Multimodal acquisition of an augmented sequence of CT parameter maps

Description

[0001] The invention relates to a method for determining a quantitative result CT parameter map in an examination region of an examination subject. Furthermore, the invention relates to a method for acquiring a quantitative result parameter map. Furthermore, the invention relates to a parameter determination device. Furthermore, the invention relates to a multimodal medical imaging system.

[0002] Modern imaging techniques often generate two- or three-dimensional image data that can be used to visualize an object under examination and for other applications. In many cases, these imaging techniques are based on X-ray acquisition, generating so-called projection measurement data. For example, projection measurement data can be acquired using a computed tomography (CT) system.

[0003] In CT imaging with spectrally resolved CT data, generated, for example, by photon-counting detectors, dual-source CT systems, or CT systems that use X-rays with different energy spectra, different CT parameter values, particularly contrast agent concentrations (in the specific example of CT, the iodine content in tissue), can be determined within a given examination area. Such spectral data sets have already proven their suitability as surrogates for determining viability and treatment response in tumor examination and therapy.

[0004] Another form of medical imaging is magnetic resonance imaging, abbreviated MRI. Magnetic resonance imaging can be used to generate high-resolution MRI datasets that provide useful information for a variety of diseases. Like CT, MRI measures the flow of blood through organs, tissues, and blood vessels, or visualizes it using contrast media. A typical examination involves the liver. Images are repeated at different times to obtain information about the blood flow and anatomy of the organ and any pathologies present.

[0005] In order to be able to describe the condition of a tumor in a comparable manner, the images must always be taken in the same blood flow phase, for example the time of maximum blood flow.

[0006] When applied to liver perfusion imaging, multiphase CT typically includes four distinct phases: the native phase, the arterial phase, the parenchymal phase, and the venous phase.

[0007] In the native phase, information about fatty deposits, but also calcifications, bleeding and residues of previous contrast agents can be obtained.

[0008] The contrast agent then reaches the liver via the hepatic artery. During the arterial phase, perfusion abnormalities and hypervascularized tumors become particularly visible.

[0009] During the acquisition phase, colloquially known as the portal venous phase, contrast medium also flows into the liver via the portal vein. Due to the increased contrast of the parenchyma, hypovascular tumors (e.g., metastases) can also be easily detected during this phase.

[0010] In the venous phase, often called the parenchymal phase, the contrast agent is evenly distributed throughout the various compartments. Liver lesions with delayed, persistent contrast agent uptake, such as focal nodular hyperplasia, can become visible during this phase. In even later images, homogeneous contrast agent uptake occurs, which is particularly relevant for assessing the extent of necrosis.

[0011] However, especially with multiphase CT, the optimal time for a measurement is often missed, which impairs the informative value of the image acquisition with regard to the desired parameter, preferably a parameter relating to the maximum contrast agent concentration, which corresponds to maximum blood flow. This problem occurs particularly when multiphase CT images are acquired at only a few time points, for example, four time points, corresponding to the four different perfusion phases.

[0012] To achieve the broadest possible temporal coverage for organ imaging, multiphase CT often uses a non-spectral perfusion CT measurement with higher temporal resolution. Spectral CT in this context refers to a CT image that differentiates between different spectral components of the detected X-rays. A spectral CT image can be used to determine the quantity or proportion of different materials or substances in an examination area. For example, the amount or location-dependent concentration of contrast agent, bone, calcium, fat, or water can be determined.

[0013] A non-spectral CT image, on the other hand, only provides information about the location-dependent attenuation of X-rays in an examination area, but cannot provide quantitative information regarding the occurrence of different materials and their proportions.

[0014] Therefore, such a non-spectral perfusion CT measurement has the disadvantage that it cannot quantify contrast agents, such as iodine, based on the CT image data alone. Physiological data such as blood flow or time to maximum contrast agent uptake can be parameterized using a pharmacokinetic model. In contrast, with spectral CT, a contrast agent quantity, particularly iodine, can be determined based on the CT image data alone. Furthermore, a high-resolution perfusion CT measurement results in a comparatively high radiation exposure and, consequently, a correspondingly reduced image quality for individual time points compared to conventional multiphase CT with only a few image acquisitions.

[0015] In contrast, new developments in magnetic resonance imaging enable the acquisition of high-resolution 3D data sets without radiation exposure, which practically always also capture the optimal time for maximum contrast agent delivery in a monitored area during organ / tumor perfusion. However, similar to non-spectral perfusion CT, quantification of physiological parameters in magnetic resonance imaging is only possible using a pharmacokinetic model.

[0016] It is common for both a CT scan in the form of a multiphasic CT and an MRI scan to be taken from a patient and, depending on therapeutic relevance, repeated.

[0017] As already mentioned, however, images of both modalities have the problem of determining an absolute quantification of a contrast agent at an optimal time as a surrogate for, among other things, an assessment of tumor viability, in particular at a defined time at which a maximum amount of contrast agent prevails in the examination area.

[0018] A method for dynamic contrast-based CT perfusion measurements is known, for example, from Van Beers BE et al., "Hepatic Perfusion Parameters in Chronic Disease: Dynamic CT Measurements Correlated with Disease Severity," AJR, 2001, vol. 176, no. 3, pp. 667-687. MR perfusion measurements are described, for example, in Jahng G.-H. et al., "Perfusion Magnetic Resonance Imaging: A Comprehensive Update on Principles and Techniques," Korean Journal of Radiology, 2014, vol. 15, no. 5, pp. 554-577.

[0019] The stated object is achieved by a method for determining a quantitative result CT parameter map in an examination region of an examination object according to patent claim 1, a method for acquiring a quantitative result CT parameter map from an examination region of an examination object according to patent claim 11, a parameter determination device according to patent claim 12 and a multimodal imaging system according to claim 13.

[0020] In particular, the features and advantages described in connection with the methods according to the invention can also be implemented as corresponding subunits of the parameter determination device according to the invention or of the multimodal imaging system according to the invention. Conversely, the features and advantages described in connection with the parameter determination device according to the invention or the multimodal imaging system according to the invention can also be implemented as corresponding method steps of the methods according to the invention.

[0021] In the method according to the invention for determining a quantitative result CT parameter map in an examination region of an examination subject, a sequence of quantitative input CT parameter maps is adopted for predetermined points in time. The predetermined points in time lie in a defined or predetermined time range, which is defined by the measurement on which the generation of the quantitative input CT parameter maps is based. The measurement comprises a spectral multiphase CT, with which the sequence of quantitative input CT parameter maps of the examination region was generated with the aid of contrast medium. As already mentioned, the defined time range comprises predetermined points in time orMeasurement points, but preferably at least four measurement points, which preferably cover the different phases of a perfusion measurement mentioned above, preferably a perfusion measurement of an organ, in particular a liver perfusion measurement. The method according to the invention is therefore preferably used for evaluating a perfusion measurement.

