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US4670840A
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Ring artifact correction for computerized tomography
Assignee: Philips Nuclear Medicine Inc
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Filing: 1986-03-26
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Granted: 1987-06-02
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Expires: 2006-03-26
Abstract
Abstract A method for the correction of ring artifacts in images obtained by rotate-rotate computerized tomography (CT) scanners includes the step of reconstructing an original image, which comprises a real image and a ring artifact image. The ring artifact image is determined by edge enhancing the original image and dividing the edge enhanced original image into rings or annuli with the annuli being concentric around the axis of rotation. The average intensity is determined for each concentric annulus. This average value of each annulus is substracted from pixels of the original image belonging to that annulus to provide a ring artifact free image.
Inventors
- 👤 David A. Freundlich
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-23 21:57
CPC Classifications
G; G06; G06T; G06T12/00; G06T12/30; H; H05; H05G; H05G1/00; H05G1/08; H05G1/60; G06T2211/00; G06T2211/40; G06T2211/412; Y; Y10; Y10S; Y10S378/00; Y10S378/901
Claims Analysis
Total Claims: 12
Claim 1
Independent Claim
202 words
A method for the correction of ring artifacts in images of an object obtained by a rotate-rotate computerized tomography (CT) scanner, said method comprising the steps of: (a) reconstructing an original image including ring artifacts, said original image comprising a real image without ring artifacts and a ring artifact image; (b) determining the ring artifact image by the steps of: (i) edge enhancing the original image to obtain an edge enhanced original image; (ii) dividing the edge enhanced original image into specified areas, said specified areas being concentric annuli which are centered about a point in the original image corresponding to the axis of rotation of the scanner; (iii) said step of dividing the image into concentric annuli comprising assigning each pixel of the enhanced original image to a specific one of said concentric annuli, said concentric annuli conforming to the ring artifacts; (iv) determining the average intensity of each concentric ring area of the said enhanced image to provide an average ring artifact pixel intensity value for each annulus; (c) subtracting the average ring artifact pixel intensity value of each annulus from each pixel of the original image that belongs to the same annulus to thereby obtain a ring artifact-free image.
Claim 2
Independent Claim
202 words
A method for the correction of ring artifacts in images of an object obtained by a rotate-rotate computerized tomography (CT) scanner, said method comprising the steps of: (a) reconstructing an original image including ring artifacts, said original image comprising a real image without ring artifacts and a ring artifact image; (b) determining the ring artifact image by the steps of: (i) edge enhancing the original image to obtain an edge enhanced original image; (ii) dividing the edge enhanced original image into specified areas, said specified areas being concentric annuli which are centered about a point in the original image corresponding to the axis of rotation of the scanner; (iii) said step of dividing the image into concentric annuli comprising assigning each pixel of the enhanced original image to a specific one of said concentric annuli, said concentric annuli conforming to the ring artifacts; (iv) determining the average intensity of each concentric ring area of the said enhanced image to provide an average ring artifact pixel intensity value for each annulus; (c) subtracting the average ring artifact pixel intensity value of each annulus from each pixel of the original image that belongs to the same annulus to thereby obtain a ring artifact-free image.
Claim 3
Dependent Claim
18 words
The method of claim 1 including the step of modifying the original image to avoid high edge values.
Claim 4
Dependent Claim
67 words
The method of claim 2 wherein the step of modifying the original image comprises the steps of obtaining a modified image in accordance with the following equations: p*(i,j)=p(i,j) for t2 p(i,j) t1; p*(i,j)=t2 for t2 p(i,j); and p*(i,j)=t1 for t1 p(i,j); where: p* (i,j) are pixel values of the modified image, p (i,j) are pixel values of the original image, and t1 and t2 are threshold values.
Claim 5
Dependent Claim
96 words
The method of claim 1 wherein the original image is divided into pixels having intensity values equal to q(i,j) and wherein the edge enhancement step includes the step of: (a) generating values q(i,j) for the pixel values p(i,j) using the following equation: q(i,j)=Ap(i,j)+B avn(p(i,j))+C where: A and B are constants, usually approximately +1 and -1 p(i,j) is the original intensity value of the pixel (i,j), avn (p(i,j)) is the average intensity of nxn pixels centered at (i,j), i,j are location determinants, n is any position integer number, and C is a constant usually equal to 0.
Claim 6
Dependent Claim
111 words
The method of claim 4 wherein the average ring artifact pixel intensity value for each ring annulus is calculated according to the following steps: (a) determining the average intensity av(Rm) of the edge enhanced pixels, q(i,j) belonging to each ring annulus that are further defined as being within threshold values such that: C+T1<q(i,j)(C-T2) (b) determining ring artifact values r(Rm) for the pixels within each annulus (Rm) such that: r(Rm)=av(Rm)-av where: av is the average value of pixels having the threshold limited values, q(i,j) in some defined area of the image such as the complete image, T1 and T2 are threshold values, and Rm, is a location determinant relating to ring numbers.
Claim 7
Dependent Claim
71 words
The method of claim 1 wherein the step of dividing the edge enhanced image into specified areas comprises futher dividing each of the concentric annuli into adjoining radial sectors, determining the average ring artifact intensity value for each radial sector, and subtracting the average ring artifact intensity value of each sector from each pixel of the original image belonging to the same radial sector to thereby obtain a ring artifact-free image.
Claim 8
Dependent Claim
127 words
The method of claim 6 wherein the average ring artifact pixel intensity value for each radial sector is calculated according to the following steps: (a) determining the average intensity av(Rm,Sn) of the edge enhanced pixels, q(i,j) belonging to each of said radial sectors that are further defined as being within threshold values such that: C+T1<q(i,j)(C-T2) (b) determining ring artifact values r(Rm,Sn) for the pixels within each of the sectors (Rm, Sn) such that: r(Rm,Sn)=av(Rm,Sn)-av where: C is a constant usually equal to zero, av is the average value of pixels having the threshold limited to values q(i,j) in some defined area of the image such as the complete image, T1 and T2 are threshold values, and Rm,Sn are location determinants relating to ring numbers and sector numbers.
Claim 9
Dependent Claim
51 words
The method of claim 6 including the steps of: a. obtaining correction values r(Rm,Sn) for each pixel of the ring artifact image, b. modifying the correction values r(Rm,Sn) to provide smooth changes in values when the pixel is near the border of another sector, such that the correction values becomes b(Rm)=r(Rm,Sn)+r(Rm,Sn-1)/2
Claim 10
Dependent Claim
35 words
The method of claim 8 wherein the real image free of ring artifacts is obtained from pixels p (i,j) in accordance with the following equations: p(i,j)=p(i,j)-b(Rm) where p(i,j) defines the pixels of the original image.
Claim 11
Dependent Claim
22 words
The method of claim 6 including the step of using a smoothing function on the pixels proximate to the adjoining radial sectors.
Claim 12
Dependent Claim
25 words
The method of claim 6 including the step of modifying the original image prior to the step of edge enhancement to avoid high edge values.
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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) | 1990-06-01 | 1990-12-01 | 1991-06-02 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1994-06-01 | 1994-12-01 | 1995-06-02 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 1998-06-01 | 1998-12-01 | 1999-06-02 | $7700.00 | Window Closed / Lapsed |