Method and apparatus for reconstructing virtual monoenergetic image

Abstract

The invention relates to a method for reconstructing a virtual monoenergetic image (B), comprising the following steps: recording spectral CT data (D) at at least two different average radiation energies; reconstructing, from the CT data (D), a plurality of virtual monoenergetic images (B), the VMI energies (E) of which vary along the patient longitudinal axis (8) and which collectively derive a 3D image of at least a portion of the patient (6); and outputting the plurality of virtual monoenergetic images (B). The invention further relates to corresponding equipment, a control device and a CT system.

Description

Technical Field

[0001] The present invention relates to a method and a device for reconstructing a virtual monoenergetic image and in particular for controlling a computed tomography system ("CT system"). Background Art

[0002] In daily clinical practice, spectral recording methods in the field of computed tomography (CT) have become a fixed component of examination practice. Dual-energy CT (DECT) or multi-energy CT has developed into a common imaging method due to its unique post-processing possibilities for improved evaluation of all body regions. In addition to the reconstruction of multi-energy images, the reconstruction of virtual mono-energy images ("VMI", in English "virtual monoenergetic images") has advantages both for contrast-free and for contrast-enhanced spectral CT recording methods.

[0003] A virtual monoenergetic image is a spectral CT record that is not based on the energy spectrum of the X-rays, but rather simulates the energy of a single (average) photon. This energy is typically in the range of tens of kiloelectronvolts (keV). The energy on which the reconstruction is based is referred to herein as "VMI energy" to better distinguish it from other energies (e.g., ray energies). The VMI energy has little to do with the X-ray energy used for recording; the VMI energy is more of a simulated energy or an energy used for reconstruction. Relatively low VMI energies (e.g., 50 keV) increase the contrast in the image, which is particularly interesting for contrast agent applications.

[0004] In CT angiography, virtual monoenergetic images are usually reconstructed. In the case of iodine as a contrast agent, these images increase the iodine contrast-to-noise ratio at a VMI energy of, for example, 55 keV. This can reduce the potential radiation dose or the concentration or amount of contrast agent.

[0005] In particular, in CT angiography, especially in CT angiography of the lungs, for example in the case of suspected pulmonary embolism, changes in the iodine contrast medium, especially in the direction of the patient's longitudinal axis (z axis), are often observed. In this case, in the shoulder region, in the region of the subclavian vein, where the contrast medium usually flows, the iodine contrast increases so strongly that the image becomes difficult to diagnose due to excessive radiation. This is a serious problem.

[0006] On the other hand, non-anatomically caused reductions in contrast agent concentration in the legs may be obtained during CT angiography ("CTA") of the entire aorta or pelvic vessels or leg vessels if the contrast agent protocol or scanning protocol is not optimal. This can occur, for example, if the scan is started too early or the feed rate is too high, and is also a serious problem.

[0007] Until now, in the case of large contrast differences within the scan volume, in particular along the longitudinal axis of the patient, it has only been possible to perform an additional reconstruction of the entire scan volume with the aid of VMIs of different VMI energies. For example, in the case of overirradiation in the shoulder area of ​​a lung CTA, an additional VMI with a higher VMI energy (and thus a lower iodine contrast) can be calculated in addition to the normally reconstructed VMI. In the case of reduced contrast in the direction of the legs in a pelvic leg CTA, an additional VMI can be reconstructed with a lower VMI energy (and thus a higher iodine contrast). It is also possible to calculate the VMI only in part of the scan volume, but for this the corresponding areas need to be manually defined by the user. In any case, the processing method described results in a plurality of image data sets that must be diagnosed together. Summary of the invention

[0008] The object of the present invention is to provide an alternative, more convenient method and a corresponding device for reconstructing virtual monoenergetic images, in particular for controlling a CT system, with which the above-mentioned disadvantages can be avoided.

[0009] The object is achieved by the method and the device according to the invention, as well as the control device and the CT system according to the invention.

