On Sun, 29 Jan 2006 04:34:10 GMT, Richard Lamb wrote:

Possibly a dumb question (tm), Martin, but while you are flipping these
electrons around - where do the go?
Do they hop out to the next valence band/shell/level ?

I'm not Martin, but yes. You're exciting those electrons with a pulse
of RF energy (in a typical MRI scanner of 1.5 Tesla magnet strength, the
RF pulse is at 63.86 MHz). That RF pulse bumps the electron up to the
next band. After a predictable time, it falls back down and gives off
an echo of the RF pulse that put it up there in the first place. That
echo is what we pick up, run through a fast-fourier transform (twice)
and come up with the image. Oversimplified but essentially true.

Or are we doing something else here?

It's not unlike sonar, but the ping and echo are with RF rather than
audio. Again way oversimplified but not a wrong way to look at it.

Dave Hinz