EP0098479A2
Expired / Lapsed
EP
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Elimination of induction decay in NMR imaging by phase alternation
Assignee: General Electric Co
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Filing: 1983-06-24
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Granted: —
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Expires: 2003-06-24
Abstract
Abstract The effects of a spurious free induction decay (FID) NMR signal due to imperfect 180° RF pulses are eliminated by phase alternating the selective 90° RF pulses and subtracting alternate ones of the NMR signals. The desired signals due to the 90° RF pulses reinforce, while the signals produced by the imperfect 180° pulses cancel. This method also has the beneficial effect of eliminating DC offset in the desired signal which, when present, causes artifacts in NMR images. Another method to eliminate the effects of spurious FID signals due to imperfect 180° pulses is to phase alternate these 180° pulses on successive NMR imaging pulse sequences and add successive NMR signals. In this case, the phase shifted spurious FID signals cancel, while the desired NMR signals reinforce. The latter method does not eliminate the effects of DC offset, however.
Inventors
- 👤 William Alan Edelstein
- 👤 Paul Arthur Bottomley
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-11-04 07:51
CPC Classifications
G; G01; G01R; G01R33/00; G01R33/20; G01R33/44; G01R33/48; G01R33/54; G01R33/56; G01R33/565
Claims Analysis
Total Claims: 24
Claim 1
Independent Claim
251 words
A method for overcoming the effects of a spurious FID NMR signals produced by imperfect 180° RF pulses so as to avoid interference between the FID signal and the desired NMR spin echo signal, said method comprising the steps of: a. maintaining a static magnetic field along a first axis of an NMR imaging sample; b. irradiating said sample with a first R F pulse during a first predetermined time interval, so as to excite a first plurality of nuclear spins in said sample; c. irradiating said imaging sample during a second time interval with a 180° RF pulse, whereupon inherent imperfections in said 180° RF pulse cause a second plurality of nuclear spins in said imaging sample to change orientation by an angle other than 180° relative to the direction of said static magntic field, said last-mentioned nuclear spins producing a spurious FID NMR signal upon termination of said 180° RD pulse; d. sampling an NMR signal composed of a first component produced by said first plurality of spins and a second component produced by said second plurality of nuclear spins in a third predetermined time interval subsequent to the application of said 180" RF pulse; and e. repeating steps (a-d) while shifting the phase of alternate ones of said first RF pulses so as to produce a 180° shift in the corresponding NMR signal produced by said first plurality of nuclear spins, such that when alternate ones of said sampled NMR signals are subtracted said spurious FID signals cancel.
Claim 2
Independent Claim
251 words
A method for overcoming the effects of a spurious FID NMR signals produced by imperfect 180° RF pulses so as to avoid interference between the FID signal and the desired NMR spin echo signal, said method comprising the steps of: a. maintaining a static magnetic field along a first axis of an NMR imaging sample; b. irradiating said sample with a first R F pulse during a first predetermined time interval, so as to excite a first plurality of nuclear spins in said sample; c. irradiating said imaging sample during a second time interval with a 180° RF pulse, whereupon inherent imperfections in said 180° RF pulse cause a second plurality of nuclear spins in said imaging sample to change orientation by an angle other than 180° relative to the direction of said static magntic field, said last-mentioned nuclear spins producing a spurious FID NMR signal upon termination of said 180° RD pulse; d. sampling an NMR signal composed of a first component produced by said first plurality of spins and a second component produced by said second plurality of nuclear spins in a third predetermined time interval subsequent to the application of said 180" RF pulse; and e. repeating steps (a-d) while shifting the phase of alternate ones of said first RF pulses so as to produce a 180° shift in the corresponding NMR signal produced by said first plurality of nuclear spins, such that when alternate ones of said sampled NMR signals are subtracted said spurious FID signals cancel.