[0010] The method according to the invention is used to reconstruct a virtual monoenergetic image (VMI). In particular, the method is designed to be performed in a computed tomography system and then in principle used to control the CT system. The method comprises the following steps:

[0011] - recording of spectral CT data at at least two different average radiation energies (i.e. in principle dual-energy CT or multi-energy CT),

[0012] - reconstructing a plurality of virtual monoenergetic images from the CT data, the VMI energy of the plurality of virtual monoenergetic images varying along the longitudinal axis of the patient and the plurality of virtual monoenergetic images together resulting in a 3D image of at least a portion of the patient, for example in the form of an image stack,

[0013] - Output virtual monoenergetic image.

[0014] The recording of spectral CT data is basically known in the art and is preferably performed within the scope of CT angiography. It is particularly preferred that a contrast agent, such as iodine, has been administered to the patient before the recording. The recording is then performed while the patient is carrying the contrast agent himself and preferably while a further contrast agent is administered to the patient. Typically, the spectral CT data are recorded at at least two (possibly more) different radiation energies.

[0015] It should be noted that the X-ray source of a CT scanner sometimes has a very broad X-ray spectrum. Here, a distinction must be made between the acceleration voltage (in kV) and the average radiation energy (the most frequently emitted energy of an X-ray quantum in keV) of the X-ray source. For lower radiation energies, the acceleration voltage is usually in the range below 100 kV, while for higher radiation energies, the acceleration voltage is usually in the range above 100 kV. For example, it follows that for lower radiation energies, the average radiation energy is in the range between 33 keV and 65 keV, while for higher radiation energies, the average radiation energy is in the range between 65 keV and 100 keV.

[0016] In a CT system with two different tube voltages, in practice, for example, for lower radiation energy, an acceleration voltage between 70-100 kV is used. Then, the corresponding average radiation energy is usually between 50 keV and 65 keV, for example. For higher radiation energy, for example, an acceleration voltage between 120 kV and 140 kV is used. Then, the corresponding average radiation energy is usually between 70 keV and 80 keV.

[0017] The reconstruction of a plurality of virtual single-energy images from CT data, which together result in a 3D image of at least a portion of the patient (at a constant standard energy) is known in principle in the prior art. However, a particularity of the present invention is the variation of the VMI energy along the longitudinal axis of the patient. The variation should approximate the real variation of the contrast agent concentration in the patient. The variation preferably decreases monotonically in one direction, in particular in the direction towards the feet of the patient, i.e. in the fore-aft direction.

[0018] The output of the virtual monoenergetic image can be display on a display or saving the image. Forwarding the image data for further calculation or post-processing is also an output.

[0019] Even though the reconstruction can be performed by a diagnostic station, the entire method is preferably performed by a CT system which records the data, reconstructs the data and outputs the data for further diagnosis or storage.

[0020] In practice, the method can be implemented as follows: a CT system that can record spectral data, i.e., in particular a CT system with a photon counting detector, but also other CT systems, such as a dual-source CT system, a CT system with a dual-layer detector, a CT system with a kV switching function or a CT system with a separate prefilter (Twin Beam), records CT data at two or more medium radiation energies at a standard voltage (e.g., 120 kV). A virtual monoenergetic image is then reconstructed from the recorded data, the VMI energy of which varies along the longitudinal axis of the patient (z direction) and is particularly preferably automatically adapted to the corresponding iodine contrast at the position. Thus, when reconstructing the scan volume, an image stack of virtual monoenergetic images with an energy that varies in the z direction is formed. Thus, for example, in a lung CTA, the VMI is calculated in the shoulder region with a high VMI energy, and the VMI energy increases rapidly in the cranial direction (scanning direction) along the z axis, but preferably without a sudden transition to the desired standard value, such as a standard value of 55 keV. On the other hand, in CTA of the aorta and pelvic leg vessels, the VMI energy can be slowly reduced along the z-axis in the direction of the leg from a preset standard value (e.g. 55 keV) to a value of, for example, 40 keV in order to compensate for a possible contrast reduction caused by a non-optimized contrast medium regimen. The VMI energy should not fall below the K threshold of the contrast medium, which in the case of iodine is, for example, 33 keV.

