5 ms·

I didn't realize they were on all the time.

by lutorm 16y ago

jarin 16y ago
You know how it takes months to power up and down the Large Hadron Collider? MRI machines are kind of like a mini version of that.
- retube 16y ago
  and it takes weeks to power them down too, else you get the dreaded quench... http://en.wikipedia.org/wiki/Superconducting_magnet#Magnet_quench http://en.wikipedia.org/wiki/Superconducting_magnet#Magnet_q...
mbateman 16y ago
Yes, you can't even turn them off by unplugging them, as it were. A certain kind of emergency shutdown dumps the helium needed to allow superconductivity (and thus maintain the magnetic field). It costs like $20,000 to do this and is quite dangerous in itself.
- india 16y ago
  It's called MRI quenching and dangerous it can be[1]. [1] http://www.youtube.com/watch?v=1R7KsfosV-o http://www.youtube.com/watch?v=1R7KsfosV-o
  - InclinedPlane 16y ago
    Modern high-field superconducting MRI magnets can hold tens of megajoules of stored magnetic field energy, equivalent to the explosive energy in several kilograms of TNT.
- lutorm 16y ago
  Yeah, I should have thought this through. I knew you kept them cool, and with a bit of thinking it should have been clear that you don't turn a superconducting coil off, since nothing's powering it... ;-)
glabifrons 16y ago
They're not. They're only magnetic when they are in operation. I've been in them, I've been with people who've been in them, and my family is in the medical field. If they were fully energized all the time, you wouldn't be able to bring people in and out of the room on a stretcher.
- mbateman 16y ago
  I'm not an MRI tech but I work on MRI image analysis academically, so I know a bit more than usual by hearsay and I see them used all the time. Why are there "MRI magnet is always on" signs in the medical imaging areas of the med school here? Are you sure they don't use titanium stretchers near the field? Why do they perpetually supply the MRI with liquid helium? My understanding is that even when they are not used there is a field of between 1 and 3 teslas, depending on the scanner. (A quick google search for things like "mri magnet is always on" and "mri safety" and such seems to confirm this.) Edit: Also, you can get closish to the scanner with a ferromagnetic object without a problem. The field doesn't encompass the whole room or anything. 10 or 15 feet away will be fine.
  - glabifrons 16y ago
    I apologize, I should clarify my point. They're not magnetic enough to pull a chair off the floor or things across the room (as the story indicates) when they are not in operation. You can walk around the room with your watch on (for example), but if they fire it up and you're in there with it, it's destroyed. Just look how close they have to get the steel oxygen bottle to the center (in the video linked above) before it moves. It's almost inside it already.
    - kressler 16y ago
      There are low field scanners that use conventional electromagnets, which aren't always on. However, the scanner shown is a 1.5 Tesla (T) or 3T scanner (difficult to say which from just the housing), and the main magnetic field is always on. The main field is supplied by a superconducting ring immersed in liquid helium. As a previous poster noted, the only way to shut one down is to dump the helium, and the cost to bring the scanner back up is in the tens of thousands of dollars. There are smaller electromagnets involved to apply field gradients across the bore of the scanner (on the order of 50 mT/m) which are off when the scanner is not operating, but these are quite small compared to the main field. My Ph.D. dissertation was on data acquisition and reconstruction techniques for MRI. I've logged hundreds of hours operating high field scanners like the one shown, and dozens of hours being scanned for various research studies. I've also (carefully) hauled a variety of strange things in and out of scanner rooms. Most of the tools we used were non-ferrous, and we had to be extremely careful with the few ferrous pieces of equipment we had to use. The magnetic field does drop off rapidly as you move away from the scanner, so objects more than 10 feet or so away are unlikely to be pulled in.
js2 16y ago
Depends on the MRI magnet type apparently, but with modern superconducting magnets, yes: http://health.howstuffworks.com/mri.htm/printable http://health.howstuffworks.com/mri.htm/printable http://en.wikipedia.org/wiki/Superconducting_magnet#Persistent_mode http://en.wikipedia.org/wiki/Superconducting_magnet#Persiste...
- deleted 16y ago
  [deleted]

