Abstract:
The invention relates to a method for three-dimensional representation of a moving structure by a tomographical method. Projection images are recorded by an image recording unit during a rotational run from recording angles between a start angle and an end angle, with a three-dimensional image data reconstructed from the projection images, with a first perfusion measurement with a first contrast agent injection and a first rotational run and with a further perfusion measurement with a further contrast agent injection and a further rotational run, which is started after the preceding perfusion measurement has concluded, with the start time and/or the start angle of the additional rotational run deviating from one another in respect of the time of the contrast agent injection. The method enables a functional three-dimensional time-resolved imaging of perfusion processes with the aid of flexible C-arm x-ray devices, which allow a functional imaging in an interventional environment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of German application No. 10 2007 037 874.4 filed Aug. 10, 2007, which is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a method for the three-dimensional representation of a moving structure by a tomography method. 
     BACKGROUND OF THE INVENTION 
     The three-dimensional imaging with C-arm x-ray devices was hitherto only implemented statically, i.e. without temporal information. 
     To this end, the C-arm rotates by approximately 200° (180°+fan angle) about the patient and in this way records between 50 and 1000 images of the patient. A three-dimensional image can be obtained from these recordings from different projection directions with the aid of a suitable reconstruction method, like for instance the filtered back projection, which is described in “Practical Cone-beam Algorithm” by Feldkamp et al., J. Opt. Soc. Amer. A, Vol. 1, No. 6, June 1984, pages 612 to 619, or the algorithm described by Klaus Mueller et al. in “A Fast and Accurate Projection Algorithm for 3D Cone-Beam Reconstruction with the Algebraic Reconstruction Technique (ART)”. 
     Two rotational runs are generally implemented in order to represent vascular trees. A contrast agent is injected into the vessel to be displayed in the second rotational run. A three-dimensional image in which only the contrast agent-filled vessels are displayed is obtained by subtracting both runs and a subsequent reconstruction. 
     The functional imaging, like for instance perfusion imaging, is generally implemented using CT and MR. These imaging modalities are however generally not available in an interventional setting. 
     DE 10 2004 057 308 A1 relates to an angiographic x-ray diagnostics device for rotation angiography comprising an x-ray emitter which can be moved on a circular path, an opposing image detector unit which can be moved on a circular path, a digital imaging system for recording a plurality of projection images, a device for reconstructing a 3D volume image and a device for correcting physical effects and/or inadequacies in the recording system such as truncation correction, scattered radiation correction, irradiation correction, ring artifact correction, correction of the beam hardening and/or of the low frequency drop for the soft tissue representation of projection images and the 3D volume images reconstructed therefrom. 
     In the former patent application DE 10 2006 035 067.7, a method for the temporal and three-dimensional representation of a periodically changeable structure is described, in which several rotational exposures are created. The necessary rotational runs are started at an identical event of the period process offset by a specific angle. New image series are combined from the rotational exposures, whereby three-dimensional representations are reconstructed to form different phase regions of the periods. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of embodying a method such that a functional three-dimensional time-resolved imaging of perfusion processes with the aid of flexible C-arm x-ray devices is enabled, which allows a functional imaging in an interventional setting. 
     The method relates to a three-dimensional representation of a moving structure by a tomography method, with which a series of projection images are recorded by an image recording unit during a rotational run from different recording angles between a start angle and an end angle, with it being possible to reconstruct three-dimensional image data from the projection images. 
     The object is inventively achieved in accordance with the claims:
         comprising a first perfusion measurement with a first contrast agent injection and at least one first rotational run and   at least one further perfusion measurement with a further contrast agent injection and at least one further rotational run, which is started after the preceding perfusion measurement has concluded,   with the start time and/or the start angle (φ 0 ) of the at least one further rotational run deviating from one another in respect of the time of the contrast agent injection.       

