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Balancing areas of varying density in a digital imageRelated Patent Categories: Image Analysis, Applications, Dna Or Rna Pattern Reading, X-ray Film Analysis (e.g., Radiography)Balancing areas of varying density in a digital image description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070009145, Balancing areas of varying density in a digital image. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001] This application is a divisional application of U.S. application Ser. No. 10/302,821, which claims priority from and incorporates by reference U.S. application Ser. No. 60/333,224 entitled "Lens Assembly and Barrel Correction Method For X-Ray System", U.S. application Ser. No. 60/333,252 entitled "Self Diagnostic System for Optically Coupled Digital Radiography", U.S. application Ser. No. 60/333,207 entitled "Positioning Stand for a Radiography Imaging Device", and U.S. application Ser. No. 60/333,206 entitled "Balancing Areas of Varying Density in a Digital Image", all filed on Nov. 23, 2001. FIELD OF THE INVENTION [0002] The present invention is directed generally to digital radiography, and in particular to an image processing apparatus, program and method for balancing areas of widely varying density in a digital image. BACKGROUND OF THE INVENTION [0003] For over a hundred years photographic films have been used to capture and display x-rays for diagnostic purposes. In the last ten years or so, digital radiography has become increasingly popular. Digital radiography refers to the application of digital image processing techniques to projection radiography (x-rays). Digitally recorded x-rays are superior to those recorded with photographic film due to the greater dynamic range of the digital recording system. Furthermore, computer image processing techniques provide a wealth of capabilities to study otherwise obscured details within the image. [0004] To take a digital radiograph, a digital radiography imaging unit is positioned behind a subject. A standard radiographic generator directs radiation through the subject to a fluorescent-imaging screen mounted just behind the front surface of the imaging unit. The imaging screen is the conversion media for radiation to visible light. The fluorescent-imaging screen absorbs the radiographic radiation and emits light of a particular wavelength which closely matches the peak sensitivity of a charge coupled device (CCD) camera. A front-surfaced mirror is positioned at a 45 degree angle inside the imaging unit to direct the radiographic image into the CCD camera. The mirror allows the CCD camera to be positioned out of the direct path of the radiation, effectively shielding it from radiation exposure and prolonging its life. A high-efficiency lens reduces the image and directs it onto the surface of the CCD. [0005] The visual image formed by the fluorescent-imaging screen is converted into a digital image by the CCD sensor. A control computer converts the image into a medical image file that can be viewed for clinical diagnosis, enhanced and electronically stored with the patient demographic information in a picture archiving system. [0006] In a digital radiographic image, it is often desirable to reduce the contrast between various parts of the image so that detailed information in all parts of the image can be viewed at the same brightness and contrast (window level and width) settings. As an example, in a chest radiograph, a patient's ribs may be obscured by differing amounts of soft tissue. The result is that some of the ribs may appear to have lower contrast even though they are the same thickness and density as others. SUMMARY OF THE INVENTION [0007] According to one aspect of the invention, there is provided an equalization method for balancing areas of varying density within a digital image. The equalization method comprises the following steps: [0008] (a) receiving an unequalized digital image having a plurality of pixels each with an unequalized intensity value; [0009] (b) associating a blurred intensity value to each pixel; [0010] (c) selecting an image enhancement strength factor for the image; [0011] (d) selecting an exposure compensation factor for the image; [0012] (e) determining a mask intensity value for each pixel by calculating the linear convex combination of the pixel's blurred intensity value and the exposure compensation factor, wherein the degree of convex combination is determined by the strength factor; [0013] (f) determining an enhanced intensity value for each pixel by dividing the pixel's unequalized intensity value by the pixel's mask intensity value, and then scaling the dividend by the exposure compensation factor; then [0014] (g) displaying an enhanced image having a plurality of pixels at their enhanced intensity values. [0015] The blurred intensity value for each pixel may be a function of the unequalized intensity values of the pixels in a surrounding kernel. In particular, the blurred intensity value for each pixel may be the average of the unequalized intensity values of the pixels in a surrounding kernel. [0016] The exposure compensation factor may be selected to be the mean blurred intensity value of the image. The mask intensity value may be determined to be the exposure compensation factor*(1-strength factor)+strength factor* blurred intensity value. [0017] According to another aspect of the invention, the equalization method described above may be incorporated into a module of a program encoded on a computer-readable storage medium. [0018] According to another aspect of the invention, there is provided a digital radiography processor that is programmed to balance areas of varying density within a digital image according to the equalization method described above. [0019] According to yet another aspect of the invention, there is provided a method of producing a mask for digital unsharp masking or for digital equalization, comprising: [0020] (a) selecting a starting kernel size K; [0021] (b) selecting a minimum kernel size M; [0022] (c) selecting a number of iterations n; [0023] (d) receiving an unprocessed digital radiograph I; [0024] (e) creating a first new image I' by copying the radiograph I; [0025] (f) creating a second new image I.sub.i by blurring the first new image I' with a kernel of the starting kernel size K; [0026] (g) replacing pixel values in the first new image I' by retrieving the pixel-wise maximum of the second new image I.sub.i and the radiograph I; [0027] (h) dividing the starting kernel size K by two; then [0028] (a) repeating steps (f) through (h) for either the selected number of iterations n or until the starting kernel size is less than or equal to the minimum kernel size. BRIEF DESCRIPTION OF DRAWINGS [0029] FIG. 1 is a schematic illustration of a digital radiography system having a computer with an image enhancing program that carries out an equalization method on captured unequalized x-ray images. [0030] FIG. 2 is a flowchart of the steps performed in the equalization method. DETAILED DESCRIPTION [0031] In one embodiment of the invention, there is provided a digital radiography system that digitally captures an x-ray image and applies to the image a digital equalization method modeled after a method known in the world of analog film processing as unsharp masking. [0032] Analog Unsharp Masking: Photographic film has a higher dynamic range than photographic paper. Therefore, high contrast images captured on a film negative may not display well on a paper print. For example, in a paper print of a person standing inside a darkened room and beside a bright window, the person may appear dark and his details may be obscured, even though the film negative from which the print was developed may have sufficient dynamic range to capture the details of the person. Similarly, a person standing outside and visible through the window may appear so bright that his details may not be clearly visible on the paper print. Special film processing techniques can be applied to bring the details captured on the film negative within the dynamic range of the paper. An example of such a film processing technique is analog unsharp masking. This process helps ease contrast problems in the paper print caused by the film capturing a high contrast image, by balancing areas of widely varying density in the image. It also has the effect of sharpening the resulting image. [0033] Analog unsharp masking involves first creating an under-exposed unsharp negative of the original image negative, known as an "unsharp mask". In other words, a washed-out, blurry positive is produced on film. Then, the unsharp mask is overlaid with the original image negative, and the mask and original negative are developed together. This process balances areas of widely varying density in the original image negative, such that when the image is displayed on a paper print, edges and details are enhanced. [0034] This process can be manipulated to vary the appearance of the printed image. For example, the amount of blurring and the degree of underexposure can be independently varied. If the mask is perfectly exposed (no underexposure) and is not blurred, then unsharp masking will produce a perfectly gray picture. If the mask is perfectly exposed and is moderately blurred, then only the edges of the image will be enhanced with the rest of the image being predominantly gray. At the other extreme, where the mask is extremely blurred--so that no shapes are discernable--the masking will serve to even out very broad changes in illumination in the captured image. Continue reading about Balancing areas of varying density in a digital image... Full patent description for Balancing areas of varying density in a digital image Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Balancing areas of varying density in a digital image patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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