Abstract:
Method for limiting an X-ray beam, wherein the X-ray beam is limited by a limiting unit comprising a couple of blades approaching each other or moving away from each other to adjust the width of a space between the blades the two blades being driven by an actuator ( 12 ) linked to both blades. The position of a symmetry axis (S) of the space between the blades is set in a shifting step by changing the distance between the blades and by impeding the motion of one blade in comparison with the motion of the other blade resulting in an asymmetric motion of the blades with respect to an initial position of the symmetry axis (S). The desired width (W) of the space between the blades is set in an adjustment step by an unimpeded symmetric motion of the blades with respect to the position of the symmetry axis (S) set in the shifting step.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the technical field of radiology, and especially to beam limiting units used to define the shape of the X-ray beam emitted by an X-ray tube during the acquisition of a radiography. 
     The X-ray tube generally emits an X-ray beam, which is limited by beam stops near the X-ray source to a generally conical or pyramidal form. The beam is then further modified to obtain a final shape, which is defined downstream the emission point by guiding the X-ray beam through an aperture allowing it to get the desired shape and dimensions with greater precision. Usually with the passage through the aperture the X-ray beam assumes a square or rectangular shape. 
     In the different types of radiological acquisition, apertures having different shapes and dimensions must be interposed between the X-ray source and the X-ray detector, with the aim of conferring to the X-ray beam the shape and the dimensions optimal for that particular acquisition. 
     The use of beam limiting units is well known in the art, and in particular the use of beam limiting units which use one or more blades which can be moved in different ways through suitable actuators. These beam limiting units sometimes also form part of a collimator. 
     Typically a beam limiting unit is made of two blades and one or two actuators moving the blades. In this way, an aperture having a fixed height (corresponding to the blades height) and variable width (the distance between the internal edge of the two blades) is obtained. Superimposing perpendicularly two beam limiting units, an aperture having variable height and width can be obtained. The superimposition of two beam limiting units is known in the art. 
     The central rays of the final X-ray beam that has passed through the aperture must be in the desired position. One of the limits of prior art is the impossibility of setting the central ray of the final X-ray beam where desired, due to limitations linked to blades position. For instance, in the beam limiting unit known from DE 32 36 082 A1, the two blades move in a symmetrical way with respect to the central ray, which, once fixed, cannot be moved. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a collimation unit capable of forming an aperture having the desired width and the central ray of the X-ray beam leaving the aperture at a desired position, which might differ from the position of the central ray of the X-ray beam impinging on the aperture. It is moreover an object of the present invention to provide a beam limiting unit settable in a short time, and which, by reducing the number of electronically actuated components, is more reliable and easier to control, and which can finally be produced at low costs. 
     With respect to prior art beam limiting units which make use of an actuator for each blade (four blades and four actuators), the present invention has the advantage of a reduction of the actuators number, in that, with two actuators only, an aperture having the desired height and width can be obtained, and moreover having the central ray of the X-ray beam in the desired position. This is possible thanks to the particular movement, which allows first to move the blades to define the position of the axis of symmetry of the aperture, and thereafter to open or close them, so as to obtain a final X-ray beam having the desired shape and position. Due to the fact that both blades are directly linked to the actuator by separate force transmission chains, the actuator exerts force on both blades simultaneously resulting in both a speedy positioning of the central beam and a swift adjustment of the width. 
     Another advantage of the present invention lies in the possibility of moving the central ray of the final X-ray beam with respect to the central ray of the X-ray beam impinging on the aperture. This concept will be better explained in the following with the help of  FIG. 3 . 
     A further advantage of the present invention lies in the fact that the opening and closing of the aperture can occur even during the X-ray emission by the X-ray tube, which can be important in order to reduce the radiation dose to the patient. Imagine for instance to use the beam limiting unit during the acquisition of a panoramic radiography wherein initially a first condyle is irradiated, then the cranial frontal portion and finally the second condyle. When incisive teeth are irradiated, reducing the vertical dimension of the X-ray beam could be very useful, so as to avoid irradiating a radiosensitive tissue as retina. 
