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Greetings,
Today in class, during a discussion of magnetism, one student mentioned
that she had heard that MRI's pulled metal objects into the cavity that
held the patient, and in at least one case had killed someone. I was quite
skeptical of this. But it turned out that another student knows the family
of a 6-year old boy who was killed in this manner. In this case, a oxygen
tank was accidentally left inside the room and it flew "at 20 - 30
feet/sec" across the room into the machine. (The quote is from a NY Times
article on the incident.) The Times article, student's testimony, and the
link below convinced me that my skepticism was unfounded. On top of that, I
felt quite dissatisfied at my level of intuition about the forces involved
outside the MRI. So I set out to "back-of-the-envelope" the forces involved.
(WARNING: The following analysis is very rough!)
Very quickly, I treated the machine and the canister like two dipoles. I
assumed that the orientations and positions would be optimal (max. force)
for an initial distance of 4 m between the tank and the machine. The
machine I treated as permanent dipole with a field that went like 5T/r^3
(from what I read, that seemed high); and the tank I treated as an induced
dipole with susceptibility on the order of 10^-5. I then determined the
force of attraction by estimating M (magnetic dipole moment/ volume), which
equalled10^-7 and is where I think my analysis is really flawed. I then
assumed it would require about at least 10 N to drag the cylinder.
Unfortunately, at the initial distance of 4 m, force of attraction that I
estimated was about 12 orders of magnitude to small! Even at an initial
distance of 1 m, I'm still 8 orders of magnitude off. (For full disclosure
of my solution, I should also mention that I estimated that the metal
volume of the tank was about 10^-3 m^3.)
Obviously, experiment trumps theory, but I don't see an easy way to
estimate what must be the necessary force.
Someone interested in taking a crack at this?
Thanks in advance,
Jeff
Here's a link with some interesting pictures of tanks that had flown into
MRIs (All the pictures are mechanical - no people or blood.)
http://www.ajronline.org/cgi/content/full/177/1/27? maxtoshow=&HITS=10&hits=10&RESULTFORMAT=1&andorexacttitle=and&fulltext =oxygen+tank +MRI&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT
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