Abstract:
A reading apparatus obtains a line of image data stored on a surface, the reading apparatus having a radiation source ( 12 ) for directing a line of stimulating radiation onto a stimulable image carrier on the surface, generating a line of image-bearing radiation. A sensing head ( 22 ) having a plurality of channels ( 66 ) obtains image data from the line of image-bearing radiation, each channel ( 66 ) sensing a segment ( 32 ) of the line of image-bearing radiation. Each channel ( 66 ) has inverting optics for inverting the segment ( 32 ) of the line of image-bearing radiation to form an inverted line segment image ( 44 ) and a sensor ( 29 ) for providing image data for the inverted line segment image ( 44 ). An image processor ( 30 ) accepts image data from sensing head channels ( 66 ) and forms a line of image data according to the line of image-bearing radiation.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to imaging systems for reading images exposed on CR plates and more particularly relates to an imaging system for scanning line images using single-stage inverting imaging optics. 
     BACKGROUND OF THE INVENTION 
     Computed radiography (CR) systems using stimulable phosphor sheets enjoy broad acceptance as clinical imaging tools. In a CR system, radiation is passed through a subject and impinges upon a stimulable phosphor sheet, commonly referred to as a CR plate, that stores a portion of the radiation energy as a latent image. After exposure to the radiation, the stimulable phosphor on the CR plate is subsequently scanned using an excitation light, such as a visible light or laser beam, in order to emit the stored image. 
     Early CR scanning systems employ a flying-spot scanning mechanism, in which a single laser beam is scanned across the phosphor plate in a raster pattern. The resulting excitation that provides the stored image is then directed to a sensor, providing a single point of image data at a time. More recent CR systems have improved upon this earlier technique by providing a full line of image data at a time, offering advantages of faster throughput and lower cost and complexity over flying-spot scanners. As just one example, U.S. Pat. No. 6,373,074 (Mueller et al.) discloses a CR system that scans a full line of image data points at a time. 
       FIG. 1  shows the basic components of an optical scanning system  10  such as that described in U.S. Pat. No. 6,373,074. A linear array of light sources  12 , typically an array of laser diodes, directs a linear scanning beam  14  onto a stimulable phosphor sheet  16  that has been irradiated and stores a latent X-ray image. One or more cylindrical lenses  18  are used to direct the highly asymmetric linear output beam along a line  20  on the surface of phosphor sheet  16 . In a sensing head  22 , collection optics  24  direct the stimulated light from line  20  on phosphor sheet  16  through an optical filter  26  and to a linear photodetector array  28 , typically a charge couple device (CCD) array. Phosphor sheet  16  is indexed in direction D by a transport mechanism  60 , such as a continuous belt or other indexing apparatus, to provide a scanning motion. In this way, phosphor sheet  16  is scanned past sensing head  22  to detect each line of the image stored thereon. The sensed image data is then processed by an image processor  30  that assembles a two-dimensional output image from each successive sensed line. The output image can then be recorded onto a writable medium such as a photosensitive film, or can be displayed. 
     There have been a number of solutions proposed for improving the overall performance of CR plate scanner optics, including the following:
         U.S. Patent Application Publication No. 2003/0010945 (Ishikawa) discloses improvements to light projection apparatus for projecting a line of stimulating light from an array of laser diodes;   U.S. Patent Application Publication No. 2002/0096653 (Karasawa) discloses the use of condenser lens chromatic characteristics for isolating stimulated light from stimulating light provided from the array of laser diodes;   U.S. Patent Application Publication No. 2002/0056817 (Furue) discloses a more compact reading apparatus for obtaining the stored image from an irradiated stimulable phosphor sheet;   U.S. Patent Application Publication No. 2002/0040972 (Arakawa) discloses an optical reading head that employs a grid pattern for sensing each line of the stored image;   U.S. Patent Application Publication No. 2002/0100887 (Hagiwara et al.) discloses an improved photodiode arrangement in a scanning head for a stimulable phosphor sheet; and   U.S. Patent Application Publication No. 2001/0028047 (Isoda) discloses a system using conventional optical techniques with improvements to line sensor componentry for obtaining a larger percentage of the stimulated light.       

