Abstract:
An electronic video camera apparatus is provided for focusing light rays from object plane proximate image intensifier of a medical x-ray imaging system onto an image plane proximate a light sensor. The electronic video camera includes a lens system located between the object and image planes to focus light rays from the object plane onto the image plane. The light rays at the object plane are representative of a patient image. An optical filter is located between the object and image planes and partially blocks light rays passing there through. The optical filter includes at least first and second filter regions having different opacity. The first and second filter regions are alignable with the lens system at different times to block differing first and second amounts of light rays, respectively, associated with differing first and second x-ray amounts transmitted at different times.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    At least one preferred embodiment of the present invention generally relates to a medical x-ray imaging system employing an electronic video camera that operates upon visible light to control brightness. At least one preferred embodiment of the present invention relates to an electronic video camera that utilizes a neutral density filter having varying opacity across the filter and that is adjustable for light attenuation.  
           [0002]    In the past, medical diagnostic imaging systems have been proposed for imaging regions of interest in patients through the use of x-ray sources and receptors positioned on opposite sides of a patient&#39;s region of interest. Typical x-ray imaging systems utilize an x-ray source and receptor that are movable to various positions relative to the patient&#39;s region of interest. The x-ray source is controlled to adjust the amount of x-rays transmitted therefrom, passed through the patient and impinged on the x-ray receptor. X-ray receptors generally include an image intensifier having an x-ray detection layer that detects x-rays passing through a patient. The image intensifier converts the x-rays to visible light which is, in turn, guided onto an object plane proximate a video camera. The video camera includes an optical lens system focusing light from the object plane onto an image plane proximate a light sensitive sensor. One example of a light sensitive sensor is a charge coupled device. The light sensitive sensor detects and converts the visible light at the image plane data that is processed and ultimately displayed to a user.  
           [0003]    Various anatomical regions attenuate x-rays to different degrees depending upon thickness, density, structure and the like of the anatomic region. These different characteristics of patient anatomy attenuate x-rays to different degrees and may degrade x-ray images where an anatomy of interest is located proximate certain other types of anatomy.  
           [0004]    Operators of x-ray imaging equipment attempt to improve image quality of x-ray images through a variety of manners. One such manner for improving x-ray image quality involves adjusting the x-ray intensity transmitted by the x-ray source. For instance, anatomical regions that highly attenuate x-rays are imaged better by increasing the number of x-rays transmitted from the source. By increasing the x-ray transmissions, the user similarly increases the photon statistics sensed at the receptor (e.g., the number of photons impingent upon the image intensifier). As the photon statistics increase, the image intensifier converts more and more x-rays to visible light, thereby increasing the brightness of the light incident on the object plane of the electronic video camera. The light brightness may rise to a level sufficient to saturate the light sensor, such as the CCD. As the sensed light becomes excessive, the resulting processed and displayed image degrades. Image degradation may appear in several forms, such as a washed out image, an image having poor contrast between adjacent anatomies, and the like.  
           [0005]    In the past, x-ray systems have attempted to prevent the light brightness from overloading the sensor by adding an iris to the electronic video camera having an adjustable opening passing only a desired amount of light. The diameter of the opening can be varied to affect the desired average attenuation of the brightness of the light at the object plane. As the system reduces the iris opening to “stop down” or partially close the iris opening, feedback sensing will detect that the average brightness of the light at the object plane is reduced, and the system can automatically increase the amount of x-rays impinging upon the receptor.  
           [0006]    In accordance with the foregoing, the quality of the ultimately displayed image is influenced by the amount of x-ray flux (intensity) that is incident upon the image intensifier. The amount of light that is allowed to pass through the optics of the electronic video camera typically controls the amount of x-ray flux. A higher quality image requires more x-ray flux and more x-ray flux is permitted by decreasing the iris aperture that passes light through the camera optics, thereby avoiding sensor saturation. Motor controlled irises precisely control the amount of light passed through the optics in order to ensure that the minimum x-ray flux necessary is used in view of patient concerns. The iris aperture diameter and thus the amount of x-ray flux may be varied during single patient imaging procedure. Hence, light intensity is typically controlled automatically by the x-ray imaging system in accordance with commands from a user entered to initiate an imaging operation.  
