On 7/23/25 7:41 PM, Joe Gwinn wrote:
On Wed, 23 Jul 2025 22:48:47 -0000 (UTC), danny burstein
<dannyb@panix.com> wrote:
In <a9328ktck8aaea6p37a9lvvkmtb9gqb7st@4ax.com> Joe Gwinn <joegwinn@comcast.net> writes:
FYI.
Man wearing heavy metallic necklace dies after being sucked into MRI
machine, Madeline Halpert, BBC, 21 July 2025.
.<https://www.bbc.com/news/articles/cx2n39dvp0po>
Freak accident for sure, but MRI magnets are *exceedingly* strong, and
no they cannot be turned off.
Umm, yes, they can. An emergency shutdown is very expensive...
How long does it take?
Joe
I did an emergency discharge on a 3 tesla 6" room temperature bore
supercon once in grad school. There was a vacuum leak in the cryostat
and the boiloff rates had gotten high enough that it had to be shut down
and we didn't have a power supply to do it. We were trying to borrow a
supply from the system vendor but couldn't get one in time. Oxford had
an emergency procedure in the manual that used cross coupled stud diodes
rated for the main coil current (30 or 40 amps?) on a heat sink along
with a little dc supply and clip leads to energize all the shim and main
coil superconducting switches. Took us an hour to scrounge the parts
and a few minutes to get the charging wand inserted and connected. The
actual discharge took about 20 minutes for the field to be weak enough
that you couldn't feel it tug on a screwdriver held against the side of
the cryostat (not inside the bore). We let it go 2 or 3 hours just to
be safe, then burped a small hit of helium gas into the cryostat to
spoil the vacuum and boil off the rest of the LN2 and LHe. It was all
room temp a week or two later when we tore it down to replace the magnet.
That was back in the early 1980's. I've seen pics of MRI machines with
the diodes on a heat sink hung on the cryostat along with a discharge
cable for the charging wand that already had all the superconducting
switch leads tied together and a 5 volt power supply for the switch
heaters so it was all ready to go in case of an emergency. That plus
the normal charging wand and down it comes. An MRI magnet would have a
lot more stored energy than our little 3 T magnet so I'd guess at least
an hour or two to get the field down to where you could remove a large
iron object like a fire extinguisher (or a person with a large ferrous
necklace :-)) and 24 hours to be sure it was completely discharged.
--
Regards,
Carl
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