Role of reference landmarks (AC&PC) in "normalization" of MR images

Dear NeuroStars list,

What does it mean exactly to normalize an MR image relative to some anatomical landmark such as the anterior/posterior commissure? Is it the case that it implies that that landmark is the only one that stays invariant when going from the source to the destination image, i.e. that, in the final registered image, the location of the AC is the same as in the source image, while all other voxels/structures have been spatially transformed? Or am I misinterpreting the meaning?

Thank you for your answers!

Hi Francesco,

Landmarks allow us to establish a point-to-point transformation between the source and destination image (without using image intensity features, etc). There is some localization error associated with placing points but this is typically quite small (< 1mm) for landmarks like AC and PC. You need at least 3 points.

The degrees of freedom can vary from one transformation method to the other from rigid registration (6 DOF), to B-splines, and all the way to much higher order nonlinear registrations. The general idea is that the same transform applied to match the points is used to transform the source–>destination with some choice of interpolation method required too. Many of the registration algorithms used nowadays are completely data-driven and do not make use of any anatomical landmarks as priors. However, the other benefit of defining AC and PC is to have an internal reference frame for the location of brain structures – this technique is still commonly used for DBS planning.

I happen to be giving a demo on some work by our team on this very topic at the INCF NeuroInformatics meeting in Montreal this week in case you happen to be around.

Hope this helps,

Hi Jclau! I won’t be in the area unfortunately, but thank you very much for a very clear answer to my question!