     It has proven advantageous in the case of an identical rotational direction of the image recording unit during N perfusion measurements for the individual delay times between the contrast agent injection and recording sequence of the nth perfusion measurement to be shifted by the n/Nth duration of a complete rotational exposure (rotation duration including return motion), compared with the first perfusion measurement. 
     In accordance with the invention, the delay time between the contrast agent injection and the recording sequence of the second perfusion measurement may be greater, but also smaller than the delay time with the first perfusion measurement. 
     The second perfusion measurement can advantageously be started temporally prior to the second contrast agent injection, i.e. the delay ΔT can be negative. As a result, a rotational exposure without a contrast agent can be created as a mask image. 
     Alternatively, in the case of opposite rotational directions of the image recording unit during the perfusion measurements, in accordance with the invention the perfusion measurements can however begin at the same start time, in respect of the injection time, however with a reverse start angle. 
     It has proven advantageous for the following steps to be implemented:
     a) Start a first contrast agent injection,   b) Await a delay time (delay) ΔT 1 ,   c) Record several rotational exposures during rotational runs with the same rotational direction and return motion between the rotational runs,   d) Optional pause of any length   e) Start a second contrast agent injection   f) Await a delay time (delay) ΔT 2 =ΔT 1 +½*(rotation time+return motion time),   g) Record several rotational exposures during rotational runs with the same rotational direction and return motion between the rotational runs,   h) Interpolation of rotational exposures at fixed times t i  from two perfusion measurements,   i) Reconstruction of three-dimensional data records at the times t i  and   j) Derivation of functional parameters and graphical representation (as is conventional with perfusion CT).   

     Alternatively, the following steps can also be implemented in accordance with the invention:
     a) Start a contrast agent injection   b) Await a delay time (delay) ΔT,   c) Record the rotational exposures during rotational runs with an alternating rotational direction from a starting position (e.g. 0°),   d) Optional pause of any length,   e) Start a second contrast agent injection,   f) Await a delay time (delay) ΔT,   g) Record the rotational exposures during rotational runs with an alternating rotational direction from a final position (e.g. 180°+fan angle),   h) Interpolation of rotational exposures at fixed times t i  from the two perfusion measurements,   i) Reconstruction of three-dimensional data records at times t i  and   j) Derivation of functional parameters and graphical representation (as is conventional with perfusion CT).   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in more detail below with reference to the exemplary embodiments shown in the drawings, in which; 
         FIG. 1  shows a schematic view of an x-ray C-arm system for implementing the method according to the invention, 
         FIG. 2  shows a graphical representation of a perfusion measurement consisting of four rotational runs in the same direction in respect of the recording angle, 
         FIG. 3  shows a graphical representation of a perfusion measurement consisting of four rotational runs in the opposite direction in respect of the recording angle, 
         FIG. 4  shows a graphical representation of two inventive perfusion measurements, each with four rotational runs in the same direction in respect of the recording angle, 
         FIG. 5  shows a graphical representation of two inventive perfusion measurements, each with four rotational runs in the opposite direction in respect of the recording angle, 
         FIG. 6  shows the inventive method sequence to create rotational exposures with the same rotational direction and return motion and 
         FIG. 7  shows the inventive method sequence to create rotational exposures with an alternating rotational direction without return motion. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described as exemplified by an x-ray C-arm system  1  with reference to  FIG. 1 , in which an x-ray tube  3  and an x-ray detector  4  are affixed opposite one another on a C-arm  2  as an image recording unit. The C-arm  2  is attached to a supporting stand  5  and can be moved by means of a control and regulating module  6 . An examination object  7 , for instance a patient, is positioned on a patient support couch  16  for examination purposes in the pivotal point of the C-arm  2 , which can be freely rotated about the examination object  7 . 
     During a movement of the C-arm  2 , a rotational run, the examination object  7  can be x-rayed from different projection angles. An image data computer  8  can reconstruct a three-dimensional image data record from a plurality of such projection images, a so-called rotational exposure and display it on a monitor  9 . 
     A contrast agent injector  10 , a stimulator  11  and an ECG device  12  or a comparable measuring instrument for recording and evaluating runs of an object of interest with a structure within the examination object  7  can be connected to the examination object  7 . The respective device is connected to the signal input  13  of the control and regulating model  6  of the x-ray C-arm system  1  by way of a suitable signal output by means of a switch  15  for instance. An angle sensor  14  for detecting the rotation angle during a rotational run is assigned to the C-arm  2 . 
     Since the number N of necessary rotational runs was determinate particularly as a function of the run speed of the periodic process and the travel speed of the C-arm  2 , a differential angle is determined from this information which determines the start angle φ 0  for each rotational run and adjusts the C-arm  2  for the rotational run by means of the control and regulating module  6  with the aid of the angle sensor  14 . Depending on which process is to be represented, the contrast agent injector  10 , the stimulator  11  or the ECG device  12  or the comparable measuring instrument for recording and evaluating sequences of the object of interest is connected to the structure within the examination object  7 . 
     In order to be able to map the propagation processes of the contrast agent in tissue and/or in organs, the so-called perfusion, the concentration of the contrast agent in the tissue is measured as a function of the time. As these propagation processes take place very quickly in the majority of tissues (5 to 30 seconds), as close a temporal scanning of the contrast agent process as possible poses a significant challenge. 
     To detect the contrast agent concentration in a quantitative fashion, it is necessary to generate an accurate three-dimensional reconstruction of the contrast agent distribution. To this end, it is necessary to record projections through the object from an angular range of approximately 200° (180°+fan angle), the so-called rotational runs and to reconstruct a three-dimensional volume image herefrom. 
     Two problems essentially arise here:
         The projection images, which are used for a reconstruction, are not recorded at the same time (data inconsistency).   The recording of a three-dimensional data record using a C-arm system requires approximately 4 to 10 seconds. A temporal scanning value can thus only be measured every 4 to 10 seconds.       

