Abstract:
In a computed tomography apparatus and operating method, a radiation source and radiation detector are rotated around a system axis, and a patient support plate and diaphragm elements of a diaphragm associated with the x-ray source are also movable in the direction of the system axis. Movement of the patient support plate and the diaphragm plates between respective end positions is coordinated during a dynamic computed tomography examination of a subject so as to reduce and homogenize the dose of x-ray radiation to which the subject is exposed during the examination.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for operating a CT (computed tomography) device and a computed tomography device for performing a dynamic CT examination on a patient, the computed tomography device being of the type having a gantry with a stationary part and a part that can be rotated around a system axis, with an x-ray radiation source and an x-ray radiation detector disposed opposite one another on the rotatable part, and a patient support plate that can be moved in the direction of the system axis. The invention also relates to a non-transitory data storage medium, on which program code is encoded that implements such a method. 
     2. Description of the Prior Art 
     In addition to conventional CT examinations, in which slice images or 3D images of a body region of a patient are reconstructed to obtain information about the morphology of the patient, so-called dynamic CT examinations are now established procedures, that are used to obtain functional information, for example about patient tissue. Contrast agents are frequently used in this type of computed tomography. 
     An example of such dynamic examination is a multiphase examination of the liver of a patient, of which images are produced in different time phases or different states, in order to be able to distinguish between different types of lesions in the liver for diagnostic purposes. In the case of the liver the different phases or states are produced by administering contrast agent, which is absorbed by the different types of lesions at different times. The multiphase examination of the liver therefore includes a so-called native phase, in which no contrast agent is present in the liver, a second arterial phase after the administration of contrast agent and a third venous phase after the administration of contrast agent, following the arterial phase. In order to be able to reconstruct images in all the liver phases, it is necessary to record x-ray projections of the body region containing the liver in all the liver phases over quite a long time period of approx. 30 to 50 seconds, for example to evaluate perfusion parameters. 
     According to a first method the body region of the patient containing the liver is positioned with the patient support plate in the measurement volume of the computed tomography device defined by the x-ray radiation source and the x-ray radiation detector and, with the patient support plate stationary, successive x-ray projections are recorded of the body region of the patient containing the liver, for image reconstruction purposes. A disadvantage of this recording technique is that the examinable body region is limited to the width of the x-ray radiation detector when viewed in the direction of the system axis or the longitudinal axis of the patient, and this cannot easily be extended, at least with existing computed tomography devices. Arbitrary patient movement and respiratory movement make it desirable to have greater coverage in the longitudinal direction of the patient when recording x-ray projections.  FIG. 1  shows the described situation, with the width of the x-ray radiation detector or the extension A 1  of an x-ray projection PR in the direction of the system axis SY defining the scan region S 1  in the direction of the system axis SY or the body region of the patient P to be examined. The dose profile D 1  of the dose of x-ray radiation applied to the patient P during the recording of x-ray projections is relatively homogeneous and is also based on the width of the x-ray radiation detector or the extension A 1  of the x-ray projections PR in the direction of the system axis SY. 
     To avoid the disadvantages of the first method, a second method was created, in which while the measurement system remains otherwise the same, but during the recording of x-ray projections the patient support plate bearing the patient is moved forward and back periodically and continuously when viewed in the direction of the system axis or the longitudinal axis of the patient within a scan region S 2 , so that x-ray projections PR of a longer body region of the patient can effectively be recorded. Compared with the first method, the dose profile D 2  of the dose of x-ray radiation applied to the patient during the recording of x-ray projections PR widens. The distribution of the dose when viewed in the direction of the longitudinal axis of the patient is however comparatively homogeneous.  FIG. 2  shows the method, with which a larger or longer body region of the patient can be examined by moving the patient support plate holding said patient, while the x-ray radiation detector remains in a fixed position. 
