Method for Magnetic Resonance Imaging

• US 20080204020A1
• Filed: 10/31/2007
• Published: 08/28/2008
• Est. Priority Date: 10/31/2006
• 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: Park, Jang-Yeon, Chamberlain, Ryan, Garwood, Michael
• Granted Patent: US 7,786,729 B2
• Time in Patent Office: Days
• US Class Current: 324/312
• CPC Class Codes: G01R 33/4835 of multiple slices
• CPC Class Codes: G01R 33/5615 Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
• CPC Class Codes: G01R 33/5616 using gradient refocusing, e.g. EPI
• CPC Class Codes: G01R 33/56527 due to chemical shift effects
• CPC Class Codes: G01R 33/56536 due to magnetic susceptibility variations
• CPC Class Codes: G01R 33/56554 caused by acquiring plural, differently encoded echo signals after one RF excitation, e.g. correction for readout gradients of alternating polarity in EPI

Abstract

A method of magnetic resonance imaging based on rapid acquisition by sequential excitation and refocusing is provided. The method comprises turning on a first time-encoding gradient and applying an excitation pulse in the presence of the first time-encoding gradient. The excitation pulse excites magnetization sequentially along one spatial axis. Thereafter, a first refocusing pulse is applied. A second time-encoding gradient is turned on followed by a second refocusing pulse. A third time-encoding gradient is turned on and a signal is acquired in the presence of the third time-encoding gradient. The third time-encoding gradient sums to zero with the first time-encoding gradient and the second time-encoding gradient for sequential points in space.

Claims

1. A method of magnetic resonance imaging comprising:

applying a frequency-swept chirp excitation pulse;

turning on an acquisition gradient in a time-encoding direction during an acquisition window;

turning on an alternating frequency-encoding gradient in a different direction to the acquisition gradient;

acquiring a signal in the presence of the acquisition gradient and the alternating gradient, the signal being time-encoded and frequency-encoded and being substantially contiguous; and

using the signal to create two dimensions of an image.

2. The method of claim 1, further comprising turning on a selection gradient and wherein the excitation pulse is applied in the presence of the selection gradient.

3. The method of claim 2, further comprising applying a first refocusing pulse, turning on a refocusing gradient, and applying a second refocusing pulse.

4. The method of claim 3, wherein the excitation pulse excites isochromats to process in a transverse plane, wherein the excitation gradient causes each isochromat to experience a different amount of dephasing, and wherein the first and second refocusing pulses and the acquisition gradient rephase the isochromats sequentially.

5. The method of claim 3, wherein the acquisition gradient sums to zero with the selection gradient and the refocusing gradient.

6. The method of claim 5, further comprising turning on a phase-encoding gradient after applying the excitation pulse and before applying the first refocusing pulse, wherein a plurality of slices are acquired.

7. The method of claim 6, wherein the slices are overlapped.

8. The method of claim 6, wherein the slices are spaced.

9. The method of claim 6, wherein the time-encoding direction of the acquisition gradient is through-plane.

10. The method of claim 5, further comprising turning on a first slice selection gradient approximately concurrently with the first refocusing pulse and turning on a second slice selection gradient approximately concurrently with the second refocusing pulse.

11. The method of claim 10, wherein the time-encoding direction of the acquisition gradient is through-plane.

12. The method of claim 5, wherein the acquisition gradient is blipped.

13. The method of claim 5, wherein the acquisition gradient is continuous.

14. A method of multi-slice magnetic resonance imaging comprising:

turning on a selection gradient;

applying an excitation pulse in the presence of the selection gradient, the excitation pulse exciting magnetization sequentially along one spatial axis;

turning on a phase-encoding gradient;

applying a first refocusing pulse;

turning on a refocusing gradient;

applying a second refocusing pulse;

turning on an acquisition gradient;

turning on an alternating frequency encoding gradient in a different direction to the acquisition gradient;

acquiring a signal in the presence of the acquisition gradient and the alternating frequency encoding gradient, the signal being time-encoded and frequency-encoded;

wherein the acquisition gradient sums to zero with the selection gradient and the refocusing gradient for sequential points in space; and

wherein a plurality of slices are acquired.

15. The method of claim 14, wherein the excitation pulse has varied frequency.

16. The method of claim 14, wherein the excitation pulse has varied gradient.

17. The method of claim 14, wherein the excitation pulse is a chirp pulse.

18. The method of claim 14, wherein the excitation pulse is a hyperbolic secant pulse.

19. The method of claim 14, wherein the selection gradient is a time-encoding gradient.

20. The method of claim 14, wherein the refocusing gradient is a time-encoding gradient.

21. The method of claim 14, wherein the acquisition gradient is a time-encoding gradient having a time-encoding direction and wherein the time-encoding direction is through-plane.

