Ultra-fast multi-section MRI using gradient and spin echo (GRASE) imaging

Fast magnetic resonance imaging uses combined gradient echoes and spin echoes. In each of one or more TR intervals, after an initial NMR RF nutation pulse, a sequence of 180.degree. RF nutation pulses is used to refocus the RF response into corresponding string of spin echoes. However, in addition, during the time that such spin echo would normally occur after each such 180.degree. RF nutation pulse, a plurality of alternating polarity read-out magnetic gradient pulses is utilized so as to very rapidly form a sub-sequence of gradient echoes. This fast multi-section MRI sequence utilizes the speed advantages of gradient refocusing while overcoming the image artifacts arising from static field homogeneity and chemical shift. Image contrast is still determined by the T2 contrast in Hahn spin echoes. A novel k-space trajectory temporally modulates signals and demodulates artifacts. The echo responses are selectively phase-encoded and time shifted in occurrence so as to smoothly distribute unwanted phase shift from field inhomogeneity and/or chemical phase shift effects over the entire phase encoded dimension in k-space. The technique can also be extended so as to provide T2-weighted multi-slab three-dimensional volume images.

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Claims

4. A method as in claim 2 further comprising, at the conclusion of each said subsequence, applying a phase-decoding magnetic gradient pulse to return nuclei phase encoding to the same point in k-space prior to application of the next 180.degree. NMR RF nutation pulse.

5. A method for generating MRI signals from NMR nuclei within an image volume, said method comprising:

generating a train of NMR spin echoes using a sequence of plural 180.degree. NMR RF nutation pulses;
for each such RF refocused spin echo, generating a sub-sequence of NMR gradient echoes by using a sequence of alternating polarity read-out magnetic gradient pulses;
phase-encoding each such gradient echo to trace different trajectories in k-space, the phase-encoding being modulated to cause gradient echoes of each said sub-sequence to have k-space trajectories which are interleaved with those of other such sub-sequences so as to more evenly distribute T2* and/or chemical phase shift effects over the phase-encoded dimension of k-space; and
after each said sub-sequence of gradient echoes, applying a phase-decoding magnetic gradient pulse to return nuclei phase encoding to the same point in the k-space prior to generation of the next NMR spin echo.

6. A method as in claim 5 wherein said train of NMR spin echoes is generated, during each of plural TR intervals, by an initial 90.degree. NMR RF nutation pulse followed by a sequence of plural 180.degree. NMR RF nutation pulses, each said RF nutation pulse occurring during a slice-select magnetic gradient pulse G.sub.z.

7. A method as in claim 6 wherein said sub-sequences of NMR gradient echoes are generated by a sequence of alternating polarity G.sub.x read-out magnetic gradient pulses occurring after each 180.degree. NMR RF nutation pulse.

8. A method as in claim 7 also including use of a dephasing G.sub.x read-out magnetic gradient pulse occurring between said initial 90.degree. NMR RF nutation pulse and the first of said 180.degree. NMR RF nutation pulses.

9. A method as in claim 7 wherein said phase-encoding during each sub-sequence is achieved by an initial G.sub.y magnetic gradient pulse of a first polarity and by subsequent G.sub.y magnetic gradient pulses of a second polarity and wherein said phase-decoding is achieved by a further G.sub.y magnetic gradient pulse of said first polarity.

10. A method for generating MRI signals, said method comprising:

(a) subjecting NMR nuclei within an image volume to a perturbing NMR RF nutation pulse;
(b) thereafter subjecting said NMR nuclei to
(i) a 180.degree. NMR RF nutation pulse following by
(ii) a plurality of alternating polarity magnetic gradient read-out pulses to generate a sequence of plural gradient echoes occurring after said 180.degree. NMR RF nutation pulse and before application of another 180.degree. NMR RF nutation pulse, and
(iii) repeating steps (i) and (ii).Iadd.at equal time intervals within the same TR interval, each of said equal intervals being substantially twice the interval between the initial said perturbing pulse and the first 180.degree. NMR RF pulse.Iaddend.to generate a further sequence of gradient echoes.Iadd.without applying alternate polarity read-out magnetic gradient pulses before the first 180.degree. RF nutation pulse.Iaddend..

