Eddy current mitigation systems and methods
Techniques for compensating for presence of eddy currents during the operation of a magnetic resonance imaging (MRI) system in accordance with a pulse sequence, the pulse sequence comprising a gradient waveform associated with a target gradient field. The techniques include: compensating for presence of eddy currents during operation of the MRI system at least in part by correcting the gradient waveform using a nonlinear function of a characteristic of the gradient waveform to obtain a corrected gradient waveform; and operating the MRI system in accordance with the corrected gradient waveform to generate the target gradient field.
1. A method of operating a magnetic resonance imaging (MRI) system, the MRI system including at least one radio frequency (RF) coil configured to apply an RF signal in accordance with a pulse sequence, the pulse sequence comprising a gradient waveform associated with a target gradient field and an RF waveform associated with a target RF signal, the method comprising:
compensating for a presence of eddy currents during operation of the MRI system at least in part by correcting the RF waveform to obtain a corrected RF waveform, the correcting comprising:
determining a shifted B 0 magnetic field strength waveform based on a corrected gradient waveform obtained at least in part by applying a first nonlinear function of a characteristic of the gradient waveform to the gradient waveform; and
correcting the RF waveform using the shifted B 0 magnetic field strength waveform to obtain the corrected RF waveform; and
operating the MRI system in accordance with the corrected RF waveform to generate the target RF signal.
2. The method of claim 1 , wherein determining the shifted B 0 magnetic field strength waveform comprises applying a first transformation to the corrected gradient waveform to obtain the shifted B 0 magnetic field strength waveform.
3. The method of claim 2 , wherein applying the first transformation to the corrected gradient waveform comprises filtering the corrected gradient waveform with a first filter determined using the first nonlinear function.
4. The method of claim 1 , further comprising determining the first nonlinear function based on first eddy current calibration data obtained from the MRI system, the first eddy current calibration data describing a B 0 magnetic field strength as a function of the characteristic of the gradient waveform.
5. The method of claim 1 , wherein the method further comprises:
compensating for the presence of eddy currents during operation of the MRI system at least in part by correcting the gradient waveform using a second nonlinear function of a characteristic of the gradient waveform to obtain a corrected gradient waveform; and
operating the MRI system in accordance with the corrected gradient waveform to generate the target gradient field.
6. The method of claim 5 , wherein the characteristic of the gradient waveform comprises an amplitude of the gradient waveform, a direction of the target gradient field, shape of the gradient waveform, and/or slew rate of the gradient waveform.
7. The method of claim 5 , wherein the second nonlinear function is a polynomial function, an exponential function, a piecewise polynomial function, or a piecewise constant function of the characteristic of the gradient waveform.
8. The method of claim 5 , wherein the second nonlinear function is determined based on second eddy current calibration data obtained from the MRI system.
9. The method of claim 8 , further comprising:
obtaining the second eddy current calibration data using a field probe to measure eddy currents in the MRI system during its operation; and
determining the second nonlinear function using the second eddy current calibration data.
10. The method of claim 5 , wherein correcting the gradient waveform comprises:
applying the second nonlinear function to the gradient waveform to obtain a scaled gradient waveform; and
applying a second transformation to the scaled gradient waveform to obtain a transformed gradient waveform, the second transformation being determined using second eddy current calibration data obtained from the MRI system.
11. The method of claim 10 , wherein applying the second transformation to the scaled gradient waveform comprises filtering the scaled gradient waveform with a filter determined using the second eddy current calibration data.
12. The method of claim 11 , wherein filtering the scaled gradient waveform is performed in a time domain at least in part by performing a convolution.
13. The method of claim 11 , wherein filtering the scaled gradient waveform is performed in a domain other than a time domain.
14. The method of claim 11 , wherein the filter is a pre-emphasis filter, and the method further comprises determining the pre-emphasis filter using an inverse Laplace transformation.
15. The method of claim 10 , further comprising:
subtracting the scaled gradient waveform from the transformed gradient waveform to obtain a correction waveform; and
combining the correction waveform with the gradient waveform to obtain the corrected gradient waveform.
16. The method of claim 15 , wherein the first nonlinear function is a nonlinear function for compensating for first eddy currents and the second nonlinear function is a nonlinear function for compensating for second eddy currents, wherein the compensating comprises:
compensating for the presence of eddy currents using multiple nonlinear functions of the characteristic of the gradient waveform to obtain a corrected gradient waveform, the multiple nonlinear functions including the first nonlinear function and the second nonlinear function.
17. The method of claim 5 , wherein operating the MRI system in accordance with the corrected gradient waveform comprises generating one or more preparation gradient field pulses prior to generating the target gradient field.
18. The method of claim 1 , wherein the pulse sequence comprises one of a diffusion weighted imaging (DWI) pulse sequence, a diffusion weighted steady-state free precession (DW-SSFP) pulse sequence, or a fast spin echo (FSE) pulse sequence.
19. At least one non-transitory computer-readable storage medium storing processor-executable instructions that, when executed by a magnetic resonance imaging (MRI) system, cause the MRI system to perform a method of operating the MRI system by applying a plurality of RF signals in accordance with a pulse sequence, the pulse sequence comprising a gradient waveform associated with a target gradient field and an RF waveform associated with a target RF signal, the method comprising:
compensating for a presence of eddy currents during operation of the MRI system at least in part by correcting the RF waveform to obtain a corrected RF waveform, the correcting comprising:
determining a shifted B 0 magnetic field strength waveform based on a corrected gradient waveform obtained at least in part by applying a first nonlinear function of a characteristic of the gradient waveform to the gradient waveform; and
correcting the RF waveform using the shifted B 0 magnetic field strength waveform to obtain the corrected RF waveform; and
operating the MRI system in accordance with the corrected RF waveform to generate the target RF signal.
20. A magnetic resonance imaging (MRI) system, comprising:
at least one controller configured to operate one or more of a plurality of magnetics components in accordance with a pulse sequence, the pulse sequence comprising a gradient waveform associated with a target gradient field and an RF waveform associated with a target RF signal, wherein the at least one controller is configured to:
compensate for a presence of eddy currents during operation of the MRI system at least in part by correcting the RF waveform to obtain a corrected RF waveform, the correcting comprising:
determining a shifted B 0 magnetic field strength waveform based on a corrected gradient waveform obtained at least in part by applying a first nonlinear function of a characteristic of the gradient waveform to the gradient waveform; and
correcting the RF waveform using the shifted B 0 magnetic field strength waveform to obtain the corrected RF waveform; and
operate the MRI system in accordance with the corrected RF waveform to generate the target RF signal.