[0021] The device for reconstructing a virtual monoenergetic image according to the present invention comprises the following components:

[0022] - a data interface designed to receive spectral CT data at at least two different average radiation energies,

[0023] a VMI unit designed to reconstruct a plurality of virtual single-energy images from the CT data, the VMI energy of the plurality of virtual single-energy images varying along the longitudinal axis of the patient and the plurality of virtual single-energy images together resulting in a 3D image of at least a portion of the patient,

[0024] - A data interface designed to output a virtual monoenergetic image.

[0025] The functional mode of the components has been described within the scope of the method. The device is preferably designed to carry out the method according to the invention. In particular, the features and advantages described in conjunction with the method according to the invention can also be designed as corresponding subunits of the device according to the invention or the control device according to the invention, the computer tomography system according to the invention and the computer program product according to the invention.

[0026] Rather, features and advantages described in conjunction with the apparatus according to the invention or the control device according to the invention, the computed tomography system according to the invention and the computer program product according to the invention may also be designed as corresponding method steps of the method according to the invention.

[0027] A control device according to the invention for controlling a computed tomography system is designed to carry out the method according to the invention and / or comprises an apparatus according to the invention.

[0028] The computer tomography system according to the invention comprises a control device according to the invention.

[0029] Most of the above-mentioned components of the device can be implemented completely or partially in the form of software modules in a processor of a corresponding computing system, for example by a control device of a computed tomography system. A largely software-based implementation has the advantage that the computing system used so far can also be converted in a simple manner by a software upgrade to work in the manner according to the invention. In this regard, the object is also achieved by a corresponding computer program product with a computer program that can be directly loaded into a computing system, the computer program product having program segments so that when the program is executed in the computing system, at least the steps that can be executed by the computer are executed. It should be noted that in the described implementation, the recording of spectral CT data corresponds to receiving CT data, for example via a data bus or by reading a storage unit. In addition to the computer program, such a computer program product can optionally include additional components (for example documentation) and / or additional components and hardware components, such as a hardware key (dongle, etc.) for using the software.

[0030] A computer-readable medium, such as a memory stick, a hard disk or other transferable or permanently installed data carrier, can be used for transfer to a computing system or a control device and / or for storage at or in a computing system or a control device, on which program segments of a computer program that can be read in and executed from a computing system are stored. For this purpose, a computing system can, for example, have one or more cooperating microprocessors or the like.

[0031] In addition, particularly advantageous designs and improvements of the present invention can be derived from the following description, wherein claims of one claim category can also be improved in a similar manner to the claims and the description parts of other claim categories and in particular individual features of different embodiments or variants can be combined into new embodiments or variants.

[0032] According to a preferred method, two different X-ray spectra are selected to record spectral CT data with at least two different ray energies. One (low energy) X-ray spectrum is preferably generated by an acceleration voltage of at most 100 kV. In particular, the X-ray spectrum has an average ray energy between 33 keV and 65 keV. The other (high energy) X-ray spectrum is preferably generated by an acceleration voltage greater than 100 kV. In particular, the X-ray spectrum has an average ray energy between 65 keV and 100 keV.

[0033] If a CT system with a broadband source and one or more detectors is available, it is preferred that the energy selective detector records data with low average radiation energies in the range of 33keV to 70keV and data with higher average radiation energies in the range of 55keV to 100keV.

[0034] According to a preferred method, the spectral CT data are recorded using a CT system with a photon counting detector, a dual source CT system, a CT system with a dual layer detector, a CT system with kV switching capability, or a CT system with a separate pre-filter.

[0035] According to a preferred method, for several locations along the longitudinal axis of the patient the contrast agent concentration prevailing there is determined.

