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 fig4 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 spring focus . in the present exemplary embodiment of the invention the x - ray tube 16 has two focuses 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 fig5 to 8 for four time points of a periodic movement . fig5 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 . fig6 shows the arrangement from fig5 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 . fig7 shows the arrangement from fig5 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 . fig8 shows the arrangement from fig5 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 fig8 to fig5 ). 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 fig5 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 fig3 ). the dose profile d shown in fig9 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 .
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 .