B1 and/or B0 mapping in MRI system using k-space spatial frequency domain filtering

• US 20100239151A1
• Filed: 05/24/2010
• Published: 09/23/2010
• Est. Priority Date: 03/19/2009
• Status: Active Grant

• Current Assignee: Toshiba Medical Systems Corporation (Canon Inc.)
• Original Assignee: Toshiba Medical Systems Corporation (Canon Inc.), Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: Wheaton, Andrew J., Dannels, Wayne R.
• Granted Patent: US 8,502,538 B2
• Time in Patent Office: Days
• US Class Current: 382/131
• CPC Class Codes: G01R 33/243 Spatial mapping of the polarizing magnetic field
• CPC Class Codes: G01R 33/246 Spatial mapping of the RF magnetic field B1
• CPC Class Codes: G01R 33/44 using nuclear magnetic resonance [NMR] G01R33/24, G01R33/62 take precedence
• CPC Class Codes: G06T 11/003 Reconstruction from projections, e.g. tomography

Abstract

Frequency filtering of spatially modulated or “tagged” MRI data in the spatial frequency k-space domain with subsequent 2DFT to the spatial domain and pixel-by-pixel arithmetic calculations provide robust data that can be used to derive B1 and/or B0 maps for an MRI system.

Claims

1. A method for generating a B0 map in an MRI system, said method comprising using an MRI system to:

(a) acquire at least one set of amplitude tagged MRI data signals in spatial frequency domain k-space from MR nuclei within an imaged volume of the MRI system;

(b) process said k-space data to produce at least two sub-sets of frequency-filtered k-space data including at least two of;

(i) a baseline low-frequency sub-set, (ii) a positive higher frequency sub-set including a harmonic version of said baseline sub-set, and (iii) a negative higher frequency sub-set including a harmonic version of said baseline sub-set;

(c) separately Fourier transform each of said at least two sub-sets to produce at least two respectively corresponding complex-valued spatial domain data sets;

(d) arithmetically combine said at least two complex-valued spatial domain data sets on a pixel-by-pixel basis to provide an off-resonance phase set of pixel data representing a B0 map value; and

(e) store said B0 map values in a computer accessible and readable memory device for subsequent use by said MRI system.

2. A method for generating a B0 map as in claim 1, wherein step (d) comprises:

calculating a difference value between the complex phases of said at least two complex-valued spatial domain data sets.

3. A method for generating a B0 map as in claim 2, wherein said at least two complex-valued spatial domain data sets comprise (i) a 2DFT of said baseline low-frequency sub-set, and (ii) a 2DFT of one of said positive and negative higher frequency sub-sets.

4. A method for generating a B0 map as in claim 2, wherein said at least two complex-valued spatial domain data sets comprise (i) a 2DFT of said positive higher frequency sub-set and (ii) a 2DFT of said negative higher frequency sub-set.

5. A method for generating a B0 map as in claim 1, wherein said sub-sets of frequency-filtered k-space data comprise windowed k-space data from substantially equally-sized windows.

6. A method for generating a B0 map as in claim 5, wherein each of said windows comprises no more than one-third of k-space data.

7. A method for generating a B0 map as in claim 1, and further comprising:

generating a B1 map using at least two of said sub-sets of frequency-filtered k-space data; and

correcting said B1 map using said B0 map.

8. A method for generating a B0 map as in claim 1, wherein said amplitude tagged MRI data signals are generated using phase-cycled tagging pulse pairs, wherein the polarity of an RF tag pulse is reversed in one pair of tagging pulses as compared to another pair of tagging pulses, thereby improving the selectivity of signals appearing in said frequency-filtered k-space data.

9. A computer-readable storage medium containing computer program code which, when executed, effects the method of claim 1.

10. An MRI system comprising:

an MRI scanner configured to acquire at least one set of amplitude tagged MRI data signals in spatial frequency domain k-space from MR nuclei within an imaged volume of the MRI system; and

at least one processor configured to;

process said k-space data to produce at least two sub-sets of frequency-filtered k-space data including at least two of;

(i) a baseline low-frequency sub-set, (ii) a positive higher frequency sub-set including a harmonic version of said baseline sub-set, and (iii) a negative higher frequency sub-set including a harmonic version of said baseline sub-set;

separately Fourier transform each of said at least two sub-sets to produce at least two respectively corresponding complex-valued spatial domain data sets;

arithmetically combine said at least two complex-valued spatial domain data sets on a pixel-by-pixel basis to provide an off-resonance phase set of pixel data representing a B0 map value; and

store said B0 map values in a computer accessible and readable memory device for subsequent use by said MRI system.

11. An MRI system as in claim 10, wherein said arithmetic combination comprises:

calculating a phase difference value between said at least two complex-valued spatial domain data sets.

12. An MRI system as in claim 11, wherein said at least two complex-valued spatial domain data sets comprise (i) a 2DFT of said baseline low-frequency sub-set, and (ii) a 2DFT of one of said positive and negative higher frequency sub-sets.

13. An MRI system as in claim 11, wherein said at least two complex-valued spatial domain data sets comprise (i) a 2DFT of said positive higher frequency sub-set, and (ii) a 2DFT of said negative higher frequency sub-set.

14. An MRI system as in claim 10, wherein said sub-sets of frequency-filtered k-space data comprise windowed k-space data from substantially equally-sized windows.

15. An MRI system as in claim 14, wherein each of said windows comprises no more than one-third of k-space data.

16. An MRI system as in claim 10 and further configured to:

generate a B1 map using at least two of said sub-sets of frequency-filtered k-space data; and

correct said B1 map using said B0 map.

17. An MRI system as in claim 10, wherein said amplitude tagged MRI data signals are generated using phase-cycled tagging pulse pairs, wherein the polarity of an RF tag pulse is reversed in one pair of tagging pulses as compared to another pair of tagging pulses, thereby improving the selectivity of signals appearing in said frequency-filtered k-space data.