[0036] The contrast agent concentration can be determined in particular by measurements (e.g. CT images) and / or by interpolation between known values. For example, the contrast agent concentration can be measured point by point at different positions of the patient's longitudinal axis and the contrast agent concentration can be interpolated at points in between, for example by fitting a curve to the measured points according to an Nth order polynomial.

[0037] Thus, when reconstructing a virtual monoenergetic image of a position along the longitudinal axis of the patient, the VMI energy of the virtual monoenergetic image is automatically adapted to the iodine concentration determined for the position. This can be done according to a table or according to a mathematical function. For example, in the case of a curve of the iodine concentration corresponding to the function f(z) in the z direction, the VMI energy follows a function a·f(x) with a constant coefficient a. However, it is preferred that a calibration curve for the VMI energy is obtained from a test measurement. This is explained in more detail below.

[0038] It is also preferred that for several positions along the longitudinal axis of the patient, a measure of the contrast agent concentration (e.g. iodine concentration) prevailing in a reference vessel or reference tissue is determined from the CT values ​​of the virtual monoenergetic image at a fixed energy or from a spectral contrast agent image (e.g. iodine image). Then, when subsequently reconstructing the virtual monoenergetic image, the VMI energy of the position along the longitudinal axis of the patient is automatically adapted to the measure of the corresponding contrast agent concentration (e.g. iodine concentration) determined for said position, so that the desired course of the CT values ​​along the longitudinal axis of the patient is obtained.

[0039] According to a preferred method, corresponding VMI energies are automatically determined to reconstruct multiple virtual monoenergetic images of the patient according to the following steps:

[0040] - Reconstruction of an image dataset consisting of multiple virtual monoenergetic images using a preset standard energy,

[0041] - (automatically) segmenting several reference vessels of the reconstructed image data set, in particular segmenting the subclavian vein and / or the superior vena cava and / or the aorta,

[0042] - measuring (automatically) the mean CT value in a pre-set region (ROI: “Region of Interest”) of several segmented reference vessels for a plurality of positions of a plurality of virtual monoenergetic images, preferably in HU (Houndsfield Units),

[0043] - determining the course of the measured CT values ​​along the longitudinal axis of the patient and calculating the VMI energy from said course,

[0044] - Reconstructing a new image data set consisting of a plurality of virtual monoenergetic images based on the measured course of the CT values ​​with the aid of the calculated VMI energy, in particular such that a desired course is obtained by means of the virtual monoenergetic images at the calculated energy.

[0045] When reconstructing an image data set consisting of a plurality of virtual monoenergetic images for the first time, a standard energy is preset. This is a pre-confirmed and unchanged VMI energy. In this respect, prior art can be relied upon. One of the VMI energies selected there can be preset as the standard energy.

[0046] The (automatic) segmentation of several reference vessels of the reconstructed image data set is known in the prior art, which can be performed with the aid of a "normal" algorithm or with the aid of a machine learning trained model with learning capabilities. Such algorithms or models are known in the prior art and are particularly suitable for identifying the subclavian vein and / or the superior vena cava and / or the aorta in the image and performing segmentation there.

[0047] For the location of the virtual monoenergetic image, it is also known in the prior art to (automatically) measure the average CT value in a preset region of several segmented reference vessels. CT values ​​are usually stated in Hounsfield units (HU).

[0048] According to the invention, the course of the measured CT values ​​along the longitudinal axis of the patient can now be determined from the measured values, for example by plotting the measured values ​​at their longitudinal axis positions and adapting a curve (for example an Nth order polynomial). The VMI energy can then be calculated from the course.

[0049] In practice, the CT values ​​of the iodine sample in the phantom can be determined in advance in VMI at various energies, for example, from 40keV to 190keV. The phantom can have a cross-section similar to that of the abdomen. The iodine sample can be a syringe filled with a specific concentration of iodine. If the CT value is plotted as a function of the VMI energy, a calibration curve is obtained therefrom, and then the corresponding target energy of the VMI can be derived from the CT values ​​measured at various positions of the patient with the aid of the calibration curve.

