Method for generating cephalometric images with reduced chatter marks of patient's head

Abstract

To provide a computer-implemented method for reducing chatter marks.SOLUTION: A computer-implemented method for generating a patient's head measurement image commprises: a step for linearly moving an X-ray cephalo detector 6 and an X-ray cephalo collimator 8 arranged on opposite sides along the head of the patient, for exposing the head of the patient with the X-ray from an X-ray source 2, for acquiring a sequence of projection images, where the X-ray cephalo collimator 8 has an aparture and at least one radiopacity marker which may be detected in the projection images, and the projection images are read sequentially at a predetermined rate from a prescribed frame of the X-ray cephalo detector 6; a step for detecting an actual position of the at least one radiopacity marker in each projection image to the target position on the X-ray cephalo detector 6; and a shift / addition reconstruction step for shifting each projection image by a prescribed shift quantity to a preceding projection image of the projection image on the basis of the predetermined rate, then adding the projection image.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a computer-assisted extraoral X-ray system and computer-implemented method for generating cephalometric images of a patient's head. [Background technology]

[0002] Cephalometric (cephalometric) images (cephalograms) are part of the orthodontic clinic's standard diagnostic procedure. Cephalometric images are needed, among other things, to determine the relative positions in the patient's jaws for planning, implementing, and following up on orthodontic treatment.

[0003] There are a variety of digital cephalometric systems in the dental market today for extraoral X-ray systems.

[0004] On the other hand, there are so-called one-shot cephalometers, in which correspondingly large and expensive cephalometric sensors are used to capture the patient's head directly with only one short static exposure. This prevents motion artifacts due to, for example, the reduced exposure time, but such cephalometric sensors are often expensive and bulky. Such one-shot cephalometric systems are not part of this application.

[0005] On the other hand, linear scanning systems are another option for creating cephalometric images. Here, the patient is scanned sequentially at the required focus-detector distance (usually about >1.5 m). In addition to the primary aperture in the X-ray source, an additional secondary aperture associated with the cephalometric detector is generally used for additional X-ray dose reduction. Although the exposure time is longer than in one-shot cephalometry, the cost of the cephalometric sensor (i.e., digital cephalometric systems based on CCD / CMOS) is significantly lower. However, due to the sequential recording technology, the requirements for the X-ray device (machine, motor) are significantly higher, since the cephalometric sensor used must be moved synchronously with the secondary aperture along the patient's head during recording.

[0006] Older linear scanning systems based on this technique use CCD-based cephalometric sensors that follow the commonly known TDI principle (i.e., time delay integration). This means that the summation of image information occurs directly in the cephalometric sensor, and one image sequence of the cephalometric image to be created is always output sequentially by such a cephalometric sensor. The integration time and readout speed are determined by the TDI frequency. However, subsequent analysis of the image information per acquisition time through software is not possible in this variant of the TDI-based linear scanning system. In the TDI method, all individual image information is generally processed directly in the cephalometric sensor.

[0007] Generally, mechanical and motor-related deviations from the target behavior of a linear scanning system during operation can lead to disturbances in the acquisition process, which in turn cause image artifacts, including so-called chatter marks, in cephalometric images. These can reduce the image quality and diagnostic suitability of the cephalometric image (i.e., risk of misdiagnosis) and lead to reexposure. When the secondary aperture and cephalometric detector are not synchronously aligned during linear scanning (e.g., when the center of the secondary aperture does not synchronously coincide with the center of the exposure frame being read out), the gray-level peaks of the distribution of image data captured by the cephalometric sensor (i.e., generally, a Gaussian-like distribution across the secondary aperture) are shifted relative to their ideal center position on the cephalometric detector, which in turn leads to noticeable gray-level intensity variations in the form of vertical and / or horizontal stripes during add-and-shift reconstruction of the cephalometric image. For this reason, the readout frame must be uniformly exposed. An example of chatter marks can be seen in cephalometric image 23' in Figure 3 by distinct, non-patient-specific variations in gray value intensity. In particular, in so-called technical cephalometric images (ie cephalometric images without a patient), chatter marks can also be seen as shown in FIG. 2 due to distinct variations in the intensity of the grey values.

