recorded in the "deflection diagram" configuration file. As described above regarding method 500, this process can be performed for each of the multiple rack angles.

[0132] The method disclosed herein can be implemented in a variety of ways. For example, although Figure 2 The diagram depicts a calibration pattern with multiple first marks positioned closer to the center and multiple second marks in the corners, but other implementations are possible, and those skilled in the art will understand this. For example, only a single mark may be positioned near the center of the pattern, which can be used to determine the intersection of projection axes in one or more images. Furthermore, this method can be used not only with radiotherapy equipment including imaging devices, but also with any imaging device capable of capturing projected images. For example, this method can be used with a stand-alone CT or CBCT imaging device.

[0133] In addition, although Figure 5 The flowchart depicts a particularly advantageous method 500, but some features and steps of the workflow are optional and can be skipped. For example, the calibration process may include acquiring a first projected image of a calibration phantom positioned at a first location within the field of view at block 503. At block 505, the projected position of the calibration phantom in the first projected image can be determined. Then, at block 508, the relative rotation angle between the source and the tube can be calculated. This calculation is performed based on the distance between the projected position of the calibration phantom and one or more pixels in the projected image that represent the (first) intersection point between the detector and the imaging radiation axis when the imaging device is in the preferred configuration.

[0134] An improved method for generating gain maps

[0135] This paper also discloses a method for generating a gain map at each rack angle, including both single-level gain (SLG) and multi-level gain (MLG) implementations.

[0136] A gain map is a mapping designed to correct for pixel gain in imaging devices that include flat panel detectors, such as CBT and CT detectors. The gain map is intended to record pixel-by-pixel gain variations that may occur on the detector, such as due to differences in the sensitivity to radiation displayed by each pixel. These differences can then be compensated for, for example, when acquiring projected images of a patient.

[0137] According to known methods, gain maps are acquired only at a single "special" gantry angle (e.g., 270 degrees or -90 degrees). Furthermore, current SLG / MLG calibration and correction processes do not account for the mechanical deflection of the panel, which varies as a function of the gantry angle. Therefore, when gain correction is applied at angles other than the aforementioned "special" gantry angles, the previous gain correction process may introduce non-uniformity issues, resulting in poor image quality. This problem is exacerbated when several suboptimal issues are combined as part of the 3D reconstruction process.

[0138] To address these issues encountered in existing technologies, the following solution is provided. First, the previous single "gain map" is split into two components: a pure gain map and an X-ray field map. This... Figure 7 The image is depicted schematically.

[0139] A “pure” or purified gain map serves the same purpose as in existing solutions; however, it only considers the gain factor. This method may include homogenizing all pixel values ​​under open air (empty field) scanning at one or more single-dose levels (SLG) or different-dose levels (MLG). When considering a purified gain map, the calibration and correction process is the same as the previous calibration and correction process, i.e., applied at each gantry angle.

[0140] X-ray field mapping is designed to account for the mechanical deflection of the kV panel. The method involves generating a new X-ray field map at each gantry angle. This has the advantage of eliminating non-uniformity issues, removing relative artifacts, and improving image quality in the reconstructed volumetric image.

[0141] Each new X-ray field map can be generated for a specific rotation angle using a single X-ray field map and flexure map file to simulate real X-ray field flexure. As discussed elsewhere in this document, the flexure map file can include translational and / or rotational offsets measured at each of multiple gantry angles.

[0142] Therefore, this method can include acquiring a single X-ray field image at a specific gantry angle and using the flexural map to generate other X-ray field images. For example:

[0143] An X-ray field image is acquired at a special gantry angle (270 degrees or -90 degrees).

[0144] Obtain the deflection map file—containing kV panel deflection parameters at each rack angle. During standard deflection map generation, the projected positions of balls placed at isocenters are acquired at each of the multiple rack angles. The X and Y differences between the panel center and the ball projection positions are recorded to generate the deflection map file. A ball detection algorithm is used to calculate the difference between the ball center and the detector center. This can be performed for each of small, medium, and large FOV configurations, and for both clockwise and counterclockwise directions.

[0145] Based on an X-ray field map and flexural map file containing the kV panel flexural parameters in the X and Y axes at each rack angle, each new X-ray field map can be simulated at any rack angle during the calibration process.

