Dear All,
I want to thank Deane McIntyre, Patrick Wheeler, Arnold Harrison, Roy
Van Vliet, David Redwine, Guy Daelen, Walt Niemczura, Jim Breeyear,
Alan Kook, Dave Vander Velde, George Sukenick, Gareth Morris, Joe
Schwartz, Lew Cary, Dave Scott, Joe Ortiz, and Woody Conover for
responding. If I missed anyone, I apologize.
This has been a tough problem because it is intermittent. It varies
from coil to coil and from day to day for a given coil. Moreover, I
now suspect that we have more than one source because sometimes it
looks frequency modulated (the radio station I talked about before)
and sometimes it appears as a series of short bursts so that it
almost looks like an echo train from a CPMG sequence with no T2
decay. Also, one morning we were running just fine with no
interference and the second form snapped on suddenly pretty close to
10 AM. Some of you suggested filtering schemes of various sorts, but
putting a narrow band 26 MHz filter (~1 MHz bandwidth) on the preamp
input didn't help. I thought this was a good idea and was surprised
that it didn't work. In the end, what did work was what many also
suggested was turning the magnet into a Faraday cage. The machine
shop had, made a copper plate (why copper and not aluminium I don't
know, it was before my time) to mechanically tie the magnet and
gradient set together. Simply draping Al foil over the bore was
insufficient. We had to remove a few of the mounting screws from the
copper plate and use those to adequately ground the Al foil. That got
rid of the interference although it is not the most convenient setup.
As an aside one of the graduate students dug up a book that I was not
aware of "Biomedical Magnetic Resonance Technology" by Chen and
Hoult. Section 4.4 has a good section on probe design for rejecting
external interference. It also discusses why large bore magnets at
low field are particularly susceptible to this problem. I think I
also better understand the problems the coils had with regard to
picking up 26 MHz, but I haven't figured out how 90 MHz gets through
the filter yet. Possibly around on the filter ground? Anyway, we can
get rid of it now.
Thanks again to everyone!
Jeff
Jeffrey H. Walton
Associate Research Physicist
One Shields Ave.
UCD NMR Facility, Bldg. MS1-D
University of California
Davis, CA 95616
(530) 752-7794 (office)
(530) 752-6480 (MS1-D lab)
(530) 754-9064 (Cruess Hall lab)
(530) 752-3516 (FAX)
jhwalton_at_ucdavis.edu
NMR Facility URL
http://www.nmr.ucdavis.edu
Received on Fri Feb 09 2001 - 11:48:30 MST