By Jo Twist
BBC News Online science and technology staff
Sensors to seek out submarines and mines have been adapted in a detection system to save lives during Magnetic Resonance Imaging (MRI) scans.
Most "MRI projectile" incidents are minor
The simple system, developed by science firm QinetiQ, will sound an alert when ferrous objects enter scan rooms.
Attracted by the MRI machines' intense magnetic fields, these objects can become dangerous projectiles.
On average, an incident occurs every 100 to 1,000 scans. Equipment may be
damaged and people injured or killed.
The Ferroguard system is cheap and easy to install. Forty hospitals in the UK have signed up for one so far, and 10 have been installed in US hospitals.
It uses military detection expertise to sound a loud warning when metals that are magnetic and could be drawn into the MRI scanner pass through the door.
"The sensors monitor the ambient magnetic field," Dr Mark Keene, from QinetiQ, told BBC News Online.
"The ambient field is the Earth's own field plus any contribution from the MRI magnetism. It is usually quite static and unchanging.
"Magnetic sensors sense disturbances in the ambient field caused by a ferromagnetic object moving."
Hospitals have experimented with conventional metal detection systems used in airport security.
But these are blind to any distinctions in metals and sound an alarm even if harmless non-magnetic instruments are taken into an MRI room.
After a while, the alarms become irritating and many systems are switched off, said Dr Keene.
The idea for Ferroguard came to Dr Keene when he heard about a six-year-old American boy who died while having an MRI scan.
"In 2001, I was at a conference in San Diego to do with landmine detection," he explained.
"I had just got back to the hotel one evening and had put the telly on, to catch up with the news.
"The news came on that a six-year-old boy had died in a New York hospital a couple of days after a gas cylinder had gone into his head."
Staff had not realised the cylinder was ferrous and it slammed into the machine and the boy.
Dr Keene, who had been developing magnetic sensor techniques for military purposes since 1991 under QinetiQ's former guise of Dera (Defence Evaluation and Research Agency), immediately knew the same technology could help prevent MRI projectile incidents.
After some preliminary research back in the UK, he was shocked to discover that there was no such effective detection technology being used in hospitals.
"All they relied on was staff training and a set of warning stickers about what not to take in," he said.
But more often than not, it was untrained or inexperienced staff who did not know the differences between ferrous and non-ferrous material, who caused incidents.
Further research revealed that of 46 medical institutions in the US, 52% reported a serious incident involving objects being "sucked up" by the massive machines.
Some of the case studies were minor, where scissors or hairpins were the attracted objects.
Others were more serious. A prisoner being scanned while accompanied by a police officer had a lucky escape when the officer's gun was pulled from him.
The system is based on mine-detection technology
Most of the time, the incidents cause damage to the machine itself rather than the patient.
"Quite apart from the danger to anyone in the machine, you can't just pull objects off the magnets," continued Dr Keene.
"The magnets are large superconductive coils and they have to be turned off - a slow process called a quench - and the item has to be removed, then field has to be switched on again."
This means the MRI scanner is out of action for two to three days, which can be costly financially as well as medically.
There are 15,000 MRI in use scanners globally.