What is the difference between an mri and a cat scan? except for the radiation factor.

Physics: The physical phenomena on which these imaging techniques are built are very different, each is best suited for different purposes. Cat scans are sophisticated x ray, they rely on the degree of absorption of gamma rays (bones, ca, other metals will contrast most). Mri is better for softer tissue imaging because it depends more on the behavior of hydrogen atoms and h2o in the strong magnetic field.