What is the difference between a pet and a mri scan?

Can be complimentary: A pet scan is sensitive to the metabolism of glucose which is relatively increased in many tumors. Mri provides anatomic detail and is predominately sensitive to water. In most cases of pathology (like in tumors) the water content is increased. Recently, these two technologies have been fused to provide the anatomic detail of the MRI scan with the functional data of the pet scan all in one image.

Very different: Pet is a technique for measuring concentrations of positron-emitting radiopharmaceuticals and presenting these as a metabolic image of the body. F18-fdg, a "radioactive sugar", is the most commonly used radiopharmaceutical in oncology. Mri uses a combination of a strong magnetic field and radio waves to produce anatomic images of the body without the use of radiation.