Reconstruction of image data using contour data

Abstract

Methods for reconstructing image data (f), where first projection data (p) were acquired during a relative rotational movement between an X-ray source (C2) of a CT device (C1) and at least one examination object (O) lying partially outside the scanning area (FOV) of the CT device (C1), where contour data (k) of the surface of the object under investigation (O) were recorded using a camera (C13), where the spatial correlation between the first projection data (p) and the contour data (k) is known, wherein the first projection data (p) are supplemented with the contour data (k) to create modified projection data (p'), wherein the reconstruction of image data (f) is carried out using the modified projection data (p'), where the subject of investigation (O) is a patient, wherein at least one anatomical landmark is identified in the contour data (k), where the pixel values ​​in the area of ​​the landmark are modified depending on a typical X-ray absorption of this landmark.

Description

[0001] The invention relates to a method as well as an imaging system for reconstructing image data using contour data.

[0002] Computed tomography (CT) is a two-stage imaging technique. Projection data is acquired by irradiating a specimen with X-rays and measuring the attenuation of the X-rays along their path from the X-ray source to the X-ray detector. This attenuation is caused by the materials irradiated along the beam path, and can therefore be understood as a line integral over the attenuation coefficients of all volume elements (voxels) along the beam path. The acquired projection data are not directly interpretable; that is, they do not provide an image of the irradiated layer of the specimen. Only in a second step, using reconstruction techniques, is it possible to calculate the attenuation coefficients of the individual voxels from the projection data and thus generate an image of the attenuation coefficient distribution.The projection data can comprise a multitude of individual projections, captured from different projection angles. Capturing a multitude of individual projections from different projection angles is also referred to as scanning.

[0003] Based on the projection data generated during a scan, image data can be reconstructed, representing, for example, a two-dimensional image in the form of a layer of the object under investigation. Image data can also include, for example, a multitude of two-dimensional images or a contiguous three-dimensional volume. Problems arise in reconstructing the image data when, during the projection data acquisition described above, the object under investigation extends beyond the scanning range of the X-ray source and detector system, at least for some projection angles. The scanning range is determined by the relative spatial arrangement of the X-rays generated by the X-ray source and the X-ray detector. Complete projection data can be obtained within the scanning range; complete projection data cannot be obtained outside the scanning range.If the object under investigation extends beyond the scanning area, the projection data acquired during the X-ray examination of the object are incomplete, leading to image artifacts during reconstruction. To enable the most accurate image reconstruction possible, the incomplete projections must be supplemented with additional projection data before reconstruction. The scanning area is also referred to as the measurement field, and methods for reconstructing image data from incomplete projection data are also called extended measurement field reconstruction methods. The terms scanning area and measurement field are used synonymously in the following.

[0004] A method for reconstructing image data from incomplete measurement data is known from DE 10 2010 006 585 A1. In this method, initial image data is reconstructed from the measurement data, and a boundary of the object under investigation is determined based on this initial image data. Subsequently, the initial image data is modified using the determined boundary, and projection data is calculated from the modified initial image data. The measurement data is then modified using the projection data, and finally, a second set of image data is reconstructed from the modified measurement data.

[0005] From the publications DE 10 2011 075 904 A1, DE 10 2005 022 540 A1, and "3D-Guided CT Reconstruction using Time-Of-Flight Camera" by Mahmoud Ismail et al., Proc. of SPIE Vol. 7964, pp. 796429-1 - 796429-11, correction methods are known for reconstructing projection images of an object taken from different projection angles, where parts of the object are not within the field of view of every projection image. In these methods, the surface of the object is optically captured and used to supplement the missing image data during the reconstruction of the projection images.

[0006] The purpose of the invention is to improve the reconstruction of image data from incomplete projection data.

