[TenTec] Induction Field [WAS - PS grounding]

Bill Fuqua wlfuqu00 at uky.edu
Fri Dec 12 14:59:35 EST 2003


MRI works by having a magnetic field gradient across the body so that the 
the hydrogen atoms in different location have different resonant 
frequencies. Then by exciting those resonances using a pulsed RF source a 
receiver can detect all the signals and using a FFT determine the 
frequencies produced.  Each frequency is associated with a particular 
location. The magnets have a very strong homogenous field but there are 
gradient coils that allow them to change the field gradient direction for a 
different view you may say.

73
Bill wa4lav
At 06:34 PM 12/12/2003 +0000, you wrote:
>Well, remember that all protons are not identical and one must use the
>'magic individual combination of RF freq and field strength' to get a proton
>to flip.
>
>That simply means oine could keep the field constant and vary the RF freq or
>keep the RF freq constant and vary the field.  Really doesn't matter to the
>protons but one way may be much, much simpler to implement than the other.
>
>BTW the percent change in one or the other is small e.g. about 0.1 % would
>cover all protons.
>
>----- Original Message -----
>From: "David McClafferty" <ve1adh at yahoo.ca>
>To: <tentec at contesting.com>
>Sent: Friday, December 12, 2003 15:22
>Subject: Re: [TenTec] Induction Field [WAS - PS grounding]
>
>
> > I thought it was just the RF field that was changing
> > within a steady magnetic field. Did some more reading
> > and found there is a smaller magnetic field that is
> > pulsed.
> > I was wondering what the RF frequency was. It doesn't
> > seem to be mentioned in anything I have read.
> >
> > Dave
> >
> >  --- Ken Brown <ken.d.brown at verizon.net> wrote: >
> > David McClafferty wrote:
> > >
> > > >To induce a current into a conductor the magnetic
> > > >field must be varying or the conductor or field
> > > must
> > > >be moving. The magnetism in an MRI machine is
> > > steady,
> > > >so unless they wheeled you in there quite fast, you
> > > >wouldn't get much current induced into your body.
> > > >
> > > >
> > > Excuse me, but if there is no change in the field,
> > > it will not make
> > > anything resonate. I'll look at the url you included
> > > later and see what
> > > they say. If they say that it is a constant, non
> > > varying magnetic field,
> > > then I say they are wrong. I took my VX-5 handheld
> > > radio and listened
> > > outside the lab while my wife was in the MRI
> > > machine. And I heard lots
> > > of varying  pulse rates and bandwidth signals around
> > > the 60 MHz area.
> > > What I heard defininately came from the MRI machine,
> > > and it is not
> > > surprising that water molecules have a resonance
> > > around 60 MHz, human
> > > bodys being mostly water. Also the spare 3CX800A7s
> > > that I have for my
> > > Titan 425 are pulls from a MRI machine. If it was a
> > > constant magnetic
> > > field and not a varying one (at RF frequencies where
> > > ther are resonances
> > > with things in the body) they could just use a
> > > switch of some sort and
> > > not need a power amplifier tube>
> > >
> > >
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