Acquisition of image data using a computed tomography device

Abstract

A method for acquiring image data (BD) of an object (P) using a computed tomography (CT) scanner (2) is described. The CT scanner (2) has at least one detector array (4) and at least one X-ray source (5) attached to a support structure (3). The support structure (3) rotates around a measurement chamber (6) such that, to acquire image data (BD), projection data (RD) from different directions through the object (P) are first acquired, and then the image data (BD) of the object (P) are reconstructed based on the projection data (RD). Rotation-dependent changes (ΔGP) of beam geometry parameter values, which occur due to the rotation of the support structure (3) around the measurement chamber (6) during the acquisition of the projection data (RD), are determined and taken into account when acquiring the image data (BD).Furthermore, an image acquisition device (20) for carrying out this procedure and a computed tomography system (1) with a computed tomography device (2) and such an image acquisition device (20) are described.

Description

[0001] The invention relates to a method for acquiring image data from the interior of an object under investigation using a computed tomography device, which has at least one detector arrangement and at least one X-ray source attached to a support structure, wherein the support structure rotates around a measuring chamber such that, to acquire image data, projection data from different directions or different projection angles through the object under investigation are first acquired, and then the image data of the object under investigation are reconstructed based on the projection data. The invention also relates to a corresponding image acquisition device with which such a method can be carried out, and to a computed tomography system with such an image acquisition device.

[0002] The following documents are considered state of the art: DE 10 2010 039 080 A1, DE 10 2010 027 671 A1 and US 2003 / 0 159 508 A1.

[0003] In the so-called gantry of a computed tomography (CT) scanner, the X-ray source is, in most current designs, attached to a mechanical support structure that rotates around an axis. In most scanner types, the detector array, located opposite the X-ray source, is also attached to the support structure and likewise rotates around the scan chamber. Other components, such as a beam collimator, may also be attached to the support structure. This support structure is typically made of cast aluminum and comes in various forms. Regardless of the exact design, the support structure is subjected to increasingly greater mechanical stresses as the rotational speed increases, which can eventually lead to deformations of varying degrees. These deformations can become so severe that they affect the image, i.e., they distort the image.The beam geometry changes so significantly with rotation that errors in image generation can occur. A particularly common example is a support structure in a so-called pot or drum shape, where a pot base is arranged perpendicular to the axis of rotation and has a correspondingly large central opening for the measuring chamber in the region of the axis of rotation. A circumferential pot wall extends around the radial edge of the pot base, parallel to the axis of rotation. With such a drum, deformation or widening can occur at the upper edge of the pot wall, the side facing away from the pot base. This deformation could then extend through the entire geometry of the component, potentially leading to a migration of the X-ray beam relative to the detector arrangement.

[0004] To avoid this, the component geometry of newer devices designed for higher rotational speeds has been modified to achieve greater resistance to deformation. In particular, the wall thickness could also be increased accordingly. Alternatively or additionally, a more robust material was chosen. However, with ever-increasing demands on rotational speed, such solutions are becoming increasingly complex. For example, further material modification is not always possible or would lead to a significant increase in the device's cost and weight.

[0005] One objective of the invention is to create a safe and cost-effective way to reduce, as far as possible, any errors that may occur in imaging, even at increasingly higher rotational speeds.

[0006] This problem is solved by a method according to claim 1 and by an image acquisition device according to the dependent claim.

[0007] In the method according to the invention, rotation-dependent changes in beam geometry parameters are determined, i.e., changes that occur due to the rotation of the support structure around the measuring space during the acquisition of the projection data. Such rotation-dependent changes are generally reversible or temporary, as they disappear when the support structure stops rotating. The beam geometry parameters here are those parameters that relate to the geometry, in particular the extent, the relative position, distortion, etc., of the X-ray beam with respect to the detector arrangement itself, but not to the change in the projection data, which inevitably changes with the projection angle because the object under investigation is irradiated from a different direction. According to the invention, the determined changes in the beam geometry parameter values ​​can then be used in the acquisition of the image data, e.g., computationally or...The software-based corrective measures will be taken into account.

[0008] Unlike previous approaches, this method does not rely on completely absorbing all mechanical and / or thermal loads resulting from rotation without any changes to the beam geometry parameters. Instead, it attempts to identify potential changes and compensate for them computationally as much as possible. This allows for a simpler and more cost-effective support structure. In particular, it also enables a lighter support structure, saving not only raw materials and energy during manufacturing but also energy during operation. This, in turn, leads to even more cost-effective manufacturing and operation.

[0009] The determination of rotation-dependent changes in beam geometry parameter values ​​can be carried out by recording, at regular intervals or quasi-continuously, only the differences compared to, for example, the original beam geometry parameter values ​​in the stationary state, or by determining the changes relative to a previous measurement of the beam geometry parameters. It is also possible to record the current beam geometry parameters themselves at regular intervals or quasi-continuously. This also (indirectly) determines the changes. All these approaches are ultimately equivalent and encompassed by the invention.