[0007] The inventive solution to the problem is described below with respect to both the claimed system and the claimed method. Features, advantages, or alternative embodiments mentioned herein are also applicable to the other claimed subject matter and vice versa. In other words, the claims (which, for example, relate to a system) can also be further developed with the features described or claimed in connection with a method. The corresponding functional features of the method are thereby implemented by corresponding material modules.

[0008] The method according to the invention is based on initial projection data acquired during a relative rotational movement between an X-ray source of a CT scanner and at least one object being examined that lies partially outside the scanning area of ​​the CT scanner. These initial projection data are therefore incomplete. The inventors have recognized that contour data of the surface of the object being examined, acquired by means of a camera, can be used to improve the reconstruction of the incomplete initial projection data. The spatial correlation between the initial projection data and the contour data is known. According to the invention, the initial projection data are supplemented with the contour data to create modified projection data, such that the modified projection data includes information about the contour of the object being examined that lies outside the scanning area of ​​the CT scanner.In contrast to conventional methods, the projection data is supplemented using contour data captured by a camera. Such camera-captured contour data has a particularly high information content, which allows for a particularly effective supplementation of the incomplete initial projection data. Consequently, the reconstruction of image data according to the invention using the modified projection data results in fewer or no artifacts compared to the reconstruction of image data using only the initial image data.

[0009] According to a further aspect of the invention, initial image data are reconstructed from the incomplete initial projection data, and then the initial image data are modified using the contour data. Furthermore, modified projection data are generated from the modified image data, so that a second image data set can be reconstructed from both the initial projection data and the modified projection data. In this aspect, the information content of the contour data is taken into account during modification in the image space, and such modification is particularly intuitive for a user of an imaging system according to the invention. Thus, the initial image data and the contour data can be displayed graphically on an output unit.Furthermore, the inventive method can be carried out iteratively according to this aspect, whereby the reconstruction of the image data through the additional iteration steps yields better results, i.e., results with fewer artifacts.

[0010] According to a further aspect of the invention, the first image data are modified such that an interface of the object under investigation is determined in the first image data by means of a correlation between the first image data and the contour data. Such determination of the interface represents a significant supplement to incomplete projection data and therefore largely eliminates artifacts resulting from the incompleteness of projection data in a subsequent reconstruction.

[0011] According to a further aspect of the invention, the interface is the surface of the object under investigation, wherein the first image data are modified such that pixel values ​​in an area outside the scanned area and within the area bounded by the surface are modified. This is because the incompleteness of projection data typically results from incomplete information near the surface of an object under investigation. Artifacts resulting from incomplete information near the surface can be reduced or avoided by taking into account the additional information about the surface by modifying the pixel values ​​in the incompletely scanned area of ​​the object under investigation, which, according to the invention, is achieved using the contour data.

[0012] According to a further aspect of the invention, the object of investigation is a patient, wherein at least one anatomical landmark is identified in the contour data, and the pixel values ​​in the area of ​​the landmark are modified depending on a typical X-ray absorption of this landmark. The landmark can be, in particular, a body region of the patient, such as the head, eyes, thorax, chest, legs, or a single knee joint. Different landmarks exhibit different X-ray absorption, for example, because they consist mainly of different materials such as bone, fat, or water. Since a specific X-ray absorption corresponds to specific values ​​in a reconstructed image data, this aspect of the invention offers a particularly precise way to supplement incomplete projection data.

[0013] According to a further aspect of the invention, a sinogram is determined from the initial projection data, wherein the initial projection data are supplemented by extrapolating the sinogram to obtain modified projection data. No reconstruction is required before supplementing the projection data, which allows the method according to the invention to be carried out with low computational effort and thus particularly quickly.

[0014] According to a further aspect of the invention, the initial projection data are supplemented such that they are weighted according to the correlated contour data. In particular, individual rays within individual projections can be weighted. For example, a weighting according to the invention provides that complete projections or rays receive a higher weight than incomplete projections or rays. A method according to this aspect of the invention can, in particular, be combined with an iterative reconstruction algorithm. The projection data are then further supplemented with each iteration step so that the resulting reconstruction of image data exhibits fewer artifacts and is thus improved.

