SYSTEM, METHOD, AND APPARATUS FOR MAGNETIC RESONANCE RF-FIELD MEASUREMENT

• US 20100315084A1
• Filed: 06/15/2009
• Published: 12/16/2010
• Est. Priority Date: 06/15/2009
• Status: Active Grant

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Sacolick, Laura I., Vogel, Mika W., Hancu, Ileana, Wiesinger, Florian
• Granted Patent: US 8,198,891 B2
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: A61B 5/055 involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• CPC Class Codes: A61B 5/318 Heart-related electrical modalities, e.g. electrocardiography [ECG]
• CPC Class Codes: G01R 33/246 Spatial mapping of the RF magnetic field B1
• CPC Class Codes: G01R 33/583 Calibration of signal excitation or detection systems, e.g. for optimal RF excitation power or frequency G01R33/246 takes precedence

Abstract

An apparatus, system, and method including a magnetic resonance imaging (MRI) apparatus includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet, and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images, and a computer. The computer is programmed to apply a first off-resonant radio frequency (RF) pulse at a first frequency different than the resonant frequency to a plurality of nuclei excited at a resonant frequency, acquire a first signal from the plurality of nuclei after application of the first off-resonant RF pulse, determine a phase shift from the first signal based on the first off-resonant RF pulse, determine a B1 field based on the phase shift, and store the B1 field on a computer readable storage medium.

Claims

1. A magnetic resonance imaging (MRI) apparatus comprising:

a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet, and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images; and

a computer programmed to;

apply a first off-resonant radio frequency (RF) pulse at a first frequency to a plurality of nuclei excited at a resonant frequency, wherein the first frequency is different than the resonant frequency;

acquire a first signal from the plurality of nuclei after application of the first off-resonant RF pulse;

determine a phase shift from the first signal, wherein the phase shift is based on the first off-resonant RF pulse;

determine a B1 field based on the phase shift; and

store the B1 field on a computer readable storage medium.

2. The MRI apparatus of claim 1 wherein the computer is further programmed to:

apply a first resonant RF pulse to the plurality of nuclei before application of the first off-resonant RF pulse to excite the plurality of nuclei at the resonant frequency;

apply a second resonant RF pulse to the plurality of nuclei to excite the plurality of nuclei at the resonant frequency;

apply a second off-resonant RF pulse to the plurality of nuclei after application of the second resonant RF pulse, wherein the second off-resonant RF pulse is at a second frequency different than the resonant frequency and the first frequency;

acquire a second signal from the plurality of nuclei after application of the second off-resonant RF pulse; and

wherein the computer, in being programmed to determine the phase shift, is programmed to determine a first phase difference between the first signal and the second signal to determine the phase shift, wherein the phase shift is further based on the second off-resonant RF pulse.

3. The MRI apparatus of claim 2 wherein the computer is further programmed to:

repeat application of the first resonant RF pulse and the first off-resonant RF pulse and repeat acquisition of the first signal until a first image data set is acquired;

repeat application of the second resonant RF pulse and the second off-resonant RF pulse and repeat acquisition of the second signal until a second image data set is acquired; and

determine a plurality of spatially dependant phase differences between the first and second image data sets, wherein the plurality of spatially dependant phase differences comprises the first phase difference.

4. The MRI apparatus of claim 2 wherein the computer is further programmed to:

apply a spatial encoding gradient to the plurality of nuclei during acquisition of the first signal; and

apply the spatial encoding gradient to the plurality of nuclei during acquisition of the second signal.

5. The MRI apparatus of claim 2 wherein the first off-resonant RF pulse is a Fermi pulse and the second off-resonant RF pulse is a Fermi pulse.

6. The MRI apparatus of claim 2 wherein the first frequency and the second frequency are substantially symmetric about a water resonance frequency.

7. The MRI apparatus of claim 2 wherein the computer is further programmed to:

determine a first B1 magnetic field map based on the first phase difference; and

display the first B1 magnetic field map to a user.

