RF controversy!

[univox_high_flyer]

Dear all,

If my understanding is correct, D.I Hoult has shown http://www.mri-q.com/uploads/3/2/7/4/327...42_ftp.pdf that the MR signal (current) is driven by a circulating electric 'near' field that follows Faraday's law of induction. In other words, it is induced by a time varying magnetic field (precessing magnetic moments).
On the other hand, the electric 'far' field induces a perpendicular magnetic 'far' field. Moreover, both these 'far' fields can be shown to be the components of an electromagnetic RF wave.
What I don't understand is why the 'near' electric field can't be induced by a fluctuating magnetic field of an RF wave.
If no RF is emitted by the spins, then does that mean that no RF was absorbed to begin with? And if that's the case why are RF pulses used in MRI in the first place?

As the title says, it is a controversial issue but I would appreciate an additional insight to the subject.

Thanks in advance.

[AndrewBworth]

If no RF is emitted by the spins, then does that mean that no RF was absorbed to begin with? And if that's the case why are RF pulses used in MRI in the first place?

As the title says, it is a controversial issue but I would appreciate an additional insight to the subject.

Thanks in advance.
[/quote]

-If it helps - I remember Slichter writes something about this in his book - Principles of Magnetic Resonance. If I recall though - only the magnetic component of the RF radiation interacts with the spin, and of that magnetic component, only half are rotating in the same sense as the precession of the spin, so only these waves matter. Thanks for bringing this up!

[aelster]

I have added some additional references by Hoult as well as a review about virtual photons from Engelke on my original web page: http://mriquestions.com/not-radio-waves.html

There is also a new (2015) book by Csaba Szantay, Anthropic Awareness: The Human Aspects of Scientific Thinking in NMR Spectroscopy and Mass Spectrometry, published by Elsevier which addresses some of these issues in Chapter 4.

[mmm]

hi
I've got a question.
Can we flip net magnetisition vector more than 180 degree?If so,what happens?
360 degree flip makes spin back into first state?After that there is no relaxation & so no voltage induce?

[embermoon]

This all seems very interesting.