System and method for reducing radio frequency peak voltage and power requirements in magnetic resonance imaging using time-shifted multiband radio frequency pulses

Patent metadata

Field	Value
Publication number	US-9689948-B2
Application number	US-201214349195-A
Country	US
Kind code	B2
Filing date	Oct 3, 2012
Priority date	Oct 3, 2011
Publication date	Jun 27, 2017
Grant date	Jun 27, 2017

Abstract

A system and method for producing images depicting a plurality of slice locations in a subject using a magnetic resonance imaging (“MRI”} system is provided. In particular, the system and method utilize time-shifted multiband radio frequency (“RF”} pulses to lower peak voltage and peak power requirements when using conventional multiband RF pulses. A time-shifted multiband RF pulse includes at least two component RF pulses, which may be single-band or multiband pulses. The component RF pulses are designed such that they do not have temporal footprints that completely overlap; although, they may have temporal foot-prints that partially overlap or do not overlap at all. The MRI system is used to acquire magnetic resonance signals formed in response to a time-shifted multiband RF pulse and, from these acquired signals, images depicting each of the plurality of slice locations in the subject are reconstructed.

First claim

The invention claimed is:

1. A method for producing images depicting each of a plurality of slice locations in a subject using a magnetic resonance imaging (MRI) system, the steps of the method comprising:
a) directing the MRI system to perform a pulse sequence that includes generating a multiband radio frequency (RF) pulse by generating at least two component RF pulses, each component RF pulse having a center frequency associated with a different slice location and each component RF pulse being generated at a different time;
b) acquiring with the MRI system, k-space data from the plurality of slice locations by sampling magnetic resonance signals formed in response to the multiband RF pulse generated in step a); and
c) reconstructing images depicting each of the plurality of slice locations in the subject from the acquired k-space data.

2. The method as recited in claim 1 in which at least one of the component RF pulses is a multiband RF pulse that is configured to manipulate spin magnetization in a plurality of slice locations.

3. The method as recited in claim 2 in which the multiband RF pulse is a time-shifted multiband RF pulse that is composed of at least two component RF pulses having temporal footprints that do not completely overlap.

4. The method as recited in claim 1 in which the different times at which each component RF pulse is generated are selected such that a temporal footprint of each component RF pulse partially overlaps with a temporal footprint of at least one other component RF pulse.

5. The method as recited in claim 1 in which the different times at which each component RF pulse is generated are selected such that each component RF pulse has a temporal footprint that does not overlap with a temporal foot print of another component RF pulse.

6. The method as recited in claim 1 in which step a) includes performing a pulse sequence that also includes generating a multiband RF refocusing pulse by generating at least two component RF refocusing pulses, each component RF refocusing pulse having a different center frequency associated with a different slice location and each component RF refocusing pulse being generated at a different time such that a temporal footprint of each component RF refocusing pulse partially overlaps with a temporal footprint of at least one other component RF refocusing pulse.

7. The method as recited in claim 6 in which step a) includes performing a pulse sequence that also includes establishing a slice-selection gradient as the multiband RF refocusing pulse is generated and asymmetrically positioning the component RF refocusing pulses with respect to the slice-selection gradient such that differential dephasing of spins in the different slice locations is mitigated.

8. The method as recited in claim 6 in which the component RF refocusing pulses are asymmetrically positioned in the temporal footprint of the multiband RF refocusing pulse so as to cancel differential dephasing caused by the multiband RF pulse generated in step a).

9. The method as recited in claim 6 in which the at least two component RF pulses each have a bandwidth that is equal to a bandwidth of each of the at least two component RF refocusing pulses, and in which the at least two component RF refocusing pulses are each separated by a time shift with a duration that is twice a duration of a time shift separating each of the at least two component RF refocusing pulses.

10. The method as recited in claim 6 in which the at least two component RF pulses each have a bandwidth that is twice a bandwidth of each of the at least two component RF refocusing pulses, and in which each of the at least two component RF pulses are separated by a time shift with a duration that is equal to a time shift separating each of the at least two component RF refocusing pulses.

11. The method as recited in claim 1 in which the component RF pulses are designed to impose a different phase pattern along different slice-encoding directions, such that differential dephasing of spins in the different slice locations is mitigated.

12. The method as recited in claim 1 in which each of the component RF pulses have at least one of a different amplitude modulation, a different frequency modulation, a different peak amplitude, and a different duration.

13. The method as recited in claim 1 in which each of the at least two component RF pulses have a different phase that is shifted relative to the other component RF pulses.

14. A magnetic resonance imaging (MRI) system, comprising:
a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject arranged in the MRI system;
a plurality of gradient coils configured to apply a magnetic gradient field to the polarizing magnetic field;
a radio frequency (RF) system including at least one RF coil configured to apply an RF field to the subject and to receive magnetic resonance signals therefrom;
a computer system programmed to:
direct the RF system to generate a time-shifted multiband RF pulse that rotates spin magnetization in a plurality of slice locations in the subject, the time-shifted multiband RF pulse being composed of at least two component RF pulses, each component RF pulse having a different center frequency associated with a different one of the plurality of slice locations and each component RF pulse having a temporal footprint that partially overlaps a temporal footprint of another component RF pulse;
direct the RF system to receive magnetic resonance signals formed in response to the time-shifted multiband RF pulse, the magnetic resonance signals being received from the plurality of slice locations; and
reconstruct images depicting each of the plurality of slice locations in the subject from the received magnetic resonance signals.

15. The MRI system as recited in claim 14 in which the computer system is programmed to direct the RF system to generate at least one of the component RF pulses as a multiband RF pulse that is configured to manipulate spin magnetization in a plurality of slice locations.

16. The MRI system as recited in claim 14 in which the multiband RF pulse is a time-shifted multiband RF pulse that is composed of at least two component RF pulses having temporal footprints that do not completely overlap.

17. The MRI system as recited in claim 14 in which the computer system is programmed to direct the RF system to generate the time-shifted multiband RF pulse such that the temporal footprint of each component RF pulse does not overlap the temporal footprint of another of the component RF pulses.

18. The MRI system as recited in claim 14 in which the computer system is programmed to generate a time-shifted multiband RF refocusing pulse that is composed of at least two component RF refocusing pulses, each component RF refocusing pulse having a different center frequency associated with a different one of the plurality of slice locations and each component RF refocusing pulse being generated at a different time such that each of the component RF refocusing pulses do not have completely overlapping temporal footprints.

19. The MRI system as recited in claim 18 in which the computer system is programmed to:
direct the plurality of gradient coils to establish a slice-selection gradient as the time-shifted multiband RF refocusing pulse is generated; and
direct the RF system to asymmetrically position the component RF refocusing pulses with respect to the slice-selection gradient such that differential dephasing of spins in the different slice locat

Assignees

• Univ Minnesota

Inventors

• Ugurbil Kamil
• Auerbach Edward

Classifications

• G01R33/4835

  of multiple slices · CPC title

• G01R33/5615 Primary

  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 title

• A61B5/055 Primary

  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging · CPC title