Method and apparatus for automated magnetic field shimming in magnetic resonance spectroscopic imaging

• US 4,899,109 A
• Filed: 08/17/1988
• Issued: 02/06/1990
• Est. Priority Date: 08/17/1988
• Status: Expired due to Fees

• Current Assignee: Toshiba America MRI Incorporated (Toshiba Corporation)
• Original Assignee: Diasonics, Inc. (General Electric Company)
• Inventors: Derby, Kevin A., Tropp, James S., Hawryszko, K. Christine
• Primary Examiner(s): Tokar, Michael J.
• Application Number:
  US07/233,021
• Time in Patent Office: 538 Days
• Field of Search: 324/300, 324/307, 324/309, 324/318, 324/320, 324/319, 324/322
• US Class Current: 324/320
• CPC Class Codes: G01R 33/3875 using correction coil assemblies, e.g. active shimming
• CPC Class Codes: G01R 33/56 Image enhancement or correction, e.g. subtraction or averaging techniques , e.g. improvement of signal-to-noise ratio and resolution

Abstract

An automated magnetic field shimming process for magnetic resonance spectroscopic imaging (MRSI) uses doubly phase encoded NMR spin echo responses in conjunction with multiple fourier transformation to derive a spectroscopic plot for each voxel in a three-dimensional array of voxels in an image volume (including portions of a living subject) which is to be subsequently subjected to MRSI using NMR FID or spin echo responses. The thus derived hydrogen peak frequency is taken as a measurement of existing static magnetic field intensity within that voxel and adjusted shim coil currents are calculated so as to reduce or minimize nonuniformity of the static magnetic field. A nonexistent pseudo shim coil having an assumed uniform shim contribution may enhance the resulting field homogeneity. The auto shimming procedure may be applied iteratively as required to achieve the predetermined degree of field uniformity.

Claims

1. A method for adjusting currents in plural magnetic shim coils in an MRI/MRSI system comprising the steps of:

(a) mapping an existing magnetic field distribution;

(b) machine calculating an adjusted set of currents for actual shim coils to improve uniformity of field distribution while also using in such calculation an assumed uniformly distributed fictitious pseudo shim coil which does not actually exist thereby permitting uniform offset adjustments to be included in the calculations; and

(c) applying said adjusted set of currents to the actual shim coils.

2. A method for adjusting currents in plural magnetic shim coils in an MRI/MRSI system comprising the steps of:

(a) acquiring an MRI image of a patient volume;

(b) defining a desired MRSI image volume therewithin; and

(c) automatically shimming a static magnetic field within the defined MRSI image volume by mapping the existing field distribution and calculating and applying adjusted shim coil currents to improve the homogeneity of the thus mapped distribution, including the step of using in such calculation an assumed uniformly distributed fictitious pseudo shim coil which does not actually exist thereby permitting uniform offset adjustments to be included in the calculations.

44. A method as in claim 2 wherein said shimming step (c) includes the step of generating spectrographic data including measured NMR response versus NMR frequency data for each of plural voxels within said patient volume, and automatically picking peak spectrographic responses by selecting a predetermined NMR species response peak frequency as the spectrographic data representing B_o intensity within a corresponding voxel.

3. A method of automated shimming of static magnetic fields for homogeneity in an MRSI system comprising:

(a) acquiring and using NMR spin echo responses to create NMR spectral responses from plural voxels within an image area and a map of existing magnetic field variation therewithin corresponding to the NMR spectral peak frequency of a predetermined NMR species and using same to automatically reduce said variations; and

(b) thereafter acquiring and using NMR FID responses to create MRSI plots of other NMR species from said image area.

4. A method of automated shimming of static magnetic fields for homogeneity in an MRSI system comprising:

(a) mapping the distribution of incremental magnetic field change for unit current to create a calibration value matrix for each of plural magnetic shim coils over a predetermined array of voxels;

(b) mapping and recording variations in existing static magnetic field distribution for only some of said voxels;

(c) using said recorded variations and a subset of said calibration value matrix corresponding to only some of said voxels to calculate adjusted shim coil currents required to reduce the magnitude of said variations, including the step of using in such calculation an assumed uniformly distributed fictitious pseudo shim coil which does not actually exist thereby permitting uniform offset adjustments to be included in the calculations; and

(d) applying said adjusted shim coil currents to respectively corresponding shim coils.

