Magnetic Resonant Imaging Mechanics

[stevmg]

MRIs are used in medicine for diagnostic imaging. I am told it has something to do with protons. But in the human body protons don't exist, only occasional hydronium ions (H₃O⁺. ) I am told that a strong magnetic field imparts a "spin" to the protons (I don't know if this in a quantum mechanics sense, which I don't understand, or an actual physical sense, in which the proton actually turns around and around.) When the magnetic field is stopped, the protons stop spinning in the given direction and revert to their random directions and energy is released.
1) What form does this released energy take? Is it electromagnetic radiation like X-rays or the like?
2) is it just hydrogen (hydronium) ions that are affected the magnetic field or is it hydrogen atom nuclei (also a "proton") in fixed compounds that are a likewise affected with an imparted "spin"
3) Since water is a polar molecule with a "negative" end and a "positive" end (although totally neutral) and water comprises seventy percent of mammalian bodies by weight, could it be that this is in fact the "spin" that is imparted to these H₂O molecules from the very strong magnetic field an MRI machine delivers which releases "energy" when the magnetic field is released?

Please help me out!

Stephen M. Garramone, M.D.
stevmg@yahoo.com

 

[krome]

1) You are correct. The released energy is electromagnetic radiation. The frequency of the radiation depends on the strength of the static magnetic field in the MRI as well as the duration of the pulsed magnetic field. In case you don't know what these two things are, here is a youtube video of a quick description of MRI mechanics. I honestly don't know what the standards in the industry are so I don't know the frequency range of the released radiation in a standard MRI machine. That would be interesting to find out.

2) It is mostly the hydrogen atoms in molecules that are affected. This is found in water, in proteins, etc. So, different types of tissue can be distinguished because of different hydrogen content. Actually one wonderful family of materials with which to do MRI is hydrocarbons because they are so dense in hydrogen. When I was an undergrad, we did MRI on glycerin and other hydrocarbons. We submerged small structures made of wood, which had very little hydrogen, and were able to image these structures as the regions which responded minimally to the applied magnetic fields.

3) The spin is quantum mechanical spin, not physical spin or angular momentum. The molecules themselves are not rotating as a result of the applied magnetic fields.