     If a number of rotational exposures are implemented consecutively, as is required for the perfusion measurement, the individual projections in respect of recording time and recording angle (φ) can be graphically represented according to  FIGS. 2 and 3 . 
       FIG. 2  shows a schematic representation of the dependency of the recording angle (φ) of the C-arm  2  plotted on the y-axis over the time t plotted on the x-axis. In a first examination sequence, the rotational exposure can always be implemented in the same direction. Each rotational run is thus represented by a slanted ascending dot and dash line  20   a ,  20   b ,  20   c  and  20   d . After injection by means of the contrast agent injector  10  at time t 1 , the first rotational run is started after an adjustable delay time (delay) ΔT, with projection images being created at different angles. This is shown by means of the line  20   a , which starts at t=ΔT as well as at a specific recording angle, the start angle φ 0 , and ends at t=T (duration of a rotational run T R  of 4 to 5 seconds for instance) and a higher recording angle of 1.25π for instance, the end angle φ T . Temporal gaps result here for the return motion, the return motion time t r . 
     In a second examination sequence, the individual projection images are recorded in opposite rotational runs, so that no empty return motions are needed, as is shown in  FIG. 3 , in which the angulation is plotted over the time t. Each rotational run is alternately illustrated by a slanted ascending and/or slanted descending dot and dash line  21   a ,  21   b ,  22   a  and  22   b.    
     To reconstruct a three-dimensional data record, a rotational exposure at a fixed time t 0  can now be interpolated from the measured rotational exposures or estimated with a suitable method, like for instance nearest-neighbor-, linear-, spline-interpolation or curve fitting with suitable functions. It should be noted here that the measured times of a projection are not equidistant in the case of an alternating rotational direction. 
     Any number of times can in principle interpolate in this way; the scanning of the perfusion process is however not changed. 
     To be able to represent temporally quick processes in a sufficiently effective fashion, it is necessary to improve the actual temporal scanning of the process. The technical limitations of the x-ray C-arm system does not render this readily possible with an individual perfusion measurement. 
     If the assumption is made that the tissue is again completely in the initial state following the perfusion measurement, additional data can be recorded in a second perfusion measurement after a second injection of contrast agent.  FIGS. 4 and 5  indicate with a dashed line how additional data can be obtained expediently using a second perfusion measurement. 
     Here the dot and dash lines  20   a ,  20   b ,  20   c  and  20   d  in  FIG. 4  show the rotational run for the first rotational exposures using a first contrast agent fill according to  FIG. 2 . 
     The delay time (delay) ΔT 1  from the first contrast agent injection to the first rotational run is determined from the time of the first appearance of the contrast agent in the region of interest (ROI). In the exemplary embodiment according to  FIG. 4 , the rotational direction of all rotational exposures is identical. The delay ΔT 2  between the contrast agent injection and the recording sequence with the second perfusion measurement is to be increased by a half a duration of a complete rotational run (rotation duration T R  including return motion t r ) compared with the first perfusion measurement so that the delay ΔT 2  with the second contrast agent injection results from: 
     
