Patent Number: 051951215
Section: description

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION FIG. 1 shows a system 1 for lower limb angiography, comprising an X-ray cone and tube assembly 3 of conventional structure, designed to generate an X-ray beam 5 in a given direction "D" towards a radiographic table 7 incorporating a radiographic film cassette 9 mounted in a changer. This kind of equipment wherein the X-ray cone and tube assembly 3 is always centered on the cassette changer is well known in the art and commonly used to take a sequence of radiographic pictures of the lower part of a patient "P". Accordingly, it does not need to be further described. As better shown in FIG. 2, the X-ray cone and tube assembly 3 incorporates an X-ray beam modulator 11 mounted in a large box 13 fixed to the bottom of the assembly 3. The box 13 has a small upper window opening 15 coaxially positioned with respect to the outlet of the X-ray tube, and a large lower window opening 17 that is also aligned with the outlet of the X-ray tube in order to let the X-ray beam 5 pass therethrough. In accordance with the invention, the X-ray beam modulator 11 located in the box 13 comprises a rotor 19 having a rotation axis "A" parallel to the direction "D". The rotor 19 is positioned close to the upper window opening 15 and thus extends adjacent the X-ray beam 5. Means are provided for driving rotor 19 about its rotation axis "A" at variable speeds. These means include an electric motor 23 whose shaft is connected via a belt 25 to the rotor 19 in order to drive the same. These means also include remote control means 21 including a rheostat or any other similar device known per se to control the rotation speed of the motor 23 and thus the rotation speed of rotor 19. These control means 27 are preferably associated to a programmable computer to allow preselection of one or more given speeds of rotation of the rotor, corresponding to some required compensation curves, as will be explained hereinafter. The X-ray beam modulator also comprises a set of at least three and preferably four blades 27 made of a material opaque to X-ray. The blades 27 are identical and each comprises a central hub 29 and a pair of symmetrical wings 31 extending away from the hub 29. The wings 31 are advantageously shaped 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, as will be explained hereinafter. The blades 27 have their hubs 29 slidably mounted on a corresponding member of radially projecting shafts 33 symmetrically positioned about the rotor 19, each shaft being fixed to the rotor by screwing of one of its ends 35 into a receiving hole 37. Guiding means are provided on each pair of hub and shaft to cause the corresponding blade 27 to pivot about the shaft 33 on which it is mounted, when the speed of the rotor 19 increases and all the blades 27 are then moved in unison radially outwardly along the shafts 33 because of the centrifugal force. The guiding means preferably comprises a set of rollers 39 freely mounted on pins 41 symmetrically fixed to and projecting from any one of the hub and shaft in such a manner as to engage a corresponding set of threads 43 made in the other one of these hub and shaft. In the illustrated embodiment, the pins 41 projects inwardly from the hub 29 touward the shaft 33 and the threads 43 are made in a cylindrical member 45 coaxially fixed to the shaft 33 away from the rotor 19, this member having an external diameter wider than the shaft 33 so as to define a bearing flange 47. The hub 29 has an internal diameter substantially identical to the external diameter of the member 45 and an opening 49 adjacent the roto 19. This opening 49 is closed by a guiding ring 51 fixed to the hub by a key 53. Of course, the ring 51 is slidably mounted onto the shaft 33 to allow the blade to slide along this shaft. Return spring means are provided for urging each of the blades 27 radially inwardly towards the rotor 19 along their respective shafts 33. These return spring means preferably consist of compression springs 55 mounted about the shafts 33 within the hub 29, each spring 55 having one end bearing against the flange 47 and another end bearing against the ring 51. The pins 41 and their rollers 39 and the threads 43 are positioned in such a manner that the blades 27 extend at a small angle with respect to the direction "D" when the rotor 19 is stopped or driven at low speed, as is shown in FIG. 1, and extends substantially horizontally when the rotor is driven at high speed. In accordance with the invention, the blades and their shafts 33 are sized to cause the blades to intersect the X-ray beam 5 when the rotor is driven. Assuming that the exposure time is 1/10 sec., the rotation speed of the rotor 19 will be selected to be equal to at least 600 rpm, to prevent the blades from being "radiographed" and thus appear onto the film. As soon as the motor is driven, the blades 27 are moved radially outwardly by the centrifugal force, and start rotating about their own axis as a result of the combined action of the pins 41 and rollers 39 within the threads 43. The more they slide radially outwardly, the more they rotate and the more they extend horizontally across the beam 5, thereby reducing the amount of irradiation. Thus, it becomes very simple to modulate the intensity of the X-ray beam 5 as a function of the angular position of the blades 27 about their shafts 33, which itself depends on the speed of the rotor 19 and allow more or less radiation to pass between the blades. As aforesaid, shape of the winfs 31 of the blades 27 may be selected to achieve variation in the modulation obtained therewith according to the skew angle of the blades 27, and thus to generate different compensaton curves as a function of the rotation speed of the rotor. Moreover, the programmable control means may be used to preselect one or more given speeds of rotation of the rotor corresponding to some required compensation curves adapted to the patient's anatomy and fatness. The X-ray beam modulator may programmed to start simultaneously with the anode rotation of the X-ray tube and cone assembly and then to operate continuously so as to be totally independent from the selected exposition sequence and duration that can be very short depending on the speed of screens use. FIG. 3 shows the kind of compensation curves that may be obtained in a lower limb angiography. It must be understood however that the X-ray beam modulator according to the invention may be used in other systems and is not restricted exclusively to be used with a lower limb angiography system.