[0010] A suitable image acquisition device for determining image data of the object under investigation, which can operate according to this procedure, has at least the following components: - an input interface for acquiring projection data, which was acquired using a computed tomography device as described above with at least one detector arrangement and at least one X-ray source attached to a rotating support structure; - an image reconstruction unit to reconstruct the image data of the object under investigation based on the projection data; - an output interface for the output of the image data.

[0011] According to the invention, the image data acquisition device is designed such that reversible rotation-dependent changes in beam geometry parameter values, which occur as explained due to the rotation of the supporting structure around the measuring space during the acquisition of the projection data, are determined and taken into account in a corrective manner when determining the image data.

[0012] For example, the image acquisition device may include, in addition to an image reconstruction unit, a corresponding beam geometry parameter value determination device and, if necessary, additional correction units, depending on how the determined changes in the beam geometry parameter values ​​are specifically taken into account during image data acquisition or reconstruction.

[0013] A computed tomography system according to the invention comprises, firstly, a computed tomography scanner and, secondly, an image acquisition device as previously described. This image acquisition device can, for example, be part of the control unit of the computed tomography scanner. Accordingly, the computed tomography scanner must have a detector arrangement and at least one X-ray source attached to a rotating support structure as described above. Preferably, as mentioned, the detector arrangement is also located on this support structure, usually opposite the X-ray source. However, the method and the use of the image acquisition device are not limited to such tomography scanners. For example, the computed tomography scanner could also have a rotating detector ring, so that it does not rotate with the support structure.It is also possible that the supporting structure has multiple X-ray sources and, if necessary, opposing detector arrangements, i.e., in a so-called dual-source system.

[0014] Key components of the image acquisition system can be implemented in software. This applies in particular to the image reconstruction unit, optionally a beam geometry parameter value determination unit, and other correction units. The aforementioned interfaces can also be implemented wholly or partially in software form, for example, as software module interfaces for transferring data from other software modules. Therefore, the entire image acquisition system can also be implemented in software.

[0015] A largely software-based implementation has the advantage that even previously used image acquisition devices or control units can be easily retrofitted via a software update to operate according to the invention. In this respect, the problem is also solved by a corresponding computer program product with a computer program that can be directly loaded into a storage device of an image acquisition device, e.g., a control unit of a computed tomography system, containing program sections to execute all steps of the method according to the invention when the program is run in the image acquisition device. In addition to the computer program, such a computer program product may optionally include additional components such as documentation and / or additional components, including hardware components such as hardware keys (dongles, etc.) for using the software.

[0016] For transport to and / or storage on or in the image acquisition device, a computer-readable medium, such as a memory stick, a hard drive, or other portable or permanently installed data carrier, can be used, on which the program sections of the computer program that can be read and executed by a computer unit of the image acquisition device are stored. The computer unit can, for example, include one or more cooperating microprocessors or similar components.

[0017] Further, particularly advantageous embodiments and developments of the invention result from the dependent claims and the following description, wherein the independent claims of a claim category may also be further developed analogously to the dependent claims of another claim category and, in particular, individual features of different embodiments or variants may be combined to form new embodiments or variants.

[0018] There are various ways to take into account the rotation-dependent changes in beam geometry parameters for compensation in image acquisition.

[0019] Firstly, the determined changes in beam geometry parameters could be used directly to correct the projection data, i.e., the raw data measured by the computed tomography scanner. This is possible, for example, by computationally reducing or increasing intensity values ​​in a preprocessing step of the projection data, depending on whether the focus of the X-ray source moves closer to or further away from the detector array.

[0020] Furthermore, changes to the beam geometry parameter values ​​could be used to correct the fully reconstructed image data, for example with the help of a correction algorithm in which correction values ​​for the geometric adjustments are entered.

[0021] The changes to the beam geometry parameter values ​​can be particularly advantageous if they are used directly within a reconstruction algorithm that reconstructs the image data based on the projection data. This is possible with most reconstruction algorithms, as they require the beam geometry parameters for virtual projection anyway as part of the reconstruction process. Changes to the beam geometry parameter values, or corresponding correction parameters, affect all projection methods—backward and forward projections, 3D beam hardening projection, etc.—that can be used in the reconstruction. Previously, it was common practice to use the beam geometry parameter values ​​known from the construction of the computed tomography system with a non-rotating support structure.Within the scope of the invention, these beam geometry parameter values ​​can be modified, with the changes to the beam geometry parameter values ​​being taken into account for the time at which the respective projection data were acquired. For this purpose, the corresponding changes to the beam geometry parameters (or the modified beam geometry parameter values) can preferably be assigned to the projection data. This is possible, for example, by linking the projection data with the corresponding data, for instance, by means of a timestamp or the like, which indicates the exact time at which the projection data were acquired, etc. Accordingly, the rotation-dependent changes to the beam geometry parameter values ​​determined at the individual times, or the current beam geometry parameter values, would have to be temporally assigned, marked, or linked.

