Patent Number: 059237244
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to a medical X-ray diagnostics installation and a method for operating same, the installation being of the type having an X-ray source, with a filter optionally introducible into the beam path, and a detector that receives the X-radiation having individual picture elements and which supplies image signals pixel-by-pixel dependent on the received radiation, and a computer for image generation and output. 2. Description of the Prior Art The exposure of an X-ray image usually ensues by first making a fluoroscopic exposure of the examination subject in order to obtain information in view of, among other things, the illumination distribution of the X-ray exposure. If it turns out that regions of excessively great illumination exist within the exposure, which can be the case due to subject locations of different density and thus different absorbency, then these can be largely eliminated by introducing a corresponding filter, for example a wedge filter or a graduated filter, into the beam path. The radiation dose applied to the subject in this region, and thus the dose ultimately reaching the detector as well thus being reduced, i.e. filtered. The positioning of the filter, for example semi-transparent diaphragm lamellae, is thereby manually carried out by the operator, who inserts the filter into the beam path in conformity with the knowledge about the over-irradiated regions. A further fluoroscopic exposure is required in order to know the correct position of the filter and its filtering effect and in order to decide whether the positioning is adequate, i.e. another radiation image must be registered. This, however, is extremely disadvantageous, particularly since the patient must be exposed to a radiation dose again. Further, the implementation of the second fluoroscopic exposure is disadvantageous for the overall examination sequence itself, particularly in view of time. PCT Application 84/04878 A1 discloses a method as well as an apparatus for the production of filters that are subsequently introduced into the beam path. The production ensues such that a filter is arranged in the beam path in the framework of a first fluoroscopic exposure. Dependent on the transmission signals identified with a pick-up unit, a filter material is then deposited on the filter with an appropriate device. After the production of this filter, it is subsequently placed into the beam path, after which the actual second exposure ensues. SUMMARY OF THE INVENTION An object of the present invention is to provide a method and apparatus which, given adequate knowledge of the effect of the filter on a radiation exposure, enable a minimization of the applied radiation dose and an acceleration of the examination sequence. This object is achieved in a method and apparatus of the invention wherein a first and only image exposure of the examination subject initially ensues without a filter introduced into the beam path. After this single exposure, information identifying a position of the filter in the beam path is supplied to a computer and image signals for those pixels which will be covered when the filter is introduced into the beam path are processed pixel-by-pixel in the computer and a calculated image is generated from these processed signals and from the remainder of the image signals in the single exposure, which will not be effected by the introduction of the filter into the beam path. The calculated image thus represents the same type of image which is produced in the aforementioned conventional systems, with the filter introduced into the beam path, but there is no necessity to again activate the x-ray source in order to produce the calculated image. With particular advantage, thus, the inventive method creates the possibility of pre-calculating the image which would be obtained if the second conventional fluoroscopic exposure were made, i.e. it is possible for the physician to obtain knowledge about the position and the effect of the filter which will be introduced into the beam path, without actually implementing a second radiation exposure. In this way, only the implementation of a single (i.e. the first) fluoroscopic exposure is required; a second exposure is no longer required for obtaining knowledge of the filtering effect. The X-ray dose applied to the patient is thus advantageously considerably reduced and the examination sequence is significantly accelerated. It has proven especially expedient from a processing-oriented point of view to process the gray scale values of the respective picture elements in the framework of the overall signal processing. The processing expediently ensues dependent on specific filter data which may relate to one or more parameters of one or more apparatus components, the number of filters, the geometry of the filter and/or the absorption properties of the filter. The parameters may be normed (normalized) to a reference material thickness dependent, for example, on the X-ray tube voltage. All specific filter data ultimately influence the nature of the filtering as well as the image regions covered by the filter are employed and within the signal processing so that knowledge and incorporation of these influences in the processing leads to adequately exact and diagnostically relevant calculated images. In addition to the specific filter data, the processing can ensue dependent on specific parameters of at least one apparatus component, the generator-side tube operating voltage with which the first radiation image was exposed being particularly relevant. Further, taking specific parameters of all image-generating and output components into consideration is meaningful since these also ultimately enter into the image shown at the respective output means, i.e. processing-specific parameters are to be taken into consideration. The voltage of the X-ray source selected in the framework of the first image exposure and/or specific parameters of the components following the X-ray apparatus can be inventively employed as such specific parameters. Expediently, all component-specific or filter specific parameters are stored in the form of a table, from which the respective parameters are selected for a given situation. From a processing-oriented point of view, it has proven expedient in the framework of the signal processing, for example of the gray scale values, to first determine correction value based on the signal obtained in the framework of the first exposure and to subsequently computationally operated on this value with the obtained signal, particularly subtracted therefrom, for generating the signal employed in the calculated image. For example in the case of the gray scale value processing, a gray scale attenuation value is identified that is then subtracted from the actual initial value in order to obtain the calculated image value. The correction value itself can be inventively determined by computational operation, particularly multiplication, of an attenuation value determined dependent on the specific filter means data and an attenuation value determined dependent on the apparatus component, the specific attenuation value for the apparatus component is preferably a multiplication coefficient. As already set forth, a pre-calculation of an anticipated X-ray image is possible with the inventive method. In an embodiment of the inventive method and apparatus the position of the filter is continuously acquired, and the calculated image is generated dependent on the position or on a change in position. It is thus possible in this embodiment to already define the region within which the processing should ensue by automatic acquisition of the filter position, so that completely automatic operation is achieved. An "on-line" operation that leads to a continuous generation and output of the calculated image can also be enabled with this embodiment for generating a calculated image given a change in the filter position. When the operator readjusts and displaces the filter, for example after output of a first calculated image, then the change in position of the filter is automatically acquired according to this inventive embodiment and a second calculated image is subsequently produced so that the operator can immediately see the effect of the manual modification, for example at the monitor. An "on-line" simulation of the X-ray image is thus possible. The inventive method X-ray diagnostics installation for implementation of the above-described method includes an X-ray source, with a filter optionally introducible into its beam path, a detector that receives the X-radiation, having individual picture elements and which produces image signals pixel-by-pixel dependent on the received radiation, and a computer for image generation and output of the image to an output device, such as a display. The computer generates the aforementioned calculated image that corresponds to an image as would be expected if a further image exposure with the filter introduced in the beam path were made. The calculated image is based on the signals obtained without the filter in a first (and only) image exposure of the examination subject. An arrangement can be provided for acquiring the position of the filter, with the computer generating the calculated image dependent on this position data. The computer can undertake generation of the calculated image dependent on specific filter data, particularly the number of filters, the geometry of the filter, the material of the filter and/or the absorption properties of the filter. Moreover, the computer can generate the calculated image dependent on specific parameters of at least one apparatus component, particularly the voltage of the X-ray source, selected in the framework of the first image exposure, and/or dependent on specific parameters of the components following the detector. In this way, it is possible to virtually take all parameters entering into the attenuation and processing of the respective signals into consideration in the generation of the calculated image. In an embodiment of the invention the acquisition of the position of the filter can be done continuously, and that the computer then can generate the calculated image dependent on a change of the position data in order to enable a continuous display of the calculated image, and thus a tracking of the result which occurs due to a positional change of the filter. Since a computational transition from the signals of the first image exposure ensues to the calculated signals, a modification of the filter position in this "on-line" image generating mode would result in the original signals (before the change in filter position) being lost. Therefore, a memory for the signals obtained in the first image exposure is provided in order to be able to have been able to retrieve these signals in the calculation of a further calculated image. This allows the original signals (for the regions that are no longer covered by the filter in its changed position) to be used in the new calculated image. These original signals would otherwise have to be generated by a back-calculation.