Method for spectrally selective B1 insensitive T2 preparation contrast enhancement for high field magnetic resonance imaging
A T 2 preparation sequence uses a segmented BIR-4 adiabatic pulse with two substantially equal delays and is insensitive to B 1 field variations and can simultaneously suppress fat signals with low specific absorption rate (SAR). An adiabatic reverse half passage pulse is applied followed by a predetermined delay. An adiabatic full passage pulse is applied followed by a substantially equal delay, followed by an adiabatic half passage pulse. Fat signal suppression is achieved by increasing or decreasing either the first delay or the second delay.
1. A magnetic resonance imaging method, comprising:
situating a sample in a longitudinal magnetic field to establish a longitudinal specimen magnetization;
applying a reverse adiabatic half passage pulse so as to produce a substantially transverse magnetization from the longitudinal magnetization;
allowing the transverse magnetization to evolve for a first time interval;
applying a matched adiabatic full passage pulse so as to substantially invert the transverse magnetization;
allowing the transverse magnetization to evolve for a second time interval; and
applying a matched adiabatic half passage pulse so as to produce a T 2 -weighted longitudinal magnetization from the evolved transverse magnetization.
2. The method of claim 1 , further comprising obtaining a T 2 -weighted image based on the T 2 -weighted longitudinal magnetization.
3. The method of claim 1 , wherein a first portion of the matched adiabatic full passage pulse corresponds to the matched adiabatic half passage pulse, and a second portion of the matched adiabatic full passage pulse corresponds to the matched reversed adiabatic half passage pulse.
4. The method of claim 1 , wherein the first time interval and the second time interval are substantially the same.
5. The method of claim 1 , wherein the specimen includes spins of a first constituent and spins of a second constituent, and a difference between the first time interval and the second time interval is selected so that the matched adiabatic half passage pulse produces a longitudinal magnetization associated with primarily the first constituent.
6. The method of claim 5 , wherein the adiabatic half passage pulse produces a transverse magnetization associated with primarily the second constituent, and further comprising applying at least one gradient field so as to reduce signal contributions from the second constituent.
7. The method of claim 5 , wherein the difference between the first time interval and the second time interval is selected based on a difference between Larmor frequencies of the spins of the first constituent and the spins of a second constituent.
8. The method of claim 7 , wherein the first constituent is water and the second constituent is fat.
9. A computer readable medium, containing computer-executable instructions for performing, on a sample situated in a longitudinal magnetic field such that a longitudinal specimen magnetization is established, the method comprising:
applying a reverse adiabatic half passage pulse so as to produce a substantially transverse magnetization from the longitudinal magnetization;
allowing the transverse magnetization to evolve for a first time interval;
applying a matched adiabatic full passage pulse so as to substantially invert the transverse magnetization;
allowing the transverse magnetization to evolve for a second time interval; and
applying a matched adiabatic half passage pulse so as to produce a T 2 -weighted longitudinal magnetization from the evolved transverse magnetization.
10. A method, comprising:
applying a series of pulse sequences comprising a reverse adiabatic half passage pulse, an adiabatic full passage pulse, and an adiabatic half passage pulse, wherein the pulses are matched;
selecting durations of a first time interval and a second time interval, wherein the adiabatic full passage pulse is applied after the first interval elapses after the reverse half passage pulse is applied and the adiabatic half passage pulse is applied after the second time interval elapses after the adiabatic full passage pulse is applied;
obtaining specimen images for a plurality of durations of the first interval and the second interval; and
identifying at least two sample constituents based on the images.
11. The method of claim 10 , wherein the constituents are water and fat.
12. A computer readable medium, containing computer executable instructions for the method comprising:
applying a series of pulse sequences comprising a reverse adiabatic half passage pulse, an adiabatic full passage pulse, and an adiabatic half passage pulse, wherein the pulses are matched;
selecting durations of a first time interval and a second time interval, wherein the adiabatic full passage pulse is applied after the first interval elapses after the reverse half passage pulse is applied and the adiabatic half passage pulse is applied after the second time interval elapses after the adiabatic full passage pulse is applied;
obtaining specimen images for a plurality of durations of the first interval and the second interval; and
identifying at least two sample constituents based on the images.
13. A method, comprising:
applying a first adiabatic half passage pulse to specimen to produce a substantial transverse magnetization;
allowing a time interval to elapse;
applying a matched adiabatic full passage pulse during the first time interval,
applying a matched second adiabatic half passage pulse to the specimen after the time interval to rotate at least a portion of the transverse magnetization into a longitudinal magnetization; and
obtaining an image based on the longitudinal magnetization.
14. The method of claim 13 , wherein the transverse magnetization includes portions associated with a first sample constituent and a second sample constituent, and the adiabatic full passage pulse is applied at a time point in the time interval so that only the magnetization portion associated with one of the sample constituents is substantially rotated by the second adiabatic half passage pulse so as to become a longitudinal magnetization.
15. The method of claim 14 , wherein the transverse magnetization includes portions associated with a first sample constituent and a second sample constituent, and further comprising dividing the time interval into a first subinterval and a second subinterval, wherein a difference between the first subinterval and the second subinterval is based on a difference between a Larmor frequency of the first sample constituent and a Larmor frequency of the second sample constituent.
16. A method, comprising;
determining a frequency shift associated with spins of first and second sample constituents in a longitudinal magnetic field;
producing a first transverse magnetization associated with the spins of the first sample constituent and a second transverse magnetization associated with the spins of the second sample constituent;
selecting a time interval based on the frequency shift so that the first and second transverse magnetizations are substantially orthogonal; and
rotating only the first transverse magnetization with an adiabatic pulse so as to be a longitudinal magnetization; and
obtaining an image based on the rotated magnetization.
17. The method of claim 16 , further comprising applying a gradient to spoil the second transverse magnetization.
18. The method of claim 16 , wherein the first and second sample constituents are fat and water, respectively.
19. The method of claim 16 , wherein the image is based on a transverse relaxation time of the first sample constituent.
20. The method of claim 16 , further comprising alternately selecting the first and second transverse magnetizations to be the longitudinal magnetization.
21. A magnetic resonance imaging apparatus, comprising:
an adiabatic pulse generator configured to produce a pulse sequence that includes a reverse adiabatic half passage pulse, a matched adiabatic full passage pulse, and a matched adiabatic half passage pulse; and
a pulse controller configured to direct the adiabatic pulse generator to produce the matched adiabatic full passage pulse at a first time after the reverse adiabatic half passage pulse and to direct the pulse controller to produce the matched adiabatic half passage pulse at a second time after the matched adiabatic full passage pulse.
22. The apparatus of claim 21 , wherein the pulse controller determines at least one of the first time and the second time based on a frequency associated with Larmor frequencies of at least a first specimen constituent and a second specimen constituent.
23. The apparatus of claim 22 , wherein the first specimen constituent and the second specimen constituent are water and fat, respectively.