Nuclear magnetic resonance signal artifact removal

Claim 2

In a nuclear magnetic resonance imaging system in which NMR signals are collected by a two-pulse excitation sequence, including a source of gradient field signals for spatially encoding said NMR signals, apparatus comprising: means for exciting an imaging subject during alternate lines of signal acquisition using a two-pulse excitation sequence and detecting resultant NMR signals during subsequent data collection periods during which undesired artifact signal components and desired NMR signals exhibit a first phase relationship; means for exciting an imaging subject during the remaining lines of signal acquisition using a two-pulse excitation sequence and detecting resultant NMR signals during subsequent data collection periods during which undesired artifact signal components and desired NMR signals exhibit a second phase relationship; and means for performing a Fourier transformation of said lines of artifact signal and NMR signal information with said artifact signal components exhibiting a phasing which varies from line-to-line so as to form an NMR image, whereby said artifact signal components are located away from the center of said NMR image.

Claim 3

The nuclear magnetic resonance imaging system of Claim 1, wherein said desired NMR signals are spin-echo signals and said artifact signal components are free decay signal components.

Claim 4

The nuclear magnetic resonance imaging system of Claim 2, wherein the phasing of said artifact signal components varies from line to line and the phasing of said spin-echo signals does not substantially vary from line to line except as a result of spatial encoding.

Claim 5

The nuclear magnetic resonance imaging system of Claim 2, wherein the phasing of said artifact signal components does not vary from line to line and the phasing of said spin-echo signals alternate from line to line in addition to variations due to spatial encoding, and further comprising: means for inverting the phasing of alternate ones of said lines prior to Fourier transformation.

Claim 6

In a nuclear magnetic resonance imaging system, in which NMR signals are generated in response to a series of two-pulse excitation sequences, each sequence comprising an alpha pulse and a beta pulse, said excitation pulse sequences being accompanied by spatially encoding gradient field signals; apparatus comprising: means for alternating the phase of said beta excitation pulse from one excitation pulse sequence to the next, whereby the phasing of free decay signals produced after each beta excitation pulse is alternated from one sequence to the next; means for detecting a desired NMR signal following each two-pulse sequence during a data collection period during which components of said free decay signals may also be present; and means for performing a Fourier transformation of said detected signals so as to form an image, said Fourier transformation being performed at least in the direction of phase alteration of said free decay signals.

Claim 7

In a nuclear magnetic resonance imaging system, in which NMR signals are generated in response to a series of two-pulse excitation sequences, each sequence comprising an alpha pulse and a beta pulse, said excitation pulse sequences being accompanied by spatially encoding gradient field signals; apparatus comprising: means for alternating the phase of said beta excitation pulse from one excitation pulse sequence to the next, whereby the phasing of free decay signals produced after each beta excitation pulse is alternated from one sequence to the next; means for detecting a desired NMR signal following each two-pulse sequence during a data collection period during which components of said free decay signals may also be present; and means for performing a Fourier transformation of said detected signals so as to form an image, said Fourier transformation being performed at least in the direction of phase alteration of said free decay signals.

Claim 8

The nuclear magnetic resonance imaging system of Claim 5, wherein said desired NMR signals are spin-echo signals which are not phase-encoded from sequence to sequence in response to the alternating phase of said beta excitation pulse.

Claim 9

In a nuclear magnetic resonance imaging system, in which NMR signals are generated in response to a series of two-pulse excitation sequences, each sequence comprising an alpha pulse and a beta pulse, said excitation pulse sequences being accompanied by spatially encoding gradient field signals; apparatus comprising: means for alternating the phase of said alpha excitation pulse from one excitation pulse sequence to the next, whereby the phasing of free decay signals produced after each beta excitation pulse is not alternated by the phase alternation of said alpha pulse one sequence to the next, and the phasing of desired NMR signals produced after each beta pulse is alternated in response to the phase alternation of said alpha pulse from one sequence to the next; means for detecting said desired NMR signals following each two-pulse sequence during a data collection period during which components of said free decay signals may also be present, said detecting means including means for alternating the relative phasing of detected signals from one sequence to the next; and means for performing a Fourier transformation of said detected signals so as to form an image, said Fourier transformation being performed at least in the direction of said phase alternation, whereby the effects of artifacts of said free decay signals and substantially constant DC signal offsets are removed from the center of said image.

Claim 10

In a nuclear magnetic resonance imaging system, in which NMR signals are generated in response to a series of two-pulse excitation sequences, each sequence comprising an alpha pulse and a beta pulse, said excitation pulse sequences being accompanied by spatially encoding gradient field signals; apparatus comprising: means for alternating the phase of said alpha excitation pulse from one excitation pulse sequence to the next, whereby the phasing of free decay signals produced after each beta excitation pulse is not alternated by the phase alternation of said alpha pulse one sequence to the next, and the phasing of desired NMR signals produced after each beta pulse is alternated in response to the phase alternation of said alpha pulse from one sequence to the next; means for detecting said desired NMR signals following each two-pulse sequence during a data collection period during which components of said free decay signals may also be present, said detecting means including means for alternating the relative phasing of detected signals from one sequence to the next; and means for performing a Fourier transformation of said detected signals so as to form an image, said Fourier transformation being performed at least in the direction of said phase alternation, whereby the effects of artifacts of said free decay signals and substantially constant DC signal offsets are removed from the center of said image.

Claim 11

The nuclear magnetic resonance imaging system of Claim 7, wherein said desired NMR signals are spin-echo signals.

Claim 12

In a nuclear magnetic resonance imaging system, in which an image is formed by acquiring a plurality of sequences of line information in response to two-pulse excitation sequences, each of which produces an undesired free decay signal and a desired NMR signal; a method for reducing the effects of said undesired free decay signal components comprising the steps of: a) controlling the phasing of said excitation pulse sequence so as to phase-encode detected free decay signal components with phasing which alternates from line to line; and b) performing a Fourier transformation of said acquired line information so as to form an image, said Fourier being performed in at least the direction of phase alternation of said detected free decay signal components, whereby artifacts of said free decay signal components are removed from the center of said image.

Claim 13

In a nuclear magnetic resonance imaging system, in which an image is formed by acquiring a plurality of sequences of line information in response to two-pulse excitation sequences, each of which produces an undesired free decay signal and a desired NMR signal; a method for reducing the effects of said undesired free decay signal components comprising the steps of: a) controlling the phasing of said excitation pulse sequence so as to phase-encode detected free decay signal components with phasing which alternates from line to line; and b) performing a Fourier transformation of said acquired line information so as to form an image, said Fourier being performed in at least the direction of phase alternation of said detected free decay signal components, whereby artifacts of said free decay signal components are removed from the center of said image.

Claim 14

The method of Claim 9, wherein step a) comprises alternating the phase of the second pulse in said two-pulse sequences from one sequence to the next.

Claim 15

The method of Claim 9, wherein step a) comprises the steps of: al) alternating the phase of the first pulse in said two-pulse sequences from one sequence to the next; and a2) alternating the phase of detected signals from one sequence to the next so as to phase-encode detected free decay signal components with phasing which alternates from line to line; whereby substantially constant DC signal offsets are also phase encoded so as to be removed from the center of said image through Fourier transformation.