Re: MRI Machines

On Wed, 25 Jun 2008 08:58:36 -0400, Spehro Pefhany
<speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Tue, 24 Jun 2008 17:33:34 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Tue, 24 Jun 2008 19:30:42 -0400, RFI-EMI-GUY
<Rhyolite@xxxxxxxxxxxx> wrote:

Jim Thompson wrote:
On Tue, 24 Jun 2008 11:34:07 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Tue, 24 Jun 2008 13:28:46 -0400, RFI-EMI-GUY
<Rhyolite@xxxxxxxxxxxx> wrote:

Well I spent a half hour of my life under the magnet of an "open" MRI
machine yesterday. Still a bit claustrophobic. While I sort of
understand the theory of the MRI, what I don't understand is what
creates all the noise and racket. Any experts out there that can tell me
whats going on?
Pulsed gradient coils. These are "small" 3-axis coils that sweep a
gradient field across your innards, modulating the nuclear resonant
frequencies of your molecules to produce the spatial resolution. These
are generally water-cooled, with ballpark 20 KW constant-current
drivers per axis, playing all sorts of weird waveforms.

They scanned my head last year, with the gradient coils around my head
like some horror mask: noisy and boring.

John

A few years ago they scanned my sinuses.

I must admit a claustrophobic feeling also :-(

...Jim Thompson

I told the tech to give me a time check every so often so I wouldn't
lose my mind, then I worked out a design problem in my head for 30
minutes as a distraction. I can put up with a lot of stuff but being
inside or under that thing creeps me out. I keep having visions of
oxygen cylinders getting sucked through the drywall and crushing me or
the magnet quenching and getting burned in a puff of steam.

What an absurd fear. It quenches, a lot of cold helium blows out, the
O2 in the room is displaced, and you die painlessly of asphixiation.

John

There are a number interesting modes of dying involving cryostats.


Yeah. And I froze my knee once, kneeling on the floor, when I spilled
some LN2.

The MRI magnets are usually vented outside, so a quench is no big
deal. The ballpark-8-feet-high NMR magnets I work around are no big
deal either; in a reasonably-sized room, they don't displace enough
oxygen to be a hazard.

Here's a 600 MHz (14T) unvented Varian/Oxford magnet:

http://www.nmrfam.wisc.edu/specs/newvosges.jpg


At Jlabs, the underground electron accelerator in Virginia, the
magnets are big, in a small tunnel, and there are alarms and blowers
and stuff. If one quenches, staff are instructed to clear out pronto.

John

.

Relevant Pages

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    ... While I sort of ... gaussmeter to maybe 10-20 feet above the magnets and almost nothing ... in 8 years of working here I've only seen 2 quench events. ...
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  • Re: Smith and Jones science (Was: Doctor Who progressive or interlaced?)
    ... Such magnets are basically one shot devices ... So, the MRI should destroy ... the moon and project it to the Earth, every point of the Earth at that level ... This is a science fiction show and has to be believably ...
    (rec.arts.drwho)
  • Re: Smith and Jones science (Was: Doctor Who progressive or interlaced?)
    ... >>> early effects of her modifications to the MRI so she obviously has ... >> very, very, limited range just like the ones generated on Earth. ... The MRI scanner has shielding and probably has focusing devices like the ... pieces of metal you put on the end of magnets to concentrate the field in ...
    (rec.arts.drwho)