           [0007]    It is preferable that the electronic video camera only focus light near the object plane onto the image plane. The compact nature of x-ray systems typically results in the object plane and image plane being in close proximity to opposite ends of the camera optics. Hence, structure within the camera optics, such as glass surfaces and the like through which the light passes are located proximate the object plane. The glass surface and other transparent structure near the object plane may be focused by the camera optics onto the image plane as the iris aperture is reduced. These transparent structures in or near the camera optics may contain blemishes, such as scratches, digs and the like and may accumulate foreign material such as dirt. The blemishes and/or dirt may be close enough to the object plane as to become at least partially focused onto the image plane when the iris aperture is stopped down. The camera optics may partially focus images of the blemishes or dirt onto the image plane sufficiently that the light sensor at the image plane detects the blemishes/dirt as data conveyed to the processor to be imaged. These projections of blemishes and dirt create unwanted artifacts at the image plane that result as artifacts appearing in the displayed image.  
           [0008]    [0008]FIG. 8 illustrates an exemplary configuration for the camera optics as formed in accordance with conventional systems. The camera optics  75  include a glass or other transparent layer  77  located at the input side to the camera optics proximate the object plane  79 . The glass or other transparent layer  77  represents any kind of structure that could be part of the camera optics  75  such that this structure presents an opportunity for its surfaces to contain blemishes or dirt that may partially be in focus. For example, structure  77  could be part of the forward lens system  81 , or structure  77  could be leaded glass installed for the purpose of reducing x-ray radiation beyond the optics such as would otherwise irradiate the optical sensor. The image intensifier directs light rays representative of an x-ray image onto the object plane  79 . A forward lens system  81  is located proximate the glass layer  77  which directs light ray traces  83  and  86 , from the object plane  79  through optical components  87  onto a rear lens system  89 . The forward lens system also directs light ray traces  84  and  85  from blemishes/dirt in the glass layer  77  onto the rear lens system  89 . The forward lens system  81  collimates the light ray traces  83 - 86 , while the rear lens system  89  reconverges the light ray traces  83 - 86 . The forward and rear lens systems  81  and  89  cooperate such that light ray traces  83  and  86  projecting from the object plane are collimated at the forward lens system  81  into a parallel manner and converged at the rear lens system  89  onto an image plane  91 . When blemishes and dirt exist on the surface of the glass layer  77 , light ray traces  84  and  85  are focused by the forward and rear lens systems  81  and  89  at a point  97   
           [0009]    An adjustable iris  93  is opened and closed based upon the desired x-ray flux to control the amount of light ray traces passed therethrough onto the rear lens system  89 . As the adjustable iris  93  reduces the opening therethrough, the shape and size of a focus region  95  proximate the image plane  91  expands. The focus region represents an area in which light rays are adequately in focus to be detectable at the image plane by the light sensor as a distinct image for which data is generated and processed (albeit possibly as an artifact). The size of the focus region  95  is relatively small when the iris  93  is open to a relatively large state. When in a relatively closed state (as illustrated in FIG. 8), the iris  93  forms a relatively large focus region  95  that includes light ray traces  84  and  85  projected from the surface of the glass layer  77 . Hence, while the projection of blemishes and dirt are not focused directly on the image plane  91 , the point  97  at which such blemishes and dirt are focused is adequately close to the image plane  91  to be sufficiently in focus at the image plane  91  that an artifact is created in the data generated by the light sensor. The partially focused images at the image plane  91  of dirt and other blemishes are detected by the sensor, processed and displayed along with the x-ray image. The image portions associated with the dirt and blemishes appear as artifacts in the resulting x-ray image. Hence, reducing the iris aperture may increase the tendency of dirt or blemishes close to the object plane to manifest themselves on the image plane.  
