System and method for minimizing MRI-imaging artifacts
View Patent ↗Methods of, and systems for, simultaneously compensating for external-magnetic-field inhomogeneity as well as radiofrequency magnetic-field inhomogeneity in an MRI system. In one method embodiment, a pulse sequence is applied when the transmitter-reference frequency is delivered on resonance. The pulse sequence includes radiofrequency pulses which may be applied at arbitrary-excitation-flip angles that are not necessarily 90° degrees. The pulse sequence also includes spin-locking pulses applied in concert with a refocusing-composite pulse. In another method embodiment, a pulse sequence is applied when the transmitter-reference frequency is delivered off resonance. This off-resonance-pulse sequence includes radiofrequency pulses which may be applied at arbitrary-excitation-flip angles that are not necessarily 90° degrees. Sandwiched between the excitation-flip angles are at least two off-resonance-spin-lock pulses applied at an inverse phase and frequency from each other.
1. A method of simultaneously compensating for external and radiofrequency (RF) magnetic-field inhomogeneities in an MRI system, having pulses delivered by RF scanner coil(s), the method comprising:
causing a first RF pulse to be applied with an arbitrary-excitation-flip angle α;
causing a refocusing-composite pulse to be applied perpendicular to the first RF pulse;
causing a second RF pulse to be applied with the arbitrary-excitation-flip angle α, wherein the second RF pulse is perpendicular to the refocusing-composite pulse, and parallel to the first RF pulse; and
causing a spin-locking pulse to be applied (i) between the first RF pulse and the refocusing-composite pulse, and (ii) between the refocusing-composite pulse and the second RF pulse.
2. The method as recited in claim 1 , further comprising causing a transmitter reference frequency for the MRI system to be delivered on resonance.
3. The method as recited in claim 1 , whereby the refocusing-composite pulse and the second RF pulse compensate for the external and RF-magnetic-field inhomogeneities.
4. The method as recited in claim 1 , whereby the spin-locking pulses compensate for RF-magnetic-field inhomogeneities.
5. The method as recited in claim 1 , whereby the spin-locking pulses relax the magnetization a time constant T 1 ρ.
6. The method as recited in claim 1 , wherein the refocusing-composite pulse has a flip angle of approximately 180°.
7. The method as recited in claim 1 , further comprising causing the refocusing-composite pulse to be applied along a +y direction of the Cartesian-coordinate system.
8. The method as recited in claim 1 , further comprising causing the first and second RF pulses to be applied along a +x direction of the Cartesian-coordinate system.
9. The method as recited in claim 1 , further comprising causing the spin-locking pulse to be applied between the first RF pulse and the refocusing-composite pulse, to be applied along a +y direction of the Cartesian-coordinate system.
10. The method as recited in claim 1 , further comprising causing the spin-locking pulse to be applied between the refocusing-composite pulse and the second RF pulse, to be applied along a +y direction of the Cartesian-coordinate system.
11. The method of claim 1 , wherein the inhomogeneities occur in a B 0 or B 1 field, or a combination thereof.
12. One or more computer-readable media having computer-readable instructions thereon which, when executed by one or more processors of a magnetic-resonance-imaging device having pulses delivered by RF scanner coil(s), cause the device to:
apply a first RF pulse with an arbitrary-excitation-flip angle α;
apply a refocusing-composite pulse perpendicular to the first RF pulse;
apply a second RF pulse with the arbitrary-excitation-flip angle α, wherein the second RF pulse is perpendicular to the refocusing-composite pulse, and parallel to the first-RF pulse; and
apply a spin-locking pulse (i) between the first RF pulse and the refocusing-composite pulse, and (ii) between the refocusing-composite pulse and the second RF pulse.
13. A system for simultaneously compensating for external and radiofrequency (RF) magnetic-field inhomogeneities in an MRI system, comprising:
means for causing a first RF pulse to be applied with an arbitrary-excitation-flip angle α;
means for causing a refocusing-composite pulse to be applied perpendicular to the first RF pulse;
means for causing a second RF pulse to be applied with the arbitrary-excitation-flip angle α, wherein the second RF pulse is perpendicular to the refocusing-composite pulse, and parallel to the first-RF pulse; and
means for causing a spin-locking pulse to be applied (i) between the first-RF pulse and the refocusing-composite pulse, and (ii) between the refocusing-composite pulse and the second RF pulse.
14. The system as recited in claim 13 , whereby the refocusing-composite pulse and the second RF-pulse compensate for the external and RF-magnetic-field inhomogeneities.
15. The system as recited in claim 13 , whereby the spin-locking pulses compensate for RF-magnetic-field inhomogeneities.
16. The system as recited in claim 13 , whereby the spin-locking pulses relax the magnetization a time constant T 1 ρ.
17. The system as recited in claim 13 , wherein the refocusing-composite pulse has a flip angle of approximately 180°.
18. The system as recited in claim 13 , further comprising causing the refocusing-composite pulse to be applied along a +y direction of the Cartesian-coordinate system.
19. A method of simultaneously compensating for external and radiofrequency (RF) magnetic-field inhomogeneities in an MRI system having pulses delivered by RF scanner coil(s), the method comprising:
applying a first RF pulse with an arbitrary-excitation-flip angle α;
applying a first off-resonance-spin-lock pulse after the first RF pulse;
applying a second off-resonance-spin-lock pulse at an inverse phase and frequency from the first off-resonance-spin-lock pulse; and
applying a second RF pulse with the arbitrary-excitation-flip angle α inverse to the arbitrary-excitation-flip angle α of the first RF pulse.
20. The method of claim 19 , wherein the inhomogeneities occur in a B 0 or B 1 field, or a combination thereof.