On Mon, 14 Mar 2005 14:53:35 -0600, Tim Williams wrote:
"Dave Hinz" wrote in message
...
You have _stronger_
magnets in your computer's hard drive, by the way.

I didn't know they could produce permanent magnets anywhere near 2T, let
alone 20.

Most MRI systems are 1.5T. Some of the newest ones are 4T, but those
are few and far between.

When I worked in MR Engineering, we, well, were playing around with
magnets, oddly enough. The permanent magnets in a Hawk hard drive
(1 gigabyte full height - these were old) came in above the measurement
capability of our gauss meter, which went up to above 4T. So, at
the surface, it's more than a 4T permanent magnet, in those drives.

Boy, those are some impressive magnets then... hey, they'd make great
replacements for those tricky superconducitng rigs in MRI's!
BG

It's strong, but the field isn't anywhere near even (parts per billion
is needed), or large enough.

RF Heating might also be a concern, but less so than the discomfort
and image problems.

That's an UL with tatoos, but as I recall it only causes a little
distortion, not really any heating or "pull".

Um, no. RF Heating is a real and measurable phenomenon in MRI imaging.
The "SAR" (specific absorption ratio) calculates just how much RF
you can pump into the patient, based on their mass. Also, RF
coupling to the body coil can increase this; some brands of MRI
scanners have "decoupling plates" where the patients' arms go,
so that they don't get the warming (and detune the system...) by
getting to close to the RF coil. Different manufacturers probably
handle decoupling in different ways, but a simple spacer does wonders.