           [0010]    A need remains for an improved x-ray imaging system and electronic video camera apparatus that avoids the disadvantages discussed above, while permitting x-ray flux to be increased when desired to obtain a higher quality image.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    In accordance with at least one embodiment of the present invention, in a medical x-ray imaging system, an electronic video camera is provided for focusing light rays from an object plane proximate an image intensifier onto an image plane proximate a light sensor. The electronic video camera includes an object plane receiving light rays representative of a patient image. A lens system is provided between the object plane and image plane to focus the light rays from the object plane onto the image plane. An optical filter is also located between the object and image planes. The optical filter attenuates or partially blocks the light rays. The optical filter includes at least first and second filter regions having different opacity. The first and second filter regions are alignable with the lens system at different times to block different amounts of light rays associated with differing x-ray intensities that are transmitted at different times.  
           [0012]    In accordance with at least one alternative embodiment, the optical filter includes a neutral density wheel having first and second sectors with differing thicknesses of opaque material deposited thereon to form the first and second filter regions. Optionally, the optical filter may include a circular filter having multiple sectors located adjacent one another. The sectors may have different opacities. Optionally, the optical filter may include filtered discs having at least two non-overlapping sectors of different opacity where the opacity is constant throughout each sector. As a further option, the optical filter may attenuate light passing there through to different degrees based on a rotational orientation of the optical filter with respect to the lens system.  
           [0013]    In accordance with at least one embodiment, at least a portion of the first filter region is formed to be highly transparent to light rays and at least a portion of the second filter region is formed to have increasing opacity at progressively larger angular orientations of the optical filter with respect to a reference plane traversing the lens system. Optionally, the optical filter may be formed with a continuously varying opacity. The optical filter may variably attenuate the amount of light rays passed through the lens system based upon the position at which the optical filter is set relative to the lens system. Optionally, the optical filter may include a wheel located such that a sector of the wheel aligns with the lens system. The sector of the wheel aligned with the lens system represents one of the first and second filter regions. The wheel may have an opacity that continuously varies as a function of the angular orientation of the wheel with respect to the lens system.  
           [0014]    Optionally, the optical filter may be formed with uniform opacity over discrete non-overlapping sectors where each discrete sector has a unique opacity that differs from other sectors by an amount based on an orientation of the optical filter with respect to the lens system. Optionally, the optical filter may include two filter wheels aligned with one another and having similar but opposite variations in opacity at progressively greater angular positions about the filter wheels.  
           [0015]    In an alternative embodiment, the lens system may include forward and rear lens assemblies spaced apart from one another with the optical filter being positioned there between. Optionally, an iris may be located between the optical filter and the object plane with the iris including an aperture controlling a brightness of the light rays impingent upon the optical filter. The iris maintains a constant aperture at numerous x-ray intensities. Optionally, an electrical motor may be provided to adjust the position of the optical filter with respect to the lens system to automatically adjust attenuation of light rays by moving the optical filter between first and second positions to move the first filter region to an unused position and the second filter region to an operative position.  
           [0016]    In accordance with at least one alternative embodiment, a medical x-ray system is provided having a support structure holding an x-ray source and receptor facing one another and aligned along a patient imaging axis. The x-ray source and receptor cooperate to obtain x-rays attenuated by a patient region of interest. The x-ray source may be controlled to vary an intensity of transmitted x-rays. The receptor converts x-rays to light rays representative of the patient region under examination, such that a brightness of the light rays varies based on the intensity of the x-rays received at the receptor. A processor processes the light rays to obtain x-ray images and a display displays processed x-ray images. A partially opaque member is provided to block a portion of the light rays to reduce a brightness of the light rays. The partially opaque member is provided with regions of different opacity.  
           [0017]    In accordance with at least one embodiment, a motor assembly is provided for automatically moving the partially opaque member to vary the amount of attenuation of the brightness of the light rays. Optionally, means may be provided for shifting the partially opaque member from a highly opaque state to a lesser opaque state causing a reduction in an intensity of x-rays transmitted from the x-ray source until the average brightness of the light incident on the image sensor is reduced to the proper level.  
