Method and apparatus for deriving an X-ray image data set - Patents.com

Abstract

The invention relates to a method for deriving an X-ray image data set of a target area of ​​an examination object, in which projection images representative of this target area are acquired by means of an X-ray detector and an X-ray image data set is reconstructed from said projection images, a first part of the projection images being acquired collimated onto the target area by means of an X-ray device, the target area being irradiated with X-rays through at least one main radiation aperture of a collimator system. Furthermore, the invention relates to a corresponding device for deriving an X-ray image data set of a target area of ​​an examination object. In general, the invention relates to the field of medical radiography, in particular for the purpose of acquiring projections for reconstructing cross-sectional images in computed tomography.

Description

[Technical field]

[0001] The invention relates to a method for deriving an X-ray image dataset of a target area of ​​an examination object, in which projection images representative of the target area are acquired by means of an X-ray detector and an X-ray image dataset is reconstructed from said projection images, in which a first part of the projection images is collimated and acquired onto the target area by irradiating the target area with X-rays through at least one main radiation aperture of a collimator system by means of an X-ray device. Furthermore, the invention relates to a corresponding device for deriving an X-ray image dataset of a target area of ​​an examination object. In general, the invention relates to the field of medical radiography, in particular for obtaining projections for reconstructing cross-sectional images in computed tomography. [Background technology]

[0002] An embodiment of the present invention includes a collimator for limiting the X-ray flux for computed tomography. The collimator is a mechanical or mechatronic device that limits the optical path between the X-ray device (X-ray tube) and the X-ray detector to a defined area, for example through the motorized or manual positioning of thin plates of material that strongly attenuates X-rays. In this case, the thin plates can be configured straight or curved, may have different thicknesses, and may include chamfers to ensure more accurate fluoroscopy. The aim is to fluoroscopically view the patient with the smallest possible radiation dose that is sufficient for image acquisition. For example, collimators are employed in X-ray photography systems (planar fluoroscopy), angiography systems (also planar fluoroscopy) and computed tomography systems. So-called multi-lamella collimators are also used in radiation therapy in various constructional ways aimed at localized irradiation of the patient.

[0003] In the initially mentioned configuration for the purpose of this imaging, a collimator is arranged directly in front of the X-ray tube in order to set the optical path between the X-ray tube and the X-ray detector in a targeted manner, and between the collimator and the X-ray detector, the object to be examined (e.g. patient, phantom, animal, part) is usually positioned on a table.

[0004] The invention described herein relates, for example, to a collimator for generating (so-called reconstruction) projection images on the basis of a 3D cross-sectional image calculation. In this type of image acquisition, the light path is usually set along the z-axis (longitudinal axis of the patient). The reason for this is that in detector systems with multiple lines, individual lines can also be illuminated. In many cases, it is not necessary to illuminate all lines in order to be able to examine the body region of interest in the reconstructed cross-sectional image.

[0005] A further application of the invention is computed tomography with so-called flat panel detectors. This is often carried out with so-called C-arm X-ray systems. In this field, methods for generating volume-of-interest (VOI) cross-sectional images are established. In this case, only partial projections are made into the patient's region of interest (target region). However, nowadays, so-called truncation artifacts in the reconstructed images are usually avoided by completely projecting the entire examined object (patient). Truncation artifacts occur when the examined object cannot be completely detected. This leads to a loss of information in the undetected regions, which further leads to a loss of essential information in the image calculation.

[0006] Therefore, although volume-of-interest CT has the advantage of reducing the dose to the examined object, it also has the disadvantage that it usually produces artifactual (incomplete) images.

[0007] For this reason, in today's CT diagnostics, it is preferable to obtain a complete CT scan, since the highest possible image quality must be achieved, given that within the scope of the diagnosis, it is often not clear in advance what the patient is suffering from and in which body part the pathology is located.

[0008] VOI imaging is employed, especially in flat panel detector CT, when the area to be examined is already known in advance (e.g., therapeutic management after implanting a stent in a cerebral artery). However, with current technology, these VOI CT images are highly affected by artifacts, especially at the edges, do not represent the actual Hounsfield values ​​of the examined object, and are affected by unknown offset values.

[0009] However, in order to take advantage of the dose reduction, various methods have already been presented to compensate for the "lost" image information. For example, a mathematical transition model from the edge region to the object is used, which assumes the object geometry and attenuation properties in the non-transparent region. Further methods make use of specific preliminary information from previously created datasets (e.g. CT, MRI, ultrasound, surface scans or other methods).