     While there have been numerous improvements to apparatus and methods for obtaining the stored image on a CR plate, there is still need for increased efficiency and overall image quality. One widely recognized problem with existing CR plate readers relates to the need for improved image quality at image sensing circuitry (generally represented as linear photodetector array  28  in  FIG. 1 ). The apparatus disclosed in U.S. Patent Application Publication Nos. 2002/0096653, 2001/0028047, 2002/0040972, and in U.S. Pat. No. 6,373,074, and elsewhere, for example, employ Selfoc™ lenses and provide 1:1 imaging. While this solution allows compact packaging of the sensing components and their support optics, it imposes a constraint on numerical aperture (NA). The Selfoc™ gradient index lens is characterized as having a low NA. The maximum f/# value for this type of lens is typically about f/2, which provides an NA of 0.25. Because collection efficiency of this lens is proportional to the square of the NA value, a low NA can significantly degrade overall system brightness. Yet another disadvantage of existing systems relates to the relatively low fill factor of the Selfoc lens array. Gaps between adjacent Selfoc™ lens elements limit the fill factor and further constrain light collection. 
     As a result of the overall inefficiency of the collection optics, the signal-to-noise (SN) ratio of conventional sensing systems is disappointing. Collecting light over a broader area, such as is disclosed in U.S. Patent Application Publication No. 2001/0028047 noted above, tends to further degrade the SN relationship, even when using two-channel sensing optics. Low collection efficiency also constrains the reading speed of the CR plate reader. In addition, these systems use 1:1 imaging, which may require two optical stages if an optical system other than a Selfoc™ lens is used, with correction for imaging aberration for each stage. 
     Thus it can be seen that while prior art solutions provide a CR plate reader with some capability, the need for improved light collection efficiency must be met for further improvements in reader sensitivity and overall performance. 
     SUMMARY OF THE INVENTION 
     With the above object in mind, the apparatus and methods of the present invention offer solutions for improving light collection efficiency of CR reading apparatus. The present invention provides a reading apparatus for obtaining a line of image data stored on a surface, the reading apparatus comprising:
         (a) a radiation source for directing a line of stimulating radiation onto a stimulable image carrier on the surface, generating a line of image-bearing radiation thereby;   (b) a sensing head for obtaining image data from the line of image-bearing radiation excited from the image carrier, the sensing head having a plurality of channels, each channel sensing a segment of the line of image-bearing radiation, each channel comprising:
           (i) inverting optics for inverting the segment of the line of image-bearing radiation to form an inverted line segment image; and   (ii) a sensor for providing image data for the inverted line segment image;   
           (c) an image processor for accepting the image data obtained from sensing head channels and forming a line of image data according to the line of image-bearing radiation.       

     It is a distinguishing feature of the apparatus of the present invention that it uses, for obtaining each of a set of inverted images, a single optical stage that provides inverted imaging at a range of magnification factors. 
     It is an advantage of the present invention that it provides a reading apparatus for a CR plate that offers improved speed and overall collection efficiency, as well as improved signal-to-noise (SN) ratio. 
     It is an advantage of the present invention that, due to the reduced number of optical stages, it allows simpler optical design for correcting image aberration. 