[0022] In principle, it is also possible to combine different correction methods or to divide the correction into different stages. For example, the adjustment of the signal amplitude could be carried out in the preprocessing stage, and further correction steps in the subsequent reconstruction, especially in the virtual projection methods used there.

[0023] The beam geometry parameter values ​​preferably include a relative arrangement of the X-ray source and the associated detector arrangement to each other. A relative arrangement can refer, for example, to the relative positions and / or orientations, in particular a relative distance between the X-ray source and the associated detector arrangement and / or an angular position to each other.

[0024] The rotation-dependent changes in the beam geometry parameter values ​​can be due to various effects. Preferably, several effects are taken into account. Particularly preferably, these effects are of at least one of the following types: - Mechanical effects: These include in particular mechanical deformations of the supporting structure and / or the detector arrangement or the X-ray source or other image-relevant components such as the apertures, etc.; - Thermal effects: These could occur, for example, due to different cooling effects depending on the rotational speed. This can also lead to rotation-dependent changes in beam geometry parameter values; - Cyclic effects: These are effects that recur more regularly and may depend in particular on the current angle of rotation, for example whether the X-ray source is currently at the bottom or top and accordingly the gravitational force and the centrifugal force are in the same or opposite directions; - Non-cyclic effects: Such effects can also occur depending on rotation and can be dynamic or reversible, meaning that the effects disappear when the rotational speed changes or the rotation stops. Dynamic changes can also occur slowly over time, for example, a gradual lowering of the bearing axis of rotation in damped bearings. For instance, there are bearing designs that incorporate a damping element (e.g., a wire bearing with rubber rings). These exhibit a settling behavior or spring action, which is accompanied by a shift in the geometric position of the bearing in the direction of its center of gravity. These effects can also be taken into account within the scope of the invention.

[0025] Likewise, a combination of the above-mentioned different effect types is also possible, i.e., mechanical cyclic effects, thermal cyclic effects or corresponding mechanical or thermal non-cyclic effects, as well as a combination or superposition of cyclic and non-cyclic effects or mechanical and thermal effects.

[0026] In particular, rotation-dependent changes can include rotation angle-dependent changes, such as the previously described dependence on the effect of gravity, and / or rotation speed-dependent changes.

[0027] To account for all these effects and the various rotation-dependent changes in the different beam geometry parameter values, such as position and / or orientation, it is particularly preferable to first calculate the rotation-dependent changes of the various beam geometry parameter values ​​and / or the rotation-dependent changes for the various effects separately. These changes can then be combined into an overall change, for example, by superimposing them.

[0028] For example, changes in the relative arrangement of the X-ray source and the detector arrangement to each other in the x, y, and z directions can be determined separately. Changes in the various angular directions can also be considered separately. This determination can be performed individually for all possible effects—mechanical effects, thermal effects, cyclic or non-cyclic effects. Subsequently, the individual changes for each position or orientation are summed to determine a total change, which is then used in the generation of image data based on the raw data.

[0029] To determine the changes in beam geometry parameter values, suitable measurement data and / or control data are preferably acquired during the acquisition of the projection data.

[0030] The measurement data could include, for example, the current rotation angle. An example of control data could be the current control data for setting the rotation speed. While the rotation speed could also be acquired as measurement data, it is generally simpler to directly use the current data transmitted to the MRI scanner for setting the rotation speed. If no separate measurement data, for example for the rotation angle, is available, the current orientation, i.e., the current projection angle, could in principle also be directly derived from the projection data. The value of the current tilt angle of the gantry, i.e.,The angle at which the rotation axis of the supporting structure is tilted relative to the horizontal system axis, along which the patient positioning table is moved relative to the gantry, can be adopted in the form of control data, since this angle also influences the changes in the beam geometry parameter values, as will be shown later.

[0031] Additional measurement data, such as the current temperature inside the gantry or at specific points in the supporting structure and / or at the X-ray source and / or the detector arrangement, can also be determined and used in the calculations.

[0032] Preferably, input data is also used to determine changes in beam geometry parameter values, based on design data of the supporting structure and, particularly preferably, also of the X-ray source and / or the detector arrangement. Input data determined using FEM calculations is especially preferred for this purpose. For example, using FEM calculations, the design data can be used to determine the specific shape of the supporting structure at each rotational speed and angle, and the associated changes in beam geometry parameter values. The data can be generated for various rotational speeds and angles, and possibly also for other parameters.The data for different tilt angles of the gantry must be stored in a database, so that, if this data is known, the corresponding current beam geometry parameter values ​​can be quickly determined from this database.

