From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Sun, 21 Dec 1997
Newsgroups: sci.physics,sci.med.physics,sci.techniques.mag-resonance

Chuck Bloch <cbloch@iucf.indiana.edu> wrote:

> Steven B. Harris <sbharris@ix.netcom.com> wrote:
> >
> >    I've already ASKED an MRI physicist, and his verdict is that YOU
> >don't know what you're talking about.  I'm waiting on other opinions,
> >from these news groups.
> >
> >   I'm sorry, but I have spoken to one bona fide expert, an MRI
> >physicist, and he says you're misusing the langauge when you say "radio
> >wave."  I agree.
> >
> >                                             Steve Harris, M.D.
>
> What is his name?  What are his credentials?  What is the source of
> his information?  What is the basis of his opinion?  Why doesn't he
> post his explanation to this newsgroup instead of having you do it for
> him? Can other people see this physicist, or just you?  (I've never
> seen a bona fide physicist who has an M.D. do his bidding before!)


	Actually it's more a matter of a bona fide physicist doing an
M.D.'s bidding (me being dragged into this by Steve).  I'm the physicist
that Steve consulted by telephone.  My credentials are that I've spent the
past four years earning a physics Ph.D. doing MRI research at the National
Research Council of Canada Institute for Biodiagnostics in Winnipeg,
Canada.

	Steve's physics explanations were generated by him alone
(being no slouch at physics himself).  As far as I can ascertain from quick
perusal before posting this message, they look right on the money.

	Steve is right that no self-respecting MR physicist (except possibly
for the crudest pedagogical purposes) would refer to what happens inside
an MR probe as involving "radio waves".  In fact, in the center of a
circularly
polarized MR coil, no E field even exists.  If we define "radio wave" as a
pattern of E and B field oscillations that will self-sustain in free space,
then MR coils do not produce radio waves.  Designed specifically to
produce oscillating B fields within a confined volume, MR coils are the
antithesis of radio antennas.

	I'm especially sensitive to this issue because one of my
colleagues at NRC was David I. Hoult, an MR physicist and engineer
whose contributions to MRI begin with the earliest papers in the field.
David had no toleranance for people talking about "radio waves" in
MR context.  You can read his thoughts on the matter the standard
reference text "Biomedical Magnetic Resonance Technology", C-N Chen,
D.I. Hoult, published by Adam Hilger, 1989.  He also wrote an editorial
on this very subject in the ISMRM (then SMR) newsletter, "MR Pulse",
a few years ago.  More recently, I believe JMR published a paper by
him and Bhakar (sp?) a theoretician from the University of Manitoba
on the distinction between real and virtual photons in MR experiments.

---Brian Wowk, PhD

From: "Brian Wowk" <wowk1@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Mon, 22 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med

meron@cars3.uchicago.edu wrote in article
<ELK0L3.21M@midway.uchicago.edu>...

> Any time dependent source broadcasts in the far field.

	No.  The whole field of MR coil engineering is dedicated
to the production of devices (often unshielded) that do not radiate
in the far field when you apply RF power to them.  They are
inherently near field devices.

> A solution of the wave equation is a wave.  An RF cavity broadcasts
> nothing in the far field, yet nobody who knows anything about the
> topic would claim that you're not dealing with waves.

	Yes, an RF wave but not a "radio wave".

> The only absurd thing I've seen on this thread so far is the
> insistence that physicists should adapt their concepts and terminology
> to suit your preconceptions.

	MR physicists and engineers do not call near-field
phenomena "radio waves".  Calling an MR probe an "antenna"
would insult the engineer who built it!  From our point of view,
you are the outsider who is insisting that we adapt our terminology
to suit your preconceptions! :)

---Brian Wowk

From: "Brian Wowk" <wowk1@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Mon, 22 Dec 1997
Newsgroups: sci.physics,sci.med.physics,sci.techniques.mag-resonance

meron@cars3.uchicago.edu wrote in article
<ELK11E.30K@midway.uchicago.edu>...

> They didn't teach you physics too well, I'm afraid.  Here is a quick
> question:  Is a solution of a wave equation a wave or isn't it?

	Are the static fields surrounding stationary charges and
currents radio waves?  Why not?  They satisfy the wave equation.
Indeed, all EM phenomena in free space satisfy the wave
equation.  Does this mean all EM phenomena are radio waves?
Of course not, or the specific term would never have been invented.