[0015] According to a further aspect of the invention, a first examination object is a patient, and a second examination object is identified using the contour data. Depending on the identity and position of the second examination object, an output is displayed on an output unit connected to the CT scanner. This makes it possible to consider further examination objects, which lie partially or even completely outside the scanning area, during the reconstruction and thus provide the user of a CT scanner according to the invention with additional information.

[0016] According to a further aspect of the invention, the second object under investigation is a medical device for intervention, wherein the output comprises a pictorial representation of at least a part of the two objects under investigation as well as information about the relative position of the objects under investigation to each other. Thus, the invention can be used to make an intervention, for example by means of a needle or an endoscope, more precise and reliable.

[0017] According to another aspect of the invention, the output is a warning message, thereby increasing safety during the continuous recording of projection data and during interventions.

[0018] Another aspect is that the camera is a 3D camera and the contour data is 3D contour data, resulting in a particularly high information content in the contour data. This allows for a very precise complement to the projection data and thus a reconstruction of image data with very few artifacts. Furthermore, due to the high information content of the contour data, results can be achieved particularly quickly during iterative reconstruction.

[0019] Furthermore, the invention can be implemented in the form of an imaging system comprising a CT scanner with a rotatable X-ray source and an X-ray detector interacting with the X-ray source, wherein the CT scanner is designed to acquire first projection data of an object under investigation both within and outside the scanning range of the CT scanner. The imaging system according to the invention further comprises a camera designed to acquire contour data of the surface of the object under investigation, wherein the spatial correlation between the first projection data and the contour data is known, and a processing unit designed to supplement the first projection data with the contour data to produce modified projection data. The imaging system according to the invention also comprises a reconstruction unit designed to reconstruct image data using the modified projection data.With such a system according to the invention, the method according to the invention can be advantageously carried out in the described variants.

[0020] The invention will now be described and explained in more detail with reference to the exemplary embodiments shown in the figures.

[0021] They show: Fig. 1: A first schematic representation of an exemplary embodiment of an imaging system, Fig. 2: a second schematic representation of an exemplary embodiment of an imaging system, Fig. 3: a section of the recording geometry perpendicular to the z-direction, and Fig. 4: a flowchart of a variant of the method according to the invention, and Fig. 5: a flowchart of another variant of the method according to the invention.

[0022] In Fig. Figure 1 schematically depicts a first imaging system with a CT scanner. This is a so-called third-generation CT scanner, to which the invention is not limited. The gantry housing C6 contains a closed gantry (not visible here) on which a first X-ray source C2, in the form of an X-ray tube, is arranged with an opposing X-ray detector C3. Optionally, the CT scanner C1 shown here includes a second X-ray tube C4 with an opposing X-ray detector C5, so that the additional available source / detector combination allows for higher temporal resolution, or, by using different X-ray energy spectra in the source / detector systems, so-called "dual-energy" examinations can be performed.

[0023] The CT scanner C1 also features a patient table C8, on which a patient can be moved along a system axis C9, also known as the z-axis, into the scanning area or field of view (FOV) during the examination. However, it is also possible for the scan itself to be performed as a simple circular scan without moving the patient, exclusively within the area of ​​interest. The movement of the patient table C8 relative to the gantry is achieved by a suitable motor. During this movement, the X-ray source C2 or C4 rotates around the patient. Simultaneously, the detector C3 or C5 rotates opposite the X-ray source C2 or C4 to acquire initial projection data p, which is then used to reconstruct image data, such as cross-sectional images.As an alternative to a sequential scan, in which the patient is moved stepwise through the examination field between individual scans, a spiral scan is also possible. In this method, the patient is continuously moved along the system axis C9 through the examination field between the X-ray tube C2 or C4 and the detector C3 or C5 during the rotating scan with the X-ray beam. Due to the patient's movement along the axis C9 and the simultaneous rotation of the X-ray source C2 or C4, a spiral scan results in a helical path for the X-ray source C2 or C4 relative to the patient during the measurement. This path can also be achieved by moving the gantry along the axis C9 while the patient remains stationary. Furthermore, it is possible to move the patient continuously and, if necessary, periodically back and forth between two points.

