Hi, I'm taking a course on medical imaging (mainly MRI), and I'm having trouble understanding NMR. Can anyone explain what this is all about in plain language? Here is what I make of it: Odd-numbered nuclei possesses a total spin of magnitude 1/2 and an associated magnetic moment (why is that, is there a classical analogue?), normally the nuclei is distributed nearly equally between spin-up and spin-down states, but this can be inverted by applying a time-varying RF-field (variying with the Larmor-frequency, why?). When the nuclei returns to their equilibrium states they emit radio waves, which can be measured and by analyzing the time from irradiation to emmision one can somehow infer something about the chemical enviroment of the nuclei in question (i.e. the type of tissue in the case of MRI). But how is this done and what is it precisely we are measuring? Also, I don't understand why the largest RF-signal is generated by a 90° pulse, since the energyshift for 180° pulse is bigger? Thanks.