Abstract:
An x-ray imaging source comprises a radiation source ( 12 ) providing x-ray radiation. A substrate comprised of a scintillating material ( 16 ) responsive to a level of incident radiation provides output light according to the level of incident radiation. A Fresnel lens ( 40 ) is disposed proximate to the substrate for directing the output light toward a second lens. The second lens directs the output light to an image sensor for converting light levels to the digital data, forming an image thereby.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention generally relates to radiographic imaging and more particularly to an imaging apparatus for providing radiographic images with improved optical efficiency.  
       BACKGROUND OF THE INVENTION  
       [0002]     Digital radiography (DR) systems are enjoying growing acceptance as clinical imaging tools. As shown in the simplified block diagram of  FIG. 1 , radiation from x-ray source  12  in a lens-coupled DR imaging apparatus  10  is passed through a subject  14  and impinges upon a scintillator screen  16 , converting the energy from ionized radiation into light radiation having different frequencies, typically within the visible spectrum. In lens-coupled DR imaging apparatus  10 , this emitted light energy is directed, through a lens system  18 , to an image sensing apparatus  20  that then forms a digital image from the emitted light. Unlike conventional x-ray film apparatus, DR imaging apparatus  10  does not require a separate processing area or image processing consumables. Another advantage of DR imaging technology is speed, since images are obtained immediately after the x-ray exposure. For medical applications, an image can be provided to medical personnel while a patient is still present at an imaging facility.  
         [0003]     While there are inherent advantages to DR imaging, however, there is a need to improve the overall performance of lens-coupled DR systems. One area of particular interest relates to the amount of light that lens system  18  channels from scintillator screen  16  to image sensing apparatus  20 , commonly characterized in terms of optical coupling efficiency. As a rule, optical efficiency directly affects the image quality of the DR system. Improvements in optical coupling efficiency may result in improved diagnostic capability and can advantageously also reduce radiation dosage requirements in many cases.  
         [0004]     To date, conventional lens-coupled DR systems, as shown in  FIG. 1 , have exhibited relatively low optical coupling efficiencies for a number of reasons. Referring to  FIG. 2 , light emission from a pixel  24  on scintillator screen  16  is divergent. Thus, a large fraction of the light from scintillator screen  16  is emitted at angles that exceed the light-gathering capability of standard lens components of lens system  18 . Also, as shown in  FIG. 3 , the size of lens system  18  is often considerably smaller than scintillator screen  16  due to cost, space, and manufacturing constraints. Rays emitting from the outer regions of scintillator screen  16 , such as pixels  25  and  25 ′, even at small emission angles, totally miss lens system  18 . Thus, for the reasons shown in  FIGS. 2 and 3 , a large portion of the emitted light from scintillator screen  16  never reaches image sensing apparatus  20 .  
         [0005]     There have been a number of efforts at improving the optical coupling efficiency of lens-coupled DR imaging systems. One approach is directed to reducing the angular spread of the emission by controlling the structure of scintillator screen  16  itself. Examples of this approach include the following: 
        U.S. Pat. No. 5,519,227 (Karellas) discloses a laser-based micro-machining process for reducing spatial dispersion and scattering;     U.S. Pat. No. 5,418,377 (Tran et al.) discloses another method for treating phosphorus sites on the scintillator screen to reduce scattered luminescent radiation; and,     U.S. Patent Application Publication No. 2004/0042585A1 (Nagarkar et al.) discloses processing a columnar structured material to reduce crosstalk and enhance collection efficiency. Medical Physics Journal article “Scintillating fiber optic screens: a comparison of MTF, light conversion efficiency, and emission angle with Gd 2 O 2 S:Tb screens” Vol. 24, Number 2, February, 1991, pages 279-285, discloses scintillating fiber optic screens having forward-directed emission distributions.        
 
         [0009]     Another approach for improving optical coupling efficiency has been to improve the performance of collection optics themselves. As shown in  FIG. 2 , the emissive surface of scintillator screen  16  broadcasts light over a wide range of angles. Any type of collection optics must collect as much of the emitted light as possible and direct the light to image sensing apparatus  20 , while keeping crosstalk between pixels to a minimum. As one example of an approach to improving collection optics, U.S. Pat. No. 6,178,224 (Polichar et al.) discloses the use of an emission modification layer positioned near the scintillation layer to limit the divergence of the emitted light. As embodiments of this emission modification layer, the Polichar et al. &#39;224 disclosure mentions using various types of brightness enhancement film (BEF) and lenslet array or microsphere array structures.  