[0024] The CT system C1 is controlled by a control and computing unit C10 with computer program code Prg1 to Prg stored in a memory. n It should be noted that these computer program codes Prg1 to Prg are of course n They may also be contained on an external storage medium and can be loaded into the C10 control and computing unit if required.

[0025] From the control and processing unit C10, 24 acquisition control signals AS can be transmitted via a control interface to control the CT scanner according to specific measurement protocols. These acquisition control signals AS relate, for example, to the X-ray tubes C2 and C4, where specifications can be made regarding their power and the times they switch on and off, as well as to the gantry, where specifications can be made regarding its rotation speed, and the table feed.

[0026] Since the control and processing unit C10 has an input console, measurement parameters can be entered by a user or operator of the CT device C1, which then control the data acquisition in the form of acquisition control signals AS. Information about currently used measurement parameters can be displayed on the output unit in the form of a screen; additional information relevant to the operator can also be displayed. The imaging system according to the invention can also include further output units, such as an acoustic output unit.

[0027] The initial projection data p or raw data acquired by detectors C3 and C5, respectively, are transferred to the control and processing unit C10 via a raw data interface C23. These initial projection data p are then further processed in a reconstruction unit C21, possibly after suitable preprocessing. In this embodiment, the reconstruction unit C21 is implemented in the control and processing unit C10 as software on a processor, for example, in the form of one or more of the computer program codes Prg1 to Prg1. n Regarding image reconstruction, as already explained regarding the control of the measurement process, the computer program codes Prg1 to Prg nThe data can also be stored on an external storage medium and loaded into the C10 control and processing unit as needed. Furthermore, it is possible for the control of the measurement process and the image reconstruction to be performed by different processing units.

[0028] The image data f reconstructed by the reconstruction unit C21 are then stored in a memory C22 of the control and processing unit C10 and / or displayed on the screen of the control and processing unit C10 in the usual manner. They can also be accessed via a Fig. 1. The data is fed into a network connected to the CT device C1, for example a radiological information system (RIS), via an interface not shown, and stored in a mass storage device accessible there or output as images.

[0029] The control and processing unit C10 can also perform the function of an ECG, with a line C12 being used to derive the ECG potentials between the patient and the control and processing unit C10. Additionally, the unit features... Fig. The CT system shown, C1, is also equipped with a contrast agent injector, C11, through which additional contrast agent can be injected into the patient's bloodstream, allowing, for example, better visualization of the patient's blood vessels, particularly the chambers of the beating heart. Furthermore, this also enables perfusion measurements, for which the proposed method is also suitable.

[0030] The control and calculation unit C10 must – unlike in Fig. Figure 1 shows that the components of the CT system C1 are not located near the other components. Instead, they can be placed in a different room or at a more distant location. The raw data p and / or the acquisition signals AS and / or the ECG data can be transmitted via cable or, alternatively, wirelessly.

[0031] Furthermore, the imaging system according to the invention comprises at least one camera C13, which is designed for contactless scanning of at least a portion of the surface of the object under investigation O. The camera C13 is designed for the detection of electromagnetic radiation, in particular for the detection of electromagnetic radiation in a low-frequency spectral range compared to X-rays, for example, in the visible or infrared spectral range. Thus, the camera C13 can be one or more still cameras or video cameras. In the example shown here, the camera C13 is a 3D camera mounted on the gantry, which is designed, for example, as a stereo camera or as a time-of-flight camera. In a further embodiment of the invention, the camera C13 is configured to scan a surface using structured illumination.In this configuration, the imaging system additionally includes an illumination unit for generating structured illumination of at least a part of the object under investigation O. Furthermore, in this case, the camera C13 and the illumination unit are positioned and configured in their emission and detection properties such that the camera C13 is designed to detect the radiation reflected from the surface of the object under investigation O.