[0146] The advantages of this solution include:

[0147] It takes into account two aspects: different dose levels and different rack angles.

[0148] It can improve the uniformity of the projected image and thus achieve better image quality in the reconstructed volume (i.e., in 3D images).

[0149] It can reuse existing flexural calibration procedures that already exist on many radiotherapy machines, so there is no need to involve many new additional functional implementations.

[0150] Radiotherapy system

[0151] Figure 8 A block diagram of one embodiment of a radiotherapy system 800 is shown. The radiotherapy system 800 includes a computing system 810, within which a set of instructions can be executed to cause the computing system 810 to perform any or more methods discussed herein.

[0152] The computing system 810 should be considered as including any number or set of machines, such as one or more computing devices, that individually or jointly execute a set (or more) of instructions to perform any one or more methods discussed herein. That is, hardware and / or software may be provided in a single computing device or distributed across multiple computing devices within the computing system. In some implementations, one or more elements of the computing system may be connected (e.g., networked) to other machines, such as in a local area network (LAN), intranet, extranet, or the Internet. One or more elements of the computing system may operate as a server or client machine in a client-server network environment, or as a peer-to-peer (or distributed) network environment. One or more elements of the computing system may be a personal computer (PC), tablet computer, set-top box (STB), personal digital assistant (PDA), cellular phone, network device, server, network router, switch, or bridge, or any machine capable of executing a set of instructions (sequentially or otherwise) specifying the actions to be taken by that machine.

[0153] The computing system 810 includes controller circuitry 811 and memory 813 (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM), such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM)). Memory 813 may include static memory (e.g., flash memory, static random access memory (SRAM)) and / or auxiliary memory (e.g., data storage devices), which communicate with each other via a bus (not shown).

[0154] The controller circuit 811 represents one or more general-purpose processors, such as microprocessors, central processing units, accelerated processing units, etc. More specifically, the controller circuit 811 may include a Complex Instruction Set Computing (CISC) microprocessor, a Reduced Instruction Set Computing (RISC) microprocessor, a Very Long Instruction Word (VLIW) microprocessor, a processor implementing other instruction sets, or a processor implementing combinations of instruction sets. The controller circuit 811 may also include one or more special-purpose processing devices, such as application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), digital signal processors (DSPs), network processors, etc. The one or more processors of the controller circuit may have a multi-core design. The controller circuit 811 is configured to execute processing logic for performing the operations and steps discussed herein.

[0155] The computing system 810 may also include network interface circuitry 818. The computing system 810 may be communicatively coupled to input device 820 and / or output device 830 via input / output circuitry 817. In some embodiments, input device 820 and / or output device 830 may be elements of the computing system 810. Input device 820 may include alphanumeric input devices (e.g., a keyboard or touchscreen), cursor control devices (e.g., a mouse or touchscreen), audio devices such as a microphone, and / or haptic input devices. Output device 830 may include audio devices such as speakers, video display units (e.g., liquid crystal displays (LCDs) or cathode ray tubes (CRTs)), and / or haptic output devices. In some embodiments, input device 820 and output device 830 may be provided as a single device or as separate devices.

[0156] In some embodiments, the computing system 810 may include image processing circuitry 819. Image processing circuitry 819 may be configured to process image data 880 (e.g., image or imaging data), such as medical images obtained from one or more imaging data sources, treatment device 850, and / or image acquisition device 840. Image processing circuitry 819 may be configured to process or preprocess image data. For example, image processing circuitry 819 may convert received image data into a specific format, size, resolution, etc. In some embodiments, image processing circuitry 819 may be combined with controller circuitry 811.

[0157] In some embodiments, the radiotherapy system 800 also includes an image acquisition device 840, also referred to as an imaging apparatus. The imaging apparatus is mounted to a rotatable gantry and includes a detector and an imaging radiation source configured to emit imaging radiation. The radiotherapy system 800 may also include a treatment apparatus 850. The image acquisition device 840 and the treatment apparatus 850 may be provided as a single device. In some embodiments, the treatment apparatus 850 is configured to perform imaging, for example, in addition to providing treatment and / or during treatment.

