Alignment of CT image data for cardiac examination

Abstract

Computer-implemented method for spectrally differentiated cardiac CT imaging, comprising the steps: - Receiving first spectrally differentiated CT projection measurement data (PMD1) relating to a heart (H) of a patient (2) in a first time interval (I1), wherein a contrast agent is mainly located in the chambers and / or vessels of the heart (H) during the first time interval (I1), wherein the first time interval (I1) comprises a diastole of the heart (H), - Receiving second spectrally differentiated CT projection measurement data (PMD2) relating to the patient's heart (H) (2) at a later second time interval (I2), wherein the contrast agent is mainly located in a muscular tissue of the heart (H) during the second time interval (I2), wherein the second time interval (I2) includes a systole of the heart (H), - Calculation of first monoenergetic image data (BD1) for a first energy value (E1) based on the first spectrally differentiated CT projection measurement data (PMD1), - Calculation of second monoenergetic image data (BD2) for the first energy value (E1) based on the second spectrally differentiated CT projection measurement data (PMD2), - Determining a second energy value (E2) and a third energy value (E3) different from the second energy value (E2), - Calculation of third monoenergetic image data (BD3) for the second energy value (E2) based on the first spectrally differentiated CT projection measurement data (PMD1), - Calculation of fourth monoenergetic image data (BD4) for the third energy value (E3) based on the second spectrally differentiated CT projection measurement data (PMD2), - wherein the second energy value (E2) and the third energy value (E3) are determined in such a way that an adjustment of a contrast agent-induced attenuation in the third monoenergetic image data (BD3) and a contrast agent-induced attenuation in the fourth monoenergetic image data (BD4) is made to each other.

Description

[0001] The invention relates to a method for spectrally differentiated cardiac CT imaging. The invention also relates to an image generation device. Furthermore, the invention relates to a computed tomography system.

[0002] The following are considered state of the art: DE 10 2016 203 257 A1, EP 4 311 496 A1, DE 10 2018 221 691 A1, DE 10 2021 201 809 A1 and US 2020 / 0 179 539 A1.

[0003] To assess potential risks associated with possible coronary artery disease, a CT scan (CT stands for computed tomography) of the heart is now frequently used in clinical routine. The most common examinations employed are imaging to determine a calcium score without contrast agent and coronary angiography, abbreviated as "CTA" (CTA stands for "CT angiography").

[0004] Both methods primarily aim to visualize and examine the coronary arteries, thereby "ignoring" the condition of the myocardium as another part of the heart that also has diagnostic relevance. With the introduction of spectral CT systems, the acquisition is extended, depending on the clinical question, by a third scan, called ECV measurement, which is delayed by 3 to 5 minutes relative to the coronary angiography. This allows for a quantitative assessment of the fibrotic component of the myocardium (ECV measurement: abbreviation for "Extracellular Volume" measurement).

[0005] In direct comparison to diagnosis based on magnetic resonance imaging (MRI), the sole use of computed tomography (CT) for a complete assessment of the heart lacks only one piece of information regarding the dynamics of the heart muscle or the estimation of changes in heart muscle thickness—that is, a quantitative comparison between images acquired during the systolic and diastolic phases. The systolic phase characterizes the contracted state of the heart muscle, in which blood is pumped from the ventricles into the pulmonary artery and the aorta, the heart muscle contracts, and the heart wall is particularly thick. The diastolic phase characterizes the relaxed state of the heart muscle, in which the ventricles are completely filled with blood and the heart is maximally expanded. During this phase, the heart wall is particularly stretched and therefore particularly thin.

[0006] This functional assessment of the heart's dynamic behavior is, in principle, readily achievable with a CT system. However, it currently requires coronary angiography to be performed with a very wide ECG window (ECG stands for electrocardiogram) to allow reconstruction at the two characteristic times of the cardiac cycle: systole and diastole. For example, such a combined CT scan requires an acquisition time of approximately 8 seconds, necessitating the use of a high-dose radiation dose. This process requires a total of about 5 to 6 cardiac phases, alternating between diastole and systole, to obtain one image during diastole and one during systole. The resulting radiation exposure is undesirably high. Furthermore, a third CT scan must be performed after approximately three to five minutes to measure the ECV (extracorporeal volume).

[0007] Due to the significantly increased radiation dose, this procedure is usually avoided, and the dynamics of the heart are examined using magnetic resonance imaging.

[0008] The task is therefore to provide sufficient image data using CT imaging for an examination of the coronary arteries, an examination of the fibrotic part of the myocardium and an examination of the dynamics of the heart muscle, whereby the radiation exposure should be lower than with a conventional approach.

[0009] This problem is solved by a method for spectrally differentiated cardiac CT imaging of a patient according to claim 1, an image generation device according to claim 12 and a computed tomography system according to claim 13.

[0010] Using spectrally differentiated projection measurement data, preferably spectrally resolved projection measurement data, a purely diastolic acquisition can, for example, be combined with a late systolic acquisition, so that an evaluation of the wall changes of a patient's heart is successfully possible and at the same time no significantly increased radiation exposure of the patient results.

[0011] In the inventive method for spectrally differentiated cardiac CT imaging, first spectrally differentiated CT projection measurement data are received, which relate to a patient's heart in a first time interval, wherein a contrast agent is mainly located in the chambers and vessels of the heart during the first time interval, wherein the first time interval comprises a diastole of the heart.

[0012] In particular, it may be possible to acquire the first spectrally differentiated CT projection measurement data with contrast enhancement. "Contrast-enhanced acquisition" refers to the acquisition of projection measurement data in the presence of a contrast agent in the respective examination area, in this case, the cardiac region of a patient. Spectrally differentiated CT projection measurement data refers to projection measurement data that were either acquired with different spectra or acquired with high spectral resolution. Photon-counting, spectrally resolved X-ray detectors are preferably used to generate such projection measurement data. Alternatively, different X-ray spectra from one or more X-ray sources can be generated with appropriate temporal or spatial differentiation. Typical systems for this are so-called dual-energy CT systems or split-filter CT systems.

