Balancing of CT image data for cardiac examination

Abstract

The invention relates to a spectrum distinguishing heart CT (computed tomography) imaging method. In the method, first spectrally differentiated CT projection measurement data relating to the heart of the patient in a first time interval during which a contrast agent, preferably iodine, is located predominantly in the chamber and / or in the blood vessel of the heart is received, the first time interval comprising the diastolic period of the heart. In addition, second spectrally distinguished CT projection measurement data is received which is related to the heart of the patient in a later second time interval during which the contrast agent is located predominantly in the muscular tissue of the heart, where the second time interval comprises the systolic period of the heart. First monoenergetic image data is calculated for a first energy value based on the first spectrally distinguished CT projection measurement data, and second monoenergetic image data is calculated for the first energy value based on the second spectrally distinguished CT projection measurement data. The invention further relates to an image generation device and a computed tomography system.

Description

Technical Field

[0001] The present invention relates to a method for spectrally differentiated cardiac CT imaging, an image generating device, and a computed tomography system. Background Art

[0002] In order to clarify potential risks due to possible coronary heart disease, CT scans of the heart (CT stands for computed tomography) are often used in clinical practice. The most common examinations are image recordings for determining calcium scores without contrast agent and coronary angiography, referred to as "CTA" (CTA stands for "CT angiography").

[0003] Both methods focus primarily on the visualization and examination of coronary vessels and, in doing so, "ignore" the condition of the myocardium, another part of the heart that is also diagnostically relevant. With the introduction of spectral CT systems, the acquisition is expanded with a third scan, called ECV measurement, delayed 3 to 5 minutes relative to the coronary angiography, depending on the clinical question. This third scan allows for a quantitative assessment of the fibrotic portion of the myocardium (ECV measurement, short for "extracellular volume" measurement).

[0004] In direct comparison with MRI-based diagnosis, the only missing aspect of complete cardiac assessment using computed tomography alone is an assessment of myocardial dynamics or changes in myocardial thickness—that is, a quantitative comparison of images recorded during the systolic and diastolic cardiac phases. The systolic cardiac phase characterizes the myocardial state of tension, in which blood is pumped from the ventricles into the pulmonary arteries and aorta, the myocardium contracts, and the heart walls are particularly thick. The diastolic phase characterizes the myocardial state of relaxation, in which the ventricles are completely filled with blood and the heart is maximally dilated. The heart walls are particularly stretched and, therefore, thin.

[0005] Functional statements about the dynamic behavior of the heart can, in principle, be made readily with CT systems. However, to date, coronary angiography has required a very wide electrocardiogram (ECG) window ("EKG" stands for "electrocardiogram") to allow reconstruction during the two characteristic phases of the cardiac cycle, systole and diastole. For example, such a combined CT scan requires a recording duration of approximately 8 seconds, necessitating a high dose. This requires a total of approximately five to six cardiac phases, including images during diastole and images during systole, with diastole alternating with systole. The resulting radiation exposure is undesirably high. Furthermore, a third CT scan must be performed approximately three to five minutes later to perform the ECV measurement.

[0006] Due to the significantly higher radiation dose thus involved, this type of treatment is usually avoided and the dynamics of the heart are correspondingly examined using magnetic resonance tomography. Summary of the Invention

[0007] The object of the present invention is therefore to provide sufficient image data with the aid of CT imaging for examining coronary vessels, examining fibrotic areas of the myocardium, and examining the dynamics of the myocardium, with the radiation exposure being lower than in conventional procedures.

[0008] This object is achieved by the method according to the invention for spectrally differentiated cardiac CT imaging of a patient, the image generation device according to the invention, and the computed tomography system according to the invention.

[0009] With the aid of spectrally differentiated, preferably spectrally resolved, projection measurement data, a purely diastolic acquisition can be combined, for example, with a later systolic acquisition, so that changes in the wall of the patient's heart can be successfully evaluated without significantly increasing the patient's radiation exposure.

[0010] In a method for spectrally differentiated cardiac CT imaging according to the present invention, first spectrally differentiated CT projection measurement data are received, which relate to the heart of a patient in a first time interval, wherein during the first time interval a contrast agent is primarily located in chambers and vessels of the heart, wherein the first time interval includes the diastole of the heart.

[0011] In particular, it can be provided that the first spectrally differentiated CT projection measurement data are acquired in a contrast agent-enhanced manner. "Contrast agent-enhanced acquisition" is to be understood as recording the projection measurement data in the presence of a contrast agent in the corresponding examination region, i.e. in this case in the cardiac region of the patient. Spectrally differentiated CT projection measurement data are to be understood as projection measurement data that have been detected using different spectra or in a spectrally resolved manner. A photon-counting, spectrally resolved X-ray detector is preferably used to generate such projection measurement data. Alternatively, different X-ray spectra can also be generated from one or more X-ray sources, correspondingly separated in time or space. Typical systems are so-called dual-energy CT systems or split-filter CT systems.

[0012] In the method according to the present invention, 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 primarily located in the heart muscle tissue during the second time interval. The second time interval includes the cardiac systole. In particular, it can be provided that the second spectrally differentiated CT projection measurement data is acquired using contrast agent enhancement.