Claim 3
Independent Claim
324 words
A method for overcoming the effects of a spurious FID NMR signal so as to avoid interference thereof with the desired NMR spin echo signal, said method comprising sequentially the steps of: a. maintaining a static magnetic field along a first axis of an NMR imaging sample: b. selectively exciting, during a first predetermined time interval, a first plurality of nuclear spins in a planar section of said imaging sample by irradiating said imaging sample with a selective RF pulse in the presence of a first magnetic field gradient pulse; c. applying for a second predetermined time interval at least one dephasing magnetic field gradient pulse along a second axis of said imaging sample to dephase said excited nuclear spins; d. irradiating said imaging sample during a third time interval with a 180" RF pulse so as to initiate the rephasing of said excited nuclear spins, whereupon inherent imperfections in said 180 0 RF pulse cause a second plurality of nuclear spins in said imaging sample to change orientation by an angle other than 180° relative to the direction of said static magnetic field, said last-mentioned nuclear spins producing a net nuclear magnetization component transverse to the direction of said static magnetic field, which component produces a spurious FID NMR signal upon termination of said 180° RF pulse; e. applying during a fourth predetermined time interval at least one imaging gradient having the same direction as said dephasing gradient such that a nuclear spin echo signal is produced by said rephasing of said excited nuclear spins; f. sampling said spin echo signal in the presence of said imaging gradient; and g. repeating steps (b-f) while shifting the phase of alternate ones of the selective RF pulses by 180° so as to produce a 180° shift in the corresponding spin echo signals such that, when alternate ones of said spin echo signals are subtracted, said spin echo signals reinforce while said spurious FID signals cancel.
Claim 4
Independent Claim
324 words
A method for overcoming the effects of a spurious FID NMR signal so as to avoid interference thereof with the desired NMR spin echo signal, said method comprising sequentially the steps of: a. maintaining a static magnetic field along a first axis of an NMR imaging sample: b. selectively exciting, during a first predetermined time interval, a first plurality of nuclear spins in a planar section of said imaging sample by irradiating said imaging sample with a selective RF pulse in the presence of a first magnetic field gradient pulse; c. applying for a second predetermined time interval at least one dephasing magnetic field gradient pulse along a second axis of said imaging sample to dephase said excited nuclear spins; d. irradiating said imaging sample during a third time interval with a 180" RF pulse so as to initiate the rephasing of said excited nuclear spins, whereupon inherent imperfections in said 180 0 RF pulse cause a second plurality of nuclear spins in said imaging sample to change orientation by an angle other than 180° relative to the direction of said static magnetic field, said last-mentioned nuclear spins producing a net nuclear magnetization component transverse to the direction of said static magnetic field, which component produces a spurious FID NMR signal upon termination of said 180° RF pulse; e. applying during a fourth predetermined time interval at least one imaging gradient having the same direction as said dephasing gradient such that a nuclear spin echo signal is produced by said rephasing of said excited nuclear spins; f. sampling said spin echo signal in the presence of said imaging gradient; and g. repeating steps (b-f) while shifting the phase of alternate ones of the selective RF pulses by 180° so as to produce a 180° shift in the corresponding spin echo signals such that, when alternate ones of said spin echo signals are subtracted, said spin echo signals reinforce while said spurious FID signals cancel.
Claim 5
Independent Claim
252 words
A method for overcoming the effects of a spurious FID NMR signals produced by imperfect 180° RF pulses so as to avoid interference between the FID signal and the desired NMR spin echo signal, said method comprising sequentially the steps of; a. maintaining a static magnetic field along a first axis of an NMR imaging sample; b. irradiating said sample with a first RF pulse during a first predetermined time interval, so as to excite a first plurality of nuclear spins in said sample; c. irradiating said imaging sample during a second time interval with a 180° RF pulse, whereupon inherent imperfections in said 180° RF pulse cause a second plurality of nuclear spins in said imaging sample to change orientation by an angle other than 180° relative to the direction of said static magnetic field, said last-mentioned nuclear spins producing a spurious FID NMR signal upon termination of said 180 0 RF pulse; d. sampling an NMR signal composed of a first component produced by said first plurality of spins and a second component produced by said second plurality of nuclear spins in a third predetermined time interval subsequent to the application of said 180° RF pulse; and e. repeating steps (a-d) while shifting the phase of alternate ones of said 180° RF pulses so as to produce a 180 0 phase shift in the said FID NMR signal produced by said second plurality of nuclear spins, such that when said samples NMR signals are added, said spurious FID signals cancel.
