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NMRI electromagnetic principles

  1. Nov 15, 2014 #1
    In nMRIs the strong magnetic field plus the radio wave aligns protons.

    1). Which protons? Just the proton ions (actually H3O+) from water; or protons in all molecules that contain hydrogen atoms such as water, fats, carbohydrates, proteins?

    2). The relaxation phase does what? As the protons go back to equilibrium there is an emission of radio waves?

    3). Isn't that a form of γ radiation? Doesn't it have inherent risks?

  2. jcsd
  3. Nov 15, 2014 #2


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    What has your research into this shown you" Please post the appropriate studies from acceptable journals that you have read that have caused your concern.
  4. Nov 16, 2014 #3


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    See - http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nmr.html
    and http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/mri.html

    Radio-frequency is not gamma (γ) radiation, not even X-ray, and in fact is lower energy than visible or infrared.

    http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/larmor.html (I recommend some calculations of Larmor frequencies)

    From - http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr1.htm
    "Note that this electromagnetic radiation falls in the radio and television broadcast spectrum. Nmr spectroscopy is therefore the energetically mildest probe used to examine the structure of molecules."

    For protons, read the section 2. Proton NMR Spectroscopy, and note the differences among protons in different compounds.

    Another very basic tutorial - http://www.cis.rit.edu/htbooks/nmr/inside.htm

    http://www.brynmawr.edu/chemistry/C...uclear_Magnetic_Resonance _Spectroscopy_2.htm

    Last edited: Nov 16, 2014
  5. Nov 16, 2014 #4
  6. Nov 16, 2014 #5
    To Astronuc - The question was does the nMRI technique as used in medicine align all hydrogen atom protons such as those in water, fats, carbohydrates, proteins or just the water ones? I cannot decipher the answer to that from the extensive discussions these websites presented - too much information.
  7. Nov 16, 2014 #6
    When a rotating field gradient is used, linear positioning information is collected along a number of different directions. That information can be combined to produce a two-dimensional map of the proton densities. The proton NMR signals are quite sensitive to differences in proton content that are characteristic of different kinds of tissue. Even though the spatial resolution of MRI is not as great as a conventional x-ray film, its contrast resolution is much better for tissue. Rapid scanning and computer reconstruction give well-resolved images of organs.
  8. Nov 16, 2014 #7


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    According to the literature one can excite water or fat, and ostensibly any organic compound by selecting the proper frequency.

    This article mentions water (bound and unbound) and fat.

    Another article describes NMR/MRI of sugar by exciting protons in the glucose hydroxyl groups as opposed to water.
    "GlucoCEST works by measuring glucose uptake through the chemical exchange of protons between glucose hydroxyl groups and water. By selectively saturating the magnetization of protons in the hydroxyl groups, using radiofrequency pulses, this exchange of protons causes a reduction in the MR signal from water."

    So the population of protons, which are to be excited, depends on the radiofrequency.
  9. Nov 16, 2014 #8
    Good answer. Explains what I need to know. I take it that medical MRI uses radio frequencies for water and fat, true?
  10. Nov 16, 2014 #9


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    Last edited: Nov 16, 2014
  11. Nov 16, 2014 #10
    Thank you. Do you know how many doctors and healthcare workers do NOT understand this basic concept? All they know is it is something to do with protons but they don't know what and where protons are. I didn't know that regular analog X-ray machines have better resolutions than MRIs.

    Stephen M. Garramine, MD
  12. Nov 19, 2014 #11


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    The resolution of an MRI depends on the intensity of the field you use. The higher the magnetic field, the higher the resolution. Typical MRIs operate at around 1.5 T, but higher field MRIs are gaining in popularity, and in research settings they have gone as high as 9T for people, and 21 T for animals. In addition, whilst proton MRI's are the norm, you can use any nucleus that has a net nuclear spin. helium-3, lithium-7, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129 have all been developed. These are for different diagnostic situations.
  13. Nov 20, 2014 #12
    Seems like the phosphorus-31 would be handy for bone MRIs
    Still, straight X-rays give better resolution. Is the CT scan still better?

    Where would I obtain a list of nuclei with net spin? Also, what is the percentage of these isotopes in human tissue and is it sufficient to detect in clinical situations?

    And, most doctors don't have a clue what you are talking about.

    Time to have a little yellow book, "MRIs for Idiots" with all this explained, and it can be quite easily.
    Last edited: Nov 20, 2014
  14. Nov 20, 2014 #13


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    Depends on what you mean by "better".

    Given that CT scans expose patients to (sometimes very) large radiation doses, if you can use an MRI you should. CT scans are also sensitive to different things to MRIs. And stuff like fMRI can't really be done with CT.

    (Also I don't know how these new 3+ T magnets stack up against CT)
  15. Nov 20, 2014 #14
    Reich, from University of Wisconsin, has on line an excellent summary of nuclei with spin and their relative abundance. I can't make this IPAD copy the link. Sorry.
  16. Nov 20, 2014 #15


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