One mistake that I often notice in medical shows like the old series House, is that in the TV shows or movies, the magnet for MRI scanners turn on and off with the scan. That isn't true. The magnet stays on. I worked on MRI machines when they first came into use in medicine. At first they were called NMR for Nuclear Magnetic Resonance, but the word "nuclear" scared people so they were renamed MRI. This goes back to the early 80s.
We had mobile units because they were too expensive for most hospitals to buy one. MRI units in hospitals normally leave the magnet active 24-7 except during maintenance. They were often charged using a pair of jumper cables. Once the required current was flowing in the magnet, the cables were disconnected. In principle the current would continue flow for as much as 10,000 years because they were superconductive. Ours used nitrogen over helium for cooling. The mobile units required that the nitrogen for the magnet be quenched when relocating to another hospital because of the power of the magnet. Once the nitrogen was purged and the supercooling was gone, the current and magnetic field disappeared almost instantly.
Even the early magnets were as powerful as 1.5 Tesla. On one occasion a D8 Cat got too close to a (parked) mobile MRI unit and got sucked in through the wall. The more iron you have present, the greater the force produced. I had a close call with a chair. Everything inside the unit was supposed to be stainless steel or other non-ferrous materials. My tools were made out of beryllium. One night I had to do something to the magnet and grabbed a chair to stand on so I could reach it. But someone had accidently mixed a regular chair made with iron or steel, in with the stainless chairs. They all looked identical so I had no idea. I had my arm slung through the back of the chair so I could carry my tools and other equipment. As I approached the magnet I felt a tug on the chair. By then it was too late. The next thing I knew, I was airborne. I slammed into the magnet and hit my head pretty hard but I was okay. But I am lucky it didn't rip my arm off or crush me. Had I gotten between the chair and the magnet it could have been deadly. It took a winch and a long cable to get the chair out.
I guess my favorite story came from a driver - the MRI units were on 40-foot busses. I arrived to work on the unit just after he arrived at a hospital. As I pulled in I saw him standing outside and looking bit out of sorts. I asked what was wrong; he didn't look so good. He said that as he was driving over, something felt strange, like there was a lot of odd crosswinds. Then he noticed other cars swerving a lot as they passed. It wasn't until he arrived at the hospital that he realized he had never quenched the magnet. It was active while he was driving. And that is why everyone kept swerving as they passed. Luckily no one got close enough to get completely sucked in. That could have been a disaster.
Worst story: On one occasion we did a scan on a very old WWI veteran. He had a pin in his leg from the war. But back then, just after the turn of the century, even the stainless used for medical purposes was very low quality and our techs didn't know that. When they put him in the magnet, it removed the pin from his leg bone.
I was in my early 20s back then and had my own fleet of million-dollar magnets to play with at night. That was so fun!
I was quick to inform doctors of a major aftereffect that MRI units were having on patients. Of course they were always anxious to know more. So I explained that due to hysteresis, the iron in the blood retains a significant magnetic field after the scan. As a result, the patients have a tendency to walk North.
Our company began with the first mobile CAT scanners. We pioneered the field. We were originally MCTS for Mobile Computerized Tomographic Services. We had shirts that read MCTS - We give good head and better body scans.