11. A method as in claim 10 wherein said perturbing NMR RF nutation pulse is a 90.degree. NMR RF nutation pulse.

12. A method as in claim 10 wherein step (b) (iii) includes plural repetitions of steps (i) and (ii) to generate plural further sequences of gradient echoes.

13. A method as in claim 10, 11 or 12 wherein steps (a) and (b) are repeated in each of plural TR intervals to generate additional sequences of gradient echoes.

14. A method in claim 10, 11 or 12 wherein.Iadd.all.Iaddend.said NMR RF nutation pulses occur during a slice volume selecting magnetic gradient pulse in a multi-slice sequence.

15. A method as in claim 14 wherein each magnetic gradient read-out pulse is preceded by a phase-encoding magnetic gradient pulse of predetermined magnitude, different from the magnitude of other such phase-encoding pulses.

16. A method for generating MRI signals, said method comprising:

(a) subjecting NMR nuclei within an image volume to a perturbing NMR RF nutation pulse;
(b) thereafter subjecting said NMR nuclei to
(i) a 180.degree. NMR RF nutation pulse followed by
(ii) a plurality of alternating polarity magnetic gradient read-out pulses to generate a sequence of gradient echoes, and
(iii) repeating steps (i) and (ii) to generate a further sequence of gradient echoes.
said NMR RF nutation pulses occurring during a slice volume selecting magnetic gradient pulse in a multi-slice sequence,
each magnetic gradient read-out pulse being preceded by a phase-encoding magnetic gradient pulse of predetermined magnitude, different from the magnitude of other such phase-encoding pulses, and
wherein the magnitude of phase-encoding magnetic gradient pulses with each repetition of steps (i) and (ii) generate MRI gradient echoes respectively corresponding to non-contiguous trajectories in k-space, the MRI gradient echoes generated from other repetitions of steps (i) and (ii) respectively filling in the remaining contiguous trajectories in k-space in an interleaved fashion.

17. A method as in claim 16 wherein the phase-encoding magnetic gradient pulses have magnitudes which generate a sequence of MRI gradient echoes in k-space having substantially reduced phase shifts between next-adjacent k-space echoes caused by field inhomogeneity and/or chemical shift effects occurring during each repetition of steps (i) and (ii).

18. A method as in claim 17 wherein the phase shifts caused by said field inhomogeneity and/or chemical shift effects increase monotonically in approximately equal amount from one gradient echo to the next throughout the phase-encoded dimension of k-space.

19. A method as in claim 18 wherein prior to each repetition of steps (i) and (ii), a phase-return magnetic gradient pulse is applied to said NMR nuclei having polarity and magnitude for substantially cancelling all prior phase-encoding magnetic gradient pulses and thus momentarily returning the NMR nuclei to the same point in k-space.

20. A method for generating MRI signals, said method comprising:

(a) subjecting NMR nuclei within an image volume to a perturbing NMR RF nutation pulse;
(b) thereafter subjecting said NMR nuclei to
(i) a 180.degree. NMR RF nutation pulse followed by
(ii) a plurality of alternating polarity magnetic gradient read-out pulses to generate a sequence of gradient echoes, and
(iii) repeating steps (i) and (ii) to generate a further sequence of gradient echoes.
said NMR RF nutation pulses occurring during a slice volume selecting magnetic gradient pulse in a multi-slice sequence,
each magnetic gradient read-out pulse being preceded by a phase-encoding magnetic gradient pulse of predetermined magnitude, different from the magnitude of other such phase-encoding pulses, and
wherein prior to each repetition of steps (i) and (ii), a phase-return magnetic gradient pulse is applied to said NMR nuclei having polarity and magnitude for substantially cancelling all prior phase-encoding magnetic gradient pulses and thus momentarily returning the NMR nuclei to the origin of k-space.