     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 
       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: 
         FIG. 1  is a schematic block diagram showing the basic component arrangement of a prior art CR plate reader; 
         FIGS. 2   a ,  2   b , and  2   c  are ray diagrams comparing 1:1, Selfoc™ lens, and 1:−1 imaging, respectively; 
         FIGS. 3   a  and  3   b  are block diagrams showing optical arrangements for imaging segments of a scanned line, using 1:1 and nominal 1:−1 imaging; 
         FIG. 3   c  is a block diagram showing an optical arrangement for imaging segments of a scanned line using a nominal 1:−1.2 imaging; 
         FIG. 3   d  is a block diagram showing an optical arrangement for imaging segments of a scanned line using a nominal 1:−0.95 imaging; 
         FIG. 4  is a schematic side view showing the basic component arrangement of a single channel of a CR plate reader in a single sensing head embodiment, according to the present invention; 
         FIG. 5  is a ray diagram showing a lens assembly for a single channel according to the present invention; 
         FIG. 6  is a cross-sectional diagram showing one arrangement for a portion of an array of lens assemblies; 
         FIG. 7  is a perspective view of a portion of an array of lens assemblies; 
         FIG. 8  is a front view showing an array of lens assemblies arranged in a linear fashion; 
         FIG. 9  is a front view showing an alternate arrangement from that shown in  FIG. 8 , in which lens assemblies are arranged in a staggered linear array; 
         FIG. 10   a  is a perspective view showing a portion of a CR sensing head, using a linear array of lens assemblies arranged as shown in the embodiment shown in  FIG. 8 ; 
         FIG. 10   b  is a diagram showing a scanned line segment obtained using the linear array of  FIGS. 10   a  and  FIG. 8 ; 
         FIG. 11   a  is a perspective view showing a staggered linear array of lens assemblies in the embodiment shown in  FIG. 9 ; 
         FIG. 11   b  is a diagram showing a scanned line segment obtained using the staggered linear array of  FIGS. 11   a  and  FIG. 9 ; 
         FIG. 12  is a perspective view showing a dual sensing head used in an alternate embodiment; 
         FIG. 13   a  is a cross-sectional diagram showing one arrangement for a dual sensing head, in which channels on each head are offset from each other with respect to the scanned line; 
         FIG. 13   b  is a graph showing the spatial relationship of sensed channels using the dual sensing head with the offset as shown in  FIG. 13   a ; and 
         FIG. 13   c  is a graph showing the additive effect of sensed channels obtained with the dual sensing head arrangement represented in  FIGS. 12 and 13   a.    
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     The description that follows employs the +/− polarity for magnification that is conventionally used among those skilled in the optical arts. That is, positive (+) magnification refers to a non-inverted image; negative (−) magnification refers to an inverted image. (For example, with a 1:−1.2 magnification ratio, the image is inverted and is magnified times 1.2.) 
     Use of Optical Stages for Imaging Line Segments 
     In order to appreciate the operation of imaging optics in the apparatus and method of the present invention, it is first useful to contrast the 1:1 imaging technique used in prior art CR reading apparatus with the nominal 1:−1 imaging technique that has been adapted to the present invention. Referring first to  FIG. 2   a , there is shown the conventional imaging technique used in CR reading apparatus of  FIG. 1  for scanning each segment of line  20 . Radiation from each object segment  32  of line  20  is inverted through a first optical stage  34  to form an intermediate inverted image segment  36 . A second optical stage  38  again inverts inverted image segment  36  to form a non-inverted image segment  40 . Each optical stage  34  and  38  is typically an arrangement of multiple lenses, but represented for simplicity as a single unit in  FIG. 2   a . For comparison,  FIG. 2   b  is the imaging diagram of lens assembly  50  for a Selfoc™ lens. The intermediate image plane (in which intermediate inverted image segment  36  is formed) is inside the lens. The Selfoc™ lens of  FIG. 2   b  provides non-inverted image segment  40 , as shown. In comparison with both two-stage and Selfoc™ optical configurations of  FIGS. 2   a  and  2   b  respectively,  FIG. 2   c  shows a single optical stage as lens assembly  50 , directing radiation from object segment  32  to form an inverted image segment  40 ′. 
     It must be observed that each optical stage  34  and  38  in the conventional arrangement of  FIG. 2   a  introduces an amount of image aberration. Thus, even though the arrangement of  FIG. 2   a  provides non-inverted image segment  40 , this arrangement inherently requires increased correction for image aberration with conventional optical design over the single optical stage arrangement of lens  50  in  FIG. 2   b.    
