Nuclear magnetic resonance multi-echo imaging method and apparatus

Claim 2

A nuclear magnetic resonance multi-echo imaging method for eliminating an artifact signal and an error signal from an echo signal sampled for formation of an image so as to make the image correct, comprising the steps of: (a) exciting a nuclear spin in a specified slice of an object placed in a static magnetic field by applying a slice selection gradient field pulse (Sn) and an RF exciting pulse (Pe) in a first direction orthogonal to said slice; (b) applying a phase encoding gradient field in a second direction to assign the phase of said spin with positional information and generating an NMR echo signal a plurality of times by applying an effective inversion pulse (Pn) a plurality of times; (c) sampling each NMR echo signal under the application of a readout gradient field (Rn) in a third direction, said steps (a) to (c) forming a series of measurement sequence wherein gradient field pulses (Sn, Rn, En) having predetermined products of intensity and time are added in at least two of said first, second and third directions to dephase components of a magnetization vector which cause said artifact signal and error signal; (d) executing said measurement sequence repetitively by changing the product of intensity and time of said phase encoding gradient field; and (e) subjecting data of each echo signal to multi-dimensional Fourier transform to form an image from a spin distribution indicated by each echo signal.

Claim 3

A nuclear magnetic resonance multi-echo imaging method according to Claim 1 wherein the Fourier transform carried out in step (e) for image formation is two-dimensional and an image thus formed is also two-­dimensional.

Claim 4

A nuclear magnetic resonance multi-echo imaging method according to Claim 2 wherein the number of applications of said inversion pulse (Pn) is four, and said gradient field pulses are applied in the following manner: S₁, S₃ = S o , S₂, S₄, = 2S o , R₁, R₄ = R o R₂, R₃ = 2R o E₁, E₂, E₃, E₄ = 0 where Sn, Rn and En respectively represent the product of intensity and time of the slice selection gradient field pulses, that of the readout gradient field pulses and that of the gradient field pulses in the same direction as that of the phase encoding gradient field, said pulses being applied before and after the n-th inversion pulse, and So and Ro are sufficiently large values.

Claim 5

A nuclear magnetic resonance multi-echo imaging method according to Claim 2 wherein the number of applications of said inversion pulse (Pn) is six, and said gradient field pulses are applied in the following manner: S₁, S₆ = S o S₂, S₅ = 3S o S₃, S₄ = 2S o R₁, R₃, R₅ = R o R₂, R₄, R₆ = 2R o E₁ ∼ E₆ = 0 where Sn, Rn and En respectively represent the product of intensity and time of the slice selection gradient field pulses, that of the readout gradient field pulses and that of the gradient field pulses in the same direction as that of the phase encoding gradient field, said pulses being applied before and after the n-th inversion pulse, and So and Ro are sufficiently large values.

Claim 6

A nuclear magnetic resonance multi-echo imaging method according to Claim 2 wherein the number of applications of said inversion pulse (Pn) is eight, and said gradient field pulses are applied in the following manner: S₁, S₃, S₅, S₇ = S o , S₂, S₄, S₆, S₈ = 2S o , R₁, R₄, R₅, R₈ = R o , R₂, R₃, R₆, R₇ = 2R o , E₁, E₃, E₆, E₈ = 0, E₂, E₄, E₅, E₇ = E o where Sn, Rn and En respectively represent the product of intensity and time of the slice selection gradient field pulses, that of the readout gradient field pulses and that of the gradient field pulses in the same direction as that of the phase encoding gradient field, said pulses being applied before and after the n-th inversion pulse, and So, Ro and Eo are sufficiently large values.

Claim 7

A nuclear magnetic resonance multi-echo imaging method according to Claim 2 wherein the number of applications of said inversion pulse (Pn) is five, and said gradient field pulses are applied in the following manner: S₁, S₃, S₅ = 2S o , S₂, S₄ = S o , R₁, R₃, R₅ = R o R₂, R₄ = 2R o E₁, E₂ = 0 E₃, E₄, E₅ = E o where Sn, Rn and En respectively represent the product of intensity and time of the slice selection gradient field pulses, that of the readout gradient field pulses and that of the gradient field pulses in the same direction as that of the phase encoding gradient field, said pulses being applied before and after the n-th inversion pulse, and So, Ro and Eo are sufficiently large values.

