[vtkusers] How to SetSampleDistance for ray casting?

[Wolfram H Volpi]

I am ray casting a slc file and I want to SetSampleDistance to 1/4 of a
voxel.

In vtkVolumeRayCastMapper it says "SetSampleDistance is in volume
coordinates".

The slc file header is: 11111256 256 128 81.0 1.0 1.01 5 0 050 50 X      
What are the volume coordinates of a slc file?

Thank you,
Wolfv