Computer-implemented method for determining torsion information of a bone, X-ray equipment, computer program and electronically readable data carrier

Abstract

The invention relates to a computer-implemented method for determining torsion information of a patient's bone, wherein an X-ray device with adjustable projection geometry of an imaging arrangement described by projection parameters is used, comprising the following steps: a) Taking a first X-ray image of an end region of the bone in a first projection geometry in which a torsion direction defined with reference to at least one torsion landmark, to be used to determine a roll angle, lies in a first computational plane formed by the focal point of the X-ray source and a reference direction determinable in the first X-ray image by means of at least one reference landmark, b) Taking a second X-ray image of the end region of the bone in a second projection geometry with a projection direction that differs from the projection direction of the first projection geometry and forming a second computational plane from the reference direction that can be determined in the second X-ray image using at least one reference landmark and the focal point, c) Determination of the three-dimensional course of the reference direction and the torsion direction from the computational planes and the roll angle as the rotation of the torsion direction about the reference direction relative to a comparison specification, - where the torsion information is determined from the roll angle.

Description

[0001] The invention relates to a computer-implemented method for determining torsion information of a bone, in particular the femur, of a patient. This method utilizes an X-ray device with an adjustable projection geometry defined by projection parameters of a recording arrangement comprising an X-ray source and an X-ray detector. The invention also relates to an X-ray device, a computer program, and an electronically readable data carrier.

[0002] For bones, especially bones of the extremities, of the human body, various torsional information is known that describes the orientation and relative alignment of bone components. These information is defined primarily by appropriate anatomical landmarks of the corresponding bone. In medicine, torsion refers to the twisting of an anatomical feature around its own longitudinal axis, so torsional information contains clues to a potentially incorrect alignment of bone components that interact with other parts of the body.

[0003] Such considerations are particularly relevant for the human femur, the thighbone. In particular, fractures of the femoral shaft occur relatively frequently, and these fractures are usually stabilized surgically by inserting a femoral nail. It is extremely important to restore the antetorsion angle between the femoral neck axis and the condylar plane to an anatomically correct level before locking the femoral nail.

[0004] In the prior art, methods have already been proposed to quantify the antetorsion angle of a patient's femur from three-dimensional computed tomography images, with the method according to Waidelich et al. in particular now being established; see, for example, the article "Computertomographische Torsionswinkel- und Längenmessung an der unteren Extremität" by H.-A. Waidelich et al., Fortschr. Röntgenstr. 157, 3 (1992), pages 245 to 251. In the prior art, due to the complex structure of the proximal femur, a bilateral computed tomography scan is required to determine the antetorsion. This scan is usually performed after surgery, since intraoperative measurement with a computed tomography device is not possible in many institutions.

[0005] If a patient experiences acute gait problems after surgery, a postoperative CT scan is used to determine the antetorsion angle on both sides. If the difference between the right and left antetorsion is more than 12°, revision surgery is recommended. If the patient has no acute problems despite a malalignment, revision surgery is often not performed. However, this can still lead to long-term health limitations for the patient.

[0006] In principle, it is conceivable to perform computed tomography determination of the antetorsion angle intraoperatively, for example, using an X-ray system with a C-arm. By rotating the C-arm, on which an X-ray source and an X-ray detector are arranged opposite each other, different projection geometries can be achieved around the relevant bone, allowing for three-dimensional computed tomography reconstruction. For example, a cone beam computed tomography scan can be performed intraoperatively in the proximal and distal end regions of the femur to independently determine the torsion of the joints relative to the horizontal, both proximally and distally.Torsion angles and directional angles are determined based on specific anatomical landmarks of the femur, for example, the femoral head and greater trochanter can be used proximally, and the condylar plane is considered distally. The antetorsion angle is then determined by the difference between the two torsions described by the corresponding directional angles.

[0007] In practice, however, such a procedure is not performed because, firstly, X-ray equipment with isocentric C-arms suitable for three-dimensional imaging is too cumbersome for this type of surgery. Furthermore, the significant time required to perform two intraoperative three-dimensional computed tomography scans would significantly increase the overall operating time.

[0008] Therefore, it is currently common practice to restore the antetorsion without imaging before locking the femoral nail. For this purpose, the positions of both legs and feet are aligned as symmetrically as possible. If the type of fracture allows, an attempt is also made to reattach the fractured parts as precisely as possible using the fracture edge to restore the original antetorsion. However, these methods are characterized by a high degree of inaccuracy, so that deviations in the antetorsion angle, i.e., malalignments, of 15° or more can occur.

[0009] The publication US 2007 / 0161929 A1 discloses a method for determining an antetorsion angle based on two fluoroscopy images.

[0010] The invention is therefore based on the object of providing a simple, time-efficient way of determining torsion information of bones by X-ray imaging.

[0011] To achieve this object, the invention provides a computer-implemented method, an X-ray device, a computer program, and an electronically readable data carrier having the features of the independent claims. Advantageous embodiments are set forth in the subclaims.

[0012] A method of the type mentioned above comprises the following steps: a) recording a first X-ray image of an end region of the bone in a first projection geometry in which a torsion direction to be used to determine a roll angle and defined in relation to at least one torsion landmark lies in a first calculation plane which is formed by the focal point of the X-ray source and a reference direction which can be determined in the first X-ray image by means of at least one reference landmark, in particular parallel to the shaft axis of the bone, b) taking a second X-ray image of the end region of the bone in a second projection geometry with a projection direction that differs from the projection direction of the first projection geometry, in particular one that is at least substantially perpendicular to this, and forming a second calculation plane from the reference direction that can also be determined in the second X-ray image by means of the at least one reference landmark, as well as the focal point, c) Determination of the three-dimensional course of the reference direction and the torsion direction from the calculation planes and the roll angle as the rotation of the torsion direction around the reference direction compared to a reference specification, - where the torsion information is determined from the roll angle.

