Abstract:
A radiation diaphragm for an X-ray apparatus has an absorber element that is seated so as to be eccentrically rotatable around a rotational axis oriented in its longitudinal direction, so that the size of the ray beam is variable by means of a rotary motion of the absorber element. The absorber element is fashioned, for example, as a drum, a roller or a cylinder. The radiation diaphragm can be implemented especially rugged and is suited for higher rotational speeds in a computed tomography apparatus.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is directed to a radiation diaphragm for an X-ray apparatus, of the type having an absorber element that is adjustable in position for variably limiting a ray beam.  
           [0003]    2. Description of the Prior Art  
           [0004]    German OS 44 37 969 discloses an example of an X-ray apparatus having a radiation diaphragm of the above type. In an X-ray apparatus fashioned as a computer tomography apparatus, an X-ray fan is generated by the primary radiation diaphragm fashioned as a slit diaphragm. This fan determines the dose profile in the patient, and thus the slice thickness in the exposure. It thus also influences the dose stress on the patient and the intensity of the detector signal from which the image data are acquired. For setting various slice thicknesses, it is necessary to set various apertures of the primary radiation diaphragm.  
           [0005]    A slit diaphragm can be realized, for example jaws, functioning as absorber elements, into the ray beam or out of this in the fashion of a parallelogram. Slut diaphragms realized in this way are composed of a multitude of moving parts.  
           [0006]    Diaphragms having rotatably seated absorber elements are also known, for example from German OS 31 36 971 or German OS 36 00 824. These diaphragms are likewise very complicated.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the present invention is to provide a radiation diaphragm for an X-ray apparatus as well as an X-ray apparatus having a radiation diaphragm that allow an especially rugged design.  
           [0008]    The above object is achieved in accordance with the principles of the present invention in a radiation diaphragm for an X-ray apparatus having an absorber element that is eccentrically mounted so as to be rotatable around a longitudinal axis, and the absorber element having a shape so that by rotation around the longitudinal axis a larger or a smaller portion of the absorber element is disposed in the path of an X-ray beam.  
           [0009]    The invention is based on the perception that previously employed radiation diaphragms are particularly unsuited for those used in a computed tomography apparatus wherein the X-radiator and/or the detector for detecting the X-radiation are rotatable along a gantry with especially high rotational velocity. The radiation diaphragm of the invention is distinguished by an especially simple and rugged design that requires only a very small number of moving parts. It is therefore only slightly susceptible to external influences or movements. In particular, rotational velocities from 3 rps (revolutions per second) up to 5 rps and more are possible in a computed tomography apparatus. It suffices in the limit case when the absorber element and, possibly, a shaft driving it, is movable.  
           [0010]    In known radiation diaphragms, the rotary motion generated by a motor must be converted into a linear motion of an absorber element that produces the actual variation of the ray beam. Various mechanical components are required for this purpose. Compared thereto, the rotary motion of a motor in the radiation diaphragm of the invention can be directly converted into a rotary motion of the absorber element so the ray beam can be varied without requiring a linearization of the motion. A simple structure that operates satisfactorily with few component parts thus is achieved.  
           [0011]    Any component that is suitable for limiting the ray beam by absorbing parts of the ray beam that are not needed is referred to as absorber element in conjunction with the invention. The absorber element particularly has an outside contour, for example a cylindrical surface, serving the purpose of blanking out parts of the X-rays that are emitted by an X-ray tube but that are not needed for the diagnosis. In other words: rays that can pass the contour of the absorber element are gated into the patient.  
           [0012]    According to a preferred embodiment, the absorber element is fashioned as an elongated, rotationally symmetrical body, particularly as a drum, a roller or a cylinder. This has the advantage of a simple and economical manufacture. However, other shapes are also possible for the absorber element that can be realized, for example, by applying or forming or attaching an eccentric element to a rotatable body.  
           [0013]    In accordance with the invention the absorber element fashioned as a rotary body can be fabricated to achieve an adequate X-ray absorption. The absorber element preferably contains a material that absorbs X-rays, particularly a material having an attenuation coefficient of more than 1 cm−1, and/or a material having an atomic number of more than 50 or 80, particularly lead or tantalum or tungsten.  
