I have two images registered together and I would like to, given the IJK coordinates of a point in the first image, get its IJK coordinates in the second one.
To do so, I did this:

Ok,
but when I do firstVolume.GetParentTransformNode() I get a null node (same thing for the second volume) so the transform matrix between these nodes is identity

That is good then, they are in the same coordinate system.

However, there is a problem with your coordinates. Homogeneous point coordinates have 4 components, not just 3. The value of the 4th component must be 1.

That works perfectly, thank you very much !
I thought I was missing some kind of transformation between the two images and I wouldn’t have thought about this.