[0013] In the method according to the invention, a second spectrally differentiated CT projection measurement data is also received, which relates to the patient's heart in a later, second time interval, wherein the contrast agent is located mainly in the muscle tissue of the heart during the second time interval. This second time interval includes a systole of the heart. In particular, it can be provided that the second spectrally differentiated CT projection measurement data is acquired with the aid of contrast agent.

[0014] The second time interval is preferably taken approximately 3 to 5 minutes after the first. After this time, the contrast agent has moved from the large blood-carrying structures, i.e., the major vessels and the heart chambers, into the muscle tissue, allowing for an ECV measurement to be performed to examine the tissue condition. This measurement can be combined with a systole, so that this second measurement, in combination with the first measurement taken during diastole, can also be used to determine the dynamic behavior of the heart muscle and to compare the thickness of the heart muscle during the diastolic and systolic phases.

[0015] Subsequently, initial monoenergetic image data are calculated based on the first spectrally differentiated CT projection measurements for a first energy value. The calculation of monoenergetic image data, also referred to as "pseudo-monoenergetic image data," is described in detail in Alvarez RE and Macovski A. "Energy-selective reconstructions in x-ray computed tomography," Phys. Med. Biol. 21, 733-744 (1976).

[0016] Crucial for the calculation of monoenergetic image data is that the CT projection measurement data are spectrally differentiated, or preferably spectrally resolved. The calculation includes a series of corrections, particularly for beam hardening effects, not only of water but also of the contrast agent, especially iodine. Furthermore, different spectral information can then be processed either within the projection measurement data space, followed by reconstruction, preferably using filtered backprojection, to obtain monoenergetic image data. Alternatively, different image data can first be reconstructed based on different spectrally differentiated projection measurement data, and then the reconstructed image data, each with different spectral information, can be processed together to generate monoenergetic image data.

[0017] Similarly, second monoenergetic image data are calculated based on the second spectrally differentiated CT projection measurement data for the first energy value. The monoenergetic image data correspond to image data that would have been reconstructed based on projection measurement data generated by the proportion of X-rays in the acquired total spectrum that exhibit only radiation with the respective underlying energy value. The calculation of the monoenergetic image data, in particular the first and second monoenergetic image data, comprises a reconstruction or image data reconstruction based on the respective assigned projection measurement data.

[0018] As a rule, the first and second attenuation values ​​differ due to the strong influx of contrast medium into the area of ​​the heart during diastole, when the heart is expanding, whereas at the time of systole, when the heart is contracting, the heart is relatively bloodless and therefore also low in contrast medium, and thus the contrasts are significantly weaker.

[0019] In particular, it may be possible to align the attenuation values, and thus the contrasts, of the image data relating to diastole and those relating to systole. This significantly improves the contrasts in the systole image data. Furthermore, aligning the contrasts of image data acquired at different times and under different cardiac conditions allows for the investigation of the heart's dynamic behavior without the need for additional imaging, which would entail additional radiation exposure.

[0020] Based on the first attenuation value and / or the second attenuation value, a second energy value is determined for which third monoenergetic image data are reconstructed based on the first spectrally differentiated CT projection measurement data. Since the first monoenergetic image data, whose corresponding first projection measurement data were acquired during diastole, exhibit very high attenuation values ​​compared to the second monoenergetic image data, the second energy value is selected to reduce these attenuation values ​​somewhat. When using common contrast agents, such as iodine, this is achieved by selecting a second energy value higher than the first energy value, with both energy values ​​typically being above the energy value of the X-ray threshold of the contrast agent, preferably iodine.In this case, the second energy value, especially in the case of the contrast agent iodine, is further away from the X-ray threshold or absorption edge associated with the contrast agent than the first energy value, so that the effect of the contrast agent is reduced.

[0021] Based on the first attenuation value and / or the second attenuation value, a third energy value, different from the second, is determined. Fourth monoenergetic image data are then reconstructed for this third energy value based on the second spectrally differentiated CT projection measurement data. Since the second monoenergetic image data, whose corresponding second projection measurement data were acquired during systole, exhibit very low attenuation values ​​compared to the first monoenergetic image data, the third energy value is selected such that these attenuation values ​​in the fourth monoenergetic image data are slightly higher than those of the second monoenergetic image data. This is achieved by selecting a third energy value lower than the first.In this case, the third energy value is closer to the absorption edge associated with the contrast agent than the first energy value; for example, with iodine as a contrast agent, it is closer to 33 keV, so that the effect of the contrast agent is enhanced.

[0022] The second energy value and the third energy value can therefore be determined in such a way that an adjustment of a contrast agent-induced attenuation in the third monoenergetic image data and the fourth monoenergetic image data is achieved in comparison to a contrast agent-induced attenuation in the first monoenergetic image data and the second monoenergetic image data.

[0023] Finally, the third monoenergetic image data for the determined second energy value are calculated based on the first spectrally differentiated CT projection measurement data, and the fourth monoenergetic image data for the determined third energy value are calculated based on the second spectrally differentiated CT projection measurement data. The calculation of the third and fourth monoenergetic image data includes, in particular, a reconstruction of the third and fourth monoenergetic image data.

[0024] The second energy value can be determined, in particular, based on the first monoenergetic image data and / or the second monoenergetic image data. The third energy value can also be determined, in particular, based on the first monoenergetic image data and / or the second monoenergetic image data.

[0025] Contrast-induced attenuation can particularly affect a representative subregion of the heart. The alignment of contrast-induced attenuation in the third monoenergetic image data and contrast-induced attenuation in the fourth monoenergetic image data can be achieved, in particular, by comparing it to the difference between contrast-induced attenuation in the first monoenergetic image data and contrast-induced attenuation in the second monoenergetic image data.

