FREQUENCY SWEPT EXCITATION FOR MAGNETIC RESONANCE

• US 20070188172A1
• Filed: 10/11/2006
• Published: 08/16/2007
• Est. Priority Date: 10/11/2005
• Status: Active Grant

• Current Assignee: Regents of The University of Minnesota (University of Minnesota)
• Original Assignee: Regents of The University of Minnesota (University of Minnesota)
• Inventors: Moeller, Steen, Garwood, Michael, Corum, Curt, Idiyatullin, Djaudat
• Granted Patent: US 7,425,828 B2
• Time in Patent Office: Days
• US Class Current: 324/310
• CPC Class Codes: G01N 24/081 Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity
• CPC Class Codes: G01N 24/10 by using electron paramagnetic resonance G01N24/12 takes precedence
• CPC Class Codes: G01R 33/441 Nuclear Quadrupole Resonance [NQR] Spectroscopy and Imaging
• CPC Class Codes: G01R 33/446 Multifrequency selective RF pulses, e.g. multinuclear acquisition mode spatially selective RF pulses G01R33/4833
• CPC Class Codes: G01R 33/4616 using specific RF pulses or specific modulation schemes, e.g. stochastic excitation, adiabatic RF pulses, composite pulses, binomial pulses, Shinnar-le-Roux pulses, spectrally selective pulses not being used for spatial selection
• CPC Class Codes: G01R 33/4816 NMR imaging of samples with ultrashort relaxation times such as solid samples, e.g. MRI using ultrashort TE [UTE], single point imaging, constant time imaging
• CPC Class Codes: G01R 33/54 Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console

Abstract

A method of magnetic resonance is provided that uses a frequency swept excitation wherein the acquired signal is a time domain signal is provided. In one embodiment, the method comprises, applying a sweeping frequency excitation and acquiring a time domain signal. The sweeping frequency excitation has a duration and is configured to sequentially excite isochromats having different resonant frequencies. Acquisition of the time domain signal is done during the duration of the sweeping frequency excitation. The time domain signal is based on evolution of the isochromats.

Claims

1. A method comprising:

applying a sweeping frequency excitation having a duration, the sweeping frequency excitation being configured to sequentially excite isochromats having different resonant frequencies; and

acquiring a time domain signal during the duration, the time domain signal being based on evolution of the isochromats; and

;

achieving a steady, oscillating, or pseudo-steady state.

2. The method of claim 1, further comprising applying a first gradient while applying a sweeping frequency excitation.

3. The method of claim 2, wherein acquiring a time domain signal comprises capturing or freezing motion along a direction of the first gradient.

4. The method of claim 2, wherein the first gradient is constant.

5. The method of claim 2, wherein the first gradient is shaped.

6. The method of claim 2, further comprising calculating a projection angle trajectory or plane using a gradient shape and frequency sweep.

7. The method of claim 2, further comprising applying a second gradient and acquiring a second time domain signal, the second time domain signal being based on evolution of the isochromats, wherein the second gradient is opposite the first gradient.

8. The method of claim 2, further comprising creating a gradient-echo, a spin-echo, or a simulated-echo.

9. The method of claim 8, further comprising filtering spin-spin relaxation times based on the created gradient-echo, spin-echo, or simulated-echo.

10. The method of claim 8, further applying a second sweeping frequency excitation having a duration wherein the first sweeping frequency excitation results in a first sequence and the second sweeping frequency excitation results in a second sequence, and further comprising subtracting the second sequence from the first sequence.

11. The method of claim 2, further comprising changing an orientation of the first gradient.

12. The method of claim 11, further comprising separating a fluctuation of an off-resonance isochromat with a consistent location of an on-resonance isochromat.

13. The method of claim 1, further comprising jittering at least one of the sweeping frequency excitation and the time domain signal.

14. The method of claim 13, further comprising unjittering the at least one of the sweeping frequency excitation and the time domain signal that was jittered.

15. The method of claim 1, wherein applying a sweeping frequency excitation is done to coincide with a motion of interest.

16. The method of claim 1, wherein achieving a steady, oscillating, or pseudo-steady state is done before applying a sweeping frequency excitation.

17. The method of claim 16, wherein achieving a steady, oscillating, or pseudo-steady state comprises applying a preparation pulse.

18. The method of claim 17, further comprising acquiring a plurality of preparation time domain signals.

19. The method of claim 17, further comprising applying a preparation gradient.

20. The method of claim 19, further comprising acquiring a preparation plurality of time domain signals wherein acquiring the plurality of preparation time domain signals is done with sequential gradient orientations.

21. The method of claim 19, further comprising acquiring a plurality of preparation time domain signals wherein acquiring the plurality of preparation time domain signals is done with random or pseudo-random gradient orientations.