[0008] The one-shot sensor described above for cephalometric imaging completely solves the chattermark problem, but is very expensive and not available for all extraoral x-ray systems.

[0009] Some extraoral x-ray systems with linear scan cephalometric technology omit the secondary aperture entirely to avoid the chattermark problem, which can lead to higher doses and therefore unnecessary patient risk. With linear scan cephalometric technology without a secondary aperture, the scan speed can be increased to keep the dose constant, which can lead to reduced image quality. Summary of the Invention

[0010] The inventor is not aware of any prior art in which the actual position of the Gaussian-like gray level distribution of the projection image data is adjusted to an ideal position during reconstruction of the cephalometric image by a shift-and-add technique to reduce the effect of chatter marks by using opaque markers placed in the cephalometric image to enable detection of the level of adjustment required due to inaccurate positioning / alignment of the cephalometric aperture relative to the cephalometric detector.

[0011] An object of the present invention is to provide an extraoral X-ray system and computer-implemented method for producing cephalometric images of a patient's head from which chattermarks are derived.

[0012] This object is achieved by a computer-implemented method according to claim 1 and an extraoral X-ray system according to claim 6. The subject matter of the dependent claims defines further developments and preferred embodiments.

[0013] The method according to the present invention is a computer-implemented method that can be realized as software, i.e., by computer readable code, to be executed in conjunction with an extraoral X-ray system, for generating a cephalometric image of a patient's head. The present invention includes a step of acquiring a sequence of projection images by exposing a patient's head to X-rays from an X-ray source while linearly moving an X-ray cephalometric detector and an X-ray cephalometric collimator on opposite sides along the patient's head, wherein the X-ray cephalometric collimator has an aperture and one or more radiopaque markers that can be detected in the projection images, and the projection images are sequentially read out from a predetermined frame of the X-ray cephalometric detector at a predetermined rate, and a step of detecting an actual position of at least one radiopaque marker in each of the projection images relative to a target position on the X-ray cephalometric detector; and a shift and add reconstruction step of shifting each projection image by a predetermined shift amount based on the predetermined rate relative to its preceding projection image and then adding the projection images, and the positions of the shifted projection images are individually adjusted by an amount corresponding to the difference between the actual position and the target position of the at least one radiopaque marker detected in the corresponding projection image prior to addition in order to reduce chatter marks. In this context, the actual position refers to the measured position, whereas the target position refers to the ideal future position where the cephalometric aperture would be symmetrically positioned / aligned with the readout frame of the cephalometric sensor. The target position can ideally be predetermined through the geometry of the x-ray system.

[0014] According to the present invention, the markers may be implemented in a variety of alternative and different ways and may be used alone or in combination. According to an embodiment of the present invention, the markers are defined through the cephalometric collimator edge surrounding the cephalometric aperture. In another alternative embodiment of the present invention, the markers comprise one or more recesses and / or protrusions formed on the cephalometric collimator edge surrounding the cephalometric aperture. In another alternative embodiment of the present invention, the markers comprise one or more holes formed near the cephalometric collimator edge surrounding the cephalometric aperture.

[0015] Generally, chatter marks are image artifacts that spatially extend in the vertical direction. However, they can also be tilted due to tilt disturbances in the machine. The present invention also optionally addresses reducing tilted chatter marks. Therefore, in an embodiment of the present invention, in the detection step, the actual position of at least one additional (second) radiopaque marker is detected in each of the projection images to determine the alignment to be adjusted, and the position and alignment of the shifted projection images are individually adjusted according to the two markers detected in the corresponding projection images prior to addition.

[0016] The present invention also provides an extraoral X-ray system comprising an X-ray source, an X-ray cephalometric detector, and an X-ray cephalometric collimator, wherein the X-ray cephalometric collimator has at least one radiopaque marker, and control means for controlling the X-ray source, the X-ray cephalometric detector, and the X-ray cephalometric collimator, the control means being configured to execute the computer-implemented method of any one of claims 1 to 5. The extraoral X-ray system may be provided solely for cephalometric imaging. Alternatively, it may be provided in combination with PAN and / or DVT imaging.