           [0014]    In a preferred development of the radiation diaphragm, a further absorber element is provided that—like the aforementioned absorber element—is preferably fashioned as an elongated, rotationally symmetrical body. The further absorber element can be seated or fabricated like the aforementioned absorber element. With two absorber elements, it is possible to vary the size of the ray beam without varying the position of the central ray. It is advantageous for this purpose if the two absorber elements are rotatable by the same rotational angle, either in the same direction or in opposite directions (symmetrical reduction or enlargement of the ray beam, symmetrical gating). To this end, it is also advantageous for the two absorber elements to have their rotational axes aligned parallel to one another.  
           [0015]    For a simple design, the two absorber elements have their rotational axes aligned essentially perpendicular to the course of the ray beam.  
           [0016]    The two absorber elements preferably are motor-rotatable independently of one another. Separate motors can be present for this purpose. As a result, a symmetrical gating as well as a variation of the position of the middle of the ray beam can be realized, particularly given rotation of the absorber elements by different rotational angles.  
           [0017]    The motor or motors for turning the absorber element or elements are, in particular, stepping motors.  
           [0018]    The above object also is achieved in accordance with the invention in an X-ray apparatus is inventively achieved by an X-ray apparatus, particularly a computed tomography apparatus, having a radiation diaphragm as described above. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 shows an X-ray apparatus of the invention in a schematic overall view.  
         [0020]    [0020]FIG. 2 shows a radiation diaphragm of the X-ray apparatus of FIG. 1 in a direction as seen proceeding from a gated X-ray beam.  
         [0021]    [0021]FIG. 3 is a section through the radiation diaphragm of FIG. 2.  
         [0022]    [0022]FIG. 4 is a longitudinal side view of the radiation diaphragm of FIGS. 2 and 3.  
         [0023]    [0023]FIG. 5 shows the radiation diaphragm of FIGS. 2 through 4 enlarged and in closed position.  
         [0024]    [0024]FIG. 6 shows the radiation diaphragm of FIGS. 2 through 4 enlarged and illustration in a maximally open position. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    [0025]FIG. 1 shows an X-ray apparatus  1  fashioned as a computed tomography apparatus, only the rotating part thereof with an X-ray radiator  3 , that has a focus  5 , and a detector  7  being shown. The X-ray radiator  3  and the detector  7  rotate around an axis  9 . A radiation diaphragm  11  fashioned as a primary radiation diaphragm generates a fan-shaped X-ray beam  13  that transirradiates a patient (not shown) through which the axis  9  passes. A central ray of the X-ray beam  13  is referenced  14 .  
         [0026]    Together with the X-ray radiator  3  and the detector  7 , the radiation diaphragm  11  rotates around the axis  9 . A gantry that is present for the mechanical realization of the rotary motion is not explicitly shown for reasons of clarity. Upon rotation of the components  3 ,  7 ,  11 , the patient is transirradiated from various directions, and a computer calculates an image of the transirradiated part of the patient from the output signals of the detector  7 . The fan plane proceeds perpendicularly to the plane of the drawing, and the detector  7  is composed of a row of individual detectors that likewise extends perpendicular to the plane of the drawing. To this end, the detector  7  is also curved around the focus  5 .  
         [0027]    The radiation diaphragm  11  is shown simplified in FIG. 1 and is explained in greater detail below on the basis of FIGS. 2 through 4.  
         [0028]    According to FIG. 2, the radiation diaphragm  11  has an oblong absorber element  15  that is fabricated of lead or a lead alloy either entirely or partly or in sections. The absorber element  15  is fashioned as a rotationally symmetrical body having a cylindrical middle region and end regions that conically taper toward respective bearing ends. The middle region and the two end regions are fabricated of one piece. For the automatic drive with a rotary motion of the absorber element  15 , a stepping motor  17  having an encoder  19  and a transmission  21  is allocated thereto. Via the transmission  21 , the stepping motor  17  drives a shaft  23  on which the absorber element  15 —which has an inside bore—is slipped and secured. The shaft  23  is fabricated of steel and has an end lying opposite the stepping motor seated at a housing  27  of the radiation diaphragm  11 . The shaft  23  proceeds eccentrically through the absorber element  15 , so that the absorber element  15  has a rotational axis  25  that is eccentrically seated. All rotational axes, i.e. the rotational axis of the motor  17 , rotational axes as may exist in the transmission  21  and the rotational axis  25  of the absorber element  15 , are parallel to one another. No linear motion occurs.  