[0050] With the aid of the calculated VMI energy, a new image data set consisting of a plurality of virtual mono-energy images can now be reconstructed according to the method according to the invention. The image at the longitudinal axis position is reconstructed with the aid of the VMI energy calculated there.

[0051] Thus, for example, an automatic determination of the VMI energy in each slice can be performed, so that first an image data set of the VMI with the determined energy is calculated. In the image data set, the reference vessels are automatically segmented (for example, the subclavian vein, the superior vena cava in the case of pulmonary CTA, the aorta in the case of abdominal / pelvic CTA), wherein the average CT value in HU is measured in the automatically determined ROI in each slice. The trend of the VMI energy of the VMI in the z direction is derived from the trend of the CT values ​​in the z direction. A new VMI image data set is then calculated, in which each VMI has the desired VMI energy at the corresponding z position.

[0052] According to a preferred method, during reconstruction, a virtual monoenergetic image is created with a VMI energy that is variable in the cranio-caudal direction. It is preferred that the corresponding VMI energy of the virtual monoenergetic image decreases in the cranio-caudal direction, in particular in a strictly monotonically decreasing manner.

[0053] According to a preferred method, an image stack of virtual monoenergetic slice images is created when reconstructing VMI. It is preferred that the slice images of the image stack are parallel to the transverse plane or parallel to the sagittal plane. Of course, these images do not necessarily have to be aligned along the z-axis, even if this represents an advantageous embodiment.

[0054] According to a preferred method, a virtual mono-energy image is reconstructed in the region of the patient's shoulder in a lung CTA with the aid of a first VMI energy. When reconstructing further virtual mono-energy images, the corresponding VMI energy is then reduced in the cranial-caudal direction along the longitudinal axis of the patient. It is preferred that the course of the VMI energy does not have abrupt changes, that is, the course is continuous and in particular also differentiable. It is preferred that the VMI energy is reduced to a value between 40 keV and 60 keV.

[0055] According to a preferred method, in a CTA of the patient's aorta and / or pelvic leg vessels, the VMI energy of the virtual monoenergetic image is continuously reduced along the longitudinal axis of the patient toward the leg from a first value, in particular a first value between 50keV and 80keV, in particular to a VMI energy between 40keV and 50keV.

[0056] The use of AI-based methods (AI: "artificial intelligence") is preferred for the method according to the invention, in particular for the segmentation of blood vessels and / or for the determination of VMI energy from contrast agent concentrations. Artificial intelligence is based on the principle of machine learning and is usually implemented with the aid of correspondingly trained algorithms with learning capabilities. The English expression "Machine Learning" is often used for machine-based learning, which also includes the principle of "deep learning".

[0057] The components of the invention, in particular the components for reconstructing the VMI, are preferably used as "cloud services". Such cloud services are used for processing data, in particular by means of artificial intelligence, but can also be services based on conventional algorithms. In general, a cloud service (hereinafter also referred to as "cloud") is an IT infrastructure in which, for example, storage space or computing power and / or application software are available via a network. The communication between the user and the cloud takes place here by means of a data interface and / or a data transmission protocol. In the present case, it is particularly preferred that the cloud service provides computing power and application software.

[0058] In the context of a preferred method, data are provided to a cloud service via a network. A cloud service comprises a computing system, such as a computer cluster, which generally does not include a local computer of the user. The cloud can be provided in particular by a medical institution, which also provides a medical technology system. For example, the data of an image record is sent to a (remote) computer system (cloud). The computing system, the network and the medical technology system of the cloud are preferably a union in the sense of data technology. In this case, the method can be implemented with the aid of a constellation of instructions in the network. The data calculated in the cloud ("result data") are then sent again to the local computer of the user via the network. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The invention is explained in more detail below based on embodiments with reference to the attached drawings. In the different figures, the same components are provided with the same reference numerals. The drawings are generally not to scale. Among them:

[0060] Figure 1 A schematic diagram of a CT system with an exemplary embodiment of a control device having a device according to the invention for carrying out the method is shown.