           [0018]    Alternatively, a motor and gear assembly may be provided to rotate the partially opaque member between first and second angular positions to move a more opaque region of the partially opaque member into alignment with the light rays. Optionally, an assembly may be provided for moving the partially opaque member between an initial position at which light rays pass through a highly transparent portion of the partially opaque member to a final position at which a portion of the light rays are blocked by a highly opaque portion of the partially opaque member. Optionally certain regions of the partially opaque member may be provided with constant opacity. Optionally, regions of the partially opaque member may be provided with continuously varying opacity. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 illustrates a mobile x-ray imaging system formed in accordance with one embodiment of the present invention.  
         [0020]    [0020]FIG. 2 illustrates an x-ray imaging system formed in accordance with an alternative embodiment of the present invention.  
         [0021]    [0021]FIG. 3 illustrates a side sectional view of an optical camera assembly formed in accordance with an embodiment of the present invention.  
         [0022]    [0022]FIG. 4 illustrates a side view of a portion of an optical camera apparatus formed in accordance with an embodiment of the present invention.  
         [0023]    [0023]FIG. 5 illustrates a front view of a portion of the optical and mechanical camera apparatus formed in accordance with an embodiment of the present invention.  
         [0024]    [0024]FIG. 6 illustrates a graphical representation of an optical assembly formed in accordance with an embodiment of the present invention.  
         [0025]    [0025]FIG. 7 illustrates a graphical representation of an optical filter formed in accordance with an embodiment of the present invention.  
         [0026]    [0026]FIG. 8 illustrates a graphical representation of a conventional optical camera apparatus.  
         [0027]    [0027]FIG. 9 illustrates a graphical representation of an alternative optical filter formed in accordance with an embodiment of the present invention. 
     
    
       [0028]    The foregoing summary, as well as the following detailed description of the preferred embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the preferred embodiments of the present invention, there is shown in the drawings, embodiments that are presently preferred. It should be understood, however, that the present invention is not limited to the arrangements and instrumentality shown in the attached drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    An embodiment in accordance with the present invention is illustrated in FIG. 1 wherein is shown a C-arm X-ray apparatus, generally designated at  10 . The apparatus  10  includes a C-arm  12  having inner and outer circumferences  14  and  16 , respectively, and terminating in opposing upper and lower distal ends  18   a  and  18   b . The C-arm  12  preferably has a uniformly circular C-shape, but may alternatively comprise any arc-shaped member.  
         [0030]    The C-arm  12  is held in a suspended position by support means such as structure, generally designated at  20 , which includes a support arm  22  mounted upon a wheeled base  24 . The support arm  22  provides for rotational movement of the C-arm  12  about an axis of lateral rotation  30 , either by a bearing assembly between the support arm  22  and the C-arm  12 , or by the support  22  itself being rotatably mounted with respect to the base  24 .  
         [0031]    The wheeled base  24  enables transport of the C-arm  12  from a first location to a second location. As such, the wheels of the base operate as transporting means coupled to the support structure  20  for transporting the support arm  22  and the C-arm  12  from a first location to a second location. It may be preferable to move X-ray equipment from one room to another. The mobile nature of the apparatus  10  as provided by the wheeled base  24  offers increased access by patients in many different rooms of a hospital, for example.  
         [0032]    The support arm  22  is slidably mounted to the outer circumference  16  of the C-arm  12  and the support structure  20  includes structure and mechanisms necessary to enable selective, sliding orbital motion of the C-arm about an axis of orbital rotation  26  to a selected position. The axis  26  preferably coincides with a center of curvature of the C-arm  12  and with the axis of lateral rotation  30 . It will be appreciated that the sliding orbital motion causes the C-arm  12  to move through various sliding points of attachment  28  to the support arm  22 . The support structure  20  further includes mechanisms for laterally rotating the support arm  22  selectable amounts about an axis of lateral rotation  30  to a selected lateral position. The combination of sliding orbital motion and lateral rotation enables manipulation of the C-arm in two degrees of freedom, i.e. about two perpendicular axes. This provides a kind of spherical quality to the movability of the C-arm  12 —the sliding orbital motion and lateral rotation enable an X-ray source  32  coupled to the C-arm to be moved to substantially any latitude/longitude point on a lower hemisphere of an imaginary sphere about which the C-arm is moveable.  