[0017] The computer-implemented method is provided as a computer program, i.e., software, comprising computer readable code that, when executed by a computer-aided extraoral x-ray system, causes the computer-aided extraoral x-ray system to perform the computer-implemented method of the present invention.

[0018] An important advantageous effect of the present invention is that it improves the diagnostic capabilities and image quality of cephalometric images on extraoral X-ray systems using linear scanning technology. This is achieved by using a frame-based cephalometric sensor, i.e., a cephalometric sensor in which X-ray projection images are read out sequentially, including subsequent reconstruction of the cephalometric images by software. The frame-based cephalometric sensor reads out multiple projection images during a cephalometric scan. Because individual projection images are made available to software during and after cephalometric acquisition, system-related disturbances (a source of image quality artifacts) can then be reduced by spatially adjusting the individual projection images based on detection of the actual positions of the markers. The deviation of the actual positions of the markers from the target (ideal) positions provides a measure for spatial adjustment of the image data of the projection images. Overall, the present invention significantly improves the determination of relevant positions on a patient's head for orthodontic treatment planning, implementation, and follow-up.

[0019] A further important advantageous effect of the present invention is that these image artifacts, i.e., chatter marks, can be significantly reduced by using software, thereby not placing higher demands on the mechanics and motors of extraoral X-ray systems with linear scan cephalometric technology. By reconstructing cephalometric images using software, the image quality and diagnostic suitability of cephalometric images can be significantly increased and re-exposure can be avoided.

[0020] A further important advantageous effect of the present invention is that it allows the reliable use of frame-based cephalometric sensors, which are cheaper and much smaller in size than one-shot cephalometric sensors, and therefore the present invention does not impose any increased demands on the mechanics and electronics of X-ray cephalometric systems with linear scanning technology, and allows the reduction of chatter marks through dedicated software.

[0021] A further important advantageous effect of the present invention is that it reliably enables the use of a cephalometric collimator for additional patient dose reduction during cephalometric acquisition, while also reducing chatter marks.

[0022] Chattermark reduction is achieved by analyzing and adjusting the projection images through software before final reconstruction, which in turn leads to improved image quality, whereby the projection images can also be pre-processed for dark current, gain, pixel defects, and the like prior to reconstruction.

[0023] The data volume of frame-based cephalometric images (projection images) is significantly higher compared to TDI-based linear scanning systems, which allows high-quality cephalometric images to be produced with reduced chatter marks.

[0024] In summary, the method of the present invention provides additional diagnostic and therapeutic value to the dentist through more reliable diagnoses resulting from cephalometric images with reduced chattermarks.

[0025] In the following description, the invention will be explained in more detail with reference to exemplary embodiments and with reference to the drawings. [Brief explanation of the drawings]

[0026] [Figure 1] 1 shows a schematic diagram of an extraoral X-ray system according to an embodiment of the present invention. [Figure 2] 1 shows a cephalometric image taken without a patient (i.e., a so-called technical cephalometric image) having chatter marks and acquired with a prior art extraoral X-ray system. [Figure 3] 1 shows a cephalometric image of a patient's head having chatter marks and acquired with a prior art extraoral X-ray system. [Figure 4A]4A shows various schematic diagrams illustrating the relative positions of an X-ray cephalometric detector, specifically its predetermined frame (area), and a cephalometric collimator edge, according to an embodiment of the present invention, with FIG. 4A showing an ideal symmetrical arrangement. [Figure 4B] 4A and 4B show various schematic diagrams illustrating the relative position of an X-ray cephalometric detector, specifically a predetermined frame (area) thereof, and a cephalometric collimator edge, according to an embodiment of the present invention, with FIG. 4B showing an incorrect placement. [Figure 4C] 4A and 4B show various schematic diagrams illustrating the relative positions of an X-ray cephalometric detector, specifically a predetermined frame (area) thereof, and a cephalometric collimator edge, according to an embodiment of the present invention, and FIG. 4C shows displacement arrows for adjusting image information due to incorrect positioning in FIG. 4B. [Figure 5A] 1A-1C show various schematic views of a multi-component cephalometric collimator with various markers (protrusions, edges) according to alternative embodiments of the present invention, where a double-sided arrow indicates a moving component. [Figure 5B] 1A-1C show various schematic views of a multi-component cephalometric collimator with various markers (recesses, edges) according to alternative embodiments of the present invention, where double-sided arrows indicate moving components. [Figure 5C] 1A-1C show various schematic views of a multi-component cephalometric collimator with various markers (holes, edges) according to alternative embodiments of the present invention, where double-sided arrows indicate moving components. [Figure 6A] 10A-10C show various schematic views of a single-piece cephalometric collimator with various markers (protrusions, edges) according to further alternative embodiments of the present invention. [Figure 6B] 10A-10C show various schematic views of a single-piece cephalometric collimator with various markers (recesses, edges) according to further alternative embodiments of the present invention. [Figure 6C] 10A-10C show various schematic views of a single-piece cephalometric collimator with various markers (holes, edges) according to further alternative embodiments of the present invention. [Figure 7A]10 shows a comparative example for a cephalometric image of a phantom head with unreduced chattermarks. [Figure 7B] 1 shows a cephalometric image of a phantom head with reduced chatter marks according to the present invention. [Figure 8] 3 shows a sequence of projection images acquired by an extraoral X-ray system according to the present invention. [Figure 9] 1 shows the course of the actual position of the marker throughout the sequence of projection images relative to the target position relative to the cephalometric sensor. DETAILED DESCRIPTION OF THE INVENTION