	Historically "radio waves" refers to self-propagating
EM waves in the far-field.  And radio waves by this definition do
not exist in MR.  Terminological preferences aside, that much is
certain.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: Which is more dangerous: radiation or electricity?
Date: Thu, 25 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med

Jonathan E. Hardis <jhardis@tcs.wap.org> wrote in article
<jhardis-1812972134290001@dialup02.wap.org>...
> In article <67ado4$ae3@sjx-ixn10.ix.netcom.com>,
> sbharris@ix.netcom.com(Steven B. Harris) wrote:
>
> > No, YOU are wrong.  I've recently asked a medical physicist who got his
> > Ph.D. working on MRI machines, to make sure.  He also confirms that MRI
> > machines do not radiate patients with RF.  What you go into are several
> > kinds of magnetic fields-- one of them a gradient field which is
> > constant, and one or more additional fields which are fast-pulsed in
> > various ways.
>
> Something must have gotten lost in the communication between you and your
> friend.

	Indeed.  Actually I said that MRI machines do not radiate patients
with radio waves.  Of course MR requires radio frequency (RF) magnetic
fields, but that is not the same as radio waves as I've argued elsewhere.

>
> The rest gets extraordinarily complicated, because what's actually being
> measured is the difference in propensity of the nuclii to spin-flip
> (slight majority up to slight majority down) because of the RF excitation
> pulse compared to the rate that they tend to return to thermodynamic
> equalibrium because of the natural fluctuating fields in the body (the
> so-called T1 and T3 rates).

	Actually that's T1 and T2.


> > If MRIs did radiate people with RF, can you imagine the RF interference
> > and general noise the turkeys would produce?  The FCC would never allow
> > them.
>
> Well, they do.  All the FCC cares about is that the RF power is low enough
> to not affect communication.  More importantly, the instrument
> manufacturer wants to keep the power low enough to not affect the
> patient.  The latter is the more stringent requirement of the two.


	Substitute "radiate people with RF" with "radiate people with
radio waves" and then Steve is right.  The power of the RF transmitters
in MR is huge-- tens of kilowatts in some cases --and would indeed
mess up the commercial FM band *if* MR coils produced significant
radio waves.  But they don't because MR coils are designed to produce
near-field B oscillations, not radio waves.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Thu, 25 Dec 1997
Newsgroups: sci.physics,sci.med.physics,sci.techniques.mag-resonance

Jonathan E. Hardis <jhardis@tcs.wap.org> wrote:

> There is no "pattern of E and B field oscillations that will self-sustain
> in free space."  Either the power drops as 1/r^2, or by some faster
> function.  In either case, should the power be absorbed from the field,
> there's no way to tell one from the other.


	By "pattern of E and B field oscillations that will self-sustain
in free space" I refer to oscillations where B only depends on E and
E only depends on B and not on source charges or currents.  In the far
field (which I assert are radio waves) E and B alone define each other.
In the near field (the operative field in MR), E and B are determined by
each other plus source current terms in Maxwell's equations.

	And there actually is a way to tell whether power is being
absorbed from the near or far field.  If you absorb power from the
near field you are engaging in a virtual photon interaction with the
source current, and the source current will react to this interaction.
If you absorb power from the far field, you are interacting with real
photons decoupled from the original source current.

	Another (reciprocal) manifestation of this principle is found
in the phenomenon of "radiation resistance".  Real antennas (as
distinct from MR coils) exhibit an electrical resistance to applied
RF that is a reflection of the antennas' constant outpouring of
energy into the far field (i.e. radio waves).  MR coils, on the other
hand, have no radiation resistance of this sort.  MR coils only
fill a small volume with RF fields, and do not dump continuous
power into space around them as antenna's do.


> > I submit that "radio wave" refers to self-propagating
> > EM radiation (wich MR does not produce), and this is certainly
> > the operative definition within the culture of MR physicists.
>
> How does the energy get from the antenna to the patient?  That seems
> rather "self propogating" to me.

	The energy gets to the patient from the MR coil (not
antenna) via a near-field coupling of the patient to currents in
the coil.  The coupling fields are intimately related to these
source currents.

