Patent US2308391A successfully refreshed from Google Patents.
EP0301629A1
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Method and apparatus for carrying out a phase correction in MR angiography
Assignee: Koninklijke Philips NV
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Filing: 1988-07-11
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Granted: —
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Expires: 2008-07-11
Abstract
Abstract A method of obtaining with MR angiography an angiogram of a body is based on the formation of a difference image of two images produced by different gradient wave forms of the body. The gradient wave forms are chosen so that essentially in a difference image to be obtained from the two images the signals originating from excited stationary spin nuclei are not present and the signals originating from moving spin nuclei - due to flow in blood vessels - form the angiogram. The problem then arising is that due to the different gradient wave forms when forming signals for the two images different eddy currents, which flow in metal parts of the apparatus, exert their influence. The two images of the body contain a phase error induced by the eddy currents. This phase error varies smoothly over the image of the body. This is the case because the body is located at a comparatively large distance from the fields produced by the eddy currents in the metal parts of the apparatus. Due to the different eddy currents, signals originating from stationary spin nuclei are not fully eliminated in the difference image, as a result of which the angiogram contains signals originating from stationary tissue. The signals originating from flowing blood yield a deviating phase at the area of a vessel. A method is proposed of eliminating the signals in the angiogram originating from stationary tissue. By estimating the smoothly varying phase difference before forming the difference image from the two images and by thus carrying out a non-linear phase correction, the smoothly varying phase difference is eliminated. The difference image no longer contains substantially any signals originating from stationary tissue.
Inventors
- 👤 Josephus Jacobus Egidius In Den Kleef
- 👤 Johannes Petrus Groen
- 👤 Rudolf Gillis De Graaf
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-10-23 12:55
CPC Classifications
G; G01; G01R; G01R33/00; G01R33/20; G01R33/44; G01R33/48; G01R33/54; G01R33/56; G01R33/563; G01R33/565; G01R33/56518
Claims Analysis
Total Claims: 16
Claim 1
Independent Claim
239 words
A method of determining a nuclear magnetization distribution or a distribution derived therefrom by subtraction of image information from images originating from a first and a second collection of resonance signals produced in a body, which is located in a stationary homogeneous magnetic field, at least comprising a first cycle for producing the first collection of resonance signals, which comprises at least one high-frequency electromagnetic pulse for excitation of spin nuclei in the body and at least one magnetic field gradient superimposed on the homogeneous magnetic field, which first cycle is repeated several times whilst varying the strength and/or duration of at least one magnetic field gradient, and at least comprising a second cycle for producing the second collection of resonance signals, which second cycle is repeated several times and differs from the first cycle in at least one gradient wave form for differently influencing in the first and in the second cycle moving spin nuclei and for influencing substantially equally stationary spin nuclei in the body, the first and the second complex image being formed by Fourier transformation of respective sample values of the respective resonance signals, characterized in that, before the subtraction of the image information, a smoothly varying contribution to the phase difference between the first and the second image occurring due to the difference in gradient wave forms is eliminated by means of a phase correction utilizing an estimation of the smoothly varying phase difference.
Claim 2
Independent Claim
239 words
A method of determining a nuclear magnetization distribution or a distribution derived therefrom by subtraction of image information from images originating from a first and a second collection of resonance signals produced in a body, which is located in a stationary homogeneous magnetic field, at least comprising a first cycle for producing the first collection of resonance signals, which comprises at least one high-frequency electromagnetic pulse for excitation of spin nuclei in the body and at least one magnetic field gradient superimposed on the homogeneous magnetic field, which first cycle is repeated several times whilst varying the strength and/or duration of at least one magnetic field gradient, and at least comprising a second cycle for producing the second collection of resonance signals, which second cycle is repeated several times and differs from the first cycle in at least one gradient wave form for differently influencing in the first and in the second cycle moving spin nuclei and for influencing substantially equally stationary spin nuclei in the body, the first and the second complex image being formed by Fourier transformation of respective sample values of the respective resonance signals, characterized in that, before the subtraction of the image information, a smoothly varying contribution to the phase difference between the first and the second image occurring due to the difference in gradient wave forms is eliminated by means of a phase correction utilizing an estimation of the smoothly varying phase difference.
