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I MRI physics question

  1. Mar 13, 2018 #1
    If this belongs to the high energy particle physics subforum please move it there.

    So since I have to take an MRI, I am curious to know how it works, I think I know the basics but the information about it out there is rather confusing.

    So first off here is what I know. A large cylindrical magnet submerged in liquid helium to achieve it's superconducting state makes a uniform B field of high strength (1.5 or 3T) where the field lines are parallel to the axial direction of the cylinder (also the direction of human body from head to foot) , now here comes the difficult bit, as the human body is inside the MRI cylinder the strong B field aligns the proton spins parallel and some antiparallel to the field lines (as I assume that is their state of lowest energy being parallel to the external field.

    Now there must be a set of different coils similar to the deflection coils in a crt tube located on the inner side of the MRI cylinder which when energized set up a B field that is perpendicular to the main external field, and I suppose yet another third set of coils similar to the second that make yet another b field which is now perpendicular to the second field? I assume these secondary field strengths are much lower than the primary axial B field?

    I also understand from what I've read is that these secondary fields are there to change the alignment of the protons for a while and as those fields are switched off only the main field stays on and so the protons now want to get back to their lowest energy state and align themselves like before and as they do they emit a certain frequency wave which is then received by coils inside the MRI and the frequency is this signal can be differentiated and interpreted as either brighter or darker pixels on a screen which then illustrates different fluids and solids in a body which have different chemical properties.

    Here is another thing I don't quite understand, how can the apparatus differentiate between protons in a belly fat and protons in say spine discs or bone matter? Because as far as I know all photons have the same properties like charge and spin and mass, so why would the protons in fat respond differently than the protons in bone matter if they are all in the same strength B field?

    Also could someone please explain the proton gyroscopic precession because in many videos the explanation involves saying that the proton physically spins about it's axis but I think that is a misconception isn't it?

    this video also talks about a spinning proton which I believe is wrong?

    So if the proton doesn't spin but it's spin is basically it's ,magnetic moment which is a property of it without it physically spinning then how does the precession takes place? does it simply wobbles around it's axis when influenced by external b fields without physically spinning around it's axis like a planet would do?
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  3. Mar 13, 2018 #2


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    As I understand it, the decay time of the resonance is different for different elements and compounds so you can tell what was in the section that the MRI excited.
  4. Mar 13, 2018 #3
    I think what is important is that they are pulse-like (short-lived) impulse fields which "tilt"/bring the spins out of equilibrium.
    Again I am no specialist in this (so maybe someone else can add to this), but I believe there are a number of different relaxation-type times which can be used to image a variety of compounds. The spin-lattice relaxation time (T1) for instance has a large response for fat.
    All of these spins are interacting generally with each other (spin-spin interactions) as well as with the "medium" (spin-lattice interactions) and the resulting signal depends on these interactions and the structure of the material etc.
  5. Mar 13, 2018 #4
    There is a classical analogy you can draw. It is an analogy which one shouldn't take too literally, but it still gives some insight. I remember seeing this video series a good while ago and enjoying it (it covers the analogy):

    I can't remember if it mentions what it happening quantum mechanically, but we can get into that later if you want.
  6. Mar 13, 2018 #5


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    There are many different procedures that can be used, but one of the most common, which has been vaguely described, is to induce a 90-degree flip of the magnetization. This will result in a spin-lattice relaxation time, whereby the bulk magnetization will decay back to its equilibrium value.

    The reason why this is useful is that the decay rate for this process depends on the surrounding environment (the "lattice"). So different types of tissues will have different types of environment, so the decay rate will be different. This will allow the signal to be resolved for different types of tissues.

    The other thing that is often done is that the external, static magnetic field may also have a spatial gradient, for example, along the length of the body. This results in a slight variation of the resonance frequency along different cross-section of the body. It allows for the operator to looks at various transverse slices of the body by picking up only the signal at a particular resonance frequency.

  7. Mar 13, 2018 #6
    thanks folks for replying, well yes I had read about the relaxation phenomenon which is basically the time it takes for the proton to get back to the state of lowest energy and align with the large axial b field after it has been tilted by 90degrees with the help of the smaller high frequency B field coming from the secondary coils, correct?
    But I read they speak about T1 and T2 relaxation times, well T1 is the time I just described but then what is T2? Is that another tilting yet in another direction to achieve some better radio frequency resolution of the picture?

    @ZapperZ , so the axial large magnet coil has a slightly varying strength along it axis so the relaxation times differ slightly from leg tissue to say head tissue and so one can slice up the body with a certain amount of slices much like higher bandwidth digital music track has more sound resolution than a lower one since it is sliced into more smaller parts?

    Now I still don't get fully the "different tissue different proton relaxation times" fact, now we know that all protons are the same, so normally the whole body would come out as one dark or light blob on the screen but do you say that the nucleus in different fluids or tissue have different number of protons in the nucleus and the number of protons a given nucleus has impacts the time it takes for the protons to "relax" in that given nucleus with respect to another nucleus in other tissue given that both are screened in the same strength b field with the same frequency secondary field?

    BTW, and I really want to understand this, so what's with the proton or electron for that matter spin? Is it just an old word that got stuck because at first when we discovered that these particles have charge and also magnetic moment we thought that they spin like little gyroscopes and that causes the magnetic moment and later found out that actually don't physically spin but simply have a magnetic moment as an inherent property much like charge?
    Because if they dont physically spin that means they are simply like tiny bar magnets and they then align with the external b field (explains why metal can be magnetized in the presence of a b field) but this then begs the question once a secondary b field changes their position and is then switched off the protons process back to their previous state of lower energy but while doing so they behave like gyros which I assume is the process by which they emit characteristic RF by which they are then "seen" by the MRI?
    Anyway what I'm asking is do they spin or not and if not then why would they process like a gyro upon such interaction because taking the analogy of a bar magnet it too can be aligned with an external b field and then its position changed but as it goes back to its previous lower energy state it does not rotate or process it simply flips back and that's it... this makes me confused.
  8. Mar 13, 2018 #7


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    The field produced by the gradient coils points in the same direction of the main magnetic field. Their job is not to change the direction of the field but to change its strength. Since the resonant (Larmor) frequency is proportional to the field strength, changing the field strength with the gradients changes the frequency.
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