[0013] The second time interval is preferably approximately 3 to 5 minutes after the first time interval. After this time, the contrast agent has moved further from large blood-filled structures, i.e., large blood vessels and ventricles, into the muscle tissue, so that an ECV measurement can now be performed to examine the tissue state. This measurement can be combined with the contraction event, so that this second measurement, in combination with the first measurement performed during diastole, can also be used to determine the dynamic behavior of the myocardium and to compare the thickness of the myocardium during diastole and systole.

[0014] Subsequently, first monoenergetic image data are calculated for the first energy value based on the first spectrally differentiated CT projection measurement data. The calculation of monoenergetic image data, also known as "pseudo-monoenergetic image data," is described in detail in Alvarez R.E. and Macovski A., "Energy-selective reconstructions in x-ray computed tomography," Phys. Med. Biol. 21, 733-744 (1976).

[0015] For the calculation of monoenergetic image data, it is important that the CT projection measurement data are present in a spectrally differentiated or preferably spectrally resolved manner. This calculation includes numerous corrections, in particular for beam hardening effects, water, and contrast agents, particularly iodine. Furthermore, different spectral information can be calculated in the projection measurement data space, which is then preferably reconstructed using filtered backprojection to obtain the monoenergetic image data. Alternatively, different image data can first be reconstructed based on different spectrally differentiated projection measurement data, and the reconstructed image data, provided with different spectral information, can then be combined to generate the monoenergetic image data.

[0016] Similarly, second monoenergetic image data are calculated for the first energy value based on the second spectrally differentiated CT projection measurement data. The monoenergetic image data corresponds to image data reconstructed based on the projection measurement data, which was generated using only a portion of the detected total X-ray spectrum that includes radiation with a corresponding basic energy value. Calculation of the monoenergetic image data, in particular the first and second monoenergetic image data, includes reconstruction or image data reconstruction based on the corresponding associated projection measurement data.

[0017] Typically, when the heart is expanded, the first and second attenuation values ​​differ due to the intense washout of contrast agent in the heart region during dilation, whereas when the heart is contracting, at the time of contraction the heart is relatively anemic and therefore also lacks contrast agent, and the contrast is therefore also significantly weaker.

[0018] In particular, it can be provided to balance the attenuation values ​​and, therefore, the contrast of the image data assigned to the diastole and the image data assigned to the systole. In this way, the contrast in the image data assigned to the systole is significantly improved. Furthermore, by balancing the contrast of image data acquired at different times and in different cardiac states, the dynamic behavior of the heart can be examined without having to perform additional scans that impose additional radiation exposure.

[0019] Therefore, a second energy value is determined based on the first and / or second attenuation values, for which third monoenergetic image data are reconstructed based on the first spectrally differentiated CT projection measurement data. Because the first monoenergetic image data (to which the associated first projection measurement data were acquired during diastole) have significantly higher attenuation values ​​than the second monoenergetic image data, the second energy value is selected such that these attenuation values ​​are slightly reduced. When using a common contrast agent, such as iodine, this goal is achieved by selecting a second energy value that is higher than the first energy value, wherein both energy values ​​are typically above the energy value of the X-ray threshold of the contrast agent (preferably iodine). In this case, the second energy value, particularly in the case of a contrast agent (iodine), is further away from the X-ray threshold or absorption edge associated with the contrast agent than the first energy value, thereby reducing the effect of the contrast agent.

[0020] Furthermore, a third energy value is determined based on the first and / or second attenuation values, for which fourth monoenergetic image data are reconstructed based on the second spectrally differentiated CT projection measurement data. Because the second monoenergetic image data (to which the associated second projection measurement data were acquired during contraction) have significantly lower attenuation values ​​than the first monoenergetic image information, the third energy value is selected such that these attenuation values ​​in the fourth monoenergetic image data are slightly increased compared to the attenuation values ​​of the second monoenergetic image data. This goal is achieved by selecting a third energy value that is lower than the first energy value. 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, approximately 33 keV in the case of iodine—thus increasing the contrast agent's effectiveness.

[0021] Therefore, the second and third energy values ​​can be determined such that the contrast agent-related attenuation in the third and fourth monoenergetic image data is balanced compared to the contrast agent-related attenuation in the first and second monoenergetic image data.

[0022] Finally, for the determined second energy value, third monoenergetic image data is calculated based on the first spectrally differentiated CT projection measurement data, and for the determined third energy value, fourth monoenergetic image data is calculated based on the second spectrally differentiated CT projection measurement data. Calculation of the third and fourth monoenergetic image data particularly includes reconstruction of the third and fourth monoenergetic image data.

[0023] The second energy value can be determined in particular based on the first monoenergetic image data and / or the second monoenergetic image information. The third energy value can be determined in particular based on the first monoenergetic image data and / or the second monoenergetic image information.

[0024] The contrast agent-related attenuation may particularly relate to a representative sub-range of the heart. In particular, the contrast agent-related attenuation in the third monoenergetic image data and the contrast agent-related attenuation in the fourth monoenergetic image data may be balanced relative to the difference between the contrast agent-related attenuation in the first monoenergetic image data and the contrast agent-related attenuation in the second monoenergetic image data.

[0025] The mutual balancing of the contrast agent-related attenuation in the third mono-energy image data and the contrast agent-related attenuation in the fourth mono-energy image data can in particular be achieved such that the difference between the contrast agent-related attenuation in the third mono-energy image data and the contrast agent-related attenuation in the fourth mono-energy image data is smaller than the difference between the contrast agent-related attenuation in the first mono-energy image data and the contrast agent-related attenuation in the second mono-energy image data.