[0022] The term "accept" is intended to encompass all types of data reception, in particular reception via a receiving unit in a technical data transmission system, which may include, for example, a data transmission unit, such as a data transmission line, a transmit / receive system, or a data reception interface. Furthermore, the term "accept" also encompasses the reception of data within a data processing system or a computer program used to evaluate measurement data, in particular raw data and image data. The data is preferably acquired from a CT scanning unit or a medical database as the source.

[0023] An examination area comprises a body region that can be imaged using medical imaging, in particular multiphase CT and magnetic resonance imaging, preferably an organ, in particular an area or sub-area of ​​the liver or brain of a human. The vessels and tissue types of the liver mentioned above are particularly relevant. However, it should be noted that the method can also be applied to body regions other than the liver, for example, the intestine, spleen, or brain of an examination subject, in particular a human or animal.

[0024] A quantitative CT parameter map comprises a 2-dimensional or 3-dimensional representation of a location-dependent value of a CT parameter at a predetermined time or for a predetermined time interval for an examination region. A CT parameter refers to a measurable quantity determined based on CT imaging. A single CT parameter map represents, in other words, a snapshot of an examination region.

[0025] CT parameters can, in particular, include so-called perfusion parameters, which quantitatively characterize a perfusion process. Perfusion parameters include a measured variable characterizing perfusion, in particular the location-dependent contrast agent concentration or amount, blood flow, blood volume, mean flow time, or vascular permeability. These measured variables are preferably determined for different tissue types, which include a tumor and healthy tissue.

[0026] The determination of the input CT parameter values, for example, values ​​representing a contrast agent concentration or contrast agent quantity, is preferably performed using a method for base material decomposition or a method for reconstructing or calculating virtual base material-weighted image data, particularly pseudo-monoenergetic image data. Such a method is known from Alvarez RE and Macovski A. "Energy-selective reconstructions in x-ray computed tomography," Phys. Med. Biol. 21, 733-744 (1976).

[0027] A particularly suitable method for reconstructing or calculating such reconstructed image data, especially pseudo-monoenergetic image data, is described in KL Grant et al., "Assessment of an Advanced Image-Based Technique to Calculate Virtual Monoenergetic Computed Tomography Images From a Dual-Energy Examination to Improve Contrast-To-Noise Ratio in Examinations Using Iodinated Contrast Media," Investigative Radiology 2014; 00: 00-00.

[0028] Advantageously, based on such a basic material decomposition, individual portions of the image data acquired by spectral multiphase CT can be used to create CT parameter maps, particularly the input CT parameter maps. In this way, in addition to the attenuation of voxels in a CT measurement, other CT parameters, such as contrast agent concentration, can also be determined.

[0029] In this context, "contrast-enhanced" means that the CT image acquisition or the MRI image acquisition mentioned below is performed in the presence of a contrast agent, and the contrast agent's presence influences the measured values. Iodine is the preferred contrast agent for CT imaging. In contrast, contrast agents containing gadolinium compounds, iron oxides, or manganese compounds are preferred for MRI imaging. Generally, CT imaging uses a substance as a contrast agent that exhibits a strong increase in absorption for a specific X-ray energy value (for iodine, this is 33 keV). In contrast, MR imaging uses contrast agents that exhibit strong paramagnetism, which allows neighboring protons, essentially water protons, to relax more quickly.The contrast agent therefore creates a strong difference between the measured values ​​recorded in an area exposed to the contrast agent compared to the measured values ​​recorded in an area not exposed to the contrast agent.

[0030] "Quantitative" in this context means that the CT parameter values ​​can be determined in terms of their absolute magnitude based on the acquired measurements. While the determination of quantitative parameter values ​​is possible with spectral CT measurements, it is not possible with an MRI measurement performed solely or exclusively.

[0031] In addition, a sequence of contrast-enhanced MRI image datasets is acquired. This sequence includes a so-called 4D MRI image acquisition. 4D MRI is an abbreviation for "four-dimensional magnetic resonance imaging." 4D MRI images a temporally variable examination region in a defined or predetermined time range using a sequence of MRI image datasets, each of which represents a snapshot. These snapshots are often referred to as a frame. Compared to the sequence of quantitative input CT parameter maps, the sequence of contrast-enhanced MRI image datasets exhibits a higher temporal resolution, down to the subsecond range, for the entire examination region, preferably the entire liver. This means that preferably one MRI image dataset is generated per second, and particularly preferably more than one MRI image dataset per second.By applying motion correction techniques, high-resolution temporal and spatial data can be acquired over an entire perfusion cycle, preferably over an entire liver perfusion cycle. Motion correction is particularly useful for moving examination areas or organs, for example, in the patient's torso area.

[0032] A higher temporal resolution means that a higher number of MRI image datasets than CT parameter maps are acquired within the same defined time range. Preferably, four CT parameter maps are acquired within the predefined time range. Then, using the above-mentioned technique or with 4D MRI imaging, a larger number of image datasets are acquired within the defined time range than with CT. The higher the sampling rate, the more accurately the different effects during contrast agent build-up and efflux can be visualized.

[0033] Thus, the sequence of contrast-enhanced MRI image data sets comprises a higher number of image data sets than the sequence of quantitative input CT parameter maps. The sequence of contrast-enhanced MRI image data sets is preferably received from an MR system or an MR scanning unit or from a medical database as a source. The sequence of contrast-enhanced MRI image data sets relates to the same time range defined for the acquisition of the quantitative result CT parameter map, i.e., the time range covered by the sequence of contrast-enhanced MRI image data sets comprises, in particular, the at least two predetermined points in time. Since the image acquisitions with the different modalities generally cannot be performed simultaneously, the defined time period between the two modalities must be synchronized.For example, the start of the respective imaging process is determined and, if necessary, a temporal normalization is also determined in order to be able to correctly assign the temporal data sets or CT parameter maps of the different modalities to one another. Correct temporal assignment means that the two data sets (one image data set and one associated CT parameter map) were acquired in the same phase of the inflow or outflow of the respective contrast agent. For example, the contrast agent injection can be used as the starting point for the respective imaging or the defined time period. If, due to changed circumstances, the inflow behavior or inflow speed of the respective contrast agent occurs, this must be detected and a temporal normalization of the data sets required for the respective imaging or CT parameter maps must be carried out.The time ranges used to generate a CT parameter map must be determined in order to be able to correctly assign the respective image data sets or CT parameter maps of the different measurements to one another in terms of time. For example, in the case of different cardiac outputs when imaging with different modalities, a temporal adjustment or normalization can be carried out by comparing the quotient ratio of relative signal intensity values ​​in the supplying and draining vessels to an examination area, in particular an organ, preferably the liver, with a quotient ratio of the CT parameters or CT parameter values ​​of the CT parameter maps. Corresponding times or phases of the measurements of different modalities occur when generating the quotient where the quotients for different modalities assume the same value.A modality is understood to be a specific type of imaging system, in particular a CT system or an MR system that is technically fundamentally different from it.