[0026] The adjustment of the contrast agent-induced attenuation in the third monoenergetic image data and the contrast agent-induced attenuation in the fourth monoenergetic image data can be carried out in particular such that a difference between a contrast agent-induced attenuation in the third monoenergetic image data and a contrast agent-induced attenuation in the fourth monoenergetic image data is smaller than a difference between a contrast agent-induced attenuation in the first monoenergetic image data and a contrast agent-induced attenuation in the second monoenergetic image data.

[0027] Thus, the attenuation values ​​of image data acquired at different times and under different cardiac states are aligned. This is achieved because acquiring spectrally differentiated image data allows for contrast alignment through the targeted selection of energy values ​​for monoenergetic image data. In this way, contrasts can be aligned between image data that exhibit significantly different intrinsic contrast due to the varying acquisition times of their respective projection measurement data and the associated natural physiological timing of cardiac function. This ensures that the acquired monoenergetic image data can be processed together in suitable further processing without the influence of systematic errors caused by the contrast difference.

[0028] By achieving comparable contrast across the different reconstructions, a standard threshold-based segmentation of the heart chambers and myocardium is possible without systematic errors. Following such an analysis, the stroke volume of the left ventricle is determined, and the relative change, and thus the ejection fraction (the proportion of blood ejected by the heart with each heartbeat relative to the total volume of the heart), is calculated from the measurements taken during systole and diastole.

[0029] This approach allows for the acquisition of additional diagnostic information via CT, particularly regarding myocardial wall thickness and the dynamic behavior of the myocardium during the transition between diastole and systole, without the need for conventional acquisition methods involving increased radiation exposure. The contrast between the myocardium and the ventricles, especially the left ventricle, is significantly improved in the systole-generated image data, enabling assessments of wall thickness and the dynamic behavior of the myocardium during cardiac activity. Furthermore, the contrast in these processed systolic image data is sufficient to also provide information about the extracellular volume of the heart and any associated fibrotic changes.In particular, cardiac deformation analysis can be performed with exceptional precision, enabling the early detection of cardiac damage, such as that which can occur during chemotherapy, allowing for timely intervention. It should be explicitly summarized here that the first monoenergetic image data is preferably used for angiography to examine the heart's vessels. A combination of the third and fourth monoenergetic image data is preferably used to investigate the heart's dynamic behavior, and the fourth monoenergetic image data is used for ECV (extracorporeal circulation) examination. The multiple use of image data for different examination types advantageously reduces the overall imaging time, thereby also reducing the radiation dose for the patient.

[0030] The image generation device according to the invention has an input interface configured to receive first spectrally differentiated CT projection measurement data. The first spectrally differentiated CT projection measurement data relates to a patient's heart in a first time interval, wherein a contrast agent is located mainly in the chambers and vessels of the heart during the first time interval, the first time interval comprising diastole of the heart. The input interface is further configured to receive second spectrally differentiated CT projection measurement data. The second spectrally differentiated CT projection measurement data relates to the patient's heart in a subsequent second time interval, wherein the contrast agent is located mainly in the muscular tissue of the heart during the second time interval, the second time interval comprising systole of the heart.

[0031] Part of the image generation device according to the invention is also a reconstruction unit, which is configured to calculate first monoenergetic image data based on the first spectrally differentiated CT projection measurement data for a first energy value. The reconstruction unit is also configured to calculate second monoenergetic image data based on the second spectrally differentiated CT projection measurement data for the first energy value.

[0032] The image generation device according to the invention also comprises an adjustment unit for determining a second energy value, for which third monoenergetic image data are reconstructed, and a third energy value different from the second energy value, for which fourth monoenergetic image data are reconstructed, such that an adjustment of a contrast agent-induced attenuation in the third monoenergetic image data and the fourth monoenergetic image data is made in comparison to a contrast agent-induced attenuation in the first monoenergetic image data and the second monoenergetic image data.

[0033] The reconstruction unit of the image generation device according to the invention is further configured to calculate the third monoenergetic image data for the determined second energy value based on the first spectrally differentiated CT projection measurement data and to calculate the fourth monoenergetic image data for the determined third energy value based on the second spectrally differentiated CT projection measurement data.

[0034] The image generation device according to the invention shares the advantages of the inventive method for spectrally differentiated cardiac CT imaging.

[0035] The computed tomography system according to the invention comprises a scan unit with a photon-counting X-ray detector and a control unit for controlling the scan unit and for evaluating the raw data or projection measurement data generated by the scan unit. For this purpose, the control unit also includes an image generation unit according to the invention. The computed tomography system according to the invention shares the advantages of the image generation unit according to the invention.

[0036] A large proportion of the aforementioned components of the image data generation device according to the invention can be implemented wholly or partially as software modules in a processor of a corresponding computer system, e.g., a control unit of a computed tomography system or a computer used to control such a system. A largely software-based implementation has the advantage that even previously used computer systems can be easily retrofitted by a software update to operate in the manner of the invention. In this respect, the problem is also solved by a corresponding computer program product with a computer program that can be directly loaded into a computer system, containing program sections to execute the steps of the inventive method for spectrally differentiated cardiac CT imaging when the program is run in the computer system.In addition to the computer program itself, such a computer program product may include additional components such as documentation and / or additional components, including hardware components such as hardware keys (dongles, etc.) for using the software.

[0037] For transport to the computer system or control unit and / or for storage on or in the computer system or control unit, a computer-readable medium, such as a memory stick, a hard drive, or other portable or permanently installed data carrier, can be used, on which the program sections of the computer program that can be read and executed by a computer system are stored. The computer system may, for example, have one or more cooperating microprocessors or similar components for this purpose.

[0038] The dependent claims and the subsequent description each contain particularly advantageous embodiments and further developments of the invention. In particular, the claims of one claim category may also be further developed analogously to the dependent claims of another claim category. Furthermore, within the scope of the invention, the various features of different embodiments and claims may also be combined to form new embodiments.

