Abstract:
The embodiments relate to generating a 2D projection image of a vascular system of a body region of interest, including: (1) acquiring a 3D dataset of the body region of interest, (2) acquiring at least one 2D projection image of the body region of interest (S 3 ), (3) generating a modified 3D dataset by eliminating vessels whose size exceeds a predetermined limit value, (4) normalizing the 2D projection image using projection data of the modified 3D dataset, (5) eliminating vessel projections in the normalized 2D projection image whose size exceeds a predetermined limit value, (6) interpolating the areas of the 2D projection image in which the vessel projections have been eliminated, and (7) denormalizing the normalized and interpolated 2D projection image using projection data of the modified 3D dataset.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of DE 10 2014 201 134.5, filed on Jan. 22, 2014, which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The embodiments relate to a method and a device for generating at least one 2D projection image of a vascular system of a body region of interest, in particular of the parenchyma, in particular of the human or animal body. 
       BACKGROUND 
       [0003]    Different technologies are known for visualizing a vascular system of the human or animal body. In a form of digital subtraction angiography (DSA), after administration of a contrast agent two-dimensional projection images are acquired repeatedly at a predetermined, fixed projection angle by an X-ray apparatus, a reference image (e.g., mask image) is subtracted from each image. During the procedure, the mask image is generated at a point in time at which no contrast agent is present. In this way, the arrival of the contrast agent in the vessels, and so the blood flow, may be visualized (e.g., perfusion). Owing to the subtraction of the individual time-series images from the mask image, the visualizations from now on contain only the vessels filled with contrast agent. The background (e.g., tissue, bone, air) is suppressed. Digital subtraction angiography plays a major role in diagnostics, for example, in the detection of pathological vascular diseases in the brain. 
         [0004]    A tool by the name of Syngo iFlow is known that generates a parametric visualization of the arrival time of a contrast agent bolus based on digital subtraction angiography. The individual images of the DSA series (e.g., time-series images) are combined into a parametric image in which the arrival time of the contrast agent bolus may be visualized in color-coded form. In this case, the arrival time is, for example, the time up until the contrast agent reaches its maximum intensity; a so-called time-to-peak map (TTP map) is produced. An early arrival time of the contrast agent may be represented, for example, in a color from the red color spectrum, and a late arrival time in a color from the green or blue color spectrum. More information on this may be found in the publication titled “syngo iFlow/Dynamic Flow Evaluation/Answers for life” published by Siemens AG, Medical Solutions, Angiography, Fluoroscopic and Radiographic Systems. 
         [0005]    In many cases, only the (e.g., smaller) vessels of the parenchyma (e.g., tissue) are of interest in the angiographic images, in digital subtraction angiography, for example, although the vessels are often obscured to a great degree by the large affarent and/or efferent vessels (e.g., arteries, veins). Currently, however, it is not possible to eliminate the large vessels from the projection images obtained by digital subtraction angiography. 
         [0006]    In addition to conventional two-dimensional subtraction angiography, imaging techniques are known that enable a three-dimensional visualization of a vascular system or of the blood flow, referred to as 3D angiography or 3D perfusion. Computed tomography (CT angiography, perfusion CT), magnetic resonance tomography (magnetic resonance angiography, magnetic resonance perfusion imaging) as well as C-arm-based imaging may be cited here by way of example. Basically, these three-dimensional techniques may also apply the subtraction principle, thus enabling pure vessel visualizations to be generated while suppressing the background. With the three-dimensional techniques, it is known to segment large vessels, (e.g., to classify them as such and where necessary to remove them from the visualization). The use of three-dimensional techniques during a treatment, however, is subject to restrictions and the temporal resolution is poorer than in the case of the 2D techniques. 
       SUMMARY AND DESCRIPTION 
       [0007]    The scope of the present invention is defined solely by the appended claims and is not affected to any degree by the statements within this summary. The present embodiments may obviate one or more of the drawbacks or limitations in the related art. 
         [0008]    The object underlying the embodiments is to disclose a method and a device that enable smaller vessels, (e.g., vessels of the parenchyma), to be visualized in the most clearly discernible manner possible in a 2D projection image, (e.g., obtained by two-dimensional dynamic subtraction angiography). 
         [0009]    The method includes: (1) acquiring a 3D dataset of the body region of interest, (2) acquiring at least one 2D projection image of the body region of interest, (3) generating a modified 3D dataset by eliminating vessels whose size exceeds a predetermined limit value, (4) normalizing the 2D projection image using projection data of the modified 3D dataset, (5) eliminating vessel projections in the normalized 2D projection image whose size exceeds a predetermined limit value, (6) interpolating the areas of the 2D projection image in which the vessel projections have been eliminated, and (7) denormalizing the normalized and interpolated 2D projection image using projection data of the modified 3D dataset. 