[0061] Figure 2 A sketch showing the course of the contrast agent concentration in the human body,

[0062] Figure 3 A sketch showing the flow of the method,

[0063] Figure 4 A VMI image stack is shown with a curve of decreasing VMI energy. DETAILED DESCRIPTION

[0064] Figure 1 An embodiment of a computed tomography system (CT system) 1 is shown having a radiation detector 4 and a radiation source 5. The radiation source 5 is designed to irradiate the radiation detector 4 with radiation. The CT system 1 shown comprises a gantry 2 having a rotor 3. The rotor 3 comprises an X-ray source as the radiation source 5 and a radiation detector 4 designed to detect X-rays.

[0065] The radiation detector 4 and / or the radiation source 5 are designed for spectral CT recording. Thus, for example, the radiation source 5 can be broadband and the radiation detector 4 can detect two or more energies in a resolved manner. However, the radiation source 5 can also emit two or more different spectra in sequence, which are then detected by the radiation detector 4. The gantry 2 can also have two or more systems consisting of a radiation source 5 and a radiation detector 4, which perform measurements in different energy spectra.

[0066] The rotor 3 can rotate about an axis of rotation 8, which can also be considered here as the patient longitudinal axis 8. The patient 6 is supported on a patient bed 7 and can be moved along the axis of rotation 8 by the gantry 2. A computing unit 9 is provided to control the imaging system 1 and / or to generate image data sets based on the signals detected by the radiation detector 4.

[0067] Typically, a (raw) X-ray image data set of the object 6 is recorded from a large number of angular directions at the corresponding radiation energy by means of a radiation detector 4, i.e. two or more raw data sets. The final image data set can then be reconstructed based on the (raw) X-ray image data set by means of mathematical methods, which include, for example, filtered back projection or iterative reconstruction methods. In this case, the image data set includes a virtual monoenergetic image B that has been reconstructed by means of the method according to the invention.

[0068] The computing unit 9 may include or be a control device 9 for controlling the CT system 1 and a generation unit for generating an X-ray image data set. In the case shown here, the computing unit 9 is a control device 9 .

[0069] Furthermore, an input device 10 and an output device 11 are connected to the computing unit 9. The input device 10 and the output device 11 can, for example, enable interaction by a user or display the generated image data set or output a determined solution to the problem.

[0070] The computing unit 9 as a control device 9 comprises here a device 14 according to the invention for reconstructing a virtual monoenergetic image B (see for example Figure 3 ). The device 14 includes a data interface 12 and a VMI unit 13.

[0071] The data interface 12 is designed to receive spectral CT data D at at least two different average radiation energies and to output a virtual monoenergetic image B.

[0072] The VMI unit 13 is designed to reconstruct a plurality of virtual monoenergetic images B from the CT data D, which together result in a 3D image of at least a part of the patient 6 . In this case, the VMI energy E of these VMI B varies along the patient longitudinal axis 8 .

[0073] The CT system shown here may be, for example, a CT system 1 with a photon counting detector, a dual-source CT system 1 , a CT system 1 with a dual-layer detector, a CT system 1 with a kV switching function, or a CT system 1 with a separate prefilter.

[0074] The following figures illustrate how the reconstruction of VMI B is performed.

[0075] Figure 2 The sketch shows the course of the contrast medium concentration K in the human body. In the background is shown the silhouette of a patient 6, to whose upper body the contrast medium is administered. Towards the feet, ie in the fore-aft direction, the contrast medium concentration K decreases, as shown by the curve.

[0076] Figure 3 A schematic diagram of the sequence of the method for reconstructing a virtual monoenergetic image B is shown in the form of a block diagram.