8. The MRI apparatus of claim 7 wherein the computer is further programmed to:

apply a third resonant RF pulse at the resonant frequency to the plurality of nuclei;

acquire a third signal from the plurality of nuclei after application of the third resonant RF pulse;

determine if a frequency center point of the first off-resonant RF pulse and a frequency center point of the second off-resonant RF pulse are asymmetric about a water resonance frequency based on the first, second, and third signals; and

determine a B0 field based on the determination.

9. The MRI apparatus of claim 2 wherein the application of the first resonant RF pulse and the first off-resonant RF pulse occurs during a first scan, and wherein the second resonant RF pulse and the second off-resonant RF pulse occurs during a second scan.

10. The MRI apparatus of claim 9 wherein the first and second scans are imaging scans.

11. The MRI apparatus of claim 2 wherein the first resonant RF pulse, the first off-resonant RF pulse, the second resonant RF pulse, and the second off-resonant RF pulse occur during a first scan.

12. The MRI apparatus of claim 11 wherein the first scan is an imaging scan.

13. A computer readable storage medium having stored thereon a computer program comprising instructions which when executed by a computer cause the computer to:

cause a pulse generator to apply a first off-resonant radio frequency (RF) pulse to a plurality of excited nuclei, the first off-resonant RF pulse configured to shift a resonant frequency of the plurality of nuclei;

receive a first signal from the plurality of excited nuclei via at least one magnetic resonant (MR) receive coil after application of the first off-resonant RF pulse;

determine a phase shift induced by the first off-resonant RF pulse;

determine a B1 field based on the phase shift; and

store the B1 field on a storage device coupled to an MR imaging apparatus.

14. The computer readable storage medium of claim 13 having further instructions to cause the computer to:

apply a first resonant RF pulse to the plurality of nuclei to excite the plurality of nuclei, the first resonant RF pulse at a first frequency substantially equivalent to a resonant frequency of the plurality of nuclei;

apply a second resonant RF pulse to the plurality of nuclei after acquisition of the first signal to excite the plurality of nuclei;

apply a second off-resonant RF pulse to the plurality of excited nuclei after application of the second resonant RF pulse, the second off-resonant RF pulse configured to shift the resonant frequency of the plurality of nuclei;

receive a second signal from the plurality of excited nuclei after application of the second off-resonant RF pulse;

determine a phase difference between the first signal and the second signal; and

wherein the instructions that cause the computer to determine the phase shift cause the computer to determine the phase shift based on the phase difference.

15. The computer readable storage medium of claim 14 having further instructions to cause the computer to generate a B I map based on the phase shift.

16. The computer readable storage medium of claim 14 having further instructions to cause the computer to determine a transmit gain based on the phase shift.

17. A method of determining a magnetic field comprising:

shifting a resonating frequency of a plurality of nuclei after the plurality of nuclei are excited at a first resonating frequency;

acquiring a first signal with a magnetic resonance (MR) receive coil from the plurality of nuclei after shifting the resonating frequency;

determining a phase shift based on the first signal; and

storing the phase shift on a computer readable storage medium coupled to an MR apparatus.

18. The method of claim 17 further comprising determining a B1 field based on the phase shift.

19. The method of claim 17 wherein shifting the resonating frequency of the plurality of nuclei comprises applying a first off-resonance RF pulse to the plurality of nuclei excited at the first resonating frequency.

20. The method of claim 19 further comprising:

applying a first resonating RF pulse to the plurality of nuclei before applying the first off-resonance RF pulse to excite the plurality of nuclei at the first resonating frequency;

applying a second resonating RF pulse to the plurality of nuclei after acquiring the second signal to excite the plurality of nuclei at the first resonating frequency;

shifting the resonating frequency of the plurality of nuclei excited at the first resonating frequency via application of a second off-resonance RF pulse after applying the second resonating RF pulse;

acquiring a second signal from the plurality of nuclei after application of the second off-resonance RF pulse; and

determining a phase difference between the first and second signals; and

wherein determining the phase shift comprises determining the phase shift based on the phase difference.

21. The method of claim 20 further comprising generating a magnetic field map based on the phase difference between the first and second signals.