         [0010]    The computer program product has a computer-readable data medium and a program code of a computer program. The program code is stored on the data medium and is configured to control the method when the computer program is executed on a computer. 
         [0011]    The control and computing unit has a program memory for storing a program code, the program code being configured to control the method. 
         [0012]    The device has a first acquisition apparatus for acquiring a 3D dataset of the body region of interest, a second acquisition apparatus for acquiring at least one 2D projection image of the body region of interest, and a control and computing unit that is configured to control the method. The first and second acquisition apparatus may be embodied as separate acquisition apparatuses or as a common or the same acquisition apparatus. 
         [0013]    The method and the device enable large vessels (e.g., vessels having a predetermined minimum size) to be eliminated in a 2D projection image in order to generate a visualization of the (e.g., smaller) vessels of the parenchyma, referred to as a virtual parenchymogram. 
         [0014]    For this purpose a 3D dataset of the body region of interest is acquired in the first instance, which operation may be carried out independently in time of the acquisition of the 2D projection image. The 3D dataset may be an angiographic image that, in particular, represents the blood volume (e.g., parenchymal blood volume (PBV)). The acquisition of the 3D dataset includes, e.g., the acquisition of two-dimensional X-ray projection images from different projection directions and the reconstruction of a three-dimensional image dataset from the two-dimensional projection images, for example, by a back projection method or some other reconstruction method. These techniques are generally known in the prior art and therefore will not be described in any further detail at this point. 
         [0015]    During, prior to, or after the acquisition of the 3D dataset, a projection image of the same body region is generated, for example, by an X-ray acquisition. 
         [0016]    The 3D dataset contains the vessels of the imaged body region as well as, where applicable, background, such as bone, tissue and/or air, for example. In particular, the 3D dataset contains the vessel geometry, (e.g., information relating to the three-dimensional structure of the vessels and the position of the vessels in space). The 3D dataset therefore contains a three-dimensional map of the vessels of the body region of interest. The body region may be in a “steady state” such that all the vessels are uniformly contrasted, e.g., because they are filled with contrast agent (e.g., static visualization). 
         [0017]    In one embodiment, the vessels in the 3D dataset are segmented, with the result that the 3D dataset exclusively contains the vessel information (e.g., without background). 
         [0018]    The 2D projection image contains a projection visualization of the vessels of the body region of interest and, in principle, may be generated from an arbitrary projection direction. In particular, the 2D projection image contains not only the parenchymal vessels, in other words, the vessels of the tissue, but also the large afferent and efferent vessels. 
         [0019]    Certain vessels whose size exceeds a predetermined limit value are segmented out of the 3D dataset, which initially also contains all of the vessels, e.g., both the parenchymal vessels and the afferent and efferent vessels (e.g., arteries, veins). In this way, a modified 3D dataset may be generated that no longer contains the afferent and efferent vessels, e.g., from this point on includes only the vessels of the so-called microcirculation. The modified 3D dataset may also be referred to as a three-dimensional dataset of the parenchymal blood volume (PBV). The 3D dataset may, if desired, be visualized by a display apparatus and also be referred to as a 3D acquisition, in particular, a 3D PBV acquisition. The modified 3D dataset therefore now contains only the section of the vascular system that is of interest, in particular without the large afferent and efferent vessels (e.g., arteries, veins). 
         [0020]    Projection data may be generated from the modified 3D dataset in a generally known manner by a forward projection. The projection data is used for normalizing the 2D projection image, (e.g., the 2D projection image is normalized using projection data obtained from the modified 3D dataset). For this purpose, the forward projection is performed along the same projection direction as that of the 2D projection image. The projection data of the 3D dataset obtained in this way forms a projection image that, in contrast to the 2D projection image, from now on contains only the regions of interest of the vascular system. By normalization in the present context is to be understood in particular a division of the 2D projection image by the projection image (e.g., projection data) computed from the modified 3D dataset. 
         [0021]    In the normalized 2D projection image, those vessel projections whose size exceeds a predetermined limit value are segmented out and the corresponding areas are filled with artificial data, e.g., by non-linear interpolation. In this case, the non-linear interpolation produces a soft image impression and eliminates the large vessels. All other information, however, in the areas is also lost. 