     Considerations for Magnification 
       FIG. 3   a  shows how an image line  64  is formed in 1:1 imaging by an array, using the two-stage arrangement of  FIG. 2   a . Here, image line  64  is formed as a series of congruent, non-inverted image segments  40  of their corresponding congruent object segments  32 . A channel  66 , represented in dotted lines, forms the image, non-inverted image segment  40 , for each individual object segment  32 , using an arrangement of optical components. Using multiple optical stages  34  in an array of channels  66 , this method first forms an intermediate image consisting of inverted image segments  36 . A second arrangement of optical stages  38  in a second array then forms the reconstructed image line  64 . Non-inverted image segments  40  have substantially the same dimensions as their corresponding object segments  32 . 
       FIG. 3   b , on the other hand, shows how image line  64  is initially formed along each channel  66  using 1:−1 imaging and one optical stage  34 . It might appear that image line  64  would simply be an inverted version of line  20 ; however, each individual image segment  36  is inverted, as is represented in  FIG. 3   b . Moreover, line  20  itself has some width dimension. This means that inverted image segments  36  are also “mirrored” with respect to object segments  32 . Thus, further processing is needed to assemble image line  64  properly, as is described subsequently. 
     While  FIG. 3   b  shows 1:−1 imaging for object segments  32 , other magnification factors are possible for the inverted image segments in each channel  66 . Referring now to  FIG. 3   c , there is shown an arrangement where magnification provides a nominal 1:−1.16 imaging. Here, inversion and magnification at a value greater than 1 are effected. Inverted image segments  44  are magnified and overlap each other slightly.  FIG. 3   d , meanwhile, shows magnification providing a nominal 1:−0.95 imaging scheme, in which inversion and magnification at a value less than 1 is effected. Here, inverted image segments  46  are reduced in size and a gap G occurs between each image segment  46 . 
     Using conventional imaging approaches, the imaging method of  FIG. 3   d  would seem to be disadvantageous over nominal 1:1 or 1:−1 imaging as shown in  FIGS. 3   a  and  3   b , respectively. However, there are unexpected advantages in separating inverted image segments  46 , as represented in  FIG. 3   d . By providing gap G between each inverted image segment  46 , the method used in  FIG. 3   d  enables each inverted image segment  46  to be readily handled as a separate unit by optical sensing and processing components in each channel  66 . That is, gap G clearly defines, for imaging logic, the end-points of each inverted image segment  46 . Thus, each inverted image segment  46  can be sensed individually, with minimal crosstalk from congruent image segments  46  in adjacent channels  66 , and can then be processed and inverted using imaging algorithms for assembling image line  64 . 
     Arrangement of a Single Channel  66   
     Referring to  FIG. 4 , there is shown an optical scanning system  100  according to the present invention, with sensing components for a single channel  66 . Radiation from light source  12  is directed onto phosphor sheet  16  through lens  18 . Image-bearing line  20  emitted from the surface of phosphor sheet  16  is then scanned as individual congruent object segments  32 , as was described with reference to  FIG. 3   d . Radiation from each object segment  32  is directed through optical filter  26  by lens assembly  50  and onto a photosensor  29  in photodetector array  28 . 
     Image processor  30  then processes the obtained image data for each inverted image segment  36  for 1:−1 imaging ( FIG. 3   b ), inverted image segment  44  for inversion and magnification ( FIG. 3   c ), or inverted image segment  46  for inversion and demagnification ( FIG. 3   d ). Image processor  30  then reconstructs image line  64  from individual image segments  36 ,  44 , or  46 . As with prior art devices, described with reference to  FIG. 1 , transport mechanism  60  is used to index phosphor sheet  16 , one line  20  at a time, in direction D for scanning. 
     Referring to  FIG. 5 , components of lens assembly  50  for each object segment  32 , that is, for each channel  66 , are shown. Lenses  52 ,  54 , and  56  provide image inversion and magnification/demagnification of radiation from line  20  on phosphor sheet  16  and form an image onto photosensor  29 . 