         [0010]     While the approach described in the Polichar et al. &#39;224 disclosure may improve total brightness from increased optical coupling, however, there are drawbacks for imaging when using the particular types of solutions proposed. In particular, with any of the disclosed embodiments of the Polichar et al. &#39;224 disclosure, an increase in brightness comes at the price of lost contrast. This is because the predominant contribution to brightness increase for BEF components comes not from the BEF&#39;s divergence narrowing action, but from recycling of light rays that have undergone total internal reflection (TIR), which causes undesirable pixel crosstalk. This is illustrated in greater details by ray trace plots in  FIGS. 5A and 5B .  FIG. 5A  shows how light from different points on the surface of scintillator screen  16  is directed through a BEF  26  having light-redirecting prisms  28 , as disclosed in Polichar et al. &#39;224. Light rays R from pixel P 1 , initially emitted over a fairly broad range of angles, are conditioned by BEF  26  and redirected toward normal, so that the divergence angle of light decreases from the original divergence angle shown as α 1  to a smaller divergence angle α 2 . On the other hand, light rays from pixel P 2  does not get redirected toward normal, in fact emerging from BEF  26  at an angle β 2  larger than the original divergence angle β 1 . It can be observed that BEF  26 , then, produces narrowing of the divergence of the emitted light from scintillator screen  16  only for certain pixel locations, depending on the relative positions of the pixels with respect to the light-redirecting prisms  28 . Even for those light cones where divergence narrowing takes place, such as from pixel P 1 , the centroid remains forward-directing and does not get bent toward the center of lens system  18 , contrary to what is discussed in Polichar et al. &#39;224. A substantial part of the light cone still misses lens system  18 , as shown in  FIG. 5A . It is thus clear that the divergence narrowing action of the BEF  26  by itself is limited in effectiveness in increasing light throughput.  
         [0011]      FIG. 5B  shows how total internal reflection (TIR) in BEF, as used in Polichar et al. &#39;224, decreases contrast. Light from pixel P 3  enters BEF  26  at a number of angles. Some rays (R 3  and R 4 ) undergo refraction at the exit surface of BEF  26  and can be imaged by lens system  18  to form the image of P 3  on image sensing apparatus  20  (not shown in  FIG. 5B ). Other rays undergo TIR at surfaces of BEF  26 , and backscatter from scintillator screen  16  at positions different from P 3 . These recycled rays (R 1  and R 2 ) re-emerge from BEF  26  to be directed to image sensing apparatus  20  by lens system  18 . However, R 1  and R 2  will be imaged to image sensing apparatus  20  at points other than the image of P 3 ; this constitutes undesirable pixel crosstalk that degrades image contrast.  
         [0012]     Emission modification layers using lenslet or microsphere array structures, as disclosed in Polichar et al. &#39;224, suffer similar deficiencies as using BEF  26 .  
         [0013]     Thus, it can be seen that while solutions for improved optical coupling have been proposed, there is room for improvement, particularly with respect to improving optical coupling efficiency while maintaining image contrast.  
       SUMMARY OF THE INVENTION  
       [0014]     Briefly, according to one aspect of the present invention, x-ray imaging system incorporates: 
        a) a radiation source providing x-ray radiation;     b) a substrate comprised of a scintillating material responsive to the level of incident radiation and providing output light according to the level of incident radiation;     c) a Fresnel lens disposed proximate the substrate, for directing the output light toward a second lens; and     d) the second lens directing the output light to an image sensor for converting light levels to digital data, forming an image thereby.        
 
         [0019]     It is a feature of the present invention that it uses a Fresnel lens to focus light from the scintillator screen surface toward the image sensing apparatus.  
         [0020]     It is an advantage of the present invention that it improves optical efficiency of the lens-coupled DR system without degrading system contrast. Improved optical efficiency means the radiographic image would have higher image quality, and potentially, lower radiation dosage would be needed for obtaining an image from a human patient.  