Claim 6
Independent Claim
252 words
A method for overcoming the effects of a spurious FID NMR signals produced by imperfect 180° RF pulses so as to avoid interference between the FID signal and the desired NMR spin echo signal, said method comprising sequentially the steps of; a. maintaining a static magnetic field along a first axis of an NMR imaging sample; b. irradiating said sample with a first RF pulse during a first predetermined time interval, so as to excite a first plurality of nuclear spins in said sample; c. irradiating said imaging sample during a second time interval with a 180° RF pulse, whereupon inherent imperfections in said 180° RF pulse cause a second plurality of nuclear spins in said imaging sample to change orientation by an angle other than 180° relative to the direction of said static magnetic field, said last-mentioned nuclear spins producing a spurious FID NMR signal upon termination of said 180 0 RF pulse; d. sampling an NMR signal composed of a first component produced by said first plurality of spins and a second component produced by said second plurality of nuclear spins in a third predetermined time interval subsequent to the application of said 180° RF pulse; and e. repeating steps (a-d) while shifting the phase of alternate ones of said 180° RF pulses so as to produce a 180 0 phase shift in the said FID NMR signal produced by said second plurality of nuclear spins, such that when said samples NMR signals are added, said spurious FID signals cancel.
Claim 7
Independent Claim
321 words
A method for overcoming the effects of a spurious FID NMR signal so as to avoid interference thereof with the desired NMR spin echo signal, said method comprising sequentially the steps of: a. maintaining a static magnetic field along a first axis of an NMR imaging sample; : b. selectively exciting, during a first predetermined time interval, a first plurality of nuclear spins in a planar section of said imaging sample by irradiating said imaging sample with a selective RF pulse in the presence of a first magnetic field gradient pulse; c. applying for a second predetermined time interval at least one dephasing magnetic field gradient pulse along a second axis of said imaging sample to dephase said excited nuclear spins; d. irradiating said imaging sample during a third time interval with a 180° RF pulse so as to initiate the rephasing of said excited nuclear spins, whereupon inherent imperfections in said 180° RF pulse cause a second plurality of nuclear spins in said imaging sample to change orientation by an angle other than 180° relative to the direction of said static magnetic field, said last-mentioned nuclear spins producing a net nuclear magnetization component transverse to the direction of said static magnetic field, which component produces a spurious FID NMR signal upon termination of said 180° RF pulse; e. applying during a fourth predetermined time interval at least one imaging gradient having the same direction as said dephasing gradient such that a nuclear spin echo signal is produced by said rephasing of said excited nuclear spins; f. sampling said spin echo signal in the presence of said imaging gradient; and g. repeating steps (a-d) while shifting the phase of alternate ones of the 180° RF pulses by 180° so as to produce a 180° shift in the corresponding spurious FID signals such that, when successive spin echo signals are added said spin echo signals reinforce while said spurious FID signals cancel.
Claim 8
Independent Claim
321 words
A method for overcoming the effects of a spurious FID NMR signal so as to avoid interference thereof with the desired NMR spin echo signal, said method comprising sequentially the steps of: a. maintaining a static magnetic field along a first axis of an NMR imaging sample; : b. selectively exciting, during a first predetermined time interval, a first plurality of nuclear spins in a planar section of said imaging sample by irradiating said imaging sample with a selective RF pulse in the presence of a first magnetic field gradient pulse; c. applying for a second predetermined time interval at least one dephasing magnetic field gradient pulse along a second axis of said imaging sample to dephase said excited nuclear spins; d. irradiating said imaging sample during a third time interval with a 180° RF pulse so as to initiate the rephasing of said excited nuclear spins, whereupon inherent imperfections in said 180° RF pulse cause a second plurality of nuclear spins in said imaging sample to change orientation by an angle other than 180° relative to the direction of said static magnetic field, said last-mentioned nuclear spins producing a net nuclear magnetization component transverse to the direction of said static magnetic field, which component produces a spurious FID NMR signal upon termination of said 180° RF pulse; e. applying during a fourth predetermined time interval at least one imaging gradient having the same direction as said dephasing gradient such that a nuclear spin echo signal is produced by said rephasing of said excited nuclear spins; f. sampling said spin echo signal in the presence of said imaging gradient; and g. repeating steps (a-d) while shifting the phase of alternate ones of the 180° RF pulses by 180° so as to produce a 180° shift in the corresponding spurious FID signals such that, when successive spin echo signals are added said spin echo signals reinforce while said spurious FID signals cancel.