         [0033]    The apparatus  10  includes an X-ray source  32  and an image receptor  34  as known generally in the X-ray diagnostic art, mounted upon opposing locations, respectively, on the C-arm  12 . The X-ray source  32  and the image receptor  34  may be referred to collectively as the X-ray source/image receptor  32 / 34 . The image receptor  34  can be an image intensifier or the like. The orbital and laterally rotational manipulation of the C-arm enables selective positioning of the X-ray source/image receptor  32 / 34  with respect to the width and length of a patient located within interior free space  36  of the C-arm  12 . The sliding orbital movement of the C-arm causes the X-ray source/image receptor  32 / 34  to move along respective arcuate movement paths. The image receptor  34  is preferably secured to the inner circumference  14  of the C-arm  12  and the X-ray source  32  may also be secured to said inner circumference  14 , the significance of which will be described below.  
         [0034]    Another C-arm support structure, exemplified in FIG. 2, includes a downwardly-extending L-arm  23  such that its point of attachment  28  with the C-arm  12  resides a distance D away from the axis of lateral rotation  30 . The image receptor  34  on the C-arms are mounted and positioned in such a way as to encumber a back convex portion  40  of the C-arm  12 , as shown by encumbering portions  42  in FIG. 2, thereby preventing the support arm  23  from slidably attaching to that portion  12   a  of the C-arm. In order to achieve complete horizontal positioning of the image receptor  34 , the L-arm was developed to attach to the C-arm the point of attachment  28  below the axis of lateral rotation  30 , thus permitting the C-arm  12  to slide the image receptor  34  to at least a horizontal orientation. This introduces an eccentric lateral moment arm D upon lateral rotation of the C-arm  12  about the axis  30 . This typically requires lateral rotation of the C-arm  12  about the axis  30  to be electrically powered to overcome the torque that results from the imbalance.  
         [0035]    [0035]FIG. 3 illustrates a side sectional view of a camera apparatus  100  included within the image receptor  32 . The camera apparatus  100  includes a housing  102  mounted to a lateral bracket  104 . The housing  102  and bracket  104  cooperate to securely locate camera optics  106  at a desired location with respect to an image intensifier  108  and a light sensor  110  (e.g., a CCD). The camera optics  106  are located between an object plane  112  and an image plane  114 . The image intensifier  108  is located to direct light rays onto the object plane  112  where such light rays are representative of an x-ray image detected by the receptor  32 . The light sensor  110  is located proximate the image plane  114  and operates to convert light rays focused on or substantially near the image plane  114  into data that is subsequently processed by a processor  116  and displayed by a display unit  118 .  
         [0036]    The camera apparatus  100  includes a leaded glass cover  122  located proximate the object plane  112 . The glass cover  122  blocks x-rays from reaching the light sensor  110 , while permitting light rays to pass therethrough. The camera optics  106  include a forward lens assembly and optical prism  121  (pechan prism) located proximate the glass cover  122  and a rear lens assembly  124  located proximate an opposite end of the camera apparatus  100 . The prism  121  enables the forward and rear lens assemblies  120  and  124  to be closely spaced. The rear lens assembly  124  is located proximate the image plane  114 . The forward lens assembly and optical prism  121  collimates light ray traces passing through the glass cover  122  and provides a compacted path length for the near-columnar light to travel, while the rear lens assembly  124  reconverges such collimated light ray traces. The forward and rear lens assembly  120  and  124  cooperate to focus ray traces from the object plane  112  onto the image plane  114 .  
         [0037]    The camera optics  106  further include optics components  126  that may be used to effect a variety of operations. An iris  128  is provided having an opening therethrough that is adjustable in diameter to control a brightness of light passing therethrough.  
         [0038]    The forward lens assembly  120  may be formed in a variety of manners. By way of example only, the forward lens assembly  120  may include a complex convex lens  130  having forward and rear portions. The rear lens assembly  124  may also include a variety of lens configurations. By way of example only, the rear lens assembly  124  may include first through fifth lenses  132 ,  134 ,  136 ,  138  and  140  arranged adjacent one another as shown with various combinations of convex and concave surfaces. A rear structure  142  isolates the rear end of the camera apparatus  100  to protect the camera optics  106  from environmental elements while permitting light rays to pass therethrough.  
