What can an mri show that a ct can't?

It depends: Both are very good at imaging various parts of the body. Depending on the body part and the question being asked, ct or MRI may be better. In general, MRI can "see" the soft tissues a little better (e.g., brain and spinal cord, rotator cuff) and ct is sometimes better for looking at the bones (e.g., facial fractures, sinuses). Doctors work closely with radiologists to determine the best test.