Slice selective MRI excitation with reduced power deposition using multiple transmit channels
Described are embodiments for slice-selective excitation for MRI that utilize multiple RF transmit coils, each of which are driven with a separate independent current waveform. These embodiments allow slice-selective excitation with slice profile and excitation time similar to other single-channel excitation, but with reduction in SAR caused by the transverse component of the RF field by a factor up to the number of excitation coils.
1. A method, of emitting radio frequency (RF) pulses into an object during magnetic resonance imaging (MRI), comprising:
emitting a first RF pulse into the object with a first RF excitation coil element, wherein the first RF pulse comprises a first RF waveform;
emitting a second RF pulse into the object with a second RF excitation coil element, wherein the second RF pulse comprises a second RF waveform that is time shifted relative to the first RF waveform by 2nπ/γG, where G is a gradient strength of a slice-select gradient, γ is a gyromagnetic ratio in units of radians per unit gradient strength, and n is an integer; and
emitting a third RF pulse into the object with a third RF excitation coil element, wherein the third RF pulse comprises a third RF waveform that is time shifted relative to the first RF waveform by −2nπ/γG.
2. The method of claim 1 , wherein RF pulses are emitted into the object by n RF coil elements.
3. The method of claim 2 , wherein n≧3.
4. The method of claim 1 , wherein the first, second and third RF waveforms are substantially identical in shape.
5. The method of claim 1 , further comprising, after emitting the third RF pulse, detecting an RF signal scattered from the object for generating an MRI image.
6. The method of claim 1 , wherein the first RF excitation coil element has a first sensitivity profile comprising a first phase that is substantially constant in a direction across an imaging volume of the object, the second RF excitation coil element has a second sensitivity profile comprising a second phase that varies substantially linearly, with an n multiple of 2π, in the direction across the imaging volume, and the third RF excitation coil element has a third sensitivity profile comprising a third phase that varies substantially linearly, with an n multiple of −2π, in the direction across the imaging volume.
7. The method of claim 6 , wherein the first phase is substantially zero across the imaging volume.
8. The method of claim 6 , wherein the sensitivity profile of each of the first, second and third excitation coil elements comprises a substantially uniform amplitude across the imaging volume.
9. A system, for emitting radio frequency (RF) pulses into an object during magnetic resonance imaging (MRI), comprising:
a transmit module, programmed to:
emit a first RF pulse into the object with a first RF excitation coil element, wherein the first RF pulse comprises a first RF waveform;
emit a second RF pulse into the object with a second RF excitation coil element, wherein the second RF pulse comprises a second RF waveform that is time shifted relative to the first RF waveform by 2nπ/γG, where G is a gradient strength of a slice-select gradient, γ is a gyromagnetic ratio in units of radians per unit gradient strength, and n is an integer; and
emit a third RF pulse into the object with a third RF excitation coil element, wherein the third RF pulse comprises a third RF waveform that is time shifted relative to the first RF waveform by −2nπ/γG.
10. The system of claim 9 , wherein RF pulses are emitted into the object by n RF coil elements.
11. The system of claim 10 , wherein n≧3.
12. The system of claim 9 , wherein the first, second and third RF waveforms are substantially identical in shape.
13. The system of claim 9 , further comprising a receive module configured to, after the third RF pulse is emitted, detect an RF signal scattered from the object for generating an MRI image.
14. The system of claim 9 , wherein the first RF excitation coil element has a first sensitivity profile comprising a first phase that is substantially constant in a direction across an imaging volume of the object, the second RF excitation coil element has a second sensitivity profile comprising a second phase that varies substantially linearly, with an n multiple of 2π, in the direction across the imaging volume, and the third RF excitation coil element has a third sensitivity profile comprising a third phase that varies substantially linearly, with an n multiple of −2π, in the direction across the imaging volume.
15. The system of claim 14 , wherein the first phase is substantially zero across the imaging volume.
16. The system of claim 14 , wherein the sensitivity profile of each of the first, second and third excitation coil elements comprises a substantially uniform amplitude across the imaging volume.
17. A method, of emitting radio frequency (RF) pulses into an object during magnetic resonance imaging (MRI), comprising:
emitting a first RF pulse into the object with a first RF excitation coil element, wherein the first RF pulse comprises a first RF waveform;
emitting a second RF pulse into the object with a second RF excitation coil element, wherein the second RF pulse comprises a second RF waveform that is time shifted relative to the first RF waveform by 2nπ cos θ/γG, where θ is an angle between a selected slice and an axial plane, G is a gradient strength of a slice-select gradient, γ is a gyromagnetic ratio in units of radians per unit gradient strength, and n is an integer; and
emitting a third RF pulse into the object with a third RF excitation coil element, wherein the third RF pulse comprises a third RF waveform that is time shifted relative to the first RF waveform by −2nπ cos θ/γG.
18. The method of claim 17 , wherein the first, second and third RF waveforms are substantially identical in shape.
19. The method of claim 17 , further comprising, after emitting the third RF pulse, detecting an RF signal scattered from the object for generating an MRI image.
20. The method of claim 17 , wherein the first RF excitation coil element has a first sensitivity profile comprising a first phase that is substantially constant in a direction across an imaging volume of the object, the second RF excitation coil element has a second sensitivity profile comprising a second phase that varies substantially linearly, with an n multiple of 2π, in the direction across the imaging volume, and the third RF excitation coil element has a third sensitivity profile comprising a third phase that varies substantially linearly, with an n multiple of −2π, in the direction across the imaging volume.