         [0039]    The camera optics  106  further include a filter member  144  positioned with a portion of the filter member  144  spanning the opening between the forward and rear lens assemblies  120  and  124 . The filter member  144  is partially opaque to attenuate light rays passing from the forward lens assembly  120  to the rear lens assembly  124 . The filter member  144  has differing amounts of opaqueness at different positions upon the filter member  144 . The filter member  144  is moved to different positions, while at least a portion of the filter member  144  remains between the forward and rear lens assemblies  120  and  124  in order to align desired regions of differing opacity along the line of sight  146  extending between the forward and rear lens assemblies  120  and  124 .  
         [0040]    [0040]FIG. 4 illustrates the filter member  144  in more detail. A bracket  148  mounts the filter member  144  to the bracket  104  and camera apparatus  100 . The bracket  148  includes a rotatable support pin  150  having the filter member  144  secured to one end thereof. An opposite end of the support pin  150  is secured to a gear  152  that is part of a larger gear assembly  154  that is driven by a motor  156 . The motor  156  is controlled by the processor  116  (or a separate and distinct control processor not shown). The motor rotates the filter member  144  through the gear assembly  154  in order to position a portion  158  of the filter member  144  between the object and image planes  112  and  114 . The portion so aligned is considered the active portion. FIG. 5 illustrates a front view of the gear assembly  154  in more detail.  
         [0041]    [0041]FIG. 6 illustrates graphically one embodiment of a camera optics  159 . An object plane  160  and an image plane  174  are located on opposite sides of the camera optics  159 . A leaded glass cover  162  is located proximate a forward lens assembly  120  which is in turn located proximate optics components  166  and iris  168 . A light attenuator  170  is provided between the forward lens assembly  164  and rear lens assembly  172 . Optionally, the iris  168  may be removed and/or the optics components  166  may be removed. The optics components  166 , iris  168  and light attenuator  170  may be reordered. Light rays  180  and  181  from an object at the object plane  160  are collimated by the forward lens assembly  164  and re-converged by the rear lens assembly  172  to be focused at the image plane  174 . Light rays  182  and  183  from a blemish  184  on the glass cover  162  are collimated and re-converged by the forward and rear assemblies  164  and  172 , respectively, to be focused at a point  176  beyond the image plane  174 . The point  176  at which the blemish  184  is focused is outside of the focus region  178  surrounding the object plane  174 . The focus region  178  is maintained small enough to exclude the focus point  178  by maintaining the diameter of the aperture through the iris  168  relatively large. Hence, the blemish  184  is not adequately in focus at the image plane  174  to be detected as an artifact by the light sensor, nor displayed as an artifact in the x-ray image.  
         [0042]    As is apparent from a comparison of FIGS. 6 and 8, for high intensity x-ray shots, the diameter  186  of the opening through iris  168  is substantially larger than the diameter  94  of the opening through iris  93  as used in conventional camera optics  75 . The diameter  186  of the opening through the iris  168  may remain constant over a wide range of x-ray intensities. As the light attenuator  170  is adjusted to increase the degree to which the light is attenuated, the system will automatically respond by increasing x-ray intensity per feedback obtained by monitoring the average amount of light intensity at the light sensor located at the image plane  174 . Similarly, as the light attenuator  170  is adjusted to decrease the degree to which the light is attenuated, the x-ray intensity is decreased.  
         [0043]    [0043]FIG. 7 illustrates a neutral density wheel  188  that, in accordance with one embodiment, may be utilized as a light attenuator  170 . The neutral density wheel  188  is circular and is positioned to rotate about a filter axis that is parallel to an imaging axis extending through the camera optics  159 . The filter axis is spaced apart from the imaging axis by a distance based on the radius of the neutral density wheel  188 . For example, filter axis may be spaced slightly outside the view of the cameral optics  159  such that a sector of the neutral density wheel  188  extends through and covers the active viewing area between the forward and rear lens assemblies  164  and  172 .  
