MAGNETIC RESONANCE DIFFUSION IMAGING WITH EDDY-CURRENT COMPENSATION

• US 20040227510A1
• Filed: 05/12/2003
• Published: 11/18/2004
• Est. Priority Date: 05/12/2003
• Status: Active Grant

• Current Assignee: The University of Queensland
• Original Assignee: The University of Queensland
• Inventors: Griffin, Mark Philip, Rose, Stephen Edward, McMahon, Katie Louise
• Granted Patent: US 6,815,952 B1
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: G01R 33/56341 Diffusion imaging
• CPC Class Codes: G01R 33/56518 due to eddy currents, e.g. caused by switching of the gradient magnetic field

Abstract

A method for eddy current compensated diffusion imaging using magnetic resonance is used to obtain a spin echo signal in a readout time window by excitation of a nuclear resonance signal using a first radio-frequency pulse and by refocusing that signal using at least one second radio-frequency pulse and a third radio-frequency pulse. Gradients fields are applied in a gradient field direction field having a strength and being activated by gradient pulses between each of said radio-frequency pulses and prior to said readout window, said gradient pulses having a polarity which is alternated between successive gradient pulses, with a totality of said gradient pulses having a gradient time integral of zero between a time of said excitation and the center of k_x or k-space. At least two of said gradient pulses have differing gradient time integrals. The gradient direction is then changed and the previous steps repeated to evenly distribute gradient direction vectors over a sphere. An eddy current compensated and optimized imaging sequence is thereby produced which achieves improved imaging through optimization of signal to noise ratios of the detected signal while avoiding distortions in imaging due to magnetic fields associated with eddy currents.

Claims

1. A method for eddy current compensated diffusion imaging using magnetic resonance, the method comprising the steps of:

a) obtaining a spin echo signal in a readout time window by excitation of a nuclear resonance signal using a first radio-frequency pulse and by refocusing that signal using at least one second radio-frequency pulse and a third radio-frequency pulse;

b) applying imaging gradient fields to spatially encode the image in slice, read and phase directions;

c) applying additional gradient fields of a specific direction and amplitude through activation of gradient pulses between each of said radio-frequency pulses and prior to said readout window, said gradient pulses having a polarity which is alternated between successive gradient pulses, a totality of said gradient pulses having a gradient time integral of zero between a time of said excitation and the center of k_x or k-space, with at least two of said gradient pulses having differing gradient time integrals;

d) changing said gradient direction; and

e) repeating steps a) to c) to evenly distribute the additional gradient direction vectors over a sphere.

2. The method of claim 1, further comprising adjustment of time locations of said radio-frequency pulses, said readout time window and said gradient fields to maximize a diffusion parameter related signal to noise ratio and to minimize eddy current field distortion at the center of k_x or k-space.

3. The method of claim 2, wherein said time locations are iteratively and systematically varied to obtain a relative maximum in said signal to noise ratio and a relative minimum in said eddy current field distortion.

4. The method of claim 3, wherein said time locations are analyzed as a function of echo times.

5. The method of claim 4, wherein said time locations are analyzed as a function of gradient field durations dependent upon echo times.

6. The method of claim 5, wherein said time locations are analyzed as a function of an eddy current decay time.

7. The method of claim 3, wherein said time locations are analyzed as a function of a number of measurements.

8. The method of claim 3, wherein said time locations are analyzed as a function of a gradient time integral.

9. The method of claim 3, wherein said time locations are analyzed as a function of the diffusivity of the sample under measurement.

10. The method of claim 3, wherein said time locations are analyzed as a function of transverse relaxation time.

11. The method of claim 3, wherein said time locations are analyzed using the following formula:

12. The method of claim 3, wherein said time locations are analyzed using the following formula

H(t)˜

e<sup>−

λ

_i(t_D1+t_D2+t_D3+t_D4+2t₁₈₀+t_s)</sup>−

e<sup>−

λ

_i(t_D2+t_D3+t_D4+2t₁₈₀+t_s)</sup>−

e<sup>−

λ

_i(t_D2+t_D3+t_D4+t₁₈₀+t_s)</sup>+2e<sup>−

λ

_i(t_D3+t_D4+t₁₈₀+t_s)</sup>−

e<sup>−

λ

_i(t_D4+t₁₈₀+t_s)</sup>−

e<sup>−

λ

_i(t_D4+t_s)</sup>+e<sup>−

λ

_i(t_s)</sup>

wherein H(t) is a magnetic field due to eddy currents, λ

_i a decay rate of eddy currents triggered by a rise and fall of gradients, i an index of a number of different decay rates generated for each gradient switching point, t_Dj a duration of a jth diffusion gradient, t₁₈₀ a time of an RF refocusing pulse, and t_s a delay before a center of an echo or a center of K-space.

13. The method of claim 1, further comprising selecting said gradient field strengths in steps c) and e) to maximize diffusion imaging sensitivity.

14. The method of claim 1, wherein steps c), d), and e) comprise simultaneous application of three gradients in three orthogonal spatial directions and changing relative field strengths of said three gradients between successive iterations of step e).

15. The method of claim 14, wherein said three gradients are expressed in Cartesian or radial coordinates.

16. The method of claim 1, wherein step c) comprises the steps of:

c1) activating a gradient pulse of a first polarity between said first radio-frequency pulse and said second radio-frequency pulse;

c2) activating two gradient pulses of differing polarity between said second radio-frequency pulse and said third radio-frequency pulse, beginning with a polarity which is opposite to said first polarity; and

c3) activating another gradient pulse between said third radio-frequency pulse and said readout window.

17. The method of claim 1, further comprising application of said slice selection gradient during a time duration of said radio-frequency pulses.

18. The method of claim 1, wherein step b) comprises the steps of:

b1) spatially encoding a spin echo signal by activating a phase-encoding gradient prior to said readout; and

b2) activating a readout gradient during said readout time window. or;

b3) activating a series of oscillating readout gradients during said readout time window, incorporating phase encoding between each readout gradient.