22. The method of claim 14, wherein the acquisition gradient is blipped.

23. The method of claim 14, wherein the acquisition gradient is continuous.

24. The method of claim 14, wherein the alternating frequency encoding gradient is in an orthogonal direction to the acquisition gradient;

25. The method of claim 14, wherein the excitation pulse excites isochromats to process in a transverse plane, wherein the excitation gradient causes each isochromat to experience a different amount of dephasing, and wherein the first and second refocusing pulses and the acquisition gradient rephase the isochromats sequentially.

26. The method of claim 14, wherein the slices are overlapped.

27. The method of claim 14, wherein the slices are spaced.

28. A method of multi-slice magnetic resonance imaging comprising:

turning on a time-encoding selection gradient;

applying a frequency-swept chirp pulse in the presence of the time-encoding selection gradient, the excitation pulse exciting magnetization sequentially along one spatial axis;

turning on a phase-encoding gradient;

applying a first refocusing pulse;

turning on a time-encoding refocusing gradient;

applying a second refocusing pulse;

turning on a time-encoding acquisition gradient;

turning on an alternating frequency encoding gradient in an orthogonal direction to the acquisition gradient acquiring a signal in the presence of the acquisition gradient and the alternating frequency encoding gradient, the signal being time-encoded and frequency-encoded;

wherein the acquisition gradient sums to zero with the selection gradient and the refocusing gradient for sequential points in space; and

wherein a plurality of slices are acquired.

29. The method of claim 28, wherein the acquisition gradient is blipped.

30. The method of claim 28, wherein the acquisition gradient is continuous.

31. The method of claim 28, wherein the slices are overlapped.

32. The method of claim 28, wherein the slices are spaced.

33. A method of single-shot magnetic resonance imaging comprising:

turning on a selection gradient;

applying an excitation pulse in the presence of the selection gradient, the excitation pulse exciting magnetization sequentially along one spatial axis;

applying a first refocusing pulse;

turning on a first slice selection gradient approximately concurrently with the first refocusing pulse;

turning on a refocusing gradient;

applying a second refocusing pulse;

turning on a second slice selection gradient approximately concurrently with the second refocusing pulse;

turning on an acquisition gradient;

turning on an alternating frequency encoding gradient in a different direction to the acquisition gradient;

acquiring a signal in the presence of the acquisition gradient and the alternating frequency encoding gradient, the signal being time-encoded and frequency-encoded;

wherein the acquisition gradient sums to zero with the selection gradient and the refocusing gradient for sequential points in space.

34. The method of claim 33, wherein the excitation pulse is a chirp pulse.

35. The method of claim 33, wherein the excitation pulse is a hyperbolic secant pulse.

36. The method of claim 33, wherein the selection gradient is a time-encoding gradient.

37. The method of claim 33, wherein the refocusing gradient is a time-encoding gradient.

38. The method of claim 33, wherein the acquisition gradient is a time-encoding gradient.

39. The method of claim 33, wherein the third time-encoding gradient is blipped.

40. The method of claim 33, wherein the third time-encoding gradient is continuous.

41. The method of claim 33, wherein the acquisition gradient is a time-encoding gradient having a time-encoding direction and wherein the time-encoding direction is in-plane.

42. The method of claim 33, wherein the alternating frequency encoding gradient is in an orthogonal direction to the acquisition gradient;

43. The method of claim 33, wherein the excitation pulse excites isochromats to process in a transverse plane, wherein the excitation gradient causes each isochromat to experience a different amount of dephasing, and wherein the first and second refocusing pulses and the acquisition gradient rephase the isochromats sequentially.

44. A method of single-shot magnetic resonance imaging comprising:

turning on a time-encoding selection gradient;

applying a 90°

frequency-swept excitation pulse in the presence of the time-encoding selection gradient, the excitation pulse exciting magnetization sequentially along one spatial axis;

applying a first 180°

refocusing pulse;

turning on a first slice selection gradient approximately concurrently with the first refocusing pulse;

turning on a time-encoding refocusing gradient;

applying a second 180°

refocusing pulse;

turning on a second slice selection gradient approximately concurrently with the second refocusing pulse;

turning on a time-encoding acquisition gradient;

turning on an alternating frequency encoding gradient in an orthogonal direction to the acquisition gradient;

acquiring a signal in the presence of the time-encoding acquisition gradient and the alternating frequency encoding gradient, the signal being time-encoded and frequency-encoded;

wherein the acquisition gradient sums to zero with the selection gradient and the refocusing gradient for sequential points in space.

45. The method of claim 44, wherein the third time-encoding gradient is blipped.

46. The method of claim 44, wherein the third time-encoding gradient is continuous.