The center of the head

Dear all,

In Power’s 2012 paper, they gave a definition of Framewise displacement (FD) calculations which is used in almost all related works later. When converted rotational displacements from degrees to millimeters, the radius of the sphere of brain surface was approximately defined as the mean distance from the cerebral cortex to the center of the head.

Here I have a question about the center of the head. Is there a specific definition of which part of the brain is the center of the head or the centroid of the brain? Did anyone know or hear about this in any textbooks or references before? It could be defined based on anatomy or neuroimaging.

Thank you so much!