Function max_displacement_mm 

fn max_displacement_mm(t: &RigidTransform, mask: Option<&[u8]>) -> f64

Largest |M·x − x| over the evaluation domain, in mm.

M maps fixed world → moving world, and both are the same scanner’s world — the scanner does not move — so for the bit of tissue the reference has at world x, M·x is where that same tissue was during the moving acquisition. The difference is the distance it travelled.