     Array Arranyement of Channels  66   
     For scanning line  20  according to the present invention, a lens array  62  provides light-handling components for each of the individual channels  66 , as shown in the cross-section of  FIG. 6 . Each object segment  32  of line  20  has a corresponding channel  66  with its lens assembly  50 , shown outlined for a single channel  66 . Photodetector array  28  itself consists of an array of individual photosensors  29 , one for each channel  66 .  FIG. 7  shows the arrangement of channel  66  optical components in a perspective view. Lenses  52 ,  54 , and  56  are provided in lens arrays  62 , with the individual optical elements suitably aligned to provide the needed components for each channel  66 . 
     Channels  66  in lens arrays  62  may be arranged in a line, as shown in the front view of  FIG. 8 . This embodiment is constrained, however, to magnification factors less than or equal to 1, primarily to minimize optical crosstalk between adjacent object segments  32 . Alternately, to provide improved fill factor and magnification factors in excess of 1, adjacent channels  66  may be spatially shifted, as shown in the front view of  FIG. 9 . In the  FIG. 9  embodiment, adjacent channels  66  are shifted, in the width direction relative to line  20 , by a distance Y, yielding an improved fill factor and allowing magnification factors exceeding 1. This effectively increases light collection efficiency and provides other advantages for image processing, as described subsequently. 
     Configuration and Operation of Sensing Head  22   
     Referring to  FIG. 10   a , there is shown a portion of sensing head  22  in optical scanning system  100  using an embodiment with channels  66  having the straight-line arrangement of  FIG. 8 . The graph of  FIG. 10   b  shows how the optical arrangement of  FIGS. 8 and 10   a  provides individual inverted line segments  46 , demagnified and slightly separated by gap G. With the more conventional arrangement of  FIG. 10   a , the width and field of each lens assembly  50  is constrained by the space available, as is noted above. 
     Referring to  FIG. 11   a , an alternate embodiment of a portion of sensing head  22  with channels  66  having the shifted arrangement of  FIG. 9  is shown. The graph of  FIG. 11   b  shows how this arrangement provides, to photodetector array  28  of photosensors  29 , individual inverted line segments  46  having a slight spatial overlap. This embodiment allows each channel  66  to provide inversion and magnification with a factor greater than 1, providing 1:−1.18 magnification for example. This arrangement can be used to help compensate for inherent fall-off of the image of each object segment  32  at the edges, as is well known to those skilled in the optical arts. 
     Embodiment Using Dual Sensing Heads  22   
       FIG. 12  shows an alternate embodiment in which optical scanning system  100  has dual sensing heads  22   a  and  22   b . One advantage of this embodiment is best represented using the cross-sectional view of  FIG. 13   a . Here, channels  66  in the respective lens arrays  62  of sensing heads  22   a  and  22   b  are offset with respect to the length of line  20 , allowing higher scan speed with improved SN ratio. As  FIG. 13   a  illustrates, adjacent channels  66  are nominally offset by one-half of the channel width W, increasing collection efficiency over that of the single sensing head  22  of the  FIG. 10   a  embodiment. 
     The graphical representations of  FIGS. 13   b  and  13   c  show how the offset arrangement of dual sensing heads  22   a  and  22   b  in  FIG. 13   a  improves the collection efficiency. One waveform  122   a  is obtained from sensing head  22   a ; another waveform  122   b  is obtained from sensing head  22   b . Waveforms  122   a  and  122   b , graphing the spatial distribution of light energy for each of five channels  66  at respective sensing heads  22   a  and  22   b , shows how cos 4  effects degrade the optical signal near the edges of each channel  66 .  FIG. 13   c  shows a combined waveform  124  that is the result of summing or otherwise combining the sensed light energy of waveforms  122   a  and  122   b , providing a relatively uniform collection efficiency along the length of sensed line  20 . 