45. A method as in claim 4 wherein said mapping step (a) includes the steps of:

(i) generating spectrographic data including measured NMR response versus NMR frequency data for each of said voxels, and

(ii) automatically picking peak spectrographic responses by selecting a predetermined NMR species response peak frequency as the spectrographic data representing B_o intensity within a corresponding voxel.

5. A multiply-tuned RF coil for use in magnetic resonance spectroscopic imaging, said coil comprising:

multiple pairs of resonant Helmholtz coils, the coils in each pair being capacitively over coupled to one another; and

plural adjustably positionable loading and coupling loops, one of said loops being inductively coupled to each of said Helmholtz pairs and feed-connected to a respectively associated separate RF transmission line.

6. A multiply-tuned RF coil as in claim 5 further comprising:

a frequency multiplexer comprising a plurality of band-pass/band-stop filters individually connected at one end to a respectively corresponding one of said transmission lines and commonly connected together at their other ends to a common RF input/output port.

7. A method for adjusting currents in plural magnetic shim coils in a magnetic resonance imaging system, said method comprising the steps of

(a) mapping the distribution of a static magnetic field B within an imaging volume with a first set of currents I₁ . . . I_n in shim coils S₁ . . . S_n by magnetic resonance spectroscopy which maps the frequency of a predetermined NMR species at an array of locations within said imaging volume;

(b) machine calculating an adjusted second set of shim coil currents AI₁ . . . AI_n using a calibration matrix of incremental B field distributions produced by the individual shim coils and the measured existing B field distribution so as to minimize variations in the B field distribution; and

(c) applying said adjusted shim coil currents AI₁ . . . AI_n to respectively corresponding shim coils S₁ . . . S_n.

8. A method as in claim 7 further comprising, after said applying step(c);

(d) again performing said mapping step;

(e) comparing the resulting uniformity of said B field distribution to a predetermined maximum permissible magnitude of variation; and

(f) if said predetermined maximum is exceeded, repeating steps (b) and (c).

9. A method as in claim 8 wherein said predetermined maximum corresponds to 0.2 ppm or less.

10. A method as in claim 7 wherein, prior to step (a), at least one magnetic resonance image is created to define the imaging volume over which the B field distribution is to be shimmed for uniformity.

11. A method as in claim 7 which is performed while a living patient is located at least partially within said imaging volume.

12. A method as in claim 11 further comprising:

thereafter taking magnetic resonance spectrographic imaging (MRSI) data on additional NMR species of nuclei without moving said patient from said imaging volume.

13. A method as in claim 12 wherein all magnetic resonance data used therein is obtained using a multiply-tuned set of RF coils coupled to the imaging volume.

14. A method as in claim 7 wherein said predetermined NMR species of nuclei comprises hydrogen nuclei.

15. A method as in claim 7 wherein all magnetic resonance data used therein is obtained from NMR spin echoes and not from FID NMR responses.

16. A method as in claim 7 further comprising generating said calibration matrix by

(i) individually changing the current flowing in each of said shim coils;

and

(ii) measuring and recording the resulting distribution of the incremental changes in the B field per unit current.

17. A method as in claim 7 wherein step (b) is performed including a non- existent pseudo shim coil S_p having an assumed uniform incremental B field distribution and calculating a corresponding pseudo-current I_p which produces a uniform offset in the actual B field but with resulting improved uniformity thereover.

18. In a magnetic resonance spectrographic imaging (MRSI) system having shim coils S₁ . . . . . S_n for adjusting the uniformity of a nominally static magnetic field B_o within an image volume, the improvement comprising:

first means for generating a map array of magnetic resonance spectrographic data representing actual B_o intensity at corresponding locations within said image volume; and

second means for using said spectrographic data to generate and supply adjusted shim currents I₁ . . . I_n to shim coils, S₁ . . . S_n to make B_o intensity substantially more uniform within the image volume, including means for performing calculations using an assumed uniformly distributed fictitious pseudo shim coil which does not actually exist thereby permitting uniform offset adjustments to be included in the calculations.

20. An improved MRSI system as in claim 18 wherein said calculation performing means generates a pseudo-current value I_n+1 for a fictitious pseudo-shim coil S_n+1 which is assumed to have the ability to add a uniform offset to B_o within said image volume thereby causing the adjusted actual shim currents I₁ . . . I_n pl to produce a more uniform actual B_o within said image volume.

