US5285157A
Expired / Lapsed
US
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Reduction of truncation caused artifacts
Assignee: General Electric Co
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Filing: 1992-04-27
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Granted: 1994-02-08
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Expires: 2012-04-27
Abstract
Abstract Elimination of truncation artifacts in Magnetic Resonance Images is accomplished by analaytically optimizing controllable parameters; i.e., filter characteristic and assymetry ratio when asymetrically sampling arid time domain filtering followed by complex conjugating the filtered data to reduce the Gibbs artifact.
Inventors
- 👤 Yuval Zur
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-11-09 19:19
CPC Classifications
G; G01; G01R; G01R33/00; G01R33/20; G01R33/44; G01R33/48; G01R33/54; G01R33/56; G01R33/565; G01R33/56545; G06; G06T; G06T5/00; G06T5/10; G06T5/70; G06T2207/00; G06T2207/10; G06T2207/10072; G06T2207/10088; G06T2207/20; G06T2207/20048; G06T2207/20056
Claims Analysis
Total Claims: 12
Claim 1
Independent Claim
335 words
A method for improving an acquired magnetic resonance image (MRI) of an object by reducing overshoot in the image which is visible as a ringing artifact, said overshoot being caused by Fourier transforming, for a finite amount of time, data obtained by sampling a signal in the time domain at a number of sampling points, said signal containing frequency and phase information of said imaged object; said method comprising the steps of: a) choosing a filter having filter coefficients; b) analytically determining an optimized value of a which controls a filter characteristics, such as the shape of the filter, and an optimized value of R which is an asymmetrical sampling ratio in order to reduce the overshoot while maintaining the spatial resolution and signal-to-noise ratio (SNR) of the image without changing the number of sampling points, where R is equal to: R=(N.sub.A -N.sub.S)/N.sub.A =(N2/2)/N.sub.A wherein N A =total number of sampling points after complex conjugation to cause symmetrical sampling, and N S =total number of sampling points in asymmetrical sampling; c) asymmetrically sampling said acquired signals to obtain a two dimensional time domain matrix of data containing rows and columns; d) Fourier transforming the obtained data matrix along the column direction; e) filtering by multiplying said transformed data matrix by said filter coefficients in the row direction in order to provide filtered transformed data, that: (1) reduces the overshoot; (2) degrades the resolution; and (3) increases the signal-to-noise ratio; f) obtaining symmetrical data by complex conjugation of the filtered data matrix along the row direction which: (1) increases the amount of data and consequently improves the resolution, (2) compresses the overshoot causing the ringing artifacts, and (3) decreases the signal-to-noise ratio; g) Fourier transforming the symmetrical data in the row direction to obtain the image data; and h) processing the image data to obtain an image over the same number of scanned points in a comparable scan and processing time with reduced ringing artifacts and with relative resolution and relative signal-to-noise substantially equal to unity.
Claim 2
Independent Claim
335 words
A method for improving an acquired magnetic resonance image (MRI) of an object by reducing overshoot in the image which is visible as a ringing artifact, said overshoot being caused by Fourier transforming, for a finite amount of time, data obtained by sampling a signal in the time domain at a number of sampling points, said signal containing frequency and phase information of said imaged object; said method comprising the steps of: a) choosing a filter having filter coefficients; b) analytically determining an optimized value of a which controls a filter characteristics, such as the shape of the filter, and an optimized value of R which is an asymmetrical sampling ratio in order to reduce the overshoot while maintaining the spatial resolution and signal-to-noise ratio (SNR) of the image without changing the number of sampling points, where R is equal to: R=(N.sub.A -N.sub.S)/N.sub.A =(N2/2)/N.sub.A wherein N A =total number of sampling points after complex conjugation to cause symmetrical sampling, and N S =total number of sampling points in asymmetrical sampling; c) asymmetrically sampling said acquired signals to obtain a two dimensional time domain matrix of data containing rows and columns; d) Fourier transforming the obtained data matrix along the column direction; e) filtering by multiplying said transformed data matrix by said filter coefficients in the row direction in order to provide filtered transformed data, that: (1) reduces the overshoot; (2) degrades the resolution; and (3) increases the signal-to-noise ratio; f) obtaining symmetrical data by complex conjugation of the filtered data matrix along the row direction which: (1) increases the amount of data and consequently improves the resolution, (2) compresses the overshoot causing the ringing artifacts, and (3) decreases the signal-to-noise ratio; g) Fourier transforming the symmetrical data in the row direction to obtain the image data; and h) processing the image data to obtain an image over the same number of scanned points in a comparable scan and processing time with reduced ringing artifacts and with relative resolution and relative signal-to-noise substantially equal to unity.
Claim 3
Dependent Claim
12 words
The method of claim 1 wherein steps d) and e) are interchanged.
Claim 4
Dependent Claim
90 words
The method of claim 1 wherein the step of analytically determining optimized values of controllable parameters α and R comprising the steps of: a) generating a table for each value of R that relates attenuation A between 22 and 50 db of the overshoot (by the filter) to: filter characteristics α, relative signal-to-noise ratio and relative width of step response α; and b) selecting the values of α and R from the tables to obtain the maximum attenuation of the overshoot while maintaining γ and β substantially equal to one.
Claim 5
Dependent Claim
90 words
The method of claim 3 including the step of determining the relative resolution γ from the relative width of step response of the filtered data as compared to the step response of the unfiltered data using the equation: ##EQU8## where: D' is the frequency width for the step response of the filter data, D O is the frequency width for the step response of the unfiltered data, and R is the sampling symmetry; i.e., the number of sampling points added by complex conjugation divided by the total number of points.
Claim 6
Dependent Claim
81 words
The method of claim 3 wherein the relative signal-to-noise ratio is calculated using the equation: ##EQU9## where: N A is the number of asymmetrical sampling, N Z is the number of sampling points acquired using symmetrical or asymmetrical sampling, N Z /2 is the number of sampling points added by complex conjugation, and f k is the filter function for each N A point, where: k=-(N.sub.A -1)/2 . . . 0 . . . +(N.sub.A -1)/2 and f.sub.k =1 for k=0.
Claim 7
Dependent Claim
33 words
The method of claim 4 where the step function applied to determine the step response is: ##EQU10## where F T (f') is the Fourier transform of the time domain filter f K .
Claim 8
Dependent Claim
11 words
The method of claim 3 wherein a Kaiser filter is used.
Claim 9
Dependent Claim
35 words
The method of claim 7 including the step of analytically determining the characteristic α relative to the attenuation of the filter by using the equation: α=0.5842(A-21).sup.0.4 +0.07886(A-21) for attenuations A between 21 and 50 db.
Claim 10
Dependent Claim
44 words
The method of claim 1 wherein the step of multiplying the acquired signals by a filter comprises multiply k sampled points by f k where k=-(N A -1)/2 . . . 0 . . . (N A -1)/2 and f k =1 for k=O.
Claim 11
Dependent Claim
16 words
The method of claim 1 where the asymmetrical sampling is accomplished in the phase encoding direction.
Claim 12
Dependent Claim
24 words
The method of claim 1 where the asymmetrical sampling is accomplished in the frequency encoding direction which are the rows of the data matrix.
References Cited (Backward Citations)
USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1997-02-07 | 1997-08-09 | 1998-02-08 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 2001-02-07 | 2001-08-09 | 2002-02-08 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 2005-02-07 | 2005-08-09 | 2006-02-08 | $7700.00 | Window Closed / Lapsed |