Claim 8

A nuclear magnetic resonance multi-echo imaging method according to Claim 2 wherein said inversion pulse (Pn) is a band-limited selective pulse, and another series of measurement sequence is executed to measure multi-echoes for a spin in a different slice parallel to said specified slice during wait time intervening between repetitions of said series of measurement sequence, thereby performing measurement of a plurality of slices which are parallel to each other.

Claim 9

A nuclear magnetic resonance multi-echo imaging method according to Claim 7 wherein said RF exciting pulse (Pe) for each measurement sequence has a slice thickness in its frequency characteristic which is smaller than a slice thickness and a center to center distance between adjacent slices that said inversion pulse (Pn) has in its frequency characteristic.

Claim 10

A nuclear magnetic resonance multi-echo imaging method according to Claim 8 wherein said inversion pulse (Pn) has in its frequency characteristic a slice selective characteristic which is substantially flat within the range of slice thickness of said RF exciting pulse.

Claim 11

A nuclear magnetic resonance multi-echo imaging method according to Claim 2 wherein said inversion pulse (Pn) is a wide band RF pulse, and said additional gradient field pulse is applied before and after said inversion pulse (Pn) so as not to overlap said inversion pulse (Pn).

Claim 12

A nuclear magnetic resonance multi-echo imaging apparatus for eliminating an artifact signal and an error signal from an echo signal sampled for formation of an image so as to make the image correct, comprising: (a) means (12, 13, 14, 15, 16, 17) for exciting a nuclear spin in a slice of an object (20) placed in a static magnetic field by applying a slice selection gradient field pulse and an RF exciting pulse in a first direction orthogonal to said slice; (b) means (12, 13, 14, 15, 16, 17) for applying a phase encoding gradient field in a second direction to assign the phase of said spin with positional information and generating an NMR echo signal a plurality of times by applying an effective inversion pulse a plurality of times; (c) means (12, 13, 14, 15, 16, 17) for sampling each NMR echo signal under the application of a readout gradient field in a third direction, said means (a) to (c) forming a series of measurement means wherein gradient field pulses (Sn, Rn, En) having predetermined products of intensity and time are added in at least two of said first, second and third directions to dephase components of a magnetization vector which cause said artifact signal and error signal; (d) means (12, 13, 14, 15, 16, 17) for executing said measurement sequence repetitively by changing the product of intensity and time of said phase encoding gradient field; and (e) means (9, 10, 11) for subjecting data of each echo signal to multi-dimensional Fourier transform to form an image from a spin distribution indicated by each echo signal.

Claim 13

A nuclear magnetic resonance multi-echo imaging apparatus for eliminating an artifact signal and an error signal from an echo signal sampled for formation of an image so as to make the image correct, comprising: (a) means (12, 13, 14, 15, 16, 17) for exciting a nuclear spin in a slice of an object (20) placed in a static magnetic field by applying a slice selection gradient field pulse and an RF exciting pulse in a first direction orthogonal to said slice; (b) means (12, 13, 14, 15, 16, 17) for applying a phase encoding gradient field in a second direction to assign the phase of said spin with positional information and generating an NMR echo signal a plurality of times by applying an effective inversion pulse a plurality of times; (c) means (12, 13, 14, 15, 16, 17) for sampling each NMR echo signal under the application of a readout gradient field in a third direction, said means (a) to (c) forming a series of measurement means wherein gradient field pulses (Sn, Rn, En) having predetermined products of intensity and time are added in at least two of said first, second and third directions to dephase components of a magnetization vector which cause said artifact signal and error signal; (d) means (12, 13, 14, 15, 16, 17) for executing said measurement sequence repetitively by changing the product of intensity and time of said phase encoding gradient field; and (e) means (9, 10, 11) for subjecting data of each echo signal to multi-dimensional Fourier transform to form an image from a spin distribution indicated by each echo signal.