Re: DeGaussing an MRI suite

["Jim Dunlap" <jim.dunlap@guardian.brooks.af.mil>]

     Are you pledging a fraternity?  Periodically, a bunch of guys from one 
     of the ivy league schools send a letter to Ann Landers describing some 
     far-out, fabricated situation for her to reply to.  She's pretty good, 
     most of the time, about figuring out it was a joke.  Your question 
     reminded me of one I occasionally would get when teaching the 
     'uninitiated' lay person; viz., "What happens to the residual medical 
     x-rays that aren't absorbed."  My initial response was, "They bounce 
     off of the patient, settle to the floor of the diagnostic radiology 
     suite following patient exposure, and eventually are either swept up 
     by the janitor or evaporate, but if you need to enter the room it's 
     O.K.. because they'll just bounce off of you."  Then I would tell them 
     the truth.  With regard to your question, I really don't have an 
     answer, except to say I've never heard of residual "static magnetic".  
     (Thats kind of catchy.  Sort of like the internal dosimetry tables 
     that Ken Skrable developed which have been referred to as the "Scrable 
     Tables").  As an aside, but just a peculiar a phenomenon:  I was once  
     involved in a case with a hot cell where high-dose gamma sources were 
     handled remotely, in which the thick glass window, after MANY years of 
     exposure, 'shattered', due to being 'supersaturated' with electrons, 
     but remained in the solid physical form.  This apparently was because 
     the glass was acting as a capacitor, and finally became 
     'supersaturated', and "exploded", similar to what happens when 
     'radiation trees' are produced when a piece of plastic is exposed to a 
     very narrow beam of high energy electrons.  So, maybe your question is 
     valid.  I too would be interested in an accurate discussion of this.  
     Thanks.
     
     jim.dunlap@guardian.brooks.af.mil


______________________________ Reply Separator _________________________________
Subject: DeGaussing an MRI suite
Author:  radsafe@romulus.ehs.uiuc.edu at guardian
Date:    1/26/99 5:09 PM


Has anyone has any experience in dealing with the residual static magnetic 
fields that remain in an MRI suite after a unit is removed?

The residual fields can reach 2 mT.  This is too low to be a real health 
concern, but it's enough to cause problems for electronics and its enough to 
upset people.

Degaussing is theoretically possible, but I can't find anyone who has actually 
done it.


John Moulder (jmoulder@its.mcw.edu)

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