[0010] DE 102016219817 A1 reveals a proposal for reducing truncation artifacts in X-ray image data sets. Summary of the Invention [Problem to be solved by the invention]

[0011] It is an object of the present invention to provide a method and a device for deriving an X-ray image data set for a limited target area of ​​an examination object, which allows achieving high image quality with the lowest possible X-ray dose. [Means for solving the problem]

[0012] This problem is solved in a method of the type mentioned at the beginning by irradiating an edge region of the target region with X-rays through one or more secondary radiation apertures surrounding or adjacent to the main radiation aperture of the collimator system, acquiring a second portion of projection images and determining the X-ray image data set from the first portion of projection images with the aid of the second portion of projection images, in order to reduce truncation artifacts in the X-ray image data set. In this way, the target region, also called the volume of interest, can be adaptively determined with sufficient image quality and at the same time the X-ray dose can be minimized.

[0013] According to the invention, one or more secondary radiation openings of the collimator system have a certain transparency for X-rays. A spatially limited further region of the object to be examined is irradiated with X-rays through this secondary radiation opening. In this case, the secondary radiation opening does not necessarily have to be a completely "open" design in the sense of a through opening, but can be made of one or a combination of materials completely or partially closed, as long as it is sufficiently radiotransparent to X-rays. The secondary radiation opening can be designed, for example, by making the material completely empty in the collimator aperture stop or by thinning the material, so that a sufficient amount of radiation can penetrate through it. It is also conceivable to design such secondary radiation openings by inserts made of a material that absorbs X-rays less than the surrounding material of the collimator aperture stop, so that a high mechanical load-bearing capacity of the collimator aperture stop can still be ensured.

[0014] The method may include one or both of the following features. Projection images showing the target area can be captured from different projection directions along the capture trajectory. From the projection images, a higher dimensional X-ray image data set can be reconstructed, i.e. having a higher dimensionality than the projection images.

[0015] According to an advantageous embodiment of the invention, it is provided that the size, position and / or radiation transmittance of one, some or all of the secondary radiation openings are varied during the X-ray examination of the target area. This allows a better adaptation of the function of the secondary radiation openings during the X-ray examination, so that the size, position and / or radiation transmittance can possibly be adapted depending on the achieved image quality of the X-ray image. For example, the collimator system can have an adjustable collimator aperture stop, by means of which one or more of the above-mentioned parameters size, position and / or radiation transmittance in the secondary radiation openings can be changed. This also allows a further reduction of the radiation load of the examined object.

[0016] According to an advantageous embodiment of the invention, it is provided that the variation of the size, position and / or radiation transmittance of one, several or all of the secondary radiation openings is adaptively performed depending on the current respective quality of the X-ray image, which makes it possible in a particularly advantageous manner to adaptively adjust the edge area of ​​the collimator system that is irradiated through the secondary radiation openings during image acquisition. On this background, if the image quality of the X-ray image falls below or exceeds a certain image quality, for example below or above an image quality determined by truncation artifacts or a particular signal-to-noise ratio characteristic, the secondary radiation openings can be adaptively adapted with respect to the aforementioned parameters, for example by opening or closing further up to a completely closed state, thus ensuring the lowest radiation dose while always ensuring a sufficient image quality.

[0017] According to an advantageous embodiment of the invention, the method is provided in such a way that in the step of determining the X-ray image data set, the step of automatically recognizing the irradiated and non-irradiated areas of the X-ray detector and the step of automatically taking into account the recognized irradiated and non-irradiated areas of the X-ray detector are carried out. In this way, the method according to the invention is also suitable for performing an image reconstruction method repeatedly. In such an image reconstruction method, the information of the individual image pixels can be taken into account or ignored in the image calculation process. For this purpose, the exposed and non-exposed areas of the X-ray detector must be automatically recognized. The recognition of the irradiated and non-irradiated areas or the exposed and non-exposed areas of the X-ray detector can be carried out, for example, by calculation based on the position of the aperture stop of the collimator system, for example depending on the setting of the parameters of the secondary radiation apertures and possibly taking into account the geometric situation between the X-ray device, the collimator system and the X-ray detector. A real-time evaluation of the image pixel information of the X-ray detector itself can also be carried out, for example, using threshold methods.

