> Hi Guys,
snip, snip
>
> In NMR/MRI, the rf frequency (frequency of nuclear magnetic
> resonance)
more snip, snip
> IMO it's more a function of desired/required "spin echo"
> signal level,
> which translates into S/N, image clarity, and ultimately
> machine throughput
Basically you want to flip the nuclei a given number of degrees from their
ground state for NMR spectroscopy/imaging. When they relax back you acquire
the signal.
The degrees of flip is simply a product of the RF signal the nucleus sees
(not just a function of what you put out, as the macro and micro environment
hides a lot of the RF) times the duration of the pulse. Pulse durations for
spectroscopy are generally in the several microsecond range (an overly
broad, sweeping generalization) and those for imaging are longer (quite a
bit, sometimes). The higher the power you put out, the shorter the pulse
duration needs to be. There are theoretical advantages to keeping the
pulses short (i.e. using more power), and this can result in improved signal
return. Its not a simple linear relationship though.
If you are interested in more NMR stuff, all medical and getting a bit
long in the tooth, you can peruse the "interesting projects" spot at
http://www.epix.net/~rrehr for titles. I would be willing to send out a
limited number of the monographs or reprints if specifically requested, to
the REALLY interested (first come first served). The Current Problems in
Cardiology is an 88 page invited, peer-reviewed monograph on NMR (including
and in fact, primarily MRI) with a moderate amount of very simplified
theory.
Sorry for the bandwidth, but my willpower really did well for the first few
days of this NMR thread (and it is still NMR, just a name change as
indicated before in this thread for (1) better public consumption (no
nuclear), and also (2) academic politics).
73 from
Roger Rehr W3SZ ex-AA3QK ex-WA3JYM
mailto:rrehr@epix.net FN20ah
http://www.epix.net/~rrehr
2 Merrymount Road, Reading, PA 19609
610.670.8687
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