[0033] Since the beam geometry parameter values, or rather the changes to these beam geometry parameter values, are only needed for the respective projection data during the correction or reconstruction of the image data, and since complex calculations are performed at this point anyway, it would also be possible, in principle, to initially store only the necessary measurement, control, and / or verification data for the individual projection data. Later, using the design data and FEM calculations, the corresponding changes to the beam geometry parameter values ​​could be determined for each projection data from the measurement, control, and / or other verification data. Alternatively or additionally to using input data based on design data, input data for determining the rotation-dependent changes in beam geometry parameter values ​​can also be obtained through test measurements.These can also be used and / or stored in the same way as the other input data described above.

[0034] The invention is explained in more detail below with reference to the accompanying figures and exemplary embodiments. The same components are designated with identical reference numerals in the various figures. The figures are generally not to scale. They show: Fig. 1 a schematic representation of an embodiment of a computed tomography system according to the invention, Fig. 2 a schematic representation of the change of forces on the rotating support structure in a gantry of a computed tomography system as a function of the current rotation angle, Fig. 3 A schematic representation of the change in forces on the rotating support structure in a gantry of a computed tomography system as a function of the current tilt angle, Fig. 4 A schematic representation of the possible deformation of a rotating support structure in a gantry of a computed tomography system in a section through the support structure, Fig. 5 A schematic representation of the possible deformation of a rotating support structure in a gantry of a computed tomography system in a top view of the rotation plane of the support structure, Fig. 6 a flowchart to illustrate a possible sequence of the inventive method within components of an embodiment of an inventive image acquisition device.

[0035] The in Fig. The computed tomography system 1 according to the invention, shown here in an exemplary and roughly schematic manner, comprises a computed tomography device 2 and a control unit 10 with a user terminal 11, 12. The control unit 10 is shown here as a unit separate from the computed tomography device 2 and connected to it via cables. In principle, however, it can also be wholly or partially integrated into the computed tomography device 2.

[0036] The computed tomography system 2 comprises a patient table 8 for positioning a patient P as the examination object P, wherein the tabletop 7 of the patient table 8 is adjustable along a system axis A so that the patient P can be moved in and through the measurement field 6 of the gantry 9 described below. The system axis A is also referred to as the z-axis.

[0037] The computed tomography system 2 further comprises, in the usual manner, a gantry 9 with a source-detector assembly rotatably mounted on a support frame 3 about the system axis A. The source-detector assembly has an X-ray source 5 and a detector assembly 4, which are aligned opposite each other on the support frame 3 such that, during operation, X-ray radiation emanating from the focus of the X-ray source 5 passes through the measurement field 6 (which is defined by the space between the X-ray source 5 and the detector assembly 4, through which the X-ray radiation passes) and strikes the individual detectors of the detector assembly 4.

[0038] The individual detectors of detector array 4 are structured into individual pixels, arranged in multiple detector rows and columns. The individual detectors are spatially arranged to provide a continuous pixelated detector area onto which the X-ray radiation strikes, enabling spatially resolved measurement of projection data (RD). However, the detector could also be configured differently.

[0039] A computed tomography system 1 of this type is known to be used for the reconstruction of image data, particularly in the form of 2D cross-sectional images and / or 3D volume images. To acquire image data of an examination area (or a so-called "region of interest" within the object P, e.g., an area containing a specific organ), projection data RD from a multitude of different projection directions are first acquired as the source-detector assembly rotates. In the case of so-called "spiral scanning," for example, the tabletop 7 of the patient table 8 is continuously adjusted in the direction of the system axis A during rotation of the source-detector assembly. With this type of scanning, the X-ray source and the detector thus move along a helical path around the patient P.In contrast, in a “sequential scan”, projection data is recorded at different positions along the system axis A during a standstill of the patient table 8.

[0040] The projection data RD represent the intensity values ​​of the X-ray radiation attenuated by the patient P, measured at the individual pixels. The projection data RD, as so-called raw data RD, are transmitted via a data channel to the system control unit 10, which includes an image acquisition unit 20 with an image reconstruction unit 21. Based on the raw data RD, the image data BD of the interior of the patient P can be reconstructed in the system control unit 21. This image data can then be displayed, for example, on a display unit 12 of the user terminal and / or stored in memory and / or sent to other systems. The user terminal also includes, among other things, a keyboard 11 as an input device, with which an operator can, if necessary, set values ​​for parameters, in particular for raw data acquisition and / or image reconstruction.

[0041] The computed tomography system 2 and its components are supplied with the required operating voltage or electrical power via a power supply line. Appropriate control commands can be transmitted from the control unit 10 to the computed tomography system 2 via one or more control lines.

