Spine normalization across patients

[NathanMolinier]

Description

In the script skeleton.py, discs positions were normalized across patients to be able to estimate the structure of the spine for new patients (pose estimation). However, this normalization was based on the position of the discs 1 (above vertebrae C1) and 5 (above the vertebrae C5) which are not present in all the field of views.

Possible solution

To fix this issue, images could be normalized in the space of an image with a "complete" field of view.