Abstract:
In recent years, x-ray imaging, which has been used to diagnose millions of illnesses and injuries, has evolved to use digital imaging instead of photographic film as a recording medium. Digital x-ray systems typically include an x-ray source, an x-ray focusing grid, and an array of light or x-ray detectors. Because of detector imperfections and other system factors, such as x-ray field non-uniformity and grid artifacts, digital x-ray images are often corrected, or compensated, before use. To this end, many digital x-ray systems include numerous application-specific correction maps, which unfortunately require regular maintenance that is not only time-consuming but expensive in terms of system downtime. Accordingly, the inventors devised new methods and systems for correcting application images that require maintenance of fewer correction maps. One exemplary implementation determines grid-only and non-grid correction maps and corrects application images based on a combination of these correction maps. Other aspects of the invention include automatic detection and correction of grid images in application images and computer-readable mediums which store software for computing and applying grid-only and non-grid correction maps.

Description:
TECHNICAL FIELD 
     The present invention concerns x-ray imaging, particularly methods of correcting digital x-ray images. 
     BACKGROUND OF THE INVENTION 
     Since its discovery in 1885, x-ray imaging has been used to successfully diagnose the illnesses and injuries of millions of people. This form of imaging generally entails passing x-rays, a form of high-energy radiation, through a body of material onto a phosphor plate. The phosphor plate glows, or luminesces, with an intensity dependent on the material the x-rays pass through. For example, x-rays that pass through bone produce a lower intensity glow than x-rays that pass through muscle. A photographic film next to the phosphor plate chemically reacts to the glow, making a two-dimensional record of its various intensities. 
     In recent years, x-ray imaging systems have gone “digital,” essentially replacing photographic film with electronic imaging arrays. A typical digital x-ray system includes an x-ray source, an x-ray focusing grid, and an x-ray or light detector consisting of an array of pixels. The x-ray source emits x-rays, or photons, of a specific energy level in a narrow spray pattern through a body and toward the detectors. After passing through the body, the spray pattern includes primary and scattered photons. The x-ray focusing grid, placed between the body and the detector, absorbs most scattered photons and passes most primary photons onto the array of detector pixels. 
     In response, each detector pixel in the array provides an electrical output signal representative of the intensity of light or x-rays striking it. Each output signal is then converted to a number known as a digital pixel value, which is in turn output as a particular color on an electronic display or printing device, enabling viewing of the x-ray image. 
     Before display, it is common to correct the x-ray image for irregularities in the array of detectors. These irregularities, stemming from the uniqueness of each detector pixel in the array, lead the detector to output different signals in response to the same incident light or x-rays. Correcting the image typically entails adjusting the digital representation of each detector output signal by an experimentally determined number for that detector. The numbers for all the detector pixels, known collectively as a correction map, are usually stored in a digital memory of the x-ray system. 
     In addition to correcting for detector irregularities, the correction map also corrects for all other system sensitivity factors, such as non-uniform x-ray field and grid artifacts, affecting formation of a particular x-ray image. Because of the complex interdependency of the many factors affecting system sensitivity, every correction map is uniquely applicable to a specific system configuration and exposure technique, that is, to a specific set of system factors. Moreover, configuration and technique changes—such as increasing or decreasing x-ray tube voltage (kVp) and x-ray beam filter, or replacing one grid with another—that are made to tailor the system to specific imaging applications require use of different correction maps. Skulls, chests, and hands, for example, generally require different exposure techniques and grid types and thus different correction maps for best results. 
     Therefore, to support a wide variety of system configurations, digital x-ray imaging systems may store and use many application-specific correction maps. For example, if a system supports N different configurations and exposure techniques and P different grid options, it may store N×P (N times P) different correction maps to correct images made under all possible grid-and-technique combinations. 
