In article ,
Tim Williams wrote:
"Dave Hinz" wrote in message
...
Most MRI systems are 1.5T. Some of the newest ones are 4T, but those
are few and far between.

Really? Then were do I keep reading 10-20T from?

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.

Huh. Anyone know offhand what the top field strength of the toughest
magnetic materials is these days? I always thought magnets topped out
around iron's saturation levels (1-2T)...

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

As I recall, MRI's use a Helmholtz coil (I think), basically two short
solenoids seperated by a distance. The space between them has a relatively
constant field. Not perfectly constant, so they must have to use extra
coils and tweaks either way, no?

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.

I'll admit you've got me on the RF part... All I know is the field aligns
hydrogen atoms (and maybe others), then RF is applied and re-radiated...how
the hell they decipher the 10^20llions of hydrogen atoms spatially is beyond
me.


Here ya go Tim. This is what took MRI from just that side of black magic to
just this side of it. Time to exercise your math skills (or just look at the
pretty pictures and you will get a good deal of the info).

http://www.cis.rit.edu/htbooks/mri/inside.htm

I believe that somewhere in Chapter 5 there is a spiffy picture that more or
less sums up how they got from RF signals to images.

Really interesting was that mid way through Chapter 9 we find:

Another frightening story was of a law enforcement officer being allowed
to go near a magnet with a loaded firearm. The handgun was pulled out of
its holster, and into the magnet. The force of the impact with the magnet
caused the gun to discharge. Luckily, no one was injured in this incident.
In addition to the damage to the MRI and the bullet lodged in the scan
room wall, the gun was magnetized.

Just how old was that original story?

-- Joe

--
Joseph M. Krzeszewski Mechanical Engineering and stuff
Jack of All Trades, Master of None... Yet