Patent Number: 059237244
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The FIGURE shows an inventive X-ray diagnostic installation for the implementation of the inventive method. An X-ray source 2 driven via a high-voltage generator 1 and emits an X-ray beam 3 that penetrates a subject 4 and strikes a detector 5, for example a matrix detector or an image intensifier. The detector 5 is followed by a signal processing chain including a computer 6 for image generation and output and a display 7, for example a monitor. A filter arrangement 8 that effect a pre-filtering of the X-rays is arranged in the X-ray beam of the X-ray source 2. The filter arrangement 8 can, for example, be one or more wedge filters. A number of such filters can be provided in the filter arrangement 8 in order to allow an optimally exact matching to the current conditions. For implementing the inventive method, the X-ray diagnostic installation first is operated to produce a fluoroscopic exposure, the subject 4 being transirradiated with low-dose X-radiation during the course thereof in order to obtain information about the positioning of the subject 4 and about the exposure quality on the basis of the exposure. The X-rays attenuated by the subject 4 strike the detector 5 and are converted therein into radiation-dependent signals that are supplied to the computer 6. This computer 6 contains (or has access to) a memory 9 in which the signals obtained during the course of the first exposure are stored. After processing the supplied signals in the computer 6, the fluoroscopic image is supplied as an output to the display 7. With reference to the output fluoroscopic image, the operator can now determine whether there are over-exposure regions that are to be compensated with the filter arrangement 8. When this is the case, the filter arrangement 8 is placed into the beam path 3 dependent on the position of the over-exposure regions recognizable at the display 7. A position sensor 10 for recognizing the position of the filter arrangement 8 are allocated to the filter arrangement 8, the position of the filter arrangement 8 relative to the detector 7, and thus relative to the image visible at the display 7, being automatically determined therewith. The introduction of the filter arrangement 8 thereby occurs without the production of X-rays. In order to then generate the fluoroscopic image which would (will) exist after introduction of the filter arrangement 8, a number of processing parameters are supplied to the computer 6, particularly the operating parameters of the high-voltage generator 1 that serve as criterion for the quality of the emitted X-radiation. Further, the position data of the filter arrangement 8 identified by the position sensor 10 are supplied to the computer 6, (arrow b). Two tables T1 and T2 that are accessible by the computer 6 (arrows c, d) are also allocated to the computer 6. The table T1 contains absorption values of the filters of the filter arrangement 8 dependent on the operating voltage of the X-ray source 2. This makes it possible to determine the appertaining absorption value for every source operating voltage. This absorption value is specific to each filter, i.e. it takes into account the required filter data such as, for example, the material of the filter, the geometry of the filter, etc. By contrast, correction values, that serve for taking signal variations and fluctuations which occur within the computer 6 into consideration, are stored in table T2. Dependent on the nature of the computer 6 and of the display 7 employed, of course, different influences on the signal processing occur that in turn influence the respective signals which are produced. The calculation of the expected image with the filter arrangement in the X-ray beam ensues pixel-by-pixel. Only the picture elements that are "affected" by the filter arrangement 8, i.e. whose signal would change if a further fluoroscopic exposure were made with the filter arrangement 8 in the beam path 3, are taken into consideration in the processing. These picture elements are known from the knowledge of the position of the filter arrangement 8 obtained from the position sensor 10. These picture elements are now calculated as follows in terms of their gray scale values, this calculation ensuing for every individual picture element: First, the calculation of the filter attenuation S.sub.Image ensues in two steps: 1. Attenuation of the detector entry dose S.sub.Dose : PA0 2. Attenuation of the gray scale value S.sub.Image : S.sub.Dose =value from absorption table T1 dependent on the source voltage. PA1 S.sub.Image =S.sub.Dose .times.correction value from table T2. PA1 G.sub.Calculate =G.sub.Transirradiation -S.sub.Image The image attenuation S.sub.Image obtained in this way is subsequently subtracted from the respective gray scale value in order to obtain the calculated gray scale value: The calculation for the next picture element ensues after determination of the calculated gray scale value G.sub.Calculate until all picture elements in the overexposed region are processed. Subsequently, the computational gray scale values are operated on the other, non-processed gray scale values of the regions that are not overexposed (and thus not covered by the filter arrangement 8) in order to obtain the overall calculated image, which is subsequently supplied as an output. With reference thereto, the operator can then recognize and if necessary correct the pre-filtering which ensues due to the manual introduction of the filter arrangement 8. When such a correction is required, then the filter arrangement 8 is correspondingly shifted, and a new generation of the calculated image ensues due to the renewed position acquisition. In this case, the computer 6 accesses the first fluoroscopic image signals stored in the memory 9 and produces the new calculated image based thereon. Although various minor modifications might be suggested by those skilled in the art, it should be understood that I wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come with the scope of my contribution to the art.