     One problem with storing many application-specific correction maps is that they all require repeated maintenance or update to adjust for wear, age, and other time-varying characteristics of components in host x-ray systems. Updating, or recalibration, of many correction maps is not only time-consuming but expensive in terms of system downtime. Moreover, new x-ray applications and grid types are continually being developed, further expanding the number of correction maps requiring storage and update. Accordingly, there is a need for better correction methods and systems. 
     SUMMARY OF THE INVENTION 
     To address this and other needs, the inventors devised new methods and apparatus for correcting images in digital x-ray imaging systems. In systems which, for example, support N different non-grid configurations and P different grids and thus would conventionally require storage and update of N×P (N times P) different correction maps, these exemplary methods and apparatus in accord with the invention facilitate the same correction capability with storage of only N+P (N plus P) different correction maps. With storage of fewer correction map, systems incorporating various embodiments of the invention, ultimately require considerably less time and expense for recalibration. 
     One exemplary method determines grid-only and non-grid correction maps and corrects images based on a combinations of these “partial” or “modular” correction maps. More particularly, this exemplary method determines a grid-only correction map from first and second flat-field images, the first made without a grid and the second with a grid. The first image is used to determine the non-grid correction map, and both images are used to determine the grid-only correction map. 
     The exemplary apparatus includes a memory which stores one or more non-grid correction maps and one or more grid-only correction maps. Also included is software for selecting one or more of the non-grid correction maps and one of the grid-only correction maps and for correcting a given image using the selected non-grid and grid-only correction maps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an exemplary digital x-ray imaging system  100  incorporating teachings of the present invention. 
     FIG. 2 is a flowchart  200  illustrating an exemplary method of determining grid-only and non-grid correction map, which also incorporates teachings of the present invention. 
     FIG. 3 is a flowchart  300  illustrating an exemplary method of correcting images, which also incorporates teachings of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description, which references and incorporates FIGS. 1-3, describes and illustrates one or more specific embodiments of the invention. These embodiments, offered not to limit but only to exemplify and teach the invention, are shown and described in sufficient detail to enable those skilled in the art to practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art. 
     FIG. 1 shows an exemplary digital x-ray system  100  incorporating teachings of the present invention. In particular, system  100  includes an x-ray source  110 , an x-ray focusing grid  120 , an imaging array  130 , a processor  140 , a memory  150 , and output devices  160 . For clarity, other x-ray system components such as a collimator, system controller, automatic exposure controller, and so forth are not shown. 
     X-ray source  110  emits x-ray photons at one or more selectable energy levels. The exemplary embodiment uses any type of x-ray source, for example one with many different intensity settings. (The invention, however, is not limited to any type of x-ray source or any type, range, or number of selectable x-ray source operational criteria.) An object or body  170  placed between x-ray source  110  and x-ray focusing grid  120  absorbs, passes, and scatters x-ray photons based on its structure and composition. 
     X-ray focusing grid  120  absorbs most scattered photons and passes most primary photons onto imaging array  130 . Grid  120 , in the exemplary embodiment, is manually or automatically movable in and out of the path of radiation from x-ray source  110 . Examples of suitable grids include those described in U.S. Pat. Nos. 5,581,592 and 5,291,539, which are incorporated herein by reference. Some embodiments use a focusing grid which carries strategic markers for locating it in x-ray images, and others include not only strategic markers, but also identification markers. The exemplary embodiment uses one of a number of grids P, generally denoted as grid  1 , grid  2 , . . . , grid P. These are generically indicated as grid P in FIG.  1 . (With adjustable or reconfigurable grids, each possible reconfiguration constitutes a separate grid P.) Primary photons that pass through grid  120  strike imaging array  130 . 
