Abstract:
In addition to a radiation source ( 6 ) attached to a C-arm  4 , and in addition to a detector ( 7 ), a rotation angiography device ( 1 ) for the angiocardiography has an evaluation unit ( 8 ), which formulates models with low resolution from the projection images supplied by the detector ( 7 ) for the moving object to be examined, and which generates movement fields for the projection images generated by the detector ( 7 ) on the basis of the model, so that movement-corrected projection images can be calculated from the projection images, which can be used to formulate a three-dimensional high-resolution model of the object to be examined.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to the German application No. 10 2004 017 478.4, filed Apr. 8, 2004 which is incorporated by reference herein in its entirety.  
       FIELD OF INVENTION  
       [0002]     The invention relates to a device for obtaining structure data of a moving object with: 
        a radiation source which can be moved about an object along a measurement path;     a detector arranged on the opposite side of the object and which can be moved according to the movement of the radiation source and,     an evaluation unit which determines the structure data of the object from the projection images.        
 
       BACKGROUND OF INVENTION  
       [0006]     A device of this type is generally known from US 2002/0186871 A1 and from the field of three-dimensional angiocardiography. Three-dimensional structure data of the moving heart is obtained in rotation angiocardiography, in which an X-ray source and an X-ray detector rotates about the patient at an angular range greater than 180° and thus records projection images of the moving heart. The structure data of the object can then essentially be determined in the form of three dimensional spacial coordinates from the projection images.  
       SUMMARY OF INVENTION  
       [0007]     One problem here is that the heart of a patient beats multiple times during the long rotation of the X-ray source and X-ray detector which typically takes 3 to 10 seconds. The movement of the heart results in major artifacts during the three-dimensional reconstruction of the structure data of the heart.  
         [0008]     Attempts have hitherto been made to solve this problem of angiocardiography in that during the recording of the projection images the relevant phase of the heart movement is determined with the aid of an electrocardiograph. Subsequently fewer projection images are selected from the projection images recorded during a complete rotation, said projection images displaying the heart in a still phase.  
         [0009]     This selection results in the substantial deterioration of both the spacial resolution and the contrast resolution of the three-dimensional structure data.  
         [0010]     Conversely, it is also possible to extract more projection images for obtaining the structure data but in this case, the temporary resolution deteriorates.  
         [0011]     An object of the invention is therefore, using this prior art as a starting point, to create a device with which structure data of a moving object can be obtained with a high spacial and time resolution.  
         [0012]     This object is achieved by the claims. Further advantageous embodiments and developments are set down in the dependent claims.  
         [0013]     In addition to a radiation source and a detector which move about the object along a measurement path, the device has an evaluation unit in which a moderator from the projection images in various time frames determines an object model valid for the relevant time frame in each instance. A movement analyzer of the evaluation unit can then derive model projection images with the aid of the object model, and determine movement fields for the projection images from the model projection images. A movement compensator in the evaluation unit is thus able to calculate movement-corrected projection images on the basis of the movement field, so that a reconstruction unit of the evaluation unit can finally determine the structure data of the object from the movement-corrected projection images.  
         [0014]     The concept behind the invention is that valuable movement information can still be obtained from the object models calculated from the modelator despite the poor local and contrast resolution. Then movement fields of the projection images can be estimated from the different object models assigned to the various time frames. This is possible in particular if the details of the moving object which initially do not appear in the structure object model, move in a similar manner to the associated roughly structured regions of the object to be examined.  
         [0015]     A large number of recorded projection images can be used since, with the device for the reconstruction of the structure data, the structure data has an improved local and contrast resolution in comparison with the prior art. It is also possible to achieve a high time resolution as the recorded projection images can essentially be movement-corrected at each time point.  
         [0016]     In a preferred embodiment the object to be examined is a cyclically moving object and the device has a movement sensor by means of which the phases of the cycle can be detected. The movement correction is facilitated in this way, since the phases of the cycle do not have to be estimated from the recorded projection images themselves.  
