Patent US5128864A successfully refreshed from Google Patents.
US5128864A
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US
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Method for computing tomographic scans
Assignee: WL SYSTEMS Inc A CORP OF TX
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Filing: 1989-08-09
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Granted: 1992-07-07
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Expires: 2009-08-09
Abstract
Abstract Apparatus and method for mounting on an existing x-ray simulator and calculating a back projected computed tomographic image. The detector array is linear and outputs signals from the photodiodes mounted therein to a preprocessor for smoothing, correcting and filtering and subsequent processing to transform the signal from that produced by an x-ray originating from a fan beam source, e.g., in a polar coordinate system, into the signal which would have been produced by a detector in an array on which a parallel beam is incident on a Cartesian coordinate system. The transformed data is converted to a gray scale value for a picture element having a specific position in the Cartesian coordinate system and output to an appropriate display. Data is taken at each incremental angle as the beam source and detector array rotate around a target object. The method of reconstructing this back projected image involves correcting and smoothing the output signals, scaling those corrected and smoothed signals, and convolving the scaled signals into data characterizing the ray which is incident on each individual detector element into the equivalent intensity data had the incident ray originated from a parallel beam source. Also provided are methods for correcting the data for incorrect detector reading and for displacement of the detector array resulting from the off-center rotation of either or both of the x-ray source or the detector array.
Inventors
- 👤 Robert G. Waggener
- 👤 Jory D. Lange
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-11-04 10:46
CPC Classifications
G; G06; G06T; G06T12/00; G06T12/20; A; A61; A61B; A61B6/00; A61B6/58; A61B6/582; A61B6/583; A61B6/584; G06T2211/00; G06T2211/40; G06T2211/421
Claims Analysis
Total Claims: 24
Claim 1
Independent Claim
161 words
A method of back projecting the image of a CT scan comprising the steps of: (a) exposing a target object to an x-ray fan beam; (b) producing a signal representing the intensity of a beam of the x-ray incident upon a detector after passing through the target object; (c) scaling the signal in accordance with the signal which would have been produced by the detector had the x-ray not passed through the target object; (d) transforming the scaled signal into the frequency domain for filtering; (e) convolving the filtered signal to produce a value in the spatial domain representing the signal which would have been produced by the detector had the beam been a ray comprising a parallel beam having a center axis which is contiguous and co-linear with the central axis of the fan beam; and (f) repeating steps (a)-(e) at a plurality of incremental angles around the target object to produce a back projected image of the target object.
Claim 2
Independent Claim
161 words
A method of back projecting the image of a CT scan comprising the steps of: (a) exposing a target object to an x-ray fan beam; (b) producing a signal representing the intensity of a beam of the x-ray incident upon a detector after passing through the target object; (c) scaling the signal in accordance with the signal which would have been produced by the detector had the x-ray not passed through the target object; (d) transforming the scaled signal into the frequency domain for filtering; (e) convolving the filtered signal to produce a value in the spatial domain representing the signal which would have been produced by the detector had the beam been a ray comprising a parallel beam having a center axis which is contiguous and co-linear with the central axis of the fan beam; and (f) repeating steps (a)-(e) at a plurality of incremental angles around the target object to produce a back projected image of the target object.
Claim 3
Dependent Claim
40 words
The method in accordance with claim 1 of rotating the beam source through a circular arc about a center so that a plurality of individual exposures are made of the target object at angular increments along the arc of rotation.
Claim 4
Dependent Claim
77 words
The method in accordance with claim 1 wherein a convolved profile is produced for each angular increment by a series of steps wherein a discrete Fourier transform is performed on each scaled signal and the resulting transforms are multiplied by increasing values selected from a ramp frequency function to provide scaled transforms for each angular increment and the inverse Fourier transform of each scaled transform is taken to obtain the value representing the back projected parallel beam.
