Daniel Haude wrote:

On Mon, 18 Aug 2003 16:23:10 -0500,
Jon Elson wrote
in Msg.



Up to this last tiny bit, everything you have replied is absolutely
perfect. The large
medical MRIs, due to the weight of the magnet, dewars and cryogens
alone, as well
as the magnetic fields, the requirement that things be incredible stable
for the MRI
imaging process to work, etc. require the magnet housings to be massive
structures.
I know of several medical MRIs in my area that have been hit by Oxygen
bottles of
various sizes, as well as carts, instruments, gurneys and a floor
polisher! As far as
I know (I have a very good friend who works on many of them) all of the
magnets basically
survived the assault.



But they did quench, no?


Oh, yeah! Any large iron-containing object that moves within a meter or
so of the
magnet is likely to make the field jump around, and a quench is likely
to happen.
There have been cases where a truck moved in the alley behind the NMR
facility,
and the magnet quenched.

Some newer magnets use external field cancelling coils to reduce the
volume of the
strong external field, and it also reduces the magnet's susceptibility
to external
influences. So, these are supposed to be more resistant to outside
influences causing
a quench.



All of them suffered total destruction of the
plastic "beauty"
covers, and a great mess to the shim and gradient coils that are inside
the main magnet
coil. But, as far as I remember, he said no magnet itself needed to be
replaced.
These incidents still can cause several hundred thousand Dollar repair
bills, as it
is almost always hospital negligence that caused the incident.



Indeed. But you're right: A heavy iron object accidentally brought near a
medical NMR is too likely an event in a hospital to permit the complete
annihilation of the single most expensive piece of equipment around. My
observation was based on my experience with scientific magnets which are
supported by rather scanty stainless-steel and plastic rods to minimize
heat transfer into the dewar.


Well, the insides of the medical ones have superinsulation, and fancy
low-thermal
conductivity supports, too, but the outer housing is pretty beefy. They
have to
vacuum test the dewars and the outside housing after a quench to detect
any leaks
that may have developed. That right there requires some heavy
structure. After
the vacuum checks, the outer housing remains evacuated.

Jon