Patent GB-1374610-A (GB)
Title: Radiographic apparatus
IPC: A61B6/03
Filed: 1971-11-29
Abstract: 1374610 X-ray apparatus MEDINOVA AB 29 Nov 1971 [30 Nov 1970] 55404/71 Heading H5R [Also in Division G1] In radiography, in order to even out the exposure of the recording medium (a film fluorescent screen or image intensifier) over the whole area of the recording medium and so compensate for differences from area to area which occur due to differences in thickness, density at different portions of the object (person) being radiographed, a plurality of X-ray detectors are arranged beyond the object to sense the average intensities at different areas, and by means of a plurality of respective control means, a compensating filter device, which has a plurality of portions corresponding in number to the different areas, is varied in thickness by different amounts for the different respective areas. If a film is present while adjustment of filter device is made, the intensity from the source is low enough to avoid exposure of the film. Alternatively, the film is absent, but a low intensity is still used to avoid unnecessary exposure of the patient. The radiograph is then carried out with full intensity. The detectors may be located beyond the image plane; alternatively they may be located above the image plane and be sufficiently translucent to avoid shadow images on the film or mounted for removal before the radiograph is taken. The filter device may be moved outside the radiation path during the adjustment and then returned to the path for the radiographic exposure. In the embodiment of Fig. 2, 1 is the X-ray source, 3 the subject being radiographed and 5 the image plane where the recording medium is located. The filter device comprises two solid wedge-shaped absorbers 11a, 11b, which may be varied in position so as to prevent different thicknesses at two areas in the radiation path. The absorbers are moved under control of servomotors receiving the outputs of differential amplifiers 13a, 13b which compare signals representing the intensity in the respective areas of detectors 14a, 14b, with that at a central area of detector 15 which acts as a reference. Fig. 3 (not shown) illustrates a more complex arrangement, with two arrays of elongated detectors (and filter device elements) on either side of a central elongated reference detector. Since a diaphragm arrangement, formed by four plates and which is adjustable in two dimensions, is provided, the control link to the filter device includes potentiometers to allow for the positions of the diaphragm plates. Construction of filter device.-Besides solid bodies, liquid and powder media may be used. The arrangement of Fig. 4, comprises a radiation absorbing liquid 24, e.g. mercury or other liquid metal, a solution, e.g. cesium acetate solution, or a suspension, contained within a rigid base member 26 and a flexible, e.g. rubber diaphragm 27 having attached thereto a plurality of wires 28 coupled to the servomotors 31 so that the thickness of the device can be varied at different points across its area. The diaphragm has higher rigidity along lines joining the wires In another arrangement, Fig. 10, the medium 36 is a powder mixed with binding agent, a paste or a jelly carried in a tray 35. A dispensing member 51 is moved along the tray and its exit slot is shaped under the control of the servomotors so as to deliver the required layer thickness of the medium at different areas actoss the tray. At the same time a slit diaphragm is moved across the image plane. The tray may be arranged for use with a powder, by constructing the base 45 to be permeable to air but not to powder, which is subject to a reduced pressure at 47, so that the powder is maintained stable. In the arrangement of Fig. 11, the powder is added to shape the filter device, by means of a powder jet 53, which is raster scanned across the tray 35 in synchronism with scanning of the image plane by a television camera 56 coupled to an image intensifier, the intensity of the powder jet being modulated to give the required filter thickness at different areas. To effect the scanning of the powder jet, it is passed through two pairs of electrostatic plates (62, 63), Fig. 12 (not shown). The intensity of the jet is modulated by a further pair of plates which direct powder out of the main beam as necessary. Alternatively, the sweep velocity of the scan may be varied. After an exposure has been made using the powder type of filter device, a flat layer is reproduced by reinstatement of removed material or smoothing out of the surface. The degree of equalization of the exposure may be varied, e.g. by feedback loop 45 in Fig. 2, or by using different X-ray energies during setting up of the filter and subsequent radiographic exposure. A discusion of the use of media in respect of their absorbtion edges is given and the use of rare earth metals suggested.