[0026] This results in an equalization of the attenuation values ​​of the image data recorded at different points in time and in different cardiac states. This objective is achieved because the recording of spectrally differentiated image data allows for contrast equalization through targeted selection of the energy values ​​of the monoenergetic image data. This method balances the contrast of image data characterized by very different intrinsic contrasts due to the different points in time of acquisition of the associated projection measurement data and the associated natural physiological timing of cardiac function. This ensures that the acquired monoenergetic image data can be combined in appropriate further processing without being subject to systematic errors caused by contrast differences.

[0027] By achieving similar contrast in the different reconstructions, the conventional threshold-based segmentation of the heart's chambers and myocardium can be performed without systematic errors. Within the scope of this evaluation, the stroke volume of the left ventricle is subsequently determined, and the relative changes are determined from the measurements in systole and diastole, thereby determining the ejection fraction (i.e., the proportion of blood ejected by the heart during a heartbeat, relative to the total volume of the heart).

[0028] This processing method allows for the acquisition of additional diagnostic information based on CT, particularly information on myocardial wall thickness and its dynamic behavior during the transition between diastole and systole, without having to employ conventional acquisitions with a higher radiation load. Advantageously, the contrast between the myocardium and the ventricles, particularly the left ventricle, in the image data generated during systole is significantly improved, allowing for the analysis of myocardial wall thickness and dynamic behavior during cardiac activity. Furthermore, the contrast in the resulting systolic image data is sufficient to allow for the analysis of the extracellular volume of the heart and, therefore, any associated changes in cardiac fibrosis. In particular, particularly accurate cardiac deformation analysis can be performed, enabling the early identification of cardiac lesions, such as those that may occur during chemotherapy, and their early remediation. It should be clearly stated that the first monoenergetic image data is preferably used for angiographic examinations to identify cardiac vessels. The combination of the third and fourth monoenergetic image data is preferably used to examine the dynamic behavior of the heart, with the fourth monoenergetic image data being used for ECV examinations. Advantageously, the multiple use of image data for different examination types reduces the overall imaging time, thereby also reducing the patient's radiation dose.

[0029] The image generation device according to the present invention has an input interface designed to receive first spectrally differentiated CT projection measurement data. The first spectrally differentiated CT projection measurement data relate to the patient's heart in a first time interval, wherein the contrast agent is primarily located in the heart's chambers and blood vessels during the first time interval, wherein the first time interval includes the heart's diastole. The input interface is further designed to receive second spectrally differentiated CT projection measurement data. The second spectrally differentiated CT projection measurement data relate to the patient's heart in a second, later time interval, wherein the contrast agent is primarily located in the heart's muscle tissue during the second time interval, wherein the second time interval includes the heart's systole.

[0030] Part of the image generation device according to the present invention is also a reconstruction unit, which is designed to calculate first monoenergetic image data for a first energy value based on the first spectrally differentiated CT projection measurement data. The reconstruction unit is also designed to calculate second monoenergetic image data for the first energy value based on the second spectrally differentiated CT projection measurement data.

[0031] The image generating device according to the present invention also includes a balancing unit for obtaining the second energy value and the third energy value, reconstructing the third monoenergetic image data for the second energy value, and reconstructing the fourth monoenergetic image data for the third energy value, so that the balance of the attenuation related to the contrast agent in the third monoenergetic data and the fourth monoenergetic data is achieved compared with the attenuation related to the contrast agent in the first monoenergetic image data and the second monoenergetic image data.

[0032] The reconstruction unit of the image generating device according to the present invention is also designed to calculate third monoenergetic image data for the obtained second energy value based on the first spectrally differentiated CT projection measurement data, and to calculate fourth monoenergetic image data for the obtained third energy value based on the second spectrally differentiated CT projection measurement data.

[0033] The image generation device according to the invention has the advantages of the method according to the invention for spectrally differentiated cardiac CT imaging.

[0034] The computed tomography system according to the present invention comprises a scanning unit with a photon-counting X-ray detector and a control device for operating the scanning unit and evaluating the raw data or projection measurement data generated by the scanning unit. For this purpose, the control device also includes an image generation device according to the present invention. The computed tomography system according to the present invention has the advantages of the image generation device according to the present invention.

[0035] Most of the aforementioned components of the image data generation device according to the present invention can be implemented in whole or in part as software modules in a processor of a corresponding computing system, for example, in the form of a control device for a computed tomography system or a computer for controlling such a system. A primarily software-based implementation has the advantage that even previously used computing systems can be easily retrofitted via a software update to operate in accordance with the present invention. In this regard, the object is also achieved by a corresponding computer program product having a computer program that can be directly downloaded into a computing system and includes program sections for executing the steps of the method according to the present invention for spectrally differentiated cardiac CT imaging when the program is executed in the computing system. In addition to the computer program, such a computer program product may also include additional components, such as documentation and / or additional components, as well as hardware components for the software, such as a hardware key (dongle, etc.).

[0036] A computer-readable medium, such as a memory stick, a hard disk, or other portable or permanently installed data carrier, can be used for transmission to and / or storage on or in a computing system or control device. The program sections of a computer program that can be read and executed by the computing system are stored on the computer-readable medium. For this purpose, the computing system can, for example, have one or more cooperating microprocessors or the like.

[0037] The dependent claims and the following description each contain particularly advantageous embodiments and improvements of the present invention. Claims from one claim category can also be improved, for example, in a similar manner to dependent claims from another claim category. Furthermore, within the scope of the present invention, different embodiments and different features of the claims can also be combined to form new embodiments.