>
> I define a radio wave as transmitted electromagnetic energy [at frequences
> of less than, say, a few hundred THz] that can be detected by a receiver.
> The protons in the patient make perfectly good receivers.
>

	By this definition, electric motors would be said to operate
by radio waves.  I think most physicists would say that electric
motors operate by near-field magnetic induction, not radio waves.
Similarly, MR physicists say that MR operates by magnetic
induction, not radio waves.  The principles are the same, only the
frequencies are different.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Thu, 25 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med

meron@cars3.uchicago.edu wrote:

> >> Any time dependent source broadcasts in the far field.
> >
> >	No.  The whole field of MR coil engineering is dedicated
> >to the production of devices (often unshielded) that do not radiate
> >in the far field when you apply RF power to them.  They are
> >inherently near field devices.
>
> Take two oscillating dipoles, ofsset them properly in phase, you get
> cancellation of most of the far field radiation.  Most, but not all.
> By adding quadrupole correctors (again, with the proper phasing) you
> cancel yet more.  No big deal here.  Mind you, at any finite order of
> correction you still broadcast in the far field.  Granted, at some
> point what you broadcast becomes pretty negligible.  It is not zero,
> though, there is no basis for this claim.

	Any object with mass produces a gravitational field.  MR
coils have mass.  This does not mean gravity plays a role in MR.
Similarly, far field radiation plays no role in MR.

> Of course you may adopt the
> technical definition that only propagating wave, reducible locally to
> a plane wave, is a radio wave.  Not terribly useful, though and not
> very meaningful, physically.

	Waves of E and B which are only determined by E and B
and nothing else *are* very meaningful physically.  See my reply
to Jonathan Hardis.

> As I wrote elsewhere, whatever happens
> at any given point depends on the power spectrum of the field in this
> point and it doesn't matter whether this power spectrum is generated
> by propagating or standing waves.

	What happens will in general not just depend on the
power spectrum, but also on the phase and magnitude
relationships between E and B.  These relationships are
quite different in an MR coil than in a radio wave.  As I've
already pointed out, there are zones within some MR coils of
zero E (thus no inductive heating) yet strong B.  The bioeffects
within these zones (at least in terms of heating!) will not
be the same as you would get by just bathing someone
in radio waves.

	This is just one example of the perils and pitfalls
of saying MR uses "radio waves", and an example of why
we in the MR field choose our language more carefully.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Fri, 26 Dec 1997
Newsgroups: sci.physics,sci.med.physics,sci.techniques.mag-resonance

Jonathan E. Hardis <jhardis@tcs.wap.org> wrote:

> "Brian Wowk" <wowk2@ibm.net> wrote:
>
> >         By "pattern of E and B field oscillations that will self-sustain
> > in free space" I refer to oscillations where B only depends on E and
> > E only depends on B and not on source charges or currents.  In the far
> > field (which I assert are radio waves) E and B alone define each other.
> > In the near field (the operative field in MR), E and B are determined by
> > each other plus source current terms in Maxwell's equations.
>
> B only depends on E and E only depends on B, and not on source charges or
> currents, in free space....
>
> This has nothing whatsoever to do with "near field" or "far field" -- it
> is generally true everywhere.   It is pretty basic stuff -- the four famous
> Maxwell equations...

	No.  It's not true that E and B determine each other everywhere
in free space (where there are no sources).  Maxwell's equations say
nothing about the value of E and B, only the curl and divergence of E
and B.  The actual values of E and B at any point in space depend on
the complete solution of the equations, inlcuding boundary conditions,
which include nearby sources.

	The only time E and B truly determine each other (and not merely
constrain each other's derivatives) is in the far field.  In the near field,
E and B are determined by source charges and currents, even at points
outside the sources.

>Please note that these (Maxwell's equations) are differential equations
> that define *local* behavior, in the neighborhood of a specific point, and
> have nothing to do with whether or not other sources are near or far away.

	Indeed.  And they are also equations that tell you absolutely
nothing about what E and B are at a point unless you fit the boundary
conditions, which include near sources.

>
> >         And there actually is a way to tell whether power is being
> > absorbed from the near or far field.  If you absorb power from the
> > near field you are engaging in a virtual photon interaction with the
> > source current, and the source current will react to this interaction.
> > If you absorb power from the far field, you are interacting with real
> > photons decoupled from the original source current.

(Criticism deleted)

	In writing the above, I'm attempting recall details from a
seminar I attended last year concerning the role of virtual and
real photons in MR.  The seminar was put together by an MR
physicist/engineer and theoretical physicist, so I assume they
knew what they were talking about.  Their conclusion was that
MR signals are generated by a virtual photon interation between
nuclei and the MR receiving coil.  However I'm also starting to
remember that this conclusion may have only applied when
induced EMF was being measured in the coil without drawing
any current.  If so, this may have had nothing to do with near
or far fields at all.