[0042] The basic structure and function of such computed tomography systems 1 and in particular the reconstruction of the image data BD from the raw data RD or projection data are known to the person skilled in the art, so that in the following only the differences essential for carrying out the invention compared to conventional methods will be explained.

[0043] In particular, the image acquisition device 20 preferably includes a modified image reconstruction unit 21 and a beam geometry parameter value determination unit 30, these components of which will be further described later by means of Fig. 6 will be explained in more detail.

[0044] As mentioned earlier, the high rotational speed of the support structure 3 around the measuring chamber 6 results in high loads on the support structure 3, which can lead to deformations. This can alter the beam geometry in such a way as to affect the imaging. This is particularly due to the fact that relatively heavy components, such as the X-ray source 5 or the detector array 4, are located at certain points within the support structure 3. Therefore, the support structure 3, with all its components used for imaging, is not rotationally symmetrical or balanced.

[0045] Examples of how the various forces can affect the beam geometry depending on the rotation of the support structure 3 around the measuring chamber 6 are shown below using the following: Fig. Figures 2 to 5 are explained by way of example. In each case, it is assumed that the supporting structure 3 has a drum or pot shape, i.e., that it is a so-called drum. This drum has a pot base 3B perpendicular to an axis of rotation R, which has an opening or cutout in the center around the axis of rotation R for the measuring chamber 6, and a pot rim 3R extending parallel from the pot base 3B to the axis of rotation R at the radially outer edge (see Figure 2). Fig. 2 and Fig. 3) In radial cross-section, the drum therefore has an L-shape, with components such as the X-ray source 5 arranged within the L-angle. However, the invention is not limited to such drums and can also be used in other support structure designs.

[0046] Based on the Fig. 2 and Fig. 3 it is easily evident how the weight force F Gon a component, here as an example the X-ray source 5, depending on the rotation, the beam geometry is influenced and the beam geometry parameter values ​​are changed.

[0047] If, for example, the X-ray source 5 is located at its maximum upper point during rotation, the centrifugal force F acts on the X-ray source 5. R , which increases depending on the rotational speed, as well as the weight force F G opposite. This changes depending on the rotation angle ω. For example, in another extreme case, if the X-ray source 5 has reached its lowest point, the rotational velocity-dependent centrifugal force F acts. R and the weight force F G in the same direction, i.e., they add up. This is in Fig. 2. This is easy to see. In other words, depending on the rotational speed and the current rotational angle ω, a different total force acts on the mass of the X-ray source 5, causing its position and / or orientation relative to a detector arrangement (not shown here) attached opposite to the support structure to change slightly. The same applies to the forces acting on the detector arrangement.

[0048] Consequently, for each projection angle ω, the beam geometry parameter values ​​change slightly depending on the current rotation speed v. For example, if the rotation speed is 0 rpm, i.e., the system is stationary, the beam geometry is effectively different than at high rotation speeds of, for example, 60 rpm up to 300 rpm or even more. These changed beam geometry parameter values ​​influence the currently measured projection data, for example, by shifting the X-ray beam pattern relative to the detector array (as will be explained in more detail later) or by increasing or decreasing the intensity, since the X-ray source is sometimes closer to the detector array and sometimes further away. Previously, attempts were made to minimize such changes by using a particularly robust support structure so that they would not affect the image.Within the scope of the present invention, such current rotation-dependent changes in the beam geometry parameter values ​​for the respective associated projection measurement data are to be recorded and subsequently taken into account in a corrective manner in the determination of the image data based on the projection data.

[0049] The influence of the weight force relative to the centrifugal force on the X-ray source 5 therefore varies between + / -1 G when the gantry is not tilted, i.e., when the pot bottom 3B of the support structure 3 is in a perpendicular plane. However, in many images, the gantry is tilted by a tilt angle α, as shown in Fig. Figure 3 shows that the tilt angle α can, for example, be between + / -30° for many devices. This has corresponding effects on the forces on the components, which is again illustrated here using the example of the X-ray source 5. For example, in the uppermost maximum position, the centrifugal force F Rand the weight force F G While the forces act diametrically oppositely (solid line), this is no longer the case with a tilted gantry (dashed line). Here, the axis of rotation R, around which the supporting structure 3 rotates, is also tilted relative to the horizontal by the tilt angle α. This means that the rotation-dependent centrifugal force F R It also acts in a direction that is inclined by the tilting angle α to the weight force F. G stands. The resulting force F res This results in a lower value, but also a different direction, namely with a component pointing away from the bottom of pot 3B. This is relevant for the upper position in Fig. 3 clearly visible. A different resultant force F results everywhere along the rotation angle ω. res , which can be composed vectorially from the individual forces in a similar way.

