Abstract:
The present invention provides a method and associated system for processing a digital medical image. The method includes defining a plurality regions of an initial digital medical image, wherein the initial digital medical image is a combined initial digital medical image formed from the digital pasting of a plurality of individual initial digital medical images, and wherein the initial digital medical image is an exposure-normalized initial digital medical image; measuring an intensity for each of the plurality of regions of the initial digital medical image; deriving an intensity weighting function using the intensity measured for each of the plurality of regions of the initial digital medical image; and applying the intensity weighting function to the initial digital medical image to form a final digital medical image.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to the medical imaging field. More specifically, the present invention relates to an enhanced image processing method for the presentation of digitally-combined medical images.  
       BACKGROUND OF THE INVENTION  
       [0002]     With applications ranging from diagnostic procedures to radiation therapy, the importance of high-performance medical imaging is immeasurable. As a result, new high-performance medical imaging technologies are continually being developed. Digital medical imaging technologies represent the future of medical imaging. Digital medical imaging systems are capable of producing far more accurate and detailed images of an anatomical object than conventional, film-based medical imaging systems. Such digital medical imaging systems also allow for image enhancement once an anatomical object has been scanned, further enhancing their usefulness.  
         [0003]     The flat-panel digital radiographic imaging detectors available today typically have a maximum imaging size of about 40 cm×40 cm. Often, an area of interest larger than 40 cm×40 cm must be imaged. In such cases, several sub-images are taken and combined to form a single, larger image of the area of interest. For example, if a 90 cm spinal image must be taken, three separate sub-images of the spine must be taken and combined to form a single, larger image. This presents a challenge because a wide range of anatomical thicknesses must be represented. Typically, a spinal image includes very thin anatomical parts, such as the c-spine, and very thick anatomical parts, such as the abdomen.  
         [0004]     When sub-images are acquired using auto-exposure techniques and processed individually, the auto-exposure techniques ensure that anatomical thickness differences are compensated for and accurately represented. The combined image will then have a sufficiently narrow dynamic range to be displayed as is. Referring to  FIGS. 1 and 2 , however, the combined image  10  will include different brightness or intensity bands, the boundaries of which correspond to the boundaries of the sub-images  12 . These low-frequency band artifacts  14  are caused by the fact that the brightness or intensity levels of each of the sub-images  12  are matched only where they are measured, namely in the center portion  16  of each of the sub-images  12 . The low-frequency band artifacts  14  are bothersome to those analyzing the combined image  10 , especially in the junction regions, and may obscure anatomical detail.  
         [0005]     When sub-images are acquired using fixed techniques, or if the sub-images are normalized with regard to exposure, the different brightness or intensity bands are not visible. Referring to  FIG. 3 , however, the dynamic range of the combined image  10  is increased and some portions  18  of the combined image  10  become saturated. Conventional image-equalization algorithms may compensate for this effect, but extreme parameters must be used, leading to potentially strong distortions in the area of interest, as such algorithms are typically designed for the specific detector size.  
         [0006]     Thus, what is needed is a pre-processing imaging method that compensates for dynamic range in the direction(s) of the combined scan, such that the desirable effects of the conventional image-equalization algorithms are preserved.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     Accordingly, the above-identified shortcomings of existing medical imaging systems and methods are overcome by the various embodiments of the present invention, which relate to enhanced image processing for the presentation of digitally-combined medical images.  
         [0008]     In one specific embodiment of the present invention, a method for processing a digital image includes defining a plurality regions of an initial digital image; measuring an intensity for each of the plurality of regions of the initial digital image; deriving an intensity weighting function using the intensity measured for each of the plurality of regions of the initial digital image; and applying the intensity weighting function to the initial digital image to form a final digital image.  
         [0009]     In another specific embodiment of the present invention, a method for processing a digital medical image includes defining a plurality regions of an initial digital medical image, wherein the initial digital medical image is a combined initial digital medical image formed from the digital pasting of a plurality of individual initial digital medical images, and wherein the initial digital medical image is an exposure-normalized initial digital medical image; measuring an intensity for each of the plurality of regions of the initial digital medical image; deriving an intensity weighting function using the intensity measured for each of the plurality of regions of the initial digital medical image; and applying the intensity weighting function to the initial digital medical, image to form a final digital medical image.  
         [0010]     In a further specific embodiment of the present invention, a system for processing a digital image includes a first algorithm for defining a plurality regions of an initial digital image; a second algorithm for measuring an intensity for each of the plurality of regions of the initial digital image; a third algorithm for deriving an intensity weighting function using the intensity measured for each of the plurality of regions of the initial digital image; and a fourth algorithm for applying the intensity weighting function to the initial digital image to form a final digital image.  