[0032] In a further embodiment of the invention, the electromagnetic sensor 31 is permanently integrated into the gantry of the CT scanner C1. In an alternative embodiment, the camera C13 is integrated into the rotatable part of the gantry 19 such that the camera C13 rotates with the X-ray source C2 or with the X-ray detector C3. This allows the surface of the object under investigation O to be scanned particularly easily and quickly from different perspectives. The additional surface information enables the contour data k of the scanned surface to be calculated with particular precision, so that the method according to the invention can also be carried out particularly easily, quickly, and precisely. To carry out the method according to the invention, the coordinates of the acquired contour data k must be convertible into the coordinates of the projection data. Then the contour data k can also be converted into the image space of the image data f.In other words, the correlation between the contour data k and the projection data, in particular the first projection data p, must be known. Such a correlation can be determined by a calibration that precedes the method according to the invention.

[0033] The Fig. Figure 2 shows an imaging system with a C-arm device, in which, unlike the CT device, the Fig. 1. The housing C6 supports the C-arm C7, to which the X-ray tube C2 and the opposing X-ray detector C3 are attached. The C-arm C7 is also pivoted about a system axis C9 for scanning, so that scanning can take place from a multitude of scanning angles and corresponding initial projection data p can be determined from a multitude of projection angles. The C-arm system C1 of the Fig. 2, like the CT system from the Fig. 1 via a control and calculation unit C10 of the Fig. 1 described species.

[0034] At the in Fig. In the embodiment of the invention shown in Figure 2, the camera C13 is mounted on a positioning device in the form of a tripod C14. Alternatively, the camera C13 can be positioned elsewhere in the room where the CT scanner C1 is located; for example, it can be mounted on the ceiling. Furthermore, the camera C13 can be positioned centrally above the object under examination O or centrally above the patient table C8. If the camera C13 is a 3D camera, then the contour data k also includes depth information about the detailed structure of the surface of the examination area. It is generally advantageous if the camera C13 is positioned such that the quality of the depth information about the scanned surface of the object under examination O, and thus also the quality of the captured contour data k, is as homogeneous as possible. The noise or error of depth information determined by scanning, or...The contour data k should depend as little as possible on the depth information or the contour data k itself, or on the position of the sampled area.

[0035] The invention is in both of the in the Fig. 1 and Fig. The system shown in the two examples is applicable. Furthermore, it is also fundamentally applicable to other CT scanners, e.g., CT scanners with a detector forming a complete ring.

[0036] For image reconstruction, the availability of a complete dataset of projection data is advantageous. "Complete" here means that every volume element of the object O under investigation, which is to be included in the reconstructed image, must be irradiated over a projection angle range of 180° if measuring in parallel beam geometry, or 180° plus the cone opening angle if measuring in cone beam geometry, and the corresponding projections must be captured by the X-ray detector. If this is not the case, image reconstruction is still possible, but the resulting image will contain artifacts due to the incompleteness of the projection dataset.

[0037] Problems arise when the size of the object under investigation O is larger than the field of view (FOV) of the CT scanner C1. Such a situation occurs in Fig. Figure 3 shows a section of a CT scanner according to Fig. 1 or Fig. 2, which comprises the X-ray source C2 and the X-ray detector C3. To improve clarity, the X-ray detector C3 has only 12 detector elements in the channel direction; in reality, their number is much greater. The object under investigation O is located between the X-ray source C2 and the X-ray detector C3. Fig. Figure 3 shows a section perpendicular to the z-axis; therefore, an axial section through the object under investigation O is visible. The field of view (FOV) of the CT scanner C1 corresponds to a specific projection angle, as shown in Fig. Figure 3 shows a section of a circle perpendicular to the z-axis. Its edges are formed by the X-rays that travel from the X-ray source C2 to the outermost edges of the detector C3.