[0038] In a preferred embodiment of the method for spectrally differentiated cardiac CT imaging according to the present invention, the step of determining the second energy value and the third energy value includes the following sub-steps:

[0039] First, a first attenuation value is determined in a representative subrange of the first monoenergetic image data. A "representative subrange" is to be understood as a subrange of the examination region, i.e., in particular of the heart, which has as much contrast agent as possible during the acquisition of the projection measurement data. Advantageous subranges preferably include the aorta or the left ventricle, in which a particularly large amount of blood flows or is present. The first attenuation value is preferably determined as the average value of the attenuation in the representative subrange of the first monoenergetic image data. Alternatively, the first attenuation value can also be determined as the maximum or minimum value of the attenuation in the representative subrange, or as a value at a predetermined position in the representative subrange.

[0040] Preferably, a representative sub-range is selected in order to obtain the largest possible homogeneous surface of the iodinated contrast agent, and the large vessel of the aorta or the left ventricle of the heart has proven to be anatomically advantageous for this purpose.

[0041] In addition, a second value of the attenuation associated with the contrast agent in a representative sub-range of the second mono-energetic image data is also obtained. The second value is preferably obtained as the average value of the attenuation in the representative sub-range of the second mono-energetic image data. Alternatively, the second value can also be obtained as a value at the same position in the representative sub-range of the second mono-energetic image data as in the representative sub-range of the first mono-energetic image data. Further alternatively, the second value can also be obtained as the maximum or minimum value of the attenuation in the representative sub-range, or as a value at a predetermined position in the representative sub-range of the second mono-energetic image data.

[0042] In particular, a second energy value can be determined based on the first attenuation value, and a third energy value can be determined based on the second attenuation value. The second and third energy values ​​are determined so that the difference between the attenuation associated with the contrast agent in a representative subrange of the third and fourth monoenergetic image data is reduced compared to the difference between the first and second attenuation values. The attenuation associated with the contrast agent in a representative subrange of the third and fourth monoenergetic image data is determined similarly to the determination of the first and second attenuation values. Preferably, the attenuation associated with the contrast agent in a representative subrange of the third and fourth monoenergetic image data is determined using exactly the same method as that used to determine the first and second attenuation values. Advantageously, the reference variables based on which the contrast in the third and fourth monoenergetic image data is balanced can be realized particularly accurately and representatively.

[0043] In a preferred variant of the method for spectrally differentiated cardiac CT imaging according to the present invention, the second and third energy values ​​are determined such that the difference between the contrast agent-related attenuation in the third monoenergetic image data and the contrast agent-related attenuation in the fourth monoenergetic image data is minimized. Advantageously, in this variant, the image data acquired during diastole and the image data acquired during systole are balanced to the greatest extent possible with respect to their contrast or their attenuation values. Advantageously, wall structures can be particularly clearly seen during systole, since the contrast in the fourth monoenergetic image data is increased, in particular, by the balancing. Furthermore, due to the maximum balancing, the third and fourth monoenergetic image data can be combined particularly well with one another, enabling a particularly realistic representation of the dynamic behavior of the myocardium.

[0044] When balancing the contrast of the first and second monoenergetic image data, the second and third energy values ​​are preferably selected based on a physical table that describes the functional relationship between contrast agent concentration, energy value, and attenuation value. Specifically for this material, this balancing can be based on, for example, the mass attenuation coefficient of X-ray radiation. This data can be retrieved, for example, from the National Institute of Standards and Technology. Advantageously, the second and third energy values ​​that can be selected to balance the contrast can be easily found in the database, without having to perform complex calculations again.

[0045] The contrast medium used for contrast medium imaging particularly preferably comprises an iodine substance. Iodine is particularly suitable for visualizing blood, which, when flushing the heart, initially makes the blood vessels and ventricles visible and then, in a later flushing phase, approximately 3 to 5 minutes later, diffuses into the myocardium and can be used to contrast the heart wall when using the method according to the invention.

[0046] Preferably, in the method according to the invention for spectrally differentiated cardiac CT imaging, spectrally resolved CT projection measurement data are detected as spectrally differentiated CT projection measurement data. Advantageously, such projection measurement data are particularly suitable for calculating monoenergetic image data having different energy values ​​due to their spectral information.

[0047] Such spectrally resolved CT projection measurement data are particularly preferably detected by a photon-counting X-ray detector. Using such a photon-counting X-ray detector, the X-rays can be decomposed into their individual spectral components. These spectrally resolved projection measurement data are particularly suitable for generating monoenergetic image data.

[0048] In the reconstruction of the third monoenergetic image data and the fourth monoenergetic image data, the following reconstruction parameters are preferably synchronized and their values ​​are selected identically:

[0049] - the slice thickness of the image data,

[0050] - the size of the reconstruction kernel used for the filtered backprojection,

[0051] -The size of the field of view (also called field of view).

[0052] Advantageously, image data recorded at different times can be easily combined with one another due to their identical structure or their identical size.

[0053] Preferably, the reconstruction of the third monoenergetic image data and the fourth monoenergetic image data is performed directly in the CT system. Advantageously, all further parameters (which may lead to systematic deviations in the different image data) are also synchronized as part of the reconstruction.

