Hi MRtrixperts,

I am trying to wrap my head around the confusion with regards to `qform`

`sform`

and transformations, and was hoping you could clear up something for me.

My understand is that:

- The
`qform`

transformation (as stored in a NIfTI header) relates the voxel co-ordinates (in mm) to scanner space (aka some frame of reference on the scanner), and is a rigid transform. - The
`sform`

transformation relates the voxel co-ordinates to some standard space (eg. MNI space), is an affine transform. - Sometimes a NIfTI image has one or both, and therefore MRtrix3 picks one based on predefined rules defined here.

My question is: When looking at the transform output my `mrinfo`

(in the absence of simplifying greater than 90 degree transformations through stride changes), is this simply the `qform`

/`sform`

transform (depending on which is available given the above rule)? Or is there some relation to this transform and the `qform`

/`sform`

?

If it is, by definition does this mean that if the transformation displayed by `mrinfo`

is affine, then it must be the `sform`

transform? Or can the `qform`

be set to an affine transform even if it is not supposed to be?

Many thanks

Matt