         [0011]     Further features, aspects, and advantages of the present invention will become more readily apparent to those of ordinary skill in the art during the course of the following detailed description of the invention, wherein references are made to the accompanying drawings which illustrate some preferred embodiments of the present invention, and wherein like characters of reference designate like parts throughout the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The systems and methods of the present invention are described herein below with reference to the accompanying drawings, in which:  
         [0013]      FIG. 1  is a digital medical image illustrating the shortcomings of conventional auto-exposure techniques for combining a plurality of sub-images into a single, larger combined image, the presence of low-frequency band artifacts obscuring portions of the combined image;  
         [0014]      FIG. 2  is a digital medical image illustrating the locations of the ion chambers associated with each of the sub-images used in conjunction with the conventional auto-exposure techniques of  FIG. 1 , the locations of the ion chambers substantially corresponding to the locations where brightness or intensity levels are measured and matched for each of the sub-images to form the single, larger combined image;  
         [0015]      FIG. 3  is a digital medical image illustrating the shortcomings of conventional fixed and related techniques for combining a plurality of sub-images into a single, larger combined image, some portions of the combined image becoming saturated;  
         [0016]      FIG. 4  is a schematic diagram illustrating one exemplary architecture of a digital x-ray imaging system, as used in preferred embodiments of the present invention;  
         [0017]      FIG. 5  is a perspective view illustrating one exemplary amorphous silicon flat-panel x-ray detector, as used in preferred embodiments of the present invention;  
         [0018]      FIG. 6  is a digital medical image illustrating the locations of a plurality of virtual ion chambers associated with a combined image used in conjunction with the medical imaging method of the present invention;  
         [0019]      FIG. 7  is a flow chart illustrating the medical imaging method of the present invention; and  
         [0020]      FIG. 8  is two digital medical images and two plots illustrating the medical imaging method of the present invention, the two plots illustrating brightness or intensity-related counts in the plurality of virtual ion chambers of  FIG. 7  and both unsmoothed and smoothed row-dependent weighting functions derived from these brightness or intensity-related counts, respectively. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     For the purposes of promoting an understanding of the present invention, reference will now be made to some preferred embodiments of the present invention, as illustrated in  FIGS. 1-8 , and specific language use to describe the same. The terminology used herein is for the purpose of description, and not limitation. The specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and for teaching one of ordinary skill in the art to variously employ the systems and methods of the present invention. Any modifications to or variations in the depicted systems and methods and such further applications of the principles of the present invention as would normally occur to one of ordinary skill in the art are considered to be within the spirit and scope of the present invention.  
         [0022]     As described above, the flat-panel digital radiographic imaging detectors available today typically have a maximum imaging size of about 40 cm×40 cm. Often, an area of interest larger than 40 cm×40 cm must be imaged. In such cases, several sub-images are taken and combined to form a single, larger image of the area of interest. For example, if a 90 cm spinal image must be taken, three separate sub-images of the spine must be taken and combined to form a single, larger image. This presents a challenge because a wide range of anatomical thicknesses must be represented. Typically, a spinal image includes very thin anatomical parts, such as the c-spine, and very thick anatomical parts, such as the abdomen.  
         [0023]     Referring to  FIG. 4 , a schematic diagram illustrates the architecture of one exemplary digital x-ray imaging system  20 , as used in preferred embodiments of the present invention. Such a digital x-ray imaging system  20  typically includes an x-ray source  22 , an x-ray detector  24 , an x-ray detector controller  26  that contains electronics operable for operating the x-ray detector  24 , and an x-ray source controller  28  that contains electronics operable for operating the x-ray source  22 . During operation, x-rays  30  are directed from the x-ray source  22  towards the x-ray detector  24 , which may include a scintillator  32  and an amorphous silicon array  34 . A system controller  36  provides power and timing signals to the x-ray source controller  28  and the x-ray detector controller  26 , which then control the operation of the x-ray source  22  and the x-ray detector  24 , respectively. After passing through an object to be imaged, such as an anatomical object  38 , the x-rays  30  impact the scintillator  32 , which converts the x-ray photons to visible light. This visible light is then converted to an electrical charge by an array of photodiodes  40  ( FIG. 5 ) contained in the amorphous silicon array  34 . Each photodiode  40  is of large enough area to ensure that it will intercept a substantial portion of the visible light produced by the scintillator  32 . Each photodiode  40  also has a relatively large capacitance that allows it to store the electrical charge that results from the photon excitation. A data acquisition system within the x-ray detector controller  26  samples analog electrical charge data from the x-ray detector  24  and converts that analog electrical charge data to digital signals for subsequent processing. The digital signals are sent to an image processor  42 , where the image is processed and enhanced. The processed, enhanced image may then be displayed on a cathode ray tube display  44  or other suitable display, and/or the image may be stored in mass storage  46  for later retrieval. The image processor  42  may also produce a brightness or intensity control signal that is applied to an exposure control circuit  48  to regulate the power supply  50 , thereby regulating the x-ray source  22  through x-ray source controller  28 . The overall operation of the digital x-ray imaging system  20  is governed by the system controller  36 , which receives commands and/or scanning parameters from an operator via an operator interface  52 . The operator interface  52  includes a keyboard, touchpad, or other suitable input device. The associated cathode ray tube display  44  or other suitable display allows the operator to view the reconstructed image and other data generated by the image processor  42 . The operator-supplied commands and/or scanning parameters are used by the system controller  36  to provide control signals and other information to the image processor  42 , the x-ray detector controller  26 , the x-ray source controller  28 , and/or the exposure control circuit  48 .  