Claim 5
Dependent Claim
112 words
The method in accordance with claim 3 of using the back projected parallel beam for each angular increment to establish a coordinate grid in the tomographic plane defined by the intersection of the fan beam and the target object, the parallel and fan beams intersecting in a locus of points which defines the diameter of a reconstruction circle which is co-planar with the fan beam and the back projected parallel beam, the coordinate grid defined by the spacing between detectors in the detector array, and geometrically scaling the back projected, scaled signal using the Jacobian of the transformation from the detector array to a line through the center of the reconstruction circle.
Claim 6
Dependent Claim
28 words
The method in accordance with claim 4 wherein the tomographic plane is divided into picture elements, the size and shape of which are determined by the coordinate grid.
Claim 7
Dependent Claim
37 words
The method in accordance with claim 5 of using each back projected parallel beam containing geometrically scaled transforms in the back projected geometry to calculate the linear attenuation coefficient for each picture element of the tomographic plane.
Claim 8
Dependent Claim
19 words
The method in accordance with claim 6 of translating each linear attenuation coefficient into an assigned shade of gray.
Claim 9
Dependent Claim
26 words
The method in accordance with claim 7 of defining a composite image of the tomographic plane by combining the gray shades for all the picture elements.
Claim 10
Dependent Claim
49 words
The method in accordance with claim 1 additionally comprising measuring the shift in the center of rotation for each of the angular increments to correct the value for each increment for errors caused when the central axis of the fan beam does not pass through the center of reconstruction.
Claim 11
Dependent Claim
48 words
The method in accordance with claim 9 wherein an x-ray opaque rod is used as a target object for yielding projection data from which the shift in the center of rotation is derived for each angular increment in terms of the number of detector widths of each shift.
Claim 12
Dependent Claim
47 words
The method in accordance with claim 1 additionally comprising correcting errors of linear displacement of the parallel beam relative to the detectors in the detector array so that a shifted, back projected parallel beam at the edge of the detector array will pass through the target object.
Claim 13
Independent Claim
189 words
A method of back projecting the image of a CT scan comprising the steps of: (a) exposing a target object to an x-ray fan beam; (b) producing a signal representing the intensity of a beam of the x-ray incident upon a detector after passing through the target object; (c) scaling the signal in accordance with the signal which would have been produced by the detector producing the signal had the x-ray incident thereon not passed through the target object; (d) convolving the scaled signal to produce a value representing the signal which would have been produced by the detector had the x-ray beam been a ray comprising a parallel beam, the central axis of which is contiguous and co-linear with the central axis of the fan beam; (e) repeating steps (a)-(d) at a plurality of incremental angles around the target object; and (f) producing a back projected image of the target object by assigning a gray scale value to the attenuation coefficient calculated from the back projected, convolved, scaled signal at each angle for each picture element defined by the intersection of the fan beam and the target object
Claim 14
Independent Claim
189 words
A method of back projecting the image of a CT scan comprising the steps of: (a) exposing a target object to an x-ray fan beam; (b) producing a signal representing the intensity of a beam of the x-ray incident upon a detector after passing through the target object; (c) scaling the signal in accordance with the signal which would have been produced by the detector producing the signal had the x-ray incident thereon not passed through the target object; (d) convolving the scaled signal to produce a value representing the signal which would have been produced by the detector had the x-ray beam been a ray comprising a parallel beam, the central axis of which is contiguous and co-linear with the central axis of the fan beam; (e) repeating steps (a)-(d) at a plurality of incremental angles around the target object; and (f) producing a back projected image of the target object by assigning a gray scale value to the attenuation coefficient calculated from the back projected, convolved, scaled signal at each angle for each picture element defined by the intersection of the fan beam and the target object
Claim 15
Dependent Claim
12 words
The method of claim 12 additionally comprising displaying the back projected image.
Claim 16
Dependent Claim
46 words
The method of claim 12 additionally comprising correcting each picture element using a transformation which is the Jacobian of the transformation from the detector array to the intersection of the fan beam and the target object for magnification caused by the spread of the fan beam.