[0038] Therefore, it is the detector's extension in the channel direction that determines the size of the field of view (FOV). The channel direction is the direction on the detector surface perpendicular to the line direction. The line direction extends perpendicular to the plane of the section of the Fig. 3 and thus along the z-direction. The detector dimension in the drawing plane of the Fig. 3 is the canal direction.

[0039] It is in Fig. 3. It can be seen that the object under investigation O is not completely within the field of view (FOV) at the projection angle shown. The components OA of the object under investigation O are positioned according to the X-ray source C2 and the X-ray detector C3. Fig. 3. Not illuminated by X-rays detected by X-ray detector C3: the components OA of the object under investigation O lie outside the field of view (FOV). If the X-ray source C2 and detector C3 rotate around the object under investigation O, then at some projection angles, the components in the constellation described above lie outside the field of view. Fig. 3. Parts OA of the object under investigation O lying outside the field of view (FOV) are within the field of view (FOV); for other projection angles, they lie outside the field of view (FOV). The same applies to the other edge regions of the object under investigation O.

[0040] This means that complete projection data is not available for some components of the object under investigation O. Generally, the total measurement field of the CT scanner C1, i.e., the area between X-ray source C2 and X-ray detector C3 for which complete projection data is acquired, is defined by the intersection of the radiation fields over a half-circle of X-ray source C2 and X-ray detector C3 – or over a half-circle of 180° plus the cone opening angle. The extended field of view of the CT scanner C1 is an area adjacent to the described total measurement field. Outside the total measurement field lies the extended field of view, which encompasses those volume elements that are only illuminated by X-rays at certain projection angles subsequently reaching the detector.

[0041] For components of the object under investigation O within the extended measurement field, such as parts OA of the Fig. 3. This means that in some of the recorded projections, the measurement data contains information about these parts of the object under investigation O, while in other projections it does not. Therefore, with regard to the components of the object under investigation O located in the extended measurement field, the data set is incomplete. This is also referred to as "limited angle" scanning.

[0042] In practice, parts of the subject O exceeding the measurement field can occur, for example, due to the size of the patient's body, or because a patient is unable to place their arms over or behind their head during a chest measurement.

[0043] Since information concerning the object under investigation O is included in some projections within the extended measurement field, it is not readily possible to reconstruct a CT image solely for the area of ​​the entire measurement field. Rather, exceeding the measurement field results in the CT image containing artifacts within the entire measurement field. This is due to the incompleteness of the data in the extended measurement field, as explained above. Therefore, the information from the extended measurement field must be taken into account during image reconstruction.

[0044] A flowchart of a variant of the inventive method is shown in Fig. Figure 4 illustrates the process. The method according to the invention is based on initial projection data p, which were acquired during a relative rotational movement between an X-ray source C2 of a CT scanner C1 and at least one object O located partially outside the field of view (FOV) of the CT scanner C1. At least parts of the initial projection data are therefore incomplete and require reconstruction in the extended field of view to avoid artifacts. For such reconstruction, contour data k of the surface of the object O were acquired using a camera C13. The contour data k comprises information about the contour of the object O, particularly outside the field of view (FOV). Advantageously, the contour data k were acquired using a 3D camera, so that the contour data includes detailed depth information about the contour of the object.The contour data k can also be preprocessed, i.e., filtered or reconstructed data. This makes it possible to determine a boundary of the investigation area O in the contour data k recorded by the camera, for example, using an edge detection algorithm. The spatial correlation between the initial projection data p and the contour data k is known. Therefore, the coordinates of the contour data k can be converted into the coordinates of the projection data p. Thus, according to the invention, the contour data k can be used to supplement the initial, incomplete projection data p to create modified projection data p'.The subsequent reconstruction of image data f using the modified projection data p' exhibits fewer artifacts according to the invention due to the addition of the first projection data than would be the case with a comparable reconstruction of the original first projection data p.