23. Apparatus for generating MRI signals from NMR nuclei within an image volume, said apparatus comprising:

means for nutating nuclei.Iadd.within.Iaddend.a slice-volume to initiate a TR interval;
means for repetitively applying 180.degree. NMR RF pulses to further nutate nuclei within the same said slice-volume by substantially 180.degree. at subsequent intervals within the same TR interval and thus to generate a train of NMR spin echoes;
means for applying a plurality of alternate polarity read-out magnetic gradient pulses between pairs of said 180.degree. NMR RF pulses to produce sub-sequences of plural gradient echoes; and
means for phase-encoding each said gradient echo within each sub-sequence to traverse a discontinuous trajectory in k-space which is interleaved with the trajectories of other sub-sequences so as to more evenly distribute field inhomogeneity and/or chemical phase shift effects over the phase-encoded dimension of k-space.

24. Apparatus as in claim 23 including means for shifting the time occurrences of gradient echoes within different said sub-sequences so as to more evenly distribute field inhomogeneity and/or chemical phase shift effects over the phase-encoded dimension of k-space.

25. Apparatus as in claim 23 further comprising means for applying a phase-decoding magnetic gradient pulse at the conclusion of each said subsequence to return nuclei phase encoding to the origin of k-space prior to application of the next 180.degree. NMR RF nutation pulse.

27. Apparatus as in claim 26 wherein said means for generating a train of NMR spin echoes generates during each of plural TR intervals, by an initial 90.degree. NMR RF nutation pulse followed by a sequence of plural 180.degree. NMR RF nutation pulses, each said RF nutation pulse occurring during a slice-select magnetic gradient pulse G.sub.z.

28. Apparatus as in claim 27 wherein said means for generating a sub-sequence of NMR gradient echoes generates a sequence of alternating polarity G.sub.x read-out magnetic gradient pulses occurring after each 180.degree. NMR RF nutation pulse.

29. Apparatus as in claim 28 also including means for generating a dephasing G.sub.x read-out magnetic gradient pulse occurring between said initial 90.degree. NMR RF nutation pulse and the first of said 180.degree. NMR RF nutation pulses.

31. Apparatus for generating MRI signals, said apparatus comprising:

(a) means for subjecting NMR nuclei within an image volume to a perturbing NMR RF nutation pulse;
(b) means for thereafter subjecting said NMR nuclei to
(i) a 180.degree. NMR RF nutation pulse following by
(ii) a plurality of alternating polarity magnetic gradient read-out pulses to generate a sequence of gradient echoes, and
(iii) repeating steps (i) and (ii).Iadd.at equal time intervals within the same TR interval, each of said equal intervals being substantially twice the interval between the initial said perturbing pulse and the first 180.degree. NMR RF pulse.Iaddend.to generate a further sequence of plural gradient echoes occurring after said 180.degree. NMR RF nutation pulse and before application of another 180.degree. NMR RF nutation pulse.Iadd.without applying alternate polarity read-out magnetic gradient pulses before the first 180.degree. RF nutation pulse.Iaddend..

32. Apparatus as in claim 31 including means for repetitively operating means (a) and (b) in each of plural TR intervals to generate additional sequences of gradient echoes.

33. Apparatus as in claim 31 wherein said means (a) and means (b) include means for generating said NMR RF nutation pulses during a slice volume selecting magnetic gradient pulse in a multi-slice sequence.

34. Apparatus as in claim 33 wherein said means (b) includes means for generating a phase-encoding magnetic gradient pulse of predetermined magnitude, different from the magnitude of other such phase-encoding pulses prior to each magnetic gradient read-out pulse.

36. Apparatus as in claim 35 wherein means (b) includes means for causing the phase-encoding magnetic gradient pulses to have magnitudes which generate a sequence or MRI gradient echoes in k-space having substantially reduced phase shifts between next-adjacent k-space echoes caused by field inhomogeneity and/or chemical shift effects occurring during each repetition.

37. Apparatus as in claim 36 wherein the means (b) includes means for causing the phase shifts of said field inhomogeneity and/or chemical shift effects to increase monotonically in approximately equal amount from one gradient echo to the next throughout the phase-encoded dimension of k-space.

