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?