     Imaging array  130  includes a two-dimensional array of sensors or detectors (not shown). In the exemplary embodiment, the array is rectangular and includes K rows and L columns of detectors, with each detector (or detector pixel) having a unique address or position based on its row and column. Though not shown, imaging array  130  also includes associated signal-conditioning electronics, such as sense amplifiers and/or analog-to-digital converters, as known or will be known in the art. In some embodiments, the detectors detect x-rays indirectly through light from a phosphor medium, and in others, the detectors detect x-rays directly. In either case, image array  130  provides a set of digital image signals, or pixel values, based on the output of the detectors to processor  140 . (As used herein, image refers to a set of one or more pixel values originating, or otherwise derived through processing signals, from at least one corresponding detector in an imaging array.) 
     Processor  140  interfaces with memory  150  and output devices  160 . Memory  150  includes a number of partial or modular correction maps  151 - 156 . Partial correction maps  151 ,  152 , and  153  are grid-only correction maps, that is, they are intended only to correct for grid effects in object, or application, images. On the other hand, partial correction maps  154 ,  155 , and  156  are non-grid-correction maps, which are intended to correct the effects of one or more other system components on application images. (As used herein, map refers to array of numerical values intended for correction of an image.) 
     In the exemplary embodiment, the non-grid-correction maps correct for all other system components except for grids. However, in other embodiments, a number of distinct non-grid-correction maps correct independently for non-uniform x-ray field, x-ray beam geometry, or any other characteristics. Memory  150  also includes one or more software modules or computer programs  157  and  158  which respectively govern how processor  140  defines and applies each of the non-grid and grid-only correction maps. Exemplary memory devices include magnetic, optical, and electronic read-only memories, random-access-memories, and combinations of these types of devices. 
     Output devices  160  include one more image displays, printers, and/or communications devices for outputting image information. The communications devices allow transmission of image information over telephone and broadcast communications channels as desired to facilitate remote processing or diagnosis. 
     Generally, in normal operation after computing and storing one or more sets of partial-correction maps  151 - 156  and acquiring an application (object or patient) image, processor  140  determines identity of grid  120  by locating grid-identifying markers in the application image, by looking at operator inputs or settings, or by using Fourier transforms of the grid image and the application image. After this determination, processor  140  selects from memory  140  the appropriate combination of two or more partial-correction maps, for example, one grid-only correction map and at least one non-grid correction map, for use in correcting the application image. The processor then registers the correction maps to the application image and corrects the application image by applying the registered partial-correction maps sequentially to the application image. Alternatively, the processor mathematically combines the selected partial-correction maps into a total-correction map and then applies the total-correction map to the application image. The corrected application image is subject to further processing (not described here) and then transferred to output devices  160  for output in one or more desired forms. 
     FIGS. 2 and 3 respectively illustrate flow charts for exemplary methods of determining partial correction maps and then applying them to correct an application image. More particularly, FIG. 2 shows an exemplary flow chart  200 , illustrating operation of system  100  and especially processor  140  in accord with software modules or computer programs  157 . Flow chart  200  includes blocks  202 - 208 , which are executed serially or in parallel in the exemplary embodiment. Some embodiments organize the exemplary process using a greater or lesser number of blocks. Other embodiments implement the blocks as two or more specific interconnected hardware modules with related control and data signals communicated between and through the modules. Thus, the exemplary process flow is applicable to software, firmware, and hardware implementations. In most, if not all instances, the process sequence can be varied from the order shown and described. 
     In addition to an assumption that grid effects and non-grid effects can be corrected separately, the exemplary process makes two basic assumptions about image data from imaging array  130 . First, the exemplary process assumes that each detector pixel output signal Y IJ  can be modeled as 
     
