Mapping eddy current fields in MRI system

1. A method for mapping eddy current fields in a magnetic resonance imaging (MRI) system, said method comprising:

using an MRI system to acquire and store MRI data from an object located in an imaging volume of the MRI system using an MRI data acquisition sequence which is preceded by a pre-sequence comprising (a) a gradient magnetic field transition which stimulates eddy current fields in the MRI system, and (b) a spatial modulation grid tag module which sensitizes a spatially resolved MR image of the acquired MRI data to the stimulated eddy current fields which existed during the spatial modulation grid tag module.

2. The method as in claim 1 , further comprising:

processing said acquired MRI data to generate and store a spatial domain magnitude image of said object having grid lines superimposed thereon as a result of said spatial modulation grid tag, wherein said grid lines exhibit distortion caused by said stimulated eddy current fields not otherwise already compensated by said MRI system.

3. The method as in claim 2 , wherein said distortion comprises local dilation or compression of the grid lines and/or local rotation of the grid lines.

4. The method as in claim 1 , further comprising processing said acquired MRI data to generate a spatially resolved phase map of the stimulated eddy current fields, which phase map is used to generate and store a spatially resolved eddy current field map.

5. The method as in claim 4 , wherein said MRI data is separately acquired for each of differently oriented gradient magnetic fields sufficient to generate MRI field map data representing self-terms and cross-terms of stimulated eddy current fields, said self-terms being eddy current fields stimulated in the same direction as the stimulating gradient and said cross-terms being eddy current fields stimulated in a different direction from that of the stimulating gradient.

6. The method as in claim 4 , wherein said phase map is generated using frequency-filtered replicants of acquired MRI k-space data.

7. The method as in claim 6 , wherein said replicants comprise a central low-pass filtered portion of acquired k-space data, a left-side high-pass filtered portion of acquired k-space data and a right-side high-pass filtered portion of acquired k-space data, said method further comprising:

Fourier-transforming two or more replicant portions of k-space to produce respectively corresponding complex-valued spatial domain images;

converting each complex-valued replicant spatial domain image to a corresponding phase image;

generating an off-resonance phase map by subtracting at least two of said phase images from each other;

calculating an eddy current, only, phase map wherein background reference phase is cancelled by subtraction of another off-resonance phase map acquired with eddy current stimulus of a different amplitude or polarity; and

differentiating eddy current phase map images to produce a spatially resolved eddy current field map.

8. The method as in claim 1 , wherein:

said pre-sequence is configured to include at least one time delay between the gradient magnetic field transition and the spatial modulation grid tag module; and,

said MRI data is acquired plural times, at least once for each of at least two different time delays, possibly including a zero time delay, between the gradient magnetic field transition and the spatial modulation grid tag module, thereby producing time-dependent measurements of the stimulated eddy current fields; and

fitting said time-dependent measurements to an exponential decay curve thereby deriving at least one decay time-constant associated with said stimulated eddy current fields.

9. The method as in claim 1 , wherein said MRI data acquisition sequence is one of the following: (a) FSE (fast spin echo), (b) FASE (fast asymmetric spin echo), and (c) FFE (fast field echo).

10. The method as in claim 1 , wherein said spatial modulation grid tag module comprises:

at least two non-selective RF nutation pulses with an intervening gradient magnetic field tagging pulse.

11. An MRI system comprising:

an MRI scanner configured to acquire and store MRI data from an object located in an imaging volume of the MRI system using an MRI data acquisition sequence which is preceded by a pre-sequence comprising (a) a gradient magnetic field transition which stimulates eddy current fields in the MRI system, and (b) a spatial modulation grid tag module which sensitizes a spatially resolved MR image of the acquired MRI data to the stimulated eddy current fields which existed during the spatial modulation grid tag module.

12. The MRI system as in claim 11 , wherein said MRI system comprises at least one data processor configured to process said acquired MRI data to generate and store a spatial domain magnitude image of said object having grid lines superimposed thereon as a result of said spatial modulation grid tag, wherein said grid lines exhibit distortion caused by said stimulated eddy current fields not otherwise already compensated by said MRI system.

13. The MRI system as in claim 12 , wherein said distortion comprises local dilation or compression of the grid lines and/or local rotation of the grid lines.

14. The MRI system as in claim 11 , wherein said MRI system comprises at least one data processor configured to process said acquired MRI data to generate a spatially resolved phase map of the stimulated eddy current fields, which phase map is used to generate and store a spatially resolved eddy current field map.

15. The MRI system as in claim 14 , wherein said MRI scanner is configured to separately acquire MRI data for each of differently oriented gradient magnetic fields sufficient to generate MRI field map data representing self-terms and cross-terms of stimulated eddy current fields, said self-terms being eddy current fields stimulated in the same direction as the stimulating gradient and said cross-terms being eddy current fields stimulated in a different direction from that of the stimulating gradient.

16. The MRI system as in claim 14 , wherein said phase map is generated using frequency-filtered replicants of acquired MRI k-space data.

17. The MRI system as in claim 16 , wherein said replicants comprise a central low-pass filtered portion of acquired k-space data, a left-side high-pass filtered portion of acquired k-space data and a right-side high-pass filtered portion of acquired k-space data, said at least one data processor being configured to:

Fourier-transforming two or more replicant portions of k-space to produce respectively corresponding complex-valued spatial domain images;

convert each complex-valued replicant spatial domain image to a corresponding phase image;

generate an off-resonance phase map by subtracting at least two of said phase images from each other;

calculate an eddy current, only, phase map wherein background reference phase is cancelled by subtraction of another off-resonance phase map acquired with eddy current stimulus of a different amplitude or polarity; and

differentiate eddy current phase map images to produce a spatially resolved eddy current field map.

18. The MRI system as in claim 11 , wherein said MRI scanner is configured to include at least one time delay between the gradient magnetic field transition and the spatial modulation grid tag module; and,

acquire said MRI data plural times, at least once for each of at least two different time delays, possibly including a zero time delay, between the gradient magnetic field transition and the spatial modulation grid tag module, thereby producing time-dependent measurements of the stimulated eddy current fields; and

fit said time-dependent measurements to an exponential decay curve thereby deriving at least one decay time-constant associated with said stimulated eddy current fields.

19. The MRI system as in claim 11 , wherein said MRI data acquisition sequence is one of the following: (a) FSE (fast spin echo), (b) FASE (fast asymmetric spin echo), and (c) FFE (fast field echo).

20. The MRI system as in claim 11 , wherein said spatial modulation grid tag module comprises:

at least two non--selective RF nutation pulses with an intervening gradient magnetic field tagging pulse.