head to mri transformation matrix from hpi locations

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Hi,

I am currently working on a MEG project where the fiducials that are saved are not meaningful. What was done instead was using a headcast with 4 hpi locations.

To compute the inverse solution i need the head <-> mri tranformation matrix which is usually extracted by coregistration. However, the coregistration seems to dependent on the fiducials.

The information i have available

  • hpi mri coordinates (4 x 3 matrix)
  • hpi meg coordinates for each session (4 x 3 matrix)

Any ideas on how to compute the head <-> mri transformation matrix? Is there enough information available to do so?