[0034] The contrast-enhanced MRI image datasets also show the examination area of ​​the subject captured by the spectral multiphase CT. As described in detail later, the sequences of MRI image datasets represent location-dependent temporal signal intensity profiles or time-dependent signal intensity values ​​in the examination area.

[0035] Furthermore, based on the sequence of quantitative input CT parameter maps and the sequence of contrast-enhanced MRI image datasets, a relationship between the MRI image datasets and the quantitative input CT parameter maps is determined. This relationship is understood to be a quantitative mathematical relationship between the signal intensity values ​​of the MRI image datasets and the CT parameter values ​​of the quantitative input CT parameter maps, which can be used to convert the differently generated measurement data.

[0036] In detail, the relationship is determined as a mathematical relationship between the time- and location-dependent signal intensity values ​​of the MRI image datasets assigned to the quantitative input CT parameter maps and the CT parameter values ​​of these quantitative input CT parameter maps at identical positions or mutually assigned positions or mutually assigned pixel or voxel positions of the MRI image datasets or CT parameter maps. As will be explained in more detail later, the positions or pixels or voxels of the image datasets or parameter maps from different modalities can be assigned to one another through registration or image registration.

[0037] The relationship, preferably a functional relationship, can itself comprise one or more parameters determined based on parameter values ​​and signal intensity values ​​of temporally and spatially associated voxels of the MRI image data sets and the input CT parameter maps. The temporal association has been described in detail above. As will be explained in more detail later, a spatial association can be achieved by registration, in particular image registration, of the temporally associated MRI image data sets and input CT parameter maps.

[0038] In the simplest case, the relation includes the calculation rule that the signal intensity values ​​of the MRI image data sets, if necessary after temporal normalization, are converted into the (temporally and spatially assigned) CT parameter values ​​by adding an offset value.

[0039] Preferably, the relationship describes at least a linear relationship. This relationship can be assumed under the premise that the intravascularly administered contrast agents exhibit identical or comparable pharmacokinetics through temporal normalization in different modalities, and that a signal intensity value of an MR image dataset behaves linearly with the concentration of a contrast agent present.

[0040] A change in the measured signal intensity values ​​of the MRI image data sets is then proportional to a change in the amount of contrast agent present in the examination region during multiphase CT at the same time or at the assigned time point of the different measurements of the different modalities. Quantitative measured values ​​from spectral multiphase CT can therefore be assigned to a relative change in the signal intensity in the MRI image data sets. This also applies if the signal intensity values ​​of the MRI image data sets change between two consecutive examinations with 4D MRI images with the same contrast agent dynamics. In this case, the relationship changes, but not the fundamental property that the change in the measured signal intensity values ​​of the MRI image data sets is proportional to a change in the amount of contrast agent present in the examination region during multiphase CT at the same time or at the assigned time point of the different measurements of the different modalities.the amount of contrast agent present at the assigned time point.

[0041] As will be explained later, this circumstance can also be used to reduce the number of modalities used in repeated examinations.

[0042] A problem with the comparability of measurements from different modalities could arise if the perfusion behavior changes temporally between the different measurements from different modalities. For example, a vessel may occlude, in which case the other or previous measurement would have to be repeated to perform "matching." "Matching" is understood as the transfer of the signal intensity values ​​from the sequence of MRI image datasets to the sequence of input CT parameter maps.

[0043] On the basis of the determined relationship and the MRI image data sets, a quantitative result CT parameter map is determined for at least one additional point in time that differs from the predetermined at least two points in time, i.e. measurement points in time, of the spectral multiphase CT. The MRI image data sets available with a higher temporal resolution are advantageously used to increase the temporal resolution with which the CT parameters are measured. This is achieved by determining the relationship between the MRI image data sets and the quantitative input CT parameter maps, which makes it possible to convert the relative values ​​of the MRI image data sets into absolute values ​​of the CT parameters, preferably CT parameters of a perfusion measurement, very particularly preferably CT parameters that indicate a contrast agent concentration, and thus to fill in the temporal gaps in the spectral multiphase CT orthe resulting input CT parameter maps.

[0044] The disadvantages of MRI image datasets, namely that they only reflect the change behavior of parameter values ​​characterizing perfusion and not absolute values, are advantageously compensated for by combining them with the absolute values ​​of the CT parameters of multiphase CT. On the other hand, the comparatively low temporal resolution of multiphase CT is also compensated for by combining them with the high-resolution sequence of MRI image datasets. Due to the increased temporal resolution, it is advantageous to determine times at which threshold values ​​for the CT parameters are exceeded. Such a threshold can include, for example, a significant iodine concentration or an iodine concentration threshold that indicates a pathology.

[0045] In the method according to the invention for acquiring a quantitative result CT parameter map of an examination region of an examination subject, first raw data are acquired by a spectral multiphase CT of the examination region for at least two time points within a predetermined time range, with contrast agent support. Based on the acquired first raw data, a sequence of at least two quantitative input CT parameter maps is reconstructed for the at least two predetermined time points.

[0046] In addition, second raw data from the examination area are acquired using contrast-enhanced 4D magnetic resonance imaging in the predetermined time range, with a higher temporal resolution than multiphase CT. Based on the second raw data, a sequence of contrast-enhanced MRI image data sets is reconstructed, which has a higher temporal resolution than the sequence of quantitative input CT parameter maps.

[0047] Finally, the inventive method for acquiring a quantitative result CT parameter map in an examination region of an examination subject is carried out based on the reconstructed sequence of quantitative input CT parameter maps and the reconstructed sequence of MRI image data sets. The inventive method for acquiring a quantitative result CT parameter map from an examination region of an examination subject shares the advantages of the inventive method for determining a quantitative result CT parameter map in an examination region of an examination subject. Advantageously, the multimodal measurement generates a type of augmented sequence of quantitative CT parameter maps that offers increased temporal resolution compared to the sequence of input CT parameter maps.A multimodal measurement is understood to be a temporally parallel or sequential measurement with at least two different types of medical imaging systems, preferably a CT system and an MRI system. Sequential measurement is technically easier to implement. Parallel measurement requires a technically complex integration of the two modalities. The advantage of the latter variant is that the measurements from different modalities relate to an identical state of an examination object or examination area, thus ensuring good comparability of the measurements from the different modalities and allowing the "matching" performed according to the invention to be carried out particularly precisely.

[0048] The parameter determination device according to the invention has an input interface for accepting a sequence of quantitative input CT parameter maps which were generated from an examination region of an examination object for at least two predetermined measurement times by a spectral multiphase CT with the aid of contrast medium.