39. Apparatus for generating MRI signals, said apparatus comprising:

(a) means for subjecting NMR nuclei within an image volume to a perturbing NMR RF nutation pulse;
(b) means for thereafter subjecting said NMR nuclei to
(i) a 180.degree.60 NMR RF nutation pulse followed by
(ii) a plurality of alternating polarity magnetic gradient read-out pulses to generate a sequence of gradient echoes, and
(iii) repeating steps (i) and (ii) to generate a further sequence of gradient echoes,
said means (a) and means (b) including means for generating said NMR RF nutation pulses during a slice volume selecting magnetic gradient pulse in a multi-slice sequence;
said means (b) includes means for generating a phase-encoding magnetic gradient pulse of predetermined magnitude, different from the magnitude of other such phase-encoding pulses prior to each magnetic gradient read-out pulse; and
means for generating a phase-return magnetic gradient pulse applied to said NMR nuclei having polarity and magnitude for substantially cancelling all prior phase-encoding magnetic gradient pulses and thus momentarily returning the NMR nuclei to the same point in k-space prior to each repetition of a 180.degree. RF pulse.

41. A method for generating MRI signals from NMR nuclei with an image volume, said method comprising:

nutating nuclei within a slice-volume to initiate a TR interval;
repetitively applying 180.degree. NMR RF pulses to further nutate nuclei within the same said slice-volume by substantially 180.degree. at subsequent intervals within the same TR interval and thus to generate a train of NMR spin echoes;
between pairs of said 180.degree. NMR RF pulses, applying a plurality of alternate polarity read-out magnetic gradient pulses to produce sub-sequences of gradient echoes; and
wherein the time occurrences of gradient echoes within different said sub-sequences are relatively shifted so as to more evenly distribute field inhomogeneity and/or chemical phase shift effects over the phase-encoded dimension if k-space.

42. A method for generating MRI signals from NMR nuclei within an image volume, said method comprising:

nutating nuclei within a slice-volume to initiate a TR interval;
repetitively applying 180.degree. NMR RF pulses to further nutate nuclei within the same said slice-volume by substantially 180.degree. at subsequent intervals within the same TR interval and thus to generate a train of NMR spin echoes;
between pairs of said 180.degree. NMR RF pulses, applying a plurality of alternate polarity read-out magnetic gradient pulses to produce sub-sequences of gradient echoes; and
at the conclusion of each said subsequence, applying a phase-decoding magnetic gradient pulse to return nuclei phase encoding to the same point in k-space prior to application of the next 180.degree. NMR RF nutation pulse.

43. Apparatus for generating MRI signals from NMR nuclei within an image volume, said apparatus comprising:

means for nutating nuclei within a slice-volume to initiate a TR interval;
means for repetitively applying 180.degree. NMR RF pulses to further nutate nuclei within the same said slice-volume by substantially 180.degree. at subsequent intervals within the same TR interval and thus to generate a train of NMR spin echoes;
means for applying a plurality of alternate polarity read-out magnetic gradient pulses between pairs of said 180.degree. NMR RF pulses to produce sub-sequences of gradient echoes; and
means for shifting the time occurrences of gradient echoes within different said sub-sequences so as to more evenly distribute field inhomogeneity and/or chemical phase shift effects over the phase-encoded dimension of k-space.

44. Apparatus for generating MRI signals from NMR nuclei within an image volume, said apparatus comprising:

means for nutating nuclei within a slice-volume to initiate a TR interval;
means for repetitively applying 180.degree. NMR RF pulses to further nutate nuclei within the same said slice-volume by substantially 180.degree. at subsequent intervals within the same TR interval and thus to generate a train of NMR spin echoes;
means for applying a plurality of alternate polarity read-out magnetic gradient pulses between pairs of said 180.degree. NMR RF pulses to produce sub-sequences of gradient echoes; and
means for applying a phase-decoding magnetic gradient pulse at the conclusion of each said subsequence to return nuclei phase encoding to the origin of k-space prior to application of the next 180.degree. NMR RF nutation pulse.

