Strong Magnetism

An interesting report at the NY Times about the consequences of the very strong magnets found in MRI machines"

M.R.I.'s Strong Magnets Cited in Accidents
The pictures and stories are the stuff of slapstick: wheelchairs, gurneys and even floor polishers jammed deep inside M.R.I. scanners whose powerful magnets grabbed them from the hands of careless hospital workers.

The police officer whose pistol flew out of his holster and shot a wall as it hit the magnet. The sprinkler repairman whose acetylene tank was yanked inside, breaking its valve and starting a fire that razed the building.

But the bigger picture is anything but funny, medical safety experts say. As the number of magnetic resonance imaging scanners in the country has soared from a handful in 1980 to about 10,000 today, and as magnets have quadrupled in power, careless accidents have become more frequent. Some have caused serious injuries and even death.

No one knows how many have occurred. But the safety experts say there is no doubt they are on the rise, and their growing frequency is prompting widespread calls for more regulation.

Safety guidelines drawn up by the American College of Radiology in 2002 and revised last year "have no teeth and are floating out there in intellectual Never-Never Land," Tobias Gilk, a Kansas City architect who designs medical scanning rooms, said.

He continued: "The X-ray in your dentist's office is more heavily regulated."

The report had an interesting thing that I did not know:

The magnets are never off, even at night, and cutting the electricity will not affect them. They draw most of their power from supercooled helium, which must be vented to shut down the magnet - a process that takes several minutes and has hazards of its own.

Makes sense -- the magnet is made from superconducting materials and as long as they are below the Curie point, they will be running at full strength. This page has some photos of what happens when ferrous objects get too close to an MRI machine: Simply Physics Here's a floor buffer: