FLIP ANGLE IMAGING WITH IMPROVED B1 MAPPING FOR MULTI-RF TRANSMIT SYSTEMS

• US 20110026799A1
• Filed: 03/26/2009
• Published: 02/03/2011
• Est. Priority Date: 03/27/2008
• 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: Voigt, Tobias Ratko, Nehrke, Kay, Katscher, Ulrich, Boernert, Peter
• Granted Patent: US 8,698,495 B2
• Time in Patent Office: Days
• US Class Current: 382/131
• CPC Class Codes: G01R 33/246 Spatial mapping of the RF magnetic field B1
• CPC Class Codes: G01R 33/3415 comprising arrays of sub-coils , i.e. phased-array coils with flexible receiver channels
• CPC Class Codes: G01R 33/5612 Parallel RF transmission, i.e. RF pulse transmission using a plurality of independent transmission channels
• CPC Class Codes: G01R 33/56518 due to eddy currents, e.g. caused by switching of the gradient magnetic field
• CPC Class Codes: G01R 33/5659 caused by a distortion of the RF magnetic field, e.g. spatial inhomogeneities of the RF magnetic field G01R33/56509, G01R33/56518, G01R33/56536 take precedence
• CPC Class Codes: G01R 33/586 for optimal flip angle of RF pulses

Abstract

A magnetic resonance method includes performing a plurality of magnetic resonance excitation operations each using a different sub-set of a set of radio frequency transmit coils (30), each sub-set including more than one radio frequency transmit coil, acquiring magnetic resonance data responsive to each said magnetic resonance excitation operation, and computing a B1 or flip angle map for each radio frequency transmit coil of the set of radio frequency transmit coils based on the acquired magnetic resonance data. A magnetic resonance method includes performing an actual flip angle mapping (AFI) sequence using a radio frequency transmit coil (32) with a ratio TR₁:TR₂ of the TR times of the AFI sequence selected to be rational, acquiring magnetic resonance data responsive to said AFI sequence, and computing a B1 or flip angle map for the radio frequency transmit coil based on the acquired magnetic resonance data.

Claims

1. A magnetic resonance method comprising:

performing a plurality of magnetic resonance excitation operations each using a different sub-set of a set of radio frequency transmit coils (30), each sub-set including more than one radio frequency transmit coil;

acquiring magnetic resonance data responsive to each said magnetic resonance excitation operation; and

computing a B1 or flip angle map for each radio frequency transmit coil of the set of radio frequency transmit coils based on the acquired magnetic resonance data.

2. The magnetic resonance method as set forth in claim 1, wherein each sub-set includes all but one radio frequency transmit coil of the set of radio frequency transmit coils (30).

3. The magnetic resonance method as set forth in claim 2, wherein the computing comprises:

computing the B1 or flip angle map for the jth radio frequency transmit coil (32) to be the same as or substantially equivalent to a map m_j(x,y) given by;

4. A magnetic resonance method comprising:

performing an actual flip angle mapping (AFI) sequence using a radio frequency transmit coil (32);

acquiring magnetic resonance data responsive to said AFI sequence; and

computing a B1 or flip angle map for the radio frequency transmit coil based on the acquired magnetic resonance data.

5. The magnetic resonance method as set forth in claim 4, wherein the performing of the AFI sequence further comprises:

performing the AFI sequence as an equidistant pulse sequence including real and dummy radio frequency excitation pulses.

6. The magnetic resonance method as set forth in claim 4, wherein the performing of the AFI sequence comprises performing said AFI sequence with a spoiling gradient area chosen to satisfy the condition:

7. The magnetic resonance method as set forth in claim 4, wherein the performing of the AFI sequence comprises performing said AFI sequence with a spoiling gradient phase incremented according to:

φ

_k−

φ

<sub>k−

1</sub>=nkφ

for even k

φ

_k−

φ

<sub>k−

1</sub>=kφ

for odd k

where φ

is a preselected phase value, n=TR₂/TR₁ denotes the ratio of the TR times of the AFI sequence and “

even” and

“

odd”

refer to pulses immediately before and after the shorter of the two TR times of the AFI sequence, respectively.

8. The magnetic resonance method as set forth in claim 4, wherein a ratio TR₁:

TR₂ of the TR times of the AFI sequence is selected to be rational.

9. The magnetic resonance method as set forth in claim 4, wherein the AFI sequence includes an arbitrary number N of repetition times TR₁, TR₂, . . . TR_n, where nε

{1, . . . , N}, N≧

2.

10. The magnetic resonance method as set forth in claim 9, wherein the repetition times TR₁, TR₂, . . . TR_n are selected such that the mutual ratio of each pair of repetition times, in the said group of repetition times, is a rational number.

11. The magnetic resonance method as set forth in claim 9, further comprising:

repeating the AFI sequence a plurality of times (P), wherein at least one TR time of the N repetition times TR₁, TR₂, . . . TR_n, nε

{1, . . . , N}, N≧

2, is different in each of the plurality of repetitions (P).

12. The magnetic resonance method as set forth in claim 9, wherein the computing further includes computing a spin-lattice relaxation time T₁ based on the acquired magnetic resonance data.

13. The magnetic resonance method as set forth in claim 6, wherein the radio frequency transmit coil (32) comprises a set of radio frequency transmit coils (30), and the performing and acquiring are repeated a plurality of times each using a different sub-set of the set of radio frequency transmit coils, each sub-set including more than one radio frequency transmit coil, and the computing comprises computing a B1 or flip angle map for each radio frequency transmit coil of the set of radio frequency transmit coils based on the acquired magnetic resonance data.

14. A magnetic resonance scanner (10) including at least one radio frequency transmit coil (32), the scanner configured to perform the method of claim 4.

15. A computer readable medium carrying software code which controls a processor or magnetic resonance system controller (44, 54, 62) to perform the method of claim 4.