Refocusing pulses and excitation pulses for NMR logging

Patent metadata

Field	Value
Publication number	US-9658358-B2
Application number	US-201113338083-A
Country	US
Kind code	B2
Filing date	Dec 27, 2011
Priority date	Dec 27, 2011
Publication date	May 23, 2017
Grant date	May 23, 2017

Abstract

Illustrative embodiments are directed to applying a nuclear magnetic resonance sequence to a substance within an inhomogeneous static magnetic field. Various embodiments include applying a series of refocusing pulses to the substance, each refocusing pulse in the series of refocusing pulses having at least two segments, and a total pulse duration less than or equal to approximately 1.414 times T 180 . Various embodiments can further include applying an excitation pulse to the substance in the inhomogeneous static magnetic field, where the excitation pulse generates an initial magnetization that is aligned with a refocusing axis produced by a refocusing cycle that is performed after the excitation pulse.

First claim

We claim:

1. A method for applying a nuclear magnetic resonance (NMR) sequence, the method comprising:
applying an excitation pulse to a substance within an inhomogeneous static magnetic field to induce a spin effect within the substance, wherein the excitation pulse includes a plurality of segments; and
applying a refocusing cycle to the substance, wherein the refocusing cycle generates a magnetization that is aligned with an effective refocusing axis;
wherein the refocusing cycle includes at least one refocusing pulse that includes an initial segment, a middle segment and a final segment,
wherein the initial segment and the final segment each have substantially equal durations,
wherein the middle segment has a duration,
wherein each of the initial segment, the middle segment, and the final segment has a substantially constant amplitude,
wherein a phase of the middle segment is shifted approximately 180 degrees with respect to a phase of each of the initial segment and the final segment, and
wherein the excitation pulse generates an initial magnetization that is aligned with the effective refocusing axis.

2. The method of claim 1 , wherein each of the plurality of segments of the excitation pulse has a substantially constant amplitude.

3. The method of claim 1 , wherein the plurality of segments of the excitation pulse modulate between phases that are a multiple of π/2.

4. The method of claim 1 , wherein the excitation pulse has a duration that is greater than or equal to approximately eight times T180.

5. The method of claim 1 , wherein the excitation pulse has a duration greater than or equal to an echo spacing.

6. The method of claim 1 , wherein the inhomogeneous static magnetic field varies by a value that is greater than or equal to a nominal amplitude of the refocusing cycle.

7. The method of claim 1 , further comprising:
performing at least 10 refocusing cycles after the applying of the excitation pulse.

8. The method of claim 1 , further comprising:
applying a second excitation pulse to the substance within the inhomogeneous static magnetic field to induce a spin effect within the substance; and
applying the refocusing cycle to the substance;
wherein the second excitation pulse includes an inverted phase with respect to the first excitation pulse.

9. The method of claim 1 , further comprising:
detecting nuclear magnetic resonance signals from the substance during the application of the series of refocusing pulses.

10. A method for applying a nuclear magnetic resonance (NMR) sequence, the method comprising:
applying an excitation pulse to a substance within an inhomogeneous static magnetic field to induce a spin effect within the substance, wherein the excitation pulse includes a plurality of segments, wherein each segment of the excitation pulse has a substantially constant amplitude with a phase shift between adjacent segments, and wherein each segment of the excitation pulse has one phase selected from no more than two distinct phases; and
applying a series of refocusing pulses to the substance within the inhomogeneous static magnetic field after the applying of the excitation pulse the series of refocusing pulses having an effective refocusing axis,
wherein the refocusing cycle includes at least one refocusing pulse that includes an initial segment, a middle segment and a final segment,
wherein the initial segment and the final segment each have substantially equal durations,
wherein the middle segment has a duration,
wherein each of the initial segment, the middle segment, and the final segment has a substantially constant amplitude,
wherein a phase of the middle segment is shifted approximately 180 degrees with respect to a phase of each of the initial segment and the final segment, and
wherein the excitation pulse generates an initial magnetization that is aligned with the effective refocusing axis.

11. The method of claim 10 , wherein the inhomogeneous static magnetic field varies by a value approximately greater than or equal to a nominal amplitude of the series of refocusing pulses.

12. The method of claim 10 , wherein the excitation pulse includes at least 10 segments.

13. The method of claim 10 , wherein the excitation pulse has a duration greater than or equal to approximately nine times T180.

14. The method of claim 10 , wherein the two distinct phases are:
(a) approximately φ+90 degrees; and
(b) approximately φ+270 degrees,
wherein φ is a phase for the series of refocusing pulses.

15. The method of claim 10 , further comprising:
detecting nuclear magnetic resonance signals from the substance during the application of the series of refocusing pulses.

16. The method of claim 1 , wherein plurality of segments of the excitation pulse have a phase shift between adjacent segments.

17. The method of claim 16 , wherein the plurality of segments of the excitation pulse modulate between two distinct phases.

18. The method of claim 16 , wherein:
the refocusing cycle includes at least one refocusing pulse having a phase φ; and
the plurality of segments of the excitation pulse modulate between two distinct phases of approximately φ+90 degrees and approximately φ+270 degrees.

Assignees

• Hürlimann Martin D
• Mandal Soumyajit
• Do Van Mai
• Song Yi-Qiao
• Schlumberger Technology Corp

Inventors

• Hürlimann Martin D
• Mandal Soumyajit
• Do Van Mai
• Song Yi-Qiao

Classifications

• G01R33/5617

  using RF refocusing, e.g. RARE · CPC title

• G01R33/3808

  Magnet assemblies for single-sided MR wherein the magnet assembly is located on one side of a subject only; Magnet assemblies for inside-out MR, e.g. for MR in a borehole or in a blood vessel, or magnet assemblies for fringe-field MR · CPC title

• G01R33/445

  MR involving a non-standard magnetic field B0, e.g. of low magnitude as in the earth's magnetic field or in nanoTesla spectroscopy, comprising a polarizing magnetic field for pre-polarisation, B0 with a temporal variation of its magnitude or direction such as field cycling of B0 or rotation of the direction of B0, or spatially inhomogeneous B0 like in fringe-field MR or in stray-field imaging · CPC title

• G01V3/32 Primary

  operating with electron or nuclear magnetic resonance · CPC title

• G01N24/081

  Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity · CPC title