       
         
           
             
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     In general, 
               Δ   ⁢           ⁢   Tn     =       Δ   ⁢           ⁢   T   ⁢           ⁢   1     +       1   N     ⁢           *     (       t   R     +     t   r       )               
can apply to N rotational runs for the nth rotational run.
 
     The slanted inclining lines  23   a ,  23   b ,  23   c  and  23   d  show the run of the second rotational exposure following a second contrast agent fill. 
     In  FIG. 5 , the instance of the opposite rotations is shown as in  FIG. 3 . The slanted descending dashed lines  24   a  and  24   b  as well as the slanted ascending dashed lines  25   a  and  25   b  show the run of the second perfusion measurement following a second contrast agent fill. With these opposite rotations, the perfusion measurements are to begin with the same delay ΔT, however with an opposite initial position. 
     The recording of two perfusion measurements with a suitable temporal position in respect of each other allows the temporal scanning rate to be doubled. This also allows for such perfusion processes, for which a simple perfusion measurement is too slow as a result of technical limitations of the C-arm  2 . 
     The method sequences according to the invention are described in summary below using the following steps with reference to the  FIGS. 6 and 7 : 
     Embodiment 1 (According to FIG.  4 ) 
     
         
         S 1  Start a first contrast agent injection, 
         S 2  Await a delay time (delay) ΔT 1 , 
         S 3  Record several rotational exposures with the same rotational direction and return motion between the exposures, 
         S 4  Optional pause of any length 
         S 5  Start a second contrast agent injection, 
         S 6  Await a delay time (delay) ΔT 2 =ΔT 1 +½*(rotation time+return motion time), 
         S 7  Record several rotational exposures with the same rotational direction and return motion between the recordings, 
         S 8  Interpolation of rotational exposures at fixed time t i  from the two perfusion measurements, 
         S 9  Reconstruction of three-dimensional data records at times t i  and 
         S 10  Derivation of functional parameters and graphical representation (as is conventional with perfusion CT). 
       
    
     Embodiment 2 (According to FIG.  5 ) 
     
         
         Sa) Start a contrast agent injection, 
         Sb) Await a delay time (delay) ΔT, 
         Sc) Record the rotational exposures with an alternating rotational direction from the initial position (e.g. 0°), 
         Sd) Optional pause of any length, 
         Se) Start a second contrast agent injection, 
         Sf) Await a delay time (delay) ΔT, 
         Sg) Record the rotational exposures with an alternating rotational direction from the final position (e.g. 180°+fan angle), 
         Sh) Interpolation of rotational exposures at fixed times t i  from the two perfusion measurements, 
         Si) Reconstruction of three-dimensional data records at the times t i  and 
         Sj) Derivation of functional parameters and graphical representation (as is conventional with perfusion CT). 
       
    
     In accordance with the invention, the delay ΔT can also be selected to be negative, i.e. that the second run begins temporally prior to the first run by comparison with the injection times. It may even be started before the second injection. An exposure without contrast agent can as a result be created as a mask image.