[0013] A basic idea of ​​the present invention is to determine a relevant roll angle, in particular with respect to the shaft direction of the shaft axis, using only two X-ray images. It was recognized that torsion directions, via which torsion information is defined, are usually defined in relation to at least one torsion landmark, with particular attention being paid to torsion directions that are used to define angles, in particular roll angles, that describe the torsion. These torsion landmarks, which may include, for example, condyles, bone heads, trochanter, and the like, are usually recognizable in X-ray images. The idea is to create geometric relationships in which two X-ray images with different projection geometries and different projection directions are sufficient to easily determine roll angles as the basis for torsion information.The roll angle is usually related to a reference direction, which in most cases, following the medical definition of torsion, will be the shaft direction of the shaft axis. If a bone is viewed longitudinally, i.e., in the direction of the shaft axis, the torsion landmarks and thus the torsion directions can be defined. While in many cases, for example, in antetorsion, differential angles are considered, i.e., in particular, angles between torsion directions in the distal end region and torsion directions in the proximal end region, applications are also conceivable where roll angles in only one end region are of interest for comparison purposes, for example, during follow-up during a procedure or the like.

[0014] A fundamental idea of ​​the present invention is to cleverly select the first projection geometry so that not only can the reference direction be recognized based on the reference landmarks, but the reference direction, together with the focal point of the X-ray source, forms a first computational plane in which the torsion direction (and, by definition, the reference direction) lies. To construct the computational planes, the starting point is the image locations of the reference direction on the X-ray detector. This means that computational planes are formed by the focal point and the image points that indicate the reference direction on the X-ray detector.

[0015] It was recognized that such a projection geometry is characterized by specific, discoverable and recognizable image impressions in the first x-ray image, which can be specifically sought out. For example, a torsion direction may lie along the outermost points of the condyles in a calculation plane, in which the condyles exactly overlap in the first x-ray image. If a shaft direction is used, the reference direction may need to be shifted parallel to the shaft axis itself, so that it touches the condyles, which appear as one condyle, for example, tangentially. In another example, for a bone with a head-bearing neck, the first projection geometry can be selected so that the head appears as an extension of the bone shaft, whereby the torsion direction in the first calculation plane points laterally from the shaft axis towards the head. In this example, the head would be a torsion landmark.Other cases of this kind are also conceivable, which ultimately concern a specific, characteristic image impression with regard to the reference landmarks and the torsion landmarks.

[0016] It should be noted at this point, which also applies in general, that due to working in the calculation plane, which is defined by the focal point of the X-ray source and the position of the image points of the X-ray detector in space at which the reference direction was recorded in the first projection geometry, and is therefore visible in the first X-ray image, and due to the reference direction being expediently set at the level of at least one torsional landmark, cone beam effects, i.e. angular deviations of other X-rays from the central beam, have no relevant influence on the observations carried out here, so that a correction in this regard is not necessary.

[0017] For the second projection geometry, it is ultimately essential that the reference direction (via at least one reference landmark) is also evident from the second X-ray image and that the projection directions are different, so that a second calculation plane can be constructed, which is defined by the reference direction in the second X-ray image, i.e., correspondingly in the detector plane, and in turn the focal point of the now differently positioned X-ray source. The reference direction also lies in this second calculation plane by definition. It defines the three-dimensional course of the reference direction and thus also the torsion direction more precisely, which forms the basis for determining the roll angle relative to the reference specification, which will be discussed in more detail below.It is particularly useful if the projection direction of the second projection geometry is at least substantially perpendicular to the projection direction of the first projection geometry, which can be advantageous, particularly with regard to the resulting image impressions and the ability to recognize the reference landmarks and the reference direction. In the above-mentioned example of the condyles overlapping in the first projection geometry, the two condyles would lie next to each other in an image taken perpendicular to them and also showing the reference direction, particularly such that the extension of the shaft axis runs right between them.

[0018] Expedient developments of the present invention, particularly with regard to producing substantially orthogonal projection directions, can provide for the second projection geometry to be automatically determined from the first calculation plane, particularly such that the central ray of the second projection geometry runs at least substantially along the surface normal of the calculation plane. However, automatic determination of the second projection geometry, particularly with knowledge of the first projection geometry, is also expedient in other ways, as this can reduce the user's workload. Taking into account the first calculation plane itself, particularly its surface normal, the second projection geometry can be adapted as ideally as possible such that the calculation planes are as perpendicular to one another as possible.However, especially when using a fan-beam geometry, as is often the case with X-ray systems, an optimal perpendicular relationship between the first and second computational planes can only be achieved in rare cases. This can be taken into account when evaluating the first and second X-ray images, as will be explained below.

[0019] A particularly easy-to-implement design for determining the roll angle, using simple vector operations, can provide that, first, to determine a measurement orthogonal system - the surface normal of the first calculation level, - from the calculation planes, in particular by forming the cross product of the surface normals of the first and second calculation planes, a three-dimensional reference vector of the reference direction describing the three-dimensional course of the reference direction and - a three-dimensional torsion vector describing the three-dimensional course of the torsion direction is determined in the first calculation plane, in particular by forming the cross product of the surface normal of the first calculation plane and the reference vector, wherein the roll angle is determined as the corresponding Euler angle of the measuring orthogonal system to a basic orthogonal system describing a basic position of the recording arrangement as a comparison specification.

[0020] This design takes particular account of the fact that the first calculation plane and the second calculation plane, as well as the surface normal of the first calculation plane and the surface normal of the second calculation plane, do not necessarily have to be orthogonal to one another, since inaccuracies in the setting as well as deviations due to the cone beam geometry and when determining the reference direction can occur. However, since the reference direction lies in both the first and second surface planes (by design), both surface normals are perpendicular to it, so their cross product necessarily results in a reference vector along the reference direction, regardless of whether the surface normals are perpendicular to one another.In any case, a torsion vector in the torsion direction is immediately known, namely due to the cross product of the surface normal of the first calculation plane, in which the torsion direction lies by construction, and the reference vector of the reference direction.