[0027] The reference numbers shown in the drawings refer to the elements listed below, which are referenced in the following description of the illustrative embodiments.

[0028] 1. Extraoral X-ray system 2.X-ray source 3. Primary X-ray detector (PAN / DVT detector) 4. Primary collimator (dashed line) 5.Cantilever arm 6. Secondary X-ray detector (CEPH detector) 7. Designated area (frame) 8. Secondary collimator (CEPH collimator) 9. Edge 10. Cephalometric aperture 11. Marker 12. Recess 13. Convex part 14. Hole 15. Operation unit (user interface) 16.16' head restraint 17. Bite Block 18. Computing Unit (Computer) 19. Display device (screen) 20. Input Device (Keyboard) 21. Input Device (Mouse) 22. X-ray projection image 23.23' Head measurement image A: Positioning of the patient's head for CEPH imaging B: Patient head position for PAN / DVT imaging X: Arrows indicating spatial adjustments to reduce chatter marks

[0029] The method according to the present invention, which will be described in more detail below, is a computer-implemented method (i.e., also referred to herein as software) that can be implemented on a computer-implemented extraoral X-ray system (1) such as that shown in the embodiment of FIG.

[0030] The present invention also includes a corresponding computer program (i.e., software) having computer readable code for carrying out the method, the computer program being provided on a computer readable storage medium accessible to the extraoral X-ray system (1).

[0031] The computerized extraoral X-ray system (1) shown in Figure 1 is a multi-function system for panoramic imaging, digital volume tomography imaging, and cephalometric imaging. Panoramic imaging and digital volume tomography imaging are optional. The extraoral X-ray system (1) can also be provided solely for cephalometric imaging.

[0032] The extraoral X-ray system (1) includes an X-ray source (2) and a primary X-ray detector (3) rotatably arranged around the patient's head for PAN / DVT imaging (see position B in FIG. 1 ). The X-ray source (2) has a primary collimator (4). The extraoral X-ray system (1) also has a cantilever arm (5) that holds a secondary X-ray detector (6) (i.e., a cephalometric detector) and a secondary collimator (8) (i.e., a cephalometric collimator). Preferably, a CMOS-based cephalometric detector is used. This CMOS-based cephalometric detector is preferably a detector with a scintillator or a direct conversion detector. The X-ray cephalometric detector (6) and the X-ray cephalometric collimator (8) can be moved linearly along the patient's head (see position A in FIG. 1 ). As shown in FIG. 8, a sequence of projection images (22) of a patient's head can be obtained by exposing the patient's head to X-rays from the X-ray source (2) while the X-ray cephalometric detector (6) and the X-ray cephalometric collimator (8) are moved linearly on either side of the patient's head while the head is positioned between them. See position A in FIG. 1 for the patient's head in cephalometric mode. In cephalometric mode, the primary X-ray detector (3) is automatically moved to a lateral position out of the X-ray beam path to avoid blocking the X-rays emitted from the X-ray source (2) toward the cephalometric collimator (8). The patient's head can be positioned in the X-ray cephalometric system (1) by a head fixture (16'). The X-ray cephalometric collimator (8) can be provided as a single-component structure as shown in FIG. 6 or as an adjustable (motorized) multi-component structure as shown in FIG. 5, with at least the vertically extending components being moved laterally by a motorized mechanism. The components are made of radiopaque materials. The X-ray cephalometric collimator (8) has a cephalometric aperture (10) and one or more radiopaque markers (11) that can be detected in the projection image (22). The radiopaque markers (11) are described in more detail below.