[0050] In Fig. Figure 4 schematically illustrates another effect that can occur particularly at very high rotational speeds. As can be seen here, the centrifugal force F can be particularly significant. R The forces acting on the components, i.e., the X-ray source 5 and the detector assembly 4, become so strong that the support structure 3 is bent outwards, i.e., the bottom 3B of the drum is bent so that the rim 3R tapers conically. This is in Fig. Figure 4 is greatly exaggerated for clarity. This expansion of the support structure 3, or drum 3, also leads to a change in the beam geometry, namely firstly because the distance Δy between the X-ray source 5 and the detector arrangement 4 changes, and secondly because the central beam ZS of the X-ray fan on the detector arrangement 4 shifts by a displacement angle ϕ, thus slightly displacing the entire X-ray fan on the detector arrangement 4. Similarly, spatial displacements Δz in the z-direction or Δx in the x-direction can occur.

[0051] In addition to the effects described in more detail above, further influences can occur, such as static imbalances, dynamic imbalances (e.g., due to the movement of various components like apertures or liquid media such as coolants), and dynamic changes over time (e.g., due to the lowering of the bearing axis of rotation, particularly in damped bearings). Thermal changes can also occur due to heat loss from the cooling system, which can also depend on the rotational speed. Further effects can arise from the system's natural vibrations, which can also be dependent on the rotational speed, as well as from thermal effects that can slightly alter the position of the rotating anode and its focus.

[0052] It should be noted here that the invention is not limited to the explicitly mentioned influences, but that in principle all rotation-dependent effects can be taken into account, as long as these can be determined by measurement or calculation.

[0053] All these different effects combine to result in a different beam geometry depending on the rotation speed and the current rotation angle ω, as shown in Fig. 5 is shown again using the dashed line (whereby this dashed line is only intended to show schematically that the geometry changes along the rotation angle ω, and does not give an example of what these changes actually look like depending on the rotation angle ω).

[0054] At the X-ray source 5, in this Fig. Figure 5 also shows the aperture system 5b of the X-ray source 5, which also has a significant influence on the beam geometry and, for example, determines the width of the fan angle φ and the direction in which the central ray ZS of the X-ray fan propagates from the X-ray source 5 (or from the focus F of the X-ray source 5). This figure also illustrates the changes in the beam geometry in the three spatial directions Δx, Δy, Δz, with the arrow in the z-direction projecting forward from the image plane.

[0055] To account for all these effects when determining the image data, the modified geometry parameter values ​​ΔGP are recorded for each of the acquired projection data RD, depending on these effects. Based on Fig. Section 6 shows a possible procedure and how it can be implemented using an example within the context already mentioned. Fig. The following components of the image acquisition device 20 mentioned in point 1 can be used.

[0056] For example, if a measurement program is started in step I, the raw data RD, i.e. the projection data RD, are recorded in the usual way, together with the information about the respective projection angle or rotation angle ω, since this is also needed in the context of the reconstruction.

[0057] Additionally, further input data is acquired during program start I, such as the rotational speed v, the current tilt angle data KW, focus position data FP (the current focus position), further thermal data TD (which can provide information about the current influence of cooling on the components), design data KD (which includes, for example, information about the masses being moved at the aperture, the detector assembly, the X-ray source, etc.), and / or vibration data SD (which contains information about the system's natural vibrations under certain conditions or at certain rotational speeds). This may only be a portion of the input data; that is, further information can also be included.

[0058] Some input data can be directly used as control data, such as the rotational speed v or the tilt angle setting KW. Other input data can be measurement data, such as thermal data TD, which can be measured by temperature sensors at various points in the gantry. Still other input data can be taken from a database, such as design data KD or vibration data SD, which are stored there for specific other parameters. It should be explicitly noted that the various input data can also be linked to each other, such as the influence of thermal data on the focus position or the system's natural frequencies, etc.In particular, FEM calculations can also be carried out taking into account the design data KD and / or test measurements to obtain other input data, such as vibration data or measurements with strain gauges or similar, to determine, for example, thermal length changes of mechanical components.

[0059] Additional control data can also be transferred from the CD and taken into account, whether in the form of measurement data or control data, such as movement between the gantry or support structure and the patient bed, especially periodic, reversible movements.

[0060] All these input data CD, V, KW, FP, TD, KD, SD are transferred together with the current rotation angle ω to a beam geometry parameter value determination unit 30, which can be implemented, for example, as a software component on a computer unit of the image acquisition device 20, here in a control unit 10 of the computed tomography system 1.