[0039] In a preferred embodiment of the inventive method for spectrally differentiated cardiac CT imaging, the step of determining a second energy value and a third energy value comprises the following sub-steps: First, an initial attenuation value is determined in a representative sub-region of the first monoenergetic image data. A "representative sub-region" is defined as a portion of the examination area, particularly the heart, that exhibits the highest possible concentration of contrast agent during the acquisition of the projection measurement data. Advantageous sub-regions preferably include the aorta or the left ventricle, where blood flow is particularly high. The initial attenuation value is preferably determined as the average attenuation value within the representative sub-region of the first monoenergetic image data. Alternatively, the initial attenuation value can also be determined as the maximum or minimum attenuation value within the representative sub-region, or as a value at a predetermined position within the representative sub-region.

[0040] Preferably, the representative sub-area is selected in such a way that the largest possible homogeneous area of ​​iodine contrast agent is obtained, and the large vessel of the aorta or the left ventricle of the heart have proven to be particularly advantageous for measurement in anatomical terms.

[0041] Furthermore, a second value for contrast agent-induced attenuation in the representative sub-area of ​​the second monoenergetic image data set is also determined. This second value is preferably determined as the average attenuation value in the representative sub-area of ​​the second monoenergetic image data set. Alternatively, the second value can also be determined as a value at the same position in the representative sub-area of ​​the second monoenergetic image data set as in the representative sub-area of ​​the first monoenergetic image data set. Alternatively, the second value can also be determined as the maximum or minimum attenuation value in the representative sub-area, or as a value at a predetermined position in the representative sub-area of ​​the second monoenergetic image data set.

[0042] In particular, the second energy value can be determined based on the first attenuation value, and the third energy value can be determined based on the second attenuation value. The second and third energy values ​​are determined in such a way that the difference between contrast agent-induced attenuation in the representative sub-area in the third monoenergetic image data and the fourth monoenergetic image data is reduced compared to the difference between the first and second attenuation values. The contrast agent-induced attenuation in the representative sub-area in the third and fourth monoenergetic image data is determined analogously to the determination of the first and second attenuation values.Preferably, the determination of the contrast agent-induced attenuation in the representative sub-area of ​​the third and fourth monoenergetic image data is carried out using exactly the same method as the determination of the first and second attenuation values. Advantageously, reference values ​​on which the adjustment of the contrasts in the third and fourth monoenergetic image data is based can be determined with particular accuracy and representativeness.

[0043] In a preferred embodiment of the inventive method for spectrally differentiated cardiac CT imaging, the second and third energy values ​​are determined such that the difference between contrast agent-induced attenuation in the third monoenergetic image data and contrast agent-induced attenuation in the fourth monoenergetic image data is minimized. Advantageously, in this embodiment, the image data acquired during diastole and the image data acquired during systole are maximally aligned with each other with respect to their contrast and attenuation values, respectively. Advantageously, wall structures during cardiac systole can be rendered particularly clearly, since the contrast in the fourth monoenergetic image data is especially enhanced by this alignment.Furthermore, the third and fourth monoenergetic image data can be combined particularly well due to the maximum alignment, in order to represent the dynamic behavior of the heart muscle in a particularly realistic way.

[0044] When adjusting the contrast of the first and second monoenergetic image data, the second and third energy values ​​are preferably selected based on a physics table that specifies a functional relationship between a contrast agent concentration, an energy value, and an attenuation value. For the specific material, this adjustment would, for example, be based on the mass attenuation coefficient for X-rays. Such data is available, for instance, from the National Institute of Standards and Technology. Advantageously, the second and third energy values ​​to be selected for contrast adjustment can be easily retrieved from a database without having to perform complex calculations again.

[0045] The contrast agent used for contrast-enhanced imaging preferably contains iodine. Iodine is particularly suitable for visualizing blood, which, upon influx into the heart, initially makes the vessels and heart chambers visible and subsequently, at a later stage of influx, after approximately 3 to 5 minutes, diffuses into the myocardium and, when using the method according to the invention, can be used to contrast the heart walls.

[0046] In the inventive method for spectrally differentiated cardiac CT imaging, spectrally resolved CT projection measurement data are preferably acquired as spectrally differentiated CT projection measurement data. Due to their spectral information, such projection measurement data are particularly suitable for calculating monoenergetic image data with different energy values.

[0047] Such spectrally resolved CT projection measurement data are particularly well-suited for acquisition using a photon-counting X-ray detector. With such a detector, X-rays can be decomposed into their individual spectral components. This spectrally decomposed projection measurement data is especially well-suited for generating monoenergetic image data.

[0048] When reconstructing the third and fourth monoenergetic image data, the following reconstruction parameters are preferably synchronized and their values ​​are chosen to be identical: - the layer thickness of the image data, - one dimension of a reconstruction kernel for filtered backprojection, - the dimension of a field of view.

[0049] Image data taken at different times can be easily combined due to their identical structure and dimensions.

[0050] Preferably, the reconstruction of the third and fourth monoenergetic image data is performed directly within the CT system. Advantageously, all other parameters that might lead to systematic deviations in the different image data are also synchronized as part of the reconstruction.

[0051] Alternatively, the reconstruction of the third and fourth monoenergetic image data is initially performed using different reconstruction parameter values. During post-processing, common image parameter values ​​for the monoenergetic image data are determined based on the maximum and / or minimum of the reconstruction parameter values ​​of the third and / or fourth monoenergetic image data. For this, access to a spectrally resolved data format, such as SPP, is required, which allows the generation of images with different energy levels (keV levels). Harmonization of the image parameter values ​​can then be achieved during post-processing based on the extreme values ​​of the different reconstruction parameter values, enabling the distortion-free combination of the third and fourth monoenergetic image data.

