Hi everybody,
Thanks to everyone who responded to my question. Most people think there
will be problems, but perhaps not insurmountable ones. A couple of people
suggested talking to the vendors and getting assurance that the
instruments won't interfere with each other.
A couple of people also corrected my assumption about what the Faraday
cage is for. Obviously I already knew that the purpose of the Faraday cage
is to screen the MRI's receiver from outside RF - I was just testing you.
Here are the responses:
**********
It sounds like a really strange arrangement to have these very
different systems in the same room! An MR scanner needs to be
enclosed in a Faraday's cage so that the receiver does not pick up RF
from the outside world. I have never heard the opposite. The proton
RF amplifier in a 1.5 T (63 MHz) scanner can typically provide a
maximum output of 15-25 kW.
What is the potential strayfield near the instrumentation? I have had
problems with some powerbooks down to almost 1 Gauss.
**********
My Varian 400 89mm vertical bore shares adjoining lab space with a 4.7T
33cm Inova. In addition to RF shielding might I suggest a bit of
acoustical insulation. They're good neighbors but raise a racket with
those gradients!
**********
My guess is that the reason for the Faraday cage is to prevent
external noise from getting in rather than to keep from broadcasting
with any power. fMRI experiments are relatively insensitive. A 10
percent change in signal is typical. The big question is this: Is the
a nuclei on the 800 that is at the same frequency (+/- 1 MHz, maybe 2
MHz) as protons (128,5 MHz?) on the fMRI machine that you would want
to do direct detection on? This might affect the high resolution
data. Conversely, if you transmit on this same unnamed nucleus in
high resolution, it may interfere with the imaging experiments. If
either is so, the "powers that be" might want to rethink this plan.
If not, and you put a Faraday cage around the whole room, your 800
spectra may possibly get better (less noise), but at that high a
frequency I would not expect much of an effect. Please talk with the
vendor.
**********
the only disturbing thing that I can imagine
are the strong gradient pulses, the MRI uses
for adressing different space elements.
Their stray fields are non-static
and cannot be shimmed away.
Perhaps the Faraday cage
have to be combined with magnetic
shielding like mu-metal.
I hope someone else can
say something more
detailed about that.
**********
I don't know if this is of any use to you or not, but I could
probably come up with a list of nearly
50 reasons NOT to site a 3.0 T MRI immediately adjacent to a nMR and
can't, for the life of me, come
up with a compelling reason TO site them next to one another, except a
possible ease of maintenance if
they're served by the same vendor (which, I suspect, isn't the
case).
My reasoning for not siting the new MRI in the same room with your
existing nMR fall into a couple of
categories, disruption to existing services during construction and
potential for interference between the
equipment.
First, disruption during construction:
A 3.0 T MRI probably weighs around 12,000 pounds and requires significant
vibration isolation. For either
or both of these reasons, it could be necessary to replace / reinforce the
existing floor structure under
the location for the new MRI. If your facility is in a basement or
bottom floor of a building
built directly on the earth, this probably means tearing-out the floor
with concrete saws. Not exactly
conducive to a research environment.
And you're right about a Ferriday screen. MRI is very sensitive to
RF interference so the enclosure
for the MRI would most likely have to be RF screened from the rest of the
world. Typically this is done
with solid copper foil in an assembly sandwiched between two walls. So
we're not talking about a
piece of new equipment dropped in the far corner of an existing lab...
More than likely we'd be
looking at walling-off a piece of the lab, creating a separate room
(or suite of rooms) for
the fMRI.
There are also significant power and ventilation requirements for clinical
MRI. Even if the floor is
strong enough, and the lab large enough to be subdivided, there would
likely be significant construction
to bring the required infrastructure to the new fMRI site.
Second, potential equipment interference:
First, I must qualify this with the caveat that I'm not a
technologist or a physicist. That said, I
would be very worried that the dynamic magnetic fields from the MRI could
interfere with sensitive
equipment that you've sited away from your nMR and, potentially,
alter the signal-noise ratio for
your nMR (please reference big fat caveat at the beginning of this
section). And though the field plot
for a nMR is much smaller than a 3.0 T MRI, with studies as sensitive as
fMRI, I would also be worried
that any changes in the field from the nMR could interfere with the
signal:noise of the MRI. What
consequences would there be to the operation of the fMRI if the nMR was
allowed to go warm? Would the MRI
have to be re-shimmed?
There are a number of questions about the potential interference from one
to another and I would want
iron-clad assurances from the vendors of BOTH systems that the proposed
siting would not diminish the
diagnostic / investigative capabilities of either device.
If you're interested in other issues pertaining to siting, design
and construction of clinical MRI
equipment, I invite you to take a look at a website we're in the
process of developing
(
http://www.MRI-Planning.com).
Please let me know if there is anything else I can do. I would very much
like to know how this plays out
for you and UCLA.
----------------------------------------
Robert Peterson, Ph.D.
Facility Manager
MBI-DOE Biomolecular NMR Facility
UCLA Dept. of Chemistry and Biochemistry
phone: (310)825-1816
fax: (310)825-0982
peterson_at_mbi.ucla.edu
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Received on Thu Nov 18 2004 - 17:51:45 MST