         [0044]    The neutral density wheel  188  may be formed from a transparent material, such as glass and the like, that is coated with an opaque material. The opaque material may be coated on the neutral density wheel  188  in a progressively thicker coating to vary the amount of opacity. In the example of FIG. 7, the opaque coating becomes thicker (and thus the amount of attenuation increases) as it moves in the clock-wise direction around the neutral density wheel  188 . Alternatively, the neutral density wheel  188  may be coated in step-wise even sections to form non-overlapping discrete sections of equal opacity. Alternatively, the neutral density wheel  188  may be formed from a mosaic of individual pieces that are secured to one another. Each mosaic piece may have an even coating thereon.  
         [0045]    In FIG. 7, radial lines  190  and  198  graphically illustrate the opacity. Hence, in region  192  with no radial lines, the opaque coating is either non-existent or very thin to render the neutral density filter  188  substantially transparent to light. At progressively greater angles along circular arc  196  away from a reference mark  194 , the opaque material is coated thicker. For instance, the radial lines  198  proximate region  200  are spaced relatively far apart, as compared to the radial lines  190  in region  202 . This illustrates that the neutral density filter  188  is more opaque in region  202  than in region  200 . Similarly, region  200  is more opaque than region  192 .  
         [0046]    The thickness of the opaque material may be varied continuously or in very small narrow step-wise sectors to afford fine resolution. The use of fine resolution enables the x-ray intensity to similarly be varied in small or fine steps to achieve close control over the amount of x-rays, to which a patient is exposed. For example, the neutral density wheel  188  may be rotated by a small amount to slightly adjust the attenuation. Once the wheel  188  is rotated, the system then adjusts the intensity of the x-rays based on the new position of the wheel  188 .  
         [0047]    A sensor, such as a potentiometer, is provided on one of the wheels  188 , the gear assembly  154  and the motor  156 . The sensor may sense the position of the axle of the wheel  188 . The sensor affords precise control over the wheel  188  position. The processor senses the axle position and drives the motor  156  until the wheel  188  is properly oriented.  
         [0048]    Optionally, multiple light attenuators may be utilized. By way of example only, FIG. 9 illustrates a second light attenuator  171  located adjacent and aligned parallel to the light attenuator  170 . The rotational axes  167  and  169  of the light attenuators  170  and  171 , respectively, may be along different axes (as shown). Alternatively, the light attenuators  170  and  171  may be formed directly in line with one another to rotate about a common axis, such as axis  167 . Optionally, a single motor may drive both light attenuators  170  and  171 . Alternatively, different motors may drive light attenuators  170  and  171 .  
         [0049]    The light attenuators  170  and  171  are coated with opaque material, the thickness of which varies in opposite directions from one another. Hence, when light attenuators  170  and  171  overlap one another in the viewing area between the forward and rear lens assemblies  164  and  172 , the opacity is substantially even across the viewing area even though the opacity varies continuously on each individual light attenuator  170  and  171 . By way of example, in the viewing area, the attenuation caused by the light attenuator  170  may increase while moving in a clock-wise direction around the light attenuator  170 , whereas the attenuation caused by the light attenuator  171  may increase while moving in a counter clock-wise direction around the light attenuator  171 . The composite attenuation caused by both light attenuators  170  and  171  is relatively even across the viewing area.  
         [0050]    The neutral density wheel  188  is described as circular with continuous or stepped sectors of opaque material coated thereon. However, other shapes may be used as well. Also, the opaque material need not be a coating. Also, the opaque material need not be sector shaped. For instance, the light attenuator  170  may be rectangular, octagonal, square, triangular, pentagonal and the like. The light attenuator need only be divided into two or more regions of differing opacity. If rectangular, the light attenuator  170  may be formed with opaque regions shaped as strips extending from the top to the bottom of the light attenuator  170 . If so structure, the light attenuator  170  would then be slid laterally in a direction transverse to the imaging axis in order to move a region of desired opacity into alignment with the forward and rear lens assemblies  164  and  172 .  
         [0051]    While the invention has been described with reference to alternative embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.