[0021] While there are, in principle, other ways to assess how much the torsion direction is rotated around the reference direction in relation to a reference specification, it proves particularly advantageous to work with the knowledge that the surface normal of the first calculation plane, the reference vector, and the torsion vector constitute an orthogonal system, referred to here as the measurement orthogonal system. This is because the corresponding vectors are known in the coordinate system of the X-ray device, specifically the acquisition setup. However, within this X-ray coordinate system, there are a multitude of possible comparison specifications, with basic positions being particularly suitable.For example, if an X-ray system with a C-arm is used, the orientation of the imaging arrangement is determined in particular by an angular angle and an orbital angle, so that vectors describing the main directions in a basic position with zero angular angle and zero orbital angle can be used as a reference. Of course, other basic orthogonal systems are also conceivable, as long as they are clearly and unambiguously defined in the X-ray coordinate system. The basic orthogonal system can, in particular, also be the orthonormal system that forms the basis of the X-ray coordinate system.

[0022] Between such orthogonal systems, a variety of easily implemented calculation techniques are now known for determining the Euler angles of one orthogonal system relative to another. The use of Euler angles is particularly useful in this case, since the rotation of the torsion vector around the reference vector relative to the reference vector is sought.

[0023] The measurement orthogonal system can be understood as describing the orientation of the end region of the bone, particularly in the X-ray coordinate system. As already mentioned, this refers in particular to the reference direction (described by the at least one reference landmark) and the torsion direction (described by the at least one torsion landmark), i.e., the directions relevant to torsion.

[0024] It should be noted at this point that the comparison specification, in this specific case the basic orthogonal system, can be cleverly selected in such a way that a desired reference, for example to the horizontal plane, is established whenever only a roll angle itself is to be determined as torsional information, although the specific position of the bone must then also be taken into account in the interpretation and / or in the selection of the basic orthogonal system.

[0025] It should also be noted that in cases where sufficient orthogonality between the first computational plane and the second computational plane can be assumed, the surface normal of the second computational plane can also be directly understood as the torsion vector of the torsion direction, since this should then be perpendicular to the reference direction in the first computational plane. For the reasons mentioned above, however, this situation cannot be reliably achieved in most applications.

[0026] However, the method according to the invention is particularly suitable when relative torsions are to be determined, in particular between the proximal and distal end regions, in which case a roll angle can be determined both proximally and distally by carrying out steps a) to c) and a corresponding difference can be formed. In other words, it can be provided that in order to determine a relative torsion angle, in particular the antetorsion angle of the femur, as torsion information - steps a) to c) for determining a distal roll angle for the distal end region and - steps a) to c) are carried out to determine a proximal roll angle for the proximal end region, - after which the relevant torsion angle is determined as torsion information by forming the difference between the two roll angles.

[0027] The basic concept proposed here can therefore be applied both proximally and distally, for example to obtain frequently requested torsion information on the antetorsion of the bone, particularly of the femur. This allows, in particular, the repeated use of certain routines, functions, and processes within an overall workflow, thus enabling efficient implementation. Another advantage of determining relative torsion information is that the choice of comparison specification has no influence on the obtained torsion information, as long as it is ensured that it is kept constant. This is particularly interesting when using imaging arrangements held by a mobile carrier, which should then ideally remain unchanged in their orientation when changing from the proximal position to the distal position or vice versa.This can usually be ensured with sufficient accuracy, for example, on a flat floor.

[0028] However, it should also be noted in general that, since the present invention only deals with orientations, linear displacements, even between the recording of the first and the second X-ray image, ultimately have no influence on the success of the measurement and determination.

[0029] The method according to the invention can be used particularly advantageously with respect to the femur, in particular with regard to determining its antetorsion angle. Specifically, it can be provided that, for the femur as a bone in the first projection geometry, the extension of the femoral shaft axis runs centrally through the femoral head as the reference direction for the proximal end region and / or the femoral condyles overlap for the distal end region, wherein the femoral shaft axis, in the case of the distal end region in the first X-ray image transversely shifted to a tangential course along the femoral condyles, is used as the reference direction. For the proximal end region, the direction from the femoral shaft as the reference landmark to the femoral head as the torsion landmark is therefore preferably used as the torsion direction.The torsion direction, projected onto a plane perpendicular to the femoral shaft axis, is thus the same as the femoral neck axis, which is used by definition to determine the antetorsion angle. This means that with regard to determining a roll angle around the reference direction, given here by the femoral shaft axis, the torsion direction thus determined is equivalent to the femoral neck axis.

[0030] For the distal end region, the direction of torsion in the condylar plane running from femoral condyle to femoral condyle as torsion landmarks is used. The torsion direction is therefore located in the condylar plane, specifically perpendicular to the shaft axis. To correctly select the first calculation plane containing the condylar tips, the reference direction is shifted parallel to the femoral shaft axis so that it runs tangentially along the femoral condyles. It should be noted that a shift in the second X-ray image is not necessary if the projection directions differ by approximately 90°, since the femoral shaft axis should then run centrally between the femoral condyles.

[0031] In a specific embodiment of the present invention for determining the antetorsion angle of the femur, it can therefore be provided that the torsion of the proximal femur is first determined. This takes advantage of the fact that the femoral shaft and the femoral head are easily recognizable as landmarks in the X-ray images. A first projection geometry is first determined or set such that the extension of the femoral shaft axis runs through the center of the femoral head in the first X-ray image. The course of the femoral shaft axis specifies the reference direction, which, together with the focus of the X-ray source, spans a first calculation plane containing both the femoral head and the femoral shaft axis. Based on the set first projection geometry and the known beam geometry of the recording arrangement, the orientation of this first plane and its surface normal in space can be automatically calculated.

