Suppression of oscillations in NMR measurement using off-resonance spin-locking pulses

• US 5,420,510 A
• Filed: 08/13/1993
• Issued: 05/30/1995
• Est. Priority Date: 08/13/1993
• Status: Expired due to Fees

• Current Assignee: Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Original Assignee: Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Inventors: Fairbanks, Ethan J., Sorenson, James A., Santyr, Giles E.
• Primary Examiner(s): Wieder, Kenneth A.
• Assistant Examiner(s): MAH, RAYMOND
• Application Number:
  US08/106,169
• Time in Patent Office: 655 Days
• Field of Search: 324/300, 324/307, 324/309, 128/653.2
• US Class Current: 324/309
• 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 33/50 based on the determination of relaxation times , e.g. T1 measurement by IR sequences; T2 measurement by multiple-echo sequences
• CPC Class Codes: G01R 33/5602 by filtering or weighting based on different relaxation times within the sample, e.g. T1 weighting using an inversion pulse
• CPC Class Codes: G01R 33/565 Correction of image distortions, e.g. due to magnetic field inhomogeneities

Abstract

Oscillations in longitudinal magnetization as a spin system recovers from a 180° inversion pulse in the presence of off-resonance spin-locking pulse are eliminated by tipping the magnetization prior to spin-locking by an amount θ and tipping it back by an amount -θ after spin-locking. A one-shot measurement of off-resonance relaxation constant T₁ρ employs the ±θ pulses to improve accuracy and a T₁ρ enhanced imaging sequence employs the ±θ pulses to reduce artifacts.

Claims

1. In an NMR system which subjects spins to a polarizing magnet field to produce longitudinal magnetization that precesses at a Larmor frequency and which subjects the spins to an off-resonance RF spin-locking pulse of amplitude f₁ and frequency offset A from the Larmor frequency, a method for reducing oscillations in the magnitude of the longitudinal magnetization which comprises:

a) applying an RF field pulse to the spins prior to subjecting them to the off-resonance RF spin-locking pulse, said RF field pulse having a frequency at the Larmor frequency and having a tip angle θ

determined by the relationship θ

=Tan^-1 (f₁ /Δ

); and

b) applying a second RF field pulse to the spins after subjecting them to the off-resonance RF spin-locking pulse, said second RF field pulse having a frequency at the Larmor frequency and having a tip angle -θ

.

2. A method for measuring the off-resonance relaxation time of a spin system, the steps comprising:

a) applying a polarizing magnetic field to the spin system;

b) applying a 180°

rf inversion field to the spin system;

c) applying a series of off-resonance spin-locking pulses of duration τ

to the spin system;

.

d) sampling the longitudinal.magnetization of the spin. system between off-resonance spin-locking pulses by acquiring NMR signals;

e) calculating the off-resonance relaxation time (T₁ρ

) of the spin system from the acquired NMR signals

f) applying an rf field pulse to the spin system prior to each off-resonance spin-locking pulse having a tip angle (θ

) determined by the ratio of the off-resonance spin-locking pulse'"'"'s amplitude (f₁) to its frequency offset (Δ

); and

g) applying an rf field pulse to the spin system after each off-resonance spin-locking pulse having a tip angle of -θ

.

3. The method as recited in claim 2 in which the tip angle θ

is determined by the relationship θ

=Tan^-1 (f₁ /Δ

).

4. The method as recited in claim 2 in which step d) is performed by applying an rf excitation pulse of tip angle α

to the spin system and acquiring the resulting NMR signal while applying a magnetic field gradient to the spin system.

5. The method as recited in claim 4 in which the acquired NMR signals are fit to a curve having a time constant T*₁ρ

, and the off-resonance relaxation time T₁ρ

is calculated by the relationship T*₁ρ

=τ

/[τ

/T₁ρ

-ln(cosα

)].

6. A method for producing an NMR image, the steps comprising:

a) applying a polarizing magnetic field to spins located in a region of interest to produce precession in a selected spin species at a Larmor frequency f_o ;

b) applying a 180°

RF inversion field pulse to the spins in the region of interest which is tuned to the Larmor frequency f_o ;

c) applying an RF field pulse to the spins in the region of interest which is tuned to the Larmor frequency f_o and has a tip angle θ

;

d) applying an RF spin-lock field pulse having an amplitude of f₁ to the spins in the region of interest, said RF spin-lock field pulse being tuned to a frequency which is offset by an amount Δ

from the Larmor frequency f_o ;

e) applying an RF field pulse to the spins in the region of interest which is tuned to the Larmor frequency f_o and has a tip angle -θ

;

f) executing an imaging pulse sequence to acquire an NMR signal by applying an RF excitation field pulse to the spins in the region of interest at a time interval TI following the application of the 180°

RF inversion field pulse and which is tuned to the Larmor frequency f_o ; and

g) reconstructing an image using the acquired NMR signal;

wherein the tip angle θ

is determined by the ratio of the Rf spin-lock field pulse'"'"'s amplitude f₁ to its frequency offset Δ

.

7. The method as recited in claim 6 in which the tip angle θ

is determined by the relationship θ

=Tan^-1 (f₁ /Δ

).