21. An improved MRSI system as in claim 18 further comprising:

means for generating a calibration map of magnetic resonance spectrographic data representing differential shim coil contributions to B_o per unit current at corresponding locations within said image volume for each of plural shim coils; and

wherein said second means utilizes said calibration maps to generate the adjusted shim currents using an approximated best fit algorithm.

23. An improved MRSI system as in claim 21 wherein said means for generating calibration maps includes a phantom positioned within said image volume and extending substantially therebeyond in the direction of B_o.

46. An improved MRSI system as in claim 18 wherein said first means includes:

means for generating spectrographic data including measured NMR response versus NMR frequency data for each of said voxels, and

means operatively coupled to said spectrographic data generating means for automatically picking peak spectrographic responses by selecting a predetermined NMR species response peak frequency as the spectrographic data representing B_o intensity within a corresponding voxel.

19. In a magnetic resonance spectrographic imaging (MRSI) system having shim coils S₁ . . . . S_n for adjusting the uniformity of a nominally static magnetic field B_o within an image volume, the improvement comprising:

first means for generating a map array of magnetic resonance spectrographic data representing actual B_o intensity at corresponding locations within said image volume; and

second means for using said spectrographic data to generate and supply adjusted shim currents I₁ . . . I_n to shim coils, S₁ . . . S_n to make B_o intensity substantially more uniform within the image volume,

wherein said first means generated NMR spin echoes in an MRSI sequence which includes two degrees of phase encoding with magnetic gradient pulses and three degrees of Fourier Transformation to produce spectrographic data representing the NMR frequency of hydrogen nuclei at said locations.

22. In a magnetic resonance spectrographic imaging (MRSI) system having shim coils S₁ . . . . . S_n for adjusting the uniformity of a nominally static magnetic field B_o within an image volume, the improvement comprising:

first means for generating a map array of magnetic resonance spectrographic data representing actual B_o intensity at corresponding locations within said image volume;

second means for using said spectrographic data to generate and supply adjusted shim currents I₁ . . . I_n to shim coils S₁ . . . S_n to make B_o intensity substantially more uniform within the image volume; and

means for generating a calibration map of magnetic resonance spectrographic data representing differential shim coil contributions to B_o per unit current at corresponding locations within said image volume for each of plural shim coils,

wherein said second means utilizes said calibration maps to generate the adjusted shim currents using an approximated best fit algorithm, and said approximated best fit algorithm comprises at least squares fit of B_o +shim coils fields to a uniform spatial distribution within said image volume using said adjusted shim currents I₁ . . . I_n and said calibration maps.

24. In a magnetic resonance spectrographic imaging (MRSI) system having shim coils S₁ . . . . . S_n for adjusting the uniformity of a nominally static magnetic field B_o within an image volume, the improvement comprising:

first means for generating a map array of magnetic resonance spectrographic data representing actual B_o intensity at corresponding locations within said image volume; and

second means for using said spectrographic data to generate and supply adjusted shim currents I₁ . . . I_n to shim coils S₁ . . . S_n to make B_o intensity substantially more uniform within the image volume;

wherein said first means generates spectrographic data for each said location which includes measured NMR response versus NMR frequency data and includes an automated peak-picker program for selecting a predetermined NMR species response peak frequency as the spectrographic data representing B_o intensity at a corresponding location.

25. In a magnetic resonance spectrographic imaging (MRSI) system having shim coils S₁ . . . . . S_n for adjusting the uniformity of a nominally static magnetic field B_o within an image volume, the improvement comprising:

first means for generating a map array of magnetic resonance spectrographic data representing actual B_o intensity at corresponding locations within said image volume; and

second means for using said spectrographic data to generate and supply adjusted shim currents I₁ . . . I_n to shim coils S₁ . . . S_n to make B_o intensity substantially more uniform within the image volume;

wherein said first means uses multiple phase encoding and multiple Fourier Transformations without any applied frequency encoding magnetic gradient during the reading of NMR signal responses to produce said spectrographic data.

26. In a magnetic resonance spectrographic imaging (MRSI) system having shim coils S₁ . . . . . S_n for adjusting the uniformity of a nominally static magnetic field B_o within an image volume, the improvement comprising:

first means for generating a map array of magnetic resonance spectrographic data representing actual B_o intensity at corresponding locations within said image volume; and

second means for using said spectrographic data to generate and supply adjusted shim currents I₁ . . . I_n to shim coils, S₁ . . . S_n to make B_o intensity substantially more uniform within the image volume,

wherein said first means elicits NMR echo responses and generates said spectrographic data therefrom.

