BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is directed to a system for the registration of radiation images of the type, having a radiation pick-up device and a control device that controls the operation thereof.  
           [0003]    2. Description of the Prior Art  
           [0004]    Systems of the above type for the registration of radiation images are utilized, for example in medical technology, for the registration of X-ray images. They can be employed in ordinary X-ray systems; however, employment in computed tomography is also possible.  
           [0005]    The central element of such a system is the radiation pick-up device. Components referred to as a-Si-panels that have a scintillator layer, mainly of CsI, are for use in such a known device. The incident x-ray quanta are converted into visible light therein, and the light is processed by a following, amorphous semiconductor layer wherein a matrix of photodiodes is fashioned. This matrix is followed by dedicated read-out electronics. Dependent on the quantity of light arising from the conversion, charge, i.e. electrons, is generated in the photodiodes, which is read out via the dedicated read-out electronics. A problem associated with such radiation pick-up devices is the present of non-variable light coupling between the scintillator and the photodiode matrix. This causes there to be hardly any possibilities for variation of the conversion efficiency (incident x-ray quanta to output voltage of the panel). This means that no variation of the conversion efficiency or of the gain can be achieved given different operating modes or different pick-up modes that operate with x-radiation having different doses. In medical technology, for example, modes for producing fluoroscopic or transillumination exposures, and digital radiography or digital subtraction angiography exposures, are often implemented in alternation.  
           [0006]    The first-cited operating mode operates with a low x-ray dose with simultaneous pick-up of many images; the latter operating modes operate with x-rays of a high dose per individual image that is registered. Since no variation of the conversion efficiency is possible given known a-Si panels, these panels must be selected such that no over-modulation occurs given pick-up of images having a high radiation dose. This, however, causes an increase in electronic noise occurs for fluoroscopic or transillumination exposures, particularly compared to known x-ray image intensifier video systems.  
           [0007]    Alternatively to such a-Si panels, radiation pick-up devices are known that employ a layer referred to as a HARP layer (HARP—high gain avalanche rushing amorphous photoconductor). Such a HARP layer is composed of a charge layer that generates electrical charges dependent on the incident x-rays and an electrode layer allocated thereto that is chargeable with high-voltage for triggering an electron-multiplying avalanche effect in the charge layer via, causing a potential to arise in the high-voltage-condition. The read-out ensues using an electron beam that scans the HARP layer. Such a radiation pick-up device is known, for example, from German OS 44 10 269. In this radiation pick-up device, a high-voltage is connected between the electrode layer and the emitter cathode that generates the electron beam. This high-voltage causes high electrical fields to arise in the charge layer, which is preferably composed of amorphous silicon. The electrical fields ultimately produce an avalanche effect in the amorphous semiconductor charge layer. This multiplies the electrical charges exponentially for increasing the electrical potentials. Strong electrical fields are required in order to generate this avalanche effect, but such fields are able to be achieved in a relatively simple way by making the charge layer extremely thin. A considerable signal gain can in fact be achieved as a result; however, this known system is likewise a rigid system that does not allow any variation in gain.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a system of the type initially described that allows the gain to be adapted to the image exposure mode which is to be implemented in a simple way.  
           [0009]    This object is inventively achieved in a system is provided for the registration of radiation images, having a radiation pick-up device and a control device controlling the operation thereof, wherein the radiation pick-up device has: a charge layer that generates electrical charges dependent on the incident radiation and an allocated electrode layer chargeable with high-voltage for triggering an electron-multiplying avalanche effect in the charge layer by producing a potential across the charge layer, a read-out device for reading out the generated charges in the charge layer by means of an electron beam, and wherein the potential across the charge layer can be modulated for varying the gain of the charge layer caused by the avalanche effect.  
           [0010]    The invention is based on radiation pick-up device as disclosed, for example, by German OS 44 10 269. For solving the aforementioned problem, the invention proceeds from the perception that a variable gain for the signals that are generated on the basis of the incident x-radiation can be achieved by modulating the potential across the charge layer. By modulating the voltage via the charge or HARP layer, the local gain due to the avalanche effect can be varied. The avalanche effect, i.e., its intensity, is dependent on how large the potential is between the free surface of the charge layer and the coupled electrode layer. The avalanche effect is more pronounced the higher the potential is and vice versa.  
           [0011]    This allows that the inventive system to adapt the gain of the radiation pick-up device to the currently selected image exposure mode in a simple way. When, for example, it is necessary to register transillumination images with a low x-ray dose and radiography images with a high radiation dose, then the gain can be switched between the two different operating modes by a corresponding modulation of the layer potential. Given image registrations with low radiation dose, a high gain is selected; a lower gain suffices given exposures with a low radiation dose.  
