What is the difference between ct and mri scan?

Mechanism: Mri is based on the spin-spin splitting of hydrogen atoms inherent to all bodily tissues, namely in the form of water. The raw data produced from an MRI scan must undergo extensive processing to be converted into a "readable" image. Ct scans are essentially fancy x-ray machines which can take "slices" of whatever is being examined (head, abdomen, chest, etc.) unlike mri, ct uses ionizing radiation.

Basically: An MRI uses magnets to generate images and a ct scan uses x-rays. They each show different things better. Depends on what part of the body is being imaged and what is being looked for as to which one is preferable.