[0033] In the PAN / DVT mode, the trajectory of the X-ray source (2) and the X-ray detector (3) during PAN / DVT imaging can describe a circular path. However, it can also deviate from this. When multiple actuators (not shown) are controlled simultaneously, trajectories that deviate from a purely circular path around the patient's head can be achieved. The patient's head can be positioned using a bite block (17) and, optionally, a head fixture (16). For the patient's head in the PAN / DVT mode, see position B in Figure 1. The trajectory (course) of the X-ray source (2) and the primary X-ray detector (3) relative to the bite block (17) and the head fixture (16) is known by the system (1). The primary X-ray detector (3) detects the X-rays emitted by the X-ray source (2) during rotation. X-ray projection images for PAN / DVT imaging are acquired, i.e., read out from the X-ray detector (3). The primary X-ray detector (PAN / DVT detector) comprises a separate PAN detector and a separate DVT detector that can each be rotated about a vertical axis to face the X-ray source (2) according to PAN / DVT imaging, or it can be a single flat panel detector capable of accommodating PAN and DVT.

[0034] The computerized extraoral X-ray system (1) also includes an operation unit (15) such as a user interface for controlling all the functions of the modes, a computing unit (18), e.g., a computer, and a display device (19), e.g., a screen, for visualizing any datasets (projection images and the like) resulting from the software. The CEPH / PAN / DVT modes can be selected by a user, for example, through the operation unit (15) and / or the computing unit (18). The computer can be connected to the extraoral X-ray system (1) via a local network (not shown) or, alternatively, via the Internet. The computer is connected to input devices such as a keyboard (20), a mouse (21), and the like. The computer can also be part of a cloud. Alternatively, the computer can be integrated into the extraoral X-ray system (1). Alternatively, all or part of the calculations can be performed in the cloud (cloud computing) or in hardware on an FPGA (Field Programmable Gate Array). The computer executes a computer program and provides data sets, e.g., cephalometric, panoramic, and / or DVT images, for visualization on a screen. The screen may be spatially separate from the extraoral X-ray system (1) or may be integrated with the extraoral X-ray system (1). Preferably, the computer also controls all functions of the extraoral X-ray system (1). Alternatively, a separate computer may be used for control, operation, and image reconstruction.

[0035] The extraoral X-ray system (1) is preferably configurable as an IoT system and can be bidirectionally connected to other dental devices (not shown), such as optical intraoral scanners, dental milling machines, additive manufacturing machines, and the like, via a local area network and / or the Internet (not shown) for cloud computing, data exchange, remote control, and the like.

[0036] The extraoral X-ray system (1) has control means for energizing the X-ray source (2) and for linearly moving the X-ray cephalometric detector (6) and the X-ray cephalometric collimator (8).

[0037] In the following description, a computer-implemented method for generating a cephalometric image (23) of a patient's head is described in more detail. The extraoral X-ray system (1) has control means configured to carry out the method. The method comprises the following steps:

[0038] In a first step, a sequence of projection images (22) is acquired by exposing the patient's head to X-rays from the X-ray source (2) while linearly moving the X-ray cephalometric detector (6) and the X-ray cephalometric collimator (8) along the patient's head. The projection images (22) are sequentially read out from predetermined frames (7) (see FIG. 4) of the X-ray cephalometric detector (6). The frame-based cephalometric detector (6) has a fixed frame rate, preferably 300 fps to 500 fps (frames per second). The projection images (22) are preferably pre-processed for dark current, gain, and pixel defects. The X-ray cephalometric collimator (8) has an aperture (10) and one or more radiopaque markers (11) that can be detected in the projection images (22) or pre-processed projection images (22). In the following description, the projection image (22) may also refer to a pre-processed projection image (22) for ease of understanding and presentation.

