Hi Guys,
In NMR/MRI, the rf frequency (frequency of nuclear magnetic resonance)
depends on the element to be imaged and, additionally, is a linear function
of the magnet strength as described by the Larmour (sp?) equation. The vast
majority of routine clinical imaging is done on hydrogen because water/H2O
is the most common material in the body. For hydrogen, the Larmour
frequency is approximately 42.6 MHz at 1T (1 Tesla = 10,000 gauss). It
scales linearly from there: at 0.5T it's around 21.3 MHz (anybody heard of
"MRII" on 15M?), at 1.5T, 63.9MHz; 2T about 85.1 MHz.
The RF power level doesn't necessarily HAVE to vary with magnet strength;
IMO it's more a function of desired/required "spin echo" signal level,
which translates into S/N, image clarity, and ultimately machine throughput
(how many scans can be run through the machine per hour or day - which in
turn relates directly to return on an investment that commonly runs $1M-3M
for big MRI facilities. OTOH, I guess if you pay the price ($250-400k??)
for a multi-ton, 1.5T cryogenic magnet it probably makes sense to have
15-20 kW of peak (highly linear) rf available to help make efficient use of
it.
Cheers, Dick W0ID
-----Original Message-----
From: Dave Kirkby [SMTP:davek@medphys.ucl.ac.uk]
Sent: Friday, May 29, 1998 11:06 AM
To: Jon Ogden
Cc: W9JA Paul Hellenberg; amps
Subject: Re: [AMPS] Re:
.... SNIP .... SNIP etc. ...
> From what I knew NMR and MRI are similar but 2 different
> technologies.
> Of course I could be wrong too. NMR I thought was used more for
> material
> research while MRI was more for diagnosis of human patients.
> The strength of the amp depends on the strength of the magnetic field
> generated. The frequency of the amp also depends on the strength
> too. A
> 2 Tesla system (the strongest that I know of) operates around 64 MHz.
> For the 2T system, I want to say it goes as high as 20 KW, but then
> that
> might be too high (7 KW also comes to mind). Anyhow, it's much higher
>
> than 1 KW for the bigger systems. And yes, average power is very low
> on
> these.
> ... SNIP ...
We have a 2.4 T clinical machine, and a 7 T reseach machine. To the best
of my knowledge, the peak powers are only 1 kW - but I'll check this.
Bruker is another manufacturer of these things, but I dont know if they
use valve PA's.
G8WRB.
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