	Forgetting whether the photons are real or virtual,
my point was simply that "loading" an MR coil by placing
a resistive conductor in the near-field produces a detectable
change in the impedance of the coil, while putting the same
object in the far field (of an antenna, since MR coils do not have
substantial far fields) produces a negligible effect by comparison.


> With respect to your notion that absorbers in the near-field load the
> source while absorbers in the far-field do not, you're doing some really
> funny bookkeeping.  The first case (the near-field) has to do with power
> that would be reflected back to the source in the absence of an absorber,
> and so of course if you stop that process (with an absorber) the source
> "loses" more power.  In the second case (the far-field) you're _presuming_
> that there is no reflector "out there" somewhere, and so of course the
> source won't know or care whether the power is absorbed or continues to
> radiate to infinity.  However, there's nothing about the basic theory that
> prevents there from being a reflector "out there" that could (in the
> far-field case, as in the near-field case) reflect power back to the
> source.  In this case, interposing an absorber would also be felt by the
> source.  An obvious example is a laser cavity, which contains almost
> nothing but far-field modes.

	By far "far field" I've been meaning power that radiates to infinity,
which does not happen if you interpose a reflector.  Another way to state
my position is that "MR does not primarily generate or make use of any
RF fields that radiate to infinity with 1/r^2 depedence".


> >         Another (reciprocal) manifestation of this principle is found
> > in the phenomenon of "radiation resistance".  Real antennas (as
> > distinct from MR coils) exhibit an electrical resistance to applied
> > RF that is a reflection of the antennas' constant outpouring of
> > energy into the far field (i.e. radio waves).  MR coils, on the other
> > hand, have no radiation resistance of this sort.  MR coils only
> > fill a small volume with RF fields, and do not dump continuous
> > power into space around them as antenna's do.
>
> So what?  Let me restate this in a bit more detail to show its lack of
> relevance.  Any antenna can be modeled electrically as an impedance Z,
> which consists of a real part R and a complex part.  The complex part
> corresponds to any energy that's stored in the near-field of the antenna.

> The real part R has several components:  a part that comes from electrical
> resistance in the antenna (in the sense that one might naively use an ohm
> meter to find), a part that comes from energy that gets radiated off to
> infinity (the "radiation resistance" that you're talking about), and a
> part that comes from *any other* dissipative (lossy) effect in the
> problem.

	A good overview.  MR coils do not have a significant contribution
from the "energy that gets radiated off to infinity" part.

>
> With respect to the statement that "MR coils ... have no radiation
> resistance," have I have to plead ignorance with respect to the particular
> coil design that you have in mind.  However, if you're talking about a
> regular loop of wire, its radiation resistance (in Ohms) is approximately
> 3.1 x 10^5 x N^2 x (a/lambda)^4, where N is the number of turns in the
> loop, a is the radius of the loop, and lambda is the wavelength of the
> radiation.

	MR coils or "probes" are much more complicated than this.
They contain multiple capacitive and inductive elements to minimize
radiated fields, minimize "conservative" electric fields (E fields not
produced by changing B), and maximize B within a confined volume.
"Biomedical Magnetic Resonance Technology" by David Hoult has a
section called "From antenna to probe" in which he step-by-step
converts a simple loop of wire to a true MR probe by breaking the
loop and adding capacitors at strategic points.

>
> The MR RF coil *is* an antenna, and what you describe pertains to E-M
> waves in all modes.
>

	This is not true within the terminology and culture of MR.
To accuse an MR engineer of making an "antenna" is to insult him,
for he will think you are criticizing his probe design by implying it
radiates in the far field.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Fri, 26 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med

Chuck Bloch <cbloch@iucf.indiana.edu> wrote:

> But clearly we all (now) agree that from a single
> point in space, it does not matter whether we are sampling a
> propagating or a standing wave.

	Agree.

>Therefore, in terms of discussing the
> effects of MR on a patient, any discussion of propagating waves vs
> standing waves, or terminology such as "radio" wave is irrelevant.

	What is at issue is the distinction between near field
and far field rather than propagating or standing.  Far field
waves can still be standing (e.g. waves from distant trasmitters
interferring).  The physical distinction between near and far
field is the phase and magnitude relationship between E and B.
And these are relevant to what happens in general at a given
point in space.