         [0017]     If the device is used within the context of angiocardiography for examining a moving heart, the movement sensor is preferably an electrocardiograph.  
         [0018]     In a further preferred embodiment, the evaluation unit of the device is designed for an iterative data reduction. In this case, the reconstruction unit creates structure data sets assigned to various time points and the movement analyzer determines refined model projection images from the structure data sets, said model projection images serving to calculate movement fields arranged around addition details for the conventional projection images. The evaluation unit can repeat the data reduction as often as necessary until the calculated structure data sets only change slightly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Further features and advantages of the invention are set down in the description below, in which the exemplary embodiments of the invention are described in detail with reference to the accompanying drawing, in which:  
         [0020]      FIG. 1  shows an angiography device for the three-dimensional rotation angiocardiography;  
         [0021]      FIG. 2  depicts a logical function unit in an evaluation unit of the angiography device in  FIG. 1 ;  
         [0022]      FIG. 3  shows a diagram displaying the temporal development of a projection angle of the angiography device and an electrocardiogram of an examined patient;  
         [0023]      FIG. 4  shows a, diagram in which the phase angle of recorded projection images is displayed in relation to the projection angle of the angiography device and the relative time of a heart cycle;  
         [0024]      FIG. 5  shows a diagram illustrating the sequence of the data reduction carried out by the evaluation unit of the angiography device;  
         [0025]      FIG. 6  shows a diagram displaying the phase location of the originally recorded projection images and the phase angle of the movement-corrected projection images in relation to the projection angle of the angiography device and the relative time of a heart cycle. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0026]      FIG. 1  shows an angiography device  1  which serves to obtain structure data from a heart of a patient  2 . This structure data can be a three-dimensional model of the heart of the patient  2 . The structure data can also be sections through the heart of the patient  2 . Furthermore, the structure data can be obtained with or without time resolution.  
         [0027]     The patient  2  typically lies on a table  3 , which can be rotated by a C-arm  4 . The C-arm  4  is attached to a support  5  which allows the C-arm  4  to rotate about the patient  2 . Furthermore, the C-arm  4  has an X-ray source  6  and an X-ray detector  7 , which serves to record projection images of the heart of the patient  2 . The projection images generated by the X-ray detector  7  are supplied to an evaluation unit  8 , which has an image processing unit  9  with connected image mememory  10  and an electrocardiograph  11  with a connected heart signal memory  12 . The electrocardiograph  11  is connected to the patient  2  by means of electrodes  13 .  
         [0028]      FIG. 2  shows logical function units of the image processing unit  9 . The image processing unit  9  has a modelator  14  which generates heart models assigned in each instance to various time frames from the heart signal data stored in the heart signal memory  12  and from the projection images stored in the image memory  10 . A movement analyzer  15  determines a series of movement fields with the aid of a model detected by the modelator  14 , with the aid of which a movement compensator  16  corrects the movement of the heart in the original projection images. A reconstruction unit  17  determines the structure data of the heart from the movement-corrected projection images from the movement compensator  16 , said structure data being stored in a structure data memory  18 . In a further iteration of the data reduction, the structure data generated by the reconstruction unit  17  can be made available to the movement analyzer  15 , from which it calculates refined movement fields.  
         [0029]     The function of the individual logical function units of the image processing unit  9  shown in  FIG. 2  is described below in detail.  
         [0030]      FIG. 3  shows a movement diagram  19 , in which the projection angle θ of the C-arm  4  is applied against the time t, whilst the C-arm  4  moves about the patient  2 . The projection angle increases continuously with time based on the continual movement of the C-arm  4 .  
         [0031]     Furthermore,  FIG. 3  contains an electrocardiogram  20  of a patient  2 . The so-called R-jags  21  are clearly noticeable based on which it is possible to assign a heart cycle  23  of the heart of a patient  2  to a rotation status  22  of the C-arm  4 . A heart cycle  23  extends in each instance from an R-jag  21  to the next R-jag  21 . As the movement of the heart is cyclically repeated, a relative time t rel  can be assigned to the movement process of the heart within a cycle, said relative time assuming values between 0 and 1.  
