Patent Number: 046776524
Section: summary

The invention relates to an apparatus for slit radiography comprising an X-ray source adapted to irradiate a body under examination through a slit diaphragm with a planar fan beam performing a scanning movement during operation, which slit diaphragm is divided into a plurality of juxtaposed sections; detection means for producing, during operation, electrical output signals and coacting with an X-ray detector mounted behind the body, which detection means are divided into sections corresponding to those of the slit diaphragm; and slit control means connected with the detection means for controlling X-ray attenuating elements coacting with each slit section. By means of such an apparatus, which is disclosed in Dutch patent application 84,00845, the X-rays transmitted can be adjusted to the variations in absorption by the body under examination for each section of the slit diaphragm, whereby the quality of the final image can be enhanced considerably. In this manner, an image can be obtained that has sufficient contrast in both dark and light regions for allowing details to be seen well. Dutch patent application 84,00845 discloses a number of embodiments in which either the position of the attenuation elements is controlled by generating a suitable magnetic field, or the attenuation elements are piezoelectric strips the position of which is determined by applying a suitable electrical voltage thereto. The instantaneous magnitude of the control signals adjusting the position of the attenuation elements is determined for each section by the difference between the signal from the detection means associated with the particular section and a predetermined reference signal. Optimum operation of an apparatus of the above type requires an unambiguous relation between the control signal applied to the attenuation elements and the position of these elements, so that a reproducible adjustment of the position of the attenuation elements is achieved. Controlling the position of the attenuation elements by means of a magnetic field has appeared to result in the occurrence of magnetic hysteresis effects. Reset springs coupled to the attenuation elements can exhibit (mechanical) hysteresis effects too. The same applies to attenuation elements in the form of spring reeds. Furthermore, piezoelectric strips have likewise appeared to exhibit hysteresis effects. Such hysteresis effects result in the absence of an unambiguous relation between the position of an attenuation element and the control signal. Furthermore, a reduction of the control signal to zero does not guarantee a return of the attenuation element into its rest position. Consequently, there is a need for slit control means capable of bringing the attenuation elements into a position resulting in the desired attenuation at any desired moment, regardless of the occurrence of hysteresis effects. It is an object of the invention to meet this need. To this end, in accordance with the invention an apparatus of the above type is characterized in that the slit control means include an energizing circuit for each pair of corresponding sections of the detection means and the slit, under the influence of which energizing circuit the position of at least one attenuation element coacting with the particular slit section is changed and which energizing circuit is included in a control circuit closed via the radiation passing through this slit section, penetrating through the body and incident upon the corresponding section of the detection means, which control circuit further includes a comparison circuit having its output coupled with the energizing circuit and having an electrical signal from the section of the detection means applied to its one input and a predetermined reference signal applied to its other input, the energizing circuit being so operated that the difference between the input signals of the comparison circuit continuously pursues zero value.