[0032] In a further step of this specific embodiment relating to the femur, the X-ray device can now automatically determine the second projection geometry such that the central ray is preferably parallel to the surface normal of the first calculation plane. After assuming this second projection geometry, a second X-ray image of the proximal femur is acquired, so that the second calculation plane can now be defined in space using the femoral shaft axis as the reference direction, for which a surface normal can also be calculated. Using the two surface normals, the reference vector and the torsion vector can then be determined, for example, as explained above. These vectors, together with the surface normal of the first calculation plane, define a measurement orthogonal system that can be understood as the basis of a measurement coordinate system. The measurement orthogonal system describes the position of the proximal femur in space.If one now calculates the Euler transformation to the reference specification, in particular the basic orthogonal system, which can be considered the reference coordinate system, one obtains the corresponding Euler angles. The roll angle defines the rotation of the femur around the shaft axis, i.e., precisely the torsion of the proximal femur, given the appropriate choice of the reference specification to the horizontal.

[0033] Then, in this specific embodiment, the torsion of the distal femur can be determined, described by a further roll angle for the same comparison specification. This is very similar to the procedure already described, since steps a) to c) are ultimately carried out again. In contrast to the femoral head, the condylar plane is used here as an anatomical marker, and thus a torsion landmark, alongside the femoral shaft as a reference landmark. When determining this roll angle, the first projection geometry is selected so that the femoral condyles are at the same height or exactly overlap. A straight line is used as the reference direction, which runs parallel to the femoral shaft axis and tangential to the condyles. The first calculation plane and the surface normal of the first calculation plane can be determined accordingly.In the following steps b) and c), the procedure is analogous to the proximal end region of the femur, whereby the calculated roll angle in this case corresponds to the torsion of the condylar plane. The femoral antetorsion angle then corresponds to the difference between the torsion of the proximal and distal femur, i.e., the difference between the two roll angles.

[0034] A general advantage of this inventive procedure is that it enables the determination of torsion information, particularly the antetorsion angle, with high accuracy based on individual X-ray images, despite its rapid and simple performance. Since only a few X-ray images are required, the patient's radiation exposure is significantly reduced. The method is also fundamentally suitable for intraoperative use, thus avoiding postoperative computed tomography scans and revision surgeries.

[0035] In principle, some of the steps can be performed by a user, for example a doctor or other medical personnel. For example, it is conceivable in principle for the first projection geometry to be found by varying the position and / or orientation of the recording arrangement so that the desired image impression is obtained. A user can also, in principle, be involved in determining the reference direction in the first or second X-ray image, for example, by being prompted to do so. However, preferred embodiments of the present invention provide for further automation by using suitable, in particular trained, evaluation functions.

[0036] According to the invention, the first projection geometry is determined at least before performing step a) by evaluating at least one orientation image of the X-ray device in at least one orientation geometry, and / or the reference direction is determined automatically in the first and / or second X-ray image in steps a) and b) using at least one, in particular trained, evaluation function. In this way, the temporal efficiency and user-friendliness of the method according to the invention are advantageously further increased.

[0037] In this context, it may be intended, in particular, to use artificial intelligence, i.e. to use evaluation functions of artificial intelligence trained at least in part by means of machine learning.

[0038] In general, a trained function replicates cognitive functions that people associate with other human brains. Through training based on training data (machine learning), the trained function is able to adapt to new circumstances and detect and extrapolate patterns.

[0039] Generally speaking, parameters of a trained function can be adjusted through training. In particular, supervised learning, semi-supervised learning, unsupervised learning, reinforcement learning, and / or active learning can be used. Representation learning (also known as feature learning) can also be used. The parameters of the trained function can be adjusted iteratively through multiple training steps.

[0040] For example, a trained function may include a neural network, a support vector machine (SVM), a decision tree, and / or a Bayesian network, and / or the trained function may be based on k-means clustering, Q-learning, genetic algorithms, and / or mapping rules. In particular, a neural network may be a deep neural network, a convolutional neural network (CNN), or a deep CNN. Furthermore, the neural network may be an adversarial network, a deep adversarial network, and / or a generative adversarial network (GAN).

[0041] Within the scope of the present invention, trained evaluation functions can be used with particular advantage for image evaluation, in particular with regard to the detection or localization of landmarks and / or for the recognition and interpretation of other image features. It is particularly advantageous if at least one of the at least one trained evaluation function comprises a convolutional neural network (CNN).

[0042] In a specific embodiment, it can be provided that, to determine the first projection geometry, at least one orientation landmark of the bone is detected by at least one trained evaluation function. In particular, at least two orientation landmarks, in particular including at least one of the at least one torsion landmark, can be detected by respective trained evaluation functions, the output data of which are combined to determine three-dimensional orientation information of the bone and, from this, the first projection geometry. It should first be noted that both reference landmarks and torsion landmarks can serve as orientation landmarks.However, it is particularly advantageous to use at least one torsion landmark as an orientation landmark, since after its detection, in particular segmentation, it is possible to calculate how the image impression that should result in the first projection geometry will be calculated, and thus also the possibility of defining the corresponding first calculation plane. For example, for the distal femur, an observation of the femoral condyles and the femoral shaft can be used as orientation landmarks, while for the proximal femur, as the proximal end region of the femur, the femoral head and / or the femoral neck and / or the femoral shaft can be used as orientation landmarks. In this way, a user can at least be given assistance in locating the first projection geometry.

[0043] In an expedient embodiment, it can also be provided that at least one of the at least one trained evaluation functions for determining the first projection geometry has been trained at least partially using computed tomography projection image sets. Such projection image sets usually scan the end region of a bone in dense angular increments, so that the projection image of the projection image set suitable for a first projection geometry can be annotated. It is then possible to learn how to infer the marked projection image from the other projection images. The evaluation algorithm trained in this way can then infer the first projection geometry as output data from an orientation image as input data, in particular internally based on orientation landmarks.

[0044] To determine the reference direction, at least one of the at least one reference landmark can preferably be detected by at least one trained evaluation function. For example, with regard to the detection of the shaft axis, methods have already been proposed in the prior art for detecting this with the aid of artificial intelligence, which can also be used within the scope of the present invention.