     Computed tomography devices are now being used that have a wider x-ray radiation detector when viewed in the direction of the system axis than previously used computed tomography devices. While some years ago so-called 16-slice detectors were still the standard for x-ray radiation detectors, the standard is now 64-slice detectors, or x-ray radiation detectors with even more slices. In the case of wider x-ray radiation detectors, assuming that the scan region does not get longer, since the anatomy of the patient does not change, the shape of the dose profile of the dose of x-ray radiation applied to the patient during the recording of x-ray projections changes. Particularly in the case of a relatively short scan region compared with detector coverage, there is a clear rise in the dose of x-ray radiation in the central body section of the body region of the patient to be examined, which is exposed almost continuously to x-ray radiation despite the movement of the patient support plate, without the additionally obtained information being necessary for diagnostic purposes.  FIG. 3  illustrates the problem. While scan region S 3  corresponds to scan region S 2  from  FIG. 2 , the width of the x-ray radiation detector or the extension A 3  of an x-ray projection PR in the direction of the system axis is much larger than the width of the x-ray radiation detector or the extension A 1  of an x-ray projection PR in the direction of the system axis from  FIG. 2 . Despite the movement of the patient support plate, the body section of the patient to be assigned to the center of the scan region is permanently exposed, so the dose profile D 3  results with a clear rise in the dose of x-ray radiation in the central region. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method and a computed tomography device for performing a dynamic CT examination on a patient and a data storage medium of the type described initially, wherein the dose of x-ray radiation applied to a patient during a dynamic CT examination is reduced and homogenized over the examined body region. 
     According to the invention this object is achieved by a method for operating a computed tomography device for performing a dynamic CT examination on a patient, having a gantry with a stationary part and a part that can be rotated about a system axis, on which rotatable part an x-ray radiation source and an x-ray radiation detector are disposed opposite one another, a diaphragm assigned to the x-ray radiation source, which has diaphragm elements that can be moved in the direction of the system axis to limit an x-ray radiation beam originating from the x-ray radiation source in the direction of the system axis and a patient support plate that can be moved in the direction of the system axis. In accordance with the invention, for a dynamic CT examination of a body region of the patient, the patient support plate is preferably moved forward and back in the direction of the system axis between a first end position and a second end position of the patient support plate, and at the same time the diaphragm elements of the diaphragm are preferably moved forward and back in the direction of the system axis between a first end position and a second end position of the diaphragm elements. 
     In accordance with the invention, the preferably periodic movement of the patient support plate in the direction of the system axis is overlaid with a preferably periodic movement of the diaphragm elements, e.g. diaphragm blades, of a diaphragm assigned to the x-ray radiation source in the direction of the system axis during the recording of x-ray projections, in order to be able to control which body section of the body region to be examined or of the scan region is to be exposed to x-ray radiation. Specific control, in particular of the movement of the diaphragm elements, not only allows the dose of x-ray radiation applied to the patient during the dynamic CT examination to be influenced and preferably reduced, but also allows the dose distribution or dose curve to be influenced, in particular homogenized. 
     The method is primarily provided for dynamic CT examinations, in which the scan region or the body region of a patient to be examined is relatively small given the width of the x-ray radiation detector of the computed tomography device or the extension of an x-ray projection completely covering the x-ray radiation detector when viewed in the direction of the system axis. This is generally the case when the scan region is, for example, shorter or smaller than double the width of the x-ray radiation detector when viewed in the direction of the system axis of the computed tomography device. 
     According to one embodiment of the invention the patient support plate and the diaphragm elements are moved simultaneously in the two opposing directions of the system axis, preferably periodically forward and back relative to one another. Both the diaphragm elements and the patient support plate are moved linearly at preferably constant movement speed in each instance, apart from the reversal points or reversal positions, in the two directions of the system axis. This also allows a higher scan speed to be achieved than with just the movement of the patient support plate. 
     According to another embodiment of the invention the diaphragm elements, viewed in the direction of the system axis, have a certain opening width to limit the x-ray radiation beam originating from the x-ray radiation source in the direction of the system axis, this opening width being selected so that the x-ray radiation beam, when it strikes the x-ray radiation detector, when viewed in the direction of the system axis, covers only part of the detector surface of the x-ray radiation detector. The x-ray radiation beam is therefore shaped or limited specifically in the direction of the system axis, so that only part of the x-ray radiation detector is covered and therefore only part of the body section of the body region to be examined that can be exposed per se with each x-ray projection. Specific control of the movement of the diaphragm elements thus allows over-scanning to be avoided in the central body section of the body of the patient to be examined. 