         [0021]     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein:  
         [0023]      FIG. 1  is a simplified block diagram of a conventional lens-coupled DR imaging system;  
         [0024]      FIG. 2  is a block diagram showing angular dispersion of light emission from a point on scintillator screen;  
         [0025]      FIG. 3  is a block diagram showing angular dispersion of light emission from three points on scintillator screen;  
         [0026]      FIG. 4  is a block diagram showing a lens-coupled DR imaging system with the incorporation of a Fresnel lens, according to the present invention;  
         [0027]      FIGS. 5A and 5B  show redirection of emitted light using a brightness enhancement film or similar article according to Polichar et al. &#39;224;  
         [0028]      FIGS. 6A and 6B  are diagrams showing ray traces of emitted light using a Fresnel lens according to the present invention;  
         [0029]      FIG. 7  is a close-up side view showing the paths of various light rays tracing through a Fresnel lens;  
         [0030]      FIG. 8A  is a side view showing ray traces through a Fresnel lens oriented with the ridge side facing lens system;  
         [0031]      FIG. 8B  is a side view showing ray traces through a Fresnel lens oriented with the ridge side facing scintillator screen;  
         [0032]      FIGS. 9A and 9B  are a set of graphs showing efficiency of light coupling, with and without the use of Fresnel lens, in DR imaging system with scintillator screen having large and small divergence;  
         [0033]      FIG. 10  shows relative optical coupling efficiencies (top graph), with and without the use of Fresnel lens, in DR imaging system with scintillator screen having a Gaussian emission distribution (bottom graph);  
         [0034]      FIG. 11  is a side view showing the basic anatomy of a Fresnel lens in cross section; and  
         [0035]      FIG. 12  is a close-up side view of a Fresnel lens adapted to prevent TIR-induced light from exiting through the draft surfaces. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0037]     Referring to  FIG. 4 , there is shown an arrangement of lens-coupled DR imaging apparatus  11  components according to the present invention. A Fresnel lens  40  is disposed adjacent scintillator screen  16  for focusing the emitted light. Fresnel lens  40  has a size preferably as large as scintillator screen  16 , but at least as large as the imaging area of interest. It bends the emitted light toward lens system  18  to increase the amount of light channeling into image sensing apparatus  20 . Lens system  18  is positioned to produce a demagnified image of scintillator screen  16  on image sensing apparatus  20 . Fresnel lens  40  is chosen to have a focal length about equal to the separation between scintillator screen  16  and lens system  18 . Optional mirrors (not shown) could be used to fold the light path going from Fresnel lens  40  to lens system  18 , and/or to fold the light path going from lens system  18  to image sensing apparatus  20 .  
         [0038]     Referring to  FIG. 11 , Fresnel lens  40  is a spherically symmetric optical element. One side of Fresnel lens  40  is flat; the other side consists of generally concentric ridges  32 , centered about its normal axis, each ridge  32  being a light-refracting structure. The Fresnel shape can be approximated as a lens cut into narrow, concentric rings and flattened. Each ridge  32  forms a refractive structure corresponding to the angle of a slope  44  of its “ring.” Moving from the outermost ridge toward center, the angle of each slope gets progressively smaller with respect to the flat side, with the center structure being flat (zero slope). As such, outer ridge  32  structures of the lens cause correspondingly more light-bending than do inner ridge  32  structures. A draft portion  42 , having an angle close to a normal, then defines the boundary between ridges  32 . Fresnel lens  40  is typically manufactured as a sheet of optical quality plastic. A Fresnel lens  40  having the size of a conventional scintillator screen  16  and having high focusing power can be readily obtained from any of a number of manufacturers, such as Reflexite Display Optics of Rochester, N.Y.  
         [0039]     Referring again to  FIG. 4 , Fresnel lens  40  acts like a field lens, intercepting the light emission of scintillator screen  16  and refracting the emitted light toward lens system  18 . A conventional lens performing the same function would be prohibitively thick, large, heavy, and costly. Advantageously, Fresnel lens  40  is considerably thinner, lighter, and lower cost than its conventional lens counterpart having the same diameter and focusing power.  
         [0040]     In the preferred embodiment, Fresnel lens  40  is positioned immediately after scintillator screen  16 . Fresnel lens  40  can be placed against scintillator screen  16  or physically separated from scintillator screen  16  by a small distance. Alternatively, Fresnel lens  40  can be glued or laminated onto scintillator screen  16  to form a single compact unit.  
         [0041]     Fresnel lens  40  is widely used in illumination and projection applications. In the present invention, it is adapted for use in a DR imaging system. As will be taught subsequently, proper implementation of the Fresnel lens  40  brings significant improvement to the DR imaging system  11 .  