[0045] According to the invention, with regard to determining the first projection geometry, when using the shaft axis (or the bone shaft) as an orientation landmark and / or reference landmark, a probability map of the shaft axis is determined using a trained evaluation function, to which a straight line is fitted to determine the probability. The probability map can, for example, output probabilities that the corresponding image point belongs to the shaft of the bone or even to the shaft axis. It is then possible to evaluate such probability maps using conventional fitting programs in order to determine a shaft axis path, which is then suitable as a straight line for defining the reference direction.

[0046] In an expedient development of the present invention, it can be provided that, upon determination of an orientation landmark and / or reference landmark describing the position of the respective end region with respect to the second projection geometry, a linear adjustment device of the recording arrangement of the X-ray device is automatically controlled to compensate for a position of the end region outside the recording area of ​​the recording arrangement in the second projection geometry while maintaining its projection direction. In some cases, it can happen that when setting a second projection geometry that was defined solely based on a suitable, modified projection direction compared to the first projection geometry, the end region of the bone actually of interest slips out of the recording area.However, since the present invention primarily concerns orientations, linear adjustment devices assigned to the recording arrangement can be used to create a suitable image section, which can also take place automatically if the position of orientation landmarks and / or reference landmarks of the bone relative to the recording arrangement is known. It should be noted that, of course, in exemplary embodiments, it is also conceivable that, if the end region of the bone is initially not located in the image in the second projection geometry, the user can also use corresponding adjustment devices. In general, suitable adjustment devices in this context are, for example, a lifting column and / or a horizontal displacement device.

[0047] In general, controllable adjustment means, in particular comprising suitable actuators, can optionally also comprise, in addition to the linear adjustment devices, adjustment means for the corresponding orientation settings of the recording arrangement. In such a case, an advantageous development of the invention provides that the adjustment of the first and / or second projection geometry is carried out at least partially automatically by controlling adjustment means assigned to the recording arrangement. In this way, a precise adjustment of the projection geometries can be achieved, further reducing the burden on users.

[0048] A flat-panel detector can be used as the X-ray detector. Flat-panel detectors have the advantage that distortions do not need to be taken into account when constructing the first and second planes, since the position of the receiving pixels along the reference line directly reflects their corresponding position in space.

[0049] The X-ray device can specifically be an X-ray device with a mobile C-arm to which the recording arrangement is attached. In this case, the position of the mobile C-arm can preferably remain unchanged during steps a) to c), especially if its position cannot be tracked with sufficient accuracy. In this context, it should be pointed out again that a change in the position of the mobile C-arm is certainly possible when switching from a measurement in the proximal end region to the distal end region or vice versa. This is because only the corresponding orientations are important, which can be assumed to be the same in both positions, at least on a flat floor.

[0050] In addition to the method, the present invention also relates to an X-ray device, in particular an X-ray device with a mobile C-arm. The X-ray device has a recording arrangement comprising an X-ray source and an X-ray detector, the projection geometry of which can be adjusted as described by projection parameters. In a C-arm, for example, the X-ray source and the X-ray detector can be located opposite each other on the C-arm. The X-ray device is characterized in that it has a control device designed to carry out the method according to the invention. All statements regarding the method according to the invention can be transferred analogously to the X-ray device according to the invention, with which the same advantages can therefore also be achieved.

[0051] The control device can comprise at least one processor and at least one storage means, wherein functional units for carrying out steps of the method according to the invention can be provided via the processor. The control device can in particular have an acquisition unit that controls the acquisition operation of the X-ray device, in particular also for acquisition of the first X-ray image and the second X-ray image. The acquisition unit can be used by roll angle determination units to carry out the acquisition of the first and second X-ray images that are respectively required. The roll angle determination unit can, for example, comprise a plane determination subunit and a roll angle calculation subunit for constructing the planes, determining surface normals, and ultimately calculating the roll angle.Furthermore, the control device can have a torsion information determination unit for determining or compiling the torsion information from the roll angle. Of course, further functional units are also conceivable according to advantageous embodiments of the present invention.

[0052] A computer program according to the invention can be loaded directly into the memory of a control device of an X-ray device and has program means that carry out the steps of a method according to the invention when the computer program is executed in the control device of the X-ray device. The computer program can be stored on an electronically readable data carrier according to the invention, which thus comprises control information stored thereon, which comprises at least one computer program according to the invention and is configured such that, when the data carrier is used in a control device of an X-ray device, the control device is configured to carry out the steps of the method according to the invention when the computer program is executed.

[0053] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings. In the drawings: Fig. 1 a distal view of a femur in the direction of its shaft axis, Fig. 2 a flow chart of an embodiment of the method according to the invention, Fig. 3 sub-steps of a repeated process step of the Fig. 2, Fig. 4 a view of a proximal end region of the femur in a first projection geometry, Fig. 5 a perspective view of a femur with indicated reference planes, Fig. 6 a perspective view of the proximal end region of the femur with a measuring orthogonal system, Fig. 7 schematically shows a first X-ray image of the proximal end region of the femur, Fig. 8 schematically shows a second X-ray image of the proximal end region of the femur, Fig. 9 schematically shows a first X-ray image of the distal end region of the femur, Fig. 10 schematically shows a second X-ray image of the distal end region of the femur, Fig. 11 a schematic diagram of an X-ray device according to the invention, and Fig. 12 the functional structure of the control device of the X-ray device.

[0054] The following describes an embodiment of the method according to the invention as applied to the femur as a bone. The method is, of course, also applicable to other bones, with the humerus being mentioned purely as an example.

[0055] In the example now discussed in more detail, the antetorsion angle of the femur is to be determined. This is to be determined by Fig. 1, which shows a view of the femur 1 from the distal side along the shaft axis. Accordingly, the femoral neck 2 and the femoral head 3 can also be seen proximally; the pelvic girdle 4 is indicated. On the knee side, i.e. distally, the femoral condyles 5 are clearly visible. It is evident that there is a twist between the condylar plane 6 and the femoral neck axis 7, which is referred to as antetorsion and can be described by an antetorsion angle 8, which describes the relative torsion in the proximal and distal end regions.