[0049] The input interface of the parameter determination device according to the invention also serves to accept a sequence of contrast-enhanced MRI image data sets which has a higher temporal resolution compared to the sequence of quantitative input CT parameter maps.

[0050] Part of the parameter determination device according to the invention is also a relation determination unit for determining a relationship between the MRI image data sets and the quantitative input CT parameter maps.

[0051] The parameter determination device according to the invention also comprises a parameter determination unit for determining a quantitative result CT parameter map on the basis of the determined relation and the MRI image data sets for at least one additional time point which differs from the at least two predetermined time points.

[0052] The parameter determination device according to the invention preferably comprises an assignment unit for temporally assigning the MRI image data sets to the associated quantitative input CT parameter maps for the at least two predetermined points in time. The parameter determination device according to the invention preferably also comprises a registration unit for spatially assigning the temporally assigned quantitative input CT parameter maps and MRI image data sets through image registration. Such image registration can comprise rigid image registration or elastic image registration, depending on the nature of the object being examined. Registration is understood to mean mapping two images or volumes (in this case, an MRI image and a CT parameter map) onto one another as precisely as possible. A geometric transformation must be found to ensure this.A registration is referred to as rigid if it exclusively involves translations and rotations as image transformations. Elastic, deformable, or flexible registration also includes the mapping of straight lines to curves.

[0053] Methods for image registration are described, for example, in Nadine Spahr et al., "Multimodal image registration for liver radioembolization planning and patient assessment", International Journal of Computer Assisted Radiology and Surgery, 14, 215-225 (2019).

[0054] The MRI image data sets assigned to at least two predetermined time points of the multiphase CT and the quantitative input CT parameter maps assigned to them are used by the relationship determination unit to determine a relationship between the MRI image data sets and the quantitative input CT parameter maps. This is possible because the temporal assignment and registration already achieve a clear assignment of voxels of individual MRI image data sets to voxels or pixels of the CT parameter maps.

[0055] The parameter determination device according to the invention shares the advantages of the method according to the invention for determining a quantitative result CT parameter map in an examination region of an examination object.

[0056] The multimodal imaging system according to the invention comprises a CT system for recording a sequence of contrast-enhanced quantitative input CT parameter maps of an examination region of an examination object.

[0057] Part of the multimodal imaging system according to the invention is also an MR system for recording a sequence of contrast-enhanced MRI image data sets of the examination area.

[0058] The multimodal imaging system according to the invention also comprises a parameter determination device according to the invention for determining a result CT parameter map based on the sequence of quantitative input CT parameter maps and the sequence of MRI image data sets. The multimodal imaging system according to the invention shares the advantages of the method according to the invention for determining a quantitative result CT parameter map in an examination region of an examination subject.

[0059] A large portion of the aforementioned components of the parameter determination device can be implemented entirely or partially in the form of software modules in a processor of a corresponding computing system, e.g., a control device of a multimodal medical imaging system or a computer used to control such a system. A largely software-based implementation has the advantage that even previously used computing systems can be easily upgraded to operate in the manner according to the invention via a software update.In this respect, the object is also achieved by a corresponding computer program product with a computer program that can be loaded directly into a computer system, with program sections for executing the steps of the method according to the invention for determining a quantitative result CT parameter map in an examination region of an examination subject and the steps, in particular the reconstruction steps, of the method for acquiring a quantitative result CT parameter map of an examination region of an examination subject when the program is executed in the computer system. Such a computer program product can, in addition to the computer program, optionally comprise additional components such as documentation and / or additional components, including hardware components such as hardware keys (dongles, etc.) for using the software.

[0060] A computer-readable medium, e.g., a memory stick, a hard disk, or another portable or permanently installed data storage device, can be used for transport to the computer system or control device and / or for storage on or in the computer system or control device, on which the program sections of the computer program that can be read and executed by a computer system are stored. For this purpose, the computer system can, for example, have one or more cooperating microprocessors or the like.

[0061] The dependent claims and the following description each contain particularly advantageous embodiments and developments of the invention. In particular, the claims of one claim category can also be 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 can also be combined to form new embodiments.

[0062] In the method according to the invention for determining a quantitative result CT parameter map in an examination region of an examination subject, the step of determining the relationship preferably comprises a step of temporally assigning the MRI image data sets to the input CT parameter maps for the at least two predetermined points in time. As already indicated, the temporal assignment can be based on the start time of the contrast agent administration in the two medical image acquisition sequences generated with different modalities. Furthermore, a temporal normalization can be performed between the two sequences to enable correct temporal assignment. Such normalization takes into account the different flow rates of the contrast agent in the images acquired with the different modalities.The determination of this type of temporal incoherence and a corresponding temporal normalization can be carried out, for example, on the basis of measured values ​​at different positions and for both modalities in the object under examination, by forming a quotient of the measured values ​​for different positions and by comparing the quotients of image data sets and CT parameter sets of the different modalities and by assigning image data sets and CT parameter sets with quotients that are as close to each other as possible or whose values ​​are closest to each other.

[0063] The step of determining the relationship preferably further comprises a step of relative spatial assignment of the temporally assigned quantitative input CT parameter maps and the MRI image data sets to one another for the at least two predetermined points in time by image registration. Relative spatial assignment should be understood as meaning that a spatial or spatial assignment of both the quantitative input CT parameter maps to the MRI image data sets temporally assigned to them and, conversely, a spatial or spatial assignment of the MRI image data sets to the quantitative input CT parameter maps temporally assigned to them can occur.

[0064] Furthermore, the step of determining the relationship includes the step of determining the relationship between the temporally and spatially associated MRI image datasets and input CT parameter maps. As already mentioned, this relationship is determined by a type of "matching" of the signal intensity values ​​of the MRI image datasets and the CT parameter values ​​of the associated input CT parameter maps. Before determining the relationship between MRI image datasets and input CT parameter maps, temporal assignment and image registration are advantageously used to compensate for temporal and spatial deviations between the image acquisitions or acquisitions of parameter maps of the different modalities, thus increasing the accuracy of determining the relationship.

[0065] In a variant of the method according to the invention for determining a quantitative result CT parameter map in an examination region of an examination object, the determination of the relation comprises a ratio between first CT parameter values ​​of the input CT parameter maps and signal intensity values ​​of the MRI image data sets temporally and spatially assigned to the input CT parameter maps.

[0066] Figuratively speaking, a kind of projection of the MRI image data sets onto the quantitative input CT parameter maps takes place. Advantageously, the higher temporal resolution sequence of MRI image data sets is projected onto the sequence of quantitative input CT parameter maps, or a kind of "matching" with them takes place, whereby the parameter values ​​of the input CT parameter maps or parameter values ​​derived from them are used as support points, so that the relative or qualitative information of the MRI image data sets is converted into quantitative information of the CT parameters. The "matching" particularly includes a temporal normalization of the MRI image data sets to the input CT parameter maps.