       
           Y   IJ   =M   IJ   *X   IJ   +OFF   I ,  Eq. (1) 
       
     
     where subscript IJ generally denotes quantities derived from or related to the detector in the I-th row and the J-th column of the array of detectors; M IJ  is the total sensitivity or gain of the detection system at detector pixel IJ; X IJ  is the x-ray exposure at detector IJ in Roentgens; and OFF IJ , is the offset, that is, the measured output at detector IJ without x-ray exposure. And second, it assumes that the total sensitivity M IJ  can be modeled as 
     
       
           M   IJ   =f ( G   IJ   , NG   IJ ),  Eq. (2) 
       
     
     where f denotes a generic functional or mathematical combination; G IJ  denotes a grid-only correction pixel value for detector IJ; and NG IJ  denotes a non-grid-correction pixel value used for correcting the output of detector IJ for one or more non-grid effects. With these assumptions, the exemplary method proceeds as indicated in process blocks  202 - 208 . 
     Process block  202  entails acquiring N first flat-field images without an object or patient and without x-ray focusing grid  120  being between x-ray source  110  and imaging array  130 . Each of the N first flat-field images corresponds to a particular one of the N possible non-grid configurations of system  100 . Prior to acquiring the first image, some embodiments manually or automatically move grid  120  out of the path of radiation from x-ray source  110 . In some embodiments, a series of flat-field images are aggregated (with uniform or non-uniform weighting) and averaged to determine each of the N first flat-field images. Averaging the series of flat-field images reduces noise. 
     Each first flat-field image includes a set of K×L pixel values Y 1   n  from imaging array  120 , with individual pixel values denoted Y 1   IJ, n , where I and J denote particular row and column indices and subscript n ranges from 1 to N, denoting the particular one of the N possible non-grid configurations associated with the first flat-field image. The exemplary embodiment models these pixel values as 
     
       
           Y   1   IJ, n   =M′   IJ, n   ,*X   1   IJ, n   +OFF   1   IJ, n   Eq. (3a) 
       
     
     where Y 1   IJ, n  denotes the pixel value for the first image at detector IJ; M′ IJ, n  denotes the sensitivity for the detection system at detector pixel IJ in the no-grid configuration; X 1   IJ, n  denotes the incident x-ray exposure at detector IJ; and OFF 1   IJ, n , that is, the detector offset determined immediately before or after acquisition of the first image. 
     After acquiring the N first flat-field images, the exemplary process proceeds to block  204  to compute and store N non-grid correction maps NG 1 , NG 2 , . . . NG N  to memory  150 . In the exemplary embodiment, computing the N non-grid correction maps entails adjusting each of the N first flat-field images for the offsets of each detector. Equation (3b) shows each pixel value in each non-grid correction map NG n  is defined as 
     
       
           NG   IJ, n   =M′   IJ, n   *X   1   IJ, n   =Y   1   IJ, n   −OFF   1   IJ, n   Eq. (3b) 
       
     
     Some embodiments of the invention normalize each of the non-grid correction maps before storing them to memory. One such embodiment normalizes each pixel value based on its mean value determined from a number of aggregated flat-field images. However, some embodiments normalize based on other measures of central tendency, based on an absolute or relative quantity for all the pixel values, or based on local or regional normalization techniques. The invention, however, is not limited to any particular normalization technique. 
     Execution then proceeds to process block  206 , which entails acquiring P secong flat-field images without an object and with one of the P possible x-ray focusing grids  120  being between x-ray source  110  and imaging array  120 . Each second flat-field image comprises a set of pixel values Y 2   p , with subscript p denoting a particular one of the P possible x-ray focusing grids and with individual pixel values denoted Y 2   IJ, p . The exemplary embodiment models these pixel values as 
     
       
           Y   2   IJ, p   =M   IJ, p   *X   2   IJ, p   +OFF   2   IJ, p ,  Eq. (4) 
       
     
     where Y 2   IJ, p  denotes the pixel value for thye second image at detector IJ; M IJ, p  denotes the total sensitivity of the detection system at detector pixel IJ with grid p in place; X 2   IJ, p  denotes the incident x-ray exposure at detector IJ for the second image; and OFF 2   IJ, p  denotes the detector offset determined immediately before or after acquisition of the second flat-field image. In some embodiments, a series of images of each grid p are aggregated (using uniform of non-uniform weighting) and averaged to determine each of the P second flat-field images. 
     