[0054] Alternatively, the third monoenergetic image data and the fourth monoenergetic image are first reconstructed using different reconstruction parameter values. Within the scope of reprocessing, a common image parameter value for the monoenergetic image data is formed based on the maximum and / or minimum values ​​of the reconstruction parameter values ​​for the third monoenergetic image data and / or the fourth monoenergetic image data. To this end, a spectrally resolved data format, such as SPP, should be accessed, which provides the possibility of generating images with different energy levels (keV levels). Within the scope of reprocessing, the image parameter values ​​can be coordinated based on the extreme values ​​of the different reconstruction parameter values ​​so that the third and fourth monoenergetic image data can be combined with each other without distortion.

[0055] Preferably, the reconstruction parameters that are thus balanced include the slice thickness and / or the size of the reconstruction kernel used to reconstruct the image data. While the maximum value of the reconstruction parameter (e.g., max(0.4 mm, 1 mm)) is preferably used as the reconstruction parameter for balancing image data with different slice thicknesses, the minimum value of the reconstruction parameter (e.g., min(Bv44, Qr36)) is preferably used when balancing the size of the reconstruction kernel (Bv stands for "body vessels" and Qr stands for "quantitative rule," which are kernels for different body regions and body components). In this way, the image size and image sharpness of the different image data are balanced against each other. A "sharper" image becomes "less sharp" or "softer" through filtering. The number assigned to the kernel reflects the spatial resolution achieved in the image and corresponds one-to-one to the modulation transfer function value at 60% of the kernel. The modulation transfer function value provides information about the relationship between the detail contrast at the edge of an object and the detail contrast of the visual representation of the object. With minimization, when comparing two images of different sharpness, the sharper image is "softened" by filtering, thereby balancing the sharper image that has been reconstructed with the Bv44 kernel with the softer or less sharp image reconstructed with the Qr36 kernel.

[0056] The common image parameter values ​​for the reprocessed third and fourth monoenergetic image data are preferably determined by applying a low-pass filter in the z-direction or in the slice direction to the first and / or second monoenergetic image data and / or the third and / or fourth monoenergetic image data. The low-pass filter can compensate for image textures by suppressing high-frequency image variations. As usual, the z-direction is understood to be the axis of rotational symmetry of the computed tomography system. The slices are typically oriented transversely to the longitudinal axis of the patient.

[0057] In a preferred design of the image generating device according to the present invention, the image generating device according to the present invention includes an image evaluation unit for determining a first attenuation value in a representative sub-range of first monoenergetic image data and for determining a second attenuation value in the same representative sub-range of second monoenergetic image data.

[0058] In this embodiment, the balancing unit is configured to determine a second energy value based on the contrast agent-related attenuation in a representative subrange of the first monoenergetic image data, and to determine a third energy value based on the contrast agent-related attenuation in a representative subrange of the second monoenergetic image data. The balancing unit is configured to select the second and third energy values ​​such that the difference between the contrast agent-related attenuation in the representative subranges of the third and fourth monoenergetic image data is reduced compared to the difference between the first and second attenuation values. The representative subranges particularly clearly reflect the contrast differences in cardiac formation between diastole and systole, so that, with appropriate selection of the representative subranges, particularly precise balancing of the contrast in the third and fourth monoenergetic image data is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] In the following, various features of the present invention are explained by way of examples with reference to the accompanying drawings, in which:

[0060] Figure 1 shows a schematic diagram of a computed tomography system,

[0061] Figure 2 shows a flow chart illustrating a method for spectrally differentiated cardiac CT imaging,

[0062] Figure 3 shows a schematic cross-sectional view of an image generating device,

[0063] Figure 4 shows a contrast-enhanced image of a heart in diastole with contrast agent in the left ventricle and blood vessels,

[0064] Figure 5 showing a contrast-enhanced image of a heart in systole with contrast agent in muscle tissue,

[0065] Figure 6 shows a contrast-enhanced image of a heart in diastole with contrast agent in the left ventricle and blood vessels, balanced by a method for spectrally differentiated cardiac CT imaging,

[0066] Figure 7 Shown is a contrast-enhanced image of a heart in systole with contrast agent in the muscle tissue, balanced by a method for spectrally differentiated cardiac CT imaging. DETAILED DESCRIPTION

[0067] Figure 1 A schematic diagram of a computed tomography system 1 is shown. Figure 1 The illustrated computed tomography system 1 includes an X-ray emitter assembly having an X-ray radiation source 3, an X-ray detector 4, and a control device 5. The X-ray radiation source 3 and the X-ray detector 4 are connected to the control device 5. The X-ray radiation source 3 and the X-ray detector 4 are movable and arranged radially relative to each other on a circular path 6. They are therefore in a fixed positional relationship relative to each other, in which the X-ray detector 4 detects radiation emitted by the X-ray radiation source 3 and thus forms a first source-detector assembly. A patient 2 serving as an examination subject is located at the center of the circular path 6. The X-ray radiation source 3 includes an X-ray tube 7 and a partition 8. The partition 8 is arranged slightly spaced apart from the X-ray tube 7 on the side of the X-ray tube 7 facing the patient 2. The partition 8 can be used to adjust the emission angle of the X-ray radiation 10 emitted by the X-ray tube 7 during operation.