[0061] For the individual effects, the changes ΔGP of the beam geometry parameter values ​​in each component can then be calculated as a function of the current rotational speed v and the current rotational angle ω, which in Fig. Figure 6 shows the individual blocks with their different curve profiles in the beam geometry parameter value determination unit 30 only schematically. This means that, for example, for the first effect, e.g., the effect of the weight force as a function of the rotational velocity v and the rotational angle ω on the changes in beam geometry, the individual directional components Δx1, Δy1, Δz1, and, if applicable, the angular displacement of the central beam ZS of the X-ray fan on the detector arrangement 4, are calculated component-wise. The same procedure is performed for the other effects, whereby all geometry parameter values ​​are also determined component-wise, so that further changes Δx2, Δy2, Δz2, Δφ2, Δx3, Δy3, Δz3, Δφ3 ... or a displacement angle ϕ can be determined accordingly. Temperature changes ΔT1, ΔT2 ... can also be determined.

[0062] Once all effects have been determined component-wise, they can then be superimposed to obtain the overall impact or the total changes ΔGP of the geometry parameter values ​​achieved by the various effects, which is shown in Fig. 6 within the beam geometry parameter value determination unit 30 is symbolically represented by the block on the right-hand side.

[0063] The various changes ΔGP of the geometry parameter values ​​are linked to the rotation angle ω and thus also to the individual data sets of the raw data RD or projection data RD, and can be assigned accordingly. It should be noted again here that instead of the changes ΔGP of the geometry parameter values, the current geometry parameter values ​​can, of course, always be determined, which is equivalent to determining the changes. If, as shown here as an example, only the changes ΔGP of the geometry parameter values ​​are determined, i.e., the deviations from the geometry parameter values ​​GP0 for a non-rotating supporting structure, these can also be passed to the reconstruction unit 21 to be taken into account in the reconstruction algorithm RA.

[0064] The changes ΔGP of the geometry parameter values ​​are then passed to an image reconstruction unit 21 of the image acquisition device 20, which usually receives the raw data RD or projection data RD with the information about the associated rotation angles ω.

[0065] The reconstruction unit 21 has an interface 27 for receiving changes ΔGP of the beam geometry parameter values, an input interface 24 for receiving the beam geometry parameter values ​​GP0 for a non-rotating support structure, an interface 22 for receiving the projection data RD with the rotation angles, an interface 23 for receiving patient data PD or the like, and further interfaces for receiving additional data, of which only one further interface 25 is shown symbolically here. Furthermore, the reconstruction unit 21 has a corresponding interface 26 for outputting the finished image data BD, which can be in the form of volume image data or tomographic image data.

[0066] The image reconstruction unit 21 can then use the changes ΔGP of the beam geometry parameter values ​​to correctively reconstruct the image data BD from the raw data RD. For this purpose, for example, based on the beam geometry parameter values ​​GP0, which apply to a non-rotating supporting structure and are typically used in such reconstruction methods, the appropriate beam geometry parameter values ​​for the respective raw data RD are determined using the calculated changes ΔGP and then inserted into a reconstruction algorithm RA to obtain the image data BD, which have been corrected according to the changes ΔGP of the beam geometry parameter values.

[0067] Changes in beam geometry parameter values ​​(ΔGP) or currently modified beam geometry parameter values ​​can be accounted for in various ways. One option is to correct the projection data during preprocessing. For example, if the focus of the X-ray source shifts away from the detector array, the intensity decreases. In such cases, the intensity must be computationally increased. Conversely, if the focus of the X-ray source moves closer to the detector array, the intensity increases and must be computationally reduced accordingly. If, on the other hand, the geometry of the X-ray source and detector array is rotated relative to each other, values ​​may need to be interpolated. Simple geometric changes can be corrected relatively easily to significantly improve image acquisition results.

[0068] Another possibility is to use the set of ray geometry parameter values, individually determined for each set of projection data using the determined changes in the ray geometry parameter values, in the reconstruction process itself, namely in the step of a virtual forward projection or the like.

[0069] As the examples above demonstrate, the inventive correction of dynamically reversible, and in particular consistently repeating, processes in the supporting structure makes it possible to design the entire mechanism more simply, cost-effectively, and lighter. This means that complex mechanical solutions for achieving the required image quality are no longer necessary to the same extent. It is also possible, of course, to implement a combination of mechanical stabilization measures for the supporting structure and a correction of the remaining deviations. In any case, savings in material, energy, and costs are possible because the supporting structure can be designed with a simpler geometry and thinner walls than before, even if the mechanical loads due to centrifugal force, etc., increase.In particular, this will make future requirements, such as further increasing the number of rotations while simultaneously increasing image resolution, easier and more cost-effective to meet.

[0070] Finally, it should be noted once again that the devices and methods described in detail above are merely exemplary embodiments, which can be modified in various ways by a person skilled in the art without departing from the scope of the invention. Furthermore, the use of the indefinite articles "a" or "an" does not preclude the possibility that the features in question may be present multiple times. Likewise, the term "unit" does not preclude the possibility that the component in question consists of several interacting sub-components, which may also be spatially distributed.