         [0022]    In order to reinstate certain parenchymal information or, as the case may be, information relating to the vessels of the parenchyma, the interpolated projection data (2D projection image) is denormalized, specifically on the basis of the modified 3D dataset, which exclusively contains the parenchymal volume, e.g., the filling of the parenchyma. Toward that end, analogously to the preceding normalization, line integrals are calculated through the 3D dataset (e.g., containing the vessels of the parenchyma) and used to denormalize the interpolated 2D projection image. In this way, anatomical information is incorporated back into the 2D projection image. Denormalization in the present context is to be understood in particular a multiplication of the 2D projection image by the projection image (e.g., projection data) computed from the modified 3D dataset. 
         [0023]    In one embodiment, the vessels of the 3D dataset whose size exceeds a predetermined limit value are projected into the, in particular normalized, 2D projection image and that the elimination of those vessel projections in the normalized 2D projection image whose size exceeds a predetermined limit value is carried out on the basis of the projected vessels of the 3D dataset. The segmented vessels from the 3D PBV acquisition are projected into the 2D projection image (the 2D iFlow projection, for example) and their two-dimensional “tracks” marked. The large vessels are identified in the 2D projection image on the basis of the marking. The large vessels are therefore segmented out in the 3D dataset. The segmented large vessels are projected into the 2D projection image in order to identify the corresponding large vessels there. In this way, the large vessels may be recognized in the 2D projection image. Because of the spatial information present, the segmentation of the large vessels is easier and more accurate in the 3D dataset than in the 2D projection image. 
         [0024]    In another embodiment, the 3D dataset is generated by computed tomography (CT), magnetic resonance tomography (MRT) or C-arm-based imaging. Examples of a computed tomography modality are DynaCT PBV or multi-slice computed tomography (MSCT). Magnetic resonance tomography may be in particular an MR perfusion scan. In magnetic resonance angiography, an atlas-based segmentation and classification may be carried out in order to compute attenuation maps from the MR data. The 3D dataset (PBV acquisition) may be produced on the basis of a subtraction so that only the fill volume (without environment, for example bone) is included. The 3D dataset may be a static visualization of the vessels in which all of the vessels are filled (without time information). 
         [0025]    The 2D projection image may be generated by the digital subtraction angiography (DSA) method. With this approach, a differential image is generated between a fill image acquired while a contrast agent is administered and a mask image free of contrast agent. The difference is calculated one pixel at a time. As a result of the DSA, there remains as the final image an acquired image depicting only the contrast-agent-filled vascular system (e.g., without tissue, bone, etc.). 
         [0026]    In a particular embodiment, a time series of 2D projection images (DSA series) is acquired. The contrast-agent-free mask image is subtracted from each projection image of the time series so that the uptake of the contrast agent may be visualized. This technique is generally known and will not be described in any further detail here. The generation of the DSA series may also be referred to as dynamic two-dimensional DSA. 
         [0027]    A particularly illustrative visualization of the perfusion may be achieved by generating or calculating, from the time series of 2D projection images (DSA series), a final or cumulative 2D projection image that contains information about the inflow of contrast agent into the vascular system, in particular the parenchyma. With this approach, the individual images of the DSA series (e.g., time-series images) are assembled into a parametric overall image in which specific contrast agent inflow parameters are included, e.g., the time of maximum contrast agent concentration (e.g., time-to-peak), the inflow time instant, the blood flow or the blood volume (e.g., time-to-peak map, 2D parametric DSA visualization). The 2D projection image may be a parametric visualization of the inflow of contrast agent into the vascular system, in particular, in the form of a time-to-peak map. 
         [0028]    In a further embodiment, an associated modified 3D dataset is generated for each image of the DSA series, those regions of the vascular system that are not filled in the respective 2D projection image being eliminated in the 3D dataset, and that the normalization and/or denormalization are/is performed for each image of the DSA series on the basis of the respective associated modified 3D dataset. The background hereto is that in an embodiment, the PBV map (e.g., 3D dataset without the large vessels) represents only one point in time at which the vascular system is completely filled. It is therefore provided at the time of segmentation or forward projection to synchronize which vessel region is filled in the 2D series at the respective point in time. A synchronization in respect of which regions of the vascular system are filled therefore takes place for each image of the 2D series. Those regions that are not filled in the 2D projection image at the respective point in time are eliminated in the 3D dataset. This may take place based on a back projection of the 2D image onto the 3D dataset. By a binary analysis (e.g., voxel filled yes/no?), a modified 3D dataset is generated for each time-series image of the DSA series, in which modified 3D dataset the unfilled voxels in each case have been removed. This act may be performed prior to the normalization and/or denormalization. Furthermore, this act may be performed before the large vessels are segmented out. 