     According to a further variant of the invention, the patient support plate and the diaphragm elements are moved relative to one another in opposing directions so that, while the patient support plate is being moved from its first end position into its second end position and at the same time the diaphragm elements are being moved from their first end position into their second end position, the x-ray radiation beam covers the x-ray radiation detector completely when viewed in the direction of the system axis. To this end, the movement speeds for the patient support plate and the diaphragm elements are to be selected inter alia as a function of the size of the scan region, the opening width of the diaphragm elements and the width of the x-ray radiation detector when viewed in the direction of the system axis. 
     As mentioned above, the diaphragm elements have a certain opening width, when viewed in the direction of the system axis, to limit the x-ray radiation beam originating from the x-ray radiation source in the direction of the system axis, this opening width remaining constant according to one embodiment of the invention during the movement of the diaphragm elements in the direction of the system axis. 
     According to a further embodiment of the invention, the focus of the x-ray radiation source during the movement of the diaphragm elements in the direction of the system axis is moved in the same direction as the diaphragm elements in respect of the system axis. Depending on the position of the diaphragm elements or the opening width of the diaphragm elements relative to the x-ray radiation source, the focus is therefore tracked on the anode of the x-ray radiation source in the direction of the system axis. 
     The focus is preferably moved spasmodically, in other words following the principle of the so-called springing focus. 
     According to a further embodiment of the invention, as the patient support plate and the diaphragm elements are being moved, x-ray projections of the body region of the patient are preferably recorded from different directions and images of the body region of the patient are reconstructed. 
     The object of the invention is also achieved by a computed tomography device for performing a dynamic CT examination on a patient, having a gantry with a stationary part and a part that can be rotated about a system axis, on which rotatable part an x-ray radiation source and an x-ray radiation detector are disposed opposite one another, a diaphragm being assigned to said x-ray radiation source, which has diaphragm elements that can be moved in the direction of the system axis to limit an x-ray radiation beam originating from the x-ray radiation source in the direction of the system axis, a patient support plate that can be moved in the direction of the system axis and a computing facility, and that has a control unit configured to implement one or all embodiments of the method described above. 
     The above object also is achieved in accordance with the present invention by a non-transitory, computer-readable data storage medium encoded with programming instructions (program code) that, when the storage medium is loaded into a computerized control unit of a computed tomography apparatus, cause the control unit to operate the computed tomography apparatus to implement any or all of the embodiments of the method described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  to  FIG. 3  show principles of dynamic CT examinations of a patient according to the prior art. 
         FIG. 4  shows a computed tomography device according to the invention. 
         FIG. 5  to  FIG. 8  show the principle of the dynamic CT examination of a patient according to the invention. 
         FIG. 9  shows the dose profile resulting during the dynamic CT examination. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Identical element or elements of identical function are shown with the same reference characters in all the figures. The diagrams in the figures are schematic and not necessarily to scale. Without restricting its generality, the computed tomography device  11  is only examined below to the extent that this is deemed necessary for an understanding of the invention. 
     The computed tomography device  11  shown in  FIG. 4  has a gantry  12  with a stationary part  13  and a part  14  that can be rotated about a system axis  15 . In the present exemplary embodiment of the invention the rotatable part  14  has an x-ray system, which comprises an x-ray radiation source  16  and an x-ray radiation detector  17 , which are disposed opposite one another on the rotatable part  14 . During operation of the computed tomography device  11  x-ray radiation  18  is emitted from the x-ray radiation source  16  in the direction of the x-ray radiation detector  17 , penetrates a measurement object and is detected by the x-ray radiation detector  17  in the form of measurement data or measurement signals. 
     The computed tomography device  11  also has a patient couch  19  to support a patient P to be examined. The patient couch  19  comprises a couch base  20 , on which a patient support plate  21  provided to actually support the patient P is disposed. The patient support plate  21  can be moved relative to the couch base  20  in the direction of the system axis  15  in such a manner that it can be introduced, together with the patient P, into the opening  22  of the gantry  12  for the recording of 2D x-ray projections of the patient P, e.g. during a spiral scan. 
     The computational processing of the 2D x-ray projections recorded using the x-ray system and the reconstruction of slice images, 3D images or a 3D data record based on the measurement data or measurement signals of the 2D x-ray projections take place using a schematically illustrated image computer  23  of the computed tomography device  11 . 
