# US 10,478,147 B2 · Calibration apparatus and method for computed tomography > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 10,478,147 B2 - Document ID: US10478147B2 - Status: Granted - Kind: B2 - Application number: 15/100,192 - Filed: 2014-11-25 - Published: 2017-01-26 - Granted: 2019-11-19 - Earliest claimed: 2013-11-28 - Assignees: Nikon Metrology NV - Applicants: Nikon Metrology NV - Inventors: Sam Hawker; Gemma Fardell; Patrick Blankaert - Web page: https://patentgrep.com/patent/US10478147B2 ## CPC - A61B 6/583: {using calibration phantoms} - A61B 6/032: {Transmission computed tomography [CT]} - A61B 6/584: {determining position of components of the apparatus or device using images of the phantom} - G01N 23/046: using tomography, e.g. computed tomography [CT] - A61B: DIAGNOSIS; SURGERY; IDENTIFICATION (usefulness limited to only animals A61D) - G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q) ## Abstract A calibration object is imaged in a radiographic system such as a CT system. The images are processed to calibrate the system, without prior measurement of the calibration object. Initial estimates are refined to improve accuracy. ## Claims ### Claim 1 A method of calibrating a radiographic system comprising a source and a detector, the method comprising: providing a calibration object comprising a plurality of markers, rotating the calibration object relative to the source and/or detector, acquiring a plurality of radiographic images of the calibration object, identifying the plurality of markers in the plurality of images, deriving an elliptical path for each of the plurality of markers using said plurality of images by assigning a nominal value representing a distance between the source and an axis of rotation of each said marker, and determining complete geometrical parameters of the system apart from said nominal distance value using the radiographic images and said nominal distance value, where said complete geometrical parameters are seven parameters including two parameters specifying a principal point, one parameter specifying a focal length, two parameters specifying the orientation of the axis of rotation and two parameters specifying the position of the axis of rotation, and using said elliptical paths to determine geometrical parameters of the system. ### Claim 2 (depends on 1) The method of claim 1 wherein the method does not require measurement of the calibration object. ### Claim 3 (depends on 1) The method of claim 1, the method further comprising identifying the location of the markers in the radiographic images. ### Claim 4 (depends on 3) The method of claim 3 wherein said deriving of the elliptical path fits an ellipse to the imaged trajectory of each of the markers. ### Claim 5 (depends on 1) The method of claim 1 comprising intersecting the ellipses to determine geometrical parameters of the system. ### Claim 6 (depends on 1) The method of claim 1 comprising deriving an initial geometry estimate of the system using said markers in said radiographic images. ### Claim 7 (depends on 6) The method of claim 6 comprising deriving an estimated model of said calibration object, and refining said initial geometry estimate and said estimated model of said calibration object. ### Claim 8 (depends on 7) The method of claim 7 wherein said refining said initial geometry estimate and said estimated model of said calibration object uses an iterative non-linear technique. ### Claim 9 (depends on 1) The method of claim 1 comprising using a circle-detection method to identify a plurality of circular shapes in each image, and selecting N or fewer of said circular shapes, where N is the number of markers in the calibration object. ### Claim 10 (depends on 9) The method of claim 9 further comprising deriving one or more of the distance from the source to detector, object offset, and rotation axis. ### Claim 11 (depends on 1) The method of claim 1 wherein said geometrical parameters comprise one or more of the image of the rotation axis, the image of a central slice and the principal point. ### Claim 12 (depends on 1) The method of claim 1 comprising compensating for perspective distortion, by projecting images of markers onto a virtual detector in which the projected images are more circular. ### Claim 13 (depends on 1) The method of claim 1 comprising refining the co-ordinates of the images of the markers (circles) using a maximum cross correlation method. ### Claim 14 (depends on 1) The method of claim 1 comprising analysing and compensating for residual errors, including image distortion. ### Claim 15 (depends on 14) The method of claim 14, wherein the distance value is determined using measurements of the calibration object, or measurements of relative positions of the calibration object. ### Claim 16 (depends on 1) The method of claim 1 comprising determining a value representing the distance between the source and the axis of rotation. ### Claim 17 (depends on 1) The method of claim 1 comprising providing the calibration object in a central position where the axis of rotation of the calibration object approximately intersects with the beam centreline, wherein a plurality of said radiographic images are obtained with the calibration object in said central position. ### Claim 18 (depends on 1) The method of claim 1 comprising providing the calibration object in an offset position where the axis of rotation of the calibration object is offset from the beam centreline, wherein a plurality of said radiographic images are obtained with the calibration object in said offset position. ### Claim 19 (depends on 1) The method of claim 1 using the calibration object for calibrating a radiographic system comprising a plurality of strongly X-ray attenuating spherical markers arranged within a weakly X-ray attenuating supporting material, whereby high contrast and high accuracy images of the markers can be obtained. ### Claim 20 (depends on 1) The method of claim 1 comprising performing scans at different speeds to compensate for changes in environmental conditions. ### Claim 21 (depends on 1) The method of claim 1 comprising assigning a nominal value representing the distance between the source and the axis of rotation, and detei mining geometrical parameters of the system using said radiographic images and said nominal distance value. ### Claim 22 (depends on 21) The method of claim 21 further comprising deriving a value for said nominal distance value using measurements of the calibration object. ### Claim 23 (depends on 1) The method of claim 1 comprising using an uncalibrated calibration object. ### Claim 24 (depends on 1) A method of measuring an object in a radiographic system calibrated using the method of claim 1. ### Claim 25 (depends on 1) A non-transitory computer-readable storage medium having computer-executable instructions for executing the method of claim 1 stored therein. ### Claim 26 (depends on 1) The method of claim 1 wherein said deriving of the elliptical path fits an ellipse to the imaged trajectory of each of the markers. ### Claim 27 A method of calibrating a radiographic system comprising a source and a detector, the method comprising: providing an object equipped with a marker, rotating the object relative to the source and/or detector, acquiring a plurality of radiographic images of the object, identifying a position of an image of the marker in each of a plurality of images, deriving a path for the marker using said plurality of images by assigning a nominal value representing a distance between the source and an axis of rotation of said marker, and determining complete geometrical parameters of the system apart from said nominal distance value using the radiographic images and said nominal distance value, where said complete geometrical parameters are seven parameters including two parameters specifying a principal point, one parameter specifying a focal length, two parameters specifying the orientation of the axis of rotation and two parameters specifying the position of the axis of rotation, and using said path to determine geometrical parameters of the system. ### Claim 28 A radiographic system having a source and a detector, the radiographic system configured to: rotate a calibration object comprising a plurality of markers relative to the source and/or detector, acquire a plurality of radiographic images of the calibration object, identify the plurality of markers in the plurality of images, derive an elliptical path for each of the plurality of markers using said plurality of images by assigning a nominal value representing a distance between the source and an axis of rotation of each said marker, and determining complete geometrical parameters of the system apart from said nominal distance value using the radiographic images and said nominal distance value, where said complete geometrical parameters are seven parameters including two parameters specifying a principal point, one parameter specifying a focal length, two parameters specifying the orientation of the axis of rotation and two parameters specifying the position of the axis of rotation, and use said elliptical paths to determine geometrical parameters of the system.