METHOD AND APPARATUS FOR GENERATING A FLIP ANGLE SCHEDULE FOR A SPIN ECHO TRAIN PULSE SEQUENCE

• US 20080319301A1
• Filed: 06/25/2007
• Published: 12/25/2008
• Est. Priority Date: 06/25/2007
• Status: Active Grant

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Busse, Reed F.
• Granted Patent: US 8,228,060 B2
• Time in Patent Office: Days
• US Class Current: 600/410
• CPC Class Codes: A61B 5/055 involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• CPC Class Codes: G01R 33/5615 Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
• CPC Class Codes: G01R 33/5617 using RF refocusing, e.g. RARE
• CPC Class Codes: G01R 33/5618 using both RF and gradient refocusing, e.g. GRASE

Abstract

A method for calculating a flip angle schedule for a train of refocusing radio frequency (RF) pulses with reduced flip angles allows control of RF power deposition and use of a longer echo train. A target signal is defined for each echo in the echo train, and flip angles are then calculated from the target signals. The target signal schedule includes two phases. In the first phase, the target signals drop asymptotically to efficiently establish a pseudo-steady state at a pre-defined minimum signal level, S_min. In the second phase, the target signal is increased monotonically for the remainder of the train to a pre-defined maximum signal level, S_max. By increasing the target signal, the effect of relaxation may be reduced, decreasing blurring and ringing artifacts. Flip angles are then calculated from the target signal schedule, using a simplified method that requires no information about the tissues'"'"' relaxation properties.

Claims

1. A method for calculating a flip angle schedule for a spin echo train with reduced refocusing flip angles, the spin echo train having a plurality of spin echoes, the method comprising:

defining a target signal schedule including target signals for each spin echo in the spin echo train, the target signal schedule comprising a first phase configured to establish a pseudo-steady state and a second phase where the target signals increase from a pre-defined minimum signal value to a pre-defined maximum signal value; and

calculating a corresponding flip angle schedule from the target signal schedule using only non-relaxation properties.

2. A method according to claim 1, wherein the first phase of the target signal schedule comprises target signals that decrease asymptotically to the pre-defined minimum signal value.

3. A method according to claim 1, wherein the target signals of the second phase of the target signal schedule increase from the pre-defined minimum signal value to the pre-defined maximum signal value.

4. A method for acquiring magnetic resonance images using a spin echo train with reduced refocusing flip angles, the spin echo train having a plurality of spin echoes, the method comprising:

applying a magnetic field to an imaging subject comprising tissues to cause a net longitudinal magnetization in the tissues;

defining a target signal schedule including target signals for each spin echo in the spin echo train, the target signal schedule comprising a first phase configured to establish a pseudo-steady state and a second phase where the target signals increase from a pre-defined minimum signal value to a pre-defined maximum signal value;

calculating a corresponding flip angle schedule from the target signal schedule using only non-relaxation properties;

generating an excitation radio frequency pulse;

generating a plurality of refocusing radio frequency pulses, each refocusing radio frequency pulse having a flip angle determined based on the flip angle schedule; and

acquiring magnetic resonance imaging data.

5. A method according to claim 4, further comprising:

calculating an equivalent TE for each spin echo in the spin echo train.

6. A method according to claim 5, further comprising:

selecting a spin echo to fill a center line of k-space based on the equivalent TE of the spin echo.

7. A method according to claim 4, wherein the plurality of refocusing radio frequency pulses is generated in accordance with a single-shot fast spin echo (SSFSE) pulse sequence.

8. A method according to claim 4, wherein the plurality of refocusing radio frequency pulses is generated in accordance with a gradient and spin echo (GRASE) pulse sequence.

9. A method according to claim 4, wherein the plurality of refocusing radio frequency pulses is generated in accordance with a three-dimensional fast spin echo (3D-FSE) pulse sequence.

10. A method according to claim 4, wherein the plurality of refocusing radio frequency pulses is synchronized to a respiratory cycle.

11. A method according to claim 4, wherein the plurality of refocusing radio frequency pulses is synchronized to a cardiac cycle.

12. A computer-readable medium having computer-executable instructions for performing a method for calculating a flip angle schedule for a spin echo train with reduced refocusing flip angles, the spin echo train having a plurality of spin echoes, the computer-readable medium comprising:

program code for defining a target signal schedule including target signals for each spin echo in the spin echo train, the target signal schedule comprising a first phase configured to establish a pseudo-steady state and a second phase where the target signals increase from a pre-defined minimum signal value to a pre-defined maximum signal value; and

program code for calculating a corresponding flip angle schedule from the target signal schedule using only non-relaxation properties.

13. An apparatus for acquiring magnetic resonance images using a spin echo train with reduced refocusing flip angles, the spin echo train having a plurality of spin echoes, the apparatus comprising:

a magnetic resonance imaging assembly comprising a magnet, a plurality of gradient coils, a radio frequency coil, a radio frequency transceiver system, and a pulse generator module; and

a computer system coupled to the magnetic resonance imaging assembly and programmed to;

define a target signal schedule including target signals for each spin echo in the spin echo train, wherein the target signal schedule comprising a first phase configured to establish a pseudo-steady state and a second phase where the target signals increase from a pre-defined minimum signal value to a pre-defined maximum signal value;

calculate a corresponding flip angle schedule from the target signal schedule using only non-relaxation properties;

generate an excitation radio frequency pulse;

generate a plurality of refocusing radio frequency pulses, each refocusing radio frequency pulse having a flip angle determined based on the flip angle schedule; and

acquire magnetic resonance imaging data.

14. An apparatus according to claim 13, wherein the first phase of the target signal schedule comprises target signals that decrease asymptotically to the pre-defined signal value.

15. An apparatus according to claim 13, wherein the target signals of the second phase of the target signal schedule increase from the pre-defined minimum signal value to the pre-defined maximum signal value.

16. An apparatus according to claim 13, wherein the plurality of refocusing radio frequency pulses is generated in accordance with a single-shot fast spin echo (SSFSE) pulse sequence.

17. An apparatus according to claim 13, wherein the plurality of refocusing radio frequency pulses is generated in accordance with a gradient and spin echo (GRASE) pulse sequence.

18. An apparatus according to claim 13, wherein the plurality of refocusing radio frequency pulses is generated in accordance with a three-dimensional fast spin echo (3D-FSE) pulse sequence.

19. An apparatus according to claim 13, wherein the plurality of refocusing radio frequency pulses is synchronized to a respiratory cycle.

20. An apparatus according to claim 13, wherein the plurality of refocusing radio frequency pulses is synchronized to a cardiac cycle.