MAGNETIZATION PRIMER SEQUENCE FOR BALANCED STEADY STATE FREE PRECESSION IMAGING

• US 20040095138A1
• Filed: 11/14/2002
• Published: 05/20/2004
• Est. Priority Date: 11/14/2002
• Status: Active Grant

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Foxall, David L.
• Granted Patent: US 6,885,193 B2
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: G01R 33/5613 Generating steady state signals, e.g. low flip angle sequences [FLASH]

Abstract

To prepare magnetization for steady state magnetic resonance imaging, magnetic resonance is primed using a parameterized priming sequence (60). The parameterized priming sequence (60) has a spectral offset parameter value (40) corresponding to a spectral offset of the steady state imaging. Subsequent to the priming, steady state magnetic resonance imaging data at the first spectral offset is acquired using a magnetic resonance imaging scanner (10). A reconstruction processor (44) reconstructs the imaging data to generate an image representation. Preferably, the parameterized priming sequence (60) includes a longitudinal priming sequence (62) that prepares longitudinal magnetization in a state approximating a steady state longitudinal magnetization. A spoiler sequence (64) spoils transverse magnetization while retaining longitudinal magnetization. A transverse priming sequence (66) prepares traverse magnetization in a state approximating a steady state transverse magnetization.

Claims

1. A steady state magnetic resonance imaging apparatus including:

a means (10, 22, 24, 30, 34, 40) for priming magnetic resonance using a parameterized priming sequence, the parameterized priming sequence having a spectral offset parameter value corresponding to a first spectral offset;

a means (10, 22, 24, 30, 42) for acquiring imaging data using steady state magnetic resonance imaging at the first spectral offset subsequent to the priming; and

a means (44) for reconstructing the imaging data to generate an image representation.

2. The apparatus as set forth in claim 1, wherein the parameterized priming sequence generating means includes:

a means (62) for generating a longitudinal priming sequence that prepares longitudinal magnetization in a state approximating a steady state longitudinal magnetization, the longitudinal priming sequence including;

a first radio frequency pulse at a selected flip angle, and

a second radio frequency pulse with a flip angle corresponding to 90°

minus the selected flip angle,

the first and second radio frequency pulses being temporally spaced by a temporal spacing of excitation pulses of the steady state magnetic resonance imaging, and

the first and second radio frequency pulses being relatively phase-shifted by an amount determined by the spectral offset parameter value;

a means (64) for generating a spoiler sequence that spoils transverse magnetization while retaining longitudinal magnetization; and

a means (66) for generating a transverse priming sequence that prepares traverse magnetization in a state approximating a steady state transverse magnetization.

3. The apparatus as set forth in claim 2, wherein the transverse priming sequence includes:

a sequence of radio frequency pulses in which successive pulses are each phase-shifted by a selected amount determined from the spectral offset parameter value.

4. The apparatus as set forth in claim 2, wherein the transverse priming sequence includes:

a series of radio frequency pulses, each radio frequency pulse having a tip angle of (2n+1)Δ

α

, where n is an integer index of successive pulses starting at n=0 and Δ

α

is a tip angle of the first pulse.

5. The apparatus as set forth in claim 1, wherein the priming sequence generating means (10, 22, 24, 30, 34, 40) and the data acquiring means (10, 22, 24, 30, 42) repeat the priming and acquiring of imaging data for at least one other spectral offset, and the apparatus further includes:

a means (38) for combining imaging data at the first spectral offset and at the at least one other spectral offset to generate combined imaging data, the reconstructing means (44) reconstructing the combined imaging data to generate the image representation.

6. The apparatus as set forth in claim 5, wherein the first spectral offset and the at least one other spectral offset include spectral offsets at θ

_o+360°

×

(n/N), where N specifies a total number of spectral offsets, n is a spectral offset index running from 0 to N−

1, and θ

_o is an angular offset value.

7. An apparatus for steady state magnetic resonance imaging, the apparatus including:

a priming sequence generator (34) that generates a parameterized priming sequence configured using a spectral offset parameter (40);

a magnetic resonance imaging scanner (10) that primes magnetic resonance using the parameterized priming sequence configured to a selected spectral offset and, subsequent to the priming, acquires imaging data using steady state magnetic resonance imaging at the selected spectral offset; and

a reconstruction processor (44) that reconstructs imaging data to generate an image representation.

8. The apparatus as set forth in claim 7, further including:

a phase-cycling processor (38) that repeats the priming and acquiring of imaging data for at least one other spectral offset, and combines imaging data at the first spectral offset and at the at least one other spectral offset to generate combined imaging data from which the reconstruction processor (44) generates the image representation.

9. A steady state magnetic resonance imaging method including:

priming magnetic resonance using a parameterized priming sequence, the parameterized priming sequence having a spectral offset parameter value corresponding to a first spectral offset;

subsequent to the priming, acquiring imaging data using steady state magnetic resonance imaging at the first spectral offset; and

reconstructing the imaging data to generate an image representation.