     The computed tomography device  11  also has a computing unit  24 , which can be and is used to execute computing programs to operate and control the computed tomography device  11 . The computing unit  24  does not have to be configured as a separate computing unit  24  here but can also be integrated in the computed tomography device  11 . 
     In the present exemplary embodiment of the invention a computing program  25  is loaded into the computing unit  24 , which implements the inventive method for performing a dynamic CT examination on a patient P. The computing program  25  here represents a specific operating mode for the computed tomography device  11  and can be loaded into the computing unit  24  from a portable data medium, for example from a CD  26  or memory stick, or even from a server  27  via a network  28 , which may be a public or internal clinic or hospital network. 
     For a dynamic CT examination of the patient P according to the invention, for example for a dynamic CT examination of the body region of the patient P containing the liver using contrast agent, in the present exemplary embodiment of the invention a diaphragm  30  is assigned to the x-ray radiation source  16 , the diaphragm  30  having two diaphragm elements or diaphragm blades  31  and  32 , which can be moved in the two directions of the system axis  15 . The movement of the diaphragm blades  31 ,  32  can be brought about by one or more electric drives (not shown), which are activated at least indirectly by the computing unit  25 . 
     During the dynamic CT examination of the body region of the patient P containing the liver, a scan region S is first defined in the direction of the system axis  15 , in which x-ray projections of the body region of the patient P are recorded from different directions over approx. 50 seconds. The scan region S, when viewed in the direction of the system axis  15 , is larger than the width B of the x-ray radiation detector  17 . In order to be able to record x-ray projections from the entire scan region S periodically, the patient support plate  21  must be moved forward and back periodically between a first end position E 1PL  and a second end position E 2PL . If in this process the x-ray radiation detector  17  were covered continuously over its entire width B when viewed in the direction of the system axis  15  by the x-ray radiation beam  18  originating from the x-ray radiation source  16 , a relatively high dose of x-ray radiation would be applied to the patient P in the central body section of the body region to be scanned or examined, since a sort of over-scanning would take place there, without being able to use the additional information usefully. 
     For this reason the diaphragm elements  31 ,  32  of the diaphragm  30  are moved by a program controller counter to the patient support plate  21  in the direction of the system axis  15  from a first end position E 1diaphragm  into a second end position E 2diaphragm . The diaphragm blades  31 ,  32  here have a selectable opening width W when viewed in the direction of the system axis  15 , so that, when it strikes the x-ray radiation detector  17 , when viewed in the direction of the system axis  15 , the x-ray radiation beam  18  originating from the x-ray radiation source  16  only covers part of the detector surface of the x-ray radiation detector  17 . As the diaphragm blades  31 ,  32  are being moved and x-ray projections are being recorded, the opening width W remains constant. 
     The patient support plate  21  and the diaphragm blades  31 ,  32  are moved by a program controller in opposite directions relative to one another so that, as the patient support plate  21  is being moved from its first end position E 1PL  into its second end position E 2PL  and at the same time the diaphragm blades  31 ,  32  are being moved from their first end position E 1diaphragm  into their second end position E 2diaphragm , the x-ray radiation beam  18  covers the x-ray radiation detector  17  completely when viewed in the direction of the system axis  15 . To this end the movement speeds for the patient support plate  21  and the diaphragm blades  31 ,  32  should be selected or set correspondingly inter alia as a function of the size of the scan region S, the opening width W of the diaphragm blades  31 ,  32  and the width B of the x-ray radiation detector  17  when viewed in the direction of the system axis  15 . These settings are assisted by the computing program  25 , which preferably also has a graphical user interface, which can be displayed on the display apparatus of the computing unit  24 . 
     In the present exemplary embodiment of the invention the x-ray radiation source  16  is an x-ray tube  16  with a springing focus. In the present exemplary embodiment of the invention the x-ray tube  16  has two foci F 1  and F 2  offset in the direction of the system axis  15 . This makes it possible, as the diaphragm blades  31 ,  32  are being moved in the direction of the system axis  15 , to move the respectively active focus, used to generate x-ray radiation, likewise in the direction of the system axis  15 , in order to be able to generate an appropriate x-ray radiation beam  18  for the scan. 