[0158] Image acquisition device 840 can be configured to acquire projected images via any of the following imaging modalities: positron emission tomography (PET), computed tomography (CT), cone-beam computed tomography (CBCT), magnetic resonance imaging (MRI), etc. Image acquisition device 840 can be configured to output image data 880, which can be accessed by computing system 810. Treatment device 850 can be configured to output treatment data 860, which can be accessed by computing system 810.

[0159] The computing system 810 can be configured to access or acquire treatment data 860, planning data 870, and / or image data 880. Treatment data 860 can be acquired from an internal data source (e.g., from memory 813) or from an external data source (e.g., treatment device 850 or an external database). Planning data 870 can be acquired from memory 813 and / or from an external source (e.g., a planning database). Planning data 870 may include information obtained from one or more of the image acquisition device 840 and treatment device 850.

[0160] The various methods described above can be implemented by a computer program. The computer program may include computer code (e.g., instructions) 910, arranged to instruct a computer to perform one or more functions of the various methods described above. The steps of the methods described above can be performed in any suitable order. The computer program and / or code 910 for performing these methods can be provided on a device (e.g., a computer), which may be located on one or more computer-readable media or more generally on a computer program product 900 (e.g., ...). Figure 9 (As shown). The computer-readable medium can be transient or non-transient. The one or more computer-readable media 900 can be, for example, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, or a propagation medium for data transmission, such as for downloading code over the Internet. Alternatively, the one or more computer-readable media can take the form of one or more physical computer-readable media, such as semiconductor or solid-state memory, magnetic tape, removable computer disk, random access memory (RAM), read-only memory (ROM), hard disk, and optical disk, such as CD-ROM, CD-R / W, or DVD. Instructions 910 may also reside wholly or at least partially within memory 813 and / or controller circuitry 811 during execution by computing system 810, which also constitute computer-readable storage media.

[0161] In one implementation, the modules, components and other features described herein may be implemented as discrete components or integrated into the functionality of hardware components such as ASICs, FPGAs, DSPs or similar devices.

[0162] A "hardware component" is a tangible (e.g., non-transient) physical component (e.g., a group or one or more processors) capable of performing certain operations and which can be configured or arranged in a physical manner. A hardware component may include dedicated circuitry or logic permanently configured to perform certain operations. A hardware component may include dedicated processors, such as FPGAs or ASICs. A hardware component may also include programmable logic or circuitry temporarily configured by software to perform certain operations.

[0163] Furthermore, modules and components can be implemented as firmware or functional circuitry within a hardware device. Additionally, modules and components can be implemented as any combination of hardware devices and software components, or solely in software (e.g., code stored in or otherwise embodied in a machine-readable medium or transmission medium).

[0164] Unless otherwise specified, it will be apparent from the following discussion that throughout the description, the use of terms such as “receive,” “determine,” “compare,” “enable,” “record,” and “identify” refers to the actions and processes of a computer system or similar electronic computing device that manipulate and transform data represented as physical (electronic) quantities within the computer system’s registers and memories into other data similarly represented as physical quantities within the computer system’s memory or registers or other such information storage, transmission, or display devices.

[0165] It should be understood that the above description is intended to be illustrative and not restrictive. Many other embodiments will be apparent to those skilled in the art upon reading and understanding the above description. Although this disclosure has been described with reference to specific exemplary embodiments, it will be appreciated that this disclosure is not limited to the described embodiments but can be practiced with modifications and changes within the spirit and scope of the appended claims. Therefore, the specification and drawings should be regarded as illustrative and not restrictive. Consequently, the scope of this disclosure should be determined by reference to the appended claims and the full scope of their equivalents.

Claims

1. A computer-implemented method for calibrating an imaging apparatus mounted on a rotatable gantry, wherein the imaging apparatus includes a detector and an imaging radiation source configured to emit imaging radiation in a field of view, the emitted imaging radiation having an imaging radiation axis; the method includes performing a calibration process, the calibration process comprising: Acquire a first projected image of the calibration phantom located at a first position within the field of view; Determine the projection position of the calibration phantom in the first projection image; as well as Based on the distance between the determined projection position of the calibration phantom and one or more pixels in the first projection image that represent the intersection point between the detector and the imaging radiation axis when the imaging device is in the preferred configuration, the relative rotation angle α between the detector and the imaging radiation source is determined.

2. The method according to claim 1, wherein, The one or more pixels in the first projected image are located at the center of the first projected image.