         [0029]    A further oblong absorber element  35  also is provided, this being fashioned analogously to the aforementioned absorber element  15 , and having a stepping motor  37 , an encoder  39 , a transmission  41  and a shaft  43  allocated to it in an analogous fashion. The further absorber element  35  also has a rotational axis  45  that is eccentrically seated.  
         [0030]    The two rotational axes  25 ,  45  of the absorber elements  15 ,  35  proceed parallel to one another and perpendicular to the ray beam  13 . The slit-like region enclosed by the middle regions of the two absorber elements  15 ,  35  defines that region of the ray beam  13  that is gated into the patient.  
         [0031]    The rotary motion of the two absorber elements  15 ,  35  for varying the size of the ray beam  13  is explained in greater detail in the cross-sectional illustration of FIG. 3. A ray beam  13  having a defined slit size is allowed to pass in the position of the absorber elements  15 ,  35  that is shown with solid lines. The radiation diaphragm  11  is closed in the position of the absorber elements  15 ,  35  that is shown with broken lines, so that no radiation proceeds to the patient.  
         [0032]    A position-variable wedge filter  51  for the variable attenuation of the X-radiation is also integrated into the housing  27  of the radiation diaphragm  11 . A filter device  61  is also present with which different spectral quantum energy distributions can be impressed on the ray beam  13 . For this purpose, four spectral filters  65 ,  66 ,  67 ,  68  that are different from one another are attached on a rotatable carrier  63  at equidistant angular spacings. A desired spectral filter  65 ,  66 ,  67 ,  68  can be positioned in the beam path with a motor  70  (see FIG. 4).  
         [0033]    It is self-evident that the focus side of the housing  27  has an aperture for the admission of the X-radiation deriving from the X-radiator  3  and comprises an exit aperture at the opposite side.  
         [0034]    As already indicated in FIG. 3, it is advantageous if the two absorber elements  15 ,  35  are arranged slightly behind one another or offset as viewed in the direction of the ray beam  13 . As explained in greater detail in FIGS. 5 and 6, namely, the advantage derives therefrom that the radiation diaphragm  11  can completely block the ray beam  13  in the position shown in FIG. 5. As a result of the attachment of the absorber element  15 ,  35  behind one another—as seen in the direction of the central ray  14  --, the radiation diaphragm  11  can close with a lateral overlap U of the absorber elements  15 ,  35 . Given an exact side-by-side arrangement of the two absorber elements  15 ,  35 , and at essentially the same distance from the focus  3 , the absorber elements  15 ,  35  would at most touch in one point in the closed condition, resulting in an inadequate absorption in this region. Due to manufacturing imprecision and for a dependable operation, a small gap would even have to remain between the absorber elements in the closed condition. Compared thereto, the illustrated overlap U (=R 1 +R 2 —D) in the closed condition of the radiation diaphragm  11  according to FIG. 5 sees to an adequate absorption of the central ray  14  as well.  
         [0035]    [0035]FIG. 6 shows the arrangement as in FIG. 5, wherein the two absorber elements  15 ,  35  each have been rotated by 180° around their respective rotational axes  25  and  45  compared to the position in FIG. 5. As a result, the major radii R 1 , R 2  of the middle regions of the absorber elements  15 ,  35  now face away from the central ray  14 , and only the minor radii r 1 , r 2  face toward the ray beam (d=R 1 +r 1 =R 2 +r 2 ). A maximum diaphragm aperture B is possible in the condition shown in FIG. 6.  
         [0036]    For a prescribed, desired overlap and a prescribed, maximum diaphragm aperture B (=D−r 1 −r 2 ), the required rotational axis spacing D between the rotational axes  25  and  45  of the absorber elements  15 ,  35  is established by  
       D   =     d   +       B   -   U     2                             
 
         [0037]    given a prescribed diameter d of the middle regions of the absorber elements  15 ,  35 .  
         [0038]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.