27. In a magnetic resonance spectrographic imaging (MRSI) system having shim coils S₁ . . . . . S_n for adjusting the uniformity of a nominally static magnetic field B_o within an image volume, the improvement comprising:

first means for generating a map array of magnetic resonance spectrographic data representing actual B_o intensity at corresponding locations within said image volume; and

second means for using said spectrographic data to generate and supply adjusted shim currents I₁ . . . I_n to shim coils, S₁ . . . S_n to make B_o intensity substantially more uniform within the image volume,

wherein said first means comprises a multiple-tuned RF coil structure including plural pairs of resonant Helmholtz coils capacitively coupled to each other and individually tuned to resonate for a respectively corresponding NMR species.

28. An improved MRSI system as in claim 27 wherein said first means comprises:

a frequency multiplexer including plural band-pass/band-stop filters connected to combine resonant NMR signals of plural frequencies from said resonant coil pairs onto a common input/output port while still isolating each resonant coil pair from other coil pair(s); and

a frequency selective transmit/receive switch means connecting said common input/output port to RF signal transmitting and receiving circuits.

29. An improved MRSI system as in claim 28 wherein said transmit/receive switch means includes an RF transmission line approximately equal to an odd multiple of a quarter wavelength for RF signals emanating from plural species of NMR nuclei.

30. In a magnetic resonance spectrographic imaging (MRSI) system having shim coils S₁ . . . . . S_n for adjusting the uniformity of a nominally static magnetic field B_o within an image volume, the improvement comprising:

first means for generating a map array of magnetic resonance spectrographic data representing actual B_o intensity at corresponding locations within said image volume;

second means for using said spectrographic data to generate and supply adjusted shim currents I₁ . . . I_n to shim coils, S₁ . . . S_n to make B_o intensity substantially more uniform within the image volume; and

means for generating a locator image using phase and frequency encoded NMR signals from a predetermined single species of NMR nuclei to define the image volume to be shimmed for more uniform B_o.

31. A magnetic resonance spectroscopic imaging (MRSI) system comprising:

RF coils tuned to resonate at plural frequencies f₁ and f₂ and adapted to couple RF signals at such frequencies to/from an object to be imaged;

RF signal transmit and receive circuits coupled (a) to transmit RF signals at said plural frequencies to said RF coils and (b) to receive RF signals at said plural frequencies from said RF coils;

a main magnet for generating a nominally static magnetic field B_o throughout a predetermined image volume;

plural shimming magnet coils S₁ . . . S_n for controllably adjusting the uniformity of said static magnetic field B_o by passing adjusted shim currents AI₁ . . . AI_n through the corresponding shim coils S₁ . . . S_n ;

magnetic gradient coils Gx, Gy, Gz for controllably generating respective gradients in the magnetic field B_o,

control means connected to control sequences of RF signal transmission and reception in conjunction with magnetic gradient pulses to generate MRSI image data;

said control means including shim control means which;

(1) generates a map of spectrographic data representative of actual B_o intensity within said image volume;

(2) generates corrective shim current values AI₁ . . . AI_n based on said spectrographic data for making B_o intensity more uniform; and

(3) adjusts the shim current values accordingly so as to make B_o intensity substantially more uniform prior to generation of said MRSI image data.

32. An MRSI system as in claim 31 further comprising an RF transmitter/receiver isolation circuit including:

an RF transmission line connected between the RF transmitter and receiver circuit and having an effective length which is simultaneously approximately equal to an odd multiple of quarter wavelengths for plural NMR species of nuclei, and

a PIN shunt diode and associated biasing circuit for selectively short circuiting the end of said line nearest the receiver circuits.

33. An MRSI system as in claim 32 further comprising:

a second RF transmission line connected serially between the first mentioned line and said receiver circuit and having an effective length less than 0.2 wavelength at all received NMR frequencies;

a second PIN shunt diode for selectively short circuiting the end of said line nearest the receiver circuits.

34. Apparatus for adjusting currents in plural magnetic shim coils in an MRI/MRSI system comprising:

(a) means for mapping an existing magnetic field distribution;

(b) means for machine calculating an adjusted set of currents for actual shim coils to improve uniformity of field distribution while also using, in such calculation a fictitious, assumed uniformly distributed pseudo-shim coil which does not actually exist thereby permitting uniform offset adjustments to be included in the calculations; and

(c) means for applying said adjusted set of currents to the actual shim coils.

