Re: transcranial magnetic stimulation
Dan Clemmensen (Dan@Clemmensen.ShireNet.com)
Sun, 19 Apr 1998 19:31:11 -0400
Anders Sandberg wrote:
>
> Nick Bostrom <bostrom@ndirect.co.uk> writes:
> > Would it in principle be possible to arrange the magnetic fields in
> > such a way that the flield strength is only significant in the
> > nucleus accumbens, say, and perhaps in some regions were the
> > stimulation doesn't have any effect?
>
> Tricky. I'm not good enough at physics to say for certain, but it
> looks like a very complicated problem; you want to add together a lot
> of dipoles (possibly time-varying) so that the induced current given
> the electromagnetic properties of the head become large in only one
> small area. It might be doable, but it would be a hack.
>
If the reponse time of the region is long enough (doubtful), you
could perhaps physically swing a single dipole around the head.
as I recall, an appropriate dipole would generate a cylinder of
approximately uniform flux along its axis between the poles,
witb the "return path" for the flux approximateing a a much larger
(and therefore less dense) sphere. If this approximation is
valid, thenswinging the entire mess around so that the opposite
poles describe a sphere about the target region will concentrate
the average flux at the target. This is the apporach that was (is?)
used in radiation treatments of cancer. the problems to solve are:
1) the diameter of the hi-flux cylinder is probably large by comparison
to the equivalent beam used in radiation therapy, and
2) the time constant for reaction to the magnetic field is likely
to be short (milliseconds?) relative to the time constant for
sensitivity to radiation (minutes to days.) You may be able to
solve the second problem by moving the field from magnet to magnet
electronically rather than by moving it physically, but as Anders
says, this could be tricky.