Phasing of spin due to a radiofrequency pulse

[mkj]

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.

 

[naima]

I found this !

 

[Jilang]

I found this !

The link is not working.

 

[naima]

try again. it works here when i click on it.
http://www.physics.utoronto.ca/~key/PHY138/Flash/MRI - the Movie.swf

 

[Jilang]

try again. it works here when i click on it.
http://www.physics.utoronto.ca/~key/PHY138/Flash/MRI - the Movie.swf

Nope no luck. It maybe because I'm using an iPad. It's doesn't like the file type.

 

[naima]

I liked very much this movie.
Humorous physics movies are rare.

 

[f95toli]

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