Claim 3
Dependent Claim
83 words
A method as claimed in Claim 1, characterized in that a non-linear phase correction is carried out by developing the smoothly varying contribution to the phase difference between the first and the second image into a series of suitably chosen base functions, of which coefficients are determined by carrying out a weighted least squares optimization procedure for minimizing a smoothly varying phase difference estimated with the series and the smoothly varying contribution to the phase difference between the first and the second image.
Claim 4
Dependent Claim
94 words
A method as claimed in Claim 1, characterized in that the non-linear phase correction is carried out by developing a function of the smoothly varying contribution to the phase difference of the first and the second image into a series of suitably chosen base functions, of which coefficients are determined by carrying out a weighted least-squares optimization procedure for minimizing the difference between a function of the smoothly varying phase difference estimated with the series and a function of the smoothly varying contribution to the phase difference between the first and the second image.
Claim 5
Dependent Claim
23 words
A method as claimed in Claim 3, characterized in that the function is a complex exponential function of the smoothly varying phase difference.
Claim 6
Dependent Claim
36 words
A method as claimed in Claim 2, 3 or 4, characterized in that in the optimization procedure weighting factors are used which depend upon the moduli of respective pixels in the first and the second image.
Claim 7
Dependent Claim
47 words
A method as claimed in Claim 2, 3, 4 or 5, characterized in that in the optimization procedure such weighting factors are chosen that the disturbing influence on the phase correction in vexels having a phase influenced by moving spin nuclei is eliminated for the major part.
Claim 8
Dependent Claim
21 words
A method as claimed in Claim 2, 3, 4, 5 or 6, characterized in that the base functions are Chebyshev polynomes.
Claim 9
Dependent Claim
17 words
A method as claimed in Claim 1, characterized in that a linear phase correction is carried out.
Claim 10
Independent Claim
26 words
A method as claimed in any one of the preceding Claims, characterized in that the linear phase correction is carried out before the non-linear phase correction.
Claim 11
Independent Claim
39 words
A method as claimed in any one of the preceding Claims, characterized in that the image information of the image obtained by subtraction is formed by the moduli of the complex image values in the pixels of the image.
Claim 12
Independent Claim
40 words
A method as claimed in any one of Claims 1 to 9, characterized in that the image information of the image obtained by subtraction is formed by phase differences in the pixels of the first and the second complex image.
Claim 13
Independent Claim
33 words
A method as claimed in any one of the preceding Claims, characterized in that upon variation of the strength of the magnetic preparation gradient the first and second cycles are alternately carried out.
Claim 14
Independent Claim
159 words
An apparatus for determining a projective nuclear magnetization distribution of a body, which apparatus comprises means for producing a stationary homogeneous magnetic field, means for producing high-frequency electromagnetic pulses, means for producing magnetic field gradients, control means for the aforementioned means and processing means which are provided with programmed calculation means for forming complex images from sample values, which are obtained by sampling means from resonance signals produced in cycles in the body, the processing means further being provided with programmed calculation means for forming a nuclear magnetization distribution by subtraction of image information from images obtained whilst applying magnetic field gradients having different gradient wave forms, characterized in that the processing means are further provided with programmed calculation means for eliminating before subtraction of the image information a smoothly varying contribution to the phase difference between the first and the second image occurring due to the difference in the gradient wave forms by means of a phase correction.
Claim 15
Independent Claim
159 words
An apparatus for determining a projective nuclear magnetization distribution of a body, which apparatus comprises means for producing a stationary homogeneous magnetic field, means for producing high-frequency electromagnetic pulses, means for producing magnetic field gradients, control means for the aforementioned means and processing means which are provided with programmed calculation means for forming complex images from sample values, which are obtained by sampling means from resonance signals produced in cycles in the body, the processing means further being provided with programmed calculation means for forming a nuclear magnetization distribution by subtraction of image information from images obtained whilst applying magnetic field gradients having different gradient wave forms, characterized in that the processing means are further provided with programmed calculation means for eliminating before subtraction of the image information a smoothly varying contribution to the phase difference between the first and the second image occurring due to the difference in the gradient wave forms by means of a phase correction.
Claim 16
Dependent Claim
85 words
An apparatus as claimed in Claim 13, characterized in that the processing means are further provided with programmed calculation means for developing the smoothly varying contribution to the phase difference in series of base functions present in the programmed calculation means and for determining coefficients of the series by means of a least-squares optimization procedure contained in the programmed calculation means for minimizing the difference between the smoothly varying contribution to the phase difference and a smoothly varying phase difference estimated with the base functions.
References Cited (Backward Citations)
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