         [0042]      FIGS. 6A and 6B  show the response of emitted light rays R that are incident on Fresnel lens  40 .  FIG. 6A  traces rays emitting from three points P 5 , P 6 , and P 7  on scintillator screen  16  and propagating through Fresnel lens  40  to lens system  18 . As is shown best in the magnified view of  FIG. 6B , rays R from off-axis point P 7  in scintillator screen  16  undergo a change in direction through light bending action of Fresnel lens  40 . That is, centroid C of light cone  30  is redirected toward the focus of Fresnel lens  40 . Similar light bending occurs for the light cone from off-axis point P 6 , but by a smaller amount, since P 6  is closer to the center of Fresnel lens  40  than point P 7 . The light cone from on-axis point P 5  experiences no bending.  
         [0043]     The focusing effect of Fresnel lens  40 , therefore, is to direct the centroids of the light cones emitting from each point on scintillator screen  16  toward the center of lens system  18 . This decreases the overall angular distribution of the scintillator screen  16  emission. Light from the outer part of the field, that otherwise would not get collected by lens system  18 , are brought within the light acceptance angle of lens system  18 . The net effect is that a substantially larger portion of the emitted light are collected, thereby increasing optical throughput of the system.  
         [0044]     Significantly, the optical coupling efficiency increase is achieved in the present invention purely from the ray bending action of Fresnel lens  40 . Because ray bending by Fresnel lens  40  does not mix the spatial ordering of the rays, unlike the TIR action relied upon by the Polichar et al. &#39;224 application, the use of Fresnel lens  40  does not cause the adverse effects of pixel crosstalk. The present invention thus realizes the optical efficiency improvement without compromising system contrast.  
         [0045]     Optimal optical efficiency of DR imaging system  11  depends on the inter-working relationship between lens system  18 , Fresnel lens  40 , and scintillator screen  16 . The f-number of lens system  18  determines the light acceptance angle. Because of manufacturing, image quality, and cost requirements, lens system  18  is limited to an f-number of about 1. With such a lens system  18 , Fresnel lens  40  is most effective in increasing light coupling when used with scintillator screen  16  having smaller divergence, or more forward- directed emission distribution.  
         [0046]     Referring to  FIGS. 9A and 9B , there are shown comparative plots of optical coupling efficiency as a function of field position (where 0 is the center of scintillator screen  16 , and  200  is the position of scintillator screen  16  200-mm away from center), when using the approach of the present invention with scintillator screen  16  having two different divergence characteristics. Data for these plots are generated from ray tracing results for a DR system having lens system  18  with f-number=1 and demagnification factor=7.6.  FIG. 9A  shows the plots of the calculated coupling efficiency, with and without Fresnel lens  40 , for a DR imaging system where scintillator screen  16  has Lambertian emission with a divergence angle of +/−90 degrees. As the graph indicates, there is little measurable efficiency improvement when scintillator screen  16  has Lambertian emission with large divergence angle.  
         [0047]     In  FIG. 9B , a dotted line curve  46  shows relative optical coupling efficiency of the DR system  11  with a Fresnel lens  40 , where scintillator screen  16  has a Lambertian emission with small divergence angle of +/−10 degrees. A solid line curve  48  shows relative optical efficiency for the same system without the use of Fresnel lens  40 . Improvement in coupling efficiency from the use of Fresnel lens  40  is clear and significant for off-axis field positions.  
         [0048]     As was noted in the background section given above, there have been attempts to reduce the angular emission spread of scintillator screen  16 . The attempts have been successful in producing scintillator screens  16  whose light emission distribution is more forward-directed. By using such scintillator screens  16  with Fresnel lens  40  in lens-coupled DR imaging apparatus  10  according to the present invention, significant additional improvements in optical coupling efficiency can be realized.  
         [0049]     Referring now to  FIG. 10 , there are shown comparative plots (top graph) of optical coupling efficiency when using a scintillator screen  16  having a forward-directed emission distribution that is in the shape of a Gaussian function, with a full-width at half-maximum of 36 degrees, as shown by curve  50  in the bottom graph of  FIG. 10 . Data for these plots are generated from ray tracing results for a DR system having a lens system  18  with f-number=1 and demagnification factor=7.6. A dotted line curve  46  shows the relative optical coupling efficiency of the DR system with a Fresnel lens  40 , as a function of field position. A solid line curve  48  shows relative optical efficiency for the same system without the use of Fresnel  40 . Use of Fresnel lens  40  brings about considerable improvements in coupling efficiency.  
