MR imaging: deconstructing timing diagrams and demystifying k-space

Magnetic resonance imaging (MRI) works on the principle that hydrogen molecules, which are abundant in organic tissue, have a magnetic moment arising from the spin of the protons in the nucleus. All atoms consist of a nucleus made of protons and neutrons. When a sample is put in a large magnet field, the hydrogen atoms become magnetized resulting in a bulk magnetization of the sample. Each of these hydrogen atoms acts like a bar magnet, spinning at a frequency about the applied main magnetic field. The frequency of spin is proportional to the applied main field and hence to encode position, we apply an additive field that increases linearly with position in a given direction. Hence, the spins in that direction will precess at a linearly increasing frequency and can be resolved by matching each resolvable frequency bin to a given position. This allows one direction to be resolved. By repeating the same procedure for the other dimension, a 2D image can be resolved by averaging over the third dimension.