"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.

Also, RF coupling to the body coil can increase this;

Basically induction heating a person? Or more likely capacitively (B and E
are related anyway, so who cares) given the lossy nature of flesh.

Tim

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