[0067] Particularly preferably, the quantitative resulting CT parameter map is determined at the at least one additional point in time by extrapolation or interpolation based on the predetermined point in time and on the basis of the determined relationship. In this context, extrapolation or interpolation should be understood as using the CT parameter values ​​as support values ​​and the projected MRI image data sets as an extrapolation function to determine additional resulting CT parameter values.

[0068] The absolute value or resulting CT parameter value at the predetermined time, for example, a time of a maximum of a resulting CT parameter, is thus determined using the CT parameter values ​​from the quantitative input CT parameter maps as support values. Advantageously, a significant value, for example, a maximum value of a CT parameter, can be determined particularly precisely. If a value of a parameter is to be determined that differs from the type of the parameter in the input CT parameter map, this can be determined based on the determined resulting CT parameter value and a previously known mathematical relationship between the resulting CT parameter value and the different parameter value.

[0069] If the input CT parameters include, for example, so-called CT attenuation values, then, based on the knowledge of the difference between the CT attenuation values ​​and the attenuation values ​​associated with a contrast agent concentration, which can be calculated for at least two time points by a base material decomposition, a corresponding value of the contrast agent concentration, in particular an iodine concentration, can also be calculated for the at least one additional time point by subtracting this difference from the CT attenuation value calculated as the result parameter value. Thus, parameter values ​​of different types can advantageously be determined based on the "matching" performed once.

[0070] Preferably, determining the quantitative result CT parameter map comprises the steps of determining the at least one additional time point based on the MRI image data sets. The higher-resolution sequence of MRI image data sets allows for more precise detection or localization of significant time points during the examination, such as a time point at a maximum of a CT parameter.

[0071] Particularly preferably, the at least one additional time point comprises a time point at which, based on the MRI image data sets, a maximum CT parameter value, preferably a CT parameter value characterizing perfusion, most preferably a maximum contrast agent concentration value, is to be expected in the examination region. Advantageously, the at least one additional time point at which the maximum CT parameter value occurs is determined more precisely based on the MRI image data sets than solely on the basis of the quantitative input CT parameter maps with a lower temporal resolution.

[0072] In addition, based on the additional knowledge of the relationship between the MRI image data sets and the quantitative input CT parameter maps, the desired maximum CT parameter value, preferably a parameter of a CT perfusion measurement, most preferably a maximum contrast agent concentration, is determined more precisely than based solely on the quantitative input CT parameter maps.

[0073] In a particularly preferred variant of the method according to the invention for determining a quantitative result CT parameter map in an examination region of an examination object, the input CT parameter maps and the result CT parameter map indicate parameter values ​​that comprise one of the following CT parameter value types: a time- and location-dependent normalized attenuation value, a time-, location- and energy-dependent attenuation value, a time- and location-dependent density value, a time- and location-dependent contrast agent concentration value, a value indicating a time- and location-dependent contrast agent quantity, a value indicating a time- and location-dependent blood quantity.

[0074] A normalized attenuation value represents a location-dependent X-ray absorption normalized to the attenuation coefficients of water and air. Such a value is often referred to as an "HU" value.

[0075] An energy-dependent attenuation value is recorded in spectral CT imaging for a specific X-ray spectrum with a specific energy.

[0076] A density value is determined based on spectral CT imaging and based on the above-mentioned basic material decomposition by determining pseudomonochromatic image data.

[0077] Particularly preferably, a contrast agent concentration is determined as the density. Most preferably, the contrast agent comprises an iodine-based contrast agent. Perfusion processes in body structures can advantageously be visualized and quantitatively evaluated by the contrast agent. In this particularly preferred variant, quantitative contrast agent maps or contrast agent concentration maps, most preferably quantitative iodine maps or iodine concentration maps, are used as quantitative input CT parameter maps. The result then obtained on the basis of multimodal imaging is contrast agent maps, in particular iodine maps with increased temporal resolution. The amount of contrast agent, particularly at a time of maximum contrast agent quantity, provides information about the location-dependent blood flow in an examination region.

[0078] It should be explicitly mentioned again at this point that the input CT parameter maps and / or the output CT parameter map preferably represent a distribution of at least one perfusion parameter. As already mentioned, this perfusion parameter can usually be determined based on a contrast agent distribution. As already mentioned, perfusion parameters comprise a measured variable characterizing perfusion, in particular one of the following variables: the location-dependent contrast agent concentration or amount, blood flow, blood volume, mean flow time, or vascular permeability. The gain in temporal resolution compared to a monomodal approach advantageously enables more precise imaging and analysis of the perfusion behavior in an examination region.

[0079] In a particularly preferred variant of the method according to the invention for determining a quantitative result CT parameter map, the generated quantitative result CT parameter map is used to determine a pharmacokinetic model. Such a pharmacokinetic model, which simulates the dynamics of transport of a pharmaceutical substance in an examination subject and is used, for example, to determine the time of a maximum contrast agent concentration in an examination region of the examination subject, can advantageously be specified.

[0080] In the method according to the invention for determining a quantitative result CT parameter map based on the determined relationship and the MRI image data sets, a plurality of quantitative result CT parameter maps are particularly preferably generated. Advantageously, the resolution of the resulting augmented sequence of quantitative CT parameter maps can be further increased due to the increased number of acquired CT parameter maps.

[0081] Thus, a temporally high-resolution sequence of CT parameter maps can be generated based on the determined relationship and the MRI image data sets. A temporally high-resolution sequence of CT parameter maps preferably comprises at least one CT parameter map per half a second, particularly preferably at least one CT parameter map per second.

[0082] Particularly preferably, a monomodal examination, preferably a follow-up examination, is carried out based on knowledge of the relationship and / or the high-resolution sequence of CT parameter maps. In this type of examination, only a single examination is performed using either a CT system or an MR system, and the missing information is supplemented using the known relationship or the known high-resolution sequence data. Figuratively speaking, an estimate for the other modality can be determined for both a future CT measurement and a future MRI measurement, so that only a single modality needs to be used. Advantageously, for frequently occurring examinations, particularly follow-up examinations, effort, time, and resources can be saved, and the burden, particularly radiation exposure, for the patient can be reduced.

[0083] One form of frequently recurring examinations is follow-up examinations, which are intended to regularly document a health condition, for example the dynamics of a tumor or the response of a tumor to therapy.

[0084] Particularly preferably, the relationship is determined based on the premise that intravascularly administered contrast agents in the CT measurement have identical pharmacokinetics to intravascularly administered contrast agents in the MRI measurement and that the changes in signal intensity in the MRI measurement are subject to a certain linearity depending on the concentration of the contrast agent, so that the change in the measured signal intensity values ​​in the MRI measurement is equivalent to the amount of contrast agent in the CT measurement at one of the predetermined times. Advantageously, the contrast agent concentration values ​​determined based on the CT measurement can be used as reference values ​​to determine further contrast agent concentration values ​​assigned to other times based on the MRI measurement and thus increase the temporal resolution of a contrast agent concentration measurement.

