Method and magnetic resonance apparatus to generate a spatially selective excitation

Patent metadata

Field	Value
Publication number	US-9797969-B2
Application number	US-201414484576-A
Country	US
Kind code	B2
Filing date	Sep 12, 2014
Priority date	Sep 12, 2013
Publication date	Oct 24, 2017
Grant date	Oct 24, 2017

Abstract

In a method to generate a spatially selective excitation in an imaging region of a magnetic resonance apparatus that precedes an acquisition of magnetic resonance data, in the course of the excitation an excitation trajectory in k-space is traversed, the excitation trajectory having a symmetry relative to the k-space center in at least one direction of k-space in the sense that a first traversed extreme value in this direction corresponds to the negative of the other extreme value traversed in this direction, so the excitation trajectory is shortened in the at least one directions on one side of the zero point between the extreme values, and the shortened excitation trajectory is used for excitation.

First claim

We claim as our invention:

1. A computerized method to generate a spatially selective excitation of nuclear spins in a subject situated in an imaging region of a magnetic resonance apparatus, preceding acquisition of diagnostic magnetic resonance data, comprising:
providing a computerized processor with a basic excitation sequence, that will excite said nuclear spins by causing said magnetic apparatus to radiate an excitation and to activate a gradient field that occurs while said excitation pulse is radiated, and thereby causing an excitation trajectory to be traversed in k-space, said excitation trajectory having a symmetry relative to a center of k-space in at least one direction of k-space, said at least one direction comprising extreme values, and said symmetry relative to said center of k-space being defined by a first-traversed extreme value of said excitation trajectory in said at least one direction and a corresponding negative of said first-traversed extreme value that forms the other extreme value in said at least one direction;
in said computerized processor, operating on said basic excitation sequence to shorten said excitation trajectory in said at least one direction on one side of a zero point between said extreme values; and
making said basic excitation, with said shortened excitation trajectory, available in electronic form at an output of said computerized processor in a control protocol having a format for operating said magnetic resonance apparatus to acquire said diagnostic magnetic resonance data with said nuclear spins being spatially selectively excited.

2. A method as claimed in claim 1 comprising shortening said excitation trajectory by shortening a portion of said excitation trajectory that occurs after traversal of said center of k-space by said excitation trajectory.

3. A method as claimed in claim 2 comprising shortening said excitation trajectory by shortening only said portion of said excitation trajectory that occurs after traversal of said center of k-space by said excitation trajectory.

4. A method as claimed in claim 1 comprising shortening said excitation trajectory by omitting planes situated orthogonally to said at least one direction and covered by partial trajectories in k-space.

5. A method as claimed in claim 1 comprising shortening said excitation trajectory by truncating a portion of said excitation trajectory that traverses successive k-space values in said at least one direction after crossing said center of k-space.

6. A method as claimed in claim 1 comprising using, as said excitation trajectory, an excitation trajectory selected from the group consisting of an echoplanar trajectory, a spiral stack, and a radial scan trajectory.

7. A method as claimed in claim 1 comprising shortening said excitation trajectory to give said shortened excitation trajectory an asymmetry described by an asymmetry factor that is less than 1 by multiplying said negative extreme value by said asymmetry factor.

8. A method as claimed in claim 7 wherein said asymmetry factor is between 0.5 and 1.

9. A method as claimed in claim 7 comprising shortening said excitation trajectory by executing an optimization method in said computerized processor that determines an asymmetry factor of the shortened excitation time by weighting an excitation quality and a time gain with regard to a duration of at least one of the excitation and echo time associated with said control protocol.

10. A method as claimed in claim 1 comprising shortening said excitation trajectory by executing an optimization method in said computerized processor by weighting an excitation quality and a time gain with regard to a duration of at least one of the excitation and echo time associated with said control protocol.

11. A method as claimed in claim 10 comprising, in said optimization method, determining and evaluating pulse responses for different shortened excitation trajectories.

12. A method as claimed in claim 10 comprising executing a Bloch simulation for each of a plurality of different shortened excitation trajectories using predetermined B1 maps, to determine said loss of excitation quality.

13. A method as claimed in claim 12 comprising also implementing said Bloch simulation with regard to an entirety of said magnetic resonance sequence, or said subject, to obtain respective values for image qualities and specific absorption ratios, and using said values and said optimization method.

14. A method as claimed in claim 10 comprising also adapting a power emitted to achieve said excitation at a point in k-space, as a further parameter in said optimization method.

15. A method as claimed in claim 10 comprising executing said optimization method dependent on at least one parameter that describes a sought imaging task.

16. A method as claimed in claim 1 comprising shortening said excitation trajectory dependent on a predetermined limitation of emitted radio frequency power for points in k-space that are traversed by said excitation trajectory.

17. A method as claimed in claim 1 comprising generating said control protocol as a protocol for parallel transmission of radio frequency pulses via different transmission channels of said magnetic resonance apparatus.

18. A method as claimed in claim 16 comprising also temporally shortening said excitation in said control protocol by undersampling with said parallel transmission.

19. A method as claimed in claim 1 comprising generating at least one radio frequency pulse in said control protocol using a windowing filter to smooth said radio frequency pulse.

20. A magnetic resonance (MR) apparatus comprising:
an MR data acquisition unit having an imaging volume in which an examination subject is situated;
an electronic memory organized as k-space;
a computerized processor provided with a basic excitation sequence, that will excite nuclear spins in said subject by causing said MR data acquisition unit to radiate an excitation and to activate a gradient field that occurs while said excitation pulse is radiated, and thereby causing an excitation trajectory to be traversed in k-space when MR data are acquired by said MR data acquisition unit and entered into said memory, said excitation trajectory having a symmetry relative to a center of k-space in at least one direction of k-space, said at least one direction comprising extreme values, and said symmetry relative to said center of k-space being defined by a first-traversed extreme value of said excitation trajectory in said at least one direction and a corresponding negative of said first-traversed extreme value that forms the other extreme value in said at least one direction;
said computerized processor being configured to operate on said basic excitation sequence to shorten said excitation trajectory in said at least one direction on one side of a zero point between said extreme values; and
said computerized processor being configured to incorporate said basic excitation, with said shortened excitation trajectory, in electronic form at an output of said computerized processor in a control protocol and to operate said MR data acquisition unit according to said protocol to acquire said diagnostic magnetic resonance data with said nuclear spins being spatially selectively excited.

Assignees

• Siemens Ag

Inventors

• Pfeuffer Josef
• Schneider Rainer

Classifications

• G01R33/5608

  Data processing and visualization specially adapted for MR, e.g. for feature analysis and pattern recognition on the basis of measured MR data, segmentation of measured MR data, edge contour detection on the basis of measured MR data, for enhancing measured MR data in terms of signal-to-noise ratio by means of noise filtering or apodization, for enhancing measured MR data in terms of resolution by means for deblurring, windowing, zero filling, or generation of gray-scaled images, colour-coded images or images displaying vectors instead of pixels (image data processing or generation, in general G06T) · CPC title

• G01R33/4836 Primary

  using an RF pulse being spatially selective in more than one spatial dimension, e.g. a 2D pencil-beam excitation pulse · CPC title

• G01R33/36

  Electrical details, e.g. matching or coupling of the coil to the receiver · CPC title

• G01R33/4818 Primary

  MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space · CPC title

• G01R33/4833

  using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices · CPC title