[0039] In a further step, the actual position of at least one radiopaque marker (11) is detected in each projection image (22) relative to a target position on the X-ray cephalometric detector (6). The target position on the cephalometric detector (6) is ideally known by the system (1). Figure 9 shows an example in which the actual position (dashed curve) of the marker (11) on the cephalometric detector (6) is detected relative to a sequence of projection images (22), for example by image processing. The horizontal solid line in Figure 9 indicates the target position on the cephalometric detector (6).

[0040] In a further shift-and-add reconstruction step, the projection images 22 are added after shifting each projection image 22 by a predetermined amount relative to its preceding projection image 22 to obtain a cephalometric image 23, as shown in FIG. 7B. The predetermined shift amount can be set based on the frame rate used and the linear velocity set for the cephalometric detector 6. Ideally, when the cephalometric aperture 10, and therefore the image data distribution, is centered relative to a given frame 7 throughout the entire sequence, no adjustments to the projection images 22 are necessary. FIG. 4a illustrates the ideal case in which the cephalometric collimator 8 is accurately positioned, i.e., centered, on a given frame 7. FIG. 4b illustrates the case in which the cephalometric collimator 8 is incorrectly positioned on a given frame 7. In such a case, the gray value distribution of the image data (see, for example, the graphical representation) is offset from the center of the frame 7. Graphical representations (a) and (a') show horizontal cross sections of the gray value distribution of the image data. Graphical representations (b) and (b') show vertical cross sections of the gray value distribution of the image data. The boundaries of the distribution can be set by a threshold gray value. However, to reduce chatter marks, before the summation procedure, the positions of the shifted projection images (22) can be individually adjusted by an amount corresponding to the difference between the actual and target positions of at least one radiopaque marker (11) detected in the corresponding projection images (22). Figure 4c shows arrows X indicating the adjustments to be made to reduce misalignment, i.e., to symmetrically match the gray value distribution of the image data with respect to the midpoint of the frame. Arrow X links the actual position of the detected marker (11) to the target position of the marker (11); in this example, edge (9) serves as marker (11). The shifted projection images (22) can be adjusted by calculating individual 2x2 matrices that correspond to a predetermined frame size (7) based on the direction and length of arrow X, and then transforming (i.e., shifting) the projection images (22) from their actual positions to their target positions using the individual matrices. Herein, matrices are used to map image data from actual locations to target locations, with the size of the matrix corresponding to the pixels in the columns and rows of the frame (7).After the shifted projection images (22) are individually adjusted, they are added in a shift-and-add reconstruction step to obtain the cephalometric image (23) shown in FIG. 7B. For comparison, FIG. 7A shows a cephalometric image (23') without the shifted projection images (22). Mapping the image data prevents asymmetric gray value distributions from becoming noticeable in the form of chatter marks, i.e., gray value stripes. Mapping is typically on the order of one pixel or a few pixels. Mapping can also be performed with subpixel accuracy, but can be combined with interpolation methods. Thus, mapping reduces chatter marks at the expense of introducing negligible motion artifacts that are not noticeable to the dentist's eye.

[0041] In the following description, the various markers (11) are described in more detail.

[0042] In a preferred embodiment, the radiopaque markers (11) are defined through the X-ray cephalometric collimator edge (9) surrounding the aperture (10) (see, for example, FIG. 4A). For example, a corner of the cephalometric collimator edge (9) or a location at a predetermined distance from the corner can serve as the marker (11). The markers (11) are therefore visible in the image data of the projection image (22), for example, through a deflection point of the grayscale values ​​of the image data, and can be located by image processing. Herein, the edges (9) ideally extend along the horizontal and vertical directions.

[0043] In another preferred embodiment, as shown in Figures 5A-5B and 6A-6B, the marker (11) comprises a recess (12) or a protrusion (13) formed on the cephalometric collimator edge (9) surrounding the aperture (10). The recess / protrusion shape of the marker (11) is visible in the image data of the projection image (22), for example, through the grayscale values ​​of the image data, and can be located by image processing. More than one recess (12) and / or protrusion (13) can be formed in the radiopaque material of the edge (9).