Claim 17
Independent Claim
261 words
A method of back projecting the image of a CT scan resulting from the rotation of a fan beam source around a target object when the center of rotation changes during rotation comprising the steps of: (a) exposing a target object to an x-ray fan beam; (b) producing a signal representing the intensity of a beam of the x-ray incident upon a detector after passing through the target object; (c) scaling the signal in accordance with the signal which would have been produced by the detector had the x-ray incident thereon not passed through the target object; (d) convolving the scaled signal to produce a vlaue representing the signal which would have been produced by the detector had the x-ray beam been a ray comprising a parallel beam having a central axis which is contiguous and co-linear with the central axis of the fan beam; (e) rotating the source of the fan beam around the target object and repeating steps (a)-(d) at a plurality of incremental angles; (f) measuring any shift in the center of rotation of the fan beam source for the angular increments to correct the convolved, scaled signal for errors caused when the central axis of the fan beam does not pass through the center of a circle of reconstruction which is defined by the co-planar intersections of the parallel and fan beams; and (f) calculating an attenuation coefficient from the corrected signal at the angular increments for each picture element defined by the intersection of the fan beam and the target object in the circle of reconstruction.
Claim 18
Independent Claim
261 words
A method of back projecting the image of a CT scan resulting from the rotation of a fan beam source around a target object when the center of rotation changes during rotation comprising the steps of: (a) exposing a target object to an x-ray fan beam; (b) producing a signal representing the intensity of a beam of the x-ray incident upon a detector after passing through the target object; (c) scaling the signal in accordance with the signal which would have been produced by the detector had the x-ray incident thereon not passed through the target object; (d) convolving the scaled signal to produce a vlaue representing the signal which would have been produced by the detector had the x-ray beam been a ray comprising a parallel beam having a central axis which is contiguous and co-linear with the central axis of the fan beam; (e) rotating the source of the fan beam around the target object and repeating steps (a)-(d) at a plurality of incremental angles; (f) measuring any shift in the center of rotation of the fan beam source for the angular increments to correct the convolved, scaled signal for errors caused when the central axis of the fan beam does not pass through the center of a circle of reconstruction which is defined by the co-planar intersections of the parallel and fan beams; and (f) calculating an attenuation coefficient from the corrected signal at the angular increments for each picture element defined by the intersection of the fan beam and the target object in the circle of reconstruction.
Claim 19
Dependent Claim
64 words
The method of claim 15 wherein a convolved profile is produced for each angular increment by performing a discrete Fourier transform on each scaled signal, multiplying the resulting transforms by increasing values selected from a ramp frequency function to provide scaled transforms for each angular increment, and taking the inverse Fourier transform of each scaled transform to obtain the value representing the parallel beam.
Claim 20
Dependent Claim
98 words
The method of claim 16 wherein the back projected parallel beam for each angular increment is used to establish a coordinate grid in the tomographic plane defined by the intersection of the fan beam and the target object, the parallel and fan beams intersecting in the locus of points which defines the circle of reconstruction, the coordinate grid being defined by the spacing between detectors in a detector array, and geometrically scaling the back projected, scaled signal using the Jacobian of the transformation from the detector array to a line through the center of the circle of reconstruction.
Claim 21
Dependent Claim
20 words
The method of claim 17 wherein the size and shape of the picture elements are determined by the coordinate grid.
Claim 22
Dependent Claim
17 words
The method of claim 18 additionally comprising assigning a gray scale value to each linear attenuation coefficient.
Claim 23
Dependent Claim
35 words
The method of claim 17 wherein an x-ray opaque rod is used as a target object for obtaining projection data from which the shift in the center of rotation is measured at each angular increment.
Claim 24
Dependent Claim
45 words
The method of claim 16 additionally comprising correcting errors of linear displacement of the parallel beam relative to the detectors in the detector array so that a shifted, back projected parallel beam at the edge of the target object will pass through the target object.
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USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1995-07-07 | 1996-01-06 | 1996-07-07 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1999-07-07 | 2000-01-06 | 2000-07-07 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 2003-07-07 | 2004-01-06 | 2004-07-07 | $7700.00 | Window Closed / Lapsed |