Magnet mistakes

This is just a short post on the many ways in which films and telly often get MRI wrong, and one thing that they tend to get right. Also, it is a good excuse to post a few interesting MRI videos.

1. The magnet is ALWAYS on. You don’t turn on an MRI. Nor do you turn it off. The machine uses a magnetic field which is always on as long as the machine is operational (whether scans are taken or not). This field can be pretty strong, and will trap ferrous metal objects in the bore, even if there is no scan running. Actually turning off the magnetic field (quenching it) is only done if the scanner is being decommissioned or in life-threatening situations, as puts it out of action for at least a week and cost a lot to restart (>£20,000), even if it has not been damaged by the quench.

2. Pressing the red button is usually bad news. There are two types of big red button. One is an emergency stop which does not turn off the magnet per se, but turns off power to consoles, lights (not emergency lights) and so on. The other quenches the magnet (rarely done, see the above point) and it looks like this:

The video above is a magnet being quenched at 1% helium capacity, which is to say that it is not nearly as big an event as it could be.

3. The magnet is as strong as it is. Variable field strength is not a thing. You cannot turn up the field, and you cannot turn it down.

4. Scans typically take time to acquire and interpret. If it was possible to put a person in and read out the data within seconds, that would be great. However, a good structural scan takes minutes, and a functional scan often even longer plus it requires additional processing steps that can take hours or days. There are also setup scans typically run before the main scan, and these also take time. It is unfortunately not plug and play. Caveat: real-time MRI is a thing, but it is mostly used for cardiac imaging and rare cases of functional MRI neurofeedback sequences. Typically, these are not the ones portrayed in the offending films.

5. Colours? Scans usually don’t come automatically in pretty colours. Structural scans are in more-or-less grainy black and white, and while functional scans can be presented in colour, this requires a lot of processing after the scan has been completed (see above). And what you get out is typically a statistical map of the signal, not the actual measurements themselves. In short: colours usually means lots of stats, stats usually means lots of time.

6. It’s noisy! And not simply high-tech whirr either: it can sound like a construction site in there.

7. There’s often a coil. At least with neuroimaging, where the coil is a cage-like structure placed around the head. This mistake can be forgiven if the scan in question would use a body coil, which can be pretty invisible and look like a part of the table.

There are plenty of films getting MRI wrong, for example Die Another Day (although the MRI bloopers are arguably not the biggest problem with that film) and Terminator Genisys that manage to get not only the turn it on and off again wrong, but also the variable field strength, plus introducing a conveniently appalling lack of shielding (meaning that the fringe field (the magnetic field that surrounds the magnet) is so large it reaches the control room). Go watch the Terminator clip over on youtube (from 1 min) to see for yourself. I feel for that poor MRI scanner.

+1. It’s strong. This is the one most get right. The magnetic field is strong – it will pull ferrous items into the bore of the magnet, wreck the item (and sometimes itself), and you’re probably not strong enough to stop it.

That being said, there are plenty of films where metal props are far too close to the magnet to be believable, and the ‘patients’ are allowed to keep on items of clothing such as underwired bras and watches, and even bring handbags or other personal items into the scan room. Even if such items are not ferrous, they can still cause image artefacts, and are typically removed. I’ve been told there is a Grey’s Anatomy episode where an MRI was requested for a patient with a fork stuck in the neck – the less said about that the better.

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