MR/CT head registration

Hello,
I have just started using 3D slicer. I would like to register 3D head CT with head MR. I have data for the whole body CT and just the head of the MR. Is there a way to remove the body slices of the CT before registration with the head MR? What registration method would you recommend?
Thanks!

you can crop the CT volume by cropVolume module.

I have been using BRAINS general registration to register MR and CT volumes. I am using MMI as the cost metric. How can I display the MMI result before and after registration?
Thanks,
Emily