Patent Number: 051951215
Section: claims

1. An apparatus for modulating the intensity of an X-ray beam generated in a given direction by an X-ray tube, said apparatus comprising: a rotor having a rotation axis parallel to said given direction, said rotor being positioned adjacent said X-ray beam;  means to drive said rotor about its rotation axis at variable speeds;  a set of at least three blades made of a material opaque to X-ray, each of said blades comprising a central hub and a pair of symmetrical wings extending away from said hub, the blades of said set having their hubs slidably mounted on a corresponding set of radially projecting shafts symmetrically positioned about said rotor;  return spring means to urge each of said blades radially inwardly towards said rotor along their respective shafts; and  guiding means on each pair of hub and shaft to cause the corresponding blade to pivot about the shaft on which it is mounted when the speed of the rotor increases and the blades are then moved in unison radially outwardly against the action of the return spring means as a result of the centrifugal force;  wherein said blades and their shafts are sized to cause said blades to intersect said X-ray beam when said rotor is driven and thus to modulate the intensity of said X-ray beam as a function of the annular position of the blades about their shafts, which allow more or less radiation to pass therebetween.  said set of threads are made in a cylindrical member coaxially fixed to the shaft away from said rotor, said member having an external diameter wider than said shaft and defining a bearing flange;  said hub has an internal diameter substantially identical to the external diameter of said member and an opening adjacent said rotor, said opening being closed by a guiding ring fixed to said hub, said ring being slidably mounted onto said shaft; and  said compression spring mounted about said shaft within said hub has one end bearing against said flange and another end bearing against said ring. 2. The apparatus of claim 1, wherein the wings of said blades are so shaped as to cause variation in the modulation obtained therewith according to their skew angle, and thus to generate different compensation curves as a function of the rotation speed of the rotor. 3. The apparatus of claim 1, wherein said guiding means comprises a set of rollers freely mounted on pins symmetrically fixed to and projecting from any one of said hub and shaft in such a manner as to engage a corresponding set of threads made in the other one of said hub and shaft. 4. The apparatus of claim 3, wherein said return spring means comprises a compression spring mounted about said shaft within said hub. 5. The apparatus of claim 4, wherein: 6. The apparatus of claim 5, wherein said pins and threads are positioned in such a manner that the blade extends at a small angle with respect to said given direction when the rotor is stopped or driven at low speed, and extends substantially horizontally when said rotor is driven at high speed. 7. The apparatus of claim 6, further comprising programmable control means operatively connected to said means to drive the rotor, said control means allowing preselection of at least one given speed of rotation of the rotor corresponding to a required compensation curve. 8. The apparatus of claim 7, wherein said X-ray tube generating the X-ray beam to be modulated is part of a system for lower limb angiography. 9. The apparatus of claim 7, wherein the wings of said blades are so shaped as to cause variation in the modulation obtained therewith according to their skew angle, and thus to generate different compensation curves as a function of the rotation speed of the rotor. 10. The apparatus of claim 1, further comprising programmable control means operatively connected to said means to drive the rotor, said control means allowing preselection of at least one given speed of rotation of the rotor corresponding to a required compensation curve.