[0056] Based on X-ray images, the antetorsion angle 8 is to be determined as torsion information in a particularly time-efficient, low-radiation, and simple manner. The method described below can also be used intraoperatively, for example, to check whether the antetorsion has been correctly restored before locking a femoral nail. However, the method can also be used for other purposes, for example, in a diagnostic context. In this case, the X-ray device used is an X-ray device with a C-arm, on which an X-ray source and an X-ray detector are arranged opposite one another, forming an imaging arrangement. A flat-panel detector is used as the X-ray detector.To accommodate different projection geometries, especially different projection directions, the X-ray system may have controllable adjustment devices. These may include adjustment options for an angular angle (rotation of the C-arm around its central axis) and an orbital angle (displacement along its circumference), as well as adjustment devices for linear adjustments, such as a lifting column for height adjustment and / or a linear guide for lateral, horizontal adjustment. The C-arm is mobile and can therefore be positioned differently in the room, the floor of which is assumed to be at least essentially flat.

[0057] According to Fig. 2, the method for determining the antetorsion angle 8 as torsion information begins in a step S1. A patient is already positioned on a patient couch of the X-ray system, for example, on an operating table.

[0058] A choice is then made as to whether to begin with the distal end region (knee area) or the proximal end region (hip area). The mobile C-arm moves to the corresponding end region of the femur in step S2, which can be performed by a user and / or at least partially automatically.

[0059] In a step S3, a sub-step sequence then takes place by which, as will be explained in more detail, a roll angle is determined for the respective end region compared to a comparison specification that is the same for both end regions.

[0060] In step S4, it is then checked whether roll angles have already been determined for both end regions of the bone, in this case the femur, and if not, steps S2 and S3 are repeated for the other end region. If so, in a step S5, the antetorsion angle θ is determined by forming the difference between the two roll angles and forms torsion information which can be output, for example, on a display means and / or stored in a storage means.

[0061] Fig. Figure 3 shows a general flowchart of substeps of step S3 for determining the roll angle. At least the measures a) to c) discussed in the general description are implemented.

[0062] Thus, in a sub-step S31, a first projection geometry is set, in which a first x-ray image of the respective end region of the femur 1 is then acquired. The first projection geometry is selected such that a torsion direction to be used to determine the roll angle and defined in relation to at least one torsion landmark lies in a first calculation plane formed by the focal point of the x-ray source and a reference direction determinable in the first x-ray image using at least one reference landmark, in particular parallel to the shaft axis of the femur 1. The reference direction is viewed in space where it was measured, i.e., at the respective pixels of the x-ray detector in the detector plane. The first projection geometry is associated with an image impression of the at least one torsion landmark in a corresponding x-ray image.

[0063] To locate this first projection geometry, a user can change the orientation of the recording arrangement until the desired image impression is obtained; however, it is also conceivable to record an orientation image from which the first projection geometry can be determined using at least one trained evaluation function, in particular a CNN. For this purpose, for example, orientation landmarks, in particular including the torsion landmark and / or the reference landmark, can be analyzed. In particular, the trained evaluation algorithm uses the orientation image as input data and can output the first projection geometry as output data, either absolutely or as adjustment instructions, i.e., relatively. The adjustment instructions can also be suitable for corresponding control if suitable controllable adjustment means are available.To train this first evaluation function, projection images of a computed tomography scan of a corresponding bone can also be used, in which the projection image, whose image impression is intended as the first projection geometry, is annotated accordingly.

[0064] It is understood that such training approaches, which use projection images from computed tomography for training, in which image impressions are available from many projection directions, thus using many projection geometries, which can be associated with orientation images, can be profitably used not only in the case of the femur, but can also be applied accordingly to other bones.

[0065] In a sub-step S32, the corresponding construction of this first calculation plane and the determination of its surface normal then take place. While it is fundamentally conceivable for a user to use the at least one reference landmark recognizable in the first X-ray image to plot the reference direction, a suitable embodiment can also provide for the use of evaluation algorithms, in particular trained evaluation algorithms, here. For example, neural networks, in particular CNNs, have already been proposed that provide a probability map for the femoral shaft axis as output data. From the probability map, the corresponding reference direction, or in this case initially the shaft axis, can then be determined by fitting a straight line.

[0066] In a sub-step S33, a suitable second projection geometry for a second X-ray image is determined and adjusted accordingly manually and / or at least partially automatically, whereby, however, the position of the mobile C-arm stand remains the same. In the present case, the second projection geometry is determined, for example, such that the central ray of the second projection geometry runs at least substantially along the surface normal of the first calculation plane. However, other projection directions of the second projection geometry can also be used, as long as they differ sufficiently from the projection direction of the first projection geometry.If the end region of the femur is not within the acquisition range of the acquisition setup after setting the second projection geometry, linear adjustment devices of the C-arm can be used to compensate, in particular a height adjustment device and / or a lateral adjustment device in the horizontal plane. This can also be done automatically, especially if anatomical landmarks have been determined automatically.

[0067] In a sub-step S34, the second x-ray image is then acquired. Accordingly, in a sub-step S35, the second computational plane can be constructed, whereby the reference direction can again be determined at least partially manually and / or at least partially automatically, preferably fully automatically, for example, using the trained evaluation algorithm that provides a probability map of the femoral shaft axis. The surface normal to the second computational plane is also determined.

[0068] With this tool, it is now possible to automatically determine the roll angle in step S36. To do this, the cross product of the surface normals of the first and second calculation planes is first formed to obtain a reference vector that describes the three-dimensional course of the reference direction. If the cross product of the surface normals of the first calculation plane and the reference vector is then formed, the torsion vector indicating the three-dimensional course of the torsion direction is obtained. From this, the roll angle can be determined as the rotation of the torsion direction and the reference direction relative to a comparison specification. Specifically, this can take advantage of the fact that the reference vector, the torsion vector, and the surface normal of the first calculation plane form an orthogonal system that can be referred to as the measurement orthogonal system. The measurement orthogonal system can be understood as the basis of a measurement coordinate system that describes the orientation of the femur.This information is all available in the X-ray coordinate system of the imaging setup, which can also contain a basic orthogonal system defined, for example, with respect to a basic position (e.g., angular angle and orbital angle 0). The roll angle can then be easily determined by determining the Euler transformation between these two orthogonal systems or coordinate systems.

