Patent Number: 051857758
Section: description

DETAILED DESCRIPTION OF THE INVENTION A radiological apparatus 10 for angiographic examination schematically comprises (FIG. 1) a table 11 having a base 12 with a sliding panel 13 mounted thereon to support a patient 14. The patient 14 is irradiated by a beam 19 of X-rays which is emitted by a radiation source 15 having a focus F and an aperture 34. After being absorbed to a greater or lesser extent by the patient's body, the X-rays are detected by a receiver 16 which may be any conventional receiver such as photographic film, a screen in combination with photographic film, or an image intensifier. If an image intensifier is used, then the light signals delivered by the image intensifier are processed in conventional manner in order to obtain images suitable for tracking the progress of the contrast substance. In order to track the progress of the contrast substance, the X-ray beam 19 needs to be displaced relative to the patient 14, either by displacing the source 15 as shown in FIG. 1, or else by sliding the panel 13. In this figure, means for controlling the displacement of the source 15 and of the receiver 16 are represented by a device 36. This device 36 is a computer which receives position data relating to the source 15 and to the table 11 and which issues instructions for displacing the source 15 and the table 11, both of which are fitted with motorized horizontal displacement means (not shown in FIG. 1). As shown in FIGS. 2 and 3, the X-rays are subject to very different degrees of attenuation since they go through portions of the patient's body having a wide range of thicknesses and/or having different absorption coefficients. Thus, the abdomen attenuates X-rays more than do the legs because of the difference in thickness, but even so the bones in the legs also attenuate strongly because they have a higher coefficient of absorption than do the tissues of the abdomen. In addition, between the legs and around the margins of the legs and the abdomen, the X-rays are not attenuated. These phenomena lead to images having very high degrees of contrast, making them difficult to analyze or else obliging the practitioner to change the exposure parameters. In order to mitigate these drawbacks, the invention proposes interposing a "homogenizing" filter between the X-ray source and the receiver, and preferably between the source and the patient and in the proximity of the source. The filter 20 is constituted by a plate 27 which is made of an X-ray absorbent material and which is of varying thickness so as to attenuate each X-ray path in such a manner that the total attenuation to which the said X-rays are subjected over their entire path all the way to the receiver is substantially the same for all paths in the X-ray beam. As a result, maximum attenuation is inserted between the legs and minimum attenuation is inserted for the abdomen zone, and the attenuation in other zones has intermediate values. An image is then obtained in which the exposure is homogenized. Perfect compensation would lead to an image that was uniformly gray on which only the contrast substance conveyed by the blood vessels would appear. The geometrical diagrams of FIGS. 2 and 3 serve to explain how the thickness of the filter is determined for each X-ray path, taking account of the respective positions of the filter 20 and of the patient 14 relative to the focus F of the X-ray source 15, which positions define a scale factor. This scale factor is used for calculating the lateral and transverse dimensions of the filter, and the nearer the filter is to the focus F, the smaller these dimensions. In FIG. 2, which corresponds to a section through the knees of the patient, the rays 26 and 26' determine the limits of the gap between the legs and define the high attenuation (or high thickness) central zone 28 of the filter. The rays 25 and 25' determine the side edges of the patient and define the outer high attenuation (or thickness) zones 29 and 30 of the filter. Finally, all of the rays such as 24 and 24' are attenuated by respective ones of the legs 22 and 23 of the patient and define zones 31 and 32 of varying attenuation (or thickness). In general, the thickness of the plate 27 is selected so as to impart attenuation on the path of each X-ray such that the total attenuation to which the X-ray is subjected on its path all the way to the receiver 16 is substantially the same for all of the paths in the beam. In FIGS. 3a and 3b, which are schematic sections through the apparatus of FIG. 1 at respective positions A and B of the source and receiver pair (the receiver not being shown), the filter is shown having the shape that it would have were it to be placed level with the patient: i.e. a half-mold 33 of the lower limbs of the patient. According to the invention, the filter is placed close to the focus F at the outlet from the source 15, thereby, as shown in FIG. 3, defining a scale factor of five between the lateral and transverse dimensions of the half-mold 33 and the corresponding dimensions of the filter 20. The thickness of the filter does not depend on the scale factor, but on the attenuation to which the X-rays are subjected passing through the patient and on the attenuation coefficient of the material used for making the filter. Since the radiological apparatus 10 is designed to take several pictures of the lower limbs, with each picture corresponding to a different zone, the filter 20 must be capable of being displaced horizontally relative to the X-ray source 15 in order to ensure that corresponding zones of the filter and of the patient lie on the paths of the X-rays. To this end, the filter 20 is carried by the X-ray source 15 in such a manner as to be capable of sliding relative to the aperture 34 through the diaphragm of the source. In FIG. 1, the device for horizontally displacing the filter 20 is not shown, but it is clear that it can be implemented in various different ways without requiring invention. Thus, the filter may be motorized and controlled synchronously with the horizontal displacement of the source-receiver pair or of the panel 13 if the radiological apparatus is of the type in which the patient is displaced. In FIG. 1, the means for controlling horizontal displacement of the filter constitute a portion of the device 36, but the motorized displacement means are not shown. According to the invention, the device 36 is also provided to control vertical displacement of the filter 20 and/or of the source 15 and/or of the panel 13 in such a manner as to vary the scale factor K=(a+b)/a, thereby adapting the filter to the size of the patient. These vertical displacement means are not shown in FIG. 1. In addition, the device 36 is designed to adapt the horizontal displacement of the filter as a function of the scale factor K and of the ratio K'=H/h between the size H of the patient and the length h of the filter. It should be observed that K=K' for a filter designed to correspond to the standard patient, and the filter is displaced by h/K between taking two pictures so as to obtain a displacement of H/K at the patient. If, in order to match the filter to the size H1 of the patient, K is changed to become K1, then it is necessary also to change the length of the horizontal displacement of the filter before taking two pictures so as to maintain correspondence in each picture between corresponding portions of the filter and of the patient. This change in displacement is determined by the device 36. The filter must match the morphology of the patient, and according to the invention it is proposed that a plurality of filters should be made depending on whether the patient is male or female, and depending on whether the patient is large, medium, or small in size, or possibly depending on the weight of the patient. The practitioner will thus have a set of filters available from which the filter most closely matching the patient to be examined will be selected. The filter of the invention thus makes it possible to homogenize image exposure, and consequently to distinguish the contrast substance more clearly. In addition, there is no need to change exposure parameters from one image to the next. Finally, the dynamic range of such a homogenized image is reduced, thereby making digital encoding possible without losing information. This gives rise to better digital processing of the image. The invention has been described with the filter placed at the outlet from the X-ray source, however it would be preferable to integrate the filter in the assembly constituted by the X-ray tube and the collimator including an iris and/or flaps. The material from which the filter of the invention is made may be an acrylic resin, for example, having a lead filler, e.g. the substance sold under the name "Kyowa Glass". The varying attenuation of the filter is obtained by the varying thickness of the plate 27, but it would also be possible to vary attenuation by varying the composition of the plate materiel, e.g. by adding more highly absorbent particles, particularly for the paths between the legs and for the outside margins. The horizontal and vertical displacement means for the source 15, the filter 20, and the table 11 are not shown in FIG. 1 so as to avoid overcrowding the figure. In any case, these means are known to the person skilled in the art and may be implemented without requiring any invention.