[0085] The invention is explained in more detail below with reference to exemplary embodiments in the accompanying figures. They show: FIG 1 a flowchart illustrating a method for acquiring a quantitative result CT parameter map of an examination region of an examination object according to an embodiment of the invention, FIG 2 a schematic representation of signal intensity curves at different positions in the examination area for a 4D MRI image acquisition, FIG 3 a schematic representation of a CT attenuation value curve, which is based on a spectral multiphase CT image and the FIG 2 shown signal intensity curve of a 4D MRI image, as well as an extrapolated iodine concentration value for the time of maximum blood flow, FIG 4 a schematic representation of an examination area comprising a human liver, FIG 5 a schematic representation of a parameter determination device according to an embodiment of the invention, FIG 6 a schematic representation of a medical imaging system according to an embodiment of the invention.

[0086] In FIG 1 a flowchart 100 is shown which illustrates a method for acquiring a quantitative result CT parameter map V-CT-QPK(t max ) from an examination region ROI of an examination object O

[0087] In step 1.I, first raw data RD1 are acquired from the ROI (region of interest) using a contrast-enhanced spectral multiphase CT scan at predetermined time points t1, t2, t3, and t4. Iodine is used as the contrast agent in the spectral multiphase CT scan. The raw data RD1 are acquired with spectral resolution, allowing the concentration of a contrast agent in the ROI to be determined as a CT parameter.

[0088] In step 1.II, a sequence S CT of quantitative input CT parameter maps CT-QPK(t 1 ),..., CT-QPK(t 4 ) is reconstructed on the basis of the acquired first raw data RD1 for the predetermined times t 1 , t 2 , t 3 , t 4 . These input CT parameter maps CT-QPK(t 1 ),..., CT-QPK(t 4 ) preferably each represent a location-dependent distribution of a contrast agent at the predetermined times t 1 , t 2 , t 3 , t 4 . The predetermined times t 1 , t 2 , t 3 , t 4 correspond to the different phases of a perfusion measurement, as explained at the beginning and also in connection with FIG 4 be mentioned again.

[0089] In step 1.III, contrast-enhanced acquisition of second raw data (RD2) from the region of interest (ROI) is performed using four-dimensional magnetic resonance imaging (MRI). This provides higher temporal resolution than multiphase CT. A gadolinium compound is preferably used as the contrast agent in 4D MRI imaging. Iron oxide compounds or manganese-containing compounds are also used.

[0090] Furthermore, in step 1.IV, a sequence S MR of contrast-enhanced MRI image data sets BD(t), which has a higher temporal resolution than the sequence S CT of quantitative input CT parameter maps CT-QPK(t 1 ),..., CT-QPK(t 4 ), is reconstructed based on the second raw data RD2. The MRI image data sets BD(t) each show location-dependent signal intensity values ​​SI (see FIG 2 ) at different times.

[0091] Subsequently, in steps 1.V to 1.VIII, the method according to the invention for determining a quantitative result CT parameter map V-CT-QPK(t max ) in an examination region ROI of an examination object O is carried out.

[0092] In step 1.V, the 4D MRI image data sets BD(t) are temporally assigned to the predetermined times t 1 , t 2 , t 3 , t 4 of the multiphase CT, wherein MRI image data T-BD temporally assigned to the predetermined times t 1 , t 2 , t 3 , t 4 are generated.

[0093] In step 1.VI, the temporally associated quantitative input CT parameter maps CT-QPK (t 1 ), ..., CT-QPK (t 4 ) and MRI image data T-BD are spatially assigned by image registration, whereby registered MRI image data R-BD are generated with the quantitative input CT parameter maps CT-QPK (t 1 ), ..., CT-QPK (t 4 ).

[0094] In step 1.VII, on the basis of the registered MRI image data R-BD and the quantitative input CT parameter maps CT-QPK (t 1 ),..., CT-QPK(t 4 ), a relation R is determined between the time- and location-dependent signal intensity values ​​SI of the MRI image data R-BD registered with the quantitative input CT parameter maps CT-QPK(t 1 ),..., CT-QPK(t 4 ) and the CT parameter values, for example attenuation values, of the quantitative input CT parameter maps CT-QPK(t 1 ),..., CT-QPK(t 4 ) temporally assigned to the registered MRI image data R-BD.

[0095] In step 1.VIII, a quantitative result CT parameter map V-CT-QPK(t max ) is determined on the basis of the determined relation R and the sequence S MR of 4D MRI image data sets BD(t) for at least one additional time t max , which differs from the predetermined times t 1 , t 2 , t 3 , t 4 of the multiphase CT.

[0096] In FIG 2 A schematic representation 20 of signal intensity curves K1, K2, K3, K4 at different positions in the examination area for a 4D MRI image is shown. The different signal intensity curves K1, K2, K3, K4 correspond to contrast agent concentrations at the FIG 4 shown positions 41, 42, 43, 44 of a patient's liver.

[0097] A first position 41 corresponds to a general location of the liver, a second position 42 corresponds to a location of a tumor, a third position 43 corresponds to the location of the hepatic artery and a fourth position 44 corresponds to the location of the vena cava.

[0098] Furthermore, the first signal curve K1 corresponds to a measurement in the native phase, the second signal curve K2 corresponds to a measurement in the arterial phase, the third signal curve K3 corresponds to a measurement in the portal venous phase and the fourth signal curve K4 corresponds to a measurement in the late venous phase, for example.

[0099] The third curve, K3, is typically used to observe tumor behavior. For the third curve, a large number of snapshots are taken at different times, particularly at times t 1 , t 2 , t max , t 3 , and t 4 .

[0100] The corresponding signal intensity values ​​SI(t 1 ), SI(t 2 ), SI(t max ), SI(t 3 ), and SI(t 4 ) are also recorded. It should be noted at this point that the 4D MRI acquisition is performed at high resolution, and in addition to the aforementioned time points t 1 , t 2 , t max , t 3 , and t 4 , MRI image data is acquired for a variety of additional time points, so that the continuously plotted curve K3 is representative.

[0101] In FIG 3 is a schematic representation 30 of a CT attenuation value curve K-CT, which is based on a spectral multiphase CT image acquisition and the FIG 2 shown signal intensity curve K3 of a 4D MRI image, as well as an extrapolated iodine concentration value J(t max ) for the time t max of maximum blood flow.