[0068] During operation, the X-ray radiation source 3 and the X-ray detector 4 rotate around the patient 2 on a circular path 6 in order to acquire projection measurement data. The detected projection measurement data can then be transmitted to an evaluation unit, for example, located in the control device 5, and reconstructed there into an image of the patient 2. In order to acquire projection measurement data from further 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 is carried out continuously with a likewise continuous table advance. The recorded projection measurement data are then divided into their spectral components for reconstruction, and a reconstruction based on spectral resolution is performed. The reconstruction takes place in a control device (not shown), which is, for example, either directly part of the computed tomography system 1 or is designed as software in a downstream computer. In particular, the control device 5 comprises Figure 330 is shown in detail in FIG. The image data generated by the image generating device 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's heart region via an injection device (not shown) to enable visualization of vascular regions and the myocardium itself at intervals of approximately 3 to 5 minutes.

[0069] Figure 2 A flow chart illustrating a method for spectrally differentiated cardiac CT imaging is shown.

[0070] In step 2.1, after a contrast agent (particularly iodine) has been flushed into the cardiac region and, in particular, into the chambers and vessels of the patient's heart, first spectrally resolved CT projection measurement data PMD1 are acquired from the patient's heart in a contrast-enhanced manner during a first time interval I1. During the first time interval I1, the contrast agent is primarily located in the chambers and vessels of the heart, and the first time interval I2 includes the cardiac diastole. That is, the heart is maximally relaxed and the ventricles are filled with blood.

[0071] In step 2.II, approximately 3 to 5 minutes after step 2.I, second spectrally resolved CT projection measurement data PMD2 are acquired from the patient's heart in a later second time interval I2, during which the contrast agent is primarily located in the heart muscle tissue. The second time interval I2 includes the heart's systolic phase.

[0072] In step 2.III, first monoenergetic image data BD1 are calculated for a first energy value E1 based on the first spectrally differentiated CT projection measurement data PMD1. This first energy value is selected such that the brightness of the areas containing the contrast agent in the first monoenergetic image data BD1 is particularly high, so that this image data BD1 is particularly well suited for angiographic presentations.

[0073] Furthermore, in step 2.IV, second monoenergetic image data BD2 are reconstructed or calculated for the first energy value E1 based on the second spectrally differentiated CT projection measurement data PMD2. Typically, the contrast in the second monoenergetic image data BD2 is now significantly weaker than the contrast in the first monoenergetic image data BD1 because, at the time of acquisition of the second projection measurement data BD2, relatively little contrast agent remained in the heart. In particular, the contrast in the second monoenergetic image data BD2 is typically so weak that the heart's wall structures cannot be reliably and accurately identified in the second monoenergetic image data BD2 or cannot be sufficiently distinguished from the ventricular region.

[0074] To balance the contrast between the first monoenergetic image data BD1 and the second monoenergetic image data BD2, in step 2.V, a first attenuation value EH1 is first determined in a representative subrange RTB of the first monoenergetic image data BD1. The central region of the left ventricle, which has particularly high brightness or attenuation values ​​during diastole, can be used as the representative subrange RTB of the first monoenergetic image data BD1.

[0075] Furthermore, in step 2.VI, a second attenuation value EH2 is determined in a representative subrange RTB in the second monoenergetic image data BD2 , ie at the same location where the first attenuation value EH1 of the contrast agent-related attenuation was determined in the first monoenergetic image data BD1 .

[0076] Typically, the second attenuation value EH2 is significantly lower than the first attenuation value EH1, because the contrast agent has largely been moved out of the heart during the second time interval I2. Contrast agent is still present only in the muscle tissue, but at a much lower concentration than the contrast agent present in the heart chamber during the first time interval I1.

[0077] In order to now achieve a balance of the contrast of the image data BD1, BD2 recorded at different times, in step 2.VII, the second energy value E2 is determined based on the first attenuation value EH1, and the third monoenergetic image data BD3 is reconstructed for the second energy value E2 based on the first spectrally differentiated CT projection measurement data PMD1.

[0078] 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 the contrast agent-related attenuation in the respective representative subranges of the third and fourth monoenergetic image data BD3, BD4 is reduced.

[0079] Figure 3 A schematic diagram of an image generating device 30 is shown.

[0080] Image generation device 30 has an input interface 31. Input interface 31 is configured to receive first spectrally resolved CT projection measurement data PMD1. Spectrally resolved CT projection measurement data PMD1 are acquired from the patient's heart in a contrast-enhanced manner during a first time interval I1. This first time interval I1 is selected such that, during a first time interval I2, the contrast agent is primarily located in the heart's chambers and vessels, and the first time interval I1 includes the heart's diastole.

[0081] The input interface 31 is further configured to receive second spectrally differentiated CT projection measurement data PMD2. The second spectrally differentiated CT projection measurement data PMD2 relate to the patient's heart in a later, second time interval I2, during which the contrast agent is primarily located in the heart muscle tissue. The second time interval I2 is typically selected, for example, so that it is located 3 to 5 minutes after the first time interval. The second time interval I2 includes the heart's systolic phase. During systolic phase, only a small amount of blood, and therefore a small amount of contrast agent, is located in the ventricles, so that the visual representation of the heart at this point in time can be expected to have less contrast.

[0082] Part of the image generating device 30 is also a reconstruction unit 32, which is designed to calculate first monoenergetic image data BD1 for the first energy value E1 based on the first spectrally differentiated CT projection measurement data PMD1, and to calculate second monoenergetic image data BD2 for the first energy value E1 based on the second spectrally differentiated CT projection measurement data.

