Depends on the vendor.  For GE the gradients are in MR physics coordinates - look at the function geCorrectBvecs(). For UIH, Bruker, Hitachi Siemens or Philips look at the function siemensPhilipsCorrectBvecs().

https://github.com/rordenlab/dcm2niix/tr...
https://github.com/rordenlab/dcm2niix/tr...
https://github.com/rordenlab/dcm2niix/tr...