3. The method according to claim 1 or 2, wherein, When the imaging device is in the preferred configuration, the first position is located on the imaging radiation axis.

4. The method according to any of the preceding claims, wherein, The imaging device forms part of a radiotherapy system, which includes a therapeutic radiation source.

5. The method according to claim 4, wherein, The first position is the isocenter of the therapeutic radiation source.

6. The method according to any of the preceding claims further comprises: Based on the determined distance between the projection position of the calibration phantom and one or more pixels in the first projected image, the relative translational offset D between the detector and the imaging radiation source is determined. ts .

7. The method according to any of the preceding claims, wherein, The first projected image is also a projected image of the calibration pattern located at a second position within the field of view.

8. The method according to claim 7, wherein, The second position is located at the imaging radiation source.

9. The method according to claim 7 or 8, wherein, The imaging radiation source includes an aperture defining the field of view, and the second position is adjacent to the aperture.

10. The method according to any one of claims 7 to 9, wherein, The second position is configured such that most of the imaging radiation entering the field of view will pass through the calibration pattern.

11. The method according to any one of claims 7 to 10, wherein, The calibration pattern includes at least a first mark, and the calibration process further includes: determining the projection position of at least one first mark in the first projection image.

12. The method of claim 11, wherein the second position is configured such that the imaging radiation axis passes through the calibration pattern, thereby defining a second intersection point; and wherein the positional relationship between the at least one first mark and the second intersection point is known.

13. The method according to claim 12, wherein, The at least one first mark is a plurality of first marks, each of the plurality of first marks being equidistant from the second intersection point in the calibration pattern.

14. The method according to claim 12 or 13, wherein, The method further includes: The projection position of the second intersection point is determined based on the projection position of the at least one first mark in the first image; Determine the distance between the projection position of the second intersection point and the projection position of the calibration phantom in the first projection image; and Based on the distance between the determined projection position of the second intersection point and the projection position of the calibration phantom in the first projection image, it is determined whether a relative translational shift has occurred between the detector and the imaging radiation source.

15. The method according to claim 14, wherein, If the distance between the determined projection position of the second intersection point and the projection position of the calibration phantom in the first projection image is higher than a threshold, then a relative translational shift has been determined to have occurred.

16. The method according to any one of claims 7 to 15, wherein, The calibration pattern includes a second plurality of marks, and the calibration process further includes: Determine the projection position of each of the second plurality of marks; and Based on the projection position of each of the determined second plurality of marks, it is determined whether a relative rotational offset has occurred between the detector and the imaging radiation source.

17. The method according to claim 16, wherein, If it is determined that a relative rotational offset has occurred between the detector and the imaging radiation source, then the relative rotation angle α is determined.

18. The method according to claim 16 or 17, wherein, Determining whether a relative rotational offset has occurred between the detector and the imaging radiation source includes comparing the distance between the projection positions of a pair of marks in the second plurality of marks in the first projected image with the expected distance between the pair of marks if the imaging device is in the preferred configuration.

19. The method according to any one of claims 16 to 18, wherein, Each of the second plurality of marks is positioned at a corresponding corner of the calibration pattern.

20. The method according to any of the preceding claims, further comprising: The calibration process is performed at each of the plurality of rack rotation angles to determine the relative rotation angle between the detector and the imaging radiation source for each of the plurality of rack angles.

21. The method of claim 20, further comprising: The relative rotation angle of each of the determined plurality of rack rotation angles is recorded in a flexural diagram file, wherein the flexural diagram file enables the adjustment of the projected image from each of the plurality of rack angles during 3D image reconstruction to take into account the relative rotation between the detector and the imaging radiation source.

22. A system comprising: Rotatable frame; One or more processors; An imaging device mounted on the rotatable frame, wherein the imaging device includes: Detector; and An imaging radiation source configured to emit imaging radiation, the emitted imaging radiation having an imaging radiation axis; The processor is configured to perform the method described in any of the preceding claims.

23. The system according to claim 22, wherein, The system is a radiotherapy system and also includes a therapeutic radiation source mounted on the gantry.

24. One or more processors configured to perform the method according to any one of claims 1 to 21.

25. A computer-readable medium comprising computer-executable instructions, which, when executed by one or more processors, cause the one or more processors to perform the method according to any one of claims 1 to 21.

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