>
> While there are such zones, I presume since you refer to them as
> "zones", these properties are not found everywhere within the MR.  In
> the regions outside those particular zones, are the bioeffects just
> the same as one gets by bathing someone in "radio" waves?

	I strongly suspect that for a given point in space,
bioeffects will depends solely on the strength and frequency
of the E field regardless of whether that E field is near or
far field.  But it is still important to appreciate the uniqueness
of far field radiation in terms of it's specific E and B relationships--
relationships that do not exist in the near-fields of MR.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Fri, 26 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med

meron@cars3.uchicago.edu wrote:

> In terms of heating, if you deliver 100 Watts to to patient, the
> heating is exactly the same as if you would deliver these 100 Watts
> using far field radiation.

	Except that the spatial distribution would be more
uniform for far-field radiation.

> The difference (and that's why you use
> near field in MRI) is that due to the different ratio of E to B you
> get way stronger B magnitude per same power delivered, and it is the B
> that you care about in this case.  Which is fine, but it is a
> technical difference, not an essential one.

	Agree 100%.

	Skimming over the origins of this thread today (EM fields from
cell phones vs. MR machines?), I think Steve Harris' point was that
near field and far field RF fields are physically distinct phenomenon
(in terms of E and B phase and magnitude relationships) and that
care must be taken in translating bioeffects of one to the other.
I personally suspect that E field strength and frequency are the
primary determinants of bioeffects, but not necessarily.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: Which is more dangerous: radiation or electricity?
Date: Fri, 26 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med

Jim Carr <jac@ibms48.scri.fsu.edu> wrote in article:

> sbharris@ix.netcom.com(Steven B. Harris) writes:
> >
> >No, YOU are wrong.  I've recently asked a medical physicist who got his
> >Ph.D. working on MRI machines, to make sure.  He also confirms that MRI
> >machines do not radiate patients with RF.
>
>  Then he "worked on" them in on only the loosest sense, and is lucky
>  no one asked about the mode of operation during the thesis defense.
>

	No need for insults.  As I've mentioned elsewhere, Steve asked
me whether MR radiates patients with radio waves (which is not the
case) which apparently got translated to "radiate with RF".  Patients
are certainly *exposed* to RF in MR.  Whether "radiate" is appropriate
terminology depends on whether you equate "radiate" to radio waves.

>  Low resolution needs only low B, while work is now
>  directed at high B magnets with large dB/dx that operate at a GHz.

	dB/dx (gradient strength) is not typically the limiting factor
in spatial resolution.  Signal-to-noise ratio is.  Stronger fields give
a larger net nuclear magnetization vector, and the more rapid
precession produces a stronger magnetic induction signal in the
receiving coil.  Stronger fields thus gives you higher SNR.  You also
get a larger chemical shift for spectroscopy and functional imaging.
These are the factors that are pushing biomedical MR to higher
field strengths.

	BTW, for a variety of reasons you will probably never see
a GHz proton human MR system.  The strongest fields now being
looked at for human systems are 8T, with a proton resonant
frequency of 340 MHz.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Sat, 27 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med,
	sci.techniques.mag-resonance

James Logajan <jamesl@netcom.com> wrote:

> Has anyone ever considered the possibility that the large static
> magnetic field may have more dangerous bio-effect than any RF E-M field?
> I have heard (via a story from a physics professor who shall remain
> nameless for now) that if one moves ones head about in an MRI machine,
> one may experience wierd psychedelic effects. This seems reasonably
> plausible to me since the functioning of neurons takes place by ionic
> transport across membranes. I could see how a strong enough B field
> might upset things. Considering all the people who have been through
> MRI, I suppose it isn't a hazard; still, anybody know if the physics
> professor who told the story of the psychedelic effects was full of it,
> or is what he said true?

	The most common reported side effect of exposure to
high static fields is vertigo.  Individual susceptibility varies.
I am very susceptible, and always dreaded crawling into our
3T system in Winnipeg because I would feel unwell for the rest
of the day.

	No one is certain of the mechanism that causes the
vertigo.  It is worse (for me) when I was moving, suggesting that
induced currents in the inner ear are the culprit.  The most
exotic hypothesis is that humans have some residual magnetic
field direction sense (similar to the one that has been verified
in birds), and the high field wreaks havoc with it.