22. The method of claim 19, wherein the preparation gradient is reversed during the preparation pulse.

23. The method of claim 17, wherein the preparation pulse is a radio frequency pulse.

24. The method of claim 17, further comprising varying radio frequency power during the preparation pulse.

25. The method of claim 1, further comprising using a digital filter while acquiring a time domain signal to limit the acquired signal.

26. The method of claim 1, wherein the sweeping frequency excitation is asymmetrical.

27. The method of claim 1, further comprising processing the acquired time domain signal.

28. The method of claim 27, wherein achieving a steady, oscillating, or pseudo-steady state includes correcting for a lack of a steady state and is done during processing the acquired time domain signal.

29. The method of claim 1, further comprising applying a magnetic field during applying the sweeping radio frequency excitation.

30. The method of claim 1, wherein applying the sweeping frequency excitation includes applying a plurality of discrete excitation pulses and wherein acquiring the time domain signal includes sampling between excitation pulses.

31. The method of claim 1, further comprising generating a first spectra based on a correlation of a Fourier transform of the applied sweeping frequency excitation and a Fourier transform of the acquired time domain signal.

32. The method of claim 31, further comprising generating a second spectra based on processing the acquired time domain signal as a frequency domain signal and further comprising averaging the first and second spectra.

33. The method of claim 1, further comprising generating a first spectra based on deconvoluting the applied sweeping frequency excitation and the acquired time domain signal to generate a deconvoluted signal and Fourier transforming the deconvoluted signal.

34. The method of claim 33, further comprising generating a second spectra based on processing the acquired time domain signal as a frequency domain signal and further comprising averaging the first and second spectra.

35. The method of claim 1, wherein applying the sweeping frequency excitation comprises applying a phase modulation.

36. The method of claim 1, wherein applying the sweeping frequency excitation comprises applying a frequency modulation.

37. A method comprising:

exciting a specimen with a sweeping frequency excitation having a plurality of alternating excitation segments and quiescent segments, the excitation segments modulated according to at least one of a phase modulation function and a frequency modulation function;

applying at least one functional pulse;

acquiring a time domain signal from the specimen based on resonance during a plurality of the quiescent segments; and

processing the acquired time domain signal.

38. The method of claim 37, wherein the at least one functional pulse is applied before exciting a specimen with a sweeping frequency excitation.

39. The method of claim 37, wherein effects of the functional pulse are localized in one or more spatial dimensions in the presence of a field gradient.

40. The method of claim 37, wherein the at least one functional pulse is applied after exciting a specimen with a sweeping frequency excitation.

41. The method of claim 37, wherein the at least one functional pulse establishes a steady or oscillating spatial magnetization state.

42. The method of claim 37, further comprising applying a gradient during the at least one functional pulse.

43. The method of claim 42, wherein a subset of gradients is used during the at least one functional pulse.

44. The method of claim 37, wherein the at least one functional pulse prepares magnetization.

45. The method of claim 37, wherein the at least one functional pulse spoils magnetization.

46. The method of claim 37, wherein the at least one functional pulse restores magnetization.

47. The method of claim 37, wherein the at least one functional pulse suppreses or enhances magnetization based on T2, T1, T1ρ

, T2ρ

, or diffusion.

48. The method of claim 37, wherein processing comprises performing conjugate multiplication of a Fourier transform of the time domain signal and a Fourier transform of the frequency excitation.

49. The method of claim 48, wherein at least one of the Fourier transformed signal and Fourier transformed excitation are windowed or filtered before processing and wherein processing comprises conjugate multiplication.

50. The method of claim 37, further comprising applying a field gradient to create an image.

51. The method of claim 50, further comprising reconstructing the image based on the time domain signal.

52. The method of claim 51, further comprising adjusting the time domain signal with a constant, linearly, quadratic, or higher order phase correction prior to reconstructing the image.

53. The method of claim 51, further comprising deleting pulse contamination during reconstruction.

54. The method of claim 37, wherein the acquired time domain signal has a poor quality segment and or edges, and further comprising eliminating the poor quality segment or edges before correlation or deconvolution.

55. The method of claim 37, wherein processing the acquired time domain signal comprises applying a phase consistency algorithm.

56. The method of claim 37, wherein processing the acquired time domain signal comprises processing complex phased projections.

57. The method of claim 37, wherein processing the acquired time domain signal comprises processing real phased projections.

58. The method of claim 37, wherein processing the acquired time domain signal comprises applying a cone beam resampling algorithm to correct between expected angles of projections and actual projections by estimating new projections from different measured projections.

59. The method of claim 37, wherein the sweeping frequency excitation is located in a radiofrequency spectrum.

60. The method in claim 37, wherein the sweeping frequency excitation is in an optical spectrum.