[0102] The CT attenuation value curve K-CT includes attenuation values ​​CT(t 1 ,), CT(t 2 ), CT(t 3 ), CT(t 4 ), which were determined by the aforementioned spectral multiphase CT image acquisition. The attenuation values ​​CT(t 1 ,), CT(t 2 ), CT(t 3 ), CT(t 4 ) result from attenuation values ​​of a spectral measurement or a dual-energy measurement for a first energy E 1 : HU 1 (t 1 ,), HU 1 (t 2 ), HU 1 (t 3 ), HU 1 (t 4 ) and a second energy E 2 : HU 2 (t 1 ,), HU 2 (t 2 ), HU 2 (t 3 ), HU 2 (t 4 ). From the values ​​of the spectral measurement, attenuation values ​​J(t 1 ,), J(t 2 ), J(t 3 ), J(t 4 ) associated with an iodine concentration or iodine quantity can also be calculated by the method of basic material decomposition already mentioned several times and a determination of pseudo-monoenergetic image data.The K-CT curve of attenuation values ​​determined on the basis of the spectral multiphase CT and the 4D MRI image was obtained by matching the attenuation values ​​CT(t 1 ,), CT(t 2 ), CT(t 3 ), CT(t 4 ) of the spectral multiphase CT and the attenuation values ​​shown in . FIG 2 shown curve K3 of a 4D MRI image.

[0103] In other words, the signal intensity curve K3 is fitted to the attenuation values ​​CT(t 1 ), CT(t 2 ), CT(t 3 ), CT(t 4 ) at the four time points t 1 , t 2 , t 3 and t 4 , for each of which an iodine concentration was determined by the spectral multiphase CT image.

[0104] The adjustment can be made, for example, by the following calculation rule CT t k = a * SI t k + b , k = 1 , … , 4 and the parameters a and b are determined using the least squares method. In the best case, the parameter a has the value 1 and, figuratively speaking, only a shift of the curve K3 in the direction of the ordinate occurs.

[0105] Based on the determined parameters, a curve K-CT(tk ) of attenuation values ​​can then be determined, where k can now assume further values ​​in addition to the values ​​1 to 4 or the resolution of the curve K-CT is now higher than only the four measured values ​​recorded by the spectral CT measurement.

[0106] Based on this curve K-CT(tk ) we can now also, as in FIG 3 shown, a maximum value CT(t max ) of the attenuation, at which a maximum contrast agent concentration in the hepatic artery is to be expected, can be read.

[0107] Furthermore, based on the knowledge of the difference d between the CT attenuation values ​​CT(t 1 ,), CT(t 2 ), CT(t 3 ), CT(t 4 ) and the attenuation values ​​J(t 1 ,), J(t 2 ), J(t 3 ), J(t 4 ) assigned to a contrast agent concentration, which can be calculated for the four times t 1 , t 2 , t 3 and t 4 by a basic material decomposition, an associated value of the contrast agent concentration, in particular an iodine concentration J(t max ) can also be calculated. The difference d is simply subtracted from the CT attenuation value CT(t max ) at the time t max of maximum blood flow.

[0108] Thus, the time t max of the maximum contrast agent concentration can be read from the fitted curve K-CT(tk ) and values ​​of the contrast agent concentration and in particular a maximum contrast agent concentration value J(t max ) or a maximum amount of contrast agent in the hepatic artery can be calculated.

[0109] In FIG 4 a schematic representation 40 of an examination area comprising a human liver 41 is shown.

[0110] The human liver 41 comprises in the FIG 4 shown illustration shows a tumor 42, which is shown in the upper left corner of the liver 41. Furthermore, the liver 41 is supplied with fresh blood by the hepatic artery 43. The hepatic artery 43 is shown in the center of the liver 41. In the background, ie partially hidden by the liver 41 and in FIG 4 The vena cava 44 is shown running vertically.

[0111] In FIG 5 is a schematic representation of a parameter determination device 50 according to an embodiment of the invention.

[0112] The parameter determination device 50 comprises an input interface 51. The input interface 51 is configured to receive a sequence S CT of quantitative input CT parameter maps CT-QPK (t 1 ), ..., CT-QPK (t 4 ) from an examination region ROI of an examination object O, which were generated by a contrast agent-assisted spectral multiphase CT for predetermined times t 1 , t 2 , t 3 , t 4 .

[0113] In addition, the input interface 51 is configured to receive a sequence S MR of contrast-enhanced MRI image data sets BD(t) which has a higher temporal resolution compared to the sequence S CT of quantitative input CT parameter maps CT-QPK(t 1 ), ..., CT-QPK(t 4 ).

[0114] Part of the parameter determination device 50 is also a relation determination unit 52. The relation determination unit 52 is configured to perform a "matching" between the sequences S MR , S CT acquired by different modalities.

[0115] For this purpose, the relation determination unit 52 comprises an assignment unit 52a for temporally assigning the MRI image data sets BD(t) to the predetermined times t 1 , t 2 , t 3 , t 4 of the multiphase CT, wherein MRI image data T-BD temporally assigned to the predetermined times t 1 , t 2 , t 3 , t 4 are generated.

[0116] The relation determination unit 52 also comprises a registration unit 52b for spatially assigning the reconstructed quantitative input CT parameter maps CT-QPK(t 1 ), ..., CT-QPK(t 4 ) and the reconstructed MRI image data sets BD(t) by image registration, wherein MRI image data R-BD registered with the quantitative input CT parameter maps CT-QPK(t 1 ), ..., CT-QPK(t 4 ) are generated.

[0117] The relation determination unit 52 further comprises a relation determination subunit 52c for determining a relation R between the signal intensity values ​​of the registered MRI image data R-BD and the CT parameter values ​​of the associated quantitative input CT parameter maps CT-QPK(t 1 ), ..., CT-QPK(t 4 ). The relation R is determined between the time- and location-dependent signal intensity values ​​SI of the MRI image data R-BD registered with the quantitative input CT parameter maps CT-QPK(t 1 ), ..., CT-QPK(t 4 ) and the CT parameter values ​​of pixels or voxels spatially assigned by registration to pixels or voxels of the MRI image data of the input CT parameter maps CT-QPK(t 1 ), ..., CT-QPK(t 4 ) assigned to the registered MRI image data R-BD by the temporal assignment.

[0118] Furthermore, the parameter determination device 50 comprises a parameter determination unit 53 for determining a quantitative result parameter map V-CT-QPK(t max ) on the basis of the determined relation R and the reconstructed image data sets BD(t) for at least one additional time t max .

[0119] In FIG 6 is a schematic representation of a medical imaging system 1 according to an embodiment of the invention. The medical imaging system 1 comprises a CT system 2, an MR system 3 and the FIG 5 The parameter determination device 50 receives from the CT system a sequence S CT of contrast-enhanced quantitative input CT parameter maps CT-QPK(t 1 ), ..., CT-QPK(t 4 ) and from the MR system 3 a sequence S MR of contrast-enhanced MRI image data sets BD(t) and generates, as in connection with FIG 1 bis FIG 5explained in detail, a result CT parameter map V-CT-QPK(t max ) for an additional time point t max .