       
           Y   3   IJ, np   =M   IJ, np   *X   3   IJ, np   +OFF   3   IJ, np   Eq. (6) 
       
     
     After acquiring the P second flat-field images, the processor proceeds to block  208  where it computes and stores P grid-only correction maps G 1 , G 2 , . . . G P  to memory  150 . If the first and second flat-field images are acquired with sufficiently low noise and approximately equal radiation exposures, that is, X 1   IJ  approximately equals X 2   IJ , then the pixel values for each grid-only correction map can be calculated as 
     
       
           G   IJ, p   =[Y   2   IJ, p   −OFF   2   IJ   ]*[NG   IJ, n ] −1   Eq. (5a) 
       
     
     where NG IJ, n  is defined in equation (3b) and where p denotes the particular grid and n denotes the system configuration used in acquiring the associated second flat-field image. Equation (5a) represents the proposition that the grid-only correction map can be calculated from a flat-field image of the grid that is itself corrected by the flat-field image without the grid. 
     In embodiments that normalize the flat-field images (or quantities based on these images), there is no restriction on the relation of the radiation exposure levels X 1   IJ  and X 2   IJ . In this case, the pixel values for each grid-only correction map can be calculated as 
     
       
           G   IJ, p   =Norm[Y   2   IJ, p   −OFF   2   IJ, p   *{Norm′[NG   IJ, n ]} −1   Eq. (   5   b) 
       
     
     where Norm and Norm′ denote respective normalization functions or techniques. In some embodiments, Norm and Norm′ are the same, and in other embodiments they are different. 
     After being computed and stored in memory  150 , the partial-correction maps are ready to be used to correct appropriate application images. (Note that these partial correction maps are subject to regular updates using the method outlined in FIG. 2, thereby ensuring that the maps reflect a reasonably current state of the x-ray system.) In some embodiments, there is substantial delay—for example, hours, days, weeks, or months—between computation and storage of the partial-correction maps and their actual use in correcting application images as shown in FIG.  3 . 
     FIG. 3 shows an exemplary flow chart  300 , illustrating operation of system  100  and especially processor  140  in accord with software modules or computer programs  158 . Like flow chart  200 , flow chart  300  is applicable to software, firmware, and hardware implementations. Flow chart  300  includes process blocks  302 - 316 , which can be executed serially or in parallel or reorganized as a greater or lesser number of blocks. In most, if not all instances, the process sequence can be varied from the order shown and described. 
     In process block  302 , system  100  acquires an application (patient or object) image with one of N possible system configurations and techniques defined by one or more non-grid operating criteria, such as tube and detector position, and one of P possible x-ray focusing grids placed between the x-ray source and the imaging array. Other non-grid operating criteria which some embodiments use to define system configurations and techniques include x-ray beam energy spectrum, kVp and beam filtration, x-ray beam spatial variation, source-to-image distance, x-ray collimation, x-ray tube focal spot, ion chamber characteristics, and tube, grid, and detector array alignment. 
     The application image comprises a set of pixel values Y 3   np , with n denoting the particular non-grid system configuration and p denoting the particular grid used in forming the application image. Each pixel value Y 3   IJ, np , has the form: 
     
       
           Y   3   IJ, np   =M   IJ, np   *X   3   IJ, np   +OFF   3   IJ, np   Eq. (6) 
       