         [0029]    The 3D dataset may be acquired likewise on the basis of a subtraction. In this case a three-dimensional mask image dataset is subtracted from a three-dimensional contrast agent dataset (e.g., voxel-by-voxel subtraction) such that only the signals due to the contrast agent remain. The corresponding method may be described as three-dimensional subtraction angiography. After the large vessels (e.g., arteries, veins) have been segmented out, only the vessels of the parenchyma still remain. The remaining differential image dataset may in this case be referred to as the parenchymal blood volume (PBV). An embodiment therefore may be seen in using a dataset of the parenchymal blood volume (PBV dataset) obtained on the basis of subtraction angiography in order to normalize and/or denormalize the 2D projection image. 
         [0030]    The normalization and/or denormalization may be performed one picture element at a time (e.g., pixel by pixel). In the normalization, the picture elements of the 2D projection image are divided by the corresponding pixels of the forward projection of the 3D dataset. In the denormalization, the picture elements of the 2D dataset are multiplied by the corresponding pixels of the forward projection of the 3D dataset. By this denormalization, in particular in the areas containing the interpolated data, anatomical information from the 3D dataset is incorporated into the 2D projection image. 
         [0031]    The advantages and effects described in connection with the method are achieved by the computer program product, the control and computing unit, and the device. In this respect reference is made to the above statements. 
         [0032]    The device is configured in particular to perform the method. In this regard the provided acquisition apparatuses may also be realized by a single acquisition unit. Said common acquisition apparatus may be for example a C-arm X-ray system. Using the latter it is possible, by rotation around the body, not only to generate a 3D dataset but also to acquire one or more 2D projection images sequentially in time. The control and computing unit is configured to control the generation and subsequent processing of the acquired images. In this way the method may be performed by the device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  depicts a flowchart intended to illustrate acts of an exemplary embodiment of the method. 
           [0034]      FIG. 2  depicts an embodiment of a device for performing the method. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    The individual method acts according to an exemplary embodiment are described below with reference to  FIG. 1 . In a first act S 1 , a 3D dataset of the vascular system is acquired by an angiography system, for example a C-arm device. The dataset may be acquired on the basis of a difference calculation between a fill image in which the vascular system is filled with contrast agent statically, e.g., uniformly or in a “steady state”, and a reference image that is free of contrast agent. The result of the 3D acquisition is a three-dimensional, static visualization of the vascular system. The acquisition contains not only the large afferent and efferent vessels (e.g., arteries, veins) but also the vessels of the parenchyma (in particular, the so-called microcirculation). All of the vessels may be segmented if necessary. 
         [0036]    In a second act S 2 , those vessels whose size exceeds a predetermined limit value are segmented. This may be carried out on the basis of known segmentation methods, for example, by threshold-value-based segmentation. Alternatively, however, a manual segmentation on the basis of a 3D visualization of the dataset is also deserving of consideration, for example. The result of the segmentation is a first modified 3D dataset (e.g., segmented dataset) containing only the smaller vessels of interest (in particular, the vessels of the parenchyma). In addition, a second modified 3D dataset is generated that exclusively contains the large vessels (e.g., arteries, veins) that have been segmented out. 
         [0037]    Acts S 1  and S 2  may be performed at an arbitrary point in time, independently of the following acts and/or independently of a perfusion measurement. 
         [0038]    In a further act S 3 , at least one 2D projection image of the body region of interest is acquired, in particular, by digital subtraction angiography. The acquisition may be performed dynamically by obtaining a plurality of projection images sequentially as a time series shortly after injection of a contrast agent bolus in order to visualize the perfusion of the vascular system dynamically (e.g., with time information). The subsequent acts may be performed on the basis of the individual images of the acquisition series or on the basis of a parametric projection image which combines the individual images of the acquisition series and visualizes them in a common image. 
         [0039]    In act S 4 , the 2D projection image is normalized on the basis of the 3D dataset of the parenchymal volume (e.g., first modified 3D dataset, PBV dataset). In this process, the 2D projection image is divided pixel by pixel by a forward projection of the 3D dataset of the parenchymal volume. The forward projection of the 3D dataset is performed in this case along the same projection direction as the 2D projection image. Toward that end a registration of the forward projection with the 2D projection image may be carried out first. 
         [0040]    In act S 5 , those vessels in the 2D projection image whose size exceeds a predetermined limit value are marked or eliminated. This is accomplished, e.g., on the basis of the segmented large vessels of the 3D dataset (e.g., second modified 3D dataset). By a forward projection of the second modified 3D dataset, which exclusively contains the segmented large vessels, the corresponding large vessels are identified in the 2D projection image and may be classified as such. 
         [0041]    In act S 6 , the regions of the large vessels in the 2D projection image are filled with artificial data, in particular by interpolation, e.g., non-linear interpolation. As a result of the interpolation the anatomical information of the areas hidden by the large vessels is initially lost. 