[0069] The following section will explain in more detail the specific procedures for the proximal end of femur 1 and the distal end of femur 1. We will begin with the proximal end, i.e., the hip-side end.

[0070] This shows Fig. Figure 4 shows a schematic, perspective view of the proximal end region 9 of the femur 1 in a view or image impression as desired for the first projection geometry. It can be seen that the extension of the femoral shaft 10 passes centrally through the femoral head 3, which also applies to the shaft axis of the femoral shaft 10. However, this means that the portion of the neck axis of the femoral neck 2 relevant for torsion runs perpendicularly, i.e., toward the observer, so that the first calculation plane will obviously contain this relevant portion—the torsion direction—by design.

[0071] In this regard, Fig. 5 schematically shows the proximal end region 9 on a patient couch 11 in the context of the C-arm 12 with the X-ray source 13 and the X-ray detector 14, which, however, are already shown in the position for the second projection geometry. The partially indicated first calculation plane 15 is defined by the focal point 16 of the X-ray source 13 in the first projection geometry and the shaft axis of the femoral shaft 10 as the reference direction. The projection direction of the second projection geometry is at least substantially perpendicular to the projection direction of the first projection geometry and allows the definition of the second calculation plane 17, again using the shaft axis of the femoral shaft 10 as the reference direction and the focal point 18 of the X-ray source 13 in the second projection geometry.

[0072] Fig. Figure 6 schematically shows the resulting measurement orthogonal system 19 in the context of the proximal end region 9. The reference vector 20 points along the reference direction, the femoral shaft axis, which was determined using the corresponding reference landmark, the femoral shaft 10. The first calculation plane allows the construction of its surface normal 21, with the torsion vector 22 indicating the torsion direction, which clearly points through the femoral head 3 (torsion landmark), shown perpendicular to both. The roll angle sought is the rotation of the torsion vector 22 about the reference vector 20 relative to a comparison specification, which in the present case is arbitrary as long as it is selected the same for both end regions, since it concerns relative torsion information, specifically the antetorsion angle θ.

[0073] Fig. Figure 7 schematically shows a first X-ray image 23 in which, as can be clearly seen, the extension of the femoral shaft axis 24 runs centrally through the femoral head 3 as the reference direction 25. If the femoral shaft axis 24 is known in the first X-ray image 23, it is also known with which pixels of the X-ray detector 14 it was recorded, so that the first plane 15 can be easily constructed using the focal point 16 and these pixels in the detector plane.

[0074] Accordingly, Fig. 8 schematically shows a second X-ray image 26, in which the femoral shaft axis 24 is again shown as the reference direction 25, wherein the femoral head 3 is now rotated outwards as expected.

[0075] A similar procedure can be used in the distal end region of femur 1. The first projection geometry is chosen so that it covers femoral condyles 5 in the first X-ray image. This means that they appear like a femoral condyle 5. A corresponding first X-ray image 27 is shown schematically in Fig. 9. Due to the superimposition, the femoral condyles 5 appear as a single condyle, but the femoral shaft 10 is also clearly visible. To ensure that the first reference plane 15 is positioned correctly, namely also in the area of ​​the condylar plane 6, the femoral shaft axis 24 is not used directly as the reference direction 25. Instead, the shaft axis 24 is shifted parallel until it lies tangentially against the condyles 5 to define the reference direction 25.

[0076] In the second projection geometry, at least when its projection direction is at least substantially perpendicular to the projection direction of the first projection geometry, in the second X-ray image 28, compare the schematic representation of the Fig. 10, the femoral shaft axis 24 is anyway at least substantially centrally between the femoral condyles 5, so that it can form the reference direction 25.

[0077] It should be noted at this point that the measurement orthogonal system can ultimately also be understood as indicating the orientation of the respective end regions 9, and that the comparison orthogonal system can also be specifically selected so that a desired reference can also be established for individual end regions 9. For example, torsions against the horizontal can be determined in this way.

[0078] If the roll angles of the femur are determined in the two passes of step S3 as described with reference to the Fig. 4 to 10, the required antetorsion angle 8 is then obtained from the difference. For example, if roll angles of -26° proximal and -3° distal are determined, the resulting antitorsion is -26° - (-3°) - - 23°.

[0079] Fig. 11 shows a schematic diagram of an X-ray device 29 according to the invention. The X-ray device comprises, as already mentioned with regard to Fig. 5 explains the C-arm 12, on which the X-ray source 13 and the X-ray detector 14 are arranged opposite one another. The C-arm 12 is designed as a mobile C-arm 12 and is mounted on a correspondingly movable stand 30. By means of adjustment means 31, only indicated here, various projection geometries of the recording arrangement formed by the X-ray source 13 and the X-ray detector 14, which is a flat detector, can be set for a patient 32. In particular, a rotation of the C-arm 12 by an angular angle, compare arrow 33, as well as a displacement of the C-arm 12 for tilting by an orbital angle according to arrow 34 is possible. The adjustment means 31 can comprise adjustment devices for linear adjustment, for example a lifting column and / or a horizontal guide.

[0080] The operation of the X-ray device 29 is controlled by a Fig. 1 only indicated control device 35, which is designed to carry out the method according to the invention and whose functional structure with regard to Fig. 12. Accordingly, the control device 35 initially comprises a storage means 36 in which the x-ray images 23, 26, 27, and 28, intermediate results, and / or final results such as the torsion information can be stored. A recording unit 37 controls the recording operation of the x-ray device 29. In particular, the recording unit 37 can also control the recording of the first and second x-ray images 23, 26, 27, 28 according to sub-steps S32 and S34. The recording of an orientation image, if provided, in sub-step S31 can also be controlled via the recording unit 37.