[0083] The image generating device 30 further comprises an image evaluation unit 33, which is designed to ascertain a first attenuation value EH1 in a representative subrange RTB of the first monoenergetic image data BD1 and to ascertain a second attenuation value EH2 in a representative subrange RTB of the second monoenergetic image data BD2. The representative subrange RTB is selected such that a particularly high amount of contrast agent is present there during the diastolic phase, in order to obtain a difference that is as meaningful as possible between the first attenuation value EH1 and the second attenuation value EH2.

[0084] Typically, the representative sub-range RTB is selected such that the largest possible homogeneous surface of iodinated contrast medium is obtained, and the large vessel of the aorta or the left ventricle of the heart has proven to be anatomically advantageous for measurement for this purpose.

[0085] The image generation device 30 further includes a balancing unit 34. The balancing 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 so as to minimize the difference in contrast agent-related attenuation in the respective representative subranges of the third and fourth monoenergetic image data BD3, BD4. Figure 3 Part of the image generating device 30 shown is also an output interface 35 , with which the third and fourth monoenergetic image data BD3 , BD4 , which are balanced with regard to their contrast behavior, are output.

[0086] Figure 4 A contrast-enhanced image 40 of a heart H in diastole with contrast agent in the left ventricle LV and the blood vessels is shown. A circular representative sub-area RTB is shown in the left ventricle LV, in which the mean attenuation value is determined. Figure 4 The heart wall HW is clearly visible in the image 40 shown, which contrasts sharply with the bright inner area of ​​the left ventricle LV. In the early phase after washout of the contrast agent, the contrast agent is still present in the Figure 4 In contrast, the actual myocardium or heart wall has not yet been penetrated by the contrast agent and is therefore not visible in the image. Figure 4 In the nomenclature used above, Figure 4 The first monoenergetic image data BD1 reconstructed with a first energy value E1 is shown. Such a first energy value E1 may be 70 keV, for example.

[0087] Figure 5 A contrast-enhanced image 50 of a heart H in systole is shown with contrast agent in the muscle tissue. Figure 5 In the phase shown, the left ventricle LV is shown in gray, because in this phase the contrast agent is distributed in the muscle. The circular representative subarea RTB is now also colored dark gray. Figure 5 is no longer visible in the left ventricle because the heart wall is no longer distinguishable from the left ventricle LV. In the nomenclature already used above, Figure 5 Second monoenergetic image data BD2 are shown, which have been reconstructed using a first energy value E1 , which, as already mentioned, can be 70 keV, for example.

[0088] In order to make the images during diastole and systole comparable and, in particular, to distinguish the heart wall from the left ventricle LV even in the representation during systole, the energy values ​​E2, E3 should now be selected to generate the third and fourth monoenergetic image data BD3, BD4 so that the contrast in the diastole and systole images is comparable. To this end, the energy values ​​E2, E3 must be selected significantly differently because the contrast agent concentration varies significantly between diastole and systole.

[0089] Figure 6 A contrast-enhanced image 60 of a heart in diastole with contrast in the left ventricle and blood vessels is shown. Figure 6 The third monoenergetic image data BD3 reconstructed for the second energy value E2 is shown. 85 keV can be used as the second energy value, for example. Therefore, the second energy value is further away from the iodine absorption edge than at a value of 70 keV, thereby attenuating the contrast associated with the contrast agent. Figure 6 As can be seen in Figure 6In the above nomenclature, the contrast of the image data referred to as the third mono-energy image data BD3 is actually the same as Figure 4 The first image data BD1 shown is attenuated in comparison, but is still sufficiently intense so that the heart wall HW and the left ventricle LV of the heart H are sufficiently distinguishable. Figure 6 The circular representative sub-range RTB shown is now no longer colored white, but gray.

[0090] Figure 7 A contrast-enhanced image 70 of a heart H in systole with contrast agent in the muscle tissue is shown. Figure 7 1 shows image data reconstructed using the third energy value E3, which is referred to as fourth energy image data BD3 in the nomenclature used above. A value of 45 keV, for example, can be used as the third energy value E3. This value is close enough to the absorption edge of iodine to be consistent with the energy value E1 of 70 keV. Figure 5 compared to, Figure 7 The contrast in the image is significantly increased. Figure 5 Compared to the second monoenergetic image data BD2 shown, the contrast is significantly increased, so that the heart wall HW is sufficiently distinguished from the left ventricle LV of the heart H, so that the heart wall can be segmented. Figure 7 The circular representative sub-area RTB shown is now no longer colored dark gray, but light gray and is at least slightly distinguished from the heart wall HW. Figure 4 and Figure 5 compared to, Figure 6 and Figure 7 The contrasts in the .

[0091] Finally, it is pointed out again that the above-described methods and devices are only preferred embodiments, and that modifications may be made by those skilled in the art without departing from the scope of the invention, as provided by the claims. For the sake of completeness, it is also pointed out that the use of the indefinite article "a" or "an" does not exclude the possibility that the relevant feature may appear multiple times. The term "unit" does not exclude the possibility that it consists of multiple components, which may also be spatially distributed if necessary. Regardless of the grammatical gender of a particular term, it includes persons with both masculine and feminine identities.