     
     where Y 3   IJ, np  denotes the uncorrected pixel value for the application image at detector IJ; M IJ, np , denotes the total sensitivity of the detection system at detector pixel IJ with non-grid configuration n and grid p; X 3   IJ, np  denotes the incident x-ray exposure at detector IJ for the application image; and OFF 3   IJ, np , denotes the detector offset immediately before or after acquisition of the application image. After adjusting for offset, each pixel value of the application image has the form M IJ, np *X 3   IJ, np . 
     In block  304 , the processor determines the non-grid system configuration used for the application image. In the exemplary embodiment, this entails examining one or more operator inputs or corresponding x-ray system settings. Some embodiments determine the non-grid system configuration through identification of two or more parameters such as tube voltage, kVp, and focal-spot specification. 
     In block  306 , the processor uses its determination of the non-grid system configuration to select one or more non-grid correction maps from memory. In the exemplary embodiment, this entails choosing an existing non-grid correction map which was generated for the non-grid system configuration used to form the subject application image. However, in other embodiments, this entails selecting two or more non-grid correction maps, each one addressing a different non-grid aspect of the system configuration. After selecting one or more appropriate non-grid correction maps, execution of the exemplary method continues at block  308 . 
     Block  308  entails determining the identity or type of grid placed between the x-ray source and the imaging array and finding the identified in the application image. There are at least three ways to determine grid identity or type and to find the grid. A first method is to look at x-ray system inputs or settings indicating the grid identity or type. A second method is to recognize identity or type markers from the grid in the application image. And, a third is to compare the distinct Fourier transforms of each of the possible grids to Fourier transforms of the application image, using signature spectrum features for discrimination. Finding the grid includes determining the rotation and translation of the identified grid in the application image. 
     The exemplary process then executes blocks  310  and  312 . Block  310  entails using the determined grid identity or grid type to select a grid-only correction map, such as grid-only correction map P, from memory. And block  312  entails registering the selected grid-only correction map to the application image. Registration entails orienting the grid-only correction map to the application image to ensure that its pixel correction values are applied to the correct pixels of the application image. The exemplary embodiment uses affine registration, Fourier spectrum analysis, or the location of visible grid features in the application image to ensure proper registration. 
     Block  314  entails correcting the application image based on the one or more selected non-grid correction maps and the selected grid-only correction map. In the exemplary embodiment, this entails applying partial correction maps G p  and NG n  to the application image as prescribed in equation (7): 
     
       
           Y   3 ′ IJ   =[f ( G   IJ, p   , NG   IJ, n )] −1   *[Y   3   IJ, np   −OFF   3   IJ, np ],  Eq. (7) 
       
     
     where Y 3 ′ IJ  denotes the corrected pixel value for pixel value IJ of the application image; f(G IJ , NG IJ )] −1  denotes the total correction map as defined in equation (2); and the quantity [Y 3   IJ −OFF 3   IJ ] denotes the offset corrected application image pixel value IJ. In implementing equation (7), the exemplary embodiment defines f(G IJ, p , NG IJ, n ) as the product of G IJ  and NG IJ , or 
     
       
           f ( G   IJ, p   , NG   IJ, n )= G   IJ, p   *NG   IJ, n   Eq. (8a) 
       
     
     In embodiments that use normalized correction maps, f(G IJ, p , NG IJ, n ) as the following form: 
     
       
           f ( G   IJ, p   , NG   IJ, n )= Norm[G   IJ, p   ]*Norm′[NG   IJ, n ]  Eq. (8b) 
       
     
     After correcting the application image as indicated in equation (7), the exemplary method executes block  316 . Block  316  entails outputting the corrected image to output devices  160 . 
     CONCLUSION 
     In furtherance of the art, the inventors devised new methods and apparatus using non-grid and grid-only correction maps for correcting application images in direct and indirect digital imaging systems. In systems which, for example, support N different non-grid configurations and P different grids and thus would conventionally require storage and update of N×P (N times P) different correction maps, exemplary methods and apparatus in accord with the invention facilitate the same correction capability with storage of only N+P (N plus P) different correction maps. 
     The embodiments described above are intended only to illustrate and teach one or more ways of practicing or implementing the present invention, not to restrict its breadth or scope. The actual scope of the invention, which embraces all ways of practicing or implementing the teachings of the invention, is defined only by the following claims and their equivalents.