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In what proton MRI is focusing ?

  1. May 4, 2013 #1
    Hello all,

    I know that MRI is focuse on Hydrogen proton,

    but I don't understand if it focuse just in the Hydrogen proton

    that in water (Liquids) of the body or in all the Hydrogen proton

    that are within the molecules in the body.

    In relation to this, how the MRI could distinguish between tissues-he doing this

    While focusing on their environment of the protons?

    Thanks
     
  2. jcsd
  3. May 4, 2013 #2

    Dale

    Staff: Mentor

    All of them, not just the ones in water. In fact, one of the primary challenges in clinical MRI is the suppression of signal from hydrogen atoms in lipids (fat), not water.


    The resonance frequency of the hydrogen nucleus is slightly shifted depending on it's environment.
     
  4. May 5, 2013 #3
    Thanks,

    Maybe you could elaborate a little more about this process-
    We can distinguish between tissues because the shifted in resonance frequency of the hydrogen nucleus that cause because of their environment?
     
  5. May 10, 2013 #4
    No one knows?
     
  6. May 10, 2013 #5

    Dale

    Staff: Mentor

    A hydrogen nucleus resonates at 42.58 MHz/T. So the question is what field does the hydrogen see? If it is a free proton then it is not shielded from the external field, but if it is bound then there are some electrons nearby. Those electrons react to the external field in a way which shields the hydrogen a small amount (a few ppm). This changes the field seen by the nucleus and therefore shifts the resonant frequency.
     
  7. May 10, 2013 #6
    Thank. Let's look at it from the perspective of a radio pulse sent to the tissue.
    It give to the proton energy changing the position of the protons.
    Now, when they make relaxation, They emit energy that received differently because of the different environment of electrons?
     
  8. May 10, 2013 #7

    Dale

    Staff: Mentor

    Yes. Although I wouldn't say changing position so much as changing orientation.
     
  9. May 10, 2013 #8
    Of course, it Effect Zeeman (English is not my native language so I'm kinda having trouble translating, sorry
    about that).
    What I don't understand is the relationship between the external magnetic field, protons and the radio pulse-
    The radio pulse is in the frequency of the hydrogen nucleus ( 42.58 MHz/T), but if we talk about pulse, then
    how can we talk about frequency???
     
  10. May 10, 2013 #9

    Dale

    Staff: Mentor

    A RF pulse typically lasts from about .5 ms to 5 ms. At typical clinical MRI frequencies that is anywhere from 32,000 to 640,000 cycles. The term "RF pulse" does not imply such a brief duration that it does not have a well-defined frequency.
     
  11. May 10, 2013 #10
    Ok, and I understand that the frequency of this pulse is 42.58 MHz/T, but the pulse must be with Frequency Range
    Because proton frequencies are slightly different from each other depending on their environment (although the intensity of the magnetic field is the same) - Otherwise he would not be absorbed by all the protons in the tissue that being scanned? (I include here the use of the gradient- that creates the difference between each point )
     
  12. May 10, 2013 #11

    Dale

    Staff: Mentor

    Yes, that is called the bandwidth. The center frequency and the bandwidth are selected so as to resonate with the protons of interest, especially in the presence of a field gradient.
     
  13. May 10, 2013 #12
    All this is calculated by the Larmor formula when inside taken into account, among other things, the external magnetic field strength and gradient strength may also-but how is take into account the environmental changes that affect the frequency of each proton and proton?
     
  14. May 10, 2013 #13

    Dale

    Staff: Mentor

    There are essentially two ways to do that. Either you can turn off the gradients so that the chemical shift is the only major effect or you can vary the gradients during your RF pulse so as to generate a desired spatial and spectral excitation.
     
  15. May 11, 2013 #14
    Ok, but now you are talking about giving the signal during the relaxation, am I right?

    I understand that the molecule in which the proton of the hydrogen, ie environment, more big - it will be different Signal relatively small molecule , such as water molecules?
     
  16. May 11, 2013 #15

    Dale

    Staff: Mentor

    In all of my comments above I was talking only about frequency shift, not changes in relaxation. We can certainly discuss relaxation if you want, but it is a little more complicated than frequency.
     
  17. May 11, 2013 #16
    I understood the physics behind NMR is similar like the physics behind the MRI.

    Is relaxation underlies more to mri then for NMR? What is the difference in a few words

    Between NMR and MRI from the Physics view?
     
  18. May 11, 2013 #17

    Dale

    Staff: Mentor

    The physics is the same. However, in MRI relaxation provides valuable clinical information while in NMR relaxation typically is not of interest and serves only to broaden the spectral peaks.

    Gradients.
     
  19. May 15, 2013 #18
    Hello again, I wanted to ask you about Pulse 90 and pulse 180 in MRI - I understand that the role of the 90 pulse is to separated the longitudinal magnetic of the proton from the external magnetic and this allowing measurement. Pulse also creates a common occurrence between the protons. Now we send a pulse of 180, which makes protons to change direction and save the proton consolidation (I do not know how this expression is called in English) - Is pulse 180 associated only T2 image?How?
     
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