[0018] According to an advantageous embodiment of the invention, the method is provided with a step of automatically recognizing scattered radiation of X-rays emitted in non-directly irradiated areas of the X-ray detector and automatically taking the recognized scattered radiation into account when determining the X-ray image data set. In this way, the image quality of the X-ray image can be further improved and / or the radiation dose required for its acquisition can be further reduced. For example, X-ray radiation incident in non-directly irradiated areas of the X-ray detector (but occurring due to scattering effects (scattered radiation)) can be measured and included in the determination of the X-ray image data set taking into account a physical model of the system. Non-directly irradiated areas of the X-ray detector are, for example, areas that are not linearly connected between the X-ray device and the X-ray detector by at least one main radiation aperture or at least one secondary radiation aperture.

[0019] The problem stated at the outset is further solved by an apparatus for deriving an X-ray image data set of a target area of ​​an examination object, comprising an X-ray detector for acquiring projection images representative of the target area and a reconstruction device for reconstructing the X-ray image data set from the projection images, the apparatus further comprising an X-ray device and a collimator system with at least one main radiation aperture for acquiring a first part of a collimated projection image acquired on the target area, the acquisition being performed by irradiating the target area with X-rays through the at least one main radiation aperture by means of an X-ray radiation means, the collimator system having one or more secondary radiation apertures surrounding or adjacent to the main radiation aperture, a second part of the projection image being acquired by irradiating an edge area of ​​the target area with X-rays through the one or more secondary radiation apertures, the reconstruction device being arranged to derive the X-ray image data set from the first part of the projection images with the aid of the second part of the projection images in order to reduce truncation artifacts in the X-ray image data set, again achieving the above-mentioned advantages.

[0020] According to an advantageous embodiment of the invention, the collimator system is equipped to adjust the width and / or length of one, some or all of the sub-apertures. For example, the device can have an automatic drive, for example via a motor, to adjust the width and / or length of the sub-apertures. In this way, certain parameters of the sub-apertures can be varied, in particular the size, position and / or radiation transparency.

[0021] According to an advantageous embodiment of the invention, the collimator system has a movable lamella system with lamellae that are adjustable in one or two spatial directions, by means of which the width and / or length of one, some or all of the sub-apertures can be adjusted, which has the advantage that the width and / or length of the sub-apertures can be adjusted relatively quickly and that the mechanism can be made adjustable with low construction costs.

[0022] According to an advantageous embodiment of the invention, the movable lamina system has at least two individual laminas arranged one behind the other in the radiation direction of the X-rays, each of which has a number of radiolucent openings and is arranged so as to be displaceable relative to one another, which allows the lamina system to be constructed simply and reliably. The radiolucent openings can be configured similarly to the previously mentioned sub-openings and can be formed, for example, in the form of a slit.

[0023] According to an advantageous embodiment of the invention, it is provided that at least the part of the collimator system with the sub-apertures is curved about a spatial axis, in particular curved with a constant radius, in this way the effect of the sub-apertures can be optimized.

[0024] According to an advantageous embodiment of the invention, an apparatus is provided which is equipped to carry out a method of the type described above, whereby the apparatus can have, for example, an electronic control device which is equipped by means of programming to carry out the steps of the method according to the invention, for example the electronic control device being arranged for acquiring the first and second parts of the projection image and for determining an X-ray image data set therefrom. The control device can also be formed to control the collimator system.

[0025] The present invention deals with the advantageous possibility of compensating for lost edge information and thus achieving high image quality with realistic Hounsfield values ​​at low X-ray doses. The invention is primarily intended for applications in therapeutic fields where the volume of interest is known in advance. However, the invention can also be used in diagnostic applications.

[0026] The particularity of the invention is that partial information of the edge area is obtained by a clever configuration of each or all collimator aperture stops. Here, the collimator aperture stops can be configured in such a way that pockets of defined geometric shapes ("slits", for example the above-mentioned apertures) are provided at defined positions in the collimator. These pockets can be configured, for example, as square, rectangular, circular or any other conceivable geometric shape. The pockets can be shaped as complete material gaps or ensured by thinning the material that allows photon detection above the electronic noise of the detector. For example, it is conceivable to shape the radiolucent areas by inserts made of a material with low X-ray attenuation to ensure even higher mechanical load-bearing capacity. The image reconstruction method takes into account this information acquired per slice, which contributes to improving the image quality.

[0027] An exemplary technical configuration of this system includes, among other things, the following components: For collimation in the z-direction (setting of the light path), a thin plate of high attenuation, preferably made of a metallic material (e.g. lead or tungsten). · A thin plate made of a highly attenuating, preferably metallic material (e.g. lead or tungsten) for collimation in the x-direction (setting of the light path). The lamella system movable in the z-direction is preferably completely structured and curved about the z-axis, but can also be slit-like or planar. A lamina system movable in the z-direction can also have a defined slit, preferably curved around the z-axis, which preferably extends parallel to the x-axis, but can also be parallel to the z-axis or extend in other free space within the lamina plane. The lamina system movable in the x-direction preferably has a defined slit, preferably curved around the z-axis, which preferably extends parallel to the z-axis, but can also be parallel to the x-axis or extend in other free space within the lamina plane. The lamina system consists of a mechanism that allows the relative movement of two lamina components to vary the size of the radiolucent area of ​​the lamina system up to its complete closure.