           [0012]    The layer potential can be modulated in a simple way by varying the high-voltage that is applied to the electrode layer, controlled by the aforementioned control device. The voltage can be modulated either before or during the registration of a radiation image, by setting the free surface of the charge layer to a pre-selected potential. Additionally, the phenomenon that the potential across the charge layer is somewhat reduced dependent on the of induced charge carriers can be used to advantage, so that a gain reduction arises by itself during the exposure, even though it is slight. Since the curve of this gain reduction is known by virtue incident quantity of charge carriers, a linear amplitude characteristic can be determined, producing the advantage that there is hardly any over-modulations; further, any modulation range can be optimally scanned in view of the signal-to-noise ratio.  
           [0013]    The amplitude of the high-voltage that is applied preferably should be continuously variable dependent on the dose of the incident radiation in order to thus be able to optimally adapt the gain to the operating mode employed.  
           [0014]    The electrode layer can be arranged on a film-like carrier, particularly on a glass film, such as by printing, and can be composed of a of essentially parallel layer strips spaced from one another. A closed electrode layer surface, however, is also conceivable.  
           [0015]    The read-out device can be a flat emitter device, so that an extremely low overall structure of the radiation pick-up device is achieved. The surface emitter device can have linear electron emitter cathodes having allocated horizontal and/or vertical deflection electrodes. Alternatively, the surface emitter device can have micro-structured electron emitter cathodes arranged in a matrix or an array, for example in the form of nano tubes or small emitter micro-tips.  
           [0016]    It is expedient for the radiation pick-up device to be integrated in a flat vacuum housing wherein stabilization elements, particularly in the form of structural webs, are provided, to intercept the significant high forces that act between the large-area sides.  
           [0017]    It is also expedient to provide at least one reset light source for the exposure of the charge layer, which should be capable of being operated in pulsed fashion via the control device. Using this reset light source and given simultaneous activation of one or more or of all electron emitters, it is possible to stabilize the free surface of the charge layer to a lower potential compared to the potential that was previously present. As a result, it is possible in a simple way to lower the sensitivity of the radiation pick-up device before the registration of radiation images having a high dose that were preceded by registrations having a low dose, wherein, thus, registration was carried out with a high gain.  
           [0018]    Overall, the inventive system offers a of advantages. First, the employment of a-Si panels provided with photodiodes having allocated switches can be foregone, since the inventive system and radiation pick-up device operate with an electron beam that scans line-by-line. This has the advantage that parasitic capacitances are minimized due to the elimination of the switch capacitances with a simultaneous increase of the fill factor and of the maximum charge that can be scanned (since the scanning electron beam allows an enhanced voltage boost of the pixels). The avoidance of the photodiodes, further, is advantageous because a significantly more beneficial inertia behavior is established, particularly when switching between the various operating modes. The afterglow behavior of known a-Si panels is essentially defined and dominated by the inertia behavior of the a-Si photodiodes, which are no longer present in the inventive system.  
           [0019]    The employment, in particular, of micro-structured flat emitter cathodes also leads to lower acquisition costs and devices having a longer service life. The greater range of dynamics established due to the possibility of varying the gain also allows employment in multi-line computed tomography detectors as well as in x-ray photon-counting detectors. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a schematic illustration of an inventive radiation image pick-up system having a radiation pick-up device shown in an exploded view.  
         [0021]    [0021]FIG. 2 is an enlarged illustration of the region  11  from FIG. 1.  
         [0022]    [0022]FIG. 3 is a schematic illustration of a micro-structured electron emitter cathode.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    [0023]FIG. 1 shows an inventive system  1  composed of a radiation pick-up device  2  and a control device  3  that controls the operation thereof. The radiation pick-up device  2  is arranged in a housing  4  (which is not shown in detail). The device  2  has a substrate  5  at the beam entry side identified with the arrow, for example in the form of a glass carrier, on which a scintillator layer  6  is applied, for example, in the form of CsI needles. This is followed by a conductive electrode layer  7  of, for example, ITO (indium tin oxide) or S n O 2 . This electrode layer  7  should be as thin as possible (in the range of a few 100 Å) in order to avoid stray effects. A charge layer  8 , preferably of amorphous silicon, is applied on this electrode layer  7 . X-ray quanta incident thereon initially penetrate through the substrate  5  and subsequently penetrate into the scintillator  6  wherein conversion into visible light occurs. This light subsequently penetrates the extremely thin electrode layer and is incident on the charge layer  8 . Dependent on the intensity of the penetrating light, charges are generated in the charge layer  8 .  
         [0024]    These charges are read out by an electron beam with a following read-out device. This read-out device has a cooperating cathode  9  followed by a of linear cathodes  10  which can, for example, be coated tungsten wires. These linear cathodes  10  serve as electron beam sources. Further, vertically converging electrodes  11 ,  12  are provided, as are vertically deflecting electrodes  13 . Further, an electron beam control electrode  14  as well as a horizontally converging electrode  15  and a horizontally deflecting electrode  16  are provided. The read-out device also has an electrode  17  that accelerates the electron beam, and a retarding electrode  18 .  
         [0025]    In the illustrated example, the linear cathodes  10  extend horizontally and enable the generation of an electron beam having a linear horizontal expanse. Of course, more than the four electrodes  10  that are shown can be provided, dependent on the size of the panel. The cooperating electrodes  9  serve the purpose of generating a potential gradient with the vertically converging electrodes  11  in order to prevent the generation of electron beams from cathodes  10  other than the cathode driven for the emission of the electron beam.  