     It will be only briefly mentioned that the blades are made of material able to stop X-rays. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in one of its embodiments with the help of the following figures, showing: 
         FIG. 1  a schematic view of the beam limiting unit of the present invention; 
         FIG. 2  a schematic view of the beam limiting unit when blades are approaching to each other (Step O); 
         FIG. 2A  a schematic view of the beam limiting unit during the setting of the symmetry axis of the aperture (Step A) ; 
         FIG. 2B  a schematic view of the beam limiting unit during the setting of aperture width (Step B); 
         FIG. 2C  a schematic view of the beam limiting unit during reset (Step C); 
         FIG. 3  a schematic view of two superimposed beam limiting units; 
         FIG. 4A  a schematic view of the beam limiting unit without filter; 
         FIG. 4B  a schematic view of the beam limiting unit with filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a beam limiting unit  1  comprising a leadscrew  2 , having a right-hand thread extending from point A to point O and a left-hand thread extending from point O to point B. A first nut  3  engages the right-hand thread and moves on the right-hand thread; a second nut  4  engages the left-hand thread and moves on the left-hand thread. The beam limiting unit  1  further comprises a first blade  5  attached to the first nut  3 , and a second blade  6  attached to the second nut  4 . The first nut  3  is further provided with a pin  7  extending through a slot  8  machined in blade  5 . A friction device (not shown), for instance a spring disposed between pin  7  and blade  5 , presses the blade  5  against the nut  3 . The beam limiting unit  1  is finally also provided with a first stop  10  and a second stop  11 , which both delimit the motion path of blade  5 . An actuator  12  drives the leadscrew  2 . 
     As a whole, pin  7  and slot  8  form an uncoupling system  9  which uncouples the movement of actuator  12  and the movement of blade  5 , in particular the movement of nut  3  and blade  5 . 
       FIG. 1  shows an aperture, having width W, formed by the internal edges of blades  5  and  6 , and having an axis of symmetry S. 
     It should be noted that the beam limiting unit  1  form the aperture of the X-ray imaging system in most cases. There might also be X-ray imaging system where the beam limiting unit  1  is just one of the baffles along the X-ray path and that the aperture of the X-ray imaging system is formed by another beam limiting unit  1 . 
     Actuator  12  moves leadscrew  2  clockwise or anti-clockwise, pushing away or drawing closer, symmetrically and at the same time, the two nuts  3  and  4 . On nut  3  pin  7  is fixed; pin  7  slides in slot  8  machined in blade  5 . Blade  5  is secured to nut  3  through the friction device. The presence of the friction device allows a more controlled sliding of blade  5  on nut  3 . 
     Blade  6  is integral with nut  4 . 
     In the following, the operation of a beam limiting unit  1  will be described. For better clarity, the operation is subdivided into steps illustrated by  FIGS. 2 .
     O) Step O: blades are drawn nearer and close
       Actuator  12  rotates leadscrew  2 . The two nuts  3 ,  4  are drawn nearer to each other, approaching the two blades  5  and  6 . Blade  6  is integral with nut  4 , therefore any movement of nut  4  entails a corresponding movement of blade  6 . Blade  5  is secured with nut  3  through the friction device and pin  7 , therefore, in this step, a movement of nut  3  entails a corresponding movement of blade  5 . At a certain point blade  5  abuts stop  11 . When blade  5  abuts stop  11 , an aperture having width W 0  forms between the internal edges of the two blades  5  and  6 . In the aperture having width W 0 , S 0  represents the initial axis of symmetry dividing it into two symmetrical parts.   
       A) Step A: setting of the axis of symmetry of the aperture (shifting step)
       Actuator  12  continues to rotate leadscrew  2  in the same direction as in Step O: the two nuts  3  and  4  continue approaching each other. Blade  6  is integral with nut  4 , therefore any movement of nut  4  entails a corresponding movement of blade  6 .   Blade  5  is pushed by stop  11  in a direction contrary to the movement of nut  3 . This thrust is greater than the friction exerted by the friction device, therefore blade  5  can slide on the underlying nut  3 . The movement is guided by pin  7  along slot  8 .   Now an aperture having width W 1  has formed; S 1  represents the axis of symmetry of this second aperture (different from S 0 ). Differentiating position S 1  from position S 0  is possible as much as allowed by the width of slot  8 .   Actuator  12  continues to rotate the leadscrew  2  until S 1  arrives at the desired position (setting of the axis of symmetry of the aperture).   
       B) Step B: setting the width of the aperture (adjustment step)
       Actuator  12  reverses the rotation of leadscrew  2  with respect to step A. The two nuts  3  and  4  move away from each other in a symmetrical manner.   Blade  6  is integral with nut  4 , therefore any movement of nut  4  entails a corresponding movement of blade  6 .   Blade  5  is secured to nut  3  through the friction device and pin  7 , and there is no other thrust, therefore, in this phase, the movement of nut  3  corresponds to the movement of blade  5 .   Now an aperture having width W 2  has been formed; S 1  represents the axis of symmetry of this second aperture; S 1  remains unaltered with respect to Step A in that, in this phase, the withdrawal of the two blades  5  and  6  from S 1  is symmetrical.   Actuator  12  continues to rotate the leadscrew  2  until the aperture reaches the desired width W 2  between the two blades.   