[0077] In step I, spectral CT data D are recorded at at least two different average radiation energies. It can be assumed that: Figure 1The CT system of the present invention is used for recording, wherein the spectral CT data D are recorded with the aid of at least two different radiation energies. For this purpose, for example, two different X-ray spectra can be selected, preferably an X-ray spectrum generated by an acceleration voltage of up to 100 kV and in particular with an average radiation energy between 33 keV and 65 keV, and an X-ray spectrum generated by an acceleration voltage of more than 100 kV and in particular with an average radiation energy between 65 keV and 100 keV. However, there can also be an energy-selective radiation detector 4, which can record data with a low average radiation energy in the range of 33 keV to 70 keV and data with a higher average radiation energy in the range of 55 keV to 100 keV.

[0078] In step II, a plurality of virtual mono-energy images B are reconstructed from the CT data D and the course of the VMI energy E, such that the VMI energy E of the VMIB varies along the patient longitudinal axis 8. These VMI B together result in a 3D image of at least a part of the patient 6.

[0079] For this purpose, for example, the contrast agent concentration K prevailing at several locations along the longitudinal axis 8 of the patient 6 can be determined there, and when reconstructing a virtual monoenergetic image B of the locations along the longitudinal axis 8 of the patient, the VMI energy E of the virtual monoenergetic image B can be automatically adapted to the contrast agent concentration K determined for these locations. Figure 2 and Figure 4 An overview is given in.

[0080] In step III, a virtual monoenergetic image B is output.

[0081] Figure 4 A VMI image stack S is shown having a total of four slice images S1, S2, S3, S4, wherein the slice images have Figure 2 The trend of the contrast agent concentration K in the VMI energy E decreases. Figure 4 Similar to Figure 2 , with the difference that here the contrast agent concentration K is not plotted but the VMI energy E. Slice images S1 , S2 , S3 , S4 are reconstructed at different positions of the longitudinal axis 8 of the patient based on the VMI energy E present there.

[0082] It can be assumed that the VMI energy at the relevant points has already been determined by measurement and that the shown course is interpolated between these points for the further slice images.

[0083] Finally, it should be pointed out again that the drawings described in detail above are only exemplary embodiments, which can be modified in various ways by those skilled in the art without departing from the scope of the present invention. In addition, the use of the indefinite article "a" or "an" does not exclude that the features involved can exist multiple times. Similarly, the terms "unit" and "device" do not exclude that the components involved can be composed of multiple cooperating subcomponents that can also be distributed in space if necessary. The expression "several" should be understood as "at least one". Regardless of the grammatical gender of a particular term, it includes people / things with masculine, feminine or other gender identities.

Claims

1. A method for reconstructing a virtual monoenergetic image (B), the method comprising: The following steps are involved: recording spectral CT data (D) at at least two different average radiation energies; reconstructing a plurality of virtual monoenergetic images (B) from the CT data (D), wherein the VMI energy (E) of the plurality of virtual monoenergetic images varies along the longitudinal axis (8) of the patient and the plurality of virtual monoenergetic images together obtain a 3D image of at least a portion of the patient (6); The plurality of virtual monoenergetic images are output (B).

2. A method according to claim 1, wherein in order to record spectral CT data (D) with at least two different radiation energies, two different X-ray spectra are selected, preferably an X-ray spectrum generated by an acceleration voltage of up to 100 kV and, in particular, with an average radiation energy between 33 keV and 65 keV, and an X-ray spectrum generated by an acceleration voltage of more than 100 kV and, in particular, with an average radiation energy between 65 keV and 100 keV, or wherein the energy-selective radiation detector (4) records data with a low average radiation energy in the range between 33 keV and 70 keV and data with a higher average radiation energy in the range between 55 keV and 100 keV.

3. The method according to any of the preceding claims, wherein for recording the spectral CT data (D) a CT system (1) with a plurality of photon counting detectors, a dual-source CT system (1), a CT system (1) with a double-layer detector, a CT system (1) with a kV switching function or a CT system (1) with a plurality of separate pre-filters is used.

