represented in fig1 is an x - ray tube 2 which is operated by a high - voltage generator 1 and emits an x - ray beam 3 , which passes through a patient 4 . the x - ray tube 2 is mechanically coupled to an x - ray image amplifier 6 by means of a c arch . instead of the rigid mechanical coupling by the c arch 5 , however , electronic coupling may also be provided if the x - ray tube 2 and the x - ray image amplifier 6 are , for example , arranged on freely positionable telescopic arms . the fluorescent output screen of the x - ray image amplifier 6 is coupled via an optical system 7 to a ccd camera 8 , the output signal of which is fed to an image system 9 connected for display purposes to a monitor 10 . the components are controlled in their time sequences by a system control and communication 11 . fastened to the patient 4 is an ecg electrode 12 , the output signal of which is connected to the control of the high - voltage generator ( as still to be explained further ). if this x - ray diagnostic system can be used to produce radiographs for rotation angiography , for example , a phase - angle sensor 13 which is likewise connected to the system control 11 can be attached to the c arch 5 . the clock and control signals of an x - ray diagnostic system according to the prior art are explained in more detail on the basis of fig2 to 5 . shown in fig2 is the triggering of the ccd camera 8 . for example , a reset pulse 14 , which brings about an automatic reset , is generated every 264 ms . this reset is intended to prevent an excessively high dark signal from accumulating on the ccd image converter . these reset pulses 14 result in a readout of the ccd image converter , shown in fig3 which is represented as a dark readout 15 . in our example , this readout time of the camera is 66 ms . shown in fig4 is the external trigger , which is generated for example from the ecg captured by the ecg electrode 12 of a person being examined or on the basis of an angle triggering controlled by the phase - angle sensor 13 . on the basis of this trigger pulse 16 , the radiation pulses 17 of the x - radiation represented in fig5 are generated , at the end of which , controlled by an exposure control which is not shown , a readout pulse 18 is set with the camera trigger , whereby the ccd image converter is read out , as shown by the readout signals 19 to 22 in fig3 . in the case represented , the interval of the external trigger pulse 16 from the automatic reset pulse 14 is very much smaller than 264 ms , so that the dark signal component makes up a very small proportion of the first readout signal 19 . if the external trigger pulse 16 then occurs at a later point in time within the reset interval of 264 ms , the dark signal increases and , in the case in which averaging takes place over almost 260 ms and which is represented by the dashed pulses 23 to 26 , is very high and disruptive . the pulses 23 to 26 occur instead of the pulses 16 to 19 . the further external trigger pulses 16 control the radiation and after that the readout of the ccd image converter in a known way . these external trigger pulses 16 may occur at irregular times , since in the first example they are controlled by the ecg and in the second example , when there is a movement of the image generating system , they depend on the speed during acceleration or braking . as a result , different times t 1 to t 3 are also obtained between the individual camera trigger pulses , so that the dark signal of the corresponding readouts can also vary correspondingly . the image converter of the ccd camera 8 is an it image converter ( interline transfer ), in which the charge accumulated in the light - sensitive region is transferred by a trigger pulse ( 14 , 18 or 23 ) within the shortest time ( for example ≦ 300 μs ) into a memory area which is shielded from the incidence of light ; after that , the actual exposure of the light - sensitive region of the ccd image converter can then immediately take place . from this memory area , the actual readout of the charge corresponding to the exposure from the shielded region also takes place immediately and is then fed to the image system 9 as a video signal . this readout operation lasts 66 ms . the operating principle of the x - ray diagnostic system according to the invention is now explained in more detail on the basis of the clock and control signals represented in fig6 to 11 . shown in fig6 is the reset triggering of the ccd camera 8 with the reset pulses 14 . on the basis of these reset pulses 14 , recurring every 264 ms , a dark readout 15 takes place , which is represented in fig7 . if in the case of the external trigger in fig8 there occurs a trigger pulse 16 , which may be triggered by the ecg electrode 12 and / or the phase - angle sensor 13 , this brings about a reset pulse 27 , which results in a dark readout 28 . triggered with an offset time span of less than or equal to 300 μs is a radiation pulse 17 ( represented in fig9 ), the length of which is dependent on the thickness of the subject under examination ( is correspondingly corrected ). generated on completion of either the dark readout 28 or the radiation pulse 17 is a readout pulse 18 ( represented in fig1 ), which brings about a readout of the image , as shown by the readout signals 29 represented in fig1 . however , this results in a dark readout 28 taking place immediately before a readout signal 29 . thus , the dark signal component contained in the useful signal is always the same and very small , irrespective of the time spans t 1 to t 3 . the it image converter of the ccd camera 8 is consequently activated in such a way that , with the reset pulse 27 , the dark charge accumulated in the light - sensitive region is transferred within a time of for example 300 ms into the memory area shielded from the incidence of light . this is followed by the exposure of the light - sensitive region of the ccd image converter during the radiation pulses 17 . at the same time , the dark signal is read out from the memory area . after this dark readout 28 , the transfer of the charge image produced during the actual exposure into the memory area takes place with the readout pulse 18 . from this memory area , the actual readout of the charge from the shielded region corresponding to the exposure then also takes place immediately , and is then fed as readout signal 29 to the image system 9 . the pulses represented in fig6 and fig1 and the pulses represented in fig7 and fig1 usually lie respectively on the same lines , as represented in the case of fig2 and 3 . they have only been shown separately to give a better overview . the activation according to an embodiment of the invention for another case is now explained in more detail on the basis of fig1 to 14 . shown in fig1 is again the external trigger signal , in fig1 the camera trigger and in fig1 the control signal for the x - radiation . if the interval between a reset pulse 14 and the external trigger pulse 16 is then less than 66 ms ( but more than 300 μs ), an x - ray trigger pulse 30 which starts the x - radiation is generated directly by the trigger pulse 16 . subsequently , the transfer of the charge of the ccd image converter is triggered by the readout pulse 18 . if , however , as represented in fig6 to 11 , the interval between the reset pulse 14 and the external trigger pulse 16 is more than 66 ms , the further reset pulse 27 is first set , bringing about a renewed dark readout 28 . only after completion of the reset pulse 27 is an x - ray trigger pulse then set , so that the x - radiation is started with something of a time offset with respect to the external trigger pulse 16 . then the transfer of the charge of the ccd image converter is triggered by the readout pulse 18 . a simpler construction of the system control 11 is obtained if , when the external trigger pulse 16 occurs , the x - ray trigger pulse 26 is delayed by the time which is required to transfer the charge from the light - sensitive region of the ccd image converter into the memory area . in the example described , this is 300 μs . as a result , this control does not need to consider whether the external trigger pulse 16 is removed from the reset pulse 14 by less or more than 66 ms . the readout pulse 18 also need not be set 66 ms after the trigger pulse 16 . in the case of the example described on the basis of fig1 to 14 , it can also be generated directly after completion of the x - radiation ( not represented in these figures ) and be triggered by it . in the case of the known x - ray diagnostic systems , the reset pulses generated every 264 ms serve for the so - called dark triggering , to prevent an excessively high dark signal on the ccd image converter when no external triggers take place . if the external triggering takes place , the dark signal is dependent on the times of the triggering . in the case of the x - ray diagnostic system according to an embodiment of the invention , on the other hand , a reset pulse 14 for dark triggering also takes place every 264 ms as long as there is no external triggering , but after that , triggered by the external trigger , a reset pulse 27 for dark triggering always takes place and the actual exposure takes place after that with a delay of about 300 μs . it is consequently ensured that all the images have the same dark signal ( tending toward zero ). only in the special situation in which the first external trigger pulse 16 comes within a window of 66 ms after the last periodic ( 264 ms ) reset pulse 14 for the dark triggering would the camera be triggered twice . however , the camera needs 66 ms after a trigger to respond again meaningfully to the next trigger . this is to do with the readout mechanism of the ccd image converter . otherwise , there are some regions in the ccd image converter that have already been read out and some that have not yet been read out . this would then lead to a different dark signal distribution in the actual image . therefore , in this case the camera trigger ( dark ) is suppressed , in order that all the images of the series have exactly the same preconditions , that is to say dark signal contents . exemplary embodiments being thus described , it will be obvious that the same may be varied in many ways . such variations are not to be regarded as a departure from the spirit and scope of the present invention , and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims .
an x - ray diagnostic system includes a ccd camera , a device for generating external trigger pulses and a system control . the system control is formed in such a way that , in the absence of x - radiation , a readout of the ccd camera without a useful signal takes place at regular time intervals . to reduce the dark signal component in the useful signal of the ccd camera , the system control is formed in such a way that , when an external trigger pulse occurs at a point in time at which no readout of the ccd camera is taking place , firstly a readout without a useful signal is triggered and then an exposure of the ccd camera takes place . further , when an external trigger pulse occurs at a point in time at which a readout of the ccd camera is taking place , a readout without a useful signal is suppressed before an exposure of the ccd camera .