         [0000]     Suppression of TIR Effects  
         [0050]     As was noted with reference to the BEF  26  solutions disclosed in U.S. Pat. No. 6,178,224, TIR has been shown to be detrimental to overall image quality. Depending on the angle of incident light, Fresnel lens  40  as used in the present invention may exhibit some amount of TIR due to its highly angular surface structures. As shown by the ray traces in  FIG. 7 , for example, Ray R 1  has undergone TIR on the surface of draft portion  42  and is directed in an unwanted direction. Ray R 2  has undergone TIR twice: once on the inner surface of slope  44  at point T 1 , then on the flat surface  34  of Fresnel lens  40  at point T 2 . Ray R 2  then exits from the surface of draft portion  42  as stray light. Both R 1  and R 2  could give rise to unwanted crosstalk if imaged onto image sensing apparatus  20  (not shown).  
         [0051]     To avoid loss in image contrast, it is desirable to suppress TIR and its image degradation effects. By proper implementation of the Fresnel lens  40 , the present invention can improve light coupling efficiency while suppressing TIR. This is possible in the present invention because it does not make use of recycled TIR light to achieve the coupling increase. It is emphasized again that Polichar et al. &#39;224 suffers from the detrimental effects of TIR on image contrast because it relies on recycled TIR light to bring about the optical coupling increase.  
         [0052]     In one embodiment of the present invention, Fresnel lens  40  is adapted in order to minimize TIR effects and unwanted transmitted light from draft surfaces as shown in  FIG. 7 . Referring now to the side cross-sectional view of  FIG. 12 , each draft portion  42  of Fresnel lens  40  has an opaque coating  36  applied for this purpose. Optionally, the surface of draft  42  can be suitably roughened to reduce its transmissivity. The opaque coating or surface roughening can considerably reduce crosstalk arised from TIR effects.  
         [0053]     As shown in  FIG. 7 , one side of Fresnel lens  40  is flat surface  34 ; the opposite featured side has ridges  32 . For the purpose of redirecting incident light from scintillator screen  16 , Fresnel lens  40  can be oriented in either of two directions: either with ridges  32  facing toward lens system  18 , or with ridges  32  facing toward scintillator screen  16 . As shown in  FIG. 8A , with ridges  32  facing toward lens system  18 , the light from point S propagates to lens system  18  in different paths. Group G 1  are rays that are refracted by the slopes of Fresnel lens  40 ; this group of rays are useful and contain the image-modulated light or signal. However, group G 2  of rays are not useful. Group G 2  rays are total internal-reflected on internal surfaces of Fresnel lens  40  to exit through draft surfaces of Fresnel lens  40  and can be considered “crosstalk noise” rather than signal. If allowed to propagate through lens system  18 , group G 2  rays diminish contrast and compromise overall image quality.  
         [0054]     In another embodiment of the present invention, as shown in  FIG. 8B , Fresnel lens  40  is oriented in the opposite orientation, with ridges  32  facing toward scintillator screen  16 . This orientation minimizes crosstalk noise at image sensing apparatus  20  by preventing the TIR rays from propagating through lens system  18 . With this arrangement, G 4  is the group of useful image data rays that have undergone refraction at the slope surfaces of Fresnel lens  40  and propagate toward lens system  18 . Rays that are refracted by the draft surfaces of Fresnel lens  40  and undergo subsequent TIR (as traced out by rays in group G 3 ), however, are directed well away from lens system  18 . The unwanted light rays of group G 3  are kept from reaching image sensing apparatus  20  and thus do not constitute crosstalk noise. Therefore, there are advantages to embodiments of lens-coupled DR imaging apparatus  10  using this orientation of Fresnel lens  40 , with ridges  32  facing scintillator screen  16 .  
         [0055]     It is noted that when Fresnel lens  40  is oriented with ridges  32  facing scintillator screen  16 , optimal image quality is obtained when the pitch between ridges  32  of Fresnel lens  40  is at least twice smaller than the resolution of the screen, for example, 50 μm. This requirement ensures that the transmissivity of Fresnel lens  40  and overall image quality of the system are not compromised.  
         [0056]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention.  
         [0057]     Thus, what is provided is an apparatus and method for DR imaging offering improved optical coupling efficiency without degrading contrast.  
       PARTS LIST  
       [0000]    
       
           10  conventional lens-coupled DR imaging apparatus  
           11  lens-coupled DR imaging apparatus with Fresnel lens  
           12  radiation source  
           14  subject  
           16  scintillator screen  
           18  lens system  
           20  image sensing apparatus  
           24  pixel  
           25  pixel  
           25 ′pixel  
           26  BEF  
           28  prism  
           30  light cone  
           32  ridge  
           34  flat surface  
           36  coating  
           40  Fresnel lens  
           42  draft portion  
           44  slope  
           46  curve  
           48  curve  
           50  curve