         [0042]    In order to reintroduce at least some of said information into the image, the 2D projection image is denormalized on the basis of the 3D dataset of the parenchymal volume (e.g., first modified 3D dataset) in act S 7 . Toward that end, the 2D projection image is multiplied pixel by pixel by a forward projection of the 3D dataset of the parenchymal volume. 
         [0043]    Act S 3  may include the acquisition of a time series of 2D projection images in order to visualize the perfusion of the vascular system. Acts S 4  to S 7  are performed for each point in time of the time series, only the regions of the vascular system filled at the respective point in time being taken into account in each case in the 3D dataset. 
         [0044]    In a further act, the post-processed individual projections may then be assembled to provide a new time-to-peak (TTP) calculation and visualization. The new visualization suppresses the large vessels. 
         [0045]    By a time-dependent denormalization of projection values, the embodiments therefore achieve an improved representation of TTP maps, which no longer contain the large and interfering vessels. A three-dimensional dataset of the parenchymal blood volume (3D PBV dataset) may be used for this purpose. This may be acquired independently in time of the acquisition of the 2D projection (DSA acquisition or DSA series) and used on the basis of the 3D information for denormalizing a 2D DSA acquired from an arbitrary angle. A 3D dataset, once present, may therefore be used subsequently for denormalizing a plurality of 2D DSA acquisitions in order to improve their visualization. 
         [0046]      FIG. 2  depicts a device by which the method may be performed and which itself may be embodied or configured. The device includes an acquisition apparatus  30 , specifically an X-ray machine, in particular, a C-arm system. The acquisition apparatus  30  includes an X-ray source  32  and an X-ray detector  33 , which are attached to the arms of a C-arm  31  that may be rotated around a patient couch  38 . The X-ray detector  33  may be a digital detector that is capable of generating digital X-ray images of a patient  40  positioned on the patient couch  38 . The C-arm is movably mounted on a pedestal  34 . 
         [0047]    The movements of the C-arm and the acquisition of X-ray images are controlled by a control and computing unit  35 . Furthermore, the digital X-ray images acquired by the X-ray detector  33  may be transmitted to the control and computing unit  35  and processed there. The control and computing unit  35  includes a data medium or program memory  36 , for example, an internal hard disk on which is stored a program code of a computer program for controlling the acquisition apparatus  30  and for processing the data obtained. The control and computing unit  35  and the program memory  36  may be part of a computer  37  that is, for example, a PC, a workstation, or a console for the acquisition apparatus  30 . In addition, a screen for displaying X-ray images, as well as input devices such as keyboard and/or mouse, may also be present. 
         [0048]    The acquisition apparatus  30  is configured for the purpose of acquiring a 3D dataset of the body region of interest, in particular, of the brain of a patient, based on acquisition of a plurality of two-dimensional projection images from different directions. In addition, the acquisition apparatus  30  is provided for the purpose of acquiring a time series of 2D projection images from a predetermined direction. The acquired series images (DSA series) may be assembled into a parametric visualization (iFlow image), which operation may be performed in the control and computing unit  35 . Alternatively, however, different and/or other acquisition devices may also be provided for the acquisition of the static 3D dataset and of the 2D projection image containing the time information. 
         [0049]    Although the invention has been illustrated and described in greater detail on the basis of an exemplary embodiment, the invention is not limited by the disclosed examples and other variations may be derived herefrom by the person skilled in the art without leaving the scope of protection of the invention. 
         [0050]    The above-described method may be implemented via a computer program product including one or more readable storage media having stored thereon instructions executable by one or more processors of the computing system. Execution of the instructions causes the computing system to perform operations corresponding with the acts of the method described above. 
         [0051]    The instructions for implementing processes or methods described herein may be provided on non-transitory computer-readable storage media or memories, such as a cache, buffer, RAM, FLASH, removable media, hard drive, or other computer readable storage media. A processor performs or executes the instructions to train and/or apply a trained model for controlling a system. Computer readable storage media include various types of volatile and non-volatile storage media. The functions, acts, or tasks illustrated in the figures or described herein may be executed in response to one or more sets of instructions stored in or on computer readable storage media. The functions, acts or tasks may be independent of the particular type of instruction set, storage media, processor or processing strategy and may be performed by software, hardware, integrated circuits, firmware, micro code and the like, operating alone or in combination. Likewise, processing strategies may include multiprocessing, multitasking, parallel processing and the like. 
         [0052]    It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification. 
         [0053]    While the present invention has been described above by reference to various embodiments, it may be understood that many changes and modifications may be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.