Claim 9
Dependent Claim
148 words
A method according to claim 1 or 2 or 3 or 4 wherein in step (c) said dephasing is in addition to nuclear spin dephasing induced by inherent inhomogeneities in said static magnetic field and in step (d) the third time interval is at a time period τα subsequent to the mean occurrence of said selective RF pulse, so as to initiate the rephasing of said excited nuclear spins and in step (e) the imaging gradient is applied at a period of time equal to said period τα following said 180° RF pulse the nuclear spin echo caused by rephasing of the nuclear spins dephased by the last-mentioned dephasing gradient coincides with the occurrence of the nuclear spin echo derived from the dephasing of the nuclear spins dephased by the inherent inhomogeneities in said static magnetic field, said nuclear spin echoes producing a composite NMR spin echo signal.
Claim 10
Dependent Claim
21 words
The method of claim 1 or 2 or 3 or 4 wherein said selective RF pulse comprises a 90° RF pulse.
Claim 11
Dependent Claim
36 words
The method of claim 1 or 2 or 3 or 4 wherein said frequency selective pulse comprises, a carrier modulated by a signal defined by (sin bt)/bt, wherein b is a constant and t is time.
Claim 12
Dependent Claim
22 words
The method of claim 1 or 2 or 3 or 4 wherein said 90° RF pulse comprises a Gaussian amplitude modulated carrier.
Claim 13
Dependent Claim
25 words
The method of claim 1 or 2 or 3 or 4 wherein said planar section is situated orthogonal to said first axis of said sample.
Claim 14
Dependent Claim
52 words
The method of claim 1 or 2 or 3 or 4 wherein said dephasing gradient in step (d) comprises a resultant dephasing gradient of the vectorial addition of two constituent dephasing gradients which are mutually orthogonal and which are coplanar with said planar section, said resultant dephasing gradient having a predetermined direction.
Claim 15
Dependent Claim
71 words
The method of claim 10 wherein said imaging gradient in step (e) is a resultant rephasing gradient of the vectorial addition of two constituent rephasing gradients which are mutually orthogonal and which are coplanar with said planar section, said constituent rephasing gradients having the same respective directions within said thin planar slab as said constituent dephasing gradients, said constituent rephasing gradient has the same predetermined direction as said resultant dephasing gradient.
Claim 16
Dependent Claim
28 words
The method of claim 11 wherein said resultant rephasing gradient is selected to exhibit a constant amplitude during the step of sampling said composite NMR spin echo signal.
Claim 17
Dependent Claim
42 words
The method of claim 12 further comprising the step of repeating the sequence of steps (a-f) for different directions of said resultant dephasing and rephasing gradients so as to cover, incrementally, at least a 180 0 arc within said thin planar slab.
Claim 18
Dependent Claim
55 words
The method of claim 1 or 2 or 3 or 4 wherein said dephasing gradient in step (c) comprises the resultant of the vectorial addition of first and second mutually orthogonal gradients which are coplanar with said planar slice, said first orthogonal gradient being amplitude-adjustable for phase encoding nuclear spin information in the direction thereof.
Claim 19
Dependent Claim
21 words
The method of claim 14 wherein said imaging gradient is selected to have the same direction as said second orthogonal gradient.
Claim 20
Dependent Claim
29 words
The method of claim 14 wherein said imaging gradient in step (e) is selected to have a constant amplitude during the step o sampling said NMR spin echo signal.
Claim 21
Dependent Claim
25 words
The method of claim 16 further comprising the step of repeating the sequence of steps (a-g) for a different amplitude of said first orthogonal gradient.
Claim 22
Dependent Claim
61 words
The method of claim 1 or 2 or 3 or 4 wherein the integral of the waveform of said resultant dephasing gradient with respect to time over said first time interval is selected to be equal to the integral of the waveform of said resultant imaging gradient with respect to time over a time interval equal to said time period τα.
Claim 23
Dependent Claim
58 words
The method of claim 1 or 2 or 3 or 4 wherein the integral of the waveform of said second orthogonal gradient with respect to time is selected to be equal to the integral of the waveform of said rephasing gradient in step (f) with respect to time over a time interval equal to said time period τα.
Claim 24
Dependent Claim
22 words
The method of claim 1 or 2 or 3 or 4 wherein said 180° RF pulse comprises a selective 180° RF Pulse.
References Cited (Backward Citations)
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