Would prolonged exposure to MRI and/or PET scans (hours on end) be hazardous to your health?

MRI scans are performed in a very strong magnetic field. There is no conclusive proof, but it is generally felt that exposure to strong magnetic fields generated by MRI scanners is harmless to humans. That being said, however, there are some theoretical risks. Without getting too technical, MRI images are generated by rapid variations in the magnetic field. These variations cause the water protons within the body to behave a certain way, which in turn generate very minute signal which can be picked up by special receiver coils that make up part of the MRI scanner. These signals are then processed by the scanner, and and image is generated. The rapid shifts in magnetic fields can theoretically produce heat as a by-product. This is especially true for any metallic object within the scanner that has the shape of a ring or closed loop. In fact, this can produce enough heat in conventional MRI scanners to cause flesh burns. This is one reason why all metal must be removed before getting an MRI scan. The actual magnetic field itself is probably safe.

As for PET scans, the physics is entirely different. PET scans generate an image by radioactive emissions that are produced in the body after getting injected with radioactive material. The PET scanner itself is really nothing but a giagantic receiver device to detect these emission. While the actual dose of radiation from a PET scan is felt to be reasonably safe, it is still radiation nonetheless. The more PET scans you have, the more radiation you will be exposed to. But there is no danger to just lying in the scanner itself without getting injected with the radioactive isotope because the scanner itself does not generate actual radiation.