Patent Number: 060884274
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As is shown schematically in FIGS. 1 and 2, an apparatus for radiological examination generally includes a radiation source 10 which is preferably and typically an x-ray generator. Radiation 13 is directed towards the body of a patient 11 lying on an appropriate examination table 12. The radiation 13, after having passed through the body of the patient, reaches an arrangement 14 which includes a holder for a film or plate 15 that is to be impressed with an image. The arrangement 14 further includes a grid that is formed by lead strips 16 separated from each other by elements of an x-ray transparent material. The grid is displaced in a reciprocating motion by an appropriate driving mechanism when the apparatus is operating. The grid 16 performs the task of filtering the x-rays scattered by the irradiated elements, i.e. the patient. Because such scattered rays would affect the quality of the image that is impressed on the photographic film or plate, it is desired to have them filtered. In order to ensure a good radiographic image, the reciprocating motion of the grid must, as far as possible, occur at a constant speed. However, the grid 16, caused to be displaced with a reciprocating motion, can give rise, due to its mass, to vibrations over the entirety of the apparatus. As a result, this can contribute to lowering the quality of the attainable radiographic image. It would also cause a further inconvenience to the patient, who is usually lying quite close to the arrangement 14 that is receiving the image. In order to eliminate such drawbacks, the present invention provides a counterweight for balancing the oscillating mass of the grid 16. According to a further preferred feature of the present invention, a stepper motor is used as the driving means for the grid and the counterweight, crank-connecting rod mechanisms are used as motion transmission mechanisms between the motor and the grid as well as the counterweight, and a power electronic control arrangement is provided for the stepper motor. This will be explained in further detail below. Turning to FIGS. 5 and 6, the arrangement 14 including the holder for holding the film or plate 15 to be impressed with the image and the moving grid 16 is housed in a box-like structure on a side of which the components of the present invention are assembled. The grid 16 is provided with a bracket 17. An end portion of a connecting rod 18 is pivotally connected with the bracket 17, and the other end of the connecting rod is pivotally attached to a crank 19. The crank 19 is also connected to an end portion of a shaft of a stepper motor 20. At the other end of the shaft of the stepper motor 20 is connected a crank 21. A connecting rod 22 is pivotally connected to the crank 21 at one end thereof, and at the other end thereof is pivotally connected to a counterweight 23. The counterweight 23 is adapted to balance the mass of the grid 16 in its reciprocating motion. The cranks 19 and 21 of the respective motion transmission mechanisms from the motor to the grid and the counterweight are, preferably, provided so as to be parallel and out of phase, i.e. out of phase by 180 degrees. With the above structure, accordingly, the counterweight 23 is displaced with a rectilinear reciprocating motion in opposite phase to the grid 16. It should be noted that the counterweight 23 is preferably guided in motion by two shafts 24 and 25 that are connected as end portions thereof to a support 26 of the stepper motor 20 and the chassy of the arrangement 14, respectively. Thus, the counterweight is able to rectilinearly slide on the support shafts 24 and 25. The stepper motor 20 ideally turns at a constant speed. If it turns at a constant speed, the curve of the reciprocating displacement speed of the grid 16, due to the law governing the motion of oscillating masses, will have a sinusoidal profile as illustrated in FIG. 3. With the apparatus according to the present invention, programmable profiles can, on the contrary, be obtained for the reciprocating speed of the grid 16 in view of the elimination of the imbalances that were in the past brought about by inertial forces. For a profile to be obtained which differs from the sinusoidal profile as shown in FIG. 3, the angular speed of the motor needs to be varied on an instant-by-instant basis in accordance with the rotation angle of the crank 19 and the speed profile that is desired to be obtained for the oscillating mass. One such profile is illustrated in FIG. 4. The variation in the angular speed of the motor is obtained by varying the times elapsing between two successive steps of the stepper motor 20. Such times are delivered to the power control unit of the motor by a microprocessor based control arrangement (which is of a per se known type, and, therefore, not illustrated). As a result, the sinusoidal pattern of the reciprocating speed of the grid 16 which is ordinarily brought about by the crank and connecting-rod type of motion transmission mechanism can be compensated for by control of the motor to provide for a constant speed of the grid 16.