[0044] In another preferred embodiment, as shown in Figures 5C and 6C, the marker (11) comprises a hole (14) formed near the cephalometric collimator edge (9) surrounding the aperture (10). More than one hole (14) may be formed. The shape of the hole in the marker (11) is visible in the image data of the projection image (22), for example, through the grayscale values ​​of the image data, and can be located by image processing.

[0045] The recesses (12), protrusions (13), holes (14) and edges (9) can be used either alone or in combination as markers (11).

[0046] In a further embodiment, the above-described adjustment is further extended to address skew alignment of the X-ray cephalometric collimator (8) relative to the cephalometric detector (6). Accordingly, in the detecting step, the actual position of at least one additional second radiopaque marker (11) is detected in each of the projection images (22) relative to a target position on the X-ray cephalometric detector (6) to determine the alignment to be adjusted. Two arrows X' (not shown) can then be derived for the two markers (11). Then, based on the two arrows X', individual matrices with translation and rotation can be derived for each projection image (22). Thus, the alignment of the shifted projection images (22) can be individually adjusted according to the two radiopaque markers (11) detected in the corresponding projection images (22) prior to summation.

Claims

1. 1. A computer-implemented method for generating a cephalometric image (23) of a patient's head, comprising: acquiring a sequence of projection images (22) by exposing the patient's head to X-rays from an X-ray source (2) while linearly moving an X-ray cephalometric detector (6) and an X-ray cephalometric collimator (8) on opposite sides along the patient's head, wherein the X-ray cephalometric collimator (8) has an aperture (10) and one or more radiopaque markers (11) that can be detected in the projection images (22), and the projection images (22) are sequentially read out at a predetermined rate from predetermined frames (7) of the X-ray cephalometric detector (6); detecting an actual position of at least one of the radiopaque markers (11) in each of the projection images (22) relative to a target position on the X-ray cephalometric detector (6); a shift-and-add reconstruction step of shifting each projection image (22) by a predetermined shift amount based on the predetermined rate relative to its preceding projection image (22) and then adding the projection images (22); and wherein positions of the shifted projection images (22) are individually adjusted by an amount corresponding to a difference between the actual position and the target position of at least one radiopaque marker (11) detected in the corresponding projection images (22) prior to the addition in the shift-and-add reconstruction step in order to reduce chatter marks.

2. 2. The computer-implemented method of claim 1, wherein the radiopaque marker (11) is defined through an X-ray cephalometric collimator edge (9) surrounding an aperture (10).

3. 3. The computer-implemented method according to claim 1 or 2, characterized in that the radiopaque marker (11) comprises one or more recesses (12) and / or protrusions (13) formed on a cephalometric collimator edge (9) surrounding the aperture (10).

4. 4. The computer-implemented method according to claim 1, wherein the radiopaque marker (11) comprises one or more holes (14) formed in the vicinity of a cephalometric collimator edge (9) surrounding the aperture (10).

5. In the step of detecting the actual position, the actual position of at least one additional radiopaque marker (11) is detected in each of the projection images (22) relative to a corresponding target position on the X-ray cephalometric detector (6) to determine an alignment to be adjusted; 5. The computer-implemented method according to claim 1, wherein the alignment of the shifted projection images (22) is also adjusted individually according to the two radiopaque markers (11) detected in the corresponding projection images (22) prior to the addition.

6. An extraoral X-ray system (1), comprising: an X-ray source (2), an X-ray cephalometric detector (6), and an X-ray cephalometric collimator (8), wherein the X-ray cephalometric collimator (8) has a radiopaque marker (11); control means for controlling the X-ray source (2), the X-ray cephalometric detector (6), and the X-ray cephalometric collimator (8); wherein the control means is configured to execute the computer-implemented method according to any one of claims 1 to 5.

7. 6. A computer program comprising computer readable code, which when executed by a computer-aided extraoral X-ray system (1) causes the computer-aided extraoral X-ray system (1) to perform the computer-implemented method of any one of claims 1 to 5.

Images