46. A method as in claim 45 wherein steps (ii) and (iii) are repeated at least once after one occurrence of step (i).

49. A method for generating MRI signals from NMR nuclei within an image volume, said method comprising:

nutating nuclei to initiate an MRI data acquisition pulse sequence;
repetitively applying a plurality of 180.degree. NMR RF pulses only at subsequent equal intervals to produce a Hahn spin echo occurrence at equal intervals after each 180.degree. NMR RF pulse; and
after each 180.degree. NMR RF pulse, applying a plurality of alternate polarity read-out magnetic gradient pulses to produce sub-sequences of gradient echoes occurring after each said 180.degree. NMR RF pulse..Iaddend..Iadd.50. A method as in claim 49 wherein a phase-encoding magnetic gradient pulse of the same polarity but different respective magnitude is applied prior to each gradient echo thus producing sub-sequences of phase-encoded gradient echoes..Iaddend..Iadd.51. A method as in claim 50 wherein prior to each repetition of a 180.degree. NMR RF pulse, a phase-return magnetic gradient pulse is applied to substantially cancel all prior phase-encoding magnetic gradient pulses and thus momentarily return NMR nuclei to the origin of k-space..Iaddend..Iadd.52. A method as in claim 50 wherein said phase-encoding magnetic gradient pulses produce phase-encoded gradient echo sub-sequences that each have non-contiguous k-space trajectories, contiguously interleaved with those

of other gradient echo sub-sequences..Iaddend..Iadd.53. A method as in claim 51 wherein said phase-encoding magnetic gradient pulses produce phase-encoded gradient echo sub-sequences that each have non-contiguous k-space trajectories, contiguously interleaved with those of other gradient echo sub-sequences..Iaddend..Iadd.54. A method for generating MRI signals from NMR nuclei within an image volume, said method comprising:

nutating nuclei to initiate an MRI data acquisition sequence;
repetitively applying a plurality of 180.degree. NMR RF pulses, at least some of which 180.degree. pulses are followed by a plurality of alternate polarity read-out magnetic gradient pulses to produce sub-sequences of gradient echoes with phase-encoding magnetic gradient pulses of different respective magnitudes being applied prior to each echo occurrence thus providing phase-encoded echo signals; and
prior to each repetition of a 180.degree. NMR RF pulse, applying a phase-return magnetic gradient pulse to substantially cancel all prior phase-encoding magnetic gradient pulses and thus momentarily return NMR nuclei to the origin of k-space..Iaddend..Iadd.55. A method as in claim 54 wherein said 180.degree. NMR RF pulses are all spaced at equal time intervals from each other to produce a Hahn spin echo occurrence at equal intervals after each 180.degree. NMR RF pulse..Iaddend..Iadd.56. A method as in claim 54 wherein the phase-encoding magnetic gradient pulses of at least some contiguous sub-sequences are of the same

polarity..Iaddend..Iadd.57. A method as in claim 54 wherein said phase-encoding magnetic gradient pulses within a given sub-sequence produce a sub-sequence of phase-encoded gradient echoes having non-contiguous k-space trajectories..Iaddend..Iadd.58. A method for generating MRI signals from NMR nuclei within an image volume, said method comprising:

nutating nuclei to initiate an MRI data acquisition sequence, and
repetitively applying a plurality of 180.degree. NMR RF pulses, at least some of which 180.degree. pulses are followed by a plurality of alternate polarity read-out magnetic gradient pulses and phase-encoding magnetic gradient pulses of the same polarity but different respective magnitudes to produce sub-sequences of phase-encoded gradient echoes, each sub-sequence having non-contiguous k-space trajectories..Iaddend..Iadd.59. A method as in claim 58 wherein said 180.degree. NMR RF pulses are all spaced at equal time intervals from each other to produce a Hahn spin echo occurrence at equal intervals after each 180.degree. NMR RF pulse..Iaddend..Iadd.60. A method as in claim 58 wherein prior to each repetition of a 180.degree. NMR RF pulse, a phase-return magnetic gradient pulse is applied to substantially cancel all prior phase-encoding magnetic gradient pulses and thus momentarily return NMR nuclei to the origin of k-space..Iaddend.
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Patent History
Patent number: RE35656
Type: Grant
Filed: Aug 15, 1995
Date of Patent: Nov 11, 1997
Assignee: Brigham & Women's Hospital, Inc. (Boston, MA)
Inventors: David A. Feinberg (New York, NY), Koichi Oshio (Brookline, MA)
Primary Examiner: Louis M. Arana
Law Firm: Nixon & Vanderhye P.C.
Application Number: 8/515,177
Classifications
Current U.S. Class: To Obtain Localized Resonance Within A Sample (324/309); Using A Nuclear Resonance Spectrometer System (324/307)
International Classification: G01V 300;