X-Ray Diagnostics Installation

1. Field of the Invention

The present invention is directed to an x-ray diagnostics installation, and in particular to such an installation having means for monitoring the average image brightness on the output luminescent screen of the x-ray image intensifier, and controlling the operation of the x-ray tube based thereon.

2. Description of the Prior Art

An x-ray diagnostics installation is disclosed in European Application 0 217 456 which includes a detector for the average image brightness of the output luminescent screen of the x-ray image intensifier in a predetermined region of the screen. The detector is a semiconductor surface on which the entire output image of the x-ray image intensifier output screen can be imaged, and includes means for selecting a predetermined region of the semiconductor surface for use in generating the control signal. The detector consists of an array of a plurality of individual detector elements, the outputs of the individual elements being combined to form a brightness signal for the selected measuring field An output amplifier must be provided for each detector element, which requires a considerable circuit outlay, and reduces the resolution obtainable by the array because of the physical space which must be occupied by each amplifier, thereby limiting the number of detector elements which can be provided in a given area.

It is an object of the present invention to provide an x-ray diagnostics installation of the type described above wherein the circuit outlay is reduced, and wherein measuring fields having variable shapes can be selected.

The above object is achieved in accordance with the principles of the present invention in an x-ray diagnostics installation wherein the detector is a large-area semiconductor detector, with a diaphragm disposed in front thereof consisting of a liquid crystal matrix. The diaphragm is controlled by a control unit, which selects a predetermined region of the total area of the semiconductor detector on which light will be permitted to be incident. A particularly low circuit outlay for signal editing results. Selection of the measuring field using the liquid crystal matrix is accomplished electronically. In contrast to the conventional array of light-sensitive discrete elements, only a single amplifier is required in accordance with the principles of the present invention, instead of the large number of amplifiers required to form the measured signal in conventional units.

FIG. 1 is a schematic block diagram of an x-ray diagnostics installation constructed in accordance with the principles of the present invention.

FIG. 2 is a plan view of a detector constructed in accordance with the principles of the present invention for use in the x-ray diagnostics installation of FIG. 1.

An x-ray diagnostics installation constructed in accordance with the principles of the present invention is shown in FIG. 1. The installation includes an x-ray tube 1 which is fed by a high voltage generator 2. A patient 3 is transilluminated by x-radiation generated by the x-ray tube. X-radiation attenuated by the patient 3 is incident on an input screen of an x-ray image intensifier 4. A light image corresponding to the x-ray image is present on the output luminescent screen of the x-ray image intensifier 4. The light from this image is supplied to a video camera 5, which generates a signal supplied to a video processor 6, for display on a TV monitor 7.

To maintain the average picture brightness in a measuring field of the output screen of the x-ray image intensifier 4 at a constant level, a semi-reflective mirror 14 directs a portion of the light from the output luminescent screen to a semiconductor detector 8. The semiconductor detector 8 functions as an actual value generator, and supplies a signal to an actual value input of a comparator 9 via a transducer 10. The comparator 9 is supplied with a rated value 11, which may be set, for example, by a potentiometer 12. Depending upon the difference between the actual value and the rated value, the high voltage generator 2 is controlled by a brightness control 13, connected to the output of the comparator 9. The operating parameters of the x-ray tube, such as filament voltage and current, can be adjusted to generate an x-ray dose to maintain a desired brightness level. The semiconductor detector 8 has a surface on which the entire output image of the x-ray image intensifier 4 can be imaged by the semi-reflective mirror 14. A control unit 15 electronically selects a region of the semiconductor detector 8 which will be used to generate the actual value signal, in accordance with a desired measuring field. The semiconductor detector 8 permits selection of a number of different measuring fields, which may vary in position, shape and size.

Further details of the semiconductor detector 8 are shown in FIG. 2. The semiconductor detector 8 is a large-area detector formed by a single detector element, for example, a single photodiode. For selecting the desired measuring field, a diaphragm is disposed in front of the semiconductor detector 8 consisting of a liquid crystal matrix formed by a plurality of liquid crystals 17. The respective light transmissivity of the individual liquid crystals 17 is controlled by the control unit 15. Each crystal can be controlled to admit or block incoming light to the detector 8. In FIG. 2, for example, the shading indicates the selection of three measuring fields, i.e., a central measuring field and two lateral measuring fields.

In the embodiment shown in FIG. 2, signal interpretation within a measuring field is also possible. In conventional devices, the measuring signal of each individual photodiode of the measuring field matrix can be electronically multiplied by a weighting factor before adding the outputs to form the total actual value signal. The selection of weighting factors may be selected, for example, depending upon the organ of the patient under examination. In the detector of the present invention, such a weighting can be achieved by driving only a portion of the liquid crystals 17 of the matrix within a measuring surface so as to be transmissive. An adaptation to the subject to be examined can thus be achieved by varying the liquid crystals driven transmissive per unit of surface area in the region of the measuring field.

Moreover, peak value control can be achieved by serially reading out the individual matrix elements of the selected measuring field with only the maximum value of the signal distribution being used to form the actual value. To match the brightness control to different subjects, known methods of pattern recognition can also be applied in selecting the matrix elements which are to contribute to the formation of the actual value signal.

The above-described techniques for forming measuring fields for a video camera can also be employed if a sheet film (photographic) camera is used.

If the semiconductor detector 8 is used for direct exposure, instead of an ionization chamber, the advantages described above of a flexibility in the formation of the measuring field, signal weighting, and average value/peak value control, can be implemented. Because the image intensifier is arranged following the film cassette in such installations, exposure corrections which, for example, can be stored in tabular form in the memory of an automatic exposure unit, are needed to compensate for the effects of increased radiation hardness occurring upon passage of the radiation through the cassette. The degree of transparency of the patient to the radiation can be calculated by a test transillumination before initiating the direct examination exposure, and the selected gating will then be entered in the automatic exposure unit. The necessary exposure correction can then be taken from the stored table.

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 his contribution to the art.