Signal Detection:
Virtual photons and coherent spontaneous emission

by Stanislav Sykora (Extra Byte, Castano Primo, Italy)

to be delivered at the 18th ISMRM,
Stockholm (Sweden), May 1, 2010, in the Weekend Course MRI Physics for Physicists.

Lecture SLIDES (369 kBytes). NMR Articles and Posters

Note: The fact that this talk has almost the same title as a 1997 article by David Hoult and B. Bhakar is neither accidental, nor a concequence of my free choice. The title was kind of imposed on me by the organizer of the ISMRM Course (Michael H.Buonocuore) and, as far as I know, approved by David himself. Which does not mean that I will necessarilly and uncritically adhere to all David's ideas ...

Addition on the evening before the talk: It is totally impossible to cover this topic reasonably in 20 minutes, nor can a reader understand what I meant by some of the slides without hearing the commentary. Consequently, I must work on this much more. There will be additions so, if you are interested, come back and check out the current status.

Lecture Syllabus

1. Interaction between a [classical] coil and the [quantum] spins
   - A brief historic sketch of the topic
      Two starting approaches: cavity (Purcell) and induction coil (Bloch)
      The impact of microwave thinking and terminology (spectroscopy)
      The heredity of atomic and molecular beam experiments (resonance)
      The complication introduced by pulsed methods (what is FID, really?)
      Formation of a pro-tempore paradigm (coherent spontaneous emission)
      The nearly forgotten spin-temperature controversy
      Reduction of alternative views to marginality
   - The chasm between physical theory and practical engineering
      Engineering is based 100% on classical induction (a near effect)
      Theory is split between classical and quantum (a must in spectroscopy)
         Quantum description can't do without energy quanta (is that radiation?)
2. The current status of our understanding (not very satisfactory):
   - Controversial aspects:
      Stimulated versus spontaneous emission (this is easy in CW, but what about FID?)
      Coherent and incoherent emission (is MR strictly an ensemble effect?)
      Is there any true radiation to start with? (why don't we see it?)
      Near versus far phenomena (induction versus radiation)
      Ontology of photons: are they real, virtual or none at all?
         Real: some would be escaping and be detectable far away
         Virtual: virtual particles are just expansion terms in QED formulas
         None: ok, but to explain NMR spectra, one still needs the quanta
         All the photon paradoxes
      Single spin versus an ensemble
         Emergence of mixed states (required for transversal magnetization)
         Can a single spin give rise to a mixed state? Not an observable one!
         The complex nature of the transition from eigenstates to mixed states
      Formation of thermal equilibrium polarization
         Crucial role of relaxation interactions (no relaxation, no polarization)
   - Common fallacies (the spherical cows of MR):
      Impossibility to describe typical MR phenomena considering just a single spin
         Unacceptability of classical single-spin spontaneous emission estimates
         Inapplicability of single-spin quantum physics to any MRI situation
      Necessity to include relaxation interactions (no relaxation, no signal!)
   - Novel ways of signal detection and the lectures they teach us:
      Magnetic force detection
         For electrons, this is the only single-spin experiment: no mixed states
      Electric field detection (controversial: is it a true electric-field measurement?)
      Thermal noise induction (or is it thermal radiation?)
      Semi-remote detection (the waveguide dilemma)
      Novel front-ends devised to clarify some of the theoretical doubts (author's work)
   - The role of the reciprocity principle:
      What is it and how is it exploited?
      What does it teach us? Is induction out and interaction in?
3. Why is MRI the least controversial among MR techniques:
   - Extremely simple Hamiltonian (compared to NMR spectroscopy):
      Typically, MRI exploits directly only the Zeeman interactions!
      Typically, MRI deals only with spin ½ particles!
   - Are there any aspects of MRI which impose a strict quantum approach?
      Many think that there are none; it depends on what one wants to discuss
4. Attempts to describe the combined {coil + spins} system:
   - This has been attempted by J.Jeener and F.Henin who claim full agreement with experiment
   - The coil and the spins made to form a common quantum Hamiltonian (author's work)
      A classical precursor of such a theory
      The combined Hamiltonian and its link to the reciprocity principle
      Coil-current quantization and where could it lead us

This section is in development; come back to check on it before the event.

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