Phasing of spin due to a radiofrequency pulse

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Discussion Overview

The discussion centers around the behavior of nuclear spins in MRI when subjected to a 90° radiofrequency (RF) pulse. Participants explore the theoretical basis for why spins, initially randomly dephased, become phased after exposure to the RF pulse, particularly in the context of precession in an external magnetic field.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant expresses interest in understanding the scientific theory behind spins precessing in phase after a 90° RF pulse, despite initial random dephasing.
  • Another participant suggests that the discussion may involve a refocusing pulse and recommends searching for additional material on "NMR" and "refocusing."
  • Several participants share links to a movie related to MRI, although there are issues with accessing the content.
  • There is a mention of the spins precessing at the same angular frequency, but the implications of this for initial dephasing are questioned.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the explanation for the phasing of spins after the RF pulse, and multiple viewpoints regarding the underlying theory and related resources are presented.

Contextual Notes

Some assumptions about the initial state of the spins and the nature of the RF pulse are not fully explored, and there may be limitations in the provided resources for further understanding.

Who May Find This Useful

This discussion may be of interest to those studying MRI technology, nuclear magnetic resonance (NMR), or the theoretical aspects of spin dynamics in magnetic fields.

mkj
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hi all,
i am interested in MRI, and i wish to know what is the scientific theory telling us that the spins will precess in phase if they were placed in an external field and exposed to a 90° RF pulse. what i know is that the nuclei will precess at the same angular frequency (or same angular speed) but if they were initially randomly dephased (each spin has an initial angle θ0) then why should be phased after exposed to the RF pulse.
 
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naima said:
I found this !

The link is not working.
 
I liked very much this movie.
Humorous physics movies are rare.
 
mkj said:
hi all,
i am interested in MRI, and i wish to know what is the scientific theory telling us that the spins will precess in phase if they were placed in an external field and exposed to a 90° RF pulse. what i know is that the nuclei will precess at the same angular frequency (or same angular speed) but if they were initially randomly dephased (each spin has an initial angle θ0) then why should be phased after exposed to the RF pulse.

I assume you are talking about a refocusing pulse. There should be plenty of material available online (try "NMR" and "refocusing" as search terms), you could e..g try http://en.wikipedia.org/wiki/Spin_echo
 

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