[0052] Preferably, the adjusted reconstruction parameters include a slice thickness and / or a dimension of a reconstruction kernel used to reconstruct the image data. While the maximum slice thickness (e.g., max (0.4 mm, 1 mm)) is preferably used as the adjusted reconstruction parameter for the different image data, the minimum dimension of the reconstruction kernel (e.g., min (Bv44, Qr36)) is preferably used for adjusting the dimensions (Bv stands for "Body vascular," Qr stands for "Quantitative regular," which are kernels for different body regions and components). In this way, the image dimensions and sharpness of the different image data are adjusted. A filtering process makes the "sharper" image "less sharp" or "softer."The number assigned to a kernel represents the achieved spatial resolution in the image and has a one-to-one relationship to the modulation transfer function (MFT) value at 60 percent of that kernel. The MFT value provides information regarding the relationship between the detail contrast at the edges of an object and the detail contrast of its image representation. Minimizing this value, when comparing two images of different sharpness, "softens" the sharper image by filtering, so that the sharper image reconstructed with the Bv44 kernel is made to resemble a softer or less sharp image reconstructed with the QR36 kernel.

[0053] The common image parameter values ​​of the post-processed third and fourth monoenergetic image data are preferably determined by applying a low-pass filter in the z-direction or in the slice direction (in-plane) to the first and / or second monoenergetic image data and / or to the third and / or fourth monoenergetic image data. The low-pass filter allows the image textures to be matched by suppressing higher-frequency image changes. The z-direction is understood to be the rotational symmetry axis of a computed tomography system, as is customary. Slices are typically oriented perpendicular to the patient's longitudinal axis.

[0054] In a preferred embodiment of the image generation device according to the invention, the image generation device according to the invention comprises an image evaluation unit for determining a first attenuation value in a representative sub-area of ​​the first monoenergetic image data and for determining a second attenuation value in the same representative sub-area in the second monoenergetic image data.

[0055] In this configuration, the adjustment unit is set up to determine the second energy value based on the contrast agent-induced attenuation in a representative sub-area of ​​the first monoenergetic image data, and to determine the third energy value based on the contrast agent-induced attenuation in a representative sub-area of ​​the second monoenergetic image data. The adjustment unit is configured such that the second and third energy values ​​are selected in such a way as to reduce the difference between the contrast agent-induced attenuation in the representative sub-area of ​​the third monoenergetic image data and the fourth monoenergetic image data compared to the difference between the first and second attenuation values.The representative sub-area particularly clearly reflects the contrast differences in heart formation between diastole and systole, so that with a suitable choice of a representative sub-area, a particularly precise matching of the contrasts in the third and fourth monoenergetic image data is achieved.

[0056] The following section explains individual features of the invention by way of examples, with reference to the accompanying figures. These show: Fig. 1 a schematic representation of a computed tomography system, Fig. 2 a flowchart illustrating a method for spectrally differentiated cardiac CT imaging, Fig. 3 a schematic representation of an image generation device, Fig. 4. Contrast-enhanced imaging of the heart during diastole with contrast medium in the left ventricle and blood vessels, Fig. 5. Contrast-enhanced imaging of the heart during systole with contrast medium in the muscle tissue. Fig. 6. A contrast-enhanced image of the heart in the stage of diastole, adapted by the method for spectrally differentiated cardiac CT imaging, with contrast medium in the left ventricle and the blood vessels. Fig. 7. A contrast-enhanced image of the heart in the stage of systole with contrast medium in the muscle tissue, adapted by the method for spectrally differentiated cardiac CT imaging.

[0057] In Fig. Figure 1 illustrates a schematic representation of a computed tomography system. Fig. The computed tomography system 1 shown here comprises an X-ray array with an X-ray source 3, an X-ray detector 4, and a control unit 5. The X-ray source 3 and the X-ray detector 4 are connected to the control unit 5. The X-ray source 3 and the X-ray detector 4 are movable and arranged diametrically opposite each other on a circular path 6. They are thus in a fixed position relative to each other, in which the X-ray detector 4 detects the radiation emitted by the X-ray source 3, and therefore form a first source-X-ray detector array. A patient 2 is located at the center of the circular path 6 as the subject of the examination. The X-ray source 3 comprises an X-ray tube 7 and an aperture 8. The aperture 8 is arranged slightly spaced from the X-ray tube 7 on the side of the X-ray tube 7 facing the patient 2.It allows the emission angle of X-ray radiation 10, which is emitted by the X-ray tube 7 during operation, to be adjusted.

[0058] During operation, the X-ray source 3 and the X-ray detector 4 are rotated around the patient 2 on a circular path 6 to acquire projection measurement data. The acquired projection measurement data can then be transmitted to an evaluation unit, located, for example, in the control unit 5, where it is reconstructed into an image of the patient 2. To acquire projection measurement data from other areas of the patient 2, the patient 2 can be moved relative to the computed tomography system 1, for example, by means of a positionable patient table (not shown here), perpendicular to the plane of the circular path 6. In so-called spiral CT, the acquisition takes place continuously with a continuous table movement. The acquired projection measurement data are then separated into their spectral components for reconstruction, and a reconstruction based on the spectral resolution is performed.The reconstruction takes place in a control unit (not shown), which is, for example, either directly part of the computed tomography system 1 or is implemented as software in a downstream computer. In particular, the control unit 5 comprises a [function / process / etc.]. Fig. Image generation unit 30 shown in detail. The image data generated by the image generation unit 30 can be further processed in a computer (not shown) electrically connected to the computed tomography system 1 and displayed on a computer screen (not shown). A contrast agent can also be administered to the patient via an injection device (not shown) into the cardiac region in order to visualize both vascular regions and the heart muscle itself at intervals of approximately 3 to 5 minutes.

[0059] In Fig. Figure 2 shows a flowchart illustrating a method for spectrally differentiated cardiac CT imaging.