4. A method according to any one of the preceding claims, wherein for several positions along the longitudinal axis (8) of the patient (6), a contrast agent concentration (K) prevailing at the several positions is determined, in particular by measurement and / or interpolation between known values, and when reconstructing the multiple virtual monoenergetic images (B) of the several positions along the longitudinal axis (8) of the patient (6), the VMI energy (E) of the virtual monoenergetic images is automatically adapted to the contrast agent concentration (K) determined for the several positions.

5. The method according to claim 4, wherein the corresponding VMI energies (E) are automatically determined to reconstruct the plurality of virtual single-energy images (B) of the patient (6) according to the following steps: Reconstructing an image data set consisting of a plurality of virtual monoenergetic images (B) by means of a preset standard energy; Segmenting several reference blood vessels of the reconstructed image dataset, in particular segmenting the subclavian vein and / or the superior vena cava and / or the aorta; For the positions of the plurality of virtual monoenergetic images (B), measuring the average CT value in a preset area of ​​the segmented plurality of reference blood vessels, preferably in HU units; Determining the direction of the measured CT values ​​along the longitudinal axis (8) of the patient (6), and calculating the VMI energy (E) based on the direction; Based on the course of the measured CT values, a new image data set consisting of a plurality of virtual monoenergetic images (B) is reconstructed with the aid of the calculated VMI energy (E).

6. A method according to any one of the preceding claims, wherein during reconstruction, multiple virtual single energy images (B) are created with the aid of a variable VMI energy (E) in the fore-aft direction, preferably wherein the corresponding VMI energies (E) of the multiple virtual single energy images (B) decrease along the fore-aft direction.

7. A method according to any one of the preceding claims, wherein an image stack (S) of multiple virtual monoenergetic slice images (S1, S2, S3, S4) is created when reconstructing the multiple virtual monoenergetic images (B), preferably wherein the slice images (S1, S2, S3, S4) of the image stack (S) are parallel to the transverse plane or parallel to the sagittal plane.

8. A method according to any of the preceding claims, wherein in CT angiography, in particular pulmonary CT angiography, a virtual monoenergetic image (B) is reconstructed in the shoulder region of the patient, in particular a virtual monoenergetic image (B) is reconstructed in the shoulder region of the patient with the aid of a first VMI energy (B), and when reconstructing another virtual monoenergetic image (B) along the longitudinal axis (8) of the patient (6), the corresponding VMI energy (E) decreases in the fore-and-aft direction, in particular in the form of a continuous path, in particular to a value between 40 keV and 60 keV.

9. A method according to any of the preceding claims, wherein in CT angiography of the aorta and / or pelvic leg vessels of the patient (6), the VMI energy (E) of the multiple virtual monoenergetic images (B) decreases continuously along the longitudinal axis (8) of the patient (6) in the direction of the legs from a first value, in particular a first value between 50 keV and 80 keV, in particular to a VMI energy (E) between 40 keV and 50 keV.

10. A device (14) for reconstructing a virtual monoenergetic image (B), include: A data interface (12) is designed to receive spectral CT data (D) at at least two different average radiation energies; a VMI unit (13) designed to reconstruct a plurality of virtual mono-energy images (B) from the CT data (D), the VMI energy (E) of the plurality of virtual mono-energetic images varying along the longitudinal axis (8) of the patient, and the plurality of virtual mono-energetic images together obtaining a 3D image of at least a part of the patient (6); A data interface (12) is designed to output the plurality of virtual monoenergetic images (B).

11. A control device (9) designed to control a computed tomography system (1), the computed tomography system comprising an apparatus (14) according to claim 10, and / or the control device (9) designed to perform a method according to any one of claims 1 to 9.

12. A computer tomography system (1) comprising a control device (9) according to claim 11.

13. A computer program product comprising instructions which, when the program is executed by a computer, cause the computer to perform the steps of the method according to any one of claims 1 to 9, wherein the recording of the spectral CT data (D) comprises receiving the data and in particular comprises sending corresponding control instructions.

14. A computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to perform the steps of the method according to any one of claims 1 to 9, wherein the recording of the spectral CT data (D) comprises receiving the data and in particular comprises sending corresponding control instructions.

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