[0120] Finally, it is emphasized once again that the methods and devices described above are merely preferred embodiments of the invention and that the invention may be varied by those skilled in the art without departing from the scope of the invention, as defined by the claims. For the sake of completeness, it is also emphasized that the use of the indefinite articles "a" or "an" does not exclude the possibility that the respective features may be present in multiple instances. Likewise, the term "unit" does not exclude the possibility that it may consist of multiple components, which may also be spatially distributed.

Claims

1. Method for determining a quantitative result CT parameter map (V-CT-QPK(tmax)) in a region of interest (ROI) of an object under examination (O), having the following steps: - acquiring a sequence (SCT) of quantitative input CT parameter maps (CT-QPK(t1), CT-QPK(t2)) for at least two predetermined points in time (t1, t2, t3, t4), wherein the sequence (SCT) was generated in a contrast-enhanced manner by spectral multiphase CT of the region of interest (ROI), - receiving a sequence (SMR) of MRI datasets (BD(t)) generated in a contrast-enhanced manner which has a higher temporal resolution compared to the sequence (SCT) of input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)), - determining a relation (R) between the MRI image datasets (BD(t)) and the input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)), - determining the result CT parameter map (V-CT-QPK(tmax)) based on the determined relation (R) and the MRI image datasets (BD(t)) for at least one additional point in time (tmax) which is different from the predetermined points in time (t1, t2, t3, t4).

2. Method according to claim 1, wherein determining the relation (R) comprises the following steps: - temporally assigning the MRI image datasets (BD(t)) to the input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) for the at least two predetermined points in time (t1, t2, t3, t4), - relative spatial assignment of the quantitative input CT parameter maps (CT-QPK(t1), CT-QPK(t2)) and the MRI image datasets (BD(t)) by means of image registration for the at least two predetermined points in time (t1, t2, t3, t4), - determining the relation (R) between the 4D MRI image datasets (BD(t)) and input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) temporally and spatially assigned to each other.

3. Method according to claim 2, wherein determining the relation (R) involves a ratio between first CT parameter values (CT(t1), CT(t2), CT(t3), CT(t4)) of the input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) and signal intensity values (SI(t1), SI(t2), SI(t3), SI(t4)) of the MRI image datasets (BD(t)) temporally and spatially assigned to the input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)).

4. Method according to claim 3, wherein the result CT parameter map (V-CT-QPK(tmax)) at the at least one additional point in time (tmax) is determined by extrapolation or interpolation on the basis of the first CT parameter values (CT(t1), CT(t2), CT(t3), CT(t4)) of the input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) and on the basis of the determined relation (R).

5. Method according to one of the preceding claims, wherein the at least one additional point in time (tmax) is determined on the basis of the MRI image datasets (BD(t)).

6. Method according to one of the preceding claims, wherein the at least one additional point in time (tmax) includes a point in time (tmax) at which, on the basis of the MRI image datasets (BD(t)), a maximum CT parameter value, preferably a maximum contrast agent concentration value (J(tmax)), can be expected in the region of interest (ROI).

7. Method according to one of the preceding claims, wherein the input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) and / or the result CT parameter map (V-CT-QPK(tmax)) indicate a parameter value of at least one of the following CT parameter value types: - a normalised attenuation value (CT(t1), CT(t2), CT(t3) CT(t4), CT(tmax)), - an energy-dependent attenuation value (HU(t1), HU(t2), HU(t3), HU(t4), HU(tmax)), - a density value, - a contrast agent concentration value (J(t2), J(t2), J(t3), J(t4) , J(tmax)) .

8. Method according to one of the preceding claims, wherein the input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) and / or the result CT parameter map (V-CT-QPK(tmax)) reflect a distribution of at least one perfusion parameter.

9. Method according to one of the preceding claims, wherein a temporally high-resolution sequence of CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4), V-CT-QPK(tmax)) is generated on the basis of the determined relation (R) and the MRI image datasets (BD(t)).

10. Method according to claim 9, wherein a monomodal examination, preferably a follow-up examination, is performed on the basis of the temporally high-resolution sequence of CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4), V-CT-QPK(tmax)) and / or the relation (R).

11. Method for acquiring a quantitative result CT parameter map (V-CT-QPK(tmax)) of a region of interest (ROI) of an object under examination (O), having the following steps: - contrast-enhanced acquisition of first raw data (RD1) by spectral multiphase CT of the region of interest (ROI) for at least two predetermined points in time (t1, t2, t3, t4), - reconstructing a sequence (SCT) of quantitative input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) on the basis of the acquired first raw data (RD1) for the at least two predetermined points in time (t1, t2, t3, t4), - contrast-enhanced acquisition by means of a 4D MRI measurement of second raw data (RD2) of the region of interest (ROI) with higher temporal resolution compared to multiphase CT, - reconstructing, on the basis of the second raw data (RD2), a sequence (SMR) of MRI datasets (BD(t)) generated in a contrast-enhanced manner having a higher temporal resolution compared to the sequence (SCT) of input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)), - performing the method according to one of claims 1 to 10 on the basis of the sequence (SCT) of input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) and the sequence (SMR) of MRI image datasets (BD(t)) generated in a contrast-enhanced manner.

12. Parameter determining device (50), having: - an input interface (51) - for importing a sequence (SCT) of quantitative input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) of a region of interest (ROI) of an object under examination (O) for at least two predetermined points in time (t1, t2, t3, t4), wherein the sequence (SCT) has been generated in a contrast-enhanced manner by spectral multiphase CT - and for importing a sequence (SMR) of MRI image datasets (BD(t)) generated in a contrast-enhanced manner which has a higher temporal resolution compared to the sequence (SCT) of input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)), - a relation determining unit (52) for determining a relation (R) between the MRI image datasets (BD(t)) and the input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) - a parameter determining unit (53) for determining a result CT parameter map (V-CT-QPK(tmax)) on the basis of the determined relation (R) and the MRI image datasets (BD(t)) for at least one additional point in time (tmax) which is different from the predetermined points in time (t1, t2, t3, t4).

13. Multimodal medical imaging system (1), having: - a CT system (2) for acquiring a sequence (SCT) of quantitative input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) of a region of interest (ROI) of an object under examination (O), - an MRI system (3) for acquiring a sequence (SMR) of MRI image datasets (BD(t)) generated in a contrast-enhanced manner of the region of interest (ROI), - a parameter determining device (50) according to claim 12 for determining a result CT parameter map (V-CT-QPK(tmax)) on the basis of the sequence (SCT) of quantitative input CT parameter maps (CT-QPK(t1), CT-QPK(t2), CT-QPK(t3), CT-QPK(t4)) and the sequence (SMR) of MRI image datasets (BD(t)) generated in a contrast-enhanced manner.

14. Computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method according to one of claims 1 to 11.

15. Computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the steps of the method according to one of claims 1 to 11.