         [0024]     Various embodiments of the present invention make use of software or firmware running on the system controller  36  to carry out the processing of data and/or images. A mouse, pointing device, or other suitable input device is employed to facilitate the entry of data and/or image locations. Various embodiments of the present invention make use of a general purpose computer or workstation having a memory and/or printing capability to store and/or print images. Suitable memories are well known to those of ordinary skill in the art and may include, but are not limited to, random-access memory, one or more hard drives, diskettes, optical media, etc. Embodiments using a general purpose computer or workstation may send/receive data via conventional electronic storage media and/or a conventional communications link, and images may be reconstructed there from.  
         [0025]     Referring to  FIG. 5 , a perspective view illustrates one exemplary amorphous silicon flat-panel x-ray detector  24 , as used in preferred embodiments of the present invention. Typically, column electrodes  54  and row electrodes  56  are disposed on a single-piece glass substrate  58 , and an amorphous silicon array  34  is defined thereby. The amorphous silicon array  34  includes an array of photodiodes  40  and field-effect transistors  60 . A scintillator  32  is disposed over the amorphous silicon array  34  and is optically coupled thereto. The scintillator  32 , which may be a dose-efficient cesium iodide scintillator or the like, receives and absorbs x-ray radiation during operation and converts the x-ray photons therein to visible light. The high-fill factor amorphous silicon array  34 , each photodiode  40  therein representing a pixel, converts the detected visible light into an electrical charge. The charge at each pixel is read and digitized by low-noise electronics, via contact fingers  62  and contact leads  64 , and thereafter sent to an image processor  42 .  
         [0026]     As described above, when sub-images are acquired using auto-exposure techniques and processed individually, the auto-exposure techniques ensure that anatomical thickness differences are compensated for and accurately represented. The combined image will then have a sufficiently narrow dynamic range to be displayed as is. Referring again to  FIGS. 1 and 2 , however, the combined image  10  will include different brightness or intensity bands, the boundaries of which correspond to the boundaries of the sub-images  12 . These low-frequency band artifacts  14  are caused by the fact that the brightness or intensity levels of each of the sub-images  12  are matched only where they are measured, namely in the center portion  16  of each of the sub-images  12 . The low-frequency band artifacts  14  are bothersome to those analyzing the combined image  10 , especially in the junction regions, and may obscure anatomical detail.  
         [0027]     As described above, when sub-images are acquired using similar fixed techniques, or if the sub-images are normalized with regard to exposure, the different brightness or intensity bands are not visible. Referring again to  FIG. 3 , however, the dynamic range of the combined image  10  is increased and some portions  18  of the combined image  10  become saturated, such that only portions of the combined image  10  may be used diagnostically. Conventional dynamic range management algorithms may be able to compensate for this effect, but extreme parameters must be used, leading to potentially strong distortions in the area of interest, as the intent of such algorithms is to compensate for effects in an area not wider than the physical detector. Again, a non-diagnostic image results.  
         [0028]     Referring to  FIG. 6 , in general, the present invention uses a plurality of virtual ion chambers  66  in conjunction with the pasting of a plurality of sub-images  12  to form a single, larger combined image  10 . These virtual ion chambers  66  allow for the matching of the brightnesses or intensities in adjacent regions of each of the sub-images  12  and the combined image  10 , thus avoiding the low-frequency band artifacts  14  described above. The resulting combined image  10  is equivalent to an image that would be acquired using a slot-scanning device, however, it does not require the use of any specific medical imaging system.  
         [0029]     The method of the present invention uses an exposure-normalized combined image  10  wherein each sub-image  12  is normalized by its exposure, thus producing a combined image  10  without low-frequency band artifacts  14 , but having a wide dynamic range. It is then possible to equalize the brightness or intensity of this exposure-normalized combined image  10  with great flexibility by placing a plurality of virtual ion chambers  66  in predetermined locations, measuring the corresponding count in each of the plurality of virtual ion chambers  66 , and then modifying the gray-level of the pixels accordingly in each of the plurality of corresponding regions. Preferably, the plurality of virtual ion chambers  66  are aligned in the scanning direction, as this is the direction in which the most brightness or intensity equalization is needed. The plurality of virtual ion chambers  66  may or may not be placed directly adjacent to one another. Based upon the count measured in each of the plurality of virtual ion chambers  66 , a row-dependent weighting function is derived and applied to the combined image  10 . Preferably, the weighting function is smoothed prior to being applied. This method is illustrated in  FIGS. 7 and 8 .  
         [0030]     The method of the present invention may also be applied to non-rectilinear acquisition geometries, where the plurality of virtual ion chambers  66  would be placed on a two-dimensional grid and a two-dimensional weighting function would be used. Three-dimensional digital medical image processing is also contemplated. A coefficient may also be introduced in order to change the gain of the weighting function, thus controlling the amount of equalization that is applied to the combined image  10 .  
         [0031]     Although the present invention has been illustrated and described with reference to preferred embodiments and specific examples thereof, it should be noted that other embodiments and examples may perform the same functions and/or achieve similar results. All such equivalent embodiments and examples are within the scope and spirit of the present invention and are intended to be covered by the following claims.