×

Shimming method for NMR magnet having large magnetic field inhomogeneities

  • US 5,539,316 A
  • Filed: 08/25/1995
  • Issued: 07/23/1996
  • Est. Priority Date: 08/25/1995
  • Status: Expired due to Term
First Claim
Patent Images

1. A method of determining the inhomogeneities in the magnetic field of a magnet which generates a magnetic field in a region containing a sample material, the method comprising:

  • A) generating an RF magnetic pulse which is incident on the sample material;

    B) generating a phase encoded gradient pulse in a gradient direction which is incident on the sample;

    C) detecting a first set of free induction decay signals s(tr)1 corresponding to a plurality of spatial points in the sample at a first time T1 ;

    D) detecting a second set of free induction decay signals s(tr)2 corresponding to said plurality of spatial points in the sample at a second time T2 ;

    E) transforming s(T1)1 and s(T1)2 from the time domain to image signal sets S(r)1 and S(r)2 in the frequency domain;

    F) converting S(r)1 and S(r)2 to polar coordinate signals Pw (r)1 and Pw (r)2, respectively;

    G) for each of Pw (r)1 and Pw (r)2, detecting a change in phase at each of a plurality of spatial points relative to a phase of a next previously detected point and, when said change in phase at a particular point exceeds a predetermined threshold indicative of a phase wrap, unwrapping the phase by adjusting the phase at said particular point by 2π

    , the phase unwrapping for all the spatial points resulting in phase unwrapped signals P(r)1 and P(r)2 ;

    H) subtracting P(r)1 from P(r)2 to obtain a phase difference signal P(r)D ; and

    I) acquiring a base frequency distribution, FB, in the gradient direction by calculating FB =(P(r)D /(t2 -t1)) for each of said spatial points.

View all claims
  • 1 Assignment
Timeline View
Assignment View
    ×
    ×