10. The imaging method as set forth in claim 9, wherein the parameterized priming sequence includes:

a longitudinal priming sequence that prepares longitudinal magnetization in a state approximating a steady state longitudinal magnetization;

a spoiler sequence that spoils transverse magnetization while retaining longitudinal magnetization; and

a transverse priming sequence that prepares traverse magnetization in a state approximating a steady state transverse magnetization.

11. The imaging method as set forth in claim 10, wherein the spoiler sequence is interleaved with the longitudinal primer sequence.

12. The imaging method as set forth in claim 10, wherein the longitudinal priming sequence includes:

a binomial sequence.

13. The imaging method as set forth in claim 10, wherein the longitudinal priming sequence produces a squared sinusoidal longitudinal magnetization having zeroes conforming with the steady state longitudinal magnetization.

14. The imaging method as set forth in claim 10, wherein the longitudinal priming sequence produces a longitudinal magnetization having zeroes conforming with the steady state longitudinal magnetization.

15. The imaging method as set forth in claim 10, wherein the longitudinal priming sequence includes:

a first radio frequency pulse at a selected flip angle; and

a second radio frequency pulse with a flip angle corresponding to 90°

minus the selected flip angle;

the first and second radio frequency pulses being temporally spaced by a temporal spacing of excitation pulses of the steady state magnetic resonance imaging; and

the first and second radio frequency pulses being relatively phase-shifted by an amount determined by the spectral offset parameter value.

16. The imaging method as set forth in claim 10, wherein the longitudinal priming sequence includes:

a sequence of radio frequency pulses in which successive pulses are each phase-shifted by a selected amount determined from the spectral offset parameter value.

17. The imaging method as set forth in claim 10, wherein the longitudinal priming sequence and the spoiler cooperate to generate a longitudinally aligned fan of magnetization isochromats.

18. The imaging method as set forth in claim 17, wherein the transverse priming sequence acts upon the longitudinally aligned fan of magnetization isochromats, the transverse priming sequence including:

a sequence of radio frequency pulses that tip the fan of longitudinally aligned magnetization isochromats away from longitudinal alignment to an angle defined by the steady state longitudinal and transverse magnetizations.

19. The imaging method as set forth in claim 18, further including:

applying a reverse transverse priming sequence including the sequence of radio frequency pulses of the transverse priming sequence in reverse order to tip the fan of magnetization isochromats back to longitudinal alignment.

20. The imaging method as set forth in claim 19, further including:

subsequent to the applying of the reverse transverse priming sequence, applying a pre-saturation magnetic resonance sequence; and

subsequent to the pre-saturating, applying the transverse priming sequence to tip the fan of longitudinally aligned magnetization isochromats back to the angle defined by the steady state longitudinal and transverse magnetizations.

21. The imaging method as set forth in claim 10, wherein the transverse priming sequence includes a series of radio frequency pulses, successive radio frequency pulses increasing in phase by an amount determined by the spectral offset parameter value.

22. The imaging method as set forth in claim 10, wherein the transverse priming sequence includes a series of radio frequency pulses, each radio frequency pulse having a tip angle of (2n+1)Δ

α

, where n is an integer index of successive pulses starting at n=0 and Δ

α

is the tip angle of the first pulse.

23. The imaging method as set forth in claim 10, wherein the acquiring of imaging data using steady state magnetic resonance imaging includes:

applying radio frequency pulses spaced at a selected inter-pulse spacing to maintain coherence of nuclear magnetization during the steady state magnetic resonance imaging.

24. The imaging method as set forth in claim 9, wherein the parameterized priming sequence includes two or more radio frequency pulses that cooperatively produce a longitudinal magnetization having zeroes conforming with the steady state longitudinal magnetization.

25. The imaging method as set forth in claim 9, wherein the parameterized priming sequence includes spatial encoding slice-select and read magnetic field gradients that are balanced and introduce no net magnetic resonance phase shift.

26. The imaging method as set forth in claim 9, further including:

repeating the priming and acquiring of imaging data for at least one other spectral offset; and

combining imaging data at the first spectral offset and at the at least one other spectral offset to generate combined imaging data, the combined imaging data being reconstructed to generate the image representation.

27. The imaging method as set forth in claim 26, wherein the combining of imaging data includes:

adding the image representations in k-space to remove bending artifacts.

28. The imaging method as set forth in claim 26, wherein the first spectral offset and the at least one other spectral offset include 0°

and 180°

.

29. The imaging method as set forth in claim 26, wherein the first spectral offset and the at least one other spectral offset include four spectral offsets angularly spaced at 90°

increments.

30. The imaging method as set forth in claim 26, wherein the first spectral offset and the at least one other spectral offset include spectral offsets at phase increments of 360°

/N, where N specifies a total number of spectral offsets.