Patent Number: 
Section: description

FIGS. 1 and 2 show respective end views of honeycomb-filter structures for limiting the dynamic range of an X-ray image formed by an X-ray detector by exposure of an object, such as a patient to be examined, to X-rays. The hexagonal cells are formed of capillary tubes, the one ends of which communicate with a reservoir containing an X-ray absorbing liquid. The adhesion of X-ray absorbing liquid to the inner sides of the capillary tubes can be adjusted by means of electrical voltages applied to the respective electrically conductive layers provided on the inner sides of the capillary tubes. In accord with the invention groups of adjacent tubes are in mono-cyclic fashion energized in a way such that in the region of interest or ROI the object to be examined is exposed to X-ray radiation transmitted through the successive groups of filter elements energized in a way such that the X-ray absorbing liquid is during exposure temporarily removed from the capillary tubes in question. FIG. 1 indicates with the respective numerals 1, 2, 3 and 4 the single cycle of energizing the respective capillary tubes. In this case the cycle consists of four phases, viz. the energization of the groups indicated with 1, 2, 3 and 4, successively. FIG. 2 shows an alternative, in which the successive phases of the cycle are indicated with seven different hatchings instead of the numerals used in FIG. 1, clearly showing that each full exposure cycle consists of seven phases. In analogy to the prior art technique of continuous slit scanning with a moving narrow slit the discrete spot scanning apparatus according to the invention can be used to generate one or more fan-like X-ray beams. The advantage of scatter reduction achieved in this way can be enhanced by generating a moving spot pattern on basis of the principles of the present invention. A scatter component is further reduced while the total surface of the exposing spots comprised of a plurality of filter elements can be equally large as the total surface of the slit pattern of a prior art slit scanning device. Spot transmission times can be adapted individually such that the dynamic range of the absorbed signal is reduced thus resulting in a better deployment of the X-ray detector""s dynamic range and a considerable reduction of the X-ray dose to which the object is exposed. Specifically in the case of medical application this is important in view of the desired limitation of the dose to which a patient is exposed. The adjustment of one phase of the sequence of the dynamic beam attenuator takes about 200 ms. The exposure time takes about 10-100 ms. In case of a number of phases of four in accord with the FIG. 1 embodiment the entire exposure time will be a maximum of (3xc3x97200)+4xc3x97100)=1000 ms or 1 s. This result shows that even in the worst case exposure time of 100 ms the purpose of the invention to make a picture within a time period of about 1 s is realized.