[0028] The advantage of this adaptive lamina system is that the projections required for the image acquisition can be cleverly adapted within the rotation or partial rotation of the CT or C-arm system to generate a data set for computing a CT data set with the required image quality at the minimum dose.

[0029] For example, a lamina system can consist of two individual laminas one above the other with radiolucent pockets. These laminas can be positioned relative to each other. One of the aims of lamina shaping is to have a free VOI area and the freedom to set variable radiolucent zones in the edge areas.

[0030] As a further variant, the individual radiolucent pockets can be set by means of decoupled displacement mechanisms which can be driven individually, for example by electric motors.

[0031] In a further advantageous configuration, the lamina system is curved around the z-axis with a constant radius, so that the pocket edges always run parallel to the X-ray radiation, thereby ensuring homogeneous attenuation, which radius can accordingly be individual for each lamina in a lamina system with two laminas stacked one on top of the other.

[0032] It is furthermore conceivable to achieve a lamella system for creating volume areas of different geometric properties, for example circular volume areas like the classical aperture stop systems in photographic imaging, the aim here too being to incorporate adaptive pockets in the lamella area, whereby well-defined radiation-transmitting areas are formed.

[0033] Hereinafter, the present invention will be described in more detail based on embodiments using the drawings. [Brief description of the drawings]

[0034] [Figure 1] FIG. 2 shows the structure of a device according to the invention as viewed along the z-axis. [Diagram 2] FIG. 2 shows the structure of a device according to the invention as viewed along the x-axis. [Diagram 3] FIG. 2 is a top view of the collimator system. [Figure 4] FIG. 2 shows the movable lamina system in a first position. [Diagram 5] FIG. 5 shows the moveable lamina system of FIG. 4 in a second position. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Here, a coordinate system is assumed in which the X-ray beam direction extends in the y-direction, and the collimator system therefore extends with its planar extent in the xz-plane or, in the case of curved design systems, in a plane that is slightly curved relative to the xz-plane.

[0036] 1 and 2 show an exemplary configuration of an apparatus for deriving an X-ray image data set of a target area 17 of an examination object 13. The apparatus comprises an X-ray device 11, e.g. an X-ray tube, by which X-rays are emitted via a collimator system 12 onto the examination object 13 and onto an X-ray detector 14 arranged behind the examination object 13 in the radial direction. The collimator system 12 has a main radiation opening 16, which can be set with respect to its position and size, and possibly also with respect to its radiation transmittance, by a movable lamella system of the collimator system 12.

[0037] It will be seen that the X-rays extend in a collimated optical path 15 through a target area 17 of the object 13 to the X-ray detector 14, where the exposed area is evaluated, for example, by means of a matrix arrangement of detector elements sensitive to X-rays, from whose signals a respective projection image can be formed. The device further comprises a control device, not shown, which serves for controlling the collimator system 12, for reading out data from the X-ray detector 14 and for determining an X-ray image data set from the projection images obtained by the X-ray detector 14.

[0038] FIG. 3 shows a schematic principle diagram of the collimator system 12. This collimator system 12 has a housing 21 in which four displaceable lamina systems 24, 25, 26, 27 are arranged. These lamina systems 24, 25 are displaceable, for example, along the x-axis, and the lamina systems 26, 27 are displaceable along the z-axis. The lamina systems 24, 25 are displaceable to the left and right and therefore can be displaced towards each other and away from each other. The lamina systems 26, 27 can be displaced up and down and therefore move closer or further away from each other. Depending on the settings, the lamina systems 24, 25, 26, 27 can define a certain radiation-transmitting area forming the main radiation aperture 16. In FIG. 3, an example of a maximum possible aperture range 23 of the lamina systems, i.e. the maximum size of the main radiation aperture 16, is shown exemplarily in dashed lines. The lamina systems can be controlled automatically, for example, by a motor drive.

[0039] In order to form secondary radiation apertures in the edge region of the target area 17, i.e. outside the main radiation aperture 16, the thin plate systems 24, 25, 26, 27 can have additional functions compared to a displaceable simple aperture stop, which will be explained on the basis of Figures 4 and 5.