Claims

[1] Method for determining image data (BD) of an object under investigation (P) using a computed tomography device (2) with a gantry, which has at least one detector arrangement (4) and at least one X-ray source (5) attached to a support structure (3), wherein the support structure (3) rotates around a measuring space (6) such that, in order to determine image data (BD), projection data (RD) are first acquired from different directions through the object under investigation (P), and then the image data (BD) of the object under investigation (P) are reconstructed on the basis of the projection data (RD), wherein rotation-dependent changes (ΔGP) of beam geometry parameter values ​​that occur due to the rotation of the supporting structure (3) around the measuring space (6) during the acquisition of the projection data (RD) are determined and taken into account during the determination of the image data (BD), wherein rotation-dependent changes (ΔGP) include rotation angle-dependent changes and rotation velocity-dependent changes, taking into account whether the X-ray source (5) is located at the bottom or top and accordingly whether the gravitational force and the centrifugal force are in the same or opposite directions, wherein input data for determining the rotation-dependent changes (ΔGP) of beam geometry parameter values ​​are based on design data (KD) of the gantry, wherein, with the help of the design data (KD) using FEM calculations, it is determined at which rotation speeds and which rotation angle the supporting structure (3) assumes which specific shape and which beam geometry parameter value changes are associated with it. [2] Method according to claim 1, wherein the rotation-dependent changes (ΔGP) of the beam geometry parameter values ​​are used to correct the projection data (RD) and / or to correct the image data (BD) and / or within a reconstruction algorithm (RA) to reconstruct the image data (BD) based on the projection data (RD). [3] Method according to claim 1 or 2, wherein the beam geometry parameter values ​​comprise a relative arrangement of the X-ray source (5) and the detector arrangement (4) to each other, in particular a distance (Δx, Δy, Δz) and / or an angle (ϕ) to each other. [4] Method according to any of the preceding claims, wherein rotation-dependent changes (ΔGP) of the beam geometry parameter values ​​are taken into account due to various effects, preferably effects of at least one of the following types: - mechanical effects, - thermal effects, - cyclical effects, - non-cyclic effects and / or a combination of these types. [5] Method according to one of the preceding claims, wherein the rotation-dependent changes (ΔGP) of different beam geometry parameter values ​​and / or for different effects are first calculated separately and then combined to form an overall change. [6] Method according to one of the preceding claims, wherein measurement data and / or control data are acquired during the acquisition of the projection data (RD) and rotation-dependent changes (ΔGP) of beam geometry parameter values ​​are determined on the basis thereof. [7] Method according to one of the preceding claims, wherein input data for determining rotation-dependent changes (ΔGP) of beam geometry parameter values ​​are determined by means of test measurements. [8] Image acquisition device (20) for obtaining image data of an object of investigation (P), wherein the image acquisition device (20) comprises at least the following components: - an input interface (22) for acquiring projection data (RD) acquired by means of a computed tomography device (2) with a gantry, wherein the computed tomography device (2) has at least one detector arrangement (4) and at least one X-ray source (5) attached to a support structure (3), wherein the support structure (3) rotates around a measuring chamber (6) such that the projection data (RD) are acquired from different directions through the object under investigation (P), - an image reconstruction unit (21) to reconstruct the image data (BD) of the object under investigation (P) based on the projection data (RD), - an output interface (26) for outputting the image data (BD), wherein the image acquisition device (20) is designed such that rotation-dependent changes (ΔGP) of beam geometry parameter values, which occur due to the rotation of the support structure (3) around the measuring space (6) during the acquisition of the projection data (RD), are determined and taken into account during the determination of the image data (BD), wherein rotation-dependent changes (ΔGP) include rotation angle-dependent changes and rotation velocity-dependent changes, taking into account whether the X-ray source (5) is located at the bottom or top and, accordingly, whether the gravitational force and the centrifugal force are in the same or opposite directions, wherein input data for determining the rotation-dependent changes (ΔGP) of beam geometry parameter values ​​are based on design data (KD) of the gantry, wherein, with the help of the design data (KD) using FEM calculations, it is determined at which rotation speeds and which rotation angle the supporting structure (3) assumes which specific shape and which beam geometry parameter value changes are associated with it. [9] Computed tomography system (1) comprising a computed tomography device (2) and an image acquisition device (20) according to claim 8. [10] Computer program product comprising a computer program which can be directly loaded into an image acquisition device (20), comprising program sections to execute all steps of the method according to any one of claims 1 to 7 when the computer program is executed in the image acquisition device (20). [11] Computer-readable medium on which program sections readable and executable by a computer unit are stored in order to execute all steps of the method according to any one of claims 1 to 7 when the program sections are executed by the computer unit.

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