         [0026]    Each vertically converging electrode  11  and  12  is plate-shaped and has a of oblong slots  19 , each slot lying opposite a linear cathode  10 . Each of the electron beams emitted by the cathodes  10  passes through a slot  19 , causing the beam to converge vertically.  
         [0027]    The vertically deflecting electrodes  13  are allocated to the respective slots  19  and are composed of upper and lower conductor  20  between which an insulator  21  is provided. When a voltage is applied between two conductors  20  lying opposite one another in two different electrodesl 3 , then an electron beam that passes therethrough is deflected.  
         [0028]    The electron beam control electrode  14  is composed of a number of individual electrodes that each have an oblong slot  22 . An electron beam can pass only through the slot of a correspondingly driven electron beam control electrode. An electron beam that passes through is employed for reading out the signals of a number of horizontally arranged pixels, for example ten pixels, i.e. distributions of electrical potential on the charge layer  8 . After the ten pixels adjacent to this currently driven electrode are read out, then the electron beam control electrode skips ahead to the next driven electrode.  
         [0029]    The horizontally converging electrode  15  is likewise plate-shaped and has a number of individual slots  23  that are respectively positioned opposite the slots  22 . This electrode  15  causes the electron beam to be contracted horizontally to form a thin ray corresponding to the size of a pixel or to a distribution of potential.  
         [0030]    The horizontally deflecting electrode  16  also has the shape of a conductive plate that is composed of individual plate segments. When a voltage is applied between two neighboring plate segments, then the electron beam can be horizontally deflected, and the allocated pixels or distributions of potential, for example ten pixels, are horizontally scanned.  
         [0031]    The acceleration electrodes  17  also are plate-shaped here and serve the purpose of accelerating the electron beam. The retarding electrode  18  has the shape of a grid conductor with numerous grid openings and serves the purpose of retarding the electron beam immediately before the charge layer  8  and of guiding the electron beam such that it strikes the charge layer at the correct angle.  
         [0032]    As shown, a high-voltage V is applied to the electrode layer  7 , the amplitude thereof being controlled via the control device  3 . As a result, a high-voltage is also present across the charge layer  8 . This induces an avalanche effect in the charge layer  8 , dependent on the amplitude of the high-voltage that is applied as well as on the number of electrons that are generated in the quanta-to-photon. By variation of the high-voltage V, the gain via the charge layer  8  can be set, so that switching can be carried out in a simple way between different operating modes that need different gains. This can ensue very quickly, particularly by using reset light  24  serving the purpose of exposing the charge layer  8 . This reset light  24  can be operated, for example, in a pulsed manner by the control device  3  and causes the potential at the free surface of the charge layer  8  to be stabilized. The reset light  24  is mainly utilized for stabilizing the potential and thus for setting a desired potential when the following image exposure was previously preceded by an image exposure having low radiation dose, and thus a high gain.  
         [0033]    [0033]FIG. 2 shows the enlarged excerpt II from FIG. 1 in the form of a schematic diagram, showing the substrate  15 , for example in the form of a glass plate, onto which the scintillator  6  is applied. An intermediate carrier  25  is in turn applied on the scintillator  6 , for example in the form of the glass plate. An intermediate carrier  25 , for example in the form of a glass film, is in turn applied thereon, the electrode layer  7 , preferably being printed on the intermediate carrier  25 , for in the form of the ITO electrode. The electrode layer can be composed of a number of parallel, preferably vertically arranged, electrode stripes. Finally, the charge layer  8  is applied onto the electrode layer  7 . As shown, the high-voltage V is applied to the electrode layer  7 .  
         [0034]    [0034]FIG. 3 is a schematic diagram of a second embodiment of an inventive system  26 . The structure at the beam entry side (substrate, scintillator, electrode layer, charge layer) is the same as in the previously described embodiment, however, a different readout device is employed in the embodiment of FIG. 3. In this read-out device, a micro-structured electron emitter cathode  27  is provided as a flat emitter device, this being shown in the form of a schematic diagram. Any micro-structured emitter cathode that allows a targeted, punctiform emission of the electrons can be utilized, for example in the form of nano-tubes or micro-tips. Here, as well, the emitted electron beam is shaped by corresponding electrodes (not shown) and strikes the charge layer for the readout, a potential due to the high-voltage V at the electrode layer also being present across the charge layer.  
         [0035]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.
A system for the registration of radiation images has a radiation pick-up device and a control device controlling the operation thereof. The radiation pick-up device has a charge layer that generates electrical charges dependent on the incident radiation and an allocated electrode layer that is chargeable with high-voltage for triggering an electron-multiplying avalanche effect in the charge layer by virtue of a potential produced across the charge layer by the high-voltage. A read-out device reads out the generated charges in the charge layer by means of an electron beam. The potential via the charge layer can be modulated for varying the gain of the charge layer caused by the avalanche effect.