Method for reducing image artifacts due to periodic signal variations in NMR imaging
First Claim
1. A method for reducing artifacts in a desired image due to substantially periodic signal variations while a portion of an object is being examined using nuclear magnetic resonance techniques, which techniques include measurement of imaging data about the object portion through the implementation of a plurality of views, each made up of at least one pulse sequence which includes irradiation of the object portion by an RF excitation pulse at the Larmor frequency to produce an NMR signal, application of a pulsed magnetic field gradient along at least one dimensional axis of the object, and acquisition of imaging data, said magnetic field gradient having a parameter value adjustable to have a different value in each view so as to encode spatial information into the NMR signal, said method comprising:
- (a) Selecting a predetermined relationship between the signal variations and the parameter value of said magnetic field gradient, wherein selection of said relationship defines a correspondence between a desired parameter value to be implemented in a given pulse sequence and the phase of said signal variations;
(b) measuring the phase of said signal variations in the course of implementing said plurality of views; and
(c) selecting a non-monotonic temporal order of application of said magnetic field gradient so as to approximate said predetermined relationship, wherein said order is based on said phase measurements, thereby to reduce artifacts in said desired image.
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Abstract
A method for reducing image artifacts due to signal variations in the course of examining a subject using nuclear magnetic resonance (NMR) techniques includes the acquisition of scan data for imaging the object. The scan data is composed of a number of views. The acquisition of each view includes the implementation of a pulse sequence to generate an NMR signal and application of a magnetic gradient along at least one dimensional axis of the object. The magnetic field gradient is characterized by a parameter (e.g., amplitude or direction) adjustable from view to view to encode spatial information into the NMR signal. The parameter value is selected just prior to implementation of the pulse sequence for that view and depends on the phase of the signal variation at that point in time. The final view order depends on the measured phase during the scan. In this manner, the view order can be continuously adjusted to obtain optimum artifact reduction. The method is applicable to various NMR imaging techniques, including Fourier transform and multiple angle projection reconstruction.
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Citations
39 Claims
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1. A method for reducing artifacts in a desired image due to substantially periodic signal variations while a portion of an object is being examined using nuclear magnetic resonance techniques, which techniques include measurement of imaging data about the object portion through the implementation of a plurality of views, each made up of at least one pulse sequence which includes irradiation of the object portion by an RF excitation pulse at the Larmor frequency to produce an NMR signal, application of a pulsed magnetic field gradient along at least one dimensional axis of the object, and acquisition of imaging data, said magnetic field gradient having a parameter value adjustable to have a different value in each view so as to encode spatial information into the NMR signal, said method comprising:
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(a) Selecting a predetermined relationship between the signal variations and the parameter value of said magnetic field gradient, wherein selection of said relationship defines a correspondence between a desired parameter value to be implemented in a given pulse sequence and the phase of said signal variations; (b) measuring the phase of said signal variations in the course of implementing said plurality of views; and (c) selecting a non-monotonic temporal order of application of said magnetic field gradient so as to approximate said predetermined relationship, wherein said order is based on said phase measurements, thereby to reduce artifacts in said desired image. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A method for reducing artifacts in a desired image due to substantially periodic signal variations while a portion of an object is being examined using nuclear magnetic resonance techniques, which techniques include measurement of imaging data about the object portion through the implementation of a plurality of views, each made up of at lease one pulse sequence which includes irradiation of the object portion by an RF excitation pulse at the Larmor frequency to produce an NMR signal, application of a pulsed magnetic field gradient along at least one dimensional axis of the object, and acquisition of imaging data, said magnetic field gradient having a parameter value adjustable to have a different value in each view so as to encode spatial information into the NMR signal, said method comprising:
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(a) Selecting a predetermined relationship between the signal variations and the parameter value of said magnetic field gradient, wherein selection of said relationship defines a correspondence between a desired parameter value to be implemented in a given pulse sequence and the phase of said signal variations, and wherein said relationship is such that when the measurements of imaging data acquired in each pulse sequence are rearranged in order of monotonically increasing parameter value, the signal variations, averaged within each view, for views having more than one pulse sequence as a function of monotonically increasing gradient parameter are at a lower frequency than the actual frequency of the signal variations; (b) measuring the phase of said signal variations in the course of implementing said plurality of views; and (c) selecting a non-monotonic temporal order of application of said magnetic field gradient so as to approximate said predetermined relationship, wherein said order is based on said phase measurements, thereby to reduce artifacts in said desired image. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19)
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20. A method for reducing artifacts in a desired image due to substantially periodic signal variations while a portion of an object is being examined using nuclear magnetic resonance techniques, which techniques include measurement of imaging data about the object portion through the implementation of a plurality of views, each made up of at least one pulse sequence which includes irradiation of the object portion by an RF excitation pulse at the Larmor frequency to produce an NMR signal, application of a pulsed magnetic field gradient along at least one dimensional axis of the object, and acquisition of imaging data, said magnetic field gradient having a parameter value adjustable to have a different value in each view so as to encode spatial information into the NMR signal, said method comprising:
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(a) Selecting a predetermined relationship between the signal variations and the parameter value of said magnetic field gradient, wherein selection of said relationship defines a correspondence between a desired parameter value to be implemented in a given pulse sequence and the phase of said signal variations, and wherein said relationship is such that when the measurements of imaging data acquired in each pulse sequence are rearranged in order of monotonically increasing parameter value, the signal variations, averaged within each view, for views with more than one pulse sequence, as a function of gradient parameter are at a higher frequency than the actual frequency of the signal variations; (b) measuring the phase of said signal variations in the course of implementing said plurality of views; and (c) selecting a non-monotonic temporal order of application of said magnetic field gradient so as to approximate said predetermined relationship, wherein said order is based on said phase measurements, thereby to reduce artifacts in said desired image. - View Dependent Claims (21, 22, 23, 24, 25, 26, 27, 32)
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28. A method for reducing artifacts in a desired image due to substantially periodic signal variations while a portion of an object is being examined using nuclear magnetic resonance techniques, which techniques include measurement of a set of imaging data about the object portion through the implementation of a plurality of views, each made up of at least one pulse sequence which includes irradiation of the object portion by an RF excitation pulse at the Larmor frequency to produce an NMR signal, application of a pulsed magnetic field gradient along at least one dimensional axis of the object, and acquisition of imaging data, said magnetic field gradient having a parameter value adjustable to have a different value in each view so as to encode spatial information into the NMR signal, said method comprising:
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(a) Selecting a predetermined relationship between the signal variations and the parameter value of said magnetic field gradient, wherein selection of said relationship defines a correspondence between a desired parameter value to be implemented in a given pulse sequence and the phase of said signal variations; (b) measuring the phase of said signal variations in the course of implementing said plurality of views; (c) selecting a non-monotonic temporal order of application of said magnetic field gradient so as to approximate said predetermined relationship, wherein said order is based on said phase measurements; and (d) selecting a subset of the imaging data acquired as a result of implementing said pulse sequences, said subset being smaller than the full set of imaging data, wherein the pulse sequences from which imaging data measurements are included in said subset are selected so as to more closely approximate said predetermined relationship, said subset being sufficient for constructing said desired image, thereby to reduce artifacts in said desired image. - View Dependent Claims (29, 30, 31, 33, 34, 35, 36, 37, 38, 39)
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Specification