         [0032]     It should be noted that the projection angle θ represents the whole set of position data, which completely describes the position of the C-arm  4 .  
         [0033]      FIG. 4  now displays a diagram, in which phase angles  24  of the projection images generated by the X-ray detector  7  are plotted in relation to the projection angle θ and the relative time t rel .  
         [0034]      FIG. 4  particularly displays the phase angles  24  of projection images which lie in a time frame  25 , which is centered on t rel =0.7 and has a width of Δ=0.2. From the projection images which can be assigned to the time frame  25 , the modelator  14  from  FIG. 2  now calculates a model for the heart of the patient  2 , which is assigned to the relative time t rel =0,. This model generally suffers from a poor local and contrast resolution.  
         [0035]     In addition, the modelator  14  in further time frames  25  calculates further models of the heart. The phase strips are therefore preferably centered on the relative times t rel =0.1/n, 2/n, 3/n . . . n-1/n and distributed equally over the heart cycle  23 . A value Δ=1/n or greater is chosen as the width of the time window  25 , whereby the latter results in a time frame  25  to be overlapped.  
         [0036]     As a result, the modelator  14  generates a four-dimensional data set  25 , as displayed in  FIG. 5 , said data set having a plurality of models  27  of the heart assigned to various time points t rel .  
         [0037]     The movement analyzer  15  mentioned in relation to  FIG. 3  generates model projection images  28  at various projection angles θ, with the aid of model  27 . To create the model projection images  28 , methods known to a person skilled in the art, for example referred to as DRR (digitally reconstructed radiograph) or MIP (maximum intensity projection) are used.  
         [0038]     A three-dimensional data set  29  is shown in  FIG. 5 , which has model projection images  28  assigned to a specific projection angle θ at various relative times t rel .  
         [0039]     The movement analyzer  15  now calculates movement fields  31  in an analysis process  30 , said movement fields describing the movement of the image structures in the two-dimensional model projection images  28 . In this way for example, search algorithms which are used in conjunction with the video coding can be accessed.  
         [0040]     After the movement fields  31  have been obtained, the movement compensator  16  performs a movement compensation of the type illustrated in  FIG. 6 .  FIG. 6  displays phase angles  33  of projection images in a time frame  32 , said phase angles being assigned to the time frame  32 . The original phase angles  33  are conveyed to the corrected phase angles  34  with the aid of the movement compensator  16 , so that each projection image corresponds to the associated corrected projection images which are recorded at a fixed time t rel  and at different projection angles θ. The generation of movement-compensated projection images at time point t rel =0.7 is illustrated for example in  FIG. 4 , said projection images being used whose phase angles  33  is located within a time frame  32  of the width Δ=0.4.  
         [0041]     The projection images movement-compensated in this manner are finally fed to the reconstruction unit  17 , which generates the structure data of the heart of a patient  2  from the movement-compensated projection images.  
         [0042]     As already mentioned, the structure data can also be three-dimensional models of the heart of a patient  2  associated with various relative times t rel , from which the movement analyzer  15  can calculate refined movement fields  31 , so that refined structure data sets result with an iteration of the method carried out by the evaluation unit  8 , which can again be fed to the movement analyzer until the structure data sets no longer change or change only slightly.  
         [0043]     It should be noted that the evaluation of the heart signal of the electrocardiograph  11  is not necessary in each case. In fact, the heart cycles  23  can also be approximately estimated from the original projection images.  
         [0044]     Furthermore, it should be noted that the device described here can essentially also be used for examining objects not moving cyclically.  
         [0045]     Finally please note that terms such as modelator, movement analyzer, movement compensator or reconstruction units are to be understood as functional. These logical units do not necessarily have to form physical units, but can also be realized in a physical unit in the form of software and conversely distributed over a plurality of physical units.