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

Claim 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 f 1 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 (f 1 /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.

Claim 3

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 (T 1 ρ) 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 (f 1 ) 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.

Claim 4

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 (T 1 ρ) 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 (f 1 ) 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.

Claim 8

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 f 1 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 f 1 to its frequency offset Δ.

Claim 9

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 f 1 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 f 1 to its frequency offset Δ.