I didn't realize they were on all the time.

by lutorm 16y ago

I didn't realize they were on all the time.

jarin 16y ago

You know how it takes months to power up and down the Large Hadron Collider? MRI machines are kind of like a mini version of that.

retube 16y ago
and it takes weeks to power them down too, else you get the dreaded quench... http://en.wikipedia.org/wiki/Superconducting_magnet#Magnet_quench http://en.wikipedia.org/wiki/Superconducting_magnet#Magnet_q...

mbateman 16y ago

Yes, you can't even turn them off by unplugging them, as it were. A certain kind of emergency shutdown dumps the helium needed to allow superconductivity (and thus maintain the magnetic field). It costs like $20,000 to do this and is quite dangerous in itself.

india 16y ago
It's called MRI quenching and dangerous it can be[1]. [1] http://www.youtube.com/watch?v=1R7KsfosV-o http://www.youtube.com/watch?v=1R7KsfosV-o
- InclinedPlane 16y ago
  Modern high-field superconducting MRI magnets can hold tens of megajoules of stored magnetic field energy, equivalent to the explosive energy in several kilograms of TNT.
lutorm 16y ago
Yeah, I should have thought this through. I knew you kept them cool, and with a bit of thinking it should have been clear that you don't turn a superconducting coil off, since nothing's powering it... ;-)

glabifrons 16y ago

They're not. They're only magnetic when they are in operation. I've been in them, I've been with people who've been in them, and my family is in the medical field. If they were fully energized all the time, you wouldn't be able to bring people in and out of the room on a stretcher.

mbateman 16y ago
I'm not an MRI tech but I work on MRI image analysis academically, so I know a bit more than usual by hearsay and I see them used all the time. Why are there "MRI magnet is always on" signs in the medical imaging areas of the med school here? Are you sure they don't use titanium stretchers near the field? Why do they perpetually supply the MRI with liquid helium? My understanding is that even when they are not used there is a field of between 1 and 3 teslas, depending on the scanner. (A quick google search for things like "mri magnet is always on" and "mri safety" and such seems to confirm this.) Edit: Also, you can get closish to the scanner with a ferromagnetic object without a problem. The field doesn't encompass the whole room or anything. 10 or 15 feet away will be fine.
- glabifrons 16y ago
  I apologize, I should clarify my point. They're not magnetic enough to pull a chair off the floor or things across the room (as the story indicates) when they are not in operation. You can walk around the room with your watch on (for example), but if they fire it up and you're in there with it, it's destroyed. Just look how close they have to get the steel oxygen bottle to the center (in the video linked above) before it moves. It's almost inside it already.
  - kressler 16y ago
    There are low field scanners that use conventional electromagnets, which aren't always on. However, the scanner shown is a 1.5 Tesla (T) or 3T scanner (difficult to say which from just the housing), and the main magnetic field is always on. The main field is supplied by a superconducting ring immersed in liquid helium. As a previous poster noted, the only way to shut one down is to dump the helium, and the cost to bring the scanner back up is in the tens of thousands of dollars. There are smaller electromagnets involved to apply field gradients across the bore of the scanner (on the order of 50 mT/m) which are off when the scanner is not operating, but these are quite small compared to the main field. My Ph.D. dissertation was on data acquisition and reconstruction techniques for MRI. I've logged hundreds of hours operating high field scanners like the one shown, and dozens of hours being scanned for various research studies. I've also (carefully) hauled a variety of strange things in and out of scanner rooms. Most of the tools we used were non-ferrous, and we had to be extremely careful with the few ferrous pieces of equipment we had to use. The magnetic field does drop off rapidly as you move away from the scanner, so objects more than 10 feet or so away are unlikely to be pulled in.

js2 16y ago

Depends on the MRI magnet type apparently, but with modern superconducting magnets, yes: http://health.howstuffworks.com/mri.htm/printable http://health.howstuffworks.com/mri.htm/printable http://en.wikipedia.org/wiki/Superconducting_magnet#Persistent_mode http://en.wikipedia.org/wiki/Superconducting_magnet#Persiste...

deleted 16y ago
[deleted]