[0045] Reconstruction can be performed using common methods, such as filtered backprojection, cone-beam reconstruction, or iterative or algebraic methods. The initial projection data p is typically supplemented by extrapolation. This primarily involves supplementing incomplete projections, but it can also be technically advantageous to supplement complete projections, for example, to ensure certain continuities between different projections. In a further embodiment of the invention, the rays of a single projection are weighted, thereby taking even more information, based on the contour data k, into account, resulting in better image data f with fewer artifacts.Furthermore, the original first projection data p can be taken into account during the reconstruction, for example by mixing the first projection data p and the modified projection data p'.

[0046] In a further embodiment of the invention, a sinogram is determined from the first projection data p, wherein the first projection data p are supplemented by extrapolating the sinogram to modified projection data p'. The sinogram can be either a 2D or a 3D sinogram, with a 3D sinogram typically being configured as a stack of a plurality of 2D sinograms. The extrapolation can be performed, in particular, along the individual rows and columns of a 2D sinogram. A sinogram supplemented by extrapolation contains additional information in the extended measurement field, so that modified projection data p' derived from the supplemented sinogram also exhibit additional information in the extended measurement field. The extrapolation is performed, for example, using polynomials or trigonometric functions.According to the invention, the extrapolation is carried out using the contour data k, i.e., taking into account information about a contour of the object under investigation O. Since the contour data k were acquired using a camera and therefore provide precise information about the contour of the area under investigation O, the invention makes it possible to extrapolate sinograms particularly accurately and efficiently, so that a corresponding reconstruction in the extended measurement field quickly leads to particularly good results in the form of image data f.

[0047] A flowchart of another variant of the invention is provided in Fig.Figure 5 shows that since the procedure can also be performed iteratively according to the variant shown here, the index n indicates the respective iteration cycle. In a first iteration, initial image data f_1 is reconstructed from the initial projection data p. Then, the initial image data f_1 is modified using the contour data k. The modification thus takes place in the image space. If modified projection data p'_1 is generated from the modified image data f', then the modified projection data p'_1 exhibits increased information content compared to the initial projection data p, particularly in the extended measurement field. Therefore, second image data f_2 can be reconstructed particularly advantageously from the initial projection data p as well as from the modified projection data p'. The specified iteration cycle can be repeated until a termination criterion is reached.For example, the termination criterion is given by an absolute maximum number of iterations or by falling below a predetermined difference in image information between two reconstructed image data sets f_n and f_n+1.

[0048] The image space enhancement can be advantageously utilized in various embodiments of the invention. In one embodiment, the image data f_n are modified such that an interface of the object under investigation O is determined in the image data f_n by means of a correlation between the initial image data and the contour data. The interface can, in particular, be the surface of the area under investigation O. Determining an interface in the image data allows the pixels on both sides of the interface to be processed differently, thereby providing additional information. In another embodiment, the image data f_n are modified such that pixel values ​​in a region outside the scanning area FOV and within the area bounded by the surface are modified.Thus, the respective pixel values ​​can assume very specific attenuation values, which are to be expected based on the contour data k or information derived from it, such as the interface.

[0049] If the object of investigation is a patient, in a further embodiment of the invention at least one anatomical landmark is identified in the contour data, so that the pixel values ​​in the area of ​​the landmark are modified depending on a typical X-ray absorption of this landmark. If the landmark is an arm, the corresponding pixel values ​​can be modified so that they correspond to the X-ray absorption resulting from a mixture of materials such as bone and muscle tissue typical of an arm.