[0040] Figures 4 and 5 show, by way of example, an advantageous configuration based on a lamina system 30 which can be used to realise one or more lamina systems 24, 25, 26, 27, said configuration comprising a radially upstream lamina 31 and a radially downstream lamina 32. Figures 4 and 5 show the lamina system in a side view at the top and in a plan view at the bottom, respectively.

[0041] The upstream lamina 31 and the downstream lamina 32 have exactly or at least substantially congruent radiation openings 33, which are, for example, slit-shaped openings 33, respectively. If the upstream lamina 31 and the downstream lamina 32 are arranged exactly aligned one above the other, as shown in FIG. 4, their radiation openings 33 are also aligned with respect to each other, so that a number of secondary radiation openings are formed in the edge area around the main radiation opening 16. If the upstream lamina 31 and the downstream lamina 32 are relatively offset with respect to each other, as shown in FIG. 5, the radiation opening of the upstream lamina 31 is covered by the downstream lamina 32, and the radiation opening of the downstream lamina 32 is covered by the upstream lamina 31, so that the secondary radiation openings formed by the aligned radiation openings 33 are closed. This is clearly shown in FIG. 5 by the opening 34 of the upstream lamina 31 being closed.

Claims

1. 1. A method for deriving an X-ray image dataset of a target area of ​​an examination object, comprising: acquiring projection images of the target area using an X-ray detector; and reconstructing the X-ray image dataset from the projection images, wherein an X-ray device is used to collimate a first portion of the projection images onto the target area by irradiating the target area with X-rays through at least one main radiation aperture of a collimator system, the method comprising: a second portion of the projection images is captured by irradiating an edge region of the target area with X-rays passing through one or more secondary radiation apertures surrounding or adjacent to the main radiation aperture of the collimator system to reduce truncation artifacts in the X-ray image data set, and the X-ray image data set is determined from the first portion of the projection images using the second portion of the projection images.

2. 2. The method of claim 1, wherein the size, position and / or radiation transparency of one, some or all of the sub-radiation apertures is varied during the course of X-ray examination of the target area.

3. 3. The method according to claim 2, characterized in that the variation of the size, position and / or radiation transparency of one, some or all of the sub-radiation openings is performed adaptively depending on the respective quality of the current X-ray image.

4. 2. The method according to claim 1, wherein the step of determining the X-ray image data set comprises automatically recognizing irradiated and non-irradiated areas of the X-ray detector, and automatically taking into account the recognized irradiated and non-irradiated areas of the X-ray detector.

5. 2. The method of claim 1, further comprising the steps of automatically recognizing scattered radiation of X-rays emitted into non-directly irradiated areas of the X-ray detector, and automatically taking the recognized scattered radiation into account when determining the X-ray image data set.

6. 1. An apparatus for deriving an X-ray image dataset of a target area of ​​an examination object, comprising: an X-ray detector for capturing projection images indicative of the target region; and a reconstruction device for reconstructing the X-ray image data set from the projection images; an X-ray device and a collimator system with at least one main radiation aperture; the at least one main radiation aperture is for collimating and capturing a first portion of the projection image onto the target area by irradiating the target area with X-rays through the at least one main radiation aperture using X-ray radiation means; the collimator system has one or more sub-radiation apertures surrounding or adjacent to the main radiation aperture; a second portion of the projection image is captured by irradiating an edge region of the target area with X-rays passing through the one or more secondary radiation apertures; the reconstruction device is configured to derive the X-ray image dataset from the first portion of the projection images using the second portion of the projection images to reduce truncation artifacts in the X-ray image dataset.

7. 7. Apparatus according to claim 6, characterized in that the collimator system is arranged to adjust the width and / or length of one, some or all of the sub-radiation apertures.

8. 8. The device according to claim 7, characterized in that the collimator system comprises a movable lamella system with lamellas that are adjustable in one or two spatial directions, by means of which the width and / or length of one, some or all of the sub-radiation openings can be adjusted.

9. 9. The apparatus according to claim 8, wherein the movable lamina system has at least two individual lamina arranged one behind the other in the direction of radiation of the X-rays, the individual lamina each having a plurality of radiolucent openings and being displaceable relative to one another.

10. 7. The device according to claim 6, characterized in that at least the part of the collimator system having the secondary radiation opening is curved about a spatial axis, in particular curved at a constant radius.

11. Apparatus according to any one of claims 6 to 10, characterized in that it is equipped to carry out the method according to any one of claims 1 to 5.

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