[0081] To perform the calculations and determinations of steps S2 to S4, the control device 35 comprises a roll angle determination unit 38, which may have various determination subunits not shown in detail here, in particular at least one plane determination subunit and one roll angle calculation subunit for performing steps S32 / S35 and S36, respectively. Further subunits may include evaluation subunits for applying, in particular trained, evaluation functions, as well as projection geometry determination subunits (e.g., step S31, step S33).

[0082] In a torsion information determination unit 39, the torsion information can then be determined from the roll angles or the at least one roll angle according to step S5.

[0083] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

[1] Computer-implemented method for determining torsion information of a bone, in particular of the femur (1), of a patient (32), wherein an X-ray device (29) with adjustable projection geometry described by projection parameters of a recording arrangement comprising an X-ray source (13) and an X-ray detector (14) is used, comprising the following steps: a) recording a first X-ray image (23, 27) of an end region (9) of the bone in a first projection geometry in which a torsion direction to be used to determine a roll angle and defined in relation to at least one torsion landmark lies in a first calculation plane (15) which is formed by the focal point (16) of the X-ray source (13) and a reference direction (25) which can be determined in the first X-ray image (23, 27) by means of at least one reference landmark, in particular parallel to the shaft axis (24) of the bone, b) recording a second X-ray image (26, 28) of the end region (9) of the bone in a second projection geometry with a projection direction that differs from the projection direction of the first projection geometry, in particular at least substantially perpendicular to this, and forming a second calculation plane (17) from the reference direction (25) that can also be determined in the second X-ray image (26, 28) by means of the at least one reference landmark, as well as the focal point (18), c) determining the three-dimensional course of the reference direction (25) and the torsion direction from the calculation planes (15, 17) and the roll angle as the rotation of the torsion direction about the reference direction (25) compared to a comparison specification, - where the torsion information is determined from the roll angle, wherein the determination of the first projection geometry is carried out automatically at least before carrying out step a) by evaluating at least one orientation image of the X-ray device (29) in at least one orientation geometry and / or the determination of the reference direction (25) in the first and / or second X-ray image (23, 26, 27, 28) in steps a) and b) by means of at least one, in particular trained, evaluation function, wherein for determining the first projection geometry at least one orientation landmark of the bone is detected by at least one of the trained evaluation functions and / or for determining the reference direction (25) at least one of the at least one reference landmark is detected by at least one of the trained evaluation functions,wherein, when using the shaft axis (24) as an orientation landmark and / or reference landmark, a probability map of the shaft axis course is determined by means of a trained evaluation function, to which a straight line is fitted into the probability map to determine the shaft axis (24). [2] Method according to claim 1, wherein the second projection geometry is determined from the first calculation plane (15), in particular such that the central ray of the second projection geometry runs at least substantially along the surface normal (21) of the first calculation plane (15). [3] Method according to claim 1 or 2, wherein for determining a measurement orthogonal system (19) - the surface normal (21) of the first calculation plane (15), - from the calculation planes (15, 17), in particular by forming the cross product of the surface normals (21) of the first and second calculation planes (15, 17), a three-dimensional reference vector (20) of the reference direction (25) describing the three-dimensional course of the reference direction (25) and - a three-dimensional torsion vector (22) describing the three-dimensional course of the torsion direction is determined in the first calculation plane (15), in particular by forming the cross product of the surface normal (21) of the first calculation plane (15) and the reference vector (20), wherein the roll angle is determined as a corresponding Euler angle of the measuring orthogonal system (19) to a basic orthogonal system describing a basic position of the recording arrangement as a comparison specification. [4] Method according to one of the preceding claims, wherein for the femur (1) as a bone in the first projection geometry for the proximal end region (9), the extension of the femoral shaft axis (24) runs centrally through the femoral head (3) as the reference direction (25) and / or for the distal end region, the femoral condyles (5) overlap, wherein in each case the femoral shaft axis (24), in the case of the distal end region in the first X-ray image (27) transversely shifted to a tangential course along the femoral condyles (5), is used as the reference direction (25). [5] Method according to one of the preceding claims, wherein for determining a relative torsion angle, in particular the antetorsion angle (8) of the femur (1), as torsion information - steps a) to c) for determining a distal roll angle for the distal end region and - steps a) to c) are carried out to determine a proximal roll angle for the proximal end region (9), - after which the relative torsion angle is determined as torsion information by forming the difference between the two roll angles. [6] Method according to one of the preceding claims, wherein, to determine the first projection geometry, at least two orientation landmarks of the bone are detected, in particular comprising at least one of the at least one torsion landmark, by respective trained evaluation functions, the output data of which are combined to determine three-dimensional orientation information of the bone and, therefrom, the first projection geometry. [7] Method according to one of the preceding claims, wherein, upon determination of an orientation landmark and / or reference landmark describing the position of the respective end region (9) with respect to the second projection geometry, an automatic control of a linear adjustment device of the recording arrangement of the X-ray device (29) takes place to compensate for a position of the end region (9) outside the recording area of the recording arrangement in the second projection geometry while maintaining its projection direction. [8] Method according to one of the preceding claims, wherein the adjustment of the first and / or the second projection geometry is carried out at least partially automatically by controlling adjustment means (31) assigned to the recording arrangement. [9] Method according to one of the preceding claims, wherein a flat detector is used as the X-ray detector (14). [10] Method according to one of the preceding claims, wherein an X-ray device (29) with a mobile C-arm (12) to which the recording arrangement is fastened is used as the X-ray device (29), wherein the position of the mobile C-arm (12) remains unchanged during steps a) to c). [11] X-ray device (29) with adjustable projection geometry, described by projection parameters, of a recording arrangement comprising an X-ray source (13) and an X-ray detector (14), having a control device (35) designed to carry out a method according to one of the preceding claims. [12] Computer program which carries out the steps of a method according to one of claims 1 to 10 when executed on a control device (35) of an X-ray device (29). [13] An electronically readable data carrier on which a computer program according to claim 12 is stored.

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