[0060] In step 2.I, after the influx of a contrast agent, particularly iodine, into the cardiac region, and especially into the chambers and vessels of the patient's heart, contrast-enhanced initial spectrally resolved CT projection measurement data (PMD1) of the patient's heart are acquired during a first time interval (I1). During this first time interval (I1), the contrast agent is primarily located in the chambers and vessels of the heart, and this interval encompasses the heart's diastole. This means the heart is maximally relaxed, and the ventricles are filling with blood.

[0061] In step 2.II, approximately 3 to 5 minutes after step 2.I, a second spectrally resolved CT projection measurement data (PMD2) is acquired from the patient's heart in a later time interval (I2), during which the contrast agent is primarily located in the heart muscle tissue. This second time interval (I2) encompasses a systole of the heart.

[0062] In step 2.III, the first monoenergetic image data BD1 are calculated based on the first spectrally differentiated CT projection measurement data PMD1 for a first energy value E1. This first energy value is chosen so that the brightness of the contrast-enhanced areas in the first monoenergetic image data BD1 is particularly high, making this image data BD1 especially suitable for angiographic imaging.

[0063] Furthermore, in step 2.IV, second monoenergetic image data (BD2) are reconstructed or calculated based on the second spectrally differentiated CT projection measurement data (PMD2) for the first energy value (E1). As a rule, the contrast in the second monoenergetic image data (BD2) is much weaker than in the first monoenergetic image data (BD1), since relatively little contrast agent remains in the heart at the time of acquisition of the second projection measurement data (PMD2). In particular, the contrast in the second monoenergetic image data (BD2) is usually so weak that the wall structures of the heart cannot be reliably and accurately identified in the second monoenergetic image data (BD2), or they cannot be sufficiently distinguished from the ventricular region.

[0064] To equalize the contrasts between the first monoenergetic image data BD1 and the second monoenergetic image data BD2, an initial attenuation value EH1 is determined in a representative sub-region RTB of the first monoenergetic image data BD1 during step 2.V. A central area of ​​the left ventricle, which exhibits particularly high brightness values ​​or attenuation values ​​during diastole, can be used as a representative sub-region RTB in the first monoenergetic image data BD1.

[0065] Furthermore, in step 2.VI, a second attenuation value EH2 is determined in the representative sub-area RTB, i.e., at the same location where the first attenuation value EH1 of a contrast agent-induced attenuation was determined in the first monoenergetic image data BD1, in the second monoenergetic image data BD2. The second attenuation value (EH2) is typically significantly lower than the first attenuation value (EH1) because the contrast agent has largely moved out of the heart during the second time interval (I2). Only the muscle tissue retains contrast agent, but at a much lower concentration than the amount present in the heart chambers during the first time interval (I1).

[0066] In order to achieve an alignment of the contrasts of the image data BD1, BD2 acquired at different times, a second energy value E2 is determined in step 2.VII based on the first attenuation value EH1, and third monoenergetic image data BD3 are reconstructed for the second energy value E2 based on the first spectrally differentiated CT projection measurement data PMD1.

[0067] In step 2.VIII, a third energy value E3 is determined based on the second attenuation value EH2, and fourth monoenergetic image data BD4 are reconstructed for the third energy value E3 based on the second spectrally differentiated CT projection measurement data PMD2. The second energy value E2 and the third energy value E3 are selected such that the difference between contrast agent-induced attenuation in the respective representative sub-regions of the third and fourth monoenergetic image data BD3 and BD4 is reduced.

[0068] In Fig. Figure 3 shows a schematic representation of an image generation device 30.

[0069] The image generation unit 30 has an input interface 31. The input interface 31 is configured to receive initial spectrally resolved CT projection measurement data PMD1. The spectrally resolved CT projection measurement data PMD1 were acquired with contrast enhancement from a patient's heart during a first time interval I1. This first time interval I1 was selected such that during the first time interval I1 the contrast agent is mainly located in the chambers and vessels of the heart and that the first time interval I1 includes one diastole of the heart.

[0070] Input interface 31 is also configured to receive a second spectrally differentiated CT projection measurement data set, PMD2. This second spectrally differentiated CT projection measurement data set, PMD2, pertains to the patient's heart during a later time interval, I2, in which the contrast agent is primarily located within the heart muscle tissue. The second time interval, I2, is typically selected to be approximately 3 to 5 minutes after the first time interval. The second time interval, I2, encompasses a systole of the heart. During systole, there is little blood, and therefore little contrast agent, in the heart chambers, resulting in reduced contrast in cardiac imaging at this time.

[0071] Part of the image generation unit 30 is also a reconstruction unit 32, which is set up to calculate first monoenergetic image data BD1 based on the first spectrally differentiated CT projection measurement data PMD1 for a first energy value E1 and to calculate second monoenergetic image data BD2 based on the second spectrally differentiated CT projection measurement data for the first energy value E1.

[0072] The image generation unit 30 also includes an image evaluation unit 33, which is configured to determine a first attenuation value EH1 in a representative sub-area RTB of the first monoenergetic image data BD1 and to determine a second attenuation value EH2 in the representative sub-area RTB of the second monoenergetic image data BD2. The representative sub-area RTB is selected such that a particularly high concentration of contrast agent is present there during the diastolic phase in order to obtain the most meaningful difference possible between the first attenuation value EH1 and the second attenuation value EH2. As a rule, the representative sub-area RTB is selected in such a way that the largest possible homogeneous area of ​​iodine contrast agent is obtained, and the large vessel of the aorta or the left ventricle of the heart have proven to be anatomically advantageous for such a measurement.