     In addition to obtaining improved uniformity, the dual sensing head  22   a  and  22   b  arrangement of  FIG. 12  also provides improved collection efficiency of optical scanning system  100  over that of the single sensing head  22  shown in  FIG. 10   a . Nominal collection efficiency of the  FIG. 10   a  embodiment is approximately 18%, whereas the improved collection efficiency of the  FIG. 12  embodiment is approximately 30%. This level of improvement is significant, allowing the residual image scanned from line  20  to be more accurately obtained, lowering the overall signal-to-noise ratio of optical scanning system  100 . The arrangement of channels  66  within each sensing head  22   a  or  22   b  of  FIG. 12  can be linear, as is shown in  FIGS. 8 and 10   a  or shifted, as is shown in  FIGS. 9 and 11   a.    
     In any of the embodiments of optical components shown in  FIGS. 4 ,  10   a ,  11   a , and  12 , the role of image processor  30  is to reconstruct each line  20  as it is scanned. With each of these embodiments, each object segment  32  is inverted as it is detected by photosensor  29  and may be at any of a range of magnifications, as is described for the various embodiments given above. As was noted with reference to  FIGS. 3   b - 3   d , each inverted image segment  36 ,  44 , or  46  is also mirrored with respect to depth and requires processing to reconstruct image line  64  using the original line  20  data. 
     Unlike prior art arrangements, the optical embodiments of the present invention shown in  FIGS. 4 ,  10   a ,  11   a , and  12 , use a single optical stage to provide inverted image segments  46 . This has the inherent advantage of reduced image aberration over conventional designs using two optical stages, as was described with reference to  FIGS. 3   a - 3   d . Additional image processing is required for reconstructing the image of line  20  from inverted line segments, but this slight disadvantage is more than compensated by an improvement in overall image quality, due to an improved SN ratio. Unlike prior art arrangements that are constrained to 1:1 imaging, the embodiments of the present invention allow magnification at other factors, depending on what is most advantageous for image quality. Each of the embodiments of  FIGS. 4 ,  10   a ,  11   a , and  12  provide a larger NA than conventional designs, with values at nearly 0.45 or better, with consequent improved collection efficiency and improved signal-to-noise ratio The dual sensing head  22  arrangement described with reference to  FIGS. 12 and 13   a - 13   c , when used in conjunction with the inverted segment imaging techniques of the present invention, offers additional advantages in both resolution and collection efficiency over prior art designs. 
     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. For example, with the dual sensing head  22   a ,  22   b  configuration of  FIG. 12 , an offset that is any fraction of channel width W could be employed. Lenslet arrays of various types could be used for fabrication of lens array  62 , as is familiar to those skilled in the optical arts. Photosensors  29  could be CCD devices, such as devices using time-delayed integration (TDI), familiar to those skilled in the optical arts. Photosensors  29  could alternately be CMOS devices, or other suitable sensor types. Any type of suitable transport mechanism  60  could be employed for indexing phosphor sheet  16  forward to effect line-by-line scanning. 
     Thus, what is provided is an apparatus and method for an imaging system for scanning line images using single-stage inverting imaging optics. 
     PARTS LIST 
     
         
           10  optical scanning system 
           12  light source 
           14  scanning beam 
           16  phosphor sheet 
           18  lens 
           20  line 
           22  sensing head 
           22   a  sensing head 
           22   b  sensing head 
           24  collection optics 
           26  optical filter 
           28  photodetector array 
           29  photosensor 
           30  image processor 
           32  object segment 
           34  optical stage 
           36  inverted image segment 
           38  optical stage 
           40  non-inverted image segment 
           40 ′ inverted image segment 
           44  inverted image segment 
           46  inverted image segment 
           50  lens assembly 
           52  lens 
           54  lens 
           56  lens 
           60  transport mechanism 
           62  lens array 
           64  image line 
           66  channel 
           100  optical scanning system 
           122   a  waveform 
           122   b  waveform 
           124  combined waveform