     The sequence of the dynamic CT examination is illustrated in  FIGS. 5 to 8  for four time points of a periodic movement. 
       FIG. 5  shows the initial situation, in which the patient support plate  21  is in its first end position E 1PL  and the diaphragm blades  31 ,  32  are in their first end position E 1diaphragm . In the present exemplary embodiment of the invention the opening width W of the diaphragm blades  31 ,  32  is selected so that approximately a quarter of the detector surface of the x-ray radiation detector  17  is covered by the x-ray radiation beam  18  originating from the focus F 1  of the x-ray tube  16 . Therefore with this configuration only part of the body region of the patient P to be scanned is penetrated by the x-ray radiation beam  18 . The patient support plate  21  is now moved first in the direction of the arrow a and the diaphragm blades  31 ,  32  are moved in the opposite direction at the same time in the direction of the arrow b. 
       FIG. 6  shows the arrangement from  FIG. 5  at a time point, when the patient support plate  17  has been moved a little in the direction of the arrow a and the diaphragm blades have been moved a little in the direction of the arrow b. 
       FIG. 7  shows the arrangement from  FIG. 5  at a time point when the change from focus F 1  to focus F 2  has taken place, so that the focus follows the movement of the diaphragm blades  31 ,  32 . 
       FIG. 8  shows the arrangement from  FIG. 5  at a time point when the patient support plate  17  has reached its end position E 2PL  and the diaphragm blades  31 ,  32  have reached their end position E 2diaphragm . The end position E 2PL  is also the reversal point for the movement of the patient support plate  17 , which now moves in the direction of the arrow b. The end position E 2diaphragm  is correspondingly the reversal point for the movement of the diaphragm blades  31 ,  32 , which now move in the direction of the arrow a, therefore once again counter to the patient support plate  21 . To this extent the sequence is now reversed (see also  FIG. 8  to  FIG. 5 ). The end positions E 1PI  and E 1diaphragm  also represent reversal points for the movements. 
     While the patient support plate  21  and the diaphragm blades are moved forward and back periodically between their end positions, x-ray projections of the body region of the patient P to be examined are recorded continuously with the rotatable part  14  rotating about the patient P, from which projections slice images are preferably reconstructed with the aid of the image computer  23 . Since the slice images generally follow one another in time, the liver can be displayed in different phases produced by the contrast agent, as described above. 
     It can be seen from  FIGS. 5 to 8  that as a result of the inventive method no over-scanning takes place in the central body region of the body region of the patient P to be examined or scanned, so that a smaller dose of x-ray radiation is applied to the patient P than with a scan, in which only the patient support plate is moved periodically between its end positions with the x-ray radiation detector being covered completely with each x-ray projection (see also  FIG. 3 ). The dose profile D shown in  FIG. 9  is also more homogeneous. 
     The simultaneous movement of patient support plate  21  and diaphragm blades  31 ,  32  also means that a higher scan speed is achieved than with the movement of the patient support plate  21  alone. Also, to achieve the same scan speed as with the method in which only the patient support plate is moved, the speed of the patient support plate can be reduced as a result of the opposing movement of the diaphragm blades, so that the patient is also exposed to slower acceleration speeds to reach the respective speed. 
     Since the movement and positioning of the diaphragm blades can take place very quickly, dynamically triggered heart recordings are also possibly with the inventive method. For these the patient is moved forward and back with the patient support plate between two end positions according to his/her heart rate. If variations occur in the patient&#39;s heart rate, which, due to the inertia of the patient support plate, cannot be compensated for by a corresponding change in the movement speed of the patient support plate, the movement speed of the diaphragm blades is matched to the changed heart rate instead, in order to achieve the desired triggering during the recording of x-ray projections. It is clear from this that the movement speeds of the patient support plate and the diaphragm blades do not have to be constant but can vary or be matched to the recording situation. 
     In contrast to the described exemplary embodiment of the invention the focus of the x-ray radiation source does not necessarily have to be a spring focus. The x-ray radiation source can therefore also have just one stationary focus. 
     The described embodiment of the invention should generally only be considered to be exemplary. In particular settings such as the opening width of the diaphragm blades, the scan region, etc. can also be selected differently. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.