Claims

1. A computer-implemented method for spectrally differentiated cardiac CT imaging, comprising the following 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, preferably iodine, being located predominantly in chambers and / or vessels of the heart (H) during the first time interval (I1), wherein the first time interval (I1) comprises the diastole of the heart (H), - receiving second spectrally differentiated CT projection measurement data (PMD2), the second spectrally differentiated CT projection measurement data relating to the heart (H) of the patient (2) in a second later time interval (I2), wherein the contrast agent is mainly located in the muscle tissue of the heart (H) during the second time interval (I2), wherein the second time interval (I2) includes the systole of the heart (H), - calculating first monoenergetic image data (BD1) for a first energy value (E1) based on the first spectrally differentiated CT projection measurement data (PMD1), - calculating second monoenergetic image data (BD2) for the first energy value (E1) based on the second spectrally differentiated CT projection measurement data (PMD2), - obtaining a second energy value (E2) and a third energy value (E3), - calculating third monoenergetic image data (BD3) for the second energy value (E2) based on the first spectrally differentiated CT projection measurement data (PMD1), - calculating fourth monoenergetic image data (BD4) for said third energy value (E3) based on said second spectrally differentiated CT projection measurement data (PMD2), - wherein the second energy value (E2) and the third energy value (E3) are determined such that the contrast agent-related attenuation in the third monoenergetic image data (BD3) and the contrast agent-related attenuation in the fourth monoenergetic image data (BD4) balance each other.

2. The method according to claim 1, wherein The step of obtaining the second energy value (E2) and the third energy value (E3) comprises the following sub-steps: - determining a first attenuation value (EH1), which relates to the contrast agent-related attenuation in the first monoenergetic image data (BD1), - determining a second attenuation value (EH2), which relates to the contrast agent-related attenuation in the second monoenergetic image data (BD1), - determining the second energy value (E2) based on the first attenuation value (EH1) and / or the second attenuation value (EH2), - determining the third energy value (E3) based on the first attenuation value (EH1) and / or the second attenuation value (EH2), -Wherein, the second energy value (E2) and the third energy value (E3) are determined so that the difference between the contrast agent-related attenuation in the third monoenergetic image data (BD3) and the contrast agent-related attenuation in the fourth monoenergetic image data (BD4) is reduced compared to the difference between the first attenuation value (EH1) and the second attenuation value (EH2).

3. The method according to claim 1 or 2, wherein: The second energy value (E2) and the third energy value (E3) are determined so that the difference between the contrast agent-related attenuation in the third monoenergetic image data (BD3) and the contrast agent-related attenuation in the fourth monoenergetic image data (BD4) is minimized.

4. A method according to any one of the preceding claims, wherein The second energy value (E2) and the third energy value (E3) are determined based on a physical table, which describes the functional relationship between energy values ​​and attenuation values, in particular the relationship with the concentration of the contrast agent.

5. A method according to any 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. The method according to claim 5, wherein: The spectrally resolved CT projection measurement data (PMD1, PMD2) are detected by means of a photon counting X-ray detector.

7. A method according to any 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 selected identically: - Slice thickness of image data (BD3, BD4), - the size of the reconstruction kernel used for the filtered backprojection, - The size of the field of view.

8. The method according to claim 7, wherein: The third monoenergetic image data (BD3) and the fourth monoenergetic image data (BD4) are calculated directly in a computed tomography system (1), which is used to acquire the first spectrally differentiated CT projection measurement data (PMD1) and the second spectrally differentiated CT projection measurement data (PMD2).

9. The method according to any one of claims 1 to 6, wherein Within the scope of reprocessing of the third monoenergetic image data (BD3) and the fourth monoenergetic image data (BD4), common image parameter values ​​of the reprocessed third monoenergetic image data (BD3) and the fourth monoenergetic image data (BD4) are formed based on the maximum value and / or minimum value of the values ​​of the reconstruction parameters of the third monoenergetic image data (BD3) and / or the fourth monoenergetic image data (BD4).

10. The method according to claim 9, wherein: The reconstruction parameters include slice thickness and / or reconstruction kernel size.

11. The method according to claim 9 or 10, wherein: The common image parameter value is formed by applying a low-pass filter in the z-direction or in the slice direction within the scope of post-processing of the third monoenergetic image data (BD3) and the fourth monoenergetic image data (BD4).

12. An image generating device (30), comprising: - an input interface (31) for 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 The contrast agent, preferably iodine, is located predominantly in the chambers and vessels of the heart (H) during the first time interval (I1), wherein the first time interval (I1) includes the diastole of the heart (H), and the input interface is used to receive second spectrally differentiated CT projection measurement data (PMD2), which relate to the heart (H) of the patient (2) in a later second time interval (I2), wherein the contrast agent is located predominantly in the muscle tissue of the heart (H) during the second time interval (I2), wherein the second time interval (I2) includes the 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 the second energy value (E2) based on the first spectrally differentiated CT projection measurement data (PMD1), and for calculating fourth monoenergetic image data (BD4) for the third energy value (E3) based on the second spectrally differentiated CT projection measurement data (PMD2), - A balancing unit (34) for determining the second energy value (E2) and the third energy value (E3) so that the attenuation related to the contrast agent in the third monoenergetic image data (BD3) and the attenuation related to the contrast agent in the fourth monoenergetic image data (BD4) are balanced with each other.

13. A computerized tomography system (1) comprising: - a scanning unit having a photon counting X-ray detector (4), - a control device (5) for controlling the scanning unit and for receiving projection measurement data (PMD1, PMD2) of the scanning unit, wherein: The control device (5) has an image generating device (30) according to claim 12.

14. 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 11.

15. 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 11.

Images