[0073] The image generation unit 30 also includes an adjustment unit 34. The adjustment unit 34 is configured to determine a second energy value E2 and a third energy value E3 based on the first attenuation value EH1 and the second attenuation value EH2. For the second energy value E2, third monoenergetic image data BD3 are reconstructed based on the first spectrally differentiated CT projection measurement data PMD1. For the third energy value E3, fourth monoenergetic image data BD4 are reconstructed based on the second spectrally differentiated CT projection measurement data PMD2. The second energy value E2 and the third energy value E3 are selected such that any difference in contrast agent-induced attenuation in the respective representative sub-regions of the third and fourth monoenergetic image data BD3 and BD4 is reduced. Fig. The image generation device 30 shown in Figure 3 also includes an output interface 35, with which the third and fourth monoenergetic image data BD3, BD4, which are matched with respect to their contrast behavior, are output.

[0074] In Fig. Figure 4 illustrates a contrast-enhanced image of the heart (H) during diastole with contrast medium in the left ventricle (LV) and blood vessels. A representative circular area (RTB) is shown in the left ventricle (LV), in which an average attenuation value is determined. Furthermore, in the Fig. In image 40, the heart wall (HW) is clearly visible, appearing dark against the bright interior of the left ventricle (LV). At this early stage after the influx of contrast medium, the contrast medium is still located in the ventricles and vessels, which are in Fig. 4 are shown in bright colors. The actual heart muscle or heart wall, however, has not yet been penetrated by the contrast medium and is in Fig. 4 is therefore shown in dark. In the nomenclature used above, in Fig. Four initial monoenergetic image data (BD1) are shown, reconstructed with a first energy value E1. Such a first energy value E1 can be, for example, 70 keV.

[0075] In Fig. Figure 5 shows a contrast-enhanced imaging 50 of the heart H during systole with contrast medium in the muscle tissue. In the Fig. In stage 5, the left ventricle (LV) is shown in gray because the contrast agent has distributed throughout the muscle by this stage. The circular representative area RTB is also now colored dark gray. The heart walls are in Fig. 5 are no longer recognizable, as they no longer stand out from the left ventricle (LV). In the nomenclature used above, in Fig. 5 second monoenergetic image data BD2 shown, which were reconstructed with the first energy value E1, which, as already mentioned, can be, for example, 70 keV.

[0076] To make the images of diastole and systole comparable, and in particular to differentiate the heart walls from the left ventricle (LV) during systole, the energy values ​​E2 and E3 for generating the third and fourth monoenergetic image data (BD3 and BD4) should be selected such that the contrasts in the diastolic and systolic images are comparable. For this purpose, the energy values ​​E2 and E3 must be significantly different, since the contrast agent concentrations vary considerably between diastole and systole.

[0077] In Fig. Figure 6 illustrates a contrast-enhanced image of the heart during diastole, using the procedure to match the contrast agent in the left ventricle and blood vessels. Fig. Figure 6 shows third monoenergetic image data BD3, which were reconstructed for a second energy value E2. For example, 85 keV can be used as the second energy value. This places the second energy value further away from the iodine absorption edge than a value of 70 keV, thus attenuating the contrast-induced contrasts. As in Fig. As can be seen in section 6, the contrasts are in Fig. 6. The image data designated as third monoenergetic image data BD3 in the preceding nomenclature are indeed also comparable to those in Fig. The first image data shown in 4 shows BD1 somewhat weakened, but still strong enough for the heart walls HW to stand out sufficiently from the left ventricle LV of the heart H. The in Fig. The circular representative section RTB shown in Figure 6 is now colored grey instead of white.

[0078] In Fig. Figure 7 illustrates a contrast-enhanced image of the heart H during systole, using contrast medium in the muscle tissue, as adapted by the procedure. Fig. In the nomenclature used above, image data BD3, designated as the fourth monoenergetic image data, are represented as image data reconstructed with a third energy value E3. For example, a value of 45 keV can be used as the third energy value E3. This value is sufficiently close to the absorption edge of iodine so that the contrast in Fig. 7 compared to the Fig. 5, where the energy value E1 was 70 keV, is significantly enhanced. This is because the contrasts are significantly greater compared to those in Fig. In the second monoenergetic image data shown in 5, BD2 is clearly enhanced, and the heart walls HW stand out sufficiently from the left ventricle LV of the heart H to allow segmentation of the heart walls. The in Fig. The circular representative sub-area RTB shown in Figure 7 is now colored light gray instead of dark gray and stands out at least somewhat from the heart walls HW. Furthermore, the contrasts in Fig. 6 and Fig. 7 compared to Fig. 4 and Fig. 5 aligned with each other.

[0079] Finally, it should be noted once again that the methods and devices described above are merely preferred embodiments and that the invention can be varied by a person skilled in the art without departing from the scope of the invention, insofar as it is defined by the claims. For the sake of completeness, it should also be noted that the use of the indefinite articles "a" or "an" does not preclude the possibility that the features in question may be present multiple times. Likewise, the term "unit" does not preclude the possibility that it consists of several components, which may also be spatially distributed. Regardless of the grammatical gender of a particular term, persons of male, female, or other gender identities are included.