       

     At this point, the aperture of the beam limiting unit has the desired width W 2  and the symmetry axis S 1  is at the desired position. It should be noted that the present invention reaches the desired setting with only two steps. Should it be necessary to modify the symmetry axis, the beam limiting unit  1  must be reset, that is the symmetry axis S of the aperture must be restored to its initial position S 0 .
     C) Step C: restoring of the position of the axis of symmetry S 0  (restoration step)
       Actuator  12  rotates leadscrew  2  so as to push away blades  5  and  6  from each other. The two nuts  3  and  4  are withdrawn in a symmetrical way.   Blade  6  is integral with nut  4 , therefore any movement of nuts  4  entails a corresponding movement of blade  6 .   Blade  5  is secured to nut  3  through the friction device and pin  7 , and there are no other thrusts, therefore, in this phase, the movement of nut  3  corresponds to the movement of blade  5 .   At a certain point blade  5  abuts stop  10 .   Blade  5  is pushed by stop  10  in a direction contrary to the movement of nut  3 . This thrust is greater than the friction exerted by the friction device, therefore blade  5  can slide on the underlying nut  3 . The movement is guided by pin  7  on slot  8 .   Actuator  12  continues to turn until pin  7  arrives at the slot end, restoring the initial arrangement of the beam limiting unit  1 .   
       

     A beam limiting unit without stop  11  can also be built. In this case, the operation is slightly modified, in that the role of stop  11  is performed by the abutment of the two blades  5  and  6  for setting the symmetry axis of S 1  (Step A). Once axis S 1  is set, the two blades  5  and  6  move away from each other forming an aperture having the desired width W (Step B). The embodiment without stop  11  has the advantage of allowing to set the symmetry axis S of the aperture having width W with a greater freedom. 
     Nonetheless, the beam limiting unit  1  having stop  11  is the preferred embodiment, in that the setting of point S 1  is quicker as blade  5  abuts stop  11  without the need to contact blade  6 . 
     Instead of the stops  10  and  11 , the motion of the blade  5  can also be impeded by other mechanical means such as braking, clutching or latching means or other mechanical, electrodynamic or hydrodynamic means which impede and in particular stop the motion of the blade  5  starting from a given point along the motion path of nut  3  and therefore blade  5 . 
     In a modified embodiment, the position of the symmetry axis S can also be set by abutting the external stop  10  instead of the internal stop  11 . Vice versa the inner stop  11  can also be used for the restoration step. 
     In the embodiments described herein, both blades  5 ,  6  are directly linked to the actuator  12  by separate force transmission chains. The blade  5  is linked to the actuator  12  by means of the leadscrew  2 , the first nut  3  and the uncoupling system  9  formed by the pin  7 , slot  8  and the friction device. The blade  6  is linked to the actuator  12  by means of the leadscrew  12  and the second nut  4 . By these separate force transmission chains—one between actuator  12  and blade  5 , and the other one between actuator  12  and blade  6 —the forces generated by the actuator  12  are transmitted to both blades  5  and  6  directly and simultaneously resulting in both a speedy positioning of the central beam and a swift adjustment of the width W of the aperture. 
     Moreover, should a rectangular aperture be needed, having a ratio between two adjacent sides very different from 1, realising a perfectly rectangular aperture, wherein the edges of blades  5  and  6  are parallel would be costly and complex using a beam limiting unit without stop  11 . The presence of stop  11  allows to get an aperture wherein the opposed sides are parallel, thanks to the fact that the presence of stop  11  recovers the clearance of blade  5  departing from a window having a non-rectangular shape. The dimensional precision of the aperture is important for the certification of the medical device. This problem in many prior art devices is overcome by limiting the fan X-ray beam with a beam limiting unit having a fixed profile, wherein the dimensions of the beam cannot be varied. 
     Obviously beam limiting unit  1  can be provided with detection devices known in the art (photoelectric cells, micro-switches, etc.) for detecting the stop positions of the various moving elements. 
       FIG. 3  shows two beam limiting units  21  and  31 , one superimposed on the other with an angle of 90°. Beam limiting unit  21  limits an aperture having width W 2  and axis of symmetry S 2 , while beam limiting unit  31  limits an aperture having width W 3  and axis of symmetry S 3 . The symmetry axes S 2  and S 3  are aligned at right angle.  FIG. 3  shows that the X-ray beam emitted by X-ray tube has a conical shape and therefore a circular cross section  32  having centre Ω, which is transformed to a quadrilateral shape  33  thanks to the superimposition of the two beam limiting units  21  and  31 , formed by the couple of blades  25  and  26  and the couple of blades  35  and  36 , respectively. 
       FIG. 3  shows what was stated in the introduction: point S 2, 3 , the intersection of the symmetry axis S 2  of beam limiting unit  21  and of the symmetry axis S 3  of beam limiting unit  31  does not coincide with central point Ω of the circular cross section of the conical beam emitted by the X-ray tube. 
