Add quick utils functions to get B and C matrices from off-resonance maps

[chaithyagr]

Currently, in

https://github.com/mind-inria/mri-nufft/blob/f5a6ba006184b96a1f73ff54be27d854797c79fe/src/mrinufft/operators/off_resonnance.py#L19-L58

we just generate the fourier_op which takes the B and C values into account.

• [x] It might help to also include a function in mrinufft.operators.off_resonance called get_interpolators_from_fieldmap.
• [ ] Also, it might further help to be able to get fourier correction possible directly from the init function itself like MRIFourierCorrected(fourier_op, b0_field, mask)
• [ ] It could be useful to allow for autodiff to pass through this new fourier op, currently we only support cupy or numpy, it might help to check if we have autograd enabled on based fourier_op and enable it as well (we just need to have tensors as torch tensors.
• [ ] A bit distant, but it could help to have the off-resonance estimation from k-space data as function. Although, this is a bit distant and I really feel it should belong to pysap-mri.

[paquiteau]

I fully agree with you.

Getting the off resonance map from the k space data could be done in the extras module, just as we propose it for the smaps estimation.

We may also wish to have autodiff support around this as well ?

[chaithyagr]

Sure. But note that the estimation isnt trivial, it is computationally expensive (for now). @Daval-G you think these codes can be open sourced though? I wanted to confirm that.

[philouc]

I agree with @paquiteau : Estimation of off-resonance effects from k-space data should be carried out separately, as it depends on the phase information usually extracted from the image domain and TE value.