	The closest thing to "psychedelic effects" are photophosphenes
(flashes of light on the retina) that have been reported by patients
exposed to rapidly changing field gradients during imaging.
There are still rarer reports of photophosphenes seen when people
turn their heads in high static fields (4T or greater).  I ran more
than 100 people in our 3T system during my PhD work, and didn't
encounter any reports of photophosphenes.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Sun, 28 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med

Jonathan E. Hardis <jhardis@tcs.wap.org> wrote:
>
> While I haven't tried to verify the idea, there's a school of thought that
> says that (1) human flesh is sufficiently conductive (i.e., through
> desolved salts), and (2) the inter- and intracelluar field strengths are
> already suffiently high (i.e., though short distances if not large
> charges) that effects of externally applied electric fields (of the types
> one might be exposed to from CRTs, power lines, etc.) have to be in the
> noise.
>

	Indeed.  Of course there are theories about how below-noise-level
effects can still have bioeffects, but they tend to be weird and
intrinsically unverifiable.  John Moulder of sci.med.physics regularly
posts a FAQ on this topic.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: Which is more dangerous: radiation or electricity?
Date: Sun, 28 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med

Steven B. Harris <sbharris@ix.netcom.com> wrote:

>meron@cars3.uchicago.edu writes:
>
> >At distances small in comparison to
> >wavelength, the ratio of the magnitudes of the electric and magnetic
> >field is of the order of distance/wavelength.
>
>    Is that for the same point in space at the same time?  Since there's
> a phase difference between E and B in the nearfield, I'm interested in
> max E values for a point over a half cycle time.
>

	In the simple case of uniform, linearly polarized B oscillations,
the maximum E and B amplitudes will be related by

	E = B c pi r/wavelegth

where r is the distance from the cavity center.  So the E values are
indeed smaller in MR machines than they would be for RF in the
far-field.

	But of course it is E that determines SAR (specific
absorption rate, or heating) in MR.  So I would expect that
RF fields in MR have the same bioeffects as far-field RF of
the same SAR, even though B will be smaller for the same
SAR in the far field.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Mon, 29 Dec 1997
Newsgroups: sci.physics,sci.med.physics,sci.techniques.mag-resonance

Jim Carr <jac@ibms48.scri.fsu.edu> wrote:

> >    Yep, but not everything that satifies Maxwell is electromagnetic
> >radiation.
>
>  Not everything that satisfies some of the equations, but everything
>  that satisfies the wave equation.
>

	There are certainly solutions to the wave equation that
are not waves or radiation.  Static fields being one example.  In
terms of time-varying fields, it's a bit of a stretch to call fields that
do not propagate outside a small volume << wavelength "radiation".
It's not a useful picture.

> >The same kind of induction loop you see in a metal detector, or the
> >gate detectors at an airport.  In short, a magnet.
>
> But not operated as an induction loop, or as a magnet, but as an
> RF antenna of a particular type.

	In contrast to conventional antennas, the induction loop
picture is actually useful for understanding MR coils.  MR coils
contain time-varying currents (with no time-varying charge), thereby
producing time-varying B within a confined volume.  Even though
the frequencies are RF, the physical picture of what's happening
is no different than low frequency electromagnets as long as the
length scale remains << wavelength.

---Brian Wowk

From: "Brian Wowk" <wowk2@ibm.net>
Subject: Re: MRI and Diathermy use RF??  (was: Which is more dangerous: 
	radiation or electricity?)
Date: Mon, 29 Dec 1997
Newsgroups: alt.folklore.science,sci.physics,sci.med.physics,sci.med

Hugh Gibbons <hgibbons@guess.where.com> wrote:

>"Brian Wowk" <wowk2@ibm.net> wrote:
>
> !         Skimming over the origins of this thread today (EM fields from
> ! cell phones vs. MR machines?), I think Steve Harris' point was that
> ! near field and far field RF fields are physically distinct phenomenon
> ! (in terms of E and B phase and magnitude relationships) and that
> ! care must be taken in translating bioeffects of one to the other.
> ! I personally suspect that E field strength and frequency are the
> ! primary determinants of bioeffects, but not necessarily.
>
> This last is something that ought to be substantiated.  There is no
> reason I know to think that electric field strength as opposed to
> power delivered to the subject is not the main thing.

	Except that electric fields are what deliver power to
the subject.  An oscillating B field alone (such as occurs at
the isocenter of a typical MR probe) transfers negligible power
to the subject.

---Brian Wowk