[0050] In a further embodiment of the invention, a second object of investigation O' is identified using the contour data k, and an output is displayed on an output unit connected to the CT scanner C1, depending on the identity and position of the second object of investigation O'. This allows a warning to be issued if a second object of investigation, for example, in the form of a C-arm or a tube, is too close to the patient. In a further embodiment of the invention, the second object of investigation O is a medical device used for interventions, and the output includes a pictorial representation of at least a part of the two objects of investigation O as well as information about the relative position of the objects of investigation to each other. This allows the method according to the invention to also be used for navigation during surgical procedures.

[0051] In further embodiments of the invention, the acquisition of the first projection data p, the acquisition of the contour data k, and further steps described here as preceding the method according to the invention can themselves be designed as part of the method according to the invention.

Claims

[1] Methods for reconstructing image data (f), where first projection data (p) were acquired during a relative rotational movement between an X-ray source (C2) of a CT device (C1) and at least one examination object (O) lying partially outside the scanning area (FOV) of the CT device (C1), where contour data (k) of the surface of the object under investigation (O) were recorded using a camera (C13), where the spatial correlation between the first projection data (p) and the contour data (k) is known, wherein the first projection data (p) are supplemented with the contour data (k) to create modified projection data (p'), wherein the reconstruction of image data (f) is carried out using the modified projection data (p'), where the subject of investigation (O) is a patient, wherein at least one anatomical landmark is identified in the contour data (k), where the pixel values ​​in the area of ​​the landmark are modified depending on a typical X-ray absorption of this landmark. [2] Method according to claim 1, where initial image data are reconstructed from the initial projection data (p), where the first image data are modified using the contour data (k), where modified projection data (p') are generated from the modified image data (f'), where the second image data (f) is reconstructed from the first projection data (p) as well as from the modified projection data (p'). [3] Method according to claim 2, wherein the first image data are modified such that an interface of the object under investigation (O) is determined in the first image data by means of a correlation between the first image data and the contour data. [4] Method according to one of claim 3, where the interface is the surface of the object under investigation (O), where the initial image data are modified such that pixel values ​​are modified in an area outside the sampling range (FOV) and within the area bounded by the surface. [5] Method according to claim 1, where a sinogram is determined from the first projection data (p), where the first projection data (p) are supplemented by extrapolation of the sinogram to modified projection data (p'). [6] Method according to claim 1, wherein the first projection data (p) are supplemented by weighting according to the correlated contour data (k). [7] Method according to any one of claims 1 to 6, where the first subject of investigation (O) is a patient, where a second object of investigation (O') is identified using the contour data (k), where, depending on the identity and position of the second subject (O'), an output is made on an output unit connected to the CT device (C1). [8] Method according to claim 7, the second object under investigation (O') is a medical device for intervention, wherein the output includes a pictorial representation of at least one part of the two objects of investigation (O') as well as information about the relative position of the objects of investigation (O, O') to each other. [9] Method according to one of claims 7 or 8, wherein the output is a warning message. [10] Method according to any one of claims 1 to 9, wherein the camera (C13) is a 3D camera and wherein the contour data (k) is 3D contour data. [11] Imaging system, comprising: - a CT scanner (C1) with a rotatable X-ray source (C2) and with an X-ray detector (C3) interacting with the X-ray source (C2), designed to acquire first projection data (p) of an object under investigation (O) inside and outside the scanning range (FOV) of the CT scanner (C1), - a camera (C13) designed to capture contour data (k) of the surface of the object under investigation (O), wherein the spatial correlation between the first projection data (p) and the contour data (k) is known, - a computing unit (C10) designed to supplement the first projection data (p) with the contour data (k) to create modified projection data (p'), - a reconstruction unit (c21) designed to reconstruct image data (f) using the modified projection data (p'), wherein the imaging system is designed to perform a method according to any one of claims 1 to 6. [12] Imaging system according to claim 11, further comprising an output unit, such that the imaging system is designed to perform a method according to any one of claims 7 to 9. [13] Imaging system according to one of claims 11 or 12, wherein the camera (C13) is a 3D camera and wherein the contour data (k) is 3D contour data.

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