35. Apparatus for adjusting currents in plural magnetic shim coils in an MRI/MRSI system comprising:

(a) means for acquiring an MRI image of a patient volume;

(b) means for defining a desired MRSI image volume therewithin; and

(c) means for automatically shimming a static magnetic field within the defined MRSI image volume by mapping the existing field distribution and calculating and applying adjusted shim coil currents to improve the homogeneity of the thus mapped distribution, including means for performing calculations using an assumed uniformly distributed fictitious pseudo shim coil which does not actually exist thereby permitting uniform offset adjustments to be included in the calculations.

47. Apparatus as in claim 35 wherein said shimming means includes:

means for generating spectrographic data including measured NMR response versus NMR frequency data for each of a plurality of voxels within said patient volume, and

means operatively coupled to said spectrographic data generating means for automatically picking peak spectrographic response by selecting, for each of a subset of said plurality of voxels, a predetermined NMR species response peak frequency as the spectrographic data representing B_o intensity within said corresponding subset of voxels.

36. Apparatus for automated shimming of static magnetic fields for homogeneity in an MRSI system comprising:

(a) means for acquiring and using NMR spin echo responses to create NMR spectral responses from plural voxels within an image area and a map of existing magnetic field variation therewithin corresponding to the NMR spectral peak frequency of a predetermined NMR species and using same to automatically reduce said variations; and

(b) means for thereafter acquiring and using NMR FID or spin echo responses to create MRSI plots of other NMR species from said image area.

37. Apparatus for automated shimming of static magnetic fields for homogeneity in an MRSI system comprising:

(a) means for mapping the distribution of incremental magnetic field change per unit current to create a calibration value matrix for each of plural magnetic shim coils over a predetermined array of voxels, including means for generating spectrographic data for each voxel which includes measured NMR response versus NMR frequency data, including automated peak-picking means for selecting a predetermined NMR species response peak frequency as the spectrographic data representing B_o intensity within a corresponding voxel;

(b) means for mapping and recording variations in existing static magnetic field distribution for only some of said voxels;

(c) means for using said recorded variations and a subset of said calibration value matrix corresponding to only some of said voxels to calculate adjusted shim coil currents required to reduce the magnitude of said variations; and

(d) means for applying said adjusted shim coil currents to respectively corresponding shim coils.

38. Apparatus for adjusting currents in plural magnetic shim coils in a magnetic resonance imaging system, said apparatus comprising:

(a) means for mapping the distribution of a static magnetic field B within an imaging volume with a first set of currents I₁ . . . I_n in shim coils S₁ . . . S_n by magnetic resonance spectroscopy which maps the frequency of a predetermined NMR species at an array of locations within said imaging volume, including means for generating spectrographic data for each voxel which includes measured NMR response versus NMR frequency data, including automated peak-picking means for selecting a predetermined NMR species response peak frequency as the spectrographic data representing B_o intensity within a corresponding voxel;

(b) means for machine calculating an adjusted second set of shim coil currents AI₁ . . . AI_n using a calibration matrix of field distributions produced by the individual shim coils and the measured existing B field distribution so as to minimize variations in the B field distribution; and

(c) means for applying said adjusted shim coil currents AI₁ . . . AI_n to respectively corresponding shim coils S₁ . . . S_n.

39. Apparatus as in claim 38 further comprising,

(d) means for again performing said mapping step;

(e) means for comparing the resulting uniformity of said B field distribution to a predetermined maximum permissible magnitude of variation; and

(f) means for repeating steps (b) and (c) if said predetermined maximum is exceeded.

40. Apparatus as in claim 38 further comprising means for creating at least one magnetic resonance image to define the imaging volume over which the B field distribution is to be shimmed for uniformity.

41. Apparatus as in claim 38 wherein:

said mapping means includes means for mapping said field distribution while a patient is disposed within said imaging volume;

said calculating means and said applying means each operate while said patient is disposed within said imaging volume; and

said apparatus further comprises means for taking magnetic resonance spectrographic imaging (MRSI) data on additional NMR species of nuclei without moving said patient from said imaging volume after said shim currents have been adjusted.

42. Apparatus as in claim 41 wherein said means for mapping comprises a multiply-tuned set of RF coils coupled to the imaging volume.

43. Apparatus as in claim 38 wherein said means for calculating includes means for including a fictitious non-existent pseudo-shim coil S_p having an assumed uniform incremental B field distribution and for calculating a corresponding pseudo-current I_p which produces a uniform offset in the actual B field but with resulting improved uniformity.