Claims

[1] Computer-implemented method for spectrally differentiated cardiac CT imaging, comprising the steps: - Receiving first spectrally differentiated CT projection measurement data (PMD1) relating to a heart (H) of a patient (2) in a first time interval (I1), wherein a contrast agent is mainly located in the chambers and / or vessels of the heart (H) during the first time interval (I1), wherein the first time interval (I1) comprises a diastole of the heart (H), - Receiving second spectrally differentiated CT projection measurement data (PMD2) relating to the patient's heart (H) (2) at a later second time interval (I2), wherein the contrast agent is mainly located in a muscular tissue of the heart (H) during the second time interval (I2), wherein the second time interval (I2) includes a systole of the heart (H), - Calculation of first monoenergetic image data (BD1) for a first energy value (E1) based on the first spectrally differentiated CT projection measurement data (PMD1), - Calculation of second monoenergetic image data (BD2) for the first energy value (E1) based on the second spectrally differentiated CT projection measurement data (PMD2), - Determining a second energy value (E2) and a third energy value (E3) different from the second energy value (E2), - Calculation of third monoenergetic image data (BD3) for the second energy value (E2) based on the first spectrally differentiated CT projection measurement data (PMD1), - Calculation of fourth monoenergetic image data (BD4) for the third energy value (E3) based on the second spectrally differentiated CT projection measurement data (PMD2), - wherein the second energy value (E2) and the third energy value (E3) are determined in such a way that an adjustment of a contrast agent-induced attenuation in the third monoenergetic image data (BD3) and a contrast agent-induced attenuation in the fourth monoenergetic image data (BD4) is made to each other. [2] Method according to claim 1, wherein the step of determining the second energy value (E2) and the third energy value (E3) comprises the following sub-steps: - Determining an initial attenuation value (EH1) which relates to contrast agent-induced attenuation in the first monoenergetic image data (BD1), - Determining a second attenuation value (EH2), which relates to contrast agent-induced attenuation in the second monoenergetic image data (BD2), - Determining the second energy value (E2) based on the first attenuation value (EH1) and / or second attenuation value (EH2), - Determining the third energy value (E3) based on the first attenuation value (EH1) and / or second attenuation value (EH2), - wherein the second energy value (E2) and the third energy value (E3) are determined in such a way that the difference between a contrast agent-induced attenuation in the third monoenergetic image data (BD3) and a contrast agent-induced attenuation in the fourth monoenergetic image data (BD4) is reduced compared to a difference between the first attenuation value (EH1) and the second attenuation value (EH2). [3] Method according to claim 1 or 2, wherein the second energy value (E2) and the third energy value (E3) are determined in such a way as to minimize the difference between a contrast agent-induced attenuation in the third monoenergetic image data (BD3) and a contrast agent-induced attenuation in the fourth monoenergetic image data (BD4). [4] Method according to one of the preceding claims, wherein the determination of the second energy value (E2) and the third energy value (E3) is carried out on the basis of a physics table which specifies a functional relationship between energy values ​​and attenuation values. [5] Method according to one of the preceding claims, wherein the spectrally differentiated CT projection measurement data (PMD1, PMD2) are spectrally resolved CT projection measurement data (PMD1, PMD2). [6] Method according to claim 5, wherein the spectrally resolved CT projection measurement data (PMD1, PMD2) are acquired using a photon-counting X-ray detector. [7] Method according to one of the preceding claims, wherein when calculating the third monoenergetic image data (BD3) and when calculating the fourth monoenergetic image data (BD4), the following reconstruction parameters are synchronized and / or their values ​​are chosen identically: - the layer thickness of the image data (BD3, BD4), - one dimension of a reconstruction kernel for filtered backprojection, - the dimension of a field of vision. [8] Method according to claim 7, wherein the calculation of the third monoenergetic image data (BD3) and the calculation of the fourth monoenergetic image data (BD4) is performed directly in the computed tomography system (1) which is used for the acquisition of the first spectrally differentiated CT projection measurement data (PMD1) and the second spectrally differentiated CT projection measurement data (PMD2). [9] Method according to any one of claims 1 to 6, wherein, in the context of post-processing the third monoenergetic image data (BD3) and the fourth monoenergetic image data (BD4), common image parameter values ​​for the post-processed third and fourth monoenergetic image data (BD3, BD4) are formed on the basis of the maximum and / or minimum values ​​of the reconstruction parameters of the third monoenergetic image data (BD3) and / or the fourth monoenergetic image data (BD4). [10] Method according to claim 9, wherein the reconstruction parameters comprise a layer thickness and / or a dimension of a reconstruction kernel. [11] Method according to claim 9 or 10, wherein the common image parameter values ​​are formed by applying a low-pass filter in the z-direction or in the layer direction as part of the post-processing of the third monoenergetic image data (BD3) and the fourth monoenergetic image data (BD4). [12] Image generation device (30), comprising: - an input interface (31) for receiving first spectrally differentiated CT projection measurement data (PMD1) relating to a patient's heart (H) (2) in a first time interval (I1), wherein a contrast agent is mainly located in the chambers and / or vessels of the heart (H) during the first time interval (I1), the first time interval (I1) comprising a diastole of the heart (H), and for receiving second spectrally differentiated CT projection measurement data (PMD2) relating to the patient's heart (H) (2) in a subsequent second time interval (I2), wherein the contrast agent is mainly located in a muscular tissue of the heart (H) during the second time interval (I2), the second time interval (I2) comprising a systole of the heart (H), - a reconstruction unit (32) for calculating first monoenergetic image data (BD1) for a first energy value (E1) based on the first spectrally differentiated CT projection measurement data (PMD1), for calculating second monoenergetic image data (BD2) for the first energy value (E1) based on the second spectrally differentiated CT projection measurement data (PMD2), for calculating third monoenergetic image data (BD3) for a second energy value (E2) based on the first spectrally differentiated CT projection measurement data (PMD1), and for calculating fourth monoenergetic image data (BD4) for a third energy value (E3) different from the second energy value (E2) based on the second spectrally differentiated CT projection measurement data (PMD2). - an adjustment unit (34) for determining a second energy value (E2) and a third energy value (E3) such that an adjustment of a contrast agent-induced attenuation in the third monoenergetic image data (BD3) and a contrast agent-induced attenuation in the fourth monoenergetic image data (BD4) is made to each other. [13] Computed tomography system (1), comprising: - a scanning unit with a photon-counting X-ray detector (4), - a control device (5) for controlling the scanning unit and for receiving projection measurement data (PMD1, PMD2) from the scanning unit, wherein the control device (5) comprises an image generation device (30) according to claim 12. [14] Computer program product comprising instructions which, when the program is executed by a computer, cause it to perform the steps of the method according to any one of claims 1 to 11. [15] Computer-readable storage medium comprising instructions which, when executed by a computer, cause it to perform the steps of the method according to claims 1 to 11.

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