# US 5,708,693 A · Image processing for noise reduction > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,708,693 A - Document ID: US5708693A - Status: Granted - Kind: A - Application number: 08/671,123 - Filed: 1996-06-20 - Granted: 1998-01-13 - Earliest claimed: 1995-06-23 - Assignees: U.S. Philips Corporation - Inventors: Til Aach; Dietmar W. Kunz; Stefan H. E. Ludeke - Web page: https://patentgrep.com/patent/US5708693A ## CPC - G06T 5/70: Denoising; Smoothing - G06T 5/20: using local operators - G06T 2207/10116: X-ray image - G06T 2207/20016: Hierarchical, coarse-to-fine, multiscale or multiresolution image processing; Pyramid transform - G06T 2207/20032: Median filtering - G06T 2207/30004: Biomedical image processing - G06T: IMAGE DATA PROCESSING OR GENERATION, IN GENERAL ## Abstract An image processing method includes multi-resolution decomposition to decompose an input image into frequency-band images, which are subsequently filtered according to an order statistics filtering. Preferably, a finite impulse response median hybrid is employed. The filtered frequency-band images are synthesized to form the filtered output image. ## Claims ### Claim 1 An image processing steps comprising the steps of: dividing an input image into several frequency-band images by means of a multi-resolution decomposition method, processing the separate frequency-band images to derive processed frequency-band images, the processing comprising filtering the frequency-band images to form filtered frequency-band images, and performing a multi-resolution synthesis method to said processed frequency-band images to form a filtered output image, characterized in that the filtering comprises an order statistics filtering. ### Claim 2 (depends on 1) An image processing method as claimed in claim 1, characterized in that the input image comprises sub-images, image lines of separate sub-images forming alternating image lines of the input image and that the filtering is performed along directions not parallel to the image lines of the input image. ### Claim 3 (depends on 1) An image processing method as claimed in claim 1, characterized in that the order statistics filtering is a finite impulse response median hybrid filtering. ### Claim 4 (depends on 3) An image processing method as claimed in claim 3, characterized in that the input image comprises sub-images, image lines of separate sub-images forming alternating image lines of the input image and that the filtering is performed along directions not parallel to the image lines of the input image. ### Claim 5 (depends on 3) An image processing method as claimed in claim 3, characterized in that the processing comprises: computing differences, for corresponding pixels, between pixel-values of a filtered frequency-band image and pixel-values of a corresponding frequency-band image, comparing said differences with a threshold value and, deriving a processed frequency-band image from pixel-values of said filtered frequency-band image for which pixel-values said difference has a magnitude less than the threshold value and from pixel-values of said frequency-band image for which pixel-values said magnitude of said difference is greater than the threshold value. ### Claim 6 (depends on 5) An image processing method as claimed in claim 5, characterized in that the input image comprises sub-images, image lines of separate sub-images forming alternating image lines of the input image and that the filtering is performed along directions not parallel to the image lines of the input image. ### Claim 7 (depends on 5) An image processing method as claimed in claim 5, characterized in that the threshold value is adjustable. ### Claim 8 (depends on 7) An image processing method as claimed in claim 7, characterized in that the input image comprises sub-images, image lines of separate sub-images forming alternating image lines of the input image and that the filtering is performed along directions not parallel to the image lines of the input image. ### Claim 9 (depends on 7) An image processing method as claimed in claim 7, wherein the input image is derived from an x-ray image that is formed by irradiating an object with x-rays, characterized in that the threshold value is derived from the x-ray dose. ### Claim 10 (depends on 9) An image processing method as claimed in claim 9, characterized in that the input image comprises sub-images, image lines of separate sub-images forming alternating image lines of the input image and that the filtering is performed along directions not parallel to the image lines of the input image. ### Claim 11 (depends on 1) An image processing method as claimed in claim 1, characterized in that the processing comprises: computing differences, for corresponding pixels, between pixel-values of a filtered frequency-band image and pixel-values of a corresponding frequency-band image, comparing said differences with a threshold value and, deriving a processed frequency-band image from pixel-values of said filtered frequency-band image for which pixel-values said difference has a magnitude less than the threshold value and from pixel-values of said frequency-band image for which pixel-values said magnitude of said difference is greater than the threshold value. ### Claim 12 (depends on 11) An image processing method as claimed in claims 11, characterized in that the input image comprises sub-images, image lines of separate sub-images forming alternating image lines of the input image and that the filtering is performed along directions not parallel to the image lines of the input image. ### Claim 13 (depends on 11) An image processing method as claimed in claim 11, characterized in that the threshold value is adjustable. ### Claim 14 (depends on 13) An image processing method as claimed in claim 13, characterized in that the input image comprises sub-images, image lines of separate sub-images forming alternating image lines of the input image and that the filtering is performed along directions not parallel to the image lines of the input image. ### Claim 15 (depends on 13) An image processing method as claimed in claim 13, wherein the input image is derived from an x-ray image that is formed by irradiating an object with x-rays, characterized in that the threshold value is derived from the x-ray dose. ### Claim 16 (depends on 15) An image processing method as claimed in claim 15, characterized in that the input image comprises sub-images, image lines of separate sub-images forming alternating image lines of the input image and that the filtering is performed along directions not parallel to the image lines of the input image. ### Claim 17 An image processor comprising: a decomposition unit for dividing an input image signal into several frequency-band image signals, a converter unit for processing the frequency-band image signals, the converter unit comprising a filter for filtering said frequency-band image signals to form processed frequency-band image signals, and a synthesizer for forming an filtered output image signal from said processed frequency-band image signals, characterized in that the filter comprises an order statistics filter. ### Claim 18 (depends on 17) An image processor as claimed in claim 17, characterized in that the converter unit comprises: an arithmetic unit arranged to receive pixel-values of the frequency-band image signals and pixel-values of the corresponding filtered frequency-band image signals and compute differences between said pixel-values of the frequency-band image signals and said pixel-values of the corresponding filtered frequency-band image signals, a comparator to compare said differences with a threshold-value, and a selection unit for selecting pixel-values of the filtered frequency band image signals difference having a magnitude less that the threshold and forming a processed frequency band image form selected pixel-values. ### Claim 19 (depends on 18) An x-ray examination apparatus comprising: an x-ray source for irradiating an object with an x-ray beam to form an x-ray image, an x-ray detector facing the x-ray detector for providing at its output an electronic image signal from the x-ray image, characterized in that the x-ray examination apparatus is provided with an image processor as claimed in claim 18 and having its input coupled to the output of the x-ray detector. ### Claim 20 (depends on 18) An x-ray examination apparatus comprising: an x-ray source for irradiating an object with an x-ray beam to form an x-ray image, an x-ray detector facing the x-ray detector for providing at its output an electronic image signal from the x-ray image, characterized in that the x-ray examination apparatus is provided with an image processor as claimed in claim 18 and having its input coupled to the output of the x-ray detector.