     Different shapes of the aperture can be obtained using blades having different shapes and/or superimposing two beam limiting units with an angle different from 90°, which makes the beam limiting unit of the present invention suitable for any radiographic apparatus. 
     The beam limiting unit of the present invention finds a particularly advantageous application in the technical field of the extraoral dental radiology. 
     In the following, reference will be made to extraoral radiographic apparatuses performing panoramic radiographies, cranial teleradiography and volumetric radiographies of the facial skeleton with cone beam technique. All these types of radiographies are well known in dental radiography. 
     Panoramic radiography (in some cases also called orthopantomography) produces a radiographic image of a curved plan approximating patient jaws, with blurring of the anatomical structures laying outside a narrow layer around the predesigned curved plane. Usually the acquisition of a panoramic radiography occurs with a fan beam X-ray beam, that is with a beam limiting unit producing a rectangular aperture wherein the ratio between two adjacent sides is very different from 1. 
     Teleradiography is a projective radiographic technique, producing radiographic images of the skull or of other anatomical areas from different projections, with minimum magnification and geometrical distortion. Usually two perspectives are represented, latero-lateral and anteroposterior. The acquisition of a teleradiography can occur with two different modalities:
     1) with a fan shaped X-ray beam, that is with the beam limiting unit producing a rectangular aperture wherein the ratio between two adjacent sides is very different from 1;   2) in one-shot modality, that is with a beam limiting unit producing a quadrangular aperture having the same proportions of the X-ray detector. In case 2) the ratio between adjacent sides of the rectangle is closer to 1 than in case 1).   

     Cone beam volumetric radiography (also known as CBCT) is the acquisition, from different projection angles, of a series of bidimensional radiographic images which will be processed post-acquisition to reconstruct tridimensional volumes. In this case, the aperture can be rectangular, square or even circular, according to the X-ray detector. In addition to the above-mentioned needs, when performing a CBCT volumetric radiography, interposing a filter between the X-ray tube and the patient might be necessary in order to improve the reconstruction quality of bones, and decreasing the quantity of harmful soft X-rays. 
     The beam limiting unit of the present invention finally allows to add or remove an X-ray filter, which can be positioned upstream or downstream of the beam limiting unit itself, as shown in  FIG. 4 . 
       FIG. 4A  shows beam limiting unit  41  in its setting for acquiring a panoramic radiography: as can be seen, the aperture having width W is of a rectangular shape wherein the ratio between adjacent sides is very different from 1, conferring to the X-ray beam the typical fan shape. During the acquisition of a panoramic radiography the X-ray beam does not need further filtration. 
       FIG. 4B  shows instead beam limiting unit  51  during the acquisition of a CBCT volumetric radiography, which needs a supplementary filtration of X-ray beam emitted by X-ray tube. To this aim, on the X-ray path, a filter  46  is interposed, which can be indifferently positioned upstream or downstream beam limiting unit  41 . 
     The superimposition of the filter to the aperture formed by collimation unit  41  is linked to the position of nut  4 . As a matter of fact, on nut  4  a cam  45  is fixed which pushes arm  43 , supporting filter  46 , so that it can rotate around pin  44 . When blades  5 ,  6  are at short distance ( FIG. 4A ), the filter  46  is pushed so as to remove it from aperture; when blades  5 ,  6  are sufficiently far away from each other ( FIG. 4B ), a spring (not shown) returns arm  43 , so that filter  46  is superimposed on the aperture. 
     Using suitable dimensions of filter  46 , arm  43  and cam  45 , it is possible that, given a pre-set width W of aperture, filter  46  covers it completely and that, given another pre-set W′ of the aperture, the filter is completely removed from the X-ray path. Although the beam limiting unit  1  can form apertures having any width W in a continuous mode, in reality, when positioned inside an extraoral radiographic apparatus, the effectively used apertures are only a discrete number, having a pre-defined width W. 
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               1  beam limiting unit 
               2  leadscrew 
               3  first nut 
               4  second nut 
               5  first blade 
               6  second blade 
               7  pin 
               8  slot 
               9  uncoupling system 
               10  first stop 
               11  second stop 
               12  actuator 
               21  first beam limiting unit 
               25  first blade of beam limiting unit  21   
               26  second blade of beam limiting unit  21   
               31  second beam limiting unit 
               32  circular section of the X-ray beam impinging on beam limiting unit 
               33  aperture 
               35  first blade of beam limiting unit  31   
               36  second